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Author: Guangyan Li
Publisher:
ISBN:
Category :
Languages : en
Pages : 416
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Book Description
The goal of this dissertation is to elastically characterize thermoelectrics and other novel materials using resonant ultrasound spectroscopy (RUS) at elevated temperatures. A "direct-contact" transducer system with test sample in direct contact with the piezoelectric elements was developed. This new transducer system with improved signal-to-noise ratio (SNR) and the conventional buffer-rod system were used for elasticity measurements and phase transition studies. The temperature dependent elastic moduli of thermoelectric materials of four nanostructured polycrystalline silicon-germanium (SiGe) samples were obtained up to 950°C. Abnormal elastic behavior (stiffening) in the temperature range of 350-550°C was observed in the two n-doped SiGe samples, which is associated with dopant (phosphorus) precipitation. The elastic moduli of the complex Zintl phase Yb 14 MnSb 11 were also measured up to 600°C. Using the quick "mode-tracking" method, the various temperature-induced phase transitions in Yb 14 MnSb 11, transition metal oxide LuFe 2 O 4 and bulk metallic glass Zr 50 Cu 31 Pd 9 Al 10 were investigated. In addition to the high-temperature RUS measurements, the continuum elastic model in RUS was also applied to numerically study several lower normal modes of vibration in carbon nanotubes (CNTs).
Author: Guangyan Li
Publisher:
ISBN:
Category :
Languages : en
Pages : 416
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Book Description
The goal of this dissertation is to elastically characterize thermoelectrics and other novel materials using resonant ultrasound spectroscopy (RUS) at elevated temperatures. A "direct-contact" transducer system with test sample in direct contact with the piezoelectric elements was developed. This new transducer system with improved signal-to-noise ratio (SNR) and the conventional buffer-rod system were used for elasticity measurements and phase transition studies. The temperature dependent elastic moduli of thermoelectric materials of four nanostructured polycrystalline silicon-germanium (SiGe) samples were obtained up to 950°C. Abnormal elastic behavior (stiffening) in the temperature range of 350-550°C was observed in the two n-doped SiGe samples, which is associated with dopant (phosphorus) precipitation. The elastic moduli of the complex Zintl phase Yb 14 MnSb 11 were also measured up to 600°C. Using the quick "mode-tracking" method, the various temperature-induced phase transitions in Yb 14 MnSb 11, transition metal oxide LuFe 2 O 4 and bulk metallic glass Zr 50 Cu 31 Pd 9 Al 10 were investigated. In addition to the high-temperature RUS measurements, the continuum elastic model in RUS was also applied to numerically study several lower normal modes of vibration in carbon nanotubes (CNTs).
Author: Lindsay Heatherly Vanbebber
Publisher:
ISBN:
Category : Elasticity
Languages : en
Pages : 114
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Book Description
The elastic behavior of materials with unusual low-temperature behavior involving structural, magnetic, and superconducting transitions has been studied using Resonant Ultrasound Spectroscopy (RUS). Two classes of materials have been examined, namely, iron-based superconductors and thermoelectric materials. A series of BaFe2As2 materials as well as TlFe1.6Se2 comprise the former class, and a series of Mo3Sb7 materials is included in the latter. The elastic behavior of BaFe2As2 reveals that a large softening is observed as the material approaches a tetragonal-to-orthorhombic structural transition accompanied by an antiferromagnetic ordering near 132K. This significant shear softening indicates that the structural transition is due to magnetically-driven nematic fluctuations. The elastic behavior of superconducting cobalt-doped BaFe2As2, with a critical temperature of 22K, supports this finding. In BaFe2As2, an unusually large amount of softening is observed as the material approaches the superconducting transition, and the material begins to stiffen immediately following this transition. The effect of chromium doping in the BaFe2As2 material is also examined. It is found that chromium doping does not produce superconductivity at any concentration. It is shown that the structural transition that occurs in the BaFe2As2 parent material is suppressed with chromium doping, but this suppression is slower than that observed in cobalt-doped compounds. The TlFe1.6Se2 material displays a slight softening at 140K, where a canting of the iron spins occurs. When the original magnetic structure is recovered at 100K, stiffening is observed. A magnetic field of 3T does not affect the elastic response despite the magnetic feature. The elastic behavior of a series of Mo3Sb7 single crystals is also examined with RUS. The temperature dependence of the elastic response of this material reveals a transition at around 53K, evidenced by a dramatic softening in c'. This softening is associated with a cubic-to-tetragonal structural transition. Single crystals that were doped with chromium, ruthenium, and tellurium were also examined. The chromium doping simply decreases the transition temperature by approximately 5K. While the addition of ruthenium and tellurium suppresses the transition, a structural instability persists, evidenced by a remaining c' softening.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 28
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Department of Energy EPSCoR The University of Mississippi Award: DE-FG02-04ER46121 Resonant Ultrasound Spectroscopy Studies of Complex Transition Metal Oxides The central thrust of this DOE funded research program has been to apply resonant ultrasound spectroscopy (RUS), an elegant and efficient method for determining the elastic stiffness constants of a crystal, to the complex and poorly understood class of materials known as transition metal oxides (TMOs). Perhaps the most interesting and challenging feature of TMOs is their strongly correlated behavior in which spin, lattice, and charge degrees of freedom are strongly coupled. Elastic constants are a measure of the interatomic potentials in a crystal and are thus sensitive probes into the atomic environment. This sensitivity makes RUS an ideal tool to study the coupling of phase transition order parameters to lattice strains. The most significant result of the project has been the construction of a high temperature RUS apparatus capable of making elastic constant measurements at temperatures as high as 1000 degrees Celsius. We have designed and built novel acoustic transducers which can operate as high as 600 degrees Celsius based on lithium niobate piezoelectric elements. For measurement between 600 to 1000 C, a buffer rod system is used in which the samples under test and transducers are separated by a rod with low acoustic attenuation. The high temperature RUS system has been used to study the charge order (CO) transition in transition metal oxides for which we have discovered a new transition occurring about 35 C below the CO transition. While the CO transition exhibits a linear coupling between the strain and order parameter, this new precursor transition shows a different coupling indicating a fundamentally different mechanism. We have also begun a study, in collaboration with the Jet Propulsion Laboratory, to study novel thermoelectric materials at elevated temperatures. These materials include silicon germanium with various doping and Zintl phase materials. Such materials show promise for increased figures of merit, vital to making thermolectrics competitive with traditional power generation mechanisms.
Author: Sumudu P. Tennakoon
Publisher:
ISBN:
Category :
Languages : en
Pages : 181
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Relaxor ferroelectric lead magnesium niobate-lead titanate (PMN-PT) material exhibits exceptional electromechanical properties. The material undergoes a series of structural phase transitions with changes in temperature and the chemical composition. The work covered in this dissertation seek to gain insight into the phase diagram of PMN-PT using temperature and pressure dependence of the elastic properties. Single crystal PMN-PT with a composition near morphotropic phase boundary (MPB) was investigated using a resonant ultrasound spectroscopy (RUS) methodologies in the temperature range of 293 K - 800 K and the pressure range from near vacuum to 3.4 MPa. At atmospheric pressure, significantly high acoustic attenuation of PMN-PT is observed at temperatures below 400 K. A strong stiffening is observed in the temperature range of 400 K - 673 K, followed by a gradual softening at higher temperatures. With varying pressure, an increased pressure sensitivity of the elastic properties of PMN-PT is observed at the temperatures in the stiffening phase. Elastic behavior at elevated temperatures and pressures were studied for correlations with the ferroelectric domains at temperatures below the Curie temperature (TC), the locally polarized nano-regions, and an existence of pseudo-cubic crystalline at higher temperatures between (TC and TB). Thermoelectric lanthanum tellurides and skutterudites are being investigated by NASA's Jet Propulsion Laboratory for advanced thermoelectric generates (TEGs). Effects of nickel (Ni) doping on elastic properties of lanthanum tellurides at elevated temperatures were investigated in the temperature range of 293 K - 800 K. A linear stiffening was observed with increasing the Ni content in the material. Elastic properties of p-type and n-type bismuth-based skutterudites were investigated in the temperature range of 293 K - 723 K. Elastic properties of rare-earth doped strontium titanate were also investigated in the temperature range of 293 K - 750 K.
Author: Kenneth A. Pestka
Publisher:
ISBN: 9780542828034
Category :
Languages : en
Pages : 169
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Author: Michael Alan McGuire
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ISBN:
Category :
Languages : en
Pages : 140
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Author:
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ISBN:
Category :
Languages : en
Pages : 5
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Author:
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ISBN:
Category :
Languages : en
Pages :
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Author: Dennis Salvatore Agosta
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ISBN:
Category : Nanocrystals
Languages : en
Pages : 246
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Author: Wanyue Peng
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ISBN:
Category : Electronic dissertations
Languages : en
Pages : 131
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The ability to predict materials with desired thermal conductivity from a large material database can significantly improve the efficiency of experimental work. Lattice thermal conductivity is controlled by the velocity and relaxation time of phonons (lattice vibrations). Phonon scattering is closely related to the anharmonic lattice vibrations of a material, while phonon velocity depends on density and, bond stiffness. In this research, the relationship between structure, bonding, and thermal properties is discussed in two classes of layered materials, AM2X2 intermetallic compounds and GeTe - Sb2Te3 alloys.First, we study the origin of the anomalously low lattice thermal conductivity of MgMg2Sb2 compare to other isostructural AMg2Pn2 compounds (A = Mg, Ca, Yb, and Pn = Sb and Bi ). By employing high-temperature X-ray diffraction (XRD) and resonant ultrasound spectroscopy (RUS) techniques, we have shown that the low lattice thermal conductivity is due to previously-unrecognized soft shear modes and highly anharmonic acoustic phonons in layered MgMg2Sb2. Combined with the phonon calculations from our collaborators, we attribute the anomalous thermal behavior of MgMg2Sb2 to the instability of the vibrational modes that originated from the weak bonding of the Mg, which is too small for the octahedral site. Second, we investigate the phase stability of the AMg2Pn2 system with mixed occupancy of Mg, Ca, Sr, or Ba on the cation (A) site. We show that the small ionic radius of Mg2+ leads to limited solubility when alloyed with larger cations such as Sr or Ba. Third, by performing in-situ high-pressure synchrotron X-ray diffraction, we showed that a few AM2X2 compounds can exhibit phase transitions at high-pressure, most of which are previously unrecognized. In addition, we observed that the compressibility of MgMg2Sb2 and MgMg2Bi2 is near-isotropic, whereas other isostructural AM2X2 compounds show clear signs of anisotropy between the in-plane and out-of-plane compressibility as is typical of layered compounds. We have analyzed the compressibility, transition pressure/temperature, anisotropy, as well as the type of phase transition to develop a deeper understanding of the stability and bond strength of different AM2X2 compounds.Lastly, we observed and explained the lattice stiffening and flattened lattice thermal conductivity curve with increasing temperature in GeTe - Sb2Te3 alloys. Unlike most compounds that soften with increasing temperature, the elastic moduli of (GeTe)17 - Sb2Te3 stiffen with increasing temperature before the phase transition. We investigate GeTe, Sb2Te3, and (GeTe)17 - Sb2Te3 from room temperature up to the phase transitions with high-temperature XRD, high-temperature RUS, and transport property measurements. We attribute the stiffening behavior to the gradual diffusion of layered vacancies to random vacancies on the cation site, which profoundly impact the elastic properties and the transport properties of the material.
