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Thermal and Electrical Transport in Thin-Film Materials for Energy Management

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With the rapidly growing global demand in energy nowadays, innovation and technology become critical for a transition to renewable energy, high energy efficiency or low-carbon emission. Thin-film materials are especially favorable in energy management due to the easy integration into devices. The transport properties for heat and electrical conduction are critical not only to achieve a better fundamental understanding of underlying physics, but also to develop proper and efficient operation of devices. In order to apply the Materials Genome paradigm to calculate the properties of possible new energy materials, the performance of existing materials must be properly characterized, first. This thesis studies three materials corresponding to three aspects of energy management: storage, efficiency, and conversion. For energy storage, metal organic frameworks (MOF) are a category of highly porous materials with large surface area for natural gas adsorption and storage. Thermal conductivity is a crucial parameter for managing the exothermal process of gas adsorption as well as the endothermal process of gas desorption in MOFs, but experimental studies up to now have been limited. As a case study of MOFs, the cross-plane thermal conductivity of a zeolitic imidazolate framework (ZIF)-8 was experimentally determined on thin films using the 3-omega technique at different partial pressures in perfluorohexane, nitrogen, air, or vacuum ambients at room temperature. The observed thermal conductivity was observed to be approximately independent of ambient gas species and pressure ranging from atmospheric pressure down to vacuum. This approach of probing MOF thermal conductivity with gas adsorption establishes a method for studying MOFs with different gas ambients for effective thermal management for adsorbed natural gas applications. In terms of energy efficiency, transparent conducting oxide InGaZnO (IGZO) is a commercialized high-performance active channel material in transparent thin-film transistors. IGZO consumes less power due to its high carrier mobility, low leakage current and good transparency. For effective thermal management of IGZO-based devices, a comprehensive study measured the thermal conductivity of various different phases of amorphous (a-IGZO), semicrystalline (semi-c-IGZO), and c-axis-aligned single-crystal-like IGZO (c-IGZO) grown by various physical deposition and chemical synthesis approaches. The atomic structures of the amorphous and crystalline films were simulated with ab initio molecular dynamics, and the film morphology was assessed by multiple X-ray techniques. The temperature-dependent thermal conductivity showed pronounced dependence on porosity, crystallinity, and shelf time. All samples are consistent with the porosity-adapted Cahill−Pohl (p-CP) model of minimum thermal conductivity. Lastly for energy conversion, pxn-type transverse thermoelectric (TTE) is a new paradigm where the material shows n-type and p-type behavior in two orthogonal crystal axes, respectively, leading to effective heat flow perpendicular to the current flow. Both thin-film and bulk materials will be relevant for energy harvesting and energy management applications. To facilitate the rapid discovery of new pxn-type TTE by employing Material Genome, we introduce the criteria to identify bulk pxn TTE's from the calculated three-dimensional Seebeck tensor. A thorough search is conducted in the past literature for ambipolor compounds as pxn-type transverse thermoelectric candidates, for which the figure of merit and critical angles are calculated to evaluate their potential performance. To better understand the underlying mechanism of anisotropy, band structure and thermopower calculation are also conducted on two representative compounds. Besides the bulk ambipolar compounds, the type-II superlattice of InAs/GaSb is another pxn-type TTE which can be prepared in thin-film form. For these novel materials, the emphasis is currently on testing and developing the appropriate characterization methods of their thermal and electrical conduction so that these advanced materials can be researched and improved. Both the in-plane and cross-plane thermal conductivities are characterized by 2-wire 3-omega method. A measurement example of T2SL is also shown from room temperature down to 15 K. To deconvolve the electrical characteristics of each carrier species in this multi-carrier system, Fourier-domain mobility spectrum analysis (FMSA) is developed in Matlab with an intuitively simple algorithm, fast convergence, low computational cost, simplicity of implementation, and good fitting accuracy. The temperature-dependent measurement examples of T2SL are shown for both thermal conductivity and FMSA.

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  • 11/01/2018
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