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Study On Doped Nanocrystalline AgSnO2 Contact Materials And Quasi-fluid State In Silicon Single Crystal

Posted on:2005-07-17Degree:DoctorType:Dissertation
Country:ChinaCandidate:J ZhengFull Text:PDF
GTID:1101360182475045Subject:Materials science
Abstract/Summary:PDF Full Text Request
Part I. Due to its non-toxicity, AgSnO2 as a new type of electric contact material has been subjected to extensive studies in recent years. However, the large contact resistance, high temperature arising in use as well as the difficulty to molding deterred its development and application. In present research, a series of novel doped nanocrystalline SnO2 materials has been prepared, which focused on improving the electric and mechanical properties of the AgSnO2 electric contact materials. The doped nanocrystalline SnO2 powders were prepared by sol-gel method with co-precipitation of Sn 4+ and doped metal cations, such as Ti4+ , Zn2+, Sb3+ and Cu2+ etc. In order to prevent the congregation of the nano-particles, dispersant PEG (polyglycol) was added in the preparation. The XRD spectra of the resulting materials showed that the doped cations entered into the lattice of SnO2 thus forming substituted solid solutions, and the average size of the particles of resulting doped SnO2 powder is ca. 10 nm; in addition, chemical plating method was adopted to modify the surface of the doped nanocrystalline SnO2 particles, which decreased the hard congregation of SnO2 particles, improved the wetting property and enhanced the cohesion between the SnO2 nano-particles and silver. As a result, the tensile strength of the AgSnO2 electric contact material increased by 50%, the ductibility by 200%. XRD, TEM and DTA measurements have been applied to study the composition, structure and heat treatment temperature of the doped nanocrystalline SnO2 powder. The superplasticity, dispersion enhancement, plastic deformation and heat treatment anneal properties of the nanocrystalline AgSnO2 electric contact materials have been theoretically analyzed. The results showed that the SnO2 nano-particles are uniformly distributed in the matrix of AgSnO2 materials. This character, on the one hand, diminish the dissevering of silver matrix by SnO2, on the other hand, decrease the enrichment of SnO2 particles, which prevents the temperature arising owing to the formation of SnO2 insulation layer, and thus significantly improves the electric properties, such as the electric life, anti-fusion welding ability, electric arc resistance etc., of the materials. The electric and mechanic properties of the obtained doped nanocrystalline AgSnO2 electric contact materials surpass that of the national standards. Part II. It has been observed on the surface of many different solid materials that there randomly exist some tissues similar to liquid. In order to understand the mechanism of this phenomenon, in the present study, the (111) surface of a silicon single crystal has been subjected to the observations with metallographic microscope under atmospheric pressure and ambient temperature. Beautiful liquid-like cells randomly appear on the surface, growth, deform and disappear. The process exhibits some character of oscillation. AFM measurement of the cell shows the motion of atoms in the surface layer. It has been thought that the cell region may be some new phases which differ little in energy and geometry with the stable one (under the normal pressure, cubic diamond structure). In order to verify the guess, single crystal X-ray structure analysis has been carried out for both crystals from cell region and the non-cell region of the same crystal. In surprising, the results revealed that the crystal from the non-cell region is cubic diamond structure as expected, however, the crystal in the cell region belongs to tetrahedral crystal system with space group I 41amd (No. 141), a β-tin structure. In the tetrahedral one, there are two silicon atoms in the asymmetric unit, and the environment of each atom is nearly the same with that in the cubic diamond structure. The difference in molar volume for both structures is only 0.4%. The total energy calculation of both structures using PBEPBE density functional method (3-21g basis set, Gaussian 03w, PBC) showed that the energy of the β-tin structure is lower than that of the cubic diamond structure, and the energy difference for both structures is 31.504 kJ/mol, which is compared with the hydrogen bond energy of medium strength. The potential energy surface near the experimental β-tin structure is V-shaped, and the bottom is quite smooth. The experimental β-tin structure is located at the bottom of the potential energy surface, but it is not a stable state. From the result of calculation, it may be expected that (1) the thermal vibration of the crystal can induce the cubic diamond to β-tin structure phase transformation, and (2) the smooth bottom of the potential energy surface enable the β-tin structure varies its dimensions in a quite large range, which in macroscopy exhibits the oscillation of the cell region.
Keywords/Search Tags:electric contact materials, sol-gel, doped nanocrystalline, surface modification, heat treatment, liquid-like state, single crystal silicon, non-linear oscillation, crystal structure, density functional method
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