Facile synthesis of hexagonal boron nitride fibers and flowers

Hexagonal boron nitride (h-BN) fibers and flowers were synthesized with a facile synthesis route. The precursor was obtained as an intermediate material by the reaction of KBH₄ and NH₄Cl at 120 °C for 48 h. h-BN was obtained by heating the intermediate material at 1250 °C for 10 h. The sample obtained was characterized by X-ray powder diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR), thermogravimetry (TG/DTA) and environment scanning electron microscopy (ESEM), which matched with h-BN. ESEM image indicated that the diameter of the BN fibers is mainly in the range of 1–2 μm.     

异质双金属复合管的弯曲工艺研究进展

发展异质双金属复合管的塑性弯曲成形工艺,是工业技术发展的必然趋势。目前有关双金属复合管弯曲行为的研究,大部分还属于材料性能测试性研究,只有少量文献尝试了异质双金属复合管的推弯、压弯成形工艺。研究发现:基-覆管界面分层、截面畸变和起皱是双金属复合管弯曲过程中频频出现的成形缺陷,且这三者之间存在耦合作用;可以通过合理的内腔填充(如施加一定的内压力,采用合理的芯模)来抑制双金属管的起皱行为;加载力对覆管的作用传递,容易因为界面层的存在而受到阻碍。而绕弯成形过程中作用在管坯上的力,分布均匀、加载位置合理,因此,绕弯成形适合弯曲塑性较好、对力的加载条件要求较高的异质双金属复合管件。近年来,由于弯管不仅要发挥传统的功用,还要满足设计性能、安装空间及轻量化上的要求,发展高精度小半径弯曲的异质双金属复合管弯曲成形技术,已经成为先进制造业发展的必然要求。     

The dielectric properties of epoxy/AlN composites

In this paper, AlN ceramic powder is chosen to be mixed with epoxy to form epoxy/AlN composites, the effects of the content of AlN filler on the physical and dielectric properties of epoxy/AlN composites are developed. From the SEM observation, the particles of self-synthesized AlN powder, obtained by using combustion method, is less uniformity and the average particle size is about 3.12 μm. Only the AlN phase can be detected in the XRD patterns of the epoxy/AlN composites. The more AlN powder is mixed with epoxy, the higher crystal intensity of AlN phase will exist in the XRD patterns. As the content of AlN powder in the epoxy/AlN composites increases from 5 to 40 wt.%, the dielectric constant increases from 6.52 to 7.28 (measured at 1 MHz). The loss tangent of epoxy/AlN composites is slightly increased as the measured frequency increases. Moreover, the epoxy/AlN composites in this investigation show less pores as compared to other literatures. The results indicate that the fabrication process has an apparent effect on the decrease of porosities, and the composites with a low porosity will lead to a low loss tangent.     

基于船体变形及齿间摩擦的斜齿轮齿根弯曲疲劳强度计算

当船舶在波浪中航行时，由于受到水浮力、水阻力、船舶重力及惯性力等作用，船体会产生中拱或中垂变形。研究表明，船体的这种变形会导致沿船体纵向布置的齿轮机构的轴承支座中心距产生变化，中心距的改变影响到轮齿的啮合传动，并最终对齿轮的寿命产生影响。另外，已有的研究也表明，齿间摩擦对齿轮强度的影响不可轻视。针对传统的船用齿轮在强度计算中不计船体变形及齿间摩擦力的问题，以沿船体纵向布置的渐开线斜齿减速齿轮机构中主动轮为研究对象，通过系统地分析、研究，推导出综合考虑船体变形及齿间摩擦的齿根弯曲疲劳强度优化计算公式。算例表明．船体变形及齿间摩擦的确对齿轮强度有着不可忽视的影响。     

Effect of laser surface melting on friction and wear behavior of AM50 magnesium alloy

In the present study, an attempt has been made to enhance tribological properties of AM50 magnesium alloy by laser surface melting (LSM) with a 2 kW continuous wave CO₂ laser. The microstructure of the laser surface melted zone consists of fine columnar dendrites growing epitaxially from the liquid-solid interface. Microhardness of the melted zone was improved to 55-75 HV as compared to 40 HV of the substrate. The friction and wear behavior of the laser surface melted layer were investigated using a ball-on-flat apparatus under dry sliding condition. It was found that the friction coefficient curve of the laser surface melted layer was similar to that of substrate. They showed a lower initial friction coefficient about 0.18 that after the running-in period increased up to about 0.38. Furthermore, compared with the AM50 substrate, the wear volume of the laser surface melted layer was decreased by 42%, the wear resistance of the laser surface melted layer was improved.     

Water dynamics inside a cathode channel of a polymer electrolyte membrane fuel cell

The present study focuses on the investigation of water dynamics inside a polymer electrolyte membrane fuel cell using two different modelling approaches: Eulerian two-phase mixture and volume of fluid interface tracking models. The Eulerian two-phase mixture model has provided overall information of species distribution inside a fuel cell and identified that the liquid water usually accumulates under the land area. The volume of fluid interface tracking model has then been implemented to investigate the emergence of water droplets from the gas diffusion layer into the cathode channel and the subsequent removal of water from the channel. Further, the effects of the location of water emergence in the cathode channel on the dynamic behavior of liquid water have been investigated. The present study shows that the water emerging into the channel near the side walls greatly reduces the surface water coverage of the channel. In order to control the water path into the channel near side walls, a further discussion has been provided that a gas diffusion layer design based on hydrophilic fibres distributed inside a hydrophobic fibre matrix could provide a precisely controlled water path through the gas diffusion layer.     

Static characteristics of a new hydrodynamic–rolling hybrid bearing

An analytic model is developed to predict static characteristics of a new hydrodynamic–rolling hybrid bearing (HRHB). Experiments are carried out to measure the equilibrium positions, the load sharing, and the rotational speeds of the rolling bearing cage. The results show that the working states of HRHB are divided into two distinct phases by a transition speed at which the hydrodynamics and contact models are decoupled. The cage speeds can be used to determine transition speed. The theoretical results agree with the experimental results, and it can be employed to design the bearing parameters according to the expected performance.     

WS2 sensitized mesoporous TiO2 for efficient photocatalytic hydrogen production from water under visible light irradiation

Nanosized WS₂ sensitized mesoporous titanium dioxide (TiO₂) was prepared to extend the adsorption of the wide band gap TiO₂ into visible light region. Compared with TiO₂ substrate without mesopores, it was found that the mesoporous substrate is beneficial for larger amount of WS₂ loading and inhibition of the light-induced detachment of WS₂ particles from the substrate during photocatalytic reaction, which leads to higher photocatalytic activity and better stability. We also found that the rate-determining step for photocatalytic hydrogen production is the transfer of electrons from TiO₂ substrate to H⁺ in solution, when the recombination between electrons and holes within TiO₂ could be neglected. In this case, deposition of Pt at the surface of semiconductor is indispensable for efficient hydrogen production.     

激光间接冲击下钛箔的微成形特性

采用具有高应变率加载特征的激光驱动飞片间接冲击微成形技术对钛箔进行了微成形实验,以解决难成形材料的微塑性成形问题。从飞片完整性、工件贴模性以及厚度减薄率等方面探究了该工艺的成形性能。实验中采用微细电火花和曲面研磨技术制造微模具,材料为AISI 1090模具钢,飞片为20μm的钛箔,成形工件为35μm的钛箔。通过KEYENCE VHX-1000C超景深显微系统对飞片和工件进行了观测和分析,结果显示:飞片具有较好的完整性,能够提供均匀的冲击压力;成形后的工件具有良好的表面质量和贴模性。借助冷镶嵌工艺测量了工件的厚度分布,分析了工件厚度减薄率,其最大值为19.8%,最小值为2%,显示工件的厚度分布比较均匀。研究表明:激光间接冲击微成形技术对于难成形材料具有良好的成形效果并且能有效抑制厚度减薄。     

激光复合织构焊管轧辊模具成形有限元模拟与试验研究

目的提高焊管成形质量,为激光复合织构轧辊模具产业化提供试验依据。方法通过数值模拟方法模拟焊管轧辊成形过程,获得不同摩擦系数组合下板料成形应力应变和径向厚度数据,并获得最优模具表面织构方案,为轧辊模具表面织构处理提供依据。依据数值模拟结果,运用激光表面织构技术对辊子模具表面进行复合织构加工处理,开展激光复合织构模具和未织构模具成形对比,并对成形件进行残余应力、应变、边缘减薄率等检测分析。结果数值模拟结果表明,下辊边缘区域应为减摩区,上辊边缘区域应为增摩区;应对下辊边缘区域进行激光微织构减摩,上辊边缘区域进行激光毛化增摩。成形试验结果表明,试验结果与数值模拟结果基本一致;与未织构模具相比,激光复合织构焊管轧辊模具优化了成形件的应力应变分布,降低了板厚边缘减薄率(5.06%),提高了成形件的均匀性(3.9%),成形件边缘区域形成了残余压应力。结论激光复合织构焊管轧辊模具相比未织构模具,可显著改善成形件的边缘稳定度和成形质量。