Showing papers by "Harry L. Tuller published in 2010"

Novel deposition techniques for metal oxide: Prospects for gas sensing

Abstract: With the growing need for high performance gas sensors, a variety of preparation methods have been introduced and investigated. In the present contribution, the focus is on novel means for preparing metal oxide gas sensor devices. First, the key gas sensor concepts are reviewed as well as the conventional deposition techniques widely used for thick and thin film deposition of metal oxides such as screen-printing and chemical and physical vapour deposition. This is followed by a review and examination of innovative deposition techniques developed within the past several years, focusing on methods with direct-write features as well as techniques offering precise control of micro- and nano-structural features of the deposited materials.

Electrospun SnO2 nanofiber mats with thermo-compression step for gas sensing applications

Abstract: SnO2 nanofiber mats fabricated through electrospinning followed by thermo-compression and subsequent calcination steps exhibited unique morphologies facilitating efficient gas transport into the layers combined with high surface area (∼73.5 m2/g, measured by BET) and small grain size (∼5–15 nm), which are well suited for ultrasensitive gas detection. Single SnO2 nanofibers were found to have a belt-like structure of closely packed nanocrystallites, facilitating excellent adhesion to the substrate and good electrical contact to the electrodes. I–V measurements of single SnO2 nanofibers displayed ohmic behavior with electrical conductivity of 1.5 S/cm. Gas sensor prototypes comprising a random network of SnO2 fibers exhibited high sensitivity when exposed to NO2 at 225°C and CO at 300°C. A detection limit of 150 ppb NO2 at 185°C was estimated by extrapolating the sensitivity results obtained on exposure to higher gas concentrations, demonstrating potential of achieving ultra-sensitive gas detection at low operating temperatures enabled by the present synthesis method.

Nonlinear thermomechanical design of microfabricated thin plate devices in the post-buckling regime

Abstract: A design approach for thermomechanically stable sub-micron plates is developed utilizing the post-buckling regime via a nonlinear plate analysis. Based on the analysis results and experimental observations, local stresses are observed to have maxima in the near-post-bifurcation regime, but then to decrease significantly in the post-buckling regime. This effect is more significant with plates of larger sidelength and smaller thickness structures, enabling microfabrication of numerous plate and membrane structures that are typically considered susceptible to failure due to buckling. Using a stress-based failure criterion, rather than the typical buckling criterion, an expanded design space for thin plates beyond the traditional pre-buckling regime is revealed. A device class that benefits in both power and efficiency from thin, large-area freestanding plates is microfabricated fuel cells, particularly high-temperature solid oxide fuel cells (µSOFCs). As a demonstration of the expanded design space, µSOFCs of submicron (450 nm) layer thickness are designed, fabricated and operated in the far postbuckled regime, verifying thermomechanical stability (up to 625 °C) and functional operation. The design approach introduced here can be applied to a range of microfabricated devices such as purification membranes, electrolysis cells and biochemical sensors.

The oxygen permeation properties of nanocrystalline CEO2 Thin Films

Abstract: The measurement of oxygen flux across nanocrystalline CeO{sub 2} cerium oxide thin films at intermediate temperature (650 to 800 C) is presented. Porous ceria support substrates were fabricated by sintering with carbon additions. The final dense film was deposited from an optimized sol-gel solution resulting in a mean grain size of 50 nm which displayed oxygen flux values of up to 0.014 {micro}mol/cm{sup 2}s over the oxygen partial pressure range from air to helium gas used in the measurement at 800 C. The oxygen flux characteristics confirm mixed ionic and electronic conductivity in nanocrystalline ceria films and demonstrate the role of size dependent materials properties as a design parameter in functional membranes for oxygen separation.

Defect Structure, Charge Transport Mechanisms, and Strain Effects in Sr4Fe6O12+δ Epitaxial Thin Films

Abstract: The defect structure and charge transport mechanisms of layered perovskite-related structure Sr4Fe6O12+δ in the form of epitaxial thin films are studied as a function of temperature, oxygen partial pressure, and the type of substrate upon which they were grown. Values for the band gap, oxidation, and reduction enthalpies are extracted by fitting the experimental data to a defect model assuming a high oxygen deficiency. The electrical conductivity was dominated by p-type and n-type electronic conductivity at high and low oxygen partial pressures, respectively, with no evidence of a measurable ionic conductivity. Specimens cooled to below 400 °C after being annealed at different pO2’s at elevated temperatures exhibited an activated conductivity whose activation energy Ea increased from ∼0.2 eV to ∼1.0 eV as the pO2 at which the specimens were annealed decreased from 1 atm to 10−4 atm. These data, which are also examined with respect to the different epitaxial strains induced by selecting substrates with dif...

MEMS-based gravimetric sensors for explosives detection

Abstract: A compact, rapid system for trace detection of explosives using MEMS-bases resonant structures is described. Various software algorithms are used to eliminate problems in previous implementations of array-based detectors. Results for detection of trace levels of explosives and other threat agents are given.