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The Effect of Interface on Material Thermodynamic Characters

Author: GuHongJu
Tutor: JiangQing
School: Jilin University
Course: Materials Science
Keywords: Nanocrystals melting Size effect Interface Embedded
CLC: TB301
Type: Master's thesis
Year: 2009
Downloads: 121
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Abstract

When the material size reduces from the bulk to a nanometer size, the material optical quality, the electricity nature, magnetism nature, the mechanical properties and so on present a new change in the physical properties different from bulk crystals and single molecules. In the above various types of properties, the melting temperature of nanocrystals has been studied hotly, because the melting phase change is an important process of material. Since 1954, Takagi confirmed in the experiment of metal nanocrystals melting temperature is lower than the corresponding bulk metal, more and more experiments had proven that not only the metal, but also semiconductors, ceramics, organic crystals, etc., which are in line with the law, namely their temperature of fusion reduces along with the size change. And one kind of situation is an exception, when the surface conditions change, there may be overheating. It means that crystal melting temperature decreases with the size increased. This is in terms of embedded particles, so the melting temperature can not be dogmatic view of the size effect, both may be lower than bulk melting temperature and higher than bulk melting temperature. Except the melting temperature of nanocrystals thermodynamic properties, the other thermodynamics performance research is also by far insuffieient, for example the melting entropy, the melting enthalpy, the binding energy etc., need more thorough research to promulgate the nanometer thermodynamics essence.Because a nanometer size is situated between macroscopic and a microscopic, there are two -pronged approach to understand the size effect of nano-crystalline materials. From a macro perspective is more simple and effective, because there’s still a lot of ready-made thermodynamic theory for reference. The thermodynamic theory has been developed for more than two centuries, the long history development, the reliable research methods, the simple theoretical basis and a broad universality, it is more suitable from the point of view to study the size effect of nanocrystals. Nanomaterials due to its ultra-fine grain, containing a large number of internal interfaces, so that a large number of atoms at the interface between the grains, as compared with the bulk material having a unique structural features. As a result of this characteristic, nanomaterials display a series different essential distinction performance with the ordinary bulk polycrystalline materials and amorphous materials. Especially its superior physical, chemistry and the mechanical properties, supplied the possibility of improving work performance and developing new high-performance materials. For this reason, nanomaterials are attracted more and more attention, and even set off a wave around the world boom.Among many melting theories, Lindemann melting criterion has so far gone through the test of nearly a century since it’s proposed, even though it is an empirical theory, it is still playing its formidable role in researching melting behavior of material. Experimental results show that, Lindemann melting criterion applies not only to study the melting process of bulk materials, but also apply to study the melting of nanocrystals. Therefore, from this theory, you can further explore the nanocrystals from melting theory and research the reducing the size impact on the melting properties. In order to describe the melting behavior Professor Shi further expanded the Lindemann melting criterion, deduced a model of melting temperature related crystal size, which well explains both the situation of reduced nanometer crystal melting point（underheating）, similarly could also explain the situation of elevated nanometer crystal melting point （overheated）. For the free surface nanoparticles and the nanoparticles substrate with non-coherent interface, the ratio of atoms at the surface （interface） increase with the crystal size decreases, at this time the oscillation amplitude of superficial atoms is bigger than that of the substrate internal atoms, therefore their melting temperature decreases with the particle size decreases, namely underheating. The melting temperature of embedded nanoparticles which has coherent or semi-coherent interface with matrix should be higher than the corresponding bulk crystal. It is because the surface atoms of the particle are constrained by the matrix, resulting in the weakening of particle vibration of surface atoms, thus the average amplitude of the whole particle decreases. Based on the Lindemann melting criterion, we must raise the temperature to make nanoparticles melt, and thus overheating happens.Based on the classical Lindemann melting criterion, the model was extended to predict the size-dependent thermal characterizations of Ge/a-SiO₂ quantum dot system, the main content are as follows: 1. System summarized the size-dependent melting temperature model of nanocrystals. According to this model, the melting temperature of nanocrystals with the free surface decreases as the size decreases, for the embedded nanocrystals which have non-coherent interface with matrix, their melting behavior is similar to the free surface ones. With the particle size decreases, the proportion of atoms at surface （interface） increases, at this time, the the amplitude of the surface atoms is larger than that of within the matrix, so its melting temperature decreases as the size decreases. According to this model, we predicted melting temperatures of different dimensions of In nanocrystals, the results show fine agreement with experimental results. Based on the above model combined with Mott’s melting entropy theory, the size dependent of melting enthalpy and the binding energy model are established. Through in comparing the model predictions of melting entropy, melting enthalpy of In nanocrystals as well as binding energy of Mo and W nanoparticles with experimental results, we can verify the correctness of the model.2. The interface structure between the matrix and the embedded nanoparticles determines the variation of melting temperature. The embedded nanoparticles which have coherent or semi-coherent interface with matrix show higher melting temperature comparing with bulk crystals, namely overheated. This is because the surface atoms receive the substrate diversion, resulting in the weakening of the vibration of surface atoms, so that the average amplitude of the particle decreases. Put the vibration amplitude of interface atoms approximately to that of the inside atoms and the substrate atoms’ average, find the unknown parameters, so that you can use a unified model to predict the melting temperature of nano-particles embedded size effect. The model predictions for different system such as Pb-Al, In-Al, Pb-Zn and Ar-Al give a good agreement with experimental evidences, the melting temperatures increase with size decreasing. According to the model, necessary conditions for superheating of the nanocrystals embedded in the matrix are that the matrix has a higher melting temperature than the embedded crystals in bulk have and there are the coherent or semi-coherent interfaces between them. It should be noting that in the study systems, the atomic diameter of the matrix is smaller than the nanocrystals. Since the surface melting of the nanocrystals is avoided as the pressure increases, a superheating arises.3. By considering vibrational characteristics of interface atoms, the size-dependent thermal properties of Ge nanoparticles （quantum dots） embedded in silica （a-SiO₂） matrix is modeled. （?）_E（D） and x（D） functions for embedded Ge quantum dots are also modeled. In terms of this model without any adjustable parameters, it is found that the matrix stabilizes the interface atoms of Ge quantum dots since coherent or semi-coherent interface effect, which results in the increase of Einstein temperature and the decrease of the linear thermal expansion coefficient with dropping D （D shows the diameter of quantum dots）. It is due to strong chemical interaction between quantum dots and their surroundings. The model predictions correspond to experimental results, which implies that size and interface conditions could be used to modulate materials properties.

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