Impedance and dielectric spectroscopy revisited: Distinguishing localized relaxation from long-range conductivity

The goal of this report is to provide a comprehensive guide for researchers in the area of measurements of lossy dielectric and magnetic materials with the intent of assembhng the relevant information needed to perform and interpret dielectric measurements on lossy materials in one place. The report should aid in the selection of the most relevant methods for particular applications. We emphasize the metrology aspects of the measurement of lossy dielectrics and the associated uncertainty analysis. We present measurement methods, algorithms, and procedures for measuring both commonly used dielectric and magnetic materials, and in addition we present the fundamentals for measuring negative-index materials.

Measuring the Permittivity and Permeability of Lossy Materials: Solids, Liquids, Metals, Building Materials, and Negative-Index Materials

Electrical and dielectric behaviors and their origins in the three-dimensional polyvinyl alcohol/MWCNT composites with low percolation threshold

Stimulated by the excellent colossal permittivity (CP) behavior achieved in In+Nb co-doped rutile TiO2, in this work we investigate the CP behavior of Ga and Nb co-doped rutile TiO2, i.e., (Ga(0.5)Nb(0.5))(x)Ti(1-x)O2, where Ga(3+) is from the same group as In(3+) but with a much smaller ionic radius. Colossal permittivity of up to 10(4)-10(5) with an acceptably low dielectric loss (tan δ = 0.05-0.1) over broad frequency/temperature ranges is obtained at x = 0.5% after systematic synthesis optimizations. Systematic structural, defect, and dielectric characterizations suggest that multiple polarization mechanisms exist in this system: defect dipoles at low temperature (∼10-40 K), polaronlike electron hopping/transport at higher temperatures, and a surface barrier layer capacitor effect. Together these mechanisms contribute to the overall dielectric properties, especially apparent observed CP. We believe that this work provides comprehensive guidance for the design of new CP materials.

Colossal Dielectric Behavior of Ga+Nb Co-Doped Rutile TiO2

We herein present comparative investigation on the dielectric properties of both ceramic and single crystal SrTiO3 samples in the temperature from room temperature to 1073 K. Two relaxations were observed in both samples. They behave as Debye-like and relaxor-like relaxations in ceramic and single crystal samples, respectively. These relaxations were found to be bulk effect related to oxygen-vacancy. In single crystal sample, the relaxations result from the long-range conduction associated with singly and doubly charged oxygen vacancies. In ceramic sample, the oxygen vacancies are more strongly localized in relation to the crystal. This leads to a new phenomenon of formation and dissociation of oxygen vacancy clusters before the vacancies make contribution to the long-range conduction. The low-temperature relaxation in ceramic sample was determined by the clustering and dissociating processes of the oxygen vacancies. The high-temperature relaxation in ceramic sample was found to share the same mechanism as that in the single crystal sample.

Oxygen-vacancy-related dielectric relaxations in SrTiO3 at high temperatures

Calculation of various relaxation times and conductivity for a single dielectric relaxation process

Porous silica gelled from colloidal silica and potassium silicate mixtures as developed by Shoup has been investigated. Various gels exhibit an average pore size from 50 to 250 nm and a porosity up to 85%. Dealkalization by solution leaching is found to depend on the average pore size and is easier for large pore sizes. A domain model has been proposed for the gel networks to account for the observation of both macropores and mesopores as measured by Hg porosimetry and N2 desorption, respectively. The pore coarsening observed during sintering is explained by the material transport from areas around interdomain pores to those around intradomain pores in order to minimize the total surface energy.

Preparation and Sintering of Colloidal Silica‐Potassium Silicate Gels

Partially sintered silica gels were immersed in different NaCI solutions to deliberately introduce a small amount of sodium ions ranging from 387 to 9900 ppm. Dielectric measurements were carried out in the frequency range 12 to 107Hz under various relative humidity conditions. Two relaxation processes were observed in the impedance and modulus planes and were assigned to two separate regions in the hydrated silica surface. The top region is expected to be rich in hydrated sodium since its modulus relaxation time depends strongly on the relative humidity as well as the sodium content. The interfacial region is believed to be composed of structured water since its dielectric conductivity is affected mainly by the amount of water adsorbed on the pore surfaces.

Effect of Sodium Ions on the Dielectric Conductivity of Porous Silica in Humid Environments

The dielectric properties of silica gels of varying density were measured as a function of frequency and relative humidity. Capacitance and dielectric loss measurements were taken from lOHz -10 ' Hz at humidities ranging from 5 to 90% RH. It is found that the electrical behavior of these materials is distinctively different when they contain adsorbed water versus molecular or bulk water. Namely, adsorbed water is manifested as an increase in the dielectric constant and dielectric loss primarily at lower frequencies while the higher frequencies remain unchanged. In constrast, the presence of bulk water leads to a uniform increase in the dielectric constant at all frequencies. This behavior is believed to be applicable not only to ceramic insulators but also to a wide range of other materials including polymers and semiconductors.

Wanqing Cao

Papers

Calculation of various relaxation times and conductivity for a single dielectric relaxation process

Preparation and Sintering of Colloidal Silica‐Potassium Silicate Gels

Effect of Sodium Ions on the Dielectric Conductivity of Porous Silica in Humid Environments

Distinguishing bulk water from adsorbed water via dielectric measurements

Preparation and Sintering of Colloidal Silica-Potassium Silicate Gels.