Elastic and dielectric properties

Dynamic models for ionic lattices recognize explicitly the force constants between ions and their polarization. In shell models, the ions are represented as a shell and a core, coupled by a spring (see Refs. 57-59), and parameters are evaluated by matching bulk elastic and dielectric properties. Application of these models to the surface region has allowed calculation of surface vibrational modes [60] and LEED patterns [61-63] (see Section VIII-2). 

In the case of polar crystals such as the P-phase of PVDF, the elastic and dielectric properties are strongly coupled. This coupling, described formally in... 

Eqs. (6)-(9), necessitates a self-consistent treatment of the thermal, elastic, and dielectric properties. 

PIEZOELECTRICITY AND PYROELECTRICITY THE COUPLING OF THERMAL, ELASTIC, AND DIELECTRIC PROPERTIES... 

Different values of (Jq correspond to different equilibrium values of r)c, as illustrated in Figure 2.22. Statistical spatial fluctuations of gq give rise to the evolution of the pore radius distribution (PRD) upon water sorption. The PRD evolution is influenced as well by dispersions in elastic and dielectric properties. Larger values of G will give smaller equilibrium pore radii due to the stronger elastic forces that constrain the swelling. Below, only the effect of fluctuations in gq is evaluated. 

There has been a great deal of work on elastic and dielectric properties of polymers. An important characteristic of elastic and dielectric properties of polymers is that relaxational phenomena take place with change of temperature and frequmtey (or time scale)of measurement. Therefore, the clastic and dielectric constants determined by dynamic measurements arc represented by com ricx quantities as c s c + fd and c c — ie. ... 

Elastic and dielcclric relaxatioas are oommonly observed for most polymers. The onset of thermal molecular motions infiueoces the elastic and dielectric properties. Pi-ezoelectridty is a cross effect of the elastic and dielectric effects. Since the plezoclec-tridty expresses the internal strain of the polymer, the piezoelectric relaxation reflects the change of the internal strain. This is the most interesting characteristic ai the piezoelectric relaxation. 

Interesting and useful coupling between optical, magnetic, elastic, and dielectric properties is expected for the compoimds R(I03)3 nH20 (Abrahams et al., 1973). These have been studied by Abrahams et al. (1976), Abrahams and Bernstein (1978), and Liminga et al. (1975, 1977). 

Other physical properties. Anisotropy of thermal and electrical conductivity, coefficient of thermal expansion, elasticity, and dielectric constant may also provide information on internal structure. These properties, however, have so far been little used in structure determination, because they are less easily measured than those already considered consequently not very much experimental evidence is available for the purpose of generalizing on the relations between such properties and structural features. For further information on these subjects, see Wooster (1938), Nye (1957). 

Theoretical analysis indicates that occurrence of such convective instabilities depends on anisotropy of electrical conductivity and dielectric properties in the initial aligned nematic material. That is, conductivity parallel to the direction of alignment must differ from conductivity perpendicular to this direction. Calculation of the stability condition requires knowledge not only of these anisotropic electrical properties but also of anisotropic elastic and viscous properties which oppose disruption of the alignment and flow. 

Joern Petersson, Julio Gonzalo, and Jinzo Kobayashi, Dielectric, Elastic and Thermal Properties, Computer Simulations and NMR of Ferroelectrics and Related Materials, Gordon 8c Breach, Amsterdam, The Netherlands, 1998. 

The volumetric, elastic and dynamic properties of internally and externally plasticised PVC were studied and compared with those of unplasticised PVC. The glass transition temperature for the plasticised samples was markedly lowered and this decrease was more important for the externally plasticised ones. The positions of the loss peaks from dielectric alpha-relaxation measurements confirmed the higher efficiency of the external plasticisation. However, the shape of the dielectric alpha-relaxation function was altered only for the internally plasticised samples. The plasticisation effect was linked with a decrease in the intensity of the beta-relaxation process but no important changes in the activation energy of this process were observed. The results were discussed. 47 refs. 

Significant variations in the magnitude of the physical properties, such as the liquid crystal transition temperatures, viscosity, birefringence and the elastic and dielectric constants, of alkenyl-substituted compounds with a carbon-... 

This is undertaken by two procedures first, empirical methods, in which variable parameters are adjusted, generally via a least squares fitting procedure to observed crystal properties. The latter must include the crystal structure (and the procedure of fitting to the structure has normally been achieved by minimizing the calculated forces acting on the atoms at their observed positions in the unit cell). Elastic constants should, where available, be included and dielectric properties are required to parameterize the shell model constants. Phonon dispersion curves provide valuable information on interatomic forces and force constant models (in which the variable parameters are first and second derivatives of the potential) are commonly fitted to lattice dynamical data. This has been less common in the fitting of parameters in potential models, which are onr present concern as they are required for subsequent use in simulations. However, empirically derived potential models should always be tested against phonon dispersion curves when the latter are available. 

In addition to calculating energies, it is also possible to calculate routinely a range of crystal properties, including the lattice stability, the elastic and dielectric and piezoelectric constants, and the phonon dispersion curves. The techniques used, which are quite standard, require knowledge of both first and second derivatives of the energy with respect to the atomic coordinates. Indeed it is useful to describe two quantities first the vector, g, whose components g are defined as ... 

The surface Fuchs-Kliewer modes, like the Rayleigh modes, should be regarded as macroscopic vibrations, and may be predicted from the bulk elastic or dielectric properties of the solid with the imposition of a surface boundary condition. Their projection deep into the bulk makes them insensitive to changes in local surface structure, or the adsorption of molecules at the surface. True localised surface modes are those which depend on details of the lattice dynamics of near surface ions which may be modified by surface reconstruction, relaxation or adsorbate bonding at the surface. Relatively little has been reported on the measurement of such phonon modes, although they have been the subject of lattice dynamical calculations [61-67],... 

Two common properties which can be calculated from the minimum-energy structure are the elastic and dielectric constants. The elastic constant matrix is used to relate the strains of a material to the internal forces, or stresses It is defined as the second derivative of the energy with respect to the strain, normalised by the cell volume. The inverse of the elastic... 

At very low temperatures, most degrees of freedom are frozen. The detailed chemical structure of the polymer chains does not remarkably influence most of the elastic and thermal properties at these temperatures. (Properties, such as mechanical strength or dielectric loss, may be influenced by the chemical structure because of factors such as steric hindrance and dielectric polarization.) Cross-linking is one structural feature of epoxy resins which might influence low-temperature properties. 

The properties of existing epoxy adhesives fail to meet a number of requirements of current engineering, bringing the problem of their modification to the forefront because, despite the good adhesion and dielectric properties of the epoxy polymers, their elasticity and resistance to impact loads are not high. 

An acoustic wave biosensor utilizes acoustic or mechanical waves as a detection mechanism to obtain medical, biochemical, and biophysical information about the analyte of interest [1,2]. It detects changes in mass, elasticity, conductivity, and dielectric properties from mechanical or electrical variations. These devices also employ the piezoelectric effect to excite acoustic waves electrically at an input transducer and to receive the waves at the output transducer [2]. 

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