Hai Feng Cheng

Reducing Thermal Conductivity of Polymer Derived SiC Ceramics via Microwave Sintering Processing

Generally, high thermal conductivity of SiC ceramics is required. However, in some circumstances, low thermal conductivity SiC is also needed, for example, for good thermoelectricity or heat insulation. In this work, the product of pyrolysis of polycarbosilane (PCS) is chosen as raw materials. The microwave sintering process was applied to obtain low thermal conductivity at a relatively low sintering temperature. The microwave sintering processes were performed at 1200 °C under Ar atmosphere with dwelling time of 10, 20 and 30 min. The morphology and phases were analyzed by electric microscopy and X-ray diffraction (XRD), respectively. The thermal diffusivities, specific heat and thermal conductivities were measured by laser-flashed method using a thermal analyzer. The thermal conductivity of samples is within the range from 0.81 to 1.37 W/m•K. The sample with dwelling time of 30 min shows potential for multiple applications, due to its good crystallization in XRD spectra and relatively low thermal conductivity in the considered temperature range. The results also suggest a phase transformation at a dwelling time near 20 min that may have occurred.

Numerical Investigations on Thermoelectric Recovery of SiC Ceramics for Supersonic Vehicles Structures

The thermoelectric SiC structural materials for supersonic vehicles can convert intense aerodynamic heat to electricity simply by temperature difference. A general model of nose tip is developed to predict the thermal-electrical energy con-version performance of the thermoelectric SiC materials. The temperature distributions of model was obtained by solving the Navier-Stokes (N-S) equations and the heat conduction equation. The largest temperature difference between the hot side and cold side of the hypothetical thermoelectric module is about 275 K. With the thermoelectric properties assumed constant in the presence of temperature gradient, the output power and thermoelectric efficiency of the model are calculated. The maximum of thermoelectric efficiency and output power of the model are 0.4×10-3 W and 1.6×10-4 %, respectively, at a current of 0.014 A. The thermoelectric performance of the model shows great potential for the application of SiC ceramic structures to thermoelectric generation from aerodynamic heat on supersonic vehicles.

Microstructure of Silicon Nitride Fibers at Elevated Temperatures

The composition and microstructure of silicon nitride fibers after heat-treatment at elevated temperatures were investigated by XRD, NMR, XPS, SEM and TEM analyses. The results show that as-received fibers consisted of amorphous silicon nitride, and a little Si-C-O structure. During heat-treatment process, α-Si3N4 and β-Si3N4 formed resulting from the crystallization of amorphous silicon nitride, and the formation of β-SiC derived from the decomposition of Si-C-O structure. As heat-treated temperature increased from 1400oC to 1600oC, the above phenomenon become obvious, indicating that the fiber would possess high serving life with serving temperature lower than 1400oC. The tensile strength of fibers stays stable when heat-treated temperature was below 1200oC, while the strength retention of fibers sharply decreased to 50% after heat-treatment at 1400°C.

Effect of Templates on the Electrochromic Properties of Tungsten Oxide Thin Films

The mesoporous tungsten oxide (WO3) films were derived from the peroxotungstic acid (PTA) sol with templates through sol-gel method. Polyethylene glycol (PEG) 400 and tri-block polymer P123 (HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H) were chosen as templates. The structural, morphological, optical and electrochromic properties of the WO3 thin films derived from different sols were studied. The composition and crystal phase of the films change at different annealing temperatures. The films derived from the sols containing templates have higher crystallization temperature than those without templates. And the morphologies are distinctly different from different sols. The addition of the templates can improve the electrochromic properties of the WO3 films, and those prepared from the 3% of P123 sol show the best electrochromic properties. The highest transmittance modulation is near 60%, and the largest ion inserted and deinserted diffusion coefficient can reach 5.706×10-12 cm2/s and 1.271×10-11 cm2/s, respectively.

Study on Thermophysical Properties of Partially Oxidized Polymer Derived SiC Composites

SiC ceramics were synthesized by polymer derived method and microwave sintering processes. The as-produced SiC composites were developed by oxidizing the amorphous Si-C raw materials derived from a polymer precursor polycarbosilane. The products analyzed by X-ray diffraction (XRD), Fourier transmittance infrared spectroscopy (FTIR), and electronic microscopye consist of excess free carbon, β-SiC and SiO2 produced by the partial oxidation of amorphous Si-C on the surface of the starting particles. The thermophysical properties of the partially oxidized SiC composites were investigated by measuring the thermal diffusivity, the specific heat and thermal conductivity. The amount of SiO2 increased as the sintering temperature is increased, resulting in a reduction of thermal diffusivity and an increase of density due to improved liquid phase sintering of SiC particles. The thermal conductivity of samples decreased, instead of increased, as the sintering temperature was increased from 1100 °C to 1200 °C. The sample dwelled at 1200 °C, in this study, has highest density and a relatively low thermal conductivity, slightly over 0.7 W/(m·K) at room temperature.

Formation Mechanism of Interior Microflaws in the Preparation of Polymer-Derived Silicon Carbide Fibers

Microflaws were detected on the cross-section of polymer-derived silicon carbide fibers and their formation mechanism was studied by varying the curing degree and the firing rate. The results show that microflaws decrease in size with increase of the curing degree due to an increased ceramic residue. The results also show that microflaws decrease in size with decrease of the firing rate. No microflaws are detectable when the firing rate is as low as 10K/h. This indicates that the microflaws are the main channels of evolution gases and the pressure of these gases leads to their formation and propagation. So a high curing degree and a low firing rate are both preferred in the preparation of dense silicon carbide fibers.

The Electrochemical Deposition of Fe Nanowires and the Influence of Current Density on the Deposition Rate

The Fe nanowire arrays are prepared by the method of alternating current, constant voltage and constant current respectively. the morphology and composition of the Fe nanowires have been characterized By scanning electron microscope (SEM), Energy-dispersive X-ray spectroscopy (EDS). The Fe nanowires growth rate with different current density was studied in anodic aluminum oxide (AAO) template for pore diameter of 85 nm, pore depth of 55 μm under the same total charge. Research shows that the deposition rate of nanowires first slightly decreased, dramatically increased again and then reduced greatly with the increase of current density from 1 mA/cm2 ~ 5 mA/cm2.

Microstructure Evolution of Hybrid TiO2 Nanowire-Nanotube Structure Fabricated by Anodization

Hybrid TiO2 nanowire–nanotube structure was synthesized by a facile anodization on Ti substrate. To study the origin and evolution of TiO2 nanotubes, the morphology of TiO2 nanotubes was investigated. It was found that nanotubes corrode gradually into nanowires, and the critical time was about 8.5 h after the beginning of anodization. Uniform nanotubes were obtained by ultrasonic cleaning. It was demonstrated experimentally that the inner diameter of TiO2 nanotube increased with the extension of anodization time, but the outer diameter of TiO2 nanotubes, which were fabricated under different anodization time, was almost the same.

Influence of Temperatures on the Formation of SiBCN Powders Prepared by CVD Using Borazine and Liquid Polycarbosilane

SiBCN powders have been prepared by chemical vapor deposition using borazine and liquid polycarbosilane as source precursors in the deposition temperature range from 800 °C to 1100 °C, with the pressure of 10 kPa. The effect of the temperatures on the surface morphologies of the as-received powders was investigated by SEM. FTIR analysis of the powders was used to obtain the composition of the powders, and the XRD analysis was used to determine the crystalline state of the deposits. The results showed the powders were amorphous and composed of normally spherical grains.

Design, Preparation and Microwave-Absorbing Properties of Sandwich-Structure Radar-Absorbing Materials Reinforced by Glass and SiC Fibres

In order to broaden the absorbing bandwidth of radar-absorbing materials (RAMs), a type of sandwich-structure RAMs (SSRAMs) derived from a Salisbury absorber and comprising two dielectric layers and one resistive sheet was investigated. In this paper, the impedance characteristics of the SSRAMs were analysed and the mechanisms of broadening microwave-absorbing bandwidth were interpreted using a Smith chart. In order to realise the study’s SSRAMs, plain-woven glass fibre fabric and silicon carbide (SiC) fibre fabric with low electrical resistivity were employed as reinforcements of the dielectric layers and lossy layer, respectively. The microwave-absorbing properties of the SSRAMs were measured and compared with simulated results. The results showed that the experimental and simulated results were in good agreement, that the SSRAMs had better wideband microwave-absorbing properties and that the microwave-absorbing bandwidth at reflectivity below −10 dB can reach 11.6 GHz.