Study On The Synthesis Of Negative Thermal Expansion ZrW_2-xMo_xO₈(0＜x≤2) Powders And Controlling Of Morphology

Negative thermal expansion (NTE) material becomes a new branch of materialsscience in recent years. Particularly, ZrW_2-xMo_xO₈ exhibits large isotropic NTE propertyover its entire stability range, their potential uses include electronic, optics, fuel cell,oxygen sensors, thermostats and dental filling products. Due to the excellent propertiesand potential applications of ZrW_2-xMo_xO₈, the synthesis and morphology controlling ofZrW_2-xMo_xO₈ powders and the NTE property were studied in this paper.The liquid-phase method and solid state method are largely utilized to synthesis ofZrW_2-xMo_xO₈ powders. Because of many advantages such as high purity, controlledmorphology, uniform composition, liquid-phase method is a kind of method havinggreat prospect. ZrW_2-xMo_xO₈(03)·5H₂O,N₅H₃₇ W₆O₂₄·5H₂O and N₆H₂₄ Mo₇O₂₄·4H₂O as raw materials by dehydration precursorroute and hydrothermal method in this paper. The reaction conditions and negativethermal expansion coefficient were investigated. Moφhology controlling of the finalproducts was manipulated by adopting certain reaction conditions.The precursors of ZrW_2-xMo_xO₈ were studied by thermogravimetric anddifferential scanning calorimetry (TG-DSC) methods. The structure of the products wasstudied by Powder X-ray diffraction (XRD) and the morphology of the resultingpowders was characterized by Scanning electron microscopy (SEM). The negativethermal expansion coefficient was calculated using the lattice constants obtained byPowderX software using the data collected at different temperatures by in Situ X-raymeasurement.The results showed that cubic ZrW_2-xMo_xO₈ (01．7 Mo_0．3 O₈ prepared bydehydration precursor route exhibits NTE property from 20℃to 600℃, and its averagethermal expansion coefficient is-6．16×10^-6 K^-1 . Cubic ZrWMoO₈ prepared bydehydration precursor route exhibits positive thermal expansion from 20℃to 10℃,and exhibits NTE property after dehydration the water, and its average thermalexpansion coefficient is-6．61×10^-6 K^-1 from 100℃to 600℃. Cubic ZrW_1．7 Mo_0．3 O₈prepared by hydrothermal method display NTE property from 20℃to 600℃, and its average thermal expansion coefficient is-6．61×10^-6 K^-1 . Cubic ZrWMoO₈ prepared byhydrothermal method possesses water, it will display NTE property after dehydrationthe water at 120℃, and its average thermal expansion coefficient is-5．73×10^-6 K^-1 from120℃to 700℃.In contrast with the dehydration precursor route, hydrothermal reaction is takenplace in higher temperature and higher pressure, so the hydrothermal method hasadvantages such as shorter synthetic period (15h), simple process, less strict operatedcondition and suitable for industrialized.Furthermore, by controlling the condition of crystal growth (gelling agents, acidmedia, additives and synthetic method), ZrW_1．7 Mo_0．3 O₈ powders with morphologies ofrod-like and square prism, ZrWMoO₈ powders with morphologies of cube-like,dumb-bell like, hexagonal prism and square prism, ZrMo₂O₈ powders withmorphologies of sphere and butterfly shape were obtained.