Title of Invention

A PROCESS FOR PREPARING NICKEL YTTRIA STABILIZED CERMET

Abstract

A process for preparing Nickel-yttria stabilized cermet by preparing a 20 to 25 gm/l stannous chloride solution in HCI, preparing 0.1 to 0.6 gm/l palladium chloride solution in HCI, mixing the two said solutions in equal volume ratio, treating ceramic particle with this mixed solution in the ratio 20 gm/l to 40 gm/l, recovering the treated powder by known methods, washing and drying the treated powder by known methods, further treating this treated powder with a solution in the ratio 20 gm/l to 40 gm/l, wherein the solution contains 15 to 220 gm/l nickel salt and 15 to 130 gm/l triammonium citrate in plating bath maintaining the pH in the range of 8 to 10 by adding ammonium hydroxide, maintaining the temperature in the range of 70 to 90°C adding hydrazine hydrate under stirring to obtain a nickel coated powder, recovering, washing and drying by known methods.

Full Text

The present invention relates to a process for preparing nickel (Ni)-yttria stabilized zirconia (YSZ) cermet. Nickel-yttria stabilized zirconia cermets (Ni-YSZ) are used as an anode material for solid oxide fuel cells (SOFC), as catalyst, and for coating of metal surfaces by ceramics. Specific requirements of anode material for solid oxide fuel cell applications are: 1) Stability: The anode must be chemically, morphologically and dimensionally stable in the fuel environment. 2) Conductivity: The anode must posses specific electronic conductivity for electron flow in the reducing environment at the operating temperature. 3) Compatibility: The anode must be chemically compatible with other components not only at the operating temperature but also much higher temperature at which the fuel cell ceramic structure is fabricated. Chemical interaction or elemental diffusion between the anode and adjoining component must be limited to minimise unacceptable occurrences such as second phase formation, stability reduction, change in thermal expansion, introduction of electronic conductivity in the electrolyte. 4) Thermal expansion: The thermal expansion of the anode must match that for other cell components to avoid cracking and delamination during fabrication and operation including thermal cycling. 5) Porosity: Anode must have sufficient porosity to allow gas transport to the reaction sites. 6) Catalytic activity: Anode must have sutticient catalytic activity thus low polari/alion for electrochemical oxidation. Yttria stabilized zirconia, alumina, spinel, aluminium silicates, silicon carbide, silicon nitride, boron nitride are used as a catalyst support especially because it has high reactivity with water, improves ability to reform hydrocarbons with steam and restrains deposition of carbon on the catalyst. The process for preparing nickel yttria stabilized cermet material essentially comprises physically dispersing nickel oxide throughout the stabilized zirconia, then reducing nickel oxide into nickel metal. Reference may be made to of U.S. Pat. No. 3,300,334, wherein mixed zirconium and yttrium oxides, obtained by precipitation from an aqueous solution, are processed with addition of nickel oxide and carbon in powder form. The nickel oxide is then reduced to nickel by carbon at a high temperature. The drawbacks of the process are: i) Inhomogeneous distribution of nickel and yltria stabilized /irconia phases within the matrix of the cermet, ii) At high temperature, carbon is converted to carbon monoxide, which reduces nickel oxide to metallic nickel. However, carbon monoxide is highly toxic and special care must be taken to handle the same, as well as the coverage by nickel over substrate becomes inhomogeneous. iii) Removal of the residual carbon, which is present within the cermet, is difficult, iv) Low active surface area of nickel, v) SOFC applications require a minimum of 30-40% by volume of metallic nickel phase for achieving required electrical conductivity. This is a relatively high metal content which causes thermal incompatibility with the YSZ electrolyte interface if prepared by this process, vi) Require high processing temperature (more than 800°C) Reference may also be made to "Morphology and Electrochemistry of Porous Nickel/Zirconia Cermet", at pages 90-98 of Proceedings of the First International Symposium on Solid Oxide Fuel Cells, S.C. Singhal, Ed. 1989, wherein they disclosed the preparation of nickel yttria stabilized cermet from a mkture of sintered powders obtained by co-milling nickel oxide and stabilized zirconia followed by reduction with hydrogen. The drawbacks of the process are: i) Complete reduction of nickel oxide (NiO) to metallic nickel is not possible, particularly, when the average particle size of NiO is more than 3 µm. ii) Inhomogeneous distribution of nickel and yttria stabilized zirconia phases within the cermet matrix. iii) Low active surface area of metallic nickel, iv) SOFC applications require a minimum of 30-40% by volume of metallic nickel phase for achieving required electrical conductivity. This is relatively high metal content which causes thermal incompatibility with the YSZ electrolyte interface, v) Require high processing temperature (more than 800°C). Reference may further be made to H.Arai in International Symposium on SOFC, Nov. 13-14, 1989, Nagoya, Japan, wherein they disclosed other techniques of nickel yttria stabilized zirconia cermet preparation based on C.V.D. (Chemical Vapour Deposition) and P.S. (Plasma Spraying). The drawbacks of the process are: i) Poor quality of the resulting material. ii) The processing is very complex and requires costly equipments, iii) Low active surface area of the metallic nickel. Reference may also be made to U.S. Pat. No. 5,261,944, which enables the production of solid nickel oxide and stabilized zirconium oxide, in the form of two distinet phases, with a phase distribution level of less than lum. In addition the nickel oxide contained in this solid can be completely, or almost completely, reduced by means of hydrogen to give a nickel cermet with a submicronic phase distribution and with a high active area of nickel. The drawbacks of the process are: i) SOFC applications require 30-40% by volume metallic nickel phase for achieving required electrical conductivity. This is a relatively high metal content which causes thermal incompatibility with the electrolyte interface, ii) Require high processing temperature (more than 800oC). Furthermore, reference may be made to U.S. Pat. No. 5,993,511, which discloses the production of the nickel/zirconia cermet by dispersing yttria stabilized zirconia (YSZ) powder in a water soluble nickel salt solution, evaporating the suspension to dryness and then calcining the dried mass where by NiO-YSZ is obtained, which is further reduced by hydrogen to get the desired cermet. The drawbacks of the process are: i) SOFC applications require 30-40% by volume metallic nickel phase for achieving required electrical conductivity. This will cause thermal incompatibility with the electrolyte interface. ii) Require high processing temperature (more than 800"C). References may also be made to U.S. Pat. No. 3,371,050, U.S. Pat. No. 4,657,888, U.S. Pat. No. 5,130,144 wherein, disclosed processes for preparation of catalyst by precipitating or infiltrating or, by coprecipitating two or more metals in the form of oxidic compounds or other water-insoluble compounds onto an inert refractory support from a suspension containing the inert support in suspension and the metals to be precipitated in the form of ions. Major disadvantages of this process are: i) The active component is initially deposited on the support in the form of oxidic compounds, which then have to be reduced. This reduction step is time consuming and, in some cases, can only be carried out by very rigorous reduction methods if complete reduction to metal is to be achieved. ii) Inhomogeneous distribution of catalyst phase within the inert support. iii) Low active surface area of the catalyst present. The overall drawbacks of the above processes are: i) Inhomogeneous distribution of nickel and yttria stabilized zirconia phases within the matrix of the cermet, ii) Inability to supply nickel with a large enough active surface area for offering interesting catalytic properties. iii) Inability to completely reduce nickel oxide particles, into nickel metal, iv) SOFC applications the minimum amount of nickel phase (30-40 volume %) required within the cermet to have adequate conductivity, which in turn produces a thermal stress at solid electrolyte and cermet electrode interface arising from thermal expansion mismatch. v) Require high processing temperature (more than 800°C). The main object of the present invention is to provide a process for preparing Nickel-yttria stabilized zirconia (Ni-YSZ) cermet which obviates many of the drawbacks as detailed above. Yet another object of the present invention is to decrease the amount of nickel in the cermet to reduce overall thermal expansion of the cermet without affecting its property. Accordingly the present invention provides a process for preparing Nickel-yttria stabilized zirconia (Ni-YSZ) cermet which comprises, preparing a 20 to 25 gm/1 stannous chloride solution in HC1, preparing 0.1 to 0.6 gm/1 palladium chloride solution in HC1, mixing the two said solutions in equal volume ratio, treating YSZ powder with this mixed solution in the ratio 20 gm/1 to 40 gm/1, recovering the treated powder by known methods, washing and drying the treated powder by known methods, further treating this treated powder with a solution in the ratio 20 gm/1 to 40 gm/1, wherein the solution contains 15 to 220 gm/1 nickel salt and 15 to 130 gm/1 triammonium citrate^ maintaining the pi I in the range of 8 to 10 by adding ammonium hydroxide, maintaining the temperature in the range of 70 to 90°C adding hydrazine hydrate under stirring to obtain nickel coated powder, recovering, washing and drying by known methods. In an embodiment of the present invention, ceramic particles (YSZ) may be of wide size distribution. In another embodiment of the present invention, ceramic particles other than YSZ used may be such as alumina, spinel, aluminium silicates, silicon carbide, silicon nitride, boron nitride. In another embodiment of the present invention the nickel salt used in plating bath may be such as nitrate, chloride, accelate, sulphate. In yet another embodiment of the present invention, hydrazine hydrate may be added drop wise. The process comprises the following steps: a) Preparation of aqueous solution of stannous chloride containing 20 gm/1 to 25 gm/1 SnCl2 in HCL b) Preparation of aqueous solution of palladium chloride containing 0.1 gm/1 to 0.6 gm/1 PdCl2 in llCI. c) Mixing the solutions prepared in step 'a' and 'b' in equal proportion. d) Yttria stabilized zirconia (YSZ) powders are then treated with the mixed solution obtained in step 'c' in the ratio 20 to 40 gm/1. e) The treated powder was then recovered by filtration, washed and dried. f) An electroless nickel bath is prepared by dissolving 15 to 220 gm/1 nickel salt, such as nitrate, chloride, acetate, sulphate and 15 to 130 gm/1 triammonium citrate. g) The bath was maintained at a temperature in the range of 70 to 90°C and at a pH in the range of 8 to 10 by adding ammonium hydroxide. h) The treated and dried particles obtained in step (e) were then added to the above nickel plating bath in the proportion 20 to 40 gm/1 of the solution and are kept suspended within the bath by magnetic stirring, i) Hydrazine hydrate was added drop wise within the bath during stirring until the coating ends. j) The powders were then recovered, washed and dried. The nickel cermet of the present invention consists 10-60% by volume of metal phase and 90-40% by volume of ceramic substrate such as zirconium dioxide stabilized in the cubic from with yttria. In the present invention ceramic substrate such as yttria stabilized zirconia powder was sensitized and catalyzed by colloidal palladium particles formed by the successive reaction with stannous chloride and palladium chloride solutions in an ultrasonic bath. These colloidal palladium particles on ceramic particle surface act as the nucleating center for metallic nickel during the subsequent step, when the treated powder is electroless plated in the plating bath. The present ceramic particles are dispersed in the plating bath and kept in suspension therein until the enveloping or encapsulating metal coating ends. In the present invention, ceramic substrate particles must be electroless plated, since they must be completely enveloped by the metal coating, or at least substantially completely enveloped with the metal coating so that there is no significant exposure of the ceramic substrate surface. Other plating or coating techniques do nut involve suspension of a powder, such as present fine powder, in a coating medium, and therefore, cannot produce the present metal enveloped ceramic substrate particle in a useful amount with an economically practical time period. In the present invention yttria stabilized zirconia powder is completely coated by a metallic nickel, thereby increases the effective active surface area of the nickel. In the present invention the nickel coating takes place at a temperature as low as 70-90°C, thereby it eliminates rigorous, time consuming reduction step for preparation of this type of material. In the present invention sub micron size YSZ particles are coated with metallic nickel, thereby produces a uniform phase distribution in the submicron range. In the present invention the percolation threshold of metal addition (for SOFC application) was found to in the range 20 to 30 volume % metal which is much lower than reported in the prior art. In the present invention the coefficient of thermal expansion of the cermet decreases substantially (for SOFC application) due to lowering the amount of nickel concentration in the cermet, which gives better thermal compatibility with the electrolyte. In the prior art, the different methods employed for coating over a ceramic substrate above parameters could not be properly addressed which was mainly due to the fact that in most of the cases ceramic substrate was primarily coated with metal oxides or hydroxides which were subsequently reduced to metals. Intrinsic problem of the prior art process is in its ability to prevent pore formation on the surfaces due to loss of hydroxyl group or oxygen during dehydration or reduction of metal compounds. This problem was solved by directly depositing metal on the ceramic substrate thus eliminating the step of reduction. The novelty of the prior art process is the elimination of the above shortcomings by an inventive step of reducing the metal salt in solution into elemental metal followed by deposition of this metal on the ceramic substrate activated by chemical means. The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention. Example -1 4 gm of yttria stabilized zirconia (YSZ) was treated with 25 cc of stannous chloride solution (25 gm/1) and 25 cc of palladium chloride solution (0.4 gm/1). The mixture is then kept in an ultrasonic bath for 2 min. YSZ powder was then recovered by known method (e.g. filtration, decanlation), dried in an oven at 110°C. This dried powder is then stirred with 200 ml plating bath containing 3.2476 gm Nickel niluile Ni(NO3)2, 6H2O, 3.2474 gm triammonium citrate (NH4)3C6H5O7 maintained at 80°C. The pH of the bath was kept at 8 by the addition of ammonium hydroxide. 20 cc of hydrazine hydrate was then added drop wise to this bath till the reaction ends. The powder was recovered from the solution by known method (e.g. filtration, decantation), and dried in a oven maintained at 110°C. This yields nickel coated zirconia powder with a nickel loading of 10 volume %. Example - 2 5 gm of yttria stabilized zirconia (YSZ) was treated with 25 cc of 'stannous chloride solution (20 gm/1) and 25 cc of palladium chloride solution (0.3 gm/l). The mixture is then kept in an ultrasonic bath for 2 min. YSZ powder was then recovered by known method (e.g. filtration, decantation), dried in an oven at 110°C. This dried powder is then stirred with 200 ml plating bath containing 9.135 gm Nickel nitrate Ni(NO3)2, 6H2O, 8.118 gm triammonium citrate (NH4)3C6H5O7 maintained at 90°C. The pH of the bath was kept at 8 by the addition of ammonium hydroxide. 50 cc of hydrazine hydrate was then added drop wise to this bath till the reaction ends. The powder was recovered from the solution by known method (e.g. filtration, decantation), and dried in a oven maintained at 110°C. This yields nickel coated zirconia powder with a nickel loading of 20 volume %. Example - 3 6 gm of yttria stabilized zirconia (YSZ) was treated with 25 cc of stannous chloride solution (25 gm/1) and 25 cc of palladium chloride solution (0.2 gm/1). The mixture is then kept in an ultrasonic bath for 2 min. YSZ powder was then recovered by known method (e.g. filtration, decantation), dried in an oven at 110°C. This dried powder is then stirred with 200 ml plating bath containing 18.792 gm Nickel nitrate Ni(NO3)2, 6II2O, 9.741 gm triammonium citrate (NH4)3C6H5O7 maintained at 90°C. The pH of the bath was kept at 9 by the addition of ammonium hydroxide. 75 cc of hydrazine hydrate was then added drop wise to this bath till the reaction ends. The powder was recovered from the solution by known method (e.g. filtration, decantation), and dried in a oven maintained at 110°C. This yields nickel coated zirconia powder with a nickel loading of 30 volume %. Example - 4 8 gm of yttria stabilized zirconia (YSZ) was treated with 50 cc of stannous chloride solution (25 gm/1) and 50 cc of palladium chloride solution (0.6 gm/1). The mixture is then kept in an ultrasonic bath 3 min. YSZ powder was then recovered by known method (e.g. filtration, decantation), dried in an oven at 110°C. This dried powder is then stirred with 200 ml plating bath containing 38.9736 gm Nickel nitrate Ni(N03)2, 6H20, 26.0 gm triammonium citrate (NH^CeHsO? maintained at 70°C. The pH of the bath was kept at 9 by the addition of ammonium hydroxide. 100 ce of hydrazine hydrate was then added drop wise to this bath till the reaction ends. The powder was recovered from the solution by known method (e.g. filtration, decantation), and dried in a oven maintained at 110°C. This yields nickel coated zirconia powder with a nickel loading of 40 volume %. Example - 5 4 gm of ytlria stabilized zirconia (YSZ) was treated with 30 cc of stannous chloride solution (25 gm/l) and 30 cc of palladium chloride solution (0.1 gm/1). The mixture is then kept in an ultrasonic bath for 4 min. YSZ powder was then recovered by known method (e.g. iillralion, decantation), dried in an oven at 110°C. This dried powder is then stirred with 200 ml plating bath containing 29.232 gm Nickel nitrate Ni(NO3)2, 6HI2O, 16.424 gm triammonium citrate (NH4)3C6H5O7 maintained at 80°C. The pH of the bath was kept at 10 by the addition of ammonium hydroxide. 90 cc of hydrazine hydrate was then added drop wise to this bath till the reaction ends. The powder was recovered from the solution by known method (e.g. filtration, decantation), and dried in a oven maintained at 110°C. This yields nickel coated zirconia powder with a nickel loading of 50 volume %. Example - 6 4 gm of yttria stabilized zirconia (YSZ) was treated with 50 cc of stannous chloride solution (25 gm/1) and 50 cc of palladium chloride solution (0.4 gm/1). The mixture is then kept in an ultrasonic bath for 4 min. YSZ powder was then recovered by known method (e.g. filtration, decantation), dried in an oven at 110°C. This dried powder is then stirred with 200 ml plating bath containing 43.848 gm Nickel nitrate Ni(NO3)2, 6H2O, 19.50 gm triammonium citrate (NHOaCeHsO; maintained at 90°C. The pH of the bath was kept at 10 by the addition of ammonium hydroxide. 120 cc of hydrazine hydrate was then added drop wise to this bath till the reaction ends. The powder was recovered from the solution by known method (e.g. filtration, decantation), and dried in a oven maintained at 110°C. This yields nickel coated zirconia powder with a nickel loading of 60 volume %. Example - 7 2 gm of boron nitride was treated with 25 cc of stannous chloride solution (25 gm/1) and 25 cc of palladium chloride solution (0.4 gm/1). The mixture is then kept in an ultrasonic bath for 4 min. Boron nitride powder was then recovered by known method (e.g. filtration, decantation), dried in an oven at 110°C. This dried powder is then stirred with 200 ml plating bath containing 6.264 gm Nickel nitrate Ni(NO3)2, 6H2O, 3.247 gm triammonium citrate (NH4)C6H5O7 maintained at 90°C. The pH of the bath was kept at 8 by the addition of ammonium hydroxide. 10 cc of hydrazine hydrate was then added drop wise to this bath till the reaction ends. The powder was recovered from the solution by known method (e.g. filtration, decantation), and dried in a oven maintained at 110°C. This yields nickel coated zirconia powder with a nickel loading of 50 volume %. The main advantages of the present investigation are i) Synthesis of nickel yttria stabilized zirconia cermet at a low temperature as low as 80 to 90°C. ii) The uniform phase distribution of nickel and stabilized zirconia is in the submicron level, iii) High active surface area of nickel within the cermet, iv) The powder obtained by the process shows electrical percolation at nickel content as low as 20% by volume, v) The powder obtained by the process gives lower thermal expansion, which will give better compatibility with the substrate, vi) Enhanced thermal compatibility with the electrolyte, vii) Enhanced electrical conductivity even at low nickel content. We claim: 1. A process for preparing Nickel-yttna stabilized zirconia (Ni-YSZ) cermet which comprises, preparing a 20 to 25 gm/1 stannous chloride solution in HC1, preparing 0.1 to 0.6 gm/1 palladium chloride solution in HC1, mixing the two said solutions in equal volume ratio, treating YSZ powder with this mixed solution in the ratio 20 gm/1 to 40 gm/1, recovering the treated powder by known methods, washing and drying the treated powder by known methods, further treating this treated powder with a solution in the ratio 20 gm/1 to 40 gm/1, wherein the solution contains 15 to 220 gm/1 nickel salt and 15 to 130 gm/1 triammonium citrate maintaining the pH in the range of 8 to 10 by adding ammonium hydroxide, maintaining the temperature in the range of 70 to 90°C adding hydrazine hydrate under stirring to obtain a nickel coated powder, recovering, washing and drying by known methods. 2. A process as claimed in claim 1 wherein ceramic particles other than YSZ such as alumina, spinel, aluminium silicate, silicon carbide, silicon nitride and boron nitride are used. 3. A process as claimed in claim l-2wherein nickel salt used in the plating bath is such as nitrate, chloride, accetate, sulphate. 4. A process as claimed in claim 1-3 wherein hydrazine hydrate is added drop wise. 5. A process for preparing nickel yltria stabilized zirconia (Ni-YSZ) cermet substantially as herein described with reference to the examples.

Documents:

306-del-2001-abstract.pdf

306-del-2001-claims.pdf

306-del-2001-correspondence-others.pdf

306-del-2001-correspondence-po.pdf

306-del-2001-description (complete).pdf

306-del-2001-form-1.pdf

306-del-2001-form-18.pdf

306-del-2001-form-2.pdf