JP5155597B2 - Method for producing ceramic thin film - Google Patents

Description

  The present invention relates to a method for producing a dense ceramic thin film.

  Conventionally, ceramic thin films having functions such as corrosion resistance, and ceramic thin films that give electromagnetic properties and optical properties have been developed. For example, a citric acid complex was prepared by an organic complex reaction, and nanoparticles obtained by a coprecipitation method, a hydrothermal method, a combustion method, etc. were calcined to obtain a complex oxide calcined powder, which was dispersed. A method of forming a film on a substrate using a sol is known. And the method of forming into a film using a citric acid complex aqueous solution, without performing calcination and sol-formation of powder is also known. In addition to such a method, a method of forming a film using a polymer precursor or a polymer precursor is known (see Patent Document 1 and Patent Document 2).

  For example, in the method for producing a ceramic membrane described in Patent Document 1, a colloidal suspension of ceramic powder and a polymer precursor are prepared, mixed, and the mixture is coated on a membrane support to form a composite structure. The composite structure is then heated to form a high density film on the support. Thus, a method for producing a thin ceramic film on a porous support has been proposed.

Moreover, according to the method for producing a metal oxide film described in Patent Document 2, a substrate coated with a polycrystalline metal oxide film can be obtained using a polymer precursor. The metal oxide film produced by the method described in Patent Document 2 has relatively few cracks and pinholes. For example, it can be used for electrolytes, electrodes, or gas separation membranes of intermediate temperature solid oxide fuel cells (SOFCs). used. On the other hand, in recent years, a member excellent in corrosion resistance in a harsh environment has been demanded. In particular, dense ceramic members have been used in various situations because they have excellent corrosion resistance against corrosive gases.
JP 2000-128545 A US Pat. No. 5,494,700

  However, in recent years, the demand for corrosion resistance has further increased, and even a dense ceramic member made of a single material has become insufficient in touch resistance. For this reason, members that are further excellent in corrosion resistance have been studied, but there are many cases where the workability of the members is poor or expensive, and the members that have been used in the past are simply made into members with excellent corrosion resistance. It's not that you just have to replace it.

  For such problems, a new ceramic thin film having high corrosion resistance is provided on the surface of a conventional member, so that a member excellent in workability and cost and having high corrosion resistance can be produced. . When a ceramic thin film as described in the above-mentioned patent document is used as a coating for a corrosion-resistant member, the corrosion resistance is reduced even from slight cracks and pinholes on the surface, and particles are generated. In the method of manufacturing a ceramic thin film as described above, sufficient corrosion resistance cannot be obtained because the density of the thin film is not sufficient.

  The present invention has been made in view of such circumstances, and is intended to provide a method for producing a dense ceramic thin film that can be applied to a corrosion-resistant member or the like and has almost no cracks or pinholes on the surface. And

  (1) In order to achieve the above object, the method for producing a ceramic thin film according to the present invention includes a film forming step of forming a film using an anhydrous citric acid complex solution on the surface of a base material to be coated. It is characterized by.

  As described above, the method for producing a ceramic thin film according to the present invention produces a dense ceramic thin film free from cracks and pinholes by performing a film forming process using an anhydrous citric acid complex solution having enhanced dispersibility of metal ions. can do. As a result, for example, a member that is excellent in workability and cost and that has high corrosion resistance can be manufactured.

  (2) Moreover, the manufacturing method of the ceramic thin film which concerns on this invention esterifies the said anhydrous citric acid complex gel by mixing an anhydrous citric acid complex gel and alcohol solvent, and produces the said anhydrous citric acid complex solution. It is characterized by comprising a solution preparation step. By esterifying the anhydrous citrate complex gel in this way, an anhydrous citrate complex solution with improved dispersibility of metal ions can be produced.

  (3) Moreover, the manufacturing method of the ceramic thin film based on this invention is equipped with the gel preparation process which mixes a metal salt and a citric acid aqueous solution, and produces the said anhydrous citric acid complex gel. Thereby, the anhydrous citrate complex gel used for preparation of an anhydrous citrate complex solution can be produced easily.

  (4) Moreover, the method for producing a ceramic thin film according to the present invention is characterized in that, in the gel preparation step, at least one of metal nitrate or metal chloride is mixed as the metal salt. Thereby, a polymer having a network in which metals are combined can be produced, and the dispersibility of metal ions can be improved. A dense ceramic thin film can be produced by improving the dispersibility.

  (5) Moreover, the manufacturing method of the ceramic thin film which concerns on this invention WHEREIN: In the said gel preparation process, 1 type of metal selected from the group of 2-6 group element of a periodic table, 12-14 group element, and a lanthanoid, or A salt of a combination of two or more metals is mixed as the metal salt. By producing a metal oxide film containing such components, a thin film having excellent corrosion resistance can be produced.

  (6) Further, in the method for producing a ceramic thin film according to the present invention, the film forming step includes a coating treatment in which the anhydrous citrate complex solution is applied to the surface of the substrate, and the applied anhydrous citrate complex solution. And heat treatment for heating at 250 ° C. or higher and 1100 ° C. or lower. Thus, the nanostructure which has a crystallite of 20 nm or less can be obtained by heat-processing at comparatively low temperature and producing | generating a solid electrolyte membrane. As a result, the denseness of the ceramic thin film can be increased.

  According to the method for producing a ceramic thin film according to the present invention, it is possible to improve the dispersibility of metal ions and produce a dense ceramic thin film free from cracks and pinholes. As a result, it is possible to manufacture a member having excellent workability and cost and having high corrosion resistance.

  Embodiments of the present invention will be described below with reference to the drawings. In addition, in order to facilitate understanding of the description, the same reference numerals are given to the same components in the respective drawings, and duplicate descriptions are omitted.

(Configuration of ceramic thin film)
The ceramic thin film according to the present invention is produced using an anhydrous citric acid complex solution obtained by esterifying an anhydrous citric acid complex gel with an alcohol solvent. The produced ceramic thin film is formed on a ceramic or metal base material, has almost no pinholes or cracks, and is extremely dense.

Since the ceramic thin film according to the present invention is produced through esterification of an anhydrous citric acid complex gel based on a metal salt, it is mainly formed of a metal oxide. The ceramic thin film is formed of a metal oxide such as yttria (Y ₂ O ₃ ), alumina (Al ₂ O ₃ ), samarium-doped ceria (SDC), nickel oxide (NiO), or the like.

  The film thickness of the ceramic thin film according to the present invention can be adjusted in the range of sub μm to several hundred μm. It is possible to form a desired thickness within the above range by repeating the film forming process in which each of coating, drying, and heat treatment is performed. The purity of the ceramic thin film is desirably 99% or higher, more preferably 99.9% or higher, and a ceramic thin film having such high purity can be obtained. Thus, the corrosion resistance can be improved by increasing the purity of the ceramic thin film. An average particle diameter is 10-50 nm, and about the ceria thin film, the nanostructure which has a particle size of 20 nm or less can be obtained by making the temperature of heat processing into 500 to 1100 degreeC.

  In addition, said purity means the ratio of the total content which included the trace amount additive intentionally doped other than the main component in the main component. Further, even when the content of other elements not intended is larger than the intentionally doped trace additive, the purity is calculated as an impurity. In this case, the purity is determined regardless of the presence or absence of unintended effects on the characteristics of other elements. The purity here is not the purity of the anhydrous citric acid complex, but the purity of the ceramic thin film after the heat treatment.

The base material for producing the ceramic thin film according to the present invention is formed of ceramic or metal. For example, nickel-combined ceria (NiO—CeO ₂ ), Si, Al, Al ₂ O ₃ , AlN, SiC, SiO ₂ , SUS304, NiO—SDC, Al-based compound, Y ₂ O ₃ , Y-based compound, etc. It is possible to form a ceramic thin film on a substrate. As the substrate, a plate or the like having a shape that can be easily formed can be used. Since a solution-type raw material is used for film formation, the film formation method of the present invention has good film formability even on a substrate having a more complicated shape as compared with other film formation methods. The ceramic substrate need not be a sintered body, and a film formed by CVD or thermal spraying may be used as the substrate.

  The base material constituted in this way and the ceramic thin film provided on the base material form a coating member in which both are combined. This coating member is used for various applications. For example, when a metal oxide having high corrosion resistance is employed as the material for the ceramic thin film, the coating member can be used as a corrosion resistant member used in a corrosive gas atmosphere. In this case, there is an advantage that it is possible to avoid the cost and processing restrictions as compared with the case where the corrosion-resistant member is manufactured using only the material having high corrosion resistance. In addition to such applications, the ceramic thin film can also be used for metals with insulating coatings, insulators with conductive coatings, and fuel cells. The method for producing a ceramic thin film according to the present invention will be described below.

(Manufacturing method of ceramic thin film)
FIG. 1 is a flowchart showing a method for producing a ceramic thin film according to the present invention. As shown in FIG. 1, first, a metal salt prepared in advance and an aqueous citric acid solution are mixed and heated (step S1). The heating temperature is, for example, about 100 ° C to 300 ° C. By using a metal salt, metal ions can be uniformly dispersed when a polymer having a three-dimensional network is produced. Examples of the metal salt include metal nitrates such as cerium nitrate, yttrium nitrate, and aluminum nitrate, and metal chlorides such as cerium chloride, yttrium chloride, and aluminum chloride. In the gel preparation process, a salt of one kind of metal selected from the group of Group 2-6 elements, Group 12-14 elements and lanthanoids of the periodic table or a combination of two or more metals is mixed as a metal salt. Is preferred. By producing a metal oxide film containing such components, a thin film having excellent corrosion resistance can be produced. At this time, moisture contained by heating is sufficiently removed. An anhydrous citrate complex gel is produced | generated by the gel preparation process which performs such mixing and a heating.

  Next, an alcohol solvent is mixed with the produced anhydrous citric acid complex gel and heated (step S2). In the solution preparation step by mixing and heating, esterification reaction is performed, and a transparent anhydrous citric acid complex solution is generated. The heating temperature is, for example, in the range of 50 ° C to 120 ° C. As the alcohol solvent, for example, ethanol or IPA can be used. As the esterification proceeds, the complex is polymerized and the metal ions are dispersed in the solvent. Thereby, a dense ceramic thin film free from cracks and pinholes can be produced. As a result, for example, a member that is excellent in workability and cost and that has high corrosion resistance can be manufactured. In addition, the strength of the dried green film is improved in the subsequent film formation step by esterifying into a polymer. In addition, in order to improve the intensity | strength of a green film, polyethyleneglycol can be mixed with anhydrous citric acid complex gel with an alcohol solvent. Further, the same effect can be obtained by adding ethylene glycol, polyvinyl pyrrolidone or the like instead of polyethylene glycol (PEG).

  Then, the anhydrous citric acid complex solution thus obtained is applied to the substrate surface (step S3). Examples of the coating method include wet methods such as spraying, dip, dip coating, spin coating, and screen printing. Next, the green film thus formed is dried (step S4). For drying, natural drying may be selected in consideration of the properties of the anhydrous citrate complex solution and the coating method, or drying may be performed in a room where the temperature is controlled.

  When the green film is dried in this manner, the dried film is placed in a furnace together with the base material, and heat treatment is performed at a predetermined temperature (step S5). By this heat treatment, the organic component of the dried film burns and the ceramic thin film is fired. The firing temperature of the heat treatment is preferably 250 ° C. or higher and 1100 ° C. or lower. By performing heat treatment at a relatively low temperature in this way, crystallites of 50 nm or less are formed in the ceramic thin film. As a result, a ceramic thin film having a nanostructure is formed, and the denseness of the ceramic thin film is enhanced. In this way, the film forming process shown in steps S3 to S5 is performed.

  When the heat treatment is finished and the ceramic thin film is formed, it is determined whether or not the film forming process is finished (step S6). When the film forming process is repeatedly performed, the process returns to step S3, and the anhydrous citric acid solution is again applied onto the formed ceramic thin film, followed by drying and heat treatment. When the film forming process is completed, a series of manufacturing processes for manufacturing the ceramic thin film is completed.

(Preparation of metal chloride)
When preparing a metal chloride in advance for the above manufacturing process, the following treatment is performed. First, a metal or metal oxide is dissolved in a hydrochloric acid solution, and excess hydrochloric acid is removed and concentrated. Then, the chloride crystals precipitated in the concentrated solution are taken out and dissolved in water, whereby an aqueous chloride solution can be generated.

  For example, when producing a thin film of yttria, an aqueous solution of yttrium chloride can be produced by dissolving yttria in a hydrochloric acid solution, concentrating the dissolved material, and dissolving the precipitated crystals in water. When a ceria thin film is prepared, an aqueous cerium chloride solution can be produced by dissolving cerium with a hydrochloric acid solution, concentrating the dissolved cerium, and dissolving the precipitated crystals in water. When an alumina thin film is produced, an aluminum chloride aqueous solution can be generated by dissolving a metal aluminum plate with a hydrochloric acid solution and performing the same treatment.

(Esterification)
Among the above production steps, the step of esterifying the anhydrous citrate complex gel is particularly important in the present invention. In this step, as shown in the following formula, a dehydrating ester reaction proceeds between the carboxyl group (COOH) of citric acid and the hydroxyl group (OH) of an alcohol solvent (dispersing alcohol or binder PEG). As a result, a transparent anhydrous citric acid solution is obtained.

  In this way, polymerization of the anhydrous citrate complex proceeds. FIG. 2 is a diagram schematically showing a polymer constituting the anhydrous citrate complex gel. As shown in the figure, a three-dimensional network in which metal ions M are uniformly dispersed is formed. As a result, dispersibility of metal ions is improved and the ceramic film is densified. In addition, the strength of the film when the green film is dried is increased by the polymerization.

  An experiment was conducted to demonstrate that a dense ceramic thin film can be obtained by the above manufacturing process. In the experiment, a scanning electron microscope (HITACHI FE-SEM (S-4300E)) and an X-ray diffractometer (Rigaku (Rotaflex RU-200B)) were used. When using an X-ray diffractometer, a diffraction peak was obtained by a thin film method using CuKα rays under the condition of an incident angle θ = 1.5 °.

(Experiment 1)
First, a yttria thin film was formed on a silicon wafer substrate. Yttrium was dissolved in a hydrochloric acid solution, the dissolved one was concentrated, and the precipitated crystals were dissolved in water to produce an aqueous yttrium chloride solution. A 0.2 mol% aqueous citric acid solution was mixed with a 0.2 mol% aqueous yttrium chloride solution. And the mixed aqueous solution was heated at 200 degreeC, and the water | moisture content was removed. And it formed into a film using a part of obtained anhydrous citric acid complex gel. On the other hand, ethanol and polyethylene glycol were mixed with the remaining anhydrous citric acid complex gel, and the ester reaction was allowed to proceed at 50 ° C. Film formation was performed using the anhydrous citric complex solution thus obtained.

  In the film forming step, the anhydrous citric acid complex solution was applied to the surface of the silicon wafer substrate by a spray method. The formed green film was naturally dried, and the film formed by drying was placed in a furnace together with the silicon wafer substrate, and heat treatment was performed at 500 ° C. And the film-forming process was repeated several times. A film having a thickness of about 200 nm could be produced in one film formation step.

  FIG. 3A is an SEM photograph showing an yttria film formed using an anhydrous citric complex solution. No holes were formed on the surface, and it was demonstrated that the yttria film was densified. On the other hand, FIG.3 (b) is a SEM photograph which shows the yttria film | membrane formed from the anhydrous citric acid complex gel, without performing esterification. It can be seen that holes are formed on the surface of the film shown in FIG. 3B, and the yttria film is not sufficiently densified.

(Experiment 2)
Next, a thin film of ceria doped with samarium was produced on a silicon wafer substrate. A cerium nitrate solution and a samarium nitrate solution were mixed at a ratio of 4: 1 to obtain a 0.2 mol% nitrate solution, and a 0.2 mol% citric acid aqueous solution was further mixed. And the mixed aqueous solution was heated at 200 degreeC, and the water | moisture content was removed. And it formed into a film using a part of obtained anhydrous citric acid complex gel. On the other hand, ethanol and polyethylene glycol were mixed into the remaining anhydrous citric acid complex gel, and the ester reaction was allowed to proceed while maintaining the solution at 50 ° C. Then, using the obtained anhydrous citric complex solution, a film was formed in the same manner as in Experiment 1 above.

  FIG. 4A is an SEM photograph showing a ceria film formed using an anhydrous citric complex solution. No pores were formed on the surface, demonstrating that the ceria film was densified. On the other hand, FIG.4 (b) is a SEM photograph which shows the ceria film | membrane formed from the anhydrous citrate complex gel, without performing esterification. It can be seen that holes are formed on the surface of the film shown in FIG. 4B, and the ceria film is not sufficiently densified.

(Experiment 3)
In addition, in the process of preparing the anhydrous citric acid complex gel, a ceria film doped with samarium was formed on a silicon wafer by changing the molar ratio of metal ions to citric acid. A metal salt and an aqueous citric acid solution were mixed at a molar ratio of metal ion to citric acid R = 1: 2, 1: 1, 1, 0.5 to form a film. FIG. 5 is a diagram showing the relationship between the molar ratio of metal ion to citric acid and the X-ray diffraction peak (crystallinity). As shown in FIG. 5, it was demonstrated that high crystallinity can be obtained when R <1.

(Experiment 4)
Next, for the ceria film doped with samarium, the relationship between the heat treatment temperature and the crystal grain size in the film forming process was examined. Prepare seven cerium anhydrous citric complex solutions coated on a silicon wafer substrate, one of which is maintained at 25 ° C. and the other six are 110 ° C., 300 ° C., 500 ° C., 700 ° C., respectively. Heat treatment was performed at ℃, 900 ℃, and 1100 ℃. And the film | membrane obtained using X-ray diffraction was analyzed. FIG. 6 is a diagram showing the experimental results of examining the relationship between the heat treatment temperature and the diffraction peak due to crystals. FIG. 7 is a diagram showing the experimental results of examining the relationship between the heat treatment temperature and the average crystal grain size.

  As shown in FIG. 6, crystallization is measured for those heat-treated at 500 ° C. or higher. Therefore, it was demonstrated that the low crystallization temperature is around 500 ° C. Further, as shown in FIG. 7, it was found that crystallites having a particle size of 20 nm or less were generated in a temperature range of at least 500 ° C. and 1100 ° C., and nanostructures were formed in the film.

  In addition, when it heat-processed at 250 degreeC or more and 500 degrees C or less, the dense thin film containing an amorphous was obtained. Therefore, although the structure of the obtained film | membrane differs, it turned out that the film | membrane heat-processed above 250 degreeC shows sufficient corrosion resistance as a dense thin film. Depending on the application, a film containing amorphous may have higher corrosion resistance than a film having a clear crystal grain boundary. In addition, in this invention, the film | membrane when heat-processed at 250 degreeC or more and 500 degrees C or less is called a ceramic thin film.

It is a flowchart which shows the manufacturing method of the ceramic thin film which concerns on this invention. It is a figure which shows typically the polymer which comprises an anhydrous citrate complex gel. (A) It is a SEM photograph which shows the yttria film | membrane of an Example. (B) It is a SEM photograph which shows the yttria film | membrane of a comparative example. (A) It is a SEM photograph which shows the ceria film of an Example. (B) It is a SEM photograph which shows the ceria film of a comparative example. It is a figure which shows the relationship between the molar ratio of a metal ion to a citric acid, and a crystallization characteristic. It is a figure which shows the experimental result which investigated the relationship between heat processing temperature and a diffraction peak. It is a figure which shows the experimental result which investigated the relationship between heat processing temperature and a crystal grain diameter.

Explanation of symbols

M Metal ion R Molar ratio

Claims (4)

A solution preparation step of esterifying the anhydrous citric acid complex gel by mixing an anhydrous citric acid complex gel and an alcohol solvent, and preparing an anhydrous citric acid complex solution;
A film forming step in which the anhydrous citric acid complex solution is applied to the surface of a substrate to be coated, and the applied anhydrous citric acid complex solution is heated at 250 ° C. or higher and 1100 ° C. or lower ; A method for producing a ceramic thin film comprising: The method for producing a ceramic thin film according to claim 1, further comprising a gel preparation step of mixing a metal salt and an aqueous citric acid solution to prepare an anhydrous citrate complex gel. 3. The method for producing a ceramic thin film according to claim 2 , wherein in the gel preparation step, at least one of metal nitrate or metal chloride is mixed as the metal salt. In the gel preparation step, a salt of one kind of metal selected from the group of Group 2-6 elements, Group 12-14 elements and lanthanoids of the periodic table, or a combination of two or more metals is mixed as the metal salt. The method for producing a ceramic thin film according to claim 2 or claim 3, wherein