合成温度对碳热还原法合成碳化硅晶须形貌的影响

以SiO2微粉为硅源,炭黑为碳源,氧化硼为催化剂,采用碳热还原法分别在1500 ℃、1550 ℃、1600 ℃制备了SiC晶须.通过扫描电镜,电子探针和透射电镜等分析手段,研究了合成温度对SiC晶须形貌的影响,探讨了晶须的生长机理.结果表明:当合成温度为1500 ℃时,所合成的SiC晶须形貌呈竹节状,选区电子衍射分析发现孪晶等面缺陷在晶须的生长方向上周期性出现;当合成温度在1550 ℃以上时,哑铃状晶须的数量会急剧增多,分析表明晶须表面包裹的串珠小球为β-SiC.在晶须的顶端发现催化剂熔球,由此推测生长机理为VLS机理,但当合成温度超过1550 ℃时,SiC会以VS生长机理沿径向沉积生成哑铃状晶须.     

Sisal cellulose whiskers reinforced polyvinyl acetate nanocomposites

Sisal nanowhiskers were used as novel reinforcement to obtain nanocomposites with polyvinyl acetate (PVAc) as matrix phase. They are seen as attractive materials due to the widespread availability and low cost of the sisal source material. Statistical analysis of the sisal whisker length and diameter resulted in average values of 250 nm and 4 nm, respectively, resulting in an average aspect ratio in the upper range of reported cellulose nanowhisker values. The high aspect ratio ensures percolation, with resulting mechanical improvements and thermal stability, at lower fiber loads. Water uptake and thermal behaviour of the sisal whisker–PAVc composites were studied. Whisker addition was found to stabilize the nanocomposites with no benefit seen when increasing the whisker content beyond the percolation threshold: For all whisker contents studied above percolation, the water uptake stays constant, and the Tg does not vary with whisker content at a given relative humidity. The water diffusion rate however increases due to water accumulation at the whisker–PVAc interface. Below whisker percolation, stabilization is only noticed at low relative humidity, whereas high humidity results in disruption of whisker–PVAc interactions. This work shows the potential of cellulose nanowhiskers to stabilize polar polymers even at high humidity conditions with minimal reinforcement addition.     

纳米纤维素的制备

在纳米尺寸范围操控纤维素分子及其超分子聚集体,结构设计并组装出稳定的多重花样,由此创制出具有优异功能的新纳米精细化工品、新纳米材料,是纤维素科学的前沿领域和热点。为了研究当前制备纳米纤维素的现状和发展方向,简述了纳米纤维素化学基础,介绍了三类纳米纤维素：纳米纤维素晶体（晶须）、纳米纤维素复合物和纳米纤维素纤维,重点综述了纳米纤维素的五种制备方法：化学法制备纳米纤维素晶体和晶须、生物法制备细菌纤维素、物理法制备微纤化纳米纤维素、人工合成纳米纤维素和静电纺丝制备纤维素纤维,讨论了各种制备方法的优点和缺点,指出开展纳米纤维素超分子的可控结构设计、立体与位向选择性控制与制备、分子识别与位点识别等自组装过程机理、多尺度结构效应的形成机理等基础理论性研究是主要研究基础,新型的、绿色、低能耗、快速、高效的制备方法是纳米纤维素制备方法的发展方向。     

Effect of alcohols additives on hydrothermal formation of magnesium hydroxide sulfate hydrate whiskers

Two different organic alcohols, ethylene glycol and 1, 2‐propanediol, were used in hydrothermal formation of MgSO₄·5Mg(OH)₂·2H₂O (MHSH) whiskers. Compared with the MHSH whiskers without alcohols, the crystallinity and defect of products were improved. It could also be observed that not only aspect ratio but also crystallinity and purity of the products were improved when 1, 2‐propanediol was used. A reasonable model was proposed to explain the growth process derived from the organic alcohol. In addition, the room temperature photoluminescence (PL) indicated that the spectrum intensity of MHSH whiskers used 1, 2‐propanediol was also significantly improved.     

Dehydration Mechanism and Crystallisation Behaviour of Lactose

The dehydration mechanism of α-lactose monohydrate was investigated by several techniques and interpreted on the basis of structural data. Whatever the dehydration conditions (heating or use of hygroscopic organic solvents), the departure of water molecules occurs cooperatively in channels parallel to the c axis of the initial structure. Subsequently, the reorganization leads to the closest packing (hygroscopic metastable form, Lα_H) under heating or to the stable anhydrous form (Lα_S), probably via a nucleation and growth process in ethanol. The use of acetone as dehydrating solvent on single crystals of α-lactose monohydrate led to the unexpected formation of single crystals of the anomeric β-lactose at room temperature, from which the crystal structure of β-lactose could be accurately re-determined. Recrystallization experiments of anhydrous lactose allowed to prepare N-methylpyrrolidinone and DMSO solvates of α-lactose. This revised version was published online in August 2006 with corrections to the Cover Date.     

Growth of tungsten whiskers oriented in <111> direction

The electron micro diffraction technique and pseudopotential ab initio calculations were implemented to study in details the formation and mechanism of growth of the tungsten whiskers during the reduction of nickel tungstate by CO gas. It has been shown that the W whiskers prefer to grow as crystals oriented in the <111> direction, and this process may be considered as the epitaxial growth on the hexagonal planes of Ni₄W particles. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal dissociation of iron carbonyls during growth of iron whiskers

Methods for the preparation of iron whiskers in chemical transport reactions of thermal dissociation of iron penta- and dodecacarbonyls and carbidocarbonyl clusters Fe₅C(CO)₁₅ were described. The morphology, structure, and chemical composition of the whiskers were studied. The main factors determining the growth rate and mechanical properties of the whiskers were revealed. A model for the mechanism of thermal dissociation of iron carbonyls was proposed. This process was shown to be a chain radical ion reaction initiated via the scheme of activating complex formation. Analogies between the thermal dissociation of iron carbonyls in the adsorption layer and the known radical ion processes in the liquid and gas phases were found.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1826–1836, September, 2004.     

Physicochemical properties of the binary system glibenclamide and polyethylene glycol 4000

Solid dispersions of the antidiabetic drug glibenclamide and polyethylene glycol 4000 (macrogol 4000) were prepared by the melting method in order to increase the solubility of this poorly water-soluble compound. The temperature/composition phase diagram of the components was analyzed by hot-stage microscopy and differential scanning calorimetry, showing a monotectic. Polarized light hot stage microscopy and X-ray-powder diffraction confirmed, that glibenclamide is mainly present in a non-crystalline state after melting and solidifying of a 10% (w/w) mixture, which results in an enhanced solubility compared to physical mixtures. The solubility and dissolution rate of the drug increases clearly with decreasing drug/polymer ratio. Moreover, it was observed for the first time that a drug could crystallize as whiskers at the surface of aged solid dispersion particles. Besides relaxation phenomena, this crystallization mechanism may be responsible for a deterioration of liberation properties and bioavailability of solid dispersion based drug products with increasing storage time. This revised version was published online in July 2006 with corrections to the Cover Date.     

十八烷基三氯硅烷表面改性钛酸钾晶须

利用十八烷基三氯硅烷(OTS)的自组装对钛酸钾晶须(PTW)进行表面改性. 通过测定苯、乙二醇、水在OTS改性的PTW压片上的接触角, 计算了OTS改性PTW后的表面能和极性分量, 并与传统方法(PMMA包裹和KH560表面改性)改性的PTW进行比较, 结果表明, OTS改性的PTW具有更低的表面能和极性分量, 以及更好的亲油疏水性能. IR分析和色散分量测定表明, 改性后OTS的烷基链倾斜在PTW表面.     

Physicochemical and functional peculiarities of metal oxide whiskers

Practical aspects of preparation and prospects for practical use of a series of the metal oxide whiskers were studied. The procedures for the synthesis were proposed, and the phase composition, micromorphology, and electrochemical and sensor characteristics of the macroscopic (up to 5–10 mm long) whiskers in the Ba-V-O, Ba-Mn-O, and Sn-O systems were analyzed. The electroconducting BaV₈O_21-δ whiskers were prepared by the hydrothermal treatment. These whiskers possess stable electrochemical characteristics appropriate for the development of novel secondary current sources. The protonated form of the Ba₆Mn₂₄O₄₈ whiskers produced by the isothermal vaporization of chloride fluxes is a mixed conductor with the proton and electron conductivity at a level of mS units at 25 °C. A new procedure by the thermal disproportionation of tin(ii) oxide under nitrogen was proposed for the growth of SnO₂ whiskers of various morphology. The produced whiskers have substantial sensor sensitivity toward a series of toxic components of the gaseous medium, such as nitrogen dioxide. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1023–1034, May, 2008.