Melting-solidification Cycles Research Articles

Copper based high temperature heat storage macrocapsules with optimized inner cavity

The development of practical high temperature metallic phase change macrocapsules still faces difficulties, because metallic phase change materials undergo volume expansion during phase transformations, which can lead to outer shell rupture. When a core made of stacked metal powders is used, a cavity is eventually formed inside the capsule that acts as a cushion for volume expansion, thus reducing the risk of capsule rupture. However, the cavity will weaken the thermal storage capacity of the capsule, and in order to obtain a capsule with good thermal storage capacity and long-time cycling stability, it is necessary to find a suitable cavity volume of the capsule. In this paper, Cu powders are used as the core raw material, which are spherelized into millimeter level spheres, and the Cu powder spheres are isostatically pressed using an isostatic presser under different pressures to achieve the optimization of the cavity volume by reducing the gap between the powders. The high strength α-Al2O3 is selected as the shell, and sintering additives MgO and SiO2 are doped into the raw material, and the Cu@MgO-SiO2/Al2O3 capsules are prepared after two-step sintering treatment. The material properties and thermal storage properties of the capsules are investigated in detail, which have strong thermal storage properties, for example, in the temperature interval of 1000-1100 ℃, capsules with a core-shell structure (named as 9@1.5-100MPa, prepared with initial diameter of Cu core of 9mm, shell thickness of 1.5mm, and isostatic pressing under 100MPa to the core precursor) exhibit a high heat storage density of 222J/g and 841 MJ/m³. In addition, we conduct melting-solidification cycle tests, which confirms the excellent durability of the capsules. The comprehensive results show that the Cu@MgO-SiO2/Al2O3 macrocapsules possess strong thermal storage capacity and can be used as effective thermal storage materials, especially in advanced high temperature thermal storage systems.

Recent Advances in Additive Friction Stir Deposition: A Critical Review

Additive friction stir deposition (AFSD) is a novel solid-state additive manufacturing method developed on the principle of stirring friction. Benefits from its solid-phase properties, compared with traditional additive manufacturing based on melting–solidification cycles, AFSD solves the problems of porosity, cracks, and residual stress caused by the melting–solidification process, and has a significant improvement in efficiency. In AFSD, the interaction between feedstocks and high-speed rotating print heads suffers severe plastic deformation at high temperatures below the melting point, ending up in fine, equiaxed recrystallized grains. The above characteristics make components by AFSD show similar mechanical behaviors to the forged ones. This article reviews the development of AFSD technology, elaborates on the basic principles, compares the macroscopic formability and material flow behavior of AFSD processes using different types of feedstocks, summarizes the microstructure and mechanical properties obtained from the AFSD of alloys with different compositions, and finally provides an outlook on the development trends, opportunities, and challenges to the researchers and industrial fields concerning AFSD.

Thermal Performance Improvement of Composite Phase-Change Storage Material of Octanoic Acid–Tetradecanol by Modified Expanded Graphite

Phase-change cold storage technology is recommended as a solution for energy conservation and carbon neutrality in air conditioning systems of buildings. This study focuses on the development of binary composite phase-change materials comprising octanoic acid–tetradecanol (OA-TD). To enhance its thermal conductivity, expanded graphite (EG) was employed as an additive carrier, and the surface modification of EG particles using hexadecyltrimethoxysilane (HDTMOS) was attempted to make up for the instability and further to improve the performance of OA-TD/EG CPCMs. The OA-TD/EG-HDTMOS CPCMs were synthesized by EG mixed with EG-HDTMOS at a 1:1 mass ratio. The thermal performance and stability of the OA-TD/EG-HDTMOS CPCMs were thoroughly evaluated by multi-cycle melting–solidification and thermal conductivity measurements. The results revealed that the OA-TD mixture, when at a mass ratio of 77:23, exhibited a phase-transition temperature of 11.4 °C and a latent heat ranging from 150 to 155 J/g. Then, the OA-TD/EG-HDTMOS composite material, at a 12:1 mass ratio of OA-TD to EG-HDTMOS, solidified and melted at temperatures of 9.2 °C and 11.2 °C, with a latent heat ranging from 138 to 143 J/g, and significantly improved the thermal conductivity to 0.7 W/(m·K), representing a remarkable 133% increase compared to that of OA-TD alone. Even after undergoing 100 melting–solidification cycles, the OA-TD/EG-HDTMOS maintained superior phase-change thermal performance and stability, making it suitable for cold storage and energy conservation in air conditioning.

Eutectic phase change composites with MXene nanoparticles for enhanced photothermal absorption and conversion capacity

Phase change composite materials with uniform nanomaterial dispersion are potential solutions for improving thermophysical properties and enhancing latent heat capacity applicable in thermal management. In this study, various transitional carbide-based MXene samples were synthesised via acid etching to augment the heat transfer capacity and efficiency of the solar thermal energy storage (TES). These samples were then uniformly dispersed in a eutectic mixture (1:1) of paraffin wax (PW) and polyethylene glycol (PEG-6000), resulting in the formation of a MePCM composite. The controllable porous structure and hydrogen interactions with the external layer of MXene and long-chain PCM benefitted in the well impregnation of PCM and compatibility impregnated into the composite. An exceptionally higher mass fraction of 1 wt % Ti3C2Tx MXene-based phase change composite unveiled a higher phase change enthalpy of 138.66 J/g in the melting phase and 139.54 J/g in the solidification phase with 89.82 % energy storage efficiency. In contrast to pure eutectic PCM (0.2593 W/mK), the thermal conductivity of MePCM composite is relatively higher and provides a thermal conductivity of 0.9209 W/mK for Ti3C2Tx MXene-based composite. The enhancement of thermal conductivity is ascribed to the increased heat conduction pathway facilitated by the MXene arrangement. The utilisation of spatially constrained eutectic phase change material (PCM) assembly resulted in the development of MePCM, which exhibited enhanced thermal conductivity, chemical and physical stability, and thermal consistency across 500 melting-solidification cycles. Benefitting from the thermal conduction path, the MePCM exhibits an excellent photothermal conversion efficiency of 98.53 %. The thermal responsive tests of Ti3C2Tx and V2CTx MXene composite samples provide better thermal response than eutectic PCM without thermal reduction. With a higher thermal storage capacity and higher heat transfer property, thermally reliable MePCM composites exhibited tremendous potential for photothermal applications.

Macroencapsulated Al-Si phase change materials for high temperature latent thermal energy storage

This research investigated the encapsulation of Al-Si alloy phase change materials (PCMs) for efficient thermal storage at high temperature. Two strategies, the direct powder formation route and in situ powder alloying formation route, were employed successfully. In the direct powder formation route, Al-Si alloy powders were spheroidized into spheres and encapsulated within Al2O3 powder shells with the help of organic binder, which were finally sintered at high temperature, forming Al-Si@Al2O3 capsules with cavities. Isostatic pressing at 50 MPa and 200 MPa were applied to the Al-Si cores, in order to optimize the cavity size. In the in situ powder alloying formation route, Al and Si powders were used as raw materials, which could be in situ transferred to Al-Si alloy inside the capsule during the sintering formation process. With the powders as the raw materials, the final capsules obtained after sintering could provide a buffer cavity inside the capsules, which could accommodate the volumetric expansion of the PCMs during melting, thereby mitigating the potential leakage and rupture of the capsules. Investigations into the melting-solidification cycles revealed the remarkable durability of the Al-Si phase change capsules. Isostatic pressure treatment decreased internal ineffective cavities, improving the heat storage density of the capsules. The Al-12Si(Ar)-200 MPa capsule exhibited superior thermal storage performance, enduring up to 1300 cycles. The Al-12Si core had a melting enthalpy of 479 J/g, while the thermal storage density of the phase change capsule within the temperature range of 500 °C–700 °C reached 496 J/g. The results underscore the significant potential of macroencapsulated Al-Si PCM, positioning it as a promising candidate for high-temperature thermal storage applications.

Shell-Driven Localized Oxide Nanoparticles Determine the Thermal Stability of Microencapsulated Phase Change Material.

Not all encapsulation techniques are universally apt for every type of phase change material (PCM), highlighting the imperative for methodological precision. This study addresses the challenges of microencapsulated PCM (MEPCM) arising from the immiscible pairing of α-Al2O3 nanoparticles with Sn microparticles. The high-speed impact blending (HIB) dry synthesis technique is employed, facilitating large-volume production of Sn@α-Al2O3 MEPCMs. The resulting MEPCMs not only seamlessly endure 100 cycles of melting-solidification but also, with the strategic incorporation of a glass frit, exhibit remarkable thermal durability, withstanding up to 1000 melting-solidification cycles. Even under ultrafast thermal fluctuations, the α-Al2O3 shell remained resilient through 100 cycles. A marked reduction in supercooling is observed, which is attributed to the formation of SnO and SnO2 nanoparticles within the α-Al2O3 crystal lattice. The atomically resolved interface dynamics between SnO2 and α-Al2O3 play a pivotal role, lowering the energy barrier for Sn nuclei formation during solidification. This affects the accelerated Sn nucleation rate, effectively suppressing supercooling. Such insights offer a deeper understanding of the interplay between nanoscale crystal lattice imperfections and their implications for energy storage applications.

Mechanical properties of lamellar-structured 18Ni300 maraging steel manufactured via directed energy deposition

The combination of repetitive melting–solidification cycles, directional laser scanning, and microstructure design using additive manufacturing (AM) is effective for enhancing the mechanical properties of metallic alloys. Because the ultrafine metastable phases in as-built metallic alloys are beneficial to their mechanical properties, there is a design window to enhance the strength–ductility combination by controlling the metastable phase distribution with AM. In this study, ultrafine lamellar-structured maraging steel consisting of martensite and austenite was manufactured using directed energy deposition. The stripe laser scan strategy induced linear (Mo, Ti, Ni)-rich segregations that formed lamellar-structured austenite by reducing the martensitic transformation starting temperature in the high-Ni-content maraging steel. The deformation-induced martensitic transformation and back-stress hardening near the austenite–martensite interfacial region enhanced the plasticity capability of the AM-processed maraging steel and achieved a better strength–ductility combination compared with conventional and recently developed AM-processed maraging steels. This result indicates that artificial microstructural design via AM is important for enhancing the performance of metallic alloys, which can also be achieved with conventional metallic alloys by distributing the metastable phase with processing variable control.

Melting-solidification cycle of finned heat storage tank: Optimization of fin structure by response surface method

The impact of the enhanced heat transfer performance of longitudinal fins on the entire process of melting and solidification in a phase change heat storage unit is investigated through experiments and numerical simulations in this paper. The position and structure of the longitudinal fins are optimized by using the response surface method while ensuring that the total volume of the heat storage medium remains unchanged. Studies have identified that some challenging zones in melting and solidification significantly impact the heat transfer of the entire heat storage cycle. Specifically, the challenging zone in melting primarily exists in the lower part of the unit during the charing process, while the challenging zone in solidification is mainly distributed around the unit during the discharging process. Through optimization, the optimized structure (fin spacing is 22.5 mm, fin width is 6.05 mm) can reduce the charging and discharging time by 16.94 % and 45.90 %, respectively. Additionally, the round trip time is significantly reduced by 39.19 %, and the mean heat absorption rate during the melting process is enhanced by 20.28 %. Moreover, the mean heat release rate during solidification is enhanced by 80.23 %.

Synthesis of Al@Al2O3 microcapsules by a triple-coating method for high-temperature thermal energy storage

Metallic aluminum (Al) has a high melting point and high thermal storage density, making it a proper candidate for high-temperature thermal storage applications including waste heat recovery and solar thermal utilization. To solve the problems of easy leakage after melting, high reactivity and high corrosiveness during application, microcapsules with a stable alumina shell were developed based on a triple coating method. The preparation process was divided into three steps: (1) generation of a boehmite layer on the surface of the Al powders via boiling water treatment, which was designed to slow down the corrosion of the precursor in the subsequent treatment; (2) production of Al(OH)3 gel layer on the surface of the particles through Sol-gel treatment and (3) formation of the final dense Al2O3 shell after thermal oxidation treatment. The synthesized [email protected]2O3 microcapsules had a melting point of 655.5 °C and a latent heat of 227 J/g. Notably, the latent heat of microcapsules remained almost constant after 100 melting-solidification cycle experiments, indicating a good cycling properties. In summary, the newly developed [email protected]2O3 microcapsules have a broad application prospect in the field of high-temperature thermal storage.

Effect of fullerene C60 on phase transition enthalpy of paraffin wax: Calorimetry and structural analysis

In this paper, the effect of minor fullerene С60 admixtures on the phase transitions enthalpy of phase change material (PCM) is studied experimentally. For this purpose, the stable molecular solutions of the paraffin wax (PW) and fullerene С60 were explored in terms of the effective heat capacity and enthalpy of diffuse phase transitions over the (27…65) °С temperature range and C60 mass fraction up to 0.000746 g∙g−1. The choice of C60 as PW admixture was based on the hypothesis that a small amount of C60 contributes to a modification of the PW internal structure and, as a consequence, to a change in its caloric properties. Some factors that may affect the uncertainty for the obtained experimental data were examined. It was shown that the phase transition enthalpy for the PW/C60 in the (40…60) °C temperature range increase from 154.0 J∙g−1 (for pure PW) to 194.7 J∙g−1 (for PW containing 0.000746 g∙g−1 of C60) during cooling and decrease from 233.2 J∙g−1 (pure PW) to 182.3 J∙g−1 (PW containing 0.000746 g∙g−1 of C60) during heating. The results obtained indicate the defining influence of С60 on the PW structure in the liquid and solid states. It was confirmed by an additional structural analysis, which reported that pure PW has a minimal cell volume compared to PW/С60. The C60 addition into PW contributes to the expansion of the cell, indicating that the molecules lose their tight packing. A clear dependence between the enthalpy of melting and unit cell volume for PW/C60 samples was found. It was observed that unit cell expansion of PW due to the addition of C60 reduces the enthalpy hysteresis in the heating-cooling cycle – a key parameter that affects the efficiency of a thermal energy storage material. This positive effect may be due to structural transformation in PW during the melting-solidification cycles.

Thermal energy saving and physico-mechanical properties of foam concrete incorporating form-stabilized basalt powder/capric acid based composite phase change material

The incorporation of phase change materials (PCM) in construction materials for the purpose of thermal energy storage (TES) can prevent temperature fluctuations and enable the conservation of thermal energy in buildings. Accordingly, this study aims to create a novel, environmentally friendly foam concrete incorporating a composite PCM composed of capric acid saturated waste basalt powder. Basalt powder, a byproduct of the grinding or crushing of basalt volcanic rock, was selected as a carrier material due to its high porosity resulting from its lightweight and porous nature. This allowed for the creation of a leak-free composite PCM, which was then integrated with foam concrete made of CEM I 42.5R cement, water, quartz sand, and a foaming agent. Basalt-PCM was substituted into the mixture at 50% and 100% by weight of quartz aggregate. Comprehensive evaluations were conducted on the novel foam concretes with regard to their morphological, mechanical, physical, thermal, and TES properties. The results indicate that the incorporation of basalt-PCM causes a minor reduction in the dry unit weight of the mixtures and a reduction in compressive and flexural strength. However, the leak-proof basalt powder/capric acid composite displayed a phase transition behavior, melting at 28.5 °C with a latent heat of fusion (47.9 J/g), which retained 99.5% capacity after 500 melting-solidification cycles. The specimen with 100 wt% basalt-PCM exhibited a melting temperature and latent heat of 27.8 °C and 17.4 J/g, respectively. Additionally, the basalt powder/capric composite exhibited high thermal stability up to 156 °C, far above its normal operating temperature. The foam concretes incorporating the basalt-PCM demonstrated solar thermoregulation properties, maintaining a more stable and comfortable indoor temperature. These results suggest that the form-stable PCM-incorporating foam concretes developed in this study can be considered as environmentally friendly building materials for thermoregulation and energy conservation.

A comprehensive study of the phase segregation of a urea-based phase change material tested under thermal cycling conditions

Eutectic mixtures are used as PCMs due to the possibility to tailor the melting temperatures and mainly because the phase transition occurs at a unique temperature. Eutectic mixtures are assumed to be congruent melting and solidification materials. Compositional segregation has rarely been reported, when have been researched for its use as PCM. However, the previous premise does not always match the observed facts. The presented work aims to deepen the knowledge regarding the use of eutectic mixtures as PCMs and to determine the influence of operation parameters in the eutectic PCMs potential phase segregation.The eutectic mixture formed by urea and sodium nitrate can be an interesting candidate for use as a phase change material for thermal energy storage in space heating and domestic hot water applications. Nevertheless, the eutectic mixture showed an unforeseen segregation phenomenon when it was exposed to repeated melting-solidification cycles using volumes in the scale of grams. As a result, the phenomenon was studied to determine the potential causes. An experimental campaign was performed to study the urea and sodium nitrate eutectic mixture under different conditions: consisting of thermal cycling using representative masses, and subsequently, the segregated materials and obtained samples were analyzed by different techniques (including XRD, HTXRD, and DSC); and the production of samples under different cooling conditions that were analyzed using microscopy (PLM and SEM). The results established a relationship between the operation conditions, with the resulting crystal structures, which explain the phase segregation in the eutectic mixture. A mitigation measure was determined consisting of mechanical stirring.

Phase transitions in alumina films during post-sparking anodising of Al alloys

Formation of the high-temperature α-Al2O3 phase during Plasma Electrolytic Oxidation of aluminium at ambient bulk temperatures has been previously attributed to local microdischarge events providing multiple melting-solidification cycles in micro-volumes of the surface oxide layer. In this work, it is demonstrated that the α phase can be formed even if the microdischarge is fully suppressed under specific processing conditions. Oxide layers produced in the post-sparking anodising mode were studied by FIB, TEM, EBSD, EDS and GDOES techniques to reveal microstructural and chemical evolutions that accompany the γ to α alumina transition. Our results provide strong evidence that the α phase can form spontaneously in regions of oxide with the appropriate temperature, grain size and impurity distributions in the γ-Al2O3 matrix that allow sufficient mobility of α/γ grain boundaries. Ionic migration within the oxide and hydrothermal dissolution/precipitation in the associated microporous network that facilitate species mobility at the grain boundaries allow the critical temperature for activation of γ→α transition to be reduced. Overall, it is suggested that oxide layer growth can be considered in terms of a relatively simple Plug Flow Reactor model. This can help predict the phase transition kinetics depending on key processing parameters such as current density and frequency of pulse polarisation, thus enabling optimum control of coating microstructure for specific application requirements.

The mechanisms study of the effect of sludge hydrolysis residual preparation on the performance of composite phase change materials

Composite phase change materials composed of hydrated salts for heat storage have become a hot research topic, in which the carrier plays a decisive role in preventing phase separation. Little sawdust was mixed into municipal sludge when directly hydrolyzed in an autoclave for the preparation of residual. The separated and dried hydrolysis residue was used as a carrier for loading sodium acetate trihydrate (SAT) with a mass ratio of 1:6 to prepare composite phase change materials (CPCM) for heat storage. The influence of hydrolysis temperature and pressure on the characteristics of the residue carrier and the heat storage performance of CPCM was then investigated. Multiple cycles of melting and solidification were undergone with CPCM in the plastic tube immersed in a water bath. The thermal stability of CPCM was evaluated by the multi-cycle step-cooling curves, DSC, and XRD. The results show that the residue hydrolyzed at 264 °C and 5 MPa (residue R5) has the highest pore volume and uniform particle size distribution compared with the residues hydrolyzed at other conditions. The latent heat storage density of the CPCM-R5 is 256.3 kJ/kg. After 100 melting-solidification cycles, the difference between the latent heat of CPCM-R5 and its theoretical value is only −2.5 %–4.4 %. The XRD patterns of CPCM-R5 are consistent before and after multiple cycles, indicating that CPCM-R5 is of better thermal and chemical stability, and its thermal conductivity is increased.

Thermal characteristics enhancement of Na2HPO4⋅12H2O/expanded graphite form-stable composite phase change material by the cationic surfactant modification

Phase change materials (PCMs) are widely used for their effectively thermal energy storage and conversion. Na2HPO4⋅12H2O (DHPD) is a promising inorganic PCM, as it possesses large heat enthalpy and suitable phase change temperature. Hexadecyl trimethyl ammonium chloride (CTAC) hydrophilic structure of composite PCM (CPCM) was formed, because of the poor sealing stability of expanded graphite (EG). Indeed, there is the solid intermolecular attraction between cations of CTAC and the water of DHPD, thereby improving the thermal characteristics. The heat enthalpy of EG/CTAC 0.1-DHPD (ECCD-0.1) CPCM was 299 J/g, and the thermal conductivity of ECCD-0.1 CPCM was 1.411 W/m⋅K. The ECCD-CPCMs showed a great cyclic stability which remained stable after 500 melting-solidification cycles.

Computational study on effect of enclosure shapes on melting characteristics of phase change material around a heated cylinder

This work investigates the melting and heat transfer characteristics from an isothermally heated circular cylinder placed inside twelve differentially shaped enclosures filled with the lauric acid (PCM). Extensive results are reported on streamlines, temperature contours, and rate of melting to delineate the influence of enclosure shape on the melting performance of thermal energy storage (TES) unit. The study is conducted at two different surface temperatures of the heated cylinder, namely, 333.15 K and 343.15 K. The time required to reach the fully melt condition largely depends on the amount of PCM above the heated cylinder and the interaction between the thermal boundary layers at the adiabatic wall and the cylinder surface. Among all shapes considered, the inverted semi-circular enclosure yields the fastest melting. Simple modifications in the geometrical shape of the enclosure are seen to significantly reduce the melting time (by ∼ four times) required to reach the fully melt condition thereby expediting the energy storage process. Thus, such elementary geometrical changes in the TES systems can hugely benefit by reducing the energy losses and yield larger industrial margins. At the end, limited results are also included for the complete melting-solidification cycle for the utilization of the results in a broader spectrum.

Nexus approach: ZSM-12 derived from industrial waste into microencapsulated in organic wax for solar energy storage system

ABSTRACT Solar energy storage is gaining scientists interest since it could overcome the key issues affecting the solar energy efficiency. However, the lack of low-cost storage tools still signifies a formidable technological barrier. Zeolite is an answer for such requirements since it could be synthesized via the thermal decomposition of alum sludge waste. The attained ZSM-12, zeolite, was systematically characterized via X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive X-Ray Analysis (EDX). Due to its structure, ZSM-12 is able to store solar energy for matching solar energy availability and demand. A composite of ZSM-12 integrated in paraffin wax in different mass segments subjected into a vertical-type heat exchanger (VHE) and applied as the thermal energy storage material that connected to parabolic trough solar collector (PTC). Water is the heat transfer carrier (HTC) flowing through the solar collector and the VHE in a mass flow rate of 1.83 g/s. The thermal analysis of the PTC is managed and its maximum energy achieved around the solar noon in the summer season. The new composite material displays excellent thermal performance in terms of its chemical stability and durability for melting-solidification cycles. Increasing the ZSM-12 inserted into the system up to 0.6 (wt. %) dispersion is leading to consistently good thermal properties. In addition, experimental charging/discharging data verified a minimum weight loss and simplicity of handling and operation that enhances heat transfer rate. Finally, a numerical optimization is conducted and the overall system efficiency is reached to 93%.

Preparation and properties of novel ceramic composites based on microencapsulated phase change materials (MEPCMs) with high thermal stability

Ceramic composites are widely used in medium/high temperature thermal energy storage (TES) and catalysis. Due to the high latent heat of phase change materials (PCMs), it is an effective method to improve the TES capacity by combining PCMs with ceramic materials. However, PCMs are easy to leak after being heated, so they need to be microencapsulated. Furthermore, for porous ceramic catalytic composites, the leakage of PCMs will block the pores, which seriously hinders their application. In this paper, a novel microencapsulated phase change material (MEPCM) with thermal expansion void was prepared using “double-layer coating, sacrificing inner layer” method. Based on that, two kinds of ceramic composites have been prepared. One is a TES material which composed of alumina, glass frit (GF) and MEPCMs. Thermal analysis results showed that the composite can still maintain stable heat storage performance after 200 melting-solidification cycles with little latent heat loss. Another is a multifunctional porous composite phase change material (CPCM) by loading Ce and Mn as catalyst via solution combustion synthesis (SCS) method, which can be used in low temperature SCR catalysis and other catalytic fields (100–300 °C). Based on MEPCMs with thermal expansion void, the two ceramic composites show great potential in energy storage and catalysis.

Preparation of a new capsule phase change material for high temperature thermal energy storage

Phase change materials (PCMs) have the function of the high temperature thermal energy storage through the phase transition of Al alloys. The liquid alloys have certain liquidity and strong corrosivity. Therefore, general metals and alloys as container would hardly assure the long-term stability in high temperature. Therefore, the encapsulation of melt under the condition of high temperature is a necessary way to fabricate the practical materials. The study seeks to a new method to enhance the stability, high-temperature resistance, high-strength shell and leakage prevention by copper plating on the Al/Al2O3 composite surface. Firstly, the Al microspheres (20–40 µm) are treated by boiling treatment in water forms the AlOOH shell. The AlOOH shell was transformed into Al2O3 shell through heating at 550 °C. And then, the electroless copper plating is implemented to become the multilayered micro-encapsulated phase change material (MEPCM). The thermal durability, cross-sectional structure, surface morphology, and stability of the MEPCM are investigated. The latent heat of Al/Al2O3@Cu MEPCM reaches 223.4 J/g. The shell surface of Al/Al2O3@Cu particles are smooth and dense, after 10 melting–solidification cycles. Moreover, the MEPCM particles maintain their integrity to prevent the leakage of liquid which can be used for high-temperature thermal energy storage such as industrial waste heat recovery. The research also provides a method for the preparation of other heat storage materials, especially for the low-temperature metal-based PCMs.

Directional solidification of photovoltaic silicon in re-useable graphite crucibles

Graphite crucibles are potentially interesting for the directional solidification processing of photovoltaic silicon, because, contrarily to standard silica crucibles, they can be used many times. In the present work, two types of graphite crucibles are studied: i) graphite directly coated with the Si3N4 powder classically used as a releasing layer for standard silica crucibles, and ii) graphite protected by a dense SiC layer before deposition of the releasing coating. In both cases, spontaneous detachment of the silicon ingot is observed to occur during cooling. Results are given and discussed concerning the impurities introduced in the silicon during the melting-solidification cycle, as well as their consequences on the photo-electrical characteristics of silicon. These results are compared with those obtained using the standard silica crucible. It is shown that re-useable SiC protected graphite crucibles lead to the same performances as standard silica. Reproducible results are found after four runs with the same crucible, and no degradation of the crucible is observed.