Densification mechanisms and microstructural evolution during spark plasma sintering of boron carbide powders

Micron-sized boron carbide (B₄C) powders were subjected to spark plasma sintering (SPS) under temperature ranging from 1700 °C to 2100 °C for a soaking time of 5, 10 and 20 min and their densification kinetics was determined using a creep deformation model. The densification mechanism was interpreted on the basis of the stress exponent n and the apparent activation energy Q_d from Harrenius plots. Results showed that within the temperature range 1700–2000 °C, creep deformation which was controlled by grain-boundary sliding or by interface reaction contributed to the densification mechanism at low effective stress regime (n = 2,Q_d = 459.36 kJ/mol). While at temperature higher than 2000 °C or at high stress regime, the dominant mechanism appears to be the dislocation climb (n = 6.11).     

Al含量对选区激光熔化Al_(x)CoCrFeNi(x=0.3,0.5,0.7,1.0)的显微组织及纳米压痕的影响EI北大核心CSCD

采用气雾化法制备预合金粉末,通过选区激光熔化(selective laser melting,SLM)制备Al_(x)CoCrFeNi(x=0.3,0.5,0.7,1.0)高熵合金。通过X射线衍射仪、扫描电镜以及纳米压痕实验,综合分析Al_(x)CoCrFeNi的物相、微观组织、硬度、杨氏模量及蠕变曲线,探讨Al含量对Al_(x)CoCrFeNi显微组织及纳米压痕的影响。结果表明:Al含量对物相组织有显著影响,其中Al_(0.3)CoCrFeNi与Al_(0.5)CoCrFeNi为FCC结构,Al_(0.7)CoCrFeNi和Al_(1.0)CoCrFeNi为BCC/B2结构。Al_(0.3)CoCrFeNi和Al_(0.5)CoCrFeNi主要由等轴晶组成,Al_(0.7)CoCrFeNi和Al_(1.0)CoCrFeNi主要由柱状晶组成。随Al含量增加,孔隙及裂纹等缺陷增加。在Al_(0.3)CoCrFeNi和Al_(0.5)CoCrFeNi中没有观察到明显的熔池形貌。随Al含量增加,样品残余应力增加。随Al含量增加,硬度增加,由Al_(0.3)CoCrFeNi的447HV增加至Al_(1.0)CoCrFeNi的567HV。Al_(0.3)CoCrFeNi杨氏模量约为273 GPa,Al_(0.5)CoCrFeNi约为233 GPa,Al_(0.7)CoCrFeNi和Al_(1.0)CoCrFeNi杨氏模量相近,分别为240 GPa和242 GPa,硬度与杨氏模量的变化主要与组织及物相有关。与传统蠕变曲线不同,Al_(x)CoCrFeNi的纳米压痕蠕变曲线只包括瞬时蠕变和稳态蠕变两个阶段,其蠕变机制主要为位错蠕变,其中Al_(0.7)CoCrFeNi具有最好的抗蠕变性能。Al_(0.3)CoCrFeNi具有最好的打印成形性,其屈服强度为702 MPa,伸长率为27.5%。     

Effects of sintering processes on the element chemical states of In,Sn and O in ITO targets

The element chemical states of In, Sn and O could affect the resistivity of ITO targets so as to affect the electrical property of ITO films. In the work, nine kinds of ITO targets were prepared in order to investigate the effects of sintering processes on the element chemical states of In, Sn and O in ITO targets by XPS. The results show that the heating rate of 60 °C/h, the sintering temperature of 1580 °C, the holding time of 5 h and the cooling rate of 240 °C/h are favorable for the preparation of ITO targets with the optimum conductivity. The change rule of the element chemical states of In, Sn and O in ITO targets sintered with different sintering process parameters has been proved by the theoretical analysis of thermal decomposition kinetics of In₂O₃ and SnO₂.     

降低燃气供销差的几个措施实践和效果

本文整理了降低燃气企业供销差率的有效措施,从计量仪表的管理到防范偷盗气、提高抄收率等.改造老旧表具,计量仪表定期进行校验,选择合理的计量表具是保证计量准确性的关键.选择与上游计量相同的表具,加强与上游沟通,降低输差.加强巡线检漏,合理调整管网运行压力对供销差影响,采用不停输作业,降低放散损耗.客户端管理在于提高抄收率,...     

凡口铅锌矿锗和镓资源与回收

对比分析凡口铅锌矿上部与深部锗、镓的含量及分布规律，研究选矿过程中锗镓在各选矿产品中的分布及锗镓的回收。深部锗、镓分布与上部相同。锗、镓主要在闪锌矿中，锗在方铅矿、黄铁矿、脉石的分布极少，镓有部分分布于黄铁矿与脉石中。锗、镓主要富集在锌精矿中，在铅精矿、硫精矿及尾矿中锗、镓的占有率很低。在电化学调控浮洗工艺中，锗、镓在锌精矿的富集得到了大幅度的提高，锗、镓的占有率由87．89％、52．80％分别提高到92．73％、62．68％。     

新型载银抗菌棉织物的研制

以γ-氨丙基三乙氧基硅烷(APTES)为偶联剂在棉织物表面连接氨基,然后利用氨基与银离子的络合作用在棉织物上负载银。探讨了偶联剂浓度、偶联温度、络合温度等因素对银负载量的影响,并测试了载银抗菌棉织物的抗菌性能。结果表明,偶联处理的优化工艺为：偶联温度为50℃,偶联剂浓度为0.4%;络合反应的优化工艺为：络合温度为35℃,AgNO3溶液的pH =3.0,反应时间100 min。经过处理得到的载银抗菌棉织物对大肠杆菌和白色葡萄球菌都具有良好的抗菌性,且经10次洗涤后,抗菌性能仍显著。     

光流法在掺气水流流场测量中的应用进展

水利设施中掺气水流运动速度估计一直是水气两相流领域重点研究的课题,由于掺气水流中气泡复杂的演变机制和剧烈的湍动瞬变,导致现有的测量技术难以准确的表征水流流场情况。基于光流法的流场可视化技术因其独特的优势而成为图像识别领域重点研究的方向。光流法能够根据像素亮度计算出细观气泡运动的高分辨率光流场,弥补了传统的基于粒子图像测速技术的不足。为了全面反映国内外研究人员应用光流技术对掺气水流流场可视化估计的进展,本文从经典光流法的原理出发,综述了目前用于掺气水流中的几种光流技术及其特点,然后总结了光流技术在处理掺气水流这种复杂场景时采用的关键技术和参数选择情况。最后,探讨了光流技术在掺气水流应用中所面临的挑战和前沿问题,并对其未来的发展做出展望。     

An efficient secure k nearest neighbor classification protocol with high-dimensional features

k Nearest neighbor (kNN) classification algorithm is a prediction model which is widely used for real-life applications, such as healthcare, finance, computer vision, personalization recommendation and precision marketing. The arrival of data explosion era results in the significant increase of feature dimension, which also makes for the increase of privacy concern over the available samples and unlabeled data in the applications of machine learning. In this paper, we present a secure low communication overhead kNN classification protocol that is able to deal with high-dimensional features given in real numbers. First, to deal with feature values given in real numbers, we develop a specific data conversion algorithm, which is used in the chosen fully homomorphic scheme. This conversion algorithm is generic and applicable to other algorithms that need to handle real numbers using the fully homomorphic scheme. Second, we present a privacy-preserving euclidean distance protocol (PPEDP), which works with the Euclidean distance computation between two points given in real numbers in a high-dimensional space. Then, based on the novelty PPEDP and oblivious transfer, we propose a new classification approach, efficient secure kNN classification protocol, (ESkNN) with low communication overhead, which is appropriate for a sample set with high-dimensional features and real number feature values. Moreover, we implement ESkNN in C++. Experimental results show that ESkNN is several orders of magnitude faster in performance than existing works, and scales up to 18 000 feature dimension in a memory limited environment.     

Selective laser melting W–10 wt.% Cu composite powders

Tungsten–Copper (W–Cu) alloys are promising materials for electrical and thermal applications. However, its forming method still remains limited in conventional powder metallurgy technique which is not suitable for manufacturing parts with intricate shapes. In this work, selective laser melting technology was introduced for fabricating W–10 wt.% Cu alloys parts. The morphological feature of a single molten track was analyzed. The results show that liquid phase sintering with complete melting of the binder (Cu), but nonmelting of the structural metal (W), acts as the main mechanism in this process. The melting conditions of single layers in different processing parameters were investigated. The results show that a moderate melting zone can be acquired from an established process map. Moreover, investigations on multilayers forming show that the final density increases with the decrease of scan speed until it reaches a plateau due to the insufficient rearrangement in liquid phase sintering and the balling effect.