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Chemical vapor deposition (CVD) of rhenium
Materials Letters North-Holland
12
( 199 I ) 43-46
Chemical vapor deposition (CVD) of rhenium K.T. Kim
‘, J.J. Wang 2 and G. Welsch
Department oficlaterials Science and Engineering, Case Western Reserve L1niversit.v.Cleveland, OH 44106, USA Received
7 June 199 1
Thin wall rhenium tubes have been successfully produced by chemical vapor deposition (CVD) on a molybdenum substrate which was subsequently etched off. Detail procedure of the deposition is discussed. The CVD rhenium deposit has a facetted surface and a columnar grain structure with preferential growth orientation along the c axis. At high temperature (2000 K) substantial grain growth occurs, and in the recrystallized structure individual grains traverse the entire width of the layer with smooth transverse boundaries between the grains.
1. Introduction
2. Experimental
The high melting point (T,=3453 K) and good ductility make rhenium attractive as a high-temperature structural material. Complex shapes such as thin wall tubes and nozzles are easily made by a chemical vapor deposition (CVD) process. The pyrolytic decomposition of rhenium pentachloride (ReCl, ) is a useful CVD system. However, ReCIS is solid at room temperature and highly reactive with air and water. It is difficult to handle, transfer, and meter. In-situ generation of ReCl, gas in the reaction chamber can help avoid this problem. A CVD process which uses chlorine as a carrier to transfer rhenium from one solid phase (powder) to another solid phase (deposit) is achieved by passing chlorine gas through heated rhenium powder to generate ReCls gas (chlorination) and decomposing the gas at the heated substrate. The direction of the reaction is governed by temperature and the partial pressures of the respective gases. By controlling the parameters, one can dictate the direction of the reaction.
The apparatus for producing CVD rhenium is sketched in fig. 1. A quartz tube reactor consisting of inside and outside chambers, was designed, built, and placed inside a resistance-heated tube furnace. A pair of 3 16 stainless-steel tubes of 6.35 mm outside diameter, sealed at one end, serve as electrical connectors for the resistance-heated molybdenum substrate. Lateral holes in the walls of these tube connectors enable argon gas to pass through and protect the connectors from attack by corrosive chlorine gas during the decomposition process. Chlorine gas is fed through a 3.175 mm outside diameter quartz tube. Pure rhenium powder is placed inside the inner reactor chamber. All meters, valves, and gas lines through which chlorine gas passes, are made of chlorine-resistant nickel alloy (Monel 400).
’ Present address: Research Institute of Industrial Science and Technology, P.O. Box 135, Pohang 790-600, Korea. * Present address: Adsorption Research and Applications Corporation, Sunnyvale, CA 94086, USA. 0167-577x/91/$
03.50 0 1991 Elsevier Science Publishers
Fig. 1. Schematic drawing of the CVD apparatus: ( 1) MO substrate, (2) furnace, (3) inlet gas line and (4) rhenium powder.
B.V. All rights reserved.
43
Volume 12, number 1,2
MATERIALS LETTERS
The ideal substrate should conform to the following criteria: (a) match the thermal expansion of the deposit, (b) have a melting point above the deposition temperature, (c) be inert to the reaction gases, (d) have negligible interdiffusion with the deposit material at deposition temperature, and (e) be removable from the deposit, e.g., by preferential etching. Molybdenum satisfies the above criteria well and was chosen as the substrate material. The acid used for dissolution of the molybdenum substrate was composed of 107 g of HN03, 88 g of H2S04, and 33 g of H,O. It has good selective etching power, leaving the deposited rhenium intact. Before the deposition procedure, the apparatus is flushed with argon, then with hydrogen, at a furnace temperature of 900°C. This is done for 1 h to reduce any possible oxide present in the rhenium powder. The reactor chamber is then flushed again with argon to remove residual hydrogen gas because the presence of hydrogen with ReCl, is detrimental due to gas-phase nucleation [ 11, i.e. no deposition occurs on the substrate. After the furnace and molybdenum substrate were set to the desired temperatures, chlorine gas is let into the reactor while keeping the argon gas flowing through the electrodes to protect the electrical conductors. This starts the chlorination reaction, and ReCl, is produced. Initially there is an incubation time for generating enough ReCl, to saturate the reactor before steady-state deposition is established. Saturation is indicated when brown smoke is observed from the outlet of the reactor chamber. Because the deposition increases the electrical cross section of the substrate/rhenium composite, the substrate temperature tends to decrease continuously during deposition. This is compensated for by increasing the electrical power, and a constant substrate temperature can be maintained. The duration of deposition is measured from the start of steadystate deposition. To terminate the deposition, the chlorine supply is stopped and the system is flushed with argon gas and is permitted to cool to room temperature in argon atmosphere. Microstructure of the deposit was characterized by SEM and TEM.
44
September 199 1
3. Results and discussion At 1 atm pressure the chlorination and the decomposition reactions are in equilibrium at temperature r, = 942 “C [ 2 1. The chlorination reaction is favorable at a temperature below r,, and the decomposition reaction is favorable at a temperature above To. An advantageous feature of the design in fig. 1 is that chlorination and decomposition reactions happen in the same reactor chamber, and each reaction can be achieved at different locations in the reactor chamber by controlling the furnace and the substrate temperatures. The molybdenum substrate was maintained at 1150°C (decomposition temperature) while the furnace, which controls the chlorination temperature, was kept at 800°C which is well above the boiling point of ReCIS and ensures adequate convection of the gas in the reactor. As a consequence, a chlorine cycle is obtained in the reactor. Chlorine gas, released by the decomposition of ReCl, at the hot molybdenum substrate, is transported by convection to the cooler rhenium powder where it reacts again to form ReCl, gas. This will again be transported to the substrate, decompose, and so on. Because of the chlorine cycle, the feeding of chlorine gas can be kept to a very small quantity, just enough to compensate for the gas that is exhausted through the small exit opening between the inner and outer chamber. The flow rate of chlorine gas was controlled about 1 cm3/ min and that of argon was about 500 cm3/min. In the manner described above, a good-quality Re layer, i.e. one that is dense and pure, could be obtained at a deposition rate of 0.2 mm/h. The microstructure of a cross section through the rhenium layer obtained at above deposition conditions is shown in fig. 2. Every columnar grain has a flat surface normal to the growth direction. The topography and surface roughness of the deposit are due to the valleys between the grains. Actually, there are two sub-layers, namely a nucleation layer of randomly oriented, equiaxed grains and a layer comprising oriented columnar grains which grew on the top of the nucleation layer. It is believed that the nucleation layer was formed initially on the polycrystalline molybdenum substrate and then the columnar layer was formed on it. Increasing the deposition (substrate) temperature and/or the flow rate of
Volume 12, number
1,2
MATERIALS
LETTERS
Fig. 2. Fracture cross section of CVD rhenium deposit on molybdenum equiaxed grains and (c) growth layer of columnar grains.
chlorine increases the deposition rate and this leads to severe surface roughness of the deposit and to voids in the CVD layer. The microstructural features are more sensitive to the chlorine flow rate than to the reaction temperature. This means that the quality of the deposit is transport controlled. Details of the growth mechanisms can be found in the literature [ 3,4]. A TEM micrograph of the deposit with the thin foil parallel to the substrate surface and corresponding area diffraction pattern, fig. 3, tells that the col-
substrate.
September
(a) Molybdenum
substrate,
(b) nucleation
199 1
layer of
umnar grains are (0001) oriented and the preferential growth direction of the deposit is [ 000 I]. Even though the preferentially oriented grains exhibit considerable resistance to grain growth [ 4,5 1, substantial grain growth occurred after high-temperature exposure. Fig. 4 shows the CVD deposit before and after annealing at 2000 K for 4 h. Annealing of the CVD rhenium deposit leads to grain growth which leaves large grains with smooth grain boundaries through the thickness of the deposit. This kind microstructural material creeps easily by grain bound45
MATERIALS LETTERS
Volume 12, number 1,2
Fig. 3. TEM cross section perpendicular to the growth direction through columnar CVD rhenium layer, (a), and corresponding area diffraction pattern, (b). Each grain has [OOOl] zone axis which is the preferential growth direction. at-y sliding
in high-temperature
service.
September 199 1
Fig. 4. SEM micrographs of fracture cross sections of CVD rhenium. (a) As-deposited and (b) after annealing (2000 K for 4 h).
Acknowledgement We would like to thank Dr. L.V. McCarty for his help in building and setting up the CVD equipment.
4. Conclusions A dense and pure CVD rhenium was produced at a deposition rate of 0.2 mm/h at the conditions with the chlorination at 800°C and the decomposition at 1150°C. The deposit consists of (0001) oriented columnar grains and the preferential growth direction is [ 000 11. After annealing the deposit at 2000 K for 4 h substantial grain growth occurs. The deposit has large grains with smooth grain boundaries through the thickness of it.
References [ 1] F.A. Glaski, IEEE Conf. Rec. Thermion Conversion Spec. PAP Annual Conf. 9th ( 1970) p. 128. [ 21 E.M. Savitskii, M.A. Tylkina and K.B. Povarava, in: Rhenium alloys, ed. D. Slutzkin, translated from Russian by C. Nisenbaum (Israel program for Scientific Translation, Jerusalem, 1970). [ 31 A. van de Drift, Philips Res. Rept. 22 ( 1967) 267. [4] W.A. Bryant, J. Mater. Sci. 2 (1977) 1285. [ 51 R.L. Heestant and C.F. Leitten Jr., in: Metallurgical Society Conferences, Vol. 30. Refractory metals and alloys III, ed. R.I. Jaffee (Gordon and Breach, London, 1964) p. 113.
12
( 199 I ) 43-46
Chemical vapor deposition (CVD) of rhenium K.T. Kim
‘, J.J. Wang 2 and G. Welsch
Department oficlaterials Science and Engineering, Case Western Reserve L1niversit.v.Cleveland, OH 44106, USA Received
7 June 199 1
Thin wall rhenium tubes have been successfully produced by chemical vapor deposition (CVD) on a molybdenum substrate which was subsequently etched off. Detail procedure of the deposition is discussed. The CVD rhenium deposit has a facetted surface and a columnar grain structure with preferential growth orientation along the c axis. At high temperature (2000 K) substantial grain growth occurs, and in the recrystallized structure individual grains traverse the entire width of the layer with smooth transverse boundaries between the grains.
1. Introduction
2. Experimental
The high melting point (T,=3453 K) and good ductility make rhenium attractive as a high-temperature structural material. Complex shapes such as thin wall tubes and nozzles are easily made by a chemical vapor deposition (CVD) process. The pyrolytic decomposition of rhenium pentachloride (ReCl, ) is a useful CVD system. However, ReCIS is solid at room temperature and highly reactive with air and water. It is difficult to handle, transfer, and meter. In-situ generation of ReCl, gas in the reaction chamber can help avoid this problem. A CVD process which uses chlorine as a carrier to transfer rhenium from one solid phase (powder) to another solid phase (deposit) is achieved by passing chlorine gas through heated rhenium powder to generate ReCls gas (chlorination) and decomposing the gas at the heated substrate. The direction of the reaction is governed by temperature and the partial pressures of the respective gases. By controlling the parameters, one can dictate the direction of the reaction.
The apparatus for producing CVD rhenium is sketched in fig. 1. A quartz tube reactor consisting of inside and outside chambers, was designed, built, and placed inside a resistance-heated tube furnace. A pair of 3 16 stainless-steel tubes of 6.35 mm outside diameter, sealed at one end, serve as electrical connectors for the resistance-heated molybdenum substrate. Lateral holes in the walls of these tube connectors enable argon gas to pass through and protect the connectors from attack by corrosive chlorine gas during the decomposition process. Chlorine gas is fed through a 3.175 mm outside diameter quartz tube. Pure rhenium powder is placed inside the inner reactor chamber. All meters, valves, and gas lines through which chlorine gas passes, are made of chlorine-resistant nickel alloy (Monel 400).
’ Present address: Research Institute of Industrial Science and Technology, P.O. Box 135, Pohang 790-600, Korea. * Present address: Adsorption Research and Applications Corporation, Sunnyvale, CA 94086, USA. 0167-577x/91/$
03.50 0 1991 Elsevier Science Publishers
Fig. 1. Schematic drawing of the CVD apparatus: ( 1) MO substrate, (2) furnace, (3) inlet gas line and (4) rhenium powder.
B.V. All rights reserved.
43
Volume 12, number 1,2
MATERIALS LETTERS
The ideal substrate should conform to the following criteria: (a) match the thermal expansion of the deposit, (b) have a melting point above the deposition temperature, (c) be inert to the reaction gases, (d) have negligible interdiffusion with the deposit material at deposition temperature, and (e) be removable from the deposit, e.g., by preferential etching. Molybdenum satisfies the above criteria well and was chosen as the substrate material. The acid used for dissolution of the molybdenum substrate was composed of 107 g of HN03, 88 g of H2S04, and 33 g of H,O. It has good selective etching power, leaving the deposited rhenium intact. Before the deposition procedure, the apparatus is flushed with argon, then with hydrogen, at a furnace temperature of 900°C. This is done for 1 h to reduce any possible oxide present in the rhenium powder. The reactor chamber is then flushed again with argon to remove residual hydrogen gas because the presence of hydrogen with ReCl, is detrimental due to gas-phase nucleation [ 11, i.e. no deposition occurs on the substrate. After the furnace and molybdenum substrate were set to the desired temperatures, chlorine gas is let into the reactor while keeping the argon gas flowing through the electrodes to protect the electrical conductors. This starts the chlorination reaction, and ReCl, is produced. Initially there is an incubation time for generating enough ReCl, to saturate the reactor before steady-state deposition is established. Saturation is indicated when brown smoke is observed from the outlet of the reactor chamber. Because the deposition increases the electrical cross section of the substrate/rhenium composite, the substrate temperature tends to decrease continuously during deposition. This is compensated for by increasing the electrical power, and a constant substrate temperature can be maintained. The duration of deposition is measured from the start of steadystate deposition. To terminate the deposition, the chlorine supply is stopped and the system is flushed with argon gas and is permitted to cool to room temperature in argon atmosphere. Microstructure of the deposit was characterized by SEM and TEM.
44
September 199 1
3. Results and discussion At 1 atm pressure the chlorination and the decomposition reactions are in equilibrium at temperature r, = 942 “C [ 2 1. The chlorination reaction is favorable at a temperature below r,, and the decomposition reaction is favorable at a temperature above To. An advantageous feature of the design in fig. 1 is that chlorination and decomposition reactions happen in the same reactor chamber, and each reaction can be achieved at different locations in the reactor chamber by controlling the furnace and the substrate temperatures. The molybdenum substrate was maintained at 1150°C (decomposition temperature) while the furnace, which controls the chlorination temperature, was kept at 800°C which is well above the boiling point of ReCIS and ensures adequate convection of the gas in the reactor. As a consequence, a chlorine cycle is obtained in the reactor. Chlorine gas, released by the decomposition of ReCl, at the hot molybdenum substrate, is transported by convection to the cooler rhenium powder where it reacts again to form ReCl, gas. This will again be transported to the substrate, decompose, and so on. Because of the chlorine cycle, the feeding of chlorine gas can be kept to a very small quantity, just enough to compensate for the gas that is exhausted through the small exit opening between the inner and outer chamber. The flow rate of chlorine gas was controlled about 1 cm3/ min and that of argon was about 500 cm3/min. In the manner described above, a good-quality Re layer, i.e. one that is dense and pure, could be obtained at a deposition rate of 0.2 mm/h. The microstructure of a cross section through the rhenium layer obtained at above deposition conditions is shown in fig. 2. Every columnar grain has a flat surface normal to the growth direction. The topography and surface roughness of the deposit are due to the valleys between the grains. Actually, there are two sub-layers, namely a nucleation layer of randomly oriented, equiaxed grains and a layer comprising oriented columnar grains which grew on the top of the nucleation layer. It is believed that the nucleation layer was formed initially on the polycrystalline molybdenum substrate and then the columnar layer was formed on it. Increasing the deposition (substrate) temperature and/or the flow rate of
Volume 12, number
1,2
MATERIALS
LETTERS
Fig. 2. Fracture cross section of CVD rhenium deposit on molybdenum equiaxed grains and (c) growth layer of columnar grains.
chlorine increases the deposition rate and this leads to severe surface roughness of the deposit and to voids in the CVD layer. The microstructural features are more sensitive to the chlorine flow rate than to the reaction temperature. This means that the quality of the deposit is transport controlled. Details of the growth mechanisms can be found in the literature [ 3,4]. A TEM micrograph of the deposit with the thin foil parallel to the substrate surface and corresponding area diffraction pattern, fig. 3, tells that the col-
substrate.
September
(a) Molybdenum
substrate,
(b) nucleation
199 1
layer of
umnar grains are (0001) oriented and the preferential growth direction of the deposit is [ 000 I]. Even though the preferentially oriented grains exhibit considerable resistance to grain growth [ 4,5 1, substantial grain growth occurred after high-temperature exposure. Fig. 4 shows the CVD deposit before and after annealing at 2000 K for 4 h. Annealing of the CVD rhenium deposit leads to grain growth which leaves large grains with smooth grain boundaries through the thickness of the deposit. This kind microstructural material creeps easily by grain bound45
MATERIALS LETTERS
Volume 12, number 1,2
Fig. 3. TEM cross section perpendicular to the growth direction through columnar CVD rhenium layer, (a), and corresponding area diffraction pattern, (b). Each grain has [OOOl] zone axis which is the preferential growth direction. at-y sliding
in high-temperature
service.
September 199 1
Fig. 4. SEM micrographs of fracture cross sections of CVD rhenium. (a) As-deposited and (b) after annealing (2000 K for 4 h).
Acknowledgement We would like to thank Dr. L.V. McCarty for his help in building and setting up the CVD equipment.
4. Conclusions A dense and pure CVD rhenium was produced at a deposition rate of 0.2 mm/h at the conditions with the chlorination at 800°C and the decomposition at 1150°C. The deposit consists of (0001) oriented columnar grains and the preferential growth direction is [ 000 11. After annealing the deposit at 2000 K for 4 h substantial grain growth occurs. The deposit has large grains with smooth grain boundaries through the thickness of it.
References [ 1] F.A. Glaski, IEEE Conf. Rec. Thermion Conversion Spec. PAP Annual Conf. 9th ( 1970) p. 128. [ 21 E.M. Savitskii, M.A. Tylkina and K.B. Povarava, in: Rhenium alloys, ed. D. Slutzkin, translated from Russian by C. Nisenbaum (Israel program for Scientific Translation, Jerusalem, 1970). [ 31 A. van de Drift, Philips Res. Rept. 22 ( 1967) 267. [4] W.A. Bryant, J. Mater. Sci. 2 (1977) 1285. [ 51 R.L. Heestant and C.F. Leitten Jr., in: Metallurgical Society Conferences, Vol. 30. Refractory metals and alloys III, ed. R.I. Jaffee (Gordon and Breach, London, 1964) p. 113.