Researches Into Preparation Of Nano-Se With Different Morphologies And Sizes And Its Thermodynamics Properties Of Phase Transitions

The thermodynamic properties of phase transformation play an important role in the preparation and application of nanomaterials,but these thermodynamic properties of nanomaterials are significantly different from those of the bulk counterpart,which depends on the morphology and the particle size of nanomaterials.However,the researches of the effects of particle morphology and size on phase transformation of nanomaterials are still imperfect,especially,the effects of morphology and particle size on melting thermodynamic properties of nanorods,nanowires and nanotubes are not clear;in addition,the theoretical and experimental studies about the effect of particle size on the thermodynamic properties of sublimation of nanomaterials have not been reported.This paper will theoretically deduced the relationship between the melting and sublimation thermodynamic properties of nanomaterials and particle size,experimentally using selenium nanomaterials with different morphologies and particle sizes as the research system,combined with theory and experiment to clarify the influence regularities and mechanisms of size and morphology on the melting and sublimation thermodynamic properties.In terms of the melting thermodynamics of nanoparticles,to begin with,the relations between the melting thermodynamic properties of nanospheres,nanorods,nanowires and nanotubes,respectively,and the particle size were derived based on premelting model,and the influence regularities and influence mechanisms of shape and size on the melting thermodynamic properties of nanoparticles were discussed.Secondly,the melting of nano-Se was selected as the research object and the Se nanoparticles with different morphologies of nanosphere,nanorods,nanowires and nanotubes were prepared by the methods of liquid reduction,precursor conversion and ultrasonic irradiation,respectively,and the formation mechanisms of nano-selenium with different morphology were discussed.The crystal structure,morphology and size of nano-Se were characterized by powder X-ray diffraction(PXRD)and scanning electron microscopy(SEM).The melting curves of Se nanoparticles with different size and shapes were determined by differential scanning calorimetry(DSC);and the initial melting temperature,the temperature at the maximum of melting rate,the melting enthalpy and the melting entropy were obtained.Finally,the effect of morphology and particle size on the thermodynamic properties of the nanoparticles were discussed and compared with the corresponding theoretical analysis.In terms of the the sublimation thermodynamics of nanomaterials,firstly,relations between the sublimation thermodynamics and the radius of nanoparticles were deduced for the first time based on the strict criterion for phase equilibrium,and the influence regularities and influence mechanisms of size on the sublimation thermodynamic properties of nanoparticles were discussed.In experiment,the vapor pressure of Se nanoparticles with different sizes at different temperature were determined by DVS automatic gravimetric vacuum adsorber,then the sublimation enthalpy was obtained according to the fitting Clausius-Clapeyron equation,and furthermore,the sublimation temperature and sublimation entropy of Se nanoparticles with different particle sizes were obtained.The effect of particle size on the sublimation thermodynamic properties of Se nanoparticles were discussed and compared with the corresponding theoretical analysis.In addition,the surface tension of nanoparticles with different sizes was obtained using the Kelvin equation,and a new method for accurately determining the surface tension of nanomaterials was established.The conclusions are as follows,(1)The amount of reducing agent is the main influencing factor for the preparation of the spherical Se nanoparticles.The growth time,solvents and reaction temperature of amorphous selenium have significant effect on the morphology and particle size of Se nanowires.In the preparation of Se nanorods and nanotubes by ultrasonic irradiation,the selenium nanorods can be prepared under high acetic acid concentration and Se nanotubes can be prepared under low acetic acid concentration.1D t-Se nanostructures complies with a “solid-solution-solid” formation mechanism,while the conversion from a-Se nanospheres into t-Se nanospheres complies with the mechanism of quick phase transformation.(2)Both the shape and size have significant effects on the melting thermodynamic properties of nano-Se.The melting temperature,temperature at maximum rate of melting,melting enthalpy and melting entropy decrease with the size decreasing,and all these physical quantities present a linear variation with inverse radius.The effect of morphology on the thermodynamic properties of nano-Se is as follow: at the same diameter,Se nanotubes?Se nanospheres?Se nanorods?Se nanowires.These influence regularities are consistent with the corresponding theoretical analysis.(3)The particle size has significant effect on the sublimation thermodynamic properties of nano-Se.The sublimation temperature,sublimation enthalpy and sublimation entropy decrease with the size decreasing,and all these physical quantities present a linear variation with inverse radius.These influence regularities are consistent with the corresponding theoretical analysis.(4)The Kelvin equation can be applied to the sublimation of nanomaterials.The surface tension of nanomaterials can be accurately obtained by the Kelvin equation.The phase transition thermodynamics theories of nanomaterials and quantitative size-dependent thermodynamics of phase transitions can be applied to explain and predict the phase transformation behaviors involved in the preparations and applications of nanomaterials.Furthermore,the preparation method,the formation mechanism of selenium nanomaterials proposed in this paper can provide a new approach for the preparation of other nanomaterials.