Porous ceramics mimicking nature—preparation and properties of microstructures with unidirectionally oriented pores

Porous ceramics with unidirectionally oriented pores have been prepared by various methods such as anodic oxidation, templating using wood, unidirectional solidification, extrusion, etc. The templating method directly replicates the porous microstructure of wood to prepare porous ceramics, whereas the extrusion method mimics the microstructures of tracheids and xylems in trees. These two methods are therefore the main focus of this review as they provide good examples of the preparation of functional porous ceramics with properties replicating nature. The well-oriented cylindrical through-hole pores prepared by the extrusion method using fibers as the pore formers provide excellent permeability together with high mechanical strength. Examples of applications of these porous ceramics are given, including their excellent capillary lift of over 1 m height which could be used to counteract urban heat island phenomena, and other interesting properties arising from anisotropic unidirectional porous structures.     

Initial stages of high-temperature CaF2/Si(001) epitaxial growth studied by surface X-ray diffraction

Surface X-ray diffraction was applied to study structure of the fluorite-silicon interface forming upon epitaxial growth of CaF2 on Si(001) surface kept at 750 degrees C. Samples with CaF2 coverage of 1.5-4 (110)-monolayers were grown and in-situ characterized using synchrotron radiation. The 3 x 1-like surface reconstruction was observed in agreement with the previous studies by electron diffraction. Interestingly, a well pronounced splitting of the fractional x 1/3 reflections was revealed. This splitting was ascribed to the effect of antiphase domain boundaries in the row-like structure of the interface layer. The in-plane integrated intensities were used to reconstruct two-dimensional atomic structure of the high-temperature CaF2/Si(001) interface.     

Synthesis of SnS nanoparticles by SILAR method for quantum dot-sensitized solar cells

SnS-sensitized TiO2 electrodes were applied in quantum dot-sensitized solar cells (QDSSCs) which are environmentally more favorable than conventional Cd or Pb-chalcogenide-sensitized electrodes. SnS nanoparticles were well-distributed over the surface of TiO2 nanoparticles by the successive ionic layer adsorption and reaction (SILAR) method. Deposited SnS nanoparticles had diameter about 3 nm. Under AM1.5 irradiation with 100 mW/cm2 light intensity (at 1 sun), the energy conversion efficiency of obtained cells reached a value of 0.21% (0.25 cm2) at SILAR coating cycles of 5. In addition, the photovoltaic performance was improved by additional ZnS coating on the surface of SnS-sensitized TiO2 electrodes.     

Second-Order Efficiency for Two-Stage Estimation of a Linear Function of Normal Mean Vectors when Covariance Matrices Have Some Structures

Abstract

We consider two-stage estimation for a fixed-span confidence region about a linear function of mean vectors from π _i : N _p (μ _i , Σ _i ), i = 1,…,k (≥2), when Σ _i 's have some structures. The purpose of this article is to investigate asymptotic efficiency of the estimation up to the second order in terms of the sample size. An adjustment of the design constant and a proper choice of the initial sample size appearing in the two-stage estimation are proposed to have asymptotic second-order efficiency. Some simulations are carried out to see moderate sample size performances of the proposed two-stage estimation. An example is given for a demonstration.     

采用局部加热法选择性制备碳纳米管和纳米金刚石晶体

通过局部加热系统中的化学气相沉积法（chemical vapor deposition,CVD）实现了碳纳米管（carbon nanotube,CNT）和金刚石晶体的选择性生长加热系统只加热硅衬底而对反应过程及担载气体没有加热作用．在衬底温度为700℃时,没有生成CNT或明显的金刚石颗粒当温度升到740℃时,仍没有CNT生成,但是在模样化的铁膜上生成许多尺寸为几十纳米的金刚石颗粒温度为770℃时,在铁膜的中央部位生成许多尺寸为几十到几百纳米的金刚石颗粒,而在铁膜的边缘部位可同时观测到一些CNT的生成．当温度达到850℃时,CNT的生长区域扩大而纳米金刚石的平均尺寸和生成密度减小.在910℃的高温下,生成了大量的CNT,其平均直径为20nm,和通常的热CVD法生成的cNT相同在较低的衬底温度下,表面催化反应占主导地位而可能诱导具有sp^3结构的纳米金刚石的生成随着衬底温度的增加,围绕在衬底周围的气体被加热,在达到其气相自聚合温度后形成不饱和碳氢链,这些生成的碳氢链在sp^2结构CNT的生成中起到了重要的作用．     

Effect of Hydrogen Atmosphere on Pyrolysis of Cured Polycarbosilane Fibers

SiC-based fibers with various chemical compositions were synthesized using an irradiation-curing process. Polycarbosilane (PCS) fibers were cured by irradiation with an electron beam in a helium atmosphere. The cured PCS fibers were pyrolyzed at 1300°C under controlled hydrogen or argon atmospheres, and SiC fibers with C/Si of 0.84 to 1.56 were obtained. The fibers consisted of <1.0 wt% O, <0.2 wt% N, <0.1 wt% H, with the balance being Si and C. The mechanism of pyrolytic transformation of cured PCS to SiC-based ceramics was investigated using TG/DTA analysis. Greater mass losses were observed during pyrolysis in a hydrogen atmosphere than in argon. This result suggests that the hydrogen atmosphere suppresses H₂ evolution and helps to remove excess carbon as CH₄ during pyrolysis. The microstructure and mechanical properties of the resulting SiC-based fibers were found to be very dependent on their C/Si chemical compositions.     

Preparation and properties of negative thermal expansion Zr2WP2O12 ceramics

Using solid-state reaction method, Zr₂WP₂O₁₂ powder was synthesized for this study. The optimum heating condition was 1200 °C for 4 h. The obtained powder was compacted and sintered. The relative density of the Zr₂WP₂O₁₂ ceramics with no sintering additive was 60%. That of samples sintered with more than 0.5 mass% MgO was about 97%. The average grain size (D₅₀), as estimated from the polished surface of a sample sintered at 1200 °C for 4 h was about 1 μm. The obtained ceramics showed a negative thermal expansion coefficient of about −3.4 × 10⁻⁶ °C⁻¹. Young's modulus, Poisson's ratio, three-point bending strength, Vickers microhardness, and fracture toughness of the obtained ceramics were, respectively, 74 GPa, 0.25, 113 ± 13 MPa, 4.4 GPa and 2.3 MPa m^1/2.     

INTERMETALLIC COMPOUNDS AND THEIR FORMATION REACTIONS IN SYSTEM Fe-Cr-Mo AT HIGH-TEMPERATURES

The stable ranges concerning the composition and temperature have been experimentally de-termined for the intermetallic compounds in system Fe-Cr-Mo at high-temperatures. Theisothermal section of the Fe-Cr-Mo ternary phase diagram at 1250℃ and the formation re-actions for the intermetallic compounds, which are different in part from those shown in theliterature, have been proposed.     

Improvement in Mechanical Properties of Powder-Processed MoSi2 by the Addition of Sc2O3 and Y2O3

Additions of 1-20 mol% Sc₂O₃ or Y₂O₃ to MoSi₂ eliminate glassy SiO₂, which improves mechanical properties at both ambient and high temperatures. In particular, only 1 mol% ScO₃ additions dramatically enhance three-point bending strength from 521 to 1081 MPa. Vickers hardness, Young's modulus, fracture toughness, and high-temperature strength are also improved by this low level of additive. The improvement of mechanical properties is attributed to the formation of crystalline silicates: Sc₂Si₂O₇, Y₂Si₂O₇, Y₂SiO₅, and Y₄Si₃O₁₂, which are analyzed by XRD, SEM-EDS, and TEM-EDS methods.     

一种微小物体精密定位的直线微工作台

提出了一种基于平滑冲击驱动原理的精密定位直线微工作台.通过采用微小强磁铁作为摩擦元件,减小了由摩擦元件磨损而产生的摩擦力的变化,从而提高了微工作台的稳定性.设计制作了微工作台样机,并对精密定位工作台的动态特性和静态特性进行了实验研究.结果表明,微工作台体积小(20mm×10 mm×3 mm),可实现6 nm的精确定位,最高驱动速度可达5.4 mm/s,最大行程可达20 mm.