Effects of carbon nanotubes additive on thermal conductivity and thermal energy storage properties of a novel composite phase change material

Abstract

Fatty acids are commonly preferred as phase change materials for passive solar thermoregulation due to their several advantageous latent heat thermal energy storage (LHTES) properties. However, further storage container requirement of fatty acids against leakage problem during heating period and also low thermal conductivity significantly limit their application fields. To overcome these drawbacks of capric acid–stearic acid eutectic mixture as phase change material, it was first impregnated with expanded vermiculite clay by melting/blending method and then doped with carbon nanotubes. The effects of carbon nanotubes additive on the chemical/morphological structures and LHTES properties of the composite phase change material and thermal enhanced change phase change materials were investigated by scanning electron microscope, Fourier transform infrared spectroscopy, X-ray diffraction, differential scanning calorimetry and thermogravimetric analysis analysis techniques. The differential scanning calorimetry results showed that the form-stable composite phase change materials and thermal enhanced composite phase change materials have melting temperatures in the range of 24.35–24.64℃ and latent heat capacities between 76.32 and 73.13 J/g. Thermal conductivity of the composite phase change materials was increased as 83.3, 125.0 and 258.3% by carbon nanotubes doping 1, 3 and 5 wt%. The heat charging and discharging times of the thermal enhanced -composite phase change materials were reduced appreciably due to the enhanced thermal conductivity without notably influencing their LHTES properties. Furthermore, the thermal cycling test and thermogravimetric analysis findings proved that all fabricated composites had admirable thermal durability, cycling LHTES performance and chemical stability.

Keywords

Carbon nanotubes vermiculite fatty acid composite phase change material thermal energy storage thermal conductivity

Introduction

The improvement of energy efficiency in buildings is recently one of the most important interests of the energy technologist because almost one-third of overall energy in the word is consumed in buildings.¹ In this regard, thermal energy storage (TES) has become a feasible alternative technique within last 30 years for solving energy and environmental pollution problems.² Phase change materials (PCMs) and composite PCMs (CPCMs) with high thermal performance and durability have been considered as attractive TES materials with energy storage/release ability.^3,4 In recent years, some parts of the energy researches have been directed to develop novel alternative PCMs and CPCMs and investigate their thermal management performances.^5–10

Fatty acids or eutectic mixtures of them have been preferred as organic PCMs for passive solar TES applications because of their favoured and re-obtainable latent heat TES (LHTES) properties.^11,12 However, direct operation of these PCMs not only allows chemical interaction with near environment but also leads to leakage problem during solid–liquid phase change. To avoid this intricacy, they have been encapsulated in macro or micro dimensions.^13,14 However, the encapsulations of PCMs are relatively more complex and needed high cost and generally resulted in low LHTES capacity. Another option is creation of leakage-resistive CPCMs by combining with lightweight, porous and low cost construction materials. On the other hand, another drawback of fatty acids is low thermal conductivity (0.16–0.20 W/m K), which strongly influences their heat charging–discharging rates.^15,16 Moreover, their incorporations with especially clay-based building matrixes in stabilized form can be eventuated in significant decrease in thermal conductivity. Therefore, doping of such a type CPCM with a high thermal conductivity material is an effective approach in terms of enhancing its thermal conductivity. Carbon-based materials, such as carbon nanotubes (CNTs),^17,18 carbon nanofibers (CNFs),¹⁹ graphite nanoplatelets (GNPs),²⁰ graphene oxide²¹ and expanded graphite²² have been mostly used to boost thermal conductivity of organic PCMs. On the other hand, some superior properties of CNTs like low density, large surface, high stability and resistance for corrosion and ultrahigh thermal conductivity (reach up 4000 W/m K for single-walled CNTs and 2000 W/m K for multi-walled CNTs) play important role on their usage as doping agents for heat transfer enhancement within PCMs or CPCMs.^23–25 At present provision, the production cost of the CNTs with >96% purity is too higher compared to industrial-grade, > 90% purity. However, CNTs are still one of the most influential means for thermal conductivity improvement of CPCMs. Moreover, the enrichment effect of CNTs on thermal conductivity could be changed seriously depending on their type (single or multiple walled), scattering degree and loading amount in PCM. The thermal conductivity of paraffin wax was increased as about 35–40% using 2.0 wt% CNTs.^26,27 Li et al.²⁸ reduced heat charging and discharging times of stearic acid (SA) as 91% with doping of 5.0% CNTs. Wang et al.²⁹ reported that the thermal conductivity of palmitic acid (PA) was boosted as 30% with addition of 1.0 wt% CNTs. Tang et al.³⁰ enhanced thermal conductivity of myristic acid–SA eutectic mixture as about 49% by using CNTs in mass fraction of 15 wt%. Zhang et al.³¹ found that thermal conductivity of PA–SA eutectic mixture was amplified by 20.2–29.7% in case of 5–8 wt% CNTs addition. Cui et al.³² reported that 10 wt% CNTs addition caused a 24.3% enhancement in thermal conductivity of soy wax likely due to its scanty distribution in wax matrix. Moreover, agglomeration case is still a burning issue in usage of CNTs as doping material. This problem has been tried to solve by chemical functionalization of CNTs, besides, by mechanic/ultrasonic mixing.^33–35

On the other hand, the studies about the effects of CNTs additive on thermal conductivity improvement of clay-based CPCMs are limited by a few studies. Thermal conductivity of paraffin/expanded perlite was enhanced as 4.27 times by introducing 5.27 wt% CNTs.³⁶ Thermal conductivity of paraffin/diatomite was enhanced as 42.45% by doping of 0.26 wt% CNTs without affecting its chemical and thermal properties.³⁷ The thermal conductivity of paraffin wax/halloysite was increased as 14 and 128% by using 10 wt% CNTs compared to paraffin wax and the composite, respectively.³⁸ By CNTs doping in mass fraction of 1.0 wt%, the increase in thermal conductivity of paraffin/montmorillonite was found as 29%.³⁹ The thermal conductivity of expanded perlite/paraffin was enhanced as 113.3% using CNTs (1.0 wt%).⁴⁰ Using CNTs in mass fraction of 1, 3 and 5 wt%, the thermal conductivity of silica fume/capric acid (CA)–PA eutectic mixture was improved as 12.9, 32.3 and 56.1%, respectively.⁴¹ As different from the literature given above, our challenge is to find an effective key for simultaneously eliminate leakage problem and low thermal conductivity drawback of CA–SA eutectic mixture without noticeably dropping its LHTES capacity. Expanded vermiculite (EV) was selected as good supporting material to eradicate seepage difficulty of the eutectic mixture because of its many advantageous properties such as high surface area, excellent compatibility, light weightiness, porosity and low cost.^42,43 In the first stage of the present work, the eutectic mixture was incorporated into EV to achieve novel CPCM in stabilized form using melting/blending method. In the next step, by considering low thermal conductivity values of the pure components of CPCM (0.16 and 0.05 W/m K for the eutectic mixture and EV, respectively), thermal conductivity of the CPCM was remarkably improved by doping of CNTs in mass fraction of 1.0, 3.0 and 5.0 wt%. It tried to minimize the aggregation adversity of CNTs by its mechanic/ultrasonic homogenization into acetone solution. The morphological and chemical structures of the prepared CPCM and thermal enhanced CPCMs (TE-CPCMs) were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) techniques. The influence of CNTs doping on thermal conductivity, LHTES properties, thermal stability and heat charging–discharging times of fabricated CPCM was investigated. According to our best knowledge, an inclusive study on EV/CA–SA/CNTs composite was not yet reported in literature.

Experimental

Materials

CA and SA were provided from Sigma-Aldrich company. Acetone used as dispersive agent was obtained from Merck Company. EV was procured from Agrekal Company (Antalya-Turkey). The granulated EV sample was sieved from 150 µm mesh and then dried at 105℃ for 48 h to remove humidity. Moreover, multi-walled CNTs specified as CNTs in this work was bought from Sigma-Aldrich Company. Some physicochemical properties of EV, fatty acids, CNTs and acetone were presented in Table 1.

Table 1.

Some physicochemical properties of the reagents used in the preparation of CPCM and TE-CPCMs.

Property	Value
EV	Composition (wt%)
SiO₂	42.0
Al₂O₃	13.0
MgO	25.5
Fe₂O₃	9.5
CaO	2.5
Humidity	7.5
Density (g/cm³)	0.114
Capric acid
Assay	≥98%
Density (g/cm³)	0.893
Stearic acid
Assay	≥95%
Density (g/cm³)	0.400–0.500
CNTs
Type	MWCNTs
Dimensions	OD (nm): 6–13; L (µm): 2.5–20
Size	10 µm, average length, TEM
Size	12 nm, average diameter, HRTEM
Assay (carbon %)	>98
Density (g/cm³)	∼2.1
Acetone
Boiling point (℃)	56.2
Flash point (℃)	<−20
Density (g/cm³)	0.79

Preparation of CPCM and TE-CPCMs

The eutectic mixture was formed by homogenously mixing CA with SA in melted state in eutectic mass fraction ratio (82–18 wt%), afore-determined in literature.¹² In the next step, EV/(CA–SA) and EV/(CA–SA)/CNTs composites were fabricated in shape-stabilized form by means of melting/blending method. EV/(CA–SA) was prepared as form-stable CPCM by following up the procedure shown in Figure 1. The eutectic PCM was melted and mixed with EV in different mass fractions (ranged from 10 to 50 wt%). Each combination was blended at 500 r/min for 15 min using a mechanical stirrer in order to provide homogeneity. The form-stable infiltration ratio for the eutectic PCM was decided by exposing the prepared composite samples to the leakage test. For this purpose, samples including different amount of eutectic PCM were heated separately onto a filter paper up to 45℃ for 10 min. By checking the leakage case on the filter paper, the EV/CA–SA composite with 60/40 wt% combination was characterized as form-stable CPCM.

Figure 1.

The schematic procedure used in the preparation of CPCM and TE-CPCMs.

On the other hand, TE–CPCMs were prepared at two steps as also shown in Figure 1. In the first step, CNTs at amounts given in Table 1 were dispersed in acetone solution (acetone/deionized water ratio: 1/5 v/v) by means of ultrasonic homogenizer (T18 digital Ultra-Turrax®model) at 10,000 r/min for 30 min. The formed sol–gel was centrifuged again at 2000 r/min for 30 min to minimize the agglomeration of CNTs. After that, the form-stable EV/(CA-SA) composite at specific weight amount was added in powder form to CNTs/acetone dispersion and then centrifuged at 5000 r/min for 60 min to obtain EV/(CA–SA)/CNTs as TE-CPCM. The finalized sample was moulded and maintained at 40℃ for 48 h to evaporate acetone. Additionally, the composite sample was kept at room temperature for 72 h until to be sure that the remaining water was evaporated completely. The doping process was replicated using CNTs in three different mass fractions: 1, 3 and 5 wt%. The last percentages of the eutectic PCM in form-stable CPCM and TE-CPCMs after leakage test were presented in Table 2. Moreover, the leakage test results of TE-CPCMs in stabilized and non-stabilized form were shown in Figure 2.

Table 2.

Weight amounts and percentages of EV, CA–SA eutectic mixture and CNTs in the prepared CPCM and TE-CPCMs after leakage test.

FS-CPCM		EV (g)	CA–SA (g)	CNTs (g)	CNTs (wt%)	CA-SA (wt%)
EV/(CA–SA)	CPCM	3.0	2.0	–		40.0
EV/(CA–SA)/CNTs (1 wt%)	TE-CPCM-1	3.0	2.0	0.052	1.0	39.6
EV/(CA–SA)/CNTs (3 wt%)	TE-CPCM-2	3.0	2.0	0.155	3.0	38.8
EV/(CA–SA)/CNTs (5 wt%)	TE-CPCM-3	3.0	2.0	0.267	5.0	38.0

Figure 2.

The leakage test results obtained for TE-CPCMs.

Instrumentation

The surface investigations were carried out using a SEM instrument (LEO 440 model; Japan). The SEM photographs of EV, CNTs, CPCM and TE-CPCM samples were taken from powder form after golden-coating process. The chemical structures and possible physicochemical interactions between the components were searched by FT-IR spectroscopy analysis (JASCO 430 model; USA). The spectroscopic results were obtained in the wavenumber range of 400–4000 cm⁻¹.

XRD analysis was conducted using a PANalytical X′-Pert³ powder diffraction metre (45 kV, 40 mA) with Cu (Kα = 1.5406 Å) irradiation in the wide range of Bragg angles 2θ (0 ≤ 2θ ≤ 70 °) at a step size of 0.0131 °.

The key LHTES properties of the eutectic PCM, CPCM and TE-CPCMs were measured using differential scanning calorimetry (DSC) analysis (Perkin Elmer-JADE model, USA) at 3℃/min heating–cooling rate under nitrogen atmosphere. The average deviation for repeated three measurements regarding phase change temperature and latent heat was calculated as ±0.13℃ for s and ±1.38 J/g, respectively. Thermal degradation temperatures of EV, eutectic PCM, CPCM and TE-CPCMs were determined using thermogravimetric analysis (TGA) technique (Perkin–Elmer TGA7 model instrument; USA) at a heating rate of 10℃/min and 30–800℃ range under argon gas atmosphere.

To study cycling LHTES performances of the form-stable CPCM and TE-CPCMs, each sample (about 10 mg) was heated to 35℃ and then cooled to 10℃ using a thermal cycler (BIOER TC-25/H model). After repeated cycling for 1000 times, the LHTES properties of the composites were measured again by DSC analysis. Moreover, the FT-IR analysis was re-performed to have knowledge about the chemical stability of the produced composites.

The thermal conductivities of EV, eutectic PCM, CPCM and TE-CPCMs were measured using a thermal property analyzer (KD2-Pro Decagon model; measurement range: 0.02–2.00 W/m K; accuracy: ± 5%). The device was calibrated with the glycerin sample provided by the manufacturer. For the measurement, sample in powder form was strictly put into a glass tube (diameter: 13 mm, length: 130 mm), and the measurement sensor was placed horizontally in centre of the sample so that there is no air space between sample and sensor. The needle of sensor was heated and read the temperature simultaneously. The measurement was replicated with 15 min intervals for three times to reach equilibrium and minimize data error. The average value was taken as final result, and the mean deviation was calculated as ±0.010 W/m K.

In order to investigate the effect of enhanced thermal conductivity on heat absorbing and releasing times of CPCM and TE-CPCMs, the experimental set-up was designed as shown in Figure 3. The heating period was carried out by circulating hot water at 50℃ from the water bath in which a glass tube including 10 g sample. This period was ended when the sample temperature was reached above the melting temperature of the eutectic PCM. After that, cooling period was started immediately by flowing water at 15℃ and continued until the sample temperature was decreased to below its freezing temperature. The sample temperature measurement was conducted using a K-type thermocouple which is connected to a logger (NOVA5000 model).

Figure 3.

The experimental set-up used to investigate the effect of enhanced thermal conductivity on heat charging and discharging rates of the prepared composites.

Results and discussion

Morphological characterization results

Figure 4(a) to (d) shows the SEM photographs of the EV, CNTs, CPCM and TE-CPCM-3 as a representing example for the other TE-CPCMs. As shown from Figure 4(a), the surface of EV is formed by rough particles without a specific geometry. Although these particles have an interlocked phase image, there are holes and cracks among some particles, which allow for retention of PCM molecules. As can be seen from Figure 4(b), the CNTs have an embrangled appearance before the implementation of dispersion process. The eutectic PCM was homogenously retained inside the holes and cracks on the surface of EV (Figure 4(c)). The CNTs were distributed almost uniformly within the EV/(CA–SA) composite (Figure 4(d)) although agglomeration cases were observed in some locations. Consequently, both CPCM and TE-CPCM kept their structural resistance without permitting PCM leakage owing to the capillary and surface tension forces.

Figure 4.

SEM photographs of (a) EV, (b) CNTs, (c) CPCM and (d) TE-CPCM-3.

Chemical characterization results

Figure 5 shows the FT-IR spectrum of CA–SA eutectic mixture, EV, CNTs, CPCM and TE-CPCM-3 selected as an example on behalf of other TE-CPCMs. In the spectrum of CA–SA, the peaks at 2947 cm⁻¹ and 2865 cm⁻¹ represent the stretching vibration of –CH₃ and –CH₂ groups, respectively. The peaks in the range of 3286 cm⁻¹ and 1710 cm⁻¹ are typical stretching vibration band of OH and C = O groups, respectively. The peak at 1468 cm⁻¹ signifies the bending vibration of alkyl groups. The similar bands were observed for SA.⁴⁴ In the spectrum of EV, the symmetric stretching vibration band of Si–OH was observed in the range of 3100–3700 cm⁻¹ as the asymmetric stretching band of Si–O–Si was recorded at 1098 cm⁻¹. The bands detected at 797 cm⁻¹ represent the asymmetric bending vibration of Si–O–Al. The similar peak data were reported by Guan et al.⁴⁵ Moreover, in the spectrum of CNTs, the peak at 1620 cm⁻¹ is assigned with C = C stretching vibration band.⁴⁶

Figure 5.

FT-IR spectra of the EV, eutectic PCM, CNTs, CPCM and TE-CPCM-3.

On the other hand, from the FT-IR spectrum of CPCM, it can be easily observed that all characteristic bands belong to its pure components. However, TE-CPCM-3 shows a further peak at 1624 cm⁻¹ regarding the stretching vibration of C = C of CNTs additive. When also compared with its constituents, it can be noted that any extra peak was not observed, confirming the nonexistence of chemical reaction among the components. Additionally, as can be seen from dotted lines, in consequence of weak physical interactions, little shifts can be noticed in the wavenumbers of some bands, especially –OH, C = O and C–O bands of CA–SA and Si–OH, Si–O–Si and O–Si–O bands of EV.⁴⁷

On the other hand, Figure 6 illustrates the XRD patterns of EV, CA–SA, CPCM and TE-CPCM-3 selected as an example. As seen from these results, EV shows several crystal diffraction peaks at °2θ values of 6.02 °, 7.74 °, 19.29 °, 26.51 °, 28.45 °, 29.89 °, 34.17 °, 36.95 °, 44.78 °, 54.61 ° and 60.10 ° as the eutectic PCM has the characteristic diffraction peaks detected at 5.68 °, 8.66 °, 18.83 °, 21.62 °, 24.14 °, 37.92 °, 40.66 °, 58.35 °and 72.60 °. Moreover, the CNTs have three diffraction peaks seen at 15.52 °, 25.91 ° and 43.12 °. On the other hand, in the XRD patterns of CPCM, the diffraction peaks recorded at °2θ values of 6.03 °, 7.70 °, 19.19 °, 26.40 ° and 36.85 ° are regarded with its EV component while the peaks at °2θ values of 8.75 °, 21.41 °, 23.52 °, 38.51 ° and 60.16 ° are due to its eutectic PCM constituent. In case of TE-CPCM-3 composite, the characteristic diffraction peaks due to CNTs component were seen at 26.41 ° and 44.73 °. Additionally, the negligible changes in intensity and °2θ values of the peaks could be owing to the restriction of the ingredients' crystals between the interlayers of the EV. Accordingly, it can be concluded that the crystal structures of the pure constituents in the composites remained unchanged during the fabrication of the CPCM and TE-CPCMs.

Figure 6.

XRD results of EV, eutectic mixture, CNTs, CPCM and TE-CPCM-3.

The LHTES properties of the prepared CPCM and TE-CPCMs

The DSC curve of CA–SA eutectic mixture is presented in Figure 7 and the determined LHTES properties are also given in Table 3. As clearly observed from the curve, the eutectic PCM has regular shaped phase change peaks, which indicates successful eutectic formation at combination of 82–18 wt%. It melts and solidifies at 24.72 and 23.12℃ while it has relatively high latent heat as 179.43 and −177.48 J/g, respectively. These properties make it promising PCM for passive solar thermoregulation.

Figure 7.

DSC curves of eutectic PCM and form-stable CPCM.

Table 3.

The measured LHTES properties of CA–SA eutectic mixture, CPCM and TE-CPCMs.

Building Material	Melting temperature(℃)	Latent heat of melting (J/g)	Freezing temperature (℃)	Latent heat of freezing (J/g)
Eutectic PCM	24.72	184.43	23.12	−183.12
CPCM	24.54	76.32	23.01	−74.56
TE-CPCM-1	24.39	74.13	22.76	−73.32
TE-CPCM-2	24.64	74.09	23.56	−73.48
TE-CPCM-3	24.35	73.13	23.71	−72.27

The DSC thermograms of the produced CPCM, TE-CPCM-1, TE-CPCM-2 and TE-CPCM-3 are shown in Figure 8. The LHTES data derived from the thermograms are also tabulated in Table 3. The melting and freezing temperatures were measured as 24.54 and 23.01℃, respectively, for CPCM whereas they were measured as 24.39 and 22.76℃ for TE-CPCM-1, 24.64 and 23.56℃ for TE-CPCM-2 and 24.35 and 23.71℃ for TE-CPCM-3. When compared to the eutectic PCM, it can be observed that minor deviations changed between 0.08–0.37℃ for melting and 0.11–0.59℃ for freezing phase change. This phenomenon could have happened as a result of the weak physical interactions among the composite components. The same findings were found for different CPCMs.⁴⁸ Moreover, their melting/freezing temperatures are proper for passive solar thermal management applications.

Figure 8.

DSC curves of TE-CPCMs.

On the other hand, as can be seen from Table 3, the latent heats of melting and freezing were determined as 76.32 and −74.56 J/g for CPCM, 74.13 and −73.32 J/g for TE-CPCM-1 and 74.09 and −73.48 J/g for the TE-CPCM-2 and 73.13 and −72.27 J/g for the TE-CPCM-3, respectively. These values nearly correspond to the calculated values, 75.36, 74.46, 73.02 and 71.59 J/g for melting and −74.54, −73.65, −72.23 and −70.81 J/g for freezing, respectively, which are calculated by multiplying the latent heat of eutectic PCM with its weight fractions in the composite. It means that the eutectic PCM can wholly melt/solidify and even storage/release latent heat up to its full capacity even though it was confined into EV and EV/CNTs structures. In addition, the latent heats of TE-CPCMs were decreased slightly compared to that of CPCM. The reductions were not lower than the theoretically expected. Thus, it can be concluded that the CNTs doping has no remarkably dropping effect on the latent heat capacities of TE-CPCMs.

Table 4 presents the comparison of latent heats of the fabricated CPCM and TE-CPCMs with those of different CPCMs.^{43,45,47–54} According to this comparison, the latent heats of the composites prepared in this study were higher than those of the most composite PCMs in the literature. Moreover, the differences between the data could be due to latent heat capacity and impregnation ratios of PCMs retained by supporting materials. However, the produced CPCM and TE-CPCMs can be considered as hopeful candidates for solar passive heating and cooling systems.

Table 4.

Comparison of melting temperatures and latent heats of CPCM and TE-CPCMs with that of different CPCMs reported in the literature.

CPCM	Melting temperature (℃)	Latent heat of melting (J/g)	References
Paraffin/expanded vermiculite	48.85	101.14	Kong et al.⁴⁵
Capric–myristic acid/vermiculite	19.75	27.46	Karaipekli et al.⁴⁷
Capric–myristic acid/expanded perlite	21.60	56.30	Kong et al.⁴⁹
Lauryl alchol/kaolin	19.14	48.08	Memon et al.⁵⁰
Capric–lauric acid/diatomite	16.74	66.80	Li et al.⁵¹
Capric–palmitic acid/attapulgite	21.71	48.20	Li et al.⁵²
RT20/montmorillonite	23.00	79.30	Fang and Zhang⁵³
Paraffin/bentonite	25.90	34.08	Li et al.⁵⁴
CPCM	24.54	76.32	This work
TE-CPCM-1	24.39	74.13	This work
TE-CPCM-2	24.64	74.09	This work
TE-CPCM-3	24.35	73.13	This work

Cycling LHTES performances and chemical stabilities of the prepared CPCM and TE-CPCMs

The observation of somewhat deviations in the LHTES properties with respect to the thermal cycling is possible. However, PCM should demonstrate long-term LHTES performance in appreciable level so that it can catch better TES performance. By taking into this consideration, the cycling LHTES reliability of the prepared CPCM and TE-CPCMs was examined by DSC analysis after 1000 phase change cycles. The DSC thermogram of only TE-CPCM-3 as an example on behalf of the others was shown in Figure 9, but the LHTES properties of all composites measured after the cycling operation were also presented in Table 5. The melting temperatures of CPCM, TE-CPCM-1, TE-CPCM-2 and TE-CPCM-3 were changed by −0.20, −0.01, −0.21 and −0.17℃, respectively, while the freezing temperatures of them were varied by −0.36, −0.62, −0.19 and −0.07℃, respectively. The reduction amount in the latent heats was in the range of 3.9–5.4% for melting and 2.8–4.3% for freezing of the composites. These results may be due to the partial agglomeration of CNTs into the composite that restricts free crystallization of the molecular chains of fatty acid. These results also mean that there is no significant effect of the CNTs addition on the cycling LHTES performances of the composites.

Figure 9.

DSC curves of TE-CPCM-3 after thermal cycling.

Table 5.

The LHTES properties of CPCM and TE-CPCMs after 1000 thermal cycling.

Material	Melting temperature(℃)	Latent heat of melting (J/g)	Freezing temperature (℃)	Latent heat of freezing (J/g)
CPCM	24.34	72.14	22.65	−72.47
TE-CPCM-1	24.38	71.25	22.14	−70.26
TE-CPCM-2	24.43	70.69	23.37	−71.16
TE-CPCM-3	24.18	70.13	23.64	−69.15

On the other hand, a newly developed PCM should keep its chemical structure even though it can be subjected to a long-term thermal cycling treatment. With this regard, this study was also focused to verify the chemical structures of CPCM and TE-CPCMs after the cycling process. Here, the FT-IR results of CPCM and only TE-CPCM-3 as an example were exhibited in Figure 10 because very similar findings were obtained for the others. As observed from the spectral findings, the profiles and wavenumbers of all key peaks were unchanged after thermal cycling and also further peak was not detected. These results are pointer of excellent cycling chemical stabilities of the CPCM and TE-CPCMs.

Figure 10.

FT-IR spectra of CPCM and TE-CPCM-3 after thermal cycling.

Thermal stabilities of the prepared CPCM and TE-CPCMs

Thermal stability is one of fundamental property to be taken into account in the selection of a pure PCM or CPCM. This property is also assumed as durability against thermal degradation depending on temperature increase. The TGA technique was conducted to estimate thermal stability of EV, eutectic PCM, CPCM and TE-CPCMs and the obtained results were shown in Figure 11. As seen from the curves, the eutectic PCM, it completely degraded until 225℃ while the EV lost only about 5 wt% – part of its total weight up to 800℃. Moreover, the upper limit temperature of first thermal decomposition step was measured to be 225℃ for CPCM, 255℃ for TE-CPCM-1, 260℃ for TE-CPCM-2 and 275℃ for TE-CPCM-3. These results mean that all of the prepared composites had thermally stable at least up to 225℃. In addition, during these steps, the weight loss was carried out as about 41.8, 40.6, 39.2 and 38.3 wt%, respectively, which almost equal to the incorporation amounts of the eutectic PCM in each composite. Thermal degradation temperature of especially TE-CPCM-3 was increased by about 20℃ compared to the CPCM. This might be due to the further mechanical strength provided by CNTs. The similar results were observed for ternary fatty acid mixture/flake graphite (5 wt%)⁵⁵ and solid–solid PCM/graphene oxide composites.⁵⁶

Figure 11.

TG curves of the EV, eutectic PCM, CPCM and TE-CPCMs.

Thermal conductivity increment

The thermal conductivities of the prepared TE-CPCM-1, TE-CPCM-2 and TE-CPCM-3 were measured to be 0.22, 0.37 and 0.43 W/m K, respectively, while it was measured as 0.05, 0.16, 0.12 W/m K for EV, eutectic PCM and CPCM, respectively. These values correspond to significant enhancements in their thermal conductivity as 83.3, 125.0 and 258.3%, respectively, when compared to the CPCM. It can be attributed to ultra-high thermal conductivity of CNTs used as doping agent that cause a remarkably elevated heat transfer area into the CPCM.^26,28,31 However, the improvements in the thermal conductivity were not as high as expected due to the hindrance of the interface thermal conductance in the composites by partial aggregation of CNTs.⁵⁷ On the other hand, the possible mechanism in thermal conductivity increment can be attributed to some reasons.⁵⁸ The increase in the compressed density of CPCM, TE-CPCM-1, TE-CPCM-2 and TE-CPCM-3, 0.953, 0.978, 1.03 and 1.16 g/cm³, respectively, was also responsible for the thermal conductivity improvement of them. The enhancement of thermal conductivity may be due to the formation of network structure in the composite.⁵⁹ Establishment of a network of continuous quasi-2D bundles by progressively pushing CNTs towards the grain boundaries is resulted in an improved thermal conductivity of the composites.⁶⁰ The self-organized bundles of CNTs created by high heat transfer pathways can lead to thermal conductivity enrichment.⁶⁰ Another probable reason is that CNTs have inducing ability to robust crystalline networks and align themselves parallel to the axis of CNT.⁶¹ Moreover, the boost in thermal conductivity of the composites depending on the increase in their densities may be because of the decreased void space within the composite PCM and escalated contact surface area of the composite particles.^62,63 The thermal conductivity change with respect to the compressed density was also confirmed for various CPCMs including expanded graphite.^64–66

In Table 6, the increment ratios in thermal conductivities of the composite PCMs with CNT doping were compared to different clay-based composite PCMs doped with carbon-based additives.^37,67–73 As obviously seen, especially in case of 5 wt% CNTs adding, the increase percentage in thermal conductivity was higher than that of most of the CPCMs. This evaluation exhibits that the improvement in thermal conductivity of the CPCMs can be changed considerably depending on the thermal conductivity of main supporting matrix, additive material and their mass fractions.

Table 6.

Comparison of the effects of different additives on the improvement of thermal conductivities of different CPCMs.

CPCM	Additive	Mass fraction (wt%)	Increase in thermal conductivity (%)	References
Paraffin/diatomite	MWCNT	0.26	42.5	Xu and Li³⁷
Lauric acid/EV	EG	10	78.0	Wen et al.⁶⁷
Palmitic–capric acid/diatomite	EG	5	25.2	Tang et al.⁶⁸
Capric–lauric acid/EV	EG	5	87.4	Wen et al.⁶⁹
Capric–lauric acid/EP	EG	5	89.1	Wen et al.⁶⁹
n-Hexadecane/Na-MMT	xGnP	5	230	Jeong et al.⁷⁰
n-Octadecane/Na-MMT	xGnP	5	254	Jeong et al.⁷⁰
PEG4000/diatomite	EG	10	97.2	Qian et al.⁷¹
PEG/SiO₂	MWCNTs	3	26.7	Tang et al.⁷²
PEG6000/diatomite	SWCNTs	2	260.0	Qian et al.⁷³
TE-CPCM-1	CNTs	1	83.3	This study
TE-CPCM-2	CNTs	3	125.0	This study
TE-CPCM-3	CNTs	5	258.3	This study

In addition, the influence of the increment in the thermal conductivity on the heat absorbing and releasing times of CPCM and TE-CPCMs was also investigated. As demonstrated in Figure 12, the heat charging period was taken as a time interval for temperature increase from 20 to 40℃ while the heat discharging period was considered as a time needed for temperature decrease from 40 to 20℃. The heat charging time was determined as about 300, 225, 200 and 150 s for CPCM, TE-CPCM-1, TE-PCM-2 and TE-CPCM-3 while the discharging time was established as 350, 205, 170 and 120 s, respectively. Compared to the CPCM, the time reductions are clear evidence of the enhancement in the thermal conductivities. On the other hand, a novel construction combination containing TE-CPCMs can be used as an outer plasterboard or external wall over ordinary wall of a building. In this case, the inside temperature will not surpass during the daytime due to the melting of TE-CPCM by absorbing heat. While the outside temperature is diminished, the TE-CPCM is solidified by discharging the stored energy to the indoor of envelope. However, the inside layer with lower thermal conductivity will not allow fast energy input. Therefore, the inside air can be thermo-regulated as well as reducing total energy expenditure of the envelope.

Figure 12.

The effect of enhanced thermal conductivity on the heat charging and discharging rates of CPCM and TE-CPCMs.

Conclusion

In order to simultaneously solve storage requirement against leakage problem and low thermal conductivity of CA–SA eutectic mixture, it was first retained into EV by means of melting/blending method and then doped with CNTs. The microstructure, physicochemical compatibility, LHTES properties, thermal conductivities, cycling performances and thermal stabilities of the prepared CPCMs and TE-CPCMs were investigated. The following conclusions can be withdrawn from the results:

The CA–SA eutectic mixture was successfully absorbed by EV in maximum fraction of 40 wt%. This composite was identified as form-stable CPCM.

TE-CPCMs were prepared by doping of form-stable CPCM with CNTs in mass fraction of 1, 3 and 5 wt%. The physicochemical compatibility between the composites' components was confirmed by using FT-IR, XRD and SEM analysis.

The DSC results indicated that the CPCM and TE-CPCMs have phase change temperature in the range of 24.35–24.64℃ for melting and 22.76–23.71℃ for freezing as they showed the LHTES capacity changed from 76.32 to 73.13 J/g for melting and from −72.27 to −74.56 J/g for freezing.

The composite PCMs with/without CNTs exhibited excellent cycling chemical stability and LHTES performance.

The TGA findings revealed that all of the composites had relatively high thermal stability.

The thermal conductivity of form-stable CPCM was increased by 83.3, 125.0 and 258.3% by CNTs doping, 1, 3 and 5 wt%, respectively.

The heat charging–discharging times of TE-CPCMs were appreciably shortened due to the improved thermal conductivity.

Especially TE-CPCMs have a great potential for the fabrication of new construction materials in different types such as brick, plaster, concrete and wallboard, which can be utilized for thermoregulation of building envelopes depending on the climate conditions.

Advance treatments should be conducted to assess real-scale thermal performance of the produced TE-CPCMs under actual climatic circumstances.

Additionally, in fabrication stage of such type composites in practice scale for realistic TES applications, the amount and purity degree of CNTs used as additive material should be cautiously chosen by bearing in mind its high cost.

Footnotes

Acknowledgements

The authors would like to thank the commission of scientific research projects in Gaziosmanpaşa University (Project Number: 2016/75) and Karadeniz Technical University (Project number: FBA-2017-6863). A Sarı also would like to thank King Fahd University of Petroleum & Mineral for their laboratory facilities.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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