JPH07196364A - Production of sintered lanthanum-chromite ceramic - Google Patents

Abstract

PURPOSE:To improve lanthanum chromite in its electric conductivity, density as well as strength and elasticity. CONSTITUTION:Sintered ceramic of lanthanum chromite mainly having the I perovskite structure represented by the general formula, (La1-xAx)a(Cr1-yBy)bO3 (A is an alkaline earth metal except Mg, B is at least one selected from Co, Ni, Zn, Fe, Cu and Mn; 0<x<=0.2, 0<y<=0.05, 0.95<=a/b<=1.05) is heat-treated under reductive conditions where the temperature and the oxygen partial pressure are set so that the volume increase of the sintered product may be less than 0.3% measured at room temperature, before and after the reductive heat treatment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel lanthanum chromite ceramic and a method for producing the composition. The lanthanum chromite ceramics and the composition thereof are stable in a high temperature atmosphere and a reducing atmosphere, have high electron conductivity, and can be used for solid oxide fuel cells, MHD power generation, and high temperature conductive materials.

[0002]

2. Description of the Related Art Lanthanum chromite (LaCrO ₃ )
Is an oxide-based ceramics which is extremely promising as a conductor material used in a corrosive atmosphere at high temperature because it has conductivity at high temperature and is excellent in oxidation resistance and reduction resistance. The conductivity can be improved by adding an alkaline earth metal such as magnesium, calcium, strontium, or barium as a trace impurity element to the lanthanum chromite. Lanthanum chromite has a perovskite structure (ABO ₃ [wherein A and B are metallic elements,
O is oxygen. ]) The added calcium, strontium, and barium are in substitutional solid solution at the lanthanum position in the lanthanum chromite lattice, while magnesium is in solid solution at the chromium position.

Although the above-mentioned lanthanum chromite containing a trace element has sufficient performance in terms of conductivity, it is difficult to obtain a dense sintered body in an atmospheric pressure atmosphere, and voids are generated, so that the gas is sufficiently blocked. There is a drawback that you cannot do it. Therefore, for example, when lanthanum chromite was used as a separator material for a solid oxide fuel cell, it was impossible to completely separate the fuel gas and the air, and the lanthanum chromite could not be used for this purpose.

In the lanthanum chromite, a dense sintered body cannot be easily obtained. Firstly, the vapor pressure of chromium oxide is high at the firing temperature, and it is difficult for pores to disappear during the densification process during firing. This is because the pores remain inside, and secondly, the diffusion of the ions of the constituent elements is extremely slow and the interface of the raw material powder is difficult to move. Therefore, the inventors of the present invention have disclosed in Japanese Patent Application Laid-Open No. 3-65517 that a dense sintered body can be easily obtained in atmospheric pressure and the electrical conductivity can be improved as compared with the conventional one. General formula La _1-x M _x Cr _1-y Co _y O ₃ (wherein M
Is an alkaline earth metal except magnesium, 0 <x
≦ 0.5 and 0 <y ≦ 0.5. ), And
Disclosed is a novel lanthanum chromite-based composite oxide having a perovskite structure, a high temperature conductive material using the same, and a solid oxide fuel cell separator.

This lanthanum chromite type composite oxide is
As represented by the above general formula, it has a perovskite structure, and most ideally, a perovskite structure (ABO ₃ ).
In the basic structure of lanthanum chromite in which La is located at the A site and Cr is located at the B site, a part of La is replaced with an alkaline earth metal, and a part of Cr is further replaced with Co. it is conceivable that.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The lanthanum chromite composite oxide disclosed in Japanese Patent No. 517 has sufficient performance in terms of conductivity and sinterability when used as a solid electrolyte fuel cell separator, but it is not suitable for solid electrolyte fuel cells. In the case of increasing the area and stacking in multiple stages, the effect of thermal stress leads to the destruction of the separator, and thus high material strength is required. The oxygen partial pressure is 1 × 10.
Under a low oxygen partial pressure of ^-14 atm or less, the lattice constant of the material crystal increases, and due to the internal stress, minute cracks occur inside the sintered body and the material strength decreases.

When used as a solid electrolyte fuel cell separator in which the atmosphere on both sides of the sintered plate is different, such as an oxidizing atmosphere and a reducing atmosphere, cracks may occur due to a decrease in strength, or the volume near the surface on the reducing side may increase. In some cases, the warp may occur in a convex shape on the reducing side of the separator. When the warp is flattened by a load, if the generated stress exceeds the material strength, it will lead to breakage.

If the strength of the separator can be greatly improved, the reliability of the stacked solid oxide fuel cells can be improved. If the material strength can be improved even when it is used as another high-temperature material, product design and manufacture will be facilitated. Therefore, an object of the present invention is to produce a lanthanum chromite-based ceramics sintered body which is a separator material for a solid oxide fuel cell and whose strength is significantly improved under a specific reduction heat treatment.

[0009]

In order to achieve the above object, the present invention provides a compound represented by the general formula (La _1-x A _x ) _a (Cr _1-y B
_y ) _b O ₃ (where A is an alkaline earth metal excluding Mg,
B represents at least one of Co, Ni, Zn, Fe, Cu and Mn, and 0 <x ≦ 0.2, 0 <y ≦ 0.0
5, 0.95 ≦ a / b ≦ 1.05),
A lanthanum chromite ceramics sintered body mainly having a perovskite structure is subjected to reduction heat treatment, and the condition of the reduction heat treatment shows that the volume of the sintered body at room temperature shows an increase rate of 0.3% or less before and after the reduction heat treatment. Provided is a method for producing a lanthanum chromite-based ceramics sintered body, which is characterized by being under such temperature and oxygen partial pressure.

The lanthanum chromite ceramics used in the present invention has a perovskite structure represented by the above general formula, and most ideally, is a perovskite type (ABO).
₃ ) In the basic structure of the lanthanum chromite in which La is located at the A site and Cr is located at the B site, a part of La is replaced with an alkaline earth metal, and a part of Cr is further Co, Ni, Zn, Fe. , Cu, or Mn, which is considered to have a structure substituted by a combination of two or more different elements.

The conductivity is improved by substituting a part of La with an alkaline earth metal. However, magnesium does not use La in the A site but Cr in the B site, and is not used in the present invention. The substitution amount of the alkaline earth metal is 0.2, preferably 0.05 to 0.
More preferably, it is 0.1 to 0.2. The more the substitution with these alkaline earth metals, the higher the conductivity becomes, but if it exceeds this range, the volume increase rate at the time of reduction becomes high, and when used in a solid oxide fuel cell separator with zirconia as the electrolyte, the warpage Occurs. Due to the occurrence of warpage, zirconia, which is an electrolyte, is broken, and the separator itself is cracked, so that a gas leak also occurs. In addition, since the coefficient of thermal expansion also increases, the difference in coefficient of thermal expansion with that of zirconia, which is the electrolyte, becomes large, and the zirconia is destroyed when a thermal cycle of temperature rising / falling is applied. Alkaline earth metal substituting a part of La is other than magnesium,
The type is not limited, but Ca and Sr are preferable.

The metal B is replaced with a part of chromium at the B site of the lanthanum chromite lattice to increase the mass transfer rate of the lanthanum chromite particles, and the vapor pressure of chromium oxide vaporized from the lanthanum chromite is reduced to obtain a fine calcination. It is a metal that makes it possible to obtain a unity. As such a metal, firstly, it is possible to have a perovskite structure and a melting point lower than that of lanthanum chromite, secondly, a diffusion rate of ions in an oxide is fast, and thirdly, chromium oxide vapor. Those that reduce the pressure are preferable. Metals satisfying this condition are Co, Ni, Zn, Fe, Cu or Mn.
And a combination of these two different elements.

The addition of metal B also has the effect of improving the conductivity of the sintered body. The substitution amount of metal B is 0 <
y ≦ 0.05, preferably 0.02 ≦ y ≦ 0.05. When the amount of the metal B added exceeds this range, the volume increase rate at the time of reduction increases, and when used in a separator of a solid oxide fuel cell using zirconia as an electrolyte, warpage occurs. Due to the warpage, zirconia, which is an electrolyte, is broken, and the separator itself is cracked, so that a gas leak occurs. Further, since the coefficient of thermal expansion also increases, the difference in coefficient of thermal expansion from that of zirconia, which is the electrolyte, becomes large, and the zirconia is destroyed when a thermal cycle of temperature rising / falling is applied.

As described above, the present applicant has replaced a part of lanthanum with an alkaline earth metal and a part of chromium with Co or the like in order to densify the lanthanum chromite and improve conductivity. However, it has been found out that this may damage zirconia in practical use due to the large difference in coefficient of thermal expansion from zirconia, which is a solid electrolyte. It was also found that the volume change during reduction is large, and when used as a solid oxide fuel cell, warpage may occur and the material may be destroyed. And
These drawbacks are due to the fact that the reduction amount of the A site and the B site and the combination of the substitution elements were not optimized, and by limiting the substitution amount as in the present invention, the difference in the thermal expansion coefficient from that of zirconia is reduced. The present invention has been made by finding that it can be made smaller, the volume increase rate at the time of reduction can be made smaller, and the denseness and the electric conductivity can be maintained high.

From the viewpoint of increasing the compactness and conductivity of the sintered body as much as possible, y in the above general formula is large (0.
(Close to 05) is preferable, but y is small from the viewpoint of the difference in thermal expansion coefficient from zirconia or the like, and for example, y ≦ 0.04 may be preferable. Even if the ratio a / b of A site and B site of lanthanum chromite is not always 1, a sintered body can be obtained. However, a / b> 1.05, a / b <0.9
In No. 5, sinterability may be affected, or a lanthanum chromite single phase may not be obtained. In particular, a / b >> 1.0
In the case of 5, due to La ₂ O ₃ segregated at the grain boundaries, even a densely sintered sintered body becomes brittle with time. Therefore, it is preferable that 0.95 ≦ a / b ≦ 1.05.

In the present invention, ceramics and metals other than the above general formulas can be contained or added up to 20% by volume within a range that does not deviate from high temperature conductivity and volume change during reduction. The metal serve this purpose, Ti-Al intermetallic compound _{(Ni 3 Al, Ni 2 Al} 3, NiA
l), Ni ₂ TiAl, Co ₃ Ti, TiAl, TiA
l ₃ , VAl ₃ , NbAl ₃ , ZrAl ₃ , FeAl,
There are MoSi ₂ , Ti ₅ Si _3, etc., but especially Ni-A
Examples include l-based intermetallic compounds. Further, as ceramics, Al ₂ O ₃ , TiO ₂ , Ta ₂ O ₅ , Nb ₂ O are used.
₅ , CoCr complex oxide such as CoCr ₂ , YCr
_{O 2, Y-Ca-Cr} -Ca-O system and the like. The addition amount of these metals and ceramics is 20% or less, preferably 10% or less, and more preferably 5% or less. If the addition amount of these exceeds 20%, the conductivity is lowered and other problems occur.

The lanthanum chromite ceramics represented by the above general formula or the composition mainly composed of the lanthanum chromite ceramics itself has high density and conductivity, and its volume change is small even when reduced. In addition, it is a novel substance having a usefulness such as a thermal expansion coefficient close to that of zirconia (see “Lanthan chromite ceramics and its manufacturing method and use” filed on the same date as the present application by the present applicant).

However, the reduction heat treatment according to the present invention can reduce the defects at the grain boundaries and improve the bending strength and elastic modulus. In addition, when used for applications such as a separator of a high-temperature fuel cell that is exposed to a high-temperature reducing atmosphere at the time of use, there is an advantage that expansion (deformation) on the reducing side is reduced during use by performing high-temperature reducing heat treatment in advance.

The atmosphere of the reduction heat treatment of the present invention is a high-temperature reducing atmosphere and changes depending on the material composition.
Oxygen partial pressure at 00 ° C, volume ratio of hydrogen to water vapor is 1/5, 6 ×
A high temperature reducing atmosphere of about 10 ⁻¹⁴ atm is used. When the heat treatment conditions are selected such that the volume increase rate before and after reduction is 0.3% or less, more preferably 0.2%, and further 0.1% or less, the material strength is significantly improved. At a temperature lower than 1400 ° C at which densification progresses, the relative density and the crystal grain size before and after the reduction hardly change, and the strength is increased. Therefore, the reduction heat treatment reduces defects such as separation of grain boundaries. It is considered that the bending strength and elastic modulus are improved due to the increased bonding strength of the crystals. However, if the volume increase before and after the reduction exceeds 0.3%, the internal stress due to the volume increase increases, and minute cracks occur inside the sintered body, which lowers the material strength and further If the applied stress reaches or exceeds the material strength, cracking or even destruction occurs.

In the lanthanum chromite ceramics represented by the general formula, the lattice constant changes substantially in proportion to the logarithm of the oxygen partial pressure and the volume after reduction increases under the condition of a constant temperature of 700 ° C. or higher. In other words, if the conditions of the reduction heat treatment temperature and oxygen partial pressure are determined, the volume increase rate before and after reduction according to the conditions is determined, and if the temperature and volume increase rate are determined, the oxygen partial pressure for reduction heat treatment can be determined. . Therefore, the heat treatment temperature and the oxygen partial pressure are selected according to the composition of the sintered body. Generally, oxygen partial pressure is 1 × 10 ⁻¹⁴ atm at 800 to 1000 ° C., 1
At 000 to 1200 ° C, the oxygen partial pressure is 1 × 10 ^-13 , 12
At 0 to 1400 ° C, 1 × 10 ^-11 atmospheric pressure, 1400 ° C to
At 1600 ° C., the pressure is 1 × 10 ⁻⁸ atm. Preferably 80
1 × 10 ⁻¹⁴ to 1 × 10 ^{−13 in} the temperature range of 0 to 1200 ° C.
The atmospheric pressure is good. If the heat treatment temperature is high, it is possible to perform treatment with a high oxygen partial pressure (weak reducing atmosphere), but it is difficult to control the volume change. When the temperature is high, it is sensitive to oxygen partial pressure and the rate at which equilibrium is reached is fast (volume change rate is fast), so when the size of the sintered body to be processed becomes large, the volume increase rate on the surface and inside the sintered body will differ and cracking will occur. It can also be a cause. If the heat treatment temperature is too low, a low oxygen partial pressure (strong reducing atmosphere) is required, the volume increasing rate is slow, and the treatment time is long. The time for the reduction heat treatment is determined by the time required for the entire sintered body to reach equilibrium depending on the temperature and the oxygen partial pressure, so it is necessary to optimize it according to the heat treatment conditions and the dimensions of the sintered body.

The lanthanum chromite ceramics obtained by the reduction heat treatment according to the method of the present invention or the sintered body of the composition having the lanthanum chromite ceramic as the main component is not limited, but is generally 85% of the theoretical density. Density above 1000
Conductivity in the atmosphere is 10 Ω ^-1 cm ^-1 or more, 50 to 1000
The coefficient of thermal expansion at ℃ is 9 × 10 ^{−6 to} 13 × 10 ⁻⁶ [1 /
K], more preferably 9.5 × 10 ^{−6 to} 11.1 × 10
^-6 [1 / K], and further 9.7 × 10 ^{-6 to} 10.9 × 1
0 ^-6 [1 / K], 4-point bending strength at room temperature is 8 [kgf / mm ² ]
Or more, preferably 10 [kgf / mm ² ] or more, and an elastic modulus of 0.6
It may have × 10 ⁴ [kgf / mm ² ]. The crystal grain size of the sintered body affects the strength. When the crystal grain size becomes large, the defect size at the crystal grain boundary becomes large and the strength decreases. The crystal grain size is 20 μm or less, preferably 10 μm or less.

The lanthanum chromite ceramics sintered body produced according to the present invention is useful as a heat-resistant conductive material, and is particularly dense and has high strength, and thus is useful as a separator in a solid oxide fuel cell.

[0023]

Example 1 11.057 g of lanthanum oxide, strontium carbonate 2.
505 g, chromic oxide 6.283 g, and cobaltous oxide 0.325 g were weighed and wet-mixed using an agate mortar. This composition is La _0.8 Sr _0.2 Cr _0.95 Co _0.05
Equivalent to O ₃ . This mixed powder was calcined at 1200 ° C. for 1 hour. The temperature rising rate is 20 ° C./min.

The lanthanum chromite powder thus obtained was analyzed by X-ray diffraction. As a result, the presence of the second phase could not be confirmed, and it was found that cobalt was solid-solved in the lanthanum chromite lattice having the perovskite structure. It was The La _0.8 Sr _0.2 Cr _0.95 Co _0.05 O ₃ was subjected to reduction heat treatment as follows. Put the sintered body in an alumina compactor capable of adjusting the atmosphere in which hydrogen and water vapor can be introduced, and leave it at 1000 ° C.
The temperature is raised up to 5 ° C./min, 1000 ° C. is reached, nitrogen gas is sufficiently replaced, and a predetermined amount of steam / hydrogen is passed through and maintained for 20 hours. Then, the temperature is lowered to 600 ° C. at a cooling rate of 20 ° C./min or more.

The volume change rate of the sintered body before and after the reduction is powder X
The lattice constant was measured by line diffraction and determined by the volume change of the unit cell. Using a sintered body with a relative density of 97%, a 4 × 3 × 40 mm test piece was used to measure the strength before and after reduction.
A 4-point bending test based on IS-R-1601 was performed.

The elastic modulus was calculated from the following equation from the relation of displacement-load at the center of the test piece. _{E 4b = (L-l)} · {3L 2 - (L-l) 2 ΔP} / 8
b · h ³ · Δu E _4b = Modulus of elasticity in 4-point bending test [kgf / mm ² ] L: Distance between fulcrums [mm] l: Distance between load points [mm] ΔP: Load change [kg] Δu: ΔP Displacement in the center of the test piece when the load changes
[mm] Table I shows the oxygen partial pressure dependence of the volume expansion coefficient, the state of the sample after the treatment, and the strength ratio before and after the reduction treatment. Table I: Volume expansion coefficient by reduction treatment and 4-point bending strength ratio before and after reduction ───────────────────────────────── Oxygen partial pressure [atm] Reduction expansion coefficient [%] Sample state 4-point bending strength ratio 3 × 10 ⁻¹⁹ 0.46 × −3 × 10 ⁻¹⁷ 0.38 ○ 0.41 3 × 10 ⁻¹⁵ 0.30 ○ 1.27 3 × 10 ^-13 0.21 ○ 1.26 ──────────────────────────────── Note) Sample State: X ... Confirmation of cracks with the naked eye. O ... No change was observed. It can be seen that the 4-point bending strength is improved under the condition that the volume expansion coefficient is 0.30% or less.

Example 2 Various lanthanum chromites were prepared in the same manner as in Example 1.
Volume expansion coefficient before and after reduction treatment at 00 ° C., oxygen partial pressure of 1 × 10 ⁻¹⁴ [atm], 4-point bending strength ratio, and relative density, electrical conductivity, thermal expansion coefficient, elastic modulus before and after reduction treatment Table II
Shown in. However, part of the sample is not powder X-ray diffraction,
The volume before and after reduction was measured by measuring the dimensions of the sample.

[0028]

[Table 1]

Example 3 In the same manner as in Example 1, La _0.8 Sr _0.2 Cr _0.97 C
Sintered bodies having different relative densities of _0.03 O ₃ were subjected to reduction heat treatment under the reduction treatment conditions of 1000 ° C. and oxygen partial pressure of 3 × 10 ⁻¹⁵ [atm]. The reduction expansion coefficient under this reduction condition is 0.18% when calculated from the extension of the lattice constant from X-ray diffraction.
Table III shows the amount of oxygen deficiency obtained from the lattice constant and the weight change of the sample, and the theoretical density assuming that there is no oxygen deficiency before reduction.

Table III ────────────────────────────────── Reduction heat treatment Crystal structure Lattice constant Oxygen deficiency δ Theory density a axis c axis [G / cm ³ ] None Hexagonal 5.493 13.192 − 6.56 Yes Hexagonal 5.503 13.316 0.05 6.50 ──────────────────────── ────────── Samples with different relative densities were prepared by changing the firing temperature.
The sample density was determined by the Archimedes method using toluene as a solvent, using a sapphire single crystal as a standard sample. Relative density calculated the sample density from the theoretical density of Table 3. Table IV shows the density before and after the reduction heat treatment, the 4-point bending strength ratio, the 4-point bending strength ratio, the elastic modulus before and after the reduction, and the thermal expansion coefficient after the reduction.

[0031]

[Table 2]

[0032]

The strength and elastic modulus of a lanthanum chromite-based ceramics sintered body which is dense and has high conductivity can be improved. Further, it has effects such as a small difference in thermal expansion coefficient with zirconia and a small volume change in a high temperature reducing atmosphere.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Motoaki Ando Nishi-tsurugaoka 1-3-3 Oi-cho, Iruma-gun, Saitama Prefecture Tonen Corporation Research Institute (72) Toshihiko Yoshida Nishitsuru Oi-cho, Iruma-gun, Saitama Prefecture 1-3-1, Kugaoka Inside Tonen Corporation Research Institute

Claims (4)

[Claims] 1. A general formula (La _1-x A _x ) _a (Cr _1-y B
_y ) _b O ₃ (where A is an alkaline earth metal excluding Mg,
B represents at least one of Co, Ni, Zn, Fe, Cu and Mn, and 0 <x ≦ 0.2, 0 <y ≦ 0.0
5, 0.95 ≦ a / b ≦ 1.05),
A lanthanum chromite ceramics sintered body mainly having a perovskite structure is subjected to reduction heat treatment, and the condition of the reduction heat treatment shows that the volume of the sintered body at room temperature shows an increase rate of 0.3% or less before and after the reduction heat treatment. A method for manufacturing a lanthanum chromite-based ceramics sintered body, characterized in that the temperature and the oxygen partial pressure are as described above. 2. The method according to claim 1, wherein in the firing step of the lanthanum chromite-based ceramics sintered body represented by the general formula, reduction heat treatment is performed at the time of temperature reduction after the firing. 3. A lanthanum chromite ceramics sintered body produced by the method according to claim 1. 4. The general formula (La _1-x A _x ) _a (Cr _1-y B
_y ) _b O ₃ [In the formula, A represents an alkaline earth metal except Mg, B represents at least one metal selected from Co, Ni, Zn, Fe, Cu and Mn, and 0 <x ≦ 0. 2,0
<Y ≦ 0.05 and 0.95 ≦ a / b ≦ 1.05. ] It is a reduction product of lanthanum chromite ceramics represented by the above formula, and is 0.3% compared with the volume of lanthanum chromite ceramics represented by the above general formula.
A lanthanum chromite-based ceramics sintered body having the following volume increase.