IMPROVEMENTS IN OR RELATING TO THE DEPOSITION OF A COATING ON A POROUS SUBSTRATE

Abstract: Apparatus for forming a chemically vapor deposited coating on a porous substrate where oxygen from a first gaseous reactant containing a source of oxygen permeates through the pores of the substrate to react with a second gaseous reactant that is present on the other side of the substrate. The apparatus includes means for controlling the pressure and flow rate of each gaseous reactant, a mano­ meter for measuring the difference in pressure between the gaseous reactants on each side of the substrate, and means for changing the difference in pressure between the gaseous reactants. Also a method of detecting and closing cracks in the coating by reducing the pressure difference between the two gaseous reactants whenever the pressure difference falls suddenly after gradually rising, then again increasing the pressure difference on the two gases. The attack by the by-products of the reaction on the substrate are reduced by maintaining the flow rate of the first reactant through the pores of the substrate.

Description: This invention relates to depositing a coating on a porous substrate. In the production of solid oxide electrochemical cells it is necessary to chemically vapor deposit thin (typically 1 to 200 microns) oxide layers onto a porous tube or substrate sealed at one end. The oxide layer can be the ionically conducting electrolyte, typically yttria-stabilized zirconia, or the electronically conducting interconnect, typically magnesium-doped lanthanum chromite, which connects one cell to another. In this modified chemical vapor deposition process, a first reactant containing a source of oxygen permeates through the pores in the substrate material and reacts with a metal halide gas on the other side of the substrate, depositing a metal oxide coating on the substrate. As the metal oxide reaction product grows on the substrate, it closes off the pores in the substrate strictly by chemical vapor deposition (CVD). The coating continues to grow by electrochemical vapor deposition (EVD) because oxygen ion transfer from the source of oxygen through the growing oxide layer takes place. This process is very sensitive and it is often difficult to produce a satisfactory product. One problem is that metal halide vapors and by-products of the reaction, namely, chlorine or hydrochloric acid, can attack the substrate before a protective coating has been formed. In addition, cracks in the substrate and coating can develop as it is being deposited, due to the above-mentioned side reactions with the substrate, and these cracks cannot be easily sealed as the coating continues to grow. While stronger and more conductive ceramic materials have been found which could be used in the cell as substrates, these materials cannot now be used because they are more subject to deleterious chemical side reactions with halides and reaction by-products, and exposure time of the substrate to reactants must be minimized. US-A-4,374,163 describes a process of chemical vapor deposition where one of the reactants permeates through a substrate to react with the other reactant on the other sides of the substrate and form a coating on the substrate. Accordingly, the present invention resides in a process for forming a ceramic coating on a ceramic substrate at elevated temperatures by chemical and electrochemical vapor deposition which comprises permeating a first gaseous reactant containing a source of oxygen on one side of said substrate through pores in said substrate to react with a second gaseous reactant in the form of a metal halide on the other side of said substrate and form there a pore-closing ceramic coating and by-products, characterized by measuring the difference in pressure between the gaseous reactants on each side of said substrate, maintaining a higher pressure on said first gaseous reactant than on said second gaseous reactant and in the face of cracking of said coating which is accompanied by a sudden decrease in said measured pressure difference from levels of pressure difference which increase as said pores close, further reducing said pressure difference to permit gradual sealing of the crack and a gradual increase and restoration of said pressure difference. The invention also relates to processes for forming ceramic coatings on porous substrates by chemical vapor deposition which involve (a) detecting the presence of breaks in the coating and closing them, (b) reducing damage to the coating from by-products, or (c) preventing the porous substrate from cracking on cooling of the latter. The aforementioned problems can be greatly alleviated by controlling the difference between the pressures of the reactant gases on the two sides of the substrate. By means of this control over the pressure on the gases, we have been able to greatly reduce the attack by metal halides, chlorine, and hydrogen chloride on the substrate. We have also found that by maintaining a higher partial pressure of oxygen in the first reactant during cooling we are able to greatly reduce cell fracture during cooling. By reducing the difference in the two pressures whenever a crack develops, we are able to promote growth in the crack and seal the crack. In order that the invention can be more clearly understood, a convenient embodiment thereof will now be described, by way of example, with reference to the accompanying drawing which is a cross-sectional side view of an apparatus for forming a coating on a porous substrate by chemical vapor deposition. In the drawing, a first gaseous reactant 1 containing a source of oxygen, such as free oxygen, water vapor, carbon dioxide, etc., passes through valve 2 and tube 3 into the bottom inside of a porous substrate tube 4. Excess reactant 1 can leave tube 4 by means of conduit 5. (This can be reversed, with the first gaseous reactant entering through the outside of tube 3 and leaving through the inside of tube 3.) The top of tube 4 is sealed by means of tube 6 and seal 7. The second gaseous reactant 8 of metal halides is admitted to the outside of tube 4 through conduit 9 controlled by valve 10. Envelope 11, sealed with seal 12, contains the second reactant around the tube 4. (Envelope 11 may enclose numerous tubes, which can be coated simultaneously, despite variations in the tubes.) Excess second gaseous reactant exits through conduit 13 and valve 14. A manometer 15 is connected between conduit 5 and the envelope 11 to measure the difference in pressure between first gaseous reactant 1 on the inside of tube 4 and second gaseous reactant 8 on the outside of tube 4. Another conduit 16 controlled by valve 17 permits the gases from the inside of tube 4 to pass to the outside of tube 4 without passing through the porous walls of tube 4, thereby reducing the difference in pressure between the two gaseous reactants. A more complete description of a solid oxide electrochemical cell tube can be found in U.S. Patent Specification No. 4,395,468. The apparatus is operated in the following manner. The apparatus is first heated to the temperature range required for layer deposition, typically from 1000 to 1400°C. Initially, valve 17 is closed or partially closed so that first gaseous reactant 1, admitted through tube 3 to the inside of tube 4, cannot pass through conduit 16 to the outside of tube 4. The walls of tube 4 are porous to first gaseous reactant 1 and the reactant permeates through these pores to the outside surface of tube 4 where it contacts second gaseous reactant 8, which is typically under a vacuum of from 0.1 to 100 mm Hg. The resulting reaction produces a solid reaction product which grows on the outside of tube 4, gradually closing the pores in the walls of tube 4. We have discovered that the attack of metal halide vapors and the by-products of this reaction on the porous substrate 4 can be greatly minimized if the pressure on the first gaseous reactant 1 inside tube 4 is sufficiently greater than the pressure on the second gaseous reactant 8 on the outside of tube 4. This difference in pressure between the two gases is preferably from 0.1 to 200 mm of Hg. At lower pressure differentials the reaction by-products tend to attack tube 3 and at higher pressure differentials the resulting coating may be porous or otherwise have unsuitable properties. The use of inert carrier gases, for both gaseous reactants, is desirable. Such gases include argon, helium, or nitrogen. By diluting both reactants with inert gases, one gains another means of controlling reaction rates during pore closure. For instance, a 1 to 1 dilution of reactants with argon or helium results in good deposits; however, other widely varying ratios of from 1 to 0.1 to 1 to 10 have been used with good results. As the pores in tube 4 close, the manometer will gradually register a growing pressure difference between the gases on the inside and the outside of tube 4. After the pores have closed, a considerable gradient of oxygen activity is created across the growing deposit. This leads to the formation of electronic carriers in the deposit which allows oxygen ions to be conducted through the coating, so that deposition reactions continue to proceed and build up the coating on tube 4. If, however, a crack should occur in the coating, the manometer will register a sudden decrease in the pressure difference between the two gaseous reactants. When this occurs, valve 17 is opened, or partially opened, to permit a portion of first gaseous reactant 1 in the inside of the tube 4 to pass through conduit 16 directly to the outside of tube 4, further reducing the difference in pressure between the gases on the inside and outside of the tubes. This will cause the coating on tube 4 to begin to grow inside the crack instead of mostly on top of the already formed coating. As the crack gradually seals, the manometer will register a gradual increase in the pressure difference between the two gaseous reactants, and valve 17 can gradually be closed. When the coating on tube 4 has formed to the desired thickness, the apparatus is gradually cooled back to room temperature. During cooling, the pressure on the first reactant can be maintained at a high level in order to prevent the loss of oxygen from the porous substrate, which is usually a conducting oxide electrode. Oxygen extraction can lead to expansion and subsequently to cracking and inferior properties. A partial oxygen pressure of at least 1 mm and preferably 10 millimeters of mercury is maintained during cooling in order to prevent cracking when doped lanthanum manganite electrodes are part of the porous substrate. If the process of this invention is used to form the electrolyte over an air electrode, the entire tube is coated, typically with yttria-stabilized zirconia (except for a thin strip along its length which is masked to form the interconnection between the cells). In that case, the first gaseous reactant is typically a mixture of water, oxygen, carbon dioxide, and a carrier gas. Initially, when the pores are open, the water reacts with the zirconium tetrachloride and yttrium trichloride to form the yttria-stabilized zirconia. While we do not wish to be bound by any theories, we believe that the equations which follow describe the chemical reactions that occur. ZrCl₄ + 2H₂O → ZrO₂ + 4HCl 2YCl₃ + 3H₂O → Y₂O₃ + 6HCl This is a CVD process. Once the pores are closed, however, the process continues by an EVD process as oxygen ions react with the zirconium tetrachloride and the yttrium trichloride. ZrCl₄ + 20 = → ZrO₂ + 4e⁻ + 2Cl₂ 2YCl₃ + 30 = → Y₂O₃ + 6e⁻ + 3Cl₂ The oxygen ions are formed when water or free oxygen reacts with electrons. H₂O + 2e⁻ → H₂ + O = O₂ + 4e⁻ → 20 = In forming the electronically conducting interconnection between the cells, the mask used during the forming of the electrolyte is removed and the entire tube is masked except for the still porous area where the previous mask was positioned. A typical interconnection material is magnesium-doped lanthanum chromite. This is formed in the same manner as the yttria-stabilized zirconia, using the appropriate mixture of chlorides such as lanthanum chloride, chromic or chromous chloride, and magnesium chloride. The product is both electronically and, to a very small degree, ionically conducting. In addition to its application to the production of solid oxide fuel cells, the apparatus and process of this invention can also be applied to the production of electrolyzers and gas sensors. The invention will now be illustrated with reference to the following Examples: EXAMPLE 1 An apparatus similar to that shown in the accompanying drawing was placed in a furnace so that porous substrate tube 4 was in the hot zone and valves 17, 14, and 2 and portions of the alumina carrier tube 6 were in a cool zone. The vacuum envelope 11 was then evacuated to nearly 0.1 mm Hg. At this point, valve 17 was open, thus equalizing the pressure inside and outside of tube 4. A gas mixture of approximately 40% water, 20% oxygen, and 40% carrier gas was passed through an oxygen sensor (not shown) so that oxygen activity could be monitored continuously. These gases were directed through tube 3 and exited through tube 5, finally returning to the vacuum envelope through valve 17. The furnace was then heated to the deposition temperature of 1200°C. Once that temperature had been reached, valve 17 was closed, and all gases entering tube 4 had to pass through its porous wall. At this point, a pressure differential was measured on manometer 15 which indicated the internal pressure required to force incoming reactants through the porous wall of tube 4. Typically, the pressure differential was in the range of 5 to 10 mm Hg. Halide vapors were then introduced through conduit 9 at a rate of 0.65 g ZrCl₄/min and 0.1 g YCl₃/min, using argon carrier gas of ∼100 cm³/min. The gases passed over the exterior surface (100-200 cm²) of tube 4 and reacted with the gases being forced through its porous walls from the inside. With the introduction of the halide vapors CVD of yttria-stabilized zirconia began. As the pores began to close, the once porous substrate became gas tight and further scale growth by the EVD mechanism took place. During this transition period, film growth was monitored and controlled by observing the pressure differential on the manometer and by manipulation of valve 17. As the pores closed, pressure began to rise in tube 4 and an increasing pressure differential was measured against time. After cooling, tube 4 was examined and showed no chemical attack. When it was subsequently made into a fuel cell, the cell performed efficiently. When this experiment was performed without maintaining a pressure differential of about 10 to about 30 mm Hg, tube 4 was chemically attacked, often resulting in cracking during further cell processing. EXAMPLE 2 In an experiment similar to Example 1, a sudden pressure drop of a few mm Hg was noted on the manometer, indicating that a crack had formed in tube 4. Valve 17 was opened, reducing pressure inside tube 4. After 30 seconds, valve 17 was partially closed. An increase in the pressure differential indicated on the manometer showed that the crack had been sealed with the yttria-stabilized zirconia. Subsequent examination of the tube showed that the crack had been filled. The tube was made into a fuel cell which performed effectively.

Claims: A process for forming a ceramic coating on a ceramic substrate at elevated temperatures by chemical and electrochemical vapor deposition which comprises permeating a first gaseous reactant containing a source of oxygen on one side of said substrate through pores in said substrate to react with a second gaseous reactant in the form of a metal halide on the other side of said substrate and form there a pore-closing ceramic coating and by-products, characterized by measuring the difference in pressure between the gaseous reactants on each side of said substrate, maintaining a higher pressure on said first gaseous reactant than on said second gaseous reactant and in the face of cracking of said coating which is accompanied by a sudden decrease in said measured pressure difference from levels of pressure difference which increase as said pores close, further reducing said pressure difference to permit gradual sealing of the crack and a gradual increase and restoration of said pressure difference. A process according to claim 1, characterized in that the pressure difference between the first and second gaseous reactants is maintained at at least 0.1 mm of Hg (0.1333 mbar). A process according to claim 2, characterized in that the maintained pressure difference is 0.1 to 200 mm of Hg (0.1333 mbar to 266.6 mbar). A process according to claim 3, characterized in that the maintained pressure difference is 10 to 30 mm of Hg (13.33 to 39.99 mbar). A process according to claim 1, 2, 3 or 4, characterized in that as the substrate is cooled a partial pressure of oxygen on the first gaseous reactant is maintained at at least 1 mm of Hg (1.333 mbar). A process according to claim 5, characterized in that the maintained partial pressure is at least 10 mm of Hg (13.33 mbar) A process according to any of claims 1 to 6, characterized in that the coating is yttria-stabilized zirconia. A process according to any of claims 1 to 7, characterized in that the first gaseous reactant is water vapor, carbon dioxide or mixtures thereof. A process according to any of claims 1 to 8, characterized in that the second gaseous reactant comprises zirconium chloride doped with yttrium chloride. A process according to any of claims 1 to 6 and 8, characterized in that the second gaseous reactant is lanthanum chloride, chromous chloride, or chromic chloride, and magnesium chloride. A process according to any of claims 1 to 10, characterized in that the substrate is a tube closed at one end. A process according to any of claims 1 to 11, characterized in that the first and second gaseous reactants are diluted with an inert carrier gas at a volume ratio of from 1 to 0.1 to 1 to 10. A process according to claim 12, characterized in that the inert carrier gas is argon, helium, nitrogen or mixtures thereof.

Inventors: ISENBERG, ARNOLD OTTO, ZYMBOLY, GREGORY EVAN
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1985
Main classification: C23C16
IPC codes: C23C16, C04B41, H01M8