Synthesis of amorphous boron nitride from the molecular precursor ammonia-monochloroborane
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Synthesis of amorphous boron nitride from the molecular precursor ammonia-monochloroborane Douglas R. Ketchum, Allison L. DeGraffenreid, Philipp M. Niedenzu, and Sheldon G. Shorea) Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 (Received 2 March 1998; accepted 16 December 1998)
Ammonia-monochloroborane, NH3 BH2 Cl, has been synthesized from the reaction of ammonia-borane with HCl in Et2 O. Decomposition of the solid under NH3 to 600 ±C produced amorphous BN in 97% yield. The 11 B magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectrum of the amorphous BN is indicative of boron in the same environment as in hexagonal BN. Subsequent pyrolysis of the amorphous BN to 1000 ±C produced turbostratic BN. Pyrolysis of NH3 BH2 Cl under vacuum to 1100 ±C led to the formation of turbostratic BN as confirmed by x-ray diffraction (XRD) analysis. Gas evolution during this pyrolysis confirmed that the precursor loses H2 and HCl.
I. INTRODUCTION 1
Since the discovery of the nonoxide ceramic boron nitride (BN), numerous studies have been reported on its synthesis, properties, and uses.2–8 The most notable properties are a high thermal conductivity, low thermal expansion, chemical inertness, and insulating properties. These properties make BN potentially useful in applications such as high-temperature crucibles, fibers, coatings, foams, and lubricants.2– 4 As a result, a search for new and improved synthetic routes has been undertaken. The traditional synthetic routes to hexagonal BN have involved high-temperature syntheses employing inexpensive nitrogen-containing compounds with boric acid or borates.6 –8 There are also high-temperature reactions for metal borides with N2 or the direct reaction of boron with nitrogen.2– 4 Commercially, the carbothermal reduction of boric acid above 1500 ±C under N2 is the preferred method of synthesis.6 Another route to ceramic materials is from molecular precursors. For BN, that means a 1 : 1 ratio of B to N is desirable in the precursor. The simplest precursor that has this ratio is ammonia-borane, NH3 BH3 . Ammoniaborane is soluble in organic solvents, and its thermal decomposition in solution and the solid state has been studied.9–12 However, these studies demonstrated that NH3 BH3 gives low yields of BN and undesirable volatile products [i.e., B2 H6 , (BNH2 )2 , and (BH2 N2 )2 ]. The pyrolysis of (CH3 )2 SBHBr2 under NH3 also produces BN.13 This process proceeds through the unstable intermediate H3 NBHBr2 and implies that ammoniahaloboranes might be useful molecular precursors for BN formation. This suggested to us that ammonia-
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J. Mater. Res., Vol. 14, No. 5, May 1999
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haloboranes might be employed in a more direct manner for the preparation of BN.15,16 The known ammonia-haloboranes are NH3 BH2 Cl, NH3 BHCl2 , and NH3 BH2 Br. These derivatives were studied earlier by 11 B NMR spectroscopy,14 but wer
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