JP2631243B2 - Heat-resistant conductive ceramics and manufacturing method thereof - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant conductive ceramic and a method for producing the same, and more particularly, it is dense, stable even at a high temperature of 800 ° C. or higher, and has high conductivity. The present invention relates to a heat-resistant conductive ceramic suitable as a required material for a heating element, an electrode, a fuel cell, and the like, and a method for producing the same.

Conventional technology and its problems R (Ca, S) in which a part of R of a sintered body represented by the formula: RCrO ₃ (R is a lanthanide; the same applies hereinafter) is solid-solution substituted with Ca or Sr.
r) CrO ₃ , particularly La (Ca, Sr) CrO _3, is known as a high melting point heat-resistant conductive ceramic having a perovskite crystal structure. Conventionally, such a La (Ca, Sr) CrO ₃ sintered body is prepared by blending La ₂ O ₃ , Cr ₂ O ₃ and CaCO ₃ or SrCO ₃ at a predetermined ratio, mixing, and then calcining to obtain a synthetic powder. Is manufactured by press molding, and firing at a temperature of 1600 ° C. or higher. However, since this material is a hard-to-sinter material, it is difficult to obtain a dense sintered body, and it is used with open pores. Since the sintered body having such open pores has a reduced conductive area corresponding to the pore portions, not only does the conductivity decrease, but also the evaporation of Cr from the open pores increases when used at high temperatures, Corrosion resistance also decreases.
Further, when such a sintered body is used as a separator of a fuel cell, there is a problem that airtightness cannot be maintained. The formula: R (Ca or Sr) MO ₃ (M is M
The sintered body represented by at least one of n, Ni, and Co; the same applies hereinafter) also has a perovskite-type crystal structure, has higher conductivity than R (Ca or Sr) CrO ₃ , and These oxides are known as oxides which are easily formed, but have a problem in that they are inferior in heat resistance and significantly lower in a reducing atmosphere in conductivity. In addition, such a sintered body is likely to evaporate and react with the component represented by M,
Low corrosion resistance and lacks stability. Furthermore, when such a sintered body is used as a separator for a fuel cell, there are also problems such as a significant decrease in conductivity, particularly on the fuel electrode side, and deterioration due to reaction with a contact material.

In order to obtain a La (Ca, Sr) CrO ₃ sintered body having few open pores, attempts have been made to perform sintering at a higher temperature. However, in this case, the amount of evaporation of Cr increases, so that it becomes difficult to obtain a sintered body having a predetermined composition,
It is not economical in terms of firing equipment and firing energy.

Furthermore, in order to lower the sintering temperature, sintering aids such as B ₂ O ₃ and SiO ₂ are also blended, but in this case, the conductivity of the sintered body is greatly reduced. Would.

Means for Solving the Problems The inventor of the present invention has conducted various studies while paying attention to the problems of the prior art as described above. As a result, it is difficult to sinter R (Ca, Sr) CrO ₃ because Not only because it is a high melting point material, but also because the Cr component, which is the main component in the sintering process, evaporates and precipitates and condenses on the crystal grain boundaries, obstructing the diffusion of each atom, In further research, by controlling the composition, composition, density, etc. of the rare earth chromite-based sintered body within a specific range, it is dense,
It has been found that ceramics excellent in heat resistance and conductivity can be obtained.

That is, the present invention provides the following heat-resistant conductive ceramics and a method for producing the same: 1. A sintered body made of a rare earth chromite system, wherein R _1-x A _x
CrO ₃ (where R is at least one of the lanthanide elements)
Species; A is at least one of Ca and Sr; 0.02 ≦ x ≦ 0.2
5) relative to 100 parts by weight, Al ₂ include O _3, at least one of MgO and Y ₂ O ₃ 0.02 to 1 wt%, the density of the theoretical density of 95
% Or more.

2. In the heat-resistant conductive ceramics of item 1, R _1-x A _x
A heat-resistant conductive ceramic, wherein 10 mol% or less of Cr in the composition represented by CrO ₃ is substituted and solid-solved with at least one of Co, Mn and Ni.

3. composition _{R 1-x A x Cr 1} -y M y O 3 of the sintered body (wherein, R is at least one lanthanide element; A is at least one of Ca and Sr; M is, Co , Mn and at least one of Ni; 0.0
2 ≦ x ≦ 0.25; 0 ≦ y ≦ 0.10) The respective compounds of R, A, Cr and M are blended in a ratio such that they are uniformly mixed and synthesized into a perovskite structure by a heat treatment. To 100 parts by weight, at least one of Al ₂ O ₃ , MgO and Y ₂ O ₃ is added in an amount of 0.02 to 1 weight, dispersed and pulverized to an average particle size of 2 μm or less, molded, and fired at 1500 to 1850 ° C. And producing a sintered body having a density of 95% or more of the theoretical density.

Hereinafter, the requirements of the heat-resistant conductive ceramics of the present invention and the effects based on the requirements will be described in detail.

The heat-resistant conductive ceramic obtained in the present invention has a perovskite structure, and has a basic composition of R _1-x A _x Cr _1-y M _y O
₃ (where R is at least one lanthanide element: A
Is at least one of Ca and Sr: M is Co, Mn and Ni
At least one of the following: 0.02 ≦ x ≦ 0.25: 0 ≦ y ≦ 0.10).

The lanthanide element means 15 elements ranging from lanthanum of atomic number 57 to lutetium of 71. As a lanthanide element source, compounds in the form of oxides, hydroxides, nitrates, carbonates, sulfates, chlorides, oxalates and the like are used. Lanthanide element may be used alone,
Alternatively, two or more kinds may be used in combination. As the lanthanide element, La, Nd and Pr are more preferable, and La alone or a mixture of La and another lanthanide element is more preferable. When La, Nd or Pr is used as the lanthanide element, the conductivity of the sintered body is improved and the heat resistance is also improved. On the other hand, when these elements are used, high-temperature sintering is required in order to obtain a high-density sintered body. (About 10 to 30 mol%) is preferably replaced with another lanthanide element.

(1) In the rare earth chromite represented by R _1-x A _x CrO ₃ (A is at least one of Ca and Sr), x is 2 to
It needs to be in the range of 25 mol%.

According to the present invention, the lack of charge of the cation caused by the substitution of trivalent R with divalent A reduces the transition metal Cr.
The structure compensates for the valency fluctuation of, and as a result, hopping conduction due to this fluctuation contributes to greatly improve the conductivity. When the substitution amount of R by A is less than 2 mol%, the conductivity is not sufficiently improved, whereas when it exceeds 25 mol%, no further improvement in the conductivity is observed. Rather, the sinterability and the like deteriorate during production. The substitution amount of R by A is more preferably 5 to 20 mol%.

(2) Al ₂ O ₃ , M contained in rare earth chromite sintered body
The total amount of gO and at least one of Y ₂ O ₃ is R _1-x A _x CrO
₃ (0.02 ≦ x ≦ 0.25) It should be within the range of 0.02 to 1 part by weight for 100 parts by weight.

According to the study of the inventor, a certain amount of Al ₂ O ₃ , MgO and
When at least one of Y ₂ O ₃ is added, the evaporation of the Cr component is reduced, and the Cr ₂ O ₃ that precipitates at the crystal grain boundaries is fixed as a solid solution to promote grain boundary diffusion and sintering. It has been found that densification is further promoted without substantially lowering the conductivity of the body, and heat resistance and durability are further improved. If the total amount of the additives is less than 0.02 parts by weight based on 100 parts by weight of the rare earth chromite synthetic powder, the sinterability is not sufficiently improved. On the other hand, when it exceeds 1 part by weight, not only no further improvement in the sinterability is recognized, but also the conductivity is lowered. The addition amount is more preferably 0.2 to 0.8 parts by weight. It is effective to uniformly disperse and add these additives during dispersion or pulverization of the synthetic powder.

(3) Cr in rare earth chromite represented by R _1-x A _x CrO ₃
It is desirable to replace 10 mol% or less with at least one of Co, Mn and Ni (hereinafter collectively referred to simply as M).

When a part of Cr is replaced by M to form a solid solution, sinterability is further improved. In general, in proportion to the substitution amount of M,
The conductivity as well as the sinterability are improved, but the stability, heat resistance, corrosion resistance, etc. of the sintered body are reduced. Each of these elements individually causes Mn to reduce its conductivity in a reducing atmosphere,
Increases the thermal expansion and also reduces the conductivity in a reducing atmosphere, and Ni shows a tendency such as not improving the conductivity as much as Mn and Co. Therefore, when such a sintered body is used at a high temperature such as a fuel cell separator, M
It is essential that the upper limit of the substitution is 10 mol%.
It is preferably at most 6 mol%.

(4) The density of the rare earth chromite-based sintered body is set to a theoretical density of 95
% Or more.

The conductivity of the rare earth chromite-based sintered body is greatly affected by its density, and the lower the density, the lower the conductivity. This is because, as the density is lower, the number of pores and open pores existing in the grains, at the grain boundaries, and the like increase, and the conduction area corresponding to the pores decreases. In addition, there arise problems such as a decrease in corrosion resistance and airtightness as well as a decrease in conductivity.

Therefore, the density of the rare earth chromite-based sintered body is set to 95% or more of the theoretical density.

In the sintered body of the present invention, (R _1-x A _x ) and (Cr _1-y M _y )
Is preferably equimolar, but the former 1
The latter range from 0.98 to 1.02 moles per mole is acceptable. If the ratio of the two is slightly different from equimolar within the above range, oxides of R, A, Cr and M are formed and eutectic at a temperature lower than the melting point of the perovskite crystal. Sintering is easier than in the case of an equimolar composition. If the proportion of Cr is less than 0.98 mol, the conductivity at the crystal grain boundaries of the sintered body is reduced, and a large amount of R and A oxides that hydrate at low temperatures are formed, which is not preferable. On the other hand, (Cr
_When the proportion of _{1-yM y} ) exceeds 1.02 mol, a small amount of Cr ₂ O ₃ or M ₂ O ₃ is formed at the crystal grain boundary of the sintered body, and the conductivity is reduced.

The method of the present invention is performed as follows. First,
R is adjusted so that the ratio of each component during the production of the sintered body is within a predetermined range
Source, A source, Cr source and M source are blended and mixed and pulverized to obtain a uniform powder mixture. As the R, A, Cr and M sources, compounds in the form of compounds such as oxides, hydroxides, nitrates, carbonates, sulfates, chlorides and oxalates are used. The particle size of the powder is not particularly limited, but is preferably about 5 μm or less. Generally, in the synthesis process or the like, M is easily diffused into a container or the like, so it is preferable to mix the M source or the M source and the Cr source in a slightly larger amount. This point may be determined as appropriate in consideration of the sintering temperature and time.
In the case of sintering at 50 ° C. for 3 hours, the Cr source or M source is added in an amount of about 0.2% by weight (Cr ₂ O ₃ or M ₂
It is preferable to mix a large amount of O ₃ ). Mixing is
The mixing may be performed by either wet mixing or dry mixing of powders, or by mixing an aqueous solution containing each element.

Next, the powder mixture obtained above is subjected to a heat treatment at a temperature of 70% or more at which a perovskite structure of R (Ca, Sr) Cr (M) O ₃ or more is obtained, to obtain a synthetic powder. This heat treatment temperature depends on the type, particle size,
Although it may vary depending on the mixing method, it is usually in the range of about 600 to 1500 ° C, more preferably in the range of 800 to 1350 ° C. If the heat treatment temperature is too low, shrinkage during sintering described later becomes large, and firing distortion is generated, which is not preferable. On the other hand, if the heat treatment temperature is too high, the particle size of the synthetic powder becomes too large and the sinterability is reduced, which is also not preferable. Although the heat treatment time is related to the heat treatment temperature, it is usually about 0.5 to 3 hours.

Next, at least one kind of powder of Al ₂ O ₃ , MgO and Y ₂ O ₃ was added to 100 parts by weight of the synthetic powder obtained as described above.
Add 2 to 1 part by weight and disperse or pulverize. By adding the auxiliary material in this manner, the sinterability can be improved without substantially reducing the conductivity and heat resistance of the sintered body during sintering described below. This dispersion or pulverization is preferably performed in the presence of water or an organic solvent such as alcohol. In dispersion or pulverization, the average particle size of the obtained powder is 2 μm.
m and a specific surface area of 2 m ² / g or more, more preferably,
It is preferable to carry out the treatment until the average particle diameter becomes 1 μm and the specific surface area becomes 4 m ² / g or more.

Next, the powder dispersed or pulverized to a predetermined particle size is formed into a predetermined shape according to a conventional method for forming ceramics, that is, by a method such as CIP, mechanical press, casting, injection, extrusion, or tape forming. At this time, it goes without saying that an additive such as a known molding aid such as an acrylic resin may be used in combination.

Next, the obtained molded body is fired. When the above-described auxiliary material is added, the hardly sinterable material of the present invention can be densely sintered at a relatively low temperature. It is presumed that the densification of the sintered body is not merely due to the reduction of the evaporation of Cr and M by the addition of an auxiliary material, but also has an effect that the diffusion energy actively works with each constituent atom. You.
The sintering temperature is preferably set to 1500 to 1850 ° C.

The density of the heat-resistant conductive ceramic produced by the method of the present invention reaches 95% or more of the theoretical density.

Effects of the Invention According to the present invention, the following remarkable effects are achieved.

(1) In the prior art was not possible manufacturing heat stability and excellent conductivity dense La (Ca, Sr) Cr ( M) O 3 sintered body, can be manufactured at low cost.

(2) Since the sintered body according to the present invention has excellent heat resistance at high temperatures, for example, when used as a heating element or an electrode,
Its use period is significantly extended.

(3) Further, because of its high density, the sintered body according to the present invention, when used as a separator of a solid fuel cell, for example, maintains a high degree of airtightness even in an oxidizing or reducing atmosphere and has a high conductivity. Sex can be maintained for a long time.

(4) Therefore, the heat-resistant conductive ceramic according to the present invention is excellent in economical use.

EXAMPLES Examples and comparative examples are shown below to further clarify the features of the present invention.

Example 1 La (OH) ₃ , Cr ₂ O ₃ and CaCO ₃ powders were mixed with La _0.9 Ca _0.1 Cr
It was blended so as to have a composition ratio of O ₃ , mixed and dispersed in a wet manner in the presence of water, dried, and heat-treated at 1100 ° C. for 2 hours, and 95% or more was synthesized into a perovskite structure. A powder was obtained.

To 100 parts by weight of the obtained synthetic powder, at least one of Al ₂ O ₃ , MgO and Y ₂ O ₃ was added at a ratio shown in Table 1,
The powder was wet-pulverized by a ball mill in the presence of water for 48 hours to obtain a powder having an average particle diameter of 0.9 μm. Next, an acrylic molding aid was added thereto, and a granular powder for molding (average particle size: 60 μm) was adjusted using a spray dryer. And mechanical press molding the granules powder with a molding pressure ^{3ton / cm 2, 60mm × 60mm} ×
After obtaining a molded body of 5 mm, it was fired under the condition of raising the temperature to 1800 ° C. and holding for 2 hours to obtain a sintered body. The characteristics of the obtained sintered body are also shown in Table 1.

Note: Nos. 3 and 5 are comparative examples.

As is clear from the results shown in Table 1, according to the present invention, a sintered body having higher density and higher conductivity is obtained as compared with the comparative example.

Example 2 The x and z amounts and the calcination temperature of the composition represented by La _1-xz Ca _a Sr _z CrO ₃ were set to the values shown in Table 2, and 100 parts by weight of this powder was Al ₂ O ₃
Was added in the same manner as in Example 1 except that 0.5 parts by weight of was added to obtain a sintered body. The characteristics of the sintered body together with the firing temperature
It is shown in the table.

Note: Nos. 6, 11, and 15 are comparative examples.

As is clear from the results shown in Table 2, x, z or x + z
However, the sintered body of the present invention in which the content is within the range of 2 to 25 mol% has high density and high conductivity.

In the same manner, except that the values shown in Table 3 the element and y of M having the composition shown in Example _{3 La 0.9 Ca 0.05 Sr 0.05 Cr} 1-y M y O 3 in Example 2 sintered body Obtained. Table 3 also shows the characteristics of the sintered body together with the firing temperature.

Note: Nos. 18 and 21 are comparative examples.

All the sintered bodies obtained according to the present invention exhibited high density and high conductivity. However, in comparison with the conductivity in the atmosphere (Table 3), the conductivity at 1000 ° C. in a hydrogen atmosphere is higher in the sintered bodies of Nos. 16, 19, 22, 23, 24 and 26.
In comparison with the sintered bodies of Nos. 17, 20 and 25, the ratio was 1/8, whereas the sintered bodies of Nos. 18 and 21 of Comparative Examples lacked thermal stability. Poor thermal shock resistance and reacted with ZrO ₂ .

Example 4 The same operation as in No. 1 of Example 1 was performed except that the firing temperature was 1800 ° C. and the following points were changed.
A sintered body of the present invention was obtained. The results are shown below.

Nd was used in place of sample No. 27 La.

Relative density of sintered body 98% Conductivity 6S / cm (1000 ° C) 20 mol% of sample No. 28 La was changed to Nd.

Relative density 97% Conductivity 13S / cm (1000 ° C) 10% mol of sample No. 29 La was changed to Gd.

Relative density 98% Conductivity 14S / cm (1000 ° C) Example 5 La (NO ₃ ) ₃ , Cr (NO ₃ ) ₃ and Ca
Using (NO ₃ ) ₂ solution, they are mixed and neutralized,
After drying the precipitate, it was heat-treated at 900 ° C. to produce a synthetic powder having a perovskite structure. Next, the same operation as in No. 1 of Example 1 was performed using this synthetic powder,
A sintered body having a relative density of 97% was obtained by firing.

Claims (3)

(57) [Claims] 1. A sintered body comprising a rare earth chromite system, wherein R _{1 -x} A _x CrO ₃ (where R is at least one of lanthanide elements; A is at least one of Ca and Sr; 0.02 ≤
x ≦ 0.25) Heat resistance characterized by containing at least one of Al ₂ O ₃ , MgO and Y ₂ O _{3 in} an amount of 0.02 to 1% by weight with respect to 100 parts by weight, and having a density of 95% or more of the theoretical density. Conductive ceramics. 2. The heat-resistant conductive ceramic according to claim 1, wherein 10 mol% or less of Cr in a composition represented by R _1-x A _x CrO ₃ is contained.
A heat-resistant conductive ceramic, which is substituted with at least one of Co, Mn, and Ni to form a solid solution. 3. The composition of the sintered body is R _1-x A _x Cr _{1- y My} O ₃ (where R is at least one of lanthanide elements; A is at least one of Ca and Sr; M Is at least one of Co, Mn, and Ni; 0.02 ≦ x ≦ 0.25; 0 ≦ y ≦ 0.10), and the respective compounds of R, A, Cr, and M are blended, uniformly mixed, and heat-treated. after synthesizing the perovskite structure, with respect to 100 parts by weight of the obtained powder, Al ₂ O _3, at least one of MgO and Y ₂ O ₃ was 0.02 wt added, dispersed and ground to average particle size of not more than 2μm After forming, 1500-18
A method for producing a heat-resistant conductive ceramic, which comprises firing at 50 ° C. to obtain a sintered body having a density of 95% or more of the theoretical density.