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University of Kentucky Center for Applied Energy Research, 3572 Iron Works Pike, Lexington KY, 40511. ABSTRACT

Thin films of niobium pentoxide were deposited on quartz plates by spin-coating a solution of niobium ethoxide in ethanol. The films were tested as potential selective gas sensors by exposing them to various reducing environments. It was found that they were selective and sensitive to ammonia. This sensitivity was attributed to the formation of donor-acceptor complexes between chemisorbed ammonia and Lewis acid sites on the solid. INTRODUCTION

There is great demand for high performance ceramic materials in all aspects of industrial technology. Electronic ceramics offer a wide range of sensing and actuating functions that can be incorporated into designs for smart sensors. Most sensor materials experience a change in their electronic properties, such as resistivity, capacitances or remanent polarization, that can be calibrated with respect to the change in the environment [1]. The operation of many solid state gas sensors is based on catalytic reactions. In the case of metal oxide semiconductor gas sensors, the oxides themselves can be catalytically active. Alternatively, catalysts can be added to the oxide to provide sensitivity, selectivity and rapid response to changes in the composition of the ambient gas. The purpose of this research is to acquire a better understanding of the mechanisms that control gas sensor response together with the relationship between preparation variables and resultant properties of oxide films. To achieve this, niobium pentoxide films prepared by spin coating of a niobium ethoxide/ethanol solution were tested as gas sensors for ammonia. Niobium pentoxide was selected because of its unique acidic properties and because it is a well studied oxide with known catalytic and electronic properties [2,3]. This paper reports on the results from the synthesis, deposition and sensorial characterization of sol-gel prepared niobia thin films with respect to reducing gases. EXPERIMENTAL The general procedure utilized to fabricate the niobia thin films was based on the methods patented by Herman [4] and the ones reported by Mehrotra, Bradley and Livage [5-7] for the preparation of titanium oxide and niobium pentoxide powders. Niobium pentaethoxide was synthesized following the ammonia method from the reaction of niobium pentachloride and ethanol in the presence of an excess of ammonia. The ethoxide solution was then diluted to 25 % volume in ethanol. The films of niobia were fabricated by spin-coating this ethoxide solution on thoroughly cleaned quartz plates using an Integrated Technologies spin coater operated at 2000 rpm for 15 s. After the solution was deposited on quartz plates the films were allowed to hydrolyze with moisture from ambient air. The films were then either dried at 120 0 C for 15 minutes and then fired at different temperatures that ranged between 400'C to 1000°C or 553 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society

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