METAL NITRIDE POWDERS

Abstract: A metal nitride powder can be made by heating a reactant powder that includes an oxide or hydroxide of Al, Ti, or Zr to a reaction temperature in a nonreactive atmosphere. The heated reactant powder is contacted with a gaseous reactant mixture comprising a nitrogen source and a carbon source. The molar ratio of nitrogen to carbon in the gaseous reactant mixture is at least about 15. The reactant powder is maintained at the reaction temperature for a sufficient time to convert a portion of it to metal nitride powder.

Description: Description Metal Nitride Powders Technical Field The present invention is directed to a method for making metal nitride powder. Background Art In recent years, there has been increasing interest in non-oxide ceramics, such as metal nitrides and carbides, that possess high temperature strength and corrosion resistance. Among these materials, aluminum nitride (AIN) is especially important because of its unique physical properties. For example, AIN has a thermal conductivity close to that of metals and more than 10 times that of alumina (Al 2 O 3 ), a coefficient of thermal expansion comparable to silicon and silicon carbide, a high electrical resistivity, and mechanical strength comparable to alumina ceramics. Metal nitride powders can be made in various ways. For example, a metal oxide powder, such as Al 2 O 3 , zirconia (ZrO 2 ), or titania (TiO 2 ), can be mixed with an excess of a carbonaceous powder and heated to a temperature above 1100°C in a nitrogen-containing atmosphere. The metal nitride powder formed by this method is, however, mixed with unreacted carbonaceous powder that detracts from the properties of the metal nitride powder. The unreacted carbonaceous powder can be removed by oxidizing it at temperatures between about 600 °C and about 700 °C. At these temperatures, however, a portion of the metal nitride powder also can oxidize. United States Patent 4,975,260 to Imai et al. teaches an alternate method for making a metal nitride powder by reacting a metal oxide or metal hydroxide powder with a gaseous mixture of ammonia (NH 3 ) and a hydrocarbon at a temperature ranging from 1300°C to 1600°C. Although this method is an improvement over some prior art methods, it still leaves residual carbon in the metal nitride product. Moreover, it requires a temperature of at least 1300°C. As with any synthesis process, it is desirable to keep energy consumption as low as possible. Therefore, what is needed in the industry is a method of making metal nitride powder at a relatively low temperature. Disclosure of the Invention The present invention is directed to a method of making metal nitride powder at a relatively low temperature. One aspect of the invention includes a method of making a metal nitride powder. A reactant powder comprising an oxide of Al, Ti, or Zr or a hydroxide of Al, Ti, or Zr is heated to a reaction temperature in a nonreactive atmosphere. The heated reactant powder is contacted with a gaseous reactant mixture comprising a nitrogen source and a carbon source. The molar ratio of nitrogen to carbon in the gaseous reactant mixture is at least about 15. The reactant powder is maintained at the reaction temperature for a sufficient time to convert a portion of it to metal nitride powder. Another aspect of the invention includes AIN, TiN, or ZrN powders made by the method described above. These and other features and advantages of the present invention will become more apparent from the following description and accompanying drawing. Brief Description of the Drawing Fig. 1 is a schematic of an apparatus used to make the powder of the present invention. Fig. 2 is an x-ray diffraction pattern for an AIN powder of the present invention formed by reacting reactants at 1050°C for 3 hr. Fig. 3 is an x-ray diffractiφn pattern for a TiN powder of the present invention formed by reacting reactants at 950 °C for 3 hr. Fig. 4 is an x-ray diffraction pattern for a ZrN powder of the present invention formed by reacting reactants at 1100°C for 9 hr. Best Mode for Carrying Out the Invention The method of the present invention can make AIN, TiN, ZrN, or YN powders at relatively low temperatures. The starting materials for the nitride powders include oxide or hydroxide powders of Al, Ti, Zr, or Y, such as Al 2 O 3 , TiO 2 , ZrO 2 , Y 2 O 3 , Al(OH) 3 , Ti(OH) 4 , Zr(OH) 4 , or Y(OH) 3 . The remainder of the application describes the method of the present invention in terms of making AIN powder from Al 2 O 3 powder. One skilled in the art, however, will understand that the following description, with appropriate adjustments to reaction temperature, applies equally to methods of making AIN, TiN, ZrN. and YN from Al, Ti, Zr, and Y oxides or hydroxides. To make AIN powder by the method of the present invention, Al 2 O 3 powder is contacted with a gaseous reactant mixture at suitable reaction conditions. The gaseous reactant mixture comprises a nitrogen source and a carbon source. If NH 3 is the nitrogen source and CH 4 is the carbon source, the reaction can be written as: Al 2 O 3 (s) + 2 NH 3 (g) + 3 CH 4 (g) → 2 AIN (s) + 3 CO (g) + 9 H 2 (g) The Al 2 O 3 powder may be any high surface area Al 2 O 3 powder, such as γ-Al 2 O 3 or Al 2 O 3 made with a sol gel method. Suitable 7-Al 2 O 3 may be purchased from commercial sources, including Alfa Products (Danvers, MA). The size of the Al 2 O 3 powder used with the present invention is not critical. Preferably, the powder will be of suitable size to permit it to react completely with the gaseous reactant mixture to form AIN powder. Sol gel Al 2 O 3 may be made as described in commonly-owned U.S. Application 07/991,929, filed on 17 December 1992. Aluminum isopropoxide [Al(O-/-CjH 7 ) 3 ] may be dispersed in water, for example water heated to about 75°C, in any suitable molar ratio of Al(O-/-C 3 H 7 ) 3 to water to make a sol. For example, the molar ratio may range from about 1:100 to about 1:1000. Suitable Al(O-/-CjH 7 ) 3 may be purchased from commercial sources, including Alfa Products. A small amount of HNO 3 or other acid may be added to the sol to bring its initial pH to about 3 to initiate reaction. The acidified sol may be allowed to sit until it is sufficiently viscous, for example about 12 hr, to make a gel. The gel may be formed into a powder by any conventional method such as heating, freeze drying, vacuum drying, or spray drying. After forming the powder, the powder may be allowed to dry for a sufficient time to partially convert it to Al 2 O 3 and to allow it to be handled more easily. The powder may then be fired for a sufficient time to convert it to Al 2 O 3 . For example, the powder may be fired at about 500°C for about 30 min in a He atmosphere. The powder also may be fired in air. The nitrogen source may be any reactive nitrogen compound that is a gas at the reaction conditions. For example, the nitrogen source may be NH 3 , N 2 H 2 , or N 2 . Anhydrous NH 3 is the preferred nitrogen source because it is readily available and easy to work with. NH 3 may be purchased from many suppliers, including Aero All Gas Company (Hartford, CT). Preferably, the nitrogen source will not contain any contaminants that would produce side reactions or otherwise interfere with the nitridation reaction. The carbon source should be a carbon-containing compound that is a gas at the reaction conditions. For example, the carbon source may be a hydrocarbon or an amine. Although any hydrocarbon that is gaseous at reaction conditions may be used, alkanes having four or fewer carbon atoms are preferred because they are easier to handle. Similarly, amines having four or fewer carbon atoms, such as methyl amine (CH 3 NH 2 ), are preferred. Most preferably, the carbon source will be CH 4 because it is easy to obtain and use. CH 4 may be purchased from many suppliers, including Aero All Gas Co. Preferably, the carbon source will not contain any contaminants that would produce side reactions or otherwise interfere with the nitridation reaction. The nitrogen and carbon sources in the gaseous reactant mixture may be supplied from separate sources or from a premixed source. In either case, it is preferable that they be mixed upstream of the reactor. The ratios of N:Al 2 O 3 and C:Al 2 O 3 in the reactor are not critical, although adequate amounts of gaseous reactants should be used to achieve a desired conversion in a desired time. The ratio of nitrogen to carbon (N:C) in the gaseous reactant mixture is critical to obtaining a substantially carbon-free product. To form such a product, the molar ratio of N:C in the gaseous reactant mixture should be at least about 13. Preferably, the N:C ratio will be between about 15 and about 2,000. Most preferably, the N:C ratio will be between about 30 and about 40. If desired, H 2 may be added to the gaseous reactant mixture to expand the range of conditions under which substantially carbon-free AIN powder can be made. Any excess of H 2 will facilitate the conversion of the oxide or hydroxide to the nitride. H 2 may be obtained from many commercial suppliers. The reaction conditions may be any conditions suitable for converting the Al 2 O 3 powder to AIN. In general, the desired reaction to AIN will occur at temperatures of at least about 850 °C. Complete conversion to AIN can be obtained at temperatures greater than about 1000°C. In the context of this application, complete conversion means the formation of substantially carbon-free product. The reaction temperature, therefore, may range from about 850°C to about 1275°C or 1299°C. Preferably, the reaction temperature will be about 1000°C to about 1275°C and, most preferable, about 1000°C to about 1100°C. The reaction pressure is not critical and may be any conveniently obtainable pressure. To make TiN, TiO 2 powder can be reacted with a gaseous reactant mixture at temperatures of at least about 750°C. Complete conversion to TiN can be obtained at temperatures greater than about 800°C. Preferably, the reaction temperature will be about 800°C to about 1275°C or 1299°C. ZrO 2 powder can be reacted with a gaseous reactant mixture at temperatures of at least about 1050 C C to make ZrN. Complete conversion to ZrN can be obtained at temperatures greater than about 1100°C. Preferably, the reaction temperature will be about 1100°C to about 1275°C or 1299°C. Fig. 1 is a schematic of an apparatus used to make the powder of the present invention. The apparatus has a carbon source cylinder 2, nitrogen source cylinder 4, and inert gas cylinder 6. The reaction takes place in a reactor 18, such as a quartz tube reactor. A suitable amount of Al 2 O 3 powder is placed into the reactor 18, which is positioned inside furnace 20. The Al 2 O 3 powder in the reactor 18 is heated to the reaction temperature at a moderate rate, for example, about 11.5°C/min. The actual rate of heating is not critical. The temperature in the reactor 18 can be controlled with a thermocouple 22 and a temperature controller 24. Preferably, the Al 2 O 3 powder will be preheated in a nonreactive atmosphere so no reactions occur until it reaches the reaction temperature. For example, the powder may be preheated in NH 3 , N 2 , H 2 , or an inert gas such as He, Ne, Ar, Kr, or Xe. The inert gas can be stored in the inert gas cylinder 6 and flowed into the reactor 18 through a molecular sieve 14, flow meter 16, and gas mixing chamber 12. Once the Al 2 O 3 is at the reaction temperature, the gaseous reactant mixture is flowed into the reactor 18 at a rate sufficient to achieve the desired N:C ratio. The gaseous reactants can be stored in the carbon source cylinder 2 and nitrogen source cylinder 4. They can be flowed into the reactor 18 through a molecular sieve 8, flow meter 10, and gas mixing chamber 12. Effluent from the reactor 18 flows through exhaust gas scrubbers 26 before being vented. Reaction conditions are maintained for a sufficient time to convert a desired amount of the Al 2 O 3 powder to AIN powder. Preferably, the time will be sufficient to obtain 100% conversion. For example, reaction conditions may be maintained for 1 hr to 2 hr or for up to more than 9 hours. Longer times may be necessary with lower temperatures. After the desired conversion is obtained, the gaseous reactant mixture is shut off and a nonreactive atmosphere is established in the reactor. The reactor and product are then cooled at a convenient rate. The following examples demonstrate the present invention without limiting the invention's broad scope. Example 1 (AIN) About 0.04 g of 7-Al 2 O 3 powder (Alfa Products, Danvers, MA) was weighed and placed in a quartz boat. The powder had an average particle size between about 1 μm and about 20 μ . The quartz boat containing Al 2 O 3 powder was placed in a 2.5 cm diameter quartz reactor. The powder was heated in flowing NH 3 to a reaction temperature of 1050°C at a rate of 11.5°C/min. The NH 3 was flowing at 400 mL/min. Once the reaction temperature was reached, CH 4 was flowed into the reactor at 30 mL/min. This provided a N:C ratio in the gaseous reactant mixture of 13.33. Reaction conditions were maintained for 9 hr after which the flow of CH 4 was stopped and the powder was cooled. X-ray diffraction showed the powder was completely converted to AIN. X-ray photoelectron spectroscopy showed the powder was substantially carbon-free. Example 2 (AIN) Example 1 was repeated with a NH 3 flow rate of 400 mL/min and a CH 4 flow rate of 18 mL/min to provide a N:C ratio of 22.22. X-ray analysis showed complete conversion to AIN. Example 3 (AIN) Example 1 was repeated with a NH 3 flow rate of 387 mL/min and a CH 4 flow rate of 11 mL/min to provide a N:C ratio of 35.19. Reaction conditions were maintained for 1 hr. X-ray analysis showed that some AIN was formed. Example 4 (AIN) Example 1 was repeated with a NH 3 flow rate of 395 mL/min and a CH 4 flow rate of 13 mL/min to provide a N:C ratio of 29.15. Reaction conditions were maintained for 3 hr. X-ray analysis showed complete conversion to AIN. Example 5 (AIN) Example 1 was repeated with a NH 3 flow rate of 407 mL/min and a CH 4 flow rate of 11 mL/min to provide a N:C ratio of 37.00. Reaction conditions were maintained for 6 hr. X-ray analysis showed complete conversion to AIN. Example 6 (AIN) Example 1 was repeated with a NH 3 flow rate of 394 mL/min and a CH 4 flow rate of 11 mL/min to provide a N:C ratio of 35.82. Reaction conditions were maintained for 12 hr. X-ray analysis showed complete conversion to AIN. Example 7 (AIN) Example 1 was repeated with a NH 3 flow rate of 387 mL/min and a CH 4 flow rate of 11 mL/min to provide a N:C ratio of 35.18. Reaction conditions were maintained for 18 hr. X-ray analysis showed complete conversion to AIN. Example 8 (AIN) Example 1 was repeated with a reaction temperature of 1000°C, a NH 3 flow rate of 402 mL/min, and a CH 4 flow rate of 12 mL/min. The N:C ratio was 33.50. X-ray analysis showed partial conversion to AIN. Example 9 (AIN) Example 1 was repeated with a reaction temperature of 950°C, a NH 3 flow rate of 391 mL/min, and a CH 4 flow rate of 12 mL/min. The N:C ratio was 32.58. X-ray analysis showed partial conversion to AIN. Example 10 (AIN) Example 1 was repeated with a reaction temperature of 850°C, a NH 3 flow rate of 406 mL/min, and a CH 4 flow rate of 11 mL/min. The N:C ratio was 36.91. X-ray analysis showed some AIN, but less than in Example 9. Table 1 summarizes the results from Examples 1-10. I/I is the ratio of x-ray reflection intensities from the (100) plane of AIN to the reflection intensities from the (100) plane of Al 2 O 3 . An I/I of 0 indicates no conversion to AIN. An I/I of oo indicates complete conversion. Table 1 Example 11 (TiN) About 0.05 g TiO 2 powder made from titanium isopropoxide [Ti(O-/-C 3 H 7 ) 2 ] by a sol gel process was weighed and placed in a quartz boat as in Example 1. The powder was heated in flowing He to a reaction temperature of 1150°C at a rate of 11.5°C/min. Once the reaction temperature was reached, NH 3 and CH 4 were flowed into the reactor. The flow rates of the NH 3 and CH 4 were 400 mL/min and 11.5 mL/min, respectively. This provided a N:C ratio of 35. Reaction conditions were maintained for 3 hr, after which the flows of NH 3 and CH 4 were stopped and the powder was cooled in He. X-ray diffraction showed the powder was completely converted to TiN. Examples 12-15 (TiN) Example 11 was repeated with reaction temperatures of 1050°C, 950°C, 850°C, and 750°C. X-ray diffraction showed complete conversion at 1050°C, 950°C, and 850°C. Partial conversion was observed at 750°C. Table 2 summarizes the results from Examples 11-15. I/I is the ratio of x-ray reflection intensities from the (101) plane of TiN to the reflection intensities from the (101) plane of TiO 2 . An I/I of 0 indicates no conversion to TiN. An I/I of oo indicates complete conversion. Table 2 Example 16 (ZrN) About 0.03 g of ZrO 2 powder made from zirconyl nitrate [ZrO(OH)NO 3 ] was weighed and placed in a quartz boat as in Example 1. The powder was heated in flowing He to a reaction temperature of 1100° C at a rate of 11.5°C/min. Once the reaction temperature was reached, NH*, and CH 4 were flowed into the reactor. The flow rates of the NH 3 and CH 4 were 400 mL/min and 11 mL/min, respectively. This provided a N:C ratio of 36.7. Reaction conditions were maintained for 9 hr, after which the flows of NH 3 and CH 4 were stopped and the powder was cooled in He. X-ray diffraction showed the powder was completely converted to ZrN. The method of the present invention improves over the prior art by allowing substantially carbon-free metal nitride powder to be made at temperatures less than 1300°C. The powder of the present invention can be used for any application known in the art, for example, as a precursor for a thermal spray coating. The invention is not limited to the particular embodiments shown and described herein. Various changes and modifications may be made without departing from the spirit or scope of the claimed invention. We claim:

Claims: Claims 1. A method of making AIN powder, comprising the steps of: (a) heating a reactant powder comprising an aluminum oxide or an aluminum hydroxide to reaction temperature between 850°C and 1299°C in a nonreactive atmosphere, (b) contacting the heated reactant powder with a gaseous reactant mixture comprising a nitrogen source and a carbon source, wherein the gaseous reactant mixture has a molar ratio of nitrogen to carbon of at least about 15, and (c) maintaining the reactant powder at the reaction temperature for a sufficient time to convert a portion of the reactant powder to AIN powder. 2. The method of claim 1 , wherein the nitrogen source comprises NH 3 or N 2 H 2 . 3. The method of claim 1, wherein the carbon source comprises a hydrocarbon that is a gas at the reaction temperature or an amine that is a gas at the reaction temperature. 4. The method of claim 1, wherein the reactant powder comprises is γ-Al 2 O 3 . 5. The method of claim 1 , wherein the reactant powder comprises sol gel-derived Al 2 O 3 . 6. The method of claim 1, wherein the reaction temperature is about 1000°C to about 1275°C and the reactant powder is converted to substantially carbon-free AIN. 7. The method of claim 1, wherein the molar ratio of nitrogen to carbon in the gaseous reactant mixture is about 30 to about 40. 8. The method of claim 1, wherein the reactant powder is maintained at the reaction temperature for at least about 3 hr. 9. The method of claim 1, wherein the reaction temperature is between about 1000°C and about 1100°C, the nitrogen source is NH 3 , the carbon source is CH 4 , the molar ratio of carbon to nitrogen is between about 30 and about 40, the reactant powder is maintained at the reaction temperature for at least about 3 hr, and the reactant powder is converted to substantially carbon-free AIN powder. 10. AIN powder made by the method of claim 1 , wherein the reaction temperature is about 1000°C to about 1299°C and the AIN is substantially carbon-free. 11. A method of making TiN powder, comprising the steps of: (a) heating a reactant powder comprising a titanium oxide or a titanium hydroxide to a reaction temperature of at least about 750°C in a nonreactive atmosphere, (b) contacting the heated reactant powder with a gaseous reactant mixture comprising a nitrogen source and a carbon source, wherein the gaseous reactant mixture has a molar ratio of nitrogen to carbon of at least about 15, and (c) maintaining the reactant powder at the reaction temperature for a sufficient time to convert a portion of the reactant powder to TiN powder. 12. The method of claim 11, wherein the nitrogen source comprises NH 3 or N 2 H 2 . 13. The method of claim 11, wherein the carbon source comprises a hydrocarbon that is a gas at the reaction temperature or an amine that is a gas at the reaction temperature. 14. The method of claim 11, wherein the reaction temperature is greater than about 800°C and the reactant powder is converted to substantially carbon-free TiN. 15. TiN powder made by the method of claim 14, wherein the TiN powder is substantially carbon-free. 16. A method of making ZrN powder, comprising the steps of: (a) heating a reactant powder comprising a zirconium oxide or a zirconium hydroxide to a reaction temperature of at least about 1050°C in a nonreactive atmosphere, (b) contacting the heated reactant powder with a gaseous reactant mixture comprising a nitrogen source and a carbon source, wherein the gaseous reactant mixture has a the molar ratio of nitrogen to carbon of at least about 15, and (c) maintaining the reactant powder at the reaction temperature for a sufficient time to convert a portion of the reactant powder to ZrN powder. 17. The method of claim 16, wherein the nitrogen source comprises NH 3 or N 2 H 2 . 18. The method of claim 16, wherein the carbon source comprises a hydrocarbon that is a gas at the reaction temperature or an amine that is a gas at the reaction temperature. 19. The method of claim 16, wherein the reaction temperature is greater than about 1100°C and the reactant powder is converted to substantially carbon-free ZrN powder. 20. ZrN powder made by the method of claim 19, wherein the ZrN powder is substantially carbon-free.

Inventors: GALASSO FRANCIS S, SUIB STEVEN LAWRENCE, TAN BENG JIT, XIAO YOUMING, GALASSO, FRANCIS, S., SUIB, STEVEN, LAWRENCE, TAN, BENG, JIT, XIAO, YOUMING
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1993
Main classification: C01B21
IPC codes: C01B21