Neuron-Inspired Flexible Phase Change Materials for Ambient Energy Harvesting and Respiration Monitoring.

The global energy crisis and climate change pose unprecedented challenges. Wearable devices with personal thermoregulation and energy harvesting hold great promise for achieving energy savings and human thermal comfort. Here, inspired by neurons, a novel phase change material (PCM) is reported for efficient energy harvesting and respiratory monitoring via a self-assembly strategy. The use of gum arabic (GA) enabled the encapsulation of polyethylene glycol (PEG) and the targeted distribution of carboxylated multi-walled carbon nanotubes (cMWCNTs) simultaneously in poly (ethylene vinyl acetate) (EVA) matrix. The material exhibits an outstanding toughness value of 14.88 MJ m-3 and high elongation at a break of 565.67%, exhibiting remarkable flexibility. The material with sufficient melting enthalpy (71.11 J g-1) demonstrates high photothermal conversion efficiency (95.27%) under 808 nm laser irradiation (105 mW cm-2). In addition, due to the synergistic effect of GA and PEG, especially the formation of microdome structures on the surface, the material demonstrates ultrasensitive humidity responsiveness for respiratory monitoring with high precision, excellent repeatability, and fast response/recovery time (50.4/50.5 ms). Notably, it shows great potential for moisture-electric generators (MEGs) with the function of non-contact sensing. This material opens the path toward next-generation wearable devices in energy conversion and health monitoring.

Energy-Efficient Geopolymer Composites Containing Phase-Change Materials-Comparison of Different Contents and Types.

The purpose of this study was to analyze the effects of phase-change components on the properties of geopolymer foams. Geopolymer foams are lightweight foamed geopolymers that are characterized by a high degree of porosity. Phase change materials, on the other hand, are compounds that, when added to a material, allow it to absorb, store, and then release large amounts of energy. Three types of PCMs, i.e., MikroCaps, GR42, and PX25, were introduced at 15% by weight. Geopolymer materials were produced based on silica fly ash, and hydrogen peroxide H2O2 was used to foam the geopolymer structure. The PCM geopolymer composites were cured at 60 °C. The produced materials were tested for physical, chemical, and thermal properties. The tests included oxide and mineral composition analysis of the base material, PCM particle size analysis, apparent density and porosity tests on the foams, water leachability tests, thermal tests (λ, Cv, Cp, α), and structural and textural analysis. The most relevant tests to confirm the performance of the phase-change materials were thermal tests. With the introduction of PCMs, volumetric heat capacity increased by as much as 41% and specific heat by 45%, and thermal diffusivity decreased by 23%. The results confirm the great potential of geopolymer composites as modern insulation materials for buildings and structures.

Preparation and Properties of Thermoregulated Seaweed Fibers Based on Magnetic Paraffin wax@calcium Carbonate Microcapsules.

In order to enhance the application of thermoregulated materials, magnetic phase change microcapsules were prepared using a self-assembly method. Paraffin wax was chosen for its fine thermoregulation properties as the core material, while Fe3O4 nanoparticles doped in calcium carbonate served as the hybrid shell material. The microcapsules were then blended with sodium alginate and processed into seaweed fibers through wet spinning. The microstructure, thermal, and magnetic properties of the microcapsules were analyzed using scanning electron microscopy, energy dispersive X-ray spectroscopy, a laser particle size analyzer, an X-ray diffractometer, a differential scanning calorimeter, a thermogravimetric analyzer, and a vibrating sample magnetometer. The thermoregulation of the fibers was evaluated using a thermal infrared imager. The results indicated that the microcapsules had a uniform size distribution and good thermal properties. When the mass fraction of Fe3O4 nanoparticles was 8%, the microcapsules exhibited a saturation magnetization of 2.44 emu/g and an enthalpy value of 94.25 J/g, indicating effective phase change and magnetic properties. Furthermore, the thermoregulated seaweed fibers showed a high enthalpy value of 19.8 J/g with fine shape, offering potential for developing multifunctional fiber products.

Hydrophilic Ultra-fine SiC Nanowires Enhance the Performance of Hydrated Salt Phase-Change Energy Storage Materials.

In this study, ultrafine linear nanostructured SiC with high wettability and large specific surface area were synthesized via the carbothermal reduction method. These nanowires were impregnated with Na2SO4·10H2O, CaCl2·6H2O, MgCl2·6H2O, and CaMg2Cl6·12H2O to obtain composite phase change materials (CPCMs), which demonstrated improved phase separation and significantly reduced supercooling. In particular, the supercooling degree of CaCl2·6H2O was minimized to 0.1 °C. The SiC nanowires effectively prevented issues of dehydration and deliquescence in hydrated salts. The thermal storage capacities of the CPCMs exceeded 90%, with Na2SO4·10H2O and MgCl2·6H2O reaching 107.10% and 103.35%, respectively. Furthermore, the CPCMs exhibited greater sensitivity to changes in temperature compared with the pure hydrated salt phase change materials (PCMs). These results indicate that ultra-fine SiC nanowires can act as a versatile carrier for hydrated salt PCMs at low and intermediate temperatures.

Flexible Highly Thermally Conductive PCM Film Prepared by Centrifugal Electrospinning for Wearable Thermal Management.

Personal thermal management materials integrated with phase-change materials have significant potential to satisfy human thermal comfort needs and save energy through the efficient storage and utilization of thermal energy. However, conventional organic phase-change materials in a solid state suffer from rigidity, low thermal conductivity, and leakage, making their application challenging. In this work, polyethylene glycol (PEG) was chosen as the phase-change material to provide the energy storage density, polyethylene oxide (PEO) was chosen to provide the backbone structure of the three-dimensional polymer network and cross-linked with the PEG to provide flexibility, and carbon nanotubes (CNTs) were used to improve the mechanical and thermal conductivity of the material. The thermal conductivity of the composite fiber membranes was boosted by 77.1% when CNTs were added at 4 wt%. Water-resistant modification of the composite fiber membranes was successfully performed using glutaraldehyde-saturated steam. The resulting composite fiber membranes had a reasonable range of phase transition temperatures, and the CC4PCF-55 membranes had melting and freezing latent heats of 66.71 J/g and 64.74 J/g, respectively. The results of this study prove that the green CC4PCF-55 composite fiber membranes have excellent flexibility, with good thermal energy storage capacity and thermal conductivity and, therefore, high potential in the field of flexible wearable thermal management textiles.

Active infrared tuning of metal-insulator-metal resonances by VO2 thin film.

VO2 is a promising phase change material offering a large contrast of electric, thermal, and optical properties when transitioning from semiconductor to metallic phase. Here we show that a hybrid metamaterial obtained by proper combination of a VO2 layer and a nanodisk gold array provides a tunable plasmonic gap resonance in the infrared range. Specifically, we have designed and fabricated a metal-insulator-metal gap resonance by inserting sub-wavelength VO2 film between a flat gold layer and a gold nanodisk resonator array. The resonance of the hybrid metamaterial is centered in the useful 3-5 µm range when VO2 is in its semiconductor state. The experimental study highlights a monotonical spectral tuning of the resonance when increasing temperature up to 50 °C above the room temperature, providing a continuous resonance shift of almost 1 µm in the mid-infrared range. Wavelength range and intensity tunability can be further optimized by modifying the thicknesses of the layers and metamaterial parameters.

Noise-Aware Active Learning to Develop High-Temperature Shape Memory Alloys with Large Latent Heat.

Shape memory alloys (SMAs) with large latent heat absorbed/released during phase transformation at elevated temperatures benefit their potential application on thermal energy storage (TES) in high temperature environment like power plants, etc. The desired alloys can be designed quickly by searching the vast component space of doped NiTi-based SMAs via data-driven method, while be challenging with the noisy experimental data. A noise-aware active learning strategy is proposed to accelerate the design of SMAs with large latent heat at elevated phase transformation temperatures based on noisy data. The optimal noise level is estimated by minimizing the model error with incorporation of a range of noise levels as noise hyper-parameters into the noise-aware Kriging model. The employment of this strategy leads to the discovery of the alloy with latent heat of -36.08 J g-1, 9.2% larger than the best value (-33.04 J g-1) in the original training dataset within another four experiments. Additionally, the alloy represents high austenite finish temperature (481.71°C) and relatively small hysteresis. This promotes the latent heat TES application of SMAs in high temperature circumstance. It is expected that the noise-aware approach can be convenient for the accelerated materials design via the data-driven method with noisy data.

Wafer-Scale Freestanding Monocrystalline Chalcogenide Membranes by Strain-Assisted Epitaxy and Spalling.

Monocrystalline chalcogenide thin films in freestanding forms are very much needed in advanced electronics such as flexible phase change memories (PCMs). However, they are difficult to manufacture in a scalable manner due to their growth and delamination challenges. Herein, we report a viable strategy for a wafer-scale epitaxial growth of monocrystalline germanium telluride (GeTe) membranes and their deterministic integrations onto flexible substrates. GeTe films are epitaxially grown on Ge wafers via a tellurization reaction accompanying a formation of confined dislocations along GeTe/Ge interfaces. The as-grown films are subsequently delaminated off the wafers, preserving their wafer-scale structural integrity, enabled by a strain-engineered spalling method that leverages the stress-concentrated dislocations. The versatility of this wafer epitaxy and delamination approach is further expanded to manufacture other chalcogenide membranes, such as germanium selenide (GeSe). These materials exhibit phase change-driven electrical switching characteristics even in freestanding forms, opening up unprecedented opportunities for flexible PCM technologies.

In situ characterization of vacancy ordering in Ge-Sb-Te phase-change memory alloys.

Tailoring the degree of structural disorder in Ge-Sb-Te alloys is important for the development of non-volatile phase-change memory and neuro-inspired computing. Upon crystallization from the amorphous phase, these alloys form a cubic rocksalt-like structure with a high content of intrinsic vacancies. Further thermal annealing results in a gradual structural transition towards a layered structure and an insulator-to-metal transition. In this work, we elucidate the atomic-level details of the structural transition in crystalline GeSb2Te4 by in situ high-resolution transmission electron microscopy experiments and ab initio density functional theory calculations, providing a comprehensive real-time and real-space view of the vacancy ordering process. We also discuss the impact of vacancy ordering on altering the electronic and optical properties of GeSb2Te4, which is relevant to multilevel storage applications. The phase evolution paths in Ge-Sb-Te alloys and Sb2Te3 are illustrated using a summary diagram, which serves as a guide for designing phase-change memory devices.

Thermal and mechanical performance assessment of composite (CPCM) for energy storage.

Phase change materials (PCMs) face obstacles in being widely used due to issues with heat transfer and maintaining their shape. In this research, instead of using binders, the Hexadecane (H) is melted in such a way that the capillary forces of the molten wax allow it to be impregnated into the low-density polyethylene (P) molecules and bind it together as a composite. It was found that the hot melt extrusion (HME) combines the two materials at the micro-scale, forming a phase change composite (CPCM) with various geometries that possesses superior latent heat and shape stability during phase transition. The structure can incorporate a higher percentage of PCM (60 %) using this method, which also results in lower costs. According to the thermal analysis, (H60P40) provides great thermal stability and can store a lot of energy per unit of weight. It has a high capacity of storing latent heat at 129.56 J/g and can also prevent Hexadecane leakage. Based on the mechanical properties results, hexadecane acts like plasticizer thus the addition of PCM decreases Young's modulus, stress in break, and stress at yield. This trend is observed as the PCM content increases. The high values of elongation at break also indicates the strong plasticizing properties of PCM. Based on the obtained results, the CPCMs as a potential candidate for an application in buildings for thermal regulation, reducing energy consumption, and reducing indoor temperature swing.

Flexible AIE/PCM composite fiber with biosensing of alcohol, fluorescent anti-counterfeiting and body thermal management functions.

Alcohol sensing plays a critical role in medical detection and personal health management. AIE materials with high sensitivity, selectivity and fast response have been widely used in biosensing, but their application in the field of alcohol sensing still needs further research and development. Furthermore, developing flexible phase change materials (PCMs) is significant for the research of human-body thermal management. In this study, a kind of flexible polyacrylonitrile (PAN)/polyvinylpyrrolidone (PVP)/polyethylene glycol (PEG)/Py-CH (pyrene-based AIE molecule)/SiO2@h-BN composite fiber textile (PAB) with alcohol sensing performance, writable fluorescence property, and human body thermal management function has been prepared via electrospinning technique. The PAN/PVP fiber matrix successfully integrated AIE fluorescent sensing material and PCM into a multi-functional composite with great shape stability. Owing to the introduction of novel pyrene-based Py-CH with AIE characteristic, this innovative textile exhibited wonderful fluorescent properties, including sensitive alcohol fluorescence sensing, writable fluorescence performance and variable temperature fluorescence. Furthermore, proposed PAB textile delivered a high energy storage density of 87∼90 J/g, excellent thermal reliability, great comprehensive mechanical flexibility and enhanced thermal conductivity for flexible human body thermal management. Hence, this flexible multifunctional AIE/PCM composite sensing textiles can be widely used in alcohol sensing, fluorescence anti-counterfeiting and flexible body thermal management.

Optimization of the thermal performance of a lobed triplex-tube solar thermal storage system equipped with a phase change material.

The thermal performance of a PCM-based triple-tube lobed heat exchanger storage system is here simulated and optimized, including performance improvements via lobed surfaces, Y-shaped fins, dispersed multi-walled carbon nanotubes, and metal foams, to be used in combination, or singly. Such computations are done with the finite volume method under different operating conditions. The reason behind this study is to look for solutions to improve the poor thermal performance of phase change materials (PCMs) as thermal energy storage materials, that limits their compactness and instantaneous heat stored/released. This is the first time that a throughout analysis of this aspect is presented. The result showed that higher modified Stefan number allow to improve melting time of a 50.88 %. The inclusion of lobes and fins resulted in a reduction of roughly 30.54 % in time needed for melting completion, compared to straight tubes. This reduction increases to 74.26 % when lobes are combined with both nanoparticles and metal foam, and to 73.60 % with just foam. The best solution also provides a 228.34 W mean heat rate. This study becomes an option to design tube-in-tube energy storage systems, where the best improvement is achieved by considering a lobed surface together with nano/PCM and foam, whereas the highest enhancement comes from using a metal foam.

Anisotropic cellulose-based phase change aerogels for acoustic-thermal energy conversion and management.

Porous materials are usually used as sound-absorbing materials to alleviate noise pollution problems. However, the heat energy conversed from the acoustic energy is wasteful. Herein, anisotropic cellulose-based phase change aerogels (MXene/CNF-C/PEG aerogels) are fabricated by facile directional freeze casting method with anisotropic porous structure, efficient sound wave absorption, acoustic-thermal conversion and thermal management capability. MXene/CNF-C/PEG aerogels with shape stability are formed by hydrogen bonding forces between carboxylated cellulose nanofibers (CNF-C) and PEG without chemical crosslinking. The addition of MXene not only increases thermal conductive performance to 150 % but also enhances acoustic-thermal conversion ability effectively. Moreover, the directional porous MXene/CNF-C/PEG aerogels (DMCPs) possess high energy storage density (143.0 J/g) and acoustic-thermal conversion performance, which open up broad application prospect in the field of acoustic to heat energy conversion and storage.

Unveiling the Boson Peaks in Amorphous Phase-Change Materials.

The Boson peak is a universal phenomenon in amorphous solids. It can be observed as an anomalous contribution to the low-temperature heat capacity over the Debye model. Amorphous phase-change materials (PCMs) such as Ge-Sb-Te are a family of poor glass formers with fast crystallization kinetics, being of interest for phase-change memory applications. So far, whether Boson peaks exist in PCMs is unknown and, if they do, their relevance to PCM properties is unclear. Here, we investigate the thermodynamic properties of the pseudo-binary compositions on the tie-line between Ge15Te85and Ge15Sb85from a few Kelvins to the liquidus temperatures. Our results demonstrate the evidence of the pronounced Boson peaks in heat capacity below 10 K in the amorphous phase of all compositions. By fitting the data using the Debye model combined with a modification of the Einstein model, we can extract the characteristic parameters of the Boson peaks and attribute their origin to the excess vibrational modes of dynamic defects in the amorphous solids. We find that these parameters correlate almost linearly with the Sb-content of the alloys, despite the nonmonotonic behaviors in glass forming abilities and thermal stabilities. In a broader context, we show that the correlations of the characteristic parameters of the Boson peaks with Tg and kinetic fragility, vary according to the type of bonding. Specifically, metallic glasses and conventional covalent glasses exhibit distinct patterns of dependence, whereas PCMs manifest characteristics that lie in between. A deeper understanding of the Boson peaks in PCMs holds the promise to enable predictions of material properties at higher temperatures based on features observed in low-temperature heat capacity.

Applying pH Modulation to Improve the Thermal Stability of Melamine-Formaldehyde Microcapsules Containing Butyl Stearate as a Phase-Change Material.

This paper presents a two-stage microencapsulation process that uses pH modulation to enhance the thermal stability of microcapsules that consist of a melamine-formaldehyde (MF) shell and a butyl stearate core. In the first stage, the pH value was modulated between 6.0 and 8.0. Rising the pH value to 8.0 slowed the polycondensation rate, allowing the MF resin with a lower degree of polymerization to migrate to the capsule surface and form a smooth shell. Lowering the pH value to 6.0 accelerated polycondensation. In the second stage, a relatively fast, continuous reduction in the pH value to 5.0 led to further MF polycondensation, hardening the shell. Post-curing at 100 °C prevented shell damage caused by the liquid-gas phase transition of the core material during the process. The microcapsules produced by increasing the pH value to 8.0 twice demonstrated improved thermal stability, with only a minimal overall weight loss of 5% at 300 °C. Significant weight loss was observed between 350 and 400 °C, temperatures at which the methylene bridges in the MF shell undergo thermal degradation. The results from differential scanning calorimetry, electron microscopy, and thermogravimetry analyses confirmed a successful optimization of the microencapsulation, showing that these microcapsules are promising for thermal energy storage and other applications that require high thermal stability.

An Experimental Study on the Thermal Performance of a Heat Sink Filled with Porous Aluminum Skeleton/Paraffin Composite Phase Change Material.

Organic phase change material is an ideal solution to solve the heat dissipation problem of electronic devices. However, its low thermal conductivity limits its application. To solve this problem, a new porous aluminum skeleton/paraffin composite phase change material (AS-PCM) was prepared. The effects of porosity and porous aluminum skeletons on temperature control performance were explored. The experimental results show that the addition of AS significantly improves the thermal conductivity of organic PCM, and the thermal conductivity of AS-PCM is 32.3-59.6 times higher than that of pure paraffin. In addition, the temperature difference in AS-PCM with 75% porosity is 1-2 °C lower than that of AS-PCM with 85%, and 5-8 °C lower than that of AS-PCM with 95% porosity. The skeleton structure has an impact on the temperature control performance. The Mcc porous aluminum skeleton/paraffin composite phase change material (MAS-PCM) yields the best thermal performance compared with the Fcc porous aluminum skeleton/paraffin composite phase change material (FAS-PCM). The temperature control time of the MAS-PCM heat sink is increased by 5.3-50.8% relative to the FAS-PCM heat sink. The research results provide a novel approach for improving the thermal conductivity of PCMs.

Aerogels for Phase-Change Materials in Functional and Multifunctional Composites: A Review.

Phase-change materials (PCMs) have gained more attention during the last few decades. As the main function of these materials is to store and release energy in the form of latent heat during phase transitions, they perfectly fulfill the direction of modern research focused on energy-related topics. Although they have basic energy-related properties, recent research shows a need to upgrade those materials in terms of improving their common drawbacks like shape stability, leakage, and poor conductivity. The research related to PCM-based composites leads to imparting some additional functional properties such as different types of conversion abilities or extra performance such as shape memory and thermal protection. Together with a new emerging material group-aerogels (AGs), extra-light and highly porous matrices-PCMs could become functional and multifunctional materials. AG-PCM composites could be implemented in a large variety of applications in different sectors like energy, buildings, medical, defense, space technologies, and more. This study aims to help summarize current trends, methods, and works on PCM-aerogel composites in terms of developing new functional materials, especially for energy conversion purposes but also for improved conductivity, mechanical properties, and flame retardancy.

Nanotechnology for thermal comfort and energy efficiency in educational buildings with a simulation and measurement approach in BSh climate.

Educational buildings have a large share and impact on urban development. While research shows a significant portion of non-industrial energy consumption in these buildings, obtaining optimal thermal comfort in educational buildings remains one of the main concerns in achieving the grounds to promote students' best performance and efficiency. Extensive research has been done in this field, however, this research presents a new approach to the diverse use of nanotechnology techniques which improve its properties and components in the buildings, aiming to reduce energy consumption and increase thermal comfort. In this paper, thermal comfort and energy consumption are evaluated in a 12-class elementary school located in Shiraz City. Aeropan and nano-Phase change materials (nano-PCMs) is used in the window glass and walls of the studied case. This evaluation presents the simulation and experimental analysis of thermal comfort (PMV) and energy consumption of three classroom alignments in the school building including the Linear-shape (LS), the Integrated Linear-shape (ILS), and the U-shaped (US) alignment. The simulation was performed using EnergyPlus 9.6 software, while the experimental data was collected using TESTO 425 device. The result of this research shows that after applying nano-PCM and Aeropan techniques in window glass and walls, the US alignment has the highest reduction in energy consumption (monthly average of 11.80%) compared to LS and ILS alignments. This alignment includes an energy consumption reduction of 12.03% in the coldest, and, 11.66% in the hottest day of the year in addition to increasing the monthly average thermal comfort of school by the use of nanomaterials.

Ultrahigh Stability and Operation Performance in Bi-doped GeTe/Sb2Te3 Superlattices Achieved by Tailoring Bonding and Structural Properties.

Changes in bond types and the reversible switching process between metavalent and covalent bonds are related to the operating mechanism of the phase-change (PC) behavior. Thus, controlling the bonding characteristics is the key to improving the PC memory performance. In this study, we have controlled the bonding characteristics of GeTe/Sb2Te3 superlattices (SLs) via bismuth (Bi) doping. The incorporation of Bi into the GeTe sublayers tailors the metavalent bond. We observed significant improvement in device reliability, set speed, and power consumption induced upon increasing Bi incorporation. The introduction of Bi was found to suppress the change in density between the SET and RESET states, resulting in a significant increase in device reliability. The reduction in Peierls distortion, leading to a more octahedral-like atomic arrangement, intensifies electron-phonon coupling with increased bond polarizability, which are responsible for the fast set speed and low power consumption. This study demonstrates how the structural and thermodynamic changes in phase change materials alter phase change characteristics due to systematic changes of bonding and provides an important methodology for the development of PC devices.