Study Of Size And Interface Effects On Phase Transition Of Nanomaterials

It is well known that the related properties (electronic, magnetic, optic, catalytic, thermodynamic and kinetic properties) of low-dimensional nanomaterials (nanoparticles, nanorods or nanowires, and thin films) are dramatically different from that of bulk due to high surface/volume ratio. These fascinating physicochemical properties of nanomaterials and their wide possibilities of using these properties in practice have attracted great interest. The current progress in nanotechnology has made the fabrication of individual nanocrystals possible, which in turn provides an opportunity to study the basic properties of nanomaterials. In the past years, following the pioneering work of Takagi in 1954, which experimentally demonstrated that ultrafine metallic nanocrystals melt below their corresponding bulk melting temperature, the thermal and phase stabilities of nanomaterials have been intensively studied experimentally and theoretically due to the industrial and scientific importance, such as the melting transition of nanocrystals, the glass transition of low-dimensional polymers and organic molecules, the Curie transition of ferromagnetic, ferrelectric nanocrystals, the Néel transition of antiferromagnetic nanocrystals and the critical transition of superconductive nanocrystals, etc.. These properties of thermal and phase stabilities are of concern for designing and governing materials for applications in practice. Thus, how to improve the thermal and phase stabilities is the significant project.Since many important physical and chemical processes of materials firstly happen at surfaces and interfaces, where the atoms or molecules are of different energetic states in comparison with that within the bulk materials, the interface situation should be the primary object to be considered. However, this effect has been ignored in the previous theoretical models, which results in the superficial understanding for the size dependence of the related properties of nanomaterials.To understand the above problems, there are two essential methods: Top-down and bottom-up methods. In this thesis, as one of top-down method, thermodynamics is introduced to study the region between macroscopic and microscopic ones. The application of thermodynamics to nanomaterials reveals a new branch of thermodynamics, i.e., nanothermodynamics.Recently, we have established simple and unified models for size and interface effects on thermal and phase stabilities of nanomaterials. In comparison with previously theoretical models, the characteristics of our model are: (1) Free of adjustable parameters; (2) full size range, especially the low size limit of several monolayers; (3) parameters within the model having clear physical meaning; (4) in a unified form for different substances. In addition, the interface and dimension effects on the related properties are introduced by the material- and interface situation-dependent parameterαand dimension-dependent critical size D0. The concrete contents are listed as following,(1). In chapter 2, extending Lindemann′s criterion for melting to vitrification transition, we establish a simple and unified thermodynamic model, without any adjustable parameter, for size and interface effects on glass transition temperature of nanoscaled glass with size D [Tg(D)]. According to this model, Tg(D) of free nanoscaled glass decreases with dropping D; while Tg(D) of thin films supported by substrates may decrease or increase with dropping D, which is determined by free surface and film/substrate interaction strength. When the film/substrate interaction is weak, such as van de Waals force, Tg(D) decreases as D is reduced; while it is contrary as the film/substrate interaction is strong, such as hydrogen bonding. Based on this model, the size- and interface-dependent thermodynamic and kinetic properties (viscosity, surface tension and thermal expansion coefficient, configuration entropy) of nanoconfined polymers has been deduced. Associated with above results, the roles of thermodynamics and kinetics in the glass transition of nanoconfined organic molecules are discussed detailedly. In addition, under the external pressure, the size and interface dependent glass transition temperature function has also been studied. The validity of model is confirmed by a large number of experimental results of nanoconfined polymers, organic molecules, and inorganic molecules.(2). In chapter 3, combining the thermal vibration instability model with the energy-equilibrium criterion between the spin-spin exchange interactions and the thermal vibration energy of atoms at Tc, a model for Tc(D) and TN(D) of FM and AFM nanocrystals has been developed. Not only the undercooling of Tc(D) and TN(D) but also the superheating phenomena induced by strong magnetic interaction at film/substrate interfaces have been interpreted. The model predictions are consistent with a number of availably experimental measurements of nanocrystals of Fe, Co, Ni, Gd, Ho, Co1Ni1, Co1Ni3, Co1Ni9, Fe3O4, MnFe2O4, CoO, NiO and CuO. Furthermore, on the basis of this model, we have deduced AFM thickness dependent blocking temperature function in exchange-biased FM/AFM bilayers (Fe3O4/CoO, NiO/NiFe, CoNiO/NiFe, IrMn/NiFe, Py/IrMn, IrMn/CoFe, FeMn/NiFe, MnPt/CoFe and FeF2/Fe bilayers) to discuss the thermal stability of exchange-biased systems. The agreements between the model predictions and available experimental results confirm the validity of this model.(3). In chapter 4, we established a simple and unified thermodynamic model, without any adjustable parameter, for size and interface effects on the spontaneous polarization [Ps2(D)] and the Curie temperature [Tc(D)] of low-dimensional ferroelectrics (thin films, nanowires and nanoparticles). In terms of this model, Ps2(D) and Tc(D) functions of perovskite ferroelectric nanosolids decrease or increase as D drops. This is determined by the mechanical interaction at ferroelectric film/substrate interfaces. It is shown that the tensile strain decreases the polarization and the Curie temperature while the compressive strain increases the polarization and the Curie temperature. The predicted results are consistent with the availably experimental evidences of BaTiO3 and PbTiO3 nanosolids.(4). In chapter 5, the finite size effect on the critical transition temperature of superconductive nanocrystals Tc(D) has also been obtained by incorporating the model of lattice vibration instability of nanocrystals into the simplified McMillan formula. According to this model, the Tc(D) of superconductive nanocrystals is predicted to decrease or increase with the dropping size of nanocrystals. This is dependent on the bond strength of interfacial atoms and the surface/volume ratio of nanocrystals. The predicated results are consistent with the available experimental results for superconductors MgBB2 and Nb thin films, Bi and Pb granular thin films and nanoparticles, Al thin films and nanoparticles.