Effect of MgF2 addition on mechanical properties and thermal conductivity of silicon nitride ceramics

Dense silicon nitride (Si₃N₄) ceramics were prepared using Y₂O₃ and MgF₂ as sintering aids by spark plasma sintering (SPS) at 1650 °C for 5 min and post-sintering annealing at 1900 °C for 4 h. Effects of MgF₂ contents on densification, phase transformation, microstructure, mechanical properties, and thermal conductivity of the Si₃N₄ ceramics before and after heat treatment were investigated. Results indicated that the initial temperature of liquid phase was effectively decreased, whereas phase transformation was improved as increasing the content of MgF₂. For optimized mechanical properties and thermal conductivity of Si₃N₄, optimum value for MgF₂ content existed. Sample with 3 mol.% Y₂O₃ and 2 mol.% MgF₂ obtained optimum flexural strength, fracture toughness and thermal conductivity (857 MPa, 7.4 MPa m^1/2 and 76 W m⁻¹ K⁻¹, respectively). It was observed that excessive MgF₂ reduced the performance of the ceramic, which was caused by the presence of excessive volatiles.     

面向航天器结构健康状态监测的数据压缩感知算法研究

航天器产品结构健康状态监测是保证其发射与在轨运行过程中安全平稳运行的重要环节,由于多个传感器在长时间监测过程中会产生大量数据并需要对其进行高效传输和存储,本文针对数据传输问题提出并证明了基于稀疏恢复技术的一种分式最小化模型算法和改进的分式筛选算法,并结合卫星振动试验数据与经典匹配追踪等算法进行了对比分析验证,对比结果表明,在航天器所处环境条件下,所提算法处理得到的数据恢复相对误差均低于两种现有算法,可以在较高压缩比的条件下保证数据恢复的精度,实现了低维数据到高维数据传输。     

非抽样多小波和Hilbert-Huang时频分析在行星减速器早期故障诊断中的应用

提出一种改进相邻系数方法和非抽样多小波变换融合的降噪方法,使用Hilbert-Huang时频分析作后处理,并将其应用于行星减速器早期故障诊断中.行星减速器具有故障响应微弱、振动的强烈非平稳性和明显的非线性、低频特征频率污染等特点,使得现有中心轴固定的传统齿轮箱诊断理论与技术不能有效解决行星减速器的故障诊断难题.非抽样多小波变换具有时间平移不变性,可以降低或消除信号中奇异点附近急剧振荡的Gibbs现象;改进相邻系数方法具有随分解层变化的邻域区间长度和灵活的阈值选取方式,能够准确提取出非平稳信号中的早期微弱故障特征信息;Hilbert-Huang时频分析可以更直观地表征信号中的非平稳、非线性特征.对实际信号的分析结果表明,该方法可以准确提取出行星减速器存在早期齿面点蚀损伤时的微弱故障特征.     

Thermal performance of a dual-sided multiple fans system with a piezoelectric actuator on LEDs

With the advantages of lower power consumption and longer lifetime, piezoelectric fans have been widely investigated for use in electronic cooling for many years. However, the application of the piezoelectric fans for LEDs is seldom explored because of its insufficient cooling ability. In this study, a dual-sided multiple fans system with a piezoelectric actuator (“D-MFPA”) is considered for use in the thermal management of LEDs. The obvious advantage is that the D-MFPA only uses one piezoelectric fan to drive eight passive magnetic fans vibrating simultaneously to provide sufficient cooling ability and two-directional air flows. Two 30-W chip-on-board (COB) LED models are adopted to investigate the thermal performance of the D-MFPA. The dimensionless heat convection number for the D-MFPA (M_D ‐ MFPA) is defined in this study to describe and quantify the thermal performance of the D-MFPA. Moreover, several horizontal orientations (x/S_l) and vertical orientations (y/S_h) are examined to investigate the effects of different arrangements. The results indicate that M_D ‐ MFPA can be improved to 3.92 under the case of x/S_l = 0 and y/S_h = 0.033; however, the single piezoelectric fan can only improve M_D ‐ MFPA to 2.82.     

基于LabVIEW的HDLC协议收发器设计

HDLC协议以其优异的性能在可靠性需求较高的场合得到了广泛应用.针对航天某领域的协议测试评估需求,解决传统协议测试设备程序复杂、开发周期长的问题.为了满足系统快速集成、协议参数灵活配置的总体要求,提出了一种基于LabVIEW软件构建HDLC协议收发器的方案,完成了HDLC核心收发模块设计,并以此为基础组成一套完整的测试系统.对HDLC协议处理及接收、发送程序设计进行了说明,着重介绍了收发器中核心循环校验码(CRC)校验模块及比特插入删除模块的实现方法,同时对收发器的主要功能模块进行了仿真和测试.使用LabVIEW设计的HDLC收发器轻量高效、运行灵活,可应用于HDLC协议设备的测试和协议性能分析等场合,提高数据通信系统的扩展性、实时性和稳定性.     

Design and quasi-static characteristics study on a planar piezoelectric nanopositioner with ultralow parasitic rotation

This paper presents a particular mechanism design which is inherently inclined to have no parasitic rotation. The flexure-based stage comprises symmetric flexure guiding mechanism and two piezoelectric stack actuators. The layout of the stage is evaluated by finite element analysis, and the results indicate that the proposed design exhibits ultralow parasitic rotation. A prototype of the stage is then fabricated, and its performance is tested. Experimental results show that the stage has a stroke of 500 μm × 500 μm and the crosstalk is less than 1%. The maximum parasitic rotation is ±1.2 arcsec at a stroke of 500 μm.     

An improved junction capacitance model for junction field-effect transistors

A new junction capacitance model for the four-terminal junction field-effect transistor (JFET) is presented. With a single expression, the model, which is valid for different temperatures and a wide range of bias conditions, describes correctly the JFET junction capacitance behavior and capacitance drop-off phenomenon. The model has been verified using experimental data measured at Texas Instruments.     

Investigation and performance evaluation of a d14 ceramic actuator

A new wedge-type of ceramic actuator using d14 is developed. Ceramic d14 is rarely seen and addressed in research articles. Compared to the shear formation of the ceramic actuator d15, the proposed d14 actuator has an advantage of its nearly square and thinner dimensions. Driven by sinusoidal signal, asymmetric Lissajous motion trajectory is gained. Associated with appropriate mechanism, effective transverse displacement is obtained to propel a linear motion stage for instance. Analytical simulation in ANSYS and experiment are carried out to investigate the resonance frequency and vibration mode shape as well as amplitude. Results of both approaches are in close approximation. The feature of the d14 ceramic actuator designed in this study is able to provide large force and medium speed.     

力限半经验系数研究

利用力限技术可以缓解由于星箭界面机械阻抗的差异所造成的过实验现象，半经验法是获取耦合界面力限的重要方法，如何确定半经验系数是关键。基于固定界面子结构模态综合法,推导了半经验系数的理论表达式，分析了半经验常数与星箭耦合系统参数之间的关系。基于此方法，以某工程型号卫星为例计算了其半经验系数并制定了正弦实验的力限条件，通过实验数据进行验证，结果表明：该方法所得力限条件能够较好的反映出耦合界面在共振峰处的真实受力。可以为后续力限半经验法的工程应用提供一定的参考。