JPH06206727A - Zirconia thin film - Google Patents

Abstract

PURPOSE:To improve electrical conductivity of a zirconia thin film by including CaO and one or more kinds of rare earth metal oxides in the form of a solid solution and forming a ZrO2 thin film having a specific film thickness, wherein the length of the part composed of one crystal grain in the direction of thickness is longer than a half of the film thickness. CONSTITUTION:A ZrO2 raw material such as ZrOCl2.8H2O is compounded with 5-18mol% of a stabilizer consisting of CaO and one or more kinds of rare earth metal oxides such as Y2O3 and Yb2O3 and optionally <=20mol% of a crystal growth accelerator such as TiO2 and CeO2. The mixture is dissolved in water, precipitated with NH3 water, filtered, washed with water, dried and calcined to obtain raw material powder. The raw material powder is incorporated with proper amounts of binder, plasticizer and solvent and kneaded to form a slurry. A green sheet is formed from the slurry with doctor-blade method, etc., dried and baked at 1300-1900 deg.C in air to obtain an electrically conductive ZrO3 thin film having a thickness of 5-300mum, wherein the length of the part composed of one crystal grain in the direction of thickness is >=50% of the film thickness.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a zirconia thin film which is widely used as a good oxygen ion conductor.

[0002]

2. Description of the Related Art Calcia (Ca
O), or so-called stabilized zirconia in which a rare earth oxide such as yttria (Y ₂ O ₃ ) is solid-solved, is not only excellent in heat resistance but also excellent in ionic conductivity, and thus is a good solid. It is used as an electrolyte in various fields such as fuel cells and oxygen sensors.

Fuel cells have been attracting attention because of their high power generation efficiency and low pollution. A solid oxide fuel cell, which is one type of fuel cell, uses zirconia as an electrolyte portion and can be roughly classified into a cylindrical type and a flat type. The solid electrolyte portion of the cylindrical type is formed as a zirconia film having a thickness of 10 to 100 μm by a thin film forming method such as a chemical vapor deposition method (CVD), an electrochemical vapor deposition method (EVD), and a thermal spraying method. On the other hand, the flat plate mold has its solid electrolyte portion formed by a conventional ceramics molding method such as a tape molding method, an extrusion molding method, and a casting molding method. However, the cylindrical type has a problem that the airtightness of the film is not sufficient and the manufacturing cost becomes high. Further, the flat plate type has a thin film thickness of 100 to 30
There is a limit to thinning to 0 μm, and there is a problem that it is difficult to increase the size and output.

As the oxygen sensor, a limiting current type sensor utilizing an oxygen pump action has been attracting attention because it has the advantages of higher sensitivity and wider measuring range than the conventional oxygen concentration battery type sensor. This sensor has a thickness of a zirconia thin film of about 100 to 300 μm, is heated by a heater to maintain a high temperature of 400 to 650 ° C., and a direct current is applied to act as an oxygen pump. However, the limiting current type sensor has room for further improvement in terms of response performance, operating temperature, reduction in heater capacity during heater preparation, and the like.

The zirconia solid electrolyte membrane used for these purposes is usually composed of a zirconia polycrystal. The reason why zirconia has the property as an electrolyte is based on the following reasons. That is, a Zr ⁴⁺ ion and its ionic radius are similar to each other, and a cation oxide having a smaller valence is solid-solved to form an oxygen ion vacancy, and the vibrational energy of the oxygen ion is increased by heating. Is caused by a current flowing as a result of moving to a vacancy by jumping and moving in the crystal. Examples of substances that exhibit such characteristics are CaO, Yb ₂ O ₃ , and Sc.
Rare earth oxides such as ₂ O ₃ are known, and these substances are called stabilizers. Y ₂ O ₃ is used as the most common stabilizer, and a Y ₂ O ₃ —ZrO ₂ type solid solution containing about 5 to 10 mol% of this is known as a typical stabilized zirconia. Has been. The electrical resistivity of this stabilized zirconia is about 10 ¹² Ω ・ at room temperature.
cm, 800 ℃ 10 ² Ω · cm, 1000 ℃ about 20
Ω · cm. This decrease in electrical resistance at high temperature is due to oxygen ion conduction, not electron conduction.

In order for the zirconia thin film to exhibit excellent effects as a solid electrolyte, (1) it must be airtight.
(2) low specific resistance, (3) high oxygen ion conductivity even at low temperature, (4) excellent responsiveness,
(5) Properties such as being chemically and thermally stable are required. A zirconia single crystal film can be cited as a film having such characteristics. The melting point of zirconia is about 280.
Since it is as high as 0 ° C. and the manufacturing cost becomes very high, it is hard to say that such a single crystal film is suitable for production on an industrial scale.

[0007]

SUMMARY OF THE INVENTION The main object of the present invention is to provide a zirconia thin film having electrical characteristics similar to a single crystal film, relatively easily and at low cost.

[0008]

The inventors of the present invention have conducted extensive studies in view of the above problems of the prior art, and as a result, adjusted the growth of crystal grains to modify the structure itself of the zirconia thin film. , That is, 1 in the thickness direction under a constant film thickness
By adopting a structure in which the length of the portion composed of crystal grains is 50% or more of the film thickness, the electrical characteristics close to those of the single crystal film can be obtained more easily than in the case of producing a single crystal film. And succeeded in completing the present invention.

That is, the present invention is a zirconia thin film in which at least one of CaO and a rare earth oxide is formed as a solid solution,
The present invention relates to a zirconia thin film having a film thickness of 5 to 300 μm and a length of a portion composed of one crystal grain in the thickness direction is 50% or more of the film thickness.

The present invention will be described in detail below.

The zirconia thin film of the present invention is mainly composed of stabilized zirconia. At least one of CaO and a rare earth oxide is solid-dissolved as a stabilizer in the stabilized zirconia. As the rare earth oxide, Y ₂ O
Those generally known as zirconia stabilizers such as ₃ , Yb ₂ O ₃ and Sc ₂ O ₃ are used. The content of the stabilizer may be usually about 5 to 18 mol%, but may be out of the above range as long as the electric resistance of the finally obtained zirconia thin film is the smallest. . In addition, SiO ₂ , Fe ₂ O ₃ , Al ₂ O ₃ , and Na ₂
Since impurities such as O may adversely affect the electrical characteristics and the like of the zirconia thin film, it is naturally preferable that the amount is small, but as long as the effect of the present invention can be obtained, it is 0.
There is no problem even if the content is up to about 01% by weight.

The structure of the zirconia thin film of the present invention is characterized in that, in the structure in the thickness direction, the length of the portion constituted by one crystal grain is 50% or more of the film thickness. By making the structure in the thickness direction a single crystal structure having substantially no grain boundaries, it is possible to improve the movement of oxygen ions in the thickness direction and reduce the electric resistance. Become. Most of the crystals constituting the zirconia thin film of the present invention are cubic crystals, but when the amount of the stabilizer is small, they may include tetragonal crystals. Also,
The grain size of crystals constituting the zirconia thin film of the present invention is usually 5 to
It is about 200 μm. Therefore, the thickness of the thin film having such a grain size is usually about 5 to 300 μm. The structure in the plane direction is a polycrystalline body composed of a large number of crystal grains.

The method for producing the zirconia thin film of the present invention will be described. First, the raw material powder is prepared by mixing the raw material zirconia, the stabilizer and the like in a predetermined ratio according to a usual method for preparing the stabilized zirconia powder. In this case, if necessary, TiO ₂ or CeO ₂ is added in addition to the above-mentioned components to promote the growth of crystal grains during firing. The addition amount is preferably within the range of 20 mol% or less. Further, the lower limit value may be appropriately determined depending on the composition of the thin film, the desired particle size, and the like. By adding such a crystal grain growth accelerating aid, a predetermined thin film structure can be formed even at a relatively low firing temperature, and it is particularly advantageous when producing a film having a film thickness of 100 μm or more.

Next, if necessary, various additives such as ordinary binders, plasticizers and solvents are blended with the above-mentioned raw material powder to prepare a slurry.

Next, a film molded body is prepared using the obtained slurry. As a forming method, a wet thin film forming method such as a doctor blade method or a tape forming method using a coater can be adopted, and a green sheet is formed by these methods. After drying, the raw sheet is fired. The firing atmosphere is usually an oxidizing atmosphere. The firing method may be atmospheric pressure sintering which is usually performed in firing ceramics. In some cases, hot isostatic pressing (HIP) is performed after pre-sintering.
You may practice the law. The firing temperature and the firing time differ depending on the composition of the slurry, the desired film thickness, the amount of the auxiliary agent added, and the like, and are not uniform, but usually about 1300 to 1900 ° C. In this case, generally, the higher the firing temperature, the larger the crystal grain size of the obtained crystal. Therefore, the firing temperature depends on the thickness of the thin film to be fired, and generally, the thicker it is, the higher the firing temperature needs to be. If TiO ₂ or CeO ₂ is added and baked as described above, a thin film having a predetermined structure can be obtained even at a relatively low baking temperature.

By thus forming a thin film using a slurry containing zirconia and setting an optimum firing temperature at which a crystal grain size commensurate with the film thickness can be expected, the structure in the thickness direction of the thin film is substantially A zirconia thin film having a single crystal structure can be obtained by making it into single particles.

[0017]

Since the zirconia thin film of the present invention has a structure in the thickness direction substantially made into a single particle, it has electrical characteristics similar to those of a single crystal film. In particular, it has a lower electric resistance than the conventional zirconia film and exhibits excellent conductivity. Therefore, it is particularly suitable for applications such as solid oxide fuel cells and oxygen sensors.

The zirconia thin film of the present invention can be easily and inexpensively produced as a large-area film as compared with the case of producing a single crystal film, and the addition of TiO ₂ or CeO ₂ makes it possible to obtain a thick film as a single film. Since it can be easily made into particles, it is suitable for production on an industrial scale.

[0019]

EXAMPLES Examples will be shown below to clarify the characteristics of the present invention. (Raw material powder preparation) zirconium oxychloride as the starting material _{(ZrOCl 2 · 8H 2 O)} , yttrium chloride _{(YCl 3 · 6H 2 O)} , titanium tetrachloride (TiCl ₄₎
And cerium nitrate (Ce (NO ₃ ) ₃ .6H ₂ O) were dissolved in water and ammonia water was added to cause coprecipitation, whereby Y ₂ O ₃ and ZrO ₂ were uniformly mixed. A precipitate was obtained. Next, this precipitate was washed thoroughly with water and then dried to obtain a dry cake. The dried cake was calcined in an electric furnace at 1100 ° C. for 3 hours, and then crushed and pulverized to prepare six kinds of raw material powders A to F having chemical compositions and impurities as shown in Table 1, respectively. . In addition,
The specific surface area of these powders was 7 to 8 m ² / g and the average particle diameter was 1 to 1.3 μm.

[0020]

[Table 1] (Formation and firing of thin film) Polyvinyl butyral (PVB) as a binder for 100 parts by weight of each raw material powder described above.
10 parts by weight of dioctyl phthalate (DO
P) 7 parts by weight, and 33 parts by weight of toluene as a solvent and 22 parts by weight of ethanol were mixed and mixed by a ball mill for 7 days to prepare slurries. After vacuum degassing each of the obtained slurries, the slurry viscosity is about 4000 CP.
The slurry was adjusted to S to form a tape forming slurry. The tape molding slurry was applied by a doctor blade method while changing the slit clearance. The green sheet thus obtained was dried and then fired. The firing schedule is 100 ° C./hr, 100 to 5 between room temperature and 100 ° C.
10 ° C / hr between 00 ° C, 60 ° C between 500 and 750 ° C
/ Hr heating rate, hold at 750 ℃ for 3 hours,
Furthermore, after holding at a predetermined firing temperature in each of the examples described below for 3 hours, the firing temperature was increased to room temperature at 250 ° C./hr.
It cooled at the cooling rate of. For some samples,
After firing, it is further processed by the HIP method, and then in air 13
It was re-fired at 00 ° C.

Example 1 The change in the number of constituent crystal grains in the thickness direction due to the difference in firing temperature was examined.

Five green sheets obtained by drying the green sheets obtained from the slurry using the above-mentioned raw material powder B were prepared. Then, these are (1) 1400 ° C., (2) 150
Those fired at firing temperatures of 0 ° C., (3) 1600 ° C. and (4) 1700 ° C. (Samples 1 to 4), and (5) 170
After firing at 0 ° C, further 1700 ° C under argon atmosphere
A sample (sample 5) processed by the HIP method was obtained. The thickness of the zirconia thin films of the obtained samples 1 to 5 was 65.
Was about 70 μm. X-ray diffraction analysis of these thin films revealed that all samples were composed of cubic zirconia only. Further, all of these zirconia thin films were densified with a density of 5.90 g / cm ³ or more, and no pinholes were observed using the discharge type pinhole tester.

Next, the sectional structure of each of the obtained zirconia thin films was observed with a scanning electron microscope. Electron micrographs of the cross-sectional structures of Samples 1 to 5 are shown in FIGS. A low-magnification scanning electron micrograph of Sample 5 is shown in FIG. Based on the results of FIGS. 1 to 5, the ratio of the portion composed of one crystal grain in each zirconia thin film was determined, and it was 0% in Sample 1.
Sample 2 is 0%, Sample 3 is 57%, Sample 4 is 83%
% And 86% in Sample 5.

As described above, in Sample 1, the thickness direction is 5 to 7
While it is composed of crystal grains (Fig. 1), the number of crystal grains decreases with increasing firing temperature (Fig. 2),
It can be seen that when fired at 1600 ° C. or higher, the thin film was substantially composed of one crystal grain as in Samples 3 to 5 (FIGS. 3 to 5).

Example 2 The structure of the thin film in the thickness direction was examined by the difference in the amount of Y ₂ O ₃ added as a stabilizer.

Samples A and C were obtained by preparing dried green sheets obtained from slurries using the above-mentioned raw material powders A and C and firing them at a firing temperature of 1600 ° C. The thickness of the obtained zirconia thin film was 65 to 70 μm. The cross-sectional structure of each of the obtained zirconia thin films was observed with a scanning electron microscope. Micrographs of Samples A and C are shown in FIGS. 7 and 8, respectively.

As a result, as can be seen from FIGS. 7 and 8,
The crystal grain growth was larger in the sample C in which the amount of Y ₂ O ₃ added was larger than that in the sample A, and the portion constituted by one crystal grain increased to occupy 61% of the film thickness. Further, although the crystal grain growth of sample A is smaller than that of sample C, the proportion of the portion composed of one crystal grain is 50%, which shows that a predetermined structure is formed.

Example 3 The effect of promoting grain growth by adding TiO ₂ was investigated.

Samples D and E were obtained by preparing dried green sheets obtained from slurries using the above-mentioned raw material powders D and E and firing them at a firing temperature of 1600 ° C. The thickness of the zirconia thin film thus obtained was 125 μm. X these thin films
Line diffraction analysis showed that both samples had Y ₂ O ₃ and Ti.
It was found that both O ₂ were solid-dissolved in zirconia and were composed of cubic zirconia single phase. The cross-sectional structure of each zirconia thin film was observed with a scanning electron microscope. Micrographs of samples D and E are shown in FIGS. 9 and 10, respectively. According to this, the length of the portion composed of one crystal grain is 7
It was 8 to 81%.

As can be seen from FIG. 9 and FIG. 10, if a certain amount of TiO ₂ is added, even a 125 μm thick film is fired at a relatively low firing temperature of 1600 ° C. to obtain the predetermined structure of the present invention. It can be seen that a zirconia thin film can be obtained.

Example 4 The effect of promoting grain growth by adding CeO ₂ was investigated.

A dried green sheet obtained from a slurry using the raw material powder F was prepared and fired at a firing temperature of 1600 ° C. to obtain a sample F. The thickness of the obtained zirconia thin film was 125 μm. X-ray diffraction analysis of sample F showed that Y ₂ O ₃ and Ce
It was confirmed that O ₂ was both solid-solved in zirconia and was composed of cubic zirconia single phase. Next, the cross-sectional structure of each zirconia thin film obtained was observed with a scanning electron microscope. A micrograph of Sample F is shown in FIG.

As a result, as can be seen from FIG. 11, the addition of a certain amount of CeO ₂ increased the portion composed of one crystal grain as compared with the case where Y ₂ O ₃ was added alone (1 It can be seen that the growth of crystal grains is promoted, with the ratio of the grain portion being 59%).

Example 5 The electrical resistance of the zirconia thin film was investigated.

Using the above raw material powder B in air at 1400 ° C.
The thin film a obtained by firing in the same manner as above, the same as the raw material powder B, was fired in air at 1700 ° C., further treated by the HIP method at 1700 ° C. under an atmospheric pressure of 1000 atm, and then re-fired in air at 1300 ° C. The electrical resistivities of the thin film b thus obtained and the thin film c obtained by firing the raw material powder E in air at 1600 ° C. were measured. The results are shown in Table 2 together with the physical properties of each thin film. The electrical resistivity was measured by applying platinum paste (“TR 7905” manufactured by Tanaka Massey) on both sides of each zirconia thin film, and
What was heat-treated at 00 ° C for 1 hour was used as a measurement sample, and
It was measured at 200 to 1000 ° C. in air by the AC impedance method using a frequency of 500 Hz.

[0036]

[Table 2] The thin film a is a zirconia thin film having a thickness of 70 μm and composed of crystal particles having a particle size of 10 μm. This has a complex impedance Cole similar to that of a normal polycrystalline zirconia film.
-The Cole plot shows two distinct half circles,
It was shown that the electrical resistance of the zirconia thin film is composed of the components inside the grain and at the grain boundary. An Arrhenius plot of the conductivity is shown in FIG. From FIG. 12, it can be seen that the grain boundary resistance tends to be higher than the intragranular resistance.

The thin films b and c are zirconia thin films having a thickness of 70 μm, which are completely made into single particles, and the thin film b is Y ₂ O ₃ film.
The thin film c contains Y ₂ O ₃ 7 mol% and TiO 2.
_It contains 210 mol%. The Cole-Cole plots of these complex impedances consisted only of distinct semicircles, indicating that they do not have the electrical resistance of grain boundary components. The conductivity plots are shown in FIGS. 13 and 14. It can be seen that no resistance of grain boundaries is observed in both thin films b and c, and the thin films b and c are composed only of intragranular resistance.

As described above, in the zirconia thin film,
It is understood that by making the crystal grains into single grains in the thickness direction, the grain boundary resistance can be eliminated and the electrical characteristics of the single crystal film can be approximated.

[Brief description of drawings]

FIG. 1 is an electron micrograph showing a ceramic material structure of a cross section of Sample 1 in Example 1.

FIG. 2 is an electron micrograph showing a ceramic material structure of a cross section of Sample 2 in Example 1.

FIG. 3 is an electron micrograph showing a ceramic material structure of a cross section of Sample 3 in Example 1.

FIG. 4 is an electron micrograph showing a ceramic material structure of a cross section of Sample 4 in Example 1.

5 is an electron micrograph showing a ceramic material structure of a cross section of Sample 5 in Example 1. FIG.

FIG. 6 is a low-magnification scanning electron micrograph showing the structure of the ceramic material of Sample 5 in Example 1.

7 is an electron micrograph showing a ceramic material structure of a cross section of Sample A in Example 2. FIG.

FIG. 8 is an electron micrograph showing a ceramic material structure of a cross section of Sample C in Example 2.

FIG. 9 is an electron micrograph showing a ceramic material structure of a cross section of Sample D in Example 3.

10 is an electron micrograph showing a ceramic material structure of a cross section of Sample E in Example 3. FIG.

FIG. 11 is an electron micrograph showing a ceramic material structure of a cross section of Sample F in Example 4.

FIG. 12 is a graph showing an Arrhenius plot of conductivity in the thin film a of Example 5.

13 is a graph showing an Arrhenius plot of conductivity in the thin film b of Example 5. FIG.

FIG. 14 is a graph showing an Arrhenius plot of conductivity in the thin film c of Example 5.

Claims (1)

[Claims] 1. A zirconia thin film in which at least one of CaO and a rare earth oxide is formed as a solid solution, and the film thickness is 5 to 300 μm.
A zirconia thin film characterized in that the length of a portion composed of one crystal grain in the thickness direction is 50% or more of the film thickness.