JP2022038260A - Manufacturing method of rare earth iron garnet sintered product - Google Patents

Abstract

To provide a rare earth iron garnet sintered product capable of preventing a generation of a different crystal phase even when a sintering material is sintered by a discharge plasma sintering method.SOLUTION: A manufacturing method of a rare earth iron garnet sintered product charges a sintering material M into a molding tool 10 and applies discharge plasma sintering while compressing the sintering material M by the molding tool 10. The molding tool 10 is formed of a silicon carbide including a dopant. A dopant amount is 2.0×10-19 atom/cm3 or more. The discharge plasma sintering is applied under an atmospheric pressure. The sintering material M is sintered while compressing the sintering material M at a pressure of 200 MPa or higher.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for producing a rare earth iron garnet sintered body.

Development of a rare earth iron garnet sintered body used for the Faraday element of an optical isolator that blocks the return light to a semiconductor laser for optical communication is underway. Examples of such a rare earth iron garnet sintered body include bismuth-substituted rare earths and iron garnet Bi _x R _3-x Fe ₅ O ₁₂ . The bismuth-substituted rare earth / iron garnet is a single crystal usually obtained by growing it on a gadolinium / gallium / garnet single crystal (GGG) substrate by the liquid phase epitactical method (LPE).

As the communication network is greatly developed as represented by 5G, there is a strong demand for improvement in the productivity of the rare earth iron garnet sintered body used for the Faraday element.

In response to this situation, in recent years, ceramic sintering technology has been increasing at an accelerating rate, and ceramics having characteristics equivalent to those of single crystals have appeared and have been put into practical use. As a method for sintering ceramics, for example, a hot isostatic pressure sintering method (HIP: Hot Isostatic Pressing), a discharge plasma sintering (SPS: Spark Plasma Sintering), and the like are known.

SPS is a method in which a solid or powder sintering raw material is filled in a molding die, and uniaxial pressurization and a DC pulse voltage / current are applied to the molding die for sintering. , SPS completes sintering in 1/10 of the time compared to HIP, so high productivity can be expected. For example, Non-Patent Document 1 describes a technique relating to CEO ₂ -doped YAG transparent ceramics using this SPS.

"Manufacturing of transparent ceramic phosphors by SPS" 22nd Current Sintering Study Group (2017) pp 105

For the rare earth iron garnet sintered body, a rare earth element (R) source, an iron (Fe) source and the like are prepared as raw materials, and a rare earth iron garnet powder (sintered raw material) is obtained by a predetermined method. A rare earth iron garnet sintered body is manufactured by undergoing a sintering process in which sintering is performed while pressurizing the sintering raw material.

By performing discharge plasma sintering on the sintered raw material in this sintering step, it can be expected to produce a rare earth iron garnet sintered body with higher productivity than HIP.

However, when discharge plasma sintering is applied to rare earth iron garnet powder (sintered raw material) under normal conditions, specific elements in the sintered raw material are removed and the sintered raw material reacts with the molding die. In some cases, a crystal phase different from that of garnet may be generated. Then, for example, there arises a problem that cracks are likely to occur during cooling of the sintered body.

An object of the present invention is to obtain a rare earth iron garnet sintered body in which the formation of different crystal phases is suppressed even when the sintered raw material is sintered by the discharge plasma sintering method.

The present inventor made it possible to perform sintering in an atmosphere under atmospheric pressure when sintering a sintered raw material by a discharge plasma sintering method using a molding die, and was composed of silicon carbide containing a dopant. We have found that the above problems can be solved by controlling the pressure applied to the sintered raw material to a predetermined pressure by using a molding die, and have completed the present invention. Specifically, the present invention provides the following.

(1) The first method of the present invention is a method for producing a rare earth iron garnet sintered body in which a sintering raw material is charged into a molding die and discharge plasma sintering is performed while the sintering raw material is pressurized by the molding die. The mold is made of silicon carbide containing a dopant, the amount of the dopant is 2 × 10 ^-19 atoms / cm ³ or more, and the discharge plasma sintering is performed in an atmosphere under atmospheric pressure. This is a method for producing a rare earth iron garnet sintered body that sinters the sintered raw material while pressurizing the sintered raw material at a pressure of 200 MPa or more.

(2) The second aspect of the present invention is, in the first invention, the discharge plasma sintering is a method for producing a rare earth iron garnet sintered body, which is performed in an atmospheric atmosphere.

(3) A third aspect of the present invention is the method for producing a rare earth iron garnet sintered body, wherein the discharge plasma sintering is performed in an inert gas atmosphere in the first invention.

According to the present invention, it is possible to obtain a rare earth iron garnet sintered body in which the formation of different crystal phases is suppressed even when the sintered raw material is sintered by the discharge plasma sintering method.

It is sectional drawing which shows the structure of the mold which constitutes the discharge plasma sintering apparatus.

Hereinafter, specific embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments, and can be carried out with appropriate modifications within the scope of the object of the present invention.

≪1. Overview ≫
The method for producing a sintered body of the present invention is a method for producing a rare earth iron garnet sintered body in which a sintered raw material is charged into a molding die and discharge plasma sintering is performed while pressurizing the sintered raw material by the molding die. In this manufacturing method, the molding die constituting the discharge plasma sintering apparatus is composed of silicon carbide containing a dopant, the amount of dopant is 2 × 10 ^-19 atoms / cm ³ or more, and the discharge plasma sintering is large. It is carried out in an atmosphere under atmospheric pressure, and is characterized in that the sintered raw material is sintered while pressurizing the sintered raw material at a pressure of 200 MPa or more.

In this way, using a molding die made of conductive silicon carbide, discharge plasma sintering is performed in an atmosphere under atmospheric pressure, and the pressure applied to the sintering raw material is controlled to a predetermined pressure. Therefore, it is possible to obtain a rare earth iron garnet sintered body in which the formation of different crystal phases is suppressed.

The rare earth iron garnet sintered body obtained by the production method of the present invention can be particularly preferably used as a Faraday element for rotating a polarization plane of linearly polarized light, for example.

≪2. Discharge plasma sintering equipment ≫
Hereinafter, an example of a discharge plasma sintering apparatus used for performing discharge plasma sintering on a sintered raw material will be described prior to the description of a method for manufacturing a sintered body.

The SPS usually uniaxially pressurizes a high voltage of less than 50 MPa on a sintered raw material, and a direct current pulse current is passed through a sintered mold composed of carbon to directly heat the sintered raw material. In this way, by performing sintering on the sintering raw material while activating sintering by the combined action of thermal, mechanical, and electromagnetic energy, 20 to 20 to 20 compared to other sintering methods. Sintering can be performed 100 times faster to obtain a rare earth iron garnet sintered body. Further, since SPS can sinter the sintered raw material without using a sintering aid, it is easy to obtain a high-purity rare earth iron garnet sintered body. Therefore, SPS is extremely suitable for producing, for example, a transparent sintered body used for a Faraday element.

FIG. 1 is a cross-sectional view showing an example of the configuration of the molding die 10 constituting the discharge plasma sintering apparatus. While pressurizing the sintering raw material M, the molding die 10 is energized to directly heat the sintering raw material M to perform sintering.

As shown in FIG. 1, the molding die 10 includes a cylinder 11 and two punches 12. Then, in the molding die 10, the sintering raw material M is sintered in a pressurized state in a space surrounded by the cylinder 11 and the two punches 12. Energization is performed by passing a current in the order of punch 12a (12b) → cylinder 11 → punch 12b (12a).

The cylinder 11 has, for example, a cylindrical shape and guides the vertical movement of two cylindrical punches 12a and 12b inserted into the hollow portion thereof. In the cylinder 11, the sintered raw material M is charged, and the sintered raw material M is pressurized by applying pressure by the two punches 12a and 12b.

The punch 12 has, for example, a cylindrical shape, and when inserted into the hollow portion of the cylinder 11, the punch 12 and the cylinder 11 pressurize and compress the sintered raw material charged inside the cylinder 11. Specifically, one punch 12 is inserted from one end of the hollow portion of the cylinder 11 in which the sintering raw material M is charged, and another punch 12 is inserted from the other end. Directly pressurize the sintered raw material M inside 11.

Here, in the molding die 10, for example, one punch 12b is inserted from one end into the hollow portion of the cylinder 11, and then the sintering raw material M is inserted into the hollow portion of the cylinder 11 from the other end. It is charged. After that, by inserting another punch 12a from the end portion on the side where the sintering raw material M is charged, a state in which pressure is applied to the sintering raw material M by the punch is set. The molding die 10 into which the sintering raw material M is charged in this way is installed in a discharge plasma sintering apparatus, and pressure is applied to the sintering raw material M.

In this discharge plasma sintering apparatus, a predetermined pressure is applied to the sintering raw material M through a spacer arranged in contact with the punch 12 of the molding die 10 and the punch 12 of the molding die 10, and a pulse voltage is applied. The molding die 10 to which an electric current is applied may be provided with a pressure ram.

The molding die 10 is characterized in that it is made of silicon carbide. The molding die constituting the conventional discharge plasma sintering device is generally made of a carbon material such as graphite, and the rare earth iron obtained by reacting Fe in the sintering raw material with carbon in the molding die. The surface of the garnet sintered body may be covered with a different phase.

Therefore, by using the molding die 10 made of silicon carbide having low reactivity with other substances, it is possible to effectively suppress the reaction between the sintering raw material and the elements constituting the molding die. Become.

Further, by using the molding die 10 made of silicon carbide having a Young's modulus higher than that of carbon (graphite), the molding die can be used even under high temperature heating conditions such as performing high temperature plasma sintering while pressurizing the sintering raw material M. The generation of 10 cracks can be effectively suppressed.

Further, the silicon carbide contains a dopant, and the amount of the dopant is 2 × 10 ^-19 atoms / cm ³ or more. Silicon carbide is a material that is semi-conductive and has excellent electrical conductivity at high temperatures, but has low electrical conductivity at low temperatures. Therefore, it becomes difficult to apply a voltage to the molding die 10 at the initial stage of the temperature rising process.

Therefore, by using a molding die made of conductive silicon carbide to which a dopant that imparts conductivity to silicon carbide is added in a predetermined amount for discharge plasma sintering, molding is performed from the initial stage in the heating process of the sintering step. The mold 10 can be energized.

The dopant contained in silicon carbide may be a p-type dopant such as boron or aluminum, or may be an n-type dopant such as nitrogen or phosphorus.

The amount of dopant is preferably 3.0 × ^{10-19 atoms / cm 3 or more, and more preferably 4.0 × 10 -19 atoms / cm 3 or more} .

If the molding die is made of conductive silicon carbide to which such a dopant is added, the molding die 10 can be energized even in the initial stage of the temperature raising process. The electrical resistivity of the mold made of this conductive silicon carbide is preferably less than 2.0 × 10 ^-3 Ω · cm, more preferably less than 1.0 × 10 ^-3 Ω · cm. preferable.

≪3. Manufacturing method of sintered body ≫
Next, a method for producing a bismuth-substituted rare earth / iron garnet sintered body using the above-mentioned discharge plasma sintering apparatus will be described more specifically as a specific embodiment of the present invention. A method for producing a bismuth-replaced rare earth / iron garnet sintered body will be described as one embodiment, but the rare earth iron garnet sintered body obtained by the production method of the present invention is replaced with cerium or the like instead of bismuth. It may be a rare earth / iron garnet sintered body.

In the discharge plasma sintering method according to the present embodiment, the sintering raw material M is charged into a molding die 10 including a cylinder 11 and a punch 12, and the sintering raw material M is subjected to discharge plasma sintering. It is a method including a step.

<Preparation process>
In the preparation process, raw materials are prepared. As raw materials, a rare earth element (R) source, a bismuth (Bi) source, and an iron (Fe) source are prepared. The raw material is not particularly limited as long as the desired rare earth iron garnet powder can be obtained. For example, compounds such as oxides, carbonates, hydroxides, nitrates, and / or alkoxides of the constituent elements of garnet (Bi, R and Fe) can be used. The raw material may contain the garnet constituent element alone. Alternatively, a plurality of constituent elements may be combined and contained in the form of a composite compound.

In the present specification, the rare earth element is a general term for scandium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39, and lanthanum (La) having an atomic number of 57 to ruthenium (Lu) having an atomic number of 71.

<Synthesis process>
In the synthesis step, the prepared raw materials are mixed and reacted to synthesize a sintered raw material (rare earth iron garnet powder). The synthetic method is not limited as long as the desired rare earth iron garnet powder can be obtained. Examples thereof include a simultaneous precipitation method, a solid phase reaction method, a hydrothermal synthesis method, and a sol-gel method.

In the case of synthesizing by the simultaneous precipitation method, for example, a nitrate containing garnet constituent elements (Bi, R and Fe) is prepared as a raw material and dissolved in water to prepare an aqueous solution. The obtained aqueous solution is mixed so as to obtain a target composition, and amine is added to the mixed solution to form a precipitate. The obtained precipitate is filtered and taken out, and then dried and calcined to synthesize a rare earth iron garnet powder. The calcining may be carried out under the condition that the calcining is held at a temperature of 800 to 1200 ° C. for 1 to 30 hours in an air atmosphere, for example.

When synthesizing by the solid phase reaction method, for example, raw materials (oxides, carbonates, hydroxides, etc.) containing garnet constituent elements (Bi, R and Fe) are prepared, and these raw materials have a target composition. Mix to be. Then, the obtained mixture is calcined to synthesize a rare earth iron garnet powder. The raw materials may be mixed by a wet and / or dry method using a known method such as a ball mill. The calcining may be carried out under the condition that the calcining is held at a temperature of 800 to 1200 ° C. for 1 to 30 hours in an air atmosphere, for example.

<Sintering process>
In the sintering process, the prepared sintering raw material (rare earth iron garnet powder) is sintered to form a sintered body. First, the sintering raw material M is charged into the molding die 10. Specifically, the cylinder 11 of the molding die 10 is filled with the sintering raw material M and pressed from above and below with the punch 12. Then, by transmitting the pressure to the punch 12 by the pressurizing ram, the sintering raw material M is fixed in a state of being pressurized with a predetermined pressure. A spacer may be provided between the pressure ram and the punch 12.

Next, the sintered raw material M is heated while being pressurized at a predetermined pressure. Specifically, the sintering raw material M is energized through the molding die 10 with a pressure ram at a pressure of 200 MPa or more while energizing the molding die 10 to raise the temperature at a predetermined speed.

When sintering is performed while pressurizing the sintered raw material at a pressure of less than 200 MPa, the sintering completion temperature becomes 900 ° C. or higher, and the bismuth element is removed from the crystal, which is a crystal phase different from that of garnet such as Fe ₂ O ₃ phase. Will be generated.

Therefore, by pressurizing the sintering raw material M at a pressure of 200 MPa or more, the sintering completion temperature drops below 900 ° C., effectively suppressing the removal of the bismuth element in the sintering raw material, and different crystal phases. It is possible to effectively suppress the generation of. It is preferable to pressurize the sintered raw material M at a pressure of 250 MPa or more, and more preferably pressurize the sintered raw material M at a pressure of 300 MPa or more.

The rate of temperature rise when heating the sintered raw material M is not particularly limited, but is preferably 50 ° C./min or more and 100 ° C./min or less.

At this time, the discharge plasma sintering is characterized in that it is performed in an atmosphere under atmospheric pressure. When discharge plasma sintering is performed in a vacuum pressure atmosphere, bismuth in the sintering raw material evaporates, and bismuth is not contained in the sintered body, so that a crystal phase different from garnet such as Fe ₂ O ₃ phase is generated. Will end up.

Therefore, by performing the discharge plasma sintering in an atmosphere under atmospheric pressure, it is possible to suppress the evaporation of bismuth in the sintering raw material during the discharge plasma sintering and suppress the formation of a crystal phase different from the garnet. ..

If the discharge plasma sintering is performed in an atmosphere under atmospheric pressure, it may be in an atmosphere of atmosphere or in an atmosphere of an inert gas such as nitrogen gas or argon gas. Among them, the discharge plasma sintering is preferably performed in an atmospheric atmosphere. Not only is the operation simpler than in the case of performing in an inert gas atmosphere, but it is also possible to obtain a rare earth iron garnet sintered body having high density in a state where bubbles are removed.

Then, when the sintering of the sintering raw material is completed, the sintering step is terminated when the volume of the sintering raw material, which has been reduced by sintering, becomes equal to or higher than the temperature at which the volume of the sintering raw material starts to increase again. For example, a sintered body can be obtained by terminating the discharge plasma sintering after holding the sintered raw material at a predetermined temperature equal to or higher than the temperature at which the volume of the sintered raw material starts to increase again for a predetermined time.

Hereinafter, examples of the present invention will be described in more detail, but the present invention is not limited to the following examples.

(Example 1)
First, a molding die having a cylinder with an inner diameter of 15 mm and a punch with an outer diameter of 15 mm and composed of conductive silicon carbide (silicon carbide containing an amount of 2 × 10 ^-19 atoms / cm ³ or more of a nitrogen element (dopant)). I prepared it.

Next, Bi ₁ Y ₂ O ₁₂ was synthesized as a sintering raw material (rare earth iron garnet powder). Sintered raw material (rare earth iron garnet) is obtained by mixing powders of Bi ₂ O ₃ , Y ₂ O ₃ and Fe ₂ O ₃ as raw materials in a predetermined ratio and firing them in the air at 900 ° C. for 10 hours for a solid phase reaction. Powder) was synthesized. As a result of IPC emission analysis, it was confirmed that the composition of the sintered raw material (rare earth iron garnet powder) was Bi _0.98 Y _2.02 Fe ₅ O ₁₂ .

The cylinder of the molding die was filled with 2 g of the sintering raw material M and pressed from above and below with the punch 12, so that the sintering raw material was charged into the molding die. After setting the vacuum degree of the sintering tank in SPS to 5 × 10 ^-2 Pa, energize the molding mold, raise the temperature to 600 ° C at a heating rate of 120 ° C / min, and hold it for 5 minutes to remove the adhering moisture. did. Next, the inert gas (nitrogen gas) was introduced while the energization was stopped and the mixture was cooled to bring the inside of the sintering tank into an atmosphere under atmospheric pressure.

Next, the punch of FIG. 1 was pressurized to 300 MPa while flowing an inert gas (nitrogen gas), and SPS was performed under the following conditions. Then, the temperature rise was stopped at a temperature at which the volume of the sintered body, which had been reduced by sintering, started to increase again, and the temperature was taken as the reached temperature. The following holding time is the holding time at the reached temperature. Two sintered bodies were manufactured under the following conditions.

・ Temperature rise rate: 100 ℃ / min ・ Reaching temperature: 900 ℃
・ Holding time: 20 minutes ・ Atmosphere: Atmosphere under atmospheric pressure (nitrogen gas)

(Example 2)
Two sintered bodies were produced in the same manner as in Example 1 above, except that the atmosphere was introduced instead of nitrogen gas and the discharge plasma sintering was performed in the atmospheric atmosphere.

(Comparative example)
Two sintered bodies were produced in the same manner as in Example 1 above, except that the punch of FIG. 1 was pressurized to 100 MPa and SPS was performed under the following conditions.

[Regarding the presence or absence of different phases in the sintered body]
The sintered bodies obtained by the manufacturing methods of Examples 1 and 2 had a size of φ15 × 1.5 mt, and no cracks could be confirmed in both of them.

Further, it was confirmed by powder X-ray diffraction that both of the sintered bodies obtained by the production methods of Examples 1 and 2 had a garnet single phase. For this reason, in the production methods of Examples 1 and 2, the discharge plasma sintering was performed in an atmosphere under atmospheric pressure and the pressure applied to the sintering raw material was controlled to 200 MPa or more to generate different crystal phases. It was possible to obtain a rare earth iron garnet sintered body in which the amount of water was suppressed.

Further, as a result of IPC emission analysis of the sintered bodies obtained by the production methods of Examples 1 and 2, both were Bi _0.97 Y _2.03 Fe ₅ O ₁₂ , which was a raw material for sintering. It was confirmed that the composition and the composition of the sintered body were substantially the same.

On the other hand, in the sintered body obtained by the production method of the comparative example, in addition to the garnet phase, a Fe ₂ O ₃ phase different from the garnet phase was confirmed by powder X-ray diffraction. From this, it was not possible to obtain a rare earth iron garnet sintered body in which the formation of different crystal phases was suppressed by the production method of the comparative example in which the pressure applied to the sintered raw material was set to less than 200 MPa and plasma treatment was performed.

[Transparency of sintered body]
Regarding the other sintered body obtained by the manufacturing methods of Example 1, Example 2 and Comparative Example, the optical surface was ground and polished to 0.4 mmt, and the light transmittance for light having a wavelength of 1.31 μm was determined. It was measured. A semiconductor laser was used as the light source, and the sintered body was magnetically saturated by adding ground to the sintered body in equilibrium with the light rays.

As can be seen from Table 1 above, the sintered body obtained by the production method of Example 2 was excellent in transparency. From this, by performing the discharge plasma sintering in an atmospheric atmosphere, a rare earth iron garnet sintered body having high density in a state where bubbles are removed can be obtained as compared with the case where the discharge plasma sintering is performed in an inert gas atmosphere. I understand.

Although the sintered body obtained by the production method of Example 1 was inferior in transparency, air bubbles in the sintered body were removed by subjecting it to an annealing treatment at 700 ° C. for 10 hours, and Example 2 was performed. A rare earth iron garnet sintered body having high transparency was obtained as in the case of the sintered body obtained by the above-mentioned production method.

10 Molding mold 11 Cylinder 12 Punch M Sintering raw material

Claims (3)

It is a method for producing a rare earth iron garnet sintered body in which a sintering raw material is charged into a molding die and discharge plasma sintering is performed while pressurizing the sintering raw material by the molding die.
The mold is composed of silicon carbide containing a dopant, and the amount of the dopant is 2.0 × 10 ^-19 atoms / cm ³ or more.
The discharge plasma sintering is performed in an atmosphere under atmospheric pressure, and a method for producing a rare earth iron garnet sintered body that sinters the sintered raw material while pressurizing the sintered raw material at a pressure of 200 MPa or more. .. The method for producing a rare earth iron garnet sintered body according to claim 1, wherein the discharge plasma sintering is performed in an atmospheric atmosphere. The method for producing a rare earth iron garnet sintered body according to claim 1, wherein the discharge plasma sintering is performed in an inert gas atmosphere.

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