JP7631611B1 - Zirconia sintered body and method for producing the same - Google Patents

Abstract

The present invention provides a zirconia sintered body having sufficient mechanical strength and excellent translucency and transparency.
[Solution] A zirconia _sintered body comprising stabilized zirconia containing zirconia and a stabilizer, the stabilizer containing _Y2O3 , the content of _Y2O3 in the stabilized _zirconia being more than 4.0 mol% and not more than 6.5 mol% in terms of oxide, the average crystal grain size being 100 nm or more and not more than 200 nm, the monoclinic phase fraction being 0.5% or less, the tetragonal phase fraction being 20.0% or more and not more than 96.0%, the cubic phase fraction being 4.0% or more and not more than 80.0%, and the relative sintered density being 99.6% or more.
[Selection diagram] None

Description

The present invention relates to a zirconia sintered body and a method for producing a zirconia sintered body.

Traditionally, efforts have been made to improve the mechanical strength, transparency (parallel light transmittance), and translucency (total light transmittance) of zirconia sintered bodies.

For example, Patent Document 1 discloses a translucent yttria-containing zirconia sintered body made of zirconia containing more than 4 mol% and not more than 7 mol% yttria, with a sintered body grain size of 2.0 μm or less, a relative density of 99.5% or more, and a total light transmittance of 40% or more for visible light with a wavelength of 600 nm at a thickness of 1 mm (Claim 1). It also discloses that the zirconia sintered body of this configuration achieves high translucency and strength (paragraph [0011]).

JP 2008-222450 A

However, in Patent Document 1, the crystal grain size is set to 0.4 μm or more (paragraph [0016]), and it is presumed that the zirconia sintered body in Patent Document 1 does have a certain degree of translucency and strength, but it cannot be said to have transparency.

Thus, while sintered bodies that combine mechanical strength with transparency or translucency have been obtained in the past, no zirconia sintered body has been obtained that combines all of mechanical strength, transparency, and translucency.

The present invention has been made in consideration of the above-mentioned problems, and its object is to provide a zirconia sintered body that has sufficient mechanical strength and is excellent in translucency and transparency. It is also an object of the present invention to provide a method for producing the zirconia sintered body.

The inventors conducted extensive research into the above-mentioned problems. As a result, they discovered that the above-mentioned problems could be solved by adopting the following configuration, and thus completed the present invention.

The present invention provides the following:
[1] A stabilized zirconia containing zirconia and a stabilizer,
The _stabilizer comprises _Y2O3 ,
The content of the Y ₂ O ₃ in the stabilized zirconia is more than 4.0 mol % and not more than 6.5 mol % in terms of oxide,
The average crystal grain size is 100 nm or more and 200 nm or less,
The monoclinic fraction is 0.5% or less,
The tetragonal fraction is 20.0% or more and 96.0% or less,
The cubic fraction is 4.0% or more and 80.0% or less,
A zirconia sintered body having a relative sintered density of 99.6% or more.

In order to improve transparency, a method of suppressing scattering at grain boundaries is considered. In order to suppress scattering at grain boundaries, it is effective to reduce the number of grain boundaries per unit thickness, that is, to increase the crystal grain size. However, if the crystal grain size is increased to a level at which transparency is obtained, the mechanical strength is reduced.
Therefore, in the present invention, the average crystal grain size is set to 200 nm or less, and high mechanical strength is maintained while other methods, namely, "scattering at grain boundaries" and "scattering at residual pores", are suppressed to achieve transparency and translucency.
There are two types of "scattering at grain boundaries": scattering at the interface between tetragonal phases/tetragonal phases and scattering at the interface between tetragonal phases/cubic phases. In order to suppress "scattering at grain boundaries", the inventors have conceived of 1. increasing the ratio of the cubic phase, which is an optically isotropic body, and 2. suppressing scattering at the tetragonal phase by reducing the crystal grain size.
Regarding the above "1.", according to the present invention, the content of _{Y2O3 in} the stabilized zirconia exceeds 4.0 mol% in terms of oxide, so that the cubic phase ratio can be 4.0% or more. Since the cubic phase ratio is 4.0% or more, scattering at grain boundaries can be suppressed, _and translucency can be improved. In addition, since the content of _Y2O3 is 6.5 mol% or less in terms of oxide, the mechanical strength is excellent.
Regarding the above "2.", according to the present invention, since the average crystal grain size is 100 nm or more and 200 nm or less, grains having a grain size smaller than the visible light wavelength are present, and the effect of scattering at grain boundaries is reduced. Furthermore, since the cubic phase ratio is 4.0% or more, scattering of the tetragonal phase is suppressed, and transparency in particular is improved.
In addition, in the present invention, since the relative sintered density is 99.6% or more, "scattering due to residual pores" is suppressed, and high light transmittance and transparency are obtained.
In addition, when the content _{of Y2O3} is in the range of more than 4.0 mol% to 6.5 mol% or less in terms of oxide, the tetragonal phase ratio and the cubic phase ratio are in a trade-off relationship, and the higher _the content of _Y2O3 , the higher the cubic phase ratio but the lower the tetragonal phase _ratio . The lower the content of _Y2O3 , the higher the tetragonal phase ratio but the lower the cubic phase ratio.
In addition, since the content of the Y ₂ O ₃ in the stabilized zirconia exceeds 4.0 mol % in terms of oxide, the monoclinic fraction can be set to 0.5% or less.
The more monoclinic phase is contained, the more the mechanical strength and transparency decrease. On the other hand, according to the present invention, the monoclinic phase ratio is 0.5% or less, and therefore the mechanical strength and transparency are excellent.

As a result, the above-mentioned configuration makes it possible to provide a zirconia sintered body that has sufficient mechanical strength and excellent translucency and transparency.

Furthermore, the present invention provides the following:
[2] The zirconia sintered body according to [1], characterized in that the content of the Y ₂ O ₃ in the stabilized zirconia is more than 5.2 mol% in terms of oxide.

When the content of the Y ₂ O ₃ in the stabilized zirconia exceeds 5.2 mol % in terms of oxide, the cubic phase ratio is easily set to 40.0% or more.

Furthermore, the present invention provides the following:
[3] The tetragonal fraction is 20.0% or more and 60.0% or less,
The zirconia sintered body according to [1], characterized in that the cubic phase ratio is 40.0% or more and 80.0% or less.

When the cubic phase ratio is 40.0% or more, scattering at grain boundaries can be further suppressed, and translucency can be further improved.

Furthermore, the present invention provides the following:
[4] The content of the Y ₂ O ₃ in the stabilized zirconia is more than 5.2 mol% in terms of oxide;
The tetragonal fraction is 20.0% or more and 60.0% or less,
The zirconia sintered body according to [1], characterized in that the cubic phase ratio is 40.0% or more and 80.0% or less.

When the content of the Y ₂ O ₃ in the stabilized zirconia exceeds 5.2 mol % in terms of oxide, the cubic phase ratio is easily set to 40.0% or more.
Furthermore, if the cubic fraction is 40.0% or more, scattering at grain boundaries can be further suppressed, and light transmittance can be further improved.

Furthermore, the present invention provides the following:
[5] The zirconia sintered body according to any one of [1] to [4], characterized in that it contains Al ₂ O ₃ in the range of 0.15 mass% or less based on the entire zirconia sintered body.

When Al ₂ O ₃ is contained in the entire zirconia sintered body, it functions as a sintering aid, and has excellent low-temperature sintering properties. In addition, if the content of Al ₂ O ₃ is 0.15 mass% or less, scattering due to the precipitation phase (grain boundary) of alumina (impurity) is reduced, so that high translucency and transparency can be maintained.

Furthermore, the present invention provides the following:
[6] The zirconia sintered body according to any one of [1] to [5], characterized in that the total light transmittance at a thickness of 1 mm is 45% or more and 60% or less.

If the total light transmittance at a thickness of 1 mm is 45% or more, it can be said to have excellent translucency.

Furthermore, the present invention provides the following:
[7] The zirconia sintered body according to any one of [1] to [6], characterized in that the parallel light transmittance at a thickness of 1 mm is 7.0% or more and 30.0% or less.

If the parallel light transmittance at a thickness of 1 mm is 7.0% or more, it can be said to have excellent transparency.

Furthermore, the present invention provides the following:
[8] The zirconia sintered body according to any one of [1] to [7], characterized in that the three-point bending strength is 500 MPa or more and 1000 MPa or less.

If the three-point bending strength is 500 MPa or more, it can be said to have high strength.

Furthermore, the present invention provides the following:
[9] The zirconia sintered body according to any one of [1] to [8], characterized in that the toughness value measured by an IF method is 2.7 MPa m ^0.5 or more and 5.0 MPa m ^0.5 or less.

When the toughness value measured by the IF method is 2.7 MPa·m ^0.5 or more, it can be said to have high toughness.

Furthermore, the present invention provides the following:
[10] A process A of molding a zirconia powder at a pressure of 1.0 t/cm2 or ^more and 2.5 t/cm2 or ^less to obtain a molded body;
A step B of pre-sintering the molded body under normal pressure at a temperature of 1150° C. or higher and 1250° C. or lower for 1 hour or longer and 5 hours or shorter to obtain a pre-sintered body;
The method for producing a zirconia sintered body according to any one of [1] to [9], further comprising a step C of sintering the pre-sintered body under conditions of a pressure of 50 MPa to 200 MPa, 1150 ° C. to 1250 ° C., and 1 hour to 5 hours to obtain a sintered body.

According to the above configuration, the molded body is pre-sintered under relatively low temperature conditions of 1150°C to 1250°C for 1 hour to 5 hours under normal pressure, so that a pre-sintered body with suppressed grain growth can be obtained. Since grain growth is suppressed, it is possible to obtain a highly transparent zirconia sintered body.

According to the above-mentioned configuration, the molded body is pre-sintered under normal pressure at 1150°C to 1250°C for 1 hour to 5 hours, so that the particles of the zirconia powder are appropriately bonded together during the pre-sintering process, and the pores (gaps between the particles of the zirconia powder) in the molded body can be made into closed pores. In other words, the pre-sintering can change the pores from a state in which the pores are connected together (open pores) to isolated closed pores surrounded by bonds of the zirconia powder.
The temporary sintered body, whose pores are closed pores, is then sintered under conditions of a pressure of 50 MPa to 200 MPa, 1150°C to 1250°C, and 1 hour to 5 hours, so that the closed pores can be almost completely eliminated. Specifically, the pressure applied during the sintering process compresses the closed pores to make them as small as possible, and the element diffusion (element flow) during the sintering process pushes the pores out of the sintered body, so that the closed pores can be almost completely eliminated. By removing the pores, the total light transmittance (translucency) and the parallel light transmittance (transparency) can be improved.
If the pores in the provisionally sintered body are not closed pores, the pores cannot be removed even if the main sintering is performed under conditions of a pressure of 50 MPa to 200 MPa, a temperature of 1150° C. to 1250° C., and a time of 1 hour to 5 hours. This is because if the pores in the provisionally sintered body are not closed pores, pressure is not easily transmitted to the pores during the main sintering.

Whether or not the pores in the provisionally sintered body are closed can be determined by whether or not the relative sintered density can be measured using the Archimedes method. If the pores are open, the pores will absorb water during measurement, making it impossible to measure the relative sintered density (resulting in an abnormal measurement). In other words, if the relative sintered density can be measured (the value is within the expected range), it can be determined that the pores are closed.

In addition, according to the above configuration, the pre-sintered body is sintered under conditions of a pressure of 50 MPa to 200 MPa, 1150°C to 1250°C, and 1 hour to 5 hours, so the relative sintered density can be 99.6% or more. However, if the molded body is sintered directly without forming a pre-sintered body, that is, if step C is performed after step A without performing step B, it is not possible to achieve a relative sintered density of 99.6% or more while maintaining an average crystal grain size of 100 to 200 nm.

In this specification, the term "preliminary sintered body" refers to a sintered body with a relative sintered density of 92% or more and less than 99.5%. In this specification, the term "preliminary sintering" refers to sintering so that the relative sintered density of the sintered body is in the range of 92% or more and less than 99.5%.

According to the present invention, it is possible to provide a zirconia sintered body containing Y ₂ O ₃ as a stabilizer, which has sufficient mechanical strength and is excellent in translucency and transparency. It is also possible to provide a method for producing the zirconia sintered body.

FIG. 2 is a schematic diagram for explaining a method for producing a zirconia powder according to the present embodiment. FIG. 2 is a schematic diagram for explaining an indentation length and a crack length.

The following describes embodiments of the present invention. However, the present invention is not limited to these embodiments. In this specification, zirconia (zirconium oxide) is a general term that includes 10% by mass or less of impurity metal compounds, including hafnia. In this specification, the expressions "contain" and "include" include the concepts of "contain", "include", "consist essentially of" and "consist only of".

The maximum and minimum values of the content of each component shown below are independently the preferred minimum and maximum values of the present invention, regardless of the contents of other components.
Furthermore, the maximum and minimum values of various parameters (measured values, etc.) shown below are independently preferred minimum and maximum values of the present invention, regardless of the content (composition) of each component.

[Zirconia sintered body]
Hereinafter, an example of the zirconia sintered body according to the present embodiment will be described. However, the zirconia sintered body of the present invention is not limited to the following example.

The zirconia sintered body according to this embodiment is
The stabilized zirconia includes zirconia and a stabilizer.
The _stabilizer comprises _Y2O3 ,
The content of the Y ₂ O ₃ in the stabilized zirconia is more than 4.0 mol % and not more than 6.5 mol % in terms of oxide,
The average crystal grain size is 100 nm or more and 200 nm or less,
The monoclinic fraction is 0.5% or less,
The tetragonal fraction is 20.0% or more and 96.0% or less,
The cubic fraction is 4.0% or more and 80.0% or less,
The relative sintered density is 99.6% or more.

As described above, the zirconia sintered body according to this embodiment contains stabilized zirconia.

The content of the stabilized zirconia is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more, when the entire zirconia sintered body is taken as 100% by mass. The content of the stabilized zirconia can be 99% by mass or less, 98% by mass or less, etc., when the entire zirconia sintered body is taken as 100% by mass. In addition, the zirconia sintered body may be composed of only the stabilized zirconia. In other words, the zirconia sintered body may be composed of only the stabilized zirconia sintered. In this case, the content of the stabilized zirconia is 100% by mass, when the entire zirconia sintered body is taken as 100% by mass.
The content of the stabilized zirconia is preferably 70 mass% or more and 99 mass% or less, more preferably 80 mass% or more and 99 mass% or less, even more preferably 90 mass% or more and 98 mass% or less, and particularly preferably 95 mass% or more and 98 mass% or less, when the entire zirconia sintered body is taken as 100 mass%.

The stabilized zirconia contains zirconia and a stabilizer.

The total content of zirconia and the stabilizer in the stabilized zirconia is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more, when the entire stabilized zirconia is taken as 100% by mass. The total content of zirconia and the stabilizer can be 99% by mass or less, 98% by mass or less, etc., when the entire stabilized zirconia is taken as 100% by mass. The stabilized zirconia may also be composed of only zirconia and the stabilizer. In this case, the total content of zirconia and the stabilizer is 100% by mass, when the entire content of the stabilized zirconia is taken as 100% by mass.
The total content of zirconia and the stabilizer in the stabilized zirconia, when the entire stabilized zirconia is taken as 100 mass%, is preferably 70 mass% or more and 99 mass% or less, more preferably 80 mass% or more and 99 mass% or less, even more preferably 90 mass% or more and 98 mass% or less, and particularly preferably 95 mass% or more and 98 mass% or less.

The _stabilizer comprises _Y2O3 .

The content of the Y ₂ O ₃ in the stabilized zirconia is more than 4.0 mol % and not more than 6.5 mol % in terms of oxide.
Since the content of _Y2O3 in the stabilized _zirconia exceeds 4.0 mol% in terms of oxide, the cubic phase ratio can be 4.0% or more. Since the cubic phase ratio is 4.0% or more, scattering at grain boundaries can be suppressed and translucency can be improved. In addition, since the content _{of Y2O3} is 6.5 mol% or less in terms of oxide, the mechanical strength is excellent.
In addition, when the content _{of Y2O3} is in the range of more than 4.0 mol% to 6.5 mol% or less in terms of oxide, the tetragonal phase ratio and the cubic phase ratio are in a trade-off relationship, and the higher _the content of _Y2O3 , the higher the cubic phase ratio but the lower the tetragonal phase _ratio . The lower the content of _Y2O3 , the higher the tetragonal phase ratio but the lower the cubic phase ratio.
In addition, since the content of the Y ₂ O ₃ in the stabilized zirconia exceeds 4.0 mol % in terms of oxide, the monoclinic fraction can be set to 0.5% or less.
The more monoclinic phase is contained, the lower the mechanical strength and transparency become. On the other hand, the zirconia sintered body has a monoclinic phase ratio of 0.5% or less, and therefore has excellent mechanical strength and transparency.

The content of Y ₂ O ₃ in the stabilized zirconia is preferably more than 5.2 mol %, more preferably 5.3 mol % or more, calculated as oxide.
The content of Y ₂ O ₃ in the stabilized zirconia is preferably 6.4 mol % or less, more preferably 6.3 mol % or less, calculated as oxide.
The content of the Y ₂ O ₃ in the stabilized zirconia is preferably more than 5.2 mol % and not more than 6.4 mol %, more preferably 5.3 mol % or more and not more than 6.3 mol %, calculated as oxide.

The stabilizer may further include another stabilizer other than Y ₂ O _3. The other stabilizer is not particularly limited, and examples thereof include Yb ₂ O ₃ , Er ₂ O ₃ , CeO ₂ , Sc ₂ O ₃ , Nd ₂ O ₃ , La ₂ O ₃ , Tb ₂ O ₃ , CaO, etc. One type or two or more types of these other stabilizers may be included.

When a stabilizer other than Y ₂ O ₃ is contained, the content is preferably within a range that does not significantly affect the transparency and translucency.
When a stabilizer other than Y ₂ O ₃ is contained, the content of the other stabilizer in the stabilized zirconia is preferably 0.01 mol % or more and 2.5 mol % or less in terms of oxide.
The content of the other stabilizer in the stabilized zirconia is more preferably 0.05 mol % or more, and even more preferably 0.10 mol % or more, calculated as oxide.
The content of the other stabilizer in the stabilized zirconia is more preferably 2.0 mol % or less, and even more preferably 1.5 mol % or less, calculated as oxide.
The content of the other stabilizer in the stabilized zirconia is more preferably 0.05 mol % or more and 2.0 mol % or less in terms of oxide, and further preferably 0.10 mol % or more and 1.5 mol % or less in terms of oxide.

The total amount of stabilizers in the stabilized zirconia is preferably 4.1 mol% or more and 6.5 mol% or less in oxide equivalent. When the total amount of stabilizers is 4.1 mol% or more in oxide equivalent, the monoclinic phase ratio can be reduced. Also, when the total amount of stabilizers is 6.5 mol% or less in oxide equivalent, the cubic phase ratio, which has low mechanical properties (strength, toughness), can be reduced, and the tetragonal phase ratio, which has high mechanical properties, can be increased.

The total amount of the stabilizer is more preferably 4.5 mol % or more, further preferably 5.0 mol % or more, and particularly preferably 5.2 mol % or more, calculated as oxide.
The total amount of the stabilizer is more preferably 6.4 mol % or less, further preferably 6.3 mol % or less, particularly preferably 6.0 mol % or less, calculated as oxide.
The total amount of the stabilizer is more preferably 4.5 mol % or more and 6.4 mol % or less, further preferably 5.0 mol % or more and 6.3 mol % or less, particularly preferably 5.2 mol % or more and 6.0 mol % or less, calculated as oxide.

The content of zirconia in the stabilized zirconia is preferably 80% by mass or more and 99% by mass or less. The content of zirconia in the stabilized zirconia is more preferably 85% by mass or more and even more preferably 90% by mass or more. The content of zirconia in the stabilized zirconia is more preferably 98% by mass or less and even more preferably 97% by mass or less.
The content of zirconia in the stabilized zirconia is more preferably 85% by mass or more and 98% by mass or less, and further preferably 90% by mass or more and 97% by mass or less.

The zirconia sintered body preferably contains Al ₂ O ₃ (alumina) in the range of 0.15 mass% or less based on the entire zirconia sintered body. When Al ₂ O ₃ is contained in the entire zirconia sintered body, it functions as a sintering aid, so that it has excellent low-temperature sintering properties. In addition, if the content of Al ₂ O ₃ is 0.15 mass% or less, scattering due to the precipitation phase (grain boundary) of alumina (impurity) is reduced, so that translucency and transparency can be maintained at a high level.

When the zirconia sintered body contains Al ₂ O ₃ , the content of Al ₂ O ₃ is more preferably 0.01 mass% or more, and further preferably 0.05 mass% or more.
When the zirconia sintered body contains Al ₂ O ₃ , the content of Al ₂ O ₃ is more preferably 0.13 mass% or less, and further preferably 0.10 mass% or less.
When the zirconia sintered body contains Al ₂ O ₃ , the content of Al ₂ O ₃ is more preferably 0.01 mass % or more and 0.13 mass % or less, and further preferably 0.05 mass % or more and 0.10 mass % or less.

The zirconia sintered body may contain, in addition to alumina, sinterable ceramics, thermosetting resins, etc., in order to improve properties such as strength.

The zirconia sintered body may contain one or more elements selected from the group consisting of Fe, V, Mn, Co, Zn, Cu, and Ti, within a range that does not significantly affect the transparency and translucency. When the zirconia sintered body contains one or more elements selected from the group consisting of Fe, V, Mn, Co, Zn, Cu, and Ti as a coloring element, the zirconia sintered body can be suitably colored.

The form of the coloring element is not particularly limited, and it can be added in the form of an oxide, a chloride, etc. Specific examples of colorants containing _the coloring element include _Fe2O3 , _V2O5 , _MnO2 , CoO _, ZnO, CuO, TiO2, _etc.

When _Fe2O3 is included as the colorant, the content of the colorant is preferably 0.005% by mass _or more and 1% by mass or less, and more preferably 0.05% by mass or more and 0.5% by mass or less, when the entire zirconia sintered body is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. In other words, it is easy to adjust the color tone.

When _V2O5 is contained as the colorant, the _content is preferably 0.005% by mass or more and 0.5% by mass or less, and more preferably 0.01% by mass or more and 0.1% by mass or less, when the entire zirconia sintered body is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. In other words, it is easy to adjust the color tone.

When _MnO2 is contained as the colorant, the content is preferably 0.005% by mass or more and 2% by mass or less, and more preferably 0.1% by mass or more and 1.1% by mass or less, when the entire zirconia sintered body is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. In other words, it is easy to adjust the color tone.

When CoO is included as the colorant, the content of the colorant is preferably 0.005% by mass or more and 2% by mass or less, and more preferably 0.01% by mass or more and 1.5% by mass or less, when the entire zirconia sintered body is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. In other words, it is easy to adjust the color tone.

When ZnO is included as the colorant, the content of the colorant is preferably 0.005% by mass or more and 1% by mass or less, and more preferably 0.1% by mass or more and 0.5% by mass or less, when the entire zirconia sintered body is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. In other words, it is easy to adjust the color tone.

When CuO is included as the colorant, the content of the colorant is preferably 0.005% by mass or more and 1% by mass or less, and more preferably 0.05% by mass or more and 0.6% by mass or less, when the entire zirconia sintered body is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. In other words, it is easy to adjust the color tone.

When _TiO2 is included as the colorant, the content of the colorant is preferably 0.005% by mass or more and 2% by mass or less, more preferably 0.01% by mass or more and 1% by mass or less, and even more preferably 0.1% by mass or more and 0.3% by mass or less, when the entire zirconia sintered body is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. That is, it is easy to adjust the color tone.

The zirconia sintered body has an average crystal grain size of 100 nm or more and 200 nm or less. Because the average crystal grain size is 100 nm or more and 200 nm or less, there are particles whose grain size is smaller than the visible light wavelength, and the effect of scattering at the grain boundaries is reduced. In addition, as described below, because the cubic phase ratio is 4.0% or more, scattering of the tetragonal phase is suppressed, and transparency in particular is improved.

The average crystal grain size is preferably 195 nm or less, and more preferably 190 nm or less.
The average crystal grain size is preferably 110 nm or more, and more preferably 120 nm or more.
The average crystal grain size is preferably 110 nm or more and 195 nm or less, and more preferably 120 nm or more and 190 nm or less.
The average crystal grain size can be controlled, for example, by adjusting the sintering conditions (particularly the sintering temperature) when sintering the zirconia powder.
The method for determining the average crystal grain size is the method described in the Examples.

The monoclinic phase ratio contained in the crystal phase of the zirconia sintered body is 0.5% or less. The more monoclinic phase is contained, the lower the mechanical strength and transparency. Since the monoclinic phase ratio of the zirconia sintered body is 0.5% or less, it has excellent mechanical strength and transparency.

The monoclinic fraction is preferably 0.3% or less, and more preferably 0.2% or less.
The smaller the monoclinic fraction, the more preferable it is, and for example, 0.05% or more, 0.1% or more.
The monoclinic fraction is preferably 0.05% or more and 0.3% or less, and more preferably 0.1% or more and 0.2% or less.
The monoclinic fraction can be controlled by, for example, the content of a stabilizer (particularly Y ₂ O ₃ ), etc.
The method for determining the monoclinic fraction is the method described in the Examples.

The tetragonal phase ratio contained in the crystal phase of the zirconia sintered body is 20.0% or more and 96.0% or less.
The more the tetragonal phase is contained, the more the mechanical strength is improved. The zirconia sintered body has a tetragonal phase ratio of 20% or more, and therefore has excellent mechanical strength. In addition, the zirconia sintered body has a tetragonal phase ratio of 96.0% or less, and therefore scattering of the tetragonal phase is suppressed, and the transparency in particular is improved.

The tetragonal fraction is preferably 60.0% or less, more preferably 58.0% or less, and even more preferably 55.0% or less.
The tetragonal fraction is preferably 25.0% or more, more preferably 30.0% or more, and even more preferably 32.0% or more.
The tetragonal phase ratio is preferably 25.0% or more and 60.0% or less, more preferably 30.0% or more and 58.0% or less, and further preferably 32.0% or more and 55.0% or less.
The tetragonal phase ratio can be controlled by, for example, the content of a stabilizer (particularly Y ₂ O ₃ ), etc.
The tetragonal fraction is determined by the method described in the Examples.

The cubic phase ratio contained in the crystal phase of the zirconia sintered body is 4.0% or more and 80.0% or less. Since the cubic phase does not have optical anisotropy (has optical isotropy), increasing the cubic phase ratio improves the total light transmittance (translucency). On the other hand, since the cubic phase has the property of being prone to grain growth, if the cubic phase ratio is made too high, the crystal grains will become large and the parallel light transmittance (transparency) will decrease. Since the cubic phase ratio of the zirconia sintered body is 4.0% or more, scattering at grain boundaries can be suppressed and translucency can be improved. In addition, since the cubic phase ratio of the zirconia sintered body is 80.0% or less, grain growth can be suppressed and the parallel light transmittance can be maintained high.

The cubic phase ratio is preferably 75.0% or less, more preferably 70.0% or less, and even more preferably 68.0% or less.
The cubic phase ratio is preferably 40.0% or more, more preferably 42.0% or more, and even more preferably 45.0% or more.
The cubic phase ratio is preferably 40.0% or more and 75.0% or less, more preferably 42.0% or more and 70.0% or less, and even more preferably 45.0% or more and 68.0% or less.
The cubic phase ratio can be controlled by, for example, the content of a stabilizer (particularly Y ₂ O ₃ ), etc.
The cubic phase ratio is determined by the method described in the Examples.

The crystal planes of the zirconia sintered body are unoriented. This is evident from the fact that no orientation can be confirmed in the XRD spectrum of the zirconia sintered body, and from the fact that the manufacturing method of the zirconia sintered body described below does not employ a molding/sintering method that will orient the crystal planes.

<Relative sintered density>
The relative sintered density of the zirconia sintered body is 99.6% or more. Since the relative sintered density of the zirconia sintered body is 99.6% or more, scattering in residual pores is suppressed, and high light transmittance and transparency are obtained. In addition, since the relative sintered density of the zirconia sintered body is 99.6% or more, it can be said to have high strength.
The relative sintered density is preferably 99.7% or more, and more preferably 99.8% or more.
The higher the relative sintered density, the more preferable it is, for example, 99.95% or less, 99.9% or less.
The relative sintered density is preferably 99.7% or more and 99.95% or less, and more preferably 99.8% or more and 99.9% or less.

<Method for measuring relative sintered density of zirconia sintered body>
The relative sintered density is represented by the following formula (1).
Relative sintered density (%) = (sintered density / theoretical sintered density) × 100 (1)
Here, the theoretical sintered density (ρ ₀ ) is a value calculated by the following formula (2-1).
ρ0=100/[(Y/3.987)+(100-Y)/ρz]...(2-1)
Here, ρz is a value calculated by the following formula (2-2).
ρz=[124.25(100−X)+[molecular weight of stabilizer]×X]/[150.5(100+X)A ² C] (2-2)
Here, the molecular weight of the stabilizer is _225.81 for _Y2O3 .
Additionally, X and Y are the stabilizer concentration (mol %) and the alumina concentration (wt %), respectively, and A and C are values calculated by the following formulas (2-3) and (2-4), respectively.
A=0.5080+0.06980X/(100+X)...(2-3)
C=0.5195-0.06180X/(100+X)...(2-4)
In formula (1), the theoretical sintered density varies depending on the powder composition. For example, the theoretical sintered density of yttria-containing zirconia is 6.117 g/ ^cm3 when the yttria content is 2 mol%, 6.098 g/ ^cm3 when the yttria content is 3 mol%, and 6.051 g/ ^cm3 when the _yttria content is 5.5 mol% (when _Al2O3 = 0 wt%).
In addition, the theoretical sintered density (ρ1) when other components (stabilizers, colorants, etc.) other than alumina are included is:
ρ1=100/[(Z/V)+(100-Z)/ρ0]...(2-5)
Furthermore, Z is the concentration (wt %) of components other than alumina, and V is the theoretical density (g/cm ³ ) of the components other than alumina.
The theoretical densities of the components other than alumina are: _Yb2O3 : 9.17 _g / ^cm3 , _Er2O3 : 8.64 g/ ^cm3 , _CeO2 : 7.22 g/ ^cm3 , _{Sc2O3 :} 3.86 g/cm3, _Nd2O3 : 7.24 g/ ^cm3 , _La2O3 : 6.51 _g / ^cm3 , _{Tb2O3 :} 7.81 g/ ^cm3 , _CaO : 3.34 g/ ^cm3 , _Fe2O3 : 5.24 g/ ^cm3 , _ZnO : 5.61 g/ ^cm3 , _MnO2 : 5.03 g/ ^cm3 , CoO: 6.10 g/ ^cm3 , and _{Cr2O3 :} 5.22 g/ ^cm3 _. ³ , _TiO2 at 4.23 g/ ^cm3 , CuO at 6.31 g/ ^cm3 , _{and V2O5} at 3.36 g/ ^cm3 .
The sintered density is measured by the Archimedes method.

<Total light transmittance>
The zirconia sintered body preferably has a total light transmittance of 45% or more and 60% or less at a thickness of 1 mm. When the zirconia sintered body has a total light transmittance of 45% or more at a thickness of 1 mm, it can be said to have excellent translucency.

The total light transmittance is preferably 46.0% or more, and more preferably 48.0% or more.
The higher the total light transmittance, the more preferable it is, and for example, 59.0% or less, 58.0% or less.
The total light transmittance is preferably 46.0% or more and 59.0% or less, and more preferably 48.0% or more and 58.0% or less.
The total light transmittance is determined by the method described in the Examples.

<Parallel light transmittance>
The zirconia sintered body preferably has a parallel light transmittance of 7.0% or more and 30.0% or less at a thickness of 1 mm. When the zirconia sintered body has a parallel light transmittance of 7.0% or more at a thickness of 1 mm, it can be said to have excellent transparency.

The parallel light transmittance is preferably 8.0% or more, and more preferably 10.0% or more.
The higher the parallel light transmittance, the more preferable it is, and for example, 29.0% or less, 28.0% or less.
The parallel light transmittance is preferably 8.0% or more and 29.0% or less, and more preferably 10.0% or more and 28.0% or less.
The parallel light transmittance is determined by the method described in the Examples.

<Mechanical strength>
The zirconia sintered body preferably has a three-point bending strength of 500 MPa or more and 1000 MPa or less.

The three-point bending strength is more preferably 550 MPa or more, and further preferably 600 MPa or more. The higher the three-point bending strength, the more preferable it is, but it can be, for example, 900 MPa or less, 850 MPa or less, etc.
The three-point bending strength is more preferably 550 MPa or more and 900 MPa or less, and further preferably 600 MPa or more and 850 MPa or less.

If the three-point bending strength is 500 MPa or more, it can be said to have higher strength. The three-point bending strength is determined by the method described in the examples.

<Toughness>
The zirconia sintered body preferably has a toughness value measured by an IF method of 2.7 MPa·m ^0.5 or more and 5.0 MPa·m ^0.5 or less.

The toughness value is more preferably 2.8 MPa·m ^0.5 or more, and further preferably 3.0 MPa·m ^0.5 or more. The higher the toughness value, the more preferable it is, but it can be, for example, 4.8 MPa·m ^0.5 or less, 4.6 MPa·m ^0.5 or less, etc.
The toughness value is more preferably 2.8 MPa·m ^0.5 or more and 4.8 MPa·m ^0.5 or less, and further preferably 3.0 MPa·m ^0.5 or more and 4.6 MPa·m ^0.5 or less.

When the toughness value is 2.7 MPa·m ^0.5 or more, it can be said that the toughness is higher. The toughness value is determined by the method described in the Examples.

<Resistance to hydrothermal deterioration>
The zirconia sintered body preferably has a monoclinic fraction of 2.0% or less after hydrothermal treatment at 134°C and 0.3 MPa (absolute pressure 0.3 MPa) for 75 hours. If the monoclinic fraction after the hydrothermal treatment is 2.0% or less, it can be said that the resistance to hydrothermal deterioration is more excellent. The monoclinic fraction after the hydrothermal treatment can be controlled, for example, by the amount _{of Y2O3} added or the average crystal grain size.

The monoclinic fraction after the hydrothermal treatment is more preferably 1.7% or less, and further preferably 1.5% or less. The smaller the monoclinic fraction after the hydrothermal treatment, the more preferable it is, and is, for example, 0.1% or more, 0.2% or more.
The monoclinic fraction after the hydrothermal treatment is more preferably 0.2% or more and 1.7% or less, further preferably 0.1% or more and 1.5% or less, and particularly preferably 0% or more and 1.5% or less.

The zirconia sintered body according to this embodiment is not particularly limited, but can be obtained, for example, by using a zirconia powder described later and a method for producing a zirconia sintered body described later.

The zirconia sintered body according to this embodiment can be used as an industrial part, an aesthetic part, or a dental material. More specifically, it can be used for jewelry, watch parts, watch dials, artificial teeth, molding parts, wear-resistant parts, chemical-resistant parts, etc.

[Zirconia powder]
The zirconia powder according to this embodiment is
The stabilized zirconia includes zirconia and a stabilizer.
The _stabilizer comprises _Y2O3 ,
The content of the Y ₂ O ₃ in the stabilized zirconia is more than 4.0 mol % and not more than 6.5 mol % in terms of oxide.

The zirconia powder according to this embodiment can be used in the manufacturing method of zirconia sintered bodies described below.

The zirconia powder contains primary particles mainly composed of zirconia. All or a part of the primary particles are aggregated to form secondary particles. That is, the zirconia powder contains non-aggregated primary particles and secondary particles formed by aggregation of the primary particles.
However, in the zirconia powder, the amount of primary particles that do not become secondary particles and exist in the form of non-aggregated primary particles is very small, for example, less than 1 mass % of the total primary particles (the total of non-aggregated primary particles and primary particles that have aggregated to become secondary particles). In other words, the zirconia powder may contain a very small amount of non-aggregated primary particles, but the majority is composed of secondary particles.
Incidentally, "zirconia as the main component" means that the primary particles contain 70 mass% or more of zirconia when the primary particles are taken as 100 mass%. That is, in this specification, primary particles containing zirconia as the main component refer to primary particles containing 70 mass% or more of zirconia. The content of zirconia contained in the primary particles is preferably 74 mass% or more, more preferably 80 mass% or more, and even more preferably 85 mass% or more.

As described above, the zirconia powder according to this embodiment contains stabilized zirconia.

The content of the stabilized zirconia is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more, when the entire zirconia powder is taken as 100% by mass. The content of the stabilized zirconia can be 99% by mass or less, 98% by mass or less, etc., when the entire zirconia powder is taken as 100% by mass. In addition, the zirconia powder may be composed only of the stabilized zirconia. In other words, the zirconia powder may be composed of only the stabilized zirconia sintered. In this case, the content of the stabilized zirconia is 100% by mass, when the entire zirconia powder is taken as 100% by mass.
The content of the stabilized zirconia is preferably 70% by mass or more and 99% by mass or less, more preferably 80% by mass or more and 99% by mass or less, even more preferably 90% by mass or more and 98% by mass or less, and particularly preferably 95% by mass or more and 98% by mass or less, when the entire zirconia powder is taken as 100% by mass.

The stabilized zirconia contains zirconia and a stabilizer.

The total content of zirconia and the stabilizer in the stabilized zirconia is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more, when the entire stabilized zirconia is taken as 100% by mass. The total content of zirconia and the stabilizer can be 99% by mass or less, 98% by mass or less, etc., when the entire stabilized zirconia is taken as 100% by mass. The stabilized zirconia may also be composed of only zirconia and the stabilizer. In this case, the total content of zirconia and the stabilizer is 100% by mass, when the entire content of the stabilized zirconia is taken as 100% by mass.
The total content of zirconia and the stabilizer in the stabilized zirconia, when the entire stabilized zirconia is taken as 100 mass%, is preferably 70 mass% or more and 99 mass% or less, more preferably 80 mass% or more and 99 mass% or less, even more preferably 90 mass% or more and 98 mass% or less, and particularly preferably 95 mass% or more and 98 mass% or less.

The _stabilizer comprises _Y2O3 .

The content of the Y ₂ O ₃ in the stabilized zirconia is more than 4.0 mol % and not more than 6.5 mol % in terms of oxide.
Since the content of _Y2O3 in the stabilized zirconia exceeds 4.0 _mol % in terms of oxide, the cubic phase ratio of the sintered body obtained by sintering the zirconia powder can be 4.0% or more. Since the cubic phase ratio of the sintered body obtained by sintering the zirconia powder is 4.0% or more, scattering at grain boundaries can be suppressed and translucency can be improved. In addition, since the content _{of Y2O3} is 6.5 mol% or less in terms of oxide, the sintered body obtained by sintering the zirconia powder has excellent mechanical strength.
In addition, since the content of the Y ₂ O ₃ in the stabilized zirconia exceeds 4.0 mol % in terms of oxide, the monoclinic fraction of the sintered body obtained by sintering the zirconia powder can be made 0.5% or less.
The more monoclinic phase is contained, the lower the mechanical strength and transparency become. On the other hand, the sintered body obtained by sintering the zirconia powder has a monoclinic phase ratio of 0.5% or less, and therefore has excellent mechanical strength and transparency.

The content of Y ₂ O ₃ in the stabilized zirconia is preferably more than 5.2 mol %, more preferably 5.3 mol % or more, calculated as oxide.
The content of Y ₂ O ₃ in the stabilized zirconia is preferably 6.4 mol % or less, more preferably 6.3 mol % or less, calculated as oxide.
The content of the Y ₂ O ₃ in the stabilized zirconia is preferably more than 5.2 mol % and not more than 6.4 mol %, more preferably 5.3 mol % or more and not more than 6.3 mol %, calculated as oxide.

The stabilizer may further include another stabilizer other than Y ₂ O _3. The other stabilizer is not particularly limited, and examples thereof include Yb ₂ O ₃ , Er ₂ O ₃ , CeO ₂ , Sc ₂ O ₃ , Nd ₂ O ₃ , La ₂ O ₃ , Tb ₂ O ₃ , CaO, etc. One type or two or more types of these other stabilizers may be included.

When a stabilizer other than Y ₂ O ₃ is contained, the content is preferably within a range that does not significantly affect the transparency and translucency of the sintered body obtained by sintering the zirconia powder.
When a stabilizer other than Y ₂ O ₃ is contained, the content of the other stabilizer in the stabilized zirconia is preferably 0.01 mol % or more and 2.5 mol % or less in terms of oxide.
The content of the other stabilizer in the stabilized zirconia is more preferably 0.05 mol % or more, and even more preferably 0.10 mol % or more, calculated as oxide.
The content of the other stabilizer in the stabilized zirconia is more preferably 2.0 mol % or less, and even more preferably 1.5 mol % or less, calculated as oxide.
The content of the other stabilizer in the stabilized zirconia is more preferably 0.05 mol % or more and 2.0 mol % or less in terms of oxide, and further preferably 0.10 mol % or more and 1.5 mol % or less in terms of oxide.

The total amount of stabilizers in the stabilized zirconia is preferably 4.1 mol% or more and 6.5 mol% or less in oxide equivalent. When the total amount of stabilizers is 4.1 mol% or more in oxide equivalent, the monoclinic phase ratio can be reduced. Also, when the total amount of stabilizers is 6.5 mol% or less in oxide equivalent, the cubic phase ratio, which has low mechanical properties (strength, toughness), can be reduced, and the tetragonal phase ratio, which has high mechanical properties, can be increased.

The total amount of the stabilizer is more preferably 4.5 mol % or more, further preferably 5.0 mol % or more, and particularly preferably 5.2 mol % or more, calculated as oxide.
The total amount of the stabilizer is more preferably 6.4 mol % or less, further preferably 6.3 mol % or less, particularly preferably 6.0 mol % or less, calculated as oxide.
The total amount of the stabilizer is more preferably 4.5 mol % or more and 6.4 mol % or less, further preferably 5.0 mol % or more and 6.4 mol % or less, particularly preferably 5.2 mol % or more and 6.0 mol % or less, calculated as oxide.

The content of zirconia in the stabilized zirconia is preferably 80% by mass or more and 99% by mass or less. The content of zirconia in the stabilized zirconia is more preferably 85% by mass or more and even more preferably 90% by mass or more. The content of zirconia in the stabilized zirconia is more preferably 98% by mass or less and even more preferably 97% by mass or less.
The content of zirconia in the stabilized zirconia is more preferably 85% by mass or more and 98% by mass or less, and further preferably 90% by mass or more and 97% by mass or less.

The zirconia powder preferably contains Al ₂ O ₃ (alumina) in the range of 0.15 mass% or less based on the entire zirconia powder. When Al ₂ O ₃ is contained in the entire zirconia powder, it functions as a sintering aid, so that it has excellent low-temperature sintering properties. In addition, if the content of Al ₂ O ₃ is 0.15 mass% or less, scattering due to the precipitated phase (grain boundary) of alumina (impurity) in the sintered body obtained by sintering the zirconia powder is reduced, so that the translucency and transparency can be maintained at a high level.

When the zirconia powder contains Al ₂ O ₃ , the content of Al ₂ O ₃ is more preferably 0.01 mass % or more, and further preferably 0.05 mass % or more.
When the zirconia powder contains Al ₂ O ₃ , the content of Al ₂ O ₃ is more preferably 0.13 mass % or less, and further preferably 0.10 mass % or less.
When the zirconia powder contains Al ₂ O ₃ , the content of Al ₂ O ₃ is more preferably 0.01 mass % or more and 0.13 mass % or less, and further preferably 0.05 mass % or more and 0.10 mass % or less.

In addition to alumina, the zirconia powder may contain sinterable ceramics, thermosetting resins, etc., in order to improve properties such as strength.

The zirconia powder may contain one or more elements selected from the group consisting of Fe, V, Mn, Co, Zn, Cu, and Ti, within a range that does not significantly affect transparency and translucency. When the zirconia powder contains one or more elements selected from the group consisting of Fe, V, Mn, Co, Zn, Cu, and Ti as a coloring element, the zirconia sintered body obtained by sintering the zirconia powder can be suitably colored.

The form of the coloring element is not particularly limited, and it can be added in _{the form of oxide, chloride, etc. Specific examples of colorants containing the coloring element include Fe2O3, V2O5 , MnO2 , CoO ,} ZnO, CuO, _TiO2 , etc. The colorant is preferably added to the zirconia powder as a mixture.

When _Fe2O3 is included as the colorant, the content of the colorant is preferably 0.005% by mass _or more and 1% by mass or less, and more preferably 0.05% by mass or more and 0.5% by mass or less, when the entire zirconia powder is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. In other words, it is easy to adjust the color tone.

When _V2O5 is contained as the colorant, _the content is preferably 0.005% by mass or more and 0.5% by mass or less, and more preferably 0.01% by mass or more and 0.1% by mass or less, when the entire zirconia powder is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. In other words, it is easy to adjust the color tone.

When _MnO2 is contained as the colorant, the content is preferably 0.005% by mass or more and 2% by mass or less, and more preferably 0.1% by mass or more and 1.1% by mass or less, when the entire zirconia powder is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. In other words, it is easy to adjust the color tone.

When CoO is included as the colorant, the content of the colorant is preferably 0.005% by mass or more and 2% by mass or less, and more preferably 0.01% by mass or more and 1.5% by mass or less, when the entire zirconia powder is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. In other words, it is easy to adjust the color tone.

When ZnO is included as the colorant, the content of the colorant is preferably 0.005% by mass or more and 1% by mass or less, and more preferably 0.1% by mass or more and 0.5% by mass or less, when the entire zirconia powder is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. In other words, it is easy to adjust the color tone.

When CuO is included as the colorant, the content of the colorant is preferably 0.005% by mass or more and 1% by mass or less, and more preferably 0.05% by mass or more and 0.6% by mass or less, when the entire zirconia powder is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. In other words, it is easy to adjust the color tone.

When _TiO2 is included as the colorant, the content of the colorant is preferably 0.005% by mass or more and 2% by mass or less, more preferably 0.01% by mass or more and 1% by mass or less, and even more preferably 0.1% by mass or more and 0.3% by mass or less, when the entire zirconia powder is taken as 100% by mass. When the content of the colorant is 0.005% by mass or more, it is easy to obtain the intended coloring. That is, it is easy to adjust the color tone.

<Pore distribution>
1. Peak top diameter of gaps between primary particles The zirconia powder preferably has a peak top diameter of pore volume distribution of 20 nm to 85 nm in the range of 10 nm to 200 nm in the pore distribution based on mercury intrusion porosimetry. The peak top diameter is preferably 25 nm or more, more preferably 30 nm, even more preferably 32 nm, and particularly preferably 35 nm or more. The peak top diameter is preferably 65 nm or less, more preferably 60 nm or less, even more preferably 57 nm or less, and particularly preferably 54 nm or less.
In addition, when there are multiple peaks in the range of 10 nm or more and 200 nm or less in the pore distribution, the phrase "the peak top diameter of the pore volume distribution is 20 nm or more and 85 nm or less" in this specification means that all peak top diameters in the range of 10 nm or more and 200 nm or less in the pore distribution are within the range of 20 nm or more and 85 nm or less.

2. Pore distribution width of the gaps between primary particles The zirconia powder preferably has a pore distribution width of 40 nm to 105 nm in the range of 10 nm to 200 nm in the pore distribution based on mercury intrusion porosimetry. The pore distribution width is preferably 43 nm or more, more preferably 46 nm or more, even more preferably 50 nm or more, and particularly preferably 55 nm or more. The pore distribution width is preferably 100 nm or less, more preferably 95 nm or less, even more preferably 90 nm or less, particularly preferably 85 nm or less, and especially preferably 80 nm or less.
Here, the pore distribution width refers to the width of the peak at which the log differential pore volume is 0.1 ml/g or more.
In addition, when there are multiple peaks in the pore distribution in the range of 10 nm or more and 200 nm or less, the term "pore distribution width is 40 nm or more and 105 nm or less" in this specification means that in a graph showing pore distribution with pore diameter on the horizontal axis and log differential pore volume on the vertical axis, the point where the pore diameter first crosses the log differential pore volume of 0.1 mL/g (the point where the pore diameter crosses while ascending) as viewed from the smaller pore diameter side, the point where the pore diameter crosses the log differential pore volume of 0.1 mL/g again (the point where the pore diameter crosses while descending) is taken as the minimum diameter, and the difference between the maximum diameter and the minimum diameter is 40 nm or more and 105 nm or less.

3. Pore volume of the gap between primary particles The zirconia powder preferably has a pore volume of 0.2 ml/g or more and less than 0.5 ml/g in the range of 10 nm or more and 200 nm or less in the pore distribution based on mercury intrusion porosimetry. The total pore volume is preferably 0.22 ml/g or more, more preferably 0.25 ml/g or more, even more preferably 0.3 ml/g or more, particularly preferably 0.35 ml/g or more, and especially preferably 0.4 ml/g or more. The total pore volume is preferably 0.48 ml/g or less, more preferably 0.46 ml/g or less, and particularly preferably 0.44 ml/g or less.

The "range of 10 nm or more and 200 nm or less in the pore distribution based on mercury intrusion porosimetry" is the range in which pores can exist as gaps between primary particles of the zirconia powder.
When the peak top diameter of the pore volume distribution is 20 nm or more and 85 nm or less and the pore distribution width is 40 nm or more and 105 nm or less in the range of 10 nm or more and 200 nm or less in the pore distribution based on mercury intrusion porosimetry, the size of each pore (each gap between primary particles) is small and uniform (the distribution is sharp).
Therefore, the primary particles constituting the secondary particles are uniformly and densely aggregated, and no large pores exist.
Here, zirconia particles (including primary particles and secondary particles) have the characteristic that they are difficult to sinter if their pore volume is large. In other words, in order to sinter at low temperatures, not only must the size of the pores originating from the gaps between primary particles in the secondary particles be small and have a sharp distribution, but the pore volume originating from the gaps between primary particles must also be small at the same time.
Therefore, when the pore volume in the range of 10 nm or more and 200 nm or less in the pore distribution based on mercury intrusion porosimetry is 0.2 ml/g or more and less than 0.5 ml/g, the pore volume originating from the gaps between primary particles is small and a configuration is obtained in which no large pores exist, making it possible to obtain a sintered body with a high sintered density.

<Particle diameter _D50 >
The particle diameter D ₅₀ of the zirconia powder is preferably 0.1 μm or more and 0.7 μm or less. The particle diameter D ₅₀ is preferably 0.12 μm or more, more preferably 0.14 μm or more, even more preferably 0.16 μm or more, and particularly preferably 0.2 μm or more. The particle diameter D ₅₀ is preferably 0.62 μm or less, more preferably 0.55 μm or less, even more preferably 0.48 μm or less, particularly preferably 0.4 μm or less, especially preferably 0.3 μm or less, and especially preferably less than 0.3 μm.
When the particle diameter _D50 of the zirconia powder is 0.7 μm or less, the particle diameter of the secondary particles is relatively small, so that the gap between the secondary particles can be made small. As a result, the low-temperature sintering property is excellent. In addition, since the gap between the secondary particles is small, a sintered body with a high sintering density can be obtained.

<Specific surface area>
The specific surface area of the zirconia powder is preferably 20 m ² /g or more and 60 m ² /g or less. The specific surface area is preferably 22 m ² /g or more, more preferably 24 m ² /g or more, even more preferably 30 m ² /g or more, and particularly preferably 35 m ² /g or more. The specific surface area is preferably 57 m ² /g or less, more preferably 54 m ² /g, even more preferably 52 m ² /g, and particularly preferably 49 m ² /g.

The zirconia powder preferably has a relative molded density of 40% or more and 55% or less when molded at a pressure of 2.0 t/ ^cm2 using cold isostatic pressing (CIP).
Here, the relative molding density is a value calculated by the following formula (3).
Relative molded density (%) = (molded density / theoretical sintered density) × 100 (3)
The theoretical sintered density (ρ ₀ ) is a value calculated by the formula (2-1) described in the above section "Method for measuring the relative sintered density of a zirconia sintered body."
When molding is performed at a pressure of 2.0 t/ ^cm2 using cold isostatic pressing, the relative molded density can easily be set to 40% or more and 55% or less. Therefore, by molding the zirconia powder at a pressure of 1.0 t/cm2 or ^more and 2.5 t/cm2 or less, and then subjecting the molded body to preliminary sintering in step B described ^below and main sintering in step C described below, it is possible to obtain a zirconia sintered body having an average crystal grain size of 100 nm or more and 200 nm or less, a monoclinic fraction of 0.5% or less, a tetragonal fraction of 20.0% or more and 96.0% or less, a cubic fraction of 4.0% or more and 80.0% or less, and a relative sintered density of 99.6% or more.

The relative molding density is more preferably 43.0% or more, and further preferably 45.0% or more.
The relative molding density is more preferably 50.0% or less, and further preferably 48.0% or less.
The relative molding density is more preferably 40% or more and 48% or less, further preferably 43.0% or more and 50.0% or less, and particularly preferably 45.0% or more and 48.0% or less.
The relative molding density refers to a value obtained by the method described in the Examples.

The above describes the zirconia powder according to this embodiment.

[Method of manufacturing zirconia powder]
An example of a method for producing a zirconia powder will be described below, however, the method for producing a zirconia powder is not limited to the following example.

The method for producing the zirconia powder according to this embodiment is as follows:
Step 1: heating the zirconium salt solution and the sulfating agent solution separately to 95° C. or higher and 100° C. or lower;
a step 2 of contacting the heated zirconium salt solution with the heated sulfating agent solution so that the concentration of the mixed solution does not change from the start to the end of the contact, thereby obtaining a basic zirconium sulfate-containing reaction solution as a mixed solution;
Step 3: aging the reaction solution containing basic zirconium sulfate obtained in step 2 at 95° C. or higher for 3 hours or more;
A step 4 of adding a stabilizer to the basic zirconium sulfate-containing reaction liquid after aging obtained in the step 3;
Step 5: Adding an alkali to the basic zirconium sulfate-containing reaction liquid obtained in step 4 to obtain a zirconium-containing hydroxide;
Step 6: heat-treating the zirconium-containing hydroxide obtained in step 5 to obtain zirconia powder.
Including,
In the step 2, from the start to the end of the contact, the SO ₄ ²⁻ /ZrO ₂ weight ratio in the mixed liquid is maintained in the range of 0.3 to 0.8, and the temperature of the mixed liquid is maintained at 95° C. or higher.
Each step will be described in detail below.

<Step 1>
In step 1, the zirconium salt solution and the sulfating agent solution, which are starting materials, are each heated separately to a temperature of 95° C. or higher and 100° C. or lower.
The zirconium salt used to prepare the zirconium salt solution may be any salt that supplies zirconium ions, and may be, for example, zirconium oxynitrate, zirconium oxychloride, zirconium nitrate, etc. These may be used alone or in combination of two or more. Among these, zirconium oxychloride is preferred in terms of high productivity on an industrial scale.

The solvent used to prepare the zirconium salt solution may be selected according to the type of zirconium salt, etc. Usually, water (pure water, ion-exchanged water, the same applies below) is preferred.

The concentration of the zirconium salt solution is not particularly limited, but in general, it is preferable that the zirconium salt solution contains 5 to 250 g, and more preferably 20 to 150 g, calculated as zirconium oxide (ZrO ₂ ) per 1000 g of solvent.

The sulfating agent may be any agent that reacts with zirconium ions to produce sulfates (i.e., a sulfating agent), and examples thereof include sodium sulfate, potassium sulfate, ammonium sulfate, potassium hydrogen sulfate, sodium hydrogen sulfate, potassium disulfate, sodium disulfate, and sulfur trioxide. The sulfating agent may be in any form, such as a powder or a solution, but a solution (particularly an aqueous solution) is preferred. The solvent may be the same as that used to prepare the zirconium salt solution.

The acid concentration of the zirconium salt solution is preferably 0.1 to 2.0 N. By setting the acid concentration within the above range, the aggregation state of the particles that make up the zirconia powder can be controlled to a suitable state. The acid concentration can be adjusted, for example, by using hydrochloric acid, nitric acid, sodium hydroxide, etc.

The concentration of the sulfating agent (sulfating agent solution) is not particularly limited, but it is generally preferable to use 5 to 250 g, and particularly 20 to 150 g, of the sulfating agent per 1000 g of solvent.

The materials of the containers for preparing the zirconium salt solution and the sulfating agent solution are not particularly limited as long as they have a capacity capable of sufficiently stirring the zirconium salt solution and the sulfating agent solution, respectively. However, it is preferable that the containers have a heating device capable of appropriately heating the solutions so that the temperatures of the solutions do not fall below 95° C.
The heating temperature of the zirconium salt solution and the sulfating agent solution may be from 95° C. to 100° C., and is preferably at least 97° C. If step 2 is performed while the temperatures of the zirconium salt solution and the sulfating agent solution are less than 95° C., the zirconium salt solution and the sulfating agent do not react sufficiently, resulting in a reduced yield.

<Step 2>
In step 2, the heated zirconium salt solution and the heated sulfating agent solution are brought into contact with each other so that the concentration of the mixed solution does not change from the start to the end of the contact, thereby obtaining a basic zirconium sulfate-containing reaction solution as a mixed solution. Here, from the start to the end of the contact, the SO ₄ ^2- /ZrO ₂ weight ratio in the mixed solution is maintained in the range of 0.3 to 0.8, and the temperature of the mixed solution is maintained at 95° C. or higher.
Step 2 will now be described with reference to the drawings.

Fig. 1 is a schematic diagram for explaining the method for producing zirconia powder according to the present embodiment. As shown in Fig. 1, a container 10 is connected to one end (left side in Fig. 1) above a T-shaped pipe 20 via a valve 12. A container 30 is connected to the other end (right side in Fig. 1) above the T-shaped pipe 20 via a valve 32. A zirconium solution heated to 95°C or more and 100°C or less is stored in the container 10. A sulfating agent solution heated to 95°C or more and 100°C or less is stored in the container 30.
In step 2, the zirconium solution is contacted with the sulfating agent solution by opening valve 12 and valve 32. The mixed liquid (basic zirconium sulfate-containing reaction liquid) obtained by the contact immediately flows into the aging vessel 40 from the bottom of the T-shaped tube 20. In step 2, this method is used to prevent the concentration of the reaction liquid (concentration of the reaction liquid in the T-shaped tube 20) from changing from the start to the end of the contact between the zirconium solution and the sulfating agent solution. In step 2, the change in concentration of SO ₄ ^2- /ZrO ₂ from the start to the end of the contact is suppressed, so that a uniform reaction product is obtained. By adopting such a step (step 2), the peak top diameter, pore volume, and pore distribution width of the primary particles can be controlled.
The SO ₄ ^2- /ZrO ₂ weight ratio in the mixed solution in step 2 is preferably within a range of 0.3 to 0.8, more preferably 0.4 to 0.7, and even more preferably 0.45 to 0.65. By making the SO ₄ ^2- /ZrO ₂ weight ratio in the mixed solution 0.3 or more, the yield of basic zirconium sulfate, which is the reaction product, can be increased. Furthermore, by making the SO ₄ ^2- /ZrO ₂ weight ratio in the mixed solution 0.8 or less, the generation of a soluble salt of zirconium sulfate can be suppressed, and a decrease in the yield of basic zirconium sulfate can be suppressed.
In step 2, in order to maintain the temperature of the mixed liquid at 95° C. or higher, it is preferable to install heaters in the pipes (e.g., T-shaped pipe 20) supplying each solution.

An example of step 2 will now be described in detail.
When a T-shaped tube having a tube diameter L1 of 10 mm at one upper end (left side in FIG. 1), a tube diameter L2 of 10 mm at the other upper end (right side in FIG. 1), and a tube diameter L3 of 15 mm at the lower end is used as the T-shaped tube 20, and 213 g of a 25 mass% aqueous sodium sulfate solution is contacted with 450 g of a 16 mass% aqueous zirconium oxychloride solution calculated as _ZrO2 , the time (contact time) from the start of contact to the end of contact (until the aqueous zirconium chloride solution in container 10 and the sulfating agent solution in container 30 are exhausted) is preferably 30 seconds to 300 seconds, more preferably 60 seconds to 200 seconds, and even more preferably 90 seconds to 150 seconds.

<Step 3>
In step 3, the basic zirconium sulfate-containing reaction liquid obtained in step 2 is aged at 95° C. or higher for 3 hours or more. In step 3, for example, the basic zirconium sulfate-containing reaction liquid flowing into the aging vessel 40 is aged at 95° C. or higher for 3 hours or more while being stirred with the stirrer 42. The upper limit of the aging time is not particularly limited, but is, for example, 7 hours or less. The temperature (aging temperature) of the mixed liquid (basic zirconium sulfate-containing reaction liquid) in step 3 is preferably 95° C. or higher, more preferably 97° C. or higher and 100° C. or lower. By setting the aging temperature to 95° C. or higher and the aging time to 3 hours or more, basic zirconium sulfate can be sufficiently produced and the yield can be increased.
The mixture contains basic zirconium sulfate as a main component and is a basic zirconium sulfate slurry.

<Step 4>
In step 4, a stabilizer is added to the basic zirconium sulfate-containing reaction liquid after aging obtained in step 3.

<Step 5>
In step 5, an alkali is added to the basic zirconium sulfate-containing reaction liquid obtained in step 4 to carry out a neutralization reaction. A zirconium-containing hydroxide is produced by the neutralization.
The alkali is not limited, and examples thereof include caustic soda, sodium carbonate, ammonia, hydrazine ammonium hydrogen carbonate, etc. The concentration of the alkali is not particularly limited, but it is usually diluted with water to 5 to 30%.
There are two methods for adding an alkali: (1) adding an alkali solution to a basic zirconium sulfate-containing reaction liquid; and (2) adding a basic zirconium sulfate-containing reaction liquid to an alkali solution. There is no particular limitation, and either method may be used.
After neutralization, the slurry is filtered to obtain a zirconium-containing hydroxide. If necessary, the zirconium-containing hydroxide is preferably washed with pure water or the like to remove impurities. After washing with water, drying or the like can be performed as necessary.

<Step 6>
In step 6, the zirconium-containing hydroxide obtained in step 5 is heat-treated (calcined) to oxidize the zirconium-containing hydroxide, thereby obtaining zirconia powder.
The heat treatment temperature (calcination temperature) and heat treatment time (calcination time) of the zirconium-containing hydroxide are not particularly limited, but are usually performed at about 600 to 1050°C for 1 to 10 hours. The calcination temperature is more preferably 650°C or higher and 1000°C or lower, and even more preferably 700°C or higher and 980°C or lower. The calcination temperature is more preferably 2 to 6 hours, and even more preferably 2 to 4 hours. By setting the heat treatment temperature to 600°C or higher and 1000°C or lower, the specific surface area of the obtained zirconia powder can be set in a suitable range (20 ^m2 /g or higher and 80 ^m2 /g or lower). In addition, by setting the heat treatment temperature to 600°C or higher and 1050°C or lower, the pore distribution of the obtained zirconia powder can be set in a suitable range. The heat treatment atmosphere is not particularly limited, but is usually performed in air or an oxidizing atmosphere.

<Step 7>
After step 6, the obtained zirconia powder may be pulverized to form a slurry, if necessary. In this case, a binder may be added to improve moldability. When the zirconia powder is not slurried (pulverized), the binder and the zirconia powder may be mixed uniformly in a kneader.
The binder is preferably an organic binder, since the organic binder can be easily removed from the molded body in a heating furnace in an oxidizing atmosphere and a degreased body can be obtained, so that impurities are less likely to remain in the final sintered body.
The organic binder may be soluble in alcohol or soluble in a mixture of two or more selected from the group consisting of alcohol, water, aliphatic ketones, and aromatic hydrocarbons. The organic binder may be, for example, at least one selected from the group consisting of polyethylene glycol, glycol fatty acid ester, glycerin fatty acid ester, polyvinyl butyral, polyvinyl methyl ether, polyvinyl ethyl ether, and vinyl propionate. The organic binder may further include one or more thermoplastic resins that are insoluble in alcohol or the mixture.
After the organic binder is added, the mixture is dried, pulverized, and otherwise treated by a known method to obtain the desired zirconia powder.
For example, the particle size _D50 of the zirconia powder can be controlled by the pulverization in step 7. For example, the particle size _D50 of the zirconia powder can be controlled by pulverizing the zirconia powder according to the state of the zirconia powder obtained in step 5.

When a sintering aid (e.g., alumina) or the like is added, it is possible to obtain zirconia powder containing the sintering aid, etc., by adding and mixing after the step 6. As a more detailed method of mixing, it is preferable to disperse the zirconia in pure water or the like to form a slurry and wet mix it.
When the step 7 is carried out, a sintering aid or the like may be added.

The above describes the zirconia powder according to this embodiment.

[Method of manufacturing zirconia sintered body]
An example of a method for producing a zirconia sintered body will be described below. However, the method for producing a zirconia sintered body of the present invention is not limited to the following example.

The method for producing a zirconia sintered body according to this embodiment is as follows:
A process A includes molding the zirconia powder under a pressure of 1.0 t/cm2 or ^more and 2.5 t/cm2 or ^less to obtain a molded body;
A step B of pre-sintering the molded body under normal pressure at a temperature of 1150° C. or higher and 1250° C. or lower for 1 hour or longer and 5 hours or shorter to obtain a pre-sintered body;
and C., a step of sintering the pre-sintered body under conditions of a pressure of 50 MPa to 200 MPa, 1150° C. to 1250° C., and a time of 1 hour to 5 hours to obtain a sintered body.

In the method for producing a zirconia sintered body according to this embodiment, first, zirconia powder is prepared. The zirconia powder described in the [Zirconia Powder] section can be used as the zirconia powder.

Next, the zirconia powder is molded under a pressure of 1.0 t/ ^cm2 to 2.5 t/ ^cm2 to obtain a molded body (step A). A commercially available molder or cold isostatic pressing (CIP) can be used for molding. Alternatively, the zirconia powder may be temporarily molded using a molder and then press molded.
The pressure is preferably 1.3 t/cm ² or more, more preferably 1.5 t/cm ² or more.
The pressure is preferably 2.3 t/cm2 or ^less , more preferably 2.2 t/ ^cm2 or less.
The pressure is preferably 1.3 t/cm ² or more and 2.3 t/cm ² or less, and more preferably 1.5 t/cm ² or more and 2.2 t/cm ² or less.

Next, the molded body is pre-sintered under normal pressure at 1150° C. to 1250° C. for 1 hour to 5 hours to obtain a pre-sintered body (step B).
In this embodiment, the molded body is pre-sintered under relatively low temperature conditions of 1150° C. to 1250° C. for 1 hour to 5 hours under normal pressure, so that a pre-sintered body in which grain growth is suppressed can be obtained. Since grain growth is suppressed, it is possible to obtain a zirconia sintered body having high translucency and high transparency.
In particular, when the proportion of the cubic phase is high, the grain growth proceeds isotropically, and the growth of adjacent crystal grains is easily promoted. However, since the molded body is pre-sintered under relatively low conditions of normal pressure, 1150°C to 1250°C, and 1 hour to 5 hours, the grain growth can be suppressed even if the proportion of the cubic phase is high.

The preliminary sintering temperature is preferably 1160° C. or higher, and more preferably 1170° C. or higher.
The preliminary sintering temperature is preferably 1245° C. or lower, more preferably 1240° C. or lower.
The preliminary sintering temperature is preferably 1,160° C. or higher and 1,245° C. or lower, and more preferably 1,170° C. or higher and 1,240° C. or lower.
The time for the preliminary sintering is preferably 1.2 hours or more, and more preferably 1.5 hours or more.
The time for the preliminary sintering is preferably 4.7 hours or less, more preferably 4.5 hours or less.
The time for the preliminary sintering is preferably 1.2 hours or more and 4.7 hours or less, and more preferably 1.5 hours or more and 4.5 hours or less.
The preliminary sintering atmosphere may be air or an oxidizing atmosphere. The preliminary sintering may be performed under normal pressure, and pressurization is not particularly required.

Next, the pre-sintered body is sintered under conditions of a pressure of 50 MPa to 200 MPa, a temperature of 1150° C. to 1250° C., and a time of 1 hour to 5 hours to obtain a sintered body (step C).
The main sintering is not particularly limited as long as the pressure is 50 MPa or more and 200 MPa or less and the temperature is 1150° C. or more and 1250° C. or less. For example, an apparatus employing a HIP (Hot Isostatic Pressing) method can be used.
In step B, the molded body is pre-sintered under normal pressure at 1150°C to 1250°C for 1 hour to 5 hours, so that the particles of the zirconia powder are appropriately bonded together during the pre-sintering process, and the pores (gaps between the particles of the zirconia powder) in the molded body can be made into closed pores. In other words, the pre-sintering can change the pores from a state in which the pores are connected to each other (open pores) to isolated closed pores surrounded by bonds of the zirconia powder.
Then, in step C, the provisionally sintered body, whose pores are closed pores, is sintered under conditions of a pressure of 50 MPa to 200 MPa, 1150°C to 1250°C, and 1 hour to 5 hours, so that the closed pores can be almost completely eliminated. Specifically, the closed pores are compressed and made as small as possible by the pressure applied in the sintering, and the pores are pushed out of the sintered body by element diffusion (element flow) during the sintering, so that the closed pores can be almost completely eliminated. By removing the pores, the total light transmittance (translucency) and the parallel light transmittance (transparency) can be improved.
Furthermore, the pre-sintered body is sintered under conditions of a pressure of 50 MPa to 200 MPa, a temperature of 1150° C. to 1250° C., and a time of 1 hour to 5 hours, so that the relative sintered density can be 99.6% or more.

The pressure of the main sintering is preferably 55 MPa or more, and more preferably 60 MPa or more.
The pressure of the main sintering is preferably 198 MPa or less, and more preferably 195 MPa or less.
The pressure of the main sintering is preferably 55 MPa or more and 198 MPa or less, and more preferably 60 MPa or more and 195 MPa or less.
The main sintering temperature is preferably 1160° C. or higher, and more preferably 1170° C. or higher.
The main sintering temperature is preferably 1245° C. or lower, more preferably 1240° C. or lower.
The main sintering temperature is preferably 1160°C or higher and 1245°C or lower, and more preferably 1170°C or higher and 1240°C or lower.
The time for the main sintering is preferably 1.2 hours or more, and more preferably 1.5 hours or more.
The time for the main sintering is preferably 4.7 hours or less, more preferably 4.5 hours or less.
The time for the main sintering is preferably 1.2 hours or more and 4.7 hours or less, and more preferably 1.5 hours or more and 4.5 hours or less.

After the preliminary sintering in step B, the material is usually cooled to 40°C or less, and then the main sintering is performed (step C is performed). This is because step B is sintering at normal pressure, while step C is sintering including pressure, and therefore the sintering equipment used is different. In other words, after the preliminary sintering, the material is cooled and removed from the equipment in which the preliminary sintering was performed, and then placed in an equipment for main sintering, and main sintering is performed. In order to perform the main sintering, the material is usually cooled to 40°C or less after the preliminary sintering in step B, and then heated again to perform the main sintering. However, the present invention is not limited to this example, and for example, when steps B and C are performed in the same equipment, the material may be sintered directly without cooling after the preliminary sintering in step B.

The above describes the method for producing the stabilized zirconia sintered body according to this embodiment.

The present invention will be described in detail below using examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. Note that the zirconia powder and zirconia sintered body in the examples and comparative examples contain 1.3 to 2.5 mass % of hafnium oxide as an inevitable impurity relative to zirconium oxide (calculated by the following formula (X)).
<Formula (X)>
([mass of hafnium oxide]/([mass of zirconium oxide]+[mass of hafnium oxide]))×100(%)

[Preparation of zirconia powder]
(Production Example 1)
213 g of a 25% by mass aqueous solution of sodium sulfate and 450 g of an aqueous solution of zirconium oxychloride (acid concentration: 1N) equivalent to 16% by mass in terms of _ZrO2 were separately heated to 95° C. (Step 1). After that, the heated aqueous solutions were brought into contact with each other for 2 minutes so that the _SO42− / _ZrO2 mass ratio ^of the mixed solution became 0.50 (Step 2).
Next, the obtained reaction liquid containing basic zirconium sulfate was kept at 95° C. for 4 hours for aging, thereby obtaining basic zirconium sulfate (Step 3).
Next, after the aged solution was cooled to room temperature, an aqueous solution of yttrium chloride having a concentration of 10 mass % in terms of Y ₂ O ₃ was added thereto so that the concentration of Y ₂ O ₃ was 4.1 mol %, and the mixture was mixed uniformly (step 4).
Next, a 25% by mass aqueous solution of sodium hydroxide was added to the resulting mixed solution to neutralize it to a pH of 13 or more, thereby forming a hydroxide precipitate (step 5).
The obtained hydroxide precipitate was filtered and thoroughly washed with water, and the obtained hydroxide was dried for 24 hours at 105° C. The dried hydroxide was heat-treated in air at 960° C. (calcination temperature) for 2 hours to obtain an unground zirconia-based powder (yttria-stabilized zirconia-based powder) (Step 6).
Alumina powder having an average primary particle size of 0.1 μm was added to the unpulverized yttria-stabilized zirconia powder obtained, in an amount of 0.1 mass% relative to the yttria-stabilized zirconia powder, and the mixture was pulverized and mixed for 40 hours in a wet ball mill using water as a dispersion medium. Zirconia beads φ5 mm were used for pulverization. The zirconia slurry obtained after pulverization was dried at 110° C. to obtain the zirconia powder according to Example 1.
Specifically, the above operation was carried out using the apparatus as explained with reference to FIG.

(Production Examples 2 to 16)
Zirconia powders of Production Examples 2 to 16 were obtained in the same manner as Production Example 1, except that the amount of yttrium chloride aqueous solution added was changed so that the content of Y ₂ O ₃ was the amount shown in Table 1, and/or the content of alumina powder was changed so that the content was the amount shown in Table 1.

(Production Example 17)
The zirconia powder of Production Example ₁₇ was obtained in the same manner as Production Example 1, except that the amount of the yttrium chloride aqueous solution added was changed so that the content of Y ₂ O ₃ was the amount shown in Table 1, and calcium carbonate (CaCO 3 ) was added at the same timing as the addition of the yttrium chloride aqueous solution so that the content was 0.5 mol % in terms of CaO.

(Production Example 18)
The zirconia powder of Production Example 18 was obtained in the same manner as Production Example 1, except that the amount of yttrium chloride aqueous solution added was changed so that the content of _Y2O3 was the amount shown in _{Table 1} , and a 10 mass% aqueous ytterbium chloride solution was added at the same timing as the addition of the yttrium chloride aqueous solution so that the content was 0.3 mol% in terms of _Yb2O3 .

(Production Example 19)
The zirconia powder of Production Example 19 was obtained in the same manner as Production Example 1, except that the amount of yttrium chloride aqueous solution added was changed so that the content of Y ₂ O ₃ was the amount shown in Table 1, and a 10 mass% aqueous erbium chloride solution was added at the same timing as the addition of the yttrium chloride aqueous solution so that the content was 0.2 mol% in terms of Er ₂ O ₃ .

[Preparation of zirconia sintered body]
<1. Molded body>
First, the zirconia powder of the manufacturing example was molded by cold isostatic pressing (CIP) to obtain a molded body. The molding pressure was as shown in Table 1. At this time, the relative molded density of the molded body was calculated by the following (3). The results are shown in Table 1.

<2. Pre-sintered body>
Next, the molded body was pre-sintered under normal pressure (1 atm) at the pre-sintering temperature and time shown in Table 2 to obtain a pre-sintered body.
The pre-sintered bodies of Examples 1 to 11 were obtained by pre-sintering the molded bodies of Production Examples 1 to 11, respectively.
The pre-sintered body of Example 12 was obtained by pre-sintering the molded body of Production Example 5. The pre-sintering conditions of Examples 5 and 12 were different (the molding conditions were the same).
The pre-sintered bodies of Comparative Examples 1 to 5 were obtained by pre-sintering the molded bodies of Production Examples 12 to 16, respectively.
The pre-sintered body of Comparative Example 6 was obtained by pre-sintering the molded body of Production Example 1. Example 1 and Comparative Example 6 differed in the pre-sintering conditions (the molding conditions were the same).
The pre-sintered body of Comparative Example 7 was obtained by pre-sintering the molded body of Production Example 2. Example 2 and Comparative Example 7 differed in the pre-sintering conditions (the molding conditions were the same).
The pre-sintered bodies of Examples 13 to 15 were obtained by pre-sintering the molded bodies of Production Examples 17 to 19, respectively.
The relative sintered density of the obtained pre-sintered body (pre-sintered body density) was determined as follows. The results are shown in Table 2.

[Relative sintered density]
The relative sintered density is represented by the following formula (1).
Relative sintered density (%) = (sintered density / theoretical sintered density) × 100 (1)
Here, the theoretical sintered density (ρ ₀ ) is a value calculated by the following formula (2-1).
ρ0=100/[(Y/3.987)+(100-Y)/ρz]...(2-1)
Here, ρz is a value calculated by the following formula (2-2).
ρz=[124.25(100−X)+[molecular weight of stabilizer]×X]/[150.5(100+X)A ² C] (2-2)
Here, the molecular weight of the stabilizer is _225.81 for _Y2O3 .
Additionally, X and Y are the stabilizer concentration (mol %) and the alumina concentration (wt %), respectively, and A and C are values calculated by the following formulas (2-3) and (2-4), respectively.
A=0.5080+0.06980X/(100+X)...(2-3)
C=0.5195-0.06180X/(100+X)...(2-4)
In formula (1), the theoretical sintered density varies depending on the powder composition. For example, the theoretical sintered density of yttria-containing zirconia is 6.117 g/ ^cm3 when the yttria content is 2 mol%, 6.098 g/ ^cm3 when the yttria content is 3 mol%, and 6.051 g/ ^cm3 when the _yttria content is 5.5 mol% (when _Al2O3 = 0 wt%).
In addition, the theoretical sintered density (ρ1) when other components (stabilizers, colorants, etc.) other than alumina are included is
ρ1=100/[(Z/V)+(100-Z)/ρ0]...(2-5)
Furthermore, Z is the concentration (wt %) of components other than alumina, and V is the theoretical density (g/cm ³ ) of the components other than alumina.
The theoretical densities of the components other than alumina are: _Yb2O3 : 9.17 _g / ^cm3 , _Er2O3 : 8.64 g/ ^cm3 , _CeO2 : 7.22 g/ ^cm3 , _{Sc2O3 :} 3.86 g/cm3, _Nd2O3 : 7.24 g/ ^cm3 , _La2O3 : 6.51 _g / ^cm3 , _{Tb2O3 :} 7.81 g/ ^cm3 , _CaO : 3.34 g/ ^cm3 , _Fe2O3 : 5.24 g/ ^cm3 , _ZnO : 5.61 g/ ^cm3 , _MnO2 : 5.03 g/ ^cm3 , CoO: 6.10 g/ ^cm3 , and _{Cr2O3 :} 5.22 g/ ^cm3 _. ³ , _TiO2 at 4.23 g/ ^cm3 , CuO at 6.31 g/ ^cm3 , _{and V2O5} at 3.36 g/ ^cm3 .
The sintered density is measured by the Archimedes method.

[Relative Molding Density]
Relative molded density (%) = (molded density / theoretical sintered density) × 100 (3)
Here, the theoretical sintered density (ρ ₀ ) is a value calculated by the above formula (2-1).

<3. Sintered body>
Next, the pre-sintered body was sintered at the pressure, temperature, and time shown in Table 2 to obtain a sintered body. The sintering was performed using an apparatus employing a hot isostatic pressing (HIP) method.

[Relative sintered density]
The relative sintered density of the sintered body was measured in the same manner as in the measurement of the relative sintered density of the pre-sintered body. The results are shown in Table 2.

[Average crystal grain size]
The average crystal grain size was determined using the SEM observation image of the sintered body sample obtained by scanning electron microscope (SEM) observation. The sample for SEM observation was prepared based on JIS R1633. The SEM observation image was obtained at a magnification such that the number of crystal grains in one field of view was 150 or more. The average crystal grain size of the zirconia sintered body of the examples and comparative examples was determined by the following formula according to the method described in J. Am. Ceram. Soc., 52[8]443-6 (1969), which is measured by scanning electron microscope observation of the polished and etched surface of the sintered body.
D=1.56L
D: average crystal grain size L: average length of grains crossing any straight line

[Identification of crystalline phases]
The X-ray diffraction spectrum of the zirconia sintered body was obtained using an X-ray diffractometer ("RINT2500" manufactured by Rigaku Corporation) under the following measurement conditions.
<Measurement conditions>
Measurement equipment: X-ray diffraction equipment (Rigaku, RINT2500)
Radiation source: CuKα radiation source Sampling interval: 0.02°
Scan speed: 2θ=1.0°/min Divergence slit (DS): 1°
Divergence vertical limit slit: 5 mm
Scattering slit (SS): 1°
Receiving slit (RS): 0.3 mm
Monochrome receiving slit: 0.8 mm
Tube voltage: 50 kV
Tube current: 300mA
Scanning speed: 2θ=10 to 80°: 4°/min
2θ=72~76°: 1°/min

Thereafter, the crystal phases were identified from the X-ray diffraction spectrum. The ratio of each crystal phase contained in the zirconia sintered body was calculated by the following formula.
Monoclinic fraction (%) = (Im(111) + Im(11-1)) / (Im(111) + Im(11-1) + It(101) + Ic(111)) × 100
Tetragonal phase ratio (%) = (100% - monoclinic phase (%)) × ((It (004) + It (220) / (It (004) + It (220) + Ic (004)) × 100
Cubic phase ratio (%) = (100% - monoclinic phase (%)) × ((Ic (004) / (It (004) + It (220) + Ic (004)) × 100
Here, Im(111) is the diffraction intensity of (111) in the monoclinic phase, and Im(11-1) is the diffraction intensity of (11-1) in the monoclinic phase.
It(101) is the diffraction intensity of (101) in the tetragonal phase, It(220) is the diffraction intensity of (220) in the tetragonal phase, and It(004) is the diffraction intensity of (004) in the tetragonal phase.
Ic(004) is the diffraction intensity of (004) in the cubic phase, and Ic(111) is the diffraction intensity of (111) in the cubic phase.
The monoclinic phase of zirconia was distinguished from the tetragonal and cubic phases at 2θ=26-36° in the XRD spectrum. The tetragonal and cubic phases were distinguished at 2θ=72-76° in the XRD spectrum. The cubic phase may be distorted depending on the amount of stabilizer added and the manufacturing method, and the peak position may shift, but in this example, the peak between (004) and (220) of the tetragonal phase was taken as the peak of the cubic phase and calculated.

[Total light transmittance]
The total light transmittance of the zirconia sintered bodies of the examples and comparative examples was measured using a spectroscopic haze meter (device name: SH-7000, manufactured by Nippon Denshoku Industries Co., Ltd.) with a D65 light source according to a method conforming to JIS K7361. The measurement samples were polished on both sides and adjusted to a thickness of 1 mm. The results are shown in Table 3.

[Parallel light transmittance]
The parallel light transmittance of the zirconia sintered bodies of the examples and comparative examples was calculated from the total light transmittance obtained according to JIS K7361-1 and the diffuse transmittance Td described in JIS K7136:2000 according to the following formula.
[Parallel light transmittance (%)] = [Total light transmittance (%)] - [Diffuse transmittance Td (%)]
Here, the diffuse transmittance Td (%) is a value calculated by: total light transmittance (%)×haze (%)×100 ⁻¹ .
[Haze (%)] was measured in accordance with JIS K7136:2000.
The measurement samples were polished on both sides to adjust the thickness to 1 mm. The results are shown in Table 3.

[Monoclinic phase ratio after hydrothermal treatment at 134° C., 0.3 MPa, 75 hours]
First, the zirconia sintered bodies of the examples and comparative examples were subjected to hydrothermal treatment at 134°C and an absolute pressure of 0.3 MPa (underwater atmosphere) for 75 hours. Thereafter, the monoclinic fraction contained in the crystal phase of the zirconia sintered bodies after the hydrothermal treatment was determined. The method for determining the monoclinic fraction of the zirconia sintered bodies after the hydrothermal treatment was the same as that described in the section [Identification of the crystal phase]. The results are shown in Table 3.

[Three-point bending strength]
The three-point bending strength of the zirconia sintered bodies of the Examples and Comparative Examples obtained above was measured in accordance with the three-point bending strength of JIS R 1601. The results are shown in Table 3.

[Toughness]
The toughness measurement by the IF method was performed under a load of 5.0 kgf (49.0 N) in accordance with JIS R1607 (Room temperature fracture toughness test method for fine ceramics). Using a Vickers hardness tester, seven indentations with a rectangular shape were selected to measure the toughness, and the toughness value was determined by averaging the toughness values of the five indentations excluding the smallest and largest values. However, the indentations to be measured were invalid if no cracks extended from the indentation, and indentations with four cracks extending from the tip of the rectangle were used.
Each toughness value was calculated by the following formula.
Kc=0.018×Hv×a ^0.5 ×[(ca)/a] ^-0.5 ×(Hv/E) ^-0.4
Kc, Hv, a, c, and E have the following meanings. The indentation lengths on the X and Y axes and the crack lengths on the X and Y axes when determining a and c are as shown in FIG.
Kc: toughness value [MPa·m ^0.5 ]
Hv: Vickers hardness [GPa]
a: Half the average value of the indentation length on the X and Y axes [μm]
c: Half the average crack length on the X and Y axes [μm]
E: Young's modulus [GPa]
The Vickers hardness was measured in accordance with JIS R 1610 (hardness test method for fine ceramics). The Vickers hardness was calculated according to the following formula.
Hv=0.001854×[F/ ^d2Sv ]
F and d have the following meanings. The X-axis indentation length and the Y-axis indentation length when calculating d are as shown in FIG.
Hv: Vickers hardness [GPa]
F: Test force [N]
d: average value of X-axis indentation length and Y-axis indentation length [mm]
The Young's modulus used was 210 GPa, which is known as the value of common yttria-stabilized zirconia.

Claims (9)

The stabilized zirconia includes zirconia and a stabilizer.
The content of the stabilized zirconia is 80% by mass or more when the entire zirconia sintered body is 100% by mass,
The _stabilizer comprises _Y2O3 ,
The content of the Y ₂ O ₃ in the stabilized zirconia is more than 4.0 mol % and not more than 6.5 mol % in terms of oxide,
The average crystal grain size is 100 nm or more and 200 nm or less,
The monoclinic fraction is 0.5% or less,
The tetragonal fraction is 20.0% or more and 96.0% or less,
The cubic fraction is 4.0% or more and 80.0% or less,
The relative sintered density is 99.6% or more,
A zirconia sintered body having a total light transmittance of 45% or more and 60% or less at a thickness of 1 mm . The zirconia sintered body according to claim 1, characterized in that the content of the Y ₂ O ₃ in the stabilized zirconia exceeds 5.2 mol% in terms of oxide. The tetragonal fraction is 20.0% or more and 60.0% or less,
2. The zirconia sintered body according to claim 1, characterized in that the cubic phase fraction is 40.0% or more and 80.0% or less. The content of the Y ₂ O ₃ in the stabilized zirconia is more than 5.2 mol% in terms of oxide,
The tetragonal fraction is 20.0% or more and 60.0% or less,
2. The zirconia sintered body according to claim 1, characterized in that the cubic phase fraction is 40.0% or more and 80.0% or less. The zirconia sintered body according to any one of claims 1 to 4, characterized in that the zirconia sintered body contains Al ₂ O ₃ in an amount of 0.15 mass% or less based on the entire zirconia sintered body. The zirconia sintered body according to any one of claims 1 to 4, characterized in that the parallel light transmittance at a thickness of 1 mm is 7.0% or more and 30.0% or less. The zirconia sintered body according to any one of claims 1 to 4, characterized in that the three-point bending strength is 500 MPa or more and 1000 MPa or less. The zirconia sintered body according to any one of claims 1 to 4, characterized in that the toughness value measured by an IF method is 2.7 MPa·m ^0.5 or more and 5.0 MPa·m ^0.5 or less. A step A of molding the zirconia powder at a pressure of 1.0 t/cm2 or ^more and 2.5 t/cm2 or ^less to obtain a molded body;
A step B of pre-sintering the molded body under normal pressure at a temperature of 1150° C. or higher and 1250° C. or lower for 1 hour or longer and 5 hours or shorter to obtain a pre-sintered body;
The temporary sintered body is sintered under conditions of a pressure of 50 MPa to 200 MPa, 1150 ° C. to 1250 ° C., and 1 hour to 5 hours to obtain a sintered body. The method for producing a zirconia sintered body according to any one of claims 1 to 4, further comprising: a step C of sintering the temporary sintered body under conditions of a pressure of 50 MPa to 200 MPa, 1150 ° C. to 1250 ° C., and 1 hour to 5 hours.