Polymer precursors for aluminum nitride

Abstract: An aluminum-nitrogen polymer is prepared by reacting an organic nitrile with a dialkylaluminum hydride to form an organoaluminum imine, and heating the imine to a temperature of 50° to 200°C. The polymeric product can be subjected to an additional heat treatment to form a more highly cross-linked polymer. After either heat treatment the polymeric product can be further reacted with a primary amine or ammonia. The organoaluminum imine as well as the aminated or nonaminated polymers can be pyrolyzed in a nonoxidizing atmosphere to form an aluminum nitride-containing ceramic.

Description: This invention relates to polymer precursors for aluminum nitride. Aluminum nitride (AlN) is a refractory material melting at 2400°C, which exhibits several unique chemical and physical properties, e.g., it has a density of 3.26 g/cm³, a Young's modulus of 280 GPa, a flexural strength of 400 MPa and a Knoop hardness of 1200 kg/mm². AlN is very stable in the presence of molten metals and therefore can be used, for example, for making crucibles to hold molten metal. Aluminum nitride is also an electrical insulator with a bandgap of 6.2 electron volts, which makes it an attractive alternative substrate material to replace alumina and beryllia in electronic packaging. The thermal expansion coefficient of AlN is nearly identical to that of silicon. This is an important property in high power applications where thermal distortion can occur between a silicon chip and the substrate due to a mismatch in the coefficients of thermal expansion of the two materials. The thermal conductivity of aluminum nitride is nearly ten times higher than alumina and approximately equal to that of beryllia. Unlike beryllia, aluminum nitride is not restricted by processing constraints because of its toxicity. There is currently a great deal of interest in polymer precursor materials that can be pyrolyzed to yield ceramic materials, including aluminum nitride. For example, reacting an organic nitrile with diisobutylaluminum hydride produced organoaluminum imines having the formula RCH=NAl(i-C₄H₉)₂, which were not isolated (L. I. Zakharkin and I.M. Khorlina, Bull, Acad. Sci. USSR , Engl . Transl ., 1959, 523-524 and Proc. Acad. Sci. USSR , 1957, 116, 879). A gas containing 85% isobutene and polymers having the repeating unit were produced on heating the organoaluminum imine to 220° to 240°C. During the formation of the polymer, both aluminum alkyl groups of the organoaluminum imine are eliminated as isobutene, and aluminum-nitrogen bonds are formed. European Patent Application 331,448 discloses that AlN can be deposited on a substrate by heating the substrate and contacting it with the vapor of an aluminum-nitrogen compound having the formula CH₃(R¹)Al-N(R²)(C₃H₇), where R¹ is alkyl and R² is H, alkyl or aryl. A polymer of this compound is claimed, but the structure of the polymer is not disclosed. A method has now been found for producing aluminum-nitrogen polymers in which at least some of the repeating units contain alternating aluminum and nitrogen atoms wherein each aluminum and nitrogen atom is substituted by an organic group. The process of this invention for preparing an aluminum-nitrogen polymer by reacting an organic nitrile with a dialkylaluminum hydride to form an organoaluminum imine and then heating the imine is characterized by (a) reacting an organic nitrile having the formula RCN, where R is a 1-12 carbon alkyl, cycloalkyl or aryl group, with a dialkylaluminum hydride having the formula R'R''AlH, where R' and R'' are the same or different 1-12 carbon alkyl groups, to form an organoaluminum imine and (b) heating the organoaluminum imine to a temperature of 50° to 200°C. The polymeric reaction product can be subjected to an additional heat treatment at 200° to 400°C to form a different, presumably more highly crosslinked, polymer. After either heat treatment the polymeric product can be further reacted with a primary amine or ammonia. Also, according to the invention, the organoaluminum imine as well as the aminated or nonaminated polymers can be pyrolyzed to form an aluminum nitride-containing ceramic. A single nitrile or a mixture of nitriles can be used in step (a) of the process of this invention. Suitable nitriles include, for example, acetonitrile, propionitrile, butyronitrile, benzonitrile, phthalonitrile, isophthalonitrile, terephthalonitrile, 1,4-dicyano-2-butene, dicyandiamide, adiponitrile and acrylonitrile. The organoaluminum imine formed by the reaction of the organic nitrile and the dialkylaluminum hydride is typically a dimer that contains a heterocyclic core and has the structure Depending on the nature of the substituents R, R' and R'', the imine can also be in the form of a monomer or a higher cyclic oligomer. The exact form of the imine is dictated by the steric and electronic properties of the substituents. In the reactions described below, the notation for labeling the carbon atoms bonded to Al atoms will be the following: Likewise, a hydrogen atom bonded to a Cα carbon atom will be called an α-hydrogen atom. An aluminum-nitrogen polymer is formed by heating the organoaluminum imine to a temperature of from about 50° to about 200°C with evolution of gaseous hydrocarbon products. When a dialkylaluminum hydride having a methyl group is used, the gaseous hydrocarbon products are alkanes. When a dialkylaluminum hydride having alkyl groups with two or more carbon atoms is used, both alkane and olefin resulting from thermal decomposition of the aluminum fragment of the organoaluminum imine are evolved to form a polymer. The following units comprise the major components of the polymer, although small amounts of other components can also be present: where R has the same meaning as described above. R² is an unreacted alkyl group R' or R'' on aluminum, introduced as part of the dialkylaluminum hydride. R³ is an organic group derived from R² in the process of heating the organoaluminum imine, e.g., the formation of isobutenyl groups from isobutyl groups in Path A of the reaction scheme illustrated below. The values of x and y depend upon the time of heating, the temperature of heating, and the structure of the aluminum hydride reactant used. The heating can be carried out with or without a solvent, although it is preferably carried out without a solvent. While not wishing to be bound by theory, the process of gas evolution in systems containing a γ-hydrogen atom on one of the organic groups bonded to aluminum is believed to proceed by the following mechanism, using a diisobutyaluminum imine as an example: In this mechanism the coordinated imine in the starting dimer (1) is reduced in the process of β-hydride migration from a neighboring isobutyl group as shown in intermediate (2) in Path A. The imine reduction is accompanied by transfer of an acidic γ-hydrogen from the isobutyl group involved in imine reduction to the other aluminum-bonded isobutyl group. Six membered ring intermediates appear to be involved. Loss of alkane results in an isobutenyl intermediate (3). Thermal polymerization to give polymer (4) then occurs. In the case of diisobutylaluminum hydride, the butenyl groups formed as a result of the elimination reaction shown in Path A further react to give C₈ alkyl groups and higher. This reaction is confirmed by polymer hydrolysis/mass spectrometric studies, which detect isobutylene and C₈ hydrocarbons as decomposition products of hydrolysis. In the case of methyl and ethyl substituents where no γ-hydrogen is present, it is presumed that α- or β-hydride transfer results in loss of alkane and formation of highly reactive intermediates that cannot be detected but that quickly react either intramolecularly or intermolecularly to give R³ moieties. Alternatively, β-hydride migration to reduce the coordinated imine can be accompanied by loss of alkene if Al-Cα bonds are broken in the process of β-hydride migration (5) (Path B). Heating the organoaluminum imine at 50° to 200°C also initiates partial crosslinking of the polymer that is formed as indicated by a substantial increase in the viscosity, e.g., from 15 to over 750,000 cps (MPa) measured at 30°C for the case where acetonitrile is reacted with diisobutylaluminum hydride. The viscosity of the aluminum-nitrogen polymer can therefore be controlled by varying the time and temperature of the heating cycle. Low temperatures and short heating times produce low viscosity polymers, while higher temperatures and longer heating times produce extremely viscous or solid polymers. For most applications, a material that is soluble or fusible is preferred. Further heating of the aluminum-nitrogen polymer at 200° to 400°C results in evolution of gaseous alkane and further evolution of gaseous olefin when 2-12 carbon alkyl groups are present. New Al-N bonds are formed to give crosslinked sites in the polymer that contain Al atoms bonded to three, rather than two, nitrogen atoms. In the polymer that is produced by this additional heating step, at least some of the units are the same as those described above. The aluminum-nitrogen polymer formed below 200°C, as well as the polymer formed by subsequently heating the first polymer to above 200°C, can be further dissolved in aprotic organic solvents such as hexane, toluene, xylene, or diethyl ether and treated with ammonia or a primary amine, R⁴NH₂, where R⁴ is a 1-12 carbon alkyl, cycloalkyl or aryl group, for a time sufficient to introduce HN- or R⁴N- groups into the polymer by transamination with concomitant release of RCH₂NH₂ or by aluminum alkyl bond cleavage. The aluminum-nitrogen polymer can also be treated with a mixture of ammonia and a primary amine. In the polymer produced upon treatment of the aluminum-nitrogen polymer with ammonia or a primary amine, at least some of the units comprise the following: where R, R², and R³ have the same meaning as described above. R⁵ is either H or R⁴. The values of a, b, c and d depend upon the original values of x and y as well as the amount of ammonia or primary amine added. Typically the aminated polymers are solids rather than viscous liquids and have a higher ceramic yield on pyrolysis compared to the nonaminated polymers. The aluminum-nitrogen polymers of this invention that contain an organic radical having a crosslinkable group, e.g., those derived from a nitrile such as acrylonitrile, can be crosslinked by supplying energy to generate free radicals. For example, the aluminum imine can be treated prior to or during heating with a radical source such as a peroxide. The aluminum imine can also be crosslinked by exposing the imine to UV light or electon beam radiation prior to or during heating. Alternatively, the crosslinkable group can be introduced by reaction of the aluminum-nitrogen polymer with an amine containing a crosslinkable group, e.g., allylamine. An aluminum nitride-containing ceramic is produced by pyrolysis of any of the materials listed below in a non-oxidizing atmosphere such as nitrogen or ammonia. Pyrolysis in nitrogen is carried out at 800° to 2200°C, preferably 1200° to 2000°C, and most preferably 1400° to 1700°C. Pyrolysis in ammonia can be accomplished at a temperature as low as 600°C. a. The organoaluminum imine formed by treating an organic nitrile with a dialkylaluminum hydride. b. The polymer formed by heating the organoaluminum imine of (a) at 50° to 200°C. c. The polymer of (b) that has been further heated to a temperature between 200° and 400°C. d. The polymer formed by treating the polymer of (b) with a primary amine or ammonia. e. The polymer formed by treating the polymer of (c) with a primary amine or ammonia. Aluminum nitride can be deposited on a substrate from the vapor phase by contacting the organoaluminum imine (a) with a substrate heated to the pyrolysis temperature. The aluminum nitrogen polymers can be used in the preparation of ceramic fibers, films, coatings, and foams; in the infiltration of a preform structure and subsequent pyrolysis to produce a composite aluminum nitride-containing structure, as a thin film for electronic applications or as a binder for ceramic or metal powders. Example 1 A 250 ml Schlenk round bottom flask is fitted with a pressure equalized dropping addition funnel and purged. Acetonitrile (50 ml, 946 mmol) is added to the flask. The funnel is charged with diisobutylaluminum hydride (100 ml, 1.0 M in toluene, 100 mmol) and the flask is cooled to 0°C. The diisobutylaluminum hydride is added dropwise over thirty minutes and stirred at 0°C for an additional hour. The flask is warmed to room temperature and the colorless solution is stirred overnight. After about twelve hours a bright yellow color develops. The solvent is removed under vacuum, leaving 18 g of the aluminum imine [CH₃C(H)=N-Al(i-C₄H₉)₂]₂. The product is vacuum distilled at 87°C at 0.467 mbar (0.35 torr). The product is characterized by proton NMR and IR spectroscopy. The aluminum imine (5.0 g) is heated at 180° to 200°C for eight hours. The gases evolved during the heating are shown by gas chromatography to be 72% isobutane and 28% isobutene. Throughout the heating the bright yellow color of the imine darkens toward a red-orange hue. The yield of polymer is 3.5 g. The viscosity of the liquid polymer is 52,000 cps (MPa) at 30°C measured with a Brookfield cone and plate viscometer. The pyrolysis yield is 14.1% measured by thermogravimetric analysis (25°-1000°C, 20°C/min., N₂). Within the carbon-13 NMR spectrum, peaks are observed at δ=39(CH₂) and δ=29(CH₃) characteristic of CH₃CH₂N-groups. Example 2 The polymer prepared as described in Example 1 (9.3 g) is dissolved in hexane (50 ml). Ammonia (0.95 l) is bubbled through the solution at room temperature for 9 minutes. The solution is stirred overnight. The solvent is removed under vacuum leaving 8.6 g of an orange solid product. The pyrolysis yield of this polymer is 32.8% measured by thermogravimetric analysis (25°-1000°C, 20°C/min., N₂). Infrared and NMR spectroscopy confirm the presence of CH₃CH₂N- as well as HN- groups on the polymer. The polymeric product is pyrolyzed by heating in nitrogen from room temperature to 1500°C at the rate of 10°C/min. and holding at 1500°C for two hours to form crystalline AlN. The nature of the product is confirmed by X-ray diffraction. Example 3 The polymeric product prepared as described in Example 1 is pyrolyzed by heating in nitrogen from room temperature to 1500°C at the rate of 10°C/min. and holding at 1500°C for two hours to form crystalline AlN. The nature of the product is confirmed using X-ray diffraction. Example 4 The polymeric product prepared as described in Example 1 is pyrolyzed by heating in ammonia from room temperature to 600°C at the rate of 10°C/minute and holding at 600°C for two hours to form AlN. The nature of the product is confirmed using X-ray diffraction. Example 5 The polymer prepared as described in Example 1 (10.2 g) is dissolved in hexane (50 ml), and n-propylamine (10 ml, 121 mmol) is slowly added at 0°C. The solution is warmed to room temperature and stirred for 16 hours. The solvent is removed under vacuum to yield 9.5 g of a viscous liquid. The viscosity of the liquid polymer is 82,000 cps (MPa) at 29°C measured with a Brookfield cone and plate viscometer. The pyrolysis yield of this polymer is 38.9% measured by thermogravimetric analysis (25°-1000°C at 20°C/min., N₂). The polymeric product is then pyrolyzed by heating in nitrogen from room temperature to 1500°C at the rate of 10°C/min. and holding at 1500°C for two hours to form crystalline AlN. The nature of the product is confirmed using X-ray diffraction. Example 6 The structure of the polymer units (4) and (7) in the reaction scheme described in the "Detailed Description of the Invention" is verified as follows. A sample of the polymer prepared as described in Example 1 (0.17 g) is charged to a glass vessel under argon. Degassed distilled water (4 ml) is added. The vessel is sealed to allow the gas headspace to be sampled. Gas evolution from the polymer is apparent. Gas chromatography/mass spectrometry reveals isobutane, isobutene and a C₈ hydrocarbon in the gas headspace. The presence of isobutane is evidence that unreacted alkyl (R²) is present on the aluminum atom in polymer unit (7). The presence of isobutene as well as C₈ hydrocarbon is evidence that unsaturated alkyl (R³) is present on the aluminum atom in polymer unit (4). The aqueous phase is shown by proton nuclear magnetic resonance spectroscopy to contain ethylamine as the only amine species present. Example 7 A 500 ml Schlenk round bottom flask is fitted with a pressure equalized dropping addition funnel and purged. Benzonitrile (19.2 ml, 188 mmol) and toluene (50 ml) are added to the flask. The funnel is charged with diisobutylaluminum hydride (150 ml, 1.0 M in toluene, 150 mmol) and the flask is cooled to 0°C. The diisobutylaluminum hydride is added dropwise over 90 minutes, stirred at 0°C for an additional 30 minutes, warmed to room temperature and stirred for sixteen hours. After the addition a clear yellow solution results. The solvent is removed under vacuum. The yield is 34.1 g of an aluminum imine compound [C₆H₅C(H)=N-Al(i-C₄H₉)₂]₂. The aluminum imine compound prepared as described above (32.2 g) is heated to 180° to 200°C under nitrogen for eight hours. The volatile products evolved during the heating are shown by gas chromatography to contain 44% isobutane and 56% isobutene. During the heating the yellow color of the imine darkens to a red-brown hue. The resulting polymer is a viscous liquid at room temperature. The yield of polymer is 25.8 g. Within the carbon-13 NMR spectrum, peaks are observed at δ=39(CH₂ group) and δ=127(C₆H₅ group) characteristic of C₆H₅CH₂N- groups. The viscosity of the liquid polymer is 148,000 cps (MPa) measured at 34°C with a Brookfield cone and plate viscometer. The pyrolysis yield is 35.7% as measured by thermogravimetric analysis (25°-600°C, 20°C/min., N₂). Example 8 The polymer product prepared as described in Example 5 is pyrolyzed by heating in nitrogen from room temperature to 1500°C at a rate of 10°C/min. and holding at 1500°C for two hours to form crystalline AlN. The nature of the product is determined by X-ray diffraction. Example 9 The polymer prepared as described in Example 7 (10.9 g) is dissolved in 50 ml hexane. The solution is treated with ammonia (0.66 l) for seven minutes. The solution is stirred overnight. The solvent is then removed under vacuum leaving 8.3 g of a yellow solid. The pyrolysis yield of this polymer is 33.5% as measured by thermogravimetric analysis (25°-1000°C, 20°C/min., N₂). Infrared and NMR spectroscopy confirm the presence of C₆H₅CH₂N- as well as HN- groups on the polymer. The polymer is pyrolyzed by heating in nitrogen from room temperature to 1500°C at a rate of 10°C/min. and holding at 1500°C for two hours to form crystalline AlN. The nature of the product is confirmed by X-ray diffraction. Example 10 This example describes a chemical vapor deposition process for depositing AlN on a substrate. A sample reservoir equipped with inlet and outlet valves and charged with 10 g [CH₃C(H)=NAl(i-C₄H₉)₂]₂, prepared by the method described in Example 1, is connected to a mullite tube positioned in a furnace. Substrate coupons of alumina and silicon are positioned in the tube in the region occupying the hot zone of the furnace. With the reservoir valves closed, the tube is purged with nitrogen and heated to 500°C. The sample reservoir valves are opened allowing the nitrogen carrier gas to pass through the solution. The sample reservoir is heated to 100°C. The nitrogen carrier gas transports the aluminum imine to the furnace hot zone where the aluminum imine thermally decomposes and AlN deposits on the substrates. The film deposited at 500°C is annealed in nitrogen at 1500°C. The resulting ceramic film is shown to be crystalline AlN by X-ray diffraction. Example 11 The polymer prepared as described in Example 1 (15.8 g) is further heated under nitrogen to a temperature of 250°C for 4 hours. The volatile products evolved during the heating are shown by gas chromatography to be 37% isobutane and 63% isobutene. The resulting polymer, isolated in 13.1 gram yield, is an extremely viscous liquid of viscosity greater than 750,000 cps (MPa) measured at 30°C. The pyrolysis yield as measured by thermogravimetric analysis (25°-600°C, 20°C/min., N₂) is 49.3%. The polymer is pyrolyzed by heating in nitrogen from room temperature to 1500°C at a rate of 10°C/min. and holding at 1500°C for two hours. The resulting ceramic is shown to be crystalline AlN by X-ray diffraction.

Claims: A process for preparing an aluminum-nitrogen polymer by reacting an organic nitrile with a dialkylaluminum hydride to form an organoaluminum imine and then heating the imine, characterized by (a) reacting an organic nitrile having the formula RCN, where R is a 1-12 carbon alkyl, cycloalkyl or aryl group, with a dialkylaluminum hydride having the formula R'R''AlH, wherein R' and R'' are the same or different 1-12 carbon alkyl groups, to form an organoaluminum imine, and (b) heating the organoaluminum imine to a temperature of 50° to 200°C. The process of claim 1, further characterized by reacting the aluminum-nitrogen polymer with a 1-12 carbon alkyl, cycloalkyl or aryl primary amine or ammonia. The process of claim 1, further characterized by heating the aluminum-nitrogen polymer to a temperature of 200° to 400°C after step (b). The process of claim 3, further characterized by reacting the aluminum-nitrogen polymer with a 1-12 carbon alkyl, cycloalkyl or aryl primary amine or ammonia after heating at 200° to 400°C. An aluminum-nitrogen polymer obtainable by a process according to claim 1 and characterized by repeating units having the formula: where R is a 1-12 carbon alkyl, cycloalkyl or aryl group, R² is a 1-12 carbon alkyl group, and R³ is an organic group derived from R² in the process of heating the organoaluminum imine. An aluminium-nitrogen polymer obtainable by a process according to claim 2, characterized in that at least some of the repeating units comprise the following: where R is a 1-12 carbon alkyl, cycloalkyl or aryl group, R² is a 1-12 carbon alkyl , R₃ is an organic group derived from R² in the process of heating the organoaluminum imine, R⁴ is a 1-12 carbon alkyl, cycloalkyl or aryl group and R⁵ is H or R⁴, characterized by reacting the polymer of claim 5 with ammonia or a primary amine having the formula R⁴ NH₂. Use of the polymer of claim 5 to prepare an AlN-containing ceramic characterized by pyrolyzing the polymer in a nonoxidizing atmosphere. Use of the polymer of claim 5 to prepare an AlN-containing ceramic characterized by reacting the polymer with ammonia or a 1-12 carbon primary alkyl, cycloalkyl or aryl amine and then pyrolyzing in a nonoxidizing atmosphere.

Inventors: JENSEN JAMES ALLEN, JENSEN, JAMES ALLEN
Applicants: LANXIDE TECHNOLOGY CO LTD, LANXIDE TECHNOLOGY COMPANY, LP
Application year: 1991
Main classification: C08G79
IPC codes: C01B21, C08G79, C07F5, C04B35