A,B-sites Substitution On The Phase Transition And Thermal Expansion Properties Of A₂Mo₃O₁₂?A=Al,In,Cr,Fe?

Negative thermal expansion(NTE)materials shrink on heating and expand on cooling.They have broad application prospects in aerospace,microelectronic chip packaging,sensors,optical fiber communication and other precision manufacturing fields due to their abnormal thermal expansion properties.The new type of NTE materials of A2B3O12(A is a trivalent main group metal,transition metal and lanthanide metal;B is Mo and W)have excellent NTE performance and chemical flexibility.A2Mo3O12(A=Al,In,Cr,Fe)have attracted much attention due to their advantages of simple preparation process,low sintering temperature,strong NTE performance and no moisture absorption at room temperature.However,A2Mo3O12(A=Al,In,Cr,Fe)materials show positive thermal expansion with monoclinic structure at room temperature,and only when they transform into orthorhombic structure after phase transition at 191 ?,327 ?,372? and 504?,respectively,can they show excellent NTE,which is confined to a narrow high temperature range,and the density of these materials is generally low,which restricts their application in many fields.In order to solve the above problems,this thesis aims to tailor the high phase transition temperature of A2Mo3O12(A=Al,In,Cr,Fe)materials via single cation substitution at A or B position and double cations substitution at A and B positions so as to obtain new series of materials with excellent NTE performance at room temerature and above and broaden the range of NTE response temperature.Considering the elemental electronegativity,ion radius,crystal structure and hygroscopicity of the corresponding molybdates,Sc3+ and W6+are respectively chosen as substituted cations of A-site and B-site.The contents are mainly divided into the following five parts:1.The effects of A-site substitution by Sc3+ on the phase transition temperature and thermal expansion behavior of A2Mo3O12(A=Al,In,Cr,Fe)solid solutionsWhen the A-site cation in A2Mo3O12(A=Al,In,Cr,Fe)was substituted by Sc3+with low electronegativity,the series of A2-xScxMo3O12(A=Al,In,Cr,Fe)solid solutions were synthesized by solid-state reaction method.The effects of substituted Sc3+ content on the crystal structure,morphology,the phase transition temperature and NTE performance of A2Mo3O12(A=Al,In,Cr,Fe)were studied and the corresponding rules and mechanisms were revealed.The results indicate that Sc3+substitution successfully decreases the phase transition temperature and broadens the NTE response temperature range of A2Mo3O12(A=Al,In,Cr,Fe).The A2-xScxMo3O12(A=Al,In,Cr,Fe)solid solutions show better NTE properties.Futhermore,the phase transition temperature of A2-xScxMo3O12(A=Al,In,Cr,Fe)depends not on the electronegativity of the A-site cations,but on the differences of the electronegativity and radius between(A2-xScx)and Mo in A-O-Mo.2.Study on the micro thermal expansion properties and densification of A2-xScxMO3O12(A=Al,In,Cr,Fe)solid solutionsA2-xScxMo3O12(A=Al,In,Cr,Fe)solid solutions have been successfully prepared at 780?,but the low synthesis temperature and the characteristics of these molybdates result in the density of products is not high.The A2-xScxMo3O12(A=Al,In,Cr,Fe)solid solutions exhibiting excellent NTE properties at room temperature were selected for the densification study by investigating micro thermal expansion properties.In order to improve the density and enhance the stability of the NTE performance,the densification of selected A2-xScxMo3O12(A=Al,In,Cr,Fe)solid solutions was carried out with the following three methods.?Two-step sintering process:after sintering at 780?,the final sintering temperature was increased to 900-1200 ? for twice-sintering.?Sintering additive:to improve the densification,3wt%MgO sintering additive was added to the raw materials,and the formation of MgMoO4 with low melting point accelerated the densification process.?Liquid synthesis:the precursors were synthesized via chemical co-precipitation method,and the products were obtained after heat-treatment.The effects of different methods on the crystal structure,microstructure,phase transition and NTE properties of A2-xScxMo3O12(A=Al,In,Cr,Fe)solid solutions were analyzed.The results indicate that the density of A2-xScxMo3O12(A=Al,In,Cr,Fe)solid solutions is improved significantly and the NTE performance of the synthesized products are more stable.3.The effects of B-site substitution by W6+on the phase transition temperature and thermal expansion properties of A2M03O12(A=Al,In,Cr,Fe)solid solutionsWhen the A-site cation of isomorphic compounds A2B3O12(A=Al,In,Cr,Fe;B=Mo,W)is occupied by A1 and In,the phase transition temperature of corresponding tungstate is lower than that of molybdate.When A=Cr,Fe,metastable Cr2W3O12 and Fe2W3O12 are difficult to be synthesized.As mentioned above,A2Mo3-xWxO12(A=Al,In,Cr)solid solutions were synthesized by solid-state reaction with the goal of tailoring the phase transition temperature and broadening the NTE response temperature range of A2Mo3O12(A=Al,In,Cr)solid solutions.The effects of substituted W6+content on the crystal structure and morphology of A2Mo3O12(A=Al,In,Cr)materials were analyzed.The thermal expansion properties and structural phase transition of A2Mo3-xWxO12 solid solutions were studied through TMA.Furthermore,the corresponding rules and mechanisms on the phase transition temperature and NTE properties of A2Mo3O12(A=Al,In,Cr,Fe)were revealed.The results indicate that W6+substitution successfully decreases the phase transition temperature to some extend and broadens the NTE response temperature range of A2Mo3O12(A=Al,In,Cr,Fe).However,only the phase transition temperature of Al2Mo3-xWxO12 can be tailored to room temperature and below among the series of A2Mo3O12(A=Al,In,Cr,Fe)solid solutions.4.The effects of A,B-sites co-substitution by Sc3+ and W6+on the phase transition temperature and thermal expansion properties of A2Mo3O12(A=In,Cr)solid solutionsThe regulation of phase transition temperature is limited to a certain range when the B-site cation in A2Mo3O12(A=In,Cr)is substituted by W6+.On the basis of previous studies,the AB-sites cations of the parent material A2Mo3O12(A=In,Cr)are co-substituted by Sc3+and W6+ in an attempt to obtain series materials with better NTE performance.The effects of Sc3+and W6+co-substitution on the crystal structure,phase composition,morphology,phase transition and NTE performance of the A2Mo3O12(A=In,Cr)materials were studied.The mechanisms of tailoring the phase transition temperature and NTE properties of A2Mo3O12(A=In,Cr)by Sc3+and W6+co-substitution were also discussed.The results indicate that double cations substitution has more significant effects on decreasing the phase transition temperature of A2Mo3O12(A=In,Cr),compared with single cation substitution.5.Simulation of the phase transition process of A2-xScxMo3O12(A=Al,In,Cr,Fe)solid solutions based on density functional theoy(DFT)The experimental results reveal that Sc3+-substitution has a more effective influence on the regulation of the phase transition temperature of A2-xScxMo3O12(A=Al,In,Cr,Fe)solid solutions.Taking A2-xScxMo3O12(A=Al,In,Cr,Fe)for an example,the effects of Sc3+substitution on the phase transition temperature of A2Mo3O12(A=Al,In,Cr,Fe)solid solutions were simulated by theoretical calculation.The total energies of both orthorhombic and monoclinic A2-xSCxMo3O12(A=Al,In,Cr,Fe)with different Sc3+content were calculated by the first-principles method within the framework of density functional theory(DFT).The process of structural phase transition for A2-xScxMo3O12(A=Al,In,Cr,Fe)was simulated through analyzing the energy changes with composition at room temperature.The effects of substituted Sc3+content on the phase transition temperature of A2Mo3O12(A=Al,In,Cr,Fe)were determined by the theoretical calculation,which can be applied to confirming the experimental results.The results indicate that the theoretical calculation results agree well with the experimental results.