Thermal Property Enhancement of Paraffin-Wax-Based Hydroxyl

Feb 14, 2018 - NanoSiO2 is also considered as a good supporting material of PW. Herein, a novel nanoSiO2-EG-PW ternary form-stable phase change ...
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Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Thermal Property Enhancement of Paraffin-Wax-Based HydroxylTerminated Polybutadiene Binder with a Novel NanoSiO2‑Expanded Graphite-PW Ternary Form-Stable Phase Change Material Xia Gao,†,‡ Tianbo Zhao,*,† Guan Luo,‡ Baohui Zheng,‡ Hui Huang,‡ Xue Han,† Rui Ma,† and Yuqiao Chai† †

Key Laboratory of Cluster Science of Ministry of Education, Beijing Institute of Technology, Beijing 100081, PR China Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621900, PR China

‡

ABSTRACT: High volume change and low thermal conductivity will impede the application of paraffin wax (PW). Expanded graphite (EG) is one of the best materials to shape-stabilize PW and improve its thermal conductivity. NanoSiO2 is also considered as a good supporting material of PW. Herein, a novel nanoSiO2-EG-PW ternary form-stable phase change material (FSPCM) with enhanced thermal properties was reported. Subsequently, the effect of nanoSiO2-EG-PW with different amounts on the chemical composition, morphology, and thermal properties of PW-based hydroxyl-terminated polybutadiene (HTPB) was investigated using Fourier transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermal gravimetric analyzer (TGA), thermal cycling test, and thermal conductivity, respectively. These results indicated that nanoSiO2-EG-PW could effectively shape-stabilize PW in the HTPB matrix without chemical reaction. The thermal conductivity of nanoSiO2-EG-PW was improved from 0.303 W·m−1·K−1 to 0.602 W·m−1·K−1, compared with that of PW. The nanoSiO2-EG-PW/PW/HTPB composites kept high latent heat capacity, delayed melting temperature, and good thermal reliability after 500-thermal-cycling. Moreover, the thermal stability and thermal conductivity of the composites increased, with the increasing content of nanoSiO2-EG-PW. Therefore, these materials have a potential application in hightemperature phase change material (PCM) and HTPB binder. absorb PW and increase its thermal conductivity. Li et al.17 prepared a novel ternary stabilized material consisting of paraffin/epoxy/EG, through the sol−gel method. Meanwhile, its mechanical and thermal properties for practical application were investigated. Sun et al.22 fabricated a novel FSPCM by adsorbing eutectics of adipic acid (AA) and stearic acid (SA) into EG. Gao et al.31 investigated the properties of a LiNO3/ KCl-EG composite PCM, concerning its long-term use for industrial waste heat storage. Moreover, nanoSiO2 is often chosen as a matrix material to prepare FSPCM, due to the porous and connected structure. Work also conducted by Gao et al.25 reported a FSPCM prepared through impregnating a capric acid−palmitic acid−stearic acid (CA−PA−SA) ternary eutectic mixture into nanoSiO2. Accordingly, a ternary FSPCM of nanoSiO2-EG-PW was conceived and produced in this work. It would provide a demonstration to prepare multifunctional FSPCM composites through compositing different supporting materials. As is known, PW has been widely used as fuel and desensitizer in hybrid propulsion and polymer-bonded explosives (PBX), owing to the high burning rate, low cost, and superior desensitization effect.32−37 In addition, hydroxylterminated polybutadiene (HTPB) was preferred to be fuel or binders since it could provide easy availability, desirable manufacture, and polymeric properties required by hybrid

1. INTRODUCTION With the increasing demand of energy and rapid consumption of traditional fossil sources, seeking efficient energy-storage materials has been a worldwide concern.1,2 Much attention has been paid to phase change materials (PCMs), due to their large energy storage capacity, chemical stability, good thermal reliability, little supercooling phenomenon, and environmentally friendly performance during the phase change process.3−7 Paraffin wax (PW) was recognized as a promising kind of PCM because of the low cost, nontoxicity, chemical stability, and controllable phase transition temperature.8−10 However, high volume change and low thermal conductivity are the major drawbacks of PW, which would impede its application. Recently, utilizations of microencapsulating PCM (MePCM) and form-stable PCM (FSPCM) are the most expected technologies. Microencapsulation of PCMs as core with shell materials means a relatively complicated synthesis process and high cost, which may limit its application in industrial fields.11 FSPCM is a method of using porous materials to absorb and support PCMs, intended to shapestabilize PCM undergoing the phase change process.12 These inserted supporting materials can be classified as polymers and inorganic materials, such as polyurethane (PU),13−15 high density polyethylene (HDPE),16 epoxy,17 polyethylene glycol (PEG),18 expanded graphite (EG),19−24 nanoSiO2,25 multiwalled carbon nanotubes (MWCNTs),12 expanded perlite (EP),10,26 fibers,27,28 nanoplatelet graphite,9 graphene,8,29 porous metal fiber sintered felt (PMFSF),4 and activated carbon (AC).30 Among them, EG is one of the best materials to © XXXX American Chemical Society

Received: December 7, 2017 Revised: January 31, 2018

A

DOI: 10.1021/acs.energyfuels.7b03856 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels propulsion and PBX.38−42 Sinha et al.43 investigated the ablation rate and thermal and mechanical properties of HTPB blended with different percentages of paraffin. Nevertheless, there is much difference in molecular weight and polarity between HTPB and PW, resulting in weak compatibility between the two phases.43 Thus, the integrity, heat conversion, thermal properties, and insensitivity of PWbased HTPB and PBX will be easily reduced with the change of environment. To solve such issues, we innovatively designed a novel nanoSiO2-EG-PW ternary FSPCM and inserted it into the HTPB matrix for improving the thermal properties and compatibility between PW and HTPB. To the best of our knowledge, FSPCM supported with inorganic porous materials has not been applied in HTPB binders and PBX before. In this paper, nanoSiO2-EG-PW FSPCM was prepared and blended with PW-based HTPB. The chemical structure, morphology, phase change properties, and thermal properties of nanoSiO2EG-PW FSPCM and nanoSiO2-EG-PW/PW/HTPB composites were investigated. The nanoSiO2-EG-PW/PW/HTPB composites displayed a great potential of thermal enhancement in PW-based HTPB.

Figure 1. DSC curves of pure PW and PCMs.

Table 1. Phase Change Properties of PW and Prepared PCMs material PW PCMs-1 (8 wt % SiO2/PW-80 °C) PCMs-2 (8 wt % SiO2/PW-180 °C) PCMs-3 (8 wt % SiO2/2 wt % EG/PW-180 °C) PCMs-4 (10 wt % SiO2/2 wt % EG/PW-180 °C) PCMs-5 (12 wt % SiO2/2 wt % EG/PW-180 °C)

2. EXPERIMENTAL SECTION 2.1. Materials. PW (Tm = 50−52 °C) was obtained from Shanghai Huashen Rehabilitation Material Co., Ltd., China. NanoSiO2 (15 nm, 99.8%) was purchased from Beijing Huaweiruike Reagents Co., Ltd., China. EG (80 meshes, 99%, expanded ratio = 300 mL/g) was supplied by Qingdao Tengshengda Carbon Graphite Co., Ltd., China. HTPB (OH value = 0.76 mmol/g) was supplied by Liming Research & Design Institute of Chemical Industry Co., Ltd., China. Toluene diisocyanate (TDI) (AR) was purchased from Xiya Chemical Industry Co., Ltd., China. Dinoctylsebacate (DOS) (AR) was purchased from Jiangyin Bolong Chemical Group Co., Ltd. All the chemical reagents were used without further purification. 2.2. Preparation of NanoSiO2-EG-PW FSPCM. The nanoSiO2EG-PW FSPCM was prepared through absorbing PW with nanoSiO2 and EG, and the detailed preparation process was as follows. PW was melted and stirred thoroughly in a steel container at 100 °C. The nanoSiO2 with different contents (8 wt %, 10 wt %, 12 wt %) was added in the above solution and agitated continuously. After mixing sufficiently, the temperature of the mixture was elevated to 180 °C, and another agitation of 1 h was conducted. Then the EG was added in and refined for 1.5 h at 190 °C. Finally, the mixture was cooled naturally to room temperature. Moreover, the 8 wt % nano-SiO2/PW composite prepared at 80 °C for 1 h was synthesized. The composite composition and phase change properties of pure PW and PCMs were shown in Figure 1 and Table 1. As shown in Figure 1, there were two phase transition peaks appearing on the DSC curves of pure paraffin and prepared PCMs. The minor peak with a temperature range of 25−35 °C was regarded as the solid−solid phase change of PW, and the main peak with a temperature range of 50−55 °C was considered as the solid−liquid phase transition of PW. As listed in Table 1, the Tm of prepared PCMs was almost consistent with that of PW. Moreover, the ΔHmthe and ΔHmobs of prepared PCMs were slightly reduced compared with that of PW, due to the introduction of nanoSiO2 and EG as supporting materials. In this work, the PCM-4 composite with optimized properties and contents of EG and nanoSiO2 was defined as nanoSiO2-EG-PW PCM and applied in the HTPB binders. 2.3. Preparation of NanoSiO2-EG-PW/PW/HTPB Composites. In consideration of the lubrication effect of PW, nanoSiO2-EG-PW was employed to partially replace 20, 40, and 60 wt % of PW (125 g) in PW-based HTPB. The preparation process of nanoSiO2-EG-PW/PW/ HTPB composites was described as follows. PW and nanoSiO2-EGPW were melted and blended exhaustively with HTPB (324.17 g), DOS (154.95 g), and TDI (20.88 g) at 60 °C. After that, the blend was

Tm (°C)a

ΔHmobs (J/g)b

ΔHmthe (J/g)c

51.86 51.84 51.95 51.83

184.10 170.17 171.11 168.3

184.10 169.37 169.37 165.69

51.86

162.11

162.01

51.94

161.02

158.33

a Melting temperature. bThe observed melting latent heat. cThe theoretical melting latent heat.

placed in a vacuum oven at 60 °C until the air bubbles disappeared. The blend was poured into aluminum molds and cured in an oven at 65 °C for 3 days. After cooling to room temperature, the nanoSiO2EG-PW/PW/HTPB samples were obtained. The scheme and structure of nanoSiO2-EG-PW PCM and nanoSiO2-EG-PW/PW/ HTPB composites were presented in Scheme 1. As shown in Scheme 1, the PW molecule was absorbed on the surface of nanoSiO2. With the addition of EG, nanoSiO2 absorbing PW particles filled in the irregular honeycomb-like pores of EG. Thus, PW was shape-stabilized with nanoSiO2 and EG, and with the addition of nanoSiO2-EG-PW PCM, the prepared nanoSiO2-EG-PW/PW/HTPB composite was blackened. 2.4. Characterizations. Fourier transformed infrared spectroscopy (FTIR, Nicolet 6700) was used to study the chemical composition of nanoSiO2-EG-PW PCM and nanoSiO2-EG-PW/PW/HTPB composites, with a wavenumber range from 350 to 4000 cm−1. A scanning electron microscope (SEM, ZEISS ULTRA 55) was used to observe the morphology of nanoSiO2-EG-PW PCM and the fracture surface of nanoSiO2-EG-PW/PW/HTPB composites. The composites were brittle fractured in liquid nitrogen, and the cross sections were selected for analysis. All the samples were sputter-coated with gold before analysis. Gas adsorption measurements (Micrometritics ASAP 2420 system) were conducted to investigate the Brunauer−Emmett−Teller (BET) specific surface area and pore size distribution of EG particles. A differential scanning calorimeter (DSC, TA DSC Q2000) was used to investigate the phase change temperature and latent heat of nanoSiO2-EG-PW PCM and nanoSiO2-EG-PW/PW/HTPB composites, performed in a nitrogen atmosphere with a temperature range of −10−75 °C, at a heating rate of 10 °C/min. To investigate the thermal reliability of nanoSiO2-EG-PW/PW/HTPB samples and leakage status of nanoSiO2-EG-PW PCM with dimension of Φ25 mm × 10 mm, an accelerated 500-thermal-cycling test was operated within 25−75 °C in a programmable high−low temperature test chamber. B

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Scheme 1. Schematic Illustration of the Preparation Process of NanoSiO2-EG-PW PCM and NanoSiO2-EG-PW/PW/HTPB Composites

bending vibration and in-plane rocking vibration of the −CH2 groups, respectively. For the nanoSiO2-EG-PW PCM, the characteristic peaks of nanoSiO2, EG, and PW could be found in the FTIR spectrum. It was illustrated that the nanoSiO2-EGPW PCM had been synthesized without chemical reaction. For the pristine HTPB, the absorption peak at 1720 cm−1 was due to the urethane groups, and the peaks at 971 and 915 cm−1 were associated with the bending vibration of the CC group,41 which were almost consistent with that of PW/HTPB and nanoSiO2-EG-PW/PW/HTPB composites. It was indicated that the nanoSiO2-EG-PW/PW/HTPB composites were prepared through physical blending. 3.2. Morphology of NanoSiO2-EG-PW PCM and NanoSiO2-EG-PW/PW/HTPB Composites. For EG, as a key material to improve the thermal conductivity, its physicochemical properties should have an important effect on the performance of PCM composites. Therefore, the BET surface area and porosity of EG were provided with nitrogen adsorption isotherm and pore size distribution profiles of EG in Figure 3. The experimental BET specific surface area was calculated to be 45.3310 m2/g for EG, with a narrow pore size distribution of 20−100 nm. The surface microstructure of EG and nanoSiO2-EG-PW PCM was studied using SEM, and the micrographs were shown in Figure 4. As displayed in Figure 4a, the EG presented a typical wormlike structure. In the magnified photograph of Figure 4b, irregular honeycomb-like pores can be observed on the surface of EG, as illustrated in Scheme 1. NanoSiO2 particles could highly absorb and support PW because of the porous and connected structure.25 For nanoSiO2-EG-PW PCM, there were no pores observed since the PW absorbed by nanoSiO2 had filled in and adhered by the large pores on the surface of EG. Correspondingly, a lot of spherical outlines, caused by the nanoSiO2, could be found on the surface of nanoSiO2-EG-PW PCM. It was indicated that the PW absorbed by nanoSiO2 covered and homogeneously distributed on the surface of EG. The SEM photographs of the nanoSiO2-EG-PW/PW/HTPB composite fracture surface were shown in Figure 5. The fracture surface of pristine HTPB was smooth with some rupture vestiges on. However, the fracture surface of PW/HTPB presented many holes and spherical particles because of the separation of PW from the HTPB matrix. With the addition of

Thermal gravimetric analyzer (TGA, TG-DTA 6200 LAB SYS) was applied for investigating the thermal stability of nanoSiO2-EG-PW PCM and nanoSiO2-EG-PW/PW/HTPB composites, with the temperature range of 25−600 °C at a heating rate of 10 °C/min in a nitrogen atmosphere. The thermal conductivity coefficient of nanoSiO2-EG-PW PCM and nanoSiO2-EG-PW/PW/HTPB composites with dimension of 100 × 100 × 5 mm were studied using a transient hot wire method and heat flow method (EKO, HC-074−200) according to GB/T 10295 at 25 °C, respectively.

3. RESULTS AND DISCUSSION 3.1. Chemical Composition of NanoSiO2-EG-PW PCM and NanoSiO2-EG-PW/PW/HTPB Composites. The FTIR spectra of nanoSiO2-EG-PW PCM and nanoSiO2-EG-PW/ PW/HTPB composites were shown in Figure 2. As presented

Figure 2. FTIR spectra of nanoSiO2-EG-PW PCM and nanoSiO2-EGPW/PW/HTPB composites.

in Figure 2, for nanoSiO2, the absorption peaks at 1106 and 809 cm−1 were ascribed to the asymmetric and symmetric stretching vibration of Si−O−Si groups.44 The peak located at 3430 cm−1 was attributed to the stretching vibration of the O−H group.45 For the EG sample, the broad band at 2350−2370 cm−1 belonged to the stretching vibration of the C−C skeleton. In the spectrum of PW, the peaks at 2917 and 2852 cm−1 were due to the stretching vibration of −CH2 and −CH3 groups.46 The peaks at 1472 and 719 cm−1 were associated with the C

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Figure 3. (a) Nitrogen adsorption isotherm of EG at 77 K and (b) pore size distribution profiles of EG.

Figure 4. SEM micrographs of (a, b) EG and (c, d) nanoSiO2-EG-PW.

20 wt % nanoSiO2-EG-PW, the microstructure of the fracture surface was almost coincident with that of PW/HTPB. As the content of nanoSiO2-EG-PW increased, the holes became smaller and even disappeared on the fracture surface of nanoSiO2-EG-PW/PW/HTPB composites. For 40 wt % nanoSiO2-EG-PW/PW/HTPB, the edge of the holes and PCM particles on the fracture surface were irregular, possibly due to the wormlike structure of the added EG in nanoSiO2EG-PW. For 60 wt % nanoSiO2-EG-PW/PW/HTPB, it could be observed that the PCM particles with irregular shape were embedded in the fracture surface with no gap between them and the HTPB matrix. The results indicated that the nanoSiO2EG-PW could shape-stabilize PW well in HTPB and evidently increase the compatibility between the two phases. 3.3. Phase Change Properties of NanoSiO2-EG-PW PCM and NanoSiO2-EG-PW/PW/HTPB Composites. The DSC curves of PW, nanoSiO2-EG-PW PCM, pristine HTPB, PW/HTPB, 20 wt % nanoSiO2-EG-PW/PW/HTPB, 40 wt % nanoSiO2-EG-PW/PW/HTPB, and 60 wt % nanoSiO2-EGPW/PW/HTPB composites were shown in Figure 6, and the relevant data were presented in Table 2. As shown in Figure 6a, the DSC curves of PW and nanoSiO2-EG-PW PCM exhibited an endothermic peak and exothermic peak during the melting and freezing processes, respectively. Additionally, the peak area of PW was larger than that of nanoSiO2-EG-PW PCM, due to that the doped nanoSiO2 and EG would decrease the relative content of PW in the prepared PCM. While the temperature at the endothermic peak (Tm) of nanoSiO2-EG-PW PCM was delayed compared with that of PW, since the shape-stabilization effect of nanoSiO2 and EG as supporting materials on PW

Figure 5. SEM photographs of (a, b) pristine HTPB, (c, d) PW/ HTPB, (e, f) 20 wt % nanoSiO2-EG-PW/PW/HTPB, (g, h) 40 wt % nanoSiO2-EG-PW/PW/HTPB, and (i, j) 60 wt % nanoSiO2-EG-PW/ PW/HTPB composites.

could protect PW from melting to some extent. As observed in Figure 6b, the endothermic peak and exothermic peak existed in all the DSC curves of composite samples except pristine HTPB, implying that HTPB had no latent heat capacity from D

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Figure 6. DSC curves of PW, nanoSiO2-EG-PW PCM, pristine HTPB, PW/HTPB, 20 wt % nanoSiO2-EG-PW/PW/HTPB, 40 wt % nanoSiO2-EGPW/PW/HTPB, and 60 wt % nanoSiO2-EG-PW/PW/HTPB composites.

Table 2. DSC Analysis of PW, NanoSiO2-EG-PW PCM, Pristine HTPB, PW/HTPB, and NanoSiO2-EG-PW/PW/HTPB Composites with Different Contents of NanoSiO2-EG-PW PCM melting process

a

freezing process

samples

Tm (°C)

ΔHm (J/g)

Tc (°C)

ΔHc (J/g)

ΔTsa (°C)

ΔH1b (J/g)

ΔH2c (J/g)

errord (%)

HTPB PW nanoSiO2-EG-PW PCM PW/HTPB 20 wt % PCM/PW/HTPB 40 wt % PCM/PW/HTPB 60 wt % PCM/PW/HTPB

 49.61 51.60 46.29 43.49 44.64 47.26

 167.80 142.80 30.22 30.03 27.45 27.14

 48.35 48.14 42.08 42.99 41.55 41.82

 168.10 141.70 32.19 32.22 30.93 29.29

 1.26 3.46 4.21 0.50 3.09 5.44

 167.95 142.25 31.21 31.13 29.19 28.22

   33.59 32.56 31.53 30.51

   7.09 4.39 7.42 7.51

ΔTs = Tc − Tm. bΔH1 = (ΔHc + ΔHm)/2. cΔH2 =

mPW 625g

× 100% × ΔH1,PW +

mMePW 625g

× 100% × ΔH1,PCM , where mPW, mMePW, ΔHPW, and

ΔH1,PCM are the mass of PW, mass of MePW, ΔH1 of PW, and ΔH1 of PCM, respectively. dError % = (ΔH2 − ΔH1)/ΔH2.

Figure 7. TGA and DTG curves of PW, nanoSiO2-EG-PW PCM, pristine HTPB, PW/HTPB, 20 wt % nanoSiO2-EG-PW/PW/HTPB, 40 wt % nanoSiO2-EG-PW/PW/HTPB, and 60 wt % nanoSiO2-EG-PW/PW/HTPB composites.

25 to 75 °C. The latent heat capacity of HTPB composites was provided by PW and nanoSiO2-EG-PW PCM. As generalized from Table 2, with the increasing contents of nanoSiO2-EG-

PW, the total percentage of PW in nanoSiO2-EG-PW/PW/ HTPB composites decreased, and thus the calculated enthalpy (ΔH2) of these composites decreased. The measured enthalpy E

DOI: 10.1021/acs.energyfuels.7b03856 Energy Fuels XXXX, XXX, XXX−XXX

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Table 3. Thermal Stability of PW, NanoSiO2-EG-PW PCM, Pristine HTPB, PW/HTPB, 20 wt % NanoSiO2-EG-PW/PW/ HTPB, 40 wt % NanoSiO2-EG-PW/PW/HTPB, and 60 wt % NanoSiO2-EG-PW/PW/HTPB Compositesa Step 1

Step 2

samples

T1,o (°C)

T1,e (°C)

T1,m (°C)

weight loss (%)

T2,o (°C)

T2,e (°C)

T2,m (°C)

weight loss (%)

total weight loss (%)

PW nanoSiO2-EG-PW HTPB PW/HTPB 20 wt % PCM/PW/HTPB 40 wt % PCM/PW/HTPB 60 wt % PCM/PW/HTPB

145.39 158.45 212.83 208.33 207.67 207.50 205.00

298.81 435.17 402.83 395.17 348.33 363.33 353.83

259.19 323.17 324.83 316.83 286.33 307.00 292.50

98.12 82.76 28.02 43.14 39.68 38.29 35.63

  418.83 403.17 402.00 398.17 401.83

  511.25 490.44 505.67 507.83 513.17

  461.17 460.83 461.00 462.83 463.00

  68.33 52.43 55.45 56.58 56.74

98.12 82.76 96.35 95.57 95.13 94.87 92.37

a T1,o and T1,e were obtained from the cross-point of two tangent lines at related bending locations during the first decomposition. T1,m was considered as the peak temperature on the derivative curve of the first decomposition. Similar methods were introduced to determine T2,o, T2,e, and T2,m.

Figure 8. Leakage status of PW and nanoSiO2-EG-PW FSPCM composites at 120 °C for 1 h, before and after 100, 300, and 500 thermal cycles.

and the weight loss of 418.83−511.25 °C was ascribed to the degradation of the polybutadiene segment.47 For nanoSiO2EG-PW/PW/HTPB composites, the first weight loss stage was associated with the degradation of PW and hard segment of HTPB, while the second degradation step corresponded to the decomposition of nanoSiO2-EG-PW PCM and the soft segment of HTPB, for the reason that the degradation temperature range of nanoSiO2-EG-PW was more consistent with the second degradation temperature range of pristine HTPB. Correspondingly, for the nanoSiO2-EG-PW/PW/ HTPB composites, with increasing contents of nanoSiO2-EGPW, the weight loss of the first degradation step decreased, while that of the second degradation step increased. Simultaneously, their total weight loss decreased, meaning that the residue weight increased, possibly caused by the increasing content of introduced nanoSiO2 and EG. Moreover, the start, peak, and end temperatures of nanoSiO2-EG-PW/ PW/HTPB composites undergoing the first degradation step were slightly lower than that of PW/HTPB. Nevertheless, the peak and end temperatures of nanoSiO2-EG-PW/PW/HTPB composites undergoing the second degradation step were higher than that of PW/HTPB and increased with the increasing content of nanoSiO2-EG-PW, due to the thermal reinforcement of added nanoSiO2-EG-PW. It was demonstrated that the addition of nanoSiO2-EG-PW could distinctly improve the thermal stability of PW-based HTPB. 3.5. Thermal Reliability of NanoSiO2-EG-PW/PW/HTPB Composites. A long-time 500 thermal-cycling test was conducted to investigate the leakage status of FSPCM and thermal reliability of nanoSiO2-EG-PW/PW/HTPB composites. The leakage status of the PW and FSPCM composite at

(ΔH1) value of nanoSiO2-EG-PW/PW/HTPB composites was lower than the ΔH2 value, possibly caused by the loss of PW during the curing process at 65 °C for 3 days. Furthermore, as the contents of nanoSiO2-EG-PW increased, the temperature at the endothermic peak (Tm) on the DSC curves of the HTPB composites was improved because of the higher Tm value of the introduced nanoSiO2-EG-PW PCM, as well the difference (ΔTs) between Tm and temperature at the exothermic peak (Tc) of HTPB composites increased. It was illustrated that the nanoSiO2-EG-PW/PW/HTPB composites kept high latent heat capacity, and the addition of nanoSiO2-EG-PW could delay the melting process of PW in HTPB composites. 3.4. Thermal Stability of NanoSiO2-EG-PW PCM and NanoSiO2-EG-PW/PW/HTPB Composites. The TGA curves and corresponding parameters of PW, nanoSiO2-EG-PW PCM, pristine HTPB, PW/HTPB, and nanoSiO2-EG-PW/PW/ HTPB composites with different contents of PCM were presented in Figure 7 and Table 3, respectively. As displayed in Figure 7a, the TGA curve of PW exhibited a weight loss stage from 145.39 to 298.81 °C, due to the degradation of PW. Compared with PW, there was a weight loss stage with enhanced temperature range of 158.45−435.17 °C existing in the TGA curve of nanoSiO2-EG-PW PCM, indicating that nanoSiO2 and EG could well support PW and delay the degradation of PW. In addition, the addition of nanoSiO2 and EG resulted in a decreased total weight loss of nanoSiO2-EGPW compared with PW. As shown in Figure 7b, all the TGA curves of nanoSiO2-EG-PW/PW/HTPB composites presented two weight loss stages. For pristine HTPB, the weight loss of 212.83−402.83 °C was attributed to the depolycondensation and the degradation of allophanate, biurate, and urea linkages, F

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Figure 9. DSC curves of PW/HTPB and 60 wt % nanoSiO2-EG-PW/PW/HTPB composites before and after 500-thermal-cycle test.

120 °C for 1 h, before and after 100, 300, and 500 thermal cycles, was displayed in Figure 8. As shown in Figure 8, the pure PW cake was melted completely at 120 °C, while the FSPCM cake maintained intrinsic shape with little leakage of paraffin. In addition, the pure PW cake was melted mostly and completely after 100 and 300 thermal cycles, respectively. However, the FSPCM cake could retain the inherent shape even after 500 thermal cycles. Therefore, it was indicated that the shape stability of FSPCM could be excellent under conditions above the phase change temperature and after many times of use. The DSC curves of PW/HTPB and 60 wt % nanoSiO2-EGPW/PW/HTPB composites before and after thermal cycling were shown in Figure 9. The temperatures at the endothermic peak and exothermic peak of PW/HTPB and 60 wt % nanoSiO2-EG-PW/PW/HTPB composites were almost consistent before and after a thermal cycling test. The enthalpies of both the composites after thermal-cycling test were slightly lower than that before, since the repetitive thermal cycles resulted in a little diffusion of PW. Moreover, the enthalpy loss of the 60 wt % nanoSiO2-EG-PW/PW/HTPB composite was less than that of PW/HTPB, suggesting a good form-stable effect of nanoSiO2-EG-PW on PW in the HTPB matrix. 3.6. Thermal Conductivity of NanoSiO2-EG-PW PCM and NanoSiO2-EG-PW/PW/HTPB Composites. Thermal conductivity is regarded as an important coefficient for PCM, which responds to the heat storage/release rate of materials.10,25,48 EG is one of the most effective materials selected to increase the thermal conductivity of composites. Thus, the thermal conductivity of PW, nanoSiO2-EG-PW PCM, pristine HTPB, PW/HTPB, and nanoSiO2-EG-PW/PW/HTPB composites was displayed in Figure 10. The thermal conductivity of nanoSiO2-EG-PW PCM was nearly twice as high as that of PW, which was mainly caused by the function of the thermal conductivity reinforcer provided by EG based on its graphite skeleton structure.19 The thermal conductivity of PW/HTPB was almost consistent with that of pristine HTPB, due to the absolutely high content of HTPB in PW/HTPB and the approximate thermal conductivities of PW and pristine HTPB. With the increasing loading of nanoSiO2-EG-PW, the thermal conductivity of nanoSiO2-EG-PW/PW/HTPB composites was increased, due to the increasing content of EG introduced into the composites. It was revealed that the addition of nanoSiO2EG-PW with enhanced thermal conductivity could improve the thermal conductivity of PW-based HTPB.

Figure 10. Thermal conductivity coefficient of (a) PW, (b) nanoSiO2EG-PW PCM, (c) pristine HTPB, (d) PW/HTPB, (e) 20 wt % nanoSiO2-EG-PW/PW/HTPB, (f) 40 wt % nanoSiO2-EG-PW/PW/ HTPB, and (g) 60 wt % nanoSiO2-EG-PW/PW/HTPB composites.

4. CONCLUSIONS A novel nanoSiO2-EG-PW with enhanced thermal properties was developed and incorporated into PW-based HTPB through physical blending, aimed to improve its thermal properties and compatibility between PW and HTPB. FTIR, SEM, DSC, TGA, thermal cycling test, and thermal conductivity test were conducted to investigate the chemical composition, morphology, and thermal properties of nanoSiO2-EG-PW and nanoSiO2-EG-PW/PW/HTPB composites, respectively. Based on the results, the doped nanoSiO2-EG-PW could effectively shape-stabilize PW and prevent PW from separating from the HTPB matrix. The comprehensive properties of this FSPCM, including melting temperature, thermal stability, thermal conductivity, and thermal reliability, were dramatically improved, compared with those of PW. It should be notable that the thermal conductivity of nanoSiO2-EG-PW was improved from 0.303 W·m−1·K−1 of PW to 0.602 W·m−1·K−1. The nanoSiO2-EG-PW/PW/HTPB composites possessed high latent heat capacity, delayed melting temperature, and good thermal reliability compared with PW/HTPB. As the content of nanoSiO2-EG-PW increased, the thermal stability and thermal conductivity of the prepared composites increased. Therefore, the FSPCM can be considered in many promising applications for energy storage and management. Especially, the FSPCM with enhanced thermal conductivity has great potential to improve the environmental suitability, thermal insensitivity and shorten the curing time of PBX. G

DOI: 10.1021/acs.energyfuels.7b03856 Energy Fuels XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-81381389. E-mail: [email protected]. ORCID

Tianbo Zhao: 0000-0002-7241-9746 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly appreciate the financial support from the National Natural Science Foundation of China (No. 20973022, 11472048, 11502249).

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DOI: 10.1021/acs.energyfuels.7b03856 Energy Fuels XXXX, XXX, XXX−XXX