B. J. Ingram

Defect chemistry and physical properties of transparent conducting oxides in the CdO-In2O3-SnO2 system

Combined solid state phase diagram studies and physical property measurements of the various n-type transparent conducting oxide (TCO) phases in the CdO-In 2 O 3 -SnO 2 system have been carried out. The 1175°C (air) subsolidus phase diagram has been established, including solid solution limits for binary and ternary compositions. From these limits and electrical property measurements vs. doping and degree of reduction, the prevailing defect mechanisms can be deduced. In addition to intrinsic (native) defects (e.g. oxygen vacancies) and extrinsic donor-doping of the end member compounds (e.g. Sn 1n in In 2 O 3 ), ternary solid solutions exhibit both isovalent doping (e.g. [Cd In ] = [Sn In ] in bixbyite, spinel) and donor-to-acceptor imbalance (e.g. [Sn In ] > [Cd In ] in bixbyite, spinel). Aliovalent doping can also lead to the formation of point defect associates, as in Sn-doped In 2 O 3 (ITO), as confirmed by combined Rietveld analyses of X-ray and neutron diffraction data. Cation exchange between sublattices in the spinel phase plays an important role in determining phase stability and band structure. The physical properties of the TCO phases in the CdO-In 2 O 3 -SnO 2 system are presented for both bulk ceramics and thin films.

Powder-Solution-Composite Technique for Measuring Electrical Conductivity of Ceramic Powders

A method for measuring the conductivity of ceramic powders has been developed using effective medium theory and impedance spectroscopy. Powders to be tested are mixed with aqueous solutions of varying NaCI concentrations to form a composite slurry. The impedance response of these powder-solution-composites (PSCs) is measured and analyzed using composite equivalent circuit and effective medium theories. Values obtained for a test powder of copper aluminate (CuAlO 2 ) match closely to four-point dc values measured on bulk sintered bars. A comparison was also made in a system with highly composition-dependent conductivity (YITO, In 2-2x-2y Y 2x Sn 2y O 3 , 0.08 ≤ x ≤ 0.20) to evaluate the upper and lower bounds for accurate measurements. Results from the PSC method matched the sintered bar four-point dc values within the range of NaCI solutions employed (0.5-10 -4 S/cm).

Impedance/Dielectric Spectroscopy of Nanoceramics

With proper attention to experimental design (i.e., electroding, cabling, stray apparatus imittances, etc.) impedance/dielectric spectroscopy is a powerful tool to study the electrical properties of nanoscale electroceramics. This study focuses on bulk non-ferroelectric materials (ZnO, CeO2, TiO2) and their frequency-dependent AC electrical properties, taken from a variety of literature sources. In particular, it is shown how to separate effective grain boundary and grain interior resistivities and also the effective capacitances associated with each region in the microstructure. This is possible even when Nyquist plots (-Z im vs. Z re ) without frequency markers are the only data supplied. A modified brick layer model (BLM) can be used to analyze the impedance/dielectric properties of nanoscale ceramics.

Impedance/dielectric spectroscopy of electroceramics in the nanograin regime

ABSTRACT In the microcrystalline regime, the behavior of grain boundary-controlled electroceramics iswell described by the “brick layer model” (BLM). In the nanocrystalline regime, however, grainboundary layers can represent a significant volume fraction of the overall microstructure andsimple layer models are no longer valid. This work describes the development of a pixel-basedfinite-difference approach to treat a “nested cube model” (NCM), which more accuratelycalculates the current distribution in polycrystalline ceramics when grain core and grainboundary dimensions become comparable. Furthermore, the NCM approaches layer modelbehavior as the volume fraction of grain cores approaches unity (thin boundary layers) and itmatches standard effective medium treatments as the volume fraction of grain cores approacheszero. Therefore, the NCM can model electroceramic behavior at all grain sizes, from nanoscaleto microscale. It can also be modified to handle multi-layer grain boundaries and propertygradient effects (e.g., due to space charge regions).