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مهندسی نقص در مواد ترموالکتریک
Defect Engineering in Thermoelectric Materials
Thermoelectric conversion technique, which can directly recover waste heat and convert it into electricity, holds a great promise in alleviating the energy sustainability issue. However, the existing thermoelectric applications are limited to niche market due to their relatively low efficiency compared to the traditional heat engines. The thermoelectric conversion efficiency is jointly determined by the Carnot efficiency and materials’ performance, which are quantified by the dimensionless figure of merit ZT. Therefore, the efficiency can be effectively improved with the ZT enhancement. Microstructural defects, which can scatter electrons as well as phonons, can greatly impact materials’ thermoelectric properties. Therefore, it is possible to tailor the thermoelectric transport properties and even enhance the thermoelectric performance by manipulating the defects, i.e., defect engineering. In this thesis, the effect of vacancies and nanoscale twin boundaries on the thermoelectric properties will be discussed.
Charged vacancies, which not only influence the carrier concentration and conduction type (n-type or p-type), can also greatly scatter the carriers. In Mg3Sb2-based materials, Mg vacancies has the lowest defect formation energy, which means that they are the dominant defects in this compound. Experimental and theoretical results have shown that the presence of the Mg vacancies leads to the persistent p-type conduction for this material. Fortunately, it is possible the reduced the concentration of Mg vacancies with the addition of excess Mg and then convert the material into n-type. However, it is noted that carrier mobility is dominated by the ionized impurity scattering (very likely due to the charged Mg vacancies) at lower temperature in the n-type materials, thus leading to a low power factor. By further reducing the concentration of Mg vacancies via transition metal elements doping and tuning the processing conditions, it is possible to shift the ionized impurity scattering to mixed scattering and greatly improve the power factor.
In addition to the scattering of electrons, vacancies which maximize the atomic mass difference between the host and guest atoms can greatly scatter phonons as well. Traditional strategies of introducing the vacancies can be realized by tuning the stoichiometry or alloying with a defect compound which contains intrinsically high concentration of vacancies. In fact, crystals usually have the tendency to lower their energy by forming point defects to counter the effects of dopants, an effect called self-compensation. Here we pointed out that by utilizing the self-compensation effect via chemical doping, it is also possible to induce vacancies and lead to a significant phonon scattering effect.
The phonon scattering effect of grain boundary has long been recognized and studied in detail. However, phonon scattering by twin boundary is much less considered. The difficulty in studying this topic lies in the fact it is usually challenging to synthesize a bulk material with high density of twin boundaries but without other defects. In our study, we synthesize three specimens with distinctly different microstructures: microscale grain boundaries, nanoscale grain boundaries, and nanoscale grain boundaries with twin boundaries. Therefore, the phonon scattering effect by the twin boundaries can be unambiguously revealed by comparing the lattice thermal conductivities of these specimens. Our results indicate that twin boundaries show a much weaker phonon scattering effect comparing to the grain boundaries.
مطالعه روی آلیاژهای حافظه دار با دمای پایین NiTiSn و پردازش آلیاژهای حافظه دار شکل با دمای بالا NTiHf
A Study on NiTiSn Low-Temperature Shape Memory Alloys and the Processing of NTiHf High-Temperature Shape Memory Alloys
Shape memory alloys (SMAs) operating as solid-state actuators pose economic and environmental benefits to the aerospace industry due to their lightweight, compact design, which provides potential for reducing fuel emissions and overall operating cost in aeronautical equipment. Despite wide applicability, the implementation of SMA technology into aerospace-related actuator applications is hindered by harsh environmental conditions, which necessitate extremely high or low transformation temperatures. The versatility of the NiTi-based SMA system shows potential for meeting these demanding material constraints, since transformation temperatures in NiTi can be significantly raised or lowered with ternary alloying elements and/or Ni:Ti ratio adjustments. In this thesis, the expansive transformation capabilities of the NiTi-based SMA system are demonstrated with a low and high-temperature NiTi-based SMA; each encompassing different stages of the SMA development process. First, exploratory work on the NiTiSn SMA system is presented. The viability of NiTiSn alloys as low-temperature SMAs (LTSMAs) was investigated over the course of five alloy heats. The site preference of Sn in near-equiatomic NiTi was examined along with the effects of solution annealing, Ni:Ti ratio adjustments, and precipitation strengthening on the thermomechanical properties of NiTiSn LTSMAs. Second, the thermomechanical processability of NiTiHf high-temperature SMA (HTSMA) wires is presented. The evolution of various microstructural features (grain size reduction, oxide growth, and nano-precipitation) were observed at incremental stages of the hot rolling process and linked to the thermal and mechanical responses of respective HTSMA rods/wires. This work was carried out in an effort to optimize the rolling/drawing process for NiTiHf HTSMAs.