arXiv subjects

Asegun Henry

Publications and source records attributed to Asegun Henry.

At least 19 recordsLinked to original sources

arXiv2025

Correlated Terahertz phonon-ion interactions control ion conduction in a solid electrolyte

Ionic conduction in solids that exceeds 1 mS/cm is predicted to involve coupled phonon-ion interactions in the crystal lattice. Here, we use theory and experiment to measure the possible contribution of coupled phonon-ion hopping modes which enhance Li+ migration in Li0.5La0.5TiO3 (LLTO). The ab initio calculations predict that the targeted excitation of individual TiO6 rocking modes greatly increases the Li+ jump rate as compared to the excitation of vibrational modes associated with heating. Experimentally, coherently driving TiO6 rocking modes via terahertz (THz) illumination leads to a ten-fold decrease in the differential impedance compared to the excitation of acoustic and optical phonons. Additionally, we differentiate the ultrafast responses of LLTO due to ultrafast heating and THz-range vibrations using laser-driven spectroscopy (LUIS), finding a unique long-lived response for the THz-range excitation. These findings provide new insights into coupled ion migration mechanisms, indicating the important role of THz-range coupled phonon-ion hopping modes in enabling fast ion conduction at room temperature.

cond-mat.mtrl-sci↗

arXiv2025

High emissivity, thermally robust emitters for high power density thermophotovoltaics

Thermal radiative energy transport is essential for high-temperature energy harvesting technologies, including thermophotovoltaics (TPVs) and grid-scale thermal energy storage. However, the inherently low emissivity of conventional high-temperature materials constrains radiative energy transfer, thereby limiting both system performance and technoeconomic viability. Here, we demonstrate ultrafast femtosecond laser-material interactions to transform diverse materials into near-blackbody surfaces with broadband spectral emissivity above 0.96. This enhancement arises from hierarchically engineered light-trapping microstructures enriched with nanoscale features, effectively decoupling surface optical properties from bulk thermomechanical properties. These laser blackened surfaces (LaBS) exhibit exceptional thermal stability, retaining high emissivity for over 100 hours at temperatures exceeding 1000°C, even in oxidizing environments. When applied as TPV thermal emitters, Ta LaBS double electrical power output from 2.19 to 4.10 W cm-2 at 2200°C while sustaining TPV conversion efficiencies above 30%. This versatile, largely material-independent technique offers a scalable and economically viable pathway to enhance emissivity for advanced thermal energy applications.

cond-mat.mtrl-sci↗

arXiv2025

Thermophotovoltaic performance metrics and techno-economics: efficiency vs. power density

Thermophotovoltaics (TPV) are a promising new approach for converting heat to electricity. Their performance is primarily characterized by two metrics: efficiency and power density. While recent works have shown high efficiency, it is important to understand how both of these metrics impact the techno-economics of a TPV system as efforts to commercialize the technology advance. In this work, we develop the first unification of efficiency and power density into a single techno-economic metric based on the levelized cost of electricity (LCOE). We find that the LCOE can be broken into two parts: heating cost, including infrastructure and inputs for providing heat to the TPV cells, and cell cost, the capital cost of the TPV cells. We show that systems with high heating costs should prioritize TPV efficiency, while systems with high cell costs should prioritize power density. We then develop a model to identify the most impactful cell properties in improving the important performance metric and reducing system LCOE. Namely, improving spectral control with increased back-surface reflectance is the most effective to reduce LCOE in systems with high infrastructural costs, while increasing the view factor and reducing front-surface reflectance are most critical in systems with high TPV cell cost. Improving just one or two of these properties can reduce the LCOE by 25-75%, reaching competitive values ~ 8 cents/kWh-e, less than the average cost of electricity in the US. This study thus elucidates which TPV performance metric is more important for system technoeconomics and how to maximize it.

physics.app-ph↗

arXiv2024

Designing for effective heat transfer in a solid thermal energy storage system

Thermal energy storage using sensible heating of a solid storage medium is a potential low-cost technology for long-duration energy storage. To effectively get heat in and out of the solid material, channels of heat transfer fluid can be embedded within the storage material. Here we present design principles to improve performance of channel-embedded thermal energy storage systems, and we apply these principles to a high-temperature system using graphite as the storage material and liquid tin as the heat transfer fluid. We first analyze the impact of geometry and material properties on the performance of the system, determining the ideal channel spacing and length to achieve high (dis)charge temperature uniformity. We then analyze how controlling the fluid flowrate, heating infrastructure, and heat engine can increase discharge power uniformity and accelerate charging. Finally, we model 100 high-temperature graphite storage blocks using a porous media approximation and implement the developed design principles to demonstrate significant improvement in performance for both discharging (constant discharge power for >90% of rated duration) and charging (>90% charged within 4 hours). Overall, the hierarchical design procedure presented here enables the design of cheap yet high-performing solid thermal energy storage systems.

physics.app-ph↗

arXiv2023

Exploring Model Complexity in Machine Learned Potentials for Simulated Properties

Machine learning (ML) enables the development of interatomic potentials that promise the accuracy of first principles methods while retaining the low cost and parallel efficiency of empirical potentials. While ML potentials traditionally use atom-centered descriptors as inputs, different models such as linear regression and neural networks can map these descriptors to atomic energies and forces. This begs the question: what is the improvement in accuracy due to model complexity irrespective of choice of descriptors? We curate three datasets to investigate this question in terms of ab initio energy and force errors: (1) solid and liquid silicon, (2) gallium nitride, and (3) the superionic conductor LGPS. We further investigate how these errors affect simulated properties with these models and verify if the improvement in fitting errors corresponds to measurable improvement in property prediction. Since linear and nonlinear regression models have different advantages and disadvantages, the results presented herein help researchers choose models for their particular application. By assessing different models, we observe correlations between fitting quantity (e.g. atomic force) error and simulated property error with respect to ab initio values. Such observations can be repeated by other researchers to determine the level of accuracy, and hence model complexity, needed for their particular systems of interest.

cond-mat.mtrl-sci↗

arXiv2023

High-temperature thermal conductivity measurements of macro-porous graphite

Graphite is a unique material for high temperature applications and will likely become increasingly important as we attempt to electrify industrial applications. However, high-quality graphite can be expensive, limiting the cost-competitiveness of high-quality graphite technologies. Here, we investigate the thermal properties of low-cost, low-quality, macro-porous graphite to determine the tradeoff between cost and thermal performance. We use laser flash analysis (LFA) to measure the thermal diffusivity of graphite at high temperatures. However, due to the large pores in the graphite samples preventing uniform laser flash heating, we must apply a thick coating to achieve the required flat, parallel surfaces for LFA measurements. The presence of the coating directly impacts the measured diffusivity, not only because of the added thickness but also because of the sample/coating interface profile generated. We therefore develop a methodology based on finite element modeling of a variety of sample/coating interface profiles to extract properties of the sample. Validating the methodology against a reference sample demonstrates a mean absolute percentage error of 8.5%, with potential improvement with better sample characterization. We show low-cost, low-quality graphite has a thermal conductivity of ~10 W/m/K up to 1000$^{\circ}$C, which is an order of magnitude lower than high-quality graphite, but contributions from photon conductivity may result in higher conductivities at higher temperatures. Overall, we demonstrate an approach for measuring thermal properties of macro-porous materials at high temperatures, and apply the approach to measuring thermal conductivity of porous graphite, which will aid in the design of high-temperature systems for cost-competitive decarbonization.

cond-mat.mtrl-sci↗

arXiv2023

Power Availability of PV plus Thermal Batteries in real-world electric power grids

As variable renewable energy sources comprise a growing share of total electricity generation, energy storage technologies are becoming increasingly critical for balancing energy generation and demand. In this study, we modeled an existing thermal energy storage unit with estimated capital costs that are sufficiently low to enable large-scale deployment in the electric power system. Our analysis emphasizes the value of using such units to cost-effectively improve renewable energy dispatchability. This study modeled an existing real-world grid rather than simulating hypothetical future electric power systems. The storage unit coupled with a photovoltaic (PV) system was modeled with different storage capacities, whereas each storage unit size had various discharge capacities. The modeling was performed under a baseline case with no emission constraints and under hypothetical scenarios in which CO$_2$ emissions were reduced. The results show that power availability increases with increasing storage size and vastly increases in the hypothetical CO$_2$ reduction scenarios, as the storage unit is utilized differently. When CO$_2$ emissions are reduced, the power system must be less dependent on fossil fuel technologies that currently serve the grid, and thus rely more on the power that is served from the PV + storage unit. The proposed approach can provide increased knowledge to power system planners regarding how adding PV + storage systems to existing grids can contribute to the efficient stepwise decarbonization of electric power systems.

math.DS↗

arXiv2021

Thermophotovoltaic Efficiency of 40%

We report the fabrication and measurement of thermophotovoltaic (TPV) cells with efficiencies of >40%, which is a record high TPV efficiency and the first experimental demonstration of the efficiency of high-bandgap tandem TPV cells. TPV efficiency was determined by simultaneous measurement of electric power output and heat dissipation from the device via calorimetry. The TPV cells are two-junction devices comprising high-quality III-V materials with band gaps between 1.0 and 1.4 eV that are optimized for high emitter temperatures of 1900-2400°C. The cells exploit the concept of band-edge spectral filtering to obtain high efficiency, using high-reflectivity back surface reflectors to reject unusable sub-bandgap radiation back to the emitter. A 1.4/1.2 eV device reached a maximum efficiency of (41.1 +/- 1)% operating at a power density of 2.39 W/cm2 under an irradiance of 30.4 W/cm2 and emitter temperature of 2400°C. A 1.2/1.0 device reached a maximum efficiency of (39.3 +/- 1)% operating at a power density of 1.8 W/cm2 under an irradiance of 20.1 W/cm2 and emitter temperature of 2127°C. These cells can be integrated into a TPV system for thermal energy grid storage (TEGS) to enable dispatchable renewable energy. These new TPV cells enable a pathway for TEGS to reach sufficiently high efficiency and sufficiently low cost to enable full decarbonization of the grid. Furthermore, the high demonstrated efficiency also gives TPV the potential to compete with turbine-based heat engines for large-scale power production with respect to both cost and performance, thereby enabling possible usage in natural gas or hydrogen-fueled electricity production.

physics.app-ph↗

arXiv2021

Technoeconomic Analysis of Thermal Energy Grid Storage Using Graphite and Tin

Energy storage is needed to enable dispatchable renewable energy supply and thereby full decarbonization of the grid. However, this can only occur with drastic cost reductions compared to current battery technology, with predicted targets for the cost per unit energy (CPE) below $20/kWh. Notably, for full decarbonization, long duration storage up to 100 hrs will be needed at such low costs, and prior analyses have shown that in such high renewable penetration scenarios, CPE is more critical than other parameters such as round trip efficiency or cost per unit power when comparing the costs of different technologies. Here, we introduce an electricity storage concept that stores electricity as sensible heat in graphite storage blocks and uses multi-junction thermophotovoltaics (TPV) as a heat engine to convert it back to electricity on demand. This design is an outgrowth of the system proposed by Amy et al. in 2019, which has been modified here to use a solid graphite medium and molten tin as a heat transfer fluid rather than silicon as both. The reason for this is two-fold: (1) the CPE of graphite is almost 10X lower than that of silicon, which derives from the lower cost per unit mass (i.e., $0.5/kg vs. $1.5/kg) and the higher heat capacity per unit mass (2000 J/kg-K vs. 950 J/kg-K); and (2) the melting point tin and solubility of tin in graphite are much lower than that of silicon, which lessens the number of issues that have to overcome along the research and development pathway. The usage of graphite also eliminates the need for a second tank, but the main disadvantage of using a solid medium is that one cannot easily provide a steady discharge rate, as the power output from the storage will change with time, as the graphite cools during discharge. Thus, the objective of this work is to examine how these changes in the system design effect the overall technoeconomics.

physics.app-ph↗

arXiv2021

A Computational Method for Studying Vibrational Mode Dynamics

The traditional picture of heat transfer in solids by atomic vibrations, also known as phonons, involves phonons scattering with each other like gas particles and is commonly referred to as the phonon gas model (PGM). This physical picture accounts for interactions among propagating (i.e., plane wave modulated) vibrational modes in an ideal crystal, but it becomes problematic when describing non-propagating modes arising in realistic non-idealized systems. Here, we introduce a more general formalism for studying phonon transport, which involves projection of the interatomic interactions themselves (i.e., not just the atom motion), onto the normal modes of the system. This shows, for the first time, how energy is exchanged between modes in real-time during molecular dynamics (MD) simulations, as opposed to other MD methods which use inferences based on correlations, or other time averaged schemes that do not preserve specific features in the real-time dynamics. Applying this formalism to the example case of modes interacting in a superlattice, we illustrate a new perspective on how phonon transport occurs, whereby individual normal modes share energy through specific channels of interaction with other modes. We also highlight that while a myriad of interaction pathways exist, only a tiny fraction of these pathways actually transfer significant amounts of energy, which is surprising. The approach allows for the prediction and simulation of these mode/phonon interactions, thus unveiling the real-time dynamics of phonon behavior and advancing our ability to understanding and engineer phonon transport.

cond-mat.mtrl-sci↗

arXiv2021

Development of Interatomic Potentials to Model the Interfacial Heat Transport of Ge/GaAs

Molecular dynamics simulations provide a versatile framework to study interfacial heat transport, but their accuracy remains limited by the accuracy of available interatomic potentials. In the past, researchers have adopted the use of analytic potentials and simple mixing rules to model interfacial systems, with minimal justification for their use. On the other hand, contemporary machine learned interatomic potentials have greater complexity, but have not seen rigorous validation of interfacial heat transport properties. Moreover, when fitting to ab initio data, it is not known whether interface systems small enough to be tractable for density functional theory calculations can produce reasonable interatomic force constants. These and related questions are studied herein using a model Ge/GaAs system, with a particular focus on the harmonic force constants (IFC2s) of the interface. The ab initio IFC2s are shown to recover near bulk-like values ~ 1-2 nm away from the interface, while also exhibiting a complex relationship across the interface that likely precludes any successful application of mixing rules. Two different spectral neighborhood analysis potentials (SNAP) are developed to model the interface. One is fit to the total forces, while the other is only used to describe the anharmonicity, with a Taylor expansion used to describe the harmonic portion of the potential. Each potential, along with their merits and issues are compared and discussed, which provides important insights for future work.

cond-mat.mtrl-sci↗

arXiv2020

Enhancement of Ion Diffusion by Targeted Phonon Excitation

Ion diffusion is important in a variety of applications, yet fundamental understanding of the diffusive process in solids is still missing, especially considering the interaction of lattice vibrations (phonons) and the mobile species. In this work, we introduce two formalisms that determine the individual contributions of normal modes of vibration (phonons) to the diffusion of ions through a solid, based on (i) Nudged Elastic Band (NEB) calculations and (ii) molecular dynamics (MD) simulations. The results for a model ion conductor of $\rm{Ge}$-substituted $\rm{Li_3PO_4}$ ($\rm{Li_{3.042}Ge_{0.042}P_{0.958}O_4}$) revealed that more than 87% of the $\rm{Li^+}$ ion diffusion in the lattice originated from a subset of less than 10% of the vibrational modes with frequencies between 8 and 20 THz. By deliberately exciting a small targeted subset of these contributing modes (less than 1%) to a higher temperature and still keeping the lattice at low temperature, we observed an increase in diffusivity by several orders of magnitude, consistent with what would be observed if the entire material (i.e., all modes) were excited to the same high temperature. This observation suggests that an entire material need not be heated to elevated temperatures to increase diffusivity, but instead only the modes that contribute to diffusion, or more generally a reaction/transition pathway, need to be excited to elevated temperatures. This new understanding identifies new avenues for increasing diffusivity by engineering the vibrations in a material, and/or increasing diffusivity by external stimuli/excitation of phonons (e.g., via photons or other interactions) without necessarily changing the compound chemistry.

cond-mat.mtrl-sci↗

arXiv2020

Understanding Phonon Transport Properties Using Classical Molecular Dynamics Simulations

Predictive modeling of the phonon/thermal transport properties of materials is vital to rational design for a diverse spectrum of engineering applications. Classical Molecular Dynamics (MD) simulations serve as a tool to simulate the time evolution of the atomic level system dynamics and enable calculation of thermal transport properties for a wide range of materials, from perfect periodic crystals to systems with strong structural and compositional disorder, as well as their interfaces. Although MD does not intrinsically rely on a plane wave-like phonon description, when coupled with lattice dynamics calculations, it can give insights to the vibrational mode level contributions to thermal transport, which includes plane-wave like modes as well as others, rendering the approach versatile and powerful. On the other hand, several deficiencies including the lack of vibrationally accurate interatomic potentials and the inability to rigorously include the quantum nature of phonons prohibit the widespread applicability and reliability of Molecular Dynamics simulations. This article provides a comprehensive review of classical Molecular Dynamics based formalisms for extracting thermal transport properties: thermal conductivity and thermal interfacial conductance and the effects of various structural, compositional, and chemical parameters on these properties. Here, we highlight unusual property predictions, and emphasize on the needs and strategies for developing accurate interatomic potentials and rigorous quantum correction schemes.

cond-mat.mtrl-sci↗

arXiv2018

Thermal interface conductance between aluminum and aluminum oxide: A rigorous test of atomistic level theories

We report the first ever accurate theoretical prediction of thermal conductance of any material interface. Thermal interfacial conductance of aluminum (Al)-sapphire (α-Al2O3) interface along crystal directions (111) Al || (0001) Al2O3 for temperature ranging from 50-500 K is calculated using two fundamentally different methods: interfacial conductance modal analysis (ICMA) and atomistic green function (AGF). While AGF overpredicts interfacial conductance, both the quantitative and qualitative predictions of ICMA are exceptional when compared with the time-domain thermoreflectance (TDTR) experimental data. The mean error in ICMA results are below 5%. We believe that the accurate theoretical prediction by ICMA can be credited to a more fundamental treatment of the interfacial heat flux in contrast to that of the phonon gas model (PGM) and inclusion of anharmonicity to full order. ICMA also gives the eigen mode level details revealing the nanoscale picture of heat transport: more than 90% of conductance is contributed by the cross correlation (interaction) between partially extended modes of Al and Al2O3 and the remaining is attributed to interfacial modes. This is a major milestone in combustion heat transfer research enabling materials scientists to rationally design propellant architectures to serve long-distance propulsion missions.

cond-mat.mtrl-sci↗

arXiv2018

Modal analysis of heat transfer across crystalline Si and amorphous SiO2 interface

We studied the anharmonic modal contributions to heat transfer at the interface of crystalline Si and amorphous SiO2 using the recently proposed interface conductance modal analysis (ICMA) method. Our results show that ~74% of the thermal interface conductance (TIC) arises from the extended modes, which occupy more than ~58% of the entire population of vibrational modes in the system. More interestingly, although the population of purely localized and interfacial modes on the SiO2 side is less than 6 times the population of partially extended modes, the contribution to TIC by these localized modes is more than twice that of the contribution from partially extended modes. Such an observation, once again proves the non-negligible role of localized modes to facilitate heat transfer across systems with broken symmetries, and reiterates the fact that neglecting the contribution of localized modes in any modal analysis approach is an over-simplification of the actual mechanisms of heat transfer. Correctly pinpointing the modal contributions is of vital importance, since these values are directly utilized in determining the temperature dependent TIC, which is crucial to silicon on insulator (SOI) technologies with myriad applications such as microelectronics and optoelectronics.

cond-mat.mtrl-sci↗

arXiv2018

Thermal Energy Grid Storage Using Multijunction Photovoltaics

As the cost of renewable energy falls below fossil fuels, the most important challenge to enable widespread sustainable power generation has become making renewables dispatchable. Low cost energy storage can provide this dispatchability, but there is no clear technology that can meet the need. Pumped hydroelectric and compressed air storage have low costs, but they are geographically constrained. Similarly, lithium-ion batteries are becoming ubiquitous, but even their lower bounding asymptote cost is too high to enable cost-competitive dispatchable renewables. Here, we introduce a concept based on thermal energy grid storage (TEGS) using a multijunction photovoltaic heat engine (MPV) with promising initial experimental results that could meet the low cost required to enable cost competitive dispatchable renewables. The approach exploits an important tradeoff between the accession of an extremely low cost per unit energy stored, by storing heat instead of electricity directly, while paying the penalty of a lower round trip efficiency. To understand why this tradeoff is advantageous, we first introduce a framework for evaluating storage technologies that treats round trip efficiency (RTE) as a variable, in addition to cost per unit energy stored (CPE) and cost per unit power (CPP). It is from this perspective that the TEGS-MPV concept offers a compelling economic proposition.

physics.app-ph↗

arXiv2017

Thermal boundary conductance across epitaxial ZnO/GaN interfaces: Assessment of phonon gas models and atomistic Green's function approaches for predicting interfacial phonon transport

We present experimental measurements of the thermal boundary conductance (TBC) from $77 - 500$ K across isolated heteroepitaxially grown ZnO films on GaN substrates. These data provide an assessment of the assumptions that drive the phonon gas model-based diffuse mismatch models (DMM) and atomistic Green's function (AGF) formalisms for predicting TBC. Our measurements, when compared to previous experimental data, suggest that the TBC can be influenced by long wavelength, zone center modes in a material on one side of the interface as opposed to the "vibrational mismatch" concept assumed in the DMM; this disagreement is pronounced at high temperatures. At room temperature, we measure the ZnO/GaN TBC as $490\lbrack +150, -110\rbrack$ MW m$^{-2}$ K$^{-1}$. The disagreement among the DMM and AGF and the experimental data these elevated temperatures suggests a non-negligible contribution from additional modes contributing to TBC that not accounted for in the fundamental assumptions of these harmonic formalisms, such as inelastic scattering. Given the high quality of these ZnO/GaN interface, these results provide an invaluable critical and quantitive assessment of the accuracy of assumptions in the current state of the art of computational approaches for predicting the phonon TBC across interfaces.

cond-mat.mtrl-sci↗

arXiv2017

Small-mass atomic defects enhance vibrational thermal transport at disordered interfaces with ultrahigh thermal boundary conductance

The role of interfacial nonidealities and disorder on thermal transport across interfaces is traditionally assumed to add resistance to heat transfer, decreasing the thermal boundary conductance (TBC).$^1$ However, recent computational works have suggested that interfacial defects can enhance this thermal boundary conductance through emergence of unique vibrations that are intrinsic to the material interface and defect atoms,$^{2-6}$ a finding that contradicts traditional theory and conventional understanding. By manipulating the local heat flux of atomic vibrations that comprise these interfacial modes, in principle, the TBC can be increased. In this work, we provide evidence that interfacial defects can enhance the TBC across interfaces through the emergence of unique high frequency vibrational modes that arise from atomic mass defects at the interface with relatively small masses. We demonstrate ultrahigh TBC at amorphous SiOC:H/SiC:H interfaces, approaching 1 GW m$^{-2}$ K$^{-1}$, that is further increased through the introduction of nitrogen defects. The fact that disordered interfaces can exhibit such high conductances, which can be further increased with additional defects offers a unique direction in controlling interfacial thermal transport that becomes important in manipulating heat transfer across materials with high densities of interfaces.

cond-mat.mtrl-sci↗