Hu Research Lab - University of California, Los Angeles

Spotlight

Ellectrically Gated Thermal Switch

Controlling heat flow is a key challenge for electronics, sustainable energy, industrial processing, and thermal therapy. We demonstrate a first-of-its-kind electronically gated three-terminal device, allowing continuously and reversibly regulate heat flow by an electric field through carefully controlled chemical bonding and charge distributions within the molecular interface. The devices have ultrahigh switching speeds above 1 megahertz, have on/off ratios in thermal conductance greater than 1300%, and can be switched more than 1 million times. We anticipate that these advances will generate opportunities in molecular engineering for thermal management systems and thermal circuit design. November 3, 2023

Thermal Energy Transport Under Extreme Conditions

We observed a new physics principle governing how thermal energy transport under extreme conditions. We performed in situ ultrafast optics, Raman spectroscopy, and inelastic X-ray scattering measurements to examine the phonon bandstructure evolution and confirmed the anomalous high-pressure behaviour from competitive heat conduction channels from interactive high-order anharmonicity physics. Our spectroscopy study combining atomistic theory represents as a powerful approach to probe complex phonon physics and provide fundamental insights for understanding microscopic energy transport in materials of extreme properties. December 8, 2022

Integration of Boron Arsenide Cooling Substrates Into Gallium Nitride Devices for High Power Electronics

Our study demosntrate heterogenous integeration of BAs and BP cooling substrates with wide-bandgap gallium nitride high-electron-mobility transistor devices. GaN-on-BAs structures exhibit a high thermal boundary conductance of 250MW/m2K, and comparison of device-level hot-spot temperatures with length-dependent scaling (from 100 um to 100 nm) shows that the power cooling performance of BAs exceeds that of reported diamond devices. Furthermore, operating AlGaN/GaN high-electron-mobility transistors with BAs cooling substrates exhibit substantially lower hot-spot temperatures than diamond and silicon carbide at the same transistor power density, illustrating their potential for use in the thermal management of radiofrequency electronics. June 17, 2021

Self-Assembled Manufacturing for Thermal Management of Wearable Electronics and Robotics

As an emerging field, thermal management for wearable electronics and robots requires advanced building blocks to provide not only efficient heat dissipation but also high softness and flexibility. Using new materials, modeling design, and self-assembled manufacturing, we report high performance thermal management with excellent elastic compliance similar to that of soft biological tissues down to 100 kPa, high flexibility, and preserves over at least 500 bending cycles a high thermal conductivity over about 100 times improvement of a typical polymer. The superior cooling performance has been verified by device integration with power LEDs to show a 45 'C reduction in the hot spot temperature. Together, this study demonstrates scalable manufacturing of a new generation of energy-efficient and flexible thermal materials that holds great promise for advanced thermal energy technologies. March 12, 2021

Ultrahigh Thermal Conductivity Identified in Ternary Compounds

Building on our previous efforts, this work investigates the fundamental lattice vibrational spectra and thermal conductivity of ternary compounds involving four-phonon process. We find an ultrahigh room-temperature thermal conductivity through strong carbon-carbon bonding up to 2100 W/mK beyond most common materials and the recently discovered boron arsenide. This study provides fundamental insight into the atomistic design of thermal conductivity and opens up opportunities in new materials searching towards complicated compound structures. March 12, 2021

Intrinsic low thermal conductivity and phonon renormalization due to strong anharmonicity of single-crystal tin selenide

This study reports the experimental measurements and theoretical calculations of phonon transport in the record-high thermoelectric material, i.e. SnSe but focuses on its undoped and fully dense single crystals. To investigate the phonon anharmonicty, we developed experimental approach combining picosecond ultrasonics and X-ray diffraction, to enable direct measurement of temperature-dependent sound velocity, thermal expansion coefficient, and Gruneisen parameter. The measured Gruneisen parameter suggests an abnormally large temperature effect on phonon dispersion that contributes to over 90% of phonon frequency shifts. Furthermore, we performed ab initio calculations using different methods: in comparison with self-consistent phonon theory, the harmonic and quasi-harmonic models that have been widely used in current phonon calculations, fail to accurately predict these important thermophysical properties. Our study reveals an extremely strong intrinsic anharmonicity in SnSe that introduces phonon renormalization near room temperature. This study represents an important research benchmark in characterizing high-performance thermal energy materials, and provides fundamental insight into advancing modern calculation methods for phonon transport theory. July 4, 2019

Observation of anisotropic thermal boundary resistance for near-junction thermal management: experiment and atomistic modeling of interfacial energy transport

Interfacial thermal boundary resistance (TBR) plays a critical role in near-junction thermal management of modern electronics. In particular, TBR can dominate heat dissipation and has become increasingly important due to the continuous emergence of novel nanomaterials with promising electronic and thermal applications. A highly anisotropic TBR across a prototype 2D material, i.e., black phosphorus, is reported through a crystal orientation-dependent interfacial transport study. The measurements show that the metal-semiconductor TBR of the cross-plane interfaces is 241% and 327% as high as that of the armchair and zigzag direction-oriented interfaces, respectively. Atomistic ab initio calculations are conducted to analyze the anisotropic and temperature-dependent TBR using density functional theory (DFT)-derived full phonon dispersion relation and molecular dynamics simulation. The measurement and modeling work reveals that such a highly anisotropic TBR can be attributed to the intrinsic band structure and phonon spectral transmission. Furthermore, it is shown that phonon hopping between different branches is important to modulate the interfacial transport process but with directional preferences. A critical fundamental understanding of interfacial thermal transport and TBR-structure relationships is provided, which may open up new opportunities in developing advanced thermal management technology through the rational control over nanostructures and interfaces. June 24, 2019

Exploring the theoretically predicted conductivity limit in defect-free crystals: Experimental observation of high thermal conductivity in boron arsenide

Improving thermal management of small scale devices requires developing materials with high thermal conductivities. The semiconductor boron arsenide (BAs) is an attractive target due to ab initio calculation indicating single crystals have an ultrahigh thermal conductivity. We synthesized BAs single crystals with undetectable defects, and measured a room temperature thermal conductivity of 1300 W/mK. Our spectroscopy study in conjunction with atomistic theory reveals that the unique band structure of BAs allows for very long phonon mean free paths and strong high-order anharmonicity through the four-phonon process. The single-crystal BAs has better thermal properties than other metals and semiconductors. Our study establishes BAs as a benchmark material for thermal management applications, and exemplifies the power of combining experiments and ab initio theory in new materials discovery. July 5, 2018

Thermal management application using emerging high thermal conductivity materials: Thermal and phonon spectral characterization of synthetic boron phosphide

We demonstrated a systematic synthesis-experimental-modeling approach to investigate high thermal conductivity material for efficient heat dissipation: we have chemically synthesized high-quality boron phosphide single crystals and measured their thermal conductivity as a record-high 460 W/mK at room temperature. We have, for the first time, experimentally measured the phonon mean free path spectra of boron phosphide and analyzed experimental results by solving three-dimensional and spectral-dependent phonon Boltzmann transport equation using the variance-reduced Monte Carlo method. The experimental results are in good agreement with that predicted by multiscale simulations and density functional theory, which together quantify the heat conduction through the phonon mode dependent scattering process. Our finding underscores the promise of boron phosphide as a high thermal conductivity material for thermal management and provides a microscopic-level understanding of the phonon spectra and thermal transport mechanisms of boron phosphide, to enable a rational design of high thermal conductivity materials and nano- to multiscale devices. Nov 8, 2017

High-performance flexible thermoelectric devices

A solid-state thermoelectric device is attractive for diverse technological areas such as cooling, power generation and waste heat recovery with unique advantages of quiet operation, zero hazardous emissions, and long lifetime. With the rapid growth of flexible electronics and miniature sensors, the low-cost flexible thermoelectric energy harvester is highly desired as a potential power supply. Here, we demonstrated a flexible thermoelectric copper selenide (Cu2Se) thin film through a low-cost and scalable spin coating process for waste heat harvesting applicatons. March 29, 2017

First in situ thermal-electrochemical characterization of 2D van der Waals materials in a lithium ion battery

This work presents the first time demonstration of in situ thermal-electrochemical characterization of a 2D van der Waals material in a lithium ion battery. A novel approach to integrate ultrafast optical spectroscopy and electrochemical control has been developed to investigate the thermal transport in the 2D material electrode (black phosphorus) during the battery's normal operation process. The study reveals intriguing anisotropic ion-phonon interactions and highly reversible electrochemical control over the thermal properties. Feb 23, 2017

Paper published in Nature Nanotechnology

This work presents a table-top pump-probe spectroscopy approach to measure phonon spectral contribution to heat transfer. Classical diffusion theory fails to describe small scale energy transport, such as to gauge the temperature rise in modern electronics and thermal solar energy harvesting devices, but the new work provides detailed quatification of such non-equilibrium energy transport down to its spetral contributions. Experimental measurement is compared with modeling resutls based on transient frequency-dependent Boltzmann transport equation and multi-scale Monte Carlo methods, which enables us to better understand heat transfer and design energy materials. June 10, 2015

Experimental study of the proposed super-thermal-conductor: BAs in Applied Physics Letters

This work reports the first thermal measurement of Boron Asenide, the new candidate material of ultra-high thermal conductivity for thermal management based on first-principles prediction. Despite high density of vacancy impurities, our measurement shows a reasonable high thermal conductivity, as an important first step towards developing high-conducting materails. Feb 20, 2015

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