厚度模压电超声换能器无源声学材料研究进展*

厚度模压电超声换能器作为超声波发射、接收以及电信号间转换的载体,是超声成像与检测系统的核心器件,一般由压电层、匹配层和背衬层3部分组成。超声换能器的性能一定程度上决定着整体超声设备的性能,影响了其在工业、医学、军事等领域的应用。该换能器的关键性能指标(带宽、灵敏度)除了受到压电层的影响,还与匹配层、背衬层等无源声学材料的设计密切相关。该文综述了近年来厚度模压电超声换能器无源声学材料(匹配层、背衬层和声透镜)的研究进展,提出了当前该类材料面临的难题和解决途径,并对其未来发展方向进行了展望。     

利用海洋环境噪声估计浅海海底表层声速

为提高反演效率,提出一种快速估计浅海海底表层声速的方法。根据噪声能流理论,垂直阵接收的海洋环境噪声数据能够用于无源提取海底反射损失,反射损失曲线中具有明显的临界角效应,从而接估计海底表层声速。以射线模型为基础,推导了噪声提取的反射损失与理论值之间的差异,并讨论实际阵列波束形成在不同角度和频率下的性能。考虑到非等声速环境下声线会发生弯曲,需对角度进行修正以提高方法的广泛适用性。不同频率的临界角与有效深度之间存在对应关系,黄海某海区数据处理结果表明,在临界角不变的有效深度内,海底表层可以视为等声速层,该海区海底表层0.5 m内声速估计结果为1547 m/s,与有源反演结果相近。     

Analysis of the generation mechanism of the S-shaped J—V curves of MoS2/Si-based solar cells

Amorphous-microcrystalline MoS$_{2}$ thin films are fabricated using the sol-gel method to produce MoS$_{2}$/Si-based solar cells. The generation mechanisms of the S-shaped current density-voltage ($J$-$V$) curves of the solar cells are analyzed. To improve the performance of the solar cells and address the problem of the S-shaped $J$-$V$ curve, a MoS$_{2}$ film and a p$^+$ layer are introduced into the front and back interfaces of the solar cell, respectively, which leads to the formation of a p-n junction between the p-Si and the MoS$_{2}$ film as well as ohmic contacts between the MoS$_{2}$ film and the ITO, improving the S-shaped $J$-$V$ curve. As a result of the high doping characteristics and the high work function of the p$^+$ layer, a high-low junction is formed between the p$^+$ and p layers along with ohmic contacts between the p$^+$ layer and the Ag electrode. Consequently, the S-shaped $J$-$V$ curve is eliminated, and a significantly higher current density is achieved at a high voltage. The device exhibits ideal p-n junction rectification characteristics and achieves a high power-conversion efficiency (CE) of 7.55%. The findings of this study may improve the application of MoS$_{2}$ thin films in silicon-based solar cells, which are expected to be widely used in various silicon-based electronic and optical devices.     

Secrecy sum rate maximization for a MIMO-NOMA uplink transmission in 6G networks

Non-orthogonal multiple access (NOMA), as a well-qualified candidate for sixth-generation (6G) mobile networks, has been attracting remarkable research interests due to high spectral efficiency and massive connectivity. The aim of this study is to maximize the secrecy sum rate (SSR) for a multiple-input multiple-output (MIMO)-NOMA uplink network under the maximum total transmit power and quality of service (QoS) constraints. Thanks to the generalized singular value decomposition method, the SSR of NOMA is compared with conventional orthogonal multiple access and other baseline algorithms in different MIMO scenarios. Due to the subtractive and non-convex nature of the SSR problem, the first-order Taylor approximation is exploited to transform the original problem into a suboptimal concave problem. Simulation results are provided and compared with some other benchmarks to evaluate the efficacy of the proposed method.     

Physical layer security using beamforming techniques for 5G and beyond networks: A systematic review

It has been predicted that by the year 2030, 5G and beyond 5G (B5G) networks are expected to provide hundreds of trillions of gigabytes of data for various emerging applications such as augmented, mixed, and virtual reality (AR/MR/VR), wireless computer-brain interfaces (WCBI), connected robotics and autonomous systems. Most of these applications share data with each other using an open channel, i.e., the Internet. The open and broadcast nature of wireless channel makes the communication susceptible to various types of attacks (e.g., eavesdropping, jamming). Thus, there is a strong requirement to enhance the secrecy of wireless channel to maintain the privacy and confidentiality of transmitted data. Physical layer security (PLS) has evolved as a novel concept and robust alternative to cryptography-based techniques, which have a number of drawbacks and practical issues for 5G and beyond networks. Beamforming is an energy-efficient PLS technique, that involves steering of the transmitted signal in a particular direction, while considering that an intruding user attempts to decode the transmitted data. Motivated from these points, this article summarizes various beamforming based PLS techniques for secure data transmission in 5G and B5G networks. We investigate the eight most promising techniques for beamforming in PLS: Non-Orthogonal Multiple Access (NOMA), Full-Duplex Networks, Massive Multiple-Input Multiple-Output (MIMO), Cognitive Radio (CR) Network, Relay Network, Simultaneous Wireless Information and Power Transfer (SWIPT), UAV Communication Networks and Space Information Networks, and Heterogeneous Networks. Moreover, various physical layer threats and countermeasures associated with 5G and B5G networks are subsequently covered. Lastly, we provide insights to the readers about constraints and challenges for the usage of beamforming-based PLS techniques in various upcoming future applications.     

分步法高速沉积微晶硅薄膜

为提高微晶硅薄膜的纵向结晶性能, 在甚高频等离子体增强化学气相沉积技术的基础上, 采用过渡参数缓变和两步法相结合的方法在普通玻璃衬底上高速沉积薄膜. 当功率密度为2.1 W/cm², 硅烷浓度在6%和9.6%之间变化时, 从薄膜方向和玻璃方向测算的Raman晶化率的差异维持在2%以内. 硅烷浓度为9.6%时, 薄膜沉积速率可达3.43 nm/s, 从薄膜方向和玻璃方向测算的Raman晶化率分别为50%和48%, 差异的相对值仅为4.0%. 合理控制过渡阶段的参数变化, 可使两个方向的Raman晶化率差值下降到一个百分点. 表明采用新方法制备薄膜, 不仅可以抑制非晶孵化层的形成, 改善微晶硅薄膜的纵向结构, 还为制备优质薄膜提供了较宽的参数变化空间. 关键词： 微晶硅薄膜 非晶孵化层 高速沉积 甚高频等离子体增强化学气相沉积     

Approximate solution of the magneto-hydrodynamic flow over a nonlinear stretching sheet

The approximate solution of the magneto-hydrodynamic (MHD) boundary layer flow over a nonlinear stretching sheet is obtained by combining the Lie symmetry method with the homotopy perturbation method. The approximate solution is tabulated, plotted for the values of various parameters and compared with the known solutions. It is found that the approximate solution agrees very well with the known numerical solutions, showing the reliability and validity of the present work.     

First Principles Study on FeAs Single Layers

在广义梯度近似(GGA)和GGA+U的框架下,用第一性原理方法研究了用FeAs单层的简单模型来研究LaFeAsO和BaFe2As2的合理性. 对未掺杂的FeAs单层,优化的几何结构及其对应的电子结构与本体的性质差异较大,并且在单层中没有发现体相中的共线反铁磁基态. 另外,在单层中,As与Fe层之间在z方向的间距随电子和空穴掺杂浓度的变化也与实验结果不符,这些结果表明,在LaFeAsO和BaFe2As2中,FeAs层与其他层之间的相互作用是不可忽略的,用简单的FeAs单层来处理Fe基超导体需要考虑更多的修正