POROUS SILICA-COATED ALPHA-FE2O3 CERAMICS FOR HUMIDITY MEASUREMENT ATELEVATED-TEMPERATURE

Porous alpha-Fe2O3 powder obtained from topotactic decomposition of al pha-FeOOH was dispersed in a tetraethyl orthosilicate solution, Amorph ous silica was coated on the surface of alpha-Fe2O3 grains, followed b y gelation and sintering, The N-2 sorption measurement indicates that the pore coalescence is strongly suppressed by coating, For instance, the coated sample sintered at 800 degrees C has a specific surface are a of 145.8 m(2)/g, much greater than the uncoated one, 5.035 m(2)/g, F urthermore, the hysteresis isotherm implies that the pores of the coat ed sample have an ink-bottle type structure, There are O and Si traces in the XPS survey scan, while no Fe peak is found, which confirms tha t the alpha-Fe2O3 grain is totally covered by silica, Organic radicals are also evident in the detail scan of the C1s peak of the fresh frac tured surface, The XRD analysis highlights that the coated ceramic is composed of alpha-Fe2O3 and amorphous silica, Because of the coating l imitation on grain growth, sintering below 950 degrees C hardly increa ses the grain size as compared with the starting powder, in terms of S cherrer's equation, The HRTEM images of the uncoated and coated sample s display a clear layer of the amorphous coating on highly porous grai ns, Many pores in the 2-5 nm size range seem to be connected in the la ttice matrix, The pore number and volume are greatly decreased after s intering, but the silica coating weakens this effect, The complex impe dance of the coated sample with porous RuO2 electrode was investigated in the frequency and temperature ranges of 1 Hz-1 MHz and 20 degrees- 400 degrees C, respectively. Artificial dry air was bubbled through de ionized water or analytical-purity ethanol to yield wet air or ethanol vapor condition, The sensitivities, defined as the ratio between impe dance under a test and dry air conditions, are 30 for 2.8% water vapor and 2 for 9.3% ethanol vapor at 200 degrees C, A high ethanol vapor s ensitivity above 200 degrees C is explained as an ''on-site'' water va por effect arising from the ethanol combustion on a silica catalyst, V arious gases like CH4, CO, NOx, and NO2 were introduced into dry and w et air at elevated temperatures, No cross-sensitivity was found for th e gas concentrations up to 5000 ppm, A model of the porous coating str ucture is proposed for a reason of high humidity sensitivity without g as cross-sensitivity.