UMD Theses and Dissertations

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    COMBINATORIAL EXPLORATION OF HALF-HEUSLER (Ta0.4 Nb0.4 Ti0.2)–Fe–Sb THIN FILMS VIA HIGH-THROUGHPUT POWER FACTOR MAPPING AND FREQUENCY-DOMAIN THERMOREFLECTANCE (FDTR)
    (2023) Kirsch, Dylan; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Thermoelectrics (TEs) are a class of materials capabl¬¬e of converting heat into electricity in the solid state. Their widespread application is limited by the low efficiency (≈ 5 %) for commercial modules in applications such as waste heat recovery and refrigeration. Half-Heusler (hH) TE intermetallic alloys have good electrical properties that are easily tuned by doping but are limited in commercial deployment due to high thermal conductivity (TC). This limits the achievable thermal gradient across a TE module, reducing the efficiency. One method to improve hH alloy performance is to decrease the lattice contribution to the TC through solid-solution alloying. Combinatorial synthesis approaches have the advantage of rapid sample fabrication and characterization over a wide range of material compositions. This approach can provide insights into materials systems that could be missed using conventional synthesis approaches. Several publications reported p-type NbFeSb hH alloys can accommodate off-stoichiometry, which could positively impact the TE properties similar to TC decrease observed via Ta-alloying. Combinatorial thin film co-sputter synthesis of hH alloy (Ta0.40Nb0.40Ti0.20)-Fe-Sb composition spread libraries coupled with high throughput (HiTp) characterization is utilized to produce maps of the composition-structure-property relationships as a function of Fe- and Sb-content in this system for the first time. Continuous spread composition gradient and homogeneous discrete co-sputtered combinatorial thin film synthesis methodologies are leveraged to investigate the hH stability region and TE performance in (Ta0.40Nb0.40Ti0.20)-Fe-Sb. Combinatorial thin film characterization requires specialized custom or commercial instrumentation capable of scanning across samples. Established HiTp tools were utilized to characterize the crystal structure, electrical transport properties (Seebeck coefficient and electrical resistivity), and chemical composition of the films. A scanning thin film TC measurement instrument was not available prior to this dissertation. Without this ability, the dimensionless TE figure-of-merit zT cannot be calculated. To address this need, a custom, automated Frequency Domain Thermoreflectance (FDTR) instrument was designed and constructed. FDTR TC measurements are presented on single-phase F¯4 3m off stoichiometric discrete combinatorial hH (Ta0.40Nb0.40Ti0.20)-Fe-Sb for the first time. Maximum zT values at 296 K are calculated to be 0.076 for compositions (Nb0.412Ta0.327Ti0.261)28.5Fe40.3Sb31.2 and (Nb0.418Ta0.328Ti0.254)35.0Fe31.7Sb33.3 having TC values around 2.25 ± 0.27 W m-1 K-1.
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    Combinatorial Investigation of Magnetic Materials
    (2004-10-28) Aronova, Maria A; Takeuchi, Ichiro; Wellstood, Fred C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Combinatorial synthesis is an efficient tool that can be used to discover new materials. It allows one to systemically study a large number of materials simultaneously as their physical properties change with the varying chemical composition. Using this technique, we study various multifunctional electronic materials. Different designs of libraries, such as discrete libraries and composition spreads, are fabricated and characterized in order to rapidly map composition-structure-property relationships in a variety of systems. We have made gas sensor device libraries to optimize the performance of gas sensing materials. We have utilized the combinatorial pulsed laser deposition (PLD) flange for fabricating the discrete device library of doped SnO2 thin films. Several libraries were made with different amounts of dopants such as In2O3, WO3, ZnO, Pt, and Pd. After exposing the whole library to chloroform, formaldehyde, and benzene gases, the compositions most sensitive to these gases were found. We have also demonstrated the use of a gas sensor library as an electronic nose where responses from different devices are multiplexed to perform pattern recognition for distinguishing different gases at concentrations down to 12.5 ppm with high repeatability of response signals. Magnetic properties of composition spreads and discrete libraries are analyzed by a scanning SQUID microscope. The in-plane and out-of-plane magnetization distributions are calculated from the magnetic field data using the inversion technique. Various parameters that control the inversion technique are discussed in detail and optimized with the help of simulated data. By applying the inversion technique to thin-film discrete libraries, we have mapped the functional phase diagram of Ni-Mn-Ga system whose Heusler composition Ni2MnGa is a well known ferromagnetic shape memory alloys (FSMAs). A large, previously unexplored compositional region of FSMAs outside the Heusler composition is found. In search for novel multiferroic materials, we have fabricated PbTiO3 (PTO)-CoFe2O4 (CFO) composition spreads using the combinatorial PLD. After calculating the in-plane and out-of-plane remanent magnetization distributions with the help of the inversion technique, it was found that when PTO is added to CFO, CFO's magnetic anisotropy changes. Furthermore, we found that the compositional region between (PTO)0.5(CFO)0.5 and (PTO)0.8(CFO)0.2 exhibits the coexistence of ferroelectricity and ferromagnetism.