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排序方式: 共有182条查询结果,搜索用时 15 毫秒
1.
David F. Sanders Ruilan Guo Zachary P. Smith Qiang Liu Kevin A. Stevens James E. McGrath Donald R. Paul Benny D. Freeman 《Polymer》2014
Thermal rearrangement of polyimides with ortho-position groups to polybenzoxazoles and related structures has been of recent interest for producing gas separation membranes. This study explores the influence of synthesis route and ortho-position functional group on the thermal rearrangement process and the fractional free volume of thermally rearranged (TR) polymers produced from polyimides derived from 3,3′-dihydroxy-4,4′-diamino-biphenyl and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (HAB–6FDA). Acetate, propanoate, and pivalate ortho-position functional groups were considered. Thermogravimetric analysis (TGA) was used to study thermal rearrangement at temperatures between 350 and 450 °C, and evolved gases from TGA were analyzed via mass spectrometry to characterize the byproducts of thermal rearrangement and thermal degradation. CO2 was the major byproduct of thermal rearrangement for all samples, and its evolution began well before the onset of thermal degradation. When non-hydroxyl ortho-position groups were present in the polymers, several byproducts other than CO2 were also observed due to the loss of these ortho-position groups before thermal rearrangement. Free volume generally increased with increasing extent of thermal rearrangement, but precise values of free volume could not be accurately determined for polymers with propanoate and pivalate ortho-position functional groups due to uncertainties in the chemical structure of partially converted materials. For polymers with acetate and hydroxyl ortho-position groups, free volume could be determined within the uncertainty of density measurements. Thermal rearrangement behavior and free volume results for acetate containing polymers synthesized via different routes were very similar. Based on these results, the chemical structure of the ortho-position functional group has a larger impact on TR polymer properties than the polyimide precursor synthesis route. 相似文献
2.
Nobuyoshi Ohno Sobahan Mia Kazuo Masuhara Kentaro Sonoda Yuji Yamashita Yoshitaka Tamura 《摩擦学汇刊》2013,56(5):722-730
The tribological properties of a new and unique thermoreversible gel lubricant (TR gel lube) were investigated. TR gel lube, which includes 10–40% of amide-type gelling agent in base fluid, is able to repeatedly alter or convert phase from gel state to liquid state at the melting point of the gelling agent. High-pressure rheological tests were performed in order to characterize the behavior of base oil and TR gel lube as a function of pressure and temperature. The effect of TR gel lube on ball bearing fatigue life was carried out by systematic tests using thrust ball bearings. The results of L 10 life tests of TR gel lube showed a longer life than the conventional greases. This result was investigated from the oil film formation. It was found that the gelling agent played a key role in the lubricating properties. Some mechanisms such as adsorbed film formation and solid-like formation are proposed. 相似文献
3.
Within a polymer thin film, free volume elements have a wide range of size and topology. This broad range of free volume element sizes determines the ability for a polymer to perform molecular separations. Herein, six permeable thermally rearranged (TR) polymers and their precursors were studied. Using atomistic models, cavity size (free volume) distributions determined by a combination of molecular dynamics and Monte Carlo methods were consistent with experimental observation that TR polymers are more permeable than their precursors. The cavity size distributions determined by simulation were also consistent with free volume distributions determined by positron annihilation lifetime spectroscopy. The diffusion, solubility and permeation of gases in TR polymers and their precursors were also simulated at 308 K, with results that agree qualitatively with experimental data. 相似文献
4.
为了消除超宽带通信中数据辅助(DA)的平均发射参考(ATR)接收机的判决错误传播风险,提出加入FEC译码反馈机制,用译码判决输出来反馈更新相关模板的思路。比较选择出适合的Turbo乘积码(TPC),据此形成了一种TPC译码反馈的改进DA-ATR接收机。对其性能进行了分析和多径信道仿真,结果表明:该接收机能有效消除上述风险,检测性能优于单纯TPC纠错的一般ATR接收机,约有1 dB信噪比增益优势。 相似文献
5.
分析了国内外交联聚乙烯(XLPE)绝缘电力电缆的使用情况,以及容易产生水树的原因机理。从如何避免水树的产生和发展,以提高电缆运行寿命,提出了抗水树的机理和抗水树XLPE电缆的设计理念,并对其试验流程和试验结果进行了统计分析。 相似文献
6.
本文介绍了时间标签PCR在MPEG-2数字电视传输系统中进行视音频同步的作用,着重说明了PCR的原理。包括PCR在编码端的生成、传输及解码端的接收和系统时钟的恢复,描述了DVB广播中PCR可能出现的错误及产生的影响,最后,按照TR101290标准借助码流分析仪介绍了检测PCR错误的基本操作。 相似文献
7.
Ken Yeang 《Architectural Design》2008,78(5):128-131
Best known as the pioneer of the green skyscraper, Ken Yeang is now applying his innovative, ecological thinking to the urban masterplan. Here he outlines how the introduction of an ecoinfrastructure can bring multiple benefits to a city, encouraging connectivity between green spaces, providing natural habitats for wildlife and alleviating the impact of climate change by offsetting CO2 emissions. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
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9.
针对线性成像精确性有限的缺点,提出了在非线性算法—对比源反演算法中采用正则化和并行频率方重建在暗室内的金属和电介质柱体构成的混合目标。二维实测微波散射数据采用了多频率、多收发设置的测量方式。通过目标的位置、形状及对比度值得展示,验证了扩展后的对比源反演算法对复杂的二维混合目标重建成像的精确性。 相似文献
10.