Bismuth telluride based compounds is known to be the best thermoelectric materials within the low temperature regime. In this study, the P-type Bi0.5Sb1.5Te3 thermoelectric alloy was synthesized by ceramic processing method. The Bi0.5Sb1.5Te3 thermoelectric materials were prepared via the ball milling, cold pressing, and sintering processes. The effects of sintering time and temperature on the microstructures and thermoelectric properties were investigated and discussed. The X-ray diffraction patterns of Bi0.5Sb1.5Te3 reveal that the compounds have the oxides after the sintering processes and the heat treatment process causes grain growth by the increased sintering temperature and time. The results of thermoelectric properties show that the optimal Seebeck coefficient 300(£gV/K) was obtained as the sample was sintered at 350¢XC for 3h and the resistivity will reach the maximum. The figure of merit of 0.15 was obtained at room temperature as the sample was sintered at 375¢XC for 3h.

Mechanical Alloying is a manufacturing process that produces alloys by cold welding of powders. Usually, a vial containing both the powder and steel balls is agitated. Due to impact between the balls and balls and the vial, the powder is mechanically deformed, crushed, and mixed at nano-scales. In this thesis, a numerical model is developed to simulate the dynamics of the vial and the grinding balls of the SPEX 8000 ball milling device, a standardized equipment in both industrial and academic investigations of ball milling. The numerical model is based on the Event Driven Method, typically used to model granular flows. The method implemented is more efficient than the discrete element method used previously to study ball milling dynamics. The numerical tool obtained is useful for scale-up and optimization of mechanical alloying of various materials. An optimization study is presented for the SPEX 8000.

Mechanical Alloying is a manufacturing process that produces alloys by cold welding of powders. Usually, a vial containing both the powder and steel balls is agitated. Due to impact between the balls and balls and the vial, the powder is mechanically deformed, crushed, and mixed at nano-scales. In this thesis, a numerical model is developed to simulate the dynamics of the vial and the grinding balls of the SPEX 8000 ball milling device, a standardized equipment in both industrial and academic investigations of ball milling. The numerical model is based on the Event Driven Method, typically used to model granular flows. The method implemented is more efficient than the discrete element method used previously to study ball milling dynamics. The numerical tool obtained is useful for scale-up and optimization of mechanical alloying of various materials. An optimization study is presented for the SPEX 8000.

No description available.

Waste streams of hydrogen and helium mixtures are produced at the Kennedy Space Center during purging of the hydrogen systems and supply lines. This process is done prior to and after hydrogen servicing. The purged waste gases are lost to the atmosphere, resulting in an annual loss of 2 million and 0.1 million standard cubic meters of helium and hydrogen, respectively. Recovery of these gases will have an economic benefit. Metals, alloys, and intermetallics are known to react with hydrogen in favorable conditions; therefore, they have the possibility of serving as separating and recovery agents. In this study, Mg2Ni, VTiNi and LaNi5 were studied for the separation of H2 from He, using differential scanning calorimetry and thermal volumetric analysis. The ability of LaNi5 to react with hydrogen reversibly at room temperature was verified, and further analysis focused on this compound. Size reduction and activation of LaNi5 by mechanical milling was investigated using different milling parameters for the purpose of activating the material for hydrogen absorption. Because it has been shown that addition of aluminum to LaNi5 resulted in improved hydriding and dehydriding properties, that system was studied further here. In this study, aluminum was added to LaNi5 by mechanical milling. Hydriding properties and elemental compositions of the samples were determined afterwards. The hydrogen absorption rate and capacity were compared to that of LaNi5. Both LaNi5 and its Al doped derivatives exhibited a reduced rate of hydrogen uptake and a reduced hydrogen capacity in the presence of helium. The effects of coating the samples with either gold-palladium or platinum were investigated. It was observed that coating the samples with Pt reduced the negative effect of He, whereas AuPd coating did not have any effect. Larger scale studies were done using a continuous U-tube hydride reactor, built and tested for separation of H2¬ from a 20:80 H2:He mixture. The amount of hydrogen retained in the bed was determined and found to be less than that for the batch systems.

No / Mechanochemical chemoenzymatic peptide and amide bond formation catalysed by papain was studied by ball milling. Despite the high-energy mixing experienced inside the ball mill, the biocatalyst proved stable and highly efficient to catalyse the formation of α,α- and α,β-dipeptides. This strategy was further extended to the enzymatic acylation of amines by milling, and to the mechanosynthesis of a derivative of the valuable dipeptide L-alanyl-L-glutamine. / We thank RWTH Aachen University for support from the Distinguished Professorship Program funded by the Excellence Initiative of the German federal and state governments. EPSRC, grant no. EP/L019655/1.

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No description available.

Thesis advisor: Zhifeng Ren / Solid-state cooling and power generation based on thermoelectric effects are attractive for a wide range of applications in power generation, waste heat recovery, air-conditioning, and refrigeration. There have been persistent efforts on improving the figure of merit (ZT) since the 1950's; only incremental gains were achieved in increasing ZT, with the (Bi1-xSbx)2(Se1-yTey)3 alloy family remaining the best commercial material with ZT ~ 1. To improve ZT to a higher value, we have been pursuing an approach based on random nanostructures and the idea that the thermal conductivity reduction that is responsible for ZT enhancement in superlattices structures can be realized in such nanostructures. The synthesis and characterization of various nanopowders prepared by wet chemical as well as high energy ball milling methods will be discussed in this dissertation. The solid dense samples from nanopowders were prepared by direct current induced hot press (DC hot press) technique. The thermoelectric properties of the hot pressed samples have been studied in detail. By ball milling ingots of bulk alloy crystals and hot pressing the nanopowders, we had demonstrated a high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this dissertation, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 can be achieved. Our material also exhibits a ZT of 0.7 at 250 °C, close to the value reached when ingot was used. This process is more economical and environmentally friendly than starting from bulk alloy crystals. The ZT improvement is caused mostly by the low thermal conductivity, similar to the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the high density grain boundaries and defects. The performance of thermoelectric materials is determined by its dimensionless figure-of-merit (ZT) which needs to be optimized within a specific temperature range for a desired device performance. Hence, we show that by varying the Bi/Sb ratio, the peak ZT can be shifted to a higher or lower temperature for power generation applications or a cooling mode operation. A peak ZT of about 1.3 is achieved from a Bi0.4Sb1.6Te3 composition which is highest among the different compositions. These nanostructured bulk samples have a significantly low lattice thermal conductivity compared to the bulk samples due to the increased phonon scattering in the grain boundaries and defects. This study shows that Bi0.5Sb1.5Te3 may potentially perform better for cooling devices, while Bi0.3Sb1.7Te3 should be able to show better power generation efficiency. Several issues related to accurate measurement of thermoelectric properties were identified and many of them were solved during my studies and these are discussed in this thesis. With the data we obtained, it is clear that nanopowder-based thermoelectric materials hold significant promise. Therefore, a review of synthesis of nanostructured materials by solution-based methods, including a hydrothermal process for the Bi2Te3, Bi2Se3, and Bi2Te2.25Se0.75 nanoparticles, a solvothermal route for Sb2Te3 nanostructures, and a polyol process for the preparation of Bi nanostructures is presented in this dissertation. These new nanostructures may find applications in enhancing the thermoelectric performance. Although small sized and well dispersed nanopowders of various thermoelectric materials could be prepared by a solution method in large scale, contamination and partial oxidation are always big challenges in a chemical approach. Hence, a high energy ball milling technique to prepare thermoelectric nanopowders in large scale and without major contamination is still found to be more efficient and preferred. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Neste trabalho, investigou-se a elaboração mecânica do composto intermetálico TiFe por moagem de bolas com alta energia. Uma forte aderência do material moído, particularmente nas paredes do recipiente de moagem, foi o principal problema verificado com tempos de moagem superiores a 1 hora (moinho agitador). Tentativas para resolver este problema foram realizadas inicialmente com o emprego de agentes controladores de processo (ACPs), como etanol, ácido esteárico, polietileno de baixa densidade, benzeno e ciclohexano, em diferentes quantidades (1 a 20% em massa) e tempos (1 a 40 h), mantendo-se constantes outros parâmetros de moagem como a razão bola:pó em massa (10:1) e o tamanho das bolas (=7mm). Os rendimentos mais elevados (em termos da massa de pó não aderido) foram obtidos quando se utilizaram grandes quantidades de benzeno e ciclohexano (101 e 103% em massa, respectivamente), porém com a formação de TiC ao invés de TiFe em razão da decomposição do ACP e reação do carbono com as partículas de titânio. As moagens foram realizadas posteriormente sem o emprego de qualquer ACP e também utilizando um moinho planetário. Várias estratégias foram investigadas para se tentar mitigar a aderência incluindo-se: (a) moagem de uma pequena quantidade da mistura de pós de Ti e de Fe, revestindo as paredes do recipiente e as bolas de moagem, antes da moagem da carga principal, (b) moagem pausada com aberturas intermediarias do recipiente em atmosfera ambiente, (c) moagem pausada para rotação e inversão da posição do recipiente de moagem (apenas no moinho agitador), (d) moagem isolada dos pós de Ti e de Fe, antes da moagem da mistura, e (e) moagem do pó de Fe com o Ti hidretado. Os melhores resultados, em termos de diminuição da aderência combinada com a formação majoritária do composto TiFe durante a moagem, foram obtidos quando se adotou o procedimento de inversão/rotação, juntamente com o processo de revestimento preliminar do recipiente e das bolas de moagem (26% em massa). Rendimentos maiores foram obtidos com a utilização do TiH2 no moinho planetário, porém sem a formação majoritária do TiFe durante a moagem. / In this work an investigation on the mechanical alloying of the intermetallic compound TiFe by high-energy ball milling was conducted. Strong adherence of milled material, particularly at the vial walls, was seen to be the main problem at milling times higher than 1 hour (shaker mill), hindering the compound synthesis. Attempts to prevent this problem were accomplished first by adding different process control agents (PCAs), like ethanol, stearic acid, low density polyethylene, benzene and cyclohexane at variable quantities (1 to 20 wt. %) and times (1 to 40 h), keeping constant other milling parameters like ball to powder mass ratio (10:1) and balls size (=7mm). Highest yields (related to the non adhered powder) were attained with larger amounts of benzene and cyclohexane (101 and 103 wt. %, respectively), but with TiC formation during milling instead of TiFe due to the PCA decomposition and the reaction of the carbon with and titanium particles. Milling was conducted further without adding any PCA and also using a planetary ball mill. Several strategies were tried to avoid or minimize the adherence including: (a) milling of a small quantity of the Ti and Fe powder mixture, dirtying the vial walls and the balls surfaces before milling the main charge, (b) stepwise milling with intermediate openings of the vial in air, (c) stepwise milling with the rotation and the inversion of the vial position between the steps (only in the shaker mill), (d) milling Ti and Fe powders (apart from each other) before milling the mixture of them, and (e) milling Fe powder with Ti hydride powder. Best results concerning both yield and major TiFe formation during milling were verified with the rotation/inversion procedure combined with preliminar dirtying of the vial and balls (26 wt.% in the shaker mill). Higher yields could be attained by using TiH2 powder in the planetary mill, but with no major TiFe formation during milling.