Thesis (Ph.D.)--University of Western Australia, 2005.

Thesis (Ph. D.)--University of North Texas, Dec., 2007. / Title from title page display. Includes bibliographical references.

O presente trabalho teve como foco principal, a preparação one pot de compostos orgânicos alquílicos e vinílicos contendo o grupamento butiltelurenila em sua estrutura, e o estudo do comportamento desses em variados tipos de reações. Uma delas foi a reação de troca Te/Li. Assim, teluretos foram utilizados como equivalentes sintéticos de vários reagentes organometálicos. Os teluretos foram submetidos à reação de troca Te/Li, e os respectivos compostos organolítio gerados, transformados em reagentes organometálicos de zinco, cobre e cério através de reações de transmetalação. Esses reagentes gerados in situ foram submetidos à reação com variados eletrófilos. Os reagentes organometálicos de zinco e cobre gerados foram submetidos à reação de acilação frente a cloreto de ácidos, resultando em 1,4-hidroxicetonas. Um exemplo dessas hidroxicetonas foi obtido em sua forma enantiopura utilizando um telureto quiral, e esse utilizado na síntese formal e enantiosseletiva da (-)-Pirenoforina. Diferentes estequiometrias de cupratos e outros reagentes organometálicos oriundos de um telureto também foram preparados, e esses testados em reações de substituição de tosilatos e abertura de epóxidos. Os substratos gerados dessas reações foram empregados na síntese das moléculas bioativas Endo-Brevicomina e Frontalina. Apresentaremos também, o resultado obtido na reação direta entre uma espécie di-litiada quiral na presença de benzonitrila. A hidroxicetona assim produzida é um análogo do Ipomeanol, um composto com atividade contra câncer de pulmão. Será apresentado também, o comportamento de entidades di-aniônicas contendo grupamentos silila em reações catalisadas por CeCl3, em que a captura desses com eletrófilos como aldeídos, forneceram 1,4-enedióis em bons rendimentos químicos e alta diastereosseletividade. Será apresentado também o estudo do comportamento de teluretos com hibridização sp2 em reações pericíclicas. A reação pericíclica estudada compreende compostos contendo telúrio (II) e (IV) em reações de Diels-Alder. / One of the main purposes of this work was to develop a new direct methodology to prepare functionalized organic tellurides, to be submitted to a series of different reactions, as for example the Te/Li exchange reaction, aimed to afford synthetic equivalents of several organometallic reagents. Zinc, copper and cerium organometallic compounds were generated via a transmetalation reaction of the lithium species, and were reacted with several electrophiles. As a result, 1,4-hydroxyketones could be successfully prepared. According to this methodology, an enantioenriched 1,4-hydroxyketone could be obtained, using a chiral telluride as starting material. This chiral nonracemic intermediate was employed in an enantioselective formal synthesis of (-)-Pyrenophorin. Cuprates and other organometallic reagents, prepared from a specific telluride and using the above described methodology, were the choosen nucleophiles for performing some aliphatic substitution reactions and, in particular, for the ring opening of epoxides. The resulting products were employed as building blocks for the synthesis of the bioactive cyclic compounds Endo-Brevicomin and Frontalin. The direct reaction of the chiral dilithiated specie with benzonitrile afforded a 1,4-hydroxyketone as an analogue of Ipomeanol, a bioactive compound for cancer therapy. Bis-anionic sililated species, prepared from a telluride, were submitted to reaction with a series of aldehydes. Such reactions, in which CeCl3 was employed as catalyst, showed to be highly diastereoselectives, affording (E)-1,4-enediols in good yields. The pericyclic reaction of insaturated tellurides was also investigated. It is worth mentioning that the same Diels-Alder reaction could be performed either with tellurium II or tellurium IV species.

Thesis advisor: Zhifeng Ren / Thermoelectric (TE) technology is an environment-friendly one due to reduction of carbon emission, which can be widely used either for power generation or for refrigeration. Basically applications of TEs are based on TE effects, which involve the transition between heat and electricity. Despite the superior advantages of being solid state and providing a clean form of energy, TE technology so far only finds its niche area of application due to the relatively less efficiency compared to traditional methods. The efficiency of a thermoelectric device is solely determined by the dimensionless figure-of-merit (ZT) of thermoelectric materials. According to the definition, ZT is equal to square of Seebeck coefficient times electrical conductivity times absolute temperature divided by thermal conductivity. Therefore, a good thermoelectric material should possess high Seebeck coefficient and electrical conductivity while low thermal conductivity, so called phonon glass electron crystal (PGEC). In bulk materials, it is challenging to further improve ZT or independently vary individual parameters without affecting others, mainly due to the interrelated relationships among these three parameters. Fortunately, nano approach gives us some independent control in parameters adjustment. One important aspect of nano idea lies in the fact that enhanced boundary scattering due to the increased intensities of interfaces arising from nano-sized grains could reduce the thermal conductivity more than the electrical conductivity, which is practically realized in our material system. Since the introduction of nano idea, large ZT as high as above two has been achieved in the superlattice system. Due to the high fabrication cost of superlattices, they are not scalable for mass production. Theoretical calculations indicate that thermal boundary resistance is the main mechanism for the low thermal conductivity in superlattices, rather than the periodicity. Basically, we hope to achieve the supplattice-like ZT in the less costly bulk nanograined materials, based on the idea that reduction of thermal conductivity which is responsible for ZT enhancement in superlattices can be realized in bulk materials with embedded nanostructures as well. Inspired by the nanocomposite idea, in my thesis work I applied the technique of ball milling and then hot press to various thermoelectric materials, from low temperature to high temperature, demonstrating the feasibility of the approach. By ball milling alloyed ingot into nanopowders and DC hot pressing them, we have achieved a 62-89% ZT improvement for p-type half-Heusler samples, mainly due to the significantly enhanced Seebeck coefficient and partially due to the moderately reduced thermal conductivity. Microstructure studies indicated that increased boundaries due to smaller nano-sized grains is the cause for change of parameters. For our ball milled samples, the trend of decreasing thermal conductivities with increasing ball milling time is observed, further substantiating our nano-approach idea because longer ball milling time gives rise to smaller grain sizes and thus stronger boundary scattering. By applying the same technique to n-type half-Heuslers, we also successfully obtained pronounced enhancement in ZT especially at medium and low temperature ranges, which might be useful in medium temperature power generation. By ball milling a mixture of individual constituent elements into alloyed nanopowders and then DC hot pressing them, we did not gain improvement in ZT initially for n-type BiTeSe system mainly due to the simultaneous reduced power factor with the thermal conductivity. Considering anisotropic properties of the n-type BiTeSe single crystal and randomization effect of ball milling process, we repressed the as-pressed bulk samples in a bigger diameter die, during which lateral flow took place, resulting in preferred grain orientation. As a result, a 22% improvement in the peak ZT from 0.85 to 1.04 at 125 oC in n-type Bi₂Te_2.7Se_0.3 has been successfully achieved, arising from the more enhanced power factor than the thermal conductivity. Compared with single crystal, we benefit from the small nano-sized grains in bulk materials. Taking into account the in-plane power factor of single crystal, we still have much room for further ZT improvement if more ab orientation is promoted into the disk plane and/or the crystal plate size and thickness are reduced. By applying our technique of ball milling and then hot press to p-type skutterudites system, we have achieved a peak ZT of 0.95 at 450 <super>o</super>C in NdFe_3.5Co_0.5Sb₁₂, which is comparable to that of the state-of-the-art ingot. Our approach has the advantage of being less costly and more time-efficient compared to traditional fabrication methods. Besides, even lower thermal conductivity and hence higher ZT can be expected, provided that the nanosize of the precursor powder is preserved during hot press. The nanocomposite idea has been substantiated and the feasibility and generality of our ball milling and then hot press approach has been demonstrated, based on the thermoelectric properties data we obtained and the microstructure studies we carried out from various thermoelectric material systems, from low temperature to high temperature. We believe that continued effort in the area of thermoelectrics by our approach should be paid with superlattice-like ZT if ingenious methods are devised to control the grain growth during consolidation. / Thesis (PhD) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Mercury cadmium telluride is important in the detection of electromagnetic radiation in the eight to twelve micron atmospheric window for infrared imaging systems. High resolution infrared imaging systems use either large (256x256 element to 1024x1024 element) staring arrays or much smaller (1-6 element) scanned arrays in which the image is optically scanned across the detectors. In scanned arrays, high resolution and sensitivity may result in the scan direction not being parallel to the detector bias current. The response of an infrared detector to uniform illumination is investigated. It is found that variations in the detector thickness result in significant changes in output voltage. Scanned detectors are modeled in five different orientations; scan parallel to bias, scan opposite to bias, scan perpendicular to bias, and two orientations of the scan diagonal to the bias. The response is analyzed for two cases: 1) the size of the scanned radiation equal to the size of the detector and 2) when the pixel width is half of the width of the detector, but of equal length. Results of the simulation show that the fastest response occurs when the scan and bias are parallel. The largest response occurs when the scan direction is diagonal to the bias, but the response time is much slower than when the bias is parallel to the scan. Therefore, a tradeoff must be made between maximum signal and speed of response. Test detectors are being fabricated and will be tested at FLIR Systems Inc., Portland, Oregon, to confirm the model predictions. / Graduation date: 1992

[Truncated abstract] Infrared radiation detectors find application in a wide range of military and civilian applications: for example, target identification, astronomy, atmospheric sensing and medical imaging. The greatest sensitivity, response speed, and wavelength range is offered by infrared detectors based on HgCdTe semiconductor material, the growth and characterisation of which is the subject of this thesis. Molecular Beam Epitaxy (MBE) is a versatile method of depositing layers of semiconductor material on a suitable crystalline substrate. In particular, MBE facilitates the growth of multilayer structures, thus allowing bandgap engineered devices to be realised. By modulating the bandgap within the device structure it is possible to improve the sensitivity or increase the operating temperature of photodetectors when compared to devices fabricated on single layer material. Furthermore, dual-band detectors may be fabricated using multi-layered HgCdTe material. The bulk of this thesis is concerned with the development of the MBE process for multilayer growth, from modelling of the growth process to characterisation of the material produced, and measurement of photoconductive devices fabricated on these wafers. In this thesis a previously published model of HgCdTe growth by MBE is reviewed in detail, and is applied to the growth of double layer heterostructures in order to determine the optimum method of changing the mole fraction between layers. The model has been used to predict the change in the temperature of the phase limit when the mole fraction and growth rate change suddenly as is the case during growth of an abrupt heterostructure. Two options for growth of an abrupt heterostructure were examined (a) modulating the CdTe flux and (b) modulating the Te flux. The change in the phase limit temperature between the layers was calculated as being 4:1±C for option (a) and 5:2±C for option (b) when growing a Hg(0:7)Cd(0:3)Te/Hg(0:56)Cd(0:44)Te heterostructure

Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 124-133). Also available on the World Wide Web.

Thesis (Ph.D.)--University of Western Australia, 2005.

Thesis (Ph.D)--University of Western Australia, 2007.

Thesis (M.S.)--University of Delaware, 2008. / Principal faculty advisor: Robert W. Birkmire, Dept. of Materials Science & Engineering. Includes bibliographical references.