Synthesis And Properties Of Negative Thermal Expansion Materials ZrV_2-xMo_xO_7+?and Zr_1+xMn_1-xMo_3-2xV_2xO₁₂

In daily life,most of the materials have the properties of thermal expansion and cold shrinkage,but there are also a few materials with the opposite properties,which are called negative thermal expansion materials.The thermal stress experienced can be fundamentally reduced or eliminated by combining the positive thermal expansion materials and the negative thermal expansion materials,or we can use the doping method to adjust the thermal expansion coefficient of the material.This can effectively avoid damage to the instrument caused by thermal stress.So far,although many negative thermal expansion materials have been found and prepared,there are still many defects in these materials.For example,most of them have defects such as water absorption,high initial temperature of NTE,meta-stability and so on,affecting the expansion of materials in the application.So they can not meet the needs of people's actual application.Therefore,solving the existing problems of materials,improving their performance and making them meet the requirements of practical application are still the top priorities in the research of negative thermal expansion materials.The main contents of this paper are as follows:1.Realizing isotropic NTE covering room temperature by breaking the superstructure of ZrV2O7.ZrV2O7 is a well-known isotropic negative thermal expansion material.However,the negative thermal expansion property of ZrV2O7 can only be observed in high temperatures above 375 K.The primary goal of the our work is to reduce the phase transition temperature and extend the negative thermal expansion of ZrV2O7 to well below room temperature by breaking the superstructure through Mo substitution for V.To realize this,we used a solid state reaction route followed by quenching of the samples.Mo instead of V has realized the 1x1x1 cubic structure of ZrV2O7 from room temperature,which broadens the negative thermal expansion temperature zone of ZrV2O7.XRD and high-resolution synchrotron radiation X-ray diffraction(Japan,?=0.495724 A)revealed detailed structure information,indicating that doping did not change the structure of ZrV2O7.With the increase of doping ratio,the structure of Pa-3 space group was kept unchanged.The results of thermal dilatometer and variable temperature XRD show that the phase transition temperature of ZrV2O7 is decreased by replacing V with Mo.The results of temperature-dependent XRD agree well with the dilatometry measurements,confirming that the Mo substitution can effectively reduce the phase transition temperature of ZrV2O7 and result in an intrinsic NTE over a wider temperature range.The onset of NTE appears at about 350,250 and 225 K,for x= 0.1,0.3 and 0.5,respectively.It is worth noting that the material of ZrV1.5Mo0.5O7+? retains actually near-zero thermal expansion below 225 K,till at least 123 K.Neutron powder diffraction revealed that the incorporation of Mo promoted the V-O2-V/Mo angle to extend from 1600 to 180°,which was beneficial to the destruction of the superstructure and made the ZrV2-xMoxO7+? have negative thermal expansion characteristics at room temperature.In order to get insight into the phonon anharmonicity and its contribution to the negative thermal expansion of the Mo substituted ZrV2O7,we carried out pressure dependent Raman spectroscopic study for ZrV2-xMoxO7+?.Pressure induced phase transition and amorphization were also revealed in the Mo substituted ZrV2O7.In contrast to previous framework structures where low frequency phonon modes mainly contribute to the NTE,several high frequency phonon modes are found to have negative Gruneisen parameters and hence contribute to the NTE as well in the our structure.2.Negative thermal expansion and phase transitions in Zr1+xMn1-xMO3-2xV2xO12.The negative thermal expansion material Zr1+xMn1-xMO3-2xV2xO12 was prepared by solid phase sintering.Our research group has developed a new type of negative thermal expansion material ZrMnMo3O12,which has a monoclinic structure at room temperature.But as the temperature goes up,the structure changes to orthogonal structure,and the space group is No.62 Pnma.For the orthogonal ZrMnMo3O12,the b-axis of the lattice parameters increase with the increase of temperature,but the a-axis and c-axis lattice parameters decrease with the increase of temperature,and the overall volume shows a trend of shrinkage.Although ZrMnMo3O12 is a good negative thermal expansion material,its phase transition temperature is too high,which seriously affects its application in real life.Therefore,we adopt the method of doping V and adjusting the Zr:Mn ratio in the compound to adjust the phase transition temperature and thermal expansion coefficient of ZrMnMo3O12.XRD test shows that the structure of Zr1+xMn1-xMo3-2xV2xO12 is consistent with that of ZrMnMo3O12.With the increase of doping ratio,the spatial structure remains unchanged,but the doping of high proportion cannot be realized.In order to obtain the expansion property of the compound,we tested the sample with thermal dilatometer.The test results showed that with the increase of doping ratio,the thermal expansion property of the material at room temperature was changed,and the phase change temperature gradually decreased,but the doping had little effect on the phase change temperature.The coefficient of thermal expansion near room temperature decreases with the increase of doping ratio,and the temperature range of low thermal expansion increases gradually.