Thesis advisor: Cyril P. Opeil / Thermoelectric (TE) materials are of broad interest for alternate energy applications, specifically waste heat applications, as well as solid-state refrigeration. The efficiency of TE materials can be improved through either the enhancement of the Seebeck coefficient and electrical conductivity, or through the reduction of the thermal conductivity, k, specifically the lattice portion of thermal conductivity, klatt. Nanostructuring has been proven to reduce klatt and therefore increase efficiency. The inability to accurately model the lattice and electronic contributions to k makes optimizing the reduction of klatt difficult. This work demonstrates that the lattice and electronic contributions to k in nanostructured materials can be directly measured experimentally by separating the contributions using magnetic field. We use this technique along with other characterization techniques to determine the effects of doping Ce, Sm, and Ho into Bi88Sb12. Along with enhancing the efficiency of the material, TE devices must be thermally stable in the temperature range of operation. Therefore we also study the effects of temperature cycling, annealing, oxidation, and diffusion barriers on TE devices. These studies are accomplished through both homemade and commercially available measurement equipment. / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Pressure is mounting in the construction industry to adopt more environmentally sustainable methods to reduce CO2 emissions. Portland cement (PC) often constitutes to more than two-thirds of the embodied energy of concrete, and its production generates 5% of global greenhouse gas emissions. One efficient strategy to reduce the cement content without sacrificing performance is the use of particle packing models (PPM) to mix- proportion concrete mixtures with low cement content, the so-called low cement content (LCC) concrete. If on the one hand LCC was seen to be an effective sustainable alternative to the construction industry, its mechanical behaviour, durability and long-term performance are still under debate and thus further research is needed in the area. In this project, continuous PPM theories were used to mix- design structural concrete mixes presenting distinct mechanical properties (i.e. 25 & 35 MPa) and cement contents. Their performance was evaluated in the fresh and hardened states, and gaps, recommendations, and further needs were highlighted. Results show that the use of PPM enables the development of LCC systems, showing impressive hardened state performance (i.e. higher compressive strength and modulus of elasticity and lower electrical resistivity) and low carbon footprint. However, challenges in the fresh state were faced, which may be potentially solved with the use of chemical admixtures, fillers and/or supplementary cementing materials (SCMs).

Doping nonmagnetic materials is known as an effective way of investigating the properties of frustrated magnets. LiCuSbO4 is one of the simplest quasi-one dimensional spin-1/2 magnets which can be modelled with ferromagnetic(FM) nearest neighbour and antiferromagnetic (AFM) next nearest neighbour interactions. Here, doping with both non-magnetic ions, Zn, Mg, and magnetic ions, Co, is investigated. LiCu1-xMxSbO4 (M=Mg, Zn, Co 0≦x≦0.1) samples were synthesized by a ceramics process. At higher doping levels (x≧0.04), paramagnetic Curie features are observed below 4 K, however the broad peak characteristic of short range ordering at 6 K is retained. Isothermal magnetization indicates that the critical field found at 12 T in LiCuSbO4 was shifted by Zn and Mg doping. While the field is increased as the amount of Mg doping, it was increased as Zn doping in the range of 0≦x≦0.02 but decreased by x≧0.04. The trend in critical field is observed to follow that of the c lattice parameter for both Zn and Mg doping. On doping with Co2+ (S = 3/2), a low temperature Curie feature was observed from x=0.02. The value of the critical field increased on doping from (x=0) 12 T for 13.5 T (x=0.10). As for non-magnetic doping the trend in Hc has the same behaviour as the lattice parameter. The effect of doping on the pyrochlore spin ice A2B2O7 is also explored. The effect of oxygen vacancies induced by the aliovalent substitution on the B site on the crystal electric field was explored in the ceramic solid solutions. The effect of aliovalent doping on the pyrochlore A2Sn2(1-x)Sc2xO7-x (A=Ho and Dy 0≦x≦0.10) Tb2B2(1-x)Sc2xO7-x (B=Sn and Ti 0≦x≦0.05) were studied. While no dramatic changes of the saturation value of isothermal magnetization and heat capacities was observed in Dy2Sn2O7 by Sc doping in the range of 0≦x≦0.1, the saturation value of isothermal magnetization and magnetic entropy in Ho2Sn2O7 was clearly increased by Sc doping more than x=0.05, This difference could be from the difference of Kramer’s and non-Kramer’s spins between Dy and Ho, as while Dy is a Kramer’s ion and its ground state is protected, Ho is a non-Kramer’s ion and its ground state could be split. While Tb2Sn2O7 is known as quantum spin ice, Tb2Ti2O7 is known as spin liquid. A peak at 6 K of heat capacity, which is assigned as being due to a crystal electric field excitation to an excited doublet in Tb2Sn2O7 and Tb2Ti2O7 was observed in the Tb2Sn2(1-x)Sc2xO7-x sample. However in Tb2Ti2(1-x)Sc2xO7-x it was not observed. This indicates that the increased strain in the ceramic solid solution has a larger impact on the crystal electric field.

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