万有引力常数G精确测量实验进展

万有引力常数G是人类历史上引入的第一个基本物理学常数,其在理论物理、天体物理和地球物理等许多领域中扮演着重要角色.两百多年来,人们共测量出了200多个G值,但G的测量精度仍然是所有物理学常数中最差的,这一现象反映了测G工作本身的复杂性和困难性.本文简要概述了G值测量的意义和测G的历史,并结合自2010年以来国际上新出现的三个高精度测G实验介绍这一领域的研究进展,以及华中科技大学引力实验中心测G工作的最新动态.     

平面电磁波在各向异性周期介质中的传输特性研究

将平面电磁波在一维介电各向同性周期介质中的传输矩阵问题扩展至一维介电各向异性周期介质,并得到了平面电磁波在此类介质中的传输矩阵.此矩阵的推导主要基于麦克斯韦方程组和电磁波在介质交界面的连续性条件,即电磁波在介质交界面的切向电场和磁场分量连续.通过此矩阵的推导可以加深学生对"电磁场与电磁波"这门课程的理解.此矩阵也是获得电磁波在一维介电各向异性周期介质中的场分布特性,色散关系和表面模式性质的基础.因此,此传输矩阵对科研和教学都有重要意义.     

单原子催化剂在CO2转化中的应用

单原子催化剂(SAC)是由互相隔离分散的原子级活性位点锚定在基底上而形成的一类新兴催化剂材料, 其具有最大化的原子利用率、 可调控的独特电子结构, 因而在热催化、 光催化及电催化等方面展现出良好的应用前景. 通过SAC的热/光/电催化CO₂转化反应(CCR)能将温室气体CO₂转化为燃料或具有附加值的化学品, 为解决严重的全球变暖和能源短缺问题提供了一种有效策略. 本文总结了近年来SAC在CO₂转化领域的研究进展, 讨论了其合成、 调控及催化各类CO₂转化反应的优缺点, 并对其未来的发展进行了展望.     

Effect of gate length on breakdown voltage in AlGaN/GaN high-electron-mobility transistor

The effects of gate length L_G on breakdown voltage VBRare investigated in AlGaN/GaN high-electron-mobility transistors(HEMTs) with L_G= 1 μm~20 μm. With the increase of L_G, VBRis first increased, and then saturated at LG= 3 μm. For the HEMT with L_G= 1 μm, breakdown voltage VBRis 117 V, and it can be enhanced to 148 V for the HEMT with L-_G= 3 μm. The gate length of 3 μm can alleviate the buffer-leakage-induced impact ionization compared with the gate length of 1 μm, and the suppression of the impact ionization is the reason for improving the breakdown voltage.A similar suppression of the impact ionization exists in the HEMTs with LG 3 μm. As a result, there is no obvious difference in breakdown voltage among the HEMTs with LG= 3 μm~20 μm, and their breakdown voltages are in a range of 140 V–156 V.     

硬脑膜受压膨出挠度的力学分析

硬脑膜是一种粘弹性材料,为控制硬脑膜在脑压作用下的膨出度,对粘弹性薄膜受压膨出挠度作力学分析。以位移为未知量,从粘弹性材料的分型本构关系出发将Foepple薄膜大挠度理论从弹性推广到粘弹性膜,得到一组非线性积分偏微分方程。先在空间上运用Galerkin方法将积分偏微分方程组化为积分常微分方程组。然后,在时间域上运用数值积分和有限差分将方程离散为非线性代数方程组。本文对四周固定夹紧的圆形、椭圆形和矩形薄膜进行了求解,并将求解结果用于颅底缺损重建膜的膨出量计算,计算值与实验值吻合,为颅底外科提供一个理论分析方法。     

超低频被动垂直隔振系统研究 *

在扭杆弹簧的扭臂上安装附加质量构成了一种超低频被动垂直隔振系统 .理论分析和实验结果表明 ,带有附加质量的扭杆弹簧隔振系统具有良好的隔振性能和较长的共振周期 .在10Hz处 ,隔振系统的检验质量对地脉动噪声的响应达到3×10 ^-12 m/Hz^1/2 .在6Hz处 ,系统的隔振率为- 60 dB ,比无附加质量的系统降低近30dB .系统的共振周期可从无附加质量时的 5.2s增加到21.3s.     

Haudorff测度与等径不等式

对于:Hausdorff维数为s>0的满足开集条件的自相似集E(?)Rn(n>1),我们引入等径不等式Hs|E(X)≤|X|s,以及使该不等式等号成立而直径大于0的极限集U(?)Rn.这里,Hs|E(·)是限制到集合E上的s维Hausdorff测度,而|X|指集合X在欧氏度量下的直径.当s=n时,n维球是唯一的极限集;当s∈(1,n)时,除去一些反面例子以外,我们对上述等径不等式的极限集的基本性质所知甚少.可以看出,这些不等式与Hs(E)的准确值的计算有密切联系.作为特例,我们将考虑Sierpinski垫片,指出计算这一典型自相似集的In2/In3维Hausdorff测度准确值的困难何在.由此可以大致推想,为什么除去平凡情形以外,至今还没有一个具体的满足开集条件而维数大于1的自相似集的:Hausdorff测度准确值被计算出来.     

Performance of an Accelerometer Suspended by a Pendulum

Performances of an accelerometer suspended by a pendulum and set on ground directly are both discussed. Theoretical analysis shows that the isolation from seismic disturbances of such a suspending accelerometer is equal to that of a narrow band-pass electronic filter. This means that the effect of seismic noises to the accelerometer can be substantially suppressed by means of a pendulum suspension during its performance tests.     

Measurement of Density Inhomogeneity for Source Masses in Time-of-Swing Method of Measuring G

A method with scanning electron microscopy （SEM） is presented to measure the density inhomogeneity of the stainless steel （SS316） sphere prepared for measuring G using time-of-swing method. The experimental result shows that the relative density inhomogeneity of the sphere is better than 5.9 × 10^-4 over the volume of 0.272 ×0.234× 0.005 mm^3. If we assume that the density inhomogeneity of the spheres used in our G measurement is the same as that of the sphere destroyed in testing, it will contribute to G value with an uncertainty of less than 0.034 ppm in our G measurement. Furthermore, the mass centre offset from the geometric centre of the sphere will be less than 4.3× 10^-4 μm due to this inhomogeneity.     

Phase Leading of Temperature Variations in Cavity Caused by Heat Conduction Between Air and Rock

It is well known that the terrestrial temperature varies with the period of 24 hours mainly due to the rotation of the earth in the field of the solar radiation. However, we observed the phenomenon that the semldiurnal(12 hour) temperature oscillation is dominant in our underground laboratory. By the spectrum analysis, the close correlation between the variations of temperature and atmospheric pressure was discovered, and the result of phase analysis showed that the phase of semidiurnal pressure lags behind that of the semidiurnal temperature. A model of heat conduction between air and rock is presented to explain the semidiurnal temperature oscillation observed in the underground laboratory.