Key Engineering Materials Vols. 602-603

Abstract: The application of B₄C as structural materials has been restricted largely because of its poor sinter-ability. Compared with pressure-less and hot pressing methods, sintering the B₄C via the reaction-bonded boron carbide (RBBC) can to great extent circumvent such problem, which can even be conducted at low temperature, when Si/Si-based alloy was used as binding phase. However molten Si/Si-based alloy infiltrated into performs of bare B₄C powders can strongly react with and significantly consume B₄C particles based on molten infiltration reaction method. The present study aims to encapsulate B₄C particles with a protective layer to block off its contact with molten Si/ Si-based alloy via chemical vapor deposition (CVD) method in CH₃SiCl₃(MTS)-H₂-Ar system at low temperature (900-1100 °C) under atmospheric pressure. The phase composition and surface morphology of encapsulated B₄C particles were studied using X-ray diffraction (XRD) , scanning electron microscopy (SEM) plus energy dispersive spectrometer (EDS), respectively. It was found that the deposition temperatures have a significant effect on microstructure and composition of deposited coating. The studies on surface morphology revealed that spine-like crystals, nodular growth, island structure and whiskers can be deposited onto the surface of boron carbide particles. When deposited at 900-1000 °C,the coatings is Si + SiC co-deposition, while pure SiC coatings form only at as high deposition temperature at 1050 °Cand 1100 °C.

Abstract: Wetting behaviors of pure nickel and nickel-based alloys on sintered silicon carbide ceramics and their interfacial microstructures were investigated in this presentation. The nickel-based alloys were mainly the commercial nickel-molybdenum-chromium products. The wetting and spreading properties were observed by a real-time thermal optical measurement system under flowing argon-5%hydrogen atmosphere. As temperature increased, the pure nickel cylinder sample had few changes before 1356°C except for the thermal expansion in size and changed to liquid drop-shape when the temperature reached 1366°C. The contact angle was about 90^o. And nickel could not contact sintered silicon carbide ceramics well. However, the introduction of molybdenum and/or chromium in the pure nickel was good for the wetting properties of pure nickel on sintered silicon carbide ceramics. The contact angles of nickel-based alloys (B-1, C-1 and C-2) on sintered silicon carbide ceramics after holding 15min at 1400°C were 25^o, 12.5^o, 11.5^o, respectively. And they hardly reduced as temperature increased. The SEM images indicated that the interfacial microstructures of B-1 and C-1 on sintered silicon carbide ceramic substrates were uniform and the dissolved interface near silicon carbide ceramics indicated that they had a good bonding. At the same time, the reciprocal diffusion was clear in the intermediate layer. Besides that, the introduction of chromium was supposed to reduce the diffusion rate of molybdenum from the alloy to the ceramic substrate.

Abstract: Nanoporous carbonaceous materials derived from polysiloxane were first prepared by pyrolysis at 1300°C followed with hydrofluoric acid (HF) etching treatment. Their thermal stability of pore structure in inert condition was investigated in this paper by nitrogen adsorption technique in detail. The specific surface area (SSA) and pore volume (total pore volume, micropore volume, mesopore volume) decreased continually in the heat-treatment temperature range of 1000~1400°C. The average pore size almost kept the same with the raw sample. However, when the temperature exceeded 1400°C, the micropore interconnection began transforming to mesopore structure, which led to the decline of SSA and the increase of average pore size. Furthermore, the pore size distributions (PSDs) curves showed that heat-treatment had an advantage on the transition process of pore structure from disorder to regularity to some extent when heat-treated in the range 1000~1400°C for the most possible reason of relief of residue strain in the carbonaceous materials.

Abstract: LED is the fourth generation of the illumination light source. The light efficiency, lifetime and reliability will be reduced, if the heat generated in LED chips couldn’t be dissipated rapidly. Thus, thermal conductivity of the substrate material is a very key factor in the LED field. Silicon Carbide with Aluminum metal composites (Al/SiC) has high thermal conductivity and ideal thermal expansion coefficient. In this paper, the main kinds of substrates for high-power LED were introduced. Al/SiC composite substrate with excellent heat conduct performance was fabricated by pressureless infiltration technology. The microstructure of Al/SiC was observed by scanning electron microscope (SEM). The coefficients of thermal expansion and thermal conductivity of Al/SiC composite substrate, AlN ceramic substrate and Al₂O₃ ceramic substrate were compared. The results showed that the coefficient of thermal expansion (CTE) of Al/SiC varied from 7.77×10^-6 K^-1 to 10.89×10^-6 K^-1, which is compatible with Silicon. And the thermal conductivity of Al/SiC is as 9 times high as that of Al₂O₃.

Abstract: The paper concerns composite materials made of silicone rubber matrix and ceramic fillers used as flame resistant coverings for electrical cables. Under fire, such materials must be able to form, relatively quickly, compact and stiff protecting coating, strong enough to maintain integrity of electrical circuit, even up to melting temperature of metal core. The residue of fired silicone rubber or silica filled elastomer exhibit a form of white powder. There is no evidence of solidification of silica particles, even after heating at 1100°C. However, the addition of some ceramic phases results in reaction with silica matrix (starting at about 900°C) producing a liquid phase, what facilitates particle binding. At lower firing temperatures (600°C) the problem of binding between the product of pyrolysis (silica) and filler is also present, what results in formation of fragile surface shield. The problem can be overcome by the addition of certain inorganic materials to the silicone rubber matrix. The paper discusses their influence on ability of silicone rubber composites, additionally containing glassy phase, wollastonite, mica, aluminium hydroxide, montmorillonite or calcined caoline, to ceramization.

Abstract: Hierarchical structure of biomaterials comprised of three dimensional organic matrix and inorganic minerals exhibit unique morphologies and outstanding properties. Herein, modified bacterial cellulose (BC) nanofibres/CaCO₃ composites are prepared using the ammonium carbonate diffusion method at room temperature. The copper coinage shaped aragonite has been synthesized in the 3D matrix and investigated by X-ray powder diffraction, Fourier transform infrared spectrometry, and scanning electron microscopy. It can be concluded that the carboxyl modified BC nanofibres can control the mineralization in vivo by physical confinement within the organic structure to allow the growth of non-equilibrium morphology and chemical interaction to influence polymorph selection and oriented nucleation. The system which was to mimic the biomineralization in vivo can help to produce bio-mimetic materials similarly in living creatures.

Abstract: Highly ordered mesoporous carbon (CMK-3) was fabricated for the adsorption of cobalt from aqueous solutions. With the high surface area of1112.7m²/g and pore size of 17.2 nm, its abundant mesopores were benefit for providing channels for liquid propagate. In order to improve the adsorption properties,CMK-3 was modified by hydroxylation and amination. Fourier transform infrared (FTIR) spectroscopy can be seen that the amino group was successfully grafted onto the CMK-3 with highly ordered mesoporous structure. The functionalized ordered mesoporous carbon (CMK-3-EDA) ,CMK-3 and CMK-3-OX were used as absents for the adsorption of Co (II) from aqueous solution. The results showed that CMK-3-EDA were more twice effective in adsorption of Co (II) compared to CMK-3, which indicated that CMK-3-EDA had great potential for the adsorption of Co (II).

Abstract: Coal tar pitch (CTP) as carbon material was studied using 1,4-Benzenedimethanol (PXG) as modifier by p-toluene sulfonic acid (PTS) catalyst and the variety of rheological properties of modified coal tar pitch (MTP) was discussed. The apparent viscosity of CTP and MTP were measured using rotating coaxial-cylinder viscometer, and the relations between the apparent viscosity and temperature were studied. Thermal behaviors of CTP and MTP were analyzed with TG-DSC. The results showed that the viscosity of the MTP gradually decreased with the temperature rising and the viscous activation energy of the MTP is 72.92 kJ·mol^-1, which was beneficial to the process for producing carbon materials and the carbon yield was greatly improved. Therefore, coal tar pitch modified with PXG was qualified to be an excellent carbon precursor.