Study On Preparation And Properties Of ZrW₂O₈ Powders, Composites And Thin Films

Since 1996 ZrW₂O₈ has become established as one of the more important materials showing negative thermal expansion（NTE）.In particular,ZrW₂O₈ has one of the highest negative coefficients of thermal expansion,-9×10^-6/℃,of NTE materials, the NTE is isotropic（because the crystal structure is cubic）,and the NTE exists over a wide temperature range,（-273 to 770℃）.A broad range of applications has been suggested for these NTE materials.One of the most obvious is the use in composites where the thermal expansion coefficient of the bulk material can be tailored to a specific value by combining a NTE material with a positive thermal expansion material.This may result in new materials with precisely matched expansion,being positive, negative or even zero,by adjusting the weight fraction of the different phases.These controlled expansion composites can be used in high precision optical mirrors,fibre optic systems,electronic devices or as a thermal package for fibre Bragg gratings,etc.The preparation and thermal expansion properties of ZrW₂O₈ powders,composite and thin film were studied with the finance support of National Nature Science Foundation of China（50372027,50442023）,Nature Science Foundation of Jiangsu Province（BK2003404）.The process of ZrW₂O₈ was optimized and explored to synthesize pure ZrW₂O₈ powders.ZrW₂O₈ composites,especially with near zero thermal expansion,were prepared.The ZrW₂O₈ thin film was prepared.And the effect of preparation parameters on synthesizing ZrW₂O₈ thin film was also discussed.The stress-temperature field of ZrW₂O₈ thin film and substrate with positive thermal expansion was simulated.The ZrW₂O₈ powder is prepared using chemical precipitation route and combustion method.The ZrW₂O₈ precursor was prepared via chemical precipitation route and heated at 1150℃for 1.5h,then high purity cubic ZrW₂O₈ can be obtained.The time is shorter than that needed in solid state method.The best molar ratio of Zr/W in raw materials is 1:1.8 to synthesize pure ZrW₂O₈ powder.The mean size of ZrW₂O₈ powder is 0.5μm.The precursor transformed to the fine-mixed monoclinic WO₃ and ZrO₂ in heating,then to cubic ZrW₂O₈ at 1150℃.Zr（OH）₄ and（NH₄）₂WO₄ were two intermediate products in chemical precipitation process and were heated at high temperature to synthesize ZrW₂O₈ powder.The effects of the difference of the furnace temperature,the contents of H₃BO₃ and（NH₂）₂CO,and the molar ratio of W⁶⁺and Zr⁴⁺on the synthesis process of ZrW₂O₈ were studied.The high pure ZrW₂O₈ powders, whose mean particle size is～0.5 microns,can be obtained via combustion method after combined analyses with XRD,SEM and infrared spectrum.The best prescription of combustion-synthesizing ZrW₂O₈ is following.The furnace temperature is 500℃. The molar fraction of H₃BO₃ is 0.10.The mass ratio of（NH₂）₂CO and （NH₄）₅H₅[H₂（WO₄）₆]·H₂O added with ZrOCl₂·8H₂O is 2:1.And the molar ratio of （NH₄）₅H₅[H₂（WO₄）₆]·H₂O and ZrOCl₂·8H₂O is 1:3.2.The linear thermal expansion coefficient of ZrW₂O₈ which was measured by thermal expansion dilatometer is -5.08×10Z^-6K^-1.The function of linear thermal expansion changing and temperature matches the equation dL/L₀/%= - 1.4×102-4.5 10^-4T/℃.In this work,ZrO₂/ZrW₂O₈ composites with very low thermal expansion coefficient were successfully synthesized using ZrW₂O₈ synthesized in our lab and ZrO₂ at various weight ratios.The coefficient of thermal expansion of composite was decreased with the increasing weight fraction of ZrW₂O₈.The thermal expansion of the composites can be controlled,being positive,negative or even zero,the composite shows almost zero thermal expansion when the weight ratio of ZrO₂ to ZrW₂O₈ is 2/1. Small amount of Al₂O₃ additive（c.a.0.25wt%） can increase the density of the composites,but decrease the NTE effect of ZrW₂O₈ in composites.The thin films were deposited in a radio freqency magnetron sputtering system on quartz and wafer substrate using high pure ZrO₂ and WO₃ mixture target.The effects of different process parameters,for example,sputtering atmosphere,sputtering power, sputtering pressure,bias voltage,and post-heat-treatment on preparing ZrW₂O₈ thin films were studied.The thickness of as-deposited thin film will be thinner with the decreasing of Argon concentration in sputtering atmosphere.The relationship of the thickness of thin film with sputtering time and the deposited rate with sputtering power is near linear.When the sputtering pressure and bias voltage is increased,the surface of film becomes roughness.The as-deposited films are amorphous and need the post-heat-treatment.The higher purity ZrW₂O₈ thin film can be obtained at 720℃more than those at other temperatures.The morphology of the film heated at 700-750℃with SEM shows that the film is very dense and there is no flaw in it.When the oxygen is flowed in heat treatment,it will improve the crystallization of film.On the basis of the variation of interplanar distance of crystal plane（2 1 1） at different temperatures,the coefficient of thermal expansion of ZrW₂O₈ thin film can be calculated.The coefficient of thermal expansion of ZrW₂O₈ thin film is -2.54×10^-5/℃.The stress field and deformation distribute are studied using Finite Elements Method on the basis of experiments.When the substrate is restrained,the system has fall-sector deformation.When the substrate is not restrained,the system has drum deformation. The restrained film-substrate system was chosen to analyze according to in practice. When the temperature is same in the whole system,the maximal（or minimal） stress value and deformation is linearly increased with the increasing temperature.The maximal stress in film-wafer system is larger than that in film-quartz system,but the maximal deformation is reverse.The film-wafer system is well chosen if the critical value of binding force between film-substance is the main law to judge the stripping of film-substance.