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991.
992.
A computer simulation has been used to predict crystallization kinetics and crystalline morphology in composite materials based on thermally nucleated crystallizable matrices. As demonstrated for athermally nucleated composites, the presence of reinforcing fibers increases the complexity of the system. Fibers are shown to have a dual effect on the spherulitic crystallization process. The influence that fibers have depends on the interplay between the enhancing effects that fibers have on nucleation and the depressing effects that fibers have on spherulitic growth. Fibers that do not provide additional nuclei to the system depress the rate of crystallization relative to an unreinforced polymer, while fibers that add nuclei to the system increase the rate of crystallization. The transcrystalline morphologies that develop in thermally nucleated fiber-reinforced polymers are controlled primarily by the relative numbers of bulk and fiber nuclei. The extent of transcrystalline regions can be suppressed either by increasing the rate of bulk nucleation, or by decreasing the rate of fiber nucleation. Finally, the qualitative appearance of the morphology in the transcrystalline region was found to be indicative of the mode of fiber nucleation. © 1995 John Wiley & Sons, Inc. 相似文献
993.
W. H. Fietz R. Quenzel H. A. Ludwig K. Grube S. I. Schlachter F. W. Hornung T. Wolf A. Erb M. Klser G. Müller-Vogt 《Physica C: Superconductivity and its Applications》1996,270(3-4):258-266
The pressure effect on Tc of polycrystalline and single crystalline YBa2Cu3Ox investigated as a function of oxygen content x by ac-susceptibility measurements under helium pressure. In the overdoped region x> 6.93 the single crystals show a negative dTc/d p, as expected from the charge transfer model. For optimally doped samples with x = 6.93 we find dTc/d P = 0.4 K/GPa which points to pressure effects on Tc aside from charge transfer. In the underdoped region x < 6.93 the dTc/d p values obtained from the experiment depend strongly on the storage temperature of the sample during the experiment. When the samples are stored at temperatures well below 240 K throughout the entire experiment including pressure application and pressure release, dTc/d p increases to approx. 7 K/GPa at x = 6.7 but with a further decrease of the oxygen content the dTc/d p drops to approx. 2 K/GPa at x = 6.4. These effects are intrinsic to the YBa2Cu3Ox structure and can be explained by considering the anisotropic structure of YBa2Cu3Ox. The decrease of the c-axis lattice parameter results in a charge transfer to the CuO2-planes mainly [1], whereas the compression of the a- and b-axis lattice parameter is known to produce different pressure effects which are responsible for the peak in dTc/d p at x = 6.7 [2]. When pressure is changed at room temperature oxygen ordering effects occur which cause a relaxation of Tc to the equilibrium value Tc(p) at this pressure with a time constant depending on the oxygen content x. A decrease x results in a peak effect in dTc/d p at x = 6.7 again, which is enhanced to approx. 12 K/GPa. If the oxygen content is decreased further, dTc/d p first drops to 5 K/GPa at x = 6.6, but the increases to values of more than 20 K/GPa for x < 6.42. These giant pressure effects at low oxygen contents are mainly caused by a reversible Tc increase (dTc/d p)O due to pressure induced oxygen ordering via oxygen motion between unit cells. 相似文献
994.
Ludwig Bröcker 《Archiv der Mathematik》1978,31(1):133-136
Ohne Zusammenfassung 相似文献
995.
996.
997.
998.
The properties of useful internal 1H-NMR reference compounds are discussed. Two new standards for measurements at high temperatures and in aqueous solutions are proposed. They are characterized by a high boiling point, thermal and chemical stability and chemical shift as better reference compounds than those used up today. 相似文献
999.
1000.