阅读说明：本技术 氧化铝质陶瓷和陶瓷加热器 (Alumina ceramic and ceramic heater ) 是由 王雨丛 于 2019-07-25 设计创作，主要内容包括：本发明的氧化铝质陶瓷含有氧化铝结晶粒子、氧化锆结晶粒子以及Ti、Mg和Si,关于Ti、Mg和Si的合计的含量,将Ti作为TiO-2换算,Mg作为MgO换算,Si作为SiO-2换算时,为1.4质量％以上,氧化锆结晶粒子中,将构成氧化锆结晶粒子的氧化锆表示为ZrO-2时,相对于ZrO-2的稀土元素的含量,以氧化物换算为2mol％以下,氧化锆结晶粒子中含有最大长度为1μm以上的单独粒子和最大长度为1μm以上的凝聚粒子中的至少一种,最大长度为1μm以上的所述单独粒子和最大长度为1μm以上的凝聚粒子的比率的合计,是氧化锆结晶粒子全体的50体积％以上。(The alumina ceramic of the present invention contains alumina crystal particles, zirconia crystal particles, and Ti, Mg, and Si, and the total content of Ti, Mg, and Si is Ti which is TiO 2 Converted Mg as MgO and Si as SiO 2 In terms of the amount of the zirconia grains, the zirconia grains constituting the zirconia grains are represented by ZrO 2 Relative to ZrO 2 The content of the rare earth element (b) is 2 mol% or less in terms of oxide, and the zirconia crystal particles contain single particles having a maximum length of 1 μm or more and single particles having a maximum length of 1 μm or moreThe ratio of the single particles having a maximum length of 1 μm or more to the aggregated particles having a maximum length of 1 μm or more is 50 vol% or more of the whole zirconia crystal particles.)

1. An alumina ceramic containing alumina crystal particles, zirconia crystal particles, and Ti, Mg and Si,

in the case of using the Ti as TiO₂In terms of MgO, Mg and Si are converted to SiO₂A total content of the Ti, the Mg and the Si is 1.4 mass% or more in terms,

in the zirconia crystal particles, the zirconia constituting the zirconia crystal particles is represented by ZrO₂When a rare earth element is present with respect to the ZrO₂The content of (b) is 2 mol% or less in terms of oxide, and the zirconia crystal particles contain at least one of individual particles having a maximum length of 1 μm or more and aggregated particles having a maximum length of 1 μm or more,

the total of the ratio of the individual particles having a maximum length of 1 μm or more and the agglomerated particles having a maximum length of 1 μm or more is 50% by volume or more of the whole zirconia crystal particles.

2. The alumina ceramic according to claim 1, wherein a total content of Ca, Ba, Sr, Zn, Mn and Ce is calculated in terms of Ca, Ba, Sr, Mn, MnO and Ce, and wherein Ba is calculated in terms of CaO, Sr is calculated in terms of SrO, and Mn is calculated in terms of MnO₂O₃In terms of equivalent, 300ppm or less.

3. The alumina ceramic according to claim 1 or 2, wherein the rare earth element is Y.

4. The alumina ceramic according to any one of claims 1 to 3, wherein a ratio of monoclinic crystals contained in the zirconia crystal particles is 25% or less.

5. The alumina ceramic according to any one of claims 1 to 4, wherein the three-point bending strength at room temperature is 800MPa or more.

6. The alumina ceramic according to any one of claims 1 to 5, wherein the three-point bending strength at 800 ℃ is 600MPa or more.

7. A ceramic heater comprising a heating element and an insulating ceramic covering the heating element, wherein the insulating ceramic is the alumina ceramic according to any one of claims 1 to 6.

Technical Field

The present invention relates to alumina ceramics and ceramic heaters.

Background

Having a semiconductorA heater for heating a substrate, a heater for a vaporizer of a kerosene hot air heater, a hot water heater, or a heater integrated with a gas sensor such as an oxygen sensor. As these heaters, ceramic heaters are used. The ceramic heater is configured such that a metal heating element is embedded in an insulating layer using insulating ceramics. Alumina is mainly used for insulating ceramics. In the case of alumina, ZrO tends to be present for the purpose of improving mechanical strength₂The particles of (a) are dispersed in a ceramic of alumina.

For example, patent document 1 discloses an alumina sintered substrate in which 2 to 6 mol% of Y is dissolved in a solid solution₂O₃ZrO of₂Adding into alumina to reduce sintering temperature.

In addition, there is a demand for miniaturization of heaters. Therefore, the insulating ceramics used for the heater are required to have higher mechanical strength and durability at high temperatures in order to achieve high durability even in small parts.

Patent document 2 discloses ZrO having a small particle size₂Added to alumina to improve mechanical strength. Containing ZrO having a small particle diameter₂As ZrO₂Has a high ratio of tetragonal crystals of metastable phase(s). In addition, when stressed, ZrO₂Easily changing from a tetragonal phase to a monoclinic phase. As a result, alumina ceramics having high mechanical strength can be obtained.

Prior art documents

Patent document

Japanese laid-open patent publication No. H03-223157

Japanese laid-open patent publication No. 2007 & 2695524

Disclosure of Invention

The alumina ceramic of the present invention comprises alumina crystal particles, zirconia crystal particles, and Ti, Mg and Si, wherein the total content of Ti, Mg and Si is such that Ti is TiO₂Conversion of the Mg into MgO and the Si into SiO₂1.4 mass% or more in terms of the amount of Zr in the zirconia crystal particlesO₂When a rare earth element is present with respect to the ZrO₂The content of (b) is 2 mol% or less in terms of oxide, the zirconia crystal particles contain at least one of individual particles having a maximum length of 1 μm or more and aggregated particles having a maximum length of 1 μm or more, and the total of the ratios of the individual particles having a maximum length of 1 μm or more and the aggregated particles having a maximum length of 1 μm or more is 50 vol% or more of the entire zirconia crystal particles.

The ceramic heater of the present invention comprises a heating element and an insulating ceramic covering the heating element, wherein the insulating ceramic is the above-mentioned alumina ceramic.

Detailed Description

The alumina ceramic of the present invention contains alumina crystal particles, zirconia crystal particles, and Ti, Mg and Si. The content of the rare earth element in the zirconia crystal particles is 2 mol% or less in terms of oxide with respect to zirconia. In this case, as the rare earth element, lanthanoid (atomic number 57 to 71) elements can be mentioned in addition to Y (yttrium). The term "converting the rare earth element into an oxide" means that when RE represents a rare earth element, RE is defined as RE₂O₃Expressed as mole percent. In this case, the zirconia is used as expressed as ZrO₂Molecular weight of (a).

In addition, the alumina ceramics contain Ti, Mg and Si. The total content of Ti, Mg and Si is 1.4 mass% or more in terms of oxides of the respective elements. In this case, the content of Ti is expressed as TiO₂Mass of the time. The Mg content is expressed as MgO by mass. The content of Si is expressed as SiO₂Mass of the time.

In the case of the alumina ceramic, the total amount of alumina and zirconia may be 93 mass% or more. In this case, the content of alumina is Al₂O₃Mass of the time. The content of zirconia being expressed as ZrO₂Mass of the time. In addition, the ratio of alumina to zirconia, for example, aluminum oxide is converted to Al₂O₃Zirconium oxide as ZrO₂In terms of conversion, Al may be used in terms of mass ratio₂O₃:ZrO₂The ratio is 95: 5-80: 20. In this case, the selected zirconia crystal particles may contain a rare earth element. The state of the rare earth element contained in the zirconia crystal particles can be confirmed by using an electron microscope equipped with an element analyzer.

The zirconia crystal particles may be present as single particles, that is, single zirconia crystal particles, among the alumina crystal particles, or may be present as aggregated particles in which at least 2 or more zirconia crystal particles are in contact with each other in at least a part thereof.

The alumina ceramic contains at least one of zirconia crystal particles which are individual particles having a maximum length of 1 [ mu ] m or more and zirconia crystal particles which are aggregated particles having a maximum length of 1 [ mu ] m or more. In this case, the total ratio of the individual particles of zirconia having a maximum length of 1 μm or more and the aggregated particles of zirconia having a maximum length of 1 μm or more among the zirconia crystal particles contained in the alumina ceramic may be 50% by volume or more.

The zirconia crystal particles among the alumina crystal particles may contain partially stabilized ZrO containing metastable tetragonal crystals₂And (4) crystallizing the particles. Thus, even if the alumina ceramic cracks, the progress of the cracks can be suppressed. In this case, crystal particles that undergo phase transition from tetragonal to monoclinic may be present among the zirconia crystal particles. When the phase of the zirconia crystal particles is changed from tetragonal to monoclinic, the volume of the zirconia crystal particles increases. The stress concentration at the front end of the crack is relaxed by the volume change of the zirconia crystal particle. This can suppress the progress of cracks occurring in the alumina ceramic. In this case, the stability of the tetragonal zirconia crystal particles increases as the amount of the rare earth element or the like dissolved increases.

However, if the stability of the tetragonal crystal is excessively high, a phase transition to a monoclinic crystal is difficult to occur at the stress concentration portion at the tip of the crack. As a result, the mechanical strength may not be improved. In this case, the amount of the rare earth element in the zirconia crystal particles may be 2 mol% or less in terms of oxide. The rare earth element may be dissolved in a solid solution in a range in which the tetragonal crystal can be stabilized, depending on the particle size of the zirconia crystal particles.

The smaller the particle diameter of the tetragonal zirconia crystal particles, the higher the stability. Therefore, by making the zirconia crystal particles fine, the ratio of tetragonal crystals can be increased. In addition, in the alumina ceramics, the dispersibility of the zirconia crystal particles is improved. When the zirconia crystal particles are fine particles, the zirconia crystal particles contain metastable tetragonal zirconia crystal particles in which the amount of solid solution of a rare earth element or the like is small or the rare earth element or the like is not solid-dissolved. However, as described above, if the stability of the tetragonal crystal is excessively high, a phase transition to a monoclinic crystal is difficult to occur at the stress concentration portion at the tip of the crack, and the mechanical strength cannot be improved.

The zirconia crystal particles contained in the alumina ceramic of the present invention contain individual particles having a maximum length of 1 μm or more, or aggregated particles having a maximum length of 1 μm or more. When the maximum length of the zirconia crystal particles is 1 μm or more, phase transition from tetragonal to monoclinic is likely to occur, and the alumina ceramic is less likely to be broken. This effect is likely to cause phase transition from tetragonal to monoclinic in the same manner not only in the individual zirconia crystal particles but also in agglomerated particles in which at least a part of 2 or more zirconia crystal particles are in contact with each other. When the above-mentioned individual particles having a maximum length of 1 μm or more or aggregated particles having a maximum length of 1 μm or more are present in a given amount or more, slippage at the phase interface between alumina and zirconia at high temperature hardly occurs, and the high-temperature strength of the alumina ceramic is improved.

In particular, when the alumina ceramic contains zirconia crystal particles, the flexural strength at room temperature (25 ℃ C.) can be increased to 800MPa or more. Further, the flexural strength at high temperature (800 ℃ C.) can be 600MPa or more. The zirconia crystal particles herein may contain a rare earth element in a proportion of 2 mol% or less. The zirconia crystal particles may be at least one of individual particles having a maximum length of 1 μm or more and aggregated particles having a maximum length of 1 μm or more. In this case, the ratio of at least one of the individual particles having a maximum length of 1 μm or more and the aggregated particles having a maximum length of 1 μm or more may be 50% by volume or more with respect to the entire zirconia crystal particles containing the rare earth element.

The average particle diameter of the zirconia crystal particles in the alumina ceramic may be, for example, 0.5 μm or more and 1.5 μm or less. The agglomerated particles of the zirconia crystal particles can be regarded as substantially one particle. The average particle diameter of the aggregated particles may be 0.5 μm or more and 2.0 μm or less, and the maximum length of the aggregated particles may be 10 μm or less.

The average particle diameter of the alumina crystal particles may be, for example, 1.5 μm or more and 5 μm or less.

Further, the maximum diameter of each particle contained in the alumina ceramic is preferably 15 μm. This is because if the maximum diameter of each particle is 15 μm or less, it is difficult to become a starting point of fracture. The various particles are alumina crystal particles and zirconia crystal particles. The zirconia crystal particles include the above-described individual particles and agglomerated particles.

The alumina ceramic contains Ti, Mg and Si. The total content of Ti, Mg and Si is that Ti is subjected to TiO₂Conversion of Mg into MgO and conversion of Si into SiO₂In terms of conversion, it may be 1.4% by mass or more. Also, if reference is made to the equilibrium phase diagram, these oxides, i.e. TiO₂MgO and SiO₂The eutectic temperature of (A) is 1300 ℃ or lower. Therefore, when a metal serving as a heating element of the heater and an alumina ceramic containing these components are simultaneously fired, at least one component of Al, Zr, Ti, Mg, Si and rare earth elements, which are constituent components of the alumina ceramic, is easily diffused into the heating element in a state of a metal oxide in some cases. Thus, the bonding strength between the alumina ceramics and the heating element can be improved.

Further, by containing Ti, Mg, and Si as oxides, respectively, zirconia crystal particles contained in the alumina porcelain can be grown. For example, even when zirconia powder having an average particle size of 1 μm or less is used as the raw material powder, individual particles of zirconia having a maximum length of 1 μm or more or aggregated particles of zirconia having a maximum length of 1 μm or more are easily formed in the alumina ceramic.

Ti content in oxide (TiO)₂) In terms of the content, the content may be 0.5 mass% or more and 1.0 mass% or less. The content of Mg may be 0.2 mass% or more and 4.0 mass% or less in terms of oxide (MgO). The content of Si is in the form of oxide (SiO)₂) It may be 0.7 mass% or more and 1.4 mass% or less in terms of conversion. The total content of Ti, Mg and Si is determined by subjecting Ti to TiO₂Conversion of Mg into MgO and conversion of Si into SiO₂The content in terms of conversion may be 6.4% by mass or less, particularly 5.0% by mass or less. When the total content of Ti, Mg, and Si in the above-described conversion is 6.4 mass% or less, excessive crystal grain formation of zirconia crystal particles can be suppressed, and an alumina ceramic having high mechanical strength can be obtained.

The alumina ceramic may have a porosity of 1.0% or less. If the porosity is 1.0% or less, the number of pores that become the starting points of mechanical fracture can be reduced. This makes it possible to obtain an alumina ceramic having high mechanical strength.

In the alumina ceramic, the total content of Ca, Ba, Sr, Zn, Mn and Ce is calculated by CaO, BaO, SrO, MnO and Ce₂O₃In terms of conversion, it may be 300ppm or less. If the total of these elements is 300ppm or less in terms of oxides, the flexural strength of the alumina ceramic at high temperatures can be further improved. Some of Ca, Ba, Sr, Zn, Mn, and Ce may be used as solid solution elements for partially stabilizing zirconia. In addition, the alumina crystal grains as the main component and/or the oxide containing Ti, Mg, and Si as the auxiliary components may react with each other to form a grain boundary phase. Ca, Ba, Sr, Zn, Mn and Ce are used as grain boundary phases contained in the metal oxide respectively, and the softening temperature is above 700 ℃. By setting the total content of Ca, Ba, Sr, Zn, Mn, and Ce to 300ppm or less in terms of oxides, an alumina ceramic having high mechanical strength at high temperatures can be obtained. Here, ppm is to be understood asIs represented by 1 x 10^-6The mass ratio of the stages.

The rare earth element contained in the zirconia crystal particles may be Y. Among rare earth elements used as solid solution elements for partially stabilizing zirconia, Y has a high effect of stabilizing a tetragonal crystal in particular. In addition, Y in the so-called zirconia crystal particles is represented by Y₂O₃2 mol% in terms of content means content based on ZrO₂Containing Y₂O₃Is 2 mol%.

The ratio of monoclinic zirconia crystal particles contained in the zirconia crystal particles may be 25% or less. When the ratio of monoclinic zirconia crystal particles is 25% or less, the ratio of tetragonal zirconia crystal particles becomes high, and the effect of improving the mechanical strength of the alumina ceramic by the transformation from tetragonal crystals to monoclinic crystals becomes greater.

Alumina ceramics, in accordance with JIS R1601: the three-point bending strength at room temperature (25 ℃) of 2008 may be 800MPa or more. When the three-point bending strength at room temperature is 800MPa or more, a ceramic heater having high mechanical strength, excellent reliability, and high vibration resistance can be obtained. In this case, the volume of the alumina ceramic (insulating ceramic) in the alumina ceramic heater may be about half (40 vol% or more and 60 vol% or less).

Alumina ceramics, according to JIS R1604: the three-point bending strength at high temperature of 2008 may be 600MPa or more. In this case, the test temperature may be 800 ℃ or higher. For example, if the three-point bending strength at a temperature of 800 ℃ is 600MPa or more, the resistance to the test of being left (held) at a high temperature or the temperature cycle test is high. In this case, the volume of the alumina ceramic (insulating ceramic) in the alumina ceramic heater may be about half (40 vol% or more and 60 vol% or less).

The ceramic heater includes a heating element and an insulating ceramic covering the heating element. Here, the heating element is formed of a metal material. When the above-described alumina ceramic is used as the insulating ceramic, a ceramic heater having high adhesion (in other words, may be referred to as adhesion or bondability) between the heating element and the insulating ceramic can be obtained. In this case, the volume ratio of the alumina ceramic in the alumina ceramic heater may be about half (40 vol% or more and 60 vol% or less). The ceramic heater thus obtained has high durability against vibration and temperature cycles.

The ceramic heater may be, for example, a flat plate-like ceramic heater in which a metal heating element is sandwiched by 2 insulating ceramics. Further, a cylindrical or columnar ceramic heater having a heating element between an inner cylinder and an outer cylinder which are insulating ceramics may be used. The ceramic heater may be provided with various gas sensors inside or outside.

The ceramic heater is not limited to the above-described embodiment. In other words, the present invention includes a heating element and an insulating ceramic covering the heating element, but the insulating ceramic may have another form as long as it is the above-described alumina ceramic. For example, a component other than the heating element and the gas sensor may be provided.

The alumina ceramic can be produced, for example, by the following method.

For example, as the main raw material powder, zirconia powder having an average particle size of 0.1 μm or more and 0.3 μm or less and alumina powder having an average particle size of 0.6 μm or more and 3.0 μm or less are prepared. The zirconia powder may be partially stabilized zirconia containing 1 to 2 mol% of a rare earth element. The ratio of the alumina powder to the zirconia powder may be, for example, in terms of mass ratio: the zirconium oxide powder is 95:5 to 80: 20.

In addition, as the sintering aid, titanium oxide powder, magnesium oxide powder (magnesium carbonate powder or magnesium hydroxide powder may be used instead of magnesium oxide powder), and silicon oxide powder are prepared. The titanium oxide powder, magnesium carbonate powder, magnesium hydroxide powder, and silicon oxide powder may have a purity of 99 mass% or more and an average particle diameter of 2 μm or less. The average particle diameter may be 0.1 to 2 μm. For convenience, the main raw material powder and the sintering aid may be represented as follows. The alumina being Al₂O₃The zirconia being ZrO₂Titanium oxide ofTiO₂Magnesium oxide is MgO, magnesium carbonate is MgCO₃Magnesium hydroxide is Mg (OH)₂The silicon oxide is SiO₂. Hereinafter, the latter expression will be used.

The following composition can be exemplified as the composition of the main raw material powder and the sintering aid. Mixing Al₂O₃Powder, ZrO₂TiO when the total of the powder and the sintering aid is 100 mass%₂0.5 to 1.0 mass%, MgO 0.2 to 4.0 mass%, SiO₂Is 0.7 to 1.4 mass%. The total content is 1.4% by mass or more. TiO 2₂MgO and SiO₂The total amount of (3) may be 6.0% by mass or less, particularly 5.0% by mass or less.

Can be prepared by mixing Al in a predetermined ratio₂O₃Powder, ZrO₂Powder, TiO₂Powder, MgO powder and SiO₂The powder is wet-mixed by, for example, a ball mill, dried, and then molded into a molded body by a known molding method such as press molding, cast molding, or injection molding. The molded article thus obtained may be subjected to, for example, Cold Isostatic Pressing (CIP) treatment.

The obtained molded body is degreased and fired at a temperature of, for example, 1400 ℃ to 1600 ℃ in the air depending on the composition and amount of the auxiliary agent, thereby obtaining an alumina ceramic.

The heating element can be, for example, a material prepared by blending a predetermined amount of tungsten (W) powder and Al₂O₃A conductive paste of powder. A conductive paste is printed on the surface of a molded body of alumina ceramics. If necessary, a conductive paste is embedded in a via hole provided in the molded body. Thus, a conductor pattern is formed. Thereafter, the conductor pattern and the molded body are simultaneously fired. In this case, the firing atmosphere may be a reducing atmosphere.

By using fine ZrO in this way₂The powder is fired together with the sintering aid to grow zirconia crystal particles, and can form single particles having a maximum length of 1 μm or more or aggregated particles having a maximum length of 1 μm or more. According to such a wayThe method can improve the bending strength (three-point bending strength) of the alumina ceramic at room temperature (25 ℃) and high temperature (800 ℃) as compared with the crystal structure in which the zirconia crystal particles having a particle diameter of 1 μm or less are uniformly dispersed.

In addition, TiO was used as a sintering aid₂MgO and SiO₂Constituent component of alumina ceramic, i.e., Al₂O₃、ZrO₂、TiO₂、MgO、SiO₂And a part of at least one component of the rare earth element substance, easily diffuses to the metal side of the heating element. Thus, the aluminum oxide ceramic and the heating element can be firmly adhered to each other.

Examples

Al having an average particle diameter (D50) of 0.6 μm and a purity of 99.8 mass% was prepared₂O₃Powder and a prescribed amount of solid solution of Y₂O₃ZrO having an average particle diameter (D50) of 0.1 μm₂And (3) powder. Further, TiO with a purity of 99.8% or more was prepared as a sintering aid₂Powder, SiO₂Powder and MgCO₃And (3) powder. Sample No.4 was supplemented with SrCO in addition to the raw materials shown in Table 1₃And (3) powder.

These were blended in predetermined amounts, and cellulose was added as a binder and toluene was added as a solvent, followed by mixing to prepare a slurry. ZrO in Table 1₂Is prepared from Al₂O₃Powder and ZrO₂The total of the powders is defined as a ratio of 100 mass%. TiO in Table 1₂MgO and SiO₂Is prepared from Al₂O₃ZrO containing rare earth element (Y)₂、TiO₂MgO and SiO₂The total of (3) is defined as a ratio of 100 mass%.

The resulting slurry was formed into a sheet by a doctor blade method and dried to prepare a green body having a thickness of 0.4 mm. Then, the obtained green sheets were laminated in 15 layers to obtain a molded article having a thickness of 6 mm.

The obtained molded body was cut into a predetermined shape, subjected to degreasing treatment at 500 ℃ for 10 hours in the air, and then fired at the highest temperature shown in table 1 for 5 hours in the air to obtain an alumina ceramic. Sample No.13 was held at the highest temperature for 2 hours.

[ TABLE 1 ]

The composition of the obtained alumina ceramic was confirmed by X-ray fluorescence (XRF). Al (Al)₂O₃、ZrO₂、TiO₂MgO and SiO₂The composition (c) is adjusted so as to be within the error range in the adjustment. The contents of Ca, Ba, Sr, Zn, Mn and Ce, which are indicated as others, were confirmed by inductively coupled high-frequency plasma emission spectroscopy (ICP). The contents of Ca, Ba, Sr, Zn, Mn and Ce in the prepared samples (sample Nos. 1 to 13) were 400ppm or less.

The particle size of the alumina crystal particles and the particle size of the zirconia crystal particles were confirmed as follows. After mirror polishing of the cross section of the alumina ceramic, heat treatment was performed at a temperature of 50 to 100 ℃ lower than the firing temperature in table 1 for 10 minutes to corrode the grain boundary. A scanning electron microscope was used to photograph a cross section of the grain boundary corroded at a magnification of 3000 times. The obtained cross-sectional photograph was subjected to image analysis, and 50 alumina crystal particles and 50 zirconia crystal particles were rounded, respectively, to calculate an average particle diameter. In addition, among the zirconia crystal particles, the average particle diameter was calculated by regarding the agglomerated particles as one particle. Further, the maximum length of the zirconia crystal particles was measured using the cross-sectional photograph, and the volume ratio was calculated from the area ratio of the individual particles having a maximum length of 1 μm or more and the aggregated particles having a maximum length of 1 μm or more. When the porosity of the alumina ceramic obtained was measured by image analysis of the cross-sectional photograph, the porosity of any sample was 1% or less.

The monoclinic ratio of the zirconia crystal particles was calculated by analyzing the X-ray diffraction (CuK α) pattern of the alumina ceramic. The monoclinic system used a diffraction peak of the (11-1) plane having a 2 θ of about 28.3 °, a diffraction peak of the (111) plane having a 2 θ of about 31.5 °, and the tetragonal system used a diffraction peak of the (111) plane having a 2 θ of about 30.2 °. The percentage of the diffraction intensity of monoclinic crystals in the sum of the diffraction intensities of monoclinic crystals and tetragonal crystals was defined as the monoclinic ratio in zirconia.

The flexural strength of the alumina ceramic was evaluated as follows: from the alumina ceramics obtained, a sample for measuring mechanical strength of a predetermined shape was prepared, and the mechanical strength was measured in accordance with JIS R1601: 2008 and JIS R1604: 2008, three-point bending strength at room temperature (25 ℃) and 800 ℃ was measured. The results are shown in table 2.

[ TABLE 2 ]

Sample Nos. 1 to 5, 7 to 10 and 12 each contained 50 vol% or more of zirconia single particles having a maximum length of 1 μm or more and in which a rare earth element was dissolved in a solid state or aggregated particles of zirconia having a maximum length of 1 μm or more, and had a three-point bending strength of 800MPa or more at room temperature (25 ℃ C.). In samples Nos. 1 to 3, 5, 7 to 10 and 12, the contents of Ca, Ba, Sr, Zn, Mn and Ce were 300ppm or less in total in terms of oxides, and the three-point bending strength at high temperature (800 ℃ C.) was 600MPa or more.

The molded body having the same composition as sample No.3 was prepared by holding the molded body at the same temperature for 2 hours, and the results of the samples are shown as sample No.13 in Table 2. The alumina ceramic of sample No.13 contained no single particles of zirconia having a maximum length of 1 μm or more and no aggregated particles of zirconia having a maximum length of 1 μm or more. The alumina ceramic of sample No.13 had a three-point bending strength of 620MPa at room temperature (25 ℃ C.) and a three-point bending strength of 420MPa at high temperature (800 ℃ C.), and was lower than those of the other samples (sample Nos. 1 to 12). This is considered to be because the alumina ceramic of sample No.13 is a fine particle of zirconia crystal particles as compared with the alumina ceramics of other samples. It is considered that at room temperature, the tetragonal stability is high, and therefore, the phase transition under stress hardly occurs. Further, it is considered that grain boundary slip easily occurs between alumina crystal particles and zirconia crystal particles or between zirconia crystal particles at high temperature. As a result, it is considered that the three-point bending strength at room temperature and high temperature (800 ℃ C.) is lowered.