Different origins of green-light photoluminescence emission in structurally ordered and disordered powders of calcium molybdate

A strong greenish-light photoluminescence (PL) emission was measured at room temperature for disordered and ordered powders of CaMoO4 prepared by the polymeric precursor method. The structural evolution from disordered to ordered powders was accompanied by XRD, Raman spectroscopy, and TEM imagery. High-level quantum mechanical calculations in the density functional framework were used to interpret the formation of the structural defects of disorder powders in terms of band diagram and density of states. Complex cluster vacancies [MoO3 x V(O(z))] and [CaO7 x V(O(z))] (where V(O(z)) = V(O(X)), V(O(*)), V(O(**))) were suggested to be responsible to the appearance of new states shallow and deeply inserted in the band gap. These defects give rise to the PL in disordered powders. The natural PL emission of ordered CaMoO4 was attributed to an intrinsic slight distortion of the [MoO4] tetrahedral in the short range.     

A theoretical study of water adsorption on (10-10) and (0001) ZnO surfaces: molecular cluster, basis set and effective core potential dependence

The adsorption of water molecule and hydroxyl ions on the (10-10) and (0001) ZnO surfaces has been studied for (ZnO)₄, (ZnO)₅ and (ZnO)₆ cluster models. Different representations for the atomic basis sets and the effective core potential (ECP) approximations have been employed and the effects of cluster size, conformation and basis sets are analyzed and discussed.     

高T_c超导体YBCO磁通钉扎的计算机模拟研究

临界电流密度Jc是影响高温超导体在强电领域应用的一个重要参数,在实际应用中,特别在外加磁场下,临界电流密度与超导材料的磁通钉扎性质密切相关.因此,磁通钉扎一直是高温超导体研究中的一个重要领域.由于高温超导体磁通钉扎力密度Fp的标度律存在,本文根据D.Dew-Hughes总结的钉扎力函数,主要存在两种主要作用类型(正常相和△K).我们将D.Dew-HugBes给出的钉扎力密度Fp标度函数改进为一个简化的具有物理意义的函数表达式.结合文献中已有的实验数据,我们对YBcO进行了计算机模拟,确定了它的磁通钉扎类型,模拟的研究结果与实际情况比较吻合.     

The use of acids,niobium oxide,and zeolite catalysts for esterification reactions

We have evaluated the influence of alcohol/fatty acid molar ratio (methanol or ethanol), water and catalyst concentrations and temperature by the esterification of palm fatty acids by heterogeneous acid catalysts (varying types, forms, and particle size). Polynaphtalene sulfonic acid (PSA) and niobium oxide (Nb₂O₅) presented better performance than zeolite catalysts. Reaction with methanol presented higher conversion than with ethanol. The experimental design showed that the most relevant variable is the catalyst concentration and all interactions become important in process. A heterogeneous kinetic model was proposed and applied to experimental data. One of the models was adequate for methanol reaction, whereas the homogeneous model was more appropriate for ethanol reaction. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis and Measurement of the Thermoelectric Properties of Multiphase Composites: ZnSb Matrix with Zn4Sb3, Zn3P2, and Cu5Zn8

We synthesized a series of samples with composition around 52 at.% zinc (Zn), 44 at.% antimony (Sb), 4 at.% phosphorus (P), and up to 3 at.% copper (Cu) by melting the elements and subsequent annealing. This resulted in dense and almost crack-free samples. X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) revealed composites with a majority phase of ZnSb containing varying amounts of Zn₃P₂ and Cu₅Zn₈, in addition to Zn₄Sb₃ in some of the samples. We measured the Seebeck coefficient, electrical conductivity, and thermal conductivity as a function of temperature. The thermoelectric performance tended to improve with increasing Cu content. At Cu content of 2 at.%, a reduced resistivity allows for the highest dimensionless figure of merit, with a maximum zT value of 0.18 at around 573 K.     

A 1.8-V 1.6-GSample/s 8-b self-calibrating folding ADC with 7.26 ENOB at Nyquist frequency

This study demonstrates for the first time the significant performance enhancement that calibration brings to folding-interpolating analog-to-digital converters (ADCs). The resulting 1.8-V ADC in 0.18-/spl mu/m CMOS achieves a conversion rate exceeding 1.6 GSample/s, since the amplifier device sizes can be minimized to maximize speed without the restriction of device matching. At 1.6 GS/s, the ADC achieves 0.15 LSB DNL, 0.35 LSB INL, 7.6 effective number of bits (ENOBs) at 100 MHz input, and 7.26 ENOB at Nyquist. At this speed, current consumption from a single 1.8-V supply is 245 mA analog, 185 mA digital, and 90 mA for the LVDS drivers. The ac performance is approximately 1.5 ENOBs higher compared to the same circuit with calibration disabled. The use of best design practices to optimize the ADC linearity prior to introducing calibration resulted in this small required dynamic calibration range, simplifying the calibrator circuitry and resulting in stable continuous performance over time without recalibration. Therefore, the fully on-chip calibration is performed automatically, just one time at power-up.     

Controlled use of excess backbone bandwidth for providing new services in IP-over-WDM networks

We study an approach to quality-of-service (QoS) that offers end-users the choice between two service classes defined according to their level of transmission protection. The fully protected (FP) class offers end-users a guarantee of survivability in the case of a single-link failure; all FP traffic is protected using a 1:1 protection scheme at the wavelength-division multiplexing (WDM) layer. The best effort protected (BEP) class is not protected; instead restoration at the IP layer is provided. The FP service class mimics what Internet users receive today. The BEP traffic is designed to run over the large amounts of unused bandwidth that exist in today's Internet. The goal is to increase the load carried on backbone networks without reducing the QoS received by existing customers. To support two such services, we have to solve two problems: the off-line problem of mapping logical links to pairs of disjoint fiber paths, and an on-line scheduling problem for differentiating packets from two classes at the IP layer. We provide an algorithm based on a Tabu Search meta-heuristic to solve the mapping problem, and a simple but efficient scheduler based on weighted fair queueing for service differentiation at the IP layer. We consider numerous requirements that carriers face and illustrate the tradeoffs they induce. We demonstrate that we can successfully increase the total network load by a factor between three and ten and still meet all the carrier requirements.     

Theoretical study of water coverage on MgO surfaces

Ab initio and semiempirical calculations have been performed on an (MgO)₁₆ cluster model in order to study the effects of water coverage on pure MgO (100) surfaces. The geometries of various adsorbed water molecules have been optimized and the binding energies, charge transfer, and preferential sites of interaction analyzed. We have used Mulliken and natural bond population analysis methods in order to analyze charge distributions and the direction of charge transfer processes. We have also investigated the effects of low and high coverage on energy gaps, density of states, self‐consistent field (SCF) orbital energies, and stretching frequencies. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 153–165, 1999