单级和多级固溶处理对7055铝合金螺旋面型材显微组织和性能的影响

采用差示扫描量热法(DSC)、光学显微镜(OM)、扫描电子显微镜(SEM)、电子背散射衍射(EBSD)和透射电子显微镜(TEM)方法,研究单级固溶处理(SST)、强化固溶处理(EST)、高温预析出(HTPP)和多级固溶处理(MST)对挤压态7055铝合金螺旋面型材的显微组织、力学性能和耐蚀性的影响.研究发现,与传统的S...     

2n序列伪随机信号中的能量分布与有效成分分析

为获取2n序列伪随机信号中的有效谐波成分,提出一种提取有效成分的方法.从能量守恒定律出发,首先提出一种主频平均幅值的估计方法,以此根据不同的勘探任务选择合适的发送波形.针对实际发送电流,通过阈值筛选获取潜在有效频率;对接收数据在频率域进行频谱包络,获取潜在有效频率所对应噪声评价数;基于该噪声评价数,通过给定阈值筛选获得...     

Noise reduction method of microseismic signal of water inrush in tunnel based on variational mode method

The extraction of effective signals of microseismic event caused by water inrush in tunnels is an important prerequisite for accurate location of event. In this paper, based on the characteristics of the main frequency components of the tunnel water burst signal, the variational modal method (VMD) with good frequency decomposition ability is adopted to effectively extract the high-frequency components of the water inrush signal. The effective modes are identified by the method of detrend analysis, and the modal decomposition number K is determined. At the same time, the boundary between noise and effective signal is determined. The method proposed in this paper solves the problem that noise in some modal components is mixed with effective signals, and further improves the ability of extracting weak high-frequency signals. Through the analysis of case data, the validity of the research results in this paper is verified, and an effective signal extraction method of tunnel water inrush microseismic event based on VMD-DFA principle is formed.

     

Effect of heat treatment condition on magnetic,electrical transport and magnetoresistance properties of La0.67Sr0.33MnO3 manganite coatings

Effects of three different heat treatment conditions (air, vacuum and nitrogen) on the structure, magnetic, electrical transport and low-field magnetoresistance behaviors of La_0.67Sr_0.33MnO₃ manganite coatings synthesized by sol-gel and screen printing methods have been systematically investigated. The detailed crystallographic analyses revealed that the diffraction peak positions of the samples subjected to annealing in vacuum and nitrogen ambients shifted to lower-angle regions, and lattice parameters and structure distortion increased as compared with those of the sample annealed in air, which were due to the introduction of oxygen deficiency under the annealing conditions of vacuum and nitrogen. The content of oxygen deficiency of the samples could be determined by the change of lattice parameters. Furthermore, the vacuum and nitrogen annealed samples exhibited a dramatic increase in the resistivity, accompanied by the reduction in ferromagnetism, the resistivity peak temperature and the ferromagnetism transition temperature. These phenomena were attributed to the decrease of average manganese oxidation state, as a result the mobile charge carriers were reduced, the ferromagnetic coupling and the double exchange effect between manganese ions decreased while the magnetic inhomogeneity increased. In the meanwhile, magnetoresistance ratios of the oxygen-deficient samples were enhanced around the percolation threshold.     

A possible prediction method to determine the top concealed karst cave based on displacement monitoring during tunnel construction

Bulletin of Engineering Geology and the Environment - The existence of concealed karst caves can cause major displacement and even large deformation of surrounding rock during karst tunnel...     

PAN基碳化纤维静电耗散材料的表面电阻率研究

制备了一种以氟碳树脂为基体,聚丙烯腈(PAN)基短切碳化纤维为导电填料的抗静电复合涂层。讨论了纤维的含量、长度、涂层厚度及外加电压对涂层表面电阻率的影响。结果表明表面电阻率和碳化纤维质量分数之间的关系符合逾渗理论,当纤维长度为4mm时,此体系的渗滤阈值为2.5‰(质量分数),其表面电阻率为106Ω,具有良好的抗静电性能;表面电阻率随纤维长度的增加而降低,但降低的程度愈来愈小;涂层厚度在不超过134μm的情况下,对表面电阻率的影响较小;涂层的表面电阻率在高外加电压下较低外加电压下小;碳化纤维/氟碳树脂复合涂层的导电机理是由导电通道、隧道效应和场致发射3种导电机理竞相作用的结果。     

Enhanced thermal conductivity and retained electrical insulation for polyimide composites with SiC nanowires grown on graphene hybrid fillers

Polyimide (PI) composites containing one-dimensional SiC nanowires grown on two-dimensional graphene sheets (1D–2D SiC_NWs-GSs) hybrid fillers were successfully prepared. The PI/SiC_NWs-GSs composites synchronously exhibited high thermal conductivity and retained electrical insulation. Moreover, the heat conducting properties of PI/SiC_NWs-GSs films present well reproducibility within the temperature range from 25 to 175 °C. The maximum value of thermal conductivity of PI composite is 0.577 W/mK with 7 wt% fillers loading, increased by 138% in comparison with that of the neat PI. The 1D SiC nanowires grown on the GSs surface prevent the GSs contacting with each other in the PI matrix to retain electrical insulation of PI composites. In addition, the storage modulus and Young’s modulus of PI composites are remarkably improved in comparison with that of the neat PI.     

材料切削加工性的综合评价方法

概述了影响材料切削加工性的各种因素及评价材料切削加工性的各项指标。介绍了常用的几种切削加工性的综合评价方法,并对基于雷达图法的材料切削加工性做了详细的说明。     

非等温热重分析玉米芯半焦CO_2气化特性

采用非等温热重法对玉米芯热解半焦CO2气化行为和动力学特性进行研究。结果表明:升温速率对整个气化过程有重要影响。随着升温速率的增大,完成反应所需的温度提高,反应速率增加,反应时间缩短,而且升温速率越大,反应速率的峰值越高且向高温区偏移。利用Kissinger微分法和Coats-Redferm积分法分别计算动力学参数,所得不同升温速率下的平均活化能为180.77kJ/mol;升温速率越大,活化能越小。研究发现,玉米芯热解半焦CO2非等温气化的活化能E和频率因子A之间存在动力学补偿效应,两者满足lnA=0.09384E+2.604。     

海水间浸下45钢/B10电偶对的电化学研究

海水干湿交替的间浸环境会加速电极表面的腐蚀。采用动电位极化、电化学交流阻抗(EIS)及激光拉曼光谱等分析方法研究了45钢/B10电偶对在海水间浸工况下的腐蚀及其产物,并与海水全浸进行对比。结果显示:45钢电偶对在海水间浸下的腐蚀较严重,B10电偶对在海水间浸下的腐蚀速率较大,生成的腐蚀产物膜较厚;45钢电偶对锈层在海水全浸下的主要成分是α-Fe2O3和α-FeOOH,在海水间浸下的主要成分是α-Fe2O3和γ-FeOOH;B10电偶对锈层在2种工况下的主要成分有CuO,Cu2O和CuCO3.Cu(OH)2。结果表明,电偶对在海水间浸环境下的腐蚀较严重,这主要与电极表面的状态变化有关。