US20050221974A1

US20050221974A1 - Ceramic clay, ceramic formed article, ceramic structure, and manufacturing methods thereof

Info

Publication number: US20050221974A1
Application number: US10/991,097
Authority: US
Inventors: Takahiro Tomita
Original assignee: NGK Insulators Ltd
Current assignee: NGK Insulators Ltd
Priority date: 2004-03-31
Filing date: 2004-11-18
Publication date: 2005-10-06
Also published as: US20060035778A1; CN1956936A; CN1956936B; US7442663B2

Abstract

There is disclosed a ceramic clay obtained by kneading a forming material containing a ceramic forming material, wherein the forming material contains layered double hydroxide, in addition to the ceramic forming material, at a ratio of 0.5 to 50% by mass with respect to a total with the ceramic forming material, and hardness measured by an NGK system hardness scale is set to 10 to 35 mm. A high-strength honeycomb structure can be obtained capable of preventing or inhibiting pollution and global warming when used in manufacturing a ceramic structure, and having few defects such as cracks.

Description

BACKGROUND OF THE INVENTION AND RELATED ART

The present invention relates to a ceramic clay, a ceramic formed article, a ceramic structure, and methods of manufacturing them. In more detail, the present invention relates to a ceramic clay, a ceramic formed article, a ceramic structure, and methods of manufacturing them, capable of preventing or reducing generation of CO₂or harmful gas during firing to thereby prevent or inhibit pollution, and global warming, when used in manufacturing a ceramic structure, and capable of obtaining a high-strength honeycomb structure having few defects such as cracks.
To form a ceramic product, in general, methods such as spinning lathe forming, extrusion forming, injection forming, press forming, and sheet forming have been used. However, since plasticity•shape retaining property and the like required for the forming cannot be obtained only by a ceramic material powder. Therefore, after adding water, organic binder and the like to thereby form a ceramic forming material, the material is formed. For example, a method of manufacturing a ceramic structure has been disclosed in which in the extrusion forming, a ceramic material, water, organic binder and the like are kneaded, and the forming material (clay) whose plasticity has been enhanced is extruded, dried, and fired (see, e.g., Japanese Patent No. 3227039).
When an added amount of the organic binder imparting the plasticity or the shape retaining property increases, the forming property of ceramic is enhanced. For example, in the extrusion forming, to form a large-sized structure or a structure having a complicated cell structure which has been increasingly demanded in recent years, kneaded clay having a satisfactory forming property is required as compared with a case where a small-sized or simple ceramic structure is manufactured. As a result, a large amount of organic binder has to be added.
However, when the added amount of the organic binder is large, the organic binder burns down during firing. Therefore, there has been a problem that a space occupied by the organic binder becomes a defect at a forming time, and mechanical strength of the structure drops. In a large-sized structure, there has been a problem that the inside of the structure is at high temperature by burning heat when burning the organic binder during the firing, defects such as cracks are generated because of thermal stress by an inner/outer temperature difference of the structure, the mechanical strength of the structure is lowered, and yield is largely lowered. Furthermore, CO₂or harmful gas is generated, and released to the atmosphere by the burning of the organic binder during the firing, and this has raised a problem in environmental respects such as pollution and global warming.
On the other hand, clay (Gairome clay, etc.) which is a pottery material has such plasticity that forming is possible, even when the above-described organic binder is not contained. As a factor for this material to develop the plasticity, particles are fine, shape is flat or needlelike, and hydrogen bond is caused with respect to water (See pp. 828 to 830 of Clay Handbook Version 2 (GIHODO SHUPPAN Co. 1987)). Attempts have been made to add a compound having such characteristic to a ceramic material powder as in a conventional organic binder, and accordingly impart plasticity to the ceramic material clay (see pages 175 to 178 of Artificial Clay (10th Anniversary Journal of Artificial Clay Research Institute), and The Use of Montmorillonites as Extrusion Aids for Alumina, Ceram. Engi. Sci. Proc. 12 [1-2] pp. 33 to 48 (1991)).
However, with regard to clay minerals disclosed in the above-described documents and having plasticity, such as bentonite and smectite, naturally produced minerals contain a large amount of impurities, and it is feared that the products run out in near future, and there has been a problem that synthetic compounds or refined natural products are expensive as compared with the organic binders.
The present invention has been developed in view of the above-described problems, and an object thereof is to provide a ceramic clay, a ceramic formed article, a high-strength ceramic structure, and methods of efficiently manufacturing them, capable of preventing or reducing generation of CO₂or harmful gas during firing to thereby prevent or inhibit pollution and global warming, when used in manufacturing a ceramic structure, and capable of obtaining a high-strength ceramic structure having few defects such as cracks.

SUMMARY OF THE INVENTION

To achieve the above-described object, according to the present invention, there are provided the following ceramic clay, ceramic formed article, high-strength ceramic structure, and methods of efficiently manufacturing them.
[1] A ceramic clay obtained by kneading a forming material containing a ceramic forming material, wherein the forming material contains layered double hydroxide, in addition to the ceramic forming material, at a ratio of 0.5 to 50% by mass with respect to a total with the ceramic forming material, and hardness measured by an NGK system hardness scale is in a range of 10 to 35 mm.
[2] The ceramic clay according to the above [1], wherein the layered double hydroxide is hydrotalcite represented by the following formula (II):
Mg_1-xAl_x(OH)₂(CO₃)_x/2.mH₂O (II)
(where x denotes a value in a range of 0.2≦x≦0.33, and m denotes an arbitrary value corresponding to the value of x).
[3] The ceramic clay according to the above [1], wherein the layered double hydroxide is hydrotalcite represented by the following formula (III):
Mg₆Al₂(OH)₁₆CO₃.4H₂O (III).
[4] A ceramic formed article obtained by forming the ceramic clay according to any one of the above [1] to [3].
[5] The ceramic formed article according to the above [4], comprising: a honeycomb formed article formed into a honeycomb shape.
[6] A ceramic structure obtained by firing the ceramic formed article according to the above [4] or [5].
[7] A method of manufacturing a ceramic clay, in which a forming material containing a ceramic forming material is kneaded to obtain the ceramic clay, comprising the steps of: using the forming material containing layered double hydroxide, in addition to the ceramic forming material, at a ratio of 0.5 to 50% by mass with respect to a total with the ceramic forming material.
[8] The method of manufacturing the ceramic clay according to the above [7], further comprising the steps of: using hydrotalcite represented by the following formula (II) as the layered double hydroxide:
Mg_1-xAl_x(OH)₂(Co₃)_x/2.mH₂O (II)
(where x denotes a value in a range of 0.2≦x≦0.33, and m denotes an arbitrary value corresponding to the value of x).
[9] The method of manufacturing the ceramic clay according to the above [7] or [8], further comprising the steps of: using hydrotalcite represented by the following formula (III) as the layered double hydroxide:
Mg₆Al₂(OH)₁₆CO₃.4H₂O (III).
[10] The method of manufacturing the ceramic clay according to any one of the above [7] to [9], wherein hardness of the obtained ceramic clay, measured by an NGK system hardness scale, is in a range of 10 to 35 mm.
[11] A method of manufacturing a ceramic formed article, comprising the steps of: further forming the ceramic clay obtained by the method according to any one of the above [7] to [10] to obtain the ceramic formed article.
[12] The method of manufacturing the ceramic formed article according to the above [11], further comprising the steps of: forming the ceramic clay into a honeycomb shape to obtain a honeycomb formed article.
[13] A method of manufacturing a ceramic structure, comprising the steps of: further firing the ceramic formed article obtained by the method according to the above [11] or [12] to obtain the ceramic structure.
According to the present invention, there are provided a ceramic clay, a ceramic formed article, a high-strength ceramic structure, and methods of efficiently manufacturing them, capable of preventing or reducing generation of CO₂or harmful gas during firing to thereby prevent or inhibit pollution and global warming, when used in manufacturing a ceramic structure, and capable of obtaining a high-strength ceramic structure having few defects such as cracks.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A best mode for carrying out the present invention will be described concretely hereinafter.
According to the present invention, there is provided a ceramic clay obtained by kneading a forming material containing a ceramic forming material, characterized in that the forming material contains layered double hydroxide, in addition to the ceramic forming material, at a ratio of 0.5 to 50% by mass with respect to a total with the ceramic forming material, and hardness measured by an NGK system hardness scale is in a range of 10 to 35 mm.
In the present invention, the forming material further containing the layered double hydroxide in addition to the ceramic forming material is used. Examples of other components include an organic binder, water which is a dispersing medium, dispersant, pore former and the like.
In a case where the ceramic clay is used in manufacturing the ceramic structure, after the ceramic clay is formed into the ceramic formed article, and the ceramic formed article is fired, the ceramic forming material in the ceramic clay constitutes the ceramic structure. Examples of the ceramic forming material include an alumina forming material, mullite forming material, zirconia forming material, cordierite forming material, aluminum titanate forming material, silicon nitride forming material, silicon carbide forming material, aluminum nitride forming material and the like. The examples of these forming materials include oxide and the like containing the elements contained in the forming materials. The examples of the cordierite forming material include oxide, hydroxide, or carbonate containing at least one element selected from a group consisting of magnesium, aluminum, and silicon, such as talc, kaolin, alumina, aluminum hydroxide, silica, and magnesia.
A content ratio of the ceramic forming material is preferably 50 to 99.5% by mass with respect to a total of the ceramic forming material and layered double hydroxide. Even when the ratio is less than 50% by mass, there is not any problem, but a composition of the obtained ceramic structure is not obtained as desired, or a problem sometimes occurs in respect of cost. When the ratio exceeds 99.5% by mass, it is difficult to form the material.
The organic binder is used if necessary, in order to enhance plasticity and forming property of kneaded clay prepared by kneading the forming material. The binder is used if necessary, in order to perform a function of a shape retainer which holds the shape of the structure, when used in manufacturing the ceramic structure. On the other hand, the organic binder has problems that pollution or global warming by generation of CO₂or harmful gas is promoted during the firing, a space occupied by the organic binder at the forming time results in defects, or defects such as cracks are generated in the honeycomb structure, and strength of the ceramic structure drops. Therefore, the content of the binder in the forming material needs to be minimized. Therefore, in the present invention, the content ratio of the organic binder is set to preferably 10 parts by mass or less, further preferably 5 parts by mass or less with respect to a total of 100 parts by mass of the ceramic forming material and the layered double hydroxide. The ratio may be 0 part by mass depending on applications (any binder may not be contained).
The examples of the organic binder include organic polymer. Concretely, the examples include hydroxypropoxyl methylcellulose, hydroxypropyl methylcellulose, methylcellulose, hydroxyethyl cellulose, carboxyl methylcellulose, polyvinyl alcohol and the like. The organic binder may be used alone or as a combination of two or more types.
In the present invention, even when the organic binder is used, as described above, the content ratio of the organic binder is preferably suppressed to 10 parts by mass or less with respect to a total of 100 parts by mass of the ceramic forming material and the layered double hydroxide. Accordingly, when the binder is used in manufacturing the ceramic structure, the binder is to solve the problem that the pollution or global warming by the generation of CO₂or harmful gas is promoted during the firing, the space occupied by the organic binder at the forming time results in defects, or defects such as cracks are generated in the ceramic structure, and the strength of the ceramic structure drops. To compensate for the drop of the plasticity or forming property of the ceramic clay, the forming material is used having both a ceramic forming function and plasticity (forming property) imparting function, and further containing the layered double hydroxide.
The layered double hydroxide means representation by the following formula (I).
[M²⁺ _1-xM³⁺ _x(OH)₂][Aⁿ⁻ _x/n.yH₂O] (I)
In the above formula (I), M²⁺, M³⁺, Aⁿ⁻ denote bivalent cation, tervalent cation, anion, respectively, n denotes anion valence (1≦n≦3), x is approximately in a range of 0.2≦x≦0.33 depending on a combination of cation or anion, and y takes an arbitrary value corresponding to the combination with cation or anion, or the value of x.
The examples of the above-described bivalent cation include bivalent metal ions such as Mg²⁺, Ca²⁺, Sr²⁺, Zn²⁺, Ni²⁺, Co²⁺, and Fe²⁺, and one type alone, or a combination of two or more types may be used. The examples of the above-described tervalent cation include tervalent metal ions such as Al³⁺, Fe³⁺, Cr³⁺, Ti³⁺, Y³⁺, Ce³⁺, and one type alone, or a combination of two or more types may be used. The examples of the above-described anion include CO₃ ²⁻, Cl⁻, NO₃ ⁻, CH₃COO⁻, PO₄ ³⁻, and one type alone, or a combination of two or more types may be used.
The layered double hydroxide for use in the present invention is contained at a ratio of preferably 0.5 to 50% by mass, further preferably 1 to 30% by mass with respect to a total with ceramic forming material. When the ratio is less than 0.5% by mass, sufficient forming property is not sometimes developed. Even when the ratio exceeds 50% by mass, there is not any problem, but a composition of the obtained ceramic structure is not easily obtained as desired. As to the ceramic clay containing the layered double hydroxide for use in the present invention, hardness measured by an NGK system hardness scale is in a range of 10 to 35 mm, preferably 12 to 33 mm. When the hardness is less than 10 mm, and the clay is used in manufacturing the ceramic structure, the shape retaining property of the ceramic formed article drops. When the hardness exceeds 35 mm, the forming property drops.
The NGK system hardness scale measures the hardness of the ceramic clay, and is constituted in such a manner that a conical tip portion is connected to a support portion by a spring material, and contained in a cylindrical sheath portion. When the clay is soft, the scale takes a small value. When the clay is hard, the scale takes a large value (see Japanese Patent Application Laid-Open No. 2003-89575).
As the layered double hydroxide for use in the present invention, the layered double hydroxide having an appropriate composition is usable depending on the type of the ceramic forming material. For example, in a case where the cordierite forming material is used as the ceramic forming material, layered double hydroxide containing Mg²⁺ which is bivalent cation, and Al³⁺ which is tervalent cation can be used. When a silicon nitride forming material is used, magnesium oxide or yttrium oxide is used as a sintering aid. Therefore, layered double hydroxide containing Mg²⁺ which is bivalent cation, and Y³⁺ which is tervalent cation is usable. As to anion, appropriate anion can be selected in accordance with synthesis conditions of the layered double hydroxide or manufacturing conditions of ceramic. It is to be noted that with regard to the layered double hydroxide for use in the present invention, one type alone, or a combination of two or more types may be used.
As the layered double hydroxide for use in the present invention, hydrotalcite represented by the following formula (II) is preferable from viewpoints of price and impurity amount. Especially, a synthesized product of hydrotalcite is preferable because it is inexpensive as compared with smectite (synthesized product, refined product of mineral) or the like:
Mg_1-xAl_x(OH)₂(CO₃)_x/2.mH₂O (II),
(where x denotes a value in a range of 0.2≦x≦0.33, and m denotes an arbitrary value corresponding to the value of x).
As to the layered double hydroxide for use in the present invention, hydrotalcite represented by the following formula (III) is further preferable, because forming property is satisfactory.
Mg₆Al₂(OH)₁₆CO₃.4H₂O (III)
When the forming material is used in manufacturing a porous ceramic structure, a pore former may be further contained in the forming material. This pore former constitutes a casting form for pores, the pores having desired shapes, sizes, and distribution are formed in the honeycomb structure, porosity is increased, and high-porosity porous honeycomb structure can be obtained. The examples of the pore former include graphite, flour, starch, phenol resin, polymathacrylic methyl, polyethylene, polyethylene terephthalate, foaming resin (acrylonitric plastic balloon) and the like. These materials form the pores, and burn out themselves. Above all, from a viewpoint of inhibiting the generation of CO₂or harmful gas and the generation of cracks, the foaming resin is preferable. It is to be noted that when the pore former is used, the total of content ratios of the organic binder and pore former is 10 parts by mass or less, preferably 8 parts by mass or less with respect to 100 parts by mass of the forming material.
A ratio at which water is contained as a dispersing, medium differs with the forming material for use, and it is difficult to uniquely determine the ratio, but the amount of water is preferably adjusted in such a manner as to achieve the above-described hardness.
A method of kneading the above-described forming material is not especially limited, and the examples include a method of using a kneader, a vacuum kneading machine or the like.
The ceramic formed article of the present invention is obtained by forming the above-described ceramic clay.
The shape of the ceramic formed article is not especially limited, and the examples include a sheet shape, tube shape, lotus root shape, honeycomb shape and the like. Above all, in the honeycomb shape, a honeycomb formed article is preferably used in which honeycomb-shaped partition walls extend through between two end faces to thereby form a plurality of cells. When the honeycomb formed article is used in filter application such as DPF, end portions of the cells are preferably alternately plugged in two end face portions. The whole shape of the ceramic formed article is not especially limited, and the examples of the shape of the honeycomb formed article include a cylindrical shape, a square pole shape, a triangular pole shape and the like. The cell shape (cell shape in a section vertical with respect to a cell forming direction) of the honeycomb formed article is not especially limited, and the examples include a quadrangular shape, a hexagonal shape, a triangular shape and the like.
A method of preparing the honeycomb formed article of the present invention is not especially limited, and forming methods which have heretofore been known are usable such as spinning lathe forming, extrusion forming, injection forming, press forming, and sheet forming. Above all, a method of extruding the ceramic clay prepared as described above using a ferrule having a desired cell shape, partition wall thickness, and cell density may be a preferable example. A drying method is not especially limited, and drying methods which have heretofore been known are usable such as hot air drying, microwave drying, dielectric drying, reduced-pressure drying, vacuum drying, and freezing drying. Above all, a drying method obtained by combining the hot air drying and the microwave drying or the dielectric drying is preferable, because the whole forming article can be quickly and uniformly dried.
The ceramic formed article obtained as described above may be calcined (degreased) to form a calcined article. The calcining means an operation of burning and removing organic materials (binder, pore former, dispersant, etc.) in the formed article. In general, since the burning temperature of the organic binder is about 100 to 300° C., and that of the pore former is about 200 to 800° C., the calcining temperature may be set at about 200 to 1000° C. A calcining time is not especially limited, and is usually about one to ten hours. A calcining atmosphere is appropriately selected in accordance with the type of the ceramic forming material, and the examples include air atmosphere, oxygen atmosphere, nitrogen atmosphere, argon atmosphere, vacuum atmosphere and the like.
The above-described ceramic formed article (calcined article if necessary) is fired (actually fired) to thereby obtain the ceramic structure of the present invention. The actual firing means an operation for sintering and densifying the forming material in the calcined article to secure a predetermined strength. Since firing conditions (temperature•time) differ with the type of the forming material, appropriate conditions may be selected in accordance with the type. In the present invention, for example, when the cordierite forming material is used, the ceramic formed article is preferably fired at 1300 to 1500° C. The article is further preferably fired at 1350 to 1450° C. When the temperature is less than 1350° C., a target crystal phase (e.g., cordierite) is not obtained in some case. When the temperature exceeds 1500° C. or less, the article is sometimes molten. A firing atmosphere is appropriately selected in accordance with the type of the ceramic forming material, and the examples include air atmosphere, oxygen atmosphere, nitrogen atmosphere, argon atmosphere, vacuum atmosphere and the like.
According to the present invention, there is provided a ceramic structure obtained by the above-described method, and the structure is mainly composed of ceramic (e.g., cordierite) having few defects or cracks and having high strength. The examples of a preferable composition of cordierite include 2MgO.2Al₂O₃.5SiO₂. The examples of the preferable composition of alumina include Al₂O₃, an example of the preferable composition of mullite is 3Al₂O₃.2SiO₂, an example of the preferable composition of zirconia is ZrO₂, an example of the preferable composition of aluminum titanate is Al₂TiO₅, an example of the preferable composition of silicon nitride is Si₃N₄, an example of the preferable content of silicon carbide is SiC, and an example of the preferable content of aluminum nitride is AlN.
The present invention will be described further concretely hereinafter in accordance with examples, and the present invention is not limited to the examples.
It is to be noted that as indexes indicating that the ceramic structure obtained in the example has the high strength, isostatic breaking strength and thermal expansion coefficient were measured. A method of measuring the isostatic breaking strength conformed to Automobile. Standard JASO-M505-87 of the Society of Automotive Engineers of Japan. Furthermore, as an index indicating that the generation of CO₂or harmful gas is reduced during the firing in the ceramic structure obtained in the example, a weight decrease during the firing was measured. In a measurement method of the weight decrease during the firing, a weight (M₁) of the honeycomb structure before fired, and a weight (M₂) of the structure after fired were measured, and the weight decrease (%)=[(M₁−M₂)/M₁]×100 was calculated.

EXAMPLE 1

Hydrotalcite which was layered double hydroxide was added and mixed to kaolin, talc, alumina, aluminum hydroxide, and silica which were ceramic (cordierite) forming materials to form a forming material. It is to be noted that a content ratio of hydrotalcite was set to 10% by mass with respect to a total with the cordierite forming material, and other amounts were adjusted in such a manner as to approach a cordierite composition. Furthermore, as dispersant, surfactant (1 part by mass with respect to 100 parts by mass of the forming material) and water (35 parts by mass with respect to 100 parts by mass of the forming material) were added and kneaded to thereby obtain a compact article of ceramic clay. When hardness of this clay was measured by an NGK system hardness scale, the hardness was 23 mm. This article was formed into a honeycomb shape by an extrusion forming machine, and it was then possible to form the article without causing clogging of a ferrule or defective forming. The obtained honeycomb formed article was dried by microwave and hot air, and fired at 1420° C. for seven hours. When a crystal phase of the obtained honeycomb structure was identified by X-ray diffraction, cordierite was a main phase. An isostatic breaking strength of the honeycomb structure was 9 MPa, and indicated a value larger than that of Comparative Example 1. A weight decrease during firing was 9%, and was smaller than that of Comparative Example 1.

EXAMPLE 2

Hydrotalcite which was layered double hydroxide, and methylcellulose which was an organic binder were added and mixed to kaolin, talc, alumina, aluminum hydroxide, and silica which were ceramic (cordierite) forming materials. It is to be noted that with regard to content ratios of these components, the ratio of hydrotalcite was set to 10% by mass with respect to a total with the cordierite forming material, and other amounts were adjusted in such a manner as to approach a cordierite composition. The ratio of methylcellulose was set to 4 parts by mass with respect to 100 parts by mass of the cordierite forming material. Furthermore, as dispersant, surfactant (1 part by mass with respect to 100 parts by mass of the forming material) and water (36 parts by mass with respect to 100 parts by mass of the forming material) were added and kneaded to thereby obtain a compact article of ceramic clay. When hardness of the clay was measured by an NGK system hardness scale, the hardness was 23 mm. This article was formed into a honeycomb shape by an extrusion forming machine, and it was then possible to form the article without causing clogging of a ferrule or defective forming. The obtained honeycomb formed article was dried by microwave and hot air, and fired at 1420° C. for seven hours. When a crystal phase of the obtained honeycomb structure was identified by X-ray diffraction, cordierite was a main phase. An isostatic breaking strength of the honeycomb structure was 6 MPa, and indicated a value slightly larger than that of Comparative Example 1. A weight decrease during firing was 12%, and was slightly smaller than that of Comparative Example 1.

COMPARATIVE EXAMPLE 1

Methylcellulose which was an organic binder was added and mixed to kaolin, talc, alumina, aluminum hydroxide, and silica which were ceramic (cordierite) forming materials. It is to be noted that a content ratio of methylcellulose was set to 8 parts by mass with respect to 100 parts by mass of the cordierite forming materials. Furthermore, potassium laurate (1 part by mass with respect to 100 parts by mass of the forming materials) which was surfactant, and water (33 parts by mass with respect to 100 parts by mass of the forming materials) were added and kneaded to thereby obtain a compact article of kneaded clay. When hardness of this clay was measured by an NGK system hardness scale, the hardness was 23 mm. This article was formed into a honeycomb shape by an extrusion forming machine, and it was then possible to form the article without causing clogging of a ferrule or defective forming. The obtained formed article was dried by microwave and hot air, and fired at 1420° C. for seven hours. When a crystal phase of the obtained honeycomb structure was identified by X-ray diffraction, cordierite was a main phase. An isostatic breaking strength of the honeycomb structure was 4 MPa. A weight decrease during firing was 13%.
The present invention is preferably used in various apparatuses, devices, and members which are effective as measures for preventing pollution and global warming in various fields such as chemistry, electric power, iron and steel, and industrial waste disposal.

Claims

1. A ceramic clay obtained by kneading a forming material containing a ceramic forming material,

wherein the forming material contains layered double hydroxide, in addition to the ceramic forming material, at a ratio of 0.5 to 50% by mass with respect to a total with the ceramic forming material, and

hardness measured by an NGK system hardness scale is in a range of 10 to 35 mm.

2. The ceramic clay according to claim 1, wherein the layered double hydroxide is hydrotalcite represented by the following formula (II):

Mg_1-xAl_x(OH)₂(CO₃)_x/2.mH₂O (II),

(where x denotes a value in a range of 0.2≦x≦0.33, and m denotes an arbitrary value corresponding to the value of x).

3. The ceramic clay according to claim 1, wherein the layered double hydroxide is hydrotalcite represented by the following formula (III):

Mg₆Al₂(OH)₁₆CO₃.4H₂O (III).

4. A ceramic formed article obtained by forming the ceramic clay according to claim 1.

5. The ceramic formed article according to claim 4, comprising: a honeycomb formed article formed into a honeycomb shape.

6. A ceramic structure obtained by firing the ceramic formed article according to claim 4.

7. A method of manufacturing a ceramic clay, comprising the steps of: kneading a forming material containing a ceramic forming material to obtain the ceramic clay,

wherein the forming material containing layered double hydroxide, in addition to the ceramic forming material, is used at a ratio of 0.5 to 50% by mass with respect to a total with the ceramic forming material.

8. The method of manufacturing the ceramic clay according to claim 7, wherein hydrotalcite represented by the following formula (II) is used as the layered double hydroxide:

Mg_1-xAl_x(OH)₂(CO₃)_x/2.mH₂O (II),

9. The method of manufacturing the ceramic clay according to claim 7, wherein hydrotalcite represented by the following formula (III) is used as the layered double hydroxide:

Mg₆Al₂(OH)₁₆CO₃.4H₂O (III).

10. The method of manufacturing the ceramic clay according to claim 7, wherein hardness of the obtained ceramic clay, measured by an NGK system hardness scale, is in a range of 10 to 35 mm.

11. A method of manufacturing a ceramic formed article, comprising the steps of: further forming the ceramic clay obtained by the method according to claim 7 to obtain the ceramic formed article.

12. The method of manufacturing the ceramic formed article according to claim 11, further comprising the steps of: forming the ceramic clay into a honeycomb shape to obtain a honeycomb formed article.

13. A method of manufacturing a ceramic structure, comprising the steps of: further firing the ceramic formed article obtained by the method according to claim 11 to obtain a ceramic structure.