Thomas O. Mason

Zinc self-diffusion, electrical properties, and defect structure of undoped, single crystal zinc oxide

Zinc self-diffusion was measured in single crystal zinc oxide using nonradioactive 70Zn as the tracer isotope and secondary ion mass spectrometry for data collection. Crystal mass was closely monitored to measure ZnO evaporation. Diffusion coefficients were isotropic with an activation energy of 372 kJ/mol. Zinc self-diffusion is most likely controlled by a vacancy mechanism. Electrical property measurements exhibit a plateau in conductivity at intermediate pO2 with an increase in reducing atmospheres. An analysis of the defect structure is presented that indicates that oxygen vacancies are probably the intrinsic ionic defects responsible for n-type conductivity in reducing atmospheres.

Hydration of alkali-activated ground granulated blast furnace slag

The hydration of ground granulated blast furnace slag (GGBFS) at 25 °C in controlled pH environments was investigated during 28 days of hydration. GGBFS was activated by NaOH, and it was found that the rate of reaction depends on the pH of the starting solution. The main product was identified as C-S-H, and, in the pastes with high pH, hydrotalcite was observed at later stages of hydration. The pH of the mixing solution should be higher than pH 11.5 to effectively activate the hydration of GGBFS. As deduced from very low electrical conductivity measurements, GGBFS pastes had very tortuous and disconnected pores. The effect of the pH of the aqueous solution on the composition, microstructure and properties of alkali-activated GGBFS pastes are also discussed.

Neutron diffraction study on the defect structure of indium--tin--oxide

The defect structure of undoped and Sn-doped In2O3 (ITO) materials was studied by preparing powders under different processing environments and performing neutron powder diffraction. The effect of tin doping and oxygen partial pressure was determined. Structural information was obtained by analyzing neutron powder diffraction data using the Rietveld method. The results include positions of the atoms, their thermal displacements, the fractional occupancy of the interstitial oxygen site, and the fractional occupancies of Sn on each of the two nonequivalent cation sites. The tin cations show a strong preference for the b site versus the d site. The measured electrical properties are correlated with the interstitial oxygen populations, which agree with the proposed models for reducible (2SnIn•Oi″)x and nonreducible (2SnIn•3OOOi″)x defect clusters.

Electrical and oxygen storage/release properties of nanocrystalline ceria-zirconia solid solutions

The electrical properties of nanocrystalline Ce0.75Zr0.25O2 solid solution in the ceria–zirconia system were investigated using four-point DC conductivity measurements and impedance spectroscopy. Conductivity measurements were carried out as a function of temperature (723–821 K) and oxygen partial pressure (pO2=10−3–1 atm). The results were compared to the properties of bulk oxide of similar composition. Both the nanocrystalline and the coarsened oxide exhibit mixed conduction where the prevailing contribution is electronic. However, the nanocrystalline oxide shows a higher ionic contribution due to the enhancement of anionic vacancy mobility, which is more than one order of magnitude higher. In contrast with pure ceria, the electronic conductivity and activation energy for diffusion is similar in both nanocrystalline and sintered material, which could explain the similar reduction behavior shown by high and low surface area samples. These results are discussed and interpreted in terms of the role of surface area and the related crystallite size in the catalytic and reduction properties of ceria–zirconia solid solutions.

Analysis of the impedance spectra of short conductive fiber-reinforced composites

The presence of small amounts of short conductive fibers in a composite of finite matrix conductivity results in the subdivision of the one matrix impedance arc into two separate low and high frequency arcs in the complex impedance plane. These features are attributable to a “frequency-switchable” interfacial impedance on the fiber surfaces, rendering them insulating at DC and low AC frequencies, but conducting at intermediate frequencies. A combination of physical simulations (single wires in tap water) and pixel-based computer modeling was employed to investigate the roles of fiber pull-out, debonding, and orientation on the impedance response of fiber-reinforced composites. The ratio of the low frequency arc size to the overall DC resistance (γ-parameter) is sensitive to pull-out and/or debonding, especially when a fiber just barely makes contact with the matrix. The γ-parameter is also quite sensitive to fiber orientation with respect to the direction of the applied field. Ramifications for the characterization of cement, ceramic, and polymer matrix composites are discussed.

Brick layer model analysis of nanoscale-to-microscale cerium dioxide

The frequency-dependent impedance/dielectric behavior of the brick-layer model (BLM) was investigated vs. grain size and local parameters (resistivity, dielectric constant, and grain boundary width). The simulation shows a maximum in capacitance vs. grain size, governed by the grain boundary-to-grain interior resistivity ratio. The BLM was employed to analyze the 500 °C impedance behavior of polycrystalline cerium dioxide from the nano- (∼15 nm grain size) to the micro- (∼4 μm grain size) regime. The grain boundary resistivity is orders of magnitude larger than that of the grain interiors in the microcystalline specimen. This contrast is significantly smaller in the nanocrystalline specimens, suggesting enhanced conduction at grain boundaries. The limitations of the BLM for simulating the behavior of complex electroceramic microstructures are discussed.

Point defects and electrical properties of Sn-doped In-based transparent conducting oxides

Abstract In-based transparent conducting oxides (TCOs) share the prevailing defect structure of indium–tin oxide (ITO), i.e. electrons, isolated Sn In ⋅ donors, and neutral associates, believed to be (2Sn In ⋅ O i ″) x . The present work reviews the state of the literature, presents calculated Brouwer diagrams vs. oxygen partial pressure and vs. dopant concentration, and reports intermediate temperature electrical property data (thermopower, conductivity) vs. p O 2 and Sn concentration for three systems — polycrystalline bulk ITO, nanocrystalline ITO, and the recently reported ternary cation TCO, Ga 3− x In 5+ x Sn 2 O 16 . The influence of non-reduceable tin–oxygen complexes at high doping levels is identified for ITO. Ramifications for In-based TCO properties are discussed.

Impedance spectroscopy of fiber-reinforced cement composites

Abstract The addition of chopped conductive fibers to cement matrices results in a characteristic “dual-arc” electrical impedance spectrum below the percolation threshold. This behavior can be explained on the basis of a “frequency-switchable fiber coating” model, in which a “coating” (e.g., passive oxide film on steel or charge transfer resistance/double layer on other conductors) insulates the fibers at DC and low AC frequencies, but is shorted out at higher frequencies, where the fibers become short-circuit paths in the composite microstructure. The present work investigates various factors governing the impedance spectra of fiber-reinforced cement composites – fiber aspect ratio, fiber volume fraction, fiber orientation (relative to field direction), and fiber shape. The “gamma” factor (ratio of the low frequency arc diameter to DC resistance) is a useful parameter to characterize the microstructure–property relationships of fiber-reinforced composites.

Evaluating dielectric impedance spectra using effective media theories

The immittance spectra (i.e., impedance and modulus representations) are calculated for various effective medium theories, i.e., the Maxwell-Wagner (MW), Hashin-Shtrikman (HS), Bruggeman Asymmetric (BA) and Bruggeman Symmetric (BS) models, with emphasis on their individual microstructures. In addition the brick-layer (BL) model is also considered. The BL and MW-HS models yield similar single impedance arcs for a relatively low volume fraction conductive matrix (coating on the low conductivity phase). The BA model yields single impedance arcs different from the MW-HS models. The BL and MW-HS models yield virtually identical dual impedance arc behavior for a low volume fraction insulating matrix (coating on the high conductivity phase). At low volume fractions of insulating matrix, the low frequency arc due to the insulating material for the BA model is much smaller than for the MW-HS model. The BS model exhibits single impedance arc behavior when the volume fraction of conductor is above or near the percolation threshold and dual arc behavior somewhat below the percolation threshold. Equivalent circuits for these model materials are discussed, and application is made to experimental data for various electroceramic systems.

Electrical impedance spectra to monitor damage during tensile loading of cement composites

Conductive fibers can reinforce concrete and monitor damage leading to the development of smart material. This research studied the correlation between the electrical and mechanical properties of cement composites reinforced with conductive carbon fibers. The tensile behavior and impedance behavior of extruded and notched composites with a fiber volume fraction of 0.5 and 3% were examined; mechanical load and electrical field were applied longitudinally. The crack growth of these composites during loading was observed and analyzed by digital image correlation. Impedance spectroscopy (IS) measurements were made under loaded and unloaded conditions to address the effect of specimen geometry, the manufacturing process, and the effect of fiber volume fraction. Using these IS measurements, along with numerical computation, the bridging area of the fibers could be extracted quantitatively from the tensile measurements. It is shown that such methods can be useful to elucidate the role of the reinforcing fibers during fracture.