Introduction

In recent years, the shortages of fossil fuels and increasing energy depletion have been a big trouble for the development of the human society. Thus, efficient ways of energy storage and recovery as well as the investigation of new energy resources have attracted much attention [1,2,3]. Phase change materials have an important role for various applications in air-conditioning systems [4,5,6,7], building energy conversation [8,9,10] and the solar heating systems [11,12,13]. The various available PCMs melt and solidify at a broad range of temperatures, which are categorized into two main groups; organic (paraffin, fatty acids/esters etc.) and inorganic (salt hydrates) materials. When organic PCMs compared with inorganic PCMs, organic PCMs melt and freeze repeatedly without degradation of their latent heat of fusion and phase segregation. Organic PCMs have high latent heat storage capacity, low vapor pressure, self-nucleating behavior, good thermal and chemical stability [14,15,16]. However, they need encapsulating to avoid leakage [17, 18], of the melted PCMs and high volume changes [19] during the phase change process. This process can be performed by encapsulating of the PCM in a polymeric structure, such as styrene–butadiene–styrene (SBS), copolymer [20] and high-density polyethylene (HDPE) [21,22,23], melamine–formaldehyde (MF) resin [24] and urea–formaldehyde (UF) resin [25]. While using polymeric shell in particular application, polymer might be inclined to degrade and release noxious gas continuously that leads to health and environmental problems. In addition of these, some applications of polymeric shells are limited because of their poor thermal stability low heat conductivity and mechanical strength [26], flammability [27]. So, using the inorganic shells has been achieved great interest recently [28]. The reactivity of the PCM within the outside environment can be decreased by encapsulating with an inorganic shell [29]. The TiO2 shell materials have high surface area and tunable pore size with good thermal stability, lower the flammability and no leakage during melting of PCMs. In situ polymerization [30], spray drying, complex conservation [31], interfacial polymerization [29], miniemulsion polymerization [32] and sol–gel method are encapsulating methods for PCMs. But, the fluid’s viscosity of the large particle sizes of the microencapsulated PCMs is not appropriate for repeated cycling in functionally thermal fluids fields. So, PCM nanocapsules must be prepared with smaller particle sizes as compared with microcapsules [29].

This study, therefore, has focused on synthesizing nanocapsules of palmitic acid with silica shell via the sol–gel method and studying the thermal characterization. Myristic acid is favorable organic PCM for thermal energy storage, melting at 62.50 °C with a latent heat of 178.7 kJ kg−1, and solidifying at 42.32 °C with a latent heat of 177.5 kJ kg−1 [33, 34]. Fly ash is used for investigation of building application of myristic acid/TiO2. Fly ash, a byproduct of coal combustion, can be recycled. While fly ash’s safety is up for debate, it is considered a sustainable material when reused in bricks or other building materials. As far as the authors are aware, there is no investigation reported in the literature on the chemical and morphological characterization, particle size distribution, thermal stability and long-term reliability of the myristic acid-fly ash/TiO2 for the potential application in buildings.

Experimental

Materials

All chemical were analytical grade and used without further purification. Myristic acid, tetra-n-butyl titanate (TNBT) and sodium dodecyl sulfate (SDS) were purchased from Alfa Aesar. Fly ash was provided from Technology Faculty in Firat University.

The preparation of MPCM/TiO2 shell

Myristic acid (10 g) and SDS (1.75 g) were added into deionized water to obtain an oil/water (o/w) emulsion in a beaker. The emulsion is kept at 60 °C for 60 min under constant magnetic stirring. After the completion of emulsification process, 12.5 TNBT and 50 mL absolute EtOH solution was added drop-wise under magnetic stirring. The solution was kept at 60 °C for 12 h to complete the hydrolysis and condensation of TNBT. The resultant microcapsules were washed and dried in a vacuum over at 40 °C for 24 h.

The preparation of MPCM/TiO2 shell–fly ash

The myristic acid/fly ash composite sample used in this work was manufactured by the direct impregnation method. During the preparation of composites, firstly phase change material and fly ash were heated at 60 °C until the myristic acid was completely melted and mixed with hand. In the next step, stirring was continued with a mechanical stirrer approximately 2 h. At the end of process, the sol–gel mixture was obtained according to the preparation of MPCM/TiO2 shell procedure.

Characterization of MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash

The structural analysis of MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash was analyzed by using FT-IR spectrometer. The spectra were recorded on a Perkin-Elmer infrared spectrometer as ATR-IR with a resolution of 4 cm−1 in the range of 650–4000 cm−1. The morphology and microstructure of the MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash were observed using a scanning electronic microscope (SEM, LEO 440 model). The thermal properties of PCMs were obtained using a differential scanning calorimeter (DSC, Shimadzu 60 WS), and DSC curve is tested with a heating rate of 10 °C/min in nitrogen atmosphere. The thermal stability of MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash was studied by means of thermogravimetry on a Shimadzu TA-60WS from room temperature to 600 °C with a heating rate of 10 °C min−1 in air atmosphere. The XRD measurements were taken in a Rigaku RadB-DMAX II diffractometer using Cu–Ka (k = 1.5405 Å) radiation. The thermal reliability of the MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash was evaluated with respect to change in the phase temperatures and latent heats after large number of thermal cycling. The thermal cycling was done by Applied Biosystem 96-well Thermal Cycler.

Results and discussion

FT-IR analysis of MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash

The FT-IR spectra of myristic acid, MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash are shown in Fig. 1. Characteristic vibration bands of myristic acid, TiO2 and fly ash can be clearly seen in the curve of MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash. In the spectrum of the myristic acid, the peaks at 2958–2851 cm−1 can be assigned to the stretching vibrations of aliphatic – CH groups. Beside these peaks, 1469–1198 cm−1 is ascribed to the bending vibrations of – CH2, – CH and C–C groups, respectively [35]. The peaks at 1685 cm−1 can be ascribed to the stretching vibration of the C=O group in myristic acid. The peak at 1654 cm−1 is the characteristic absorption peak for the stretching vibration of C=C group [36]. The peak at 1533 cm−1 is the characteristic absorption peak for TiO2. According to these data, there is no chemical reaction between myristic acid, TiO2 shell and fly ash.

Fig. 1
figure 1

FT-IR spectrum of myristic acid (MA), MPCM–TiO2 shell and MPCM/TiO2 shell–fly ash

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XRD patterns of the microencapsulated myristic acid with TiO2 shell and MPCM/TiO2 shell–fly ash

Figure 2 illustrates the XRD patterns of myristic acid, TiO2, MPCM–TiO2 shell and MPCM/TiO2 shell–fly ash. The XRD pattern of myristic acid is present in Fig. 2a. The two sharp peaks at 8° and 14° are attributed to the crystallization of the myristic acid [37]. In Fig. 2b, there is a flat peak at around 24° that indicates that TiO2 shell has an amorphous structure. As seen in Fig. 2c, d, the XRD patterns of the MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash include both the peaks of the myristic acid and the flat peak of the TiO2. This result illustrates that crystal structure of the myristic acid remains unchanged during the synthesis process.

Fig. 2
figure 2

XRD spectra of a myristic acid, b TiO2, c MPCM–TiO2 shell and d MPCM/TiO2 shell–fly ash

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Microstructure and morphology of the microencapsulated myristic acid with TiO2 shell and MPCM/TiO2 shell–fly ash

Figure 3 shows the SEM photograph of MPCM–TiO2 shell and MPCM/TiO2 shell–fly ash. As shown in Fig. 3, the myristic acid is well encapsulated in the TiO2 shell and the TiO2 shell avoids the myristic acid from leaking although the myristic acid was melted by the electric beam when SEM analyses were taken. The SEM photographs of all two samples illustrate the spherical structure of nanocapsules and also their smooth surfaces. Comparing Fig. 3b, c, it could be seen that the surfaces of the MPCM/TiO2 shell–fly ash are more rippled than that of the MPCM/TiO2 shell. The reason could be explained as follows: contraction coefficient of fly ash and the lower density of the TiO2 compared with the MPCM/TiO2 may lead to the shrinkage of the shells when the microcapsules cooled down, and thus ripples appeared. Beside these, the excessive TiO2 oligomers agglomerate on the surface of the microcapsules.

Fig. 3
figure 3

SEM images of a, b MPCM/TiO2 shell, c, d MPCM/TiO2 shell–fly ash

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Thermal properties of the MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash

The thermal properties of the MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash were determined by using differential scanning calorimetry. The DSC curves, the phase change temperatures and enthalpies of the PCMs are illustrated in Figs. 3–5 and Table 1. The melting and solidifying temperatures of the myristic acid are 62.50 and 42.32 °C, respectively. The peak temperature values of MPCM–TiO2 shell and MPCM/TiO2 shell–fly ash were determined as 56.95 and 52.22 °C for melting phase change; 47.56 and 26.68 °C for freezing phase change. As shown in Table 1, MPCM–TiO2 shell has higher latent heat value than MPCM/TiO2 shell–fly ash in the melting and solidifying processes.

Fig. 4
figure 4

DSC spectrum ofthe myristic acid

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Fig. 5
figure 5

DSC spectrum of the MPCM/TiO2 shell

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Table 1 The changes in thermal properties of the MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash
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According to the DSC results, phase change temperatures of the MPCM/TiO2 shell–fly ash are slightly lower for freezing. This is the result from the high thermal conductivity of the matrix material. As seen in Table 1, the melting and solidifying latent heats of the composites drop with the decrease in the myristic acid content because in thermal energy storage systems, myristic acid plays an important role in operating systems.

For these reasons, the thermal properties of PCMs must be determined by using the encapsulating ratio and encapsulation efficiency. They can be defined according to equations shown below [38,39,40,41]

$$\omega = \frac{{\Delta H_{{{\text{m}},{\text{MPCMs}}}} + \Delta H_{{{\text{c}}, {\text{MPCMs}}}} }}{{\Delta H_{{{\text{m}},{\text{myristic acid}}}} + \Delta H_{{{\text{c}}, {\text{myristic}}\;{\text{acid}}}} }}$$

where \(\Delta H_{{{\text{m}},{\text{MPCM}}{-}{\text{TiO}}_{2} }}\) and \(\Delta H_{{{\text{s}},{\text{MPCM}}{-}{\text{TiO}}_{2} }}\) are the latent heat of melting and crystallization of the microencapsulated MPCM/TiO2 shell, ΔHm,myristic acid and ΔHc,myristic acid latent heat of melting and solidification of the pure myristic acid, respectively, for the encapsulating ratio calculations of the MPCM/TiO2 shell–fly ash. The encapsulating ratio indicates the effective performance of the PCMs inside the capsules, so by increasing the encapsulation ratio, leaks proof characteristic of microcapsules decrease and the mechanical strength. The melting and crystallizing enthalpies of the MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash were found to be 96.64 and 97.72 kJ kg−1, respectively, which was slightly lower than the expected values according to the myristic acid content (54.11%). The difference might be explained by thermal isolation effect of the titanium shell. It is notable in Table 1 that the encapsulating values of MPCM/TiO2 shell–fly ash are much lower than MPCM/TiO2 shell indicating that not all of titania precursors (TBT) have converted into TiO2 shell in the sol–gel process. Actually, some of TBT were directly precipitated as solid particle rather than assembled on the surfaces of myristic acid-fly ash droplets through the synthesis process, resulting in the lower encapsulating efficiency of myristic acid. According to these results, the latent heats of melting and freezing of MPCM/TiO2 shell–fly ash are 23.43 and 22.57 kJ kg−1, respectively.

Thermal stability of the myristic acid, MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash

TGA was used to determine thermal stability of the prepared composite PCMs. Figure 7 illustrates the mass loss curves of myristic acid, MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash. There is a single degradation process 75.4–270 °C for myristic acid because of the decomposition of fatty acid. MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash rarely in the temperature range, in the temperature range, and the initial decomposition temperatures around 240 and 260 °C. Furthermore, no decomposition is observed for both MPCM/TiO2 shell and MPCM/TiO2 shell–fly within 100 °C, showing high thermal stability in low-temperature applications. Moreover, the mass loss ratio of the MPCM/TiO2 shell and MPCM/TiO2 are about 76.6 and 72.8%, respectively.

Fig. 6
figure 6

DSC spectrum of the MPCM/TiO2 shell–fly ash

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Thermal reliability of the MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash

Leakage must be avoided during the phase change, if one would like to use PCM-containing microcapsules for heat storage. To analyze the potential leakage, the microcapsules were 1000 times heated and subsequently cooled. The heat storage and chemical decompositions of the MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash were investigated by DSC and FT-IR after thermal cycles. As seen in Fig. 8, the frequency values of the characteristic peaks unchanged after 1000 thermal cycling. These results show that the chemical structures of the PCMs were not affected by thermal cycling. It was presumed that if the heat capacity of microcapsules did not change after thermal cycling test, the PCM did not leakage from the composites.

Fig. 7
figure 7

TGA spectrum of the Myristic acid, MPCM–TiO2 shell, MPCM/TiO2 shell fly ash

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In the event of leakage, significant decrease in heat capacity could be observed. In contrast, the melting and crystallizing enthalpies of MPCM/TiO2 shell (Fig. 9) decreased only slightly after 1000 heating–cooling cycles, although the heat storage capacity reduction of MPCM/TiO2 shell–fly ash was not negligible. This result confirms that the myristic acid-fly ash adsorption is weak than that of the myristic acid, and the leakage of latter MPCM/TiO2 shell–fly ash may be higher than acceptable for real use (Table 2).

Fig. 8
figure 8

FT-IR spectrum of myristic acid (MA), MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash after 1000 thermal cycling

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Fig. 9
figure 9

DSC spectrum of the MPCM/TiO2 shell, myristic acid and MPCM/TiO2 shell-fly ash after 1000 thermal cycling

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Table 2 The changes in thermal properties of the MPCM/TiO2 shell and MPCM/TiO2 shell–fly ash after 1000 thermal cycler
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Conclusions

In this work, shape-stabilized microencapsulated phase change materials were prepared by using sol–gel method. In the microcapsules, the myristic acid acted as phase change material for thermal energy storage, TiO2 was used as shell material to prevent the myristic acid from leakage and improve the thermal stability of the microcapsules, and fly ash was added to determine of using as a building material. The myristic acid was well microencapsulated with TiO2 shell, and there was no chemical interaction between the myristic acid, TiO2 and fly ash and the crystal structure of them remained unchanged. In accordance with FT-IR, SEM and XRD results, the myristic acid is well encapsulated, melts at 56.95 °C with a latent heat of 96.64 kJ kg−1 and solidifies 47.56 °C with a latent heat of 97.72 kJ kg−1. Beside these, MPCM/TiO2 shell–fly ash melts at 52.22 °C with a latent heat of 23.43 kJ kg−1 and solidifies 26.68 °C with a latent heat of 22.57 kJ kg−1. The well formed TiO2 shell has good thermal reliability and can prevent the leakage of the microencapsulated MPCM/TiO2 shell while MPCM/TiO2 shell–fly ash is not revealed good thermal reliability. Based on all results, MPCM/TiO2 shell could be used as shape-stabilized PCMs for thermal energy storage.