[ACS Symposium Series] Inorganic and Organometallic Polymers II Volume 572 (Advanced Materials and Intermediates) || Organoaluminum Precursor Polymers for Aluminum Nitride Ceramics

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  • Chapter 32

    Organoaluminum Precursor Polymers for Aluminum Nitride Ceramics

    James A. Jensen

    Lanxide Corporation, Newark, DE 19714-6077

    An overview of synthetic routes to aluminum-nitrogen polymers including recent results describing the use of alkylaluminum imine precursors to aluminum nitride will be presented. Inorganic polymer precursors provide a non-traditional route to ceramic and inorganic composite materials utilizing plastic forming techniques. Polyalazanes or polyaminoalanes, compounds which possess polymer backbones consisting of Al-N bonds, have been shown to be effective precursors to AlN-containing ceramics. The latter sections of the chapter focus on alazane preceramic polymers prepared from alkylaluminum imine feedstocks. The aluminum imine complexes are synthesized from aluminum hydride reagents and nitriles. This synthetic approach is quite versatile due to the commercial availability of a large number of nitriles of various functional group substitution. Consequently, the resulting polymers have a range of properties encompassing curable liquids to thermoplastic solids.

    Aluminum nitride, A1N, is a refractory ceramic with low dielectric constant, high intrinsic thermal conductivity, and a low coefficient of thermal expansion very close to that of silicon. In fact, aluminum nitride is one of only few materials which is both a good electrical insulator and a good thermal conductor. Consequently, there is widespread interest in its use as a replacement for alumina and beryllia in electronic packaging applications. Such a high degree of interest in A1N stems from its high thermal conductivity, which has been measured at 319 W/nvK on a single crystal (7). The theoretical value is 320 W/m-K.

    A1N is also very stable to molten metals (in particular aluminum) and has good corrosion resistance. Additional applications for A1N include parts which come into contact with molten metal or other corrosive materials, and parts which require good heat transfer characteristics, thermal shock resistance or wear resistance.

    0097-6156/94/0572-0427$08.00/0 1994 American Chemical Society

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  • 4 2 8 INORGANIC AND ORGANOMETALLIC POLYMERS II

    Consequently, considerable research on a variety of A1N processing techniques has ensued (2-4).

    Recently, there has been a great deal of research activity in polymer precursors which convert to ceramic materials upon pyrolysis (5). The use of polymer technology to prepare ceramic materials offers certain processing advantages over conventional ceramic processing routes. The hallmark of preceramic polymers is the ability to fabricate complex shapes using a variety of plastic forming methods including molding, fiber spinning, film casting, coating, and infiltration. Consequently, preferred preceramic polymers are those that are curable liquids or soluble, thermoplastic solids. When used in powder processing, preceramic polymer binders obviate extended binder burnout cycles since the polymer "burns in" to the consolidated ceramic or composite phase.

    The desire to prepare oxygen-free A1N requires the use of oxygen-free reagents and precursors. Aluminum alkyls, aluminum hydrides, or anhydrous aluminum halides are thus logical starting materials for aluminum-nitrogen polymers. Since these reagents are typically very air sensitive or pyrophoric, the resulting polymers also tend to be quite oxygen and moisture sensitive. Aluminum nitrogen bonds, in general, are hydrolytically unstable.

    This chapter provides an overview of routes and reagents which have been utilized to prepare aminoalane polymers as precursors for A1N ceramics. Throughout the text, the terms polyalazane (analogous to the silicon nomenclature), polyaminoalane, and aluminum-nitrogen polymer will be used interchangeably, and do not represent any structural features other than an aluminum-nitrogen polymer backbone. Several research groups have been very active in the area. The cited references are not intended to be comprehensive, but rather to highlight the various approaches which have been employed. The synthetic descriptions will be divided by type depending on the aluminum reagent used. Generally, the systems undergo successive condensation reactions to form polymeric species upon heating. Crosslinking continues as long as replaceable groups are present on aluminum and nitrogen. Replaceable groups consist of alkyl, aryl or hydride bound to aluminum and hydrogen bound to nitrogen. Thus [H2NA1R2] species would be expected to further condense, but species of the type [RNA1C1] and [RNA1R] have been reported to be stable to further condensation (6,7). However, [RNA1C1]X polymers have recently been converted to A1N and the latter half of this chapter will describe the preparation of [RNAlR]n polymers and their conversion to AIN. A number of reports have appeared utilizing aminoalane precursors in the preparation of composite materials such as AIN/SiC and A1N/S3N4 (8-12). These will not be addressed in this chapter.

    General Synthetic Methods

    Polymers Derived from Trialkylaluminum. The most widely studied approach to alazane polymers utilizes ammonia or amine derivatives of trialkylaluminum reagents. The method, following the steps of equation 1, which was first described over fifty years ago (75), was recently reinvestigated (14-17). Trialkyl and triarylaluminum compounds form 1:1 adducts with Lewis bases such as ammonia or amines. These adducts readily undergo intermolecular condensation with elimination of a

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  • 32. JENSEN Organoaluminum Precursor Polymers 429

    i ii iii Me 3 Al-NH 3 - [Me 2AlNH 2] 3 -> [MeAINH]x - AIN (1)

    -CH4 -CH^ -CH4

    hydrocarbon molecule as long as the amine has available hydrogen atoms bonded to the nitrogen.

    Conversion of [Me 2AlNH 2] 3 and [terf-B^AlNr^ta prepared by ammonolysis of Me 3 Al and ferf-Bu3Al, respectively, to A1N, has been extensively studied (75). These oligomers are crystalline solids which sublime under vacuum, serving as MOCVD precursors to A1N (75,77). In the condensed state, the cyclic trimers convert to a polymeric structure on thermolysis between 110-200C in N H 3 (step ii in equation 1). The resulting aluminum imide polymer is a white, insoluble solid. Pyrolysis occurs above 1000C to yield A1N, which is reported to contain less than 0.5 wt% residual carbon and oxygen. Utilization of ethylenediamine as the nitrogen source rather than ammonia results in soluble thermoplastic polymers from which films can be cast and fibers spun (18).

    A melt formable, soluble organoaluminum polymer is formed by thermolysis of 212]3 in the presence of small amounts of Et 3 Al (19-21). The polymer [(EtAINH) I1(Et2AlNH2)m(AlEt3)x] can be prepared as a colorless viscous fluid, a glassy solid, or an infusible solid by controlling the ratio of reagents and reaction conditions. Fibers could be melt spun from the precursor. Heating the polymer under ammonia affords a "cure" to an infusible polymer which converts to A1N on pyrolysis with retention of shape. High aspect ratio flakes were prepared by freeze-drying a benzene solution of the polymer. This polymer is air sensitive and all manipulations were done in a dry box.

    Alkylaluminum reagents have also been treated with aminotriazines, such as melamine, to form precursors whose structure has not been described, but which convert to A1N at 1500C in nitrogen. The resulting A1N has A1(NH2)3 -> A1(NH)NH2 -> A 1 N 1 2 7 H X (2)

    - H 2 -NH 3 - H 2

    -80C -45C 20C 150C A1H 3 +NH 3 -> H 3A1-NH 3 -> H 2A1NH 2 -> HAINH -> Ho.23AlNHo 2 3 (3)

    - H 2 - H 2 - H 2

    reinvestigated by several groups (24,25). A soluble gel was formed using the stoichiometry of equation 3. However, on drying and calcination at 1100C in

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  • 430 INORGANIC AND ORGANOMETALLIC POLYMERS II

    vacuum, the A1N was contaminated with carbon residues from the tetrahydrofuran solvent (25).

    Alternatively, lithium aluminum hydride may be treated with ammonium halides as shown in equation 4 (26). NH 4Br gave better results than NH4C1, since

    LiAlH 4 + N H 4 X -> 1/n [HAlNH]n + 3 H 2 + LiX (4)

    the LiBr by-product is soluble in the reaction solvent, Et 2 0, and only a trace of salt is retained in the insoluble polymeric aminoalane product. However, the polymer does entrap 5-10 wt% of the diethyl ether solvent. The polyaminoalane of equation 4 converts to A1N in 85-87% yield by TGA. Pyrolysis in nitrogen or under vacuum at 950C gives crystalline A1N which is black, indicating residual carbon. Pyrolysis in ammonia gave white A1N.

    Cage compounds of the form [HAlNR]n are readily prepared from L1AIH4 and primary aminehydrochlorides according to equation 5 or by use of primary amines, equations 6 and 7, (27). These cage compounds have been converted to A1N (9,28).

    L1AIH4 + R N H 3 C I -> 1/n [HAlNR]n + 3 H 2 + LiCl (5)

    LiAlH 4 + RNH 2 - 1/n [HAlNR]n + 2 H 2 + LiH (6)

    AlH 3-OEt 2 + RNH 2 - 1/n [HAlNR]n + 2 H 2 + Et 2 0 (7)

    Pyrolysis of [HA1N(/-Pr)]6 at 1000C for two hours in argon gave a black product in 43% yield which was amorphous by X-ray diffraction. Heating the black solid to 1600C resulted in an additional 50% weight loss and formation of crystalline A1N.

    Polymers Derived from Aluminum Halides. The compound [Cl2AlNHSiMe3]2 is prepared from AICI3 and hexamethyldisilazane (29). Thermolysis at 170-200C results in formation of a polymer [ClAlNH]n and evolution of Me 3SiCl (equation 8). Pyrolysis of [ClAlNH]n in ammonia or vacuum at 700C is claimed to give crystalline A1N (30).

    heat [Cl 2AlNHSiMe 3] 2 -> 2/n [ClAlNH]n + 2 Me3SiCl (8)

    The action of potassium amide on AlBr 3 results in an infusible aluminum amideimide [Al(>iH2)NH]n, equation 9, which was also prepared by ammonolysis of AlH 3-OEt 2, (equation 2) (24). Pyrolysis at 800C under vacuum results in A1N.

    AlBr 3 + 3 K N H 2 -> 1/n [Al(NH2)NH]n + 3 KBr + N H 3 (9)

    Polymers Derived Electrochemically from Elemental Aluminum. Aminoalane polymers have also been prepared by electrolysis of aluminum in a solution consisting of an alkyl amine, a solvent, and a supporting electrolyte (31-33) or in a solution of liquid ammonia (24,34). Anodic dissolution of an aluminum electrode under an inert

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  • 32. J E N S E N Organoaluminum Precursor Polymers 431

    atmosphere in an electrolyte consisting of a primary amine, acetonitrile, and tetrabutylammonium bromide leads to aluminum-nitrogen polymers, the idealized structure of which is given in equation 10 where R is alkyl or H. The proposed

    -3/2 H 2 Al + 3 RNH 2 -> [A1(NHR)3] -> 1/n [N(R)Al(NHR)]n + RNH 2 (10)

    A1(NHR)3 intermediate was not isolated or observed spectroscopically. When R is an alkyl group such as -propylamine, the solution is heated under vacuum on completion of electrolysis to drive off excess amine. At about 80C, a high viscosity liquid results; at 150C, an insoluble gel-like solid forms. Pyrolysis at 850C gives a crystalline A1N product. Pyrolysis in argon results in a product with up to 28 wt% carbon. However, pyrolysis in ammonia gave A1N with about 2% oxygen and 0.8% carbon impurities (31,32). SiC fibers were coated by dispersing the fibers in the reaction medium after electrolysis followed by pyrolysis to give AIN-coated SiC (33).

    A modification of the above procedure which simplifies the process is the use of liquid ammonia as both solvent and nitrogen source. Ammonium bromide is used as the supporting electrolyte, so carbon contamination of the final A1N product should be minimized since none of the starting materials contain carbon (34). After electrolysis, the ammonia is allowed to evaporate and the mixture of insoluble aluminum polymer/ammonium bromide is calcined at 1100C in flowing ammonia to give submicron sized A1N powder with no apparent bromine impurities.

    Polymers Derived from Alkylaluminum Hydrides. All of the above synthetic routes utilize ammonia or an amine as the nitrogen source for the polymers. Significantly less effort has been spent on the study of the interaction of aluminum reagents with other nitrogen donor molecules. For example, addition of A1H3-NR3 adducts to acetonitrile or aziridine solutions yields polymeric products with imine end groups according to equation 11 (35,36). The product is soluble in benzene and has a

    A1H3-NR3 + nCH 3 CN -> [H^tNOEOAl^Jn.^CHC^] + nNR 3 (11)

    degree of polymerization = 3 to 35. The conversion of this precursor to A1N was not reported.

    Trialkylaluminum reagents form adducts with nitriles, which can be converted thermally into imine complexes as shown in equation 12 (37,38). The complexes

    2A1R3 + 2R'CN -> 2R3A1-NCR* [RR'C=NA1R2]2 (12)

    [PhCH=NAlEt2]2 and [(CH3)3CC(CH3)=NA1(CH3)2]2 prepared by the method of equation 12 were not observed to undergo rearrangement to form [RC(R)2NAl(R)]n polymers upon heating to 200C and 280C, respectively.

    Aluminum hydride reagents reduce nitriles directly to aluminum imine complexes, equation 13, without adduct formation as observed with trialkylaluminum reagents (39). The aluminum imine, [C6H5CH=NAl(i-C4H9)2]2, prepared from

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  • 432 INORGANIC AND ORGANOMETALLIC POLYMERS II

    2R 2A1H + 2R'CN-> [R2A1N=CHR']2 (13)

    benzonitrile and diisobutylaluminum hydride is claimed to lose two equivalents of isobutene upon heating to 280C to form an aluminum nitrogen polymer of the form [-(RCH2)NAl-]n (40). The structural assignment was made based upon the recovery of benzylamine on hydrolysis. No attempt was made to identify the polymer or the inorganic residue on hydrolysis. None of these dimeric or polymeric species derived from nitriles have been investigated as A1N precursors.

    The conversion to A1N of alazane polymers derived from nitrile and alkylaluminum hydride reagents has been investigated. The commercial availability of a wide range of nitrile reagents with a variety of molecular weights and functional groups provides a single synthetic route to polyaminoalanes with a spectrum of physical properties. Such flexibility has not been obtained by use of the previously described routes.

    Experimental

    All manipulations were carried out using standard vacuum line and dry box techniques under inert atmospheres or vacuum. Solvents were dried over molecular sieves and stored under nitrogen. Diisobutylaluminum hydride and the nitrile reagents were used as received. CAUTION: Diisobutylaluminum hydride (DIBAL-H) is very moisture sensitive in dilute solutions and pyrophoric when neat. The alkylaluminum imine complexes were prepared by literature methods (41). Thermal polymerization of the aluminum imine complexes was accomplished as previously reported (42-45). Controlled headspace analysis was conducted by holding sealed 0.5 gram samples of the aluminum imine in a bath at the appropriate temperature for 1 hour. The sample was cooled to room temperature and the headspace samples analyzed on an Hewlett Packard 5890 Series II Gas Analysis system using a 50 m 0.5 mm chrompak column. Pyrolyses were conducted in an Astro Model 1000 furnace using graphite heating elements and crucibles.

    Results and Discussion

    Synthesis. The diisobutylaluminum imine complexes, [RCH=NAl(i-C4H9)2]2, were prepared by diisobutylaluminum hydride reduction of the appropriate nitrile according to equation 13. The reaction proceeds cleanly at room temperature in quantitative yield. Proton NMR spectra of the reaction products show no side product formation. The product imines are dimeric in structure; [(C6H3-2,6-(CH3)2)CH=NAl(i-C4H9)2]2 displays a four-membered heterocyclic Al-N core with a trans geometry in the solid state (41). Equation 13 is general for nitriles containing a wide range of functional groups. Aluminum imines have been prepared with R= alkyl, aryl, and olefin-containing groups.

    DIBAL-H reduction of simple nitriles such as acetonitrile and benzonitrile results in liquid products, while substituted alkyls such as trimethylacetonitrile and 2,6-dimethylbenzonitrile form molecular solids with discrete melting points. Polymerization of the alkylaluminum imine complexes is accomplished thermally by a

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  • 32. J E N S E N Organoaluminum Precursor Polymers 433

    reductive condensation process. The imine double bond is reduced and alkane or olefin volatiles are eliminated from the aluminum center during the polymerization. In general, the product alazane polymers have the form [R,CH2NAl(C4H9)]n. The alkyl group, R', introduced with the nitrile has a pronounced effect on the polymer properties. For example, simple R' groups such as methyl and phenyl yield liquid or solid thermoplastic polymers; olefln-containing R' groups form liquid or meltable solid polymers which can be thermoset by heating.

    Thermolysis of the aluminum imine complexes at temperatures below about 200C generates alazane polymers substituted at both the Al and atoms with single alkyl or alkenyl groups, i.e. one aluminum alkyl group has been lost on thermolysis. Prior reports had indicated that polymeric units of this type would not undergo further condensation reactions (6,7). However, we find this not to be the case. The "replaceable groups" criteria, as described in the introduction to this chapter is not met in the dimer [RCH=NAl(i-C4H9)2]2 or the polymer [RNAlR]n. The nitrogen atoms of these species do not possess the acidic hydrogen earlier thought to be prerequisite for alkane loss. Yet condensation, ultimately to A1N, proceeds upon additional heating to temperatures above 200C.

    The following examples describe the versatility of this synthetic approach. The olefin-containing aluminum imine, [CH2=CHCH2CH=NAl(i-C4H9)2]2,1, prepared from 3-butenenitrile and DIBAL-H is a low viscosity liquid (approximately 30 cp) (43,44). Thermolysis of I below about 160C generates a viscous liquid alazane polymer, , which retains pendant carbon-carbon double bonds as evidenced by 1 3 C NMR and Raman spectroscopy. Above about 180C, alazane polymer undergoes additional crosslinking through carbon-carbon double bonds, as well as thermal condensation through aluminum-nitrogen bond formation, to form rigid nonmelting solids, which retain their shape upon pyrolysis. No evidence of carbon-carbon double bonds remain in spectra of the cured polymer.

    In contrast, thermolysis of the alkylaluminum imine, [CH3CH=NAl(i-C4H9)2]2, , at about 200C results in a viscous liquid polymer {[(C2H5)NAl(C4H9)]x[(C2H5)NAl(R,)]y}n, IV. Continued heating of IV to 300C results in a solid thermoplastic alazane polymer which has a softening point near 110C and is represented by the formula {[(C2H5)NAl(C4H9)]x[(C2H5)NAl(R,)]y-[(C 2H 5)NAl] z} n, V, where x+y=0.65, z=0.35, the value of ranges from 6-20 and R' is an octyl group bridging two aluminum centers. The values of and y in both IV and V can be varied as a function of thermolysis time and temperature. There are two types of gross aluminum environments in polymer V; those retaining one Al-C bond as in units and y, and those bonded only to nitrogen as in unit z. Polyalazane V shows low moisture and oxygen sensitivity; it can be handled in air for short periods of time without appreciable degradation of properties. Polymer V can be processed using typical thermoplastic fabrication techniques.

    Thermal Analysis. The char yield, at 1000C, as measured by thermal gravimetric analysis, of a number of aluminum imine-derived polymers, has been obtained. The values range from 15 to 85 weight % retention depending upon the starting nitrile and the polymerization conditions. Primary weight loss typically occurs between 250 and 500C (42).

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  • 434 I N O R G A N I C A N D O R G A N O M E T A L L I C P O L Y M E R S II

    Gas chromatography of the gas headspace on polymerization of reveals that isobutane and isobutene are formed in varying ratio depending on temperature. Above 300C ethylene begins to appear. A compilation of relative gas ratios for the evolved gasses as a function of temperature is shown in Table I. Below 150C alkane

    Table I. Gas Headspace Analysis of [CH3CH=NAl(i-C4H9)2]2 Thermolysis

    Temperature (C) Relative Percent in Headspace

    Isobutane Isobutene Ethylene 50 68 32

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  • 436 INORGANIC AND ORGANOMETALLIC POLYMERS II

    into an aluminum-carbon bond to give octyl groups bridging metal centers. The amine substituent formed in Path A is an ethyl group.

    Alternatively, following Path B, -hydride migration to reduce the coordinated imine can be accompanied by loss of alkene if A l - C a bonds are broken as shown in 5. The second alkyl group bound to aluminum is unaffected in Path so 6 retains an aluminum alkyl fragment which is unchanged from the starting dimer. The amine substituent formed in Path is also an ethyl group.

    Although the mechanism shown in Figure 1 illustrates an intramolecular process, an intermolecular process involving -hydride elimination may also occur. In this process, -hydride elimination of the olefin isobutene would generate an aluminum hydride species. The aluminum hydride would further react with another molecule of aluminum imine to reduce the imine with concomitant formation of a new aluminum-nitrogen bond. The only volatile product to be observed during the thermolysis via this mechanism would be the olefin, isobutene. Hydrolysis of the polymer formed by this process would generate only isobutane and ethyl amine products. Since a -hydride elimination mechanism can only explain evolution of isobutene during polymer formation, it alone does not explain the observed product distribution. However, it is probable that a combination of intramolecular and intermolecular processes are responsible for polymer formation.

    Radical pathways are not anticipated to contribute to the butane formation on imine thermolysis due to the low temperature range where isobutane evolution predominates. Decomposition studies of triisobutylaluminum on Al surfaces show insignificant quantities of butane or butyl radicals in the 450-600 temperature range studied (46).

    Pyrolysis Studies. The imine complexes or alazane polymers, I-V, can be pyrolyzed in a non-oxidizing atmosphere to crystalline aluminum nitride. Figure 2 shows the X -ray powder diffraction pattern of polyalazane V fired from room temperature to 1000C in ammonia and from 1000C to 1800C in nitrogen. Bulk elemental analysis of the powder shows only Al and present with

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    I n I n o r g a n i c a n d O r g a n o m e t a l l i c P o l y m e r s I I ; W i s i a n - N e i l s o n , P . , e t a l . ;

    A C S S y m p o s i u m S e r i e s ; A m e r i c a n C h e m i c a l S o c i e t y : W a s h i n g t o n , D C , 1 9 9 4 .

  • 438 INORGANIC AND ORGANOMETALLIC POLYMERS II

    fabricated into complex shapes using plastic forming methods. The use of aluminum imine complexes as feedstock reagents for alazane polymer formation has been demonstrated. The imine complexes can be thermally coverted to aluminum-nitrogen polymers. During the preparation of such polymers, both saturated and unsaturated hydrocarbon by-products are evolved, providing evidence for non-trivial imine decomposition pathways. This synthetic approach is quite versatile due to the commercial availability of a large number of nitriles of various molecular weights and functional group substitution. Consequently, the resulting polymers have a spectrum of properties encompassing curable liquids to thermoplastic solids. When unsaturated nitriles are used to prepare these imine complexes, liquid polymers which thermoset to solid materials via crosslinking through centers of unsaturation can be prepared. All of the alkylaluminum imine-derived polymers prepared above convert to A1N on pyrolysis in inert atmospheres.

    Acknowledgments

    The author wishes to thank L. J. Barret, and J. A. Mahoney for technical assistance, and Dr. J. M. Schwark and Dr. A. Lukacs HI for helpful discussions.

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    RECEIVED April 8, 1994

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    Chapter 32 Organoaluminum Precursor Polymers for Aluminum Nitride CeramicsGeneral Synthetic MethodsExperimentalResults and DiscussionConclusionAcknowledgmentsLiterature Cited

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