PROCESS FOR THE PREPARATION OF REFLECTIVE PYROLYTIC GRAPHITE

Abstract: A process for preparing reflective pyrolytic graphite by the chemical vapor deposition of graphite on a suitable support from methane or other short chain hydrocarbons at a suitable pressure and temperature followed by annealing the pyrolytic graphite at a temperature in excess of 2600°C and pressure of 5 to 10 torr.

Description: The invention relates to a process for the preparation of reflective pyrolytic graphite by the surface nucleation of pyrolytic graphite prepared by a chemical vapor deposition process using a low molecular weight hydrocarbon gas. The formed graphite is annealed at a temperature in excess of 2600°C. The invention relates to reflective graphite. The reflective graphite is prepared by preparing a surface nucleated pyrolytic graphite by a chemical vapor deposition technique. The chemical vapor deposition is carried out by the decomposition of low molecular weight hydrocarbon gases at a temperature of 1700 - 2200°C. The pyrolytic graphite deposited is annealed at temperatures in excess of 2600°C and pressures in excess of 666.5 Pa (5 torr). U.S-A- 4,608,192 describes a process for forming graphite intercalates containing metal charge transfer salts. U.S-A- 3,900,540 describes a method for preparing a thin film of substantially defect-free pyrolytic graphite by vapor deposition on an inert liquid substrate surface followed by separation of the graphite. U.S-A- 3,547,676 describes the preparation of pyrolytic carbon structures by chemical vapor deposition from a mixture of methane and inert gas in a temperature of about 2100°C. There are several applications where reflective pyrolytic graphite is useful. Reflective graphite can be used for making high temperature reflectors in vacuum equipment and in high temperature heater insulation where the heater is used in inert atmospheres. The prior art methods of preparing reflective graphite included deposition of metal inclusions in the graphite or deposition of a coating on the graphite to give the desired reflectivity to the product. More generally it is an object of this invention to provide an improved process for the preparation of reflective graphite by pyrolysis of methane gas or other short chain hydrocarbons wherein the pyrolytic graphite formed on the substrate is annealed at a high temperature and pressure. In accordance with the preferred embodiment of this invention pyrolytic graphite is deposited on a suitable support such as commercially available graphite designated CS graphite by the vendor, tungsten metal or hafnium metal. In the first step the substrate is positioned in a furnace chamber and the furnace chamber is evacuated to a pressure of 133 to 399 Pa (1 to 3 torr), preferably 133 Pa (1 torr). When the pressure is reduced to 133-399 Pa (1 to 3 torr), preferably 133 Pa (1 torr), the furnace chamber is heated to 700-1000°C preferably 900°C at a rate of about 300°C per hour. When the temperature is reached it may be desirable to pass hydrogen gas through the furnace chamber at a rate of about 424.74 1/hour (15 standard cubic feet per hour). The heat up at the furnace chamber is continued at a rate of about 300°C/hr until a temperature of 1850-2250°C preferably 2150°C is reached. When this temperature is reached the hydrogen flow, where hydrogen is used, is discontinued and methane is passed through the furnace chamber at a flow of about 849.48 - 1132.64 l/hour (30-40 standard cubic feet per hour) preferably 1019.37 l/hour (36 standard cubic feet per hour). When the temperature and gas flow are stabilized at about 2150°C and 1019.37 l/hour (36 standard cubic feet per hour) the pressure in the furnace chamber is increased to about 666.5 ± 200 Pa (5 torr ± 1.5 torr). These conditions are maintained until the deposit of pyrolytic graphite of a thickness of 0.1 to 0.6 cm is achieved. Thicknesses of 0.2 to 0.6 cm are preferred by most applications. When the pyrolytic graphite deposit reaches the desired thickness the methane flow is discontinued and the furnace chamber is cooled to atmospheric pressure and room temperature. Optionally nitrogen gas may be passed through the furnace chamber while it is being cooled. The pyrolytic graphite is converted to reflective graphite by positioning the pyrolytic graphite on the support in a furnace chamber and evacuating the furnace chamber to a pressure of about 133 Pa (1 torr). The furnace chamber temperature is increased to 700 to 1000°C preferably about 900°C at a rate of 150°C to 250°C preferably about 200°C per hour. When the desired temperature is reached the pressure is increased gradually to about 666.5 to 1333 Pa (5 to 10 torr), preferably 1333 Pa (10 torr) by feeding nitrogen gas into the furnace chamber. The furnace chamber temperature is increased to 2600°C to 3300°C preferably 2900-2950°C and these conditions are maintained for 10 minutes to 2 hours preferably about 1 hour. At the end of this time the furnace chamber is cooled to room temperature and the reflective graphite product is recovered. The process of the invention can also be used to impregnate reflective graphite into a rigidized cloth or felt. The process is essentially the same except that after cloth or felt is positioned in the furnace chamber, the furnace chamber is evacuated to a pressure of 133 Pa (1 mm Hg) and the furnace chamber is heated to a temperature of 700°C to 1000°C preferably about 900°C at a rate of about 300°C per hour. When the desired temperature is reached hydrogen gas is passed through the furnace chamber at a rate of about 424.74 1/hour (15 standard cubic feet per hour). When the furnace chamber temperature reaches 900°C to 1200°C preferably about 1100°C, methane or other short chain hydrocarbon gas is feed into the furnace at the rate of about 549.48 l/hour (30 standard cubic feet per hour). When the temperature is stabilized at about 900-1200°C preferably about 1090 - 1100°C the pressure in the furnace chamber is adjusted to 266.6 Pa (2 torr). These conditions are maintained for about 24 hours. At the end of this time the methane flow is discontinued and the furnace chamber temperature is increased to 2100 - 2150°C at a rate of about 200°C per hour. When this temperature is reached the hydrogen flow is discontinued and methane is passed into the furnace chamber at a rate of about 849.48 - 1132.64 l/hour (30 - 40) preferably 1019.37 l/hour (36 standard cubic feet per hour) and the furnace chamber pressure is adjusted to 466.5 to 666.5 Pa (3.5 to 5 torr). These conditions are maintained until the thickness of the cloth of felt is increased to 0.1 to 0.6 cm preferably 0.2 to 0.6 cm. The cloth or felt impregnated with pyrolytic graphite is annealed using the same technique as described above. The invention is illustrated by the following specific but non limiting examples : EXAMPLE I This example illustrates the preparation of reflective pyrolytic graphite on a graphite support. The substrate, a graphite designated CS graphite by the vendor, was positioned in a suitable furnace. The furnace chamber was evacuated to a pressure of about 133 Pa (1 mm Hg) and the temperature was increased at the rate of about 300°C/hr until a temperature of 2150°C was reached. When this temperature was reached methane gas was feed into the furnace chamber at the rate of about 1019.37 l/hour (36 standard cubic feet per hour) and the furnace chamber pressure was increased to 639.8 - 666.5 Pa (4.8 - 5 torr). These conditions were maintained until the pyrolytic graphite was deposited to a thickness of 0.2 - 0.6 cm. When this thickness was reached the methane flow to the furnace chamber was discontinued and nitrogen was feed into the furnace chamber and the furnace chamber was allowed to cool to room temperature. The pyrolytic graphite was converted to reflective graphite. The furnace chamber was evacuated to a pressure of about 133 Pa (1 torr) and heated at the rate of 200°C per hour until a temperature of 900°C was reached. When the temperature reached 900°C the pressure was increased to 666.5 Pa (5 torr) and then gradually to about 1333 Pa (10 torr) by feeding nitrogen into the furnace chamber. The heating was continued until the temperature reached 2900-2950°C and these conditions were maintained for a period of one hour. The furnace chamber was allowed to cool to room temperature and atmospheric pressure and the reflective graphite part was recovered. The reflectance of the part was compared with the reflectance of a standard pyrolytic graphite. The data collected is set out on the table below. In this table the percentage of reflectance of the standard pyrolytic graphite and the reflecting pyrolytic graphite are compared and the percentage of advantage is set out. REFLECTANCE COMPARISON REFLECTING PYROLYTIC GRAPHITE/REGULAR PYROLYTIC GRAPHITE (PG) WAVELENGTH (micrometers) STANDARD PG REFLECTING PG % ADVANTAGE 1.0 0.35 0.455 30 2.0 0.410 0.605 48 3.95 0.470 0.705 50 4.0 0.505 0.780 54 5.0 0.540 0.815 51 6.0 0.565 0.835 48 7.0 0.585 0.845 44 8.0 0.605 0.860 42 9.0 0.630 0.879 38 10.0 0.640 0.880 38 10.6 0.655 0.880 34 11.0 0.655 0.880 34 It is apparent from the data that there is a substantial advantage in the reflectance in the product prepared by the technique described above. EXAMPLE II This example illustrates preparing reflecting graphite on rigidized cloth or felt. A piece of felt was positioned in a furnace chamber and the furnace chamber was evacuated to a pressure to about 133 Pa (1 mm Hg). When this pressure was reached the furnace chamber was heated to approximately 900°C at a rate of 300°C/hr. Hydrogen was feed to the furnace chamber at the rate of about 424.74 l/hour (15 standard cubic feet per hour) and the heat up was continued until the temperature reached 1100°C. When the temperature in the furnace chamber reached 1100°C methane was feed to the furnace chamber at the rate of approximately 849.48 l/hour (30 standard cubic feet per hour) and the temperature was maintained at 1100°C. The methane flow, hydrogen flow, pressure and temperature were maintained for a period of 24 hours. After 24 hours the temperature was increased to 2150°C and the hydrogen flow was discontinued. The methane flow was increased to 1019.37 l/hour (36 standard cubic feet per hour) and the pressured was equalized at 666.5 Pa (5 torr). These conditions maintained until the deposit reached a thickness of about 0.2 to about 0.6 cm. The pyrolytic graphite was converted to reflective graphite using the same technique as described in example 1.

Claims: A process for preparing reflective graphite characterized in that it comprises : (a) preparing pyrolytic graphite by : positioning a suitable support in a furnace chamber and evacuating said furnace chamber to a pressure of 133 to 399 Pa (1 to 3 torr) ; introducing in the furnace chamber a short chain hydrocarbon while heating the furnace chamber to a temperature of 1850 - 2250°C and at a pressure of 133 to 666.5 ± 200 Pa (1 to 5 torr ± 1.5 torr); maintaining these conditions until a deposit of pyrolytic graphite on said support achieves a thickness of 0.1 to 0.6 cm and cooling to room temperature ; (b) positioning said support and pyrolytic graphite in a furnace chamber evacuating said furnace chamber to a pressure of about 133 Pa (1 torr) and increasing the temperature to 700-1000°C at a rate of 150 to 250°C, (c) increasing the pressure in the furnace chamber to 666.5 to 1333 Pa (5 to 10 torr) while increasing the temperature to 2600°C to 3300°C, (d) maintaining said temperature and pressure for 10 minutes to 2 hours, and (e) cooling and recovering the reflective graphite product. The process according to claim 1 wherein in step (a) the furnace chamber is heated at a rate of about 300°C/hr. The process according to claim 1 or 2, wherein the support is selected from the group consisting of graphite, tungsten metal and hafnium metal The process according to claim 1 or 2 wherein the hydrocarbon is selected from the group consisting of methane, ethane, propane and natural gas. The process according to claim 1 or 2 wherein the pyrolytic graphite deposit is 0.2 to 0.6 cm in thickness. The process according to claim 1 or 2 wherein, in step (b), the temperature is increased to about 900°C at the rate of about 200°C per hour. The process according to claim 1 or 2 wherein, in step (a) the temperature is increased to 2150°C. The process according to claim 1 or 2 wherein, in step (c) the temperature is increased to 2900°C to 2950°C. The process according to claim 1 or 2 wherein, in step (a) the hydrocarbon is methane and is fed to the furnace at a rate of about 1019.37 l/hour (36 standard cubic feet per hour). The process according to claim 1 or 2 wherein, in step (e) the furnace is cooled in a nitrogen atmosphere. A process for preparing reflective graphite on a graphite, tungsten or hafnium substrate which comprises the steps of : (a) positioning the said substrate in a furnace chamber ; (b) evacuating the furnace chamber to a pressure of about 133 Pa (1 torr) (c) heating the furnace chamber to about 2150°C at a rate of about 300°C per hours ; (d) passing methane into said furnace chamber at a rate of about 1019.37 l/hour (36 standard cubic feet per hour) and adjusting the pressure in said furnace chamber to 639.8 to 693.16 Pa (4.8 to 5.2 torr) ; (e) maintaining the temperature at about 2150°C, the methane flow at about 1019.37 l/hour (36 standard cubic feet per hour) and the pressure at about 639.8 to 693.16 Pa (4.8 to 5.2 torr) until pyrolytic graphite is deposited to a thickness of 0.1 to 0.6 centimeters ; (f) discontinuing the methane flow and heating and passing nitrogen through the furnace chamber until the temperature in the furnace is at room temperature and the pressure is at atmospheric, and recovering the substrate with the pyrolytic graphite deposited thereon ; (g) annealing said pyrolytic graphite structure by (1) evacuating said furnace chamber to a pressure of about 133 Pa (1 torr), (2) heating the furnace chamber to about 900°C at a rate of about 200°C per hour, (3) increasing the pressure in said furnace chamber by passing nitrogen into the furnace until the pressure reaches about 666.5 to 1333 Pa (5 to 10 torr), (4) maintaining the pressure at 666.5 to 1333 Pa (5 to 10 torr) while increasing the temperature to about 2900 to 2950°C and maintaining the temperature for about 1 hour, (5) cooling said furnace chamber to room temperature and recovering the reflective graphite structure. A process for impregnating rigidized fabric or felt with reflective graphite which comprises the steps of : (a) positioning said fabric or felt in a furnace chamber and evacuating said chamber to a pressure of about 133 Pa (1 torr), (b) heating the furnace chamber to about 900°C at a rate of about 300°C per hour, (c) passing hydrogen into said furnace chamber at a rate of about 424.74 1/hour (15 standard cubic feet per hour) while heating said furnace chamber, (d) when the temperature in said furnace chamber reaches 1090°C - 1100°C, passing methane into said furnace at a rate of about 849.48 1/hour (30 standard cubic feet per minute), and adjusting the pressure to 266.6 Pa (2 torr), (e) maintaining the methane flow at about 849.48 l/hour (30 standard cubic feet per hour), the hydrogen flow at about 424.74 l/hour (15 standard cubic feet per minute), the pressure at about 266.6 Pa (2 torr) and the temperature at 1090°C to 1100°C for a period of 24 hours, (f) after said 24 hours discontinuing methane flow while maintaining hydrogen flow and increasing the furnace temperature to 2100 - 2150°C at a rate of about 200°C per hour ; (g) after the temperature reaches 2100 - 2150°C discontinuing hydrogen flow and passing methane into said furnace chamber at a rate of 849.48 - 1132.64 1/hour (30 - 40 standard cubic feet per hour) ; (h) adjusting the pressure in said furnace chamber to 466.5 to 666.5 Pa (3.5 to 5 torr) and maintaining the temperature at 2100 to 2150°C, the methane flow at 849.48 - 1132.64 l/hour (30 - 40 standard cubic feet per hour) until 0.1 to 0.6 centimeters of graphite is impregnated into said cloth or felt, (i) discontinuing methane flow and heating allowing said furnace chamber to cool to room temperature and recovering the pyrolytic graphite impregnated fabric or felt product, annealing said pyrolytic graphite impregnated fabric or felt by : (1) evacuating said furnace chamber to a pressure of about 133 Pa (1 torr), (2) heating said furnace chamber to 700 - 1000°C at a rate of 150 to 250°C per hour, (3) increasing the pressure in said furnace chamber to 666.5 to 1333 Pa (5 to 10 torr) while continuing to heat said furnace chamber to 2900 to 2950°C (4) maintaining the furnace chamber temperature at 2900°C to 2950°C and furnace chamber pressure at 666.5 to 1333 Pa (5 to 10 torr) for about 1 hour ; (5) allowing the furnace chamber to cool to room temperature and atmospheric pressure and recovering the reflective graphite impregnated fabric or felt product.

Inventors: GEBHARDT, JOSEPH JOHN, MULVEY, ROBERT FARRELL, YODSNUKIS, JOHN JOSEPH
Applicants: GENERAL ELECTRIC COMPANY
Application year: 1988
Main classification: C23C16
IPC codes: C23C16, C01B31, C04B35