Electrons are the major heat carriers in metals, as are phonons in semiconductors. The role of spin waves (magnons) in thermal transport problems has attracted attention in recent years with the discovery of spin Seebeck effects (SSE) in spintronics. Interactions among these particles or excitations are the origin of many fascinating phenomena and the focus of this work. Despite the computational cost, first-principles calculations use fewer approximations and no fitting parameters in comparison to semi-classical methods; therefore they produce more reliable results. Chapter 1 considers the theory of electron-electron, electron-phonon and phonon-magnon couplings from first principles. Chapter 2 reports first-principles calculations of electron-phonon coupling in bilayer graphene and the corresponding contribution to carrier scattering. At the phonon Γ point, electrons with energies less than 200 meV are scattered predominantly by LA′ and TA′ modes while higher- energy electron scattering is dominated by optical phonon modes. Based on a two-temperature model, heat transfer from electrons with an initial temperature of 2000 K to the lattice (phonons) with an initial temperature of 300 K is computed, and in the overall relaxation process, most of this energy scatters into K-point phonon optical modes due to their strong coupling with electrons and their high energies. A Drude model is used to calculate photoconductivity for bilayer graphene with different doping levels. Good agreement with prior experimental trends for both the real and imaginary components of photoconductivity confirms the model’s applicability. The effects of doping levels and electron-phonon scattering on photoconductiviy are analyzed. We also extract acoustic and optical deformation potentials from average scattering rates obtained from density functional theory (DFT) calculations and compare associated photoconductivity calculations with DFT results. The comparison indicates that momentum-dependent electron-phonon scattering potentials are required to provide accurate predictions. Chapter 3 combines first-principles calculations, spin-lattice dynamics and the non-equilibrium Green’s function (NEGF) method to compute thermal boundary conductance at a three-dimensional Co-Cu interface, considering spin-lattice interactions. Spin-lattice interactions are quantified through exchange interactions between spins, and the exchange constants are obtained from first principles. Equilibrium molecular dynamics (EMD) is used to calculate heat flux across the interface after the spin and lattice subsystems are in equilibrium. Because of the weak interaction between Co and Cu layers adjacent to the interface, spin-wave transmission is low. Spins are scattered by phonons inside the Co contact, and interfacial thermal conductance is reduced. We also compare the results to the NEGF method. Phonon and magnon scattering rates are incorporated into Buttiker probes attached to the device. NEGF results exhibit a similar trend in thermal boudary conductance with spins included. The Green’s function is solved recursively; therefore it can be applied to large devices. Chapter 4 investigates electronic and optical properties of single layer and bilayer armchair graphene nanoribbons using the first-principles method. An increase of nanoribbon width reduces the band gap and causes a redshift in photon absorption energy. We find that the 3n + 2 family nanoribbons have the smallest band gaps and lowest onset photon absorption energy among all three families considered due to the most π-conjugation indicated by the exciton wavefunctions. We also compare the bilayer α and β alignments of armchair graphene nanoribbons with their single-layer counterparts.The extra layer of these nanoribbons reduces the band gap and the onset photon absorption energy, and the difference between the α alignment and the single-layer configuration is more significant than that of the β alignment and the single layer. Our calculations indicate that the optical properties of graphene nanoribbons depend on the details of atomic structures, including nanoribbon width, edge alignment, and number of layers. Chapter 5 investigates the photo-thermal effect in the methane decomposition process. By calculating electronic transitions in polycyclic aromatic hydrocarbons from TDDFT, we extract the absorption coefficients. Further, the absorption coefficients are mapped to the experimental light intensity profile to predict the total absorption spectra. Temperature rise can be further induced by calculating heat capacities of the polycyclic aromatic hydrocarbons using frequencies of the vibrational modes.

Spectroscopy methods are widely used for system characterization. Typically a perturbationsignal is introduced to the system, and corresponding outputs are detected. The underlying properties of a system can be determined by comparing the output to the input. This work employs advanced statistical methods to improve overall performance of such methods applied to energy transport and storage, in terms of accuracy, time efficiency and cost for spectroscopy methods to characterize thermal and electrochemical systems.

The rest part of this work reports a custom spectroscopy system that employs periodicheating to characterize thermal diffusivity under ambient conditions; the technique is also known as Angstrom's method. We employ forced convection to reduce variation in convective heat transfer losses along the heat propagation direction to enable the experiment to be conducted outside vacuum. We employ IR thermography to for data-rich temperature detection and further introduce a Bayesian framework for uncertainty quantification and uncertainty reduction with increased data. We demonstrate accurate results (< 5% error) for multiple short metal strips.

In the second part we extend the room temperature spectroscopy approach to high temperatures.For high-temperature characterization, the testing environment is difficult to control precisely; therefore, additional unknowns exist in the physical model as compared to low-temperature measurements. In addition, nonlinear radiation loss are present, and the transient heat transfer process must be modeled numerically. Solving the inverse problem using conventional regression approaches is challenging because the solutions are not unique and lack uncertainty estimates. Therefore, we develop a Bayesian framework to solve the inverse problem. The main challenges overcome are: (1) probing the posterior distribution given a computationally expensive forward model and (2) achieving convergence in the sampling process for a model with a relatively high number of unknowns. In this study we report a computationally efficient parametric surrogate model to accelerate the Bayesian analysis and employ a No-U-Turn sampler to achieve good convergence in the sampling process. The custom instrument exhibits high accuracy (approx. 5% error) and requires only 10 min. obtain steady high-temperature results, compared to several hours using conventional methods and commercial instruments.

Lastly we demonstrate a broadband spectroscopy instrument for electrochemical impedancespectroscopy (EIS) measurements for supercapacitors. Measuring EIS is challenging because (1) low-frequency scans are time consuming and (2) impedance of supercapacitors typically changes by several orders of magnitude across a wide frequency band. The custom instrument employs a broadband exponential chirp signal for rapid frequency scanning. To account for large impedance variations, we report a custom circuit for automatically adaptable impedance matching during measurements. The custom instrument is accurate and exhibits four times measurement time reduction and ten times cost reduction compared to leading commercial instruments.

Flash cooling is a promising high-heat-flux cooling solution that utilizes pressure-controlled boiling of liquid coolant. This two-phase cooling solution is inherently transient and therefore less understood than traditional cooling approaches. Flash cooling has been studied previously based on individual cooling pulses rather than a continuous convective cooling solution. Convective cooling is an integral part of high-performance computing infrastructure with emerging needs for two-phase cooling due to increasing heat fluxes. The primary objective of this work is to enable dynamic thermal management of high-heat flux high-performance systems through foundational understanding of the transient mechanisms associated with pulsed, pressure-driven flash cooling techniques. A thermal testbed is developed for continuous, pulsed experiments for flash cooling with sufficient flexibility to adapt to several cooling architectures and electronic devices. A set of surrogate models are developed to enable system-design and run-time prediction for flash cooling that can be deployed for specific applications. The emphasis is on the integration of thermal data for optimal and predictive flash cooling. A next-generation high-heat-flux application that requires dynamic cooling is Silicon Interconnect Fabric, which is a platform that allows heterogeneous integration and system scaling on a silicon wafer. This platform requires large heat removal over a large area without any accommodation for heat spreading outside of the active zones. Three innovative architectures are developed to provide flash cooling for this application. The fabrication of individual prototypes and comparison of preliminary steady-periodic testing provides vital information for selection of architecture. A detailed analysis of one of the architectures – the segmented pin-fin cooling chambers, is performed with a set of thirty experiments spanning wide range of heating and cooling conditions. Several representative surrogate models are developed from this thermal data for this application that demonstrates the efficacy of flash cooling for dynamic cooling. This work lays a strong and wide foundation for understanding transient behavior of flash cooling for high-performance electronic systems, and also provides significant support material for enabling further research to enable adoption of flash cooling for broader commercial applications.

As demand for high performance computing continues its rapid growth, the required digital processing capabilities necessitate more powerful integrated circuits. High power density chips produce a challenging thermal engineering problem by generating heat fluxes greater than 0.5 W/mm2 while sometimes providing diminishing provisions for heat spreading. A pressure-driven boiling approach, termed flash cooling, is considered here as a solution for high heat flux thermal management. The thesis study aims to improve the Technology Readiness Level of flash cooling for high heat flux electronic cooling applications, especially targeted for wafer-scale systems. Furthermore, the objective is also extended to understand the physics involved in flash cooling and associated limitations. A closed-loop flash cooling system is developed with evaporator, accumulator, vacuum pump, condenser, and reservoir. Experiments are conducted by varying the heat flux from 0.2 W/mm2 to 1 W/mm2. The important control parameters are determined to be heat flux, pulse cycle time and flow rate. The results are interpreted to identify dominant parameters and to develop basic correlations among them. The experiments not only prove the dynamic and transient cooling ability of pulsed flash cooling but also exhibit dry-out recovery characteristics under short pulse cycle times. Further research directions are also suggested to improve the flash cooling technology.

Current hydrogen and carbon production technologies emit massive amounts of CO2 that threaten Earth’s climate stability, especially as demands for these materials continue to grow. Compared to alternative clean hydrogen production technologies, solar methane pyrolysis has lower energy requirements, produces carbon materials of commercial interest, and provides higher process efficiencies. In this work, a new solar-thermal methane decomposition process involving flow through a fibrous carbon medium to co-produce hydrogen gas and high-value graphitic carbon product with zero CO2 emissions is presented and thoroughly investigated. A 10 kWe custom-designed and built solar simulator is used to instigate the methane decomposition reaction with direct irradiation in a custom solar reactor. In contrast to prior work on solar methane pyrolysis, the present process reaches steady-state thermal and chemical operation from room temperature within the first minute of irradiation due to localized, direct solar heating of fibrous medium. Additionally, the present approach provides enhanced thermal transfer and efficiency, delivers graphitic carbon product in an easy to handle and extract form, and prevents undesired carbon deposition within the reactor that would otherwise lead to process interruption. These aspects are ongoing challenges reported in prior literature. In contrast to similar methane decomposition prior work that reports production of amorphous carbon black, this work produces high-quality graphite with production rates that are order(s) of magnitude higher. Parametric variations of methane inlet flow rate (10-2000 sccm), solar power (0.92-2.49 kW), operating pressure (1.33-40 kPa), and medium thickness (0.36-9.6 mm) are thoroughly presented, with methane conversions as high as 96% and graphite Raman D/G peak ratios as low as 0.06. A pathway to process scale-up for continuous production is presented by implementing a roll-to-roll processing method, which was effective in achieving continuous processing with methane conversion enhancements up to 1.5 times higher. The optical design of the solar reactor was then optimized using a secondary concentrator, by which solar-to-chemical efficiencies increased by up to 62% to reach a maximum demonstrated efficiency of 6.5%. Realizing this process at scale would avoid emissions of 10 kg of CO2 per kg of H2 and 5 kg of CO2 per kg of graphite.

Supercritical CO2 (sCO2) Brayton cycles are promising power cycles due to their high the- oretical efficiencies and power densities. However, these cycles require the development of highly compact and effective sCO2 heat exchangers in order to achieve these potential ben- efits. This study experimentally characterizes the heat transfer performance of superalloy finned tubes in sCO2 cross-flow for eventual use in shell and tube heat exchangers. Two finned tube designs, with disc and parabolic fins, were tested against a bare tube in cross- flow with with Reynolds numbers ranging from 3,500 ≤ Re ≤ 8,000. Both finned tube designs significantly outperformed the bare tube, and fins were shown to increase the Nus- selt number of the tube by more than double in every case. The disc-finned tubes achieved higher overall heat transfer, but the parabolic fins are shown to have higher fin efficiency in every case. Lastly, this study compares the experimental results with correlations for Nusselt number and fin efficiency developed in previous studies.

High-temperature supercritical CO2 Brayton cycles are promising candidates for future stationary power generation and hybrid electric propulsion applications. Supercritical CO2 thermal cycles potentially achieve higher energy density and thermal efficiency by operating at elevated temperatures and pressures. Heat exchangers are indispensable components of aerospace systems and improve efficiency of operation by providing necessary heat input, recovery, and dissipation. Tubular heat exchangers with unconventionally small tube sizes (tube diameters less than 5 mm) are promising components for supercritical CO2 cycles and provide excellent structural stability. Accurate and computationally efficient estimation of heat exchanger performance metrics at elevated temperatures and pressures is important for the design and optimization of sCO2 systems and thermal cycles. In this study, new Colburn and friction factor correlations are developed to quantify shell-side heat transfer and friction characteristics of flow within heat exchangers in the shell-and-tube configuration. Using experimental and CFD data sets from existing literature, multivariate regression analysis is conducted to achieve correlations that capture the effects of multiple critical geometric parameters. These correlations offer superior accuracy and versatility as compared to previous studies and predict the thermohydraulic performance of about 90% of the existing experimental and CFD data within �15%. Supplementary thermohydraulic performance data is acquired from CFD simulations with sCO2 as the working fluid to validate the developed correlations and to demonstrate application to sCO2 heat exchangers. A computationally efficient and accurate numerical model is developed to predict the performance of STHXs. The highly accurate correlations are utilized to improve the accuracy of performance pre- dictions, and the concept of volume averaging is used to abstract the geometry for reduced computation time. The numerical model is validated by comparison with CFD simulations and provides high accuracy and significantly lower computation time compared to exist- ing numerical models. A preliminary optimization study is conducted, and the advantage of using supercritical CO2 as a working fluid for energy systems is demonstrated. A microtube heat exchanger is fabricated, and essential design and fabrication guidelines of a compact shell-and-tube heat exchanger with microtubes (with inner diameters of 1.75 mm) are provided. A heat exchanger test rig is used to evaluate the thermohydraulic performance of this heat exchanger with supercritical CO2 and air as working fluids. Thermohydraulic data are reported for more than forty sets of experiments with varying Reynolds numbers for shell and tube flows. Critical performance metrics are calculated from the data and compared with predictions from the numerical model. The average deviations between the experimental and model results fall within 10% for all critical metrics. This excellent agreement validates the numerical model for supercritical CO2 heat exchanger optimization and scale-up. A generalized costing model is developed to estimate the capital costs incurred to manufacture microtube shell-and-tube heat exchangers. This model is utilized in conjunction with an accurate and efficient 2D numerical shell-and-tube heat exchanger performance prediction model to conduct optimization studies with two key objectives - minimization of cost and maximization of heat exchanger power density - on supercritical CO2 microtube heat exchangers utilizing superalloy Haynes 282 as the solid material. A methodology is then demonstrated to optimize these heat exchangers for aerospace applications, and highly compact and cost-effective optimal designs with power density around 20 kW/kg and cost per conductance less than 5 $ � K/W are obtained.

Flash boiling is a two-phase cooling method based on the phenomenon of flash boiling in which a working fluid is rapidly vaporized upon sudden depressurization, allowing for rapid cooling through conversion of the fluid’s sensible heat to latent heat. In this process, the sensible heat of surrounding environment around the working fluid decreases. Therefore, flash cooling is a promising candidate for transient thermal management. The phase-change process associated with flash boiling differs from traditional thermally-driven boiling, as flash boiling inception is controlled by a different thermodynamic variable – pressure. Consequently, by allowing pressure to be the driving factor, temperature becomes a variable performance metric for flash cooling in practical devices. To better understand the nature of flash boiling incipience and measure flash cooling rates, experiments were constructed to measure depressurization wave propagation through a tube and transient flash cooling in a vapor chamber for various configurations and heating loads. Additionally, a metric for flash cooling based on time constants is presented to help quantify transient cooling performance.