超声场中湿法制备纳米粉末的原理和方法

介绍了超声波在湿法制备纳米粉末中的应用机理，探讨了超声波对纳米颗粒的生成和团聚的影响，综述了超声场下湿法制备纳米粉末常用的方法，同时对超声在纳米材料制备上的应用作了展望。     

纳米粉体制备的新方法

长期以来行星式高能球磨机一直是制备亚微米粉末的最佳设备。但是对于许多工业领域,亚微米是远远不够的，制药行业、材料学领域需要制备纳米粉末用于研究。这不仅需要较高的能量,同时还要求更高的旋转速度,德国FRITSCH公司生产的premiumline行星式高能球磨机采用革新型设计解决了上述问题。     

高能球磨制备纳米掺La硬质合金粉末

用XRD、SEM和DTA等分析检测手段，研究了高能球磨过程中，掺La硬质合金粉末的结构、形貌和相的变化。结果表明：微量La的加入，有利于硬质合金粉末晶粒的细化；高能球磨10h，即可获得晶粒度为30nm的掺La硬质合金粉末；高能球磨20h,Co相的X－ray衍射峰消失，说明Co相已完全固溶或亚固溶于WC相中。高能球磨30h，可获得晶粒度约10nm的掺La硬质合金粉末。DTA分析表明，与高能球磨前的粉末相比，在300－600℃温度范围内加热时，高能球磨掺La硬质合金粉末出现了明显的结构弛豫，说明粉末晶粒内部产生了许多缺陷，如：内应力、位错、晶格畸变等。     

SiC纳米粉体的高能球磨制备方法

以工业SiC粉末为原料，用高能球磨机制备了SiC纳米粉。用扫描电镜、X射线衍射仪对球磨后的粉末进行了形貌观察和相结构分析，统计了SiC粉体尺寸随球磨时间的变化规律。结果表明，随着时间的延长，粉末逐渐细化至纳米级，可以细化到30nm左右，但球磨时间超过25h后粉末颗粒继续细化的速度明显放慢。     

用正交实验法优化纳米铋粉制备工艺参数

采用高能球磨的方法，直接分散铋粒制备纳米铋粉。实验选用无水乙醇为液相介质、聚乙烯吡咯烷酮（PVP）和硬脂酸作为复合修饰剂，通过正交实验研究铋粒的加入量、复合修饰剂的相对用量、转速和时间等工艺参数对纳米铋粉的产量、产率和粉体的特征数据D50的影响。研究表明：本方法能够制备出纯净的纳米铋粉；适当增加铋粒的加入量，减少修饰剂的相对用量，降低转速，适当缩短时间，有利于降低颗粒的平均粒径，提高纳米铋粉的产量和产率。在本试验条件下，制备纳米铋粉的优化参数为铋粒加入量取0．06mol，复合修饰剂与铋粒的质量比为1：3，转速取250r／min，时间为4h。     

超声作用下原位反应制备Mg_2Si/AM60复合材料的显微组织

以AM60镁合金和硅为原材料,在原位反应的基础上,采用高能超声处理来改善原位反应的条件,制备出以Mg_2Si为增强相的镁基复合材料;采用XRD对复合材料物相进行分析,用光学显微镜对其显微组织进行观察,同时与未施加超声处理的复合材料进行了对比。结果表明:超声作用下原位生成的Mg_2Si增强相更加细小,团聚现象得到改善;熔体在高能超声作用下,声空化产生的瞬态高温、高压及声流效应是增强相细化的主要原因。     

高能聚焦超声清洗聚能器和工具的有限元分析

依据高能聚焦超声清洗原理,研究了高能聚焦超声清洗技术,并且对安装工具后的聚能器进行了有限元分析.试验表明,高能聚焦超声清洗是一种高效、洁净、可连续清洗的先进清洗方法.     

碳纳米管增强铝基复合材料的研究进展

介绍了碳纳米管增强铝基复合材料的研究现状,通过对比不同制备工艺以及碳纳米管增强铝基复合材料的抗拉强度、硬度、伸长率等力学性能,总结了粉末冶金法、高能球磨法、热喷涂法以及其它新技术的优缺点,同时阐述了复合材料制备过程中存在的问题及解决方法;此外还介绍了碳纳米管的强化机制;最后讨论了碳纳米管增强铝基复合材料未来的发展方向和研究重点。     

熔体超声处理对A356合金铸态显微组织和力学性能的影响

采用高能超声装置对A356合金熔体进行处理,利用扫描电镜、万能材料试验机等研究了高能超声功率及处理时间对A356合金铸态显微组织和力学性能的影响,并分析了其作用机理。结果表明:对A356合金熔体进行超声处理后合金铸态组织中的硅相逐渐由树枝状变成颗粒状,-αAl相细小圆整;当超声功率为1.2 kW、处理时间为600 s时,处理效果最好;处理时间一定后,随着超声功率的提高,A356合金的抗拉强度、屈服强度和伸长率都增加,最大值可分别达到294.37,207.98 MPa和6.85%,分别为未施加高能超声处理的1.56,1.93和1.10倍;随着高能超声处理时间的延长,A356合金的抗拉强度、屈服强度以及伸长率呈现先升后降的趋势。     

高能球磨法制备纳米铋粉的研究

使用高能球磨法液相分散铋粒,通过制备与修饰同步,制得高浓度的黑色胶体溶液,纳米粉体稳定均匀分散其中。本制备方法实现了粗细颗粒的分离,也首次成功解决了纳米铋粉的分散稳定性问题。取此溶液高速离心、真空干燥进行XRD分析,表明所制粉体为纯净的铋粉,并采用聚沉法解决了溶液中纳米粉体的收集问题。为了提高制备效率,采用正交实验进行参数优化,其优化方案为PVP与铋粒的质量比为1∶1,转速为350 r/m in,放入铋粒的质量为20 g,时间1 h。     

Analysis of stir die cast Al–SiC composite brake drums based on coefficient of friction

The work reported here is to analyze the suitability of Aluminum alloy–Silicon Carbide MMC (Al–SiC MMC) in the automobile brake drum applications in comparison with cast iron (CI) brake drum. A brake drum dynamometer test rig was developed for the purpose. Al–SiC MMC was reinforced with 10% and 15% SiC particle by weight. The effect of heat treatment of the Al–SiC MMC brake drum was also studied. Performance was mainly evaluated on the basis of brake drum coefficient of friction (μ). Scanning electron microscope was also used to study the effect of braking on the sliding surface of the brake drum.     

Frictional Behaviors of Some Nitrogen Ceramics in Conformal Contact with Tin Coated Al-Si Alloy,Steel and MMC

The frictional behavior of certain nitrogen-containing ceramics, such as silicon nitride, alpha sialons, and beta sialons as journal materials were studied in conformal contact with a tin-coated Al-Si alloy (Al-Si/Sn), forged 1141 steel and a cast aluminum matrix composite with silicon carbide reinforcement (cast MMC) as bearing materials while lubricated with SAE WW30. A case-hardened 1016 steel was also tested with the Al-Si/Sn and cast MMC bearings under the same conditions. The friction values of the ceramic and the steel journal wear pairs were compared and their frictional behaviors were evaluated.

Silicon nitride and one of the beta sialons exhibited higher load-supporting capacities than the others when they were in contact with the 1141 steel bearings. The journal surface roughness was found to be very important when the journals were in contact with the Al-Si/Sn bearings. The frictional behavior of the ceramics and cast MMC pairs and the steel and cast MMC pairs were controlled by different wear mechanisms, namely for the former, hard particle pull-out and matrix plowing, and for the latter, iron transfer from the journal to the cast MMC bearing surface.     

Isogeometric topology optimization based on energy penalization for symmetric structure

We present an energy penalization method for isogeometric topology optimization using moving morphable components (ITO–MMC), propose an ITO–MMC with an additional bilateral or periodic symmetric constraint for symmetric structures, and then extend the proposed energy penalization method to an ITO–MMC with a symmetric constraint. The energy penalization method can solve the problems of numerical instability and convergence for the ITO–MMC and the ITO–MMC subjected to the structural symmetric constraint with asymmetric loads. Topology optimization problems of asymmetric, bilateral symmetric, and periodic symmetric structures are discussed to validate the effectiveness of the proposed energy penalization approach. Compared with the conventional ITO–MMC, the energy penalization method for the ITO–MMC can improve the convergence rate from 18.6% to 44.5% for the optimization of the asymmetric structure. For the ITO–MMC under a bilateral symmetric constraint, the proposed method can reduce the objective value by 5.6% and obtain a final optimized topology that has a clear boundary with decreased iterations. For the ITO–MMC under a periodic symmetric constraint, the proposed energy penalization method can dramatically reduce the number of iterations and obtain a speedup of more than 2.     

交流暂态故障对柔性直流系统影响的评判研究

在模块化多电平电压源换流器高压直流输电(MMC-HVDC)系统中,发生在交流输电线路的各类暂态故障或扰动不但会引起与交流系统连接的MMC换流站内上下桥臂不同程度过流、过压现象,也会对对向站的正常运行产生一定程度的负面影响。如何利用MMC换流站内各电气信息实现交流暂态故障或扰动对其运行状态影响程度的评判将是今后MMC-HVDC研究领域的热点问题。针对上述问题,首先分析了交流输电线路暂态故障对站内电压、电流的影响,指出站内各电气信息之间的联系。而后鉴于MMC-HVDC系统阻尼对暂态故障能量传递的抑制作用,考虑故障扰动时站内电压、电流具有变化无规律、频率复杂且扰动特征能量等级低等特点,提出利用Renyi小波包能量熵对交流暂态故障发生时站内电气信息进行特征提取,并根据特征提取结果构建了系统运行状态评判指标集。基于Renyi熵权和灰色关联分析理论,提出了一种双端MMC-HVDC系统运行状态综合评判方法。最后,利用双端31电平MMC-HVDC系统进行了各类交流暂态故障的模拟实验,并基于该方法实现了不同交流暂态故障对MMC-HVDC系统运行状态影响程度的评判,判断结果证明了该方法的有效性。     

基于嵌入式Linux的MMC/SD卡驱动开发

在三星ARM9-S3C2410上实现了MMC/SD卡的硬件扩展,设计了此硬件平台上的MMC/SD卡在Linux下的驱动,并给出了相应的关键代码。     

基于模块化多电平的统一潮流控制器拓扑设计

为了降低模块化多电平换流器的开关频率,提出了基于模块化多电平(MMC)技术的新型统一潮流控制器(UPFC)的拓扑结构,将调制技术与子模块电容电压均压技术相结合,提出了一种可有效减低开关频率的脉宽调制方法。在对模块化多电平换流器环流问题分析基础上,提出了环流抑制控制器的设计思路。仿真结果表明,该控制器可在不增加桥臂电抗的前提下减少桥臂电流中的谐波成分,提高波形质量,同时降低各子模块电容电压波动,具有一定的工程意义;该拓扑结构具有模块化程度高、可靠性好、便于维护及容量拓展等特点,是一种极具发展潜力的拓扑结构。     

多模型控制方法及其研究进展

多模型控制是一种处理复杂系统的有效方法,尤其对于不确定性系统,可以提高控制系统的响应速度和控制品质;作为一种智能控制方法,多模型控制在理论和实践中都取得了丰富的成果,但仍有很多需要完善之处。该文从多模型控制算法、建模方法、模型集构造、模型调度策略、鲁棒性和全局稳定性等方面,比较详细地介绍了当今研究的最新进展;回顾了近几年多模型控制在控制领域的应用情况,指出多模型控制研究中还存在的问题,并展望今后发展方向。     

Ultrasonic assisted turning of an aluminium-based metal matrix composite reinforced with SiC particles

In this study, diamond turning of an aluminium-based metal matrix composite (MMC) reinforced with SiC particles was conducted with the assistance of ultrasonic vibrations. A simple and small size vibration device was designed to mount a piezoelectric transducer and a diamond insert onto a CNC machine. Turning experiments were carried out using a factorial design to investigate the effects of vibration and turning conditions on the surface finish of the turned MMC samples. The experimental results show that surface roughness of the MMC sample turned with ultrasonic vibrations is better than that of the MMC sample turned without ultrasonic vibrations. Turning with vibrations creates regular surface profiles along the turning and vibration direction, resulting in a light dispersion phenomenon, which is not reported in other articles. This phenomenon can only be observed when the pitch of the regular profiles is within a particular range. The pitch is determined by the cutting speed when the vibration frequency is fixed.     

Forced air cooling by using manifold microchannel heat sinks

A study of manifold microchannel (MMC) heat sinks for forced air cooling was performed experimentally. The manifold microchannel heat sink differs from a traditional microchannel (TMC) heat sink in that the flow length is greatly reduced to a small fraction of the total length of the heat sink. In other words, the MMC heat sink features many inlet and outlet channels, alternating at a periodic distance along the length of the microchannels while the TMC heat sink features one inlet and one outlet channels. The present study primarily focused to investigate the effects of geometrical parameters on the thermal performance of the manifold microchannel heat sinks for optimal design. Also, the thermal resistances of the MMC heat sinks were compared with those of the TMC heat sinks. Experimental results showed the thermal resistances of MMC heat sinks were affected strongly by the pumping power, the microchannel width and the manifold inlet/outlet channel width, but weakly by the microchannel thickness-width ratio and the microchannel depth coorporated with the manifold inlet/outlet channel width. However, it was found that there existed the optimum values of the latter parameters. Under the optimum condition of geometrical parameters in the present study, the thermal resistance of the MMC heat sink was approximately 35% lower than that of a TMC heat sink, which clearly demonstrated the effectiveness of using a manifold.     

金属基复合材料加工的进展

介绍了金属基复合材料（ＭＭＣ）的加工发展状态,包括各种传统的切削加工方法和特种加工方法在金属基复合材料上的应用。用刀具切削仍然是ＭＭＣ的主要加工方法,复合材料增强体硬度越高、含量越大,其切削加工性能越差,对刀具硬度的要求也越高。增强体的存在使有色金属基复合材料比基体材料有更好的可磨削性。除电火花成型加工外,各种非传统加工法在ＭＭＣ上的应用多见于板料切割。非传统加工也会在复合材料中留下多种表层缺陷。