Positive temperature coefficient conduction mechanisms

It is an interesting fact that the activation energy for electrolytic conductance is almost identical with that for the viscous flow of water, viz., 3.8 kcal. at 25 hence, it is probable that ionic conductance is related to the viscosity of the medium. Quite apart from any question of mechanism, however, equality of the so-called activation energies means that the positive temperature coefficient of ion conductance is roughly equal to the negative temperature coefficient of viscosity. In other words, the product of the conductance of a given ion and the viscosity of water at a series of temperatures should be approximately constant. The results in Table XVI give the product of the conductance of the acetate ion at... 

In lieu of experimental data, the principle of corresponding states in quantum mechanics has been applied to the light molecular species to predict the liquid-state thermal conductivities and viscosities along their coexistence curves. The positive temperature coefficient of thermal conductivity for He , He", H2, and D2is shown to be part of a consistent pattern of quantum deviations. This effect is also predicted for tritium. The existing data for Ne... 

Resistivity measurements of doped, alpha-siUcon carbide single crystals from —195 to 725°C showed a negative coefficient of resistivity below room temperature, which gradually changed to positive above room temperature (45). The temperature at which the changeover occurred increased as the ionization of the donor impurity increased. This is beUeved to be caused by a change in conduction mechanism. 

The specific conductivity of a large number of oxide melts is in excess of 1 ohm-1 cm-1 and the temperature coefficient of conductance is positive. Two types of experiments from which conclusions are drawn regarding the mechanism of conduction are to be found in the literature. In one, the applicability of Faraday s laws is directly tested. In the second case, the conductivity of both the crystalline solid up to the melting point and that of the melt is measured if the change in specific conductivity on fusion is negligible, and if the so-called activation energy EK defined by... 

The conductivity mechanism in cermet films is not well understood however, the available evidence indicates that conduction occurs hy chains or networks of touching conductive particles embedded in an insulating matrix. In these films, the temperature coefficient of resistance varies from negative at low temperatures to positive at high temperatures. 

An information flow diagram for a drying model appropriate for this method is shown in Figure 4.16. This model can calculate the material moisture content and temperature as a function of position and time whenever the air humidity, temperature, and velocity are known as a function of time, together with the model parameters. If the model takes into account the controlling mechanisms of heat and mass transfer, then the transport properties (moisture dif-fusivity, thermal conductivity, boundary heat and mass transfer coefficients) are included in the model as parameters. If the dependence of drying conditions (material moisture content, temperature, and thickness, as well as air humidity, temperature, and velocity) on transport properties is also considered, then the constants of the relative empirical equations are considered as model parameters. In Figure 4.16 the part of the model that contains equations... 

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