The Synthesis Of Borazine And Its Conversion Into Boron Nitride

Borazine is an important precursor for boron nitride, which is liquid at room temperature and has the molecular formula of B₃N₃H₆. It can be used as the precursor for the preparation of both boron nitride ceramic matrix composites by PIP and boron nitride coatings by CVD. However, borazine has been rarely studied and reported domestically. In this dissertation, borazine was successfully synthesized. The reaction mechanism for the synthesis of borazine was studied. A catalytic synthesis of borazine was disclosed. The cross-linking reaction of borazine and its pyrolysis into boron nitride ceramic was studied. The boron nitride ceramic matrix composites were prepared by PIP and the boron nitride interphase coatings were prepared by CVD, using borazine as precursor.Based on the comprehensive review of the synthesis of borazine, the synthetic routine for borazine was designed. Borazine was synthesized by the routine in which the metal hydrides and ammonium salts in liquid phase. The effects of synthesis process parameters such as boron sources, nitrogen sources, solvents, and ratios of reactants, concentrations of reactants, and temperatures on the synthetic reaction were studied. The optimized synthetic process parameters (NaBH₄ as boron source, (NH₄)₂SO₄ as nitrogen source, diglyme as solvent, the concentration of NaBH₄ 8mol/L, (NH₄)₂SO₄ : NaBH₄ 0.8:1, temperatre 110℃) were obtained. The yield of borazine is 37%. The mechanism for the liquid phase reaction of NaBH₄ and (NH₄)₂SO₄ for the synthesis of borazine was studied. The reaction of NaBH₄ and (NH₄)₂SO₄ produced H₃B·NH₃ firstly. At low temperatures (40⁶0℃), H₃B·NH₃ is the main product and can exist stably without thermolysis. At moderate temperatures (60¹10℃), H₃B·NH₃ converted into H2B=NH2 by thermolysis dehydrogenation and other species includingμ-B₂H₅(NH₂) and B2H6. H2B=NH2 turned into B₃N₃H₁₂ and polyaminoborane (PAB) by addition reaction. B₃N₃H₁₂ turned into borazine B₃N₃H₆ by further dehydrogenation. At high temperatures (110¹60℃), borazine underwent serious cross-linking and there were more quantities of by-product PAB produced. Hence, the yield of borazine decreased seriously. A catalytic synthesis of borazine with high efficiency and high yields was disclosed. The addition of catalysts LiCl, MgCl₂ or AlCl₃ could lower the reaction temperature and time, and improved the yields of borazine. The catalyst AlCl₃ was the most efficient. With the addition of 1mol% of AlCl₃, the optimized synthetic reaction temperature was lowered from 110℃to 45℃, and the yields of borazine were improved from 37% up to 76%.The cross-linking of borazine and its pyrolysis conversion into boron nitride ceramics was studied. Without catalysts or curing agent, borazine could cross-linked thermally at 70¹00℃. The dynamic reaction study indicated that the cross-linking reaction was stepwise polymerization at initial stage and the order of the reaction was second. The composition and structure of the polymerized product polyborazylene were characterized. Its elemental composition was B_3.00N_3.06H_3.87 B_3.00N_3.03H_2.64. The B-N six-member rings were reserved in the structures of polyborazylene. The B-N six-member rings were cross-linked in biphenyl and fused-ring types. The content of B-H and N-H decreased with cross-linking time. The cross-linking reaction was performed by the dehydrogenation of B-H and N-H. The development of composition and structure of polyborazylene during its pyrolytic conversion into boron nitride ceramics were studied. The pyrolysis of polyborazylene developed in four stages. At the first stage (R.T.¹50℃), the components with low molecular weights volatilized. At the second stage (150⁴00℃), the weight-loss were completed mostly, the main reaction was the dehydrogenation cross-linking of B-H and N-H, and a small quantity of B-N bonds broke leading to the opening of B-N six-member rings. At the third stage (400¹000℃), pyrolysis was almost completed, B-H and N-H bonds broke and formed active H atoms and radical, the active H atoms added into H₂ which escaped, the radical migrated and rearranged. At the fourth stage (1000¹500℃), the structure of the pyrolytic product became more and more ordered. All the pyrolytic products under different temperatures had a B/N ratio of 1/1. As the pyrolysis temperature increased, the content of B-H and N-H decreased, with complete disappearance of B-H at 800℃and for N-H 1000℃. The crystalline degree of the pyrolytic products increased with the increase of pyrolysis temperature. The products pyrolyzed under 800℃were amorphous sp²-hybrid boron nitride. The products pyrolyzed at 1000¹200℃were turbostratic hexagonal boron nitride which was dense and composed of nano spherical particles. The products pyrolyzed over 1400℃crystalline hexagonal boron nitride which was loose and composed of lamellar particles. The dynamics of the pyrolytic reaction of polyborazylene was studied. The first and second stages of pyrolytic reaction had active energies of 23 and 117kJ/mol separately, which were both controlled by diffusion complying with Jander and Ginstling-Brounshtein equations separately. The third stage had an active energy of 230kJ/mol, being diffusion-controlled process complying with Zhuralev-Lesokin-TemPelman equation and radical reaction. The densities, hydrolytic and oxidative stability of the pyrolyzed boron nitride ceramics were studied. The densities improved with the increase of pyrolysis temperature. The densities raised sharply between 400℃and 1000℃, and then increased gradually upward 1000℃. The pyrolyzed product at 1600℃had a density of 2.01g/cm³. The boron nitride ceramics pyrolyzed under 1000℃would hydrolyze in moisture air, producing the ammonium borate hydrates (NH₄)₂O·xB₂O₃·yH₂O. While those pyrolyzed upward 1200℃were stable in moisture air and get only slight weight due to physical absorption. The oxidative stabilities of pyrolyzed boron nitride ceramics improved with the increase of pyrolysis temperature also. The onset temperatures of oxidation of boron nitride ceramics pyrolyzed at 1000℃, 1200℃and 1400℃were 835℃, 937℃and 987℃, forming the oxydates B₂O₃ and BN_xO_y.Using borazine as the precursor, the C_f/BN and SiC_f/BN composites were prepared by PIP method. The relationships between the pyrolysis temperature and the mechanical properties and microstructure of the composites were studied. The mechanical properties of C_f/BN were improved with the increase of pyrolysis temperature between 1000℃and 1200℃, mainly due to the weakening of the interphase bonding between fibers and matrix. While the mechanical properties of C_f/BN decreased with the increase of pyrolysis temperatures between 1200℃and 1600℃, since the boron nitride matrix pyrolyzed at high temperatures were soft, and could not transfer loading and restrict the strengthening fibers effectively. The C_f/BN composites prepared at 1200℃had the best mechanical properties, with flexural strength and modulus being 221.6MPa and 71.3GPa. The thermal physical properties, oxidation resistance and frictional properties of C_f/BN composites were studied. The thermal conductivity, specific heat capacity and thermal expansion coefficient of C_f/BN composites at room temperature was 1.2W·m^-1·K^-1, 1.07kJ·kg^-1·K^-1 and 1.3×10^-6/K, separately. The residual ratios of flexural strength of C_f/BN after oxidation at 1100℃, 1200℃and 1300℃for 30 minutes were 72.7%, 66.4% and 50.2%. The C_f/BN had stable frictional coefficients and low abrasion rates. The frictional coefficients were 0.22⁰.23. The line abrasion rates were 0.4⁴.3μm/cycle. The mass abrasion rates were 0.8⁸.7mg/cycle. The relationship between the pyrolysis temperature and the mechanical properties of SiC_f/BN composites was studied. The mechanical properties of SiC_f/BN composites decreased with the increasing pyrolysis temperature, since the embrittlement of SiC fibers was pricked up. The flexural strength and modulus of SiC_f/BN composites were brought down to 48.3MPa and 28.9GPa from 166.4MPa and 53.8GPa, as the pyrolysis temperature was elevated from 1000℃to 1400℃.Using borazine as the precursor, the boron nitride interphase coatings on carbon fibers, silicon carbide fibers and silica fibers were prepared by CVD method. The relationship between the preparation parameters and the micro-morphology, composition and structures of the CVD BN coatings were studied. The effects of substrates on the growth of CVD BN coatings were studied. When the CVD BN coatings grew on carbon fibers which had similar crystal structure with BN, no induction period were observed. However, when the CVD BN coatings grew on fibers whose crystal structures were different far from BN such as silicon carbide fibers and silica fibers, induction periods did exist. The growth dynamics of CVD BN coatings was studied. The growth was controlled by chemical reaction dynamics with an active energy of 72kJ/mol between 900℃and 1000℃. The controlling step turned to be mass transport at 1000℃. When the temperature further increased (above 1100℃), the deposition rate decreased drastically. This was due to the increased occurrence of gas-phase nucleation. The effects of CVD BN interphase coatings on the mechanical properties of C_f/BN and SiC_f/BN composites were studied. The CVD BN interphase coatings controlled the bonding between fibers and matrix and improved the mechanical properties of the composites. The flexural strength of C_f/BN and SiC_f/BN composites were increased by 41.4% and 31.4% separately.