Study On Thermodynamic Properties Of Nanocrystalline Metal Materials

The thermodynamic properties of several nanocrystalline metals and their corresponding coarse grained metal materials such as nanocrystalline ingot iron, copper, aluminum and 304 stainless steel and their corresponding conventional polycrystalline counterparts were studied by differential scanning calorimeter (DSC) and physics property measurement system (PPMS). The experiments in this thesis mainly included in specific heat in low and middle temperature range and heat stability range from room temperature to 900?. The specific heat experimental results agree with Debye model and lattice vibration theory, but there are a few experimental results that do not agree with the theoretical model, the possible factors for their difference were proposed in this thesis.The progresses on some basic concepts of nano-material structure and the related thermodynamic properties are first briefly introduced in the introduction of this thesis, they included in the Debye theory, lattice vibration theory and the lattice dynamics. Then the experimental results in this thesis were explained by these methods and theories.The thermal stability and specific heat of nanocrystalline iron were studied by the differential scanning calorimeter. These results basically meet the present basic theory such as Debye theory, lattice vibration theory and the lattice dynamics. Nanocrystalline iron have a good stability in the temperature range 300oC~900?.The main reason is nano-iron have a lower energy and the difference between both sides of the interfaces and the adjacent grain is smaller than the coarse grains. This behavior is beneficial to control the move of grain boundary and can control the grain's growing up. At the same time due to the disorder of nano-grain boundary structure, the grain interior atoms of grain boundary atoms exhibit some excess volume relative to the corresponding coarse grained atoms. Due to the inharmonic effect of nano-crystal interface materials are stronger than that of coarse-grained materials and nano-crystal's Debye temperature is lower than those corresponding coarse crystal. This value will decrease with the grain size decreases. These reasons determine the specific heat of nano-materials higher than that of the corresponding coarse grain materials. The theoretical conclusions are well validated in my present experiments.For the nano-copper and coarse-grained copper our experimental results show that there is a big difference between nano-copper and coarse grained copper in specific heat.But thermal stability have basically no difference. Anocrystalline iron have a good stability in the temperature range from300? to 900?.The ordinary pure copper have a tiny exothermic peak, this is a phase transition peak comes from lattice relaxation. There is another reason that mainly determined by the internal structure of nano-copper. A low-energy excited states which closely linked to the structural disorder characteristic exists in nano-copper. It can be aroused at very low temperatures. these can make nano-structured solid specific heat capacity of copper appears an additional item, so that the specific heat of copper nano-structures and coarse grained copper shows a great difference.The nano-crystalline aluminum and the corresponding coarse grained aluminum experimental results show that the thermal stability makes no difference while the specific heat results have a big difference. The nano-crystalline aluminum scarcely exist dislocation and defects, this kind of structure is similar to the corresponding coarse grained aluminum's structure. The nano-crystalline aluminum has a lower dislocation. In this paper, I used the Debye theory and lattice dynamics theory and combined the special structure of nano-materials to explain the particularity structure of nano-aluminum, originality introduce temperature squared item in order to make a reasonable explanation of specific heat's particularity.I have made three groups of thermodynamic properties of pure metals in the experiments, but at the same time I explained their internal reasons lead to their discriminations. The introduction of alloy 304 stainless steel is anther item, and I have made some explanations about its special thermodynamic phenomenon.