ISSN:0005-9021    

DOI:10.1002/bbpc.19890931103

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1989

数据来源: WILEY

摘要:

AbstractMacroscopic considerations on the internal dynamics of matter deal with densities (for instance mass or charge densities) rather than with particles. The properties of materials are characterized by certain constants like for instance the electric conductivity or transport coefficients like the diffusion constantD, the viscosity η and the heat conductivityX. For a semi‐microscopic approach, these coefficients sometimes are generalized into so‐called memory functions, for instance η (r, t). Models for the internal dynamics of materials are formulated often in terms of such functions; a few attempts have been made to measure such functions directly. — Microscopic considerations depend, of course, on the desired “resolution”; taking into account as particles: nuclei and electrons or ions and “free” electrons or merely atoms or molecules (rigid — “globular” or “elongated” — or flexible molecules) etc. With high resolution, quantum mechanics must be applied. With relaxed resolution, quantum mechanics still may be necessary for periodic motions, even in case of collective dynamic states. The different views are illustrated with examples: for individual and collective rotational motions, for collective translational tunneling, for a pseudo‐spin glass transition and for the generalized hydrodynamic behaviour of a simple fluid. Non‐linear phenomena presentl

ISSN:0005-9021    

DOI:10.1002/bbpc.19890931104

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1989

数据来源: WILEY

摘要:

AbstractThe ability to resist shear is a defining characteristic of a solid, and distinguishes it from liquids and gases. Stress is one of the major additional features, in addition to periodicity and anisotropy, in the physical chemistry of solids. Diffusion, phase changes, and reactions alter the state of stress, and this in turn affects the course of the reactions. The stressed state can be an equilibrium state under the constraints of the available mechanisms. The current view of the physical chemistry of stressed solids will be surveyed with emphasis on the many new concepts about equilibria and how stresses evolve and influence reaction paths and mechanisms.

ISSN:0005-9021    

DOI:10.1002/bbpc.19890931105

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1989

数据来源: WILEY

摘要:

AbstractThe close correlation between microstructure and properties is the basis for well‐defined development of advanced materials. The higher the requirements on a material are the more exacting are the demands on the microstructure, i.e. the more precise the microstructure must be tuned. The knowledge of the microstructure and its property interaction is the focus of interest of materials science. The advanced ceramics are an example for a directed microstructural optimization being an important prerequisite for technological application. The utilization of scientific knowledge of the microstructure of this class of materials still being at the developmental stage resulted in a considerable improvement of their properties. This is true particularly for the mechanical properties which can be improved considerably by toughening concepts such as transformation toughenin

ISSN:0005-9021    

DOI:10.1002/bbpc.19890931106

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1989

数据来源: WILEY

摘要:

AbstractMolecular rotation in solids involves elastic forces for which no description has existed at the quantum level. An example is a rotating methyl group which may be compared with the propeller of an elastic powered model aeroplane. How does the lattice surrounding the methyl group play the role of the twisting elastic in restraining and counting rotation? The answer is found in gauge theory whose relevance to dynamics was realised following Berry's discovery of the topological phase. Through the introduction of the concept of curved space, the missing forces are revealed in the curvature which is a consequence of the dynamic interaction between the rotor and its thermal environment. The symmetries associated with dynamic isolation are broken as in the dynamic Jahn‐Teller effect, with the appearance in the quantum regime of new coordinates which are embryonic versions of the classical Euler angles. Incorporating both quantum and classical features, a description is obtained of the smooth transition with increasing temperature from quantum tunnelling to classical hopping rotation. Rotation of tetrahedral ammonium ions raises a further problem since the number of distinguishable orientations is either six or twelve according to quantum or classical mechanics. These are reconciled by symmetry breaking, the former pertaining to dynamic isolation and the latter to interaction with a heat bath. Various nuclear magnetic resonance and neutron scattering experiments demonstrate the symmetry breaking and the simple predictable character of the evolution of molecular rotation between the extremes of free quantum tunneling and thermally driven hopping motio

ISSN:0005-9021    

DOI:10.1002/bbpc.19890931107

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1989

数据来源: WILEY

摘要:

AbstractDifferent approaches of 1 D and 2 D solid state NMR spectroscopy for the detection of ultraslow molecular reorientation are reviewed with examples from13C NMR: 2 D exchange spectroscopy of static powders is applicable generally for simple molecules only, because of spectral overlap. But the resulting spectra are readily interpreted in terms of jump and diffusive motion. More complex systems can be investigated with 1 D “inverse hole burning” spectroscopy or the 1 D magic angle spinning (MAS) method of exchange induced sidebands (EIS). 2 D MAS exchange spectra provide more specific information but are hard to interpret without computer simulations. The combination of 1 D EIS spectroscopy with rotor synchronization leads to 2 D spectra which correlate order and motion. The phase cycle for the TOSS (total suppression of sidebands) sequence needed in these experiments is given. The advantages of MAS and static exchange spectroscopy are combined in 2 D spectroscopy with sample spinning off the magic angle. 2 D exchange spectra similar to those of static samples are obtained for each chemical shift, however, the anisotropy is scaled down by the offset from the magic angle, resulting in increased spectral resolut

ISSN:0005-9021    

DOI:10.1002/bbpc.19890931108

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1989

数据来源: WILEY

摘要:

AbstractAvoided level crossing muon spin resonance effects are observed with polycrystalline norbornene in its plastic phase and analysed in terms of axial hyperfine anisotropy. The results agree with those of previous single crystal measurements. It is shown that the technique is sensitive to the detection of small anisotropies, and that it is ideally suited for the study of reorientational dynamics in polycrystalline or amorphous environments.

ISSN:0005-9021    

DOI:10.1002/bbpc.19890931109

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1989

数据来源: WILEY

摘要:

AbstractThe elementary steps of translational motion of ions in ionic crystals are described on atomic scales of space and time. — After a brief presentation of complementary experimental techniques, some clear‐cut examples are being discussed: jump diffusion in fluorite‐type anion conductors and a non‐hopping channel diffusion in α‐Ag2Se. — The jump‐relaxation effect is exemplified in RbAg4I5and α‐AgI. It is shown to cause a marked deviation from random jump diffusion and to explain the anomalies known by the name of “universa

ISSN:0005-9021    

DOI:10.1002/bbpc.19890931110

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1989

数据来源: WILEY

摘要:

AbstractWe study diffusion of Brownian particles on random fractal structures, self‐avoiding random walks and percolation clusters at criticality, which serve as model systems for polymers in a good solvent and random two‐component mixtures, respectively. Using numerical simulations and scaling arguments we find that the distribution functionP(r, t) of the particles is a stretched Gaussian and scales as log[P(r, t)/P(r, 0)] ∼ —[r/R(t)]u, whereR(t) ∼t1/dwis the root‐mean‐square displacement,u=dw/(dw‐1), anddw= 2dffor self‐avoiding walks anddw= 3df/2 for percolation;dfis the fractal dimension of the structure. In the presence of an external constant bias field, diffusion is drastically reduced andR(t) ∼ logtevolves logarithmically in time, in exactly the same way in both

ISSN:0005-9021    

DOI:10.1002/bbpc.19890931111

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1989

数据来源: WILEY

摘要:

AbstractWe monitored the nuclear quadrupole interaction (NQI) of181Ta by perturbed angular correlation in AgxTaS2crystals both in situ during electrointercalation and upon subsequent homogenisation up to 1100 K. The electrointercalation starts with the simultaneous formation of two 2ndstage phases differing in Ag lattice site occupation. Homogenisation of Ag0.18TaS2and Ag0.38TaS2yields valuable information on the miscibility gaps and gives evidence for Daumas‐Hérold defec

ISSN:0005-9021    

DOI:10.1002/bbpc.19890931112

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1989

数据来源: WILEY