Sol-Gel Coatings on Acoustic Wave Devices: Thin Film Characterization and Chemical Sensor Development
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SOL-GEL COATINGS ON ACOUSTIC WAVE DEVICES: THIN FILM CHARACTERIZATION AND CHEMICAL SENSOR DEVELOPMENT GREGORY C. FRYE, C. JEFFREY BRINKER, ANTONIO J. RICCO, STEPHEN J. MARTIN, JANICE HILLIARD, AND DANIEL H. DOUGHTY Sandia National Laboratories, Albuquerque, NM 87185 ABSTRACT We have investigated the use of porous oxide coatings, formed using sol-gel chemistry routes, as the discriminating elements of acoustic wave (AW) chemical sensors. These coatings provide several unique advantages: durability, high adsorption capacity based on large surface areas, and chemical selectivity based on both controlled pore size and acid/base, ion exchange or chelation chemistry. The porosity of these coatings is determined by performing nitrogen adsorption isotherms using the AW device response to mass changes to monitor the uptake of nitrogen at 77 K. These studies demonstrate how sol-gel chemistry and film deposition can be combined to tailor the microstructure of thin oxide coatings. The chemical sensitivity and selectivity obtained with this class of coatings will be demonstrated using several examples: hydrous titanate ion exchange coatings, zeolite/silicate microcomposite coatings, and surface-modified silicate films. INTRODUCTION Sol-gel processing involves the hydrolysis and condensation of metal organic or inorganic precursors to form inorganic polymers in solution [1]. By varying the reaction conditions (e.g., reaction protocol, concentration of catalyst, water or metal precursor), the structure of these polymers can be varied from weakly branched chains to highly ramified structures (i.e., resembling a tumbleweed) to dense colloidal particles [2]. Prior to gelation, films can be prepared from these solutions by either spin- or dip-coating. The final film structure is dictated by the film forming procedure and the polymer structure: dense films are formed from weakly branched polymers, while high surface area porous films are formed from highly ramified polymers or dense colloidal particles [3]. The pore size distribution in the porous films can be tailored by varying the size and shape of the precursor polymers as well as by varying the coating procedure or thermal treatment [4]. In this paper, the application of these controlled microstructure oxide coatings for chemical sensor coatings will be discussed. Coatings that provide both chemical selectivity and increased sensitivity are critical for the development of effective chemical sensors [5,6]. Chemical selectivity is achieved by selective sorption of a single species or a class of species while increased sensitivity is obtained by high sorption capacities. Some examples of the types of sensor coatings that have been previously employed are organic polymers [5,7], phthalocyanines [5] and biological agents [8]. For sensors based on AW devices, gas phase sensors utilizing surface acoustic wave (SAW) devices [5,7] and liquid phase sensors utilizing either acoustic plate mode (APM) devices [9] or Lamb wave devices [8] have been developed. Compared to these coatings, contro
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