| The negative thermal expansion materials contract with the increasing temperature and these materials become a new branch of materials science in recent years. Particularly, AM2O8(A=Hf or Zr、M=W or Mo) has stimulated considerable interest in this topic, because it exhibits large isotropic NTE property over it s entire stability range from0.3K to1050K,the thermal expansion coefficient is-9×10-6K-1. It has various potential applications in electronics, optics, communication, machine, biomedicine, sensor etc.Studies mainly focused on the synthesis method and characterization of ZrW2O8ceramic, powders, thin films and the ZrW2O8as a filler material was used to fabricate the controllable thermal expansion composites, but fewer reports on HfW2O8ceramic, powders, thin films were seen in the international wide. However, the properties of films may greatly differ from powder due to the dimensional effect, further research on the HfW2O8thin films is promising, and the NTE thin films also have various potential applications in aerospace vehicles, microelectronics, optics and micro machine. In this work, the negative thermal expansion materials HfW2O8and HfW1.6Mo0.4O8including both ceramic and thin films were prepared, and we can control the thermal expansion coefficient by careful adjustment of the Mo ration.The microstructure and morphology of the films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscope (AFM). The stress, elastic ratio and hardness were investigated by laser interference phase-shift technique and nano-indentation. The NTE coefficients of the HfW2O8and HfW1.6Mo0.4O8thin film were calculated using the lattice constants obtained by PowderX software using the data collected at different temperatures by inSitu X-ray measurement. The transmittance, thicknesses and cohesion of the samples were measured by Ultraviolet-visible spectr ophotometer (V570). The experimental results indicate that:(1) Cubic HfW2O8ceramic was successfully synthesized by the solid state reaction method using HfO2and WO3as raw materials. Results indicate that the cubic ceramic can be successfully prepared by sintering at1200℃for6h and then quenching in deionized water. The cubic HfW2O8ceramic is compact, and it crystallizes in small square grains. It exhibits strong negative thermal expansion (NTE) property, and an а to б structure phase transition occurs at182.5℃. This phase transformation leads to a decrease in NTE coefficient of cubic HfW2O8ceramic. The NTE coefficient for α-HfW2O8was measured to be-12.90×10-6K-1and for β-HfW2O8is-10.09×10-6K-1using HTXRD method. The average NTE coefficient for the cubic HfW2O8ceramic is-11.46X10-6K-1from25℃to600℃. The magnitudes of NTE coefficients for the cubic HfW2O8ceramic are smaller using TMA, which might be caused by the presence of pores and microcracks in HfW2O8ceramic.(2) HfW2O8thin films were deposited by pulsed laser deposition method using HfW2O8ceramic target and annealing subsequently. The as-deposited HfW2O8thin film is an amorphous phase, and the stoichiometry of the film is close to that of the HfW2O8ceramic target. The thin film deposited on the unheated substrate has a large surface roughness, and the roughness and cohesion decrease with the increased substrate temperature. The grain size of the film grows bigger with the increasing of the gas pressure. The cubic HfW2O8thin film can be obtained after annealing at1200℃for5min in air after quenching in water, and the film has larger grain size, with some cracks along the grain boundaries and some in the grains. The average thermal expansion coefficient of the resulting cubic HfW2O8film is-11.834×10-6K-1in the testing temperature range.(3) Cubic HfW1.6Moo.4O8ceramic was successfully synthesized by the solid state reaction method using HfO2、WO3and Mo as raw materials. Results indicate that the cubic HfW1.6Mo0.4O8ceramic can be successfully prepared by sintering at1150℃for6h and then quenching in deionized water. The cubic HfW1.6Mo0.4O8ceramic is compact. It exhibits strong negative thermal expansion (NTE) property, and an a to β structure phase transition occurs at100℃. It was found that the structural phase transition temperature decreased with the introduction of Mo, which is similar to the Closmann’s reports. This is mainly due to the difference in bond strengths of Mo-O and W-O. With the introduction of the weaker Mo-O bond, the reversal of adjacent MO4becomes easier. This phase transformation leads to a decrease in NTE coefficient of cubic HfW1.6Moo.4O8ceramic. The coefficient from25℃to100℃was measured to be-11.88×10-6K-1, and from100℃C to600℃is-10.24×10-6K-1using HTXRD method. The average NTE coefficient for the cubic HfW1.6Mo0.4O8ceramic is-11.51×10-6K-1from25℃to600℃.(4) HfW1.6Mo0.4O8thin films were deposited by pulsed laser deposition method using HfW1.6Mo0.4O8ceramic target and annealing subsequently. The as-deposited HfW1.6Mo0.4O8thin film is an amorphous phase, and the stoichiometry of the film is close to that of the HfW1.6Mo0.4O8ceramic target. The thin film deposited on he unheated substrate has a large surface roughness, and the roughness and cohesion decrease with the increased substrate temperature. The grain size of the film grows bigger with the increasing of the gas pressure. Crystallized cubic HfW1.6Mo0.4O8thin films were prepared by heating at1150℃for7min. The growth of the HfW1.6Mo0.4O8thin films was strongly influenced by the substrate temperature and oxygen pressure. The HfW1.6Mo0.4O8thin film deposited at500℃with an oxygen pressure of10Pa was smooth and compact, and its thickness was about540nm. The high temperature X-ray diffraction analyses demonstrated that the cubic HfW1.6Mo0.4O8thin film exhibited strong negative thermal expansion and its thermal coefficient was calculated to be-11.925×10-6K-1from100℃to600℃.This is mainly due to the difference in bond strengths of Mo-O and W-O. With the introduction of the weaker Mo-O bond, the reversal of adjacent MO4becomes easier. |
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