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Study on Prepartion, Thermodynamic Properties and Phase Stability of Metal Nanocrystalline Materials

Author: GaoJinPing
Tutor: ZhangJiuXing
School: Beijing University of Technology
Course: Materials Science
Keywords: Nanocrystalline Thermodynamics High energy ball milling Annealing Phase content
CLC: TB383
Type: Master's thesis
Year: 2005
Downloads: 556
Quote: 1
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Abstract

Nanomaterials as a new material in the aerospace, electronics, metallurgy, chemical, biological and medical fields show a broad application prospects. For nanocrystalline materials, due to the fine grain size, its internal content within the interface of the grain boundaries, phase boundaries or domain boundaries is very high, which significantly affects the physical and mechanical properties of nanocrystalline it with traditional materials are not have excellent characteristics. Respect to the coarse grain blocks polycrystalline material, the nano-material specific heat value increases, the thermal expansion coefficients exponentially increases, and the crude crystalline material with the same composition differ larger phase transition characteristics and phase stability and other characteristics, show that for the traditional thermodynamic model of the study bulk materials can not reasonably explain the phase transition behavior of nanomaterials. First of all, on the basis of the application the interface expansion model and universal equation of state of nano-interface thermodynamic characteristics, the development of a computational model of the thermodynamic functions describe the entire nanocrystals, deduce the expression of the basic thermodynamic functions of metal nanocrystals. Thereby further determine the the nanocrystalline stable phase formation temperature and grain size between the dependencies, and generates phase existed in the thermodynamic conditions and critical dimension conditions. Second, we explore the energy ball milling method Co nanocrystalline powders preparation process, namely Ar atmosphere using a 10:1 ball to powder ratio, dry mill 30-34h, you can get an average grain size of about 40nm Co nanocrystalline powder. XRD, SEM, TEM, morphology, sub-structure and grain size of the nanocrystalline comprehensive observation and analysis. Finally, the average grain size to prepare a 39 nm of Co nanocrystal for research object, subjected to annealing heat treatment at a certain temperature, and after annealing of the powder phase composition, phase content, and grain size, Determination proved Co. Nanocrystals trends and phase change of the phase change of the consistency of the temperature range and predicted results. Co nanocrystalline powders isobaric heat capacity Cp determination calculated the material enthalpy, entropy and Gibbs free energy and temperature relationship, the curve of the theoretical models predict the thermodynamic functions two not only the same trend and the specific numerical values ??of the same order of magnitude, and thereby further validate the correctness of the model prediction. Created nanocrystalline thermodynamic functions of the single-phase model, can more accurately predict single-phase basic thermodynamic properties of nanocrystalline materials, as well as the stable phase and grain size between quantitative relationship structure so as to study the microstructure of nanocrystalline materials thermodynamic characterization parameters to provide a scientific basis for the relationship between behavior and performance phase change.

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