Solid state oxide chemistry of ruthenium
The oxygen stoichiometry of RuO2+atop -x was investigated by thermogravimetric and isopiestic techniques, in the temperature range 200-800oC. RuO2+atop -x could be made hyperstoichiometric to a maximum of RuO2.014 at 500oC and hypostoichiometric to a maximum of RuO1.9808 at 600oC. Above 800oC loss of ruthenium as the volatile species RuO3(g) and RuO4(g) prevented any reasonable determination of oxygen stoichiometry. Solid solutions were prepared in the Ru-V-O, Ru-Nb-O, Ru-W-O and Ru-Mo-O systems by solid state reaction. Complete solid solution was observed in the Ru-V-O system. Unit cell parameters were determined by X-ray powder diffraction for a range of compositions in the Ru-V-O system. Large deviations from the linearity predicted by Vegard's law were found for both a and c, e.g. at 5 wt.% vanadium a = 4.489, c = 3.089 and at 75 wt.% vanadium a = 4.592, c = 2.873. Conductivity of polycrystalline pressed pellets of RuO2:VO2 solid solutions was measured using a four point probe technique. All small vanadium concentrations conductivity increased with percent dopant i.e. resistivity decreased from 8.5 k Ω cm at 2 wt.% vanadium to 4.5 k Ω cm at 5 wt.% vanadium. Solid solutions containing 25-85 wt.% vanadium were found to be semiconducting and increasing amounts of vanadium in this range produced no significant change in resistivity. Solid solutions containing 95 wt.% vanadium showed a transition, at -13oC, from metallic to semiconducting behaviour as the temperature was lowered. In the Ru-Nb-O, Ru-W-O and Ru-Mo-O systems only 5 wt.% dopant could be successfully incorporated into the RuO2 lattice. On the basis of the Ru-V-O system however these materials are considered to be potentially good conductors. Resistor inks dilute in ruthenium could prove economically viable. The high vanadium containing materials may have potential as temperature sensitive electronic switching devices. The temperature programmed reduction of an RuO2:VO2 solid solution containing 25 wt.% vanadium showed that the onset of reduction occurred at a much higher temperature (234oC) than for RuO2 itself (63oC). This suggests that vanadium containing solid solutions of RuO2 are more resistant to reduction than RuO2 itself, a much sought after property in the production of thick film resistors.