[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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<ul><li><p>Chapter 32 </p><p>Organoaluminum Precursor Polymers for Aluminum Nitride Ceramics </p><p>James A. Jensen </p><p>Lanxide Corporation, Newark, DE 19714-6077 </p><p>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. </p><p>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. </p><p>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. </p><p>0097-6156/94/0572-0427$08.00/0 1994 American Chemical Society </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>GU</p><p>ELPH</p><p> LIB</p><p>RARY</p><p> on </p><p>Oct</p><p>ober</p><p> 29,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h032</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>4 2 8 INORGANIC AND ORGANOMETALLIC POLYMERS II </p><p>Consequently, considerable research on a variety of A1N processing techniques has ensued (2-4). </p><p>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. </p><p>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. </p><p>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. </p><p>General Synthetic Methods </p><p>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 </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>GU</p><p>ELPH</p><p> LIB</p><p>RARY</p><p> on </p><p>Oct</p><p>ober</p><p> 29,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h032</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>32. JENSEN Organoaluminum Precursor Polymers 429 </p><p>i ii iii Me 3 Al-NH 3 - [Me 2AlNH 2] 3 -&gt; [MeAINH]x - AIN (1) </p><p>-CH4 -CH^ -CH4 </p><p>hydrocarbon molecule as long as the amine has available hydrogen atoms bonded to the nitrogen. </p><p>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). </p><p>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. </p><p>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 -&gt; A1(NH)NH2 -&gt; A 1 N 1 2 7 H X (2) </p><p>- H 2 -NH 3 - H 2 </p><p>-80C -45C 20C 150C A1H 3 +NH 3 -&gt; H 3A1-NH 3 -&gt; H 2A1NH 2 -&gt; HAINH -&gt; Ho.23AlNHo 2 3 (3) </p><p>- H 2 - H 2 - H 2 </p><p>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 </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>GU</p><p>ELPH</p><p> LIB</p><p>RARY</p><p> on </p><p>Oct</p><p>ober</p><p> 29,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h032</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>430 INORGANIC AND ORGANOMETALLIC POLYMERS II </p><p>vacuum, the A1N was contaminated with carbon residues from the tetrahydrofuran solvent (25). </p><p>Alternatively, lithium aluminum hydride may be treated with ammonium halides as shown in equation 4 (26). NH 4Br gave better results than NH4C1, since </p><p>LiAlH 4 + N H 4 X -&gt; 1/n [HAlNH]n + 3 H 2 + LiX (4) </p><p>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. </p><p>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). </p><p>L1AIH4 + R N H 3 C I -&gt; 1/n [HAlNR]n + 3 H 2 + LiCl (5) </p><p>LiAlH 4 + RNH 2 - 1/n [HAlNR]n + 2 H 2 + LiH (6) </p><p>AlH 3-OEt 2 + RNH 2 - 1/n [HAlNR]n + 2 H 2 + Et 2 0 (7) </p><p>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. </p><p>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). </p><p>heat [Cl 2AlNHSiMe 3] 2 -&gt; 2/n [ClAlNH]n + 2 Me3SiCl (8) </p><p>The action of potassium amide on AlBr 3 results in an infusible aluminum amideimide [Al(&gt;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. </p><p>AlBr 3 + 3 K N H 2 -&gt; 1/n [Al(NH2)NH]n + 3 KBr + N H 3 (9) </p><p>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 </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>GU</p><p>ELPH</p><p> LIB</p><p>RARY</p><p> on </p><p>Oct</p><p>ober</p><p> 29,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h032</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>32. J E N S E N Organoaluminum Precursor Polymers 431 </p><p>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 </p><p>-3/2 H 2 Al + 3 RNH 2 -&gt; [A1(NHR)3] -&gt; 1/n [N(R)Al(NHR)]n + RNH 2 (10) </p><p>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). </p><p>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. </p><p>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 </p><p> A1H3-NR3 + nCH 3 CN -&gt; [H^tNOEOAl^Jn.^CHC^] + nNR 3 (11) </p><p>degree of polymerization = 3 to 35. The conversion of this precursor to A1N was not reported. </p><p>Trialkylaluminum reagents form adducts with nitriles, which can be converted thermally into imine complexes as shown in equation 12 (37,38). The complexes </p><p>2A1R3 + 2R'CN -&gt; 2R3A1-NCR* [RR'C=NA1R2]2 (12) </p><p>[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. </p><p>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 </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>GU</p><p>ELPH</p><p> LIB</p><p>RARY</p><p> on </p><p>Oct</p><p>ober</p><p> 29,</p><p> 201</p><p>2 | ht</p><p>tp://p</p><p>ubs.ac</p><p>s.org </p><p> Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h032</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>432 INORGANIC AND ORGANOMETALLIC POLYMERS II </p><p>2R 2A1H + 2R'CN-&gt; [R2A1N=CHR']2 (13) </p><p>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 w...</p></li></ul>