Scholarly article on topic 'Tailoring thermal properties via synergistic effect in a multifunctional phase change composite based on methyl stearate'

Tailoring thermal properties via synergistic effect in a multifunctional phase change composite based on methyl stearate Academic research paper on "Materials engineering"

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{"Phase change materials" / "Flame retardant" / "Thermal stability" / "Thermal energy storage" / "Electro-to-thermal conversion"}

Abstract of research paper on Materials engineering, author of scientific article — Hassina Tabassum, Xinyu Huang, Renjie Chen, Ruqiang Zou

Abstract Organic phase change materials (PCMs) have poor conductivity and are naturally flammable. In this effort, we report a multifunctional phase change composite in which electro-to-thermal energy conversion could be realized by applying very small voltages as low as 1.4 V. Methyl stearate (MeSA) is used as the phase change material, with expanded graphite (EG) as the support, nano-organophilic montmorillonite (nOMMT) and ammonium polyphosphate (APP) as the flame retardants. Uniform interpenetration of PCM in the composite coupled with synergistic effect of the additives performs superb properties in improving the thermal stability and latent heat of composite. The thermal conductivity of the composite is increased up to 3.6 W m−1 K−1, while the thermal enthalpy is retained and an electro-to-thermal conversion efficiency of 72% is reached. Our tailored approach gives rise to a new avenue for practical application of PCM with facile preparation.

Academic research paper on topic "Tailoring thermal properties via synergistic effect in a multifunctional phase change composite based on methyl stearate"

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Journal of Materiomics 1 (2015) 229-235

www.journals.elsevier.com/journal-of-materiomics/

Tailoring thermal properties via synergistic effect in a multifunctional phase

change composite based on methyl stearate

Hassina Tabassum1, Xinyu Huang1, Renjie Chen, Ruqiang Zou*

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China

Received 20 May 2015; revised 28 June 2015; accepted 10 July 2015 Available online 22 July 2015

Abstract

Organic phase change materials (PCMs) have poor conductivity and are naturally flammable. In this effort, we report a multifunctional phase change composite in which electro-to-thermal energy conversion could be realized by applying very small voltages as low as 1.4 V. Methyl stearate (MeSA) is used as the phase change material, with expanded graphite (EG) as the support, nano-organophilic montmorillonite (nOMMT) and ammonium polyphosphate (APP) as the flame retardants. Uniform interpenetration of PCM in the composite coupled with synergistic effect of the additives performs superb properties in improving the thermal stability and latent heat of composite. The thermal conductivity of the composite is increased up to 3.6 W m-1 KT1, while the thermal enthalpy is retained and an electro-to-thermal conversion efficiency of 72% is reached. Our tailored approach gives rise to a new avenue for practical application of PCM with facile preparation. © 2015 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Phase change materials; Flame retardant; Thermal stability; Thermal energy storage; Electro-to-thermal conversion

1. Introduction

To cope with energy crisis, various methods have been developed to explore new energy resource or enhance service efficiency [1—6]. Thermal energy storage is one of the most effective ways to ease peak demand and increase energy conversion efficiency. By storing surplus energy produced during low demand period and release it when the need is high, a more equalized energy supply can be accomplished [7—10]. Thermal energy can be stored in three ways: sensible heat, latent heat and thermochemical methods [11,12]. Phase change materials (PCMs) are used for heat storage because they can absorb and release thermal energy during phase transition. The usage of PCMs is called latent heat storage because the phase change process occurs at a constant

* Corresponding author. E-mail address: rzou@pku.edu.cn (R. Zou). Peer review under responsibility of The Chinese Ceramic Society.

1 These authors contributed equally to this work.

temperature or in a small temperature range [13—19]. Due to this fabulous trait and a relatively large energy density, PCMs give a promising way to utilize thermal energy coming from the surroundings, solar irradiation, waste heat from vehicle engine and electronic devices [5,20—27].

PCMs can be classified into inorganic, organic and polymeric PCMs. Organic PCMs have drawn extensive attention owing to their wide range of melting points, high phase change enthalpy, vast availability, chemical compatibility for containers and cycle usage endurance. Paraffin and fatty acid are the two main types of organic latent heat storage materials, while recently eutectic mixture and fatty acid ester are broadly studied [4,5,28—31]. Fatty acid ester can retain the high latent heat of fatty acid and avoid some weakness such as corrosivity and smell. Palmitic acid and stearic acid are usually used to synthesize the corresponding ester for their wide application in many aspects [29]. Nevertheless, some disadvantages of organic PCMs including low thermal conductivity and high flammability have blocked them from practical applications such as building [32—34]. Paraffin and fatty acid are easily

http://dx.doi.org/10.1016/j.jmat.2015.07.002

2352-8478/© 2015 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

ignited at high temperature. When they are used in residential application, the containers should be isolated from heat source. Besides keeping distant from fire, the addition of flame retardants (FRs) can effectively retard flames. Melmine, intercalated kaolinite, silsesquioxane and silicon dioxide are used as FRs in phase change composites [35—38]. Multiple flame retardants are also investigated to control flammability [39—41]. Ammonium polyphosphate (APP) is an intumescent flame retardant that can produce CO2, NH3 and phosphoric acid during combustion. It is used with iron, pentaerythritol and melamine to induce a synergistic effect in HDPE/paraffin composite, studied in previous report [42]. The addition of iron improves the flame retardance efficiency by participating in the char formation.

In this research, we adopt methyl stearate (MeSA) as the phase change material, with expanded graphite (EG), APP and nano-organophilic montmorilonite (nOMMT) as the additives. EG can work as the framework and enhance the thermal conductivity. APP and nOMMT are used to control the flame and effect of variation of these two FRs on the thermal stability of the composite is studied. The composites also display some superior property comparing with pristine MeSA such as higher thermal conductivity and comparable latent heat. Moreover, the composites own the ability to convert and store electricity with thermal energy form. Our experiment reveals a shape-stabilized multi-functional phase change composite (PCC) for practical thermal energy storage.

2. Materials and methods

2.1. Materials

MeSA (Ci9H38O2) was chemical pure, purchased from Sinopharm Chemical Reagent Co., Ltd. EG was supplied by Qingdao Jinrilai graphite Co. Ltd. Technical grade APP, and nOMMT were used without further purification.

2.2. Preparation

The PCCs were prepared by using N,N-dimethylformamide (DMF) as solvent in a simple solution method. The samples were composed of constant weight percentage of EG and MeSA with variable ratios of nOMMT/APP. The sample mixture was heated and stirred at 40 °C for 1 h, and then was moved to an oven to evaporate the solvents at 150 °C for 12 h to obtain the homogenous composites.

2.3. Characterizations

The microstructure of PCCs was characterized by a field emission scanning electron microscope (FESEM) Hitachi-s4800 operated under high vacuum. Thermal conductivity was measured by thermal conductance meter Xiatech TC3010. Differential scanning calorimetry (DSC) data were collected with a differential scanning calorimeter of Setaram DSC 131 evo model in Al 30 mL pan. The thermal degradation curves were obtained by thermo-gravimetric analysis (TGA) TA SDT-Q600 instrument. Fourier transform infrared (FTIR) absorption spectra were measured by using thermo-Fisher Nicolet from 4000 to 300 cm-1 on KBr pellet. The electro-to-thermal energy conversion property of the composites was measured using a two-electrode system by Zahner ennium/ IM6 electrochemical workstation. A particular bias voltage (1.4—1.7 V) was applied to composite bulk for about 1600 s and then stopped and in the meantime flowing current is recorded. Temperature variation of samples was recorded by ICDAM-7033 and ICDAM-7520 (data acquisition system) connected to Pt100 thermal resistance.

3. Results and discussion

The facile fabrication process of target PCC containing MeSA, EG, nOMMT and APP is summarized in Scheme 1. The content of MeSA and EG is invariable as 0.8 g and 0.15 g in weight percentage, respectively, and the remaining 0.05 g components are contributed from the FRs of APP and nOMMT with various weight percentages entitled as PCC1 to PCC5 (Table 1). PCC6 with 0.80 g MeSA and 0.15 g EG is prepared as the control. DMF is used as solvent to uniformly disperse an appropriate amount of MeSA, EG, nOMMT and APP. After drying, the initial powder is pressed into rectangular bulk.

The microstructure of the composite is revealed with FESEM image (Fig. 1), in which EG consists of many small

Table 1

Compositions of six PCCs in weight percentage.

Sample name MeSA/g EG/g nOMMT/g APP/g

PCC1 0.80 0.15 — 0.05

PCC2 0.80 0.15 0.02 0.03

PCC3 0.80 0.15 0.025 0.025

PCC4 0.80 0.15 0.03 0.02

PCC5 0.80 0.15 0.05 —

PCC6 0.80 0.15 — —

Scheme 1. The preparation of PCCs.

-Î.GÔ nm

|| 3.00 n

Fig. 1. Microstructure of EG and the composite. The FESEM images of (a) EG and (b) PCC4.

flakes with 3—4 mm width (Fig. 1a), and MeSA was uniformly wrapped by EG flakes and stuck adjacent flakes together in return (Fig. 1b). The results clearly demonstrate that MeSA has successfully coated EG flakes and the sheets can hold MeSA during phase transition process by surface tension, which provides the shape-stable property for the composite.

Besides loading PCM, EG can also enhance the thermal conductivity of the composites. It is known that organic PCM has low thermal conductivity, which hinders its practical application. EG has a thermal conductivity value of 7.5 W m"1 K"1, much higher than raw organic PCMs, which usually ranges from 0.1 to 1 W m"1 K"1 [43]. With the introduction of EG, the thermal conductivity of the composites can be increased up to 3.6 W m 1 K"1, which is much higher than that of pure MeSA. The detailed thermal conductivities of

Fig. 2. The thermal conductivities of PCCs and pure MeSA at different temperatures.

all six PCCs and pure MeSA are summarized in Fig. 2. PCC6 with MeSA and EG in it attains a conductivity less than 3 W m"1 K"1, due to a comparably higher MeSA content in it. The composites with FRs in them have similar thermal conductivity values, while the different compositions of FRs cause slight variation from 3.3 to 3.6 W m 1 K"1. All thermal conductivity values of PCCs decreased about 0.2 W m"1 K"1 during the phase transition of MeSA from solid to liquid state. The decrease of thermal conductivity during phase transition is observed in other works [44,45]. This phenomenon in this work could be attributed to a more active molecular movement of MeSA molecules, disturbing the thermal diffusion in the PCCs. A higher thermal conductivity benefits the transfer of heat flow, so the composites can absorb energy more easily.

The thermal enthalpies of PCCs are investigated by DSC experiments as shown in Fig. 3. The onset temperature of the DSC curves and melting enthalpy (DHm) are listed in Table 2. The phase transition temperature of composites mildly differs from pure MeSA, suggesting that the existence of additives doesn't obviously affect the phase change process of MeSA. Though all samples have same content of MeSA, the latent heats they stored are different. A comparative enthalpy of 80% MeSA should be 150 J g"1, and enthalpy values of all samples lie in the range of 135—150 J g"1. The composites possess at least 90% enthalpy of the MeSA they contains, which proves good crystalline behavior of MeSA in the system. Furthermore, we compare the enthalpy of our composites with two published work using MeSA as phase change material.

Thermal stability of PCCs is evaluated using TGA technique under N2 as shown in Fig. 4 and Table 3. All the composites decompose at a higher temperature than pure MeSA and leave various amounts of residues after

Fig. 3. Thermal enthalpy results comparison between composites and MeSA. (a) DSC curves. (b) Thermal enthalpy comparison.

Table 2

Thermal enthalpies of MeSA and PCCs.

Sample T /° C m DHm/J g- Tf/°C DHf/J g-1

MeSA 32.5 188 32.7 199

PCC1 32.5 152 31.8 138

PCC2 32.5 136 33.6 139

PCC3 33.0 138 31.7 137

PCC4 33.4 147 33.1 147

PCC5 32.4 138 33.7 138

PCC6 33.5 145 34.4 146

MES/PAN [46] 35.5 107 33.3 106

CPCM [47] 25.5 164 31.1 155

Tm: the peak onset temperature when melting. Tf: the peak onset temperature when freezing. DHm: melting enthalpy. DHf: freezing enthalpy.

Fig. 4. TGA curves of MeSA and PCCs in nitrogen.

Table 3

Thermal degradation data under nitrogen atmosphere by TGA.

Char residue/wt%

Sample

Degradation temperature/°C

MeSA PCC1 PCC2 PCC3 PCC4 PCC5 PCC6

135 178 187 181 209 172 170

3.8 11.9 24.5

17.7 31.3

12.8 20.1

nOMMT and APP meets its decomposition plateau at a lower temperature, implying the combination of two FRs doesn't ensure good flame retardancy or with only either nOMMT or APP and the ratio of FRs needs optimization to obtain best flame impediment.

The formation and structure of char are very important parameters affecting the thermal stability and flame retardancy properties of PCM composites. Char residue under nitrogen condition is further studied through FTIR analysis for PCC4 since it exhibits the best thermal stability (Fig. 5). Compared to the untreated sample, the peak of C=O of char shifted from 1644 to 1746 cm-1. It suggests that the ester group of methyl stearate has decomposed completely. Three new peaks at 1435, 1467 and 1630 cm-1 from FTIR curve of char are attributed to the benzene skeleton and double bond vibration, which means that strong carbon bonding forms during ignition. It retains more char in the residue, hindering the transfer of heat. Furthermore, the peaks at 714, 804 and 884 cm-1 are attributed to the C—H vibration on the alkene group, implying that unsaturated carbon structure has emerged in condensed phase due to the effect of APP.

The morphology of char residue for PCC4 is characterized by FESEM. As shown in Fig. 6a, the surface of PCC4 is smooth because MeSA highly disperses on the surface of EG. During the decomposition of MeSA, APP decomposes and produces some gas that can adsorb heat [48]. nOMMT bonds with the decomposed product and expand, leaving a thick layer in the residue with some obvious holes appeared on the surface of char residue (Fig. 6b). The morphology is in compliance with the results of FTIR, in which MeSA decomposes completely and char residue is formed in the assist of FRs. It reveals that the synergism of APP and nOMMT could contribute to form char residue when the composites begin to decompose.

Since the high electronic conductivity of EG, these PCCs could be used for electro-to-thermal conversion via latent heat storage [49]. Current vs. voltage (I— V) measurement shows that current flows through pressed pristine EG bulk under voltage 1.6 V and stabilizes at about 97 mA with low resistance of 16.5 U. The current value of PCC4 remains similar

decomposition in nitrogen (Fig. 4). These TGA curves clearly illustrate the stability difference of PCCs. The control, PCC6, displays an earlier decomposition temperature than other PCCs. PCC1 and PCC5 with either nOMMT or APP, display similar feature and leave similar weight percentage of char residue. The existence of single flame retardant could enhance the flame retardance, but the synergistic effect between appropriate amounts of flame retardants result in better performance. PCC2 and PCC4 produce different amounts of char residues, in which PCC4 has the highest degradation temperature and more residue among all PCCs. This is attributed to a better synergism of APP and nOMMT in PCC4. Different from the other PCCs, PCC3 with the same weight ratio of

Absorbancy

Fig. 5. The FTIR plots of PCC4 and its char residue.

under the same voltage whether frozen or molten state, which suggests that the presence of MeSA do not significantly disturb the conductive framework as proven in Fig. 7a. PCC can be heated under applied bias and store thermal energy. When a constant voltage is applied to pristine EG flakes, an abrupt temperature change happens and then reaches equilibrium ultimately. The temperature of flakes drops to room temperature rapidly after the voltage is removed. As expected, PCC4 demonstrates a distinct feature of temperature—time curve from EG flakes. The comparison between Fig. 7b and c reveals that the composite has periods where temperature is inactive to time during heating and relaxation during the phase transition process of MeSA. Thus thermal energy is stored or released via a phase change process when voltage is applied or turned off. Because application of voltage leads to a change in temperature of the composite, the effect of varying voltage on composite temperature is also studied as a function of applied bias. The efficiency (h) of electro-to-thermal energy conversion was calculated using the following relation

(mDH) Ult

where 'EPCM' stands for energy stored in PCM, 'EElectro' stands for energy provided by applying bias, 'm' is the mass of the MeSA in composites, 'DH' is enthalpy, T is current, 't' is time obtained from curves by tangential method, depicted in the inset of Fig. 7d, and 'U is voltage. The calculated h of PCC was from 47% to 72% under the voltage of 1.4—1.7 V, respectively (Fig. 7d). A higher applied voltage results in higher conversion efficiency, while the conversion can be accomplished at voltage as low as 1.4 V. The results indicated that electrical potential could influence the PCC4 composites. Using proper potentials, the PCC4 composites can be provided with energy which is stored as thermal energy in the composite.

4. Conclusion

In summary, we prepared a series of shape-stabilized PCCs with fatty acid ester MeSA infiltrated in porous EG, APP and

Fig. 7. The electro-to-thermal conversion of EG and PCC4. (a) Current—time plots under 1.6 V of EG and PCC4. Temperature variations versus time under different voltages of (b) EG and (c) PCC4. (d) Energy conversion efficiency of PCC4 with different voltages.

nOMMT serve as the flame retardant additive. The thermal conductivity and phase change enthalpy of composites can be tailored by the respective fractions of APP and nOMMT. The synergism between appropriate FRs provides better thermal stability with the most char residue and highest degradation temperature. Notably, the composites could store thermal energy by applying voltage as low as 1.4 V, which opens ways for wide applications in areas related to electro-to-thermal thermal energy conversion and storage.

Acknowledgment

This work was supported by the National Natural Science Foundation of China 51322205 and 21371014, the New Star Program of Beijing Committee of Science and Technology (2012004).

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Hassina Tabassum received M.Sc. (2009) in Physics from Islamia University of Bahawalpur. She acquired her MS degree in Physics (2012) from International Islamic University Islamabad. She is currently a PhD candidate in Prof. Ruqiang Zou's group in the Department of Materials Science and Engineering, Peking University Beijing. China. Her current research focuses on the synthesis of MOF based heteroatom doped carbon hybrids for energy storage applications.

Xinyu Huang obtained his B.Sc. in Materials Science and Engineering in 2013 from Peking University. He is currently a PhD candidate in Prof. Ruqiang Zou's group at Peking University. He has been working on phase change material for thermal energy storage since 2012.

Renjie Chen obtained his PhD at Beijing Institute of Technology. After that, he worked as a postdoctoral fellowship in Prof. Ruqiang Zou's group at Peking University from 2012 to 2014. His work is mainly engaged in porous materials and phase change materials.

Ruqiang Zou is currently the Professor of Materials Science and Engineering at the College of Engineering, Peking University. He received his PhD from Kobe University and the National Institute of Advanced Industrial Science and Technology (AIST), Japan. He was awarded the JSPS Younger Scientist award during his doctoral course in Japan. After graduating from Kobe University in 2008, he was awarded a Director's Postdoctoral Fellowship at Los Alamos National Laboratory in the United States, the New Century Excellent Talents in University from the Ministry of Education of China, the New-Star of Science and Technology in Beijing, and the Excellent Young Scientist Foundation of NSFC. His research interests focus on the controllable preparation of nanopo-rous materials for green energy utilization. He proposed a new strategy to construct hierarchically porous materials for single molecule adsorption, and extended their potential applications in hydrogen storage, carbon capture, and energy storage materials.