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Boron Carbide (boron + carbide)
Selected AbstractsMechanism studies on CVD of boron carbide from a gas mixture of BCL3, CH4, and H2 in a dual impinging-jet reactorAICHE JOURNAL, Issue 3 2009Mustafa Karaman Abstract Nearly pure boron carbide free from impurities was produced on a tungsten substrate in a dual impinging-jet chemical vapor deposition reactor from a BCl3, CH4, and H2 mixture. The Fourier Transform Infrared (FTIR) analysis proved the formation of reaction intermediate BHCl2, which is proposed to occur mainly in the gaseous boundary layer next to the substrate surface. Among a large number of reaction mechanisms proposed only the ones considering the molecular adsorption of boron carbide on the substrate surface gave reasonable fits. In the proposed mechanism dichloroborane is formed in the gas phase only as a by-product. Boron carbide, on the other hand, is formed through a series of surface reactions involving adsorbed boron trichloride, adsorbed methane and gas phase hydrogen. The simultaneous fit of the experimental rate data to the model expressions gave correlation coefficient values of 0.977 and 0.948, in predicting the B4C and BHCl2 formation rates, respectively. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Catalyst-Free Synthesis and Characterization of Metastable Boron Carbide NanowiresADVANCED FUNCTIONAL MATERIALS, Issue 24 2009Aruna Velamakanni Abstract Catalyst-free growth of boron carbide nanowires is achieved by pyrolysis of diborane and methane at 650,750,°C and around 500 mTorr in a quartz tube furnace. Electron-diffraction analysis using a novel diffraction-scanning transmission electron microscopy (D-STEM) technique indicates that the crystalline nanowires are single-crystal orthorhombic boron carbide. TEM images show that the nanowires are covered by a 1,3,nm thick amorphous layer of carbon. Elemental analysis by electron energy loss spectroscopy (EELS) shows only boron and carbon while energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) show the presence of oxygen as well as boron and carbon. [source] Production of B4C coatings by CVD method in a dual impinging-jet reactor: Chemical yield, morphology, and hardness analysisAICHE JOURNAL, Issue 11 2009Mustafa Karaman Abstract ,-rhombohedral boron carbide (B4C) was deposited on a tungsten substrate from a BCl3H2CH4 gas mixture in a dual impinging-jet chemical vapor deposition reactor. On-line FTIR analysis of the product stream proved the formation of BHCl2 and HCl as by products, in a competing parallel reaction. A maximum of 13% chemical yield of boron carbide was observed, and the yield was found to have increasing trend with an increase in temperature. XRD analysis proved the existence of rhombohedral B4C phase at 1300°C without any other B4C phases or impurities. At this temperature, the formation of 5-fold icosahedral boron carbide crystals up to 30 micron sizes was observed. Such highly symmetric crystalline regions were observed to have a very high hardness value of 4750 kg/mm2 as revealed from the microhardness analysis. The change in product morphology at low substrate temperatures resulted in a decrease in the hardness values. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Mechanism studies on CVD of boron carbide from a gas mixture of BCL3, CH4, and H2 in a dual impinging-jet reactorAICHE JOURNAL, Issue 3 2009Mustafa Karaman Abstract Nearly pure boron carbide free from impurities was produced on a tungsten substrate in a dual impinging-jet chemical vapor deposition reactor from a BCl3, CH4, and H2 mixture. The Fourier Transform Infrared (FTIR) analysis proved the formation of reaction intermediate BHCl2, which is proposed to occur mainly in the gaseous boundary layer next to the substrate surface. Among a large number of reaction mechanisms proposed only the ones considering the molecular adsorption of boron carbide on the substrate surface gave reasonable fits. In the proposed mechanism dichloroborane is formed in the gas phase only as a by-product. Boron carbide, on the other hand, is formed through a series of surface reactions involving adsorbed boron trichloride, adsorbed methane and gas phase hydrogen. The simultaneous fit of the experimental rate data to the model expressions gave correlation coefficient values of 0.977 and 0.948, in predicting the B4C and BHCl2 formation rates, respectively. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Pressureless Sintering of Zirconium Diboride: Particle Size and Additive EffectsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2008William G. Fahrenholtz Zirconium diboride (ZrB2) was densified by pressureless sintering using <4-wt% boron carbide and/or carbon as sintering aids. As-received ZrB2 with an average particle size of ,2 ,m could be sintered to ,100% density at 1900°C using a combination of boron carbide and carbon to react with and remove the surface oxide impurities. Even though particle size reduction increased the oxygen content of the powders from ,0.9 wt% for the as-received powder to ,2.0 wt%, the reduction in particle size enhanced the sinterability of the powder. Attrition-milled ZrB2 with an average particle size of <0.5 ,m was sintered to nearly full density at 1850°C using either boron carbide or a combination of boride carbide and carbon. Regardless of the starting particle size, densification of ZrB2 was not possible without the removal of oxygen-based impurities on the particle surfaces by a chemical reaction. [source] Hypothetically superhard boron carbide structures with a B11C icosahedron and three-atom chainPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2009Sezgin Aydin Abstract The structural and mechanical properties are investigated for the phases of superhard boron carbide, B4C, by performing DFT/PW91 level calculations. In addition to B12 icosahedra and CCC chains, we suggest new types of hypothetically stable rhombohedral structures of boron carbide, configurations that consist of one icosahedron, B11C, and a three-atom chain such as CBC, CCB and BCC. Our results indicate that all phases are promising superhard materials, the hardness of all of them being greater than 40 GPa. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||