WKB近似下的Fourier衍射成象方法

对于介质目标微波衍射成象，本文引入了ＷＫＢ近似来模拟目标内部总场。基于这种近似，我们导出了Ｆｏｕｒｉｅｒ衍射公式，并采用了广义滤波逆传播方法由目标空间谱实现目标特性的重建。计算机模拟结果表明采用ＷＫＢ近似重建目标特性较Ｂｏｒｎ近似有明显改善。     

A preliminary analysis of the biomass energy production potential in Africa in 2025 considering projected land needs for food production

This paper provides a preliminary examination of present and projected land use in Africa to estimate the potential availability of land in 2025 for use in producing biomass energy. Fifty countries are included in the analysis. Future cropland requirements are projected on the basis of average African cereal crop yield improvements since 1972, and minimum nutritional requirements are assumed to be met in 2025 without increasing imports above present absolute levels. Cropland, natural forests and other wilderness areas are excluded from consideration for biomass energy use. Woody biomass energy yields are estimated on the basis of nationally averaged precipitation, using a yield-precipitation correlation for commercial eucalyptus plantations in Brazil. The total African bioenergy production potential in 2025 is estimated to be about 18 EJ per year for a set of baseline assumptions that includes planting only 10% of the available non-crop, non-forest, non-wilderness area with biomass energy crops. A preliminary cost assessment suggests that much of this biomass could be produced for $1–2 GJ⁻¹. A number of uncertainties in the modelling assumptions are examined through a sensitivity analysis. Despite limitations in the model used here, one robust conclusion is that Africa as a whole has a significant biophysical potential for producing biomass energy. This result suggests that more detailed country and sub-country level assessments would be worthwhile to understand better the practical prospects for future biomass energy production in Africa.     

Cumulant calculations of thermodynamic quantities for the Hubbard and the emery model

The calculations of the first paper suggesting phase separation in the Hubbard model [2] are performed under modern computer facilties for the one and three-band Hubbard model. High-temperature approximations for specific heat and static correlation functions are obtained. The latter show, at these high temperatures, no tendency to phase separate in the same way as in the cited paper because our spin parallel nearest-neighbor charge correlation function is negative for all doping values. The spin antiparallel correlation function has positive and negative values for different doping regimes.Extension of this work to higher levels of iterations and to other correlation functions is in progress.     

Residual stresses and transformation toughening in MoSi2 composites reinforced with partially stabilized zirconia

The results of study of the effects of yttria stabilization (0–6 mol.%) on the room-temperature fracture behavior and toughening mechanisms in zirconia-reinforced MoSi₂ are presented in this paper. Transformation toughening is shown to occur only in composites reinforced with zirconia particles stabilized with 2 mol.% yttria. However, the fracture toughness levels are comparable in the other composites with yttria levels between 0 and 6 mol.%. Toughening in the other composites is attributed to the combined effects of residual stress, microcrack shielding/anti-shielding and/or crack deflection. A rigorous micromechanics-based model is presented for the estimation of residual stress levels in brittle materials reinforced with phases that can transform during cooling or under stress. The model is applied successfully to the rationalization of the observed fracture and toughening phenomena.     

Effect of type of processing on the microstructural features and mechanical properties of Al-Cu/SiC metal matrix composites

In this study, an aluminum based metallic matrix (Al-2wt.% Cu) was reinforced with SiC particulates using a conventional casting technique and a new disintegrated melt deposition technique. Microstructural characterization studies conducted on the samples taken from disintegrated melt deposition technique revealed a more uniform distribution of SiC particulates and good interfacial integrity between SiC particulates and metallic matrix when compared to the conventionally cast composite samples. Results of ambient temperature mechanical tests demonstrate an increase in 0.2% YS and ultimate tensile strength of samples taken from disintegrated melt deposition technique when compared with the unreinforced and conventionally cast composite samples. The results of microstructural characterization and mechanical testing were finally rationalized in terms of the nature of processing technique employed to reinforce Al-2wt.% Cu metallic matrix with SiC particulates.     

C2 用于声化学研究的大功率超声系统的研制

根据声化学反应的特殊要求，我们研制了上前用于声化学研究的大功率超声系统，由参数可调节的超声发生器配合不同频率的换能器工作，并用它组合成杯式幅杆结构声化学反应器，解决了有关声化学设备的一些技术问题。为声化学技术从实验室规模向工业应用过渡做了必要的准备。     

Importance of out-of-plane dispersion of the excitation spectrum of superconducting bilayers with Josephson coupling

We study the effect of Josephson coupling between adjacent superconducting layers on the BCS energy spectrum. We find that the interference between the gap functions of two layers can lead to vanishing condensation energy for perpendicular momenta corresponding to the formation of standing waves. We therefore predict a conventional energy spectrum for large interlayer spacings, if the gap of the single layers has no nodes, and in all cases a gapless spectrum for small spacings. Within the experimental error, our numerical results account for the low-temperature dependence of the penetration depth reported in Nd_1.85Ce_0.15CuO₄ and YBa₂Cu₃O_6.9.     

Superconductivity in nearly single-phase Bi-Pb-Sr-Ca-Cu-O samples with different nominal compositions

The preparation conditions, phase composition, and superconducting properties of Bi-Pb-Sr-Ca-Cu oxide materials from different nominal compositions have been investigated. Nearly single-phase samples from Bi₂Pb_0.4Sr₂Ca₃Cu₄O _y , as well as from the proposed compositions Bi_1.8Pb_0.4Sr₂Ca₃Cu₄O _y and Bi_1.8Pb_0.4Sr₂Ca_2.5Cu_3.5O _y were obtained by a solid-state reaction in air. Samples with the third nominal composition showed the best superconducting properties (T _on=111 K and zero resistance atT ₀=103 K). A possible mechanism for the 2223 phase formation in the three investigated compositions has been discussed.     

混酸PEG酯的合成及性能研究

用从大豆油脚水解制得的脂肪酸合成混酸聚乙二醇酯，通过测定酸值的方法确定了最佳反应时间为16h。与油酸聚乙二醇酯相比，混酸聚乙二醇酯具有更优良的乳化性、润湿性，可以替代油酸聚乙二醇酯用于纺织、农药和化妆品工业中。