Study On The Adjustment Of Negative Thermal Expansion Properties Of Fe₂M₃O₁₂ By （NaM）³⁺（M=Mn,Ni,Mg,Co） Dual-cations Substitution

Thermal expansion of solid materials can be divided into positive thermal expansion and negative thermal expansion(NTE).With the increase of temperature,the volume of solid material will have the phenomenon of "thermal shrinkage and cold expansion",that is,NTE materials.NTE materials have been widely concerned in the fields of microelectronics,optics,materials science,and aerospace due to their unique NTE properties.Among the discovered NTE materials,A2M3O12 series materials have attracted much attention due to their numerous members,flexible A3+site chemical regulation and excellent and stable NTE performance.Although A2M3O12(A=Fe,Al,In,Cr;M=Mo,W)materials are not hygroscopic,it will undergo a phase transition from monoclinic to orthrhombic on heating.After the phase transition,it will show NTE properties.This phase transition will cause a sudden change in thermal expansion performance from positive to negative,affecting the actual application of the material.Among them,the phase transition temperature of monoclinic Fe2Mo3O12 is the highest,up to 499 ℃,so Fe2Mo3O12 is a monoclinic structure with positive thermal expansion coefficient in a large temperature range.How to reduce or eliminate the phase transition temperature of Fe2Mo3O12 and extend its NTE temperature range has become a subject worthy of study.In order to solve the high temperature phase transition of Fe2Mo3O12 and broaden its NTE temperature range,this work aims to solve these problems by dual-cations substitution of Fe3+.In this paper,the(NaM)xFe2-xMo3O12(M=Mn,Ni,Mg,Co)series materials were prepared by solid-state reaction using commercially available analytically pure Na2CO3,MO(M=Mn,Ni,Mg,Co),Fe2O3 and MoO3 as raw materials.The phase composition,crystal structure,morphology and thermal expansion property of(NaM)xFe2-xMo3O12(M=Mn,Ni,Mg,Co)ceramics were studied using X-ray diffraction,X-ray photoelectron spectroscopy,high-resolution transmission electron microscopy with EDX spectrometer system,scanning electron microscopy,thermo-mechanical analyzer and X-ray diffraction equipped with high-temperature accessories.The results are as follows:(1)A series of(NaM)xFe2-xMo3O12(M=Mn,Ni,Mg,Co)ceramics materials have been successfully prepared by the solid-state reaction.The results show that(NaM)3+(M=Mn,Ni,Mg,Co)dual-cations have successfully partially replaced Fe3+.With the increase of the doping ratio of(NaM)3+dual-cations,the crystal structure has undergone component-induced structural phase transformation.Fe2Mo3O12 presents a monoclinic structure at room temperature.With the incorporation of(NaM)3+dual-cations,when x=0.25,(NaM)3+dual-cations partially replaces Fe3+,and the monoclinic Fe2Mo3O12 partially transforms into hexagonal(NaM)xFe2-xMo3O12,and the monoclinic Fe2Mo3O12 and hexagonal(NaM)xFe2-xMo3O12 coexist in the sample.With the continuous increase of(NaM)3+dual-cations doping to 0.5,the monoclinic Fe2Mo3O12 has been completely transformed into a single hexagonal(NaM)xFe2-xMo3O12 ceramics.When x=0.75,the solid solution limit is reached,and monoclinic MMoO4(M=Mn,Ni,Co)impurities appear in samples.However,when the amount of(NaMg)3+dual-cations is increased to 0.75,the hexagonal(NaMg)0.75Fe1.25Mo3O12 has no impurity peak and is still a pure single phase.When x=1,the solid solution limit is reached and monoclinic MgMoO4 impurity appears.SEM analysis shows that the successful substitution of(NaM)3+dual-cations for Fe3+can effectively improve the density of Fe2Mo3O12 ceramics.(2)The monoclinic Fe2Mo3O12 undergoes a phase transition from monoclinic to orthorhombic around 508.1℃,and shows a diametrically opposite thermal expansion behavior before and after the transformation.It shows positive thermal expansion before phase transition,and the coefficient of thermal expansion of monoclinic Fe2Mo3O12 is 13.48×10-6℃-1 in the range of 30-500℃.After phase transition,it shows NTE,and the coefficient of thermal expansion of the orthorhombic Fe2Mo3O12 is-13.27×10-6℃-1 in the range of 530-650℃.When the amount of(NaM)3+dual-cations is 0.25,the monoclinic Fe2Mo3O12 and hexagonal(NaM)xFe2-xMo3O12 coexist,and the NTE performance is unstable.Because Fe2Mo3O12 content is small,its phase transition is hardly observed.When x=0.5,the monoclinic Fe2Mo3O12 is completely transformed into a single hexagonal(NaM)xFe2-xMo3O12 ceramics.The hexagonal(NaM)0.5Fe1.5Mo3O12 shows excellent and stable NTE properties,and its average linear expansion coefficient is-8.64×10-6 ℃-1,-7.69×10-6℃-1,-8.44×10-6℃-1 and-8.56×10-6℃-1 in a wider temperature range of 30-650℃.When the amount of(NaM)3+dual-cations is increased to 0.75,the sample still maintains stable NTE.Because monoclinic MMoO4(M=Mn,Ni,Co)impurities formed in the samples and monoclinic MMoO4(M=Mn,Ni,Co)shows positive thermal expansion,so the overall NTE coefficients of the samples decreases.In addition,when the(NaMg)3+dual-cations increases to 0.75,no impurity phase appears in the sample.The hexagonal(NaMg)0.75Fe1.25Mo3O12 ceramic shows excellent and stable NTE performance in a wider temperature range of 30-650℃,with an average linear expansion coefficient of-12.33×10-6℃-1.Because the M2+site ion with larger ion radius than Fe3+has successfully replaced Fe3+,and Na+is located in the channel of crystal structure,resulting in lattice distortion and formation of new phases,hexagonal(NaM)xFe2-xMo3O12 materials show more excellent and stable NTE properties.