Phase Change Composite Microcapsules with Low-Dimensional Thermally Conductive Nanofillers: Preparation, Performance, and Applications

Phase change materials (PCMs) have been extensively utilized in latent thermal energy storage (TES) and thermal management systems to bridge the gap between thermal energy supply and demand in time and space, which have received unprecedented attention in the past few years. To effectively address the undesirable inherent defects of pristine PCMs such as leakage, low thermal conductivity, supercooling, and corrosion, enormous efforts have been dedicated to developing various advanced microencapsulated PCMs (MEPCMs). In particular, the low-dimensional thermally conductive nanofillers with tailorable properties promise numerous opportunities for the preparation of high-performance MEPCMs. In this review, recent advances in this field are systematically summarized to deliver the readers a comprehensive understanding of the significant influence of low-dimensional nanofillers on the properties of various MEPCMs and thus provide meaningful enlightenment for the rational design and multifunction of advanced MEPCMs. The composition and preparation strategies of MEPCMs as well as their thermal management applications are also discussed. Finally, the future perspectives and challenges of low-dimensional thermally conductive nanofillers for constructing high performance MEPCMs are outlined.

Fabrication and Thermal Performance of 3D Copper-Mesh-Sintered Foam/Paraffin Phase Change Materials for Solar Thermal Energy Storage

Due to its large latent heat and high energy storage capacity, paraffin as one of the phase change materials (PCMs) has been widely applied in many energy-related applications in recent years. The current applications of paraffin, however, are limited by the low thermal conductivity and the leakage problem. To address these issues, we designed and fabricated form-stable composite PCMs by impregnating organic paraffin within graphite-coated copper foams. The graphite-coated copper foam was prepared by sintering multilayer copper meshes, and graphite nanoparticles were deposited on the surface of the porous copper foam. Graphite nanoparticles could directly absorb and convert solar energy into thermal energy, and the converted thermal energy was stored in the paraffin PCMs through phase change heat transfer. The graphite-coated copper foam not only effectively enhanced the thermal conductivity of paraffin PCMs, but also its porous structure and superhydrophobic surface prevented the paraffin leakage during the charging process. The experimental results showed that the composite PCMs had a thermal conductivity of 2.97 W/(m·K), and no leakage occurred during the charging and discharging process. Finally, we demonstrated the composite PCMs can be readily integrated with solar thermoelectric systems to serve as the energy sources for generating electricity by using abundant clean solar-thermal energy.

Performance of Nanocomposites of a Phase Change Material Formed by the Dispersion of MWCNT/TiO2 for Thermal Energy Storage Applications

Thermal energy storage technology is an important topic, as it enables renewable energy technology to be available 24/7 and under different weather conditions. Phase changing materials (PCM) are key players in thermal energy storage, being the most economic among those available with adjustable thermal properties. Paraffin wax (PW) is one of the best materials used in industrial processes to enhance thermal storage. However, the low thermal conductivity of PW prevents its thermal application. In this study, we successfully modified PW based on multi-walled carbon nanotubes (MWCNT) with different concentrations of TiO₂—3, 5 and 7 wt.%. The morphology of PCM and its relationship with the chemical structure and stability were characterized using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) and Thermogravimetric analysis (TGA). As a result, the composites achieved a highest latent heat enthalpy of 176 J/g, in addition to enhanced thermal stability after 15 thermal cycles, and reliability, with a slight change in latent heat observed when using a differential scanning calorimeter (DSC). The thermal conductivity of the composites could significantly be enhanced by 100%. Compared to pure paraffin, the PCM composites developed in this study exhibited an excellent preference for thermal energy storage and possessed low cost, high reliability, and phase change properties.

Paraffin Wax [As a Phase Changing Material (PCM)] Based Composites Containing Multi-Walled Carbon Nanotubes for Thermal Energy Storage (TES) Development

Thermal energy storage (TES) technologies are considered as enabling and supporting technologies for more sustainable and reliable energy generation methods such as solar thermal and concentrated solar power. A thorough investigation of the TES system using paraffin wax (PW) as a phase changing material (PCM) should be considered. One of the possible approaches for improving the overall performance of the TES system is to enhance the thermal properties of the energy storage materials of PW. The current study investigated some of the properties of PW doped with nano-additives, namely, multi-walled carbon nanotubes (MWCNs), forming a nanocomposite PCM. The paraffin/MWCNT composite PCMs were tailor-made for enhanced and efficient TES applications. The thermal storage efficiency of the current TES bed system was approximately 71%, which is significant. Scanning electron spectroscopy (SEM) with energy dispersive X-ray (EDX) characterization showed the physical incorporation of MWCNTs with PW, which was achieved by strong interfaces without microcracks. In addition, the FTIR (Fourier transform infrared) and TGA (thermogravimetric analysis) experimental results of this composite PCM showed good chemical compatibility and thermal stability. This was elucidated based on the observed similar thermal mass loss profiles as well as the identical chemical bond peaks for all of the tested samples (PW, CNT, and PW/CNT composites).

Leakage-proof microencapsulation of phase change materials by emulsification with acetylated cellulose nanofibrils

We use acetylated cellulose nanofibrils (AcCNF) to stabilize transient emulsions with paraffin that becomes shape-stable and encapsulated phase change material (PCM) upon cooling. Rheology measurements confirm the gel behavior and colloidal stability of the solid suspensions. We study the effect of nanofiber content on PCM leakage upon melting and compare the results to those from unmodified CNF. The nanostructured cellulose promotes paraffin phase transition, which improves the efficiency of thermal energy exchange. The leakage-proof microcapsules display high energy absorption capacity (ΔH_m = 173 J/g) at high PCM loading (up to 80 wt%), while effectively controlling the extent of supercooling. An excellent thermal stability is observed during at least 100 heating/cooling cycles. Degradation takes place at 291 °C, indicating good thermal stability. The high energy density and the effective shape and thermal stabilization of the AcCNF-encapsulated paraffin points to a sustainable solution for thermal energy storage and conversion.

Enhanced Thermal Buffering of Phase Change Materials by the Intramicrocapsule Sub per Mille CNT Dopant

Microencapsulation of a carbon nanotube (CNT)-loaded paraffin phase change material, PCM in a poly(melamine-formaldehyde) shell, and the respective CNT-PCM gypsum composites is explored. Although a very low level (0.001-0.1 wt %) of intramicrocapsule loading of CNT dopant does not change the thermal conductivity of the solid, it increases the measured effusivity and thermal buffering performance during phase transition. The observed effusivity of 0.05 wt % CNT-doped PCM reaches 4000 W s^-0.5 m^-2 K^-1, which is higher than the reported effusivity of alumina and alumina bricks and an order of magnitude larger than the solid, CNT-free PCM powder. The CNT dopant (0.015 wt %) in a 30 wt % PCM-plaster composite improved the effusivity by 60% compared to the CNT-free composite, whereas the addition of the same amount of CNTs to the bulk of the plaster does not improve either the effusivity or the thermal buffering performance of the composite. The thermal enhancement is ascribed to a CNT network formation within the paraffin core.

A novel forced separation method for the preparation of paraffin with excellent phase changes

A novel forced separation method based on driving force vacuum sweating was used to prepare high melting point paraffin with high phase-change enthalpies. The effects of the vacuum pressure and final separation temperature on the forced separation of the paraffin components were investigated. The research results showed that the optimal vacuum pressure for forced separation was 80.0 kPa. The performance of forced separation was improved with the increase in the final temperature. Increasing the final temperature increased the driving force of the separation of solid-state components and liquid components during sweating, which improved the product yield, shortened the production cycle, and reduced the oil content. The phase changes exhibited by the separation products were tested at 70 °C under optimal conditions. The raw materials and forced separation products were analyzed through Fourier transform infrared spectroscopy analysis (FT-IR), gas chromatography analysis (GC), differential scanning calorimetry analysis (DSC), and X-ray diffraction analysis (XRD). The results of these analyses showed that as the forced separation temperature was increased, the carbon atom number distribution range of the products narrowed, and the content of n-paraffin was drastically improved. The content of n-paraffin in the final fraction obtained through the forced separation of paraffin was 89.8% with a phase-transition temperature of 69.74 °C and a phase-transition enthalpy of 214.71 J g⁻¹. A significant solid–solid phase transition peak was observed in the final fraction obtained through the forced separation of paraffin, which verified that paraffin was an excellent phase-change material for energy storage.

A novel forced separation method based on driving forced vacuum sweating was used to prepare phase-change paraffin with the carbon atom distribution of the paraffin wax and its fractions as per the following.

Carbon-Filled Organic Phase-Change Materials for Thermal Energy Storage: A Review

Phase-change materials (PCMs) are essential modern materials for storing thermal energy in the form of sensible and latent heat, which play important roles in the efficient use of waste heat and solar energy. In the development of PCM technology, many types of materials have been studied, including inorganic salt and salt hydrates and organic matter such as paraffin and fatty acids. Considerable research has focused on the relationship between the material structure and energy storage properties to understand the heat storage/emission mechanism involved in controlling the energy storage performance of materials. In this study, we review the application of various carbon-filled organic PCMs in the field of heat storage and describe the current state of this research.

Zeolitic imidazolate framework-67 for shape stabilization and enhanced thermal stability of paraffin-based phase change materials

Zeolitic imidazolate framework-67 (ZIF-67), a new kind of metal-organic framework, has large surface area as well as outstanding thermal and chemical stability. In this paper, micro-sized ZIF-67 crystals were prepared and further employed as the reinforcing material to design novel paraffin-based composite phase change materials (PCMs) with a polymethyl methacrylate (PMMA) shell. The composite PCMs were fabricated by using a ZIF-67 crystal-stabilized oil-in-water (O/W) Pickering emulsion as a template. Morphologies and thermal properties of the prepared composite PCMs with different contents of ZIF-67 crystals were determined by SEM, DSC and TGA. Results showed that the ZIF-67 concentration in the emulsion system has a significant effect on the microstructure, phase change behavior and thermal stability of the resultant composite PCMs. When adding 1.5 g of ZIF-67 crystals, the resultant composite PCMs achieved a stable sphere-like structure and had about 106.06 J g-1 of latent heat. The prepared composite PCMs also exhibited a good thermal stability. Compared with pure paraffin, the thermostability of the shape-stabilized paraffin was significantly enhanced at a low content of ZIF-67 crystals.

3D Arrays of Super-Hydrophobic Microtubes from Polypore Mushrooms as Naturally-Derived Systems for Oil Absorption

Porous materials derived from natural resources, such as Luffa sponges, pomelo peel and jute fibres, have recently emerged as oil adsorbents for water purification, due to their suitability, low environmental impact, biodegradability and low cost. Here we show, for the first time, that the porosity of the fruiting body of polypore mushrooms can be used to absorb oils and organic solvents while repelling water. We engineered the surface properties of Ganoderma applanatum fungi, of which the fruiting body consists of a regular array of long capillaries embedded in a fibrous matrix, with paraffin wax, octadecyltrichlorosilane (OTS) and trichloro(1H,1H,2H,2H-perfluorooctyl)silane. Morphological and wettability analyses of the modified fungus revealed that the OTS treatment was effective in preserving the 3D porosity of the natural material, inducing super-hydrophobicity (water contact angle higher than 150°) and improving oil sorption capacity (1.8⁻3.1 g/g). The treated fungus was also inserted into fluidic networks as a filtration element, and its ability to separate water from chloroform was demonstrated.

An Environmentally Friendly Approach for the Fabrication of Conductive Superhydrophobic Coatings with Sandwich-Like Structures

A large amount of research has been devoted to developing novel superhydrophobic coatings. However, it is still a great challenge to pursuean environmentally friendly method that leads to superhydrophobic coatings. Herein, we demonstrate for the first time, an environmentally friendly method for the preparation of conductive superhydrophobic coatings with sandwich-like structures by using aminoethylaminopropyl polydimethylsiloxane modified waterborne polyurethane (SiWPU) and N-octadecylamine functionalized multi-wall carbon nanotubes. These environmentally friendly coatings with the sheet resistance of 1.1 ± 0.1 kΩ/sq exhibit a high apparent contact angle of 158.1° ± 2° and a low sliding angle below 1°. The influence of the surface texture before and after heat treatment on the wetting properties is discussed. In addition, the coatings can be electrically heated by 3~113 °C with a voltage of 12~72 V, and thus, can be used for deicing. Furthermore, the resulting coatings demonstrate good performance of wear resistance and ultraviolet resistance, which will have broad application potential in harsh environments.

The effect of 3D carbon nanoadditives on lithium hydroxide monohydrate based composite materials for highly efficient low temperature thermochemical heat storage

Lithium hydroxide monohydrate based thermochemical heat storage materials were modified with in situ formed 3D-nickel-carbon nanotubes (Ni-CNTs). The nanoscale (5–15 nm) LiOH·H₂O particles were well dispersed in the composite formed with Ni-CNTs. These composite materials exhibited improved heat storage capacity, thermal conductivity, and hydration rate owing to hydrogen bonding between H₂O and hydrophilic groups on the surface of Ni-CNTs, as concluded from combined results of in situ DRIFT spectroscopy and heat storage performance test. The introduction of 3D-carbon nanomaterials leads to a considerable decrease in the activation energy for the thermochemical reaction process. This phenomenon is probably due to Ni-CNTs providing an efficient hydrophilic reaction interface and exhibiting a surface effect on the hydration reaction. Among the thermochemical materials, Ni-CNTs–LiOH·H₂O-1 showed the lowest activation energy (23.3 kJ mol⁻¹), the highest thermal conductivity (3.78 W m⁻¹ K⁻¹) and the highest heat storage density (3935 kJ kg⁻¹), which is 5.9 times higher than that of pure lithium hydroxide after the same hydration time. The heat storage density and the thermal conductivity of Ni-CNTs–LiOH·H₂O are much higher than 1D MWCNTs and 2D graphene oxide modified LiOH·H₂O. The selection of 3D carbon nanoadditives that formed part of the chemical heat storage materials is a very efficient way to enhance comprehensive performance of heat storage activity components.

3D carbon modified LiOH·H₂O particles were well dispersed into nanoparticles (5–15 nm) and tested using in situ DRIFT spectroscopy.

Imine-linked micron-network polymers with high polyethylene glycol uptake for shaped-stabilized phase change materials

Micron-network polymers with high free interstitial space show high adsorption of PEG (up to 85 wt%) for shape-stabilized phase change materials with high energy storage density.

Preparation and thermal properties of shape-stabilized composite phase change materials based on polyethylene glycol and porous carbon prepared from potato

A shape-stabilized composite phase change material comprising PEG and porous carbon was prepared by absorbing PEG into porous carbon.

Preparation of spherical agglomerates from potash alum

Spherical crystallization proved to be feasible for the preparation of spherical salt hydrate particles as core material for microencapsulation.

Paraffin confined in carbon nanotubes as nano-encapsulated phase change materials: experimental and molecular dynamics studies

The characteristics of paraffin confined in carbon nanotubes (CNTs) were investigated using experimental and molecular dynamics (MD) methods.

A form-stable phase change material made with a cellulose acetate nanofibrous mat from bicomponent electrospinning and incorporated capric–myristic–stearic acid ternary eutectic mixture for thermal energy storage/retrieval

An innovative type of form-stable PCM was prepared by incorporating CMS ternary eutectic mixture with CA nanofibers that was derived from electrospinning of binary mixture of CA/PVP and subsequent selective dissolution of PVP from bicomponent mat.