Analysis of thermal properties of gypsum materials incorporated with microencapsulated phase change materials based on silica

Renewable Energy 149 (2020) 400e408

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Renewable Energy journal homepage: www.elsevier.com/locate/renene

Analysis of thermal properties of gypsum materials incorporated with microencapsulated phase change materials based on silica Yi Zhang a, *, Wen Tao a, Kehan Wang a, Dongxu Li b a b

School of Material Science and Engineering, Anhui University of Technology, Anhui, Maanshan, 243002, PR China College of Material Science and Engineering, Nanjing Tech University, Jiangsu, Nanjing, 211816, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 August 2019 Received in revised form 19 November 2019 Accepted 10 December 2019 Available online 11 December 2019

Microencapsulated phase change materials (MePCMs) were prepared in this study through sol-gel synthesis; these materials consisted of SiO2 as shell and paraffin as core materials, and were subsequently incorporated in gypsum. Further, this study examined the thermal regulation performance of gypsum materials. Fourier transform infrared spectra confirmed the encapsulation of the paraffin with SiO2 shell. Scanning electron microscope (SEM) results showed that the microcapsules obtained at a pH of 2.5 exhibit a regular spherical structure, excellent dispersibility, and compact surface structure. The MePCMs were found to reveal high thermal performance and encapsulation efficiency even in this synthetic condition. This is because the SiO2 shell with its high compactness imparts high thermal stability and a good anti-osmosis performance to the microcapsules. Further, gypsum materials achieved high mechanical strength, compactness, thermal conductivity, and a good thermal regulation performance with the weight ratio of MePCMs at 10 wt%. The results showed that the MePCMs in the gypsum matrix exhibited advantageous application prospects for the thermal regulation in buildings. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Microencapsules Phase change materials Micromorphology Latent heat Mechanical performance Temperature regulation

1. Introduction Phase change materials (PCMs) absorb heat during daytime and release it during nighttime [1,2]; this can provide a high density of heat storage within small temperature ranges. PCMs are effectively used to adjust the time difference between energy supply and demand. Thus far, different PCMs such as inorganic salt hydrate, paraffin, and fatty acids have been widely studied [3e5]. An inorganic salt hydrate has high thermal conductivity and phase change latent heat; however, some problems such as high corrosivity, apparent phase separation, and supercooling phenomenon during phase transitions still exist [6e8]. Organic paraffin and fatty acids exhibit high latent heat, high thermal stability, and little supercooling to provide a wider application scope for these materials [9,10]. The excellent latent heat of the PCMs renders them suitable for application in building materials while minimzing the use of energy. Further, the incorporation of PCMs into building materials can increase the thermal inertia of building envelopes [11,12], reduce temperature fluctuations, and improve indoor thermal comfort [13]. PCMs can be compounded with gypsum or concrete through

* Corresponding author. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.renene.2019.12.051 0960-1481/© 2019 Elsevier Ltd. All rights reserved.

impregnation or direct mixing method [14,15]; however, the dispersion and leakage are the primary problems that need to be resolved for improving the thermal storage performance. To improve the dispersibility of the PCMs in the building matrix, some researchers have employed the use of porous minerals materials [16,17]; further, they have used graphite [18] or inorganic polymers with a framework [19] as supporting materials to prepare shapestabilized PCMs. PCMs can be absorbed effectively through these methods, and this could reduce the probability of their leakage from the building matrix [20,21]. Compared with shape-stabilized PCMs, the microencapsulated PCMs (MePCMs) in a shell are an effective way to overcome the aforementioned problems. PCMs can be converted to small particles with a particle size of 1e1000 mm through the microencapsulation method; this could improve the dispersion performance and the heat transfer area of PCMs, and prevent them from leakage in the phase change process [22,23]. Zhang et al. [24] prepared gypsum boards that were incorporated with MePCMs; the results showed that the MePCMs were well dispersed, and the gypsum boards displayed good thermal stability and high thermal capacity when 50 wt% of MePCMs were incorporated. Toppi et al. [25] experimentally investigated the thermal performance of the gypsum materials blended with the commercial MePCMs (MICRONAL).

Y. Zhang et al. / Renewable Energy 149 (2020) 400e408

Nomenclature c T

specific heat capacity temperature

Greek symbols l thermal conductivity a thermal diffusivity r density DH enthalpy

401

research, MePCMs with SiO2 shell and paraffin core were synthesized through the sol-gel method. The chemical structure, particle size distribution, micromorphology, and phase change latent heat of the prepared MePCMs were investigated. Further, the pH value on the encapsulation efficiency and thermal stability of the MePCMs was also investigated. The MePCMs comprising a good morphology, high phase change latent heat, and high thermal stability were susbequently prepared. The mechanical strength and thermal performance of the gypsum materials were elucidated after the incorporation of MePCMs. 2. Experimental methodology

Abbreviations Lr leakage rate E encapsulation efficiency PCMs phase change materials MePCMs microencapsulated phase change materials

With the increasing amount of MePCMs, the thermal conductivity of the gypsum material was reduced but its specific heat increased. Joulin et al. [26] researched the thermophysical properties of the cement mortar incorporated with commercial MePCMs (DS 5001 X). The result showed that the incorporation of the MePCMs is an effective way to increase the apparent specific and energy storage capacity of the cement mortar. PCMs can be encapsulated by an organic shell such as melamine-formaldehyde resin [27], polyurea resin [28], and acrylic resin [29] using different synthesis methods including in-situ polymerization, interfacial polymerization, and suspension polymerization [30]. However, there are some disadvantages of organic shells that impede their application in the building materials; these disadvantages include low thermal conductivity, poor thermal stability, and flammability. An inorganic shell possesses better thermal conductivity and thermal resistance than the organic ones. Furthermore, an inorganic shell showed better compatibility with the gypsum and cement building materials; this increased the durability of the MePCMs in the building matrix. Zhang et al. [31] synthesized the MePCMs with a SiO2 shell to enhance the thermal conductivity and phase change performance; further, tetraethyl orthosilicate (TEOS) was used as an inorganic source and the silica shell was synthesized through the sol-gel process. He [32] used sodium silicate (Na2SiO3) as a precursor to prepare MePCMs with SiO2 shell. The results showed that the SiO2 shell with a structure that exhibits better compactness and permeability resistance was obtained by regulating the pH value. Yu et al. [33] prepared the MePCMs with inorganic CaCO3 shells through a self-assembly method; it was found that the thermal conductivity and durability of the prepared microcapsules were significantly improved under the encapsulation of the CaCO3 shell. The good dispersibility of the MePCMs is beneficial for the absorption and release of the latent heat storage when combined with the building materials. The inorganic microcapsule shell material exhibits better compatibility with building materials and good thermal conductivity as well as thermal stability; this is the trend observed in the development of building energy-saving PCMs. The shell material should have a higher encapsulation ratio to ensure higher latent heat of the prepared MePCMs, and the phase change energy building materials should have a better phase change energy storage performance when the MePCMs are incorporated. Currently, there is no research that focuses on the MePCMs with the SiO2 shell and their composite in the building materials. Extensive studies have proposed that the structure and properties of microcapsules can be regulated by adjusting the pH value. In this

2.1. Materials The paraffin PCMs were made of the mixture of solid paraffin (melting temperature: 56
C, Yonghua Paraffin Co., LTD) and liquid paraffin (Xilong Chemical Co., LTD) mixed in the mass ratio of 1:1. Tetraethoxysilane (TEOS, analytical reagent grade, Sinopharm Chemical Reagent Co., LTD) was used as the precursor. Anhydrous ethyl alcohol (EtOH, analytical reagent grade, Sinopharm Chemical Reagent Co., LTD) was used as solvent while hydrochloric acid (chemical pure grade, Sinopharm Chemical Reagent Co., LTD) was adopted as the pH regulator. Cetyl trimethyl ammonium bromide (CTAB, analytical reagent grade, Aladdin Chemical Reagent Co., LTD), with an HLB (hydrophile-lipophile balance) value of 15.6, was used as the paraffin emulsifier. The hydrochloric acid solution was diluted to a concentration of 1 wt%. The gypsum was offered by Jiangsu Efful Science and Technology Co., LTD. 2.2. Preparation of the MePCMs with SiO2 shell 2.2.1. Preparation of paraffin core emulsion Initially, 5 g solid and 5 g liquid paraffin were melted and mixed evenly to obtain the phase change paraffin. CTAB was added to the mixed paraffin and the 30 ml of deionized water was slowly added to the mixture in 30 min while under 70
C constant water bath. The mixture was stirred using a high-shear dispersion homogenizer at a speed of 2000 r/min; as a result, a stable oil in water (O/ W) paraffin emulsion was obtained. 2.2.2. Synthesis of MePCMs To synthesize MePCMs, 10.4 g of TEOS, 20 g of distilled water and 40 g ethyl alcohol were mixed and stirred at a rate of 400 r/min in a sealed 3-neck flask under 35
C water bath; the pH values of the mixture were respectively adjusted to 2, 2.5, 3 by regulating the HCl solution. The mixture was continuously stirred for about 30 min until a transparent sol solution was formed, and in this process, parts of TEOS were hydrolyzed to form oligomers as the silica precursors. The sol solution was added dropwise into the paraffin emulsion and stirred for 24 h at a constant stirring speed of 400 r/ min while the condensation reaction of the silica sol was performed, resulting in the formation of the silica shell. After the reaction, the suspension of MePCMs was filtered, washed with EtOH repeatedly, and dried to obtain the MePCMs particles. MePCMs that were prepared with different pH values are denoted as MeP-2, MeP-2.5, and MeP-3. 2.3. Preparation of the gypsum materials incorporated with MePCMs As shown in Table 1, gypsum materials incorporated with different contents of the MePCMs were prepared. Samples with a size of 40  40  160 mm were prepared, and the mechanical performance of the gypsum composite materials was measured.

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Y. Zhang et al. / Renewable Energy 149 (2020) 400e408

Table 1 Composition of the gypsum materials incorporated with the MePCMs. Sample code

MeP-G0

MeP-G3

MeP-G5

MeP-G10

MeP-G20

Proportion of microcapsules (wt %) Proportion of gypsum (wt %)

0 100

3 97

5 95

10 90

20 80

2.4. Characterization

3. Results and discussion

The chemical structure of the prepared MePCMs was characterized by Fourier-transform infrared spectroscopy (FTIR; Nicolet 6700), with a scanning speed of 10 cm1/s and a wavelength of 4000e400 cm1. The particle size distribution of the MePCMs was tested by a laser particle size analyzer (Malvern Mastersizer Hydro 2000MU) at a detection range of 20 nme2000 mm. Thermal stability of the MePCMs was evaluated by TGA (Shimadzu DTG-60H) at a heating rate of 10
C/min from room temperature to 900
C under nitrogen atmosphere. The surface morphology of the MePCMs and the gypsum materials were observed using a field emission scanning electron microscope (FESEM; FEI, NANO SEM430). Thermal properties of the MePCMs were determined by a differential scanning calorimeter (DSC; Netzsch 200f3) in the range of 0e70
C at a heating or cooling rate of ±10
C/min under nitrogen atmosphere. The thermal stability of the MePCMs was measured in the drying oven. Samples with a certain quality m0 (z1.5 g) were put on filter papers and placed into the oven at a temperature of 60
C. The samples were taken out and put back into the oven every few hours repeatedly to weigh their mass and replace the filter papers. The mass of the samples after each cycle was labeled as mn, and the leakage rate of the samples was calculated by the following equation:

3.1. FTIR spectra

Lrð%Þ ¼

m0  mn m0

(1)

The thermal conductivity of the gypsum materials incorporated with the MePCMs was measured by a hot disk thermal conductivity instrument (DRE-2C, Xiangtan Xiangyi instrument Co., LTD) based on the transient plane source method using a probe with a diameter of about 20 mm. Before the test, the probe was placed between 2 slabs of each sample and compacted. The thickness of the sample was not less than 10 mm, and each sample was tested 3 times to obtain the average value. The thermal conductivity l is calculated as follows:

FTIR spectra of the paraffin and the MePCMs are shown in Fig. 1. From the spectra of paraffin, the transmittance peaks at 2900 cm1 and 2850 cm1 correspond to the stretching vibration of eCH3 and eCH2 groups, respectively. The peaks at 1490 cm1 and 1360 cm1 represent the bending vibration of eCH2 group. The characterization absorption peak at 720 cm1 reflects the rocking vibration of eCH2. From the spectra of the MePCMs, the peak at 464.3 cm1 is assigned to the bending vibration of SieOeSi stretching vibrations, and the peaks at 1081.5 cm1 and 800.1 cm1 represent the asymmetrical and symmetrical stretching vibration of SieOeSi, respectively. The peak at 962.3 cm1 is attributed to the bending vibration of SieOH, and the absorption bands at 3390 cm1 and 1625 cm1 are characteristic absorption peaks of the SieOH stretching vibrations. The absorption peaks of SiO2 and paraffin can be clearly seen from the spectra of the prepared MePCMs, indicating that there was no chemical reaction between the core and shell materials. The above results confirmed the successful encapsulation of the SiO2 shell. 3.2. Phase change performance Phase change temperature and latent heat of the paraffin and the MePCMs in melting and solidification processes are shown in Fig. 2. The upper curves are the endothermic curves while the lower curves are the exothermic ones. The encapsulation efficiency(E) was used to describe the phase change performance of the microencapsulated PCMs. The parameter can be calculated through the following Equation (3.1):

E ¼

DHm; MePCMs þ DHc; MePCMs  100% DHm; PCMs þ DHc; PCMs

(3.1)

where, DHm,MePCMs and DHc,MePCMs are the melting and

Paraffin

(2)

where, a represents the thermal diffusivity of a sample; c represents the specific heat capacity of a sample; r indicates the density of a sample. For the thermal regulation performance test of the gypsum materials, the K-type thermocouples were pre-inserted into the gypsum blocks (40  40  40 mm) before the hardening of the gypsum. The samples were put into an air-blast drying oven at a constant temperature of 50
C; after the melting process, all of the samples were taken out and put into a sealed bag. The cooling process was carried out by immersing the sealed bag containing the gypsum blocks in a 10
C water environment. The temperature changes were monitored throughout the process by the thermocouples and recorded every 10 s by a data recording instrument that connect to the thermocouples.

Transmittance(%)

l ¼ a,c,r

MeP-2

MeP-2.5

MeP-3

4000

3500

3000

2500

2000

-1 Wavenumber(cm )

1500

1000

500

Fig. 1. FTIR patterns of the phase change paraffin and the prepared MePCMs.

Y. Zhang et al. / Renewable Energy 149 (2020) 400e408

6.5 6.0

6.0

(a)

Exothermic process

5.5

Paraffin

5.5

Heat Flow (W/g)

Heat Flow (W/g)

MeP-2

4.0 3.5

MeP-2.5

3.0

Paraffin

4.5

MeP-2

4.0 3.5

MeP-2.5

3.0 2.5

MeP-3

1.5

2.0 1.5

Endothermic process

2.0

MeP-3

2.5

(b)

5.0

5.0 4.5

403

1.0 0

10

20

30

40

50

60

70

0

10

20

Temperature ( C)

30

40

50

60

70

Temperature ( C)

Fig. 2. DSC curves of MePCMs prepared at different pH conditions (a) heating process, (b) cooling process.

crystallization enthalpy of the MePCMs, respectively; DHm,PCMs and DHc,PCMs are the melting and crystallization enthalpy of the paraffin, respectively. The phase change enthalpy in the melting and crystallization processes and the encapsulation efficiency of paraffin core material are shown in Table 2. The melting and crystallization enthalpies of the pure paraffin core are 70.13 J/g and 73.61 J/g, respectively. The latent heat of the MePCMs had a close relationship with the pH conditions. The latent heat of MeP-2.5 exhibited a higher encapsulation efficiency of about 83.9%, while the melting enthalpy and crystallization enthalpies are 61.12 J/g and 59.52 J/g, respectively. MeP-3 also presented a high encapsulation efficiency of 74.7% as well as encapsulation of the silica shell. The latent heat of the MeP2 is the lowest compared with the other two samples, which was ascribed to the incomplete encapsulation of the MePCMs that synthesized in this pH condition. The results showed that the silica condensation performed under the condition of a pH of 2.5 is more effective for the encapsulation of the paraffin materials.

the surface of the paraffin core because some of the silica were selfcondensed, leading to the inadequate encapsulation and smaller latent heat of the MePCMs. The rough surface of MeP-3 was caused by the fast condensation reaction of silica oligomers that deposited on the surface of the paraffin core. The PCMs were fully encapsulated and the prepared microcapsules were displayed with a high encapsulation efficiency. For the MeP-2.5, the regular surface morphology of the MePCMs was showed and a thick silica shell was formed under this condition. The MePCMs with a dense structure were synthesized under this condition. It can be inferred that the condensation of the silica oligomers was done stepwise in this pH condition, and the MePCMs with better compactness of the silica shell were formed. Table 3 compares the thermal properties of the prepared

16

MeP-2 MeP-2.5 MeP-3

14 12

The particle size distribution of MePCMs is shown in Fig. 3. The average particle sizes for MeP-2, MeP-2.5, and MeP-3 were 7.3 mm, 9.6 mm, and 15.6 mm, respectively. The MePCMs prepared under the condition of a pH of 2 (MeP-2) had a smaller particle size and narrow distribution, while the MeP-3 prepared under a pH of 3 had a larger particle size and wider distribution. In contrast, the MePCMs prepared under the condition of pH of 2.5 had a more concentrated particle size distribution than the two samples. SEM images of MePCMs are shown in Fig. 4; the prepared microcapsules presented a spherical morphology. MeP-2 exhibited a smooth surface and demonstrated a small deposition amount of the silica shell on the paraffin droplets. The shell of the MePCMs is thin, and the condensation of silica sol was not completely deposited on

10

Volume (%)

3.3. Particle size distribution and micromorphology of the MePCMs

8 6 4 2

0 0.1

1

Particle size (μm)

10

100

Fig. 3. The particle size distribution of MePCMs prepared at different pH conditions.

Table 2 Phase change temperature and latent heat of the paraffin and the MePCMs. Sample

Tom (
C)

Tpm (
C)

Tem (
C)

DHm (J/g)

Tec (
C)

Tpc (
C)

Toc (
C)

DHc (J/g)

Encapsulation efficiency (%)

Paraffin MeP-2 MeP-2.5 MeP-3

21.9 24.2 23.9 23.3

34.8 33.4 35.6 37.2

40.7 42.1 42.3 42.1

70.13 38.75 61.12 57.78

19.1 21.1 19.7 19.3

30.4 31.5 30.7 29.4

40.1 37.9 35.6 38.8

73.61 38.98 59.52 52.97

100 52.8 83.9 74.7

*Tom, onset melting temperature; Tpm, the peak temperature of melting point; Tem, end melting temperature; DHm, melting enthalpy; Toc, onset crystallizing temperature; Tpc, the peak temperature of onset crystallization point; Tec, end crystallizing temperature; DHc, crystallization enthalpy.

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Y. Zhang et al. / Renewable Energy 149 (2020) 400e408

Fig. 4. SEM image of the MePCMs prepared under different pH conditions.

Table 3 Comparison of thermal properties of the MePCM prepared with the results in the literature. MePCMs

Melting point (
C)

Freezing point (
C)

Latent heat (J/g)

Encapsulation ratio (%)

Reference

n-octadecane/SiO2 n-octadecane/SiO2 n-octadecane/CaCO3 Paraffin/SiO2 Capric-stearic acid/SiO2 Palmitic acid/SiO2 Paraffin/SiO2

27.1 27.9 23.4 58.3 21.4 61.6 35.6

22.1 23.7 29.1 57.02 22.2 57.08 30.7

185.2 86.1 40.0 165.6 91.4 181.06 59.52

86.4 41.4 83.2 87.5 56.7 88.32 83.9

[31] [32] [33] [34] [35] [36] Present study

MePCMs in this study with different MePCMs in literature [31e36]; the results indicate that the encapsulation efficiency of the MePCMs with paraffin/SiO2 are as high as those of previous studies. The phase change temperature range of the composite paraffin meets human comfort requirements, and possess a lower price compared with these PCMs in the literature such as n-octadecane and fatty acids, which are more suitable and have a wide application in the field of building energy conservation.

3.4. Thermal stability TG curves of the paraffin and MePCMs are shown in Fig. 5. The pure paraffin revealed that one degradation step started at 158
C; the complete decomposition of the pure paraffin is about 310
C. The MePCMs exhibited two-stage degradation behavior in the

temperature range of 200e490
C. The first stage of weight loss began at about 200
C corresponding to the weight loss of the paraffin; the second stage of weight loss that was ascribed to the silica condensation occurred at 355e490
C. Fig. 5 shows that the initial decomposition temperature of the microcapsules is higher than the pure paraffin; the SiO2 shell improved the degradation temperature of the MePCMs. Moreover, the degradation of the microcapsules synthesized under the pH of 2.5 occurred at a higher temperature owing to the better shell material compactness at this pH. This showed an improvement in the thermal stability of MePCMs.

3.5. Anti-osmosis performance The leakage rate of the MePCMs in the heating process reflect

Y. Zhang et al. / Renewable Energy 149 (2020) 400e408

100

100

80 60

80

40

TG (%)

20

60

0 150

200

250

300

350

Paraffin MeP-2 MeP-2.5 MeP-3

40

20

0 100

200

300

400

500

600

Temperature ( C)

700

800

Fig. 5. TGA curves of MePCMs prepared at different pH conditions.

the shell compactness of the MePCMs. Paraffin would easily leak out from the thin or porous silica shell, and the silica shell with a more compact and thicker shell would achieve low leakage rate, resulting in an improvement in the thermal durability performance. Fig. 6 represents the leakage rate of the MePCMs prepared under different pH conditions at different testing times. As observed, the microcapsules prepared under the pH of 2.5 exhibited the lowest leakage rate compared with the microcapsules under the other two pH conditions; this indicates a good encapsulation of the silica shell that formed in this pH condition. A higher leakage rate of the MeP-3 confirmed a loose and porous structure of the silica shell. The rough surface of the silica shell cannot effectively prevent the PCMs from permeating. The inadequate encapsulation of the MeP-2 also results in a thinner silica shell than the other two samples, thus leading to a high leakage rate of the MePCMs prepared under the pH condition. This indicated that a compact and thick silica shell can effectively prevent the paraffin from leaking and provide good anti-osmosis performance for the microcapsules. 3.6. Mechanical performance of gypsum composite materials incorporated with the MePCMs

405

strength of the gypsum materials, different content of MeP-2.5 prepared with higher thermal properties was incorporated into the gypsum. The mechanical performance of the gypsum materials is shown in Fig. 7. The results showed that the compressive and flexural strength of the gypsum materials decreased with the increased contents of the MePCMs in gypsum. When the MePCMs contents were 3 wt%, 5 wt%, 10 wt%, and 20 wt%, compared with the pure gypsum, the compressive strength of the gypsum composites decreased by 24.7%, 30.8%, 41.4%, and 56.1%, respectively. The reason for the reduction of the mechanical strength can be attributed to the compatibility problems between the organic PCMs and the inorganic gypsum. In the mixing process of the gypsum materials, the inevitable rupture of the MePCMs led to the leakage of paraffin from the silica shell; the paraffin in the gypsum is incompatible with it and this caused the structural damage of the gypsum materials. Fig. 7 shows that the compressive strength and flexural strength of the prepared gypsum samples MeP-G10 are 11.6 MPa and 3.8 MPa, respectively, which still maintained a high mechanical performance in 10 wt % content of the MePCMs. 3.7. The microstructure of the gypsum composite materials SEM images of the gypsum materials incorporated with different contents of the MePCMs are shown in Fig. 8. The MePCMs dispersed homogeneously in the gypsum matrix. There is still some difference between the samples with different contents of the MePCMs. As shown in MeP-G3 and MeP-G5, the microcapsules are clearly seen with no agglomeration; a small amount of the MePCMs in the gypsum materials can be dispersed properly among the gypsum matrix. The number of the microcapsules increased with an increase in the MePCMs as shown in MeP-G10, and more microcapsules embedded in the gypsum matrix were seen. The agglomeration of the microcapsules is still not explicit, but the gypsum crystals are explicitly seen; this means that the microcapsules in 10 wt% content also maintained a good dispersibility in the gypsum matrix. As the contents of the MePCMs increased to 20 wt%, the agglomeration of the MePCMs covered on the surface of the gypsum crystal is apparent; the hydration of the gypsum materials was inhibited by the MePCMs to some extent, thus resulting in a low mechanical strength of the materials. 3.8. Thermal conductivity of gypsum composite materials The thermal conductivity of the gypsum materials in terms of

To investigate the effect of the MePCMs on the mechanical 70

25

MeP-2 MeP-2.5 MeP-3

Compressive strength

Mechanical strength (MPa)

60

Leakage rate (%)

50 40 30 20 10 0

0

20

40

60

80

100

120

140

Time (h) Fig. 6. The leakage rate of MePCMs prepared under different pH conditions.

20 Flexural strength

15

10

5

0

0

3

5

10

20

Weight ratio (%) Fig. 7. Mechanical strength of gypsum materials incorporated with the MePCMs.

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Y. Zhang et al. / Renewable Energy 149 (2020) 400e408

50

MeP-G0 MeP-G5 MeP-G10 MeP-G20

Temparature  &

40

30

20

10 0

1000

2000

3000

4000

5000

6000

7000

8000

9000

Time (s) Fig. 10. Temperature curves of gypsum composite materials incorporated with different contents of the MePCMs in the heating and cooling process.

3.9. Temperature regulation performance of gypsum composite materials Fig. 8. SEM images of the gypsum matrix incorporated with different,MePCMs content.

the contents of the MePCMs is shown in Fig. 9. The test values of the gypsum materials are 0.4707 W/(m$K), 0.3361 W/(m$K), 0.3011 W/ (m$K) and 0.2537 W/(m$K) and they correspond to the 0, 5 wt%, 10 wt% and 20 wt% contents of the MePCMs. The thermal conductivity of the gypsum matrix decreased as the incorporation of MePCMs did; this could be explained by the smaller thermal conductivity of the paraffin in the MePCMs (approximately 0.15 W/ m$K) [24], which would reduce the heat transfer rate of the gypsum materials in the heat and cooling process. Owing to the incorporation of the MePCMs in the gypsum materials, the heat transfer performance of the MECMs is enhanced as the higher thermal conductivity of the gypsum materials. For the prepared sample, the MeP-G10 preserved the thermal conductivity more than 0.3 W/ (m$K), indicating the improvement of the thermal conductivity compared with the bulk MePCMs.

Thermal conductivity (W/m– K)

0.50

Thermal conductivity

0.45 0.40 0.35 0.30 0.25 0

5

10

15

20

MePCMs contents (wt% ) Fig. 9. Thermal conductivity of gypsum materials incorporated with different contents of MePCMs.

To explore the temperature regulation performance of the gypsum materials incorporated with different contents of MePCMs, the temperature curves of the gypsum composite materials in the heating and cooling processes are investigated as shown in Fig. 10. During the heating process, with an increase in the MePCMs content, the heating rate of the sample decreased significantly compared with the pure gypsum. The heating process took 1270 s to reach 40
C for the pure gypsum, while for the gypsum incorporated with 5 wt%, 10 wt%, and 20 wt% MePCMs, the heating time was about 1590 s, 1830 s, and 2090 s, respectively. The gypsum materials with MePCMs took longer times than the pure gypsum materials to reach the same heating temperature. The elongation of the heating time implies that the gypsum materials possess an excellent temperature regulation property. An obvious phase change of the MePCMs in the gypsum matrix in the 27e32
C temperature range was observed in accordance with the phase change temperature of the MePCMs. When 5 wt% contents of the MePCMs was incorporated (MeP-G5), the samples in the temperature range lasted 420 s. As the content of the MePCMs increased to 10 wt%, the samples in the temperature range lasted 560 s. When the weight ratio of the MePCMs increased up to 20 wt%, the time of the sample was prolonged to 740 s; the results showed that with the increased weight contents of the MePCMs, the gypsum materials had much better temperature regulation performance. The cooling rate of gypsum composite materials decreased significantly with an increase in the MePCMs content in the cooling process. Owing to the faster heat transfer efficiency for the gypsum samples immersed in the cooling water, the cooling rate of the MePCMs was fast, and the phase change of the MePCMs occurred in the smaller temperature range of 22e28
C. The corresponding time for pure gypsum cooled from 28
C to 22
C is 330 s, while for the samples incorporated with 5 wt%, 10 wt% 20 wt% MePCMs, the phase change time are 570 s, 750 s, and 1080s, indicating that the incorporation of the MePCMs in the gypsum materials delayed the cooling rate as the thermal energy storage capacity of the MePCMs. 4. Conclusions MePCMs, which employed the use of a SiO2 shell and a paraffin core, were prepared by sol-gel method using tetraethoxysilane as

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the precursor. The obtained MePCMs were incorporated into the gypsum at different contents and the effect of the MePCMs on the mechanical and thermal properties of these gypsum materials was studied. The FTIR spectra confirmed the encapsulation of the paraffin core in the silica shell. The microstructure of the MePCMs strongly depends on the pH value of the silica sol solution; the microcapsules obtained at a pH of 2.5 presents a regular spherical structure, excellent dispersibility, and compact surface structure. The microcapsules synthesized at a pH of 2.5 also exhibited a high encapsulation efficiency and phase change enthalpy. The shell material with high compactness imparted good thermal stability and anti-osmosis performance to the microcapsules. The gypsum materials with 10% weight ratio of the MePCMs showed high mechanical strength; the MePCMs can be dispersed completely in the gypsum matrix with little agglomeration. The heat transfer performance of the MePCMs in the gypsum matrix was enhanced due to the higher thermal conductivity of the gypsum materials. The gypsum materials achieved good thermal regulation performance with MePCMs weight ratio of 10 wt%. The weight ratio of 10 wt% of the MePCMs in the gypsum matrix is appropriate for the achievement of high mechanical strength, high thermal conductivity, and good thermal regulation performance. MePCMs can improve the thermal capacity of the gypsum wall building materials, decrease the temperature fluctuation of the indoor temperature, and adjust the time difference between energy supply and demand, which is a potential application for the thermal energy storage in buildings. Author contribution statement Yi Zhang: Conceptualization, Methodology, Writing - Original Draft, Writing - Review & Editing. Wen Tao: Validation, Investigation, Resources. Kehan Wang: Resources, Data Curation, Investigation, Validation. Dongxu Li: Methodology, Supervision, Project administration. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51702001) and the Natural Science Foundation of Anhui Province (Grant No. 1808085QE119). References [1] A. Jamekhorshid, S.M. Sadrameli, M. Farid, A review of microencapsulation methods of phase change materials (PCMs) as a thermal energy storage (TES) medium, Renew. Sustain. Energy Rev. 31 (2014) 531e542. [2] H. Nazir, M. Batool, F.J.B. Osorio, M. Isaza, X.H. Xu, K. Vignarooban, P. Phelan, Inamuddin, A.M. Kannan, Recent developments in phase change materials for energy storage applications: a review, Int. J. Heat Mass Transf. 129 (2019) 491e523. [3] M. Kenisarin, K. Mahkamov, Salt hydrates as latent heat storage materials: thermophysical properties and costs, Sol. Energy Mater. Sol. Cells 145 (3) (2016) 255e286. [4] Y.P. Yuan, N. Zhang, W.Q. Tao, X.L. Cao, Y.L. He, Fatty acids as phase change materials: a review, Renew. Sustain. Energy Rev. 29 (2014) 482e498. [5] S. Lashgari, H. Arabi, A.R. Mahdavian, V. Ambrogi, Thermal and morphological studies on novel PCM microcapsules containing n-hexadecane as the core in a flexible shell, Appl. Energy 190 (2017) 612e622. [6] Y.X. Lin, G. Alva, G.Y. Fang, Review on thermal performances and applications of thermal energy storage systems with inorganic phase change materials, Energy 165 (2018) 685e708. [7] W.L. Cui, Y.P. Yuan, L.L. Sun, X.L. Cai, X.J. Yang, Experimental studies on the supercooling and melting/freezing characteristics of nano-copper/sodium acetate trihydrate composite phase change materials, Renew. Energy 99

407

(2016) 1029e1037. [8] Y.P. Wu, T. Wang, Hydrated salts/expanded graphite composite with high thermal conductivity as a shape-stabilized phase change material for thermal energy storage, Energy Convers. Manag. 101 (2015) 164e171. [9] L. Zhang, W.B. Yang, Z.N. Jiang, F.F. He, K. Zhang, J.H. Fan, J.Y. Wu, Graphene oxide-modified microencapsulated phase change materials with high encapsulation capacity and enhanced leakage-prevention performance, Appl. Energy 197 (2017) 354e363. [10] M. Li, M.R. Chen, Z.S. Wu, J.X. Liu, Carbon nanotube grafted with polyalcohol and its influence on the thermal conductivity of phase change material, Energy Convers. Manag. 83 (2014) 325e329. [11] S.G. Jeong, S.W. Wi, S. Chang, J. Lee, S. Kim, An experimental study on applying organic PCMs to gypsum-cement board for improving thermal performance of buildings in different climates, Energy Build. 190 (2019) 183e194. [12] Y. Konuklu, M. Ostry, H.O. Paksoy, P. Charvat, Review on using microencapsulated phase change materials (PCM) in building applications, Energy Build. 105 (2016) 134e155. [13] S.P. Singh, V. Bhat, Performance evaluation of dual phase change material gypsum board for the reduction of temperature swings in a building prototype in composite climate, Energy Build. 159 (2018) 191e200. [14] D. Feldman, D. Banu, D.W. Hawes, Develop and application of organic phase change mixtures in thermal storage gypsum wallboard, Sol. Energy Mater. Sol. Cells 36 (2) (1995) 147e157. [15] N. Zhang, Y.P. Yuan, Y.G. Yuan, T.Y. Li, X.L. Cao, Lauric-palmitic-stearic acid/ expanded perlite composite as form-stable phase change material: preparation and thermal properties, Energy Build. 82 (2014) 505e511. [16] N. Xie, J.M. Luo, Z.P. Li, Z.W. Huang, X.N. Gao, Y.T. Fang, Z.G. Zhang, Salt hydrate/expanded vermiculite composite as a form-stable phase change material for building energy storage, Sol. Energy Mater. Sol. Cells 189 (2019) 33e42. [17] X.F. Kong, C.Q. Yao, P.F. Jie, Y. Liu, C.Y. Qi, X. Rong, Development and thermal performance of an expanded perlite-based phase change material wallboard for passive cooling in building, Energy Build. 152 (2017) 547e557. [18] X.M. Cheng, G. Li, G.M. Yu, Y.Y. Li, J.Q. Han, Effect of expanded graphite and carbon nanotubes on the thermal performance of stearic acid phase change materials, J. Mater. Sci. 52 (20) (2017) 12370e12379. [19] H.Z. Yang, L.L. Feng, C.Y. Wang, W. Zhao, X.G. Li, Confinement effect of SiO2 framework on phase change of PEG in shape-stabilized PEG/SiO2 composites, Eur. Polym. J. 48 (4) (2012) 803e810. [20] J. Zhang, C. Narh, P.F. Lv, Y.B. Cai, H.M. Zhou, X.B. Hou, Q.F. Wei, Preparation of novel form-stable composite phase change materials with porous silica nanofibrous materials for thermal storage/retrieval, Colloids Surf., A 570 (2019) 1e10. [21] Z.G. Luo, H. Zhang, X.N. Gao, T. Xu, Y.T. Fang, Z.G. Zhang, Fabrication and characterization of form-stable capric-palmitic-stearic acid ternary eutectic mixture/nano-SiO2 composite phase change material, Energy Build. 147 (2017) 41e46. [22] A.M. Borreguero, A. Serrano, I. Garrido, J.F. Rodríguez, M. Carmona, PolymericSiO2-PCMs for improving the thermal properties of gypsum applied in energy efficient buildings, Energy Convers. Manag. 87 (2014) 138e144. [23] C.E. Li, H. Yu, Y. Song, Z.Y. Liu, Novel hybrid microencapsulated phase change materials incorporated wallboard for year-long year energy storage in buildings, Energy Convers. Manag. 183 (2019) 791e802. [24] H.Z. Zhang, Q.Y. Xu, Z.M. Zhao, J. Zhang, Y.J. Sun, L.X. Sun, F. Xu, Y. Sawada, Preparation and thermal performance of gypsum boards incorporated with microencapsulated phase change materials for thermal regulation, Sol. Energy Mater. Sol. Cells 102 (2012) 93e102. [25] T. Toppi, L. Mazzarella, Gypsum based composite materials with microencapsulated PCM: experimental correlations for thermal properties estimation on the basis of the composition, Energy Build. 57 (2013) 227e236. [26] A. Joulin, L. Zalewski, S. Lassue, H. Naji, Experimental investigation of characteristics of a mortar with or without a micro-encapsulated phase change material, Appl. Therm. Eng. 66 (1e2) (2014) 171e180. [27] X.F. Wang, C.H. Li, T. Zhao, Fabrication and characterization of poly(melamineformaldehyde)/silicon carbide hybrid microencapsulated phase change materials with enhanced thermal conductivity and light-heat performance, Sol. Energy Mater. Sol. Cells 183 (2018) 82e91. [28] S.P. Zhan, S.H. Chen, L.M. Chen, W.M. Hou, Preparation and characterization of polyurea microencapsulated phase change material by interfacial polycondensation method, Powder Technol. 292 (2016) 217e222. [29] X.L. Qiu, L.X. Lu, J. Wang, G.Y. Tang, G.L. Song, Preparation and characterization of microencapsulated n-octadecane as phase change material with different n-butyl methacrylate-based copolymer shells, Sol. Energy Mater. Sol. Cells 128 (2014) 102e111. n, A. Gutie rrez, M. Gra geda, S. Ushakabet, A review on encapsulation [30] Y.E. Milia techniques for inorganic phase change materials and the influence on their thermophysical properties, Renew. Sustain. Energy Rev. 73 (2017) 983e999. [31] H.Z. Zhang, S.Y. Sun, X.D. Wang, D.Z. Wu, Fabrication of microencapsulated phase change materials based on n-octadecane core silica shell through interfacial polycondensation, Colloids Surf., A 389 (1e3) (2013) 104e117. [32] F. He, X.D. Wang, D.Z. Wu, New approach for sol-gel synthesis of microencapsulated n-octadecane phase change material with silica wall using sodium silicate precursor, Energy 67 (2014) 223e233. [33] S.Y. Yu, X.D. Wang, D.Z. Wu, Microencapsulation of n-octadecane phase change material with calcium carbonate shell for enhancement of thermal

408

Y. Zhang et al. / Renewable Energy 149 (2020) 400e408

conductivity and serving durability: synthesis, microstructure, and performance evaluation, Appl. Energy 114 (2014) 632e643. [34] G.Y. Fang, Z. Chen, H. Li, Synthesis and properties of microencapsulated paraffin composites with SiO2 shell as thermal energy storage materials, Chem. Eng. J. 163 (2016) 154e159. [35] S.K. Song, L.J. Dong, Z.Y. Qu, J. Ren, C.X. Xiong, Microencapsulated caprice-

stearic acid with silica shell as a novel phase change material for thermal energy storage, Appl. Therm. Eng. 70 (2014) 546e551. [36] S.T. Latibari, M. Mehrali, M. Mehrali, T.M.I. Mahlia, H.S.C. Metselaar, Synthesis, characterization and thermal properties of nanoencapsulated phase change materials via sol-gel method, Energy 61 (2013) 664e672.