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An NMR study in the cubic phase of D2 which exhibits long-range orientational order is reported. The spin systems in both para D2 (with angular momentum J=1 and spinI=1) and in ortho D2 (with J=0 andI=2) were investigated. The integrated intensity ratio of theI=1 andI=2 components in the solid echo, the NMR line shapes obtained by Fourier transforming the solid echoes, and the longitudinal relaxation times for theI=1 andI=2 systems are presented. Samples with D2 concentrationX between 0.69 and 0.62 were studied over the temperature range 0.12<T<4 K. Measurements were carried out before and after repeated thermal cycling through the ordering transition to study the effect of this cycling on the temperature dependence of the relaxation times. The complex behavior of these relaxation times was found to be similar to that in cubic H2 at a comparable concentration, and the interpretation of this behavior is discussed. Over most of the temperature range, the relaxation time of theI=2 spins is larger than that of theI=1 spins and is believed to be determined by cross-relaxation with the latter, whose intrinsic spin-lattice relaxation is observed. However, at low enough temperatures, the relaxation time for theI=2 spins is found to be shorter than that for theI=1 spins, and this observation cannot be understood on the basis of predictions. Furthermore, theI=2 relaxation time in this temperature region is found to depend on the position within the NMR line. Another observation not yet understood is that the ratio of the integrated intensitiesS(I=1)/S(I=2) in the solid echo is smaller than the theoretically predicted one by about 20%. The NMR line shapes for theI=1 spins in the ordered phase obtained from pulse measurements are compared with those from continuous-wave methods and to those from H2. 相似文献
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We report an analysis of NMR lineshape measurements in hcp single crystals of H2 at ortho concentrationsX between 0.15 and 0.55 over the temperature range where the orientational ordering becomes substantial. The normalized rms order parameter
is obtained from the second momentM
2 and is plotted against the reduced orientational entropyS
red derived from calorimetric measurements. For comparison, we present calculations for a single-particle model, for isolated pairs and triangles ofo-H2 molecules. They confirm that single-particle models as used by previous authors cannot satisfactorily account for the orientational ordering in the hcp phase. Rather, cluster configurations where the lowest excited states have a quite different ordering arrangement from that of the ground state, yet only a small energy change, need to be invoked. In a
versus 1–S
red plot, the hcp phase ordering is contrasted with that in the ordered cubic phase. In the Appendix, the experimental sample cell design at Duke University and the observed NMR line shapes are compared with those of the groups at Saclay and Harvard University under similar conditions ofT andX and the differences in results are discussed. 相似文献
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Using a coherent pulsed NMR spectrometer operating at 5.9 MHz, measurements have been made of the free induction decay and of the solid echo in hcp solid D2. From these data, and using Fourier transform techniques, the NMR line shapes and longitudinal relaxation times have been found for both o-D2 (angular momentumJ=0 and spinI=2) and p-D2 (withJ=1 andI=1) separately. The concentration and temperature extended over the ranges 0.05X(J=1)0.56 and 0.04<T<3 K, respectively. For both o-D2 and p-D2, the second moment of the NMR line shape rises smoothly as the temperature decreases, while the relaxation times pass through a minimum. No evidence of a thermal hysteresis could be found in the line shape or relaxation times. The orientational ordering increases continuously as the temperature decreases and the results lead to the conclusion that there is no evidence for a well-defined transition into a phase that has the characteristics of a glassy state, at least above 0.05 K. Calculations of the rms order parameter for p-D2 as a function ofX andT are presented and the results compared with those for solid H2. Above 0.5 K, the agreement is very good, while below 0.3 K, (H2)>(D2). The orientational polarization of theJ=0 molecules in D2 by the surroundingJ=1 molecules is measured from the ratio of the lineshape second moments and is found to be in order-of-magnitude agreement with the predictions by A. B. Harris in the high-temperature limit. An analysis and correlation of the various measured relaxation times via energy diffusion models is presented. From the spin-lattice relaxation times of theJ=1 molecules, the orientational fluctuation rates are estimated for various concentrations as a function ofT and compared with the results from H2. Good agreement is found. Some unusual features in the intensity ratio of theI=1 andI=2 signals that is different from the expected one are described. In an Appendix, corrections made to the observed line shapes to compensate for the instrumental limitations of the pulse spectrometer are outlined. 相似文献
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An analysis has been conducted on the final products obtained in attempts to prepare single phase gallium nitride from single crystal gallium arsenide. When the intermediate oxide phase was nitrided in pure ammonia it was found that (i) the lowest temperature at which rate of conversion of-Ga2O3 to GaN became significant was in the range 600 to 700°C, (ii) over the temperature range 700 to 1000°C GaN was found to be the only crystalline phase present, (iii) above 1100°C-Ga2O3 was the main constituent. In comparison, when the oxide phase was nitrided in a 50% NH3-50% N2 atmosphere it was found that (i) the lowest temperature at which conversion to GaN occurred lay between 700 and 750°C, (ii) there was only a narrow range of temperatures, 750 to 870°C, in which the final products were found to contain GaN as the only crystalline phase, (iii) samples nitrided above 870°C exhibited both GaN and-Ga2O3 phases, the proportion of-Ga2O3 increasing with increasing temperature. 相似文献
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The solid echoes for both I = 1 and I = 2 nuclear spins from a two-pulse sequence are studied forp-D2 concentrations between 0.06 and 0.5 and for 0.06 < T < 1.5 K. The drop of the I = 1 echo amplitude from Curie's law reported earlier is confirmed and is discussed. The density matrix method, previously developed to describe the I = 1 echo properties in solid H2 as a function of various parameters, is applied to p-D2, and similarly accounts for most of the observations. An analysis of the I =2 echoes for o-D2 is carried out in similar fashion in the absence of a complete density matrix theory. Studies of the echo amplitude for the I = 1 and I = 2 spins versus r.f. pulse widths are reported and discussed. Various measurements associated with the echo decay as a function of the time between the two r.f. pulses are described. They include the I = 2 echo maximum shift respective to that of I = 1, the echo amplitude ratio S (I = 1)/S(I =2), the intermodular nuclear interaction magnitude, the local inhomogeneous field distribution, and the absence of expected satellite echoes for I =2. A comparison is made with the results of a similar study on solid H2. 相似文献
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The time-dependent effects in the NMR spectrum of ortho-H2 impurities in two single crystals of H2 are investigated over the temperature range between 0.02 and 3 K and for an ortho-H2 mole fraction X between 0.1% and 2%. The disappearance of the signal from isolated o-H2 impurities, after cooling the crystals to a given temperature, and the simultaneous increase of the signal from isolated o-H2 pairs indicates a clustering process. The characteristic times from both spectra are comparable and pass over a flat maximum near T = 0.3 K. The clustering process then accelerates drastically as T is decreased below 0.1 K. The characteristic times are only weakly dependent on the concentration X. Furthermore, the subsequent decay of the o-H2 pair signals with time suggests a slow diffusion of pairs and formation of larger clusters. It is found that the period associated with the decay of in-plane pairs (having their axis parallel to the basal plane) is considerably shorter than that for the out-of-plane pairs. The results are discussed in the light of the theory of hopping impurities (resonant ortho-para conversion) and possible coherent tunneling effects at low temperatures. Finally we briefly report some studies of the longitudinal relaxation time for the o-H2 pair NMR lines.Research supported by grant DMR 78-05378 from the National Science Foundation. 相似文献
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Phillip Yu Sejong Kim Harris L. Marcus Fotios Papadimitrakopoulos 《Journal of Materials Science》2008,43(3):803-812
A creation of micron-size narrow line defects in 2D colloidal crystal array was successfully accomplished using a focused pulsed laser. The 2D colloidal crystal arrays were obtained by a convective assembly of polystyrene latex micro-spheres (1.8 μm) and then immobilized onto the glass substrate by DNA hybridization process. To release a micro-sphere from the crystal, a focused laser beam was utilized to de-hybridize the DNA by raising the temperature above its melting point. From previous studies (Geiss E, Fabrication and defect design in two-dimensional colloidal photonic crystals. Ph.D dissertation thesis, 2003; Kim et al., Mater Sci Eng 26:1401–1407, 2006; Geiss et al., sudmitted), using a continuous argon ion laser, 514 nm, line-writing in colloidal crystal with 2–4 micro-sphere widths were achieved. In this study using the Nd:YAG pulsed laser’s second harmonic, 532 nm, line-writing of 1 micro-sphere width was accomplished. The presence of EB (ethidium bromide) dye that intercalates into the ds-DNA strands that bind the micro-spheres to the substrate facilitated the micro-sphere releasing process by reducing the laser energy required to release a micro-sphere. Factors that influenced controlled defect insertion were the medium containing the colloidal crystal, the dyes imposed on the crystal and the self-assembled micro-sphere groups. The most effective medium condition were determined at 1 mM global EB dye in 0.1 M NaCl for removing an individual micro-sphere in single micro-spheres, groups of micro-spheres and the close-packed arrays. Single micro-sphere width simple curves and corners in the photonic crystals were then created. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
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An etchant of 5% alcohol in trichloroethylene (TCE) allows greater resolution of dislocation etch pits in vapour-grown single crystals of Hgl2 than the traditional alcohol etches. The improved etchant has made possible the identification of etch pits, on the (001) faces of single-crystal Hgl2, that are associated with dislocations on {100}-type slip planes. The evidence for this conclusion includes (a) the geometric and crystallographic features of the etch pits, (b) the manner in which they are induced by deformation, and (c) their regularity in passing through the crystal, perpendicular to the (001) plane.This work is based upon part of the Ph.D. dissertation of T. W. James. 相似文献
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The photorefractive effect in a diamondlike single crystal of ZnGeP2 has been studied for the first time. This phenomenon can be used for holographic data recording in ZnGeP2 crystals. Two-wave mixing in the bulk of ZnGeP2 has been studied. The diffusion length of photogenerated charge carriers, the Maxwell relaxation time, and the product μτ of the charge carrier mobility and lifetime have been determined. 相似文献
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Both single crystal X-ray diffraction techniques and convergent beam electron diffraction have been employed to examine the structure of theT
2 (Al6CuLi3) phase in three particular Al-Li-Cu alloys. It is shown thatT
2 displays icosahedral symmetry both in a high purity laboratory melt and in two impure alloys which had been processed industrially. Possible reasons for the five-fold symmetry ofT
2 are discussed. 相似文献
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S. Dinakaran J. Mary Linet C. Justin Raj S.M. Navis Priya S. Jerome Das 《Materials Research Bulletin》2008,43(8-9):2010-2017
Solubility, metastable zonewidth and induction period have been estimated for the growth of sodium paranitrophenolate dihydrate (NPNa) single crystal. The interfacial energy has been calculated using experimentally determined induction period values. Nucleation parameters such as Gibbs free energy, critical energy barrier, radius, number of molecules in the critical nucleus and growth rate have been calculated. The evaluated interfacial tension values are found to be well agreed with the theoretical value derived from the solubility data. Bulk single crystals of NPNa have been grown with an aid of methanol as a solvent by slow evaporation technique. 相似文献
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An-Chou Yeh Akihiro Sato Toshiharu Kobayashi Hiroshi Harada 《Materials Science and Engineering: A》2008,490(1-2):445-451
The present article examines microstructure stability and creep resistance of a 5th generation superalloy, which has Cr content at 4.6 wt%, 6.4 wt% Re and 5.0 wt% Ru, in comparison with that of a 4th generation superalloy (3.2 wt% Cr, 5.8 wt% Re and 3.6 wt% Ru). The aim is to elucidate the implication of increasing Cr, Re and Ru contents for future alloy developments. Experimental results have concluded that high Re + Ru content could promote formation of hexagonal δ phase at 900 °C; additional Cr and Re could enhance the precipitation of TCP phase at 1100 °C. Although an increase in lattice misfit between γ and γ′ in the 5th generation superalloy could strengthen the alloy against creep deformation under conditions at high temperatures (≥1000 °C) and low stresses (≤245 MPa) whilst the microstructural stability remained, the tendency to raft should be avoided during creep at lower temperatures and higher stresses. 相似文献
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The spontaneous phase II to phase I crystal-crystal transformation of polybutene-1 was investigated by electron diffraction and bright- and dark-field imaging of solution and thinfilm grown single crystals. Whole single crystals were observed to transform with a single phase II to phase I orientational relationship and, in the case of multiple orientations, the transformed areas were not dependent on growth sectors. These results do not support the views of Holland and Miller of a twinned phase II to I transformation determined by growth sectors but are consistent with a transformation scheme introduced by Fujiwara. Nucleation and growth of the transformation are further discussed. 相似文献