Recent progress on the morphology and thermal cycle of phase change materials (PCMs)/conductive filler composites: a mini review

Nomenclature

AgNPs

silver nanoparticles

Aℓ₂O₃

alumina nanoparticles

BPC

biological porous carbon

CBCF

carbon bonded carbon fiber

CNF

carbon nanofibers

CNTs

carbon nanotubes

CVD

chemical vapour deposition

DSC

differential scanning calorimetry

EG

expanded graphite

EGP

expanded graphite plate

ENG-TSA

expanded natural graphite treated with sulfuric acid

EVM

expanded vermiculite

F-CB

functionalized carbon black

Fe₃O₄

nanomagnetite

F-MWCNTs

functionalize multiwalled carbon nanotubes

FTIR

Fourier transform infra-red

GNPs

graphene nanoplatelets

GNS

graphene nanosheets

GO

graphite oxide

L-MWCNTs

long multiwalled carbon nanotubes

OD

1-octadecanol

O-GNs

oriented graphite nanosheets

PCMs

phase change materials

PEG

polyethylene glycol

RD

raw diatomite

R-GNs

randomly distributed graphite nanosheets

SA

sebacic acid

SCFs

short carbon fiber

SEM

scanning electron microscopy

SiC NWs

silicon carbide nanowires

S-MWCNT

short multi-walled CNTs

SSL

sodium stearoyl lactylate

TGA

thermogravimetric analysis

UFG

ultrathin-graphite foam

xGnP

exfoliated graphite nanoplatelets

XRD

X-ray diffraction

1 Introduction

The utilization of renewable and sustainable energy resources to reduce the greenhouse gases emission and dependence on the fossil fuels has been amongst the major research subjects in the past decades [1], [2], [3]. One of the best solutions in reducing the greenhouse emission and utilizing ecofriendly energy resources is to implement PCMs. Figure 1 illustrates the rapid increase in studies on phase change materials since the year 2015 due to the energy crisis caused by the increasing demands of energy, resulting from the rising population growth, wars, and market manipulations. PCMs are recognized by absorbing or releasing energy when they undergo a change in physical state, which includes solid–liquid, solid–solid, and gas–liquid. Figure 2 shows the top 10 countries that are actively involved in utilization of PCMs for energy storage. The analysis reveals that the dominant country is the People’s Republic of China (36.8%). In second position is the United States of America (10.7%) which is closely followed by India (10%). Figure 3 illustrates citation network by countries. The bibliometric network further proves the dominance of Chinese on PCMs for energy storage. The size of the frame is positively correlated with the number of citations. The thickness of the lines and proximity of the frames represent the strength of the citation link between countries. The People’s Republic of China is the most cited country regarding this research topic (42358 citations) with total link strength of 75717.

Figure 1:

Annual number of scientific publications from 2015 to 2021, using the search terms “phase change materials for energy storage” and “thermal conductivity of phase materials”. Data analysis was completed using the Web of Science core collection databases on 06 April 2021.

Figure 2:

Top ten leading countries active on research based on PCMs as sustainable energy storage materials (2015–2021).

Figure 3:

PCMs as sustainable energy storage materials bibliometric network visualization of citations by country using VOS viewer version 1.6.15 software. Data was extracted from the WoS core collection database.

Phase change materials are categorized as inorganic, organic, or eutectic mixture of inorganic–organic (Figure 4) [4], [5], [6], [7], [8]. Organic materials can be subdivided into non-paraffins and paraffins [4], [5], [6, 9, 10]. Most of the organic phase change materials undergo self-nucleation, meaning crystallization with negligible or no super cooling. Paraffins include paraffin wax, which is composed of straight chain n alkanes. Paraffin wax may be regarded as an effective heat storage material because it is reliable, safe, predictable, cheap, noncorrosive, and shows little volume change during melting [6, 11]. Non-paraffins (viz: formic acid, caprylic acid, glycerin, phenol, etc.) have a variety of properties, when compared with paraffins, which have indistinguishable properties. Some properties of non-paraffins, include: (i) inflammability, (ii) excellent heat of fusion, (iii) a low flash point, (iv) a low thermal conductivity, (v) varying degree of toxicity, and (vi) unstable at elevated temperatures. Inorganic phase change materials are divided into metallics and salt hydrates [12], [13], [14]. The inorganic PCMs do not super cool in an appropriate manner and there is no degradation during cycling. Salt hydrates are consisted of inorganic salts and water that produce a more crystalline solid. Salt hydrates have been used because of: (i) high thermal conductivity (ii) small volume changes during melting and (iii) high latent of fusion. Another type of inorganic PCMs is metallics, which has not been widely investigated due to their weight problems and corrosiveness. However, one major advantage of the metallic PCMs in comparison to other PCMs is their excellent thermal conductivity with acceptable heat of fusion and volume changes during melting. Besides other PCMs classification mentioned above, there is another type, in the form of eutectic PCM, which has a minimum melting temperature between mixtures of components where melting and freezing occur congruently, producing a mixture of crystals during crystallization. One important feature of eutectics PCMs is that they melt and freeze without segregation.

Figure 4:

Classification of phase change materials.

Most PCMs (except for the metallics) have low thermal conductivity, irrespective of their classification [15], [16], [17], [18], [19], [20], [21]. This property hinders their widespread applications because of little heat transfer during storing or releasing of thermal energy. Numerous studies were conducted (Figure 1) to overcome this issue by incorporating highly conductive fillers into PCMs. Conductive fillers are added to PCMs for two reasons, viz. (i) to facilitate thermal conductivity within PCM system and (ii) to adsorb and prevent PCMs leakage during transition. Out of the top 10 countries conducting research incorporating conductive filler to promote thermal conductivity within PCM systems, the People’s Republic of China (39.2%), United States of America (15%), and India (9.2%) are amongst the leading countries, with the People’s Republic of China as leaders (see Figure 5). The bibliometric analysis further reveals that the People’s Republic of China (58472 citations) is the most cited country regarding the inclusion of conductive filler in order to improve overall thermal conductivity of PCMs (see Figure 6). The United States of America (28 620 citations) is ranked second and it is strongly linked with third placed India regarding citations.

Figure 5:

Top ten leading countries on thermal conductivity of phase change materials research (2015–2021).

Figure 6:

Thermal conductivity of phase change materials bibliometric network visualization of the citations by country using VOS viewer version 1.6.15 software. Data was extracted from the WoS core collection database.

Reviews and research papers on thermal conductivity and thermal storage properties of PCMs are well documented, while reports on their thermal stability as well as their reliability are fairly scarce [4], [5], [6], [7], [8], [9], [10]. The success for widespread usage of phase change materials is limited by poor long-term thermal stability. The long-term performance of the storage system can be established by monitoring the differences in the thermo-physical properties of the PCMs after being subjected to a numerous number of thermal cycles. The current review paper reports on the morphology, preparation methods and thermal stability of the various types of PCMs in the presence of a conductive filler.

2 Preparation and morphology of PCMs/conductive filler systems

The development of conductive PCM composites was done not only to enhance the thermal conductivity of the ensuing system (viz. resulting in a rapid energy storage and release), even so for prevention of paraffin leakage in the molten state [22], [23], [24]. The morphological characterization is an important tool because it can provide more information on the interaction and compatibility between pure PCMs and the conductive nanofillers [25]. A number of factors, such as: different preparation methods, content of the filler, type of the PCM, type of the conductive filler, orientation, and the functionalization of the conductive nanoparticles, will naturally, affect the morphology of conductive PCM composites [25], [26], [27], [28], [29]. Table 1 summarizes the preparation methods and morphology of different PCMs/conductive filler systems. In most PCMs/conductive filler systems, the relevant methods for preparation of such composites is the immersion of the conductive filler (viz. expanded graphite, ultrathin graphite foams etc.) in a hot liquid wax and/or cold compression method, as summarized in Table 1. In the compression method, solid PCMs and fillers are mixed and then compressed with or without heat [26]. The method is preferred for the preparation of conductive PCMs composites because the fillers normally occupy the voids between the PCM particles during melt compression and this process facilitating the formation of a percolating network within the system. Irrespective of the classification of PCMs (Figure 4) and the preparation method (Table 1), in most cases, if expanded graphite (EG) was used as a conductive filler and/or matrix for PCMs, the PCMs were found to occupy the pores within the honeycomb network of EG. This indicates that EG has a strong affinity for the PCMs [27]. It is believed that the EG consists of two types of pores i.e., the macro-pores and meso-pores. The macro pores are mainly distributed in the gap between two different worm-like skeletons and the meso-pores are found through the separation of graphite layers. This means that the PCMs are, basically, absorbed in either the meso-pores or macro-pores structures of EG. Huang et al. [28] investigated wood alloy/expanded graphite composite for electronic thermal management. The authors reported absorption of wood alloys into the micro-pores due to capillary forces, while less absorption occurred into the meso-pores structures at 70 wt% of PCM in the composite. The reason for less absorption into the mesoporous is attributed to the closing of the mesopores after being through the physical mixing process. The same method of adding the conductive filler (viz. metal foam, carbon nanotubes, and carbon nanofiber etc.), other than expanded graphite into the melted PCMs was investigated elsewhere in the literature and the resultant morphology differed depending on the type of conductive filler used [29]. The ultrathin-graphite foam (UFG)-erythritol composite was investigated in a study by Ji et al. [29]. This was done by submerging the foam in hot liquid wax. In this study, the conductive filler in the form of ultrathin-graphite foam (UFG) was synthesized on nickel foam as a template through chemical vapor deposition (CVD) method. The template was then removed using wet chemical etching process, thereby, leaving behind ultrathin-graphite struts that were connected in continuous foam. The grown UFG are hydrophobic and as a result, they can easily be filled into the hydrophobic paraffin wax. Scanning electron microscopy (SEM) and micro-CT scan sheets of the UFG-erythritol composites, revealed that the hollow interior of the UFG struts were occupied with PCM.

Furthermore, the effects of different carbon nanofillers i.e., short multiwalled carbon nanotubes, long multi-walled carbon nanotubes, carbon nanofibers and graphene nanoplatelets were explored for enhancing thermal conductivity of paraffin-based nanocomposite PCMs [30]. PCM/carbon nanofillers were prepared via a two-step process. Firstly, the nanofillers were incorporated into the molten paraffin wax and secondly, the suspension was poured into a rectangular mold and then allowed to solidify into composite samples. SEM images of raw carbon nanotubes (CNTs) and carbon nanofibers (CNF) (Figure 7(a)–(c)) were found to be agglomerated, whereas CNPs were confirmed [30] to be planar and stacked together (Figure 7(d)). However, in the PCM/carbon nanofillers composites, carbon nanotubes and carbon nanofibers were well distributed within paraffin wax, and the large clusters were broken down into smaller particles during preparation (Figure 8(a)–(c)). The stacked GNPs were also separated into thin graphene sheets that were well distributed in the PCM system (Figure 8(d)).

Figure 7:

SEM images of the raw (a) S-MWCNTs, (b) L-MWCNTs, (c) CNFs, and (d) GNPs with different sizes and shapes. (d) Insert showing an individual GNP sheet and its thickness [30]. Copyright Elsevier.

Figure 8:

TEM images showing the sizes and distributions of: (a) S-MWCNTs, (b) L-MWCNTs, (c) CNFs and (d) GNPs filled in the paraffin wax [30]. Copyright Elsevier.

The influence of the filler content was reported by Ren, et al. [31]. The system was composed of Ca(NO₃)₂-NaNO₃/expanded graphite with EG loadings being varied throughout the preparation process. The prepared composite proved that the content of EG had an influence on the resultant morphology of the system [31]. Macro-cracks were observed in the 5 wt% of EG composite, whereas the leakage of PCM in a liquid form was observed for 6 wt% EG composite. Finally, the surface of 7 wt% of EG was smooth and homogenous without cracks and liquid leakage. It is well known that the morphology of any composites system affects the final properties of the system. It was reported in the literature that conductive particles experience increases in surface interaction and are more likely to undergo a particle adhesion through immediate contact, via three forces i.e. electrostatic, van der Waals, and magnetic forces due to high surface area and energy [32]. Chemical functionalization was also used to enhance the compatibility between conductive filler and phase change material. Chemical functionalization became a popular method because it is more effective in improving the compatibility when compared to the physical absorption of surfactants [33]. Functionalized carbon black (F-CB)/octadecane composites were investigated for room temperature electrical and thermal regulation [33]. The F-CB particles were combined with various amounts of octadecane and rigorously stirred at 70 °C for 2 h in order to produce the F-CB/octadecane composites. Carbon black nanoparticles were functionalized through hydroxylation and esterification and it was found that the modified carbon black particles dispersed uniformly in octadecane. Similarly, Sun and co-workers [34] fabricated carbon nanotubes (CNT)/hexadecane composites for room temperature electrical and thermal switching. In order to functionalize multiwalled carbon nanotubes (F-MWCNTs), oxidation process was employed in order to introduce carboxylic acid groups on the surface of the CNTs and then functionalized MWCNTs (F-MWCNTs) were obtained from thermal mixing of oxidized MWCNT and octadecylamine (ODA). An obvious multi-layered structure with smooth surface in the nonfunctionalized MWCNT, however for the functionalized MWCNT, an amorphous substance that was attached onto the surface of MWCNT was seen, which was attributed to the grafting of long-chain amine groups. Nonfunctionalized MWCNT and functionalized MWCNT were dispersed in hexadecane and the samples were allowed to stand for a month to analyse the stability of the two types of carbon nanotubes in hexadecane. After a month, the resultant micrographs showed sediment in the nonfunctionalized MWCNT/hexadecane composite, whereas for the functionalized MWCNT/hexadecane composite, there was no sediment, which proved that F-MWCNT had a better stability in hexadecane.

The specific surface area and orientation of the conductive filler can also affect the morphology of the conductive PCMs composite. The specific surface area of conductive filler plays a significant role in shape stabilization. In addition, a conductive filler with a high specific surface area, is expected to be well dispersed in the paraffin as a result, this makes it convenient for the particles to form a mechanically interconnected structure, even at lesser contents and provide better mechanical reinforcement. The overall idea is to obtain better reproducibility and uniformity of the conductive PCMs composite systems. Shi et al. [41] investigated the shape stabilized PCMs using two nanographite fillers (viz. exfoliated graphite nanoplatelets and graphene) with different specific surface area. The specific surface area of graphene and exfoliated graphite nanoplatelets were reported to be 876.2 and 13.8 m² g⁻¹, respectively. Because of the higher specific surface area of graphene nanoplatelets, SEM images, at 2 wt% loading of the nanofiller, showed no white areas which proved that paraffin was well immersed into the spaces between the graphene nanoplatelets. The result is an indication that graphene, as a conductive filler can assist in terms of reducing the content of additives required for shape stabilization. However, there were no SEM images for an exfoliated paraffin composite system; however, it can be deduced from the composite’s low dropping points (67.0 °C), when compared to the paraffin/graphene composite (185.2 °C) that it must have experienced some degree of liquid leakage, during its solid change. This symbolizes excessive paraffin not being absorbed by the exfoliated graphite. Depending on the type of application required, the ideal orientation of the conductive nanoparticles is very important. The assumption of random distribution and orientation of the conductive filler was demonstrated by Chen et al. [42]. In this study, the authors fabricated two types of paraffin composites, with both the randomly distributed as well as oriented nanosheets termed R-GPNs and O-GPNs, respectively. Irrespective of graphite nanosheets (GPNs) orientation, SEM images of the composites showed that GPNs well dispersed in paraffin, even at higher GPNs content. In the case of R-GPNs/paraffin composites, wrinkles were observed at higher GPNs content, i.e. 5 wt%, whereas, for the O-GPNs/paraffin composite, the GPNs were oriented relatively parallel, due to the shearing effect during the ball rolling process. Based on the morphology above, it can be expected that the wrinkled dispersion of the GPNs will have better thermal conductivity in comparison to the parallel orientation of the nanographite; due to the better GPNs contact, resulting in better pathways for heat transfer.

3 Stability test of PCMs-conductive filler composites

One of the most important requirements of PCM/conductive filler composites is their ability to withstand multiple melting and solidification cycles. A satisfactory thermal reliability is essential for the application of PCM-based composite, since it ensures a more stable phase change in temperature and the latent heat of PCM composites after a numerous number of cycles. The idea is such that there must be insignificant or no change in the thermal property for a long-term usage. Ji et al. [29] studied thermal cycle stability of the composite by analysing the electrical resistance after numerous melting and solidification cycles. In their study, two types of PCM composites (viz: UGF-wax and UGF-erythritol) were investigated. During melting process, the liquid PCM remained within the pores of the ultrathin-graphite foams (UGF), due to surface tension. This simply implies that there was no significant mass loss recorded during electrical measurement, even after 100 melting-solidification cycles. The use of other techniques, such as: XRD, FTIR, TGA, and DSC are usually employed to confirm whether there are any changes in crystallinity, chemical, and thermal properties of the PCM composites, after the stability test. In most cases, the crystallinity, thermal stability, and chemical structure of the resulting composite materials do not change after the stability test. However, no changes were recorded regarding latent heat, as well as melting and freezing temperatures, depending on the system, as summarized in Table 2. The changes in these properties are within experimental error, which makes it difficult to conclude whether the PCM composite system is losing its crystallinity, thermal and chemical integrities during the stability test. On the other hand, in most of these studies, the test methods differ, with some been in-house constructed testers, which make it difficult to compare the results obtained with other studies. The standardization of the stability test of PCM composites is needed to evaluate thoroughly and compare the results regarding the ability of the conductive filler to absorb the PCM material without observable leakage. For instance, the use of DSC to study the stability of the composites is controversial regarding the fact that the PCM composite is closed in a pan and even if there is leakage from the composite material, the PCM will still be in the pan. Thus, these types of methods must be carefully evaluated to establish their reliability towards stability testing of PCM composites. Moreover, the morphological characterization of the PCM composites after thermal cycle must be done and compared with unanalysed samples in order to thoroughly understand whether there is any leakage after testing. FTIR serves as important tool to evaluate any chemical changes, especially for PCM materials that change their chemical structures during charging and discharging, whereas for other PCMs, e.g., paraffin wax, since leakage is the only important aspect, researchers’ can overlook such techniques. Nonetheless, a simple method for the evaluation of leakage of the PCM composites was conducted by heating the composite material to a temperature above the melting temperature of the PCM, and then hold it for 20 min. It was found that the presence of halloysite is important to prevent seepage, as shown in Figure 9, while additional graphite and CNTs improves the thermal conductivity of the resulting PCM composite product [51].

Figure 9:

Photographs of (a) pure wax, (b) wax/halloysite (50/50%), (c) wax/graphite (90/10%), (d) wax/halloysite/graphite (45/45/10%) and (e) wax/halloysite/graphite/carbon nanotubes (45/45/5/5%) held at 90 °C for 20 min [51]. IOP Publishing (open access).

4 Effect of conductive filler on melting enthalpy

Often, the thermal performance of PCMs is investigated by the melting temperature (T_m), heat of fusion and thermal conductivity. PCMs systems are fabricated in such a way that there should be a specific melting temperature and high heat of fusion in to be in tandem with the energy density requirements for a large adoption of PCM-based thermos-physical storage technologies. Several studies on the incorporation of conductive fillers into a phase change material have shown to change the melting enthalpy (of PCM) because the dispersed conductive fillers may interrupt the local bonding environment of the PCM molecules, as summarized in Table 3. It is noteworthy to note that two observations, viz.: (i) some studies reported a decrease in the melting enthalpy values after filler loading were made in relation to the melting enthalpy [59, 68] whereas (ii) other studies reported a negligible change in melting enthalpy values. Sani and Etesani [69] investigated the effect of TiO₂ nanoparticles on the thermal properties of paraffin. In their study, surfactant, i.e., sodium stearoyl lactylate (SSL) was employed to facilitate the dispersion of the nanoparticles in the chosen paraffin. It was found that the PCM composite with or without surfactant showed similar phase change behavior. However, maximum latent heat was obtained for samples prepared with surfactant, especially at high mass ratio of nanoparticles, when compared to PCM composites without surfactant (Figure 10(a)). This was attributed to the ability of the surfactant to facilitate the dispersion of the nanoparticles in the paraffin. In the case of the melting temperature, it was found that the increase in nanoparticles from 0.5 wt% to 1 wt% in both samples, with or without surfactant, decreased the melting temperature, however, a further increase led to an increase in melting temperatures (Figure 10(b)). This was ascribed to the aggregation of the particles at higher loadings and reduction of the interaction between the paraffin molecules and the nanoparticles. The type and/or strength of interaction between the conductive fillers and PCM material, has the most significant influence on the thermal phase change behavior of the resulting PCM composite material, regardless of conductive-filler type employed. If there is no chemical interaction between the filler and PCM, that means it is only physical interaction, hence the melting temperature and/or freezing temperatures of the PCM composites are often similar, as depicted in Figure 11(a) and (b) [70]. It was reported that there was a weak interaction between the filler and PCM materials.

Figure 10:

(a) Latent heat and (b) melting temperature changes for TiO₂/paraffin composites, composed different TiO₂ nanoparticles loadings with (and without) SSL [69]. Copyright Elsevier.

Figure 11:

Melting and crystallization temperatures for SSPCM-filled with different concentrations of (a) expanded graphite plate (EGP) or (b) MWCNT [70]. MDPI (open access).

5 Future trends based on phase change materials

Majority of phase change materials have been hosted by porous carbon-based materials, which seemed to have performed well as host materials. However, there is need for researchers explore the possibility of hybridization of the fillers that can absorb PCMs more than single conductive fillers to avoid their leaching during change of state. These fillers can be chosen based on the structural properties as well as their thermal conductivity. Such modification can improve overall properties of the ensuing composite material. Furthermore, in order for PCMs/conductive filler(s) to meet certain requirements in some applications, the PCMs composite must have great thermal reliability, shape, and structural stability. To achieve these properties, it is very important for scientist/researchers to try and modify the conductive fillers in future in order to enhance the properties, as a result expanding the applications of the PCMs. It is well-known that the storage of PCMs composites depends on the composition of the PCM in the composite; as such there is a need to enhance the storage system to about 70% of the PCMs without compromising the strength of the composite. Furthermore, it has been recognized that PCMs tends to reduce the thermal stability and mechanical properties, as a result researchers might have to explore other possibilities of adding fillers such as natural fibers into the PCMs system, to enhance both thermal stability and mechanical properties.

6 Conclusions

The review paper discussed an in-depth overview of the morphology and thermal cycle of the various PCMs/conductive filler(s) composites. It has been highlighted from different studies that factors such as preparation methods, content of the filler, type of the PCM, type of the conductive filler, orientation, and the functionalization of the conductive nanoparticles affect the morphology of the resultant composite, as a result influences on the overall properties of the composites. Of all conductive fillers, carbon-based filler, such as graphite, has good properties, not only to improve the overall conductivity of the resulting composite material, but also the ability to absorb more PCM material, especially paraffin wax into its porous structure and prevent leakage during the charging and releasing processes. It was further realized in this review paper that, as much as PCMs are employed for energy storage, there is a need for ensuring that the PCM systems can withstand long term usage, which makes the studies on the thermal cycle of such systems very important. In most cases, the crystallinity, thermal stability, and chemical structure of the composites do not change after the stability test. However, insignificant changes were reported regarding latent heat, as well as melting and freezing temperatures, depending on the system. In future, standardization on the stability test should be considered for comparison of the results, not only in the same study, but also in the overall studies that are based on PCM-composite materials.