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101.
Flash calcination was used to dehydroxylate kaolinite and produced calcines kinetically frozen at various stages during metakaolinite formation. High speed magic-angle spinning27Al NMR, in magnetic fields of 14.1 and 11.8 T, enabled new features to be observed in the metakaolinite spectrum. The dependence on magnetic field of simulated and experimental spectra is discussed, and the effects of a distribution of electric field gradients is examined. The evolution of the metakaolinite peaks is followed during dehydroxylation. 相似文献
102.
J. P. Bell P. E. Slade J. H. Dumbleton 《Journal of Polymer Science.Polymer Physics》1968,6(10):1773-1781
Either of the two endothermic melting peaks found by differential thermal analysis of nylon 66 may be converted to the other by appropriate choice of annealing conditions. The two peaks are considered due to the melting of two morphological species, forms I and II. Form I is relatively fixed in melting temperature, while the form II melting temperature varies with annealing conditions and can be either above or below form I. The two forms can be distinguished by whether or not the conversion I → II takes place; if the sample is in form II no change in the thermogram is observed under suitable conversion conditions. The conversion of form I to form II also takes place during cold drawing. It has been previously shown that form I results from rapid cooling from the melt, and form II results from slow cooling. Form I appears to be kinetically favored, while form II is thermodynamically preferred. The variability in the form II melting point is attributed to variable crystal size and/or perfection. 相似文献
103.
We give an elementary new method for obtaining rigorous lower bounds on the connective constant for self-avoiding walks on the hypercubic lattice
d
. The method is based on loop erasure and restoration, and does not require exact enumeration data. Our bounds are best for highd, and in fact agree with the first four terms of the 1/d expansion for the connective constant. The bounds are the best to date for dimensionsd 3, but do not produce good results in two dimensions. Ford=3, 4, 5, and 6, respectively, our lower bound is within 2.4%, 0.43%, 0.12%, and 0.044% of the value estimated by series extrapolation. 相似文献
104.
Christian Borgs Jennifer T. Chayes Remco van der Hofstad Gordon Slade Joel Spencer 《Random Structures and Algorithms》2005,27(2):137-184
We study random subgraphs of an arbitrary finite connected transitive graph ?? obtained by independently deleting edges with probability 1 ? p. Let V be the number of vertices in ??, and let Ω be their degree. We define the critical threshold pc = pc (??, λ) to be the value of p for which the expected cluster size of a fixed vertex attains the value λV1/3, where λ is fixed and positive. We show that, for any such model, there is a phase transition at pc analogous to the phase transition for the random graph, provided that a quantity called the triangle diagram is sufficiently small at the threshold pc. In particular, we show that the largest cluster inside a scaling window of size |p ? pc| = Θ(Ω?1V?1/3) is of size Θ(V2/3), while, below this scaling window, it is much smaller, of order O(??2 log(V?3)), with ? = Ω(pc ? p). We also obtain an upper bound O(Ω(p ? pc)V) for the expected size of the largest cluster above the window. In addition, we define and analyze the percolation probability above the window and show that it is of order Θ(Ω(p ? pc)). Among the models for which the triangle diagram is small enough to allow us to draw these conclusions are the random graph, the n‐cube and certain Hamming cubes, as well as the spread‐out n‐dimensional torus for n > 6. © 2005 Wiley Periodicals, Inc. Random Struct. Alg., 2005 相似文献
105.
Proton NMR relaxation times (T2, T1, T1? and T1D) are reported for hydrous tin oxide (SnO2·nH2O) and hydrous titania (TiO2·nH2O) in the temperature range 135 K<T<336 K at 60 and 20 MHz. The data show proton transport including exchange between three environments: (1) surface hydroxyl groups, (2) “acid solution” in micropores (diameter <100 Å) and (3) “acid solution” in macropores (diameter>1000 Å). The NMR behaviour has many features in common with that of adsorbed water systems. A consistent interpretation of both NMR and conductivity data is presented. 相似文献
106.
107.
An experimental investigation was made of the xenon excimer laser. Several laser cavities have been employed with excitation by electron beams of cross section (15×2) cm2 and (55×4) cm2 and current density 100–150 A cm-2 injected transverse to the cavity axis. A numerical model of the xenon laser is compared with experimental results. The laser pulse width was found to be dependent on laser intensity and cavity mirrors. Early termination of the laser pulse was observed consistent with changing reflectivity of the cavity mirrors. An uncoated MgF2 retroreflector produced a laser output of significantly longer pulse width. Quasi-cw laser action was observed. 相似文献
108.
Robert C.T. Slade 《Solid State Communications》1985,53(11):927-931
A critique of studies of transport in photon-conducting solid electrolytes by NMR relaxation time measurements, pulsed field gradient NMR diffusion measurements and quasielastic scattering of neutrons is presented. The capabilities of each method are outlined and the various results are interrelated. Interpretational problems specific to proton conductors are highlighted. 相似文献
109.
The occurrence of a thermally-induced 2H → 4H inter-polytype conversion in PbI2 has been observed via the appearance, in Raman scattering, of a shear-motion rigid-layer line at 14 cm-1 and a Davydov splitting of an intra-layer line. Analysis of these data has yielded a comparison of the layer-layer coupling strength in PbI2 with that in other layer crystals, as well as an estimate of the stiffness of the iodine-iodine interlayer bond. 相似文献
110.
Proton NMR relaxation times (T2, T1, T1?) are reported for powder samples of MoO3 · 2H2O and yellow MoO3 · H2O in the temperature range 150–325 K and at 20 and 60 MHz. No translation of hydrogen atoms is detected but the spin-lattice relaxation behavior indicates reorientation of H2O molecules. The waters coordinated to Mo atoms undergo 180° flips (about their C2 axes) with similar motional parameters in both compounds. The interlayer waters in MoO3 · 2H2O undergo 180° flips with different parameters. An assumed Arrhenius-type temperature dependence of correlation times leads to preexponential factors which are “anomalously” low. The possible involvement of temperature-dependent activation barriers is discussed. 相似文献