The structure and continuous stoichiometry change of 1DTbBrx@SWCNTs

HRTEM and HAADF STEM of 1DTbBr_x@SWCNT meta‐nanotubes reveal three structural modifications of 1D nanocrystals within single wall carbon nanotube channels attributed to a different stoichiometry of the guest crystal. For SWCNTs with diameters D_m > 1.4 nm a most complete tetragonal unit cell is observed. When crystallization occurs inside SWCNT with D_m < 1.4 nm 1D TbBr_x crystal deforms a nanotube to elliptical shape in cross section. In this case the 1D crystal unit cell becomes monoclinic, with possible loss of a part of bromine atoms. Two modifications of a monoclinic unit cell appear. One of them is characterized by single or pair vacancies in the structure of the 1D crystal. Another structure is explained by peripheral and central bromine atoms loss. An appearance of such modifications can be stimulated by electron irradiation. The loss of bromine atoms is in agreement with chemical analysis data. Electronic properties of obtained meta‐nanotubes are investigated using optical absorption and Raman spectroscopy. It is shown that intercalation of terbium bromide into SWCNTs leads to acceptor doping of SWCNTs. According to local EDX analysis and elemental mapping this doping can arise from significant stoichiometry change in 1D nanocrystal indicating an average Tb:Br atomic ratio of 1:2.8 ± 0.1.     

Reduced chemistries with the Quantemol database (QDB)

Typical feed gas mixtures used in technological and other plasmas may give rise to reaction networks involving several hundred reactions. Such chemistries are often too large to be used in full reactor simulations and it is therefore desirable to construct reduced chemistry networks which mimic as closely as possible the behavior of the full chemistry but employ far fewer individual reactions and species. Constructed chemistries are available from the Quantemol database (QDB) and two approaches to constructing reduced chemistry from these chemistries based on (a) physical intuition and (b) sensitivity analysis of dominant reaction pathways, are explored. In doing this it is necessary to consider different pressure and power regimes. Reduced chemistry sets are presented for CF₄ /O₂/N₂/H₂, for which 396 reactions and 52 species are reduced to 71 reactions and 26 species, and for pure O₂, for which 45 reactions and 10 species are reduced to 34 reactions.     

电子束蒸发沉积碲锗铅薄膜的微结构和化学组分研究

为了探寻碲锗铅(Pb_(1-x)Ge_xTe)薄膜的最佳沉积方式,在硅基片上采用电子束蒸发沉积碲锗铅(Pb_(0.78)Ge_(0.22)Te)薄膜。使用X射线衍射(XRD)、电子扫描显微镜(SEM)、能量散射X射线分析(EDAX)等手段对薄膜的微结构和化学配比特性进行了分析。发现碲锗铅薄膜为多晶结构,具有明显的择优取向,晶粒多为矩形,薄膜中未出现其它相关氧化物。与热蒸发膜层相比,电子束蒸发沉积的膜层有更为完善的晶体结构。     

小量偏离正分对多晶石榴石材料基本性能参数的影响

用普通陶瓷工艺制备了不同饱和磁化强度的一系列石榴石材料Y3-aRaFe5-bMebO12,测试了从富铁(缺钇)到缺铁(富钇)整个范围内每种材料的基本参数.研究了小量偏离正分(偏离量小于0.15)对石榴石材料基本性能参数的影响,并分析了材料介电损耗在富铁和富钇情况下的变化趋势差别如此巨大的原因.研究表明,对于小量偏离石榴石材料来说,介电损耗和剩磁比是最敏感的两个参数,而介电常数是最不敏感的参数.     

聚碳硅烷制备近化学计量SiC纤维的研究进展

综述了聚碳硅烷制备近化学计量比SiC纤维的研究进展,总结了H2在PCS纤维裂解过程中的作用机理,比较了纯H2气氛、分阶段不同气氛、全过程H2/Ar混合气氛条件下得到的SiC纤维的组成和性能变化规律.纯H2气氛800℃以上保温4h,可得到近化学计量比SiC纤维;分阶段气氛下,气氛转换温度为800℃,烧结至1300℃保温1h,可得到近化学计量比SiC纤维;混合气氛下,氢气浓度为60%左右,烧结至1300℃保温1h,可得到近化学计量比SiC纤维.在这些气氛条件下得到的近化学计量比SiC纤维,烧结后的高温力学性能均优于非化学计量比SiC纤维.     

Electrode characteristics of non-stoichiometric Ml(NiMnAlFe)_x alloys

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A Direct Comparison of the Fragmentation Test and the Microbond Pull-out Test for Determining the Interfacial Shear Strength

Experiments were conducted to compare the fragmentation test with the microbond pull-out test for determining the interfacial shear strength between carbon AS4 fibers and a thermoset matrix consisting of a Di-Glycidyl Ether of Bisphenol A (DGEBA) resin cured with a diamine (meta-phenylenediamine, m-PDA) curing agent. The results indicate that, for the microbond test, diffusion of the rather volatile m-PDA curing agent at early stages of cure leads to low values of interfacial shear strengths when compared with results obtained for the same system with the fragmentation test.

With the microbond test, a distinct relationship between the glass transition temperature of the droplets and their size is noticed. Smaller (< 150 μm) droplets have very low Tg's and are incompletely cured. While changing to a modified curing cycle and/or using a m-PDA-rich curing environment alleviates the diffusion problem, the interfacial shear strength values are still not in good agreement with the fragmentation test results. Microbond data from another system consisting of DGEBA resin cured with a different, less volatile dimaine curing agent indicates that diffusion of the curing agent becomes less severe as the volatility of the curing agent decreases and the corresponding microbond interfacial shear strengths agree better with fragmentation test results.     

Systematic study of back pressure and anode stoichiometry effects on spatial PEMFC performance distribution

A segmented cell system was applied to investigate the effects of the anode and cathode back pressure and hydrogen stoichiometry on fuel cell performance in terms of overpotential distributions along the flow field. The segmented cell system was designed with closed loop Hall sensors and a data acquisition system allowing simultaneous spatial electrochemical impedance spectra (EIS) measurements. It was determined that an increase in back pressure for the tested serpentine flow field design results in an improvement of the cell performance and uneven improvement of individual segments’ performance. In general, the performance and the overpotentials become more uniform downstream with an increase in the back pressure due to a decrease in activation and mass transfer losses. Spatial EIS data for the PEMFC operated at different back pressures support the overpotential analysis. Hydrogen stoichiometry variations do not affect the performance of the cell or the individual segments at low current density because there is no significant hydrogen concentration gradient in the flow field. However, at high current densities a reduction in hydrogen stoichiometry produces a slight decrease in performance for inlet segments while outlet segments showed a noticeable performance loss. The decrease in performance is attributed to an increase in mass transfer losses due to nitrogen diffusion from the cathode to the anode. This effect becomes more pronounced for the outlet segments due to a downstream nitrogen accumulation. Under high current density conditions, the cell is locally fuel starved even with a high fuel stoichiometry creating conditions leading to cell degradation by carbon corrosion. More importantly, this local degradation is masked by the overall cell performance which remains largely unaffected.     

Reaction of Zirconia with Silica at the Stoichiometry of Zircon

Zirconia was combined with silica and halide mineralizers and fired in a nitrogen atmosphere. X-ray analysis showed zircon was formed, demonstrating that an oxygen atmosphere is not necessary for the formation of zircon. The reaction mechanism is the same as that forming zircon in air, involving transport of the silica as a volatile halide, followed by diffusion of silica and oxygen across the product layer to a reaction site on the zirconia.     

Thermogravimetric method for determination of oxygen stoichiometry in superconducting 1:2:3 and metal-substituted 1:2:3 compounds

The paper presents a modified thermogravimetric method for determination of oxygen stoichiometry of the oxide superconductors YBa₂Cu₃O_7−δ and metal-substituted compounds of the type YBa_2−x La _x Cu₃O_7+δ and YBa₂(Cu_1−x M _x )₃O_7−δ (M=Mg/Zn) between 25° and 930°C.