Influence Of The Oxygen Vacancy On The Thermal Expansion Properties Of Molybdenum-containing Oxides And Ruthenates

Negative thermal expansion is a rare and abnormal physical phenomenon in nature.Compared with common positive thermal expansion and negative thermal contraction,the occurrence of negative thermal expansion exceeds the expectation of traditional lattice dynamics and it is also a new challenge for the traditional lattice dynamics.It is hoped that the materials and devices related to production and life have stable thermal expansion performance,that is,high thermal shock resistance,in order to prevent the adverse effects of material volume change with temperature.The thermal shock resistance of the material is inversely proportional to its thermal expansion coefficient,so the negative thermal expansion material or near-zero expansion material which can be used to offset the thermal expansion and cold contraction is very important.However,the negative thermal expansion materials that can meet the demand and be directly applied are very limited.Some materials have been reported have excellent negative thermal expansion properties,but their application in devices is seriously hindered due to the existence of inappropriate phase transition temperature or water absorption.The development of new materials often requires the discovery of new mechanisms.At present,people’s understanding of negative thermal expansion is limited.Especially,the new thermal shrinkage phenomenon and the change of phase transition temperature were found in some new ternary oxides,which cannot be explained by the existing thermal shrinkage mechanism.Therefore,the research of new physical mechanisms for thermal shrinkage is important.In this paper,the main clue is the effect of oxygen vacancy on the thermal expansion property of materials.The positive thermal expansion material molybdenum trioxide,the new type of giant negative expansion material calcium ruthenate system,the typical negative thermal expansion material scandium molybdate,and new material ZrMnMo3O12 with phase transition temperature higher than room temperature are selected as the research objects.By introducing oxygen atoms defects or changing the average effective electronegativity of samples,their effects on thermal expansion properties and phase transition temperature will be explored,and this will expand the understanding of the mechanism of negative thermal expansion.The details are as follows:1.Improve the thermal expansion and capacitance performance of molybdenum trioxide by introducing oxygen vacanciesThe original commercial molybdenum trioxide powder is annealed in a mixed atmosphere of hydrogen and argon for different times to prepare samples with different oxygen vacancies.Oxygen vacancies play an important role in modulating the electronic and optical properties of molybdenum trioxide.The results show that the unit cell volume of molybdenum trioxide powder has the property of positive thermal expansion when heated,and its a-and b-axes show normal thermal expansion,but the c-axis shows abnormal thermal contraction.By introducing oxygen vacancies,the expansion coefficients of a-,b-,and c-axes were reduced by about 33.60%,27.83%,and 4.84%,respectively,while the overall linear expansion coefficient was reduced by about 30.0%of the sample without oxygen vacancies.The decrease in thermal expansion is due to the introduction of oxygen vacancies at the Oa and Ot sites,which leads to the weakening of the Mo-O bond and the deformation of the MoO6 octahedron,which will lead to the enhanced stability of molybdenum trioxide.Due to the combined effect of band gap,conductivity and interlayer spacing,the introduction of oxygen vacancies also enhances electrochemical performance.As the content of oxygen vacancies increases,the light absorption of the sample in the visible to near-infrared region increases significantly,and the optical band gap decreases from 2.93 eV to 2.73 eV.In the energy storage properties of pseudocapacitors,the specific capacitance of the samples with oxygen vacancies increased by about 323%compared with the samples without oxygen vacancies.The introduction of oxygen vacancies provides an effective way to reduce the thermal expansion coefficient of materials without reducing material properties,at the same time,it will improve the thermal shock resistance of materials and make the application of them safer.2.The effect of oxygen vacancies on the thermal expansion properties of Ca2Ru0.9Ti0.1O4By annealing the original sintered sample Ca2Ru0.9Ti0.1O3.67 with oxygen vacancies at different temperatures with different times under oxygen atmosphere,samples with different oxygen vacancy concentrations were obtained.X-ray photoelectron spectroscopy and X-ray diffraction show that samples with oxygen vacancies have a S-CRO phase structure,while with excess oxygen have a L-CRO structure.The sample with excess oxygen exhibites huge positive thermal expansion characteristics,while the samples with oxygen vacancies exhibite thermal shrinkage properties from 100 to 673 K obtained from a dilatometer.According to the expansion properties of the samples annealed under different conditions,the oxygen vacancies in the material remain stable below 973 K.Compared with Ca2RuO4,where the oxygen vacancies will disappear after annealing at 773 K in air for 50 hours,the stability of oxygen vacancies is greatly improved.The orthogonal strain is reduced by an order of magnitude after doping 10%Ti and the intrinsic coefficient of thermal expansion is near zero.The intrinsic linear expansion coefficients before and after 260 K were 0.53×10-6 and 3.44 ×10-6 K-1,respectively.Compared with pure phase calcium ruthenate,the resistivity of Ca2Ru0.9Ti0.1O3.67 in the temperature range of 200 to 400 K has no obvious sign of metal-insulator transition,and the change in magnetic properties is very little.It shows antiferromagnetic order at about 110 K,and the ferromagnetic track order component also appears in the antiferromagnetic phase below 110 K.This work further promoted the understanding of the relationship between oxygen vacancies and negative thermal expansion,and demonstrated the important research and development value of calcium ruthenate series materials.3.Effectively reduced phase transition temperature and expansion coefficient of SC2Mo3O12 by introducing oxygen vacanciesOxygen vacancies were generated in SC2Mo3O12 by hydrogen thermal annealing.Detailed oxygen content,structure information and the mechanism of reduce phase transition temperature and CTE are revealed by XPS,X-ray diffraction,and Raman spectral analyses.X-ray photoelectron spectroscopy shows that annealing under 5%hydrogen atmosphere at different temperatures could form Sc2Mo3O11.97 and Sc2Mo3O11.27.When the oxygen atoms removed,the remaining excess electrons in the material will be mainly concentrated around the Mo atom,which is equivalent to increase the averaged effective electronegativity of the position of M in the A2M3O12 series materials.It is found that the introduction of oxygen vacancies decreases significantly its phase transition temperature from 178 to below 93 K,favoring the NTE extending to lower temperatures.The introduction of oxygen vacancies reduced CTE from-4.28 ×10-6 to 0.13 × 10-6 K-1 to realize a near zero expansion properties after phase transition.It is further confirmed by hole doping experiments that oxygen vacancy has an important effect on the phase transition temperature of the sample.The results show that the phase transition temperature of Sc2Mo3O12 can be effectively reduced by introducing oxygen vacancy,and the thermal shock resistance can be further increased.This study provides a new and effective way to reduce the phase transition temperature of negative thermal expansion materials.4.The change of averaged effective electronegativity on the thermal expansion and phase transition property of Zr1+xMn1-xMo3-2xV2xO12In order to further confirm the influence of the change of averaged effective electronegativity on the phase transition temperature of the sample,ZrMnMo3O12 with a phase transition temperature greater than room temperature was selected as the original material.The solid solution of Zr1+xMn1-xMo3-2xV2xO12(0 ≤ x≤0.5)was synthesized by solid phase reaction.The phase transition temperature of ZrMnMo3O12 is effectively reduced from 362 K to 160 K by introducing V5+and controlling the proportion of other elements.When x=0 and 0.1,the material conforms to the monoclinic phase(P21/a space group)structure at room temperature,and when x≥ 0.2,it is the orthogonal phase(Pbcn space group)structure with negative thermal expansion properties.The main reason for the significant decrease in phase transition temperature is the decrease in averaged effective electronegativity,which can increase the repulsive force between them by accumulating more charges on oxygen ions,so that the structure of the material can be at a lower temperature collapse and reconstruction.Variable temperature Raman spectroscopy test shows that the negative thermal expansion mechanism of the material is caused by low-frequency phonons,and the change in the averaged effective electronegativity enables Zr1.5Mn0.5Mo2VO12 to achieve coverage in a wide temperature range(from 160 to 673 K)with a negative thermal expansion performance at room temperature.On the other hand,the research also proves that the increasing the repulsive force between oxygen atoms caused by the generation of oxygen vacancies and doping is the important factor in the regulation of the phase transition temperature of the material.