Porous silica-coated alpha-Fe2O3 ceramics for humidity measurement at elevated temperature

Porous alpha-Fe2O3 powder obtained from topotactic decomposition of alpha-FeOOH was dispersed in a tetraethyl orthosilicate solution, Amorphous silica was coated on the surface of alpha-Fe2O3 grains, followed by 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 area of 145.8 m(2)/g, much greater than the uncoated one, 5.035 m(2)/g, Furthermore, the hysteresis isotherm implies that the pores of the coated 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 that 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 fractured surface, The XRD analysis highlights that the coated ceramic is composed of alpha-Fe2O3 and amorphous silica, Because of the coating limitation on grain growth, sintering below 950 degrees C hardly increases the grain size as compared with the starting powder, in terms of Scherrer's equation, The HRTEM images of the uncoated and coated samples display a clear layer of the amorphous coating on highly porous grains, Many pores in the 2-5 nm size range seem to be connected in the lattice matrix, The pore number and volume are greatly decreased after sintering, but the silica coating weakens this effect, The complex impedance 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 deionized water or analytical-purity ethanol to yield wet air or ethanol vapor condition, The sensitivities, defined as the ratio between impedance 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 sensitivity above 200 degrees C is explained as an ''on-site'' water vapor effect arising from the ethanol combustion on a silica catalyst, Various gases like CH4, CO, NOx, and NO2 were introduced into dry and wet air at elevated temperatures, No cross-sensitivity was found for the gas concentrations up to 5000 ppm, A model of the porous coating structure is proposed for a reason of high humidity sensitivity without gas cross-sensitivity.