Surface conductance

The tangential electric field drives electric currents around the particle surface. In most electroldnetic theories it is assumed that this current is dominated by the contribution from the diffuse layer ions, for the material inside the shear plane is taken to be immobile and non-conducting. While this assumption seems to be valid for many colloids, including oxide systems, for some systems there is significant electrical conduction in the region between the shear plane and the particle surface. Dielectric dispersion and ESA studies have shown this to be the case in kaolinite [21], bentonite [20], lattices [22,23] and in emulsions [2]. We refer to this phenomenon as stagnant layer conduction. 

The surface current leads to a build up of charge over one portion of the double layer and a depletion over the remainder. This in turn leads to a back field, and this alters the average tangential field around the particle. 

From Equation 6.10 in O Brien s dynamic mobility paper [13], it can be shown that the average tangential field around a spherical dielectric particle is given by  

Here K is the conductivity of the electrolyte, a is the particle radius and fQ is the surface conductance of the particle, a quantity that includes contributions from the electrolyte both inside and beyond the surface of shear. 

The effects of surface conductance are most pronounced for small particles and at low electrolyte concentrations, for then the denominator of Equation 4.20 is small and so the X factor is increased. For particles that do not exhibit stagnant layer conduction, the surface conductance is due to the motion of ions in the diffuse layer outside the shear surface. In this case JQ is determined by the zeta potential. This quantity is also larger at low electrolyte concentrations. 

---

The surface conductivity of glass makes long creeping distances necessary. 

The presence of surface conductance behind the slip plane alters the relationships between the various electrokinetic phenomena [83, 84] further complications arise in solvent mixtures [85]. Surface conductance can have a profound effect on the streaming current and electrophoretic mobility of polymer latices [86, 87]. In order to obtain an accurate interpretation of the electrostatic properties of a suspension, one must perform more than one type of electrokinetic experiment. One novel approach is to measure electrophoretic mobility and dielectric spectroscopy in a single instrument [88]. 

Tucceri R I and Posadas D 1990 The effect of surface charge on the surface conductance of silver in surface inactive electrolytes J. Electroanal. Chem. 283 159-66... 

Fig. 3. An overview of atomistic mechanisms involved in electroceramic components and the corresponding uses (a) ferroelectric domains capacitors and piezoelectrics, PTC thermistors (b) electronic conduction NTC thermistor (c) insulators and substrates (d) surface conduction humidity sensors (e) ferrimagnetic domains ferrite hard and soft magnets, magnetic tape (f) metal—semiconductor transition critical temperature NTC thermistor (g) ionic conduction gas sensors and batteries and (h) grain boundary phenomena varistors, boundary layer capacitors, PTC thermistors.

Surface conduction is monitored in most humidity sensors through the use of porous ceramics of MgCr204—Ti02 that adsorb water molecules which then dissociate and lower the electrical resistivity. 

The surface conductivity of vitreous siUca is also low compared to other siUcate glasses. Because vitreous siUca is not hydroscopic, water films containing exuded alkaUes do not readily form on its surfaces. The surface conductivity, however, can increase significantly with increasing relative humidity. A change in the relative humidity from 20 to 80% produces a millionfold increase in the surface conductivity (174). 

Although it has been generally demonstrated (15,77,86—89) that antistaticaHy treated fabrics exhibit increased surface conductivities, many examples have been found where static behavior is not always related to the conductivity of the fabric (90) or the material. One of the main reasons for this is the fact that static charges decay not only by conduction but also by radiation. 

Heat is transferred by radiation, condurtion, and convection. Radiation is the primaiy mode and can occur even in a vacuum. The amount of heat transferred for a given area is relative to the temperature differential and emissivity from the radiating to the absorbing surface. Conduction is due to molecular motion and occurs within... 

Trace contaminants are also significant at charged solid surfaces, affecting both the charging process and the surface conductivity. In ambient air atmospheres their effect is often determined by interaction with adsorbed water vapor, whose dominant concentration may be sufficiently large to form a monolayer. Topical antistatic agents for solids typically rely on interaction with adsorbed water and can lose effectiveness at low relative humidity (4-2.1). 

Arc Resistance ASTM D495. When an electric current is allowed to travel across the surface of an insulator the surface will become damaged over time and become more conductive. The arc resistance is a measure of the time (in seconds) required to make an insulating surface conductive under a high-voltage, low-current arc. 

Antistatic Agents. Most polymers, because they are poor conductors of current, build up a charge of static electricity. Antistatic agents attract moisture from the air to the plastic surface, improving its surface conductivity and reducing the likelihood of a spark or a discharge. 

Table 2 provides different fillers and approximate concentrations at which composites have surface conductance of about 1 Ohm-1 which corresponds to a shielding level of about 40 dB [14]. 

Table 2. Types of Conducting Fillers and Concentrations Corresponding to Shielding Level of 40 dB and Surface Conductivity of 1 Ohm-1...

Range of Application of the Equations Deduced The equations reported above are not entirely rigorous. A number of assumptions and approximations have been made when deducing them, and hence the range of application of the equations is somewhat restricted. Motion of the solution has always been regarded as laminar. It was assumed that the second phase is an insulator, and hence will not distort the electrical field existing in the solution. It was assumed that an enhanced surface conductivity is not found close to the interface (this could, for instance, be caused by the higher concentration of... 

Departures of the electrokinetic behavior of real systems from that described by the equations reported occurs most often because of breakdown of two of the assumptions above because of marked surface conductivity (particularly in dilute solutions, where the bulk conductivity is low) and because of a small characteristic size of the disperse-phase elements (e.g., breakdown of the condition of bg <5 r in extremely fine-porous diaphragms). A number of more complicated equations allowing for these factors have been proposed. 

In general, the peculiarities of the surface effects in thin semiconductors, for which application of semi-infinite geometry becomes incorrect were examined in numerous papers. As it has been shown in studies [101, 113, 121 - 123] the thickness of semiconductor adsorbent becomes one of important parameters in this case. Thus, in paper [121] the relationship was deduced for the change in conductivity and work function of a thin semiconductor with weakly ionized dopes when the surface charge was available. Paper [122] examined the effect of the charge on the temperature dependence of the work function and conductivity of substantially thin adsorbents. Papers [101, 123] focused on the dependence of the surface conductivity and value of the surface charge as functions of the thickness of semiconductor and value of the surface band bending caused by adsorption and application of external field. 

Clarke, E.N. Oxygen induced surface conductivity on germanium. Physic. 

The surface oxidation products dete ted by the decrease in contact angle upon photolysis of PVCa films may dominate the photoconductivity of t. is polymer. Work is underway to confirm this relatio. ship and measure surface conductivity simultaneously with bulk conductivity as a function of photodegradation. 

Carbon-coating is an effective way to improve the performance of electrode materials for lithium batteries, particularly with graphites [11-14], It is also known to aid in the surface conductivity for LiFeP04 as a cathode material [27], There are many ways to coat powders with carbon, but in this study, we have chosen to decompose a hydrocarbon vapor of propylene in a nitrogen carrier gas at a moderate temperature of 700 °C. Criteria for using this process include a material that is stable at this temperature and under a reducing environment. 

Figure 3. CSAFM images of surface conductance (right-hand panel, black areas are conductive) and topography (left-handpanel) of a 5 x 5 pm region of the cathode surface at 1.0 V tip-sample voltage difference.

In the conductance image, a dark color represents high electronic conductance, whereas a white color corresponds to areas of low or zero electronic conductance. Taking into account that the tip is in physical contact with the oxide, the magnitude of the current is determined by the local electronic properties of the electrode and the tip, and the tip-sample voltage difference. The surface conductance image of the cathode from the fresh cell... 

For powders and granular solids, there are two types of antistatic agents surface- and volume-active additives. Surface-active agents, which increase the surface conductivity of individual particles, are effective because triboelectric charge is always situated on the surfaces of individual particles. Most if not all surface-active agents are hygroscopic and thus attract a thin film of water to the surface it is this moisture that is responsible forthe increased surface conduction (van Drumpt, 1991). The effective bulk resistivity of the particles % -may be estimated by assuming that the particles are spherical and of radius R (Jones, 1995). 

---