指纹图像增强算法研究

本文根据笔者近年来的研究试验结果，指出了传统指纹图像增强算法的不足，以两种典型滤波算法为主，对近年来流行的基于方向场估计的滤波增强算法进行了分析和评价，并针对这两种算法的不足之处提出了一种新的算法思想。     

新型热电材料及其新应用

热电材料，由于在清洁能源、光电子探测等诸多方面有巨大的应用前景，因而受到科学上和技术上的广泛关注．最新的理论进展和若干新材料的发现，为人们提供了新的应用机会．一类新材料就是基于强关联电子系统，典型例子是过渡族金属氧化物，这类材料以前从热电材料的角度并未受到重视．我们将讨论几点最近的进展，着重讨论生长在倾斜切割衬底上薄膜的新的应用．这种情况下热电电压的产生是基于各向异性的塞贝克效应．当光辐射照射到薄膜表面时，薄膜上下表面的温差引起横向的电势差，从而可用于光、热辐射的探测．我们论证：这类探测器有很具特色的优点，宽的光谱响应，快的时间响应，同时不须施加偏压或偏流，因而十分节能．我们还讨论了几种典型材料的性能．     

基于总变差的图像放大和增强方法

利用小波的多分辨分析和总变差极小原理,提出了一种实现图像放大和增强的新方法。该方法把原图像作为放大图像的小波子带,对放大图像强加了一种小波系数型的限制。放大图像利用图像总变差极小模型进行正则化。经求变分产生带限制的非线性扩散方程作为总变差极小的必要条件,求解偏微分方程得到增强的放大图像。对人工合成图像、医学MRI心脏切片和人物图像进行了实验。实验结果说明该方法同时实现图像放大和增强的有效性。     

Effective stiffness and microscopic deformation of an orthotropic plate containing arbitrary holes

Many engineering materials and structures, such as cellular structures, sandwich core structures and laminated plates with holes, can be modeled by an inclusion problem with anisotropic matrix. The paper studies the effective properties and the microscopic deformation of anisotropic plates with periodic holes by using direct and mathematical homogenization. The effective stiffnesses are calculated by different homogenization methods and the microscopic deformation of a RVE is modeled by the finite element method for the plate with arbitrarily shaped holes. All of the effective stiffness coefficients, especially stretching-shear coupling coefficients are evaluated.     

An accurate method to determine the through-plane electrical conductivity and to study transport properties in film samples

The through-plane conductivity of a film sample is critically important because it largely affects the performance of batteries, capacitors, and thermoelectric devices. In this study, we developed a modified four-probe through-plane electrical conductivity measurement method using a coaxial structure. This method is general and works for free-standing film samples. We studied different samples including a steel sheet, highly oriented pyrolytic graphite, and conducting polymers. We confirmed metallic transportation in the steel sheet and hopping transportation in graphite in the through-plane direction by conducting low temperature measurements at 100 K. In the case of a conducting polymer poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate, the conductivity anisotropic ratio decreases with increasing in-plane conductivity. Temperature dependent measurements show two distinct activation energy regimes in the through-plane direction in PEDOT/PSS but almost no change in the in-plane electrical conductivity activation energy. This could be due to additional carrier paths that occur through the more disordered region (the PSS-rich region) in the through-plane direction. We also examined the Meyer–Neldel rule in PEDOT/PSS and concluded that PEDOT/PSS follows the anti-Meyer–Neldel rule, likely due to the high carrier density in the film.     

Wicking into porous polymer-derived ceramic monoliths fabricated by freeze-casting

Silicon oxycarbide monoliths of different pore size distribution were fabricated by freeze-casting. The samples revealed a lamellar pore structure with an axial anisotropy. To evaluate the capillary transport abilities we performed wicking experiments. The sample weight measurement method was applied during the imbibition. The samples show deviations in permeability from 10% to 49% at different sample orientations that quantifies the impact of the anisotropy in the axial direction. The deviations were larger for the samples with smaller pore size. For these samples we also observed larger differences in the wicking behaviour. The samples with bigger pore size demonstrated higher permeability and faster wicking. Imbibition results at both sample orientations showed a good agreement with a prediction via the Lucas–Washburn equation with gravity effects. We demonstrate hereby, that our approach of macroscopic modelling predicts wicking behaviour in anisotropic structures reasonably well, providing a simple tool for further porous material investigations.     

An optimization-driven approach for computing geodesic paths on triangle meshes

There are many application scenarios where we need to refine an initial path lying on a surface to be as short as possible. A typical way to solve this problem is to iteratively shorten one segment of the path at a time. As local approaches, they are conceptually simple and easy to implement, but they converge slowly and have poor performance on large scale models. In this paper, we develop an optimization driven approach to improve the performance of computing geodesic paths. We formulate the objective function as the total length and adopt the L-BFGS solver to minimize it. Computational results show that our method converges with super-linear rate, which significantly outperforms the existing methods. Moreover, our method is flexible to handle anisotropic metric, non-uniform density function, as well as additional user-specified constraints, such as coplanar geodesics and equally-spaced geodesic helical curves, which are challenging to the existing local methods.     

Anisotropic electrical and abrasion-sensing properties of cement-based composites containing aligned nickel powder

The electrical properties of aligned nickel powder-filled cement-based composites and the abrasion-sensing properties of these composites, as evaluated based on the change in their resistance at varying abrasion depths, were investigated in this paper. Micrograph characterization of nickel powder distribution indicates that the electrical conduction path preferentially forms along the direction of the nickel powder alignment, which leads to increasingly anisotropic electrical properties under greater magnetic field strengths. The level of anisotropy was also determined to be strongly dependent on the nickel powder content. The maximum anisotropic electrical properties were achieved at the percolation threshold content of the nickel powder, which is the critical point for the formation of an effective conductive network. Based on the cement-based composites anisotropic electrical properties and a sectionalized electrode design, the composites filled with aligned nickel powder demonstrated good abrasion-sensing properties, with humidity and temperature self-compensation abilities.     

Oriented total variation l1/2 regularization

Total Variation (TV) is a widely used image restoration/decomposition model. It is observed that the classical TV l1 and TV l2 regularization, on the one hand, do not favor higher-gradient structures over lower-gradient details as expected for structure preserving image processing, and on the other hand, tend to reduce the horizontal and vertical gradients, and thus inevitably blur the oblique edges in images. In this paper, we address these two problems by defining Oriented Total Variation l1/2 (OTV l1/2). It is theoretically and experimentally demonstrated that applying l1/2 regularization to the directional derivatives of images leads to superior structure preservation. OTV l1/2 regularization can be applied to image denoising and video compression, and the experimental results verify that OTV l1/2 outperforms other similar models.     

Macropore formation in p-type silicon: toward the modeling of morphology

The formation of macropores in silicon during electrochemical etching processes has attracted much interest. Experimental evidences indicate that charge transport in silicon and in the electrolyte should realistically be taken into account in order to be able to describe the macropore morphology. However, up to now, none of the existing models has the requested degree of sophistication to reach such a goal. Therefore, we have undertaken the development of a mathematical model (phase-field model) to describe the motion and shape of the silicon/electrolyte interface during anodic dissolution. It is formulated in terms of the fundamental expression for the electrochemical potential and contains terms which describe the process of silicon dissolution during electrochemical attack in a hydrofluoric acid (HF) solution. It should allow us to explore the influence of the physical parameters on the etching process and to obtain the spatial profiles across the interface of various quantities of interest, such as the hole concentration, the current density, or the electrostatic potential. As a first step, we find that this model correctly describes the space charge region formed at the silicon side of the interface.