Growth and characterization of L‐alanine doped KDP crystals

Potassium Dihydrogen Phosphate (KDP) is an excellent inorganic nonlinear optical (NLO) material with different device applications. Most of amino acids possess NLO property; therefore, it is of interest to dope them in KDP crystals. In the present study, amino acid L‐alanine doped KDP crystals were grown by slow aqueous solvent evaporation technique. The doping of L‐alanine was confirmed by the paper chromatography, the CHN analysis and the FT‐IR spectroscopy. The powder XRD was carried out to assess the single phase nature of the samples. The effect of doping on thermal stability of the crystals was studied by TGA and the kinetic and thermodynamic parameters of dehydration were evaluated. As the amount of doping increased the thermal stability of crystals decreased. However, the second harmonic generation (SHG) efficiency and the UV‐Vis spectroscopy studies indicated that as the L‐alanine doping increased the SHG efficiency and optical transmission percentage increased. The dielectric behavior of the samples has been studied. The variation of dielectric constant, dielectric loss (tanδ), a.c.resistivity and a.c.conductivity with frequency of applied field in the range from 100 Hz to 100 kHz was studied. The dielectric constant and dielectric loss decreased with increase the value of frequency of applied field. The dielectric constant and the dielectric loss values of L‐alanine doped KDP crystals were lower than the pure KDP crystals. The results are discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Neutron diffraction study on composite compound Nd2Co7

The crystallographic and the magnetic structures of the composite compound Nd₂Co₇ at 300 K are investigated by a combined refinement of X-ray diffraction data and high-resolution neutron diffraction data. The compound crystallizes into a hexagonal Ce₂Ni₇-type structure and consists of alternately stacking MgZn₂-type NdCo₂ and CaCu₅-type NdCo₅ structural blocks along the c axis. A magnetic structure model with the moments of all atoms aligning along the c axis provides a satisfactory fitting to the neutron diffraction data and coincides with the easy magnetization direction revealed by the X-ray diffraction experiments on magnetically pre-aligned fine particles. The refinement results show that the derived atomic moments of the Co atoms vary in a range of 0.7 μ_B-1.1 μ_B and the atomic moment of Nd in the NdCo₅ slab is close to the theoretical moment of a free trivalent Nd³⁺ ion, whereas the atomic moment of Nd in the NdCo₂ slab is much smaller than the theoretical value for a free Nd³⁺ ion. The remarkable difference in the atomic moment of Nd atoms between different structural slabs at room temperature is explained in terms of the magnetic characteristics of the NdCo₂ and NdCo₅ compounds and the local chemical environments of the Nd atoms in different structural slabs of the Nd₂Co₇ compound.     

Design and construction of multicrystal analyser detectors using Rowland circles: application to MAD26 at ALBA

A concept is given for describing multicrystal analyser detectors (MADs), as they are in use for synchrotron powder diffraction, on the basis of the Rowland circle construction. The Rowland circle is typically used to describe focusing geometries and can be adapted for the case of MADs working at a single energy as well as in a limited energy range. With this construction it is also possible to quantify and optimize the walk of the beam along non‐central crystals which is inevitable in certain detector designs. The results of this geometrical inspection are correlated with a real detector design that is implemented at the ALBA synchrotron facility in Spain. An error budget is given to estimate the influence and amount of tolerance of the manufacturing process.     

激波诱导下纳米铝粉与微米铝粉的爆炸特征对比研究

对比研究了入射激波诱导下纳米铝粉和微米铝粉与环氧丙烷混合物快速反应系统中的爆炸特征.利用多台单色谱仪同步采集技术实验测定了二种反应混合物在不同诱导激波中强度作用下的点火延迟时间.为获得爆炸系统内部信息利用扫描电子显微镜(SEM),X射线衍射分析仪(XRD),X射线能谱(XPS)对相应铝粉反应生成物的结构、态貌、表面氧化层厚度进行了表征和分析.结果表明:TEM结果表明纳米铝粉生成物为絮状、针状和纤维状,而微米铝粉生成物为球状且体积增大;XRD结果显示在压缩区、点火区、燃烧区、爆炸区、传播区、碎片压缩致冷区生 关键词： 诱导激波 点火 XPS能谱 铝粉     

全谱拟合法研究聚丙烯腈基碳纤维形成过程中晶态结构演变

通过全谱拟合法对碳纤维制备过程中不同阶段纤维的XRD谱图进行处理,得到不同阶段纤维的微观结构参数,研究了聚丙烯腈(PAN)基碳纤维制备过程中晶态结构的演变.全谱拟合法基于晶体衍射的严格物理理论,拟合目标为整个衍射谱,并不是个别衍射峰,所得结果具有更高的可信度.研究结果表明:PAN原丝中的高分子链沿纤维轴高度取向,表观晶粒尺寸为6.5 nm左右;经过预氧化处理,纤维中的有序结构遭到破坏,表观晶粒尺寸锐减.纤维中逐渐形成梯形结构并沿纤维轴取向,从而形成新的有序结构;经过碳化处理后,环状梯形结构转变为碳的层状结 关键词： 碳纤维 晶体结构 XRD 全谱拟合法     

Quantitative analysis of cement powder by laser induced breakdown spectroscopy

Determination of elemental composition of cement powder plays an important role in the cement and construction industries. In the present paper, Laser induced breakdown spectroscopy (LIBS) is used for measuring the concentration of cement ingredient. Cement powder samples are pressed into pellets. Laser pulses are focused on the surface of pellets. A microplasma is formed in the front of samples. The plasma emission contains information about the elemental composition of the samples. By assumption of local thermodynamic equilibrium (LTE) and using several standard cement samples, a calibration curve is prepared for each element. The major and minor elements of cement such as Ca, Si, K, Mg, Al, Na, Ti, Mn and Sr are qualitatively and quantitatively determined. For verification of LTE conditions, plasma parameters such as plasma electron temperature and electron density are computed. According to the obtained results, the LIBS technique could be a suitable method for determination of elemental composition in the cement production industries.     

阴离子交换树脂分离甲苯萃取ICP-AES测定铋系超导粉中的痕量镍

建立了ICP-AES测定超导粉中痕量镍的新方法。优化了仪器工作参数,对超导前驱粉中痕量镍测定时的基体干扰及干扰消除方法进行了系统研究,结果表明:大量存在的基体元素产生严重基体效应,导致测定重现性及准确度变差,实验采用分离富集方法以提高测定结果的准确度和精密度。实验中首先用阴离子交换树脂,除去Bi3+及部分Pb2+,Sr2+,Ca2+,Cu2+等基体元素;在碱性条件下再通过甲苯萃取镍与丁二酮肟的络合物、稀盐酸反萃取富集镍;对分离富集条件进行了优化。对模拟标准样品进行测定,分析结果与理论值一致,相对标准偏差为1.9%,方法检出限为0.19μg.g-1。该方法已用于超导前驱粉样品中痕量镍的分析,结果令人满意。     

Atmospheric pressure chemical vapour deposition of selenium and tellurium films by UV laser photolysis of diethyl selenium and diethyl tellurium

Excimer laser‐induced photolysis of gaseous diethyl selenium and diethyl tellurium (C₂H₅)₂M (M = Se, Te) is controlled by cleavage of both M? C bonds, it yields C₁–C₄ hydrocarbons (ethene as major product) and results in chemical vapour deposition of selenium films and nanosized tellurium powder. The selenium and tellurium properties were characterized by X‐ray photoelectron spectroscopy and Scanning electron Microscopy techniques. Copyright © 2001 John Wiley & Sons, Ltd.     

Controlled preparation of aluminum borate powders for the development of defect-related phosphors for warm white LED lighting

The optimization of the elaboration conditions of a new family of highly emissive white phosphors based on glassy yttrium aluminum borates (g-YAB) compositions is presented. Their preparation from solutions is based on the polymeric precursor method (modified Pechini process), involving non-toxic and low cost precursors. The resulting resins were first dried at moderate temperatures followed by two-step annealing treatments of the obtain powders under controlled atmospheres: a first pyrolysis under nitrogen followed by a calcination under oxygen. This favored the gradual oxidation of organic moieties coming from starting materials, avoiding uncontrolled self-combustion reactions, which generate localized hot spots. This prevented phase segregations and the formation of pyrolytic carbon or carbonates, which are strongly detrimental to the luminescence properties. Thus, coupled chemical analyses and luminescence characterizations showed the high chemical homogeneity of the resulting powders and their intense emissions in the whole visible range. These emissions can be tuned from blue to warm white by adjusting the calcination temperature that is an important advantage for the development of LED devices. We showed that impurities of monovalent and divalent cations act as quenching emission centers for these phosphors. Therefore, by increasing the purity grade, we significantly enhanced the PL emissions leading to high internal quantum yields (80–90%). Finally, cathodoluminescence emissions showed the homogeneous dispersion of emitting centers in the g-YAB matrix.     

Formation mechanisms of Si3N4 and Si2N2O in silicon powder nitridation

Commercial silicon powders are nitrided at constant temperatures (1453 K; 1513 K; 1633 K; 1693 K). The X-ray diffraction results show that small amounts of Si₃N₄ and Si₂N₂O are formed as the nitridation products in the samples. Fibroid and short columnar Si₃N₄ are detected in the samples. The formation mechanisms of Si₃N₄ and Si₂N₂O are analyzed. During the initial stage of silicon powder nitridation, Si on the outside of sample captures slight amount of O₂ in N₂ atmosphere, forming a thin film of SiO₂ on the surface which seals the residual silicon inside. And the oxygen partial pressure between the SiO₂ film and free silicon is decreasing gradually, so passive oxidation transforms to active oxidation and metastable SiO(g) is produced. When the SiO(g) partial pressure is high enough, the SiO₂ film will crack, and N₂ is infiltrated into the central section of the sample through cracks, generating Si₂N₂O and short columnar Si₃N₄ in situ. At the same time, metastable SiO(g) reacts with N₂ and form fibroid Si₃N₄. In the regions where the oxygen partial pressure is high, Si₃N₄ is oxidized into Si₂N₂O.