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Enhancing thermal conductivity of palmitic acid based phase change materials with carbon nanotubes as fillers
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Solar Energy 84 (2010) 339–344 www.elsevier.com/locate/solener
Enhancing thermal conductivity of palmitic acid based phase change materials with carbon nanotubes as fillers Jifen Wang a,b, Huaqing Xie a,*, Zhong Xin b, Yang Li a, Lifei Chen a a
School of Urban Development and Environmental Engineering, Shanghai Second Polytechnic University, Shanghai 201209, China b State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Received 28 February 2009; received in revised form 30 November 2009; accepted 2 December 2009 Available online 29 December 2009 Communicated by: Associate Editor Halime Paksoy
Abstract Multi-walled carbon nanotubes (CNTs) as produced are usually entangled and not ready to be dispersed into organic matrix. CNTs were treated by mechano-chemical reaction with ball milling the mixture of potassium hydroxide and the pristine CNTs. Hydroxide radical functional groups have been introduced on the CNT surfaces, which enabled to make stable and homogeneous CNT composites. Treated CNTs were successfully dispersed into the palmitic acid matrix without any surfactant. Transient short-hot-wire apparatus was used to measure the thermal conductivities of these nanotube composites. Nanotube composites have substantially higher thermal conductivities than the base palmitic acid matrix, with the enhancement increasing with the mass fraction of CNTs in both liquid state and solid state. The enhancements of the thermal conductivity are about 30% higher than the reported corresponding values for palmitic acid based phase change nanocomposites containing 1 wt% CNTs treated by concentrated acid mixture. Ó 2009 Elsevier Ltd. All rights reserved. Keywords: Phase change material; Palmitic acid; Carbon nanotube; Nanocomposite; Thermal conductivity
1. Introduction Among the available techniques suitable for storing thermal energy and for controlling temperature in systems subjected to periodic heating, the use of solid–liquid phase change has attracted considerable attention (Alkan et al., 2008; Shenogin et al., 2004; do Couto Aktay et al., 2008). As phase change materials (PCMs), due to the high storage
Abbreviations: CNT, carbon nanotube; PCM, phase change material; PA, palmitic acid; PCNT, pristine carbon nanotube; TCNT, treated carbon nanotube; FTIR, Fourier transformation infrared spectroscopy; PA/ TCNT, nanocomposite containing palmitic acid and treated multi-walled carbon nanotubes; Tm, melting temperature; Ls, latent heat capacity; K0, the thermal conductivities of composites; Kc, the thermal conductivities of pure PA. * Corresponding author. Tel./fax: +86 21 50217331. E-mail address: [email protected] (H. Xie). 0038-092X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2009.12.004
density and small temperature variation from storage to retrieval and low melting temperature, organic materials including paraffin waxes and fatty acids have been employed as PCMs for thermal energy storage in many applications (Sari and Kaygusuz, 2002; Sari et al., 2008; Sari and Karaipekli, 2008; Dimaano and Watanabe, 2002; Wang et al., 2009). However, low thermal conductivities, the major drawback which leads to decreasing the rates of heat storage and retrieval during melting and solidification processes, limit their utility areas. To overcome this problem, a wide range of investigations were carried out to enhance the thermal conductivity of the organic PCMs. The often used method is to disperse solid particles with high thermal conductivity, such as expanded graphite, copper and other metal particles and so on, to form composite PCMs. However, uses of such types of heat transfer promoters considerably increase the weight and volume of latent heat thermal energy storage systems. Moreover the
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PCM composites often show poor stability because of the large difference between the solid inclusions and the organic matrix. Carbon nanotubes (CNTs) with extremely high thermal conductivity have shown tremendous potential for heat transfer applications (Carlborg et al., 2008; Kumara et al., 2007; Shaikh et al., 2008). Compared with the aforementioned heat transfer promoters, CNTs as additives have smaller density difference to the organic matrix than the metal or metal oxide particles, which makes CNTs have better stability in the organic matrix. Several researches have been reported wherein CNTs are embedded in the base fluids and phase change materials to increase their thermal conductivities (Elgafy and Lafdi, 2005; Wang et al., 2008; Bonnet et al., 2007; Zeng et al., 2008). Numerical simulations were carried out to analyze the effects of chemical functionalization on the thermal transport of carbon nanotubes and their composites (Berber et al., 2000). However, to the best knowledge of the authors, there is no theoretical or experimental study that deals with the effect of CNTs with alkaline treatment on the thermal transport of the composite as PCMs. The objective of this study is to provide a novel technical approach for preparing thermal performance enhanced PCMs. Multi-walled CNTs are used as fillers for tailoring the thermal properties of the matrix material, palmitic acid. Mechano-chemical reaction is employed to treat the pristine CNTs (PCNTs) with poor dispersibility. The current study investigates the thermal conductivity of the nanocomposites and discusses the mechanism of the thermal transport enhancement.
2. Experimental 2.1. Materials Palmitic acid (PA, 98%) and potassium hydroxide (A.R.) were obtained from Sinopharm Chemical Reagent Co., Ltd. The PA was used without further purification and the melting point of it is about 62.5 °C. Multi-walled carbon nanotubes (CNTs) were commercially supplied by Chendu Organic Chemicals Co., Ltd., Chinese Academy of Sciences. The average diameter, average length, and specific surface area of the CNTs were 30 nm, 50 lm, and 60 m2/g. 2.2. Synthesis of the composites The mechano-chemical reaction treatment described in Pan et al. (2003) was applied to tailor the CNT surfaces to enhance the dispersibility of the CNTs. In a typical treatment, 1 g of PCNTs was mixed with 20 g of potassium hydroxide and the mixture was ball-milled for 720 min. Then, the sample was diluted by distilled water, filtered, and washed repeatedly till the washings show no alkalescence. The cleaned CNTs were collected and dried at 100 °C for 24 h to remove the attached water. Fig. 1 shows the scanning electronic microscope (SEM) images of the PCNTs, treated CNTs (TCNTs) and the nanocomposites containing PA and TCNTs (PA/TCNTs). It is observed that TCNTs after mechano-chemical treatment look shorter compared with the PCNTs as obtained. During the mechano-chemical treatment, the surfaces of
Fig. 1. SEM images of PCNTs (A), TCNTs (B), and PA/TCNT (C).
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the CNTs, have been modified. As shown in Fig. 2, the TCNTs show a remarkably different Fourier transformation infrared spectroscopy (FTIR) from that of the PCNTs. Consistent with the previously reported results (Pan et al., 2003; Koshio et al., 2001), no detectable transmission band was observed for the PCNTs in the wavenumber range covered in this study. In contrast, the corresponding FTIR spectrum for TCNTs shows a very broad transmission band centered at 3411 cm1, characteristic of hydrogen bonded –OH. The band at 1095 cm1 is corresponding to the stretching band of C–O, while the band at 782 cm1 can be interpreted to the bending stretching band of hydroxyl groups. The strong transmission band at around 1629 cm1 can be attributed to the stretching mode of – C@C– in an enol form (Pan et al., 2003). It is indicated that the –OH groups have been added onto the surface of the CNTs. The nanocomposites were obtained by adding the TCNTs into melting PA in a mixing container. Intensive sonication was further used to make well dispersed and homogeneous mixtures. As shown in Fig. 1(c), TCNTs have been dispersed well in PA matrix. 2.3. Thermal conductivity measurement Short-hot-wire method was used to measure the thermal conductivities (k) of pure PA and the PA/TCNT composites and the detailed procedure of the method has been described elsewhere (Xie et al., 2006). Briefly a platinum wire with a diameter of 70 lm was used for the hot wire, and it served as both a heating unit and as an electrical resistance thermometer. Initially the platinum wire immersed in media was kept at equilibrium with the surroundings. When a regulation voltage was supplied to initiate the measurement, the electrical resistance of the wire changed proportionally with the rise in temperature. The thermal conductivity was calculated from the slope of the rise in the wire’s temperature against the logarithmic time interval. The uncertainty of this measurement is estimated
to be within ±1.0%. In our measurements, the PCM sample was melted and poured into a stainless steel cylinder container kept at a temperature higher than the melting point of the sample. A waterproof lid with pre-positioned hot wire and thermocouple was used to seal the container after sample encapsulation. The container was then put into a temperature-controlled thermostat bath with temperature variation less than 0.1 °C. The tested temperature was increased every 10 °C from 15 °C to 55 °C while increased every 5 °C from 65 °C to 75 °C. Instead of monitoring the temperature of the bath, we monitored the temperature on the spot with the pre-positioned thermocouple inside the sample. When the temperature of the sample reached a steady value, it waited further 20 min to make sure that the initial state is at equilibrium. At every tested temperature, the thermal conductivity were measured three times and taken the average values as the final results. Twenty minutes interval was needed between every measurement. After the above-mentioned careful check on the measurement condition and procedure, it could obtain the thermal conductivities around the melting point. Melting temperature (T m ) and latent heat capacity (Ls ) of pure PA and composites were measured using a differential scanning calorimetric (DSC) instrument (Diamond DSC, Perkin-Elmer, USA). The DSC measurements were performed at a heating rate of 5 °C/min and in a temperature range 15–70 °C. 3. Results and discussion Thermal conductivity is one of the most important properties of PCMs. The relationship between the thermal conductivities and temperatures is also important for PCMs for applications. Fig. 3 shows the temperature dependent thermal conductivity of pure PA and the PA/ TCNT composites. It is clearly observed from the figure that the thermal conductivity of the PA/TCNT composites
50 1 2
PCNTs TCNTs
Transmittance (%)
40
2
30
20 1
10
1000
1500
2000
2500
3000
3500
4000
-1
Wavenumber cm
Fig. 2. FTIR spectra of PCNTs and TCNTs.
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Fig. 3. Temperature dependent thermal conductivity of PCMs.
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is increased when compared to the corresponding value of pure PA. At the same time, the thermal conductivities of the PA/TCNT composites increase with the mass fraction of the TCNTs, /w . Furthermore, thermal conductivity of PA and the PA/TCNT composites is weakly depended on the temperature when the temperature is lower than 55 °C or higher than 65 °C. Interestingly, the thermal conductivities of PA and PA/TCNT composites increase suddenly near the melting point in solid state. This may be caused by the accelerated molecular vibration in the matrix of the orderly solid structure because of the increase in the temperature. The phenomenon, which the thermal conductivity suddenly falling down when the PCMs turn to liquid state, could be caused by the microstructure turning from the orderly structure in solid state into disorderly structure in liquid state. As phase change materials, high thermal conductivity close to the phase change temperature is desirable for thermal energy storage application. The thermal conductivities of the PA/TCNT composite with /w of 0.01 are 0.33 W/(m K) in the solid state and 0.21 W/ (m K) in the liquid state. The average thermal conductivities of the pure PA are 0.22 W/(m K) in the solid state and 0.16 W/(m K) in the liquid state. For PA/TCNT composite at a mass fraction of 0.01, its thermal conductivity is higher than PA by 0.12 W/(m K) at 25 °C and 0.07 W/ (m K) at 70 °C. The thermal conductivity enhancement ratios of the PA/ TCNT composites were calculated in order to show clearly the effect for the thermal transfer with adding TCNTs to PA. Figs. 4 and 5 show the thermal conductivity enhancements as functions of temperatures of the nanocomposites in solid state and in liquid state, respectively. k c and k 0 represent the thermal conductivities of composites and pure PA, respectively. ðk c k 0 Þ=k 0 is the thermal conductivity enhancement ratio of the nanocomposites. The smallest thermal conductivity enhancements of the PA/TCNT composites are about 4.0% at 25 °C and 2.0% at 65 °C for the composite with 0.2 wt% TCNTs in solid state and liquid
Fig. 4. Thermal conductivity enhancement in solid.
Fig. 5. Thermal conductivity enhancement in liquid.
state, respectively. The largest thermal conductivity enhancements of the composites are 46.0% at 25 °C and 38.0% at 65 °C for the composite with 1.0 wt% TCNTs in solid state and liquid state, respectively. The enhancements of each PA/TCNT composite at different temperatures fluctuate slightly. For PA/TCNT composite with TCNT mass fraction of 0.01, it is seen from Figs. 4 and 5 that the thermal conductivity enhancements are at an approximate level at the temperatures lower than 55 °C or higher than the melting point. The average enhancement of the composite was further calculated at temperatures lower than 55 °C and those at the temperatures higher than the melting point, respectively. It is found that the average enhancement in solid state is higher than that in liquid state. This may be caused by the orderly structure of the material in solid. The orderly structure is propitious to the heat transfer between TCNTs and PA, which reduces the thermal resistance between the included carbon tubes and the matrix (Zhong and Lukes, 2006; Shenogin et al., 2005). As CNTs have very high thermal conductivity and TCNTs participate in thermal transfer in composites, the composites are expected to have higher thermal conductivity than that of pure PA. In order to find out the relationship between the TCNT loadings and the thermal conductivities of the PA/TCNT composites, the data for 25 °C and 70 °C were select as typical values for solid state and liquid state, respectively. Fig. 6 depicts the thermal conductivity ratios at different CNT loadings. It is clearly seen from Fig. 6 that the thermal conductivity enhancements increase with the CNT mass fraction at both 25 °C and 70 °C. For the PA/TCNT composite with 0.2 wt% TCNTs, the enhancements at the given temperature points are close to each other. For the PA/TCNT composite with 0.5 wt% TCNTs, the enhancement at 70 °C is a little higher than that at 25 °C while the enhancement of the composite with 1.0 wt% TCNTs at 25 °C is higher than that at 70 °C by about 10.0%. Two factors might account for the phenomenon. One is that –OH groups on the surfaces of the
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respectively. In the preparation of the composites described in Wang et al. (2008), the pristine CNTs were pretreated by chemical oxidation based on a concentrated acid mixture. The marked discrepancy in the thermal conductivity enhancement in these two CNT composites may be ascribed to the different degree in the surface treatment. Despite the fact that the acid treatment can enhance the dispersibility of the CNTs in the organic materials, the treatment is very strong and it would heavily damage the structure of the carbon tube surface and thus deteriorate the intrinsic thermal transport capacity of the CNTs (Shenogin et al., 2004). 4. Conclusion Fig. 6. Thermal conductivity enhancement at different /w .
TCNTs decrease the thermal resistance between the CNTs and the base matrix. The other one is that the well dispersed TCNTs could contribute more thermal conductivity in the composite. Melting temperature (T m ) and latent heat capacity (Ls ) of pure PA and composites depending on the CNT loadings are summarized in Table 1. With an increase in the mass fraction of TCNTs, Ls is decreased. The melting temperatures of all given composites are lower than that of the pure PA. The composite with 1.0 wt% TCNTs is lower than that of pure PA by 1.3 K. The latent heat capacity of TCNT/PA composite with 1.0 wt% TCNTs is 184.0 J/ g which lower than that of the pure PA by 24.0 J/g. In our previous study (Wang et al., 2008) we prepared stable, homogeneous, and thermal performance enhanced heat storage nanocomposites consisting of PA and multiwalled CNTs. Here we present the CNT mass fraction dependent thermal conductivity enhancement ratios referred from Wang et al. (2008) in Fig. 6 together with the measured data in this study. It is clearly observed that the prepared PA/TCNT composites in this study have much higher thermal conductivity than the composites described in Wang et al. (2008). For example the thermal conductivity enhancements of the present composite at CNT mass fraction of 0.01 amount up to 46.0% and 38.0% in solid state and liquid state, respectively. While for the composite at CNT mass fraction of 0.01 described in Wang et al. (2008), the thermal conductivity enhancements are only 11.0% and 10.0% in solid and liquid states,
Table 1 Melting temperature (T m ) and latent heat capacity (Ls ) of PA and the composites.
/w /w /w /w
¼ 0:00 ¼ 0:002 ¼ 0:005 ¼ 0:01
Tm (°C)
Ls (J/g)
62.4 62.1 62.3 61.1
208.0 200.4 197.7 184.0
Palmitic acid (PA) dispersible multi-walled carbon nanotubes (CNT) were obtained by mechano-chemical reaction treatment through ball milling the mixture of potassium hydroxide and pristine CNTs. FTIR analysis revealed that hydroxy groups have been added onto the surfaces of treated CNTs (TCNTs). The surface-attached functional groups greatly shift the surface properties of TCNTs and play the dominant role on the dispersibility of TCNTs in PA. The TCNT addition leads to substantial enhancement in the thermal conductivity of the composite phase change material with the enhancement ratio increases with the TCNT loading. For PA/TCNT composite with TCNT mass fraction of 0.01, the thermal conductivity enhancements are 46.0% at 25 °C and 38.0% at 65 °C, respectively. The thermal conductivity enhancement of the composites only changes slightly with the temperature both in solid state, at temperatures lower than 55 °C, and in liquid state, at temperatures higher than the melting point. Interestingly, the thermal conductivities of the composite PCMs suddenly increase near the melting point in solid state and suddenly fall down when the PCMs turn to liquid state. The as prepared PA/TCNT composites in this study were proved to have much higher thermal conductivity than our previously reported composites composed of PA and CNTs treated by concentrated acid mixture. Our experimental findings demonstrated that the thermal properties of CNT composite phase change materials could be manipulated by tailoring the included CNTs. Thus the prepared PA/TCNT composites with considerably enhanced thermal conductivity are very promising in phase change heat storage applications. Acknowledgements This work was supported by the National Science Foundation of China (50876058), Shanghai Educational Development Foundation and Shanghai Municipal Education Commission (07SG56), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.
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References Alkan, C., Kaya, K., Sari, A., 2008. Preparation and thermal properties of ethylene glycole distearate as a novel phase change material for energy storage. Mater. Lett. 62, 1122–1125. Berber, S., Kwon, Y.K., Toma´nek, D., 2000. Unusually high thermal conductivity of carbon nanotubes. Phys. Rev. Lett. 84, 4613–4616. Bonnet, P., Sireude, D., Garnier, B., Chauvet, O., 2007. Thermal properties and percolation in carbon nanotube–polymer composites. Appl. Phys. Lett. 91, 201910. Carlborg, C.F., Shiomi, J., Maruyama, S., 2008. Thermal boundary resistance between single-walled carbon nanotubes and surrounding matrices. Phys. Rev. B 78, 205406. Dimaano, M.N.R., Watanabe, T., 2002. Performance investigation of the capric and lauric acid mixture as latent heat energy storage for a cooling system. Sol. Energy 72, 205–215. do Couto Aktay, K.S., Tamme, R., Mu¨ller-Steinhagen, H., 2008. Thermal conductivity of high-temperature multicomponent materials with phase change. Int. J. Thermophys. 29, 678–692. Elgafy, A., Lafdi, K., 2005. Effect of carbon nanofiber additives on thermal behavior of phase change materials. Carbon 43, 3067–3074. Koshio, A., Yudasaka, M., Zhang, M., Iijima, S., 2001. A simple way to chemically react single-wall carbon nanotubes with organic materials using ultrasonication. Nano Lett. 1, 361–363. Kumara, S., Alam, M.A., Murthy, J.Y., 2007. Effect of percolation on thermal transport in nanotube composites. Appl. Phys. Lett. 90, 104105. Pan, H., Liu, L., Guo, Z.X., et al., 2003. Carbon nanotubols from mechanochemical reaction. Nano Lett. 3, 29–32. Sari, A., Karaipekli, A., 2008. Preparation and thermal properties of capric acid/palmitic acid eutectic mixture as a phase change energy storage material. Mater. Lett. 62, 903–906.
Sari, A., Kaygusuz, K., 2002. Thermal performance of palmitic acid as a phase change energy storage material. Energy Convers. Manage. 43, 863–876. ¨ nal, A., 2008. Preparation, Sari, A., Alkan, C., Karaipekli, A., O characterization and thermal properties of styrene maleic anhydride copolymer (SMA)/fatty acid composites as form stable phase change materials. Energy Convers. Manage. 49, 373–380. Shaikh, S., Lafdi, K., Hallinan, K., 2008. Carbon nanoadditives to enhance latent energy storage of phase change materials. J. Appl. Phys. 103, 094302. Shenogin, S., Bodapati, A., Xue, L., Ozisik, R., Keblinski, P., 2004. Effect of chemical functionalization on thermal transport of carbon nanotube composites. Appl. Phys. Lett. 85, 2229. Shenogin, N., Shenogin, S., Xue, L., Keblinski, P., 2005. On the lack of thermal percolation in carbon nanotube composites. Appl. Phys. Lett. 87, 133106. Wang, J., Xie, H., Xin, Z., 2008. Thermal properties of heat storage composites containing multiwalled carbon nanotubes. J. Appl. Phys. 104, 113537. Wang, J., Xie, H., Xin, Z., 2009. Thermal properties of paraffin based composites containing multi-walled carbon nanotubes. Thermochim. Acta. doi:10.1016/j.tca.2009.01.022. Xie, H., Gu, H., Fujii, M., Zhang, X., 2006. Short hot wire technique for measuring thermal conductivity and thermal diffusivity of various materials. Meas. Sci. Technol. 17, 208–214. Zeng, J.L., Liu, Y.Y., Cao, Z.X., Zhang, J., Zhang, Z.H., Sun, L.X., Xu, F., 2008. Thermal conductivity enhancement of MWNTs on the PANI/tetradecanol form-stable PCM. J. Therm. Anal. Calorim. 91, 443–446. Zhong, H.L., Lukes, J.R., 2006. Interfacial thermal resistance between carbon nanotubes: molecular dynamics simulations and analytical thermal modeling. Phys. Rev. B 74, 125403.
Solar Energy 84 (2010) 339–344 www.elsevier.com/locate/solener
Enhancing thermal conductivity of palmitic acid based phase change materials with carbon nanotubes as fillers Jifen Wang a,b, Huaqing Xie a,*, Zhong Xin b, Yang Li a, Lifei Chen a a
School of Urban Development and Environmental Engineering, Shanghai Second Polytechnic University, Shanghai 201209, China b State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Received 28 February 2009; received in revised form 30 November 2009; accepted 2 December 2009 Available online 29 December 2009 Communicated by: Associate Editor Halime Paksoy
Abstract Multi-walled carbon nanotubes (CNTs) as produced are usually entangled and not ready to be dispersed into organic matrix. CNTs were treated by mechano-chemical reaction with ball milling the mixture of potassium hydroxide and the pristine CNTs. Hydroxide radical functional groups have been introduced on the CNT surfaces, which enabled to make stable and homogeneous CNT composites. Treated CNTs were successfully dispersed into the palmitic acid matrix without any surfactant. Transient short-hot-wire apparatus was used to measure the thermal conductivities of these nanotube composites. Nanotube composites have substantially higher thermal conductivities than the base palmitic acid matrix, with the enhancement increasing with the mass fraction of CNTs in both liquid state and solid state. The enhancements of the thermal conductivity are about 30% higher than the reported corresponding values for palmitic acid based phase change nanocomposites containing 1 wt% CNTs treated by concentrated acid mixture. Ó 2009 Elsevier Ltd. All rights reserved. Keywords: Phase change material; Palmitic acid; Carbon nanotube; Nanocomposite; Thermal conductivity
1. Introduction Among the available techniques suitable for storing thermal energy and for controlling temperature in systems subjected to periodic heating, the use of solid–liquid phase change has attracted considerable attention (Alkan et al., 2008; Shenogin et al., 2004; do Couto Aktay et al., 2008). As phase change materials (PCMs), due to the high storage
Abbreviations: CNT, carbon nanotube; PCM, phase change material; PA, palmitic acid; PCNT, pristine carbon nanotube; TCNT, treated carbon nanotube; FTIR, Fourier transformation infrared spectroscopy; PA/ TCNT, nanocomposite containing palmitic acid and treated multi-walled carbon nanotubes; Tm, melting temperature; Ls, latent heat capacity; K0, the thermal conductivities of composites; Kc, the thermal conductivities of pure PA. * Corresponding author. Tel./fax: +86 21 50217331. E-mail address: [email protected] (H. Xie). 0038-092X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2009.12.004
density and small temperature variation from storage to retrieval and low melting temperature, organic materials including paraffin waxes and fatty acids have been employed as PCMs for thermal energy storage in many applications (Sari and Kaygusuz, 2002; Sari et al., 2008; Sari and Karaipekli, 2008; Dimaano and Watanabe, 2002; Wang et al., 2009). However, low thermal conductivities, the major drawback which leads to decreasing the rates of heat storage and retrieval during melting and solidification processes, limit their utility areas. To overcome this problem, a wide range of investigations were carried out to enhance the thermal conductivity of the organic PCMs. The often used method is to disperse solid particles with high thermal conductivity, such as expanded graphite, copper and other metal particles and so on, to form composite PCMs. However, uses of such types of heat transfer promoters considerably increase the weight and volume of latent heat thermal energy storage systems. Moreover the
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PCM composites often show poor stability because of the large difference between the solid inclusions and the organic matrix. Carbon nanotubes (CNTs) with extremely high thermal conductivity have shown tremendous potential for heat transfer applications (Carlborg et al., 2008; Kumara et al., 2007; Shaikh et al., 2008). Compared with the aforementioned heat transfer promoters, CNTs as additives have smaller density difference to the organic matrix than the metal or metal oxide particles, which makes CNTs have better stability in the organic matrix. Several researches have been reported wherein CNTs are embedded in the base fluids and phase change materials to increase their thermal conductivities (Elgafy and Lafdi, 2005; Wang et al., 2008; Bonnet et al., 2007; Zeng et al., 2008). Numerical simulations were carried out to analyze the effects of chemical functionalization on the thermal transport of carbon nanotubes and their composites (Berber et al., 2000). However, to the best knowledge of the authors, there is no theoretical or experimental study that deals with the effect of CNTs with alkaline treatment on the thermal transport of the composite as PCMs. The objective of this study is to provide a novel technical approach for preparing thermal performance enhanced PCMs. Multi-walled CNTs are used as fillers for tailoring the thermal properties of the matrix material, palmitic acid. Mechano-chemical reaction is employed to treat the pristine CNTs (PCNTs) with poor dispersibility. The current study investigates the thermal conductivity of the nanocomposites and discusses the mechanism of the thermal transport enhancement.
2. Experimental 2.1. Materials Palmitic acid (PA, 98%) and potassium hydroxide (A.R.) were obtained from Sinopharm Chemical Reagent Co., Ltd. The PA was used without further purification and the melting point of it is about 62.5 °C. Multi-walled carbon nanotubes (CNTs) were commercially supplied by Chendu Organic Chemicals Co., Ltd., Chinese Academy of Sciences. The average diameter, average length, and specific surface area of the CNTs were 30 nm, 50 lm, and 60 m2/g. 2.2. Synthesis of the composites The mechano-chemical reaction treatment described in Pan et al. (2003) was applied to tailor the CNT surfaces to enhance the dispersibility of the CNTs. In a typical treatment, 1 g of PCNTs was mixed with 20 g of potassium hydroxide and the mixture was ball-milled for 720 min. Then, the sample was diluted by distilled water, filtered, and washed repeatedly till the washings show no alkalescence. The cleaned CNTs were collected and dried at 100 °C for 24 h to remove the attached water. Fig. 1 shows the scanning electronic microscope (SEM) images of the PCNTs, treated CNTs (TCNTs) and the nanocomposites containing PA and TCNTs (PA/TCNTs). It is observed that TCNTs after mechano-chemical treatment look shorter compared with the PCNTs as obtained. During the mechano-chemical treatment, the surfaces of
Fig. 1. SEM images of PCNTs (A), TCNTs (B), and PA/TCNT (C).
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the CNTs, have been modified. As shown in Fig. 2, the TCNTs show a remarkably different Fourier transformation infrared spectroscopy (FTIR) from that of the PCNTs. Consistent with the previously reported results (Pan et al., 2003; Koshio et al., 2001), no detectable transmission band was observed for the PCNTs in the wavenumber range covered in this study. In contrast, the corresponding FTIR spectrum for TCNTs shows a very broad transmission band centered at 3411 cm1, characteristic of hydrogen bonded –OH. The band at 1095 cm1 is corresponding to the stretching band of C–O, while the band at 782 cm1 can be interpreted to the bending stretching band of hydroxyl groups. The strong transmission band at around 1629 cm1 can be attributed to the stretching mode of – C@C– in an enol form (Pan et al., 2003). It is indicated that the –OH groups have been added onto the surface of the CNTs. The nanocomposites were obtained by adding the TCNTs into melting PA in a mixing container. Intensive sonication was further used to make well dispersed and homogeneous mixtures. As shown in Fig. 1(c), TCNTs have been dispersed well in PA matrix. 2.3. Thermal conductivity measurement Short-hot-wire method was used to measure the thermal conductivities (k) of pure PA and the PA/TCNT composites and the detailed procedure of the method has been described elsewhere (Xie et al., 2006). Briefly a platinum wire with a diameter of 70 lm was used for the hot wire, and it served as both a heating unit and as an electrical resistance thermometer. Initially the platinum wire immersed in media was kept at equilibrium with the surroundings. When a regulation voltage was supplied to initiate the measurement, the electrical resistance of the wire changed proportionally with the rise in temperature. The thermal conductivity was calculated from the slope of the rise in the wire’s temperature against the logarithmic time interval. The uncertainty of this measurement is estimated
to be within ±1.0%. In our measurements, the PCM sample was melted and poured into a stainless steel cylinder container kept at a temperature higher than the melting point of the sample. A waterproof lid with pre-positioned hot wire and thermocouple was used to seal the container after sample encapsulation. The container was then put into a temperature-controlled thermostat bath with temperature variation less than 0.1 °C. The tested temperature was increased every 10 °C from 15 °C to 55 °C while increased every 5 °C from 65 °C to 75 °C. Instead of monitoring the temperature of the bath, we monitored the temperature on the spot with the pre-positioned thermocouple inside the sample. When the temperature of the sample reached a steady value, it waited further 20 min to make sure that the initial state is at equilibrium. At every tested temperature, the thermal conductivity were measured three times and taken the average values as the final results. Twenty minutes interval was needed between every measurement. After the above-mentioned careful check on the measurement condition and procedure, it could obtain the thermal conductivities around the melting point. Melting temperature (T m ) and latent heat capacity (Ls ) of pure PA and composites were measured using a differential scanning calorimetric (DSC) instrument (Diamond DSC, Perkin-Elmer, USA). The DSC measurements were performed at a heating rate of 5 °C/min and in a temperature range 15–70 °C. 3. Results and discussion Thermal conductivity is one of the most important properties of PCMs. The relationship between the thermal conductivities and temperatures is also important for PCMs for applications. Fig. 3 shows the temperature dependent thermal conductivity of pure PA and the PA/ TCNT composites. It is clearly observed from the figure that the thermal conductivity of the PA/TCNT composites
50 1 2
PCNTs TCNTs
Transmittance (%)
40
2
30
20 1
10
1000
1500
2000
2500
3000
3500
4000
-1
Wavenumber cm
Fig. 2. FTIR spectra of PCNTs and TCNTs.
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Fig. 3. Temperature dependent thermal conductivity of PCMs.
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is increased when compared to the corresponding value of pure PA. At the same time, the thermal conductivities of the PA/TCNT composites increase with the mass fraction of the TCNTs, /w . Furthermore, thermal conductivity of PA and the PA/TCNT composites is weakly depended on the temperature when the temperature is lower than 55 °C or higher than 65 °C. Interestingly, the thermal conductivities of PA and PA/TCNT composites increase suddenly near the melting point in solid state. This may be caused by the accelerated molecular vibration in the matrix of the orderly solid structure because of the increase in the temperature. The phenomenon, which the thermal conductivity suddenly falling down when the PCMs turn to liquid state, could be caused by the microstructure turning from the orderly structure in solid state into disorderly structure in liquid state. As phase change materials, high thermal conductivity close to the phase change temperature is desirable for thermal energy storage application. The thermal conductivities of the PA/TCNT composite with /w of 0.01 are 0.33 W/(m K) in the solid state and 0.21 W/ (m K) in the liquid state. The average thermal conductivities of the pure PA are 0.22 W/(m K) in the solid state and 0.16 W/(m K) in the liquid state. For PA/TCNT composite at a mass fraction of 0.01, its thermal conductivity is higher than PA by 0.12 W/(m K) at 25 °C and 0.07 W/ (m K) at 70 °C. The thermal conductivity enhancement ratios of the PA/ TCNT composites were calculated in order to show clearly the effect for the thermal transfer with adding TCNTs to PA. Figs. 4 and 5 show the thermal conductivity enhancements as functions of temperatures of the nanocomposites in solid state and in liquid state, respectively. k c and k 0 represent the thermal conductivities of composites and pure PA, respectively. ðk c k 0 Þ=k 0 is the thermal conductivity enhancement ratio of the nanocomposites. The smallest thermal conductivity enhancements of the PA/TCNT composites are about 4.0% at 25 °C and 2.0% at 65 °C for the composite with 0.2 wt% TCNTs in solid state and liquid
Fig. 4. Thermal conductivity enhancement in solid.
Fig. 5. Thermal conductivity enhancement in liquid.
state, respectively. The largest thermal conductivity enhancements of the composites are 46.0% at 25 °C and 38.0% at 65 °C for the composite with 1.0 wt% TCNTs in solid state and liquid state, respectively. The enhancements of each PA/TCNT composite at different temperatures fluctuate slightly. For PA/TCNT composite with TCNT mass fraction of 0.01, it is seen from Figs. 4 and 5 that the thermal conductivity enhancements are at an approximate level at the temperatures lower than 55 °C or higher than the melting point. The average enhancement of the composite was further calculated at temperatures lower than 55 °C and those at the temperatures higher than the melting point, respectively. It is found that the average enhancement in solid state is higher than that in liquid state. This may be caused by the orderly structure of the material in solid. The orderly structure is propitious to the heat transfer between TCNTs and PA, which reduces the thermal resistance between the included carbon tubes and the matrix (Zhong and Lukes, 2006; Shenogin et al., 2005). As CNTs have very high thermal conductivity and TCNTs participate in thermal transfer in composites, the composites are expected to have higher thermal conductivity than that of pure PA. In order to find out the relationship between the TCNT loadings and the thermal conductivities of the PA/TCNT composites, the data for 25 °C and 70 °C were select as typical values for solid state and liquid state, respectively. Fig. 6 depicts the thermal conductivity ratios at different CNT loadings. It is clearly seen from Fig. 6 that the thermal conductivity enhancements increase with the CNT mass fraction at both 25 °C and 70 °C. For the PA/TCNT composite with 0.2 wt% TCNTs, the enhancements at the given temperature points are close to each other. For the PA/TCNT composite with 0.5 wt% TCNTs, the enhancement at 70 °C is a little higher than that at 25 °C while the enhancement of the composite with 1.0 wt% TCNTs at 25 °C is higher than that at 70 °C by about 10.0%. Two factors might account for the phenomenon. One is that –OH groups on the surfaces of the
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respectively. In the preparation of the composites described in Wang et al. (2008), the pristine CNTs were pretreated by chemical oxidation based on a concentrated acid mixture. The marked discrepancy in the thermal conductivity enhancement in these two CNT composites may be ascribed to the different degree in the surface treatment. Despite the fact that the acid treatment can enhance the dispersibility of the CNTs in the organic materials, the treatment is very strong and it would heavily damage the structure of the carbon tube surface and thus deteriorate the intrinsic thermal transport capacity of the CNTs (Shenogin et al., 2004). 4. Conclusion Fig. 6. Thermal conductivity enhancement at different /w .
TCNTs decrease the thermal resistance between the CNTs and the base matrix. The other one is that the well dispersed TCNTs could contribute more thermal conductivity in the composite. Melting temperature (T m ) and latent heat capacity (Ls ) of pure PA and composites depending on the CNT loadings are summarized in Table 1. With an increase in the mass fraction of TCNTs, Ls is decreased. The melting temperatures of all given composites are lower than that of the pure PA. The composite with 1.0 wt% TCNTs is lower than that of pure PA by 1.3 K. The latent heat capacity of TCNT/PA composite with 1.0 wt% TCNTs is 184.0 J/ g which lower than that of the pure PA by 24.0 J/g. In our previous study (Wang et al., 2008) we prepared stable, homogeneous, and thermal performance enhanced heat storage nanocomposites consisting of PA and multiwalled CNTs. Here we present the CNT mass fraction dependent thermal conductivity enhancement ratios referred from Wang et al. (2008) in Fig. 6 together with the measured data in this study. It is clearly observed that the prepared PA/TCNT composites in this study have much higher thermal conductivity than the composites described in Wang et al. (2008). For example the thermal conductivity enhancements of the present composite at CNT mass fraction of 0.01 amount up to 46.0% and 38.0% in solid state and liquid state, respectively. While for the composite at CNT mass fraction of 0.01 described in Wang et al. (2008), the thermal conductivity enhancements are only 11.0% and 10.0% in solid and liquid states,
Table 1 Melting temperature (T m ) and latent heat capacity (Ls ) of PA and the composites.
/w /w /w /w
¼ 0:00 ¼ 0:002 ¼ 0:005 ¼ 0:01
Tm (°C)
Ls (J/g)
62.4 62.1 62.3 61.1
208.0 200.4 197.7 184.0
Palmitic acid (PA) dispersible multi-walled carbon nanotubes (CNT) were obtained by mechano-chemical reaction treatment through ball milling the mixture of potassium hydroxide and pristine CNTs. FTIR analysis revealed that hydroxy groups have been added onto the surfaces of treated CNTs (TCNTs). The surface-attached functional groups greatly shift the surface properties of TCNTs and play the dominant role on the dispersibility of TCNTs in PA. The TCNT addition leads to substantial enhancement in the thermal conductivity of the composite phase change material with the enhancement ratio increases with the TCNT loading. For PA/TCNT composite with TCNT mass fraction of 0.01, the thermal conductivity enhancements are 46.0% at 25 °C and 38.0% at 65 °C, respectively. The thermal conductivity enhancement of the composites only changes slightly with the temperature both in solid state, at temperatures lower than 55 °C, and in liquid state, at temperatures higher than the melting point. Interestingly, the thermal conductivities of the composite PCMs suddenly increase near the melting point in solid state and suddenly fall down when the PCMs turn to liquid state. The as prepared PA/TCNT composites in this study were proved to have much higher thermal conductivity than our previously reported composites composed of PA and CNTs treated by concentrated acid mixture. Our experimental findings demonstrated that the thermal properties of CNT composite phase change materials could be manipulated by tailoring the included CNTs. Thus the prepared PA/TCNT composites with considerably enhanced thermal conductivity are very promising in phase change heat storage applications. Acknowledgements This work was supported by the National Science Foundation of China (50876058), Shanghai Educational Development Foundation and Shanghai Municipal Education Commission (07SG56), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.
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References Alkan, C., Kaya, K., Sari, A., 2008. Preparation and thermal properties of ethylene glycole distearate as a novel phase change material for energy storage. Mater. Lett. 62, 1122–1125. Berber, S., Kwon, Y.K., Toma´nek, D., 2000. Unusually high thermal conductivity of carbon nanotubes. Phys. Rev. Lett. 84, 4613–4616. Bonnet, P., Sireude, D., Garnier, B., Chauvet, O., 2007. Thermal properties and percolation in carbon nanotube–polymer composites. Appl. Phys. Lett. 91, 201910. Carlborg, C.F., Shiomi, J., Maruyama, S., 2008. Thermal boundary resistance between single-walled carbon nanotubes and surrounding matrices. Phys. Rev. B 78, 205406. Dimaano, M.N.R., Watanabe, T., 2002. Performance investigation of the capric and lauric acid mixture as latent heat energy storage for a cooling system. Sol. Energy 72, 205–215. do Couto Aktay, K.S., Tamme, R., Mu¨ller-Steinhagen, H., 2008. Thermal conductivity of high-temperature multicomponent materials with phase change. Int. J. Thermophys. 29, 678–692. Elgafy, A., Lafdi, K., 2005. Effect of carbon nanofiber additives on thermal behavior of phase change materials. Carbon 43, 3067–3074. Koshio, A., Yudasaka, M., Zhang, M., Iijima, S., 2001. A simple way to chemically react single-wall carbon nanotubes with organic materials using ultrasonication. Nano Lett. 1, 361–363. Kumara, S., Alam, M.A., Murthy, J.Y., 2007. Effect of percolation on thermal transport in nanotube composites. Appl. Phys. Lett. 90, 104105. Pan, H., Liu, L., Guo, Z.X., et al., 2003. Carbon nanotubols from mechanochemical reaction. Nano Lett. 3, 29–32. Sari, A., Karaipekli, A., 2008. Preparation and thermal properties of capric acid/palmitic acid eutectic mixture as a phase change energy storage material. Mater. Lett. 62, 903–906.
Sari, A., Kaygusuz, K., 2002. Thermal performance of palmitic acid as a phase change energy storage material. Energy Convers. Manage. 43, 863–876. ¨ nal, A., 2008. Preparation, Sari, A., Alkan, C., Karaipekli, A., O characterization and thermal properties of styrene maleic anhydride copolymer (SMA)/fatty acid composites as form stable phase change materials. Energy Convers. Manage. 49, 373–380. Shaikh, S., Lafdi, K., Hallinan, K., 2008. Carbon nanoadditives to enhance latent energy storage of phase change materials. J. Appl. Phys. 103, 094302. Shenogin, S., Bodapati, A., Xue, L., Ozisik, R., Keblinski, P., 2004. Effect of chemical functionalization on thermal transport of carbon nanotube composites. Appl. Phys. Lett. 85, 2229. Shenogin, N., Shenogin, S., Xue, L., Keblinski, P., 2005. On the lack of thermal percolation in carbon nanotube composites. Appl. Phys. Lett. 87, 133106. Wang, J., Xie, H., Xin, Z., 2008. Thermal properties of heat storage composites containing multiwalled carbon nanotubes. J. Appl. Phys. 104, 113537. Wang, J., Xie, H., Xin, Z., 2009. Thermal properties of paraffin based composites containing multi-walled carbon nanotubes. Thermochim. Acta. doi:10.1016/j.tca.2009.01.022. Xie, H., Gu, H., Fujii, M., Zhang, X., 2006. Short hot wire technique for measuring thermal conductivity and thermal diffusivity of various materials. Meas. Sci. Technol. 17, 208–214. Zeng, J.L., Liu, Y.Y., Cao, Z.X., Zhang, J., Zhang, Z.H., Sun, L.X., Xu, F., 2008. Thermal conductivity enhancement of MWNTs on the PANI/tetradecanol form-stable PCM. J. Therm. Anal. Calorim. 91, 443–446. Zhong, H.L., Lukes, J.R., 2006. Interfacial thermal resistance between carbon nanotubes: molecular dynamics simulations and analytical thermal modeling. Phys. Rev. B 74, 125403.