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991.
Solid state 1H, 29Si and 31P MAS NMR have been used to investigate the microstructure of phosphosilicate gels prepared by a modified sol-gel method involving hydrolysis of silicon precursors in a solely aqueous environment at 50 °C. Gels with molar compositions 5, 10, 20 and 30 mol% P2O5 in P2O5-SiO2 were studied. After drying to 400 °C the gels have very similar structures formed by a siloxane framework containing silanol groups and trapped molecules of orthophosphoric acid together with a very small amount, of pyrophosphoric acid. Unlike the gel samples previously synthesized by the hydrolysis of the silicon precursor in alcoholic solution at room temperature, the co-polymerization of phosphorus and silicon is much reduced. Although co-polymerization increases with phosphorus content, it still represents less than 50% of the phosphorus in the 30 mol% P2O5 gel. Furthermore there is no evidence for six-coordinated silicon in the glassy matrix. 相似文献
992.
993.
Zhangxin Chen 《Journal of Differential Equations》2002,186(2):345-376
In this paper, we analyze a coupled system of highly degenerate elliptic-parabolic partial differential equations for two-phase incompressible flow in porous media. This system involves a saturation and a global pressure (or a total flow velocity). First, we show that the saturation is Hölder continuous both in space and time and the total velocity is Hölder continuous in space (uniformly in time). Applying this regularity result, we then establish the stability of the saturation and pressure with respect to initial and boundary data, from which uniqueness of the solution to the system follows. Finally, we establish a stabilization result on the asymptotic behavior of the saturation and pressure; we prove that the solution to the present system converges (in appropriate norms) to the solution of a stationary system as time goes to infinity. An example is given to show typical regularity of the saturation. 相似文献
994.
995.
In the two‐dimensional (2D) homochiral structure of [cadmium(II) bis(S‐(?)‐lactate)]n, the lactate ligand adopts a µ3‐bridging mode to connect two cadmium atoms, leading to the formation of a 2D network. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
996.
James V. Crivello Jinseo Ahn 《Journal of polymer science. Part A, Polymer chemistry》2003,41(16):2570-2587
A study of the photoinitiated and thermally initiated cationic polymerizations of several monomer systems with S,S‐dialkyl‐S‐(3,5‐dimethylhydroxyphenyl)sulfonium salt (HPS) photoinitiators bearing different lengths of alkyl chains on the positively charged sulfur atom has been conducted. HPS photoinitiators are capable of photoinitiating the cationic polymerization of a wide variety of epoxy and vinyl ether monomers directly on irradiation with short‐wavelength UV light. Aryl ketone photosensitizers are effective in extending the spectral response of these photoinitiators into the long‐wavelength UV region. Kinetic studies with real‐time infrared spectroscopy show that HPS photoinitiators exhibit good efficiency in the polymerization of epoxide and vinyl ether monomers. Comparative studies also demonstrate that S,S‐dimethyl‐S‐(3,5‐dimethyl‐2‐hydroxyphenyl)sulfonium salts are more active photoinitiators than their isomeric S,S‐dimethyl‐S‐(3,5‐dimethyl‐4‐hydroxyphenyl)sulfonium salt counterparts. Both types of HPS photoinitiators display reversible photolysis as a result of facile termination reactions that take place between the growing chains ends with the photogenerated sulfur ylides. Preliminary studies have shown that HPS photoinitiators can also be employed as thermal initiators for the cationic ring‐opening polymerization of epoxides at moderate temperatures. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2570–2587, 2003 相似文献
997.
制约S校正电容器耐电流能力的主要因素是tgδ。tgδ愈小,耐电流能力愈强。通过改进S校正电容器的设计与工艺,并在生产中控制tgδ的上限值,将tgδ较大的产品剔除,从而保证了电容器的耐电流能力。 相似文献
998.
999.
A ring R is called right zip provided that if the right annihilator rR(X) of a subset X of R is zero, rR(Y)=0 for a finite subset Y⊆X. Faith [5] raised the following questions: When does R being a right zip ring imply R[x] being right zip?; Characterize a ring R such that Matn(R) is right zip; When does R being a right zip ring imply R[G] being right zip when G is a finite group? In this note, we continue the study of the extensions of noncommutative zip rings based on Faith's questions. 相似文献
1000.
合理使用SACETRA是维护大容量S1240程控交换机的一项重要工作。本文对如何提高SACETRA的使用效率进行了有益的探讨。 相似文献