Structural phase transition of tungsten-doped vanadium dioxide nanopowders prepared by thermolysis.

Tungsten-doped vanadium dioxide (VO2) nanopowders were prepared by thermolysis of (NH4)5[(VO)6(CO3)4(OH)9] x 10H2O at low temperature, with active white powdery tungstic acid used as a substitutional dopant. The composition and microstructure of the powders were examined by X-ray diffraction, transmission electron microscope, and differential scanning calorimetry. The change in electrical resistance due to the S-M transition was measured from 0 to 150 degrees C by the four-probe method. Hysteresis loops and differential scanning calorimetry analysis of the samples indicated that the phase-transition temperature of VO2 nanopowders was 67.15 degrees C. For tungsten-doped VO2 nanopowders, the temperature was reduced to 26.46 degrees C. After sintering the nanopowders, Tc rose from 26.46 degrees C to 34.85 degrees C with the sizes increasing to the bulk. A significant direct correlation between particle size and Tc was confirmed. The results indicated that white powdery tungstic acid is exceptionally effective as a dopant for reducing transition temperature.