Characterization study of platinum-doped stannic oxide ceramics for methane sensing in air

Abstract

Pure SnO2 and Pt-SnO2 ceramics were prepared by the dry-pressing method using a pressure of 40 MPa and sintered at various temperatures between 100-000oC from a mixture of powders of (100-x)SnO2.xPt (0 = x wt % = 5). The electrical properties of the ceramics were studied using a home-made Sensor Element Characterization System (SECS) and an Impedance Analyzer. The sensing probe of the SECS was modified so it was much slimmer with most of the electrical connections concealed and could measure either the bulk or surface resistance of the ceramic. The optimum composition for detecting methane in air was 0.5 wt.% Pt-SnO2 sintered at 1000oC and the optimum operating temperature was at 400oC. The resistance of the 0.5 wt.% Pt-SnO2 in 25000 ppm methane decreases from ~ 54.0 kO to ~ 4.6 kO at temperatures of 200oC up to 440oC respectively. The activation energies were between 0.30 eV and 0.45 eV for temperatures between 200oC and 400oC. The corresponding conductance (G) decreased with Pt loading and the gas partial pressure (p) or methane flow rate (?). As such, it indicates that the doped SnO2 is an n-type semiconductor. The conductance power law takes the form G ~ p-0.5 and this indicates that the chemisorbed ions on the doped ceramics depended only on temperature. The conductance (G)- methane concentration (c) takes the form G = kc0.35. A linear relationship ln G = 0.35 ln c – 11.9 was obtained when plotting ln G against ln c. The relative conductance change (?G/G) and the square root of methane concentrations (c½) obey the relationship ?G/G = 0.08c ½ which indicates the doping with 0.5 wt.% Pt increased the sensitivity of the base material (SnO2) to methane by a factor of 133. The response and recovery times were affected by the methane flow rate, operational temperature, level of doping with values between 30 s up to 154 s and between 600 s up to 1317 s respectively. The doping of Pt at 0.1 wt.% up to 5 wt.% in SnO2 produced ceramics with densities of 7.01g/cm3 up to 7.03 g/cm3 which exceeds the full density of pure SnO2 (6.90 g/cm3). The strength and stability were indicated from the doped SnO2 measurements of Vickers hardness (10 GPa and up to 19 GPa), Young modulus (20 GPa and up to 55 GPa) and Bulk modulus (20 GPa and up to 80 GPa) for Pt loadings between 0.1wt.% and 2.5 wt.%. High resolution X-ray diffraction showed that the mean crystallite size ranges between 25 nm and 55 nm for Pt loadings from 1 wt% up to 5 wt.% in SnO2. The strain in doped samples could not be eradicated by either sintering at high temperature (1000oC) or high Pt loading (5 wt.%). X-ray photoemissions spectroscopy (XPS), Mössbauer and nuclear magnetic resonance (NMR) analysis showed that the doped SnO2 has additional chemical environment (compared to pure SnO2) can be attributed to the ease of detecting methane in air via electrical measurements.