[show abstract][hide abstract] ABSTRACT: Boron nitride (BN) was prepared by nitriding pure boron (B) deposited on carbon substrates by chemical vapor deposition (CVD). Thermodynamic analysis of preparing BN by nitriding CVD B at 1200–1550 °C was firstly performed. And then, the effects of nitridation conditions, including temperature, nitridation atmosphere and CVD B microstructure, on the conversion of B to BN were analyzed by scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Results show that the conversion degree of B to BN firstly increased and then slightly decreased with rising temperature. The nitridation degree was controlled by mutual actions between the nitridation of B and consumption of the effective nitrogen source (NH3). The morphology of products and the reaction mechanism between B and N were influenced by nitridation temperature. At high temperatures (1400–1500 °C), BN with highly ordered microstructure was produced. On using N2–H2 as nitridation atmosphere instead of NH3–H2–N2, no BN was obtained in the studied temperature range. The microstructure and component of BN obtained in nitridation process were little affected by the microstructure of CVD B.
Progress in Natural Science: Materials International. 10/2012; 22(5):433–439.
[show abstract][hide abstract] ABSTRACT: The deposition rate, phase, chemical composition and microstructure of deposits were determined from 950 to 1100 °C. With increasing temperature, the deposition rate increases, and the morphology changes from smooth to coarse, meanwhile, the concentration of silicon increases while that of boron decreases. The deposition process is controlled by chemical reactions, and the activation energy is 271 kJ/mol. At relatively lower temperature (below 1000 °C), the deposition process is dominated by formation of B4C. While at higher temperature (above 1000 °C), it is governed by formation of SiC. B4C and SiC disperse uniformly in the Si–B–C co-deposition system and form a dense network structure.
Journal of Materials Science & Technology. 09/2012; 28(9):793–798.
[show abstract][hide abstract] ABSTRACT: Polyboronsilazane (PBSZ) precursors for SiBCN ceramics were prepared by using 9-borabicyclo-[1,3,3] nonane (9-BBN) and copolysilazanes (CPSZ) as starting materials, involving the hydroboration reaction between vinyl groups of PSZ and BH groups of 9-BBN under mild conditions. The as-synthesized PBSZ was obtained as a soluble liquid, which was characterized by FT IR and NMR. The polymer-to-ceramic conversion of PBSZ at a ceramic yield of 62.2–79.9% was investigated by means of FT IR and TGA. The crystallization behavior and microstructures of PBSZ-derived SiBCN ceramics were studied by XRD, SEM and HRTEM. The SiBCN ceramic began to crystallize at 1600 °C. Further heating at 1800 °C induced partial crystallization to give mixed XRD patterns for SiC, Si3N4, and BN(C). It is observed that the introduction of boron improves the thermal stability of SiBCN ceramics, especially under high temperatures of 1600–1800 °C. In addition, the introduction of boron significantly improves the ceramic density while inhibits the SiC crystallization.
[show abstract][hide abstract] ABSTRACT: TiC–TiB2–SiC ceramics with enhanced yield and microstructure homogeneity were synthesized from hybrid polymer precursors. The polymer reactions and phase evolution were characterized through a variety of techniques. It had been found that the ceramic yield of allylhydridopolycarbosilane was increased after introduction of borazine and was further enhanced by subsequently adding proper amount of tetrabutyl titanate. The borazine favored cross-linking and induced B containing phases to facilitate the densification, while the Ti source was competent to fix B and excess C (which was redundant for the formation of SiC) as uniformly distributed TiB2 and TiC through sintering, respectively. An optimally thermal-stable TiC–TiB2–SiC structure was constructed by consuming the excess C as the Ti content rose to 5 wt.%, at the cost of a slightly reduced ceramic yield.
Materials Chemistry and Physics. 04/2012; 133(s 2–3):946–953.
[show abstract][hide abstract] ABSTRACT: Boron nitride thin layers are in situ fabricated on chemical vapor-deposited boron in ammonia gas. Characterization by X-ray photoelectron spectroscopy and transmission electron microscopy reveals that the nitridation is dominated by different processes with varying temperatures. Below 1300°C the surface reaction is in control and leads to the formation of uniformly thin layer with mostly sp3 boron nitride. As the temperature rises, the nitridation is gradually turned as a diffusion-determining process, after which a thicker but uneven layer with hexagonal sp2 boron nitride is produced.
Journal of the American Ceramic Society 01/2011; 94(3):679 - 682. · 2.11 Impact Factor
[show abstract][hide abstract] ABSTRACT: The fabric architecture and porosity of three-dimensional (3D) Cf/SiCm composites are characterized using commercial X-ray microcomputed tomography (microCT). The non-destructive observation exhibits an inhomogeneous structure of the carbon fiber performs with gradiently distributed porosity. The shape of fiber bundles and porosity are investigated with respect to the gas transport during chemical vapor infiltration (CVI). Difference in growth rate of deposition between outer surface and inner porosity are identified through reconstructing the porosity morphology, which coincides well with the “node-bond” theoretical model. Moreover, in the light of the porosity features, gas retention and viscous flow is revealed to play key roles in the formation of the inner structure of Cf/SiCm.
Composites Part A-applied Science and Manufacturing - COMPOS PART A-APPL SCI MANUF. 01/2011; 42(11):1645-1650.
[show abstract][hide abstract] ABSTRACT: An applicable method to prepare transmission electron microscopy specimens from ceramic fibers for longitudinal and cross-sectional observations is investigated. The method includes novel embedding processes to fix fibers, a polishing process using a self-manufactured device to get uniformly low thickness (40 μm for L-fiber, 60 μm for C-fiber), a one-side dimpling process to grind the specimen to near electron transparency (about 5 μm in thickness for both L-fiber and C-fiber) and an efficient ion milling process using calculated parameters. These techniques are reliable to accomplish the preparation with high quality in a relatively short time. Many factors related to the preparation processes are discussed.
[show abstract][hide abstract] ABSTRACT: A two-layer boron carbide coating is deposited on a graphite substrate by chemical vapor deposition from a CH4/BCl3/H-2 precursor mixture at a low temperature of 950 degrees C and a reduced pressure of 10 KPa. Coated substrates are annealed at 1600 degrees C, 1700 degrees C, 1800 degrees C, 1900 degrees C and 2000 degrees C in high purity argon for 2 h, respectively. Structural evolution of the coatings is explored by electron microscopy and spectroscopy. Results demonstrate that the as-deposited coating is composed of pyrolytic carbon and amorphous boron carbide. A composition gradient of B and C is induced in each deposition. After annealing, B4C crystallites precipitate out of the amorphous boron carbide and grow to several hundreds nanometers by receiving B and C from boron-doped pyrolytic carbon. Energy-dispersive spectroscopy proves that the crystallization is controlled by element diffusion activated by high temperature annealing, after that a larger concentration gradient of B and C is induced in the coating. Quantified Raman spectrum identifies a graphitization enhancement of pyrolytic carbon. Transmission electron microscopy exhibits an epitaxial growth of B4C at layer/layer interface of the annealed coatings. Mechanism concerning the structural evolution on the basis of the experimental results is proposed. (C) 2010 Elsevier B.V. All rights reserved. National Natural Science Foundation of China [50532010, 90405015]
[show abstract][hide abstract] ABSTRACT: Two types of coating-modified 3D C/SiC, coated with CVD SiC/SiC/SiC (type I) and CVD SiC/amorphous-BC/SiC (type II), are subjected to a 14vol.% H2O/8vol.% O2/78vol.% Ar atmosphere at 700, 1000 and 1200°C up to 100h. Microstructure and corrosion behaviour are investigated using a variety of characterization techniques. The type II shows a better oxidation resistance than type I during annealing at relatively low temperatures. Nevertheless, residual strength of the type I annealed above 1000°C is enhanced by healing of many micron-sized defects. Interfacial bond strength of the composites is reasonably improved after annealing.
[show abstract][hide abstract] ABSTRACT: Boron nitride (BN) coatings were prepared by low pressure chemical vapor deposition (LPCVD) using boron trichloride (BCl3)–ammonia (NH3)–hydrogen (H2)–argon (Ar) mixture gases. Thermodynamic analysis indicated that BN was the only solid product when the amount of NH3 was in excess at the temperature of 1000°C and the total pressure of 1000Pa. The deposited BN coating had a turbostratic structure, and the highly ordered graphite substrate may have an effect on the order degree of BN coating near coating/substrate interface. The turbostratic BN was transformed into well-crystallized hexagonal boron nitride (h-BN) after heat treatments at temperatures above 1300°C. The deposition kinetics of LPCVD BN were investigated at the total pressure of 1000Pa and temperatures of 650°C, 800°C and 1000°C, respectively. It was concluded that the deposition process was controlled by the diffusion of BCl3 with an apparent activation energy of 1.3eV.
[show abstract][hide abstract] ABSTRACT: Amorphous boron carbide (α-B4C) coatings were prepared on SiC substrates by chemical vapor deposition (CVD) from CH4/BCl3/H2/Ar mixtures at low temperature (900–1050 °C) and reduced pressure (10 kPa). The deposited coatings were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that two kinds of α-B4C coatings were deposited with different microstructures and phase compositions, and the effect of deposition temperature was significant. When deposited at 1000 °C and 1050 °C, the coatings exhibited a nodular morphology and had a relatively low content of boron. The free carbon was distributed in them inhomogeneously; in contrast, when deposited at 900 °C and 950 °C, the coatings presented a comparatively flat morphology and had a uniform internal structure and high boron content. They did not contain free carbon. At the last of this paper, the pertinent mechanisms resulting in differences in microstructure and phase composition were discussed.
[show abstract][hide abstract] ABSTRACT: Boron carbide was prepared by low pressure chemical vapor deposition (LPCVD) from BCl3–CH4–H2 system. The deposition process conditions were optimized through using a uniform design method and regression analysis. The regression model of the deposition rate was established. The influences of deposition temperature (T), deposition time (t), inlet BCl3/CH4 gas ratio (δ), and inlet H2/CH4 gas ratio (θ) on deposition rate and microstructure of the coatings were investigated. The optimized deposition parameters were obtained theoretically. The morphologies, phases, microstructure and composition of deposits were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman micro-spectroscopy, transmission electron microscopy (TEM), energy dispersive spectra (EDS), and Auger electron spectra (AES), the results showed that different boron carbides were produced by three kinds of deposition mechanisms.
[show abstract][hide abstract] ABSTRACT: A technique based on melt spinning of precursor was introduced to produce continuous freestanding SiC films. An equipment including spinneret, mandril, tank and seal groove was designed and manufactured for melt spinning. The polycarbosilane (PCS) precursors were deaerated, melt spun, crosslinked (by oxidation or irradiation), and pyrolyzed at high temperature in order to convert the initial PCS into freestanding SiC films. Our results revealed that the continuous freestanding SiC films, approximately 8 μm to 190 μm in thickness depended on setting, were uniform and dense. Their microstructure consisted of amorphous SiOxCy, β-SiC nano-crystals and free carbon. The photoluminescence (PL) spectrum showed two blue emissions at 416 nm and 435 nm. The continuous freestanding SiC films with high modulus, high density, high surface hardness and optoelectronic properties may have potential applications in microelectromechanical systems (MEMS), advanced optoelectronic devices and such complex-shaped materials.
Journal of the European Ceramic Society 01/2009; 29(10):2079-2085. · 2.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: Hi-Nicalon fibers were exposed in 8% O2/78% Ar/14% H2O atmosphere for 1h at 1300, 1400, 1500, 1600°C, respectively. Residual tensile strength was evaluated by tensile test, phases in the fibers were identified using an X-ray diffractometer (XRD), morphology of the fracture surfaces and microstructure was observed by scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. Results indicated that residual tensile strength increased with increasing temperature from 1300 to 1500°C, then decreased after annealing in 1600°C. The grain size of β-SiC and the amount of the stacking faults increased under the elevated temperature as well. After annealing, a passive film with a structure of α-cristobalite crystals dispersed in amorphous SiO2 phase formed on the fiber surface, the thickness of the film increased with the annealing temperature from 1300 to 1500°C, after annealing in 1600°C, fractional silica film spalled. Finally, relationship between the structural changes and the mechanical properties, the control effect of water vapor on formation and structural evolution of the passive film were discussed.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing - MATER SCI ENG A-STRUCT MATER. 01/2008; 487(1):424-430.
[show abstract][hide abstract] ABSTRACT: Thin titania film with accessible and grid-like porosity was formed via structural transformation from a large-pore (~ 10 nm) 3D hexagonal (P63/mmc) mesoporous titania thin film. The intermediate 3D hexagonal mesoporous titania thin film (MTF) was synthesized using a tetrabutyl titanate (TBT)–P123–BuOH–HCl system by the combination of dip-coating and evaporation-induced self-assembly (EISA). The MTF calcined at 350 °C exhibited an ordered honeycomb arrangement over the entire top surface and an ABAB stacking sequence in the cross-section. After being calcined at 450 °C, the 3D hexagonal mesostructure was transformed to a grid-like mesostructure with quasi-perpendicular porosity through sintering–diffusion and pore merging along the c-axis. The accessibility of this grid-like structure estimated by the adsorption of TIRON (disodium 1,2-dihydroxybenzene, 3,5-disulfonate) was higher than that of intermediate structure. And an interesting finding was that, after structural transformation, the surface wettability changed from hydrophilic to superhydrophilic even without UV irradiation. This phenomenon could be properly explained by the increased accessibility and surface roughness of grid-like structure.
Surface and Coatings Technology 207:34–41. · 1.94 Impact Factor