Solid dispersions electrical properties

The magnitude of the errors in determining the flat-band potential by capacitance-voltage techniques can be sizable because (a) trace amounts of corrosion products may be adsorbed on the surface, (b) ideal polarizability may not be achieved with regard to electrolyte decomposition processes, (c) surface states arising from chemical interactions between the electrolyte and semiconductor can distort the C-V data, and (d) crystalline inhomogeneity, defects, or bulk substrate effects may be manifested at the solid electrode causing frequency dispersion effects. In the next section, it will be shown that the equivalent parallel conductance technique enables more discriminatory and precise analyses of the interphasial electrical properties. 

The electrical properties of polyelectrolyte complexes are more closely related to those of biologically produced solids. The extremely high relative dielectric constants at low frequencies and the dispersion properties of salt-containing polyelectrolyte complexes have not been reported for other synthetic polymers. Neutral polyelectrolyte complexes immersed in dilute salt solution undergo marked changes in alternating current capacitance and resistance upon small variations in the electrolyte concentration. In addition, their frequency-dependence is governed by the nature of the microions. As shown in... 

A suspension of finely dispersed particles is thermodynamically unstable this system is inclined to lower its free energy through flocculation. The stability and rheology of a powder in a suspension depend on the nature of the solid/ solution interface, particularly on the electrical properties of this region [70]. This interface may be described as consisting essentially of two layers (Fig. 3) ... 

The dimensions of the added nanoelements also contribute to the characteristic properties of PNCs. Thus, when the dimensions of the particles approach the fundamental length scale of a physical property, they exhibit unique mechanical, optical and electrical properties, not observed for the macroscopic counterpart. Bulk materials comprising dispersions of these nanoelements thus display properties related to solid-state physics of the nanoscale. A list of potential nanoparticulate components includes metal, layered graphite, layered chalcogenides, metal oxide, nitride, carbide, carbon nanotubes and nanofibers. The performance of PNCs thus depends on three major attributes nanoscopically confined matrix polymer, nanosize inorganic constituents, and nanoscale arrangement of these constituents. The current research is focused on developing tools that would enable optimum combination of these unique characteristics for best performance of PNCs. 

Complex fluids composed of several pseudophases with a liquid-liquid interface (emulsions, macroemul-sions, cells, liposomes) or liquid-solid interface (suspensions of silica, carbon black, latex, etc.) can, from a dielectric point of view, be considered as classical heterogeneous systems. Several basic theoretical approaches have been developed in order to describe the dielectric behavior of such systems. Depending on the concentration, the shape of the dispersed phase, and the conductivity of both the media and disperse phase, different mixture formulas can be applied to describe the electric property of the complex liquids (11-15). 

X-rays cause ionization in gases, liquids, and solids, thereby altering the electrical properties of these materials. Gas ionization is the basis of gas-filled detectors such as ionization chambers, Geiger-Miiller counters and proportional counters are used in conjunction with X-ray dispersion. The ionization of silicon atoms in the solid state forms the basis of the lithium-drifted (Si(Li)) semiconductor detector, making energy-dispersive detection of X-rays possible. 

The technological importance of various suspensions of solid particles is enormous. The literature is therefore extensive and difficult to survey. It is, in principle, possible to make a suspension of practically all solid substances, provided that they are sufficiently insoluble in the liquid. The methods used to make them take advantage of a large number of different physical principles. However, in a brief overview such as this it is not possible to give a detailed account of all available preparation methods. In the following, we will present a short outline of the most frequently used preparation methods and references to more detailed accounts. The surface electrical properties are then described and this is subsequently used to give an account of how dispersions can be stabilized and how they aggregate. 

Eq. (1) indicates that the critical electric field strength decreases with the increase of particle concentration. The critical electric field concept was also used in several other theoretical treatments [4-6]. However, the critical electric field was assumed to be a constant only depending on the electric property of the dispersing medium. The liquid-solid phase transition induced by the external electric field is supposed to happen at the critical electric field that is in the order of several hundred to one thousand volts per millimeter. In consideration of electric-field-induced particle aggregation in ER fluids, Khusid [7J suggested that the critical electric field is the function of particle volume fraction, the dielectric constant and conductivity of both the particle and dispersing medium. It should be as low as 14 V/mm, as confirmed experimentally [8]... 

Since the plasma polymers are known as highly crosslinked materials, so their degree of crosslinking was felt to be an important factor which may influence strongly the dispersion component. This assumption may be considered in terms of nature of the dispersion forces which, in general, depend on electrical properties of the volume elements involved and the distance between them. The volume element here means atom or molecule and in the case of polymer, it may be structural unit in its linear or crosslinked bulk structure. The potential for the interaction between two volume elements in liquid or solid is given by the Equation ... 

The existence of electrical charges at any interface will give rise to electrical effects, which will, in many cases, determine the major characteristics of that interface. Those characteristics will affect many of the properties of a multicomponent system, including emulsion and foam formation and stability, solid dispersions, and aerosols. The theoretical and practical aspects of electrical double layers are the subject of a vast amount of literature and for that reason have not been addressed in any detail so far. Such details can be found in bibliographic references cited for this chapter. 

An electrical double layer (edl) existing on the solid-solution interface is essentially connected with the surface properties of the system. The amount of accumulated charge influences the adsorption of ions and molecules. In the latter case it also influences the configuration of the adsorbed species. On the other hand, the adsorption of the ions and molecules varies surface properties of the interface (functional groups) and thus, the distribution of the charge in the interfacial region. The existence of the electric charge at the interface influences the dispersed system stability. 

The structure electrical double layer at the silica-aqueous electrolyte interface was one of the earlier examined of the oxide systems. At the beginning the investigations were performed with application of electrokinetic methods next, with potentiometric titrations. The properties of this system were very important for flotation in mineral processing. Measurements proved that pHpZC and pHiep are equal to 3, but presence of some alkaline or acidic contaminants may change the position of these points on pH scale. Few examples, concerning edl parameters are shown in Table 3. Presented data concern a group of systems of different composition of the liquid phase and solid of a different origin. The latest measurements of this system takes into account the kinetics of the silica dissolution [152], and at zeta measurements, also the porosity of dispersed solid [155]. 

---