Title of Invention	"IMPROVED PLASMA SPRAY PROCESS FOR ELECTROLYTE AND INTERCONNECT COATINGS ON SOLID OXIDE FUEL CELL CATHODE TUBES"
Abstract	It was analyzed that the breaking of LSM tubes was due to the high preheating temperature, which was used before plasma spraying. Preheating the tubes externally gradually helped in avoiding the breaking or cracking of LSM tubes. A procedure was evolved to preheat the sample slowly by plasma flame so that even larger samples can be preheated within a short span of time. Apart from the heating procedure, the mandrel also plays an important role. Also, the presence of a highly heat absorbing cushioning layer in between the mandrel and the tube was essential to solve the problem of tube breaking. The gradual cooling of the substrate using plasma flame after coating the electrolyte/interconnect was also found to be crucial in avoiding the cracking/breaking. The electrolyte and interconnect powders used in the fabrication of solid oxide fuel cells were successfully plasma sprayed on the LSM tubes without any breaking or cracking of tubes.

Title of Invention

"IMPROVED PLASMA SPRAY PROCESS FOR ELECTROLYTE AND INTERCONNECT COATINGS ON SOLID OXIDE FUEL CELL CATHODE TUBES"

Abstract

It was analyzed that the breaking of LSM tubes was due to the high preheating temperature, which was used before plasma spraying. Preheating the tubes externally gradually helped in avoiding the breaking or cracking of LSM tubes. A procedure was evolved to preheat the sample slowly by plasma flame so that even larger samples can be preheated within a short span of time. Apart from the heating procedure, the mandrel also plays an important role. Also, the presence of a highly heat absorbing cushioning layer in between the mandrel and the tube was essential to solve the problem of tube breaking. The gradual cooling of the substrate using plasma flame after coating the electrolyte/interconnect was also found to be crucial in avoiding the cracking/breaking. The electrolyte and interconnect powders used in the fabrication of solid oxide fuel cells were successfully plasma sprayed on the LSM tubes without any breaking or cracking of tubes.

Full Text	FIELD OF THE INVENTION The present invention relates to an improved plasma spray process for the electrolyte and interconnect coatings on solid oxide fuel cell cathode tubes. The present invention more particularly relates to an improved process for plasma spraying of Yttria Stabilized Zirconia (electrolyte) and lanthanum chromate (interconnect) ceramic powders on Strontium doped lanthanum manganite (LSM) ceramic tubes having open end and closed end without crack formation or breaking of LSM tubes. BACKGROUND OF THE INVENTION A solid oxide fuel cell is an energy conversion device that produces direct-current electricity by electrochemically reacting a gaseous fuel (e.g., hydrogen) with an oxidant (e.g., oxygen) across an oxide electrolyte. The key features of current SOFC technology include all solid-state construction, multi-fuel capability, and high-temperature operation. Because of these features, the SOFC has the potential to be a high-performance, clean and efficient power source and has been under development for a variety of power generation applications. A solid oxide fuel cell comprises an anode, a cathode opposite to the anode, and an electrolyte between the anode and cathode. A tubular Solid Oxide Fuel Cell (SOFC) consists of a porous ceramic inner cylinder as the cathode, to which a dense electrolyte layer is applied, followed by the anode as a casing. To produce electrical connection to the cathode, there is what is known as an interconnect, which is directly connected to the cathode, the electrolyte layer and the anode being interrupted in the region of the interconnect. Each type of SOFC has its own advantages and disadvantages but tubular cells are more stable against mechanical and thermal stresses than planar cells. With the advances in development of solid oxide fuel cells (SOFCs) in recent years, the biggest challenge facing the industrial application of SOFCs is to reduce the manufacturing cost. Therefore, the manufacturing processes for SOFCs components with a low cost are more attracting. In the most advanced SOFCs at present, Yttria stabilized Zirconia (YSZ) is still the most commonly used electrolyte material. Many different processes have been applied to fabricate YSZ electrolyte in the form of coating, including chemical vapor deposition (CVD), electrochemical vapor deposition (EVD), sol-gel method, spray pyrolysis method, thermal spraying, physical vapor deposition, tape-casting, and screen printing techniques. The wet chemical and thermal spray routes (mainly atmospheric plasma spraying) are the two processes that have the potential of low cost production due to high deposition rates and the ability to coat large surfaces. The production of dense coatings of the electrolyte (8 mol% Y2O3 stabilized zirconia (YSZ)) becomes increasingly difficult for wet chemical methods with subsequent sintering steps if substrates show only reduced or no shrinkage during the co-firing process ("constraint sintering"). Thermal spray techniques do not need shrinkage of the substrates and are hence especially favorable for the coating of substrates with reduced shrinkage as used in cathode supported or metallic substrate concepts. A prerequisite for the deposition of dense ceramic layers by thermal spray processes is the complete melting of the powder particles during spraying. YSZ is a material with a high melting point of about 3000° K. In most cases thermal spray processes with high process gas temperatures are used to deposit YSZ. Especially plasma spraying with gas temperatures above 10000K is advantageous for this application. The commercialization efforts on SOFC systems are recently oriented to cost reduction in order to compete more effectively with other traditional power generating methods. Atmospheric plasma spray (APS) is quite commonly used for the integrated fabrication of different components of a solid oxide fuel cell (SOFC) like cathode (LSM), anode (Ni-YSZ cermets), electrolyte (YSZ) and interconnect (LaCrO3). Siemens Power corporation tubular design and cell-in-series design (Mitsubhishi Industries) makes use of plasma spraying technique for the fabrication. The state-of-the-art materials used in tubular SOFC and respective manufacturing processes used by Siemens Westinghouse Power corporation are as follows: (Table Removed) One of the few activities, or better said the only activity where plasma technology for SOFCs reached already the level of considerable technical use represents the APS application of Siemens Power Generation Inc. (SPG) in Pittsburgh, PA, USA. There, the interconnect, the electrolyte and the anode are sprayed in atmosphere by DC plasma torches on the about 2 m long cathode tubes of about 2 cm of diameter. The electrochemical vapor deposition (EVD) process has been recently replaced with the atmospheric plasma spray (APS) process. It is believed that this process has low cost to bring tubular SOFC to a viable commercial product. The plasma spray process is most commonly used in normal atmospheric conditions and referred as APS. Some plasma spraying is conducted in protective environments using vacuum chambers normally back filled with a protective gas at low pressure, this is referred as Vacuum Plasma Spraying (VPS) or Low Pressure Plasma Spraying (LPPS). Even though this method gives dense pore free coating, it is restricted by the chamber size and high cost. The plasma spraying process is also rapid and easy to automate, making the process potentially very well suited for mass production. Depositing dense interconnect and electrolyte coatings is the biggest challenge for plasma spray processing of SOFCs. With plasma spraying, a dense coating is only achievable with completely molten particles, which are accelerated to a high velocity in the plasma jet and therefore flatten into dense lamellae on impact with the substrate. LaCrO3 / YSZ usually require very high temperature plasma with hydrogen gas in order to melt the powder feedstock. Present invention provides an improved plasma spray process for depositing electrolyte and interconnect coatings on strontium doped Lanthanum manganite (LSM) ceramic cathode tubes without any breaking or cracking of tubes. RELATED ART Prior-art search was made in patent as well as non-patent literature. There is no direct literature, which can provide solution for avoiding the cracking of ceramic substrates during plasma spraying. Following patents are referred due to their relevance to field of present invention. Reference may be made to a research article entitled "Thermal plasma spraying for SOFCs: Applications, potential advantages, and challenges" by Rob Hui et al, Journal of Power Sources 170 (2007) 308-323 wherein, LSM tubes used as SOFC cell support layers (LSM) generally have high porosity, and were found to have low thermal shock resistance, and cracked when exposed to a plasma flame. However, further referred cross-reference do not reflect any solution to avoid the cracking of ceramic tubes during plasma spraying. Reference may be made to USPTO patent 5436426, provides a novel-cooling fixture for cooling a tubular ceramic substrate during thermal spraying to avoid cracking of ceramic substrate (zirconia). The cooling fixture comprises of a coolant tube carrying pressurized coolant. This is cumbersome and may not be suitable for porous supports (substrate) such as LSM. Reference may also be made to USPTO patent 3631835, which describes a fixture for holding and cooling magnesium part to be flame sprayed to maintain the magnesium below its ignition point. Reference may be made to USPTO patent 4297388 where the graphite or copper substrate is typically rotated and cooled by cooling water to the rear surface to get uniform deposit. Above referred prior art discloses the problem of ceramic tube (Zirconia) (it is not clear whether the tube is porous or dense) cracking/breaking during thermal spraying and a cooling fixture to solve this problem and the other cooling procedures are not relevant to ceramics. No literature is available in the public domain regarding the solution for avoiding the breaking of the LSM tubes during plasma spraying. Also none of the prior art addresses the problem of breaking of porous LSM tubes during plasma spraying of electrolyte. It is a challenging task to get a very dense 8YSZ coating on a porous tubular substrate having open end and closed end. To get a dense coating of the electrolyte, the plasma flame temperature should be very high. Higher plasma flame temperatures will provide more heat to the substrate. This heat even with conventional cooling methods and that reported in the above-referred patents can result in cracking of the LSM ceramic substrates, which are porous. OBJECT OF THE INVENTION The main object of the present invention is to provide an improved process for plasma spraying on strontium doped lanthanum manganite ceramic tubes for SOFC application, which obviates the drawbacks of the hitherto known prior arts. Another object of present invention is to avoid the cracking of ceramic substrate in particular during plasma spraying of electrolyte (8YSZ) powder. Another object of present invention is to avoid the cracking of ceramic substrate in particular during plasma spraying of interconnect (LaCrO3) powder. Yet another object of present invention is to allow plasma spraying on small tubular LSM substrates at high plasma power. Yet another object of present invention is to provide a simple and economical process for coating of electrolyte and interconnect coating on SOFC ceramic LSM tubes without breaking and cracking of tubes during plasma spraying BRIEF DESCRIPTION OF DRAWINGS The present invention is illustrated in figures 1 to 4 of the drawing(s) accompanying this specification. Fig.1. LSM tubes which were broken during (a) preheating and (b) plasma spraying of 8YSZ electrolyte. Fig. 2. LSM tubes plasma sprayed with 8YSZ electrolyte without any visible cracks. Fig.3. Cross-section of plasma sprayed 8YSZ coating on LSM substrate (a) at lower magnification and (b) at higher magnification. Fig.4. Photograph of a LSM tube cracked where cooling procedure was not used. SUMMARY OF THE INVENTION Accordingly, present invention provides an improved plasma spray process for the electrolyte and interconnect coatings on solid oxide fuel cell cathode tubes which comprises the steps of: a. grit-blasting the surface of LSM tubular substrate, b. fitting a mandrel with a cushion layer to the LSM substrate, c. fixing the mandrel in the lathe, d. rotating the LSM substrate assembly at 100 rpm, e. preheating of LSM substrate with plasma flame gun, f. plasma spraying of electrolyte/ interconnect on the preheated LSM substrate, g. cooling of coated LSM substrate with plasma flame from 400- 100 °C, h. rotating the electrolyte/interconnect coated LSM tube at 100 rpm until it reaches the room temperature, characterized in that, preheating of LSM tube gradually to 400 °C in four thermal zones (Room temperature to 100 °C; 100-200 °C; 200-300 °C; 300-400 °C) with a heating rate of 60-100 °C/min by adjusting plasma power in between 5 to 45 kW and hydrogen flow rate in between 3 to 13 NLPM and gradual cooling of electrolyte/interconnect coated LSM substrate to 100 °C in three thermal zones (400-300 °C; 300-200 °C; 200-100 °C) using the plasma flame gun by varying the plasma power from 45 kW to 5 kW hydrogen flow rate 13 NLPM to 3 NLPM with rate of cooling in between to 100-60 °C/min followed by rotating the electrolyte coated LSM tube at 100 rpm until it reaches the room temperature ,where NLPM stands for Normal Liters Per Minute Novelty of present invention lies in providing crack free electrolyte/interconnects ceramic powders coated LSM tubes by plasma spraying. Novelty also lies in providing a simple, economical and user-friendly process for plasma spraying of electrolyte on cathode tube. Above said novelty of the present invention has been achieved by adopting following non-obvious inventive steps in order to avoid the breaking or cracking of LSM ceramic tube during plasma spraying. 1. Selection of mandrel material 2. Providing a cushion layer in between mandrel and LSM tube 3. Preheating the LSM tube gradually to 400 °C in four thermal zones with a heating rate of 60-100 °C/min. 4. Controlled heating of LSM tubes in a linear fashion in four different temperature zones with a heating rate of 60-100 °C/min by adjusting plasma power in between 5 to 45 kW and hydrogen flow rate in between 3 to 13 NLPM. 5. Controlled and gradual cooling of electrolyte/interconnect coated LSM substrate gradually to 100 °C in three thermal zones using the plasma flame by varying the plasma power from 45 kW to 5 kW; hydrogen flow rate 13 NLPM to 3 NLPM and rate of cooling in the range of 100 to 60 °C/min. 6. Rotating the electrolyte coated LSM tube at 100 rpm until the temperature of the LSM tubes reaches room temperature. In an embodiment of present invention, mandrel is selected from materials such as mild steel, stainless steel alloy, invar, graphite, aluminum, and ceramic. In another embodiment of present invention, preferably the mandrel is made of stainless steel alloy material. In yet another embodiment of present invention, the mandrel is provided with a central axial aperture for releasing the trapped hot air. In yet another embodiment of present invention, cushion layer should have thermal conductivity more than that of mandrel. In an embodiment of present invention, cushion layer between mandrel and substrate is selected from glass wool, metal foils and ceramic wool. In still another embodiment of present invention, preferably the cushion layer between mandrel and substrate is metal foil. In still another embodiment of present invention, the substrate is selected from ceramic materials like zirconia, NiO-YSZ, preferably strontium doped lanthanum manganite (LSM). In yet another embodiment of present invention, the preheating of the substrate tubes can be done in a furnace, ordinary flame heating, Bunsen burner heating, preferably by plasma flame. DETAIL DESCRIPTION OF THE INVENTION Plasma spraying has the advantages of fast deposition rate and easy masking for deposition of patterned structures, compared to the electrochemical vapor deposition technique, which has been widely used for the fabrication of tubular solid oxide fuel cells. Sidemen's Westinghouse has used plasma spraying for the fabrication of electrolyte, interconnect and anode layers. However, the cost will come down if the productivity goes up. Conventional coating technologies, such as tape casting, screen-printing and dip molding produces sintering defects like warp, crackle and pore in large area SOFCs. For SOFC application, the APS electrolyte should be dense, gas tight and as thin as possible in order to minimize the cell resistance. It is achievable when completely molten 8YSZ particles are completely flattened and solidified without defects during impacting on the substrate. Therefore, low particle velocity and high temperature is required. Some of the unique features of plasma spraying are as follows; (i) a wide range of materials from metals to ceramics to polymers and any combination of them and graded coatings can be deposited, (ii) homogeneous coatings with time invariant changes in composition i.e. without compositional changes across the thickness can be produced, (iii) Microstructures with fine, equiaxed grain but without columnar defects can be deposited in contrast to electron beam deposition, (iv) high deposition rates of the order of mms-1 can be achieved with only modest investment in capital equipment, (v) free-standing thick forms can be sprayed in near-net shape fashion of pure and mixed ceramics, (vi) the process can be carried out in any conceivable environment such as air, reduced pressure, inert gas, or underwater. The flow rates of H2 and Ar are the two determining variables for coating porosity. The optimum appears to be at reasonably lower Ar flow rate and high H2 flow rate. The plasma spraying involves the following important steps: surface preparation of the sample, plasma preheating and then powder spraying. During plasma spraying the powders are melted in the plasma flame and get deposited on the substrate. During deposition, the tubular ceramic substrate is rotated to ensure uniform deposition and the substrate is generally maintained at relatively low temperature. The gun is traversed repetitively across the substrate being coated so as to distribute the coating particles evenly and prevent local hot spots. Although the material, which is depositing on the work piece, heats the work piece the work piece is generally maintained at relatively very low temperature compared to the plasma flame temperature. The gun is traversed across the substrate being coated so as to distribute the coating particles and prevent local hot spots. Excessive or uneven heating can result in oxidation of a metal substrate or cracking of a ceramic substrate. Also, when the porosity of a ceramic body is increased there is generally a corresponding decrease in the thermal shock properties of the ceramic body. Thus in case of the cathode substrate i.e. Strontium doped Lanthanum Manganite (LSM) ceramic tube which is porous (35-40%) there is a decrease in the thermal shock properties and hence the tubes break during thermal spraying (plasma spraying). The present invention involves the following steps for avoiding the cracking/breaking of LSM ceramic tubular substrates during plasma spraying. In order to enhance the adhesion of the coating, the substrate was blasted with 40 µm alumina grits and cleaned with compressed air. During spraying, the tube was rotated in order to coat the all surfaces of the tube. Argon was used as the primary gas and hydrogen as the secondary plasma gas. The following examples are given by way of illustrations and therefore, should not be construed to limit the scope of the present investigation. Example 1 : The LSM substrate tube of 68.59 mm length, 18.10 mm outer diameter and 13.61 mm inner diameter, was fitted to a stainless steel alloy mandrel. The substrate was rotated at 100 rpm and the plasma gun speed was set at 800 mm/min. The spray distance was 15 cm. The LSM tube was preheated with plasma flame using the following plasma spray parameters: 45 kW plasma power; hydrogen flow rate of 13 NLPM; Argon flow rate of 42 NLPM. The LSM tube broke with the first pre-heating step itself (Fig. 1). Example 2: The LSM substrate tube of 58.13 mm length, outer diameter of 15.50 mm and 11.5 mm inner diameter was fitted to a stainless steel alloy mandrel with a cushioning layer (metal foil) in between the tube and mandrel with a tolerance of (Table Removed) Indigenously prepared YSZ powder was plasma sprayed using the following plasma spray parameters: 45 kW with a powder feed rate of 30 g/min. The tubes were slowly cooled to 100 °C using the plasma spray parameters as given in the following table: (Table Removed) Then the coated LSM tube was kept on rotating at 100 rpm until it reaches the room temperature. This resulted in crack-free 8YSZ coated LSM substrate, which can be seen visually in Fig. 2. Example 3: In another example, LSM tube with 45.23 mm length, 17.6 mm outer diameter and 14.32 mm inner diameter was plasma sprayed using 8YSZ powder with the following plasma spray parameters: 45 kW plasma power, powder feed rate of 30 g/min and spray distance of 15 cm. All the experimental parameters were kept constant as given in example-2. This resulted in crack-free 8YSZ coated LSM tube. The cracks were not even seen in the substrate as seen in the cross-sectional SEM image at lower magnification (Fig. 3a) and at higher magnification (Fig. 3b) shows dense 8YSZ coating without any cracks in the substrate or in the coating. Example 4: In another example, LSM tube with the following dimension 62.82 mm length, outer diameter of 16 mm and 12.22 mm inner diameter was fitted to a suitable stainless steel alloy mandrel with a cushioning layer in between the mandrel and the LSM tube as discussed in example 2. Before plasma spraying, the tube was preheated with the plasma flame using the same procedure as used in the above example. Then 8YSZ powder was plasma sprayed using the following plasma spray parameters: 45 kW plasma power, powder feed rate of 30 g/min and spray distance of 15 cm. However, the tube was not cooled slowly to room temperature using the plasma flame. This resulted in cracked 8YSZ coated LSM tube (Fig.4). Main advantages of present invention are as follows: 1. The invention provides crack-free electrolyte and interconnect coatings 2. The invention provides a simple and economical procedure 3. Readily available materials can be used as cushioning and mandrel materials 4. This process can be used for any ceramic coating on any ceramic substrate 5. This process can be scaled up for coating any longer ceramic tubes 6. This process does not use any complicated cooling set-up as reported in prior art We Claim: 1. An improved plasma spray process for the electrolyte and interconnect coating on solid oxide fuel cell cathode tubes, comprising the steps of: a. grit-blasting the surface of a Strontium doped Lanthanum Manganite (LSM) tubular substrate; b. fitting a mandrel with a cushion layer to the above said LSM tubular substrate; c. fixing the above said mandrel in a lathe; d. rotating the LSM tubular substrate assembly at about 100 rpm; e. preheating the LSM tubular substrate with a plasma flame in the range of room temperature to 400°C in four thermal zones ; f. plasma spraying of electrolyte/ interconnect on the preheated LSM tubular substrate; g. cooling the coated LSM tubular substrate with plasma flame in the range of 400 to 100 °C; h. rotating the electrolyte/interconnect coated LSM tubular tube at about 100 rpm until it reaches the room temperature; 2. An improved process as claimed in claim 1, characterized in preheating the LSM tube gradually to 400 °C in four thermal zones wherein the first thermal zone being in the range of room temperature to 100 °C, followed by second zone in the range of 100 to 200 °C , third zone in the range of 200 to 300 °C and fourth zone in the range of 300 to 400 °C, the four zones having heating rate in the range of 60-100 °C/min by adjusting plasma power in the range of 5 to 45KW and hydrogen flow rate in the range of 3 to 13 NLPM. 3. An improved plasma spray process as claimed in claim 1 , characterized in cooling the electrolyte/interconnect coated LSM substrate gradually to 100° C in three thermal zones, wherein the first thermal zone being in the range of 400 to 300 °C , second zone in the range of 300 to 200°C and third zone in the range of 200 to 100 °C using the plasma flame by varying the plasma power in the range of 45 kW to 5 kW hydrogen flow rate in the range of 13 NLPM to 3 NLPM, the three zones having a rate of cooling in the range of 100 to 60 °C/min followed by rotating the electrolyte coated LSM tube at 100 rpm until it reaches the room temperature . 4. An improved plasma spray process as claimed in claim 1, wherein mandrel used is selected from the group of materials consisting of stainless steel, mild steel, invar, graphite, aluminum and ceramic. 5. An improved plasma spray process as claimed in claim 1, wherein mandrel used preferably is made of stainless steel alloy material. 6. An improved plasma spray process as claimed in claim 1, wherein the mandrel used is provided with a central axial aperture for releasing the trapped hot air. 7. An improved plasma spray process as claimed in claim 1, wherein cushion layer between mandrel and substrate used is selected from group consisting of glass wool, metal foils and ceramic wool. 8. An improved plasma spray process as claimed in claim 1, wherein cushion layer used between mandrel and substrate is a metal foil. 9. An improved plasma spray process as claimed in claim 1, wherein the substrate used is selected from the group consisting of ceramic materials like zirconia, NiO-YSZ, preferably strontium doped lanthanum manganite (LSM). 10. An improved plasma spray process as claimed in claim 1, wherein the electrolyte used is Yttria Stabilized Zirconia (8YSZ) powder and interconnect used is lanthanum chromate( LaCrO3) 11. An improved process as claimed in claim 1, wherein the grit blasting of the substrate is blasted with about 40 µm alumina grits. 12. An improved plasma spray process for the electrolyte and interconnect coating on solid oxide fuel cell cathode tubes substantially as herein described with reference to the examples and the drawings accompanying this specification.

Full Text

FIELD OF THE INVENTION
The present invention relates to an improved plasma spray process for the electrolyte and interconnect coatings on solid oxide fuel cell cathode tubes.
The present invention more particularly relates to an improved process for plasma spraying of Yttria Stabilized Zirconia (electrolyte) and lanthanum chromate (interconnect) ceramic powders on Strontium doped lanthanum manganite (LSM) ceramic tubes having open end and closed end without crack formation or breaking of LSM tubes.
BACKGROUND OF THE INVENTION
A solid oxide fuel cell is an energy conversion device that produces direct-current electricity by electrochemically reacting a gaseous fuel (e.g., hydrogen) with an oxidant (e.g., oxygen) across an oxide electrolyte. The key features of current SOFC technology include all solid-state construction, multi-fuel capability, and high-temperature operation. Because of these features, the SOFC has the potential to be a high-performance, clean and efficient power source and has been under development for a variety of power generation applications. A solid oxide fuel cell comprises an anode, a cathode opposite to the anode, and an electrolyte between the anode and cathode.
A tubular Solid Oxide Fuel Cell (SOFC) consists of a porous ceramic inner cylinder as the cathode, to which a dense electrolyte layer is applied, followed by the anode as a casing. To produce electrical connection to the cathode, there is what is known as an interconnect, which is directly connected to the cathode, the electrolyte layer and the anode being interrupted in the region of the interconnect. Each type of SOFC has its own advantages and disadvantages but tubular cells are more stable against
mechanical and thermal stresses than planar cells. With the advances in development of solid oxide fuel cells (SOFCs) in recent years, the biggest challenge facing the industrial application of SOFCs is to reduce the manufacturing cost. Therefore, the manufacturing processes for SOFCs components with a low cost are more attracting. In the most advanced SOFCs at present, Yttria stabilized Zirconia (YSZ) is still the most commonly used electrolyte material. Many different processes have been applied to fabricate YSZ electrolyte in the form of coating, including chemical vapor deposition (CVD), electrochemical vapor deposition (EVD), sol-gel method, spray pyrolysis method, thermal spraying, physical vapor deposition, tape-casting, and screen printing techniques.
The wet chemical and thermal spray routes (mainly atmospheric plasma spraying) are the two processes that have the potential of low cost production due to high deposition rates and the ability to coat large surfaces. The production of dense coatings of the electrolyte (8 mol% Y2O3 stabilized zirconia (YSZ)) becomes increasingly difficult for wet chemical methods with subsequent sintering steps if substrates show only reduced or no shrinkage during the co-firing process ("constraint sintering"). Thermal spray techniques do not need shrinkage of the substrates and are hence especially favorable for the coating of substrates with reduced shrinkage as used in cathode supported or metallic substrate concepts. A prerequisite for the deposition of dense ceramic layers by thermal spray processes is the complete melting of the powder particles during spraying. YSZ is a material with a high melting point of about 3000° K. In most cases thermal spray processes with high process gas temperatures are used to deposit YSZ. Especially plasma spraying with gas temperatures above 10000K is advantageous for this application.
The commercialization efforts on SOFC systems are recently oriented to cost reduction in order to compete more effectively with other traditional power generating methods. Atmospheric plasma spray (APS) is quite commonly used for the integrated fabrication of different components of a solid oxide fuel cell (SOFC) like cathode (LSM), anode (Ni-YSZ cermets), electrolyte (YSZ) and interconnect (LaCrO3). Siemens Power corporation tubular design and cell-in-series design (Mitsubhishi Industries) makes use of plasma spraying technique for the fabrication. The state-of-the-art materials used in tubular SOFC and respective manufacturing processes used by Siemens Westinghouse Power corporation are as follows:

(Table Removed)
One of the few activities, or better said the only activity where plasma technology for SOFCs reached already the level of considerable technical use represents the APS application of Siemens Power Generation Inc. (SPG) in Pittsburgh, PA, USA. There, the interconnect, the electrolyte and the anode are sprayed in atmosphere by DC plasma torches on the about 2 m long cathode tubes of about 2 cm of diameter.
The electrochemical vapor deposition (EVD) process has been recently replaced with the atmospheric plasma spray (APS) process. It is believed that this process has low cost to bring tubular SOFC to a viable commercial product.
The plasma spray process is most commonly used in normal atmospheric conditions and referred as APS. Some plasma spraying is conducted in protective environments using vacuum chambers normally back filled with a protective gas at low pressure, this is referred as Vacuum Plasma Spraying (VPS) or Low Pressure Plasma Spraying (LPPS). Even though this method gives dense pore free coating, it is restricted by the chamber size and high cost.
The plasma spraying process is also rapid and easy to automate, making the process potentially very well suited for mass production. Depositing dense interconnect and electrolyte coatings is the biggest challenge for plasma spray processing of SOFCs. With plasma spraying, a dense coating is only achievable with completely molten particles, which are accelerated to a high velocity in the plasma jet and therefore flatten into dense lamellae on impact with the substrate. LaCrO3 / YSZ usually require very high temperature plasma with hydrogen gas in order to melt the powder feedstock.
Present invention provides an improved plasma spray process for depositing electrolyte and interconnect coatings on strontium doped Lanthanum manganite (LSM) ceramic cathode tubes without any breaking or cracking of tubes.
RELATED ART
Prior-art search was made in patent as well as non-patent literature. There is no direct literature, which can provide solution for avoiding the cracking of ceramic substrates during plasma spraying. Following patents are referred due to their relevance to field of present invention.
Reference may be made to a research article entitled "Thermal plasma spraying for SOFCs: Applications, potential advantages, and challenges" by Rob Hui et al, Journal of Power Sources 170 (2007) 308-323 wherein, LSM tubes used as SOFC cell support layers (LSM) generally have high porosity, and were found to have low thermal shock resistance, and cracked when exposed to a plasma flame. However, further referred cross-reference do not reflect any solution to avoid the cracking of ceramic tubes during plasma spraying.
Reference may be made to USPTO patent 5436426, provides a novel-cooling fixture for cooling a tubular ceramic substrate during thermal spraying to avoid cracking of ceramic substrate (zirconia). The cooling fixture comprises of a coolant tube carrying pressurized coolant. This is cumbersome and may not be suitable for porous supports (substrate) such as LSM.
Reference may also be made to USPTO patent 3631835, which describes a fixture for holding and cooling magnesium part to be flame sprayed to maintain the magnesium below its ignition point.
Reference may be made to USPTO patent 4297388 where the graphite or copper substrate is typically rotated and cooled by cooling water to the rear surface to get uniform deposit.
Above referred prior art discloses the problem of ceramic tube (Zirconia) (it is not clear whether the tube is porous or dense) cracking/breaking during thermal spraying and a cooling fixture to solve this problem and the other cooling procedures are not relevant to ceramics. No literature is available in
the public domain regarding the solution for avoiding the breaking of the LSM tubes during plasma spraying. Also none of the prior art addresses the problem of breaking of porous LSM tubes during plasma spraying of electrolyte.
It is a challenging task to get a very dense 8YSZ coating on a porous tubular substrate having open end and closed end. To get a dense coating of the electrolyte, the plasma flame temperature should be very high. Higher plasma flame temperatures will provide more heat to the substrate. This heat even with conventional cooling methods and that reported in the above-referred patents can result in cracking of the LSM ceramic substrates, which are porous.
OBJECT OF THE INVENTION
The main object of the present invention is to provide an improved process for plasma spraying on strontium doped lanthanum manganite ceramic tubes for SOFC application, which obviates the drawbacks of the hitherto known prior arts.
Another object of present invention is to avoid the cracking of ceramic substrate in particular during plasma spraying of electrolyte (8YSZ) powder.
Another object of present invention is to avoid the cracking of ceramic substrate in particular during plasma spraying of interconnect (LaCrO3) powder.
Yet another object of present invention is to allow plasma spraying on small tubular LSM substrates at high plasma power.
Yet another object of present invention is to provide a simple and economical process for coating of electrolyte and interconnect coating on SOFC ceramic LSM tubes without breaking and cracking of tubes during plasma spraying
BRIEF DESCRIPTION OF DRAWINGS
The present invention is illustrated in figures 1 to 4 of the drawing(s) accompanying this specification.
Fig.1. LSM tubes which were broken during (a) preheating and (b) plasma spraying of 8YSZ electrolyte.
Fig. 2. LSM tubes plasma sprayed with 8YSZ electrolyte without any visible cracks.
Fig.3. Cross-section of plasma sprayed 8YSZ coating on LSM substrate (a) at lower magnification and (b) at higher magnification.
Fig.4. Photograph of a LSM tube cracked where cooling procedure was not used.
SUMMARY OF THE INVENTION
Accordingly, present invention provides an improved plasma spray process for the electrolyte and interconnect coatings on solid oxide fuel cell cathode tubes which comprises the steps of:
a. grit-blasting the surface of LSM tubular substrate,
b. fitting a mandrel with a cushion layer to the LSM substrate,
c. fixing the mandrel in the lathe,
d. rotating the LSM substrate assembly at 100 rpm,
e. preheating of LSM substrate with plasma flame gun,
f. plasma spraying of electrolyte/ interconnect on the preheated
LSM substrate,
g. cooling of coated LSM substrate with plasma flame from 400-
100 °C,
h. rotating the electrolyte/interconnect coated LSM tube at 100 rpm until it reaches the room temperature,
characterized in that, preheating of LSM tube gradually to 400 °C in four thermal zones (Room temperature to 100 °C; 100-200 °C; 200-300 °C; 300-400 °C) with a heating rate of 60-100 °C/min by adjusting plasma power in between 5 to 45 kW and hydrogen flow rate in between 3 to 13 NLPM and gradual cooling of electrolyte/interconnect coated LSM substrate to 100 °C in three thermal zones (400-300 °C; 300-200 °C; 200-100 °C) using the plasma flame gun by varying the plasma power from 45 kW to 5 kW hydrogen flow rate 13 NLPM to 3 NLPM with rate of cooling in between to 100-60 °C/min followed by rotating the electrolyte coated LSM tube at 100 rpm until it reaches the room temperature ,where NLPM stands for Normal Liters Per Minute
Novelty of present invention lies in providing crack free electrolyte/interconnects ceramic powders coated LSM tubes by plasma spraying. Novelty also lies in providing a simple, economical and user-friendly process for plasma spraying of electrolyte on cathode tube.
Above said novelty of the present invention has been achieved by adopting following non-obvious inventive steps in order to avoid the breaking or cracking of LSM ceramic tube during plasma spraying.
1. Selection of mandrel material
2. Providing a cushion layer in between mandrel and LSM tube
3. Preheating the LSM tube gradually to 400 °C in four thermal zones with a heating rate of 60-100 °C/min.
4. Controlled heating of LSM tubes in a linear fashion in four different temperature zones with a heating rate of 60-100 °C/min by adjusting plasma power in between 5 to 45 kW and hydrogen flow rate in between 3 to 13 NLPM.
5. Controlled and gradual cooling of electrolyte/interconnect coated LSM substrate gradually to 100 °C in three thermal zones using the plasma flame by varying the plasma power from 45 kW to 5 kW; hydrogen flow rate 13 NLPM to 3 NLPM and rate of cooling in the range of 100 to 60 °C/min.
6. Rotating the electrolyte coated LSM tube at 100 rpm until the temperature of the LSM tubes reaches room temperature.
In an embodiment of present invention, mandrel is selected from materials such as mild steel, stainless steel alloy, invar, graphite, aluminum, and ceramic.
In another embodiment of present invention, preferably the mandrel is made of stainless steel alloy material.
In yet another embodiment of present invention, the mandrel is provided with a central axial aperture for releasing the trapped hot air.
In yet another embodiment of present invention, cushion layer should have thermal conductivity more than that of mandrel.
In an embodiment of present invention, cushion layer between mandrel and substrate is selected from glass wool, metal foils and ceramic wool.
In still another embodiment of present invention, preferably the cushion layer between mandrel and substrate is metal foil.
In still another embodiment of present invention, the substrate is selected from ceramic materials like zirconia, NiO-YSZ, preferably strontium doped lanthanum manganite (LSM).
In yet another embodiment of present invention, the preheating of the substrate tubes can be done in a furnace, ordinary flame heating, Bunsen burner heating, preferably by plasma flame.
DETAIL DESCRIPTION OF THE INVENTION
Plasma spraying has the advantages of fast deposition rate and easy masking for deposition of patterned structures, compared to the electrochemical vapor deposition technique, which has been widely used for the fabrication of tubular solid oxide fuel cells. Sidemen's Westinghouse has used plasma spraying for the fabrication of electrolyte, interconnect and anode layers. However, the cost will come down if the productivity goes up. Conventional coating technologies, such as tape casting, screen-printing and dip molding produces sintering defects like warp, crackle and pore in large area SOFCs. For SOFC application, the APS electrolyte should be dense, gas tight and as thin as possible in order to minimize the cell resistance. It is
achievable when completely molten 8YSZ particles are completely flattened and solidified without defects during impacting on the substrate. Therefore, low particle velocity and high temperature is required. Some of the unique features of plasma spraying are as follows; (i) a wide range of materials from metals to ceramics to polymers and any combination of them and graded coatings can be deposited, (ii) homogeneous coatings with time invariant changes in composition i.e. without compositional changes across the thickness can be produced, (iii) Microstructures with fine, equiaxed grain but without columnar defects can be deposited in contrast to electron beam deposition, (iv) high deposition rates of the order of mms-1 can be achieved with only modest investment in capital equipment, (v) free-standing thick forms can be sprayed in near-net shape fashion of pure and mixed ceramics, (vi) the process can be carried out in any conceivable environment such as air, reduced pressure, inert gas, or underwater. The flow rates of H2 and Ar are the two determining variables for coating porosity. The optimum appears to be at reasonably lower Ar flow rate and high H2 flow rate.
The plasma spraying involves the following important steps: surface preparation of the sample, plasma preheating and then powder spraying. During plasma spraying the powders are melted in the plasma flame and get deposited on the substrate. During deposition, the tubular ceramic substrate is rotated to ensure uniform deposition and the substrate is generally maintained at relatively low temperature. The gun is traversed repetitively across the substrate being coated so as to distribute the coating particles evenly and prevent local hot spots. Although the material, which is depositing on the work piece, heats the work piece the work piece is generally maintained at relatively very low temperature compared to the plasma flame temperature. The gun is traversed across the substrate being coated so as to
distribute the coating particles and prevent local hot spots. Excessive or uneven heating can result in oxidation of a metal substrate or cracking of a ceramic substrate. Also, when the porosity of a ceramic body is increased there is generally a corresponding decrease in the thermal shock properties of the ceramic body. Thus in case of the cathode substrate i.e. Strontium doped Lanthanum Manganite (LSM) ceramic tube which is porous (35-40%) there is a decrease in the thermal shock properties and hence the tubes break during thermal spraying (plasma spraying).
The present invention involves the following steps for avoiding the cracking/breaking of LSM ceramic tubular substrates during plasma spraying. In order to enhance the adhesion of the coating, the substrate was blasted with 40 µm alumina grits and cleaned with compressed air. During spraying, the tube was rotated in order to coat the all surfaces of the tube. Argon was used as the primary gas and hydrogen as the secondary plasma gas.
The following examples are given by way of illustrations and therefore, should not be construed to limit the scope of the present investigation.
Example 1 :
The LSM substrate tube of 68.59 mm length, 18.10 mm outer diameter and 13.61 mm inner diameter, was fitted to a stainless steel alloy mandrel. The substrate was rotated at 100 rpm and the plasma gun speed was set at 800 mm/min. The spray distance was 15 cm. The LSM tube was preheated with plasma flame using the following plasma spray parameters: 45 kW plasma power; hydrogen flow rate of 13 NLPM; Argon flow rate of 42 NLPM. The LSM tube broke with the first pre-heating step itself (Fig. 1).
Example 2:
The LSM substrate tube of 58.13 mm length, outer diameter of 15.50 mm and 11.5 mm inner diameter was fitted to a stainless steel alloy mandrel with a cushioning layer (metal foil) in between the tube and mandrel with a tolerance of
(Table Removed)
Indigenously prepared YSZ powder was plasma sprayed using the following plasma spray parameters: 45 kW with a powder feed rate of 30 g/min. The tubes were slowly cooled to 100 °C using the plasma spray parameters as given in the following table:
(Table Removed)
Then the coated LSM tube was kept on rotating at 100 rpm until it reaches the room temperature. This resulted in crack-free 8YSZ coated LSM substrate, which can be seen visually in Fig. 2.
Example 3:
In another example, LSM tube with 45.23 mm length, 17.6 mm outer diameter and 14.32 mm inner diameter was plasma sprayed using 8YSZ powder with the following plasma spray parameters: 45 kW plasma power, powder feed rate of 30 g/min and spray distance of 15 cm. All the experimental parameters were kept constant as given in example-2. This resulted in crack-free 8YSZ coated LSM tube. The cracks were not even seen in the substrate as seen in the cross-sectional SEM image at lower magnification (Fig. 3a) and at higher magnification (Fig. 3b) shows dense 8YSZ coating without any cracks in the substrate or in the coating.
Example 4:
In another example, LSM tube with the following dimension 62.82 mm length, outer diameter of 16 mm and 12.22 mm inner diameter was fitted to a suitable stainless steel alloy mandrel with a cushioning layer in between the mandrel and the LSM tube as discussed in example 2. Before plasma
spraying, the tube was preheated with the plasma flame using the same procedure as used in the above example. Then 8YSZ powder was plasma sprayed using the following plasma spray parameters: 45 kW plasma power, powder feed rate of 30 g/min and spray distance of 15 cm. However, the tube was not cooled slowly to room temperature using the plasma flame. This resulted in cracked 8YSZ coated LSM tube (Fig.4).
Main advantages of present invention are as follows:
1. The invention provides crack-free electrolyte and interconnect coatings
2. The invention provides a simple and economical procedure
3. Readily available materials can be used as cushioning and mandrel materials
4. This process can be used for any ceramic coating on any ceramic substrate
5. This process can be scaled up for coating any longer ceramic tubes
6. This process does not use any complicated cooling set-up as reported in prior art

We Claim:
1. An improved plasma spray process for the electrolyte and
interconnect coating on solid oxide fuel cell cathode tubes, comprising
the steps of:
a. grit-blasting the surface of a Strontium doped Lanthanum
Manganite (LSM) tubular substrate;
b. fitting a mandrel with a cushion layer to the above said LSM
tubular substrate;
c. fixing the above said mandrel in a lathe;
d. rotating the LSM tubular substrate assembly at about 100 rpm;
e. preheating the LSM tubular substrate with a plasma flame in
the range of room temperature to 400°C in four thermal zones ;
f. plasma spraying of electrolyte/ interconnect on the preheated
LSM tubular substrate;
g. cooling the coated LSM tubular substrate with plasma flame in
the range of 400 to 100 °C;
h. rotating the electrolyte/interconnect coated LSM tubular tube at about 100 rpm until it reaches the room temperature;
2. An improved process as claimed in claim 1, characterized in
preheating the LSM tube gradually to 400 °C in four thermal zones
wherein the first thermal zone being in the range of room temperature
to 100 °C, followed by second zone in the range of 100 to 200 °C ,
third zone in the range of 200 to 300 °C and fourth zone in the range
of 300 to 400 °C, the four zones having heating rate in the range of
60-100 °C/min by adjusting plasma power in the range of 5 to 45KW
and hydrogen flow rate in the range of 3 to 13 NLPM.
3. An improved plasma spray process as claimed in claim 1 , characterized in cooling the electrolyte/interconnect coated LSM substrate gradually to 100° C in three thermal zones, wherein the first thermal zone being in the range of 400 to 300 °C , second zone in the range of 300 to 200°C and third zone in the range of 200 to 100 °C using the plasma flame by varying the plasma power in the range of 45 kW to 5 kW hydrogen flow rate in the range of 13 NLPM to 3 NLPM, the three zones having a rate of cooling in the range of 100 to 60 °C/min followed by rotating the electrolyte coated LSM tube at 100 rpm until it reaches the room temperature .
4. An improved plasma spray process as claimed in claim 1, wherein mandrel used is selected from the group of materials consisting of stainless steel, mild steel, invar, graphite, aluminum and ceramic.
5. An improved plasma spray process as claimed in claim 1, wherein mandrel used preferably is made of stainless steel alloy material.
6. An improved plasma spray process as claimed in claim 1, wherein the mandrel used is provided with a central axial aperture for releasing the trapped hot air.
7. An improved plasma spray process as claimed in claim 1, wherein cushion layer between mandrel and substrate used is selected from group consisting of glass wool, metal foils and ceramic wool.
8. An improved plasma spray process as claimed in claim 1, wherein cushion layer used between mandrel and substrate is a metal foil.
9. An improved plasma spray process as claimed in claim 1, wherein the substrate used is selected from the group consisting of ceramic materials like zirconia, NiO-YSZ, preferably strontium doped lanthanum manganite (LSM).
10. An improved plasma spray process as claimed in claim 1, wherein the electrolyte used is Yttria Stabilized Zirconia (8YSZ) powder and interconnect used is lanthanum chromate( LaCrO3)
11. An improved process as claimed in claim 1, wherein the grit blasting
of the substrate is blasted with about 40 µm alumina grits.
12. An improved plasma spray process for the electrolyte and
interconnect coating on solid oxide fuel cell cathode tubes
substantially as herein described with reference to the examples and
the drawings accompanying this specification.

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Patent Number

270319

Indian Patent Application Number

459/DEL/2009

PG Journal Number

51/2015

Publication Date

18-Dec-2015

Grant Date

11-Dec-2015

Date of Filing

09-Mar-2009

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Applicant Address

ANUSANDHAN BHAWAN, RAFI MARG, NEW DELHI - 110 001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	SINGANAHALLI THIPPAREDDY ARUNA	NATIONAL AEROSPACE LABORATORIES, POST BOX NO. 1779 KODIHALLI BAGALORE - 560 017 KARNATAKA, INDIA.
2	ARUN VENKATESH AINAPUR	NATIONAL AEROSPACE LABORATORIES, POST BOX NO. 1779 KODIHALLI BANGALORE - 560 017 KARNATAKA, INDIA.
3	NARAYANASWAMY BALAJI	NATIONAL AEROSPACE LABORATORIES, POST BOX NO. 1779 KODIHALLI BANGALORE - 560 017 KARNATAKA, INDIA.
4	KARAIKUDI SANKARANARAYANA RAJAM	NATIONAL AEROSPACE LABORATORIES, POST BOX NO. 1779 KODIHALLI BANGALORE - 560 017 KARNATAKA, INDIA.

PCT International Classification Number

C23C4/06

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA