Research:

Scanning Electron Microscope (SEM) Thermometry

High-resolution thermal imaging is important for the characterization of many modern day solid state devices, including transistors and optoelectronic devices. Such imaging is also helpful for fundamental understanding of microscale heat transfer where classical continuum equations break down, as in nanostructured materials such as thin films, nanowires, and nanocomposites. One effort in nanothermometry is to utilize a scanning electron microscope (SEM) as a tool to map temperature at the nanoscale. SEM has spatial resolution in the nanometer range because it uses a high energy electron beam as the probe. There are several signals from the primary electron beam interacting with the sample. Some of these signals contain thermal information and can be used to extract the temperature of the sample. If successful, this technique will have significant scientific as well as engineering impact due to the wide availability of SEMs in research labs worldwide.

Publications: Khan et al.

Transmission Electron Microscope (TEM) Thermometry

We are developing temperature measurement techniques in the scanning transmission electron microscope (STEM). STEM offers the potential for ultra-high spatial resolution temperature mapping, since the electron beam is focused down to < 5 nm. Using the facilities at the National Center for Electron Microscopy, we have measured temperature-dependent thermal diffuse scattering (TDS) in diffraction patterns and annular dark field signals. We will use these TDS signals to map temperature at the nanoscale in the STEM.

Publications: Wehmeyer et al.

Thermal Diodes, Regulators, and Switches

We research new thermal devices with functionalities that exceed the capabilities of traditional thermal resistors and capacitors. As surveyed in our review article [1], thermal diodes can rectify heat currents, thermal regulators can maintain a desired temperature, and thermal switches actively control the heat transfer. To demonstrate a possible application of these thermal devices, we showed that using thermal diodes to rectify an oscillating temperature profile improves the efficiency of thermoelectric waste heat scavenging [2].

Publications: Wehmeyer et al.; Chen et al.; Dames; Miller et al.

Thermal Diode Bridge

Maximally utilizing the innately periodic availability of sunlight as a constant source of energy is a long-standing challenge. Inspired by the idea of the electrical diode bridge, used to convert AC signals into DC signals, we have developed a thermal analogue to address this problem. As shown in figure 1a, the thermal diode bridge harvests solar energy during the day to provide heat to the hot mass (thermal storage), and then releases heat to the night sky to provide cooling for the cold mass. This converts the time-periodic temperature input into two relatively constant temperatures, T₁ and T₂, across the heat engine. For a sine wave temperature input, T_max - T_min is the maximum possible ΔT over the heat engine, and the power output is proportional to (ΔT)². The thermal diode bridge improves the ΔT over the heat engine and hence significantly improves the power output. As shown in figure 1b, the thermal diode bridge increases the power output of an otherwise unchanged system by a factor of four.

Shape Memory Alloy (SMA) Thermal Regulator

An important application we see for thermally functional devices is the thermal management of lithium ion batteries (LIBs), which perform poorly in extreme temperatures. In cold weather, LIBs have reduced capacity and power capability. A hot day, on the other hand, will make batteries degrade faster, or even lead to thermal runaway and fire if not well thermally managed. We have developed a thermal regulator based on a shape memory alloy (SMA) for this application. This device can passively regulate the temperature of a battery pack within the optimal temperature range without requiring any external stimulus or energy input. Results show that the battery capacity in a -20°C environment is increased by three-fold compared with standard thermal solutions.

Publications: Hao et al.

Ferroelectric Thermal Switch

Thermal properties of ferroelectric materials, such as the thermal conductivity and thermal entropy, have recently attracted significant interest. Electric fields can be used to control the thermal properties of ferroelectrics, leading to potential applications of these materials as thermal switches and for electrocaloric cooling. Using first-principles calculations, we built an iterative method to apply electric fields in bulk ferroelectric materials and calculated the thermal properties under different electric fields. For some ferroelectric materials, we find large intrinsic thermal conductivity switching ratios and electrocaloric coefficients.

Publications: Liu et al.; Liu et al.

Biomedical Thermal Measurements

Many biomedical applications require accurate knowledge of the thermophysical properties of biological tissues. For example, cryosurgery, thermal ablation, and cryopreservation all use extreme heat and cold to preserve living tissue or destroy diseased tissue. Traditional techniques for measuring the thermal conductivity of biological specimens require sample sizes of around 1 centimeter, but many tissues of interest have much smaller characteristic lengths of ~1 mm or even smaller. We have adapted an electrothermal technique known as the "3 omega" method (a), traditionally used on rigid inorganic solids, for use with soft and hydrated biological tissues (b). This enables us to measure the thermal conductivity of biological tissue samples two orders of magnitude thinner than usually possible (100s of microns rather than cm). Because many important tissues are layered, we are also developing techniques to measure the effective anistropic thermal conductivity tensor of such materials. This research is being done in collaboration with Prof. J. Choi at East Carolina University and Prof. J. Bischof at the University of Minnesota.

Publications: Lubner et al.

Plot: Liver and H₂O reference data:

• [1, 2, 3, 4]
• J.W. Valvano, Low temperature tissue thermal properties. in: J.J. McGrath, and K.R. Diller, (Eds.), Low temperature biotechnology: emerging applications and engineering contributions, ASME, New York, 1988, pp. 331-345.
• CRC Handbook of Chemistry and Physics. 90th ed. CRC Press.

Pyroelectric Property Measurements and Energy Conversion

According to Lawrence Livermore National Lab, about 66% of our annual energy consumption is lost as waste heat during the conversion process from raw energy source to useful human energy. Pyroelectric materials offer a way to convert some of this waste heat back into useful electrical energy. We have developed electrothermal methods to quantify the properties of thin-film pyroelectric materials and also to simulate their efficiency in converting heat to electricity through thermodynamic cycles. Working collaboratively with the Lane Martin group in the Materials Science department at Berkeley, we aim to identify both the most effective pyroelectric materials and also understand the benefits/limitations of using these materials in thin-film energy-conversion processes.

Publications: Pandya et al.; Pandya et al.; Velarde et al.

Anisotropic Microstructurally-Engineered Polycrystals for Increased Laser Energy (AMPLE)

The ultimate power limit of high power lasers scales directly with the thermal conductivity of the lasing media, due to issues of thermal lensing and/or thermal stresses. As shown in the Figure, the traditional lasing material Nd:YAG has a thermal conductivity of around 14 W/m-K at room temperature. In contrast, we have recently measured the thermal conductivity of aluminium nitride doped with rare earth ions (RE:AlN) as 94 W/m-K. Thus, if RE:AlN can also be shown to lase efficiently it would be a very promising material for higher power lasers. In this project the optical and thermal performance of RE:AlN will be simultaneously optimized by the use of a highly anisotropic microstructure. We will measure the aniostropic thermal conductivity of these materials, and develop models for the effects of anisotropic grain boundary scattering on the phonon thermal conductivity. In addition, the material’s expected thermal and mechanical performance in a laser application will be predicted using finite element method (FEM) simulations. This research is in collaboration with the groups of Prof. J. Garay (synthesis and lead) from UC San Diego and Prof. E. Haberer (optical performance) from UC Riverside.

Publications: Wieg et al.; Mishra et al.; Wieg et al.

Nanoparticle Luminescence Thermometry

Nanoparticle luminescence thermometry is a far-field, non-contact thermometry technique that enables single point temperature measurements with high spatial resolution. We excite individual nanoparticles with a laser that is still diffraction limited, but ensure that only one nanoparticle is within the laser spot. Thus all emission comes from that single particle, and the spatial resolution of the temperature measurement is determined by the nanoparticle size rather than the laser beam diameter. We have demonstrated the first temperature measurements using sub-50 nm individual upconverting nanoparticles. This technnique can be used to probe thermal transport in nanostructures, microelectronics, plasmonic devices, and living cells. This work is done in collaboration with The Molecular Foundry at Lawrence Berkeley National Laboratory and is supported by an NSF GOALI grant in partnernship with Seagate.

Publications: Kilbane et al.; Pickel et al.

Transient Electrothermal Phase Front Tracking

Our lab is working on developing nondestructive detection for buried features, such as defects or phase change boundaries. For this, we use the electrothermal “3 Omega” technique, traditionally used to measure thermal conductivities of solids. Using this technique to probe multiple heating frequencies continuously allows for the location of phase change boundaries to be tracked in real time. The figures above shows an example of a moving ice/water freezing front, and 3 Omega voltage changes corresponding to a freezing and thawing event. Applications for this work include cryosurgery and cryopreservation, where this technique can be applied to prevent damage to healthy tissue as well as allowing the freezing and thawing processes to be better understood. This research is being done in collaboration with Prof. J. Bischof from University of Minnesota, and currently is supported by X-Therma.

Publications: Hodges et al.; Natesan et al.; Hodges and Dames

Cost Analysis of Thermoelectrics

Thermoelectric materials can convert heat directly into electricity with no moving parts, making them appealing for distributed power generation. However, due to issues of cost, manufacturability, abundance, and material performance, the full potential of thermoelectrics has yet to be realized. One potential path towards further improvements is to take advantage of unique transport characteristics at organic-inorganic interfaces in a new class of hybrid thermoelectric materials. Measurements of films of such hybrid materials show some transport properties superior to those of either component (led by the R. Segalman group). Polymer-based materials are also appealing because they may have substantially lower manufacturing costs than traditional solid thermoelectric materials. We are developing new performance metrics for thermoelectric materials that take into account the tradeoffs between raw thermodynamic performance (ZT) and the costs of materials and manufacturing (preliminary results shown in the figure; collaboration with S. LeBlanc and the K. Goodson group at Stanford.)

Publications: Yee et al.; LeBlanc et al.; Dames

Ray Tracing Simulations

The thermal conductivity of nanostructures is size-dependent due to energy carrier mean free path suppression from boundary scattering. We developed a new numerical phonon ray technique to quantify this size effect for structures such as nanograined materials [1] and 3D nanofabricated silicon [2]. By combining ray tracing of silicon nanomeshes with our collaborator’s experimental measurements, we showed that phonon transport in nanomeshes is dominated by the boundary scattering size effect, not by phonon wave interference effects [3].

Publications: Lee, Lee, Wehmeyer et al.; Wei et al.; Hori et al.

Highly Anisotropic Thermal Transport

We model heat conduction in anisotropic materials, such as mica or graphite. Our models showed that typical calculations of the thermal boundary conductance can vastly overestimate the conductance of anisotropic materials [1], and that anisotropic materials can have a minimum thermal conductivity even lower than the “lower-bound” estimate of isotropic models [2]. We used molecular dynamics simulations to show that increasing the bonding strength along one direction surprisingly decreases the thermal conductivity in the orthogonal direction [3], an effect referred to as “phonon focusing”. We also derived a new Onsager reciprocity relation for anisotropic thin films.

Publications: Chen et al.; Chen et al.; Wei et al.