二维弹性力学问题的光滑无网格伽辽金法

计算效率低的问题长期阻碍着无网格伽辽金法(element-free Galerkin method, EFGM)的深入发展.为了提高EFGM的计算速度,本文提出一种求解二维弹性力学问题的光滑无网格伽辽金法.该方法在问题域内采用滑动最小二乘法(moving least square, MLS)近似、在域边界上采用线性插值建立位移场函数;基于广义梯度光滑算子得到两层嵌套光滑三角形背景网格上的光滑应变,根据广义光滑伽辽金弱形式建立系统离散方程.两层嵌套光滑三角形网格是由三角形背景网格本身以及四个等面积三角形子网格组成.为了提高方法的精度,由Richardson外推法确定两层光滑网格上的最优光滑应变.几个数值算例验证了该方法的精度和计算效率.数值结果表明,随着光滑积分网格数目的增加,光滑无网格伽辽金法的计算精度逐步接近EFGM的,但计算效率要远远高于EFGM的.另外,光滑无网格伽辽金法的边界条件可以像有限元那样直接施加.从计算精度和效率综合考虑,光滑无网格伽辽金法比EFGM具有更好的数值表现,具有十分广阔的发展空间.     

化学腐蚀对混凝土材料力学特性影响的试验研究

对不同水化学环境腐蚀过程中砂浆试样物理力学性质进行了试验研究,分析了溶液pH值、浓度及水化学成分对砂浆的腐蚀程度,探讨了砂浆试样的化学腐蚀机制。试验结果表明:在相对封闭的环境条件下,无论是哪种水化学溶液,随着化学侵蚀时间的加长,溶液的pH值均趋于碱性;同时,溶出的Ca2+、Mg2+离子具有累积性特点。在其他条件相同的情况下,水化学溶液的离子成分对砂浆试样的腐蚀程度各不相同。酸性条件下,Na2SO4溶液砂浆试样物理化学性质的影响要比NaHCO3溶液大;中性条件下,与纯净的蒸馏水相比,Na2SO4溶液对砂浆试样的腐蚀作用要强一些。     

Adsorption Behaviors of 4-Mercaptobenzoic Acid on Silver and Gold Films

The adsorption behaviors of 4-mercaptobenzoic acid on silver and gold nanoparticles were studied by surface-enhanced Raman scattering (SERS) and density functional theory. The silver and gold films by electrodeposition have the same excellent characteristics as SERS-active substrates. At the same, the SERS spectra indicate that 4-mercaptobenzoic acid molecules are adsorbed on the surfaces of gold nanoparticles through the S atom, and that the carboxyl group is far away from surface of gold nanoparticles, and that there is a certain angle between the plane of benzene ring and gold film. However, 4-mercaptobenzoic acid molecules are adsorbed on the surfaces of silver nanoparticles through the carboxyl group, and the S atom is far away from surface of silver nanoparticles, and there is also a certain angle between the plane of benzene ring and the surface of silver nanoparticles. Here it is demonstrated the calculated Raman frequencies are in good agreement with experimental values, and the calculated Raman frequencies are also helpful to infer the adsorption behaviors of 4-mercaptobenzoic acid molecules.     

奇A 核的多声子γ振动带实验研究进展

对于近年来清华大学与美国合作研究组在原子核结构实验研究中有关奇A 核的多声子 振动带方面的进展进行了评述。研究人员通过有效的国际合作,识别了质量数A = 100 丰中子核区的奇A核103Nb,105Mo,107Tc 和109Tc中的两声子 振动带,这是迄今为止国际上在奇A 核的结构研究中发现的仅有的4 例这样的结构。对于实验方法、研究结果以及这些多声子 振动带的特性进行了评述,并对目前在多声子带其他方面的研究及今后进一步的工作进行了概述。The experimental research on multi-phonon γ-vibrational bands in odd-A nuclei by our international collaboration has been reviewed. The two phonon -vibrational bands in 103Nb, 105Mo, 107Tc and 109Tc in the A = 100 neutron-rich nuclear region were identified, which are the only four such bands discovered in nuclear structure studies until now. The experimental methods and results are summarized, and the characteristics for these band structures are discussed. The other aspects for our research on the multi-phonon vibrational bands as well as our future research work are also briefly reviewed.     

迁移学习在OCT视网膜图像自动分类上的应用

利用光学相干断层技术(OCT)产生的视网膜疾病图像是分类眼科疾病的重要措施。目的是利用四种不同分类模型的迁移学习方法对糖尿病黄斑水肿、老年性黄斑变性、玻璃疣三种病变的OCT视网膜图像进行自动分类,实现迁移学习在OCT图像分类上的应用。将VGG-16、Inception V3、MobileNet V2、ShuffleNet-V2这四种神经网络模型在大规模图像分类数据集预训练好后,进行模型微调和训练参数更新,进而找到实现上述三种眼科疾病自动分类的最优模型,达到高效的OCT视网膜病变自动分类效果。实验结果表明,四种模型中轻量型MobileNet V2经模型微调后的评价指标优于其他模型。     

新型水溶性荧光标示剂吲哚方酸菁染料的合成及光谱性能

用方酸与不同的N烷基取代吲哚啉季铵盐缩合制备了一系列对称的水溶性方酸菁染料. 通过核磁共振氢谱和质谱对合成的染料结构进行了表征, 研究了它们在不同溶剂中的吸收和发射光谱. 结果表明, 随着溶剂极性的增大, 染料的吸收光谱发生蓝移, 表现为负向溶剂化效应, 在极性溶剂中的荧光量子产率比在水中的大. 考察了N位取代基对染料水溶液光稳定性的影响, 结果表明在吲哚环N原子上引入较大的苄基有助于提高光稳定性, 且随着苄基上取代基吸电子能力的增强, 染料的光稳定性增强.     

SPIN-1 (3-LEVEL) MODEL OF NMR LASER DRIVEN BY AN EXTERNAL SIGNAL

A set of NMR laser equations is presented for a spin-1 model under the assumption that the system is homogeneously broadened and spatially independent. The steady state solutions are obtained and their instabilities are revealed. The results are compared with the spin-1/2 model. Multi-codimensional bifurcation points are found.     

INFLUENCE OF γ-RADIATION ON THE DIELECTRIC CHARACTERISTICS IN Rb2ZnCl4 SINGLE CRYSTALS AT INCOMMENSURATE-COMMENSURATE PHASE TRANSITION

In this paper the influence of γ-radiation on the dielectric constants of Rb₂ZnCl₄ crystal at incommensurate-commensurate phase transition (hereafter abbreviated as INC-C transition) are studied. The thermal hysteresis occurs upon both cooling and heating runs, irrespective of whether the samples have been treated with γ-radiation or not. For the γ-irradiated sample, its transition point, T_c, between the INC and C phases is not changed, but the peak value of the dielectric constant at T_c increases abruptly, compared with that before γ-irradiation, When this sample is annealed at 40℃, the peak value restores to the incipient value for the sample free from γ-irradiation. The origin of the phenomenon of the thermal hysteresis of the dielectric constant may be due to the pinning effect of dejects or impurities in the samples.     

用于移动通信的平面型紧凑极化分集天线

目前在移动通信中，基站广泛采用分集天线来提高通信质量。但是，只有移动台到基站上行链路通信质量的提高，双向通信的问题才解决了一半。为了获得更好的链接质量，可在移动台采用分集天线。极化分集是抗多径衰落的一种重要方法，能够有效提高系统链路性能。然而，研制可用于手机的分集天线是一项非常具有挑战性的工作，因为这种天线必须满足体积、带宽、效率以及天线单元(或者是天线端口)之间隔离度的要求。     

HL—1装置热脉冲传播的傅里叶分析

本文采用一种新提出的数值方法，即傅里叶变换分析法，研究了ＨＬ－１装置中用软Ｘ射线测量得到的锯齿振荡诱导的热脉冲传播行为。此方法不仅可以计算扰动调制的热传系数，而且可以分析各次谐波的幅值和相应随频率及空间的演变，并与热脉冲峰值时间延迟分析法进行了比较，两者推算的热传导系数符合较好。