Papers by Author: Hai Feng Cheng

Abstract: Thermochromic vanadium dioxide (VO2) exhibits a semi-conducting to metallic phase transition at about 68°C, involving strong variations in electrical and optical properties. A simple method was proposed to prepare VO2 thin films from easily gained V2O5 thin films. The detailed thermodynamic calculation was done and the results show that V2O5 will decompose to VO2 when the post annealing temperature reaches 550°C at the atmospheric pressure of less than 0.06Pa. The initial V2O5 films were prepared by sol-gel method on fused-quartz substrates. Different post annealing conditions were studied. The derived VO2 thin film samples were characterized using X-ray diffraction and X-ray photoelectron spectroscopy. The electrical resistance and infrared emissivity of VO2 thin films under different temperatures were measured. The results show that the VO2 thin film derived from the V2O5 thin film annealed at 550°C for 10 hours is pure dioxide of vanadium without other valences. It was observed that the resistance of VO2 thin film with thickness about 600nm can change by 4 orders of magnitude and the 7.5-14μm emissivity can change by 0.6 during the phase transition.

Abstract: In this paper, effect of heat treatment on the SSA of Si-C-O fibers was investigated and morphologies of the treated fibers were studied using SEM. The results revealed that weight loss was proportional to the treatment time at 1573K and the specific surface area (SSA) increased sharply when the weight loss reached above 6wt%. A rough and porous ceramic fiber with SSA of 23.76m2/g could be obtained at the weight loss of 9.1wt%, as a result of the treatment at 1573K for 32h.

Abstract: In the preparation of polymer-derived SiC fibers, nanochannels are believed to be formed in the early pyrolysis stages due to loss of large volumes of pyrolysis gases. In this paper, small angle X-ray scattering (SAXS) was applied to the characterization and calculation of nanochannels in partiallypyrolyzed SiC fibers. The SAXS measurements showed that nanochannels with a radius of 1.0-20 nm were formed for fibers heat-treated at 973K and 1173K. But their distributions were not continuous and at lower part of the distribution, a peak value was observed at about 1.2nm. This means if the nanochannels are finely controlled, the partially-pyrolyzed SiC fibers have great potential application in the fields of hydrogen storage, gas separation, and so on.

Abstract: 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.