激光网格化淬火表面渗硫/纳米微粒复合层的摩擦学性能研究

利用MFT-R4000摩擦磨损试验机考察了激光网格化淬火表面渗硫复合层(简称SLD层)和激光网格化淬火表面渗硫/纳米微粒复合层(简称NLD层)在不同试验载荷时的减摩抗磨性能,利用SEM和XPS分析了不同试验条件下磨损表面形貌和所含元素及其化学价态.结果表明:NLD层不仅在重载荷下的减摩性能比SLD层的优异,其在干摩擦条件下的摩擦系数仅为0.075左右,而且在较低试验载荷下具有很长的使用寿命.磨损表面元素分析发现S元素以硫化物和硫酸盐形式存在,但没有发现Si元素,这表明磨损表面形成了主要由硫化物、硫酸盐和氧化物组成的摩擦化学反应膜,而n-SiO2颗粒在摩擦表面可能起到了"微纳滚珠"的作用.     

倾斜摆动条件下衬垫改性对自润滑关节轴承摩擦学性能的影响

本文中通过对PTFE纤维/芳纶纤维混合编织衬垫分别进行稀土处理和丙酮处理,研究了在倾斜摆动条件下衬垫改性对自润滑关节轴承摩擦学性能的影响,并用扫描电子显微镜(SEM)以及三维形貌仪分析了衬垫磨损表面微观形貌.研究结果表明:与未经改性处理衬垫的轴承相比,经稀土处理和丙酮处理后自润滑关节轴承减磨耐磨性能均得到提高,其中在高摆频工况下,衬垫经稀土处理后自润滑关节轴承减摩耐磨性能得到更大程度的提高;稀土处理轴承衬垫仅出现轻微黏着磨损,这是由于稀土处理过程增强了衬垫层的自润滑功能.     

酚醛纤维增强纸基摩擦材料的性能研究

采用湿式成型工艺制备出5种不同酚醛纤维含量的纸基摩擦材料,研究了酚醛纤维含量对纸基摩擦材料各项性能的影响.通过力学试验机测试了材料剪切强度和压缩-回弹性能,利用同步热分析仪和湿式摩擦试验机测试了材料热性能及摩擦磨损性能,通过扫描电镜观察材料摩擦表面和剪切断裂面的微观形貌.结果表明:随着酚醛纤维含量的增加,材料层间剪切强度逐渐提高,而压缩率变化较小,回弹率先增大后减小;材料耐热性随酚醛纤维含量的增加而降低;随着酚醛纤维含量的增加,磨损率逐渐降低,同时摩擦系数保持稳定.     

高压干气密封扭转变形结构因素影响分析

基于圆环理论以及气体润滑理论,以复杂截面机械密封环为研究对象,建立密封扭转变形无量纲分析模型;提出了干气密封端面膜压引起端面扭转变形的计算方法,并基于该法,采用多因素优化法研究密封环结构和辅助密封圈安放位置等因素对其扭转变形的影响规律,以期使结构影响规律化.结果表明:采用解析法与集中法计算膜压扭转变形是可行的;各结构参数对扭转变形的影响规律及程度不同;扭转角随台阶半径比π2、长度比π3、O型圈位置的长度比π5的增大而增大,随宽度比π4、内径ri的增大而减少;扭转角在π4≤3.5时随长宽比π1的增大而减小,当π43.5时,随π1的增大先增大后减小,在π1=2~3间形成最大值;π1值相比π2值,对扭转角影响不大;相比π1、π2、π3和π4的影响,ri和π5影响较小,相比密封结构,O型圈位置影响较小.     

相变微胶囊改性UHMWPE复合材料的摩擦学性能

以石蜡为囊芯,蜜胺树脂为高分子囊壁材料,采用原位聚合法制备了相变微胶囊,并将其作为填料添加入超高分子量聚乙烯基体中,制得相变微胶囊改性UHMWPE复合材料.分析了该复合材料的硬度和物相组成,并研究了其在室温,低速和高速试验条件下的摩擦磨损性能.结果表明:微胶囊填料的加入可以起到较好的减摩降磨作用,填料的最适宜添加比例为20%,在低速试验条件下经改性的复合材料摩擦系数较纯UHMWPE降低60%以上,高速试验条件下改性后的复合材料耐磨性较之纯UHMWPE有明显提高,不同试验条件下材料呈现不同的磨损机理.     

激光表面织构对不同材料干摩擦特性的影响

采用环-块线接触摩擦磨损试验,研究了经激光处理后不同织构面密度的45钢和12Cr凹坑形织构试件的干摩擦磨损特性.借助高精度天平分析了试件磨损量,采用三维形貌仪和扫描电镜对试件表面形貌进行了分析.研究结果表明:激光表面织构化使得45钢试件表面形成可以改善摩擦性能的高硬度质点;在相同试验条件下,与无织构试件相比,45钢织构环试件磨损量明显降低,而12Cr织构环试件磨损量却有所升高;织构试件的磨损量在一定范围内随着织构面密度的增加而降低;激光织构化对摩擦系数的稳定值影响不大.     

CO2 reforming of methane over nickel catalysts supported on nanocrystalline MgAl2O4 with high surface area

In this paper dry reforming of methane (DRM) was carried out over nanocrystalline MgAl2O4-supported Ni catalysts with various Ni loadings. Nanocrystalline MgAl2O4 spinel with high specific surface area was synthesized by a co-precipitation method with the addition of pluronic P123 triblock copolymer as surfactant, and employed as catalyst support. The prepared samples were characterized by X-ray diffraction (XRD), N2 adsorption, H2 chemisorption, temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), temperature- programmed desorption (TPD) and transmission and scanning electron microscopies (TEM, SEM) techniques. The obtained results showed that the catalyst support has a nanocrystalline structure (crystal size: about 5 nm) with a high specific surface area (175 m2 g-1) and a mesoporous structure. Increasing in nickel content decreased the specific surface area and nickel dispersion. The prepared catalysts showed high catalytic activity and stability during the reaction. SEM analysis revealed that whisker type carbon deposited over the spent catalysts and increasing in nickel loading increased the amount of deposited carbon. The nickel catalyst with 7 wt% of nickel showed the highest catalytic activity.     

Long-term correlations in hourly wind speed records in Pernambuco, Brazil

We study the statistical properties of hourly wind speed time series detected at four weather stations in the state of Pernambuco, Brazil, in the period 2008-2009. We find that the average and maximum hourly wind speeds deviate from a mutual linear relationship, and that they may be well explained individually by a Weibull distribution, however, with different shape parameter values. On the other hand, the long-term correlations of both of these observables obey the same power-law behavior, with two distinct scaling regimes. Our results agree with previous studies on wind speed series correlations in other regions of the world, which is suggestive of universal behavior.     

Directed transport in equilibrium: A model study

It is generally believed that sustained directed transport in mechanical systems is not achievable under equilibrium conditions. In this article, by analyzing a simple model, we will show that directed motion under equilibrium conditions can take place and is not inconsistent with conservation of energy and the second law of thermodynamics. Our model demonstrates a novel symmetry breaking mechanism sustainable under equilibrium conditions and average uniform motion of the center of mass is a consequence of the sustained broken symmetry. The most important consequence of our results is that, no current condition for mechanical equilibrium is possibly non-universal.     

Evolutionary model of the growth and size of firms

The key idea of this model is that firms are the result of an evolutionary process. Based on demand and supply considerations the evolutionary model presented here derives explicitly Gibrat’s law of proportionate effects as the result of the competition between products. Applying a preferential attachment mechanism for firms, the theory allows to establish the size distribution of products and firms. Also established are the growth rate and price distribution of consumer goods. Taking into account the characteristic property of human activities to occur in bursts, the model allows also an explanation of the size–variance relationship of the growth rate distribution of products and firms. Further the product life cycle, the learning (experience) curve and the market size in terms of the mean number of firms that can survive in a market are derived. The model also suggests the existence of an invariant of a market as the ratio of total profit to total revenue. The relationship between a neo-classic and an evolutionary view of a market is discussed. The comparison with empirical investigations suggests that the theory is able to describe the main stylized facts concerning the size and growth of firms.