Optimization, Yield Studies and Morphology of WO3Nano-Wires Synthesized by Laser Pyrolysis in C2H2and O2Ambients—Validation of a New Growth Mechanism

Laser pyrolysis has been used to synthesize WO₃nanostructures. Spherical nano-particles were obtained when acetylene was used to carry the precursor droplet, whereas thin films were obtained at high flow-rates of oxygen carrier gas. In both environments WO₃nano-wires appear only after thermal annealing of the as-deposited powders and films. Samples produced under oxygen carrier gas in the laser pyrolysis system gave a higher yield of WO₃nano-wires after annealing than the samples which were run under acetylene carrier gas. Alongside the targeted nano-wires, the acetylene-ran samples showed trace amounts of multi-walled carbon nano-tubes; such carbon nano-tubes are not seen in the oxygen-processed WO₃nano-wires. The solid–vapour–solid (SVS) mechanism [B. Mwakikunga et al., J. Nanosci. Nanotechnol., 2008] was found to be the possible mechanism that explains the manner of growth of the nano-wires. This model, based on the theory from basic statistical mechanics has herein been validated by length-diameter data for the produced WO₃nano-wires.

Phase Equilibria as Related to Crystal Structure in the System Niobium Pentoxide-Tungsten Trioxide

The phase equilibrium diagram for the binary system niobium pentoxide-tungsten trioxide has been constructed from results of x-ray diffraction studies on both single crystals and powders and from fusion characteristics. Twelve stable compounds have been found in the system. The exact composition of eight of these compounds has been established by single crystal analyses at the Nb2O5:WO3 ratios of 6:1, 13:4, 7:3, 8:5, 9:8, 1:1, 4:9, and 2:7. The approximate compositions of the remaining four other phases are 30:1, 6:11, 1:11, and 1:15. The 6:1, "6:11", 4:9, and 2:7 phases melt eongruently at 1476, 1378, 1380, and 1357 °C, respectively. The "30:1", 7:3, 8:5, 9:8, "1:11", and "1:15" phases melt incongruently at 1470, 1440, 1385, 1375, 1356, and 1358 °C, respectively; and the 13:4 and 1:1 phases decompose before melting at 1435 and 1115 °C, respectively. The 8:5, 9:8, "6:11", 2:7, "1:11", and "1:15" compounds are shown on the phase diagram as having minimum temperatures of stability. One metastable phase having a narrow range of composition near the 3:8 ratio was also encountered. Although Nb2O5 apparently exhibits no solid solution, WO3 was found to accept a maximum of three mole percent niobia in solid solution enabling all the reported polymorphs of WO3 to be obtained at room temperature.