Study On Preparation,structural Phase Transitions And Performances Of Nano-CuFe₂O₄ And Nano-Fe₂O₃ With Different Particle Sizes And Morphologies

The structural phase transitions of nanomaterials are often involved during the process of preparation and application of nanoparticles.But the structural phase transitions of nanomaterials exhibit a remarkable difference compared with their corresponding bulk materials,and the difference depends on the size of nanoparticles that compose the nanomaterials.Consideration of phase transitions has typically focused on size-dependence of solid-liquid phase transition,whereas relatively little research has been conducted on size-dependence of solid-solid phase transition.Although there are experimental data on the phase transition temperature,little is known about the quantitative relationships between the enthalpy and the entropy of structural phase transition of nanoparticles and the particle size.Furthermore,the influence regularities of morphologies effect on the thermodynamic properties of structural phase transition have been unclear so far.Therefore,it is of great value to study the thermodynamics of structural phase transition in nanoscale from theoretical as well as practical point of view.At first,in this paper,the general equations of thermodynamic properties of phase transitions for nanoparticles were presented by defining the surface chemical potential.Then the relations of the thermodynamic properties（the temperature,the enthalpy and the entropy）of structural phase transition and the particle size were derived based on a thermodynamics model of structural phase transition,which begins on the surface of nanoparticles.More,the influence of particle size and morphology on the thermodynamic properties of structural phase transition was discussed.Then,the structural phase transition of nano-CuFe₂O₄ and nano-Fe₂O₃ were taken as research systems in experiments.Comparing of different methods,the tetragonal CuFe₂O₄ with spherical morphology and different sizes were prepared by high temperature solid phase method,the γ-Fe₂O₃ with different particle sizes and morphologies were prepared by hydrothermal method and solvothermal method.The average particle size and the crystal structure were characterized by X-ray diffractometer,the morphology was characterized by scanning electron microscopy,the structural phase transitions of nano-CuFe₂O₄ from tetragonal to cubic and nano-Fe₂O₃ from γ to α with different sizes were determined by means of differential scanning calorimetry（DSC）,then the temperature,the enthalpy and the entropy of structural phase transitions were obtained,respectively.Furthermore,regularities of influence of particle sizes and morphologies on the thermodynamics of structural phase transition were obtained and compared with the corresponding theoretical relations.Finally,the effects of particle size on magnetic properties and electrochemical properties of nano-CuFe₂O₄ and nano-Fe₂O₃ were studied,respectively.The theoretical results indicate that,（1）The particle sizes of nanoparticles have a significant influence on the thermodynamic properties of structural phase transitions: the temperature,the enthalpy,the entropy of structural phase transition decrease with the decrease of particle sizes,and are linearly related to the reciprocal of particle sizes,respectively.（2）The morphologies of nanoparticles have a significant influence on the thermodynamic properties of structural phase transitions: with same equivalent particle size of nanoparticle,the order of thermodynamic properties of structural phase transition is sphere >icosahedron >dodecahedron >octahedron >cube >tetrahedron.When the particle size tends to infinity,the fitted lines of thermodynamic properties of the structural phase transitions of different morphologies and reciprocal of particle size have a tendency to meet at one point.The experimental results indicate that,（1）The prepared CuFe₂O₄ nanoparticles were nearly spherical by high temperature solid phase method.The CuFe₂O₄ nanoparticles with average particle sizes ranging from 31.7 to 79.0 nm were obtained by changing the calcination temperature,the calcination time and the types of solvent.The prepared Fe₂O₃ nanoparticles were nearly spherical by hydrothermal method.The Fe₂O₃ nanoparticles with average particle sizes ranging from 19.3 to 60.3 nm were obtained by changing the hydrothermal temperature,the amount of hydrazine hydrate,the types of surfactant,the types of reducing agent and the reaction concentration.By solvothermal method,the octahedral Fe₂O₃ nanoparticles with average particle sizes ranging from 78.8 to 650.9 nm were prepared by changing the amount of sodium hydroxide.（2）The particle sizes of nano-CuFe₂O₄ and nano-Fe₂O₃ have a significant influence on the thermodynamic properties of structural phase transitions,and the regulations are in agreement with the theoretical results.The morphologies of nano-Fe₂O₃ have a significant influence on the thermodynamic properties of structural phase transition: with same equivalent particle size,the temperature,the enthalpy and the entropy of structural phase transition of spherical nano-Fe₂O₃ are higher than those of octahedral,which are in agreement with the theoretical results.（3）Nano-CuFe₂O₄ and nano-Fe₂O₃ are ferromagnetic materials.The particle sizes have a significant effect on the magnetic properties.With the decrease of particle size of nano-CuFe₂O₄,the saturated magnetization decreases,the coercivity increases,the remanence decreases.With the decrease of particle size of nano-Fe₂O₃,the saturation magnetization decreases,the coercivity and the remanence decrease initially and increase afterwards.（4）Both nano-CuFe₂O₄ and nano-Fe₂O₃ are pseudo-capacitors.Their electrode reaction kinetics is controled by diffusion.The particle sizes have a significant effect on its electrochemical performance.With the decrease of particle size of CuFe₂O₄,the peak current increases gradually,the oxidation peak potential and the reduction peak potential shift positively,the potential difference increases and the contact resistance decreases.With the decrease of particle size of Fe₂O₃,the peak current increases gradually,the oxidation peak potential shifts positively and the reduction peak potential shifts negatively,the potential difference increases and the contact resistance decreases.The regulations of particle sizes and morphologies on the thermodynamic properties of structural phase transitions,magnetic properties and electrochemical properties can provide theoretical guidance for the study of preparation and application of nanomaterials.