Synthesis of negative thermal expansion HfW2O8 thin film using pulsed laser deposition

HfW2O8 thin films were grown on quartz substrates using a pulsed-laser deposition method at various temperatures from room temperature to 500 °C and different oxygen pressures from 5 Pa to 20 Pa. The effects of substrate temperatures, oxygen pressures and post-annealing on the phase compositions and morphologies of the obtained HfW2O8 thin films were systematically investigated using X-ray diffraction (XRD) and a field emission scanning electron microscope (FESEM). The thermal expansion properties of the cubic HfW2O8 thin films were characterized using high temperature X-ray diffraction (HTXRD). Results indicate that the as-deposited HfW2O8 thin films show amorphous phases. Crystallized cubic HfW2O8 thin films can be prepared by heating at 1200 °C for 7 min. HfW2O8 thin film grown at 500 °C with an oxygen pressure of 5 Pa displayed the smoothest and uniform surface morphologies. The high temperature X-ray diffraction analyses demonstrate that the cubic HfW2O8 thin film exhibits strong negative thermal expansion and an α to β structure phase transition occurred between 100 °C and 150 °C. Its average linear thermal expansion coefficient was calculated to be −9.33×10−6 K−1 from 25 °C to 600 °C.