Effect of the preparation conditions on the formation of asymmetric poly(vinylidene fluoride) hollow fibre membranes with a dense skin

Integrally skinned asymmetric poly(vinylidene fluoride) hollow fibre membranes were prepared and characterized. The effects of phase inversion methods (dry-wet or wet) and spinning conditions, such as the type of solvent (NMP, DMAc), the concentration of polymer in dope solution, temperature of the external coagulation bath and the composition of the inner coagulant on the morphology and on the formation of a dense skin layer were investigated. The structure of the membranes was analyzed by scanning electron microscopy and the gas permeation properties with six different gases (He, H₂, N₂, O₂, CH₄ and CO₂) were measured at 25 °C to confirm the integrity of the selective skin layer. Under the proper conditions highly selective and permeable PVDF hollow fibre membranes were thus obtained by dry-wet spinning of a 30 wt.% PVDF solution in DMAc, using hot water (50 °C) as the external coagulant and a bore fluid of pure water as the internal coagulant. The best membrane had a selective outer skin with an effective thickness of approximately 0.2 μm. The ideal selectivity of the hollow fibres approached or even exceeded the intrinsic ideal selectivity of a dense PVDF film, for instance the selectivity for He over N₂ was 86.2 for the hollow fibre, whereas it was 83.5 for a dense PVDF reference film. DSC and FT-IR/ATR analysis indicated a higher fraction of the β-crystal phase in the selective skin and a high overall crystallinity than in the melt-processed film. The latter explains the relatively high selectivity and low permeability of the membranes. Intrinsic polymer properties make the membranes also suitable for vapour transport than for gas separation.     

Solvothermal indium fluoride chemistry: Syntheses and crystal structures of K5In3F14, β-(NH4)3InF6 and [NH4]3[C6H21N4]2[In4F21]

The solvothermal syntheses and crystal structures of three indium fluorides are presented. K₅In₃F₁₄ (1) and β-(NH₄)₃InF₆ (2) are variants on known inorganic structure types chiolite and cryolite, respectively, with the latter exhibiting a complex and apparently novel structural distortion. [NH₄]₃[C₆H₂₁N₄]₂[In₄F₂₁] (3) represents a new hybrid composition displaying a unique trimeric metal fluoride building unit.     

稀土氟化物微纳米材料制备方法的研究进展

稀土氟化物纳米材料由于其特殊的光、电、磁性质,在光学器件、显示、生物标记、光学晶体等领域有着广泛的应用,已成为材料科学领域的研究热点之一。目前研究者已用沉淀法、微乳液法、水热与溶剂热法、溶胶-凝胶法、微波法、超声波法、前驱体热解法、静电纺丝法等成功地制备出了纳米颗粒、纳米线、纳米膜、多面体纳米晶、复合结构纳米晶、核壳结构纳米材料等稀土氟化物纳米材料。本文总结了上述几种制备方法的研究进展,讨论了其优缺点,并结合课题组在稀土氟化物纳米材料制备方面的工作,对纳米稀土氟化物制备方法的发展方向做了展望。     

高质量CdTe量子点的水相快速合成

系统考察了水相合成CdTe量子点的主要影响因素, 通过改变无水乙醇-水体系的体积, 提高NaHTe的合成质量, 并调整反应温度, 改变反应的初始pH值, 在水相中快速合成了量子产率高、粒径分布范围窄的CdTe量子点, 实现了对量子点发光性质的调控, 在最佳条件(无水乙醇3 mL, 水1 mL, 反应初始pH 8.0, 反应温度40 ℃)下, 最高量子产率达68%. 量子点胶体溶液在回流过程中有时产生白色沉淀, 放置7 d后, 未过滤白色沉淀的量子点比过滤后的量子点荧光强度提高15%, 白色沉淀还有减小粒径分布的作用.     

EFFECT OF GAS FLOW RATE ON CRYSTAL STRUCTURES OF ELECTROSPUN AND GAS-JET/ELECTROSPUN POLY(VINYLIDENE FLUORIDE) FIBERS

The effect of gas flow rate on crystal structures of electrospun and gas-jet/electrospun poly(vinylidene fluoride) (PVDF) fibers was investigated.PVDF fibers were prepared by electrospinning and gas-jet/electrospinning of its N,N-dimethylformamide (DMF) solutions.The morphology of the PVDF fibers was investigated by scanning electron microscopy (SEM).With an increase of the gas flow rate,the average diameters of PVDF fibers were decreased.The crystal structures and thermal properties of the PVDF fibers w...     

Synthesis of transparent silica aerogels using tetraalkylammonium fluoride catalysts

Tetraalkylammonium fluoride salts have been employed as catalysts for the synthesis of silica aerogels by a two-step, sol–gel method. Aerogel materials were characterized by N₂ physisorption and SEM. The effect of the type of catalyst on the optical transparency of obtained aerogels has been examined. It has been found that such compounds allow the synthesis of silica aerogels with the highest optical transmittance ever reported for such materials. The optimal catalysts are tetrabutyl and tetraoctyl ammonium fluoride, with which aerogels with transparency as high as 96% and extinction coefficient as low as 3.5 m⁻¹ can be prepared.     

Synthesis of nanosized silicon particles by a rapid metathesis reaction

A solid-state rapid metathesis reaction was performed in a bed of sodium silicofluoride (Na₂SiF₆) and sodium azide (NaN₃) powders diluted with sodium fluoride (NaF), to produce silicon nanoparticles. After a local ignition of Na₂SiF₆+4NaN₃+kNaF mixture (here k is mole number of NaF), the reaction proceeded in a self-sustaining combustion mode developing high temperatures (950–1000 °C) on very short time scales (a few seconds). Silicon nanoparticles prepared by the combustion process was easily separated from the salt byproducts by simple washing with distilled water. The structural and morphological studies on the nanoparticles were carried out using X-ray diffractometer (XRD) and field emission scanning electron microscope (FESEM). The mean size of silicon particles calculated from the FESEM image was about 37.75 nm. FESEM analysis also shows that the final purified product contains a noticeable amount of silicon fibers, dendrites and blocks, along with nanoparticles. The mechanism of Si nanostructures formation is discussed and a simple model for interpretation of experimental results is proposed.