Computational Studies on the Sc<Subscript>n</Subscript>N<Subscript>m</Subscript> (n + m=10) Clusters: Structure,Electronic and Vibrational Properties

Besides the size and structure, compositions also dramatically affect the properties of clusters. In fact, the increased degree of freedom poses much more challenges to determine the global minimum structure of multi-component clusters. In this thesis, based on the CALYPSO structure searching method, the global minimum structures are obtained for Sc_nN_m (n + m=10) clusters at PW91/6-311+G(d) level. The growth behavior indicates that the cage unit tends to arrange into the compact configurations, and the occupied positions of N atoms shift from the surface towards the center of coordination site with the increasing number of Sc atoms. The relative stabilities have been discussed by analyzing the average binding energies and HOMO–LUMO gaps. In addition, the molecular orbitals, dipole moments, polarizability, hyperpolarizabilities, natural population, natural electron configuration, and Infared and Raman spectra calculations allow complete characterization of the electronic and vibrational properties for the global minimum structural clusters.     

Phase field simulation of the columnar dendritic growth and microsegregation in a binary alloy

This paper applies a phase field model for polycrystalline solidification in binary alloys to simulate the formation and growth of the columnar dendritic array under the isothermal and constant cooling conditions. The solidification process and microsegregation in the mushy zone are analysed in detail. It is shown that under the isothermal condition solidification will stop after the formation of the mushy zone, but dendritic coarsening will progress continuously, which results in the decrease of the total interface area. Under the constant cooling condition the mushy zone will solidify and coarsen simultaneously. For the constant cooling solidification, microsegregation predicted by a modified Brody- Flemings model is compared with the simulation results. It is found that the Fourier number which characterizes microsegregation is different for regions with different microstructures. Dendritic coarsening and the larger area of interface should account for the enhanced Fourier number in the region with well developed second dendritic arms.     

Inversion of two-band superconductivity at the critical electron doping of (Mg, Al)B2

Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)Al(x)B(2). A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximately 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T(c) < 7 K up to about x approximately 55. Only the pi band had detectable density of states for 0.33 < or =x < or = 0.55, implying an inversion of the two-band hierarchy of MgB(2) in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T(c) are discussed.     

GaN MOS-HEMT Using Ultra-Thin A1203 Dielectric Grown by Atomic Layer Deposition

We report a GaN metal-oxide-semiconductor high electron mobility transistor （MOS-HEMT） with atomic layer deposited （ALD） Al2O3 gate dielectric. Based on the previous work [Appl. Phys. Lett. 86 （2005） 063501] of Ye et al. by decreasing the thickness of the gate oxide to 3.5nm and optimizing the device fabrication process, the device with maximum transconductance of 150mS/mm is produced and discussed in comparison with the result of lOOmS/mm of Ye et al. The corresponding drain current density in the 0.8-μm-gate-length MOS-HEMT is 800mA/mm at the gate bias of 3.0 V. The gate leakage is two orders of magnitude lower than that of the conventional A1GaN/GaN HEMT. The excellent characteristics of this novel MOS-HEMT device structure with ALD Al2O3 gate dielectric are presented.     

Synthesis of diverse N,O-bridged calix[1]arene[4]pyridine-C(60) dyads and triads and formation of intramolecular self-inclusion complexes

Starting from both the bridging nitrogen atom-functionalized and the upper rim-functionalized N,O-bridged calix[1]arene[4]pyridine reactants, different types of heteracalixaromatics-C(60) dyads and triads of varied spacers were expediently synthesized using mainly the click reaction as the key step. By means of various spectroscopic methods, the heteracalixaromatics-C(60) dyads and triads obtained have been shown to form intramolecular self-inclusion complexes rather than oligomers or polymers in solution because of a flexible spacer in between the heteracalixaromatic ring and C(60) moiety. The current study, coupled with previous investigations, would provide the guideline for the construction of supramolecular fullerene motifs based on molecular design of the dyads and triads.     

射流影响下气固两相流动试验的定量分析

为了研究受限空间内的射流影响下的气固两相流动,本文采用数码颗粒图像测速仪（DpIV）系统测量颗粒分布,并开发了一套用于处理DPIV颗粒图片的算法,可用于准确地定量分析通道内的颗粒分布。使用该算法能够获得颗粒在拍摄平面内的二维相对浓度数据,定量分析该数据发现,在同样的流场条件下,射流对气流场和不同粒径颗粒的影响区域存在明...     

定向凝固过程中枝晶侧向分枝生长行为与强制调控规律

采用相场法数值模拟研究了定向凝固过程中随机噪声条件下枝晶侧向分枝生成行为与强制扰动条件下侧向分枝调控规律. 模拟结果表明: 随机噪声条件下, 侧向分枝整体上并无规则性, 但产生频率存在一定分布范围, 且在一定时间段内会出现生成频率一致且具有极强相关性的一组侧向分枝, 即波包; 不同波包之间不具有相关性, 但不同波包内部的侧枝生成频率基本相同, 且与侧枝整体频谱曲线峰值位置处的频率基本相当; 强制周期扰动条件下, 当扰动频率处于侧向分枝整体生成频率范围内时, 可激发枝晶产生规则侧向分枝, 且扰动频率与波包内侧枝生成频率一致时侧向分枝最发达. 研究结果可为向定向凝固枝晶形态的调控提供理论指导.     

Optimization and Finite Element Analysis of the Temperature Field in a Nitride MOCVD Reactor by Induction Heating

A susceptor structure with a ring channel for a vertical metalorganic chemical vapor deposition reactor by induction heating is proposed. Thus the directions of heat conduction are changed by the channel, and the channel makes the heat in the susceptor redistribute. The pattern of heat transfer in this susceptor is also analyzed. In addition, the location and size of the channel in the susceptor are optimized using the finite element method. A comparison between the optimized and the conventional susceptor shows that the optimized susceptor not only enhances the heating efficiency but also the uniformity of temperature distribution in the wafer, which contributes to improving the quality of the film growth.     

First-principles study on the structural and electronic properties of ultrathin ZnO nanofilms

Using first-principles density-functional calculations, we have studied the structural and electronic properties of ultrathin ZnO {0001} nanofilms. The structural parameters, the charge densities, band structures and density of states have been investigated. The results show that there are remarkable charge transfers from Zn to O atoms in the ZnO nanofilms. All the ZnO nanofilms exhibit direct wide band gaps compared with bulk counterpart, and the gap decreases with increased thickness of the nanofilms. The decreased band gap is associated with the weaker ionic bonding within layers and the less localization of electrons in thicker films. A staircase-like density of states occurs at the bottom of conduction band, indicating the two-dimensional quantum effects in ZnO nanofilms.