Investigation and development of advanced models of thermoelectric generators for power generation applications

With developing interest in power generation applications of thermoelectrics and the growing influence of advanced materials on thermoelectric device fabrication, there is an increased demand for better understanding of module-level behavior. Likewise, novel module geometries are being explored for higher performance and require sophisticated modeling methods. In addition to new geometrical design, transport phenomena, such as Thomson heating and contact resistances, aggravate the complexity of modeling thermoelectric modules (TEMs) and thus limit design capability. Typically, these effects are either approximated (or in some cases neglected entirely) with little exploration in to the validity of the underlying assumptions associated with the approximation. As such, standard models are often predicated on assumptions that cannot be made beyond very limited operating regimes. Consequently, most TEM analysis generally utilizes simplistic methods of modeling on a module-level scale, which introduce inaccuracies that must be redressed. Particularly with larger temperature gradients, typically negligible effects could begin to impact overall system performance. Material property temperature-dependency, combined with leakage effects, leave much to be desired of the simple property-average-based models. Additionally, one-dimensional (1-D) models neglect the contribution of three-dimensional (3-D) module facets that can significantly impact TEM performance. To compound the analytical issue, complex material technologies are emerging that will require robust models for module design. With burgeoning focus in using thermoelectrics for waste heat recovery in automobiles, industrial processes and power plants, new application and commercial development of high temperature TEMs is imminent. However, modeling design and optimization of TEMs has been piecemeal at best. Hence, it is imperative that a comprehensive model be developed for TEMs that addresses some of the analytical problems stemming from over-simplification. The primary intention of this work is to develop and validate a comprehensive model that can be used as a TEM design tool and to quantify the error in the simple 1-D analytical models. The scope of this work is multifaceted. First, several models are developed, implemented and compared to each other as design tools that are useful for determining material performance and also for optimizing TEM performance. An improved 1-D analytical model, a unique asymptotic model and a comprehensive 3-D finite element (FE) model are created and established. These models are compared to each other for both validation and for quantification of error in the analytical models. Secondly, the quantification of error in 1-D analytical models based on module parameters, called error mapping, can be used as a design tool in and of itself to either identify regimes where a 1-D model is inaccurate (and thus establish when 3-D FE modeling is required), or as a corrective factor to a 1-D model. Thirdly, an experimental test stand is developed for device characterization, to be used either for system-level integration or for future model validation. Finally, the Thomson effect is analytically explored and detailed, and its contribution to the overall performance of a TEM is quantified. The role of the Thomson effect in previous analyitical models is nebulous, but has been elucidated in this thesis both with derivation and the development of the asymptotic model, which is the first analytical solution to the non-linear thermoelectric governing equations. Ultimately, this thesis defines the advantages and limitations of current TEM models, quantifies their error and provides several new design tools that can be used for material selection, module optimization and system-level design. These new design tools will provide new leverage to advance thermoelectrics as a robust power generation technology at a time when such capability is critical

The tribological behavior of graphene and its role as a protective coating

The scope of this thesis is to explore the fundamental tribological behavior of graphene as a two-dimensional (2-D) nanomaterial and evaluate its performance as a protective coating. Graphene is the strongest material ever measured, gas-impermeable, chemically and thermally stable, and atomically-thin, making it an excellent candidate as a protective coating. The fundamental tribological behavior of graphene and other 2-D materials under sliding conditions has only just begun to be explored. In particular, the wear of graphene has hardly been explored. The objective of this work is to investigate the tribological behavior of graphene through atomistic simulation as well as experimental testing under various sliding regimes and length scales. Wear in a graphene monolayer, after scratch tests with a nanoindenter, was characterized for the first time using Raman spectroscopy, revealing new insights into the failure of graphene after sliding. These sliding tests revealed a new frictional phenomenon where friction increased linearly with sliding length over large distances. This was caused by delamination likely due to the coalescence of small bubbles of gas trapped between the graphene monolayer and substrate during sliding, confirmed with atomic force microscopy. Furthermore, atomistic simulations of an asperity sliding over a graphene bubble mimicked experimental results, further supporting this bubble coalescence hypothesis. Graphene's potential as an anti-corrosive coating was demonstrated for macro-scale, commercially-available electrical connectors. It was demonstrated that even a monolayer of graphene can prevent oxide and reduce electrical contact resistance by orders of magnitude

Quantifying High-Performance Material Microstructure Using Nanomechanical Tools with Visual and Frequency Analysis

High-performance materials like ballistic fibers have remarkable mechanical properties owing to specific patterns of organization ranging from the molecular scale, to the micro scale and macro scale. Understanding these strategies for material organization is critical to improving the mechanical properties of these high-performance materials. In this work, atomic force microscopy (AFM) was used to detect changes in material composition at an extremely high resolution with transverse-stiffness scanning. New methods for direct quantification of material morphology were developed, and applied as an example to these AFM scans, although these methods can be applied to any spatially-resolved scans. These techniques were used to delineate between subtle morphological differences in commercial ultra-high-molecular-weight polyethylene (UHMWPE) fibers that have different processing conditions and mechanical properties as well as quantify morphology in commercial Kevlar®, a high-performance material with an entirely different organization strategy. Both frequency analysis and visual processing methods were used to systematically quantify the microstructure of the fiber samples in this study. These techniques are the first step in establishing structure-property relationships that can be used to inform synthesis and processing techniques to achieve desired morphologies, and thus superior mechanical performance

Background data for modulus mapping high-performance polyethylene fiber morphologies

The data included here provides a basis for understanding “Interior morphology of high-performance polyethylene fibers revealed by modulus mapping” (K.E. Strawhecker, E.J. Sandoz-Rosado, T.A. Stockdale, E.D. Laird, 2016) [1], in specific: the multi-frequency (AMFM) atomic force microscopy technique and its application to ultra-high-molecular-weight Polyethylene (UHMWPE) fibers. Furthermore, the data suggests why the Hertzian contact mechanics model can be used within the framework of AMFM theory, simple harmonic oscillator theory, and contact mechanics. The framework is first laid out followed by data showing cantilever dynamics, force-distance spectra in AC mode, and force-distance in contact mode using Polystyrene reference and UHMWPE. Finally topography and frequency shift (stiffness) maps are presented to show the cases where elastic versus plastic deformation may have occurred

Elastic properties of 2D ultrathin tungsten nitride crystals grown by chemical vapor deposition

3D transition metal nitrides are well recognized for their good electrical conductivity, superior mechanical properties, and high chemical stability. Recently, 2D transition metal nitrides have been successfully prepared in the form of nanosheets and show potential application in energy storage. However, the synthesis of highly crystalline and well-shaped 2D nitrides layers is still in demand for the investigation of their intrinsic physical properties. The present paper reports the growth of ultrathin tungsten nitride crystals on SiO2/Si substrates by a salt-assisted chemical vapor deposition method. High-resolution transmission microscopy confirms the as-grown samples are highly crystalline WN. The stiffness of ultrathin WN is investigated by atomic force microscopy–based nanoindentation with the film suspended on circular holes. The 3D Young's modulus of few-layer (4.5 nm thick or more) WN is determined to be 3.9 × 102 ± 1.6 × 102 GPa, which is comparable with the best experimental reported values in the 2D family except graphene and hexagonal boron nitride. The synthesis approach presented in this paper offers the possibilities of producing and utilizing other highly crystalline 2D transition-metal nitride crystals.Accepted versio