Formation of ceramic thin film on ceramic body

Abstract: A process is disclosed for forming a ceramic thin film on a surface of a ceramic body (3), comprising the steps of preparing an annular body (2) from a ceramic green sheet (1), inserting the ceramic body (3) 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.

Description: 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 suitable to be used as a solid electrolyte film, an interconnector film or the like of a cylindrical solid oxide fuel cell. 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, and creates less pollution. A variety of fuels (naphtha, natural gas, methanol, coal-reformed gas, etc.) can be used, 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 cell (hereinafter referred to briefly as "SOFC"s) operates at high temperatures around 1,000°C, the 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 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 it is difficult to increase the area of the solid electrolyte film. 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. For this reason, 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 in 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 raises, and the mass productivity becomes difficult. In the field of fuel cells, it can be mentioned that it is known from EP-A-166445 to provide alumina protective layers on opposite faces of a flat zirconia solid electrolyte sheet, by applying the layers by paste printing or thermocompression bonding and then sintering. EP-A-454924 describes formation of a lanthanum chromite film on a region of a tubular air electrode structure in a two-stage process of first applying a thin LaCrO₃ particle-fugitive polymer film and then electrochemical-vapour-depositing a dense skeletal structure around the LaCrO₃ particles. It is an object of the present invention to reduce or 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 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 µm to hundreds of µm, e.g. 10µm - 500µm. Such a green ceramic body by be produced by preparing a green ceramic sheet having the thickness of tens of µm to hundreds of µm, 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 certain strength great enough 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. It is not necessary that the whole inner surface of the annular body is adhered to the ceramic body. These and other optional 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. For a better understanding of the invention, reference is made to the attached drawings, wherein: Figs. 1 (a) through (e) are views for illustrating an example of the process for producing the ceramic thin film according to the present invention in the order of producing steps; and 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 which an electrolyte and an interconnector are simultaneously formed. 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 x 10⁻⁶ to + 1.0 x 10⁻⁶. 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, Cr₂O₃, MnO₂, Mn₃O₄, LaMnO₃, LaCoO₃, ZrO₂ stabilized with Y₂O₃, or a mixture thereof, which exhibits electroconductivity at 1,000°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 lanthane 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 the thickness of hundreds of µm to few mm and can allow sufficient permeation of a gas. For example, the porous electrode support may be made of lanthanum manganate LaMnO₃ doped with Sr, Ca or the like. The interconnector may be made of lanthanum chromite LaCrO₃ doped with Ca, Mg, Sr or the like. The solid electrolyte may be made of zirconia ZrO₂ stabilized with yttria in an amount of 3-16 mol%, typically made of ZrO₂ 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°C to 1,600°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 µm were prepared from a green sheet composed of partially stabilized zirconia containing 8 mol% Y₂O₃ (hereinafter referred to as "8YSZ") or a green sheet composed of Al₂O₃. 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 Al₂O₃ powder was used in the case of the Al₂O₃ annular body. Thereafter, the annular body was fitted around the solid rod, which were fired at 1,400°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. 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 x 10⁻⁶ to 1.0 x 10⁻⁶, 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 process in 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°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. Before firing After firing Inner diameter of annular body (mm) Thickness of annular body (µm) Thickness of annular body (µm) State of 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 La₂Zr₂O₇ 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.

Claims: A process for forming a ceramic thin film on a surface of a ceramic body (3), comprising the steps of preparing an annular body (2) from a ceramic green sheet (1), inserting the ceramic body (3) into said annular body (2), and firing the thus assembled annular body and ceramic body, whereby the annular body is shrunk and joined to the ceramic body. The process according to claim 1, wherein the firing shrinkage factor of the annular body (2) is greater than that of the ceramic body (3). The process according to claim 1 or claim 2, wherein said firing is effected when an adhesive is interposed between the ceramic body and the ceramic green sheet. The process according to any one of claims 1 to 3 wherein the value of the difference (coefficient of thermal expansion of said ceramic body - coefficient of thermal expansion of said annular body) is in a range of -2.0 x 10⁻⁶ to +1.0 x 10⁻⁶. The process according to any one of claims 1 to 4, wherein said ceramic thin film and said ceramic body are a solid electrolyte thin film and a porous electrode body or electrode-supporting body of a solid oxide fuel cell, respectively. The process according to claim 5, wherein said ceramic thin film includes said solid electrolyte thin film (2) and an interconnector thin film (6) of the solid oxide fuel cell, said interconnector film being arranged between opposite ends of the solid electrolyte thin film.

Inventors: TANAKA RITSU, YAMADA HIROTAKE, TANAKA, RITSU, YAMADA, HIROTAKE
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1993
Main classification: C04B41
IPC codes: C04B37, C04B41, H01M8, B28B11