超流氦噪声沸腾过程中的压力振动分析: 小试样几何尺寸对噪声沸腾状态的影响

小试样的几何形状尺寸是影响噪声沸腾状态的一个重要的因素. 对于不同几何尺寸的小试样, 沸腾发生时所产生的气泡的尺寸不同, 因而所产生的压力振动的幅度和特征频率就不同. 采用一个归一化因子将不同几何形状尺寸下的实验数据进行统一, 发现振动的特征频率与几何尺寸因子之间存在对数曲线的关系.     

基于 MAT LA B的音频信号处理

当今社会已进入了高速信息化时代, 用现代科技手段更加有效地产生、 传输、 存储和应用语音信息对于 促进信息化发展意义重大. 利用 MA T L A B对所采集的语音信号进行滤波前的时域波形和频谱图分析, 再接入笔者设 计的低通滤波器进行滤波, 以及滤波前后的语音变化分析和播放对比, 发现滤波后的语音效果得到很好的改善     

A Strength Softening Phase Transition Observed in Shocked （Mg0.92,Fe0.08）SiO3 Perovskite at About 83 GPa

We report the experimental data of Hugoniot longitudinal sound velocity VL for natural （Mg0.92,Fe0.08）SiO3 enstatite sample at about 40-140 GPa, consisting of three new data and five previously reported data but revised by our new Hugoniot equation of state parameters. Three segments, separated by two discontinuities, appear in the VL-PH （shock pressure） plot. Analyses show that the first discontinuity at about 64 GPa, with a sharp increase of VL of about 21%, is judged to be a phase transition from enstatite to Pbnm perovskite （PV）; while the second one at about 83 GPa, with a dramatic decrease of VL of about 23%, is likely caused by a subtle structural change from Pbnm PV to tetragonal PV, accompanied by material strength softening due to melting of oxygen sublattices. This strength softening evidence is obtained first from shock wave experiments, and probably has profound implications for probing into the origin of low seismic velocity anomaly in the Earth＇s lower mantle and thus constraining the geophysical and geochemical models for the Earth＇s lower mantle.     

A simulation study of a dual-plate in-room PET system for dose verification in carbon ion therapy

During carbon ion therapy, lots of positron emitters such as 11C, 15O, 11C are generated in irradiated tissues by nuclear reactions, and can be used to track the carbon beam in the tissue by a positron emission tomography （PET） scanner. In this study, an dual-plate in-room PET scanner has been designed and evaluated based on the GATE simulation platform to monitor patient dose in carbon ion therapy. The dual-plate PET is designed to avoid interference with the carbon beamline and with patient positioning. Its performance was compared with that of four-head and full-ring PET scanners. The dual-plate, four-head and full-ring PET scanners consisted of 30, 60, 60 detector modules, respectively, with a 36 cm distance measurements. Each detector module consisted of a between directly opposite detector modules for dose deposition 24~24 array of 2 mm×2 mm×18 mm LYSO pixels coupled to a Hamamatsu H8500 PMT. To estimate the production yield of positron emitters, a 10 cm×15 cm×15 cm cuboid PMMA phantom was irradiated with 172, 200, 250 MeV/u 12C beams. 3D images of the activity distribution measured by the three types of scanner are produced by an iterative reconstruction algorithm. By comparing the longitudinal profile of positron emitters along the carbon beam path, it is indicated that use of the dual-plate PET scanner is feasible for monitoring the dose distribution in carbon ion therapy.     

含扭转晶界位错Al金属拉伸强度第一性原理预测

本文基于密度泛函理论第一原理方法,从影响力学性能本质的电子结构计算上,对含Σ 5{001}扭转晶界位错Al金属拉伸强度进行了预测,发现其理论拉伸强度达到8.73 GPa,临界应变为 24%.拉伸强度低于文献报道(Phys. Rev. B 75, 174101 (2007))的倾斜晶界位错Al金属的理论拉伸强度9.5 GPa,但其临界应变却远大于倾斜晶界的16%.本研究结果表明,通过工艺参数控制,改变缺陷形态,可极大地改变其力学性能.进一步地,从电子结构层次上, 分析了含晶界位错Al金属拉伸断裂行为的实质,通过分析电荷密度分布、键长变化等,发现其断裂处发生在晶界处;理论计算结果将对Al金属结构设计及力学性能改善具有重要的指导作用.     

具有人工晶界的Sr2FeMoO6粉末磁电阻的温度特性

利用固相反应合成了纯相的Sr2FeMoO6多晶块体，并通过机械球磨方法引入了人工晶界，研究了具有此种晶界的Sr2FeMoO6粉末磁电阻的温度特性．X射线衍射分析表明，机械球磨过程没有改变Sr2FeMoO6的晶体结构，但却在晶粒间界处引入了SrMoO4绝缘相，其量随着球磨时间的增加而增加．不同磁场下的磁电阻测量结果表明，由于一定量SrMoO4绝缘相的存在，晶粒间的绝缘隧穿势垒得到加强，更有利于自旋极化电子在晶粒间的隧穿，从而提高了Sr2FeMoO6多晶粉末的低温磁电阻值．然而随着温度的升高，磁电阻值迅速下降，表现出较强的温度依赖关系．这种现象是由于随着温度的升高，电子在晶界局域态间的非弹性跳跃逐渐增强引起的，而晶界局域态是由在晶界附近的大量缺陷构成．分析表明，晶界状态对Sr2FeMoO6多晶粉末磁电阻的温度特性有十分重要的影响．     

等离子体光谱的多重结构

利用空间分辨的掠入式光栅谱仪和晶体谱仪等，获得了等离子体发射的软Ｘ射线多重连谱和伴线结构。软Ｘ射线多重谱主要由等离子体中多重电离态离子所致，而电子密度大小和杂质粒子对伴线丰度起主要作用。实验结果显示原子序数和入射激光强度与光谱结构的紧密联系以及等离子体发射源的非平衡性质。     

激光轮廓测量中的光强自动控制研究

脉宽调制技术目前已广泛应用于高频电磁加热系统、开关电源等领域1，本文首次将脉宽调制技术用于激光扫描测量系统，解决了激光扫描测量中国不同材料对激光的散射光强不同、不同曲面对激光光强反射各异以及物体表面镜面反射等问题给测量精度带来的严重影响2，利用激光扫描系统中的摄象机直接作为光强反馈系统，实现了测量过程中的光强自动控制，有效地降低了光强饱和等问题给测量带来的误差．     

液晶矩阵大屏幕显示

人们早已盼望实现平板显示,因此发明了各种新型显示器件,液晶显示也作为一种很好的显示方式,在国内外进行了研究。 一般,液晶显示的特点有如下几点:1)耗电量少,较低电压可以驱动;2)被     

原子吸收限附近Bragg情况的衍射波和透射波的观测

在Ｇａ的Ｋ吸收限附近的强反常散射条件下，测定了ＧａＡｓ（６００）对称Ｂｒａｇｇ情况的衍射波和透射波的摇摆曲线，并将其和Ｘ射线衍射动力学理论的相应计算结果作了比较，指出了原子吸收限附近Ｂｒａｇｇ情况的透射波在研究反常散射中的应用价值 关键词：