连续调谐太赫兹回旋管非稳定振荡状态研究

太赫兹回旋管可实现高功率输出,并具有一定的频率调谐范围,是核磁共振波谱系统理想的高功率太赫兹辐射源。设计了263 GHz,TE_5,2基波连续调谐回旋管,通过磁场调节实现频率调谐范围为1.39 GHz,利用时域多模多频自洽非线性理论对设计的连续调谐回旋管非稳定振荡状态进行了研究。结果表明,在低次纵向谐波模式工作磁场范围内,当工作电流大于起振电流时,连续调谐回旋管先进入稳定状态,高次纵向谐波模式被抑制,工作模式TE_5,2的输出功率随时间不变;当电流增大,纵向谐波模式间的竞争引起回旋管由稳定状态进入到非稳定振荡状态,工作模式TE_5,2的输出功率随时间呈振荡变化且互作用效率大大降低;随着电流的进一步增大,回旋管又回到与低电流不同的稳定状态,互作用效率进一步降低。同时发现非稳定振荡状态的起始电流随着磁场增加而增大。本研究对需工作于稳定状态的面向DNP-NMR应用的连续调谐太赫兹回旋管的研制具有一定指导意义。     

新型负刚度吸能结构力学特性分析

负刚度结构作为一种具有广泛应用前景的力学超材料, 在吸能、减振及降噪等领域呈现出显著的优势, 但传统负刚度结构较低的比能吸收效率以及多稳态非自主回弹等特征, 严重限制了其工程应用. 为解决该问题, 通过单胞构型设计, 提出了一种新型可自主回弹的三维负刚度结构. 该结构利用串联的负刚度单胞在加载?卸载过程中, 曲梁胞元的自主反弹, 实现结构循环加载和多次重复利用; 通过凹槽深度设计抑制单胞多稳态的出现, 并且通过调整侧壁厚度, 控制曲梁屈曲模态的形式, 从而增大负刚度临界载荷差值, 实现吸能效率的显著提升. 随后为实现在复杂载荷环境下的高吸能, 对结构尺寸进行梯度设计, 提出了一种梯度负刚度结构, 利用有限元方法比较分析梯度负刚度结构与均匀负刚度结构在不同载荷作用下的吸能效果. 研究结果表明, 该梯度结构因微结构尺寸的不同, 具有不同的负刚度临界载荷最大值, 从而使其在不同的冲击载荷环境下, 在实现自主回弹的基础上, 均呈现出较好的吸能效率. 该新型负刚度结构为振动控制和结构重组等工程应用提供了技术支持.      

温度对GaSb/GaAs量子点尺寸分布的影响

采用低压金属有机物化学气相沉积 (LP-MOCVD) 法制备GaSb/GaAs量子点。通过对不同生长温度的样品进行分析发现温度的变化对GaSb/GaAs量子点的相位角无明显影响,量子点的形状是透镜型。由于量子点特殊的应力分布,可实现量子点的"自限制"生长。量子点的化学势不连续性以及Ostwald熟化机制的影响使得量子点尺寸分布在一定范围内不连续,会出现两种尺寸模式的量子点生长。Sb原子的表面迁移率对GaSb/GaAs量子点生长有较大的影响。升高温度可有效改善量子点的分立性,在升温过程中量子点体现出其熟化过程,高温时表面原子的解析附作用对量子点尺寸和密度的影响较大。     

High-crystalline GaSb epitaxial films grown on GaAs(001) substrates by low-pressure metal–organic chemical vapor deposition

Orthogonal experiments of Ga Sb films growth on Ga As(001)substrates have been designed and performed by using a low-pressure metal–organic chemical vapor deposition(LP-MOCVD)system.The crystallinities and microstructures of the produced films were comparatively analyzed to achieve the optimum growth parameters.It was demonstrated that the optimized Ga Sb thin film has a narrow full width at half maximum(358 arc sec)of the(004)ω-rocking curve,and a smooth surface with a low root-mean-square roughness of about 6 nm,which is typical in the case of the heteroepitaxial single-crystal films.In addition,we studied the effects of layer thickness of Ga Sb thin film on the density of dislocations by Raman spectra.It is believed that our research can provide valuable information for the fabrication of high-crystalline Ga Sb films and can promote the integration probability of mid-infrared devices fabricated on mainstream performance electronic devices.     

Nucleation of GaSb on GaAs （001） by low pressure metal-organic chemical vapor deposition

The initial growth stage of GaSb on GaAs(001) by low pressure metal–organic chemical vapor deposition(MOCVD)is investigated. The dependence of the nucleation on growth temperature, growth pressure, and vapor V/III ratio is studied by means of atomic force microscopy. The nucleation characteristics include the island density, size, and size uniformity distribution. The nucleation mechanism is discussed by the effects of growth temperature, growth pressure, and vapor V/III ratio on the density, size, and size uniformity of GaSb islands. With the growth temperature increasing from 500℃ to610℃ and the growth pressure increasing from 50 mbar to 1000 mbar(1 mbar = 105Pa), the island density first increases and then decreases; with the V/III ratio increasing from 0.5 to 3, the trend is contrary.     

基于路径分析方法探究核耗散的形变关系

利用动力学加统计模型对200Pb 系统的断前中子的发射进行了模拟。计算结果表明,在核耗散的形变关系(DDND) 确定的情况下,断前中子多重性会随着能级密度坐标关系(DDLDP) 的增强而减少,且该规律主要由第一阶段内断前中子的发射决定。此外,选定DDLDP 而采用核耗散的不同形变关系也都很好地重现了实验数据。为了解释上述现象,提出了研究热核退激的路径分析方法,其主要内容是把核耗散看成是实验粒子在拉伸维度上的阻尼力,而核裂变的动力学过程是该阻尼力和自由能产生的驱动力之间相互竞争的结果。利用该方法对200Pb 系统的断前中子多重性进行了分析,并指出在确定DDND时应考虑DDLDP。Within a certain deformation dependence of nuclear dissipation (DDND), the simulation results about the deexcitation process of 200Pb show that the prescission neutron multiplicities decrease with increasing deformation dependence of level density parameter (DDLDP). Moreover, this pattern is determined by the emission of prescission neutron in the first period of nuclear fission. On the other hand, all of the results from two kinds of different DDNDs can reproduce the experiment data perfectly with a certain DDLDP. In order to illustrate those phenomena, the pathwise analysis method (PAM) is presented in this paper. In the PAM, nuclear dissipation is treated as the damping force on the elongation dimension, whereas the fission dynamics process results from the competition between damping force and the driving force stemming from the nuclear free energy. The prescission neutron multiplicity in the deexcitation process of 200Pb is analyzed, and the results point out that the DDLDP needs to be taken into account in studying the DDND.     

The design and numerical analysis of tandem thermophotovoltaic cells

In this paper, numerical analysis of GaSb （Eg = 0.72 eV）/Gao.84Ino.16Aso.14Sbo.86 （Eg = 0.53 eV） tandem thermopho- tovoltaic （TPV） cells is carried out by using Silvaco/Atlas software. In the tandem cells, a GaSb p-n homojunction is used for the top cell and a GalnAsSb p-n homojunction for the bottom cell. A heavily doped GaSb tunnel junction connects the two sub-cells together. The simulations are carried out at a radiator temperature of 2000 K and a cell temperature of 300 K. The radiation photons are injected from the top of the tandem cells. Key properties of the single- and dual-junction TPV cells, including I-V characteristic, maximum output power （Pmax）, open-circuit voltage （Voc）, short-circuit current （/~sc）, etc. are presented. The effects of the sub-cell thickness and carrier concentration on the key properties of tandem cells are investigated. A comparison of the dual-TPV cells with GaSb and GalnAsSb single junction cells shows that the Pmax of tandem cells is almost twice as great as that of the single-junction cells.