超声轴向传播技术评价长骨的研究

用超声轴向传播技术评价长骨状况已成为近年来的一个研究热点。本文首先介绍了第一到达波(FAS)法、超声Lamb波及柱面导波法在评价长骨皮质骨状况方面的研究进展,最后分析和讨论了当前研究中存在的问题及应该研究的方向。     

一维链状配位聚合物[Cd{5-(NO2)sal}2(2,2′-bipy)]n的合成与晶体结构

A coordination polymer of [Cd{5-(NO₂)sal}₂(2,2′-bipy)]_n(5-(NO₂)sal=5-nitrosalicylate, 2,2′-bipy=2,2′-bipyridine) was synthesized by hydrothermal reaction and characterized by elemental analysis, IR and X-ray diffraction single crystal structure analysis. The title complex crystallizes in monoclinic with space group C2/c, a=2.730 8(16) nm, b=1.272 3(5) nm, c=0.674 5(3) nm, β=96.73(2)°, V=2.327(2) nm³, Z=4, R=0.022 2, wR=0.057. The 5-nitrosalicylate anions doubly bridge the Cd(Ⅱ) atoms to form one-dimensional polymeric chain with the repeated eight-membered ring units (Cd-O-C-O)₂. The crystal structure is stabilized by intra- and interchain hydrogen bonds interactions. CCDC: 694568.     

Cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of several members of the 1,ω-bis(4-cyanobiphenyl-4′-yl) alkane (CBnCB) and the 1-(4-cyanobiphenyl-4′-yloxy)-ω-(4-cyanobiphenyl-4′-yl) alkane (CBnOCB) homologous series are reported. The new odd members described CB5CB, CB13CB, CB4OCB, CB8OCB and CB10OCB all exhibit twist-bend nematic and nematic phases. The members of these series already reported in literature, CB7CB, CB9CB, CB11CB and CB6OCB, were also prepared in order to allow for a direct comparison of their transitional properties. The properties of these dimers are also compared to those of the corresponding members of the 1,ω-bis(4-cyanobiphenyl-4,-yloxy) alkanes (CBOnOCB). For any given total spacer length, for odd members of these series, the nematic–isotropic transition temperatures and associated entropy changes are greatest for the CBOnOCB dimer and lowest for the CBnCB dimer. These trends are understood in terms of molecular shape. For short spacer lengths, the twist-bend nematic–nematic transition temperature (T_NTBN) is higher for the CBnOCB series than for the CBnCB series but this is reversed as the spacer length increases. Of the CBOnOCB dimers, a virtual value of T_NTBN was estimated for CBO3OCB and T_NTBN was measured for CBO5OCB. These values are considerably lower than those observed for the corresponding members of the CBnCB or CBnOCB series. The dependence of T_NTBN on molecular structure is discussed not only in terms of the molecular curvature but also in the ability of the molecules to pack efficiently. As the temperature range of the preceding nematic phase increases, so the twist-bend nematic–nematic transition entropy change decreases and the transition approaches second order for the longer spacers. For comparative purposes, the transitional behaviour of the even-membered dimers CB6CB, CB5OCB and CBO4OCB is reported and differences accounted for in terms of molecular shape.     

Structural characteristics of different types of nanoparticles synthesised in mesomorphic metal alkanoates

The class of thermotropic ionic liquid crystals (LCs) of the metal alkanoates possesses a number of unique properties, such as intrinsic ionic conductivity, high dissolving ability and ability to form time-stable mesomorphic glasses. These ionic LCs can be used as nanoreactors for the synthesis and stabilisation of different types of nanoparticles (NPs). Thus, some semiconductors, metals and core/shell NPs were chemically synthesised in the thermotropic ionic liquid crystalline phase (smectic A) of the cadmium octanoate (CdC₈) and of the cobalt octanoate (CoC₈). By applying the scanning electron microscopy, the cadmium and cobalt octanoate composites containing CdS, Au, Ag and core/shell Au/CdS NPs have been studied. NPs’ sizes and dispersion distribution of the NPs’ size in the nanocomposites have been obtained.     

Solvothermal Synthesis and Characterization of Sheet Structure Containing Adamantane [Hg2Sn2S10]8– Anions

Sheet Rb2HgSnS4 was synthesized solvothermally and characterized by X-ray single crystal diffraction. The compound is comprised of sheets with admantane-like [Hg2Sn2S10]8– units. The crystals belong to the space group C2/c, with the unit cell parameters a=1.1063(2) nm, b=1.1071(2) nm, c=1.5741(3) nm, a=90°, β=100.13(3)°, γ=90°. A reflectance spectroscopy study reveals the nature of the semiconductor with an energy of 2.1 eV for the compound.     

骨小梁材料特性对超声背散射信号的影响

基于时域有限差分法(FDTD)建立了松质骨的超声背散射仿真系统,研究了骨小梁材料特性对超声背散射信号的影响。首次得到松质骨中的超声背散射系数(BSC)和积分背散射系数(IBC)随骨小梁材料参数(密度、拉梅常数、黏度系数及声阻抗系数)的变化关系。研究结果表明,IBC随骨小梁密度的增加而增加;BSC和IBC随拉梅常数的增加而增加、随第一黏度系数的增加而近似线性地减小,第二黏度的变化对背散射信号的影响很小;背散射参数随阻抗系数的增加而减小。说明松质骨中的超声背散射特性不仅受骨矿密度(BMD)和骨微结构的影响,还与骨小梁的材料参数密切相关。研究结果有利于理解松质骨中超声的背散射特性,对松质骨骨质状况的评价有一定帮助。      

时频域独立元分析方法实现超声兰姆波的多模式分离

为方便兰姆波信号分析与模式定征,提出一种将短时傅里叶变换(Short-Time Fourier Transform,STFT)与独立元分析(Independent Component Analysis,ICA)相结合的多模式超声兰姆波识别方法。首先通过STFT将兰姆波时域信号投影至时频域,基于各模式信号在时频域相对独立的特点,利用ICA实现混叠模式分离。根据分离模式时频能量脊提取各模式群速度曲线,进而估计板厚。将方法运用于时域有限差分(Finite-Difference Time-Domain,FDTD)法仿真与钢板实验,分离得到A0、A1和S0三种模式。仿真与实验中平均群速度估计误差约为1.5%和2.0%,板厚估计误差约为0.3%和2.0%。仿真结果表明,在信噪比(Signal-to-Noise Ratio,SNR)不小于0 dB的情况下,时频域独立元分析方法均可实现兰姆波多模式分离、群速度曲线提取及板厚估计。      

多通道超声兰姆波检测板状结构中的裂纹

超声兰姆(Lamb)波在结构缺陷检测方面愈来愈受到重视,但目前Lamb波的应用局限于单信号源激励,单通道接收的方法,容易受到噪声干扰,其后续的信号分析处理也比较复杂。本文旨在采用多通道Lamb波对板状结构中的裂纹进行定量分析与诊断。在铝板的表面凿刻出不同深度的凹槽作为裂纹,通过线阵换能器采集在一定传播距离内的多通道Lamb波信号,并采用二维傅里叶变换分析在不同深度的裂纹下,Lamb波模式能量的变化规律。结果表明,相对于完好铝板中的Lamb波信号,裂纹的存在会使Lamb波发生模式转换现象,并且转换模式能量百分比随裂纹深度的增加而线性增加。其结果为Lamb波评价板状结构中的裂纹状况提供了一种可能的方法。     

Laplace series expansion of the internal potential of a homogeneous circular torus

The internal potential of a homogeneous circular torus first is represented by a series expansion in spherical functions (Laplace series). Exact analytical formulas for the coefficients of this series are derived and it is shown that they can be expressed through the standard Gauss hypergeometric function depending only on the geometric parameter of the torus. Convergence of the series is proved and the radius of convergence is determined. The relation of the radius with the torus geometrical parameter is found. A special spherical shell, where the problem of expansion of the torus potential should be solved in additional investigations, is detected.     

二氧化碳速冻喷射器的优化设计及实验研究

本文在文献［３］的基础上作了优化计算的改进，利用气固两相流［１］和系统的优化计算模型编制程序，从最佳面积比、喷射个数和套筒尺寸之间的紧密联系出发，获得了优化设计速冻喷射系统的方法，用于设计速冻系统的套筒、喷嘴的结构尺寸。本文还将理论计算结果与试验数据进行比较，获得了满意的结果。