High resistivity measurement of SiC wafers using different techniques

To establish fast, nondestructive, and inexpensive methods for resistivity measurements of SiC wafers, different resistivity-measurement techniques were tested for characterization of semi-insulating SiC wafers, namely, the four-point probe method with removable graphite contacts, the van der Pauw method with annealed metal and diffused contacts, the current-voltage (I-V) technique, and the contactless resistivity-measurement method. Comparison of different techniques is presented. The resistivity values of the semi-insulating SiC wafer measured using different techniques agree fairly well. As a result, application of removable graphite contacts is proposed for fast and nondestructive resistivity measurement of SiC wafers using the four-point probe method. High-temperature van der Pauw and room-temperature Hall characterization for the tested semi-insulating SiC wafer was also obtained and reported in this work.     

块对角子空间聚类中成对约束的主动式学习

针对块对角表示(block diagonal representation, BDR)子空间聚类算法在对子空间重叠的高维数据聚类时效果较差的问题,提出成对约束的块对角子空间聚类(constrained subspace clustering with block diagonal representation, CBDR)算法,设计主动式学习策略,获取用户提供的少量数据点成对信息,以改进BDR算法的性能,给出CBDR算法的目标函数和求解过程。在测试集上的试验结果表明,CBDR算法的聚类错误率和归一化互信息指标比BDR和SBDR(structured block diagonal representation)算法好,而且主动式选取点对方法优于随机选取点对方法,使用少于5‰的约束信息可降低BDR的聚类错误率达到5%以上。     

加斜撑冷弯薄壁型钢组合墙体的抗震性能

通过对竖向荷载作用下加斜撑冷弯薄壁型钢组合墙体的抗震性能试验研究,考察了墙体的受力特性、破坏模式和滞回性能,分析了覆面板对组合墙体滞回性能、延性和耗能能力的影响。结果表明：相同竖向荷载作用下,单面覆OSB板墙体较无面板墙体的抗剪承载力提高了38.79%;墙架柱轴压比为0.24较轴压比为0.16的单面覆OSB板组合墙体抗剪承载力提高了7.5%,屈服位移降低了8.1%,墙体延性系数μ提高了4.5%,耗能系数E提高了4.1%,加斜撑冷弯薄壁型钢组合墙体体现出较好的抗震性能。在对有限元模型可靠性进行验证的基础上,通过变参数分析考察了墙架柱轴压比、钢龙骨屈服强度对墙体受力性能的影响规律,结果表明：随墙架柱轴压比的增大,组合墙体的抗剪承载力有所提高;减小钢龙骨的屈服强度,组合墙体的抗剪承载力下降明显。依据《低层冷弯薄壁型钢房屋技术规程》（JGJ 227—2011）和美国规范AISI S400-15,推导出墙体的抗力分项系数,确定了水平地震作用下此类墙体的抗剪承载力设计值。     

Improvement of flexural bond strength of zirconia-resin cement by surface patterning using sub-nanosecond UV laser

Zirconia is a dental material that shows excellent biocompatibility and high strength in clinical applications. This study aims to evaluate the effects of ultrafast laser applications. The surface nanostructures were classified into three groups. Group 1 was generated using the burst mode, with three different distances between dots: 52 µm (Group 1a), 104 µm (Group 1b), and 156 µm (Group 1c). Group 2 was processed using the scanning mode configuration, with a set of parallel lines. Group 3 was also processed using this scanning configuration creating a set of square-shaped patterning. Group 4 was the control group. After the surface treatments, a pair of zirconia specimens was bonded end to end with resin cement. Flexural bond strength (FBS) test was applied in a universal test machine. Multiple comparisons were performed using a one-way analysis of variance and the Tukey's HSD test. All the samples that were treated with the laser showed higher FBS values than the untreated surface. Using the burst mode, preformed circular-shaped surface on an angle of 90⁰ at 52 µm distance (Group 1a) showed the highest FBS values among all groups (p < .05). Groups 2 and 3 had significantly higher values than 1b and 1c.     

高强箍筋混凝土简支梁抗剪承载力试验研究

通过对14根500 MPa细晶粒钢筋、1根400 MPa及1根600 MPa钢筋做箍筋的混凝土简支梁在集中荷载作用下的试验,观测了不同混凝土强度、不同剪跨比、不同箍筋强度、不同配箍率、不同截面尺寸条件下试件的裂缝、挠度、承载力及破坏形态;并将实测值与有关公式的计算值进行了比较.试验结果表明,构件的斜截面承载力仍可按现行《混凝土结构设计规范》的相关公式进行计算,并有足够的安全储备.     

非奇异H矩阵的一个判定条件及一类实矩阵逆的无穷范数估计

通过理论证明,给出了判断一个矩阵为H矩阵的实用充分条件及一类实矩阵逆的无穷范数估计公式,并给出数值例子进行了说明.此结果对于判定系统的稳定性、特征值分布、线性方程组迭代解及矩阵范数估计等都具有重要的理论和实际意义.     

Adaptive detectors in the Krylov subspace

The validity of the application of the Krylov subspace techniques in adaptive filtering and detection is investigated. A new verification of the equivalence of two well-known methods in the Krylov subspace, namely the multistage Wiener filters （MWF） and the auxiliary-vector filtering （AVF）, is given in this paper. The MWF and AVF are incorporated into two well-known detectors, namely, the adaptive matched filter （AMF） and Kelly＇s generalized likelihood ratio test （CLRT） including their diagonally loaded versions, which form new detectors. Compared to the conventional AMF, CLRT, and their diagonally loaded versions as well as the reduced-rank AMF and GLRT, the probabilities of detection （PDs） of the new detectors are improved especially when the sample support is low. More importantly, the new detectors are robust of the rank selection of the clutter subspace compared to the reduced-rank AMF and GLRT. These new detectors all possess asymptotic constant false alarm rate （CFAR） property.     

四点弯曲U形切口应力集中系数的数值分析

四点弯曲实验是适用陶瓷材料断裂韧性的测试方法和标准,可以研究具有切口脆性材料构件的断裂.借助于ANSYS有限元分析,得到了不同的切口尖锐度(切口深度与切口根部半径的比值a/ρ)和宽度比值(试样宽度和切口深度的比值w/a)下,单边U形切口四点弯曲的应力集中系数.利用Matlab软件拟合不同宽度比值,得到四点弯曲U形切口应力集中系数与切口尖锐度的关系式,为四点弯曲U形切口的断裂分析提供了理论基础.     

对数周期天线阵的一种盲波束形成算法研究

对水平极化对数周期圆形天线阵中单元天线的方向图与相位特性进行了分析,指出阵列流形失配是导致传统波束形成算法性能下降甚至失效的主要原因.对此,结合最小二乘恒模算法（LSCMA）和对角加载技术（DL）的优点,给出了一种基于对角加载的最小二乘恒模算法.仿真结果表明该算法对阵列流形失配具有较强的鲁棒性,并在低信噪比下具有较好的合成性能.