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Properties and Applications of Pr_xCe_1-xO_2-d

Gas Sensors

There are many commercial and industrial applications of solid state gas sensors including toxic gas detection, atmospheric monitoring during food storage, and active feedback for combustion systems (ie, automotive exhaust). Four primary considerations in the design of a sensor are sensitivity (which determines the lowest gas concentration the sensor can detect), selectivity between different gasses, the speed at which the sensor responds to changes in gas concentration, and stability of the sensor over time and cyclic exposure to different gasses. These "4 S’s" must all be addressed in the design of a commercially useful sensor

Semiconducting gas sensors rely on a change of surface conductivity with the adsorption of a gas species. For instance, if oxygen chemisorbs at an interface in an n-type semiconducting oxide, electrons will be consumed, thus depleting the semiconductor of charge carriers and giving rise to a Schottky barrier to conduction. As oxygen adsorbs, the conductivity of the sensor will decrease. The same is true for any oxidizing species that adsorbs, yielding poor gas selectivity.

Praseodymium Cerium Oxide

Praseodymium cerium oxide solid solutions exhibit electrical unique electrical properties and extensive deviations from stoichiometry which make them appealing candidates as low temperature gas sensors. Through control of the Fermi level in the bulk Pr_xCe_1-xO_2-d ceramic via composition changes, it should be possible to selectively adsorb gas species, rather than adsorbing all species simultaneously, thus producing a selective gas sensor.

Praseodymium cerium oxide (PCO) powders are producing using coprecipitation of metal nitrates in oxalic acid. Calcination of the resulting oxalate powders produces homogeneous, fine grained PCO. These powders are pressed and sintered into dense samples. Control of microstructure is acheived using hot pressing to avoid excessive grain growth while allowing densification. Thin film samples are also produced using pulse laser ablation.

Samples are characterized using a variety of analytical techniques. X-ray diffraction and electron microprobe analysis are used for phase and composition determination. Scanning electron microscopy and BET measurements are used for microstructural characterization. Bulk samples are characterized using four point DC conductivity, AC impedance, and thermoelectric power measurements in order to gain insight into the defect chemistry of the PCO system. Diffusion kinetics and non-stoichiometry are studied using coulometric titration. Finally, sensor testing of the "4 S’s" is carried out by monitoring electrical response during exposure to a variety of test atmospheres.