T. O. Mason

Impedance spectra of hydrating cement pastes

Complex impedance spectra were obtained over the frequency range 5 Hz to 13 MHz on Portland cement pastes with water/cement ratios of 0.3, 0.35, and 0.4 at various hydration times from 6 h to 24 days. Features of the spectra which could be associated with the bulk material and which could be separated from the electrode arc, were studied. The overall bulk resistance of each paste was thereby determined as a function of hydration time. Bulk features evolved from a simple high-frequency intercept to an intercept with a single arc, then an intercept with two overlapping arcs, and back to an intercept with a single arc. A plausible equivalent circuit was developed involving an electrodeR/C network and constant phase element in series with one or two bulkR/C networks and a bulk resistor. Possible physical interpretation is discussed but assignment of equivalent circuit elements to microstructural features and/or processes will require microstructural characterization and a knowledge of pore-phase chemistry and properties.

Impedance spectroscopy of grain boundaries in nanophase ZnO

Sintered compacts of nanophase ZnO ({similar_to}60 nm average grain size, presintered at 600 {degree}C) were made from powders ({similar_to}13 nm) prepared by the gas-condensation technique. Impedance spectra were taken as a function of temperature over the range 450--600 {degree}C and as a function of oxygen partial pressure over the range 10{sup {minus}3}--1 atm (550 and 600 {degree}C only). The activation energy was determined to be 55 kJ/mole (0.57 eV) and was independent of oxygen partial pressure. The oxygen partial pressure exponent was {minus}1/6. Impedance spectra exhibited nonlinear {ital I}-{ital V} behavior, with a threshold of approximately 6 V. These results indicate that grain boundaries are governing the electrical properties of the compact. Ramifications for oxygen sensing and for grain boundary defect characterization are discussed.

Interpretation of the impedance spectroscopy of cement paste via computer modelling - Part III Microstructural analysis of frozen cement paste

The d.c. conductivity, σ, and low-frequency relative dielectric constant, k, of Portland cement paste were monitored, using impedance spectroscopy, during cooling from room temperature down to -50 °C. Dramatic decreases in the values of σ and k, as great as two orders of magnitude, occurred at the initial freezing point of the aqueous phase in the macropores and larger capillary pores. This result provides strong experimental support for the dielectric amplification mechanism, proposed in Part II of this series, to explain the high measured low-frequency relative dielectric constant of hydrating Portland cement paste. Only gradual changes in the electrical properties were observed below this sudden drop, as the temperature continued to decrease. The values of σ and k of frozen cement paste, at a constant temperature of -40 °C, were dominated by properties of calcium-silicate-hydrate (C-S-H) and so increased with the degree of hydration of the paste, indicating a C-S-H gel percolation threshold at a volume fraction of approximately 15%–20%, in good agreement with previous predictions. Good agreement was found between experimental results and digital-image-based model computations of σ at -40 °C. Freeze-thaw cycling caused a drop in the dielectric constant of paste in the unfrozen state, indicating that measurements of k could be useful for monitoring microstructural changes during freeze-thaw cycling and other processes that gradually damage parts of the cement paste microstructure.

Interpretation of impedance spectroscopy of cement paste via computer modelling Part I Bulk conductivity and offset resistance

Computer simulation of impedance spectroscopy (IS) of hydrating cement paste, using a three-dimensional, four-phase model, is described. Two puzzling features of experimental IS results, the possible offset resistance in the Nyquist plot and the sharp decrease in normalized conductivity within the first 50 h of reaction, have been studied using the computer simulation model. Insight is provided into these features using the ability of the model to compare quantitatively microstructure and properties. It is concluded that the offset resistance is an experimental artefact, and does not directly relate to microstructure. The drop in conductivity during the first 50 h is shown to be a consequence of a gradual shift from parallel-dominated to series-dominated behaviour of the electrical conductivity, as microstructural modifications take place during hydration, causing the capillary pore structure to become more tortuous. This tortuousity can also explain the high-frequency impedance behaviour in terms of a two-arc response.

Deposition and properties of yttria-stabilized zirconia thin films using reactive direct current magnetron sputtering

Yttria‐stabilized zirconia (YSZ) thin films were deposited by reactive magnetron sputter deposition from a composite Zr–Y target in Ar–O2 mixtures. Hysteresis was observed as a function of oxygen flow rateu2009 f. For a discharge current of 0.4 A and a total pressure P of 5 mTorr, for example, the target oxidized atu2009 f>2.3 ml/min, with the reverse transition from an oxidized to a metallic target surface occurring at 1.95 ml/min. The deposition rate was 2.7 μm/h in the metallic mode and 0.1 μm/h in the oxide mode. Fully oxidized (Y2O3)0.1(ZrO2)0.9 was obtained for f>2.0 ml/min, even in the metallic mode. While films deposited with P=3–20 mTorr were continuous, for P>20 mTorr crazing was apparent as expected for a ceramic film in a tensile stress state. For P<3 mTorr, the films delaminated due to excessive compressive stress. X‐ray diffraction and electron microscopy results showed that the films were polycrystalline cubic YSZ with a columnar structure and an average grain diameter of 15 nm. Fully dense films w...

Interpretation of the impedance spectroscopy of cement paste via computer modelling - Part II Dielectric response

Dielectric properties of cement pastes are measured using impedance spectroscopy, and the effective dielectric constants of the low frequency bulk arcs are reported. The unusually high values thereby obtained, and their dependence on reaction time and water:cement ratio, are explained by the presence of microstructural features that serve to amplify the dielectric constants of the individual material phases. The dielectric properties of three-dimensional cement paste models and of simple two-dimensional models of the hypothesized microstructural features are analysed. The model results provide insight into the proposed dielectric amplification mechanism in real cement paste.

NANOPHASE NI PARTICLES PRODUCED BY A BLOWN ARC METHOD

Nanophase Ni particles (<10 nm in diameter) were produced by a blown arc method. A helium gas stream directed at the arc reduces the Ni vapor concentration and increases the quench rate. The helium gas velocity is the predominant factor influencing the size of the Ni particles. Gas velocities of 20 m/s and 56 m/s (at 26.6 kPa total helium pressure) resulted in Ni particle sizes of 13 nm and 7 nm, respectively.

Electrode configurations and impedance spectra of cement pastes

Electrode effects on impedance spectra of cement pastes were investigated by two-, three-, and four-point measurements without a potentiostat over the frequency range 0.01 Hz–10 MHz. Electrode immittance effects arising from highly resistive/capacitive contacts cannot be fully corrected by nulling procedures. Two-point measurements are much more susceptible to such effects than three- or four-point measurements. The three- and four-point results on pastes suggest that there is negligible high-frequency “offset” resistance, and that bulk paste arcs are not significantly depressed below the real axis in Nyquist plots. The important impedance-derived equivalent circuit parameters are bulk resistance and capacitance; offset resistance and arc depression angle may not be physically meaningful parameters. Whereas all electrode configurations give reliable values of bulk paste resistance, only the three-point configuration provides the total paste/electrode dual arc spectrum involving a single electrode. Multielectrode (three- or four-point) measurements may be necessary to establish the true bulk paste dielectric constant.

Research needs and opportunities in highly conducting electroceramics

Abstract A workshop on Highly Conducting Electroceramics was held at Teton Village, Wyoming (USA) from July 9 to 13, 1990, under the auspices of the Division of Materials Sciences, Office of Basic Energy Sciences, US Department of Energy. The workshop covered emerging applications, new materials, the latest experimental observations, theory/modeling, and the current understanding related to electronic and ionic transport in ceramic materials. One of the major conclusions was that a better understanding of charge transport and reaction mechanisms at and along interfaces and boundaries is a critical research need. The highlights of the workshop are reviewed and recommendations are made for future research.

Cation intersite distributions in iron-bearing minerals via electrical conductivity/seebeck effect

The equilibrium high temperature cation intersite distributions in iron-bearing minerals which exhibit small polaron “hopping” conduction can be determined by a technique which combines electrical conductivity and Seebeck effect. The procedure is demonstrated for ferrospinels (Fe3O4) - FeAl2O4 and Fe3O4 - CoFe2O4) and applied to the olivine Fe2SiO4 - Mg2SiO4 system.