JP2005259556A - Solid electrolyte and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a Sc<SB>2</SB>O<SB>3</SB>-ZrO<SB>2</SB>system solid electrolyte superior in ion conductivity especially in low temperature regions (in the vicinity of 600°C to 800°C), a manufacturing method of the solid electrolyte, and furthermore a solid oxide fuel cell and an oxygen sensor using such solid electrolyte. <P>SOLUTION: By molding a powder having a composition expressed by (1-x)ZrO<SB>2</SB>+xSc<SB>2</SB>O<SB>3</SB>(x=0.03 to 0.15) by an isostatic pressing, and after calcining it within temperature ranges over 1,400°C and below 1,650°C, and by furthermore applying a heat treatment by hot isostatic pressure sintering under high temperature and high pressure atmosphere containing oxygen, a nanocrystal layer of the same component having the thickness of 1 to 10 nm is formed in the crystal grain boundary of the zirconia sinter. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a zirconia solid electrolyte used for an oxygen sensor, a fuel cell, and the like, and a method for producing such a solid electrolyte.

After Nernst discovered a solid electrolyte (SE) in 1899, Baur and Preis operated a ceramic fuel cell at 1000 ° C in 1937. Batteries) continue to advance, and several kW zirconia ceramic fuel cells have been operating for thousands of hours.
Since the SOFC is operated at a high temperature of about 1000 ° C., the hydrocarbon fuel can be reformed in the cell (internal reforming), and for example, high combustion efficiency exceeding 60% can be obtained. It is believed that.

Such SOFC is basically composed of a solid electrolyte, an anode, and a cathode, and in order to improve the performance by improving the adhesion between them, or to improve the durability by improving the thermal shock resistance, An intermediate layer is also formed between the electrode layer and the electrolyte layer.
All these constituent materials are required to be stable with respect to the redox atmosphere and have appropriate ion conductivity. In addition, the thermal expansion coefficients of the constituent materials are close to each other, and the anode and cathode are required to be porous and have gas permeability. Furthermore, from the standpoint of stability during operation, a material system sintered at the same time is desirable as a basic requirement of the conductor. In addition, as a matter of course, the battery material is desired to be excellent in strength and toughness and inexpensive.

  In addition, since the output of a single cell in SOFC is limited to about 1.1 V, it is necessary to adopt a laminated structure or a parallel structure in order to obtain high power. Therefore, it is very difficult to select a manufacturing technique and a system for a solid electrolyte and a ceramic material that supports the solid electrolyte. Therefore, effective use of metal parts such as ferrite-based stainless steel is desirable for the combustor body and the like, and in order to effectively use such metal members as support members, the operating temperature of the fuel cell is lowered. For this purpose, a solid electrolyte material having an ionic conductivity at a low temperature (600 ° C. to 800 ° C.) as excellent as that at a high temperature is desired.

For materials used as such SOFC solid electrolytes, stabilized zirconia (ZrO ₂ ) is the mainstream, and as stabilizers, divalent alkaline earth metal oxides such as CaO and MgO, Sc ₂ Rare earth oxides such as O ₃ and Y ₂ O ₃ are used. For example, the characteristic value when doped with CaO, which is an alkaline earth metal oxide, is 0.01 S / cm ion conduction at 800 ° C. Showing gender.
Moreover, although ion conductivity when rare earth oxides such as Y ₂ O ₃ and Yb ₂ O ₃ are doped alone shows a certain value, it is known that the ion conductivity is greatly lowered at 650 ° C. or lower. Yes. Incidentally, practically stabilized zirconia by adding a rare earth oxide alone is almost known by 1970.

On the other hand, recently, zirconia stabilized with scandium oxide (Sc ₂ O ₃ ) has been studied as a solid electrolyte for a fuel cell operating at 700 ° C. or higher. It is considered as the most promising solid electrolyte material.

However, in Sc ₂ O ₃ stabilized zirconia, when the amount of Sc ₂ O ₃ added is 6 to 9 mol%, a crystal phase including cubic crystals and orthorhombic phases is observed, and Y ₂ O ₃ While the ion conductivity of the same level as other rare earths such as stabilized zirconia is shown, when the addition amount of Sc ₂ O ₃ is 10 mol% or more, the conventional ionic compound as described above is used at a temperature of 700 ° C. or more. Although the ionic conductivity is several times higher than the value of the rare earth stabilized zirconia, a crystal phase represented by Zr ₇ Sc ₂ O ₁₇ appears in the low temperature range from 500 ° C. to 700 ° C. There is a report that the ionic conductivity is reduced by 2 orders of magnitude compared to the value of zirconia (FM Spiritonovet al., J. Solid State Chemistry, 2 (1970) p. 430-). 38).

On the other hand, Patent Document 1 proposes that by adding a sub-dopant such as In, Ga, or Ti in addition to Sc, the phase structure is stabilized and the destruction due to the difference in expansion coefficient based on the structural change is prevented. Has been. Furthermore, Patent Document 2 describes that by applying the sol-gel method, the Sc ₂ O ₃ and Al ₂ O ₃ -added ZrO ₂ -based electrolyte film excellent in ionic conductivity is thinned to improve power generation efficiency. ing.
Also reported is a method of reducing the pores by re-sintering Sc ₂ O ₃ stabilized zirconia in an argon atmosphere by a sintering method using a hot isostatic press, thereby improving strength and conductivity. (See Non-Patent Document 2).
Japanese Patent Laid-Open No. 06-150943 Japanese Patent Laid-Open No. 10-097860 Hirano et al., Solid State Ionics, 133 (2000) p. 1-9

  However, the techniques described in the above-mentioned patent documents and non-patent documents cannot sufficiently improve the ion conductivity, and further improve the ion conductivity in the low temperature region (around 600 ° C. to 800 ° C.) of the solid electrolyte. Is required.

The present invention has been made in view of the above-mentioned problems in conventional Sc ₂ O ₃ stabilized zirconia, and is excellent in ion conductivity particularly in the vicinity of 600 ° C. to 800 ° C., and enables low-temperature operation of a solid oxide fuel cell. It is an object of the present invention to provide a Sc ₂ O ₃ —ZrO ₂ -based solid electrolyte that contributes to the above, a method for producing such a solid electrolyte, and a solid oxide fuel cell and an oxygen sensor using the solid electrolyte. .

The present inventors have reported that in Y ₂ O ₃ -stabilized zirconia, the ionic conductivity at the grain boundary is about 2 orders of magnitude lower than the bulk conductivity in the grains (Acta Materia 51 (2003) p. 2539-). 2547), based on the assumption that a resistance layer that inhibits ionic conduction is also present at the grain boundaries of Sc ₂ O ₃ stabilized zirconia, in order to eliminate such a conduction-inhibiting layer, As a result of intensive investigations on material composition, starting materials, firing conditions, heat treatment, etc., heat treatment by hot isostatic pressing was performed on the fired Sc ₂ O ₃ —ZrO ₂ material in a high temperature and high pressure atmosphere containing oxygen. As a result, a very thin nanocrystal layer was formed at the grain boundary, and the ionic conductivity was found to be improved as compared with that before the heat treatment, and the present invention was completed.

The present invention is based on the above findings, and the solid electrolyte of the present invention is represented by the general formula: (1-x) ZrO ₂ + xSc ₂ O ₃ (wherein x is 0.03 to 0.15). It is characterized by comprising a nanocrystal layer of the same component having a thickness of 1 to 10 nm at a grain boundary.
In the method for producing a solid electrolyte of the present invention, the powder having the above components is formed by isostatic pressing, fired in a temperature range of more than 1400 ° C. and less than 1650 ° C., and then hot in an atmosphere containing oxygen. Heat treatment is performed by isostatic pressing.

  Furthermore, in the solid oxide fuel cell and the oxygen sensor of the present invention, the solid electrolyte of the present invention is used.

The solid electrolyte of the present invention comprises a zirconia sintered body having a composition represented by (1-x) ZrO ₂ + xSc ₂ O ₃ (wherein x is 0.03 to 0.15), and the grain boundaries thereof The nanocrystal layer having a thickness of 1 to 10 nm, the nanocrystal layer having the same composition as that in the crystal grains, and without inhibiting the ionic conductivity at the grain boundary, the ionic conductivity of the entire solid electrolyte Can be improved.
In the solid electrolyte production method of the present invention, the powder having the above components is subjected to isostatic pressing, fired at a temperature exceeding 1400 ° C. and less than 1650 ° C., and then hot isostatically in a high temperature and high pressure atmosphere containing oxygen. A nanocrystalline layer that bufferes the grain boundary resistance at the grain boundaries that are an impediment to ionic conduction by introducing heat treatment into the grain boundaries of the fired body through the heat treatment by hydrostatic sintering. In particular, it is possible to obtain a Sc ₂ O ₃ —ZrO ₂ -based solid electrolyte excellent in ionic conductivity at around 600 ° C. to 800 ° C.

  Such a solid electrolyte can be suitably used for a fuel cell, an oxygen sensor, and the like. In particular, when the solid electrolyte is applied to a solid oxide fuel cell, the operating temperature can be lowered. In addition to the battery element, it can greatly contribute to the improvement of durability and cost reduction of peripheral members.

  Hereinafter, embodiments of the present invention will be specifically described.

The solid electrolyte of the present invention comprises a zirconia sintered body having a composition represented by (1-x) ZrO ₂ + xSc ₂ O ₃ as described above (wherein x is 0.03 to 0.15), The crystal grain boundary is provided with a nanocrystal layer having a thickness of 1 to 10 nm. Such a nanocrystal layer is formed, for example, in a high-temperature and high-pressure atmosphere containing oxygen after molding and baking the powder material of the above components. It can be obtained by applying a hot isostatic pressing process.

  Thus, the solid electrolyte of the present invention is provided with a nanocrystal layer having a thickness of 1 to 10 nm at the crystal grain boundary of the zirconia sintered body, and the nanocrystal layer in the solid electrolyte is a transmission electron. Using a microscope (200 kV or more), it can be observed at a magnification of 100,000 times or more, and its components can be confirmed by EDS analysis irradiated with an electron beam focused to 1 to 5 nm.

Hereafter, the manufacturing method of the solid electrolyte of this invention is demonstrated in order of a process.
The manufacturing process of the solid electrolyte is the same as the conventional method through the raw material mixing process, the synthesis process, the pulverization process, the sintering process, and the molding process, but after normal firing, an oxygen-containing high-temperature and high-pressure atmosphere It is characterized by performing hot isostatic pressing.

(1-x) ZrO ₂ + xSc ₂ O ₃ (x = 0.03 to 0.15) As a starting material for a solid electrolyte having a composition represented by the general formula, it is produced from an aqueous citric acid solution of Zr and Sc. It is desirable to use the prepared powder. Here, the value of x falls within the above range when the value of x is smaller than 0.03, that is, when the Sc ₂ O ₃ content in the zirconia solid electrolyte is less than 3 mol%, On the other hand, when the monoclinic phase having low conductivity is increased, the temperature is higher than 0.15, that is, when the content of Sc ₂ O ₃ in the electrolyte exceeds 15 mol%, a high-temperature phase excellent in ion conductivity can be stably obtained. By disappearing.

  First, the citrate salt of Zr and Sc is calcined at a temperature of about 900 ° C. to 1000 ° C. to obtain the above composition. Further, the calcined product is pulverized in alcohol for about 24 hours using a ball mill, and then dried using a rotary evaporator. The calcination powder is preferably pulverized so that the particle size of the calcined powder is 0.5 μm or less. That is, when the particle diameter of the powder exceeds 0.5 μm, sintering in the subsequent process tends to be difficult.

  Next, the powder of the above component is molded by a hydrostatic pressure press at a pressure of about 98 MPa to 196 MPa, and then the molded body is fired for about 1 to 8 hours in the air in a temperature range of over 1400 ° C. and less than 1650 ° C. Apply.

  The molded body that has been fired in the above temperature range is further subjected to a heat treatment by hot isostatic pressing in an oxygen-containing atmosphere, and the heat treatment has an oxygen partial pressure of 5 to 20%. It is preferably carried out by maintaining in a high-temperature high-pressure oxygen atmosphere at a pressure of 0.9 to 200 MPa and a temperature of more than 1200 ° C. and less than 1650 ° C. for about 30 to 240 minutes.

As for the atmospheric gas other than oxygen in the heat treatment by hot isostatic pressing, argon is usually used as an inert gas, but is not particularly limited as long as it is not corrosive or flammable, For example, nitrogen gas or the like can be used.
In addition, the upper limit values of the oxygen partial pressure and sintering pressure are not particularly limited as long as there are no restrictions on the apparatus, and the processing time can be shortened by using such a high pressure / high oxygen concentration atmosphere. However, since an apparatus capable of such a sintering treatment is considered to increase manufacturing costs and operating costs, it is desirable that the apparatus be 20% and 200 MPa or less, respectively.

  The fine structure of the solid electrolyte thus obtained, that is, the presence of the nanocrystal layer having a thickness of 1 to 10 nm, is thin as described above by ion milling using a transmission electron microscope (200 kV or more). The observed sample can be confirmed by observing the sample at a magnification of 100,000 times or more, and its component can be obtained by EDS analysis irradiated with an electron beam focused to 1 to 5 nm.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited at all by these Examples.

(Example 1)
In the composition component represented by the general formula (1-x) ZrO ₂ + xSc ₂ O ₃ , a solid electrolyte material in which the molar ratio x of Sc ₂ O ₃ is 0.05 is temporarily sintered from citrate. After that, the calcined product was pulverized in alcohol using a ball mill for 24 hours, and then dried using a rotary evaporator.
Next, the powder was molded at a pressure of about 100 MPa, and then fired in the atmosphere at 1450 ° C. for 6 hours.

The molded body after firing was subjected to a heat treatment for 2 hours by hot isostatic pressing in an atmosphere of oxygen partial pressure: 20% (balance: Ar), pressure: 1.0 MPa, temperature: 1300 ° C. The obtained Sc ₂ O ₃ —ZrO ₂ solid electrolyte was examined for density, particle diameter, and cubic rate, and ion conductivity at 600 ° C. and 800 ° C. was measured. Furthermore, the structure of the sample thinned by ion milling was observed with a transmission electron microscope to confirm the presence of the nanocrystal layer.
These results are shown in Table 1 together with the production conditions.

(Example 2)
The solid electrolyte material was prepared so that the molar ratio x of Sc ₂ O ₃ was 0.07, and the same operation as in Example 1 was repeated except that the pressure was 98 MPa. Obtained. And the same investigation and measurement were performed, and the results are also shown in Table 1.
As a representative example of the embodiment of the present invention, a microstructural photograph (600,000 times) of the solid electrolyte according to the second embodiment by a transmission electron microscope is shown in FIG.

(Example 3)
The solid electrolyte material was prepared so that the molar ratio x of Sc ₂ O ₃ was 0.08, and the same operation as in Example 1 was repeated except that the pressure was 126 MPa. Obtained. And the same investigation and measurement were performed, and the results are also shown in Table 1.

Example 4
The solid electrolyte material was prepared so that the molar ratio x of Sc ₂ O ₃ was 0.07, except that the firing temperature of the molded body and the heat treatment temperature by hot isostatic pressing were 1500 ° C., respectively. The same operation as in Example 1 was repeated to obtain a solid electrolyte of this example. And the same investigation and measurement were performed, and the results are also shown in Table 1.

(Example 5)
The same operation as in Example 4 (molar ratio x = 0.07) was repeated except that the oxygen partial pressure in the heat treatment atmosphere by hot isostatic pressing was 5% and the pressure was 10 MPa. A solid electrolyte was obtained. And the same investigation and measurement were performed, and the results are also shown in Table 1.

(Example 6)
Except that the calcining temperature of the compact was 1550 ° C. and the pressure was 49 MPa, the same operation as in Example 4 was repeated to obtain a solid electrolyte of this example. And the same investigation and measurement were performed, and the results are also shown in Table 1.

(Example 7)
The same operation as in Example 6 (molar ratio x = 0.07) was repeated except that the heat treatment atmosphere pressure by hot isostatic pressing was 48 MPa to obtain a solid electrolyte of this example. And the same investigation and measurement were performed, and the results are also shown in Table 1.

(Example 8)
Except that the firing temperature of the molded body was 1600 ° C. and the pressure was 196 MPa, the same operation as in Example 4 was repeated to obtain a solid electrolyte of this example. And the same investigation and measurement were performed, and the results are also shown in Table 1.

Example 9
Except that the oxygen partial pressure of the heat treatment atmosphere by hot isostatic pressing was set to 5%, the same operation as in Example 8 (molar ratio x = 0.07) was repeated to obtain a solid electrolyte of this example. It was. And the same investigation and measurement were performed, and the results are also shown in Table 1.

(Comparative Example 1)
In the composition component represented by the general formula of (1-x) ZrO ₂ + xSc ₂ O ₃ , a solid electrolyte material in which the molar ratio x of Sc ₂ O ₃ is 0.07 is temporarily sintered from citrate. After that, the calcined product was pulverized in alcohol using a ball mill for 24 hours, and then dried using a rotary evaporator.
Next, the powder is molded at a pressure of about 100 MPa, and then fired in the atmosphere at 1450 ° C. for 6 hours, so that the solid electrolyte of this comparative example is not subjected to heat treatment by hot isostatic pressing. Obtained. And the same investigation and measurement were performed, and the results are also shown in Table 1.
Moreover, the fine structure photograph (300,000 times) of the solid electrolyte concerning the said comparative example 1 by the transmission electron microscope is shown in FIG.

As is apparent from the description in Table 1, in the solid electrolytes of Examples 1 to 9, as shown as a representative example in FIG. 1, the presence of a nanocrystal layer having a width of 1 to 10 nm was confirmed at the crystal grain boundary. In addition, it was confirmed that good ion conductivity was exhibited even in a low temperature range (around 600 ° C. to 800 ° C.).
On the other hand, in the solid electrolyte which is not subjected to the heat treatment by hot isostatic pressing, the nanocrystal layer as described above is not recognized as shown in FIG. It was also found that the ionic conductivity at ℃ and 800 ℃ was about 10% lower than that in the above example.

It is a transmission electron micrograph which shows the fine structure in the solid electrolyte concerning Example 2 of this invention. It is a transmission electron micrograph which shows the fine structure of the solid electrolyte concerning the comparative example 1 which has not been heat-processed by hot isostatic pressing.

Claims (5)

The following general formula (1)
(1-x) ZrO ₂ + xSc ₂ O ₃ (1)
(Wherein x is 0.03 to 0.15), and is composed of a zirconia sintered body having a composition represented by the following formula, and has a nanocrystalline layer of the same component having a thickness of 1 to 10 nm at a grain boundary. A solid electrolyte characterized by that.   In producing the solid electrolyte according to claim 1, the powder having the above components is molded by a hydrostatic pressure press, fired in a temperature range of more than 1400 ° C. and less than 1650 ° C., and further hot-static under an oxygen-containing atmosphere. A method for producing a solid electrolyte, characterized by performing a heat treatment by hydrostatic sintering.   In the heat treatment step by hot isostatic pressing, the oxygen partial pressure is 5 to 20%, the pressure is 0.9 to 200 MPa, and the temperature is maintained in an atmosphere in the temperature range of more than 1200 ° C. and less than 1650 ° C. for 30 to 240 minutes. The method for producing a solid electrolyte according to claim 2.   A solid oxide fuel cell comprising the solid electrolyte according to claim 1. An oxygen sensor using the solid electrolyte according to claim 1.

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