JPH0235702B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention is a ZrO ₂
Y ₂ O ₃ series zirconia porcelain with temperature range from room temperature to approximately 800
The present invention relates to a solid electrolyte with no hysteresis phenomenon in its thermal expansion curve up to ℃, and a method for producing the same. (Prior art) Conventionally, the manufacturing method for ZrO ₂ −Y ₂ O ₃ -based zirconia porcelain, which is used as a solid electrolyte for oxygen concentration batteries such as oxygen sensors, is to produce fully stabilized zirconia consisting only of cubic crystals. porcelain manufacturing method,
Methods for producing partially stabilized zirconia porcelain composed of cubic crystals and monoclinic crystals are known, and zirconia porcelains obtained by these production methods are all used as heat-resistant materials, solid electrolytes, and the like. (Problem to be solved by the invention) Of these, fully stabilized zirconia porcelain is stable in the temperature range from room temperature to approximately 1500°C and hardly deteriorates over time due to long-term use, but on the other hand, it has low strength. For example, when used as a solid electrolyte for an oxygen sensor that detects the oxygen concentration in automobile exhaust gas, it has the disadvantage of being extremely susceptible to damage due to thermal shock. On the other hand, partially stabilized zirconia porcelain made of cubic and monoclinic crystals has higher strength and better thermal shock resistance than fully stabilized zirconia porcelain, but its strength deteriorates over time in a specific temperature range of 200°C to 300°C. is extremely large, and when used for a long time at such temperatures, it has the serious drawback that many minute cracks occur on the porcelain surface, it becomes water-absorbent, its strength decreases significantly, and it eventually breaks. It was hot. (Means for solving the problem) This means that in ZrO ₂ −Y ₂ O ₃ system partially stabilized zirconia porcelain, the crystal grains, which are tetragonal at a firing temperature of about 1500°C, change to 500°C during cooling from about 1500°C to room temperature. ℃
A phase transformation to monoclinic occurs in the vicinity, and the volume change that occurs at this time applies excessive stress to the porcelain, resulting in many extremely small cracks within the crystal grains, and these cracks occur at a specific temperature of 200°C to 300°C. It is thought that if left in the area for a long time, it will expand and eventually lead to porcelain destruction. Furthermore, when partially stabilized zirconia porcelain consisting of cubic and monoclinic crystals is repeatedly heated and cooled from room temperature to approximately 800°C, the thermal expansion curve becomes heated due to phase transformation between monoclinic and tetragonal crystals that occurs around 500°C. The so-called hysteresis curves are different in the direction and the cooling direction, and the dimensions when returned to room temperature are different before and after heating and cooling, so there is a drawback that highly accurate dimensions cannot be maintained. The present invention eliminates the drawbacks of partially stabilized zirconia porcelain as a solid electrolyte used in conventional oxygen concentration batteries, and has excellent strength and
It is mainly composed of ZrO ₂ and Y ₂ O ₃ and the molar ratio of Y ₂ O ₃ /ZrO ₂ is in the range of 2/98 to 7/93 and has significantly improved the aging deterioration of strength in a specific temperature range of 200℃ to 300℃. The crystal grains are mainly composed of tetragonal or tetragonal and cubic crystal grains, and the X-ray diffraction lines of each of the tetragonal (200) plane, the cubic (200) plane, and the monoclinic (111) plane The peak intensity of T
200, C200 and M111, the following formula 1≧T(200)+C(200)/T(200)+C(200)+
A solid electrolyte and its manufacturing method characterized by satisfying M(111)≧0.72, having an average crystal grain size of 2μ or less, and having no hysteresis phenomenon in its thermal expansion curve from room temperature to high temperature. Zirconium oxide or amorphous zirconium oxide having a diameter of 1000 Å or less, particularly preferably a zirconium oxide and yttrium compound obtained by thermally decomposing zirconium hydroxide.
A molded article of a mixture in which the molar ratio of Y ₂ O ₃ /ZrO ₂ is in the range of 2/98 to 7/93, preferably the mixture.
The molded body is thermally decomposed in a temperature range of 200 to 1200℃, crushed, and then molded, and then fired in a temperature range of 1000 to 1550℃ to form mainly tetragonal crystal particles or tetragonal crystal particles and cubic Consisting of crystal grains,
When the peak intensities of the X-ray diffraction lines of the (200) plane of the tetragonal crystal, the (200) plane of the cubic crystal, and the (111) plane of the monoclinic crystal are respectively T200, C200, and M111, the following formula 1≧T(200 )+C(200)/T(200)+C(200)+
This is a method for producing a solid electrolyte that satisfies M(111)≧0.72, has an average crystal grain size of 2 μ or less, and has no hysteresis phenomenon in its thermal expansion curve from room temperature to high temperature. That is, the present invention uses Y ₂ as a solid electrolyte made of zirconia porcelain that has very little deterioration in strength over time in a specific temperature range of 200 to 300°C and that does not change in size due to heating and cooling in a temperature range of room temperature to 800°C. The molar ratio of O ₃ /ZrO ₂ is 2/98 to 7/93, and the crystal phase of each crystal particle is mainly composed of tetragonal crystal particles, or tetragonal crystal particles and cubic crystal particles, We also determined that it is important that the average crystal grain size is 2μ or less, that is, the Y ₂ O ₃ /ZrO ₂ molar ratio, the crystal phase of the crystal grains, and the average crystal grain size. It is most important that the crystallite size of the zirconium oxide constituting the molded body be below a certain particle size or be amorphous, and that the amount of stabilizer and firing temperature etc. be within a certain range. It is based on something that has been discovered as a result of numerous studies. The invention will be explained in detail below. In the present invention, "having excellent durability at 200℃ to 300℃" means
This means that there is little deterioration over time at any temperature between ℃. As an example of a specific measurement method, as described in the examples, 200℃ to 300℃ in the atmosphere
It is best to conduct a durability test in which heating and cooling are repeated at a rate of temperature rise and fall of 10°C/min, and to measure the change in bending strength before and after durability. The longer the durability time, the greater the degree of deterioration, but after about 1500 hours, the difference between the zirconia porcelain that has been used as a conventional solid electrolyte and the zirconia porcelain obtained by the present invention becomes clear. In order for the solid electrolyte made of zirconia porcelain after sintering to be more stable with mainly tetragonal crystal grains, or tetragonal crystal grains and cubic crystal grains, the oxidation that constitutes the molded body must be Zirconium preferably has a specific crystallite diameter of 1000 Å or less or is amorphous, preferably a crystallite diameter of 700 Å to 300 Å. In other words, the relationship between the crystallite diameter of zirconium oxide constituting the molded body and the crystal phase of zirconia porcelain is expressed as
Expressed in terms of line diffraction intensity ratio, for example, as shown in Figures 1 and 2, in the range where the crystallite diameter is 700 Å or less or in the amorphous form, there are mainly tetragonal crystal grains (H region) or tetragonal crystal grains and cubic crystal grains. In the range of 700 to 1000 Å, there are only a few monoclinic crystal grains mixed in (I region), but beyond 1000 Å, the monoclinic crystal grains suddenly become monoclinic. The number of crystal grains increases (J region). Note that a crystallite diameter of 0 μ indicates that it is amorphous zirconium oxide. However, if amorphous zirconium oxide is used, the firing shrinkage will be excessive, so crystalline zirconium oxide is preferable. Here, in FIGS. 1 and 2, T200, C
200 and M111 are tetragonal (200) planes, respectively.
Shows the X-ray diffraction line intensities of the cubic (200) plane and the monoclinic (111) plane. Therefore, in order to stably maintain the crystal phase of a solid electrolyte made of zirconia porcelain into mainly tetragonal crystal grains or tetragonal crystal grains and cubic crystal grains with little deterioration over time, it is necessary to form a compact. It is clearer from FIGS. 1 and 2 that the constituting zirconium oxide must have a crystallite diameter of 1000 Å or less or be amorphous. What is important here is that zirconium oxide having a specific crystallite diameter is not dissolved in solid solution with a stabilizer such as yttrium oxide. If a raw material that is not in solid solution is used, zirconium oxide and the stabilizer will react and sinter during firing.
If solid solution is present at the raw material stage, it will simply be solid phase sintering. In particular, in the case of the solid electrolyte of the present invention, when reaction sintering occurs, the firing temperature can be lowered than that of solid phase sintering, suppressing the grain growth of porcelain, resulting in more stable tetragonal crystal grains, and increasing the temperature at 200°C. Good durability at ~300°C. Here, even if the raw material is made into zirconium oxide and yttrium oxide by coprecipitation from a mixed solution of a zirconium compound and a yttrium compound during raw material preparation, there is no problem as long as the zirconium oxide and yttrium oxide are not solidly dissolved. Note that zirconium oxide with a crystallite diameter of 1000 Å or less or amorphous can be obtained by thermal decomposition of zirconium chloride, zirconium nitrate, etc., but preferably zirconyl hydroxide (ZrO(OH) ₂ · nH ₂ O) is
Zirconium oxide powder thermally decomposed at a temperature of 500 to 1050°C is preferable than 1100°C. In this case, if the thermal decomposition temperature of zirconyl hydroxide is less than 200°C, water in the zirconyl hydroxide cannot be completely removed, and if it exceeds 1100°C, the crystallite diameter will exceed 1000 Å, which is not preferable. In the production method of the present invention, first, zirconium oxide and a yttrium compound are mixed such that the molar ratio of Y ₂ O ₃ /ZrO ₂ falls within the range of 2/98 to 7/93. In this case, it is extremely important for the mixing ratio of zirconium oxide and yttrium compound to be within the range of 2/98 to 7/93 in terms of Y ₂ O ₃ /ZrO ₂ molar ratio to improve aging deterioration. be. This is 2/98
If it is less than 7/93, tetragonal crystal grains will not be produced to improve aging deterioration, and if it exceeds 7/93, it will contain almost no tetragonal crystal grains, resulting in zirconia porcelain with cubic crystal grains. In addition, the yttrium compound is preferably yttrium oxide, yttrium chloride, yttrium nitrate, yttrium sulfate, etc. In this case, the yttrium compound is about 30 mol% or less in terms of oxide, such as Yb ₂ O ₃ , Sc ₂ O ₃ , _{Nb2O3 ,}
Rare earth element oxides such as Sm ₂ O ₃ and CeO ₂ or
It may also be substituted with CaO, MgO, etc. Then,
The mixture is molded into a predetermined shape using a molding method such as rubber press molding, extrusion molding, or casting molding, and then heated in air.
Fire within the temperature range of 1000-1550℃. Firing is
The temperature is maintained at a maximum temperature of 1000 to 1550°C, preferably 1100 to 1450°C, for 1 to 20 hours. Generally, it is better to make the firing time longer when firing at a lower temperature. The relationship between firing temperature and crystal phase of porcelain is as follows:
If the firing temperature is less than 1,000℃ or exceeds 1,550℃, the formation of monoclinic crystals will increase rapidly, which is undesirable.If the temperature is within the temperature range of 1,000 to 1,550℃, the firing temperature will mainly be tetragonal or a mixed phase of tetragonal and cubic crystals. Generate stably. Furthermore, when the molar ratio of Y ₂ O ₃ /ZrO ₂ is preferably in the range of 4/96 to 7/93, a porcelain mainly composed of tetragonal and cubic crystals is obtained, which has high oxygen ion conductivity and resists aging at 200 to 300°C. becomes a solid electrolyte with less In addition, the yttrium compound is thermally decomposed by heating a mixture of zirconium oxide and a yttrium compound at a temperature of 200 to 1,200 degrees Celsius for about 1 to 10 hours, and if necessary, the material is crushed using a ball mill etc. When used, a homogeneous mixture of zirconium oxide and yttrium oxide is obtained,
When this is formed and fired, a finer porcelain can be produced, which is preferable. The particle size of the raw material after crushing using a ball mill or the like is approximately 0.1 to 10μ. Furthermore, SiO ₂ , Al ₂ O ₃ , clay, etc. may be added as a sintering aid to the mixture of zirconium oxide and yttrium compound in an amount of 30% by weight or less of the total weight of the porcelain. The diameter was determined by X-ray diffraction method using CuKα rays, and the formula D
It was determined by =0.89λ/(B-b)cosθ. Here, D is the desired crystallite diameter of zirconium oxide, and λ is
The wavelength of the CuKα ray is 1.541 Å, B is the larger value of the half-value width (in radians) of the diffraction line of the monoclinic (111) plane or the tetragonal (111) plane of zirconium oxide, and b is added as an internal standard. Crystallite diameter of 3000
The half-life width (in radians) of the (101) plane of α-quartz of Å or more, θ is the value of 1/2 of the diffraction angle 2θ of the diffraction line used to measure the half-life width of zirconium oxide. The bending strength is determined by the commonly used three-point bending method or four-point bending method, but the initial measurement and the measurement after exposure to a temperature range of 200°C to 300°C are performed using the same method. After forming into a test piece shape, the test piece is exposed to a temperature range of 200℃ to 300℃. Next, an example will be described. As shown in Table 1, zirconium oxide and yttrium compounds were mixed in a ball mill to have the compositions shown in the table. Then, if the mixture is listed as thermally decomposed in the table, it is thermally decomposed under the conditions specified, a sintering aid is added, the mixture is wet-pulverized in a ball mill, and after drying, each powder is press-molded. It was fired under the temperature conditions listed in Table 1. The resulting porcelain was measured for its average crystal grain size, the intensity ratio of tetragonal, cubic, and monoclinic crystals by X-ray diffraction, and the bending strength. The crystallite diameter was measured using the mixture used as a compact, and the X-ray diffraction line intensity ratio of the porcelain was measured using the mirror-polished surface of the porcelain.
The ratio of the X-ray diffraction line peak intensities on the (200) plane of the tetragonal crystal and the (111) plane of the monoclinic crystal was determined. Moreover, the bending strength was determined by the three-point bending method after finishing a rod shape of 3.5 x 3.5 x 50 mm. Also, the durability test in Table 1 is 200
This was a durability test in which heating and cooling were repeated between ~300°C and a heating/cooling rate of 10°C/min, and the bending strength was measured after 1500 hours had elapsed. Furthermore, the ratio of the bending strength after the durability test to the bending strength before the durability test is shown in percentage. Note that Table 1 also lists examples outside the numerically limited range of the present invention as reference examples.

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[Table] As is clear from Table 1, the solid electrolyte made of zirconia porcelain produced by the production method of the present invention mainly consists of tetragonal crystal particles or tetragonal crystal particles and cubic crystal particles, and the average crystal grain The diameter is
It was confirmed that the solid electrolyte has an extremely high strength of 2μ or less, and shows a change in bending strength of 80% or more after the durability test at 200 to 300°C compared to before the durability test, which shows little deterioration over time. Furthermore, in all of the solid electrolytes of the present invention shown in Table 1, the thermal expansion curve from low temperature to high temperature was almost linear, and there was no hysteresis phenomenon, as shown in FIG. As described above, the present invention has very little deterioration over time in a specific temperature range of 200 to 300°C, and Y ₂ O ₃ is a solid electrolyte made of zirconia porcelain that has no hysteresis phenomenon in its thermal expansion curve from room temperature to high temperature. /ZrO ₂ molar ratio of 2/98 to 7/93, the crystal phase is mainly composed of tetragonal crystal grains or tetragonal crystal grains and cubic crystal grains, and the average crystal grain size is 2 μ or less. In order to achieve this, the zirconium oxide constituting the compact must have a crystallite diameter of 1000 Å or less, be amorphous, have a Y ₂ O ₃ /ZrO ₂ molar ratio of 2/98 to 7/93, and have a firing temperature of 1000 Å. It was determined that the temperature must be ~1550℃, and the manufacturing method of the present invention can produce a zirconia solid electrolyte with strong mechanical strength that exhibits little deterioration over time in a specific temperature range and no dimensional change due to heat treatment. Solid electrolytes can be used as solid electrolytes for automobile oxygen sensors, iron and steel oxygen meters, measuring oxygen pumps, oxygen concentration batteries such as solid electrolyte fuel cells, or as thermistors, making them extremely useful in industry.

[Brief explanation of drawings]

Figures 1 and 2 are explanatory diagrams showing the relationship between the crystallite diameter of zirconium oxide powder and the crystalline phase of porcelain, and Figure 3 shows the thermal expansion curve during heating and cooling of the solid electrolyte of the present invention. FIG.

Claims (1)

[Claims] 1 Mainly composed of ZrO ₂ and Y ₂ O ₃ , Y ₂ O ₃ /
The molar ratio of ZrO ₂ is in the range of 2/98 to 7/93,
It is mainly composed of tetragonal crystal grains, or tetragonal crystal grains and cubic crystal grains, and has an average crystal grain size of 2 μ or less, and has a tetragonal (200) plane, a cubic (200) plane, and a cubic crystal grain. When the peak intensity of each X-ray diffraction line of the monoclinic (111) plane is T200, C200 and M111, the following formula 1≧T(200)+C(200)/T(200)+C(200)+
A solid electrolyte that satisfies M(111)≧0.72 and has no hysteresis phenomenon in its thermal expansion curve from room temperature to high temperature. 2 Consisting of zirconium oxide or amorphous zirconium oxide and yttrium compound with a crystallite diameter of 1000 Å or less, and a molar ratio of Y ₂ O ₃ /ZrO ₂ of 2/98 ~
7/93 range of 1000 to 1550
By firing at a temperature range of , when the peak intensities of the X-ray diffraction lines of the (200) plane of the cubic crystal and the (111) plane of the monoclinic crystal are respectively T200, C200 and M111, the following formula 1≧T(200)+C(200)/T (200)+C(200)+
A method for producing a solid electrolyte, characterized by producing a solid electrolyte that satisfies M(111)≧0.72 and has no hysteresis phenomenon in its thermal expansion curve from room temperature to high temperature.