Thermal Cycling Stability Research Articles

Photo- and magneto-responsive highly graphitized carbon based phase change composites for energy conversion and storage

Phase change material (PCM) as latent heat storage unit shows great potential in various scenes, including solar energy storage system and magnetically induced energy conversion and storage. But insufficient optical activity and electromagnetic response capability seriously hinder its wider applications. Herein, we reported an advanced high-performance photo- and magneto-driven composite phase change material (CPCM), where metal-organic framework (MOF) derived magnetic highly graphitized carbon (MGC) served as a fast-charging photo- and magneto-responsive supporting skeleton, and octadecanol (ODA) acted as a latent heat storage unit. In graphitic structure, there were numerous sp2 carbon atoms and π electrons. Under solar irradiation, heat is quickly discharged by lattice vibration resulting from π-π∗ transition and subsequent electron-phonon coupling in highly graphitized carbon. Moreover, in situ anchored magnetic cobalt nanoparticles contribute to another exothermic effect triggered by light and alternative magnetic field owing to its surface plasmon resonance (SPR) and eddy current effect, respectively. Additionally, cobalt nanoparticles can also serve as the heterogenous nucleation sites to promote the crystallinity of ODA molecules, thus profiting the increase of phase change enthalpy. Therefore, our developed cobalt-decorated highly graphitized carbon based photo- and magneto-responsive CPCM synergistically reached an enhanced photothermal efficiency of 88.1%, thermal energy storage capacity of 131.14 J/g, and excellent thermal cycling stability and reliability after undergoing multiple charging-discharging cycles, showing great potential in latent heat storage, solar and magnetic energy harvesting.

Significant improvement in the thermal cycling stability of Ni44.8Ti45.8Hf5Cu5 shape memory alloy by high pressure torsion and post-deformation annealing

A newly developed quaternary Ni44.8Cu5Ti45.2Hf5 shape memory alloy (SMA) was processed by high-pressure torsion (HPT) for five turns under a pressure of 6 GPa at room temperature and then subjected to post deformation annealing (PDA) at 450 °C for 10 min. Martensitic phase transformation behavior, thermal cycling stability of the transformation, and shape memory response of the alloy were investigated after HPT and subsequent annealing and compared with the initial as-cast material. The results showed that HPT followed by annealing decreased forward (Ms and Mf) and reverse (As and Af) transformation temperatures as compared to the as-cast state, meanwhile, significantly improved thermal cycling stability of the transformation. After 50 differential scanning calorimetry (DSC) cycles, the shift in the peak temperature (Mp) of the forward B2 (austenite) → B19′ (martensite) transformation for the as-cast alloy was measured to be 28 °C, while that for the sample processed by HPT and PDA was almost null. In addition to the substantial thermal cycling stability improvement, thermal cycling experiments under a constant stress (σ = 200 MPa) revealed that processing by HPT and subsequent annealing improved shape memory behavior of the alloy, decreased the width of thermal hysteresis from 79 °C to 53 °C, and completely removed irrecoverable strain.

In-situ deposition preparation of n-octadecane@Silica@Polydopamine-doped polypyrrole microcapsules for photothermal conversion and thermal energy storage of full-spectrum solar radiation

Phase change materials （PCMs） play an essential role in utilizing solar energy. However, there exist inherent defects of pure PCMs, such as low solar energy utilization efficiency and easy leakage. In this work, we successfully prepared a novel type of n-octadecane@Silica@Polydopamine-doped polypyrrole photothermal phase change microcapsules (SCN@PAP micro-PCMs), contained n-octadecane (n-OD) as the core material, silica as the intermediate layer, and polydopamine and polypyrrole composite (PDA-PPY) as the wall material. The microcapsules' morphology, chemical structure, photothermal and thermal storage properties, and thermal stability were investigated. This study confirmed that polydopamine (PDA) doping positively regulates the optical properties of polypyrrole (PPY). The PDA-PPY(PAP) was uniformly deposited on the surface of n-OD@SiO2 phase change microcapsules (SCN micro-PCMs) in granular form with the appropriate doping ratio. Further, tuning the ratio of SCN micro-PCMs to PAP and pyrrole (PY) to dopamine (DA) could endow SCN micro-PCMs with good photothermal properties and retain high thermal storage properties. When the mass ratio of PY: DA = 1:0.032 and SCN: PAP = 6:1, the prepared SCN@PAP micro-PCMs exhibited high photothermal conversion efficiency up to 97.31%, with a melting enthalpy of 145.3 J/g. The enthalpy value decreased by less than 1 J/g after 100 heating and cooling cycles demonstrating excellent thermal energy storage cycling stability. The SCN@PAP micro-PCMs are expected to be promising in promoting the efficient utilization of solar radiation.

Stabilized multifunctional phase change materials based on carbonized Cu‐coated melamine foam/reduced graphene oxide framework for multiple energy conversion and storage

AbstractThe leakage of organic phase change materials (OPCMs) at temperatures above their melting point severely limits their large‐scale application. The introduction of porous supports has been identified as an efficient leakage‐proofing method. In this study, a novel carbonized Cu‐coated melamine foam (MF)/reduced graphene oxide (rGO) framework (MF/rGO/Cu‐C) is constructed as a support for fabricating stabilized multifunctional OPCMs. MF serves as the supporting material, while rGO and Cu act as functional reinforcements. As a thermal energy storage material, polyethylene glycol (PEG) is encapsulated into MF/rGO/Cu‐C through a vacuum‐assisted impregnation method to obtain PEG@MF/rGO/Cu‐C composite with excellent comprehensive performance. PEG@MF/rGO/Cu‐C exhibits high phase change enthalpies of 148.3 J g−1 (melting) and 143.9 J g−1 (crystallization), corresponding to a high energy storage capability of 92.7%. Simultaneously, MF/rGO/Cu‐C endues the composite with an enhanced thermal conductivity of 0.4621 W m−1 K−1, which increases by 463% compared to that of PEG@MF. Furthermore, PEG@MF/rGO/Cu‐C displays great light‐to‐thermal and electric‐to‐thermal conversion capabilities, thermal cycle stability, light‐to‐thermal cycle stability, and shape stability, showing promising application prospects in different aspects.

Emerging PEG/VO2 dual phase change materials for thermal energy storage

Emerging PEG/VO 2 dual phase change materials (PCM) with phase transition temperature gradients were prepared with polyethylene glycol (PEG) and vanadium dioxide (VO 2 ) through the vacuum impregnation method. To improve the stability, thermal conductivity, and thermal storage capacity of PEG/VO 2 , expanded graphite (EG) with different mass gradients was introduced. PEG was successfully confined as 54.48, 63.91, and 74.31 wt% in the dual composite phase change materials including 2.60, 3.28, and 3.32 wt% EG, respectively. The chemical composition and microstructure characteristics of the composite phase change material were measured by XRD, FTIR, and SEM. The thermal storage capacity of 70–113 J g −1 and phase transition temperature range of 30–70 °C for PEG/VO 2 /EG were determined by DSC. Moreover, compared with pure PEG, the thermal conductivity of PEG/VO 2 /EG 0.10 was increased by up to 93.75%, and the thermal storage capacity was increased by up to 59.35% compared with PEG/VO 2 . Compared to pure PCM, due to the improvement of thermal conductivity, the response to ambient temperature was also significantly shortened. The thermal cycling stability and thermal durability were demonstrated by the thermal cycle test and TG. Besides, PEG/VO 2 /EG 0.10 was used to simulate the thermal management of lithium-ion battery, indicating that the material had certain advantages in cooling and could improve the safety of the battery during use. PEG/VO 2 /EG 0.10 has good application potential in the field of thermal management due to the unique dual phase transition interval. • Dual phase change materials with phase change temperature gradient were successfully prepared. • The thermal conductivity of PEG/VO 2 /EG 0.10 was increased by 93.75% compared PEG and achieves a rapid heat transfer rate. • The simulated thermal management experiments of PEG/VO 2 /EG 0.10 demonstrated excellent performance.

Salt in matrix for thermochemical energy storage - A review

Thermal energy storage (TES) method plays a vital role in storing and retrieving thermal energy from various sources (e.g. solar energy, industrial waste heat). Among all viable options for the TES, the reversible reaction pathway of thermochemical energy storage (TCES) has proved an experimentally much efficient form of energy storage over the latent and sensible energy storage. This review article focuses on a solid-gas system with water as sorbate for the TCES. At the material level, emphasis is put on comparing various pure thermochemical materials (TCMs) and salt in matrix (SIM), a thermochemical compound with porous material as a host. The basis of comparison among the compounds has been energy density and cyclic stability. It has been found that a pure TCM offers higher energy storage density compared to the SIM, while the latter exhibits better thermal cyclic stability. The porous host increases the surface area of the TCM leading to better mass and energy transfer and also prevents the deliquescence of the salt during the hydration process. This study also focuses on the synthesis of SIM, properties of host materials for SIM, and the use of various characterization methodologies for the analysis of SIM.[copyright information to be updated in production process]

Li4(OH)3Br/MgO shape stabilized composite as novel high temperature thermal energy storage material

Li4(OH)3Br/Porous-MgO shape stabilized composites were developed in this study as novel high temperature thermal energy storage materials. Li4(OH)3Br, as storage material, owns a large reaction enthalpy (247 J/g) at 288 °C and excellent thermal cycling stability over 600 cycles. Solid MgO nanopowder was selected in a previous study among several metal oxides as the most promising shape stabilizer for Li4(OH)3Br salt satisfying the criteria of wettability, thermochemical compatibility, structural stability and cycling stability. However, this material ensures the structural stability of the composite at a minimum oxide loading of 50 wt%. This relatively high oxide loading will drastically decrease the overall storage capacity of the composite, which is not practical for TES applications. In order to reduce the MgO loading, new mesoporous MgO particles were tested as supporting materials. The idea is to benefit from the mesoporosity in improving the antileakage efficiency of the composite. To do so, three different porous MgO samples were synthesized and tested. Namely, i) Porous MgO (PMgO) synthesized by combustion using Magnesium nitrate, giving a BET surface area of 40 m2/g and a pore volume of 0.217 cm3/g. ii) MgO synthesized by calcination of basic magnesium carbonate (MgO-BMC), giving a high BET surface area of 129 m2/g and a pore volume of 0.294 cm3/g. iii) nanocrystalline MgO (MgO-BM64h) obtained by ball-milling process of commercial MgO micropowder, giving a BET surface area of about 55 m2/g and pore volume of 0.088 cm3/g. The three porous MgO materials exhibit various pore structures. The composites were synthesized following a simple fabrication method by cold compression, mixing and sintering. The results were promising for PMgO based composites where appreciable thermal and structural stability were achieved as 30 wt% oxide loading, whereas MgO-BMC and MgO-BM64h showed poor cycling stability at the same loading. SEM-EDS analyses of PMgO based composite showed an improvement of the homogeneity of the composite structure over 50 melting/solidification cycles. Moreover, the overall thermal conductivity of the composite was enhanced by 33% over pure salt.

Thermo-mechanical stability of concrete containing steel slag as aggregate after high temperature thermal cycles

Thermal energy storage represents a crucial element to increase solar power dispatchability. Within sensible heat storage in solid media, concrete is considered a low-cost alternative to be further developed and therefore, this has been addressed in this paper. Four concrete dosages were designed, combining each type of considered cement, ordinary Portland and calcium aluminate cement, with each type of considered aggregate, silico-calcareous and a steel slag. Thermo-mechanical properties of concrete were studied before and after 10 thermal cycles from 290 °C to 700 °C. Maximum operating temperature and heating rates were selected accordingly to the targeted application, a concentrating solar power (CSP) tower plant. At macro-level, results show thermal cycle stability of concrete with steel slag aggregate in both cement types. On the contrary, at micro-level, the petrography analysis shows the lack of bonding between steel slag aggregate and the cement paste. In contrast, concrete mixtures containing silico-calcareous aggregates collapse after thermal cycling.

Ceramic–molten salt composites (CPCMs) for high-temperature thermal energy storage: Improving sinterability and thermal stability by using solid wastes as skeletons

Molten salts are ideal high-temperature thermal energy storage materials for solar energy generation; however, disadvantages such as leakage, corrosive behavior, and low thermal conductivity limit their widespread application. The use of ceramic skeletons to encapsulate molten salts by the mixing-sintering method is a promising approach to overcome the above shortcomings. However, the CPCMs fabricated by the mixing-sintering method usually have high porosity and are easily deformed at temperatures above the melting point of the loaded salt. In this study, two solid wastes, ferrotitanium slag and waste glass, were first used as skeleton materials to encapsulate molten Na2SO4–NaCl salt; herein, waste glass was also introduced as a modifier to promote the sinterability and thermal stability of the CPCMs. The results demonstrated that the waste glass addition not only improved the sinterability via the viscous flow mechanism but also enhanced the compressive strength, thermal conductivity, thermal cycling stability, and high-temperature structural stability of the CPCMs. Additionally, ferrotitanium slag had good compatibility with the Na2SO4–NaCl salt. The waste glass reacted with the molten salt to form a new compound, (Na0.8Ca0.1)2SO4. Although the synthesis of (Na0.8Ca0.1)2SO4 increased the melting temperature of the CPCMs by 3–4 °C, it improved the melting enthalpy of the CPCMs that was beneficial for compensating the salt loss during thermal cycling. In summary, an eco-friendly and low-cost thermal energy storage material was developed in this study, and more importantly, a promising approach to improve the sinterability and thermal stability of CPCMs was proposed.

Polyimide/phosphorene hybrid aerogel-based composite phase change materials for high-efficient solar energy capture and photothermal conversion

• We developed a new type of composite PCM for high-efficient solar energy capture. • The composite PCM was based on polyimide/phosphorene hybrid aerogel and PEG . • The introduction of phosphorene enhances solar photothermal absorption and conversion. • The composite PCM presents high solar light-to-heat conversion efficiency. • The composite PCM exhibits great potential for efficient utilization of solar energy. Solar energy is the cleanest and most abundant renewable energy source that can be utilized to displace fossil fuels and provide an effective solution for future energy shortage and climate change. Aiming at enhancing the capture efficiency of solar photothermal energy, we developed a novel type of phase-change composites based on polyimide (PI)/phosphorene (PR) hybrid aerogel and polyethylene glycol (PEG). The composites were prepared by fabricating a series of PI/PR hybrid aerogels with different loadings of PR nanosheets using freeze-drying and thermal imidization techniques, followed by vacuum impregnation of PEG as a phase change material into the aerogel framework. The obtained hybrid aerogels exhibited a lightweight nature with a density of 21.9 mg·cm −3 when 16 wt% PR nanosheets were incorporated, resulting in an extremely high PEG loading of 4067% in the aerogel system. The combination of PI and PR nanosheets leads to a significant enhancement in solar light absorption and photothermal energy conversion for the hybrid aerogel/PEG composites. The loading amount of PEG was also improved remarkably due to an increase in the volume capacity of the hybrid aerogels resulting from the introduction of PR nanosheets. The resultant hybrid aerogel/PEG composites not only exhibit a high latent-heat capacity of over 170 J·g −1 and a high photothermal conversion efficiency of 82.5%, but also show good thermal impact resistance to keep their original shape and form well after heating at 80 °C for 20 min and maintain a high thermal cycle stability after thermally cycled for 500 times. These superior performances make the hybrid aerogel/PEG composites capable of dealing with a wide range of applications in solar photothermal energy capture and storage. This study provides a promising strategy for the design and development of high-performance and lightweight composite PCMs to meet the requirement of applications for efficient utilization of solar energy.

Dimensional Stability of Mirror Substrates Made of Silicon Particle Reinforced Aluminum.

In the present study, the thermal cycling stability of mirrors made of silicon particle reinforced aluminum compounds, containing an amount of 42 ± 2 wt.% silicon particles, is investigated with respect to thermal loading. The compound is processed by single-point diamond turning to optical mirrors that were subsequently thermally cycled in a temperature range between 40 °C to −60 °C and between 20 °C and −196 °C, respectively. The residual shape change of the optical surface was analyzed using Fizeau interferometry at room temperature. The change of shape deviation of the mirrors is compared with dilatometric studies of cylinders using the same temperature regime. Due to different coefficients of thermal expansion of the two constituents of the compound, thermal mismatch stresses in the ductile aluminum matrix and the brittle silicon particles are induced by the investigated thermal loads. The plasticity that occurs causes the formation of dislocations and stacking faults as substantiated by Transmission Electron Microscopy. It could be shown that the silicon particles lead to the cold working process of the reinforced aluminum matrix upon thermal cycling. By using interferometry, a higher dimensional stability of mirrors made of silicon particle reinforced aluminum due to thermal loads is demonstrated.

Microstructure, martensitic transformation, mechanical properties and shape memory effect of (TiBw+TiCp)/Ti-V-Al shape memory alloy composites

In the present investigation, the effect of the volume fraction of B4C ceramic particles on the microstructure, martensitic transformation, mechanical properties and strain recovery characteristics of Ti-V-Al shape memory alloy composites was explored. The results revealed that the addition of B4C ceramic particles resulted in the formation of in-situ TiB whiskers and TiC particles as well as grain refinement. Based on the introduction of in-situ reinforcement and grain refinement, the martensitic transformation temperature of Ti-V-Al shape memory alloy composites gradually increased, as the B4C ceramic particle content increased from 0.5 vol.% to 10.0 vol.%. In summary, a lightweight Ti-V-Al shape memory alloy composite with high performance, including a higher martensitic transformation temperature, improved thermal cycling stability, higher strength and larger elastic modulus, can be obtained by adjusting the volume fraction of B4C ceramic particles.

Evaluation of NdBaCo2O5+δ oxygen electrode combined with negative expansion material for reversible solid oxide cells

The mismatch of thermal expansion between the oxygen electrodes and the electrolytes is still a critical issue, which leads to the degradation of performance for solid oxide cells (SOCs) or even the damage to cells during the long-term operation. Here, we propose a strategy to achieve complete thermomechanical compatibility between NdBaCo2O5+δ (NBC) electrode and other cell components by introducing NdMnO3-δ (NM), a negative thermal expansion (NTE) oxide. During the thermal cycling process, the negative expansion material alleviates the shear and tensile stresses of the NBC electrode in a three-dimensional space. Whereafter, the “buffer effect” significantly improves the thermal cycling stability of the composite electrode. The results of electrochemical impedance spectroscopy and microstructure observation also confirmed that the significant improvement in thermal stability is mainly related to a better bonding ability between the oxygen electrode and the electrolyte.

Polyimide/MXene hybrid aerogel-based phase-change composites for solar-driven seawater desalination

Aiming to explore a high-efficient seawater desalination technique based on solar photothermal energy as the cleanest and most abundant renewable energy source, we developed a novel type of phase-change composites based on a polyimide (PI)/MXene hybrid aerogel as a supporting material and polyethylene glycol (PEG) as a phase change material. The introduction of Ti3C2Tx MXene nanosheets as a high-efficient sunlight absorber not only significantly enhanced solar photothermal energy-conversion efficiency up to 87.28% for the aerogel/PEG composites, but also improved the volume capacity of the hybrid aerogels. This resulted in an extremely high PEG loading rate of up to 97.68 wt% and a high enthalpy efficiency of up to 95.16%. The hybrid aerogel/PEG composites also exhibit an outstanding thermal energy-storage capability under a high latent heat capacity of over 170 J·g−1, high thermal cycle stability, good leakage-resistant performance, excellent thermal impact resistance, and good shape/form stability to deal with the use in seawater desalination. More importantly, the hybrid aerogel/PEG composites show a high seawater evaporation mass of 6.07 kg·m−2, a high evaporation rate of 1.24 kg·m−2·h−1 under one-sun illumination, and a high evaporation efficiency of 50.6% when used in a solar-driven evaporation device. The PI/MXene hybrid aerogel/PEG composites developed by this work exhibit a great potential for application in sustainable seawater desalination.

Form-stable phase change materials based on hierarchically channel-like silica nanofibrous mats for thermal energy storage

Hierarchical channel-like SiO2 nanofibrous mats were prepared via microemulsion electrospinning in combination with calcination for thermal energy storage. The interior channel number of SiO2 nanofibers was flexibly regulated by tuning the proportion of continuous phase to dispersion phase. Microemulsion electrospinning, as a method of single-spinneret electrospinning, makes it versatile, simple and convenient for the preparation of hierarchical SiO2 nanofibers with multi-channel structure and high porosity. Quinary fatty acid eutectics were encapsulated in multi-channel SiO2 mats to obtain form-stable phase change materials (FSPCMs). The results indicated the crystallization enthalpy and melting enthalpy of as-fabricated FSPCMs could be up to 115.7 and 112.9 J·g−1, respectively. After 40 thermal cycles, there is no significant difference for the phase change temperature and enthalpy of FSPCMs, revealing outstanding thermal cycling stability. The study proves that microemulsion electrospinning is exactly a straightforward and universal method to fabricate the supporting substance of FSPCMs.

Stable salt hydrate-based thermal energy storage materials

Heating and cooling systems in building infrastructure utilize conventional materials that account for a considerable amount of energy usage and waste. Phase change material (PCM) is considered a promising candidate for thermal energy storage that can improve energy efficiency in building systems. Here, a novel salt hydrate-based PCM composite with high energy storage capacity , relatively higher thermal conductivity , and excellent thermal cycling stability was designed and developed. The thermal cycling stability of the PCM composite was enhanced by using dextran sulfate sodium (DSS) salt as a polyelectrolyte additive, which significantly reduced the phase segregation of salt hydrate . The energy storage capacity and the thermal conductivity of the composite were enhanced by the addition of various graphitic materials along with Borax nucleator. A significant increase in thermal cycling stability was observed for the DSS-modified composite, with over 100 thermal cycles without degradation. The final PCM composite exhibited as much as 290% increase in energy storage capacity relative to the pure salt hydrate, and approximately 20% increase in thermal conductivity. In addition, the PCM composite developed can be produced at larger scale, and can potentially change the future of heating/cooling system in building infrastructure. • Polyelectrolyte-stabilized salt hydrate phase change material (PCM). • Reduced phase separation of sodium sulfate decahydrate upon thermal cycling. • Significant increase in thermal cycling stability up to 100 thermal cycles. • PCM composite exhibited 290% increase in energy storage capacity. • High throughput processing technique.

Form-Stable phase change composites based on porous carbon derived from polyacrylonitrile hydrogel

Three-dimensional hierarchical porous carbon (3D-HPC) carriers were fabricated by pyrolyzing porous polyacrylonitrile hydrogels at high temperature, which were further applied to achieve simultaneous guarantee of the high energy storage density, thermal conductivity, shape stability and optical/electric-thermal energy conversion performance of PCMs. • Synthesis of PAN gels is green and facile; AMPS helps polymerization and pyrolysis. • Carbonization of PAN aerogel produces high-performance 3D-HPC carriers. • 3D-HPC can load PEG > 85 wt% with shape stability and thermal cycling stability. • 3D-HPC/PEG CPCMs present 10 times increase in thermal conductivity than PEG. • CPCMs possess optical/electric-thermal energy conversion capability. How to improve thermal conductivity as well as energy conversion efficiencies and to prevent melting leakage of solid–liquid phase change materials (PCMs) has been the essential challenges for the development of energy storage technology for storing a tremendous amount of waste heat and renewable and sustainable solar energy. Here, we reported the fabrication of high-performance form-stable composite PCMs (CPCMs) using the thermal conductive 3D-HPC carriers produced from the pyrolysis of porous polyacrylonitrile (PAN) copolymer hydrogels to confine the polyethylene glycol (PEG) PCMs. The porous 3D network PAN gel templates generate hierarchical cavities with micro/mesopores on the skeleton after pyrolysis which provides more space as well as the capillary adsorption and chemical interaction for the storage of PEG PCMs. The 3D-HPC/PEG CPCMs show high energy storage density (as high as 173.5 J g −1 ), high phase transformation efficiency and excellent thermal cycle stability. The carbon scaffold highly enhanced the thermal conductivity of CPCMs ∼ 10 times of the pure PEG and ensured its optical/electric-thermal energy conversion performance. This study demonstrates a facile, efficient and green way to fabricate hierarchical porous carbon with controllable porosity and degree of graphitization for improving the energy storage density, thermal conductivity, shape stability and energy conversion performance of PCMs.

(Bi0.5Na0.5)TiO3-based relaxor ferroelectrics with enhanced energy-storage density and efficiency under low/moderate - fields via average ionic polarizability design

• Utilize an average ionic polarizability design strategy to balance W rec and η. • The high-polarization gene is maintained and the polar order is disrupted. • Achieve excellent energy-storage properties both in low-field and moderate-field. • The existence of dynamic PNRs is confirmed by PFM measurements. • The temperature-dependent Raman spectra reveal the variation of local structure. Electrostatic capacitors are promising candidates in the application of pulse-power systems due to their high power density and fast charge-discharge rate. However, the insufficient recoverable energy density ( W rec ) and energy efficiency ( η ) under low/moderate electric field is still the bottleneck for their applications in miniaturized equipment and portable devices. In this work, we utilize an average ionic polarizability design strategy to enhance W rec by sustaining the high-polarization gene of the system and simultaneously improve η by disrupting the polar order in (1- x )[0.7Bi 0.5 Na 0.5 TiO 3 -0.3Sr 0.7 Bi 0.2 TiO 3 ]- x Sr(Zn 1/3 (Nb 0.85 Ta 0.15 ) 2/3 )O 3 ceramics. As a result, excellent energy-storage properties have been achieved both in low-field ( W rec ∼ 2.7 J/cm 3 and high η ∼ 93% at 200 kV/cm) and moderate-field ( W rec ∼ 5.2 J/cm 3 and η ∼ 91% at 330 kV/cm) for x = 0.12 sample. Additionally, the x = 0.12 sample exhibits good thermal stability (−50 ∼ 200 ℃), frequency stability (0.5 ∼ 500 Hz) and cycle stability (10 5 cycles). Hence, the current work may provide some novel perspectives for improving the W rec and η of dielectric capacitors under low/moderate electric fields.

Hierarchical microencapsulation of phase change material with carbon-nanotubes/polydopamine/silica shell for synergistic enhancement of solar photothermal conversion and storage

Aiming at improving the utilization efficiency of solar photothermal energy, this study focuses on a novel phase-change microcapsule system based on an n-docosane core and a carbon-nanotubes (CNTs)/polydopamine (PDA)/silica hierarchical shell. The system was fabricated by encapsulating n-docosane in a silica shell and then depositing a PDA layer on the shell surface, followed by conglutinating CNTs onto the PDA layer. The resultant microcapsule system shows a regular spherical morphology together with desired core-shell hierarchical microstructure and chemical compositions. The presence of CNTs/PDA coating layer can impart an efficient solar light-to-heat energy conversion capability to the microcapsule system through photon capture and sunlight absorption. The microcapsule system not only shows a good latent heat-storage capability with satisfactory phase-change enthalpies of over 130 J/g but also exhibits an optimal photothermal conversion efficiency of 90.1%. The microcapsule system also exhibits good leakage-prevention performance, high thermal cycle stability, excellent thermal impact resistance, and good shape/form stability to deal with a wide range of solar photothermal energy applications. Through integrating CNTs and PDA into a phase-change microcapsule system, this work provides a promising approach for the development of PCMs-based functional materials with enhanced solar light-to-heat energy conversion and storage performance for efficient utilization of solar energy.

Controlling microstructure evolution and phase transformation behavior in additive manufacturing of nitinol shape memory alloys by tuning hatch distance

Laser powder bed fusion (L-PBF), categorized as additive manufacturing technique, has a capability to fabricate NiTi (Nitinol) shape memory alloys with tailorable functional properties and complex geometries. An important processing parameter, hatch distance (h), is often related to macroscale structural defects; however, its role on controlling the microstructure and functional properties is usually underestimated in L-PBF of NiTi. In this work, equiatomic NiTi (50.0 at% Ni) parts were fabricated with various hatch distances to tailor the microstructure and their shape memory characteristics. Contrary to what is observed in Ni-rich NiTi alloys, in this work, we demonstrate that phase transformation temperatures of L-PBF equiatomic NiTi do not decrease proportionally with hatch distance but rather relate to a critical hatch distance value. This critical value (120 μm) is derived from the synergistic effect of thermal stress and in situ reheating. Below this value, epitaxial grain growth and in situ recrystallization are enhanced, while above, irregular grains are formed and dislocations induced by thermal stresses decrease. However, the critical value found herein is characterized by high dislocation density and fine grain size, resulting in a superior thermal cyclic stability. The proposed finite element model is proven to be an effective tool to understand and predict the effect of hatch distance on grain morphology and dislocation density evolutions in L-PBF NiTi SMAs. In the present study, we provide a comprehensive understanding for in situ controlling L-PBF NiTi microstructure and functional characteristics, which contributes to designing 4-dimensional shape memory alloys.