PVT法钨坩埚系统中自发生长自支撑AlN单晶及其表征分析

采用物理气相传输(PVT)法在AlN原料表面自发生长出大量毫米级尺寸的AlN单晶.本文对该工艺下AlN单晶的自然形貌、极性、杂质含量等进行了分析.实验及分析结果表明,在实验工艺条件下,原料表面生长的AlN晶粒具有规则的六方外形,晶粒沿C向择优生长且具有高的生长速率(约200～ 250 μm/h),但径向生长受限于{10-10}(m面).不同颜色的AlN晶粒经机械切割及化学机械抛光(CMP)后,形成高表面质量的C轴取向抛光片.通过化学湿法腐蚀和SEM表征发现,淡黄色晶粒为Al极性晶体,暗棕色晶粒为N极性晶体,淡黄-暗棕混合色晶粒为Al/N混合极性晶体,其内部可以观察到清晰的两种极性分界.通过GDMS与EGA对不同颜色晶粒内部的主要杂质元素含量进行了分析,结果表明,淡黄色晶粒内氧元素的含量相比暗棕色晶粒的含量低,而碳含量则相反.     

PVT法AlN单晶生长技术研究进展及其面临挑战

氮化铝(AlN)具有超宽禁带宽度(6.2 eV)、高热导率(340 W/(m·℃))、高击穿场强(11.7 MV/cm)、良好的紫外透过率、高化学和热稳定性等优异性能,是氮化镓基(GaN)高温、高频、高功率电子器件以及高Al组分深紫外光电器件的理想衬底材料.物理气相传输(PVT)法是制备大尺寸高质量AlN单晶最有前途的方法.本文介绍了AlN单晶的晶体结构、基本性质及PVT法生长AlN晶体的原理与生长习性.基于AlN单晶PVT生长策略,综述了自发形核工艺、同质外延工艺及异质外延工艺的研究历程,各生长策略的优缺点及其最新进展.最后对PVT法生长AlN单晶的发展趋势及其面临的挑战进行了简要展望.     

A Mathematical Model for Determining Oil Migration Characteristics in Low-Permeability Porous Media Based on Fractal Theory

Shale reservoirs are characterized by very low permeability in the scale of nano-Darcy. This is due to the nanometer scale of pores and throats in shale reservoirs, which causes a difference in flow behavior from conventional reservoirs. Slip flow is considered to be one of the main flow regimes affecting the flow behavior in shale gas reservoirs and has been widely studied in the literature. However, the important mechanism of gas desorption or adsorption that happens in shale reservoirs has not been investigated thoroughly in the literature. This paper aims to study slip flow together with gas desorption in shale gas reservoirs using pore network modeling. To do so, the compressible Stokes equation with proper boundary conditions was applied to model gas flow in a pore network that properly represents the pore size distribution of typical shale reservoirs. A pore network model was created using the digitized image of a thin section of a Berea sandstone and scaled down to represent the pore size range of shale reservoirs. Based on the size of pores in the network and the pore pressure applied, the Knudsen number which controls the flow regimes was within the slip flow regime range. Compressible Stokes equation with proper boundary conditions at the pore’s walls was applied to model the gas flow. The desorption mechanism was also included through a boundary condition by deriving a velocity term using Langmuir-type isotherm. It was observed that when the slip flow was activated together with desorption in the model, their contributions were not summative. That, is the slippage effect limited the desorption mechanism through a reduction of pressure drop. Eagle Ford and Barnett shale samples were investigated in this study when the measured adsorption isotherm data from the literature were used. Barnett sample showed larger contribution of gas desorption toward gas recovery as compared to Eagle Ford sample. This paper has produced a pore network model to further understand the gas desorption and the slip flow effects in recovery of shale gas reservoirs.

     

New nickel(II) diimine complexes bearing phenyl and sec‐phenethyl groups: synthesis,characterization and ethylene polymerization behaviour

A series of nickel(II) catalysts containing phenyl and chiral sec‐phenethyl groups, {[(4‐R₁‐2‐R₂C₆H₂N?C)₂Nap]NiBr₂} (Nap: 1,8‐naphthdiyl, R₁ = Me, R₂ = Ph ( 3a ); R₁ = Me, R₂ = sec‐phenethyl ( 3b ); R₁ = Cl, R₂ = sec‐phenethyl ( 3c ); R₁ = Me, R₂ = Me ( 3d ) were synthesized and characterized. All organic compounds were fully characterized by FT‐IR and NMR spectroscopy and elemental analysis. The single crystal for X‐ray crystallography was isolated from 3a in CH₂Cl₂/n‐hexane under air; the crystal structure showed a binuclear complex 3a ′, in which each nickel atom was six‐coordinate. The two nickel atoms together with two bromine atoms form a planar four‐membered ring, with a bromine and H₂O axial ligands. These complexes, activated by diethylaluminum chloride and chiral nickel pre‐catalysts rac‐ 3c , exhibited good activities (up to 2.85 × 10⁶ g PE (mol Ni h bar)^?1) for ethylene polymerization, and produced polyethylene products with a high degree of branching (up to 117 branched per 1000 carbons) at high temperature. The type and amount of branches of the polyethylenes obtained were determined by ¹H and ¹³C NMR spectroscopy. Copyright © 2014 John Wiley & Sons, Ltd.     

Rechargeable Room‐Temperature Na–CO2 Batteries

Developing rechargeable Na–CO₂ batteries is significant for energy conversion and utilization of CO₂. However, the reported batteries in pure CO₂ atmosphere are non‐rechargeable with limited discharge capacity of 200 mAh g^?1. Herein, we realized the rechargeability of a Na–CO₂ battery, with the proposed and demonstrated reversible reaction of 3 CO₂+4 Na?2 Na₂CO₃+C. The battery consists of a Na anode, an ether‐based electrolyte, and a designed cathode with electrolyte‐treated multi‐wall carbon nanotubes, and shows reversible capacity of 60000 mAh g^?1 at 1 A g^?1 (≈1000 Wh kg^?1) and runs for 200 cycles with controlled capacity of 2000 mAh g^?1 at charge voltage <3.7 V. The porous structure, high electro‐conductivity, and good wettability of electrolyte to cathode lead to reduced electrochemical polarization of the battery and further result in high performance. Our work provides an alternative approach towards clean recycling and utilization of CO₂.     

Evaluation and application of a mixed‐mode chromatographic stationary phase in two‐dimensional liquid chromatography for the separation of traditional Chinese medicine

In this study, two mixed‐mode chromatography stationary phases (C8SAX and C8SCX) were evaluated and used to establish a two‐dimensional liquid chromatography system for the separation of traditional Chinese medicine. The chromatographic properties of the mixed‐mode columns were systematically evaluated by comparing with other three columns of C8, strong anion exchanger, and strong cation exchanger. The result showed that C8SAX and C8SCX had a mixed‐mode retention mechanism including electrostatic interaction and hydrophobic interaction. Especially, they were suitable for separating acidic and/or basic compounds and their separation selectivities could be easily adjusted by changing pH value. Then, several off‐line 2D‐LC systems based on the C8SAX in the first dimension and C8SAX, C8SCX, or C8 columns in the second dimension were developed to analyze a traditional Chinese medicine—Uncaria rhynchophylla. The two‐dimensional liquid chromatography system of C8SAX (pH 3.0) × C8SAX (pH 6.0) exhibited the most effective peak distribution. Finally, fractions of U. rhynchophylla prepared from the first dimension were successfully separated on the C8SAX column with a gradient pH. Thus, the mixed‐mode stationary phase could provide a platform to separate the traditional Chinese medicine in practical applications.     

A continuous process for the reductive deoxygenation of aromatic ketones over Cu30Cr10/γ-Al2O3

A continuous process for reduction of aromatic ketones to the corresponding aromatic alkanes over Cu₃₀Cr₁₀/??-Al₂O₃ was established in a fixed-bed reactor. A series of aromatic ketones were examined, and all of the corresponding aromatic hydrocarbons were obtained in more than 95% yield. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and H₂-temperature programmed reduction. Cu⁰ is believed to be the active center of the catalyst. Cr was found to have an important effect on catalytic activity and stability.     

用定量会聚束电子衍射技术对二十面体AlPdMn准晶结构的优化

对n0，n1，b1和d0等4个位置上的7个原子面，用位置对称性允许的面谐函数描述原子面边界的形状和大小，用定量会聚束电子衍射技术对描述这7个原子面的形状与大小的15个参数进行了精修，得到了一个新的描述二十面体AlPdMn准晶结构的高维空间模型．由这个模型，在三维物理空间用切割法得到了比不考虑棱边中点d0处原子面的球形模型和非球形模型更加完整的原子结构，原子间距不合理的情况得以消除．通过解析式表述的这一模型，与用菱形多面体和91个参数描述原子面的6维团簇模型相比适用性更强．     

二氧化锰纳米材料水热合成及形成机理研究进展

不同晶型和形貌MnO2纳米材料由于具有离子筛、分子筛、催化和电化学等许多特殊的物理和化学性质,因而在吸附材料、催化材料、锂离子二次电池的正极材料和新型磁性材料等领域显示了广阔的应用前景。纵观合成MnO2纳米材料的各种方法,水热合成由于简单、易于控制,并且能够有效控制其晶型、形貌和尺寸,深受研究者的青睐。本文结合国内外的...