未知主轴各向异性介质的热导张量光热测量方法研究

以在连续调制光辐照下的各向异性介质的热传导理论为基础,通过So(3)群转动理论,导出不透明各向异性介质的热导张量和热导主轴方向的光热测量方法。     

一种设计双正交小波基Hilbert变换对的新方法

幅频条件、相频条件以及完美重构条件构成了双正交小波基Hilbert变换对的设计基础.本文总结出两双正交低通滤波器消失矩差和相位差之间的关系.该关系表明选择适当的消失矩是满足相频条件的唯一途径,同时提供了选择方法.幅频条件则通过一个有约束条件的优化过程近似满足,其中完美重构条件用作约束条件,选择原滤波器和对偶滤波器幅频响应之差的平方的积分为损失函数.仿真实例表明该方法能够得到较其它方法更接近Hilbert变换对的结果.     

通道均衡器的算法及其在雷达数据采集分析中的应用

从时域和频域两个方面分析了通道均衡器的算法，并对雷达数据采集通道均衡前后的幅频、相频误差进行了比较，结果表明采集通道的失真是可以通过通道均衡器进行补偿的。     

基于张量模式噪声补偿的室内可见光通信系统的信道估计

针对现有中继转发式室内可见光通信系统中信道估计算法存在的导频数量过大、估计效率和精度低的问题,提出基于张量模式噪声补偿的信道估计方法。首先,充分利用可见光通信系统发射数据的特点设计了一种适合在接收数据中进行噪声补偿的导频结构。然后,在PARATUCK2张量分解框架下,构造了这种导频模式的含噪通信系统模型。最后,结合张量分解方法,设计了一种以导频所得噪声补偿对实际噪声进行估计的方法,完成所有信道参数的计算。仿真实验结果表明,将基于张量模式噪声补偿的估计算法应用在中继转发式室内可见光通信系统中,可以在加快寻优迭代速度的同时提高估计精确度,充分验证该算法的有效性和可行性。     

工艺误差对SAW叉指换能器频率响应影响的蒙特洛分析

本文用蒙特长洛方法分析了工艺误差对SAW叉指换能器幅频及相频响应的影响.结果表明了具有不同指标的SAW叉指换能器受工艺误差影响的不同程度.     

合成孔径雷达数据传输系统失真分析

本文推导了合成孔径雷达数据传输系统视频技术信号的频谱特性，利用数传信号二维谱分析了数传系统幅频失真，相频失真合成孔径雷达距离向，方位向压缩性的影响，这一分析已成功应用于机载多条带多极化合成孔径雷达数据传输系统的研制中。     

相控阵体制SAR系统的误差建模与补偿方法

在基于相控阵天线体制的合成孔径雷达(SAR)系统中,中央电子设备和相控阵天线的非理想特性,会引入幅相误差,造成SAR信号幅相特性畸变,影响SAR图像等数据产品的质量。本文建立了相控阵体制SAR系统误差的模型,并设计了误差校正方法。研究结果表明：雷达中央电子设备的非理想特性会引入固定的幅频、相频误差;相控阵天线的非理想特性所引入的幅频、相频误差会随着波束指向的变化而变化,该误差主要根源于有源器件在不同频点处的性能差异,并会受到T/R模块移相衰减量的调制;可通过测量或分析计算对相控阵SAR的系统误差进行提取,并在SAR成像处理阶段实施误差补偿。     

相频扫描阵列通道幅相测试方法研究北大核心CSCD

相频扫描阵列具有频扫低成本和相扫控制灵活等优点,被广泛用于军事和民事。该类阵列的增益、相位扫描面波束副瓣等受通道的幅度相位一致性影响较大,需要经过校准补偿以获得高性能波束。准确高效的通道幅相测试方法是进行校准的前提和基础。文中首先分析了组成相频扫描阵列的波导缝隙天线近场区域的辐射特性,得出在柱面波区域进行取样的可行性。在此基础上提出了扫描架逐行移动取样以及中场取样两类共五种测试方法,并给出了程差的幅相补偿方法。计算了各测试方法所引入的系统误差,分析了它们各自的适用场合。最后对比其中两种测试方法在某相频扫描阵列样机中的幅相测试结果,并给出了补偿校准后的波束测试结果,验证了文中所提出测试方法的准确性和有效性。     

基于单片机和FPGA的频率特性测试仪的设计

该系统基于扫频外差基本原理,以单片机和FPGA构成的最小系统为控制核心.可在任意指定频段内测量被测网络的幅频和相频特性并显示相应曲线.系统分DDS扫频信号源、被测网络、幅度和相位检测、控制模块及幅频、相频特性曲线显示等部分,在100 Hz～100 kHz范围内可自动步进测量被测网络的幅频特性和相频特性并自动设置频段范围,观察不同频段内网络的幅频特性和相频特性,并在示渡器上同时显示幅频曲线和相频曲线.     

基于单片机和FPGA的频率特性测试仪的设计

该系统基于扫频外差基本原理,以单片机和FPGA构成的最小系统为控制核心．可在任意指定频段内测量被测网络的幅频和相频特性并显示相应曲线。系统分DDS扫频信号源、被测网络、幅度和相位检测、控制模块及幅频、相频特性曲线显示等部分,在100Hz～100kHz范围内可自动步进测量被测网络的幅频特性和相频特性并自动设置频段范围,观察不同频段内网络的幅频特性和相频特性,并在示波器上同时显示幅频曲线和相频曲线。     

调制激光束加热下各向异性材料的光热特性研究

利用傅里叶变换求解三维热传导方程得到调制激光束照射下各向异性材料的交变温度场分布,研究了各向异性材料表面和不同深度的温度场幅度与相位,并讨论了材料的光热属性对温度场分布的影响,此外对如何测量各向异性材料的热扩散张量得到了一些有用结论。     

Anisotropic thermal conductivity of nanoporous silica film

In this paper, thermal conductivity of porous silica film with porosity from 21 to 64% was studied comprehensively. The corresponded dielectric constant is from 2.5 to 1.5. It is observed that the porous silica material has strong anisotropic characteristic. A serial-parallel hybrid model is proposed to explain the correlation between porosity and thermal conductivity in both in-plane and cross-plane components. The pores in the higher porosity silica film tend to distribute horizontally. This distribution of the pores in the dielectric film is the main factor that induces the anisotropic characteristic. The nonuniform distribution of pores also makes the conventional two-dimensional model of 3u/spl grave/ method inappropriate for extracting the in-plane thermal conductivity. A new method based on the hybrid model was proposed to extract the in-plane thermal conductivity successfully. The anisotropic characteristic of the thermal conductivity may be accompanied by the anisotropic dielectric constant, which will greatly complicate the thermal management and resistance-capacitance delay simulation of the circuits and should be avoided. The proposed model would be helpful on evaluation of new porous low dielectric constant materials.     

A new method to derive white matter conductivity from diffusion tensor MRI

We propose a new algorithm to derive the anisotropic conductivity of the cerebral white matter (WM) from the diffusion tensor MRI (DT-MRI) data. The transportation processes for both water molecules and electrical charges are described through a common multicompartment model that consists of axons, glia, or the cerebrospinal fluid (CSF). The volume fraction (VF) of each compartment varies from voxel to voxel and is estimated from the measured diffusion tensor. The conductivity tensor at each voxel is then computed from the estimated VF values and the decomposed eigenvectors of the diffusion tensor. The proposed VF algorithm was applied to the DT-MRI data acquired from two healthy human subjects. The extracted anisotropic conductivity distribution was compared with those obtained by using two existing algorithms, which were based upon a linear conductivity-to-diffusivity relationship and a volume constraint, respectively. The present results suggest that the VF algorithm is capable of incorporating the partial volume effects of the CSF and the intravoxel fiber crossing structure, both of which are not addressed altogether by existing algorithms. Therefore, it holds potential to provide a more accurate estimate of the WM anisotropic conductivity, and may have important applications to neuroscience research or clinical applications in neurology and neurophysiology.      

光热辐射技术测量钴涂层热学参数及厚度

为获得涂层材料特性,使用光热辐射(PTR)技术进行无损检测,并提出一种基于一维 三层PTR 理论模型的标定方法,完成了对Al基体钴涂层热学参数、厚度及分布的检测。三层PTR 理论模型,由涂层、 空气缺陷层和基体组成。搭建了实验平台,对两种不同厚度的钴(Co)涂层Al基体样品、无涂 层Al基体参考样品 进行了多点PTR测量。利用参考样品对带涂层样品PTR相位信号进行规格化,以消除检 测系统的影响。 通过多参数优化算法,对规格化后相位信号进行拟合,完成三层理论模型公式热参数的标定 ,求出了涂层 材料的热传导率和热扩散系数,进而计算得到各检测点涂层厚度及分布。实 验结果验证了理论 模型的正确性,以及材料作为基体和涂层时其热学性能的值会发生改变。     

Propagation of ELF waves below an inhomogeneous anisotropic ionosphere

The ionospheric anisotropy is considered with horizontal magnetic field either for transverse (East-West or West-East) or for longitudinal (South-North) propagation. For transverse propagation in a vertically stratified medium the differential equations of the various field components are uncoupled and a closed form solution is given for identical exponential height variation of the components of tensor conductivity. For arbitrary height variation of the tensor conductivity numerical solutions are obtained after expressing the surface impedance below the ionosphere in terms of a Riccati-type differential equation. The West-East direction of propagation exhibits a lower attenuation constant than the East-West direction forf < 1000cps. This is contrary to the expectations based on a model of a homogeneous anisotropic ionosphere. For longitudinal propagation the differential equations of the various field components are coupled, with the coupling being particularly strong above theDregion. The differential equations are simplified by assuming no coupling in the lower ionosphere and strong coupling above a pre-selected altitudey_{1}. For exponential height variation of the tensor conductivity components the closed form solution differs negligibly from the isotropic case. For arbitrary height varition of the tensor conductivity numerical solutions are obtained similarly as for the transverse propagation. Over most of the frequency range the attenuation figures for South-North propagation are intermediate between the corresponding figures for West-East and East-West propagation.     

Image reconstruction in magnetic resonance conductivity tensor imaging (MRCTI)

Almost all magnetic resonance electrical impedance tomography (MREIT) reconstruction algorithms proposed to date assume isotropic conductivity in order to simplify the image reconstruction. However, it is well known that most of biological tissues have anisotropic conductivity values. In this study, four novel anisotropic conductivity reconstruction algorithms are proposed to reconstruct high resolution conductivity tensor images. Performances of these four algorithms and a previously proposed algorithm are evaluated in several aspects and compared.     

Anisotropic Thermal Conductive Composite by the Guided Assembly of Boron Nitride Nanosheets for Flexible and Stretchable Electronics

Owing to the growing demand for highly integrated electronics, anisotropic heat dissipation of thermal management material is a challenging and promising technique. Moreover, to satisfy the needs for advancing flexible and stretchable electronic devices, maintaining high thermal conductivity during the deformation of electronic materials is at issue. Presented here is an effective assembly technique to realize a continuous array of boron nitride (BN) nanosheets on tetrahedral structures, creating 3D thermal paths for anisotropic dissipation integrated with deformable electronics. The tetrahedral structures, with a fancy wavy shaped cross‐section, guarantee flexibility and stretchability, without the degradation of thermal conductivity during the deformation of the composite film. The structured BN layer in the composites induces a high thermal conductivity of 1.15 W m^?1 K^?1 in the through‐plane and 11.05 W m^?1 K^?1 in the in‐plane direction at the low BN fraction of 16 wt%, which represent 145% and 83% increases over the randomly mixing method, respectively. Furthermore, this structured BN composite maintains thermal dissipation property with 50% strain of the original length of composite. Various electronic device demonstrations provide exceptional heat dissipation capabilities, including thin film silicon transistor and light‐emitting diode on flexible and stretchable composite, respectively.     

Development of conducting adhesive materials for microelectronic applications

Electrically and/or thermally conducting adhesive materials are classified into two categories depending on their conduction modes: isotropic and anisotropic materials. Silver-particle filled epoxy is the most common example of the class of isotropic materials which are conductive in all directions. This material has been long used in the electronic applications as a die-bonding material, where its good thermal conduction rather than its electrical conduction property is utilized. The silver-filled epoxy material has several limitations for high performance electrical interconnections, such as low electrical conductivity, increase in contact resistance during thermal exposure, low joint strength, corrosion issue due to silver migration, difficulty in rework, and so forth. The anisotropic conducting material provides electrical and/or thermal conduction only in one direction. An anisotropic conducting film (ACF) is used for interconnecting TAB mounted chips to a liquid crystal display panel, where fine pitch interconnection and low temperature assembly are required. In this paper, a brief review of the state-of-art conducting adhesive technology is provided. Subsequently, development of new conducting adhesive materials is presented for several different applications, which include high temperature materials for ceramic substrates, and low temperature materials for organic substrates.