JP2001072465A - Solid electrolyte, its production, and fuel cell and oxygen sensor each using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a solid electrolyte active over a wide temperature range, e.g. at >=600 deg.C, high in ionic conductivity, highly stable in crystal structure even at high temperatures and having high mechanical strength, to provide a method for producing such a solid electrolyte, and to provide an oxygen sensor or fuel cell using the above electrolyte. SOLUTION: This solid electrolyte is represented by the formula: (1-x) ZrO2-xLn2O3 (Ln is a rare earth element; x is 0.05-0.16); wherein Ln2O3 is at least two kinds selected from the group consisting of Y2O3, Gd2O3 and Yb2O3. The 2nd objective method for producing the above solid electrolyte comprises a mixing step for zirconia powder and at least two kinds of rare earth metal oxides, firing synthesis step for the mixture, grinding step for the resultant fired product, molding step for the resultant powder, and sintering step for the powder thus molded; wherein the firing synthesis is carried out at 900-1,150 deg.C for 2-8 h. This solid electrolyte is usable in the other objective oxygen sensors or fuel cells.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a solid electrolyte and an oxygen sensor or a fuel cell using the solid electrolyte, and more particularly, to a solid electrolyte having activity over a wide temperature range from low to high temperatures, and a method for producing the same. And an oxygen sensor or a fuel cell using the solid electrolyte.

[0002]

2. Description of the Related Art A solid electrolyte is extremely effective as an electronic material for various elements such as a battery and a gas sensor because a solid electrolyte has a specific ion that contributes to conduction without fear of liquid leakage. It is being actively promoted.

In particular, ceramic solid electrolyte fuel cells (S
OFC) is being developed and several kW of zirconia ceramic fuel cells have been operating for thousands of hours. Since SOFCs are operated at high temperatures (> 1000 ° C), hydrocarbon-based fuels can be reformed (internal reforming) in the cell.
It is believed that high combustion efficiency (> 60%) can be obtained.

[0004] In general, a SOFC is composed of a solid electrolyte, an anode and a cathode. All constituent materials must be stable in an oxidation-reduction atmosphere, have an appropriate ionic conductivity, and have close thermal expansion coefficients, and the anode and cathode must be porous and gas permeable. .
In addition, it is desirable that the battery material has high strength and toughness and that it is inexpensive. Further, from the viewpoint of stability during operation, a material system that is simultaneously sintered as a basic requirement of the conductor is desirable.

At present, stabilized ZrO ₂ is mainly used as a material for a solid electrolyte, and divalent alkaline earth metal oxides such as CaO, MgO, Sc ₂ O ₃ and the like are used as stabilizers. Rare earth oxides such as Y ₂ O ₃ are used. The ionic conductivity characteristic value of ZrO ₂ doped with the alkaline earth metal CaO shows 0.01 (Ωcm) ⁻¹ at 800 ° C. Also H.Tannen
Proc. Int'l Etude Piles Combust, 19-26 (1
965), rare earth oxides, for example,
The ion conductivity of ZrO ₂ doped with Y ₂ O ₃ , b ₂ O ₃ or Gd ₂ O ₃ alone is about 1 × 10 ⁻¹ to 1 × 10 ⁻² S / cm at 800 ° C. It is reported that when the temperature is lowered below 2 ° C., it is considerably reduced to 2 × 10 ⁻² S / cm or less.

As for rare earth and alkaline earth stabilized zirconia, for example, Japanese Patent Publication No. 57-50748 and Japanese Patent Publication No. 57-50
749.

Further, a solid electrolyte material for an oxygen sensor is
The effects of the grain boundary phase and aging in the above temperature range are not so important, but in a solid electrolyte for fuel cells, stabilization of the crystal phase at high temperatures and prevention of strength reduction at high temperatures are performed in an oxidation-reduction atmosphere. It is a big problem.

[0008] Since the output of a single cell is limited to about 1 V, such a conventional cell needs to have a laminated structure in order to obtain high power. Such ceramic batteries have become large in size, and it has become very difficult to select a ceramic material system and to manufacture large batteries. For such a large ceramic battery, a container such as a combustor body or the like requires effective use of metal parts such as ferritic stainless steel from the economical aspect. In order to make effective use of metals, a wide range of temperatures, especially at low temperatures (600-800 ° C)
However, there is a need for a solid electrolyte material that is active and has an ionic conductivity equivalent to high temperatures (> 1000 ° C.).

The solid electrolyte is heated at a temperature of about 650 ° C.
Since the crystals are easily broken, the crystal phase is low over a wide temperature range.
Establishing stabilization technology and preventing strength reduction at high temperatures are major issues
It has become. In this regard, Japanese Patent Application Laid-Open No. H5-225820 discloses
Is used to stabilize the crystal structure._TwoO _Three The method of adding
It is shown.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to overcome the above-mentioned conventional problems and to provide a wide temperature range, for example, at 600 ° C.
Provided are a solid electrolyte having activity at the above temperature, high ionic conductivity, excellent stability of crystal structure even at high temperature, and high strength, a method for producing the same, and an oxygen sensor or a fuel cell using the solid electrolyte. It is in.

[0011]

Means for Solving the Problems As a result of intensive studies, the present inventors have found that it is important to control the ionic conductivity at low temperatures in the solid electrolyte material between the ionic conductivity in the particles and the ionic conductivity in the grain boundary. It found that is, a rare earth element to be added for stabilization, O ₂ flowing to the grain boundaries ^- by simultaneously adding two or more rare earth elements and rare earth elements to reduce, by the particle O ₂ ^- is The present inventors have found that O ₂ ⁻ can be prevented from flowing due to flow and grain boundaries, and have reached the present invention.

The solid electrolyte according to claim 1 has a general formula: (1)
-x) ZrO ₂ · xLn ₂ O ₃ (where: Ln is a rare earth element, x = 0.05 to
0.16).

According to a second aspect of the present invention, in the solid electrolyte according to the first aspect, Ln ₂ O ₃ comprises Y ₂ O ₃ , Gd ₂ O ₃ and
It is characterized by being at least two kinds selected from the group consisting of Yb ₂ O ₃ .

The solid electrolyte according to a third aspect of the present invention is the solid electrolyte according to the _{second aspect} , wherein Ln ₂ O ₃ comprises Y ₂ O ₃ , Gd ₂ O ₃ and Yb
It is characterized by being at least two members selected from the group consisting of ₂ O ₃ and Gd ₂ O ₃ , and Y ₂ O ₃ and Yb ₂ O ₃ .

[0015] The solid electrolyte according to claim 4 is characterized in that:
The solid electrolyte according to any one of the three items, wherein the average particle diameter is 2 to 12 μm.

According to a fifth aspect of the present invention, there is provided a method for producing a solid electrolyte, comprising:
It comprises a mixing step of zirconia powder and two or more rare earth oxides, a firing synthesis step, a pulverizing step, a forming step and a sintering step, wherein the firing synthesis is performed at a temperature of 900 to 1150 ° C. for 2 to 8 hours. And

[0017] The method for producing a solid electrolyte according to claim 6 comprises:
The method for producing a solid electrolyte according to claim 5, wherein the sintering step is performed at a temperature of 1400 to 1600C for 2 to 8 hours.

[0018] The method for producing a solid electrolyte according to claim 7 comprises:
The method for producing a solid electrolyte according to claim 5,
The sintering and sintering steps are performed in the atmosphere.

[0019] The method for producing a solid electrolyte according to claim 8 comprises:
The method for producing a solid electrolyte according to any one of claims 5 to 7, wherein the sintering step uses particles whose diameter is adjusted to 0.5 to 0.8 µm.

According to a ninth aspect of the present invention, an oxygen sensor or a fuel cell uses the solid electrolyte according to any one of the first to fourth aspects.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The solid electrolyte of the present invention is represented by the following formula (1-x) ZrO ₂ .xLn ₂ O ₃ (where Ln is a rare earth element and x = 0.05 to 0.16) In particular, Ln ₂ O ₃ is preferably at least two kinds selected from the group consisting of Y ₂ O ₃ , Gd ₂ O ₃ and Yb ₂ O ₃ , and Y ₂ O ₃ and Gd ₂ O ₃ , Y ₂ O _{3 3} and Yb ₂ O ₃ , Gd ₂ O ₃ and Yb ₂ O ₃ and
A combination of Yb ₂ O ₃ , Gd ₂ O _3, and Yb ₂ O ₃ can be used, because this is preferable from the viewpoint of forming a continuous solid solution in the above range of x.

By adding two or more kinds of Ln ₂ O ₃ to zirconia in this way, the solid electrolyte is stabilized and O ₂ ⁻ flowing to the grain boundaries is reduced, so that more O ₂ ⁻ flows in the particles. You can do so.

The solid solution to which the rare earth oxide is added has a "fluorite" type structure. Lattice distortion depending on the atomic radius of the added element occurs, and there is an optimum value of the added amount for each element. Since the optimum value is narrow, it is important to add two or more elements to obtain flat electric characteristics. When the composition is partially deviated due to the thermal diffusion of the added element, the characteristics are greatly changed. Therefore, it is important to maintain the crystal structure.

In the above formula, x is from 0.05 to 0.16. x is 0.
If it is less than 05, cubic zirconia will not be mixed with tetragonal zirconia and it will become a single phase, while if it exceeds 0.16, it will have a cubic zirconia structure, but will have an order / disorder transition. It is not preferable because the ion conduction performance is lowered.

The method for synthesizing the solid electrolyte represented by the above formula according to the present invention is not particularly limited, and the solid electrolyte can be synthesized using known and commonly used techniques. The main technology as a manufacturing condition of the zirconia material is a reaction firing method using a solid phase.

Specifically, for example, first, zirconia powder,
Rare earth oxide (stabilizer) (Rare earth oxide: Y ₂ O ₃ , Gd ₂ O ₃
And two or three selected from the group consisting of Yb ₂ O ₃ )
Is weighed and mixed so as to be 8 to 16 mol%. At this time,
It is preferable to add two or more rare earth oxides at the same time from the viewpoint of stabilizing the characteristics. This mixture is then
A slurry is obtained by pulverizing in water so that the average particle size becomes 2 μm or less by a ball mill. The powder average particle size at this time was 2
μm or less, and if it exceeds 2 μm,
The solid-state reaction in the firing synthesis step becomes insufficient or the solid solution concentration varies, resulting in unstable characteristic values.

After the slurry is dried, the slurry is calcined at about 900 to 1150 ° C. for 2 to 8 hours in the atmosphere, depending on the type of the stabilizer, to cause a solid phase reaction. The material after the solid phase reaction is again pulverized in water so that the average particle size becomes 0.5 μm or less by a ball mill, and dried by a spray dryer or the like to obtain stabilized zirconia granular powder. The particle size of the powder at this time must be 0.5 to 0.8 μm as measured by a laser beam. If the thickness is less than 0.5 μm, the pressing process cannot be performed. If the thickness exceeds 0.8 μm, sintering becomes insufficient and pores remain in the solid electrolyte, causing a reduction in strength.

The stabilized zirconia granules are used, for example, in a mold
And 2 ton / cm with hydrostatic press ^Two ~ 4ton / cm^TwoPressure
At a predetermined temperature (1400 to 1600 ° C for 2 to 8 hours)
Sintered solid electrolyte is obtained by sintering in air
Can be During sintering, co-fabric torch and alumina
It is preferable to use a sheath made of Aya, but Tochi may be made of alumina.
No.

As for the size of the crystal grains after sintering, the average particle size is 2 to 12 μm, preferably 2 to 10 μm. The average particle size is 10 parallel straight lines in the electron micrograph (arbitrary)
The average of the particle lengths traversed by is determined using an imaging device. By setting the average particle diameter to 2 to 12 μm, a dense and high-strength solid electrolyte can be provided. If the average particle diameter is> 12 μm, the strength is reduced, and if it is <2 μm, it is difficult to obtain high toughness. More preferably, it is 2 to 10 μm.

Further, the solid electrolyte battery of the present invention can exhibit stable characteristics when used in a fuel cell having different temperatures depending on the parts. In addition, a small-sized oxygen sensor used in an automobile exhaust pipe may be a device that can stably exhibit characteristics even under a situation where the temperature is drastically changed, the crystal structure changes with time, and the performance is easily changed.

Further, the solid electrolyte battery of the present invention has a high strength and a high toughness which can be used as a material for a fuel cell used at a high temperature, and can be sufficiently utilized for increasing the size of the battery. In general, it can be used not only as a material for an oxygen sensor attached to an exhaust pipe of an automobile but also as an oxygen sensor used in molten metal.

[0032]

The present invention will be described below with reference to the following examples and comparative examples. Examples 1 to 8 and Comparative Examples 1 to 4 Commercially available zirconia powder (EP grade, manufactured by Daiichi Kagaku Kagaku Kogyo) and a stabilizer (Shin-Etsu Chemical RU grade rare earth oxide; Gd ₂ O ₃ and Y ₂ O ₃₎ 5 to 20 mol% at the ratio shown in Table 1)
And weighed so that the average particle size is
Grinding in water was carried out for 24 hours so as to be 2 μm or less. Next, after drying the slurry, a firing reaction was carried out at 1100 ° C. for 4 hours. The slurry was again pulverized in water so as to have an average particle diameter of 0.8 μm or less by a ball mill, and dried with a spray dryer or the like to obtain a stabilized zirconia powder. The primary particle size of the powder at this time was 0.6μ.
m. The powder is compacted in a mold, and
molded at a pressure of 1 ton / cm ² , and a specified temperature (1300
で 1650 ° C. for 6 hours) to obtain a solid electrolyte.

Examples 9 to 16 and Comparative Examples 5 to 8 Solid electrolytes were obtained in the same manner as in Example 1 except for using Gd ₂ O ₃ and Yb ₂ O _{3 as} stabilizers. .

Examples 17 to 25, Comparative Examples 9 to 12 Solid electrolytes were obtained in the same manner as in Example 1 except that the stabilizers were changed to Y ₂ O ₃ and Yb ₂ O ₃ . .

Examples 26 to 34, Comparative Examples 13 to 14 According to the same procedures as those in Example 1 except that the stabilizers were Gd ₂ O ₃ , Y ₂ O ₃ and Yb ₂ O _3. A solid electrolyte was obtained.

Comparative Examples 15 to 23 Solid electrolytes were obtained in the same manner as in Example 1 except that the stabilizers shown in Table 5 were used alone.

Test Examples Each of the solid electrolytes obtained in Examples 1 to 34 and Comparative Examples 1 to 23 was processed into a JIS standard bending test piece, and ionic conductivity, bending strength, and fracture toughness were measured. Was. After the test, a durability test was performed for 100 hours in a furnace kept at 800 ° C. while flowing a current of 1.0 mA, and a change in crystal phase was measured by X-ray diffraction.

However, the characteristic evaluation was measured by the following method. (1) Bending strength: A four-point bending test described in JIS-R1601 was used. (2) Fracture toughness: measured using the SEPB method described in JIS-R1607. (3) Ionic conductivity: DC four-terminal method was used. Using JIS bending test pieces, platinum wires were fixed at regular intervals with a platinum paste, and then fired at 1000 ° C. to obtain test pieces. After measuring at a predetermined temperature, the resistivity is measured, and the reciprocal is calculated as the ion conductivity (σ).
And Conduction was assumed to be 100% oxygen ion conduction. The following equation was used for the calculation. sigma = current (A) × specimen cross-sectional area / voltage (V) × effective specimen length (4) crystalline phase: cubic and percentage cubic content of cubic peak in monoclinic peak = (I _{C (111) / I C (111) + I} M (
_{111) + I M (111)} × 100 I C (111); cubic (111) integrated intensity of the peak I _{M (111);} monoclinic (111) integrated intensity of the peak I _{M (111);} monoclinic ( 111) Integrated intensity of peak

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

[0042]

[Table 4]

[0043]

[Table 5]

The overall evaluation in the table is based on the following criteria. Mechanical properties Bending strength (MPa)? 600MPa Fracture toughness (MPa√m)? 6MPa√m Electrical properties Ion conductivity (S / cm)? 600, 700 ?? 0.02 at 600 ° C Structural stability Cubic content> 95 % When all the above conditions are satisfied, the results are shown in the table as 上 記, when the above two conditions of any one of the crystal structure, mechanical properties, and electrical characteristics are satisfied, Δ, and otherwise, ×. .

In the table, the composition range and the production conditions are such that the bending strength, the fracture toughness value, and the ionic conductivity are compatible in the examples, and the effect of adding a plurality of rare earth elements is confirmed without changing the cubic crystal ratio after durability as before. Was. When the rare earth oxide group is small, the strength toughness is high and the strength is high, but the ionic conductivity is low. On the other hand, when the amount is large, the strength toughness is lowered and the stability of the crystal is also lowered. Even when the ratio of the type of the added rare earth oxide is changed, the compatibility of the characteristic values is satisfied. If the sintering temperature is lower than the range of 1400 to 1600 ° C., the sintering is insufficient and the strength toughness is low and the ionic conductivity is low. On the other hand, if the sintering temperature is high, the crystal grains become too large and the toughness decreases.

When the amount of the rare earth oxide group is small, the toughness is high and the strength is high, but the ionic conductivity is low. When the amount is large, the strength toughness is lowered and the stability of the crystal is also lowered. Gd ₂ O ₃ and
Even if the ratio of Y ₂ O ₃ is changed, the compatibility of the characteristic values is satisfied. In addition, it was confirmed that when one rare earth oxide was used, the cubic crystal ratio after the durability test was reduced, and the crystal stability was changed.

[0047]

The solid electrolyte of the present invention has activity even at a low temperature of 600 ° C., has a high ionic conductivity, is effective in a wide temperature range, has excellent crystal structure stability and strength even at a high temperature. be able to.

According to the method for producing a solid electrolyte of the present invention, the solid electrolyte of the present invention can be produced effectively and economically.

In the oxygen sensor or fuel cell of the present invention, since the solid electrolyte of the present invention is used as a sensor material or a battery material, metal parts such as ferrite stainless steel can be used for the container body, and the cost is low. It can be.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01M 8/02 H01M 8/12 8/12 G01N 27/58 A (72) Inventor Fumio Munakata Yokohama-shi, Kanagawa 2F, Takaracho, Kanagawa-ku Nissan Motor Co., Ltd. F-term (reference) 4G031 AA07 AA08 AA12 BA03 CA04 GA03 GA09 GA11 5G301 CA26 CA28 CA30 CD01 CE02 5H026 AA06 BB01 BB06 BB08 EE13 HH01 HH08 HH10

Claims (9)

[Claims] 1. The general formula: (1-x) ZrO ₂ .xLn ₂ O ₃ (wherein, Ln
Is a rare earth element, x = 0.05 to 0.16). 2. The solid electrolyte according to claim 1, wherein Ln ₂ O ₃ is at least two members selected from the group consisting of Y ₂ O ₃ , Gd ₂ O ₃ and Yb ₂ O ₃ . 3. Ln ₂ O ₃ includes Y ₂ O ₃ and Gd ₂ O ₃ , and Yb ₂ O ₃ and Gd ₂ O
_3. The solid electrolyte according to claim 2, wherein the solid electrolyte is selected from the group consisting of Y ₂ O ₃ and Yb ₂ O ₃ . 4. The solid electrolyte according to claim 1, wherein the average particle diameter is 2 to 12 μm. 5. A method comprising the steps of mixing a zirconia powder and two or more rare earth oxides, firing and synthesizing, pulverizing, forming and sintering steps, wherein the firing and synthesis are performed at a temperature of 900 to 1150 ° C.
For 2 to 8 hours. 6. The sintering step is performed at a temperature of 1400 to 1600 ° C.
The method for producing a solid electrolyte according to claim 5, wherein the method is performed for up to 8 hours. 7. The method for producing a solid electrolyte according to claim 5, wherein the firing synthesis step and the sintering step are performed in the atmosphere. 8. The solid electrolyte according to claim 5, wherein particles having a diameter of 0.5 to 0.8 μm are used in the sintering step. 9. An oxygen sensor or a fuel cell using the solid electrolyte according to claim 1.