Dielectric Properties of Ionic Crystals

Haase and co-workers investigated electro-optic and dielectric properties of ferroelectric liquid crystals doped with chiral CNTs [495, 496]. The performance of the doped liquid crystal mixture was greatly affected even by a small concentration of CNTs. The experimental results were explained by two effects (1) the spontaneous polarization of the ferroelectric liquid crystal is screened by the 7t-electron system of the CNT and (2) the CNT 7i-electrons trap ionic impurities, resulting in a significant modification of the internal electric field within liquid crystal test cells. 

As discussed in the previous section, the dielectric properties of materials at microwave frequencies are strongly dependent on the ionic polarization. Theoretical dielectric constants of materials can be obtained from the dielectric polarizabilities of composing ions through the understanding of crystal structure. Let us consider the basic relationships between the dielectric polarizabilities and dielectric constants and how the control of dielectric properties and the search for new materials can be achieved by the additive rule. 

A new approach to the ideas of electronegativity and ionicity in solids, based on the spectroscopic and dielectric properties of crystals, has been proposed by Phillips . By making judicious simplifications in the theoretical framework many properties of tetrahedrally bonded crystals may be rationalized. 

The introduction of dye molecules into the liquid crystalline host does not change the majority of the properties of the host, provided that not too much dye is introduced (not more than 1-2%). The N I transition temperature of the liquid crystal, the viscous and elastic properties, the electrical conductivity (provided the dye is not ionic and does not contain ionic impurities), the dielectric permittivities, (provided the dye molecule does not have a large dipole moment), and even the refractive indices all remain the same. The only significant change in the properties of the crystal is the appearance of absorption bands in the visible region of the spectrum and a slight increase in viscosity [151]. 

The physical properties of sulfuric acid are listed in Table 10.3. The dielectric constant is even higher than that of water, making it a good solvent for ionic substances and leading to extensive autoionization. The high viscosity, some 25 times that of water, introduces experimental difficulties Solutes are slow to dissolve and slow to crystallize. It is also difficult to remove adhering solvent from crystallized materials. Furthermore, solvent that has not drained from prepared crystals is not reudily removed by evaporation because of the very low vapor pressure of sulfuric acid... 

It would be quite natural in terms of our picture of the electronic structure to represent the dielectric response of an ionic crystal as the sum of the dielectric responses of the individual ions, and this is the way in which these properties have traditionally been understood. (See, for example, Kittel, 1967, p. 384.) Recently, however, Pantelides (1975a) has pointed out that this representation is not consistent with the view that the principal peaks in the optical absorption spectra correspond to transitions from valence-band slates concentrated on the nonmetal-lic ion to conduction-band states concentrated on the metallic ion. Recall that in Section 4-A we saw that the same oscillator strengths that determine the optical... 

Because charge defects will polarize other ions in the lattice, ionic polarizability must be incorporated into the potential model. The shell modeP provides a simple description of such effects and has proven to be effective in simulating the dielectric and lattice dynamical properties of ceramic oxides. It should be stressed, as argued previously, that employing such a potential model does not necessarily mean that the electron distribution corresponds to a fully ionic system, and that the general validity of the model is assessed primarily by its ability to reproduce observed crystal properties. In practice, it is found that potential models based on formal charges work well even for some scmi-covalent compounds such as silicates and zeolites. 

As a polar liquid, water is the most powerful solvent known. This is partly a result of its high dielectric constant and partly its ability to hydrate ions. This latter property also accounts for the incorporation of water molecules into some ionic crystals as water of crystallization. 

TABLE 16.2 Comparison between Two Mechanical Properties of Different Actuating Materials Skeletal Muscles, Thermomechanical (Thermal Liquid Crystals and Thermal Shape Memory Alloys), Electrochemomechanical (Conducting Polymers and Carbon Nanotubes) and Electromechanical (Ionic Polymer Metal Composites, Field Driven Liquid Crystal Elastomers, Dielectric Elastomers)... 

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