Preparation, characterisation and processing of carbon fibre/polyamide-12 composites for selective laser sintering

Aiming at developing carbon fibre/polyamide-12 (CF/PA) composite powders for manufacturing high-performance components by selective laser sintering (SLS), the preparation, characteristics and sintering process of the composite powders and mechanical properties of sintered components were studied. Surfaces of the carbon fibres were treated by the oxidation modification and coated with polyamide-12 through the dissolution-precipitation process to provide good interfacial adhesion and homogenous dispersion within the polyamide-12 matrix. The particle size and micro-morphology analyses show that the CF/PA composite powders with 30 wt%, 40 wt% and 50 wt% carbon fibres present the suitable powder sizes and format for SLS. The incorporation of carbon fibres into the polyamide-12 matrix decreases the initial melting temperature and consequently lowers the SLS part bed temperatures, implying lower energy requirement and less thermal degradation in the sintering process. The CF/PA composites also represent higher thermal stability than the pure polyamide-12. The CF/PA sintered components with 30 wt%, 40 wt% and 50 wt% carbon fibres exhibit the greatly enhanced flexural strengths by 44.5%, 83.3%, 114%, and the flexural modulus by 93.4%, 129.4%, 243.4%, respectively, as compared with the pure polyamide-12 sintered parts. Fractured surface analysis shows that the carbon fibres are encapsulated and bonded well with the polyamide matrix. The complex SLS parts with the thinnest wall of 0.6 mm, the density of 1.09 ± 0.02 g/cm³ and the relatively density of 94.13 ± 1.72% were manufactured using the CF/PA composite powder with 30 wt% carbon fibres. This study demonstrates that the CF/PA composite powders prepared by the surface treatment and dissolution-precipitation method represent suitable interfacial adhesion, filler dispersion, particle sizes and sintering behaviours for SLS and enable the manufacture of complex components with high performance.     

进口E52100轴承钢盐浴热处理、磨削后表面出现凹坑原因分析

由美国进口牌号为E52100、线径为φ1.6mm的轴承钢盘圆,加工成小型精密轴承零件,经中性盐浴热处理并在圆端面磨削到成品要求后,被磨削的表面出现了极细小的黑色凹坑,凹坑数量及位置无规律可循。经光学显微镜和扫描电镜观察及X射线能谱分析后发现,在E52100轴承钢材料基体内部存在着大小不等、形状各异、位置不确定的异常组织颗粒,这些颗粒均含有较高含量的氧元素,为氧化物夹杂;在磨削过程中,这些硬而脆的氧化物颗粒极易从基体中剥落而出现凹坑;这些非金属氧化物夹杂是轴承钢材料中原有的,与盐浴热处理无关。     

聚乙二醇辅助电沉积ZnO纳米棒阵列的研究

以聚乙二醇（PEG-400）为添加剂,采用恒电位方法直接在ITO导电玻璃上制备了ZnO纳米棒阵列膜,用扫描电子显微镜（SEM）、X射线衍射仪（XRD）等分析手段对ZnO纳米棒阵列的形貌和结构进行了表征。结果表明所得ZnO纳米棒为六方纤锌矿型单晶结构,沿着垂直于基底的c轴极性生长,呈现出均匀的六方棒状形貌;在电沉积初期,PEG-400通过其大分子链的空间位阻作用及包覆作用对ZnO成核粒子的形貌和分散性产生显著影响,进而可以有效调整进一步生长所得ZnO纳米棒阵列的密度和垂直度,当PEG-400在电沉积液中的体积分数为2%时,可以得到取向良好垂直度高的纳米棒阵列;光电性能测试表明,ZnO纳米棒阵列膜电极的光电流随着阵列规整度提高而增大。     

弯游梁抽油机平衡臂失效分析及结构改进

针对16型弯游梁抽油机在吐哈油田使用过程中平衡臂支座出现底板焊缝开裂、连接螺栓断裂、加强筋板撕裂、游梁后端板压溃等问题,进行了游梁的受力计算和有限元分析,得出平衡臂支座底板与游梁上翼板焊缝处局部应力超过其许用应力、游梁后端板挤压应力超过其许用挤压应力是导致上述问题的主要原因。对比了平衡臂连接结构改进前后的受力分析,改进后的结构能够满足强度要求和抽油机安全可靠运行。     

一种基于比例因子的分形图像编码方法

比例因子是分形图像编码算法中的一个重要变量。给出了一种计算比例因子的方法,根据最小均方误差值的大小来选择比例因子的计算方法,通过对基本算法中比例因子的计算方法进行简化来达到加快分形编码的目的。与基本分形编码算法相比,在峰值信噪比下降约0．03dB的情况下,编码时闻节约了45％。     

6SE70变频器在连铸机拉矫系统中的应用

结合西门子6SE70变频器及西门子S7-400控制器在连铸机拉矫系统中的应用,介绍了拉矫机控制系统的基本原理,阐述了使用Profibus-Dp通讯方式实现S7-400控制器与变频器的通讯以及变频器的读取与应用。     

聚乙二醇丁二酸酯对铝电解电容器工作电解液性能的影响

以聚乙二醇(400)和丁二酸酐为原料,合成了聚乙二醇丁二酸酯(PEGS).考察了所制PEGS的添加量和pH值对工作电解液的闪火电压.电导率以及铝电解电容器性能的影响.结果表明:PEGS可以提高工作电解液的闪火电压,并有效抑制电导率的降低:当添加PEGS的质量分数为3%时,工作电解液的闪火电压最大可升高45V,电导率只降...     

Exergy analysis of helium liquefaction systems based on modified Claude cycle with two-expanders

Large-scale helium liquefaction systems, being energy-intensive, demand judicious selection of process parameters. An effective tool for design and analysis of thermodynamic cycles for these systems is exergy analysis, which is used to study the behavior of a helium liquefaction system based on modified Claude cycle. Parametric evaluation using process simulator Aspen HYSYS® helps to identify the effects of cycle pressure ratio and expander flow fraction on the exergetic efficiency of the liquefaction cycle. The study computes the distribution of losses at different refrigeration stages of the cycle and helps in selecting optimum cycle pressures, operating temperature levels of expanders and mass flow rates through them. Results from the analysis may help evolving guidelines for designing appropriate thermodynamic cycles for practical helium liquefaction systems.     

Experimental study on the sealing clearance between the labyrinth sealing displacer and cylinder in the 10 K G-M refrigerator

The labyrinth sealing displacer has been optimal designed to improve the operating stability and life-time of 10 K G-M refrigerator. The displacer was made of stainless steel 304 or inconel 718, coated with PTFE on its outer surface. Compared to the traditional piston-ring sealing displacer, the sealing clearance between the ridge of the labyrinth sealing displacer and cylinder is critical to the cooling performance of the G-M refrigerator. The displacers with different sealing clearances were experimentally studied, and the optimal clearance was given. The effects of the materials of the displacers and the system charge pressures on the performance of the labyrinth sealing were also tested and analyzed.     

Multi-scale temporal and spatial variation in genotypic composition of Cladophora-borne Escherichia coli populations in Lake Michigan

High concentrations of Escherichia coli in mats of Cladophora in the Great Lakes have raised concern over the continued use of this bacterium as an indicator of microbial water quality. Determining the impacts of these environmentally abundant E. coli, however, necessitates a better understanding of their ecology. In this study, the population structure of 4285 Cladophora-borne E. coli isolates, obtained over multiple three day periods from Lake Michigan Cladophora mats in 2007-2009, was examined by using DNA fingerprint analyses. In contrast to previous studies that have been done using isolates from attached Cladophora obtained over large time scales and distances, the extensive sampling done here on free-floating mats over successive days at multiple sites provided a large dataset that allowed for a detailed examination of changes in population structure over a wide range of spatial and temporal scales. While Cladophora-borne E. coli populations were highly diverse and consisted of many unique isolates, multiple clonal groups were also present and accounted for approximately 33% of all isolates examined. Patterns in population structure were also evident. At the broadest scales, E. coli populations showed some temporal clustering when examined by year, but did not show good spatial distinction among sites. E. coli population structure also showed significant patterns at much finer temporal scales. Populations were distinct on an individual mat basis at a given site, and on individual days within a single mat. Results of these studies indicate that Cladophora-borne E. coli populations consist of a mixture of stable, and possibly naturalized, strains that persist during the life of the mat, and more unique, transient strains that can change over rapid time scales. It is clear that further study of microbial processes at fine spatial and temporal scales is needed, and that caution must be taken when interpolating short term microbial dynamics from results obtained from weekly or monthly samples.