The field of them1oelectrics started in early 19th century. Since the discovery of the Seebeck effect and the Peltier effect, thermoelectric modules have found their way into, mostly, niche applications such as radioisotope thermoelectric generators on space missions. Thermoelectric modules can also be used for cooling, utilizing the Peltier effect. Thermoelectrics are promising materials due to the operation nature of the modules. That is, they have no moving parts, no exhaust, long lifetime without maintenance, features that make them attractive for many applications. Despite these promising properties, thermoelectric modules are mostly used in niche applications. The main reason for this is conventional modules with the highest efficiency are commonly made of expensive and/or rare elements which prevents mass production. To tackle this problem, new materials are investigated to find a module that can be made widely available. Oxides are one possibility, where an added benefit is that they are chemically stable even at elevated temperature. The perovskite CaMnO3 is one of the more promising oxides, with elements that are abundant on earth and cheap. The material does suffer from low electrical conductivity which results in a low electrical conductivity and efficiency. A substantial effort has been put in to increase the efficiency of CaMnO3, hut it still needs improvement. In my thesis, I have investigated the CaMnO3 system. CaMnO3 was synthesized using co-reactive RF-magnetron sputtering and post annealing. The synthesis method is already known hut has not been used for deposition of perovskites. I have also demonstrated that this synthesis method can be used to dope CaMnO3 with niobium at appropriate levels for enhancing the efficiency.

No description available.

This thesis is devoted to a theoretical study of local environment effects in alloys. A fundamental property of a disordered system is that all chemically equivalent atoms are different due to their different chemical environments, in contrast to an ideal periodic solid where all the atoms that occupy equivalent positions in the crystal have exactly the same physical properties. The local environment effects have been largely ignored in earlier theories of disordered systems, that is the system has been treated as a whole and average properties have been derived. Moreover, inhomogeneous systems, such as surfaces and interfaces, induce local environment effects that are not necessarily present in the bulk. The importance and presence of local environment effects are illustrated by calculating observable physical properties in various systems. In particular, by employing the complete screening picture the effects of local environments on the core-level binding energy shifts as well as Auger shifts in random alloys are in- vestigated. This so-called disorder broadening effect has recently been observed experimentally. It is shown that there are different contributions to the disorder broadening that vary with the local chemical environment. Furthermore, the influ- ence of inhomogeneous lattice distortions on the disorder broadening of the core- level photoemission spectra are considered for systems with large size-mismatch between the alloy components. The effects of local chemical environments on physical properties in magnetic systems are illuminated. A noticeable variation in the electronic structure, local magnetic moments and exchange parameters at different sites is obtained. This reflects the sensitivity to different chemical environments and it is shown to be of qualitative importance in the vicinity of magnetic instability. The local environment effects due to the presence of surfaces and interfaces are also considered. The effect is explicitly studied by considering the concentration profile of a thin Ag-Pd film deposited on a Ru substrate. Two computational approaches are utilized to calculate the relative composition in each layer of the thin film as a function of temperature in a theoretically consistent way. It is shown that, opposed to the situation in the bulk, where a complete solubility between Ag and Pd takes place, a non-uniform distribution of the alloy components across the film is observed. In another study it is investigated whether the presence of TiN interfaces changes the dynamical and thermodynamic stability of B1 SiN. Phonon calcula- tions show that TiN interfaces have a stabilization effect on the lattice dynamics. On the other hand, calculations of the Si vacancy formation energy show that the structures are unstable with respect to composition variations.

Examination of quantum critical points of itinerant electron systems will aid with understanding of d-electron magnetism that exhibits both local and itinerant characteristics in different families of compounds. Doping-induced quantum phase transition of the itinerant ferromagnet Sc 3.1 In that is composed of non-magnetic elements is the focus of our work. Polycrystalline samples of (Sc 1-x Lu x ) 3.1 In with 0≤ x≤ 0.08 were prepared by arcmelting and then annealing for an extended period of time. Susceptibility measurements were performed in an applied magnetic field H = 0.1 T for temperatures T = 1.85 K to 300 K. Linearity of Arrott plots in low-field region was significantly improved by implementing the non-mean-field Arrott-Noakes technique where plotting M 1/β vs. ( H/M ) 1/γ is used to determine both the Curie temperature and composition. Modified Arrott plot approach was used in order to determine the new critical exponents β, γ and δ that better describe this compound. The Curie temperature of the Sc 3.1 In compound was found to be T C = 4.4 K and the critical composition x c = 0.02. This work was supported by NSF DMR 0847681.

The magnetic properties of a 15uc thick LSMO thin film on SrTiO3 (STO) substrate atT=150K was investigated using the technique of ferromagnetic resonance (FMR). The FMRmeasurement of the 15uc thick LSMO thin film at a frequency f = 9.75GHz and powerP = 0.6325mW as a function of the angle between the static magnetic field H, and theeasy direction of magnetization within the sample plane in the "in-plane" (IP) configuration displayed an FMR spectrum. This resonance spectrum shows unequal resonance field peaks. The unequal peaks in the resonance field may be attributed to the uniaxial anisotropy field which satisfies the conditions for ferromagnetic resonance. The unequal peaks in the resonance field shows a maximum and minimum with negative and positive curvature which either increases or decreases with respect to the resonance field respectively. This increase or decrease in the resonance field depends on the magnetization direction. It has been shown that for a thick 15uc LSMO thin film at T = 150K the center position and the full width half maximum (FWHM) of the resonance field were 1070.1875 Oe and 159.3125 Oe respectively.

The recovery and the relaxation behaviors of the exchange bias (HE) in a Co/Cu/FeNi/FeMnspin valve has been studied via point contact technique with high field scan rates ranging from 1050 to 13500 Oe/s. The evolution of the exchange bias with the number of field cycles(n) shows a one over the square root of (n) dependence as against ln(n) in a previous study. A recovery of the exchange bias when the field cycling is stopped has been observed and the recovery ratio (R) is found to be a function of the logarithm of the recovery time (log(t)) in agreement with previous studies. The relaxation time of the exchange bias is estimated to be 6.62, 20.85, 43.10, 109.02, and 297.24 s for 13500, 5400, 2700, 1350, and 1050 Oe/s respectively. The one over the square root of (n) dependence of HE and the log (t) dependence of R have been discussed using thermal activation model.

ABSTRACTPart IThe shear rheology of nano-clay particles when dispersed in liquid CO2 underelevated pressure using the Couette geometry is studied. A high pressure attachmentwith two different measuring systems - the double gap and Mooney systems- is used for this purpose. The clays are Laponite, Quick clay, Na-fluorohectoriteand Na-montmorillonite. The shear viscosities of the dispersions are measuredunder increasing shear rates or shear stresses. In addition an ageing test is carriedout for six days for a laponite-liquid CO2 dispersion. Inertia of the system iscompensated for by control shear stress tests.Part IIE-field induced chain formations of the nano-clay particles when dispersed in theair are recorded by a camera. A glove bag is prepared in such a way as to providea Nitrogen atmosphere and the procedure is repeated in this glove bag. In bothsituations the chain formations are observed. An attempt to measure the leakageof current within the electrode is also made by means of an operational amplifierand feedback resistor, even though the measurement of the current is uncertain.

We have presented density functional theory calculations of electronic and optical properties of ZnS, MnS, CrS, and effects of Mn and Cr impurities on electronic properties of pure ZnS using the periodic program BAND. In our calculations, we have used the generalized gradient approximation (GGA) of PW91 to approximate the exchange and correlation energies. The calculated electronic band structure results show that zinc-blende ZnS is a direct band gap material. The calculated electronic band structure and DOS results are in good agreement with previous theoretical and experimental results. Our results indicate that zinc-blende CrS shows a half-metallic behavior, while MnS shows semiconducting behavior. Mn and Cr impurity substitution for Zn result in an expansion of the ZnS crystal structure and creates bulk defect states within the band gap. Mn impurities mainly affect the deep valence bands and the shallow conduction bands of ZnS. Cr impurities mainly affect the shallow and the deep valence bands. Our results for band structure and DOS show that a half-metallic behavior is found in Cr substituted ZnS while semiconducting behavior is found in Mn substituted ZnS. Finally, the frequency dependent dielectric function was calculated to study optical properties. Our results for the dielectric function show that strong and direct absorption at the sharp peak in the spectral region around 6.05 eV is due to a high density of states at the X-point within the Brillouin zone. The calculated results showed intraband absorption in the low frequency region within the band gap, due to Mn and Cr impurity levels.

Even though in the past the photovoltaic industry was dominated by single-crystalline silicon this days multi-crystalline silicon is consider to be on of the most promising material for application in low manufacturing cost solar photovoltaic arrays, consequently it has a huge potential to dominate the single-crystalline silicon in the photovoltaic industry in the next decades. However the presence of crystal defects such as dislocation and grain boundaries in multi-crystalline solar cells hugely reducing the conversion efficiency of this material compared to single crystalline silicon solar cell. Hence realizing the widespread utilization of this material will require understanding and control of the effects of this defects on the photovoltaic cell performance. Therefor we will examine the local resistivity of a given multi-crystalline sample in a dark and how it is affected by the presence of grain boundaries and the electrical activity when we cross or with in the grain boundary or twin boundaries. We have measured the resistivity of a number of samples of multi-crystalline silicon using a multi-height four-point probe.

Thesis (Ph.D.)--Brown University, 2008. / Advisor: Xinsheng S. Ling. Includes bibliographical references (leaves 111-125).