Formation of ceramic thin film on ceramic body

A process for forming a ceramic thin film on a surface of a ceramic body, including the steps of preparing an annular body from a ceramic green sheet, inserting the ceramic body into the annular body, and firing the thus assembled annular body and ceramic body, whereby the annular body is shrunk and joined to the ceramic body.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a process for forming a ceramic thin film 
on a ceramic body. More particularly, the present invention relates to the 
process for forming a ceramic thin film to be suitably used as a solid 
electrolyte film, an interconnector film or the like of a cylindrical 
solid oxide fuel cell. 
(2) Related Art Statement 
Recently, fuel cells have attracted public attention as power generators. 
This is because the fuel cell is a device capable of directly converting 
chemical energy possessed by a fuel to electric energy, and is free from 
the limitations of the Carnot cycle. Therefore, the fuel cell has an 
essentially high energy conversion efficiency, causes less pollution, can 
use a variety of fuels (naphtha, natural gas, methanol, coal-reformed gas, 
etc.), and the power-generating efficiency is not influenced by the scale 
of the power-generating plant. Thus, the fuel cell is an extremely 
promising technique. 
In particular, since the solid oxide fuel cells (hereinafter referred to 
briefly as "SOFC"s) operate at high temperatures around 1,000.degree. C., 
their electrode reaction is extremely active. Thus, a catalyst of noble 
metal such as precious platinum is not required at all. Further, since 
polarization is small, and output voltage is relatively high, energy 
converting efficiency is far greater as compared with those in other fuel 
cells. Furthermore, since the SOFC is entirely constituted by solid 
structural materials, the SOFC has long stability and long use life. 
In the above-mentioned SOFC, it is necessary that a solid electrolyte thin 
film and/or an inter-connector thin film is provided on a cylindrical 
porous electrode-supporting body. The term "porous electrode-supporting 
body" may include a porous air electrode itself, a porous fuel electrode 
itself, and a porous supporting body on which a porous air or fuel 
electrode is formed. The porous electrode-supporting body may be solid or 
hollow. Heretofore, thin film-forming processes such as a chemical vapor 
deposition process (CVD process) and an electrochemical vapor deposition 
process (EVD process) have been known to form such solid electrolyte thin 
films. However, these conventional processes have problems in that thin 
film-forming devices become bulky, and the treatable area and treating 
speed are too small. For this reason, these processes have defects in that 
cost is high, and the area of the solid electrolyte film is difficult to 
increase. In the case of the EVD process, the substrate is limited to the 
cylindrical shape only. 
Since plasma spraying advantageously enables simple formation of thin and 
relatively dense films at high film-forming speeds the plasma spraying 
process has been conventionally used for the production of the SOFCs 
(Sunshine 1981, Vol. 2, No. 1). However, since the plasma-sprayed film has 
generally a problem that the gas-tightness is poor, if a solid electrolyte 
film of the above-mentioned SOFC is formed by plasma spraying, the 
gas-tightness of the film is insufficient. Consequently, a fuel leakage 
occurs, that is, hydrogen, carbon monoxide or the like leaks through the 
solid electrolyte film, during the operation of the SOFC. Thus, the 
electromotive force per cell unit of the SOFC becomes smaller than, for 
example, lV in ordinary cases, so that output drops and convention rate of 
the fuel to the electric power decreases. 
In addition, a technique is known, which forms an interconnector thin film 
by using the EVD process. However, as is the same with the formation of 
the solid electrolyte thin film, the film-forming device becomes bulky, 
and the treating area and the treating speed are too small, with the 
result that the cost rises, and the mass productivity becomes difficult. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to eliminate the above-mentioned 
problems, and to provide a process capable of easily and inexpensively 
forming a ceramic thin film having good performance. 
The process for producing the ceramic thin film according to the present 
invention is directed to the formation of a thin ceramic film on a surface 
of a ceramic body, and is characterized by comprising the steps of 
preparing an annular body from a ceramic green sheet, inserting the 
ceramic body into the annular body, and forming the ceramic thin film on 
the surface of the ceramic body by shrinking and joining the annular body 
to the ceramic body by firing in this state. 
The term "joining" used in the specification and the claims means that 
since the annular body is shrunk on firing, the fired annular body (thin 
ceramic film) is formed on the surface of the ceramic body such that the 
annular body is physically and/or chemically joined to the surface of the 
ceramic body with no substantial gap at an interface between the annular 
body, and the ceramic body so that the annular body may not be peeled from 
the ceramic body. As mentioned later, the term "joining" includes bonding 
the annular body to the ceramic body by using an adhesive. The term 
"joining" may include "physical joining" and "both physical joining and 
chemical joining". 
In the above-mentioned construction, the annular body is prepared from the 
ceramic green sheet, the ceramic body is inserted into the annular body, 
and the assembled ceramic body and annular body are fired in this state, 
so that the annular body is shrunk and bonded to the ceramic body. The 
present invention is based on the discovery that the dense ceramic thin 
film having good performance can be formed on the surface of the ceramic 
body by such a simple process. 
The annular body means a green ceramic body in the form of a thin annular 
film, which may have generally the thickness of tens of .mu.m to hundreds 
of .mu.m. Such a green ceramic body may be produced by preparing a green 
ceramic sheet having a thickness of tens of .mu.m to hundreds of .mu.m, 
cutting a rectangular or square sheet from this green ceramic sheet, 
rolling the cut green sheet in an annular shape after applying an adhesive 
(glue) on opposite edges of the green sheet, and press bonding the 
adhesive-applied opposite ends. Alternatively, no adhesive is applied to 
the cut green sheet, and the green sheet is merely shaped in an annular 
shape, and the opposite ends are pressed to each other to join them. For 
example, the annular body may include a thin film of a solid electrolyte 
and a thin film of an interconnector. The ceramic body means green or 
fired ceramic bodies, solid or hollow, which have a certain strength great 
enough to serve as substrates for thin films. 
According to the present invention, it is preferable that the firing 
shrinkage factor of the annular body is greater than that of the ceramic 
body. It is further preferable that the firing is effected in the state 
that an adhesive is interposed between the ceramic body and the annular 
body. 
These and other objects, features and advantages of the invention will be 
appreciated upon reading of the following description of the invention 
when taken in conjunction with the attached drawings, with the 
understanding that some modifications, variations or changes of the 
invention could be easily made by the skilled person in the art to which 
the invention pertains, without departing from the spirit of the invention 
or the scope of claims appended hereto.

DETAILED DESCRIPTION OF THE INVENTION 
An example of the process for producing a ceramic thin film according to 
the present invention is illustrated in FIGS. 1 (a) through (e) in the 
order of the producing steps. In the example of FIGS. 1 (a) through (e), a 
solid electrolyte thin film only is formed on a porous 
electrode-supporting body. First, as shown in FIG. 1 (a), a zirconia sheet 
1 is formed by a tape-casting process or the like. It is necessary to 
determine the shape of the zirconia sheet 1 depending upon the diameter 
and the length of a cylindrical body to be produced. Next, as shown in 
FIG. 1 (b), an annular body 2 is obtained from the zirconia sheet 1 by 
joining opposite ends of the zirconia sheet 1 through bonding with 
adhesive or press bonding. It is necessary that the inner diameter of the 
annular body 2 is set to be substantially equal to or slightly smaller 
than the outer diameter of the ceramic body as measured after shrinkage, 
considering the firing shrinkage factor thereof. 
The term "porous electrode-supporting" in the case of the SOFC includes a 
porous air electrode, a porous fuel electrode, a porous air electrode 
formed on a porous supporting body and a porous fuel electrode formed on a 
porous supporting body. The porous electrode-supporting body may be solid 
or hollow. 
Then, as shown in FIG. 1 (c), an adhesive is applied onto the porous 
electrode-supporting body 3 preliminarily prepared, such that an outer 
surface 4 of the supporting member upon which the annular body 2 contacts 
is dipped into a slurry or the slurry is sprayed onto the outer surface 4. 
At that time, it is preferable that a difference of (a coefficient of 
thermal expansion of the supporting body 3--a coefficient of thermal 
expansion of the annular body 2) is in a range of -2.0.times.10.sup.-6 to 
+1.0.times.10.sup.-6. With respect to both the ceramic body and the 
annular body, the coefficient of thermal expansion means a coefficient of 
thermal expansion in a temperature range in which the ceramic body or the 
annular body can be reversibly and repeatedly expanded and shrunk. 
Further, it is preferable that the firing shrinkage factor of the annular 
body 2 is greater than that of the supporting body 3. Further, it is 
preferable to use, as the adhesive, Cr.sub.2 O3, MnO.sub.z, Mn.sub.3 
O.sub.4, LaMnO.sub.3, LaCoO.sub.3, ZrO.sub.2 stabilized with Y.sub.2 
O.sub.3, or a mixture thereof, which exhibits electroconductivity at 
1,000.degree. C. Next, as shown in FIG. 1 (d), the annular body 2 is 
fitted around a given location of the porous electrode-supporting body 3 
which is coated with the adhesive. Finally, the assembled annular body and 
supporting body are fired in this state, so that the annular body 2 is 
shrunk and joined to the porous electrode-supporting body 3. Thereby, the 
porous electrode-supporting body 3 having the solid electrolyte thin film 
2 can be obtained as shown in FIG. 1 (e). As to the ceramic supporting 
body, the supporting body may be used according to the present invention 
in either fired or green state. 
FIGS. 2 (a) through (e) are views for illustrating another example of the 
process for producing the ceramic thin film according to the present 
invention in the order of producing steps. In the example of FIGS. 2 (a) 
through (e), a solid electrolyte thin film and an interconnector thin film 
are simultaneously formed on a porous electrode-supporting body. In FIGS. 
2 (a) through 2 (e), the same reference numerals are given to the same 
parts or members as those in FIGS. 1 (a) through (e), and their 
explanation is omitted. The example of FIGS. 2 (a) through 2 (e) differs 
from that in FIGS. 1 (a) through (e) in that a sheet 6 for an 
interconnector made of lanthanum chromite doped with calcium is prepared 
separately from an zirconia sheet 1, and an annular sheet 2 is formed by 
arranging the interconnector sheet 6 at an opening defined between opposed 
ends of the zirconia sheet 1. By following the same steps as in FIGS. 1 
(a) through 1 (e) excluding the above difference, the porous 
electrode-supporting body 3 having the solid electrolyte film 2 and the 
interconnector thin film 6 can be obtained. 
The cylindrical porous electrode-supporting body may generally have a 
thickness of hundreds of .mu.m to few mm and can allow sufficient 
permeation of a gas. For example, the porous electrode support may be made 
of lanthanum manganate (LaMnO.sub.3) doped with Sr, Ca or the like. The 
interconnector may be made of lanthanum chromite (LaCrO.sub.3) doped with 
Ca, Mg, Sr or the like. The solid electrolyte may be made of zirconia 
(ZrO.sub.2) stabilized with yttoria (Y) in an amount of 3-16 mol %, 
generally made of ZrO.sub.2 stabilized with 8 mol % yttria (8 mol % YSZ). 
In the following, actual examples of the process for producing the ceramic 
thin films according to the present invention will be explained. 
Experiment 1 
A solvent and a binder were added to a powder given in Table 1, which was 
extruded. Then, an extrudate was fired at 1,200.degree. C. to 
1,600.degree. C. and worked, thereby obtaining a solid rod having a 
diameter of 20 mm and a length of 100 mm. Independently, annular bodies 
having a diameter of 22 mm, a length of 80 mm, and a thickness of 100 
.mu.m were prepared from a green sheet composed of partially stabilized 
zirconia containing 8 mol % Y.sub.2 O.sub.3 (hereinafter referred to as 
"8YSZ") or a green sheet composed of Al.sub.2 O.sub.3. Then, an adhesive 
was applied onto an outer surface of the solid rod upon which the annular 
body was to contact. As the adhesive, partially stabilized zirconia powder 
was used in the case of the 8YSZ annular body, and Al.sub.2 O.sub.3 powder 
was used in the case of the Al.sub.2 O.sub.3 annular body. Thereafter, the 
annular body was fitted around the solid rod, which were fired at 
1,400.degree. C. A joined state and any crack occurring in the thin film 
were observed. A clearance between the outer diameter of the solid rod and 
the inner diameter of the annular body was set at 2.0 mm before the 
firing. Results are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Annular body 
8YSZ (coefficient of thermal 
Al.sub.2 O.sub.3 (coefficient of thermal 
expansion, 10.5 .times. 10.sup.-6) 
expansion, 8.6 .times. 10.sup.-6) 
Difference in Difference in 
coefficient coefficient 
Solid rod (coefficient of 
of thermal of thermal 
thermal expansion, .times. 10.sup.-6) 
expansion* 
Joined state 
Cut 
expansion** 
Joined state 
Cut 
__________________________________________________________________________ 
Mullite 
4.5 -6.0 .times. 10.sup.-6 
.DELTA. 
.DELTA. 
-4.1 .times. 10.sup.-6 
.DELTA. 
.DELTA. 
Titania 
8.0 -2.5 .times. 10.sup.-6 
.DELTA. 
.DELTA. 
-0.6 .times. 10.sup.-6 
.largecircle. 
.largecircle. 
Spinel 8.2 -2.3 .times. 10.sup.-6 
.largecircle. 
.largecircle. 
-0.4 .times. 10.sup.-6 
.largecircle. 
.largecircle. 
Al.sub.2 O.sub.3 
8.6 -1.9 .times. 10.sup.-6 
.largecircle. 
.largecircle. 
0 .largecircle. 
.largecircle. 
La(Sr)CrO.sub.3 
8.9 -1.6 .times. 10.sup.- 6 
.largecircle. 
.largecircle. 
+0.3 .times. 10.sup.-6 
.largecircle. 
.largecircle. 
Beryllia 
9.1 -1.4 .times. 10.sup.-6 
.largecircle. 
.largecircle. 
+0.5 .times. 10.sup.-6 
.largecircle. 
.largecircle. 
Forstelite 
9.5 -1.0 .times. 10.sup.-6 
.largecircle. 
.largecircle. 
+0.9 .times. 10.sup.-6 
.largecircle. 
.largecircle. 
8YSZ 10.5 0 .largecircle. 
.largecircle. 
+1.9 .times. 10.sup.-6 
.DELTA. 
.largecircle. 
La(Sr)MnO.sub.3 
11 +0.5 .times. 10.sup.-6 
.largecircle. 
.largecircle. 
+2.4 .times. 10.sup.-6 
.DELTA. 
.largecircle. 
Calcia 13.0 +2.5 .times. 10.sup.-6 
.DELTA. 
.DELTA. 
+4.4 .times. 10.sup.-6 
.DELTA. 
.DELTA. 
Magnesia 
14.2 +3.7 .times. 10.sup.-6 
.DELTA. 
.DELTA. 
+5.6 .times. 10.sup.-6 
.DELTA. 
.DELTA. 
__________________________________________________________________________ 
Note) 
.largecircle.: good, .DELTA.: partially good 
*Coefficient of thermal expansion of solid rod coefficient of thermal 
expansion of 8YSZ 
**Coefficient of thermal expansion of solid rod coefficient of thermal 
expansion of Al.sub.2 O.sub.3 
It is seen from the results in Table 1 that almost excellent ceramic thin 
films can be obtained from any ceramic according to the present invention. 
Further, it is also seen that when a difference in coefficient of thermal 
expansion between the thin film and the supporting body is in a range from 
-2.0.times.10.sup.-6 to 1.0.times.10.sup.-6, more excellent results can be 
obtained. Thus, this difference in thermal expansion is preferred. 
Experiment 2 
In order to examine the relationship between the outer diameter of the 
porous electrode-supporting body and the inner diameter of the annular 
body made of a solid electrolyte, an annular body made of 8 mol % YSZ was 
fitted around a porous electrode-supporting body through the 8 mol % YSZ 
adhesive. The porous electrode-supporting body had an outer diameter of 20 
mm and a thickness of 2 mm. The annular body was prepared from a solid 
electrolyte made of 8 mol % YSZ according to the producing processing 
Experiment 1, and had an inner diameter and a thickness given in Table 2. 
Then, the assembled annular body and porous electrode-supporting body were 
fired at a firing temperature of 1,300.degree. C. After the firing, the 
thickness of the annular body and the state of the interface between the 
annular body and the supporting body were examined. Results are shown in 
Table 2. 
TABLE 2 
______________________________________ 
Before firing After firing 
Inner diameter 
Thickness of 
Thickness 
of annular body 
annular body 
of annular body 
State of 
(mm) (.mu.m) (mm) interface 
______________________________________ 
20 100 65 good 
21 100 70 good 
22 100 75 good 
23 100 80 good 
24 100 80 good 
25 100 80 slight gap 
present 
26 100 80 slight gap 
present 
______________________________________ 
It is seen from the results in Table 2 that when the inner diameter of the 
annular body was not more than 23 mm, gas-tightness was kept, and no gap 
was observed at the interface. On the other hand, it is seen that when the 
inner diameter of the annular body was 25 mm or more, a gap due to 
insufficient shrinkage was observed at the interface. From this, it is 
seen that a desirable range is also present with respect to a clearance. 
Further, with respect to each of the annular bodies, an insulating 
reaction product La.sub.2 Zr.sub.2 O.sub.7 was not produced at the 
interface, and gas leakage did not occur in a leakage-judging test at all. 
As is clear from the above explanation, according to the present invention, 
the dense ceramic thin film having good performance can be formed on the 
surface of the ceramic body by preparing the annular body from the ceramic 
green sheet, fitting the annular body around the ceramic body, and firing 
the assembled annular body and ceramic body. Accordingly, the 
self-supporting type SOFC having the solid electrolyte thin film and/or 
the interconnector thin film formed on the porous electrode-supporting 
body can be simply and inexpensively obtained by the present invention.