Two-phase heat exchangers are widely used in the power and process industries, heating ventilation and air conditioning, refrigeration, and electronics thermal management. This work presents the design, manufacturing, and performance evaluation of an additively manufactured (AM) water-cooled R134a condenser heat exchanger. The condenser design includes internal three-dimensional (3D) structures enabled by AM that are not possible from traditional manufacturing technologies. Our novel design methodology uses a physics-based model to rapidly search the large design space, followed by detailed computational fluid dynamics (CFD) simulations that verify performance. The design employs alternating channels for water and refrigerant flow, with optimized 3D shapes that enhance the heat transfer with wavy fins on the water side and chevrons on the refrigerant side. Experiments demonstrate that the AM condenser has a heat transfer rate of 3 kW to 8 kW for refrigerant saturation temperatures of 35 to 49 degrees C. The AM condenser has a power density as high as 6.2 MW/m3, outperforming traditional shell-tube designs by 30-50% with comparable normalized pumping power. The developed methods provide a robust framework for the design of high performance and high volumetric power density AM heat exchangers.

Shape memory alloys (SMAs) absorb and release large amounts of latent heat during martensitic transformation, making them ideal candidates for applications involving thermal energy storage and management. In this study, Cu–Zn–Al SMAs were investigated as lower-cost alternatives to NiTi-based SMAs for solid–solid phase change materials. The alloys were fabricated using an unconventional method of melting and solidification of the constituent elements sealed in quartz tubes under a pressurized Ar atmosphere. The alloys synthesized were found to exhibit superior figure of merit values for thermal energy storage, as compared to conventional solid–liquid phase change materials and NiTi-based SMAs, with thermal conductivity between 59 and 75 W/mK and latent heat values ranging from 3 to 6.5 J/g. Transformation temperature ranges (Af–Mf) less than 20 °C were achieved within a wide operating temperature between − 145 °C and 100 °C. In addition, select CuZnAl compositions yielded excellent cyclic stability with only ± 2 °C shifts in transformation temperatures after 20 thermal cycles. The present study demonstrates the feasibility of CuZnAl SMAs for use in high heat flux thermal energy storage and management applications at a wider range of temperatures.

Thermal buffering via phase change materials (PCMs) has been proposed as a method to reduce peak temperature in high power switching and pulsed power applications. To improve efficacy at peak temperature suppression, researchers have focused on developing high power and high capacity PCM-based thermofluidic component technologies, and in recent years, additive manufacturing has emerged as a promising fabrication technique for PCM-integrated components.To this end, a new thermal buffering component is proposed, based on additively manufactured porous cubes of nickel titanium (PCNT) exhibiting an above-ambient-temperature structural phase transition (Austenite finish temperature of 92 o C). PCNT thermal buffering components exhibit unique thermal energy storage properties owing to their high latent heat (>150 MJ/m 3 ) and high thermal conductivity (>12 Wm -1 K -1 ). The proposed thermal buffering architecture leverages the high thermal conductivity of PCNT and its ability to undergo a reversible phase transformation between austenite and martensite phases, which enables efficient energy storage and release under the appropriate working fluid temperatures.The PCNTs were fabricated using a selective laser melting (SLM) technique, which allows for precise control of the porosity and surface area of the material. The PCNTs were printed to have half of the cube volume occupied by solid material while retaining edges that minimize disruption to the fluid flow. Given the decrease in flow resistance (dropping from ~0.1 kPa for parallel plate flow to ~0.02 kPa through PCNTs), building on prior work by the authors, the fluidic test setup was reconfigured to reduce thermal time constant of the heat input to the test section, and to greatly reduce ambient heat losses by improving insulation and moving to a two-reservoir system.The proposed SLM-printed PCNT structures offer a promising solution for high power, high-capacity thermal buffering of high-power systems, with high potential to reduce size, weight, and power draw in fast switching and high power electronics applications.

Phase change materials (PCMs) can provide thermal buffering to systems that experience transient heat loads, including electronics and optoelectronics packaging. Placing the PCM in the primary path of heat rejection decreases the thermal resistance between the heat source and the PCM volume, but increases the total thermal resistance between the heat source and heat sink. In systems that operate in both steady-state and transient regimes, this introduces tradeoffs between cooling performance in these distinct regimes. Employing a conductive finite volume model, Parapower, we investigate those tradeoffs considering the impact of adding a layer of gallium (Ga), a low melting point metal, and a layer of copper (Cu) between a planar heat source and a convective boundary condition heatsink. We demonstrate: (1) side-by-side comparisons of latent (Ga) and sensible (Cu) heat storage layers must consider different layer thicknesses to account for the different thermal storage mechanisms, (2) for short periods of time, conditions exist in which a PCM outperforms a traditional heat sink for transient thermal buffering at an equivalent steady-state temperature rise, and (3) under these conditions, the Ga layer is approximately an order of magnitude thinner than the equivalent Cu, leading to significant mass and volume savings.

The use of phase change materials (PCMs) has gained much attention for applications of transient ther-mal management of electronic systems due to their high latent heat and ability to absorb heat near -isothermally. Because of their low thermal conductivity, PCMs are usually integrated with heat sinks to be used more efficiently. In this study a PCM-based heat sink with a generic structure of plate fins was investigated experimentally and numerically. The intentionally simple fin topology allowed focusing on the effects of various material properties rather than the commonly investigated geometry effects. Hence, two types of heat sink material, copper and aluminum, were examined, and two PCMs with similar melt-ing temperatures but distinctly different thermal properties - a metallic alloy (Field's metal) and an or-ganic paraffin (n-Octacosane), were used. Experimental findings allowed for validation of the numerical approach, used for a comprehensive parametric numerical study. The latter facilitated a more detailed investigation of the transient heat transfer processes, such as melting patterns and heat accumulation analyses, where the superior thermal properties of the metallic PCM manifested in more efficient latent heat accumulation, resulting in reduced system peak temperatures. It was found that systems with Field's metal were able to accommodate up to 80% of the energy in the form of latent heat, which is 10 percent-age points higher than achieved using the organic paraffin. A dimensional analysis accounting for power inputs and material properties was conducted, and a generalized behavior was achieved for a normalized time in terms of Fourier and Stefan numbers, and thermal diffusivities ratio. (c) 2023 Elsevier Ltd. All rights reserved.

Shape memory alloys (SMAs) have been demonstrated as effective phase change materials (PCMs) for thermal energy storage (TES) applications. NiTi and NiTiHf SMAs have shown high TES performance, as quantified by PCM figure of merit (FOM) but their use in applications requiring narrow operation temperature windows is limited by large overall phase transformation ranges (OTR). This work investigates NiTiCu SMAs as PCMs with high FOM and low OTR. A full-factorial design of experiments is used to examine 24 NiTiCu compositions. The compositions were fabricated using vacuum arc melting and their phase transformation and thermophysical properties were characterized using calorimetry, thermal diffusivity, and density measurements. The NiTiCu compositions spanned martensitic transformation temperatures between -22 and 84 °C and exhibited greater FOM (250–1050 106J2K−1s−1m−4), compared to traditional PCMs (typically <100 106J2K−1s−1m−4), with major benefits associated with higher density and higher thermal conductivity values. In addition, the NiTiCu compositions in this study show ultra-low OTR (12–20 °C) compared to NiTi and NiTiHf SMAs (>50 °C), enabling utility in narrow operating temperature windows. Thermal cycling was also performed revealing extreme stability of martensitic transformation with only 0.04 °C shift in transformation temperatures after 80 thermal cycles, which is the lowest reported to date in SMA literature.

In order for electronics packaging power density to increase, innovations and improvements in heat transfer are required. Electrification of transportation has the potential for significant fuel and energy savings. Changing to an electrified drive train requires reliable and efficient power electronics to provide power conversion between alternating current motors and direct current energy storage. For high power transportation systems like aircrafts or heavy vehicles, the power density of power electronics needs to be improved. Power density is also an enabler for high power military devices that must be used and transported via air, ground, and sea. This paper summarizes the outcome of a collaborative and multidisciplinary research effort aimed at co-designing a novel electronics cooling device that utilizes two-phase fluid flow. Two-phase flow cooling has been known for decades as well as the risks associated with it: critical heat flux (CHF), dryout, and thermal runaway. Our research de-risks the two-phase cooling technology by swirling the flow to remove the bubbles from the wall and confining them at the core of the cooler. The combined effects of gas phase removal, enhanced nucleation, and dramatic liquid film agitation and rupture have been quantified by our experiments: double the heat transfer coefficient with only 13% increase in pressure drop. Besides advanced fluid-dynamics, our Package-Integrated Cyclone Cooler (PICCO) utilizes cutting edge packaging and additive manufacturing technology such as direct deposition of a metal substrate and circuits (dies) on a complex helical cooler that can only be manufactured via three-dimensional printing. By co-designing and testing the cooler, we have quantified the impact of the swirled flow on the junction temperature with respect to a conventional (non-swirl) two-phase-flow-cooled power electronics package. At steady-state, our post-test thermal simulations predict a junction temperature reduction from 185 degrees C to 75 degrees C at the same power dissipation. When the heat load is unsteady (United States Environmental Protection Agency Urban Drive Cycle), the junction temperature reduction is 140 degrees C to 60 degrees C.

This paper describes inter- and intra-device phase change material integration approaches using solid-solid Ni 50.28 Ti 49.36 metallic alloys. First, we characterize Ni 50.28 Ti 49.36 solid-solid phase change material using differential scanning calorimetry and Xenon Flash to reveal a material transformation temperature of 75°C, latent heat of 28 J/g, and thermal conductivity between 12 and 16 W/mK. Next, we perform electroless copper plating studies as a prerequisite for downstream printed circuit board integration. Finally, we design, fabricate, and test three unique board designs using x-ray imaging and electronic device heating via infrared thermography. The tested boards include a state of the art copper via board, a first-of-its-kind NiTi-impregnated printed circuit board, and a copper via board with backside NiTi integration. Results from this study demonstrate the ability to electroless plate NiTi, integrate into IPC-6012 E Class 3 printed circuit boards, and (in the test conditions used herein) provide up to a 65% increase in device on-time prior to reaching a critical device temperature. These results set the stage for leap-ahead improvement in the practical implementation of solid-solid thermal energy storage materials.

Laser powder bed fusion is a promising additive manufacturing technique for the fabrication of NiTi shape memory alloy parts with complex geometries that are otherwise difficult to fabricate through traditional processing methods. The technique is particularly attractive for the biomedical applications of NiTi shape memory alloys, such as stents, implants, and dental and surgical devices, where primarily the superelastic effect is exploited. However, few additively manufactured NiTi parts have been reported to exhibit superelasticity under tension in the as-printed condition, without a post-fabrication heat treatment, due to either persistent porosity formation or brittleness from oxidation during printing, or both. In this study, NiTi parts were fabricated using laser powder bed fusion and consistently exhibited room temperature tensile superelasticity up to 6% in the as-printed condition, almost twice the maximum reported value in the literature. This was achieved by eliminating porosity and cracks through the use of optimized processing parameters, carefully tailoring the evaporation of Ni from a Ni-rich NiTi powder feedstock, and controlling the printing chamber oxygen content. Crystallographic texture analysis demonstrated that the as-printed NiTi parts had a strong preferential texture for superelasticity, a factor that needs to be carefully considered when complex shaped parts are to be subjected to combined loadings. Transmission electron microscopy investigations revealed the presence of nano-sized oxide particles and Ni-rich precipitates in the as-printed parts, which play a role in the improved superelasticity by suppressing inelastic accommodation mechanisms for martensitic transformation.

High power fiber optic waveguides are emerging as a critical component in high energy laser systems, optical relay of electrical power, and optoelectronic devices. To make use of the advantages of higher power density in these systems, a thorough understanding of the heat dissipation characteristics and thermal management strategies must be developed. With this aim in mind, we report a parametric multi-physics analysis to guide in the materials selection and design of thermal management strategies that would enable operation at previously unobtainable powers and wavelengths. We offer practical design considerations for packaging materials, heat sink design, and containment geometries, and offer insights into how to increase thermally limited power output beyond commercially available state of the art strategies. The results of this parametric analysis suggest that substrate/containment material thermal conductivity should exceed 100 Wm -1 K -1 , and that under that condition, both heat sink cooling strategy and filler thermal conductivity substantially affect peak temperature. High filler thermal conductivity and high heat sink convective coefficient are required to the point that cutting-edge strategies for conductivity and convective enhancements may be required to allow operation under the desired parameters.

In this work, we utilize COMSOL Multiphysics to model the heat flow into and out of a Nickel-Titanium (NiTi) Shape Memory Alloy (SMA) as it bends around a rotating copper disk, which acts as a thermal mass for heating or cooling. The model is verified using a novel experimental system capable of capturing the thermal gradient through the disk. We then interrogate the impacts of the wire’s cross-sectional geometry, contact thermal resistance at the disk junction, rotational speed of the disk, and the disk’s radius of curvature on the cycle’s coefficient of performance (COP). Our results suggest that this modeling technique can accurately capture the endothermic and exothermic reactions associated with the phase change. We therefore provide a useful tool for developing and optimizing next-generation passive cooling and heating systems.

High-temperature silicon carbide (SiC) die are the most critical and expensive component in electric vehicle (EV) power electronics (PE) packages and require both active and passive methods to dissipate heat during transient operation. The use of phase change materials (PCMs) to control the peak junction temperature of the SiC die and to buffer the temperature fluctuations in the package during simulated operation is modeled here. The latent heat storage potential of multiple PCM and PCM composites is explored in both single-sided and dual-sided package configurations. The results of this study show that the addition of PCM into two different styles of PE packages is an effective method for controlling the transient junction temperatures experienced during two different drive cycles. The addition of PCM in a single-sided package also serves to decrease temperature fluctuations experienced and may be used to reduce the necessary number of SiC die required for EVs, lowering the overall material cost and volume of the package by over 50%. PCM in a single-sided package may be nearly as effective as the double-sided cooling approach of a dual-sided package in the reduction of both peak junction temperature of SiC as well as controlling temperature variations between package layers.

Abstract The demand for high power density, therefore high heat dissipation, power electronics modules is propelled by applications such as hybrid transportation and asynchronous power generation, among others. Besides a low thermal resistance, these applications require high thermal capacitance to manage transient operations. The Package Integrated Cyclone COoler (PICCO) is an additively manufactured, thermal energy storing cooler codesigned by GE Research (GRC) in collaboration with the US Army Research Lab (ARL). The key aspect of PICCO is its capability to swirl a two-phase coolant, i.e. liquid-gas. The centrifugal field creates a radial pressure gradient inducing buoyancy. The strong radial acceleration to which the fluid is subject forces relatively cold flow outward to reach the hot wall, thus boosting the heat transfer, while hot flow and bubbles migrate inward and the two-phase system is nearly isothermal (thermal storage). The proposed study models the swirled flow in terms of liquid film heat conductance and critical heat flux predictions. The resulting heat transfer coefficient can be applied to the walls of the cyclone and used as a boundary condition for the heat conduction problem through the cyclone wall and the module layers.

Phase change materials (PCMs) offer a passive methodology to store thermal energy and mitigate transient temperature increases. Those that go from one solid-state phase to another remove the need for confinement structures imposed by PCMs which melt or sublimate. The use of solid-to-solid state phase change materials is an emerging topic of interest, and the parameters governing the use of such materials in high power applications (namely, thermal conductivity, latent heat, volumetric specific heat, and transformation temperature) are not well-understood in solidto-solid metallic PCMs (such as those exhibiting reversible Martensitic transformations). It has been demonstrated that thermal conductivity, latent heat, and transformation temperature in these systems can be significantly tailored by adjusting the processing conditions, but little data exists on the impact of compositional variants of ternary or quaternary elements exhibiting the shape memory effect. A main limitation is the lack of rapid, high-throughput experimental methodologies capable of characterizing these quantities to better inform design of materials. In this work, we demonstrate efficacy of a finite element method to explore parametric spaces for designs of test structures capable of simultaneously resolving thermal conductivity, specific heat, latent heat, and transformation temperature. Test structures will be fabricated using photolithography and thin film deposition techniques capable of combinatorial thin film fabrication. The goal of this work is to determine whether the proposed design - a thermally isolated suspended platform will be capable of resolving transient and frequency-domain thermal signal to recover the desired properties. This work relies on finite element modeling using multiphysics software COMSOL v5.5 to predict experimental sensitivity to the input parameters, and will ultimately rely on coupling finite element models to fitting algorithms to determine unknown material properties. This combined thermal characterization strategy represents a leap-ahead for PCM characterization capabilities. Rapid characterization of wide compositional variations will enable accelerated progress in leveraging PCMs for high power applications.

This study reports the use of numerical topology optimization schemes to design heat sinks for steady-state and transient operation. Using COMSOL 5.6, a circular heat sink design space is considered to model temperature as a function of design fill factor. An initial parameterization of Helmholtz filter (minimum feature size) from 125 to 2000μm and penalization factor from 2 to 7, shows that smaller feature size improves performance. Using a moderate selective laser melting tolerance of 250μm, we perform solid-isotropic-material-with-penalization (SIMP) simulations to show the impact of heat transfer coefficient (from 5 to 100 Wm -2 K -1 ) on fill factor. Intuitively, higher heat transfer coefficient requires less aluminum fin material for a given design point. The transient results are less intuitive and represent a unique progression from a preference for larger quantities of material near the heat source with sparse or nonexistent fins to designs that converge to steady state solutions at large times. It is found that the transient approach is preferred over steady state, particularly for low time steps, where significant weight savings can be realized. These results demonstrate the benefits of transient topology optimization approaches using COMSOL 5.6 and set the stage for future numerical transient mitigation approaches.

This Special Issue of the ASME Journal of Electronic Packaging (JEP) is dedicated to Prof. Avram (Avi) Bar-Cohen, who sadly passed away on October 10, 2020. Over the course of his career, Avi held faculty positions at the Naval Postgraduate School, the Massachusetts Institute of Technology, the University of Minnesota, the Ben Gurion University (Beer Sheva, Israel), and the University of Maryland, College Park as the Distinguished University Professor in the Department of Mechanical Engineering. Avi also served as a Program Manager in the Microsystem Technology Office at the Defense Advanced Research Projects Agency and held a variety of roles related to thermal packaging at Raytheon.Much has already been written about Avi's life [1], and this is not the first Special Issue to have been dedicated to him within the wider scientific literature [2,3], which highlights just how seminal his contributions have been to multiple electronics packaging communities (including the ASME EPPD, the ASME HTD, and the IEEE EPS) as well as the impact he had on those who were fortunate enough to be around him. In fact, Avi is widely credited as a founder of this specific field, having served as the inaugural chair of the IEEE Intersociety Conference on Thermal Management in Electronic Equipment (ITherm) in 1988. For these accomplishments (and others), Prof. Bar-Cohen was awarded the ASME Heat Transfer Memorial Award (1999), the ASME Curriculum Innovation Award (1994), the Worcester Reed Warner Medal (2000), and the InterPACK Achievement Award (2007), among many others.In this Editorial, we briefly highlight a handful of perspectives from several of Avi's former students and colleagues that detail the impact he had on their own lives. While this is by no means an exhaustive account of such impacts, these personal stories reflect the incredible person we all knew him to be.One thing we all universally adore, and can only hope to one day replicate ourselves, is Dr. Bar-Cohen's unyielding, unapologetic, and playful enthusiasm for science and technology. As described by Dr. JR Saylor, “The thing that I remember most about him was the joy that he took in the work. Avi worked hard, always had a lot on his plate, and put in a lot of hours. But I never got the sense that any of this was a burden to him. Rather, he seemed to just relish whatever he was working on. Whether it was organizing a meeting, sussing out the meaning of some experimental data, or getting a journal article submitted, Avi always seemed to be doing exactly what he wanted to be doing, with the people he wanted to do it with. But I think he was also a person who taught us how to live well. Avi taught us that we should do the thing that we truly love doing and to then embrace it wholeheartedly.”He used this enthusiasm to inspire and bring joy to others. When he wasn't writing papers, presenting at conferences, or leading his research teams, he could often be found writing poetry about two-phase cooling and physics—encouraging a sense of excitement and pride in our mutual technical endeavors. And as many others can attest, he never shied away from penning witty, punny, and/or genuinely heartfelt messages to the team and collaborators emphasizing his love for his craft and his appreciation for ‘us’. As an example, when D. J. Sharar successfully defended his Ph.D. (UMD 2016) project titled “Flow regime drive thermal enhancement in internally grooved tubes”—he sent out the following message:Throughout his groovy defense, Darin was unphased by the heated, though stratified, comments and coolly displayed his ability to intermittently slug it out with the best.Please join me in congratulating him on this well-earned and auspicious milestone.Signed – ABCHis profound love for mentoring and the community was also reflected by his genuine 1:1 interactions with others, particularly meeting others for the first time. Dr. R. Warzoha recalls, “I remember the first time that I met him was in an elevator at InterPACK (just him and I)—he asked me who I was and what I was doing, and I told him I was going to present my first conference paper as a graduate student - he immediately laughed and said not to worry, that everyone in the room was only there to be helpful, and just to be confident. He told me he was sure I'd do well, and that it was just another opportunity to be part of a wonderful community. He didn't have to say anything at all, but he made me infinitely less fearful in that moment.” And a great message by Dr. Emil Rahim, a grad student of Avi's at the University of Maryland, “I remember the first time and the last time I met Avi in person. The first time was in his UMD office back in the summer of 2005. I was an independent graduate student looking for an advisor to hire me and offer me an opportunity to work on a research project. Avi greeted me at his office, and within seconds made me feel welcome and at ease. This was and will continue to be the friendliest job interview of my life. He knew how to make people comfortable and welcome to speak, even in the toughest situations. The last time I saw him was at ITherm 2019 organizing committee dinner in Las Vegas. Sometime after dinner, he pulled a chair and sat next to me, and we had an uninterrupted 1:1 conversation for a few minutes. Despite his busy schedule, he always made time for everyone, and made sure everyone was heard and felt safe and supported.”Moreover, he taught us that diversity, through unique cultural and academic perspectives, inspires creativity, drives innovation, and represents a vision of a more-perfect world. As stated by Dr. Karl Geisler, “For Avi, diversity was an action, a practice. This is one of the clearest lessons I took away from him. During my time in Avi's group, we had team members representing all of the world's major religions and more. We came from many places, including Europe, Africa, India, China, Japan, Korea, Texas, and Minnesota. We were male and female, young and midcareer. We were open and inviting to friends and guests and found value in collaborating internally and externally. Avi worked to make our graduate school experience reflect his vision of an ideal world.” And it's in part due to this profound respect for diversity that he was such a successful mentor/leader and was capable of working so well with others. According to Gary Solbrekken, “he allowed me to develop my own approaches while guiding me through our conversations. I am certain that it was not the most efficient way to complete projects, but that style has influenced the way I advise my own graduate and undergraduate students, where I attempt to allow them to find their own creative path for solving problems.” Additionally, Dr. Mehmet Arik said, “At Ozyegin University, I had over 25 graduate students and I shared stories of ABC team and his leadership in our [now fabled] biweekly [bagel-lunch] meetings. My students, coming from different countries, were always amazed by his enthusiastic style and leadership. I always emphasize his kind personality, engineering vision, and leadership for my young students.”As our advisor, friend, and mentor, he made work fun and inspired us to be the absolute best versions of ourselves. He has motivated us, enabled us, and always supported our professional and personal development in ways that this Editorial will never do justice. We, as a collective group, hope to represent his leadership legacy not only through our technical and academic achievements but in living our lives and carrying on a community that was so central to who he is. Dr. Bar-Cohen - Thank you for teaching us to love what we do, to love and appreciate the people we do it with, to work hard and with integrity and remain unphased to adversity and challenges, and for always reminding us to have some fun along the way. You'll be deeply missed but never forgotten.We are honored to present this Special Issue to the wider electronics packaging community.The AE and GEs are extraordinarily grateful for the written contributions of Dr. J. R. Saylor, Dr. Mehmet Arik, Dr. Karl Geisler, Dr. Gary Solbrekken, and Dr. Emil Rahim.

Elastocaloric cooling has been identified as a promising alternative to high global warming potential vapor compression cooling. Two key bottlenecks to adoption are the need for bulky/expensive actuators to provide sufficient uniaxial stress and inadequate elastocaloric material fatigue life. This paper defines the physics that govern performance of axisymmetric flexural bending for use as an emerging low-force and low-fatigue elastocaloric heating and cooling mechanism and further demonstrates a continuous rotary-driven cooling prototype using polycrcrystalline Ni50.7Ti48.9. Elastocaloric material performance is determined using infrared thermography during uniaxial-tension and four-point bending thermomechanical testing. A systematic study reveals the effects of strain rate (from 0.001 to 0.025 s-1), maximum strain (from 2 to 8%), and strain mode on the temperature evolution, mechanical response, and coefficient of performance. Four-point bending experiments demonstrate a temperature reduction up to 11.3{\deg}C, material coefficients of performance between 2.31 and 21.71, and a 6.09- to 7.75-fold reduction in required actuation force compared to uniaxial tension. The absence of L\"uders bands and reduced mechanical dissipation during flexure represent reduced microstructure degradation and improved fatigue life. The rotary-based elastocaloric cooling prototype is shown to provide similar thermomechanical performance with the added benefit of discrete hot and cold zones, continuous cooling, inexpensive rotary actuation, and scalability, which represents a significant advancement for compact, long lifetime, and inexpensive elastocaloric cooling.

Energetic barriers to nucleation can result in metastable liquids, which require additional undercooling to initiate solidification. Gallium, a low melting point metal of potential use as a phase change material and for liquid metal electronics, exhibits a well-documented temperature-dependent undercooling that can exceed 60 °C in small volumes (10 μl) cooled at moderate cooling rates (10 °C/min). Here, we use an epitaxial lattice-matching technique to identify cubic carbide and nitride phases that could serve as nucleation catalysts for gallium and gallium-based eutectics. We demonstrate multiple cubic carbides and nitrides that reduce undercooling and show that the relationship between the lattice mismatch and observed undercooling conforms with the heterogeneous nucleation theory. HfC and ZrN result in the smallest reported undercooling to date, <20 and <10 °C, respectively, across all equilibration temperatures after aging. These materials remain stable, even after aging for 120 days in liquid Ga. The carbide and nitride phases described here are commonly used as hard coatings and diffusion barriers, suggesting their practical applicability as thin coatings that both protect an underlying device or component and simultaneously reduce undercooling of gallium or gallium-based eutectics.

Adding thermal conductivity enhancements to increase thermal power in solid-liquid phase-change thermal energy storage modules compromises volumetric energy density and often times reduces the mass and volume of active phase change material (PCM) by well over half. In this study, a new concept of building thermal energy storage modules using high-conductivity, solid-solid, shape memory alloys is demonstrated to eliminate this trade-off and enable devices that have both high heat transfer rate and high thermal capacity. Nickel titanium, Ni50.28Ti49.36, was solution heat treated and characterized using differential scanning calorimetry and Xenon Flash to determine transformation temperature (78deg-C), latent heat (183 kJm-3), and thermal conductivity in the Austenite and Martensite phases (12.92/12.64 Wm-1K-1). Four parallel-plate thermal energy storage demonstrators were designed, fabricated, and tested in a thermofluidic test setup. These included a baseline sensible heating module (aluminum), a conventional solid-liquid PCM module (aluminum/1-octadecanol), an all-solid-solid PCM module (Ni50.28Ti49.36), and a composite solid-solid/solid-liquid PCM module (Ni50.28Ti49.36/1-octadecanol). By using high-conductivity solid-solid PCMs, and eliminating the need for encapsulants and conductivity enhancements, we are able to demonstrate a 1.73-3.38 times improvement in volumetric thermal capacity and a 2.03-3.21 times improvement in power density as compared to the conventional approaches. These experimental results are bolstered by analytical models to explain the observed heat transfer physics and reveal a 5.86 times improvement in thermal time constant. This work demonstrates the ability to build high-capacity and high-power thermal energy storage modules using multifunctional shape memory alloys and opens the door for leap ahead improvement in thermal energy storage performance.