Showing papers by "Harry L. Tuller published in 1996"
TL;DR: In this article, the control of grain boundary segregation through purity, microstructure, and thermal history is discussed from the objective of engineering the grain-boundary impedance of polycrystalline ionic conductors.
Abstract: Solute segregation at grain boundaries has been correlated with grain-boundary conductivity in high-purity 15-mol%-CaO-stabilized ZrO2. STEM measurements of solute coverage show that the segregation of impurity silicon (present at bulk levels 103 at 500°C. At the lowest levels of segregation achieved, <0.1 monolayer, σspgb remains ∼102 less, and possibly represents an “intrinsic” limiting value for the grain boundary. Comparison with Y2O3-doped ZrO2 suggests similar behavior in this system. The control of grain-boundary segregation through purity, microstructure, and thermal history is discussed from the objective of engineering the grain-boundary impedance of polycrystalline ionic conductors.
415 citations
TL;DR: In this article, it was shown that unique defect thermodynamics and transport properties result for oxides of a few nanometers crystallite size, which are attributed to a dominant role of interfacial defect formation.
Abstract: It is shown that unique defect thermodynamics and transport properties result for oxides of a few nanometers crystallite size. Fully‐dense CeO2−x polycrystals of ∼10 nm grain size were synthesized, and their electrical properties compared with those of samples coarsened from the same material. The nanocrystals showed reduced grain boundary resistance, 104 higher electronic conductivity, and less than one‐half the heat of reduction of its coarse‐grained counterpart. These properties are attributed to a dominant role of interfacial defect formation.
354 citations
TL;DR: In this article, the electrical conductivity of nanocrystalline cerium was investigated by impedance spectroscopy and compared with literature data for single-crystal CeO 2.
Abstract: Nanocrystalline cerium was synthesized by magnetron sputtering and inert gas condensation. Controlled post-oxidation of the metallic nanoclusters resulted in formation of oxygen-deficient cerium oxide. Catalytic redox reactions such as SO 2 reduction by CO and CO oxidation in air were investigated. A comparison of activity with conventional stoichiometric ceria catalysts was used as a probe to distinguish different surface reactions. Electrical conductivity of nanocrystalline CeO 2− x was investigated by impedance spectroscopy and compared with literature data for single-crystal CeO 2 . The results are discussed in terms of the microstructure and defect chemistry of nanocrystalline cerium oxide.
67 citations
TL;DR: In this paper, electrical conductivity measurements were performed on Gd2Sn2O7 as a function of temperature, Po2 and Ca doping concentration, and an effective frenkel constant and oxygen vacancy mobility were derived.
53 citations
TL;DR: In this article, the mixed ionic and electronic conductivity was obtained in Gd2Ti2O7 by doping with Mn, which was confirmed by measuring the nonstoichiometry and the electrical conductivity as a function of oxygen partial pressure, temperature and Mn concentration.
15 citations
TL;DR: In this article, a photo-assisted electrochemical silicon micromachining method was proposed for the fabrication of stress-free submicrometer thick beams, membranes, and high aspect ratio 3D structures.
Abstract: Micromachining methods allow for the fabrication of small, three-dimensional structures including membranes, micro-valves, and pumps, channels, etc., in materials such as silicon. When integrated with microcircuitry this provides for the opportunity of creating unique miniature ‘smart’ chemical sensing devices. We present a novel method of photo-assisted electrochemical silicon micromachining which allows for the fabrication of stress-free submicrometer thick beams, membranes, and high aspect ratio three-dimensional structures, whose complex surface topologies were previously inaccessible by conventional processing routes. We discuss a number of gas and chemical sensor structures which benefit from MEMS technology and would benefit further from the processing versatility provided by our new micromachining fabrication method.
13 citations
TL;DR: In this paper, the oxygen nonstoichiometry in Mn-doped Gd{sub 2}Ti(Ti{sub 0.975}Mn{sub 1.025}O{sub 7+x} was measured as a function of temperature and oxygen partial pressure, with the aid of an oxygen titration cell.
Abstract: The oxygen nonstoichiometry in Mn-doped Gd{sub 2}Ti{sub 2}O{sub 7}, Gd{sub 2}(Ti{sub 0.975}Mn{sub 0.025}){sub 2}O{sub 7+x}, was measured electrochemically, as a function of temperature and oxygen partial pressure, with the aid of an oxygen titration cell. The analysis of the data shows that the defect equilibrium can be described by considering the dominant point defects to be neutral oxygen interstitials, doubly charged oxygen vacancies, and trivalent and quadrivalent Mn ions substituted in the Ti sites. The enthalpies for the formation of neutral oxygen interstitials and trivalent Mn are determined.
10 citations
TL;DR: In this paper, a solid state oxygen sensor based on the impedance of the metal/mixed ionic-electronic conductor interface is proposed, where porous Pt electrodes were examined in contact with (Gd1−xCax)2Ti2O7 with x = 0.02, 0.1 and 0.
Abstract: A solid state oxygen sensor based on the impedance of the metal/mixed ionic-electronic conductor interface is proposed. In this study, we examine porous Pt electrodes in contact with (Gd1−xCax)2Ti2O7 with x = 0.02, 0.1. Impedance measurements monitored the electrode impedance within the ranges, 650°C < T < 1010°C and 10−25 atm < Po2 < 1 atm. For both materials in oxidizing environments, a simple +12 power law dependence of electrode conductance on Po2 is found at temperatures greater than 950°C, with the behavior more complex at lower temperatures. Under reducing conditions, the x = 0.1 sample showed a −16 power law, while the x = 0.02 sample showed a −14 power law. Advantages and limitations of this arrangement as an oxygen sensor are discussed.
8 citations
TL;DR: In this paper, the authors used coulometric titration to measure the oxygen nonstoichiometry of nanocrystalline ceria, x ∈ Po 2 −1/6 and Po 2 = 0.21 − 10 −5 atm.
Abstract: Oxygen nonstoichiometry measurements in nanocrystalline ceria, x in CeO 2-x , were performed using coulometric titration. The measurements reveal large apparent deviations from stoichiometry, of the order of 10 −3 − 10 −4 at T = 405 − 455 °C and Po 2 = 0.21 − 10 −5 atm, as compared to levels of ∼10 −9 for coarsened materials under the same conditions. The level of nonstoichiometry is, however, larger then expected from previous electrical conductivity data of nanocrystalline ceria. In addition, x ∝ Po 2 −½ while Σ ∝po 2 −1/6 . The observed dependence of x(Po 2 , T) can be explained by either the formation of neutral oxygen vacancies at or near the interface, or by surface adsorption.
6 citations
TL;DR: In this paper, the A3+2B4+2O7 pyrochlores have fast-ion conduction properties that make them attractive candidates for applications in fuel cells.
Abstract: The A3+2B4+2O7 pyrochlores have fast-ion conduction properties that make them attractive candidates for applications in fuel cells. Rietveld powder-profile analysis of x-ray diffraction data has been used to determine structural parameters for Gd2(Ti1-ySny)2O7 solid solutions (y = 0.20, 0.40, 0.60, and 0.80) for correlation with composition-dependence of the specimens’ electrical conductivity. In accord with Vegard’s law, the lattice constants increase linearly with y as the larger Sn ion replaces Ti4+. The structures in the system remain remarkably ordered in view of the fact that it had been suggested that increasing the average radius of the ions that occupy the B4+ site relative to A3+ drives the system toward a disordered defect-fluorite state. A small but significant increase in mixing of the occupancy of the cation sites was found with increasing y to a maximum of [GdB] = 0.05. We could detect no significant disorder in the anion array. The activation energy and pre-exponential term for oxygen ion conduction were found to increase linearly and exponentially with y, respectively, such that at a given temperature, the ionic conductivity changes by less than an order of magnitude across the system. The behavior is in marked contrast to other systems where the substitution of a larger cation in the B site increases conductivity by up to three orders of magnitude as a consequence of substantial disorder that is introduced in both the cation and anion arrays.
2 citations
31 Jan 1996
TL;DR: In this paper, the same optical nonlinearity will affect the propagation of beams, and it should be considered in the design of a cw-laser, and experiments demonstrate such effects using the solid state laser material Cr:YAlO3, and show how studies of the spectroscopy and kinetics can be used to understand the results.
Abstract: Long metastable lifetimes in laser and related materials combined with a large index change makes possible large nonlinear optical effects using cw lasers. We describe experiments demonstrating such effects using the solid state laser material Cr:YAlO3, and we show how studies of the spectroscopy and kinetics can be used to understand the results. Since the same optical nonlinearity will affect the propagation of beams, it should be considered in the design of lasers.
TL;DR: In this article, the authors studied the oxygen nonstoichiometry of Pr0.545Ce0.455O2-x with solid state coulometric titration and was found to be extensive (0 ≤ y ≤ 0.28) at temperatures of 400-600 °C.
Abstract: The oxygen nonstoichiometry of Pr0.545Ce0.455O2-x was studied by solid state coulometric titration and was found to be extensive (0 ≤ y ≤ 0.28) at temperatures of 400-600 °C. The Po2 - x curves showed evidence of the existence of a number of single and two phase regions. Electrical conductivity measurements, performed under similar conditions, exhibited a p-n transition for temperature of 500 °C and below. The activated p-type conductivity was modeled in terms of the small polaron hopping mechanism. The p-n transition was correlated with the onset of phase separation.