Static and dynamic oxidation behaviour of silicon carbide at high temperature

SiC has extensive applications in high-temperature oxidation environments. However, few studies have investigated the differences between the static and dynamic oxidation behaviour. In this study, the static and dynamic oxidation of SiC were investigated in air and in plasma wind tunnels, respectively. The results demonstrated that the activation energy of static oxidation was ～68.02 kJ/mol at 1300–1600 ℃, which was approximately ten times that of dynamic oxidation ～7.05 kJ/mol at 1290–1534 ℃. The observed Si-O-C transition layer located at the SiO₂/SiC interface, and its thickness after dynamic oxidation for 300 s was thicker than that after static oxidation for 30 h. In dynamic oxidation, high-speed flowing atomic oxygen reacted directly with SiC, whereas molecular oxygen needed extra energy to break the OO bond and react with SiC in static oxidation. Atomic oxygen also migrated easier in the amorphous SiO₂ coating, contributing to a thicker Si-O-C layer and lower activation energy.     

基于KD树的k-means聚类算法优化

作为模式识别最基本的分类方法之一,聚类在各个科学领域的数据分析中都扮演着重要的角色.然而随着大数据的出现,聚类分析在前沿发展中不断地面临着计算复杂度和计算成本等新的问题和挑战.通过研究k-means聚类算法的时间复杂度O(nk),针对迭代过程中大量的最近邻计算和其特殊场景,引入KD树作为索引,提出了基于单KD树的近似近邻算法和基于多KD树的交叉搜索算法.将k-means聚类算法的时间复杂度降为O(nlog k),并通过实验验证,基于多树的交叉搜索算法具有与k-means聚类算法相当的聚类质量.     

跑气保用轮胎在静负荷和稳态侧偏工况下力学性能的研究

基于ABAQUS软件建立225/40R18跑气保用轮胎和普通轮胎的三维有限元模型,分析标准负荷下2种轮胎在静负荷和稳态侧偏工况下的力学性能。结果表明,静负荷工况下,高应变能密度主要集中在支撑胶、带束层和冠带层端部,且在缺气状态下表现更明显,跑气保用轮胎胎肩处的接地压力大于普通轮胎,且径向刚度高于普通轮胎,缺气状态下跑气保用轮胎胎侧变形小于普通轮胎,且胎肩接地压力集中程度高;稳态侧偏工况下,在标准充气压力和缺气状态下,随着侧偏角的增大,跑气保用轮胎的接地印痕从矩形变为梯形,普通轮胎的接地印痕从矩形变为三角形。     

有机硅压敏胶的性能及影响因素

对有机硅压敏胶的几个影响因素进行了考察,在此基础上研制出一种有机硅压敏胶。乙烯基含量为0.3%的MQ硅树脂和相对分子质量为4.0×105的硅橡胶等比例配合,经1.0%的二丁基二月桂酸锡催化缩合,再以1.5%的BPO交联剂200℃下交联10min制得的有机硅压敏胶性能优异,初黏力25#,室温持黏力960min,剥离强度13.45N/25mm。     

Comparative study on the crystallization behavior of β‐isotactic polypropylene nucleated with different β‐nucleation agents —effects of thermal conditions

In this study, the crystallization behavior of the β‐isotactic polypropylene (β‐iPP) samples nucleated by a rare earth based β‐nucleating agent (β‐NA) WBG‐II and a metal salts compound β‐NA NAB83 (denoted as WPP and NPP, respectively) under different cooling conditions were comparatively investigated. The thermal conditions such as the cooling rate, isothermal crystallization temperature, isothermal crystallization time, and the subsequent cooling to room temperature. The results of WAXD, SEM, and nonisothermal crystallization reveal that under the same processing conditions, the crystallite size of NPP is smaller, which arrange more compactly as compared with WPP. Meanwhile, NPP has shorter crystallization rate and higher β‐nucleation selectivity, but WPP can crystallization at wider temperature range. The results of isothermal crystallization showed that NPP has higher selectivity and higher β‐nucleation efficiency, which favors the formation of high proportion of β‐phase at the isothermal crystallization temperature of 110–130°C with and without subsequent cooling; WPP has lower selectivity, which can only induce high content of β‐phase under isothermal crystallization without subsequent cooling to 25°C. In tuning the crystallization behavior and the properties of β‐PP, the joint influence of the efficiency and selectivity of the β‐NA, and the thermal conditions should be taken into consideration. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40115.