| The volume of some solid materials in nature shrink with increasing temperature and expand with decreasing temperature,a phenomenon known as negative thermal expansion.A2M3O12 series materials are the most promising negative thermal expansion materials,and Sc2Mo3O12 is one of the most excellent materials.Its phase transition temperature is as low as-93℃,far below room temperature.No phase transition at room temperature and no hygroscopicity,with a large potential for application.At present,Sc2Mo3O12 is mostly prepared by the solid phase reaction,but this method has the disadvantages of long cycle time and high energy consumption.In contrast,the hydrothermal method is simple,with low reaction temperature and high preparation purity.Meanwhile,the reaction and crystal growth can be effectively controlled during the hydrothermal method by changing the hydrothermal temperature,time and adding surfactant,which is a good method for preparing crystals at low temperature.Therefore,in this paper,Sc2Mo3O12 is prepared by hydrothermal method and the growth mechanism of Sc2Mo3O12 is investigated.At the same time,the preparation process of small size SC2Mo3O12 and the effect of different morphological dimensions on the properties of Sc2Mo3O12 is investigated.In this paper,Sc2Mo3O12 powder particles were prepared from commercially pure Sc(N03)3·xH2O and(NH4)6Mo7O24·4H2O with the addition of different surfactants assisted by hydrothermal method.Crystal growth is modified by controlling the experimental reaction by changing the hydrothermal temperature,time,surfactant content and other conditions.The phase composition,elemental distribution and microscopic morphology of the Sc2Mo3O12 samples were characterized by a combination of XRD,SEM,HRTEM and EDX.The negative thermal expansion properties of the Sc2Mo3O12 samples were characterized by variable temperature XRD and TMA.(1)Orthorhombic Sc2Mo3O12 was prepared using polyvinylpyrrolidone(PVP)-assisted hydrothermal reaction.PVP plays a key role in the crystal growth process.At the early stage of the reaction,PVP reacts with Sc3+to form micelles to guide crystal growth.At the later stage,Sc3+is released to combine with MoO42-to form precursor nuclei,and PVP attaches to the precursor surface to prevent the particles from aggregating and attaching during growth.After annealing at 800℃,pure chocolate-like orthorhombic Sc2Mo3O12 powder particles were obtained with regular particle morphology and uniform distribution.Sc2Mo3O12 particles exhibited stable and anisotropic NTE in the same temperature range.The axial CTEs of Sc2Mo3O12 are αa=-4.37×10-6 ℃-1,αb=3,61×10-6 ℃-1 and αc=-2.96×10-6 ℃-1,and the linear CTE is-1.22×10-6 ℃-1.(2)Sc2Mo3O12 was successfully prepared using sodium dodecyl benzene sulfonate(SDBS)-assisted hydrothermal reaction.Experimental investigation revealed that changing the Mo/Sc ratio has a large effect on the growth of Sc2Mo3O12.Control other conditions constant and increase the content of Mo.When MoO42-is excessive,a large amount of MoO42-adheres to the precursor surface and changes the surface energy of the crystal surface,which makes the crystal selectively grow to get round flaky particles.Sc2Mo3O12 particles exhibited stable and anisotropic NTE in the temperature range.The average CTEs along the a,b and c axes are-7.04×l0-6 ℃.1,5.62×10-6℃-1 and-4.77×10-6℃-1,respectively.The linear CTE is-2.07×10-60℃-1.(3)Sc2Mo3O12 was successfully prepared hydrothermally by adding cetyltrimethylammonium bromide(CTAB).The experimental investigation revealed that the content of CTAB has a great influence on the microscopic morphology and size of Sc2Mo3O12 particles.When the CTAB content was 0.05 g and 0.1 g,most of the Sc2Mo3O12 particles showed irregular nanoflake shape and a small amount of rhombic flake particles with small size.When the CTAB content was further increased to 0.4 g and 0.8 g,the irregular nanoflake particles basically disappeared and the rhombic flake particles increased in size.By increasing the hydrothermal time and temperature,the powder particles grow into rhombic flake particles with similar shape and size and uniform distribution through Oswald ripening.At the same time,when the Mo:Sc ratio was changed to 3:1,the particles showed an elliptical flake stacked morphology and larger size.The average CTEs along the a,b and c axes are-6.64×10-6℃-1,5.71×10-6℃-1 and-4.59×10-6℃-1,respectively.The average linear CTE is about-1.85×10-6℃-11.(4)Flower-like Sc2Mo3O12 powder particles were successfully prepared by the hydrothermal method assisted by the addition of ethylene glycol.It was found that without the addition of ethylene glycol(EG),the single crystal nanosheets formed flower-like particles by surface-surface contact superposition.The addition of EG effectively coated the Sc2Mo3O12 precursor as an organic solvent and surfactant,inhibited the adsorption of NO3-and NH4+,shielded the surface charge of the crystal,and caused the single crystal nanosheets to undergo disordered self-assembly to form hollow flower-like cluster particles.The morphological analysis of the products at different reaction time stages showed that the formation of clustered flower-like Sc2Mo3O12 particle structures followed the Oswald ripening.Sc2Mo3O12 particles of 1-2μm were successfully prepared by adding 60 ml EG at a hydrothermal temperature of 120℃ for 2 h.Meanwhile,the results of variable temperature XRD shows that the Sc2Mo3O12 samples exhibited anisotropic NTE property and remained stable in the temperature interval.The average CTEs of Sc2Mo3O12 along the a,b and c axes are-6.73×10-6℃-1,4.98×10-6℃-1 and-4.78×10-6℃-1.The mean linear CTE is about-2.21×10-6℃-1. |
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