Comparison of gas sensing properties based on hollow and porous α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanotubes

Hollow and porous α-Fe₂O₃ nanotubes were successfully synthesized by single nozzle electrospinning method followed by annealing treatment. The crystal structures and morphologies of the as-prepared materials were characterized by X-ray diffraction and scanning electron microscopy, respectively. The as-prepared materials were applied to construct gas sensor devices which gas sensing properties were further investigated. The obtained results revealed that porous α-Fe₂O₃ nanotube gas sensors exhibit a markedly enhanced gas sensing performance compared with hollow α-Fe₂O₃ nanotube gas sensors, which was about three times higher to 100 ppm acetone at 240 °C. Interestingly, hollow and porous α-Fe₂O₃ nanotube gas sensors both showed fast response–recovery time and good selectivity, but the porous ones possessed the shorter recovery time. The improved properties could be attributed to the unique morphology of porous nanotubes. Thus, further improvement of performance in metal-oxide-semiconductors materials could be realized by preparation the unique porous structures of nanotubes. Moreover, it is expected that porous metal-oxide-semiconductors nanotubes could be further design as promising candidates for gas sensing materials.     

Measurement of second-order nonlinear optical susceptibility of cBN crystal synthesized at high pressure and high temperature

We demonstrate a transverse electro-optical modulator based on a tiny and irregular octahedral wafer of cubic boron nitride (cBN) crystal that is prepared by hexagonal boron nitride at high pressure and high temperature using nitride as the catalyst. A continuous wave semiconductor laser at the wavelength of 650 nm is used as a light source. A novel electrode fabrication is designed, a developed method different from the conventional transverse electro-optical modulator is introduced and the expression of the intensity of output beam is thought over. We obtain the half-wave voltage based on experiments of transverse electro-optical modulation. The second-order nonlinear optical susceptibility χ_ijk⁽²⁾(ω,0)=1.919×10⁻¹² m/V of cBN crystal is calculated by means of the half-wave voltage.     

金刚石生长相关技术的讨论

文章简单回顾了石墨、催化剂合金、叶蜡石的性能与金刚石生长的关系。石墨是转变成金刚石的唯一材料,石墨纯度是核心指标。元素Mn、Al、Cr等金属与碳形成碳化物稳定,不适合做催化剂,称难形成合成金刚石的金属;元素Ni、Co、Si等与碳形成碳化物不稳定,升高Fe-C中C活度,适合做催化剂,故称易形成合成金刚石的金属。高压下叶蜡石的密封性能来源于片状结构的紧密排列。     

热稳定金刚石聚晶的制备及表征

在高温高压条件下（5.6GPa,1200℃～1480℃）,以含硼金刚石微粉为原料,镍基合金为烧结助剂,采用熔渗法成功制备了热稳定金刚石聚晶（Thermally stable PCD）。通过X射线衍射（XRD）和扫描电镜（SEM）,研究了烧结温度对热稳定金刚石聚晶的物相成分、微观组织形貌的影响;并与普通金刚石聚晶进行了差热（DTA）、热重（TG）的对比分析测试,结合X射线光电子能谱（XPS）测试结果给出了相应解释。实验结果表明：在压强为5.6GPa条件下,温度在1300℃～1450℃区间内,才能实现热稳定PCD的烧结;此时形成的热稳定PCD的耐热性和抗氧化性相比普通PCD均有较大幅度提高。     

Nanostructured SiC ceramics prepared by a new process—Crystallization of interfacial glass

Large bulk and fully dense SiC based nanoceramics with average grain size of 50 nm and 20–30 wt.% nanometer sized α-Sialon, Al_xSi_{3 − x}O₆ and α-SiO₂ interfacial phases were prepared by a new process, crystallization of interfacial glass, using LMAS glass-coated SiC powder as starting material. The process involves two major steps: densification by hot pressing, and crystallization of interfacial glass by annealing treatment. The densification was controlled by interfacial glass content, hot pressing temperature, and hot pressing pressure; density 99.8% theoretical being reached for SiC/30 wt. % glass nanoceramics hot-pressed at 1520 °C and 22 MPa for 30 min. The crystallization was complete and nearly all the interfacial glass was transformed into nanocrystalline phases after 800 °C and 900 °C for 5 h annealing treatments. Plastic flow and rearrangement of particles and interfacial glass infiltration are densification mechanisms. A large number of nanometer sized SiC powder particles serve as nucleating agents, e.g. hetero-nucleation, and are responsible for interfacial glass crystallization. A characteristic of the present process is that there is no SiC grain growth during densification and interfacial glass crystallization.     

The tensile properties of a particulate reinforced al alloy in the temperature range -196–300 °c

The tensile properties of a particulate reinforced Al- Si-Mg alloy were determined in the temperature range — 196–300 °C. Microstructural coarsening of a peak aged (T61) material occurs at the higher temperatures (i.e. 200 and 300 °C), which, together with the test temperature, results in a considerable decrease in flow strength for both the composite and unreinforced alloy. However, the strengthening ratio (i.e. the ratio of composite to matrix strength, σ_c/σ_m actually increases with temperature. This effect is also observed when investigating the influence of test temperature alone (i.e. a stable matrix microstructure over the full temperature range). These results suggest that the primary relaxation mechanisms are damage accumulation and relaxation by plastic deformation of the matrix. There is no evidence of unrelaxed behaviour, even at −196 °C, and the strain rate used during testing is likely to be too high to allow diffusional relaxation to be operative at the higher temperatures