JP3863207B2 - Lanthanum chromite ceramics, separator containing the same, and solid oxide fuel cell using the separator - Google Patents

Description

【００５１】
表１中、×印は還元雰囲気中で分解すること、または、ひび割れすることを示し、また、寸法変化率欄において、＊印を付したものは前記（ｉ）の方法で、また＊印のない無印のものは前記（ｉｉ）の方法で求めたことを示す。
【００５２】
実施例１〜３、比較例６及び比較例９で得た各セラミックスから成る２００ｍｍ×２００ｍｍ×５ｍｍのサイズの成形体に所定原料ガス流路を設けて各セパレータを形成した。また、セパレータと同じ方形で厚さ０．２ｍｍの、イットリアを８モル％添加したジルコニア（以下８ＹＳＺという）から成る固体電解質板を用意し、その片面にＬａ_0.8Ｓｒ_0.2ＭｎＯ₃を０．１ｍｍ厚にスラリー塗布法で被着してカソードとし、他方の面にＮｉ−８ＹＳＺ（重量比７：３）サーメットを０．１ｍｍ厚にスラリー塗布法で被着してアノードとした。
【００５３】
この各電極を設けた固体電解質板と各セパレータを積層して各種３段スタックセルを作製した。各種セルを次の条件で運転した。
燃料ガス：Ｈ₂＝３．４０リットル／ｈｒとＨ₂Ｏ＝１．１３リットル／ｈｒとの混合ガス
酸化剤ガス：空気＝１２．８０リットル／ｈｒ
セル温度：１０００℃
荷重：１００ｋｇ
各種セルの性能を所定電流密度下での電圧で示した。その結果を表２に示す。
【００５４】
【表２】

【００５５】
これより、各比較例のセルは性能不良であり、ＯＣＶも低く、降温後にセパレータの割れが認められるため、電流密度を０．３Ａ／ｃｍ²まで使用することはできないのに対し、各実施例のセルは性能良好であり、ＯＣＶも理論値とほぼ一致し大きなリークはなく、降温後にもセパレータの割れは認められなかった。
【図面の簡単な説明】
【図１】 本発明のセラミックスをセパレータに用いた平板型固体電解質燃料電池の１例の概略図。
【図２】 電解質の熱サイクル後の割れを観察するための装置の１例の概略図。
【図３】 各セラミックスの反り量を測定するための装置の１例の概略図。
【符号の説明】
１ 固体電解質板
２ カソード
３ アノード
４ セパレータ
５ 外部出力端子
６ 空気流路
７ 燃料流路
１１ 試料
１２ ガラス
１３ ８ＹＳＺ
１４ 試料
１５ 重り
２１ 試料
２２ アルミナ製容器
２３ アルミナ管
２４ 燃料ガス
２５ 空気
２６ アルミナロッド
２７ ダイヤルゲージ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a novel lanthanum chromite ceramic, a flat plate solid electrolyte fuel cell separator containing the ceramic, and a flat plate solid electrolyte fuel cell using the separator.
[0002]
[Prior art]
Lanthanum chromite (LaCrO_Three) Is an oxide-based ceramic that is considered promising as a high-temperature conductive material because it has conductivity at high temperatures and is excellent in oxidation resistance and reduction resistance.
[0003]
The conductivity can be improved by adding alkaline earth metals such as calcium, strontium, and magnesium as trace impurities to the lanthanum chromite. Lanthanum chromite has a perovskite structure [ABO_Three(Wherein A represents lanthanum, B represents chromium, and O represents oxygen)], calcium and strontium added thereto are substituted and dissolved in the lanthanum chromite lattice and magnesium is substituted in the chromium position. ing.
[0004]
The above-mentioned trace element-added lanthanum chromite can have sufficient performance in terms of conductivity, but it is difficult to obtain a dense sintered body in atmospheric pressure, and it is a fuel when used as a separator in a flat plate type solid electrolyte fuel cell. The gas and air could not be completely separated, which was unsatisfactory.
[0005]
The reason why a dense sintered body with additive lanthanum chromite cannot be easily obtained in this way is that the diffusion of ions of structural elements is extremely slow and the interface of the raw material powder crystals is difficult to move, making it difficult to densify during the sintering process. In addition, the vapor pressure of chromium oxide is high at the sintering temperature, and vacancies hardly disappear during the densification process and remain in the sintered body.
[0006]
[Problems to be solved by the invention]
Therefore, the present inventors have previously obtained a general formula in order to easily obtain a dense sintered body in the atmosphere and to improve the conductivity as compared with the prior art.
La_1-xM_xCr_1-yCo_yO_Three
(Wherein, M is an alkaline earth metal excluding magnesium, 0 <x ≦ 0.5, 0 <y ≦ 0.5)
And a novel lanthanum chromite-based composite oxide characterized by having a perovskite structure, a high-temperature conductive material using the same, and a solid oxide fuel cell separator (Japanese Patent Laid-Open No. 3-65517). issue).
[0007]
However, the lanthanum chromite complex oxide has a volume change due to an increase in the lattice constant of the crystal under a low oxygen partial pressure in a reducing atmosphere when there are a large number of substitutional elements. In some cases, microcracks are generated inside the sintered body or cracks in the sintered body occur, resulting in a decrease in strength.
[0008]
In addition, when used as a flat-type solid electrolyte fuel cell separator in which the atmosphere on both sides is different from the oxidizing atmosphere and the reducing atmosphere, the strength decreases and cracks occur. There is a problem that the warpage of the separator occurs, leading to poor contact between the electrode and the separator, induction of electrolyte cracking, and deterioration of sealing performance.
[0009]
Furthermore, when used as a separator, it has sufficient performance in terms of conductivity and sinterability, but the thermal expansion coefficient consistency with the electrolyte that is a constituent material of the flat plate type solid electrolyte fuel cell is not always sufficient. Rather, for example, when a battery is configured with a flat structure and a large area of 20 cm square or more, a tensile or compressive stress is generated on the electrolyte due to a difference in thermal expansion coefficient with the electrolyte when the temperature is raised or lowered, and the electrolyte is destroyed. It is difficult to obtain sufficient sealing performance.
[0010]
In addition, when the lanthanum lattice position in the crystal lattice is substituted with Ca, if the Ca addition amount is excessive, a high resistance composite oxide of Ca and Cr is formed on the reduction side, or Ca diffuses into the cathode electrode. There are problems such as. That is, in order to obtain a stable sintered body, in the case of Ca-substituted lanthanum chromite, fine adjustment of the composition is required. Furthermore, Ca-substituted lanthanum chromite has an orthorhombic crystal structure at room temperature, and the temperature dependence of the thermal expansion coefficient is 300 to 500 ° C., and the change of the thermal expansion coefficient associated with the phase transformation is observed. A difference in thermal expansion coefficient with the temperature rises and becomes a factor of generating extra stress in the electrolyte during the temperature raising and lowering process.
[0011]
The object of the present invention is easily obtained in the atmosphere with high density under such circumstances, and has high density and good conductivity, low temperature coefficient of thermal expansion coefficient, and small dimensional change rate. Specific lanthanum chromite ceramics with excellent dimensional stability, improved sealing properties and electrode contact properties, good thermal expansion coefficient consistency with electrolyte, low oxygen partial pressure Another object of the present invention is to provide a flat-type solid electrolyte fuel cell separator having a small dimensional change in a reducing atmosphere and having excellent oxidation resistance and reduction resistance, and a flat-type solid electrolyte fuel cell using this separator.
[0012]
[Means for Solving the Problems]
As described above, the present inventors have previously replaced a part of lanthanum with an alkaline earth metal and a part of Cr with Co or the like in order to densify lanthanum chromite and improve conductivity. However, since this product has a large difference in thermal expansion coefficient from the stabilized zirconia that is a solid electrolyte, it may damage the zirconia in practical use, and the dimensional change during reduction is large. When used as a solid electrolyte fuel cell separator, warpage may occur and breakage, and these disadvantages are due to the inadequate amount of substitution at A site and B site and the combination of substitution elements. By limiting the amount of substitution and the combination of substitution elements, the difference in thermal expansion coefficient from the electrolyte is small, and the temperature dependence is also from room temperature. There is no drastic change up to 000 ° C, the dimensional change ratio of each ceramic in the air atmosphere and the reducing atmosphere, and hence the volume change ratio is small, excellent in dimensional stability, and high density and conductivity can be maintained, and further reproduced It has been found that a solid electrolyte fuel cell can be constructed with good performance, and the present invention has been made based on these findings.
[0013]
  That is, the present invention
  (1) General formula
  (La_1-xSr_x)_a(Cr_1-pCo_p)_bO_Three
  (Where 0.02 ≦ x ≦0.150.01 ≦ p ≦ 0.03, 0.95 ≦ a / b ≦ 1.05)
It is hexagonal at room temperature, and the ceramic has an oxygen partial pressure of 5 × 10 5 ° C. with respect to the target ceramic obtained by firing in air.^-16The dimensional change rate at room temperature of a heat-treated product for 20 hours in a reducing atmosphere at atmospheric pressure is 0.1% or less, the relative density is 90% or more, and the conductivity is 10Ω in air at 1000 ° C.^-1cm^-1Above, 1000 ° C. oxygen partial pressure 1 × 10^-151Ω in a reducing atmosphere at atmospheric pressure^-1cm^-1The thermal expansion coefficient from 50 ° C. to 1000 ° C. is 9.8 × 10.^-6K^-110.7 × 10^-6K^-14 point bending strength in accordance with JIS R1601 at room temperature98 MPa (10 kgf / mm ² )Lanthanum chromite ceramics characterized by the above,
  (2)A flat plate solid electrolyte fuel cell separator containing the lanthanum chromite ceramic according to (1),
  (3)A solid oxide fuel cell using the separator for a flat plate solid electrolyte fuel cell according to (2) above
Is to provide.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
  As a preferred embodiment,
  (4)The subscript p indicating the amount of substitution of Co in the general formula indicating the composition is represented by the following formula:
0.015 ≦ p ≦ 0.25
The lanthanum chromite ceramic according to the above item (1) in a range satisfying,
  (5)The subscript x indicating the substitution amount of Sr in the general formula indicating the composition is the following formula:
0.08 ≦ x ≦ 0.15
The lanthanum chromite ceramic according to the above item (1) in a range satisfying,
Is mentioned.
[0015]
BookIn the invention, “1000 ° C. oxygen partial pressure 5 × 10 5 of the ceramic for the target ceramic obtained by firing in air”^-16"The rate of dimensional change at room temperature of a 20-hour heat-treated product in a reducing atmosphere at atmospheric pressure" is the percentage of the change in the size of the latter heat-treated product at room temperature relative to the size of the former target ceramic at room temperature. The dimension means the cubic root of the volume of the unit cell obtained from the lattice constant measured by the X-ray diffraction method.
[0016]
  The lanthanum chromite ceramic of the present invention has a perovskite structure (ABO) represented by the above general formula._ThreeIn the basic structure of lanthanum chromite in which La is arranged at the A site and Cr is arranged at the B site, a part of La is replaced by Sr, and a part of Cr is furtherCoIt is necessary to have a substituted, predominantly perovskite structure.
[0017]
In the lanthanum chromite ceramic of the present invention, the conductivity is improved by replacing part of La with Sr. The substitution amount of Sr is selected in the range of 0.02 to 0.2, preferably 0.04 to 0.2, and more preferably 0.08 to 0.15 in molar ratio with respect to the total amount of the A site. The greater the amount of substitution of Sr, the higher the conductivity at 1000 ° C., but if it exceeds 0.2, the rate of dimensional change during reduction increases, and it is used as a separator for flat-type solid electrolyte fuel cells using stabilized zirconia as the electrolyte. As a result, warpage occurs, which causes destruction of zirconia as an electrolyte, gas leakage due to cracking of the separator itself, and an increase in thermal expansion coefficient. Therefore, there is a large difference in thermal expansion coefficient from stabilized zirconia as an electrolyte. When the thermal cycle of increasing and decreasing temperature is applied, it causes destruction of stabilized zirconia.
[0018]
On the other hand, when the substitution amount of Sr is less than 0.02, the linearity of the relationship between the thermal expansion coefficient and the temperature deteriorates, and the thermal expansion coefficient changes abruptly in the region of 300 ° C. or lower. Further, when La is excessively present in the sintered body or when A / B is increased from 1.05, La may segregate at the crystal grain boundary to absorb water vapor and collapse the sintered body. There is. Conversely, if A / B is less than 0.95, the second component may be precipitated.
[0019]
  It is a partial substitution element of Cr at B site of lanthanum chromite latticeCoThe lanthanum chromite, which has the effect of lowering the sintering temperature or improving the conductivity of the sintered body, does not have this substitution element, is 95% or more even when fired at 1700 ° C. in the atmospheric pressure atmosphere. Although a relative density cannot be obtained, a dense sintered body having a relative density of 95% or more can be obtained even at an equivalent firing temperature by dissolving this element in the B site. This is presumably because the rare earth chromite crystal particles, which are raw material powders, improve the mass transfer rate during the sintering process, and also lower the sintering temperature due to the lowering of the melting point.
[0020]
  Lanthanum chromite B siteCo substitution amount ofIn molar ratio to the total amount of B siteNext rangeChosen by. 0. 01 ≦ p ≦ 0.03, preferably0.015 ≦ p ≦ 0.025Is. PlaceIf the exchange amount is too large, the sinterability and conductivity will be improved, but the change ratio between each dimension in the air atmosphere and reducing atmosphere will be too large, and it will be used as a separator for flat plate type solid electrolyte fuel cells. Warping occurs. Due to the occurrence of warping, the sealing performance may be reduced due to the destruction of the electrolyte and the cracking of the separator itself, leading to a reduction in battery performance. Moreover, when there is too much substitution amount, the thermal expansion difference with the zirconia which is electrolyte will become large by the increase in a thermal expansion coefficient, and when the thermal cycle of raising / lowering temperature is applied, destruction of a zirconia will be caused. On the other hand, if the substitution amount is too small, the sinterability is lowered and it becomes difficult to obtain a high density.
[0021]
The effect of specifying the substitution element and the composition in this way is apparent from the examination of the physical properties of the conventionally known lanthanum chromite ceramics.
For example, Meadow craft et al. CERAMIC BULLETIN 1979 Vol. 58, no. 6 p610-615 reports that the amount of Cr evaporation in lanthanum chromite is suppressed by Al substitution at the B site._0.8Ca_0.2Al_0.25Ar_0.75O_ThreeThe thermal expansion coefficient from 100 ° C. to 1000 ° C. is 9.6 × 10^-6[1 / K] and the thermal expansion coefficient of zirconia 10.4 × 10^-6Compared to [1 / K], the difference in thermal expansion coefficient is 0.5 × 10^-6Exceed [1 / K]. Moreover, there is a temperature at which the thermal expansion coefficient changes abruptly in the vicinity of 400 ° C. In addition, the firing density at normal pressure of 1600 ° C. has less effect of improving the sinterability by 85% compared with the above substitutional elements such as Co, Ni, Zn, Fe, or Mn. Therefore, this composition is not suitable as a flat plate type solid electrolyte fuel cell separator.
[0022]
In addition, in Niels Christiansen et al., In PROCEEDING OF THE THIRD INTERNATIONAL SYMPOSIUMON SOLID STATE FUEL CELLS (p401), the amount of dimensional change during reduction varies depending on the amount of Co substitution, and La_0.7Ca_0.3Cr_0.975Co_0.025O_ThreeHowever, this composition has a large difference in thermal expansion coefficient from the electrolyte, and is not suitable for a flat plate type solid electrolyte fuel cell separator.
[0023]
On the other hand, disclosed in Japanese Patent Laid-Open No. 51-150692, (La_1-xM_x) (Cr_1-yNi_y) O_3-v[In the case of a composite oxide represented by 0 <x, y <1], the flat solid electrolyte fuel cell is usually exposed to 1000 ° C. when x = 0.2 and y = 0.9. In a reducing atmosphere, decomposition occurs and the difference in thermal expansion coefficient from the electrolyte is large, so that it cannot be used as a flat-type solid electrolyte fuel cell separator.
[0024]
B. Boven et al., In Zeitschiff for Naturforschung, Vol. 27, No. 2, pages 363-365 (1972), La_0.8Sr_0.2Cr_0.8Ni_0.2O_ThreeHowever, this composition has a large dimensional change before and after the reduction and is not suitable for use as a flat plate type solid electrolyte fuel cell.
[0025]
In the lanthanum chromite ceramic of the present invention, the B site substitution amount is large in order to increase the density and conductivity. For example, when the B site substitution element is Co, p is preferably close to 0.05. In order to reduce the difference in thermal expansion coefficient, the B site substitution amount is small. For example, when the B site substitution element is Co, p may be 0.03 or less.
The ratio a / b between the A site and B site of lanthanum chromite is not necessarily 1, but a sintered body can be obtained, but is selected so as to satisfy the following formula: 0.95 ≦ a / b ≦ 1.05. When a / b> 1.05 or a / b <0.95, the sinterability may be affected, or a lanthanum chromite single phase may not be obtained. In particular, when a / b >> 1.05, La segregates at the grain boundary.₂O_ThreeTherefore, even a sintered body that is densely fired becomes brittle over time.
[0026]
In the lanthanum chromite ceramic of the present invention, the crystal structure is hexagonal at room temperature, and the 1000 ° C. oxygen partial pressure of the ceramic with respect to the target ceramic obtained by firing in air is 5 × 10.^-16It is important that the rate of dimensional change at room temperature of a 20-hour heat-treated product in a reducing atmosphere at atmospheric pressure is 0.1% or less and the relative density is 90% or more. Among these, those having a perovskite structure are preferable.
Examples of the crystal structure analysis method include a powder X-ray diffraction method. Since the change in dimensions correlates with the change in volume, and the solid electrolyte fuel cell is also exposed to a high-temperature reducing atmosphere, the rate of change in dimensions is as important as the coefficient of thermal expansion. Only when both the rate of dimensional change and the coefficient of thermal expansion are optimized, it becomes useful as a separator for a solid oxide fuel cell.
[0027]
The crystal grain size affects the strength. When the crystal grain becomes larger, the defect size at the grain boundary becomes larger and the strength is lowered. The crystal grain size is preferably selected in the range of 20 μm or less, particularly 10 μm or less.
[0028]
  It is important that the lanthanum chromite ceramic of the present invention further has the following physical properties.
  1) Conductivity in air at 1000 ° C. and oxygen partial pressure of 1000 ° C. 1 × 10^-15Conductivity in a reducing atmosphere at atmospheric pressure Ω^-1cm^-1Σ in units_{1000, air}And σ_{1000, red}In the following formula
10 ≦ σ_{1000, air}, Preferably 15 ≦ σ_{1000, air}And 1 ≦ σ_{1000, red}, Preferably 10 ≦ σ_{1000, red}
Meet.
  2) The coefficient of thermal expansion from 50 ° C to 1000 ° C is the reciprocal of absolute temperature, ie 1 / K or K^-1In units of α,
9.8 × 10^-6≦ α ≦ 10.7 × 10^-6, Preferably 10.0 × 10^-6≦ α ≦ 10.5 × 10^-6
Meet.
  3) 4-point bending strength in accordance with JIS R1601 at room temperature is98 MPa (10 kgf / mm ² )Above, preferably137 MPa (14 kgf / mm ² )That's it.
[0029]
In general, in a fuel cell, the reducing atmosphere is usually an oxygen partial pressure of 1 × 10 on the fuel electrode side.^-15It means that the atmosphere is below atmospheric pressure. Furthermore, in order to increase battery performance such as battery output and efficiency, the oxygen partial pressure is 5 × 10 5.^-16In some cases, the operation is performed under atmospheric pressure. For example, when steam reforming is performed using LPG as a fuel under the condition of a steam / carbon ratio of 3, the gas composition is about 49% hydrogen, 32% steam, 13% carbon monoxide, 5 carbon dioxide at an operating temperature of 1000 ° C. %, And the oxygen partial pressure at this time is 1 × 10^-15Become atmospheric pressure. When the water vapor / carbon ratio is 2, the gas composition under the above operating conditions is about 58% hydrogen, 19% water vapor, 19% carbon monoxide, 4% carbon dioxide, and the oxygen partial pressure at this time is 3 ×. 10^-16Become atmospheric pressure.
[0030]
In lanthanum chromite, a part of oxygen constituting the perovskite-type crystal is released under such a high-temperature reducing atmosphere. Oxygen in the crystal exists as negative divalent ions. When oxygen escapes from the crystal lattice, repulsive force acts between nearby positive ions, or Cr is a positive ion at the B site. For example, the valence of ions changes from trivalent to tetravalent, and positive and negative charges move in the crystal lattice, and the balance between the attractive and repulsive forces of the positive and negative ions that are maintained until oxygen is released is lost. Since the distance between the atoms increases, the length of the unit cell increases, and as a result, the volume of the sintered body increases.
[0031]
The lanthanum chromite ceramic of the present invention is excellent in stability in this reducing atmosphere. As a result, oxygen partial pressure 5 × 10^-16Even if a high temperature heat treatment at about 1000 ° C. is performed in a reducing atmosphere at atmospheric pressure, there is little dimensional change, the dimensional change rate before and after this reduction heat treatment is 0.1% or less, and there is no decrease in 4-point bending strength. In addition, it has excellent oxidation resistance, the density is 90% or more of the theoretical density, and further 95% or more, and the electrical conductivity in air at 1000 ° C. is 10Ω.^-1cm^-1Or more, preferably 15Ω^-1cm^-1Above, oxygen partial pressure 1 × 10^-15Conductivity in a reducing atmosphere at atmospheric pressure is 1Ω^-1cm^-1Or more, preferably 10Ω^-1cm^-1That's it.
Therefore, the lanthanum chromite ceramic of the present invention is useful as a high-temperature conductive material that requires both corrosion resistance and conductivity at high temperatures.
[0032]
In general, in the flat plate type solid electrolyte fuel cell separator, in addition to the above high-temperature conductive material characteristics including reduction resistance, the warp is small from the viewpoint of sealing properties and reduction of contact resistance with the electrode, A warp during operation is preferably 200 μm or less with a 20 cm square plate, and characteristics such as a thermal expansion coefficient close to that of stabilized zirconia as an electrolyte are required.
[0033]
The lanthanum chromite ceramic of the present invention sufficiently has such required characteristics. That is, the lanthanum chromite ceramic of the present invention is formed into 5 cm square and 20 cm square plates, and at 1000 ° C., one surface is air and the other surface is oxygen partial pressure 5 × 10.^-16When the amount of warpage was measured without load under exposure to a reducing atmosphere at atmospheric pressure, the amount of warpage was 1 μm or less in the case of a 5 cm square, and the amount of warpage was 300 μm or less in the same measurement in the case of a 20 cm square plate. If the amount of warpage increases, the contact between the batteries worsens, the resistance of the battery increases, and the cell performance decreases. Therefore, the amount of warpage is preferably as small as possible, but even a 20 cm square plate with a warpage amount of 300 μm is more than 50 kg. When the load was applied, it could be flattened and no cracks were seen. Therefore, if the warp amount is 300 μm or less, more preferably 200 μm or less, the flattening can be sufficiently achieved without causing cracks by stacking and applying a load in a solid electrolyte fuel cell at an operating temperature of 1000 ° C. .
[0034]
In addition, the thermal expansion coefficient of the lanthanum chromite ceramic of the present invention is close to that of stabilized zirconia, so a flat solid electrolyte fuel cell is stacked in a large size of about 20 cm square and heated up to 1000 ° C. and then cooled down. However, the stabilized zirconia is not cracked and the sealing portion is not broken.
[0035]
The present invention also provides a solid oxide fuel cell separator containing such lanthanum chromite ceramics. For example, this ceramic can be used as it is or in combination with other suitable materials to obtain a desired separator by performing a required forming process.
[0036]
The present invention also provides a flat-type solid electrolyte fuel cell using such a separator.
A schematic diagram of an example of this fuel cell is shown in FIG. In the figure, reference numeral 1 denotes a solid electrolyte plate having a cathode 2 on the upper surface and an anode 3 on the lower surface. 4 is a separator made of the lanthanum chromite ceramic of the present invention, and 5 is an external output terminal made of the lanthanum chromite ceramic of the present invention, which is formed with grooves constituting the air flow path 6 and the fuel flow path 7. Yes. The separator separates air and fuel, and also serves to electrically connect the cathode 2 and the anode 3 of the adjacent unit cells. The external output terminal is a member that forms an air flow path or a fuel flow path at both ends of the integrated unit cell and that is electrically connected to the anode or the cathode. Although FIG. 1 shows a fuel cell in which two unit cells are integrated, three or more unit cells can be integrated through a separator.
[0037]
The lanthanum chromite ceramic of the present invention can be produced by a conventional ceramic production method.
For example, first, a powder mixture in which a lanthanum source, a strontium source, a chromium source, and a substitution metal source of the group B are mixed at a predetermined ratio is temporarily set at a predetermined temperature, generally 1000 to 1600 ° C, preferably 1000 to 1200 ° C. Baking to obtain a calcined powder. The calcining time is usually 1 to several tens of hours, preferably 1 to 10 hours. Calcination is performed in an oxygen-containing atmosphere, usually air. The pressure during calcination may be atmospheric pressure.
[0038]
Next, the desired ceramics can be obtained by forming the calcined powder by pressure molding or the like and firing it. The firing temperature is usually 1300 ° C. or higher, preferably 1500 to 1700 ° C., and the firing time is usually 1 to 10 hours, preferably 1 to 5 hours, depending on the shape of the molded body, and firing is an oxygen-containing atmosphere. Usually done in air. As the pressure during firing, a dense sintered body can be obtained at normal pressure, but may be under pressure.
[0039]
【The invention's effect】
The lanthanum chromite ceramic of the present invention has a dense and good conductivity, low temperature dependence of thermal expansion coefficient, small dimensional change rate, excellent dimensional stability, etc. It is useful as a conductive material, and can be easily prepared with high density in the atmosphere.
In addition, the flat plate type solid electrolyte fuel cell separator of the present invention contains this ceramic and has the characteristics thereof, the sealing property and the contact property with the electrode are improved, and the consistency of the thermal expansion coefficient with the electrolyte is good. Thus, since the dimensional change in a reducing atmosphere with a low oxygen partial pressure is small, and the oxidation resistance and reduction resistance are excellent, a flat solid electrolyte fuel cell using the separator can be provided.
[0040]
【Example】
EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited at all by these examples.
The characteristics of various ceramic samples and the influence characteristics of the samples on the electrolyte were evaluated as follows.
(1) Dimensional change rate before and after reduction
A sample having a size of about 4 × 3 × 40 mm is placed in an alumina container, nitrogen gas is circulated at 200 ml / min, the temperature is raised to 1000 ° C. at 5 ° C./min, and after reaching 1000 ° C. for 1 hour, steam / hydrogen Oxygen partial pressure 5 × 10^-16A reducing atmosphere at atmospheric pressure was circulated and held for 20 hours, then rapidly cooled to room temperature at a rate of temperature decrease of 20 ° C./min or more to prepare a reduced sample, and its dimensions relative to the sample before such treatment The degree of change was determined by the following two methods.
(I) A lattice constant is obtained by X-ray diffraction for each sample before and after the reduction treatment. Since the sintered body used as a separator is a polycrystal, the crystal axes are not oriented, and the dimensional change of the separator is observed as an average of the changes of the crystal axes. Therefore, in order to average the change in the measured lattice constant, the measured lattice constant of each sample is converted to the volume of the unit cell, and the volume change amount after the reduction treatment with respect to the volume before the reduction treatment is obtained. This change amount was expressed as a percentage of the cube root of the volume before the reduction.
(Ii) The size of each sample before and after the reduction treatment was measured with 1 μ accuracy, and the amount of change in the size of the sample after the treatment relative to the size of the sample before the treatment was shown in percentage. This dimension is for the longest side of the sample.
[0041]
(2) 4-point bending strength before and after reduction
The samples before and after the reduction treatment described in (1) were measured according to JIS-R-1601.
[0042]
(3) Thermal expansion coefficient (α)
Using a quartz vertical push rod type thermal expansion measuring device, the change in the length of the sample accompanying the temperature rise was determined as follows, and calculated according to the following formula.
α = (L_x-L₀) / [L₀× (T_x-T₀]]
[L in the formula_xIs the temperature T_xSample length in (° C), L₀Is the temperature T₀Shows sample length at (° C)]
A sample of 5 × 5 × 10 mm in size is placed in a quartz sample chamber in an electric furnace, sandwiched by a quartz displacement measuring rod from above, and a load of 10 g is applied up to 1000 ° C. so that the contact of the rod is maintained. The temperature is raised at a rate of 10 ° C./min, held at 1000 ° C. for 2 hours, lowered to room temperature at a rate of temperature drop of 10 ° C./min, and the temperature rise and fall is repeated twice. Using.
[0043]
(4) Probability of electrolyte cracking after thermal cycling
As shown in FIG. 2, a sample of an arbitrary size and 8 mol% Y₂O_ThreeDope ZrO₂The electrolyte (8YSZ) was stacked in the order of sample 11 / glass 12 / 8YSZ13 / sample 14 / weight 15. The glass was attached to four sides with a width of 10 mm from the end. Weight is 0.5g / cm²It was made to change according to the area. What was arranged in this way was heated up to 1000 ° C. at a heating rate of 1 ° C./min, held at 1000 ° C. for 2 hours, then cooled down to room temperature at a cooling rate of 1 ° C./min, and cracking of the electrolyte was observed. . Those that were not cracked by the temperature rise and fall were left as is, and when the electrolyte was cracked, it was replaced with a new electrolyte without cracking, and such temperature rise and fall and crack observation were repeated twice. The results of the observation performed three times in this way are shown with a probability of n / 3 where the number of cracks is n.
[0044]
(5) Conductivity
Conductivity in air atmosphere is measured by conductivity (AIR) (Ω^-1cm^-1), And the conductivity in a reducing atmosphere is defined as conductivity (RED) (Ω^-1cm^-1) Respectively.
[0045]
(6) Relative density
It was determined by dividing the measured density calculated from the dimensions and mass by the theoretical density. The theoretical density is 6.5 (g / cm) for strontium lanthanum chromite ceramics.^Three) And 6.2 (g / cm) for calcium-based lanthanum chromite ceramics^Three).
[0046]
(7) Warpage amount
3 is used. In this apparatus, an alumina container 22 corresponding to the size of the sample 21 is attached up and down, and the container 22 is connected to an alumina tube 23 that also serves as a gas introduction port. The fuel gas 24 adjusted to an arbitrary oxygen partial pressure by the hydrogen / water vapor ratio and the air 25 are sent from above, and glass is used for sealing the alumina tube / alumina container / sample.
The sample in the alumina container was heated to 1000 ° C. at 5 ° C./min, the oxygen partial pressure on the fuel gas side was changed at 1000 ° C., and the alumina rod 26 in contact with the center of the sample lanthanum chromite ceramic plate was The amount of warpage was measured by a dial gauge 27 at the top of the apparatus.
[0047]
  Lanthanum oxide (La₂O_Three4383 g, Strontium carbonate (SrCO)_Three442.8g, dichromium trioxide (Cr₂O_Three2324.4 g and tricobalt tetroxide (Co)_ThreeO_Four) 48.15 g was wet mixed using a ball mill for 24 hours. After drying this mixed powder, the temperature was raised at a rate of temperature rise of 20 ° C./min and calcined at 1200 ° C. for 2 hours. The calcined material thus obtained is ground again,196 MPa (2000 kgf / cm ² )Was subjected to cold isostatic pressing (CIP), heated to 1700 ° C. at a heating rate of 1 ° C./min, and fired at this temperature for 4 hours to obtain a desired lanthanum chromite ceramic. As a result of analyzing the powder prepared by pulverizing this powder by X-ray diffraction, the presence of the second phase could not be confirmed, and strontium and cobalt were dissolved in the lanthanum chromite lattice having a perovskite structure. I found out.
[0048]
Example 24Comparative Examples 1-13
  A lanthanum chromite ceramic having a perovskite structure was obtained in the same manner as in Example 1, using a predetermined raw material at a predetermined ratio. However, the calcining temperature was set to 1500 ° C. for calcium-based materials.
[0049]
Table 1 shows the composition and various properties of the ceramic thus obtained.
[0050]
[Table 1]

[0051]
In Table 1, x indicates that it decomposes or cracks in a reducing atmosphere, and in the dimensional change rate column, those marked with * are the methods of (i) above and marked with * No mark indicates that it was obtained by the method (ii).
[0052]
Each separator was formed by providing a predetermined raw material gas flow path in a compact of 200 mm × 200 mm × 5 mm made of each ceramic obtained in Examples 1 to 3, Comparative Example 6 and Comparative Example 9. In addition, a solid electrolyte plate made of zirconia (hereinafter referred to as 8YSZ) having the same square shape as the separator and having a thickness of 0.2 mm and containing 8 mol% of yttria (hereinafter referred to as 8YSZ) is prepared._0.8Sr_0.2MnO_ThreeWas deposited to a thickness of 0.1 mm by a slurry coating method to form a cathode, and Ni-8YSZ (weight ratio 7: 3) cermet was deposited to the thickness of 0.1 mm by a slurry coating method to form an anode.
[0053]
The solid electrolyte plate provided with each electrode and each separator were laminated to prepare various three-stage stack cells. Various cells were operated under the following conditions.
Fuel gas: H₂= 3.40 liters / hr and H₂Mixed gas with O = 1.13 liter / hr
Oxidant gas: air = 12.80 liters / hr
Cell temperature: 1000 ° C
Load: 100kg
The performance of each cell is shown as a voltage under a predetermined current density. The results are shown in Table 2.
[0054]
[Table 2]

[0055]
As a result, the cells of each comparative example have poor performance, the OCV is low, and cracks in the separator are observed after cooling down, so the current density is 0.3 A / cm.²However, the cell of each example had good performance, the OCV was almost the same as the theoretical value, there was no big leak, and no cracking of the separator was observed even after the temperature was lowered.
[Brief description of the drawings]
FIG. 1 is a schematic view of an example of a flat plate type solid electrolyte fuel cell using ceramics of the present invention as a separator.
FIG. 2 is a schematic diagram of an example of an apparatus for observing cracks after thermal cycling of an electrolyte.
FIG. 3 is a schematic view of an example of an apparatus for measuring the amount of warpage of each ceramic.
[Explanation of symbols]
1 Solid electrolyte plate
2 Cathode
3 Anode
4 Separator
5 External output terminal
6 Air flow path
7 Fuel flow path
11 samples
12 Glass
13 8YSZ
14 samples
15 weights
21 samples
22 Alumina container
23 Alumina tube
24 Fuel gas
25 Air
26 Alumina rod
27 Dial gauge

Claims (3)

Formula _{(La 1-x Sr x)} a (Cr 1-p Co p) b O 3
(Wherein 0.02 ≦ x ≦ 0.15 , 0.01 ≦ p ≦ 0.03, 0.95 ≦ a / b ≦ 1.05)
Of the heat-treated product for 20 hours in a reducing atmosphere of 1000 ° C. oxygen partial pressure of 5 × 10 ⁻¹⁶ atmospheres of the ceramic with respect to the target ceramic obtained by firing in air. The rate of dimensional change at room temperature is 0.1% or less, the relative density is 90% or more, the conductivity is 10Ω ^-1 cm ^-1 or more in air at 1000 ° C, and the oxygen partial pressure is 1 × 10 ^-15 atm at 1000 ° C. 1Ω ⁻¹ cm ⁻¹ or more in a reducing atmosphere with a thermal expansion coefficient from 50 ° C. to 1000 ° C. in the range of 9.8 × 10 ⁻⁶ K ⁻¹ to 10.7 × 10 ⁻⁶ K ⁻¹ . A lanthanum chromite ceramic having a four-point bending strength of 98 MPa or more in accordance with JIS R1601 at room temperature. A flat plate type solid electrolyte fuel cell separator containing the lanthanum chromite ceramic according to claim 1 . A solid oxide fuel cell using the separator for a flat plate solid electrolyte fuel cell according to claim 2 .

Images