J Am Cerom Soc zyx 73 zyx [8] zyx 2412-18 (1990) Silicon Nitride Derived from an Organometallic Polymeric Precursor: Preparation and Characterization Wayde R. Schmidt,*,*Vijay Sukumar,*'* William J. Hurley, Jr.,*** Roberto Garcia,*,* Robert H. Doremus,*,*and Leonard V. Interrante*,+ Departments zyxwvut of Materials Engineering and Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 Gary M. Renlund* General Electric Corporate Research and Development, Schenectady, New York 12301 z Partially crystalline Si3N4,with nanosized crystals and a specific surface area greater than 200 m2/g, is obtained by pyrolysis of a commercially available vinylic polysilane in a stream of anhydrous NH3 to 1000°C. This polymer does not contain N initially. Crystallization to high-purity wSi3N4 proceeds with additional heating above 1400°C under Nz. The changes in crystallinity, powder morphology, infrared spectra, and elemental compositions, for samples annealed from 1000" to 1600°C under N,, are consistent with an amorpbous-to-crystalline transformation. Although macroscopic consolidation and local densification occur at 1400"C, volatilization and accompanying weight loss limit bulk densification. The effect of temperature on specific surface area is examined and related to the sintering proc- ess. These results are applicable to pyrolysis, decomposition, and crystallization studies of ceramics synthesized by poly- meric precursor routes. [Key words: silicon nitride, precur- sor, polymer, pyrolysis, surface area.] I. Introduction HE properties of silicon nitride, such as resistance to ther- T mal shock, corrosion, creep, and oxidation, continue to generate interest in its application as a high-temperature structural material, an insulating diffusion mask in electronics, and as a refractory. Dense silicon nitride has excellent high- temperature strength, high electrical resistivity, a low coeffi- cient of thermal expansion, and low density relative to other high-temperature ceramics and s~peralloys.'-~ Considerable attention has been directed recently to the use of organometallic polymers as precursors to high- temperature, high-performance ceramics such as Si3N4.4-7 Some of the potential advantages in the preparation of ce- ramic materials from these precursors include (1) improved compositional homogeneity in the final product because of mixing of elemental components on the molecular and atomic levels, (2) high-purity ceramic products with uniform, small- grained microstructures, (3) the ability to produce high- surface-area powders of controlled morphology, and (4) the preparation of refractory ceramics at relatively low tempera- tures. Furthermore, the solubility and fluidity of organometal- G. Messing-contributing editor Manuscript No. 198090. Received October 3, 1989; approved March 6, 1 aan Supported by the National Science Foundation under Materials Chemistry 'Member, American Ceramic Society. *Department of Materials Engineering. 'Department of Chemistry. Initiative Grant No. CHE-8706131. lic polymers afford potential processing routes to binders, sintering aids, thin films, and fibers which are often difficult or impossible to produce by the more traditional ceramic proc- essing techniques. Ammonia is often used for the preparation of silicon ni- tride from precursor^.^^" Burns and Chandra' described the thermal cross-linking of polycarbosilanes, chlorinated polysi- lanes, and polysilazanes in argon and the subsequent pyroly- sis in argon-diluted ammonia. Samples were further annealed to 1500°C to yield mixtures of a-Si3N4 and amorphous ma- terial. Laine zyxwv et al.' monitored the increase in the ceramic yield for polysilazanes of various molecular weights. Signifi- cant contamination by 15% to 20% carbon was detected. Seyferth et aL8 reacted silanes with gaseous NH3 to produce silazane oils, which upon pyrolysis in N2 yielded a-Si3N4, P-Si3N4, and elemental silicon. Preliminary work in our labo- ratory demonstrated that the pyrolysis under NH3 of a mix- ture containing polymeric precursors to SIC and AlN leads to a Si3N4/AlNmi~ture.~~" This report describes the preparation of crystalline silicon nitride by pyrolysis in ammonia of a commercially available polymer, which was previously employed as a Sic precur- sor.9,11-14 The advantages of the described method include the following: (1) the polymer is cross-linked in situ at 250°C under ammonia during the pyrolysis, (2) the pyrolysis product has a high specific surface area, (3) partially crystalline Si3N4 is obtained at lOWC, and (4) a-Si3N4 results as the major crystalline phase following additional heat treatment. These features are useful in the preparation of ceramic composites which contain Si3N4, in the development of non-oxide binders for sintered or hot-pressed Si3N4, or in the sintering of Si3N4- based ceramics, where the high surface area and low degree of crystallinity could facilitate the densification of monolithic ceramic objects. 11. Experimental Procedure (1) Sample Preparation The organometallic precursor chosen for this investigation was a vinylic polysilane* (VPS) of the type [(Me3Si),- (CH2= CHSiMe)x(HSiMe)y(SiMe2)ZJ,, where Me represents a methyl group. VPS is a thermosetting polymer that is reported to undergo thermally induced cross-linking reactions involv- ing the silicon-vinyl, silicon-hydrogen, and/or vinyl groups." For pyrolysis, samples of the as-received polymer were poured into a molybdenum boat and inserted into a quartz furnace tube, equipped with an elastomer ring-sealed end cap and vacuum stopcocks, that was subsequently sealed by 'Y-12044, Union Carbide Corp., Specialty Chemicals Division, Tarry- town, NY. 2412