非奇异H-矩阵的一个简捷判别定理

设A=(aij)∈Cn×n,若存在α∈(0,1),使■i∈N,有|aii|≥Rαi(A)S1-αi(A)成立,则称A为α链对角占优矩阵。利用α-链对角占优矩阵、不可约α-链对角占优矩阵、广义严格α-链对角占优矩阵等概念及性质,给出了非奇异H-矩阵的一个简捷判别定理。从而改进和推广了相应的一些结果,并给出相应的数值例子说明结果的有效性。     

Preparation and characterisation of α-methylstyrene–butadiene latexes for paper coating applications

The purpose of this work is to demonstrate how α-methylstyrene (AMS) can replace styrene in preparing styrene–butadiene (SB) type latexes and to compare the properties of the paper coating of the prepared α-methylstyrene–butadiene emulsion with the commercial styrene–butadiene latex reference sample. A lot of work is nowadays being conducted on different biorefinery concepts replacing fossil oil with biomass based raw materials due to the expected rise of the fossil oil cost. Aromatics can in principle be produced from renewable raw materials, such as lignin, sugars and terpenes for example. The potential methods include thermochemical conversions, catalytic fast pyrolysis, metabolic engineering, catalytic aromatisation and dehydrogenation among others. Terpenes, such as α-limonene and pinene, are possible sources of aromatics, and they can indeed be catalytically converted to p-cymene. Industrial hydrodealkylation and disproportionation processes developed by major petrochemical companies can further convert p-cymene to BTX aromatics or simultaneously dehydrogenate the alkyl chain of p-cymene to styrenic monomers such as α-methylstyrene. Based on the measured paper properties for uncalendered and calendered coated samples, AMS proved to be adequate to replace the oil based styrene in commercial reference SB latexes. Even though the emulsion polymerisation for the α-methylstyrene–butadiene latex was not optimised, almost all tested properties were at least equally good as in the commercial reference sample. α-Methylstyrene containing coating colours had slightly higher viscosity than the other coating colours. Coating colours containing α-methylstyrene seems to have an improved water retention compared to the commercial reference styrene–butadiene latex coating colour and the laboratory prepared styrene–butadiene coating colour. The paper coated with the commercial reference latex containing coating colour was less porous than the other coated papers. Despite of that, both dry and wet surface strength were at least equally good as in the case of the commercial reference latex. The results are promising when thinking of the future development of the bio-based latexes.     

Unveiling crystallization mechanism for controlling nanocrystalline structure in glasses

Controlling nanocrystalline structure in glasses renders the exploration of new composite multiphase (glass-ceramic) materials with novel functionalities that determined by the precipitated nanocrystals and residual glassy matrix. Previous microstructural investigation of glass-ceramics focused only on one aspect of nanocrystalline structures, e.g., nano-polycrystalline or single nanocrystalline. The recognition of the microscopic mechanism of nanostructure formation in glasses is absent. Here, we use advanced microscopic techniques to show the formation of different nanocrystalline structures composed of nano-polycrystals and single nanocrystals in 80GeS₂·20In₂S₃ and 72.5GeS₂·14.5Sb₂S₃·13RbCl glasses, respectively. Crystallization mechanism for controlling the nanocrystalline structure in glasses was revealed to depend on whether the glass network former participates in crystallization process. The results may shed light not only on glass crystallization mechanism, but also on the fundamental nature of the network structure of chalcogenide glasses.     

Grain size dependence of hardness in nanocrystalline silicon carbide

The response of nanocrystalline silicon carbide (nc-SiC) to nanoindentation is investigated using molecular dynamics (MD) simulation. It is found that the hardness of the nc-SiC decreases with decreasing grain size, showing an inverse Hall-Petch relationship. The behavior is primarily attributed to the reduced number of intact covalent bonds with grain refinement. Dislocation nucleation and growth in nc-SiC are strongly suppressed by the grain boundaries (GBs). In addition to the dislocation region in the grains, the indentation-induced amorphization of nanograins proceeds preferentially from the GBs, leading to grain shrinkage until the grains are fully amorphized. The results provide an improved understanding of the mechanical properties in nc-SiC and other nanostructured covalent materials.     

Thermal stability and heat flux investigation of neutron-irradiated nanocrystalline silicon carbide (3C–SiC) using DSC spectroscopy

Nanocrystalline silicon carbide (3C–SiC) particles have been irradiated by neutron flux (2 × 10¹³ n∙cm⁻²s⁻¹) up to 5 h at the TRIGA Mark II type research reactor. At the present work, thermal properties of nanocrystalline 3C–SiC are comparatively investigated before and after neutron irradiation at the 300 K < T < 1300 K ranges. Simultaneously, the DSC (Scanning Calorimetry), TGA (Thermogravimetric Analysis) and DTG (Differential Thermogravimetric Analysis) experiments were conducted from 300 K up to 1300 K. Oxidation mechanism of nanocrystalline 3C–SiC particles have been theoretically and experimentally studied before and after neutron irradiation. The kinetics of mass and heat flux were analysed at the heating and cooling processes using DSC spectroscopy.     

鱼眼草抗氧化、α-糖苷酶抑制活性和抑菌活性研究

通过对DPPH自由基的清除能力对鱼眼草的抗氧化活性进行评价;考察了鱼眼草的抗氧化、α-葡萄糖苷酶抑制活性和抑菌活性。应用体外α-葡萄糖苷酶筛选模型进行鱼眼草酶抑制活力的测定;采用KB纸片法和肉汤稀释法测定鱼眼草的抑菌活性。结果:鱼眼草的化学成分及各部位提取物的抗氧化活性并不理想;化合物脱镁叶绿素甲酯(IC50=19.23 mg.L-1)和柳穿鱼素-7-葡萄糖甙(IC50=30.27 mg.L-1)的体外抑制α-糖苷酶抑制活性远高于对照acarbose(IC50=1081.27 mg.L-1);KB纸片法表明部分化合物和提取物具有较好的抑菌活性,肉汤稀释法表明乙酸乙酯部位的抑菌活性较好(金黄色葡萄球菌、肺炎克雷伯氏菌、铜绿假单胞菌的MIC分别为32、64、128μg.mL-1)。     

有无混合器对圆形直管内空气传热膜系数α影响的研究

研究了空气分别在有无混合器的圆形直管内湍流流动时的传热膜系数α,比较了两种情况下α及准数关联式的差异。结果表明:在相同条件下,空气在有混合器的强化传热管中其α远高于无混合器的圆形直管;且空气在强化管内湍流时的α其准数关联式不同于圆形直管。     

有机催化β-酮酸酯不对称α-羟基化反应研究进展

手性α-羟基β-酮酸酯是多种天然产物及医药产品的重要中间体,获得这一类结构单元最简单的方法是β-酮酸酯的直接不对称α羟基化,因此该反应在医药工业和精细化工领域具有重要的研究价值。本文针对小分子金鸡纳碱衍生物、二萜类生物碱、芳氧基氨基醇及手性相转移催化剂直接氧化β-酮酸酯不对称α-羟基化反应研究,该反应的催化机理被认为是氢键、π-π键等多种分子间力协同作用,同时还需要适当的位阻基团遮蔽以获得羟基化反应立体选择性。     

保护的3,6位支化葡萄三糖选择性合成

以2,3,4,6-四-O-苯甲酰-α-D-吡喃葡萄糖三氯乙酰亚胺酸酯为糖基供体,与不保护的烯丙基-α-D-吡喃葡萄糖苷为糖基受体进行一锅糖苷化法反应制得标题化合物.同时,对反应溶剂及配比进行了考察,目标产物及重要中间体的结构经MS、1HNMR、13CNMR及1H-1H COSY表征.     

New LiNi0.5PrxFe2−xO4 nanocrystallites: Synthesis via low cost route for fabrication of smart advanced technological devices

Praseodymium substituted nano-crystalline Li-Ni spinel ferrites with different Pr³⁺ contents were synthesized by micro-emulsion method. X-ray diffraction (XRD), scanning electron spectroscopy (SEM) and vibrating sample magnetometery (VSM) techniques were employed to study the impact of substitution of the Pr³⁺ on the structure, surface morphology and magnetic parameters. XRD confirmed the formation of the single phase spinel ferrites of all compositions of LiNi_0.5Pr_xFe_2−xO₄ nanocrystallites. The crystallite size determined from XRD data by Scherrer formula was calculated in range from 40 nm to 70 nm. However the nanoparticles size estimated by SEM was found 35–115 nm. The room temperature VSM measurements were carried out in the applied field range from “−10,000 Oe” to “10000” Oe. Saturation magnetization (M_S) (41 emu/g) and coercivity (H_C) values (156.9 Oe) of LiNi_0.5Fe₂O₄ were improved by the addition of rare earth Pr³⁺ cations. The value of Hc is low, which is a strong indication of soft ferrites. The synthesized LiNi_0.5Pr_xFe_2−xO₄ ferrites may be utilized for low core losses on transformers.