Study On The Phase Transition And Negative Thermal Expansion Property Of A₂Mo₃O₁₂?A=Sc,Al,Fe,Cr?Prepared By Co-precipitation

Negative thermal expansion(NTE)materials have become a rapid growing research field due to its thermal shrinkage property,which have great application value in aerospace engineering,precision optics,chip packaging et al.Among the discovered NTE materials,A2Mo3O12(A=Sc,Al,Fe,Cr)have great potential application.Because they show a steady NTE property and have no moisture absorption in air.However,these NTE materials are usually prepared using solid phase method whose defects of high sintering temperature,long sintering time and high energy consumption are not conducive to the large-scale industrial preparation of these NTE materials.In addition,impurities are easily introduced during the ball milling process,and the volatilization of raw material MoO3 at high temperature will result in impure products.When A-site is occupied by Al,Fe and Cr,these molybdates adopt monoclinic structure above room temperature and exhibit positive thermal expansion.However,A2Mo3O12(A=Al,Fe,Cr)will take a phase transition to orthorhombic structure at a higher temperature and show negative thermal expansion,which limits the application of these materials.Therefore,it is significant to explore the low temperature preparation process of Sc2Mo3O12 and reduce the phase transition temperature of A2Mo3O12(A=Al,Fe,Cr)to extend their NTE temperature range.The Sc2Mo3O12 powder and A2-xScxMo3O12(A=Al,Fe,Cr)ceramics samples were prepared by the co-precipitation method using the commercially available Sc(NO3)3·6H2O,Al(NO3)3,Fe(NO3)3·9H2O,Cr(NO3)3·9H2O and(NH4)6Mo7O24·4H2O as raw materials.The low temperature preparation process of Sc2Mo3O12 was studied.The effect of scandium-substitution quantity on the crystal structure,morphology,phase transition temperature and thermal expansion properties of A2-xScxMo3O12(A=Al,Fe,Cr)ceramics were investigated.The crystal structure and the phase evolution of the samples were characterized using X-ray diffractometer(XRD)and Raman spectrometer(Raman spectra).Scanning electron microscope(SEM),X-ray photon-electron spectroscopy(XPS),high resolution transmission electron microscope(HRTEM)and energy dispersive X-ray detector(EDX)were performed for morphology analysis and confirmation of elemental valence,type and mapping.Thermal expansion analysis and phase transition temperature of samples were conducted by thermal mechanical analyzer(TMA).The Topas 5.0 software combined with variable temperature XRD were used to calculate the lattice parameters of samples at various temperature.The results indicate that:(1)Well-crystallized pure SC2Mo3O12 with an orthorhombic structure were synthesized by co-precipitation method followed by heat-treating at 200?.In contrast to the solid state reaction,this method is simple and needs lower temperature.The obtained SC2Mo3O12 shows sheet-like morphology.SC2Mo3O12 shows an anisotropic NTE property.The average CTEs along a,b,c axis are-6.28×10-6?-1,5.53×10-6?-1 and-4.28×10-6?-1 in 25?-700?.The average volumetric CTE ?v of Sc2Mo3O12 is-5.05×10-6?-1.The average linear CTE ?1 is calculated to be-1.68×10-6?-1(?v=3?1).(2)The series of A2-xScxMO3O12(A=Al,Fe,Cr)ceramics were successfully prepared by the co-precipitation method.The obtained samples have higher density compared with the samples prepared using the solid state reaction method.With increasing Sc3+ content,the phase transition temperature of A2-xScxMo3O12(A=Al,Fe,Cr)was reduced to below room temperature and the crystal structure has translated from monoclinic phase to orthorhombic phase.The NTE temperature ranges of A2Mo3O12(A=Al,Fe,Cr)were extended successfully.In detail,at room temperature,Al2-xScxMo3O12(0?x?0.3),Fe2-xScxMo3O12(0?x?0.8)and Cr2-xScxMo3O12(0?x?0.8)crystalize in a monoclinic structure;Fe0.8Sc1.2Mo3O12 and Cr0.8Sc1.2Mo3O12 consist of two phases;Alz-xScxMo3O12(0.4?x?0.5),Fe0.4Sc1.6Mo3O12 and Cr0.6Sc1.4Mo3O12 take orthorhombic structure.Especially,Al1.6Sc0.4Mo3O12,Feo.4Sc1.6Mo3O12 and Cr0.6Sc1.4Mo3O12 maintain stable orthorhombic structure and show great NTE property from 25? to 600?.Their average linear coefficients of thermal expansion are-2.02×10-6?-1,-5.80×10-6?-1 and-4.74×10-6?-1 respectively.The obtained three N T E materials have widely prospect in application.