Size,Dimension And Composition Effects On Several Properties Of Low Dimension Nanomaterials

Nanomaterials have attracted great interest due to their significant physical and chemical properties which different from those of the bulk materials, including optic, magnetic, electronic and thermodynamic properties. The research on state changes of condensed matter as melting and freezing is very important. Following the pioneering experimental work of Takagi in 1954, which verified 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, such as melting temperature, freezing temperature, cohesive energy, Debye temperature and order-disorder transition temperature of metallic nanomaterials, etc. These properties of thermal and phase stabilities are of concern for designing and governing materials for applications in practice. Thus, it is necessary to investigate how to improve the thermal and phase stabilities.In virtue of the extension of the classical thermodynamics theory to nanometer scale, a new interdisciplinary theory – “nano-thermodynamics” can be generated, which is a bridge of bulk and nanomaterials. In this thesis, in view of the nano-thermodynamics, the size, dimension and composition effects on thermal and phase stabilities of nanomaterials are investigated.The main contents of this paper are as follows:1. Liquid-to-crystal transition temperature Tf(r)(freezing temperature) and the crystal-to-liquid transition temperature Tm(r)(melting temperature) are important. A clear understanding of the structure and thermal behavior of these nanomaterials is worthwhile. The most significant structural change originates from a solid-to-liquid phase transformation, namely, reasonable information on the solid-to-liquid transition is necessary. A thermodynamically quantitative model has been developed to calculate Tf(r) of metallic and semi-metallic nanoparticles. With the size dropping, Tf(r) decreases and the difference between Tm(r) and Tf(r) also decreases until it disappears at the critical radius r = 2r0. Both the phenomena attribute to the increasing surface/volume ratio with the size dropping and surface atoms plays an important role. Consistency between the model predictions and experimental results confirms that the model could be expected to be a valid approach to analyze the freezing behavior of nanomaterials.2. With the miniaturization of devices, one has to envisage an insurmountable stability problem and understanding their thermodynamic properties is essential both for practical applications and from a fundamental point of view. To resolve this limit, nanoscaled bimetallic alloys have been of special interest because of higher stability. The effect of size, dimension and composition on melting temperature of nanoscaled bimetallic alloys was invested by considering the interatomic interaction. The established model without any arbitrarily adjustable parameters can be used to predict the melting temperature of nanoscaled bimetallic alloys. It is found that, the melting temperature of nanoscaled bimetallic alloys decreases with the size decreasing. The interatomic interaction will change with the changing of size, composition and dimension, which determines the variation of the melting temperature. Moreover, for the nanoscaled bimetallic alloys with the same size and composition, the dependence of melting temperature on the dimension can be sequenced as follows: nanoparticles > nanowires > thin films. The accuracy of the developed model is verified by the recent experimental and computer simulation results.3. Melting temperature Tm is an important physical quantity accounting for the thermodynamics of materials, by which we can derive almost all thermodynamic properties of materials. According to the established model for size, dimension and composition effects on melting temperature, the cohesive energy, Debye temperature, and order-disorder transition temperature of nanoscaled bimetallic alloys are investigated. The model predictions are consistent with availableexperimental data and other theoretical results. This study has provided new insights into the basic understanding of the physicochemical properties in nanoscaled bimetallic alloys and potential applications in nano-devices.