Martin A. Seitz

Complex plane analysis of trapping phenomena in zinc oxide based varistor grain boundaries

The origin of the frequency‐dependent Mott–Schottky behavior observed in a wide range of ZnO‐Bi2O3 varistor systems has been investigated. Lumped parameter/complex plane analysis of two‐probe ac electrical data indicates that several trapping relaxations contribute to the measured MOV grain‐boundary admittance in the frequency range, 10−2 Hz≤f≤107 Hz. Furthermore, this approach allows the development of an equivalent circuit representation which incorporates these trapping phenomena in a systematic manner.

Defect characterization in CeO2−x at elevated temperatures—I: X-Ray diffraction

Abstract The atomic defect structure of nonstoichiometric ceria was studied by means of X-ray diffraction. Polycrystalline samples of CeO 2−x (0⩽ x ⩽0.21) have been examined at 900 and 1000°C, with the stoichiometry controlled by adjusting the oxygen partial pressure between 1 and 10 −21 atm. It was observed that the lattice expands as a function of increasing defect concentration and exhibits only fluorite-like diffraction peaks. The integrated intensities of the Bragg reflections were analyzed for CeO 2 and CeO 1.91 , at 900°C by difference electron-density techniques. It was concluded that the cation sublattice is essentially intact, and that the oxygen sublattice must be defective in nonstoichiometric ceria. Least-squares analyses on CeO 2−x (0⩽ x ⩽ 0.21) at 900 and 1000°C supported the electron-density results and also showed that the temperature factors of both cations and anions increase with an increase in defect concentration, implying greater mean-square displacement of the atoms from their equilibrium positions.

Specific Impedance of Canine Blood

The specific impedance of canine erythrocytes suspended in plasma was measured in the frequency range from 5 kHz to 1 MHz in samples from three animals in the hematocrit range from zero to packed cells at a temperature of 39°C; measurements were made with a conductivity cell using four electrodes and a current density of 21 μA/cm2. With the use of impedance spectroscopy, data were fitted to an equivalent circuit model; model parameters in turn were fitted as functions of hematocrit. The resultant model can be used to predict specific impedance (real and reactive components) as a function of hematocrit and frequency over a frequency range from 5 kHz to 1 MHz and a hematocrit range from 0 to 80. Over a normal range of hematocrits and at frequencies less than 100 kHz., the current is almost exclusively confined to the plasma, and the specific impedance is nearly equal to the real component; however, at higher frequencies, the complex nature of specific impedance becomes important.

Annealing and Microstructural Characterization of Tin-Oxide Based Thick Film Resistors

The sheet resistance of tin oxide based thick-film resistors exhibits two regions of temperature-dependence, described by hopping (23°C–200°C) and diffusion mechanisms (200°C–350°C), respectively. Annealing these samples causes the sheet resistance to increase in both regions. In the post-annealed samples, the hopping conduction range is extended by 50°C (23°C–250°C) while the hopping parameter, T0, is decreased by more than 50%. The activation energy of diffusion (0.60 eV) is the same for both pre- and post annealed samples, but the magnitude of resistance in the diffusion controlled region is increased significantly as a result of annealing. These changes are explained in terms of a net decrease in the concentration of tin ions in the glass matrix. From a careful microstructural study it was found that a conduction path composed of tin-oxide grains or their clusters in contact with each other does not exist in the present system. HREM micrographs showed the presence of nanocrystalline tin-oxide particles in the glass phase separating the tin-oxide grain clusters. Estimated average separation between the nanocrystals in 4 nm, consistent with a variable-range hopping conduction via the dissolved tin ions in the glass matrix.

Immittance Spectroscopy of Smart Components and Novel Devices

AC small-signal immittance spectroscopy is employed as a viable tool to demonstrate electrical characterization, performance improvement, and quality assurance issues of smart materials-based components and novel devices. The variation in the ac response, complemented via dc measurements within a range of tolerating temperature, delineates competing phenomena occurring in the microstructures of these engineering material systems. The results are presented in a generic manner with possible explanations on the mechanisms for two selected Debye-like (nearly ideal) and non-Debye (non-ideal) low-capacitance resistors. This spectroscopic approach allows systematic development of a representative equivalent circuit, considered to be the characteristic of the devices and components, for specific applications.

Thermal Runaway in Metal Oxide Varistors

The conditions leading to thermal runaway in metal oxide varistors are discussed in terms of its power dissipation. An equivalent circuit is used to rationalize the similarities and differences between the DC and 60 Hz ac behavior.

Electrical behavior of biological materials

The lumped-parameter AC equivalent circuit is used as a systematic means of representing the impact of multiple conduction and polarization processes, along with the role of a microstructure, on a biological materials electrical behavior. Systems are encounted that lead to overlapping semicircular plots, which indicates the presence of multiple polarization events having relaxation times close to one another. In this instance the data analysis becomes more complex. In situations where there exists a continuous distribution of relaxation times, the semicircular plots are found to have their centers depressed below the abscissa. This can be an indication of the interaction of polarizing units or of heterogeneous variations within the system microstructure.<<ETX>>