nanoconfined spaces for thermal storage and applications

Renewable and Sustainable Energy Reviews 82 (2018) 2319–2331

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Thermodynamics behavior of phase change latent heat materials in micro-/ nanoconfined spaces for thermal storage and applications

MARK

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Shudong Zhang, Zhenyang Wang

Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui, 230031, PR China

A R T I C L E I N F O

A BS T RAC T

Keywords: Phase change materials Confinement effect Thermodynamics behavior Thermal storage Micro-/nanoconfined systems

Phase change materials (PCMs), including inorganic and organic PCMs, play an important role in thermal storage and applications of both industry and academia. Several unique properties of PCMs, such as phase change temperature, latent heat, and thermal conductivity etc. are crucial parameters to control. By introducing a micro-/nanoconfined space as a restricted matrix around PCMs has recently been shown to be a reliable method to regulate and improve crucial parameters of inorganic and organic PCMs. Specially, the physical/ chemical interaction occurring at the surface or interface when phase transitions are involved, has appreciable or significant influences on the thermodynamics of heterogeneous interactions. Herein, the aim of this review is to focus on recent advances in micro-/nanoconfined strategies to encapsulate PCMs and to provide the analysis about the influence of the different restricted matrixes on the crucial phase-change parameters of encapsulated PCMs. As we will discuss, confinement effect can polish and improve intrinsic drawbacks of PCMs that are impossible or difficult to be achieved by other traditional methods. Meanwhile, we will describe the surfactant micelle-based soft-template approach, electronic spinning technology, as well as the vacuum filtration method to obtain such micro-/nanoconfined systems in 1D, 2D, and 3D matrixes and discuss how the thermodynamics behavior of PCMs is enhanced by confining it in a micro-/nanoconfined container. Finally, we review the distinctive applications of the PCMs in various confined matrixes for stabilizing temperature, collecting and supplying heat for thermoelectric device and thermal rectifier.

1. Introduction For the past decades, energy shortage and environmental pollution have stimulated the research for developing and exploiting new sources of clean, sustainable and renewable energy to prevent the negative effects on the global ecosystems. However, improving the energy utilization efficiency of existing energy systems is considered as one of the simplest and cheapest way [1,2]. For this purpose, as a type of latent heat storage material, phase change materials (PCMs) not only have been proven to be very effective in the utilization of solar and industrial waste heat, but also have exhibited such good performance of thermal regulation due to their large heat storage capacity and isothermal behavior during the processes of decalescence and discharge heat [3,4]. For example, inorganic hydrate salt is one of the most promising materials for latent heat storage and utilization due to its high latent heat per unit volume, phase-change near room temperature, nonflammability, and cost-effect [5,6]. As thermal energy storage materials, nevertheless, they usually exhibit many intrinsic drawbacks, such as phase segregation caused incongruent and super-

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Corresponding author. E-mail address: [email protected] (Z. Wang).

http://dx.doi.org/10.1016/j.rser.2017.08.080 Received 18 June 2016; Received in revised form 27 May 2017; Accepted 22 August 2017 Available online 04 September 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

cooling because of their weak nucleation properties, which will badly weaken their capability of heat storage-release in practical utilization [7–10]. Thus, various traditional strategies have been used to overcome these above predicaments, including the addition of nucleating species, thickening agents and extra water to minimize phase segregation and supercooling of the inorganic PCMs in their repeated dissolvingcrystallizing cyclic processes [11,12]. To some degree, although the thermodynamics performance of hydrated salts was partly improved, the addition of several kinds of exotic agents would inevitably reduce the latent heat and the corresponding conversion efficiency. In contrast to the inorganic PCMs, the organic PCMs, without the intrinsic drawbacks of the inorganic PCMs, should first be selected based on their phase change temperature, owing to a large number of organic PCMs are available in a temperature range from −5 °C to 190 °C [13– 16]. However, practical applications of all solid-liquid PCMs have been hindered by a number of challenges including their leakages, very poor thermal conductivity (∼0.4 to 0.6 Wm−1K−1), and considerable volume expansion during the phase transition.

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Various review articles have been published in the field of encapsulation methods for PCMs, providing a large heat transfer area, reducing subcooling and mitigating the volume change of PCMs during the phase transformation [1–3,17–24]. Recently, the increasing progress in micro-/nanoscale preparation and synthesis technology may provide an alternative solution to solve these problems by changing the structure and morphology of the material system. One of the most efficient ways to overcome these disadvantages is using micro-/ nanoconfinment technology, which not only can effectively prevent liquid leakage, but can realize the functionality of PCMs. Besides, the micro-/nanoconfinment effect is often related to physical interaction and/or chemical reaction at the surface or interface, and thus the specific surface area, surface energy, and surface chemistry play a critical role in the micro-/nanoconfinment effect. The surface influences are not only limited to the kinetics and rate, the surface energy and surface chemistry occurring at the interface when phase transitions are involved [25], can have appreciable or significant impacts on the thermodynamics of heterogeneous reactions. The smaller dimensions of PCMs may also offer more favorable mass and heat transfer, as well as accommodate dimensional changes associated with some phase transitions. For example, impregnating hydrate salts into various mesoporous matrixes have been proved to mitigate their supercooling and phase segregation through effective mesopores confinement [26,27]. Meanwhile, confinement of hydrate salts within nanoparticles will increase thermal conductivity by tremendously reducing the heat transmitting distance in nano scale [28]. Moreover, the structure of micro-/nanoconfinement framework would modulate and influence the phase transition behavior, such as latent heat and phase change temperature, etc. Hence, this review article mainly focuses on relatively current progress in those fields and provides the reader with slightly different perspectives on the thermodynamics behavior of PCMs under micro-/ nanoscale confinement, including thermotropic properties and unique influences of anisotropic confinement on inorganic and organic PCMs, etc.

3. Anisotropic confinement to impact on thermodynamics behavior of PCMs The basic design principle for thermal energy storage and release is first and foremost to mitigate intrinsic drawbacks of PCMs. Additionally, the crucial parameters of PCMs can be controlled, such as increasing phase transition latent heat and/or reducing phase change temperature of PCMs. Last but not least, the liquid leakage can not only be effectively prevented during phase transition, but the functionality of PCMs can be realized. Micro-/nanoconfined technology can be employed to improve the performance of PCMs by (1) generating new mechanisms, such as nucleation and phase transformations under nanoscale confinement, (2) providing large surface area and associated surface energy, such as the vapor pressure of materials changes exponentially with the curvature of the surface, (3) bringing about unique thermodynamics behavior of PCMs to either reduce the phase transition temperature or enhance the latent heat, and (4) improving the heat transport and/or collection through the use of different dimensional nanostructures or purposely designed threedimensional structures, such as 3 D interconnected netlike skeleton composite structures. We are to show that the tailoring of the micro/ nano-confined technology to optimize thermodynamics behavior of PCMs is an effective way of creating high performance composite materials for thermal storage. 3.1. 0 D of PCMs In this section, 0 D of PCMs is defined that quantum dots or nanoclusters of PCMs are uniformly dispersed into a matrix with different structures. Previous investigations of the crystallization of ice [29], metals [30] and organic solids [31] in nanoporous media, have been confirmed that the dimension and size constraints imposed by confinement in matrix can dramatically affect the crystal properties, such as melting points and enthalpies of fusion, etc [32]. Based on our knowledge and related research experience, this special crystallization behavior is a direct consequence of the increasing surface area-to volume ratio when crystal size is reduced; surface free energy is always positive and destabilizing whereas volume free energy is always negative and stabilizing. The competitive balance between surface and volume free energies will directly impact the thermotropic properties of confined crystals in a restricted matrix [33]. Consequently, confinement provides an alternative to conventional approaches – changing solvent and temperature, introducing additives to the growth medium – for inhibiting and mitigating the intrinsic drawbacks of inorganic PCMs such as phase segregation and supercooling or discovering new confined crystal mechanisms.

2. Characteristic dimensions of micro-/nanoconfined spaces There is convincing evidence of a close link between the restricted matrixes and thermal properties of the PCMs. In general, the restricted matrixes around PCMs should consider their interface feature (surface functional group), wall nature (composition, crystal structure and crystalline size), and pore topology (pore geometry and pore size), etc. These geometrical parameters would have some significant effects on the novel chemical/physical properties, especially when the structures exist in the nanometer scale. Hence, nanoscale dimensions would have a crucial effect on the efficient realization of basic progresses during phase change progress of PCMs and offer a new avenue for the highly efficient heat storage system based on PCMs. Meanwhile, a good design for the PCMs requires that their phasechange processes, especially the melting and solidification processes, should be performed in a container. This container acts as a barrier to protect PCMs from harmful interaction with the environment, provides a stable structure and sufficient surface for heat transfer, and makes the processes easy to handle. Encapsulation of PCMs in micro-/nanoconfined spaces is an attractive method of containment, and can supply a large heat capacity to control the volume changes when the phase change occurs. It can also protect PCMs from being in contact with the outside environment during their applications. With this in mind, herein, according to characteristic dimensions and the basic geometrical motifs of PCMs embedded in different matrixes, PCMs are defined as four categories (Fig. 1), zero-dimensional (0 D), one-dimensional (1D), two-dimensional (2D), and threedimensional (3D) of PCMs, respectively. Recently, many investigations have focused on the nano-dimensionality effect due to the dimensionality will lead to special influence on physical properties of PCMs.

3.1.1. Adjusting freezing point to 0 D of PCMs The different materials have been investigated to encapsulate PCMs, and SiO2 is highlighting among them. [34] For example, 0 D of Glauber’s salt (Na2SO4·10H2O) [35], Na2SO4·10H2O@SiO2 solid nanobowls structured as Glauber’s salt nanoclusters dispersed in SiO2 matrix (Fig. 2A, B), has greatly improved their heat storage properties; the mesoporous confinement effect inhibited the phase segregation of Glauber’s salt and mitigated the supercooling of Glauber’s salt as well (Fig. 2C, D). The Na2SO4·10H2O@SiO2 solid nanobowls are in situ prepared via synchronous hydrolysis reactions of TEOS and APTS in a reverse microemulsion system in which Glauber’s salt is dissolved. The Glauber’s salt nanoclusters are uniformly dispersed into the SiO2 matrix. Namely, the Glauber’s salt nanoclusters are in situ confined by mesopores correspondingly, which leads to the mesopores suitable for the Glauber’s salt nanoclusters in both shape and chemistry. Interestingly, in the DSC curves, the confined Glauber’s salt displays an exothermal peak in the cooling crystallization stage, whereas dual peaks always appear in an exothermal curve for their bulk Glauber’s 2320

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Micro-and nano-multiple structures

Ordered 0 D of PCMs

Random

1 D of PCMs

Encapsulation 2 D of PCMs

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Cross-section drawings Fig. 1. Examples of characteristic dimensions and existing morphology of PCMs embedded in a matrix, PCMs are defined as four categories, 0 D, 1D, 2D, and 3D of PCMs, respectively.

3.2. 1 D of PCMs

salt (Fig. 2C, D). Apparently, confinement effect has changed the nucleation and crystallization properties of Glauber’s salt in a restricted SiO2 matrix. Recently, it has been suggested that a crystal nucleus can proceed through a two-step mechanism wherein dense metastable clusters with sizes of several hundred nanometers form prior to formation of a nucleus [36,37]. Therefore, firstly, Glauber’s salt nanoclusters dispersed in SiO2 matrix could be easily obtained using our current designed synthesis method. Secondly, the walls of SiO2 matrix provided an ideal heterogeneous nucleation site to favor the nucleation of Glauber’s salt nanoclusters during the crystallization process. Thirdly, the melting Glauber’s salt will generate a higher pressure in the mesopores to shorten their intermolecular spacing and thus enhance the weak interactions including H-bonding and electrostatic forces between H2O molecules and SiO2 mesopore walls, which will further strengthen the confinement of mesopores to H2O molecules.

Beside 0 D of PCMs, 1 D of PCMs can be easily prepared and obtained under 1D of confined spaces, in that various porous materials can afford fully effective 1D of confined spaces. Porous materials can be classified into those with irregular and ordered porous networks and those with disordered porous networks. Therefore, confinement of PCMs in this manner provides a possibility of the earliest stages of nucleation, size-dependent polymorphism, and thermotropic behavior of nanoscale PCMs. 3.2.1. Adjusting freezing point of 1 D of PCMs Such as inorganic hydrate salt of Na2HPO4·12H2O [38] was confined into random porous CaCO3 matrix, which have disordered pores with average diameters of 13.4, 17.4, 22.3 and 55 nm (Fig. 3A, B, C) under other identical conditions [39]. When the Na2HPO4·12H2O/ CaCO3 nanocomposite materials were heated above the melting temperature of inorganic hydrate salt and then cooled, the embedded

Fig. 2. (A) TEM image and (B) schematic illustration of the solid Na2SO4·10H2O@SiO2 nanobowls. (C) DSC thermal spectra of pure Na2SO4·10H2O@SiO2 and Na2SO4·10H2O@SiO2 solid nanobowls and (D) DSC spectra of Na2SO4·10H2O@SiO2 solid nanobowls with cycling of 1, 15, 30, 45, and 60 times [35].

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Fig. 3. (A) A schematic illustration exhibits the formation of porous CaCO3 tetragonal prisms and disordered pores. (B) A typical SEM image of as-decomposed CaCO3 microporous tetragonal prisms. The corresponding enlarged structures are shown in image (C). (D) Being the supporter of 1.0 g Na2HPO4·12H2O, DSC thermal spectra of 20 mg porous CaCO3 with different pore sizes of 13.4, 17.4, 22.3 and 55 nm, respectively. Heat rate is 5 K min−1 [39].

the transition process (Fig. 4A). The DSC result for the neat PEG sample presented an endothermic peak at 22 °C for a melting temperature of PEG (Fig. 4B). In contrast, 1a⊃PEG clearly exhibited a transition at −36 °C, which is considerably lower than the melting temperature of neat PEG. Variable temperature infrared measurements of 1a⊃PEG indicated that the endothermic peak observed in the DSC scan corresponds to a transition of PEG itself confined in the channels (Fig. 4C). The transition temperature of PEG in PCPs was attributed to the manipulation of the pore size and the pore–PEG interactions. Therefore, the pore size and surface functionality of PCPs can be tailored to influence the transition behaviour of confined PEG.

melting Na2HPO4·12H2O was recrystallized as solid phase. There is no apparent difference in melting temperature of Na2HPO4·12H2O during melting progress in current 1 D irregular porous CaCO3 spaces with four different size of pore. However, interesting, the pore size of the CaCO3 matrix is lower than 22.3 nm, the embedded crystals of Na2HPO4·12H2O show only one peak centered in the exothermal cures. Meanwhile, the ternary sets of exothermal peaks were observed in macroprous CaCO3 matrix (55 nm) and the free bulk of Na2HPO4· 12H2O throughout the recrystallization progress (Fig. 3D). Obviously, the confined Na2HPO4·12H2O depressed the super-cooling and phase separation characteristic of Na2HPO4·12H2O monoliths. The preference for Na2HPO4·12H2O nanocrystals in smaller pores was attributed to a smaller critical nucleus size compared with other forms, reflecting size-dependent polymorph stability. These results suggest that it is possible to repress the phase separation phenomena of hydrated inorganic salts by controlling the supporting matrix with the appropriate pore diameter.

3.3. 2 D of PCMs 3.3.1. Controlled heat release from 2 D of thermal storage materials Two-dimensional (2D) nanomaterials with single/few-atomic thickness can be considered as promising materials to cater for people’s increasing requirement of next-generation flexible and transparent nanodevices. These brand-new materials can provide the most effective bridge to combine the microscopic superior optical, electronic, and magnetic characters with the macroscopic ultrathin, flexible and transparent features, and thus guarantee a maximum functionality with a minimized size. Planar monolayer materials, such as graphene, graphene oxide, or reduced graphene oxide, have been extensively used

3.2.2. Adjusting melting point of 1 D of PCMs The same report demonstrated the melting and crystallization of polyethylene glycol (PEG) of OPCMs in 0.75 × 0.75 nm2 of porous coordination polymers (PCPs) with high regularity [40]. The PEG molecules prefer to form aggregated (bundled) structures in the 1-D channels rather than to be dispersed throughout the channels during 2322

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Fig. 4. (A) Typical MD structure of PEG 14-mer (Mw = 634.7) encapsulated in the subnanometer channel. (B) DSC heating curves of neat PEG, 1a and 1a⊃PEG. (C) Variable temperature infrared spectra of 1a⊃PEG. Mw of PEG used in these measurements was 600 [40].

Fig. 5. (A) Fabricating the artificial GO-PEG composite paper with the ability of heat storage and release by vacuum filtration. (B-D) Morphology and structure of GO-GEG composite paper. Digital camera images of GO paper. (B)~10 μm-thick (The panda image is beneath the GO paper). (C)the separation process between GO paper and “panda” membrane demonstrated that resulting paper was easily separated from the membrane substrate. (D)tailored and folded strip-like paper. (E) XRD patterns of papers exhibited the addition of the amount of PEG molecules with enlarging the basal spacing from 0.85 to 0.98, 1.05, 1.15 and 1.4 nm, accordingly. (F) DSC measurements on GO composite papers with different PEG20000 compositions (Curves a-e stand for pure PEG20000, 75 wt%, 50 wt%, 25 wt%, and 5 wt% of PEG20000, respectively.) [41].

ment is further enhanced by the shortening of the intermolecular spacing. As a result, the transition behavior of PCM can be systematic controlled and tailed via the interlayer distance and surface effects of 2D matrix materials.

to construct 2 D of macroscopic paper-like or foil-like materials. The large interaction surfaces between these sheets is of great help for forming paper-like or foil-like materials composed of stacked platelets. Thus, these layer structures are ideal 2D of confined spaces to design and fabricate multi-functional paper-like composite materials. For example, we have reported the in situ synthesis of shaped graphene oxide (GO) composite papers with ability of heat storage and release [41]. The polyethylene glycol (PEG) macromolecules were intercalated into the interior of the GO sheet layers to form sandwich-like structure via a simple co-filtration method (Fig. 5A). And the basal spacing– phase change behavior relationship was also surveyed. Interestingly, compared with bulk PEG, while the interlayer distance decreased (Fig. 5E), the freezing point of confined PEG decreased (Fig. 5F). For example, the freezing point of 2D PEG confined into the different interlayer distance of GO sheets could be successively decreased down to nearly room temperature from 43 °C to 29 °C (Fig. 5F). The current unusual phase change behavior of paper-interior PEG was considered to the hydrogen bonding effect from the strong interactions among the GO nanosheet surfaces and PEGs. And the smaller interlayer distance of GO sheets provided a high pressure to defined PEG. Therefore, the hydrogen bonding effect caused by a defined GO nanosheet environ-

3.3.2. Synchronously decreasing melting point and freezing point of PCMs In this section, we will use a shape-stabilized phase change material of stearic acid-graphene oxide (SA-GO) as example to explain the significant decreases on melting and freezing point of SA [42]. Based on the capillary action and interfacial interaction relating to the GO layer structure, SA-GO composites will be easily fabricated by imprisoning SA in the interlayer spaces of the multilayer GO. Undoubtedly, the orientation structure of SA molecules inside GO layers has an important influence on the thermal behaviors of the composites. Such as, in the same mass ratio of SA: GO, nearly all the SA molecules can intercalate into the interlayer spaces of 2 D GO nanosheet layers. Obviously different with SA chains in the bulk crystal, the spatial configuration of the confined SA molecules almost linearly lies down on the basal planes of GO (Fig. 6 A), due to an increasing pressure and the attractive interactions from the 2D 2323

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Fig. 6. (A) Schematics of the formation and probable structures of SA-GO composites with increasing SA amount. DSC curves of (B) pure SA, (C) Sample 1, (D) Sample 2, and (E) Sample 3 [42].

interlayer confinement. Therefore, the phase change temperatures of SA-GO composites decrease greatly compared with those of the pristine SA (Fig. 6 B). In addition, the thermodynamic behavior of other organic molecules, including methanol, ethanol, acetone, and DMF, enclosed interlayer interior between GO sheets is strongly affected by the presence of a 2D confinement effect [43].

known, the crystallinity of SA in the composite has significant influences on the latent heats of the composite [44]. Therefore, SA in the SA/GBm has higher crystallinity than that in SA/GB, which results in higher latent heats [45].

3.3.3. Enhancing latent heat of PCMs Two-dimensional layered materials are potential building blocks for packing PCMs due to the capacity of incorporating a great diversity of organic/inorganic species in the interlayer region. The established host-guest interaction will be beneficial for producing materials with new chemical, physical and optical properties due to high surface area, good chemical and environmental stability, and strong adhesion to other materials. For example, two-dimensional layered graphite and microwave-acid treated bentonite mixture (GBm) have larger loading capacity to package stearic acid (SA). However, under the combined effects of capillary forces, surface polarities and hydrogen bonding, the interaction of the layered bentonite with SA is much stronger than that of graphite with SA. Consequently, SA in the SA/GBm has higher crystallinity than that in the SA/B (Fig. 7). Thus, the SA/GBm composite showed an enhanced thermal storage capacity and latent heats for melting and freezing (84.64 and 84.14 J/g) which were higher than that of SA/GB sample (48.43 and 47.13 J/g, respectively). As we

Encapsulation of PCMs in a hollow micro/nano-sphere forming a core-shell structured particle, on behalf of 3 D PCMs, is a beneficial design for the PCMs, in which their phase-change processes perform in a container. There are several main reasons accounting for their benefits. Firstly, the shell can fully encapsulate a bulk volume of PCMs and supply a large heat capacity, and thus control the volume changes when the phase change occurs. Secondly, it provides a stable structure and sufficient surface for heat collection, transfer and release later. Thirdly, it protects PCMs from being in contact with the outside environment and prevents the interior PCMs from leaking during the solid-liquid phase change for their applications. In fact, the cores of the conventional micro-PCMs are always enwrapped with organic polymers and inorganic compounds as the shell material. Organic polymers, including melamine-formaldehyde resin, urea-formaldehyde resin, and polyuria and so on, give the potential for structural flexibility and convenient processing. On the contrary, inorganic compounds provide high thermal and mechanical stability, considering a wellknown fact that the thermal conductivity of inorganic materials is always significantly higher than that of organic materials. Additionally, the chemical inertness, thermal stability and uninflammability of inorganic materials are better than those of organic materials. Last but not least, inorganic microcapsules offer thin closed structure within a restricted diameter range, and their internal van der Waals surfaces will regulate the phase change behavior of encapsulated PCMs in a precise fashion. The structure of PCMs results from a balance between short-range geometric factors and high pressure inside microcapsules compared to the unconfined state.

3.4. 3 D of PCMs

3.4.1. Modifying phase transition of high-melting-point PCM to room temperature The size-dependent melting behavior of nanosized materials is a special phenomenon [46–48], such as, the melting points of nanoscale particles usually exhibit much lower than that of the corresponding bulk materials attributed to the increasing surface effects, as observed

Fig. 7. Schematic illustration of improving the thermal storage capacity (the amorphous SA in the left of blue dash line represents confined SA while the crystallization SA in the right of blue dash line represents crystallizable SA) [45].

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temperature (~ 21 °C), showing that about 50 °C decrease of their phase change temperature has been achieved. In the light of the Gibbs– Thomson thermodynamic equation, the change in phase transition temperature can be related to not only the pore width but host-guest interactions under the confinement condition. Such host-guest interaction is an important influence factor on the phase transformation temperature of PCMs. There are strong interface interactions between the COOH group of the SA and a large number of alkaline regions from the surface of AlOOH shell, which can change the melting temperature of encapsulated SA.

in different kinds of metal nanoparticles [49,50]. The classical GibbsThomson thermodynamic equation [51] can be applied to explain the size-dependent melting point depression of nanoscale materials confined in a solid matrix. ΔT = 4T0 σ / ρLr Where ΔT is deviation of melting point from the bulk value, T0 is the normal (bulk) melting point, σ is the surface tension coefficient for the liquid–solid interface, ρ is particle density, L is the bulk enthalpy of fusion, and r is particle radius. The Gibbs-Thomson equation shows that (1) lowering of the melting point is proportional to 1/r. (2) ΔT can be as large as couple of hundred degrees when the particle size decreased below 10 nm! (3) For nanoparticles confined into a matrix, melting point may be lower or higher, depending on the strength of the interaction between the particle and matrix. Therefore, this uncommon phenomenon can inspire us new ideas and offer its great versatility in designing heat storage systems. In other words, the melting point of PCMs is not completely restricted by the intrinsic material properties. In fact, it can be controlled and tailored by adjusting the size of PCMs. Therefore, encapsulation of PCMs into confined systems will be an effective way of combining the function of shape stabilization and the thermal behavior tuning effect resulted from confinement effect. Taking the stearic acid (SA)@boehmite (gAlOOH) microcapsules as an example (Fig. 8) [52], when the highmelting-point SA crystals (Tm = 70.8 °C) were capsulated into airtight boehmite nanoshells by a microemulsion with metastable interface, the melting point of SA@AlOOH microcapsule could be dropped to room

~ 85 nm

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3.4.2. Increasing phase change latent heat of PCM via nanocapsule confinement Latent heat is a very important reference index for evaluation of the PCM heat storage ability. And latent heat of PCMs has long been regarded as constant value. But it is found that this parameter can be tuned especially in nanoscale size. Thus, we rationally design a hydrophobic interaction driven soft templating approach to synthesize SA@SiO2 nanocapsules (NCs) with controllable SiO2 shell sizes (Fig. 9) [53]. Latent heat of the obtained SA@SiO2 NCs could be increased up to 372.4 kJ/kg, about 36.9% more than that of the unconfined SA (273.3 kJ/kg). The phase change latent heat of SA@SiO2 NCs is attributed to the following reasons: for onething, nonpolar tails of the SDS in the SiO2 inner surface are oriented toward the center of the nonpolar droplet, which is beneficial for inducing SA macromolecules rearranged to form a steady and order array structure (Fig. 9). For another, the diameter of nanocapsules is smaller in the size than the reported other microcapsules. According to the Young−Laplace equation Δp=pα−pβ= 2γ/R, where pα and pβ refer to the internal and external pressures of the spherical surface, respectively; γ is surface tension; and R is its radius. In general, a smaller curvature effect will provide a higher superimposed stress in nanometer-scale spheres than microcapsules. And the high superimposed pressure should significantly shorten the intermolecular spacing to construct multiple stable hydrogen bond networks. Therefore, the multiple stable hydrogen bond networks of encapsulated SA are found under high superimposed stress from a curvature effect inside SiO2 nanoshells, which is helpful to significantly shorten the SA intermolecular spacing as compared to the unconfined state. Besides, the interaction energy from the nonpolar chains of the SA partly contributed to an increase in heat storage capacity of the obtained SA@SiO2. Furthermore, our results not only provide useful way to increase the latent heat of PCMs but also are helpful to understand the thermodynamics of phase change under nano scale. In addition, a novel microencapsulation process is used to encapsulate n-eicosane (C20) into a blend of ethyl cellulose (EC) and methyl cellulose (MC) to produce C20-loaded EC/MC microspheres. Interestingly, the obtained C20-loaded EC/MC microspheres also exhibit an increased heat storage capacity compared to the free C20 [54]. The absolute enthalpy value of the C20-loaded EC/MC microsphere was increased by 29% and 24% during crystallization and melting progress for the encap-C20 with the content of 91% (w/w) over that for the unencapsulated C20 (Fig. 10). The proposed significant interaction between the hydrophobic ethoxy groups of the EC chains inside interface of microspheres and the C20 molecules causes a different phase transition path for the encap-C20 and thus affects the enthalpy of the encap-C20 compared to the free C20. Conseqently, the new path contains multiple broad overlapped transition steps and releases more heat energy than two-step crystallization of the pure C20 does, which is attributed to confinement effects inside the microcapsules [55,56]. Besides, the paraffin wax (PW)/carbon nanotubes (CNTs) composites are fabricated by involving solvent-assisted infiltration of PW into the inner pores of a nanotube sponge. The PW completely or partially filled into a three-dimensional CNTs scaffold. A 10% increase of PW enthalpy is independent of the PW loading, which is also based on the

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Fig. 8. Typical SEM and TEM images of the obtained SA@AlOOH samples. (A) Lowmagnification and high-magnification (upper inset) SEM images and size distribution (bottom inset) of SA@g-AlOOH (21.6 wt% of SA) microcapsules. Low-magnification and high-magnification (upper inset) TEM images of SA@g-AlOOH (21.6 wt% of SA) microcapsules (B) and SA@g-AlOOH (28.5 wt% of SA) microcapsules (C). (D) DSC thermogram of the pure SA, g-AlOOH and the as-prepared SA@g-AlOOH microcapsules with a heating rate of 10 °C min−1, respectively. (E) DSC thermogram of the as-prepared SA@g-AlOOH (39 wt% of SA) microcapsules with a different heating rate of 5, 10 and 20 °C min−1, respectively [52].

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A

Stearic Acid droplet

Micelle

Hydrophilic segments Hydrophobic segments Emulsifier: SDS H+

Silica shell

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SDS emulsifier template SA H-bonding networks SA nonpolar chains

T T Fig. 9. Schematic Formation of the SA@SiO2 NCs. SEM and TEM images of SA@SiO2 NCs with different shell thickness. Nonpolar tails of the SDS in the SiO2 inner surface areoriented toward the center of the nonpolar droplet. Curvature effectresults in high superimposed stress to form multiple stable hydrogenbond networks [53].

wax-nanotube interface intermolecular interaction between CNTS and PW. Recent research has showed that the addition of nanotubes can effectively and greatly improve the enthalpy of PCMs, in that intermolecular attraction due to the Lennard-Jones potential is the main reason [57,58].

3.4.3. Enhancing thermal conductivity of PCMs A major drawback of organic PCMs is their low thermal conductivity (κ) which leads to large temperature gradients during heat transfer in or out of the material, and thus reduces heat transfer rates and large time constants. Consequently, enhancing the thermal conductivity of organic PCMs without causing a major reduction about latent heat is one of the key to satisfy practical application as latent heat storage/ release units for thermal management and thermal protection. For example, the most straightforward way to tailor the PCM thermal conductivities is to introduce nanoscale or microscale fillers to constitute additional thermal transport paths within the organic matrix. And the PCM thermal conductivities have been effectively enhanced by adding carbon nanotubes (an increase of 121%) [59], graphite, carbon

Fig. 10. Representative DSC curves of unencapsulated C20 (pure C20) and encap-C20/ EC/MC microparticles prepared with 9% and 20% (w/w) EC/MC polymer contents [54].

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Fig. 12. Measured thermal conductivity of the sample vs filler content for graphene/ PCM composites (Insert picture: Scanning electron microscopy (SEM) image of fracture surface of ∼4% by weight graphene/1-octadecanol composite) [63].

4. Applications 4.1. Stabilizing temperature As mentioned above, PCMs can absorb, store, and release large amounts of latent thermal energy during phase transitions at their phase change points. Meanwhile, the phase change temperature is a constant value during phase transition, which allows for the storage of energy and also gives a function stabilization of temperature. For example, shape-stabilized, 1 D phase change nanofibers prepared via the meltcoaxial electrospinning technique are able to absorb, stain, and release large amounts of thermal energy by taking advantage of the large latent heat of various long-chain hydrocarbons [64]. The capability of these PCM nanofibers to stabilize temperature is demonstrated by monitoring the water temperature in the glass vial. The PCM nanofibers play a positive effect in insulating the water temperature than the commercial fiberglass fibers. More importantly, the PCM fibers based film allows to stabilize the temperature (close to the melting point of octadecane) in the vial for a long time (Fig. 13). Similarly, 2 D graphene oxide–polyethylene glycol (PEG) composite papers with brick-and-mortar microstructures also demonstrate the same capability to stabilize temperature as 1D nanofibers [41,65].

Fig. 11. (A) Illustration of the fabrication process in which paraffin wax solution was infiltrated to the porous carbon nanotube sponge to make a composite. (B) Thermal conductivities of pure PW and two composites with PW weight percentages of 80% and 90%, respectively, were recorded across a temperature range of 5–40 °C [58].

fibers (240%) [60] or other materials [45]. Chen and co-works have reported a carbon nanotube sponge of high thermal conductivity encapsulated paraffin wax [58]. The obtained composites can store and release thermal energy by electro-thermal and light-thermal conversion. The thermal behavior of PW-CNTs composites could have been importantly influenced by the presence of nanotube networks. After encapsulation, the composite thermal conductivities reach to about 1.2 W•m−1•K−1, which represent a nearly 6-fold enhancement than the pure PW of a very low conductivity (0.16–0.20 W•m−1•K−1) (Fig. 11). In addition, the shape-stabilized composite phase change materials (CPCMs) were fabricated via impregnation of Cetyl alcohol (CtA) into high density polyethylene (HDPE). The thermal conductivity of CPCMs was enhanced by carbon fiber (CF). The thermal conductivity of the prepared CPCM with 5 wt% of CF was 0.33 W/(m K) and 0.47 W/(m K) in liquid and solid state respectively, which was 1.25 and 1.22 times higher than that of original CPCM without CF [61] . It is well known that graphene is a promising thermally conductive filler because of its ultrahigh thermal conductivity and low density. Nanosheets of graphene have previously been used to improve thermal conductivity of different organic materials [62]. Yavari and co-workers have made use of nanosheets of graphene to improve the thermal conductivity of PCMs. It is shown that addition of graphene nanosheets into organic PCMs can significantly enhance their thermal conductivity. An addition of a small amount of graphene (∼4% by weight) to 1octadecanol will cause a ∼140% increase in thermal conductivity (Fig. 12). It is clear that graphene were markedly superior to other nanofillers such as multiwalled carbon nanotubes and silver nanowires [63].

Fig. 13. Demonstration of stabilizing temperature capability of octadecane@TiO2-PVP nanofibers, where 1 cm3 of water at 60 °C was allowed to cool in a 4 °C environment in glass vials covered with different PCM nanofibers. Sample A had no PCM nanofibers, sample B was half covered with the PCM nanofibers, and sample C was fully covered by the PCM nanofiberst. Sample D is covered by a jacket of conventional fiberglass [64].

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Heat Source

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Fig. 14. (A) Schematic illustration of the thermoelectric device coated with the obtained G-PEGs. (B) Digital photographs of the assembled G-PEGs providing heat flow through the thermoelectric device to light up a red LED. (C) I-t curves were G-PEGs (1 cm of thickness) assembled with the amount of PEG of 93 wt%, 85 wt%, 70 wt% and blank, respectively. (D) I-t curves were G-PEGs (93 wt%) assembled with different thickness of 1 cm, 0.5 cm and blank, respectively [66].

a negative thermal bias it undergoes poor thermal conduction, thus effectively acting as a thermal insulator. Currently, there are two main modes realizing thermal rectification; i. e. an asymmetry of structure and saltation of thermal conductivity originated in the structural phase transition. For example, the perovskite cobalt oxide thermal rectifier resulted from the structural phase transition in a small temperature difference exhibits different of thermal conductivity [67]. A further example, under the ultralow temperature, a thermal rectifier constructed by MnV2O4 and La1.98Nd0.02CuO4 can induce thermal conductivity saltation at 57 K, which is driven by the structural phase transition [68]. In contrast to the thermal diodes mentioned above, a new type of thermal rectifier can realize the thermal rectification in the light of the thermal conductivity saltation through the solid-liquid phase transition of PCMs around room temperature. By taking binary eicosane/PEG4000 stuffed rGO aerogels as an example (Fig. 15) [69], the new thermal rectifier was built from binary solid-liquid PCMs, eicosane and PEG4000, which was full of the two ends of the same nanoporous rGO networks. The thermal conductivity of eicosane is 0.409 in solid state and rapidly drop to 0.170 W•m−1•K−1 after solid-toliquid phase transition. However, the difference of the thermal conductivity of PEG4000 is smaller before and after the phase transition [70]. The difference in thermal conductivity of two ends, derived from the phase transition of PCMs, instantaneously results in the thermal conductivity saltation in one end, and therefore switching on or off the thermal rectification. The thermal rectifier constructed by binary PCMs can not only work around room temperature, but also

4.2. Collecting and supplying heat for thermoelectric device Both creating a significant temperature difference and obtaining steady heat energy supply play important roles in thermoelectric power conversion systems. Fortunately, PCMs can provide steady heat energy supply to stabilize temperature by releasing large amounts of latent heat during phase transformation period. Therefore, the graphene/ polyethyleneglycol composites (G-PEGs) were proposed for heat supply for thermoelectric device [66]. The G-PEGs not only offered a large quantity of conductive pathways for heat transfer but also gave a highly thermally conductive reservoir to help phase-change materials for thermal energy collection, storage and release. The prepared G-PEGs can collect and store heat energy from waste heat etc., and then providing the heat to a thermoelectric device. Impressively, the thermoelectric device can generate electricity speedily and thus effectively light up a light-emitting diode (LED) with highly bright light (Fig. 14). And the current output of thermoelectric device has a longer plateau of steady-state current (I) and time (t) delay than that of blank device. Therefore, this work demonstrates a significant contribution to control heat current for electronic devices. 4.3. Thermal diode Rectifying heat flow, a thermal rectifier/diode, is of great significance for photonics. Such a device acts as a thermal conductor if a positive thermal bias is applied. On the contrary, in the opposite case of 2328

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Fig. 15. Schematic structure of the simulation model by FEM method, positive direction (A) and negative direction (B). The left part represented eicosane and the right was PEG4000. The arrow shown the direction of heat flux, and we heated the rectifier at the red side. (C) Thermal rectification coefficient with different proportion of PCMs. (D) Thermal rectification coefficient with different heater temperatures [69].

release. Although the advantages of confinement effect have been well documented, there are still several issues of phase change composite materials that need to be addressed.

provide a general design principle for controlling the working temperature. Recently, a single material with an asymmetric structure can also employ to construct the thermal rectifier device. For example, the single asymmetric reduced graphene oxide (rGO) may have a heat recitation phenomenon with relation of thermal conductivity of rGO on temperature [71]. Moreover, we also have designed and constructed a novel framework of binary asymmetric triangular structural graphene thermal rectifier device, which has thermal rectification coefficient up to 1.44 [72]. The asymmetric structure graphene composite, possessing both a macro- and micro-asymmetric structure, was arranged between parallelly aligned and interconnected structures. The different microstructure resulted in both ends of the thermal conductivity saltation. Therefore, the novel structure plays an important role for adjusting the thermal rectification of graphene composites to control the direction of heat flow.

(1) The synthesis technology of PCMs in a restricted matrix is still facing a challenge in establishing controllable nanostructures with the fully desired morphology, structure, and the size-dependent behaviour, etc. (2) New characterization methods are needed to further reveal microscopic properties otherwise hidden in the general measurements of statistical ensembles. The thermodynamics performance of PCMs confined in the different matrixes need to be measured by the variation with changes in temperature, or some exterior factors, such as pressure, most of which presented the macroscopic properties of bulk or microstructural ensembles, therefore the fine structural perturbations were hardly detected. (3) With the aim to further develop the merits of confinement effect for enhancing and adjusting the PCMs thermodynamics behaviors in terms of phase change temperature, latent heat, cyclic performance, etc, more insightful understanding of the relationship between the thermodynamics performance of PCMs and the matrix material structure, size and interface nature is needed. (4) Developing new phase change composite materials and structures is always expected. New mechanisms relying on micro-/nanoconfined effect are anticipated to increase the latent heat, adjust phase transition temperature to proper temperature range requirements and other new applications related to waste heat recovery and utilization.

5. Conclusions and outlook This review summarizes some of representative encapsulated PCMs with the aim of controlling the crucial phase-change parameters via the different restricted matrixes. Recent advances in micro-/nanoconfined strategies to encapsulate PCMs were reviewed in this paper, namely soft-template approach, electronic spinning technology, as well as the vacuum filtration method, etc.. The different restricted matrixes tighter with the crucial phase-change parameters of confined PCMs are introduced. Applications of the PCMs in various confined matrixes in aspects like stabilizing temperature, supplying heat for thermoelectric device and thermal rectifier. As illustrated in this paper, confinement effect has been proven to be effective in improving intrinsic drawbacks of PCMs and enhancing the performance of thermal storage and 2329

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