Our main emphasis in this thesis work is on the fundamental aspects of different types of magnetic ground states seen in the intermetallic system known as Heusler alloy. On those grounds, discovering new magnetic materials and new magnetic spin-based experimental probes are more and more essential. At present, our investigations of magnetic phenomena are guided mainly by the need for more energy-saving technologies which also demand extensive work to achieve a basic understanding of different aspects of magnetism in condensed matter. Later, the properties of the magnetic domain walls were studied in detail by Bloch, Landau, and N`eel, paving the way to modern magnetism. Weiss postulated the existence of magnetic domains, and molecular field, thus providing an explanation for how material could be magnetized, and nevertheless, have a net-zero magnetization. It is mainly in the 20th century when the foundations of magnetism-based modern technology were laid as Curie and Weiss successfully described the phenomenon of spontaneous magnetization and its temperature dependence. Based on the calculated results we conclude that one can design high efficiency thermoelectric materials by considering 18 VEC rule with aliovalent substitution. Among the considered systems, the calculated phonon spectra and heat capacity show that La 0.25 Hf 0.5 V 0.25 NiSn has more optical-acoustic band mixing which creates more phonon-phonon scattering and hence lower the value and maximizing the ZT. The calculated for Hf containing systems La 0.25 Hf 0.5 V 0.25 NiSn and non Hf containing system La 0.25 Zr 0.5 V 0.25 NiSn calculated from Phono3py (Slack's equation) are found to be 0.37 (1.04) and 0.16 (0.95) W/mK, at 550 K, respectively and the corresponding ZT value are found to be 0.54 (0.4) and 0.77 (0.53). The substitution of atoms with different mass creates more phonon scattering centers and hence lower the value. The calculated values decreases from parent TiNiSn to pentanary substituted TiNiSn systems as expected due to fluctuation in atomic mass. At high temperatures the calculated and ZT values from both these methods show very good agreement.
HALF HEUSLER PHONO DISPERSIO CODE
Two approaches have been used for calculating the lattice thermal conductivity (), one by fully solving the linearized phonon Boltzmann transport (LBTE) equation from first-principles anharmonic lattice dynamics calculations implemented in Phono3py code and other using Slack's equation with calculated Debye temperature and Grüneisen parameter using the calculated elastic constant values. From our calculated band structures and density of states we show that by preserving the 18 VEC through aliovalent substitutions at Ti site of TiNiSn semiconducting behavior can be achieved and hence one can tune the band structure and band gap to maximize the thermoelectric figure of merit (ZT) value. We have performed the detailed analysis of electronic structure, lattice dynamics, and TE transport properties of selected systems from this family. We have modeled the pentanary substituted TiNiSn by supercell approach with the aliovalent substitution, inspite of the traditional isoelectronic substitution. In this study we have reported electronic structure, lattice dynamics, and thermoelectric (TE) transport properties of a new family of pentanary substituted TiNiSn systems using the 18 valence electron count (VEC) rule. 35% compared to the zT of the parent compound TiNiSn. The best performing samples with around 5% of TiNi2Sn phase exhibit maximum figures of merit of almost 0.6 between 750 K and 800 K which is an increase of ca. This trend shows that phonons are scattered effectively as a result of the microstructure of the materials with full-Heusler inclusions in the size range of microns to tens of microns. We observe a clear correlation between the amount of full-Heusler phase and the lattice thermal conductivity of the samples, resulting in decreasing total thermal conductivity with increasing TiNi2Sn fraction. Synchrotron powder X-ray diffraction and microprobe data confirm the presence of a secondary TiNi2Sn full-Heusler phase within the half-Heusler matrix. Spark Plasma Sintering plays an important role in the process by being a part of the synthesis of the material rather than solely a densification technique.
HALF HEUSLER PHONO DISPERSIO SERIES
In this contribution, we used an energy- and time-efficient process involving solid-state preparation in a commercial microwave oven and a fast consolidation technique, Spark Plasma Sintering, to prepare a series of Ni-rich TiNi1+xSn with small deviations from the half-Heusler composition. TiNiSn is of particular interest and - with its relatively high electrical conductivity and Seebeck coefficient - allows for optimization of its thermoelectric figure of merit, reaching values of up to 1 in heavily-doped and/or phase-segregated systems. Half-Heusler thermoelectrics offer the possibility to choose from a variety of non-toxic and earth-abundant elements.
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