N. M. Tallan

Origin of Apparent Negative Impedance in Three Terminal Measurements

A brief discussion is presented of the occurrence of spurious negative dissipation factors, which appear to correspond to negative resistances, in the measurement of three terminal low loss specimens. The origin of these negative values is traced to series resistance in the guard ring lead and a technique, involving the use of a four electrode specimen, is described which obviates the need for extremely low resistance electrodes and leads.

Defect Structure and Electrical Properties of Some Refractory Metal Oxides

A brief description of the equilibrium thermodynamic approach to the characterization of defect concentrations in refractory metal oxides as a function of temperature, oxygen partial pressure, and impurity content is given. Techniques for the determination of ionic and electronic transport numbers by a blocking electrode polarization measurement and for the measurement of conductivity, thermoelectric power, and weight change are reviewed. The application of this approach and these measurements to the determination of the extent of deviation from stoichiometry, the nature and ionization state of the defects which predominate, and the mechanism of charge transport are illustrated by detailed consideration of several specific examples.

Electrode and Insulation Materials in Magnetohydrodynamic Generators

The operation and service environment of open and closed cycle MED generators is described. The performance characteristics of candidate electrode and insulation materials and their peculiar limitations with respect to conductivity, mechanical and electrochemical erosion, thermal shock resistance, maximum allowable surface temperature, compositional stability (polarization and contamination), thermionic emission, and overall service life are discussed. Special emphasis is given to problems associated with the performance of various ceramic insulators and electrodes used in open cycle generators. Recommendations for research studies on specific ceramic compositions and systems showing potential as MED generator channel materials are offered.