ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 136–142 Contents lists available at ScienceDirect Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets Sumin Kim a,, Lawrence T. Drzal b a b Department of Architecture, College of Engineering, Soongsil University, Seoul 156-743, Republic of Korea Composite Materials and Structures Center, College of Engineering, Michigan State University, East Lansing, MI 48824-1226, USA a r t i c l e in f o a b s t r a c t Article history: Received 24 April 2008 Received in revised form 8 September 2008 Accepted 16 September 2008 Available online 1 November 2008 Using exfoliated graphite nanoplatelets (xGnP), paraffin/xGnP composite phase change materials (PCMs) were prepared by the stirring of xGnP in liquid paraffin for high electric conductivity, thermal conductivity and latent heat storage. xGnP of 1, 2, 3, 5 and 7 wt% was added to pure paraffin at 75 1C. Scanning electron microscopy (SEM) morphology showed uniform dispersion of xGnP in the paraffin wax. Good dispersion of xGnP in paraffin/xGnP composite PCMs led to high electric conductivity. The percolation threshold of paraffin/xGnP composite PCMs was between 1 and 2 wt% in resistivity measurement. The thermal conductivity of paraffin/xGnP composite PCMs was increased as xGnP loading contents. Also, reproducibility of paraffin/xGnP composite PCMs as continuous PCMs was manifested in results of electric and thermal conductivity. Paraffin/xGnP composite PCMs showed two peaks in the heating curve by differential scanning calorimeter (DSC) measurement. The first phase change peak at around 35 1C is lower and corresponds to the solid–solid phase transition of the paraffin, and the second peak is high at around 55 1C, corresponding to the solid–liquid phase change. The latent heat of paraffin/xGnP composite PCMs did not decrease as loading xGnP contents to paraffin. xGnP can be considered as an effective heat-diffusion promoter to improve thermal conductivity of PCMs without reducing its latent heat storage capacity in paraffin wax. & 2008 Elsevier B.V. All rights reserved. Keywords: Exfoliated graphite nanoplatelets (xGnP) Phase change material (PCM) Paraffin wax Latent heat storage Thermal conductivity 1. Introduction Solid–liquid phase change materials (PCMs) are often used for heat-storage applications. Examples include water, salt hydrates, paraffins, certain hydrocarbons and metal alloys. Salt hydrate PCMs used for thermal storage in space heating and cooling applications have low material costs, but high packaging costs. A more economic installed storage may be possible with medium priced, high latent heat organic materials suitable for low-cost packaging, i.e. those that are insoluble in water and un-reactive with air and some of the common packaging films [1,2]. In recent times, several candidate inorganic and organic PCMs and their mixtures have been studied as PCMs for latent heat thermal energy storage (LHTES) applications [3–6]. PCMs that are used as storage media in latent thermal energy storage can be classified into two major categories: inorganic and organic compounds. Inorganic PCMs include salt hydrates, salts, metals and alloys, whereas organic PCMs are comprised of paraffin, fatty acids/esters and polyalcohols. Paraffin is taken as the most Corresponding author. Tel.: +82 2 820 0665; fax: +82 2 816 3354. E-mail address: [email protected] (S. Kim). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.09.010 promising PCM because it has a large latent heat and low cost, and is stable, non-toxic and not corrosive [7,8]. Among the investigated PCMs, paraffins have been widely used for LHTES applications due to their large latent heat and proper thermal characteristics such as little or no super cooling, varied phase change temperature, low vapor pressure in the melt, good thermal and chemical stability, and self-nucleating behavior [5,9–11]. Portable electronic devices such as notebook computers and wearable electronic devices possess unique characteristics that nearly eliminate the use of traditional methods of thermal management [12]. Cooling by heat transfer to PCMs is one of the promising directions. This cooling technology has been widely regarded overseas in recent years, and it also has had certain applications in some high-tech systems, such as aviation, microelectronics and military electronic systems [13]. Therefore, electric conductivity of PCMs is one of the important factors for electric device application. In spite of these desirable properties of paraffins, the low thermal conductivity (0.21–0.24 W/m K) is its major drawback decreasing the rates of heat stored and released during melting and crystallization processes which in turn limits their utility areas [14]. These drawbacks reduce the rate of heat storage and extraction during the melting and solidification cycles and restrict their wide applications, respectively ARTICLE IN PRESS S. Kim, L.T. Drzal / Solar Energy Materials & Solar Cells 93 (2009) 136–142 [8]. To overcome the low thermal conductivity problem of paraffin as PCMs, studies have been carried out with the purpose of developing LHTES systems with unfinned and finned configurations, dispersing high conductivity particles and inserting a metal matrix into paraffin wax [14–16]. Expanded graphite (EG) is generally produced by using H2SO4–graphite intercalation compounds (GICs). H2SO4–GICs are widely used for the exfoliation process, because they can give a high expansion volume during the thermal treatment. The electrochemical intercalation of H2SO4 as well as the chemical one were described in the Tryba’s works [17,18]. The EG maintains the layered structures similar to natural graphite flake but produces tremendously different sizes of pores and nanosheets with very high aspect ratio [19,20]. Research in the Drzal group has shown that exfoliated graphite nanoplatelets (xGnPTM), which combine the layered structure and low price of nanoclays with the superior mechanical, electrical and thermal properties of carbon nanotubes, are very cost effective and can simultaneously provide a multitude of physical and chemical property enhancements [21–23]. Nanocomposites prepared with xGnP in thermosetting and thermoplastic polymer systems showed excellent mechanical properties and electrical conductivity [24–26]. To increase thermal conductivity, EG has been used to insert into the paraffin wax [3,8,12,27,28]. However, Zhang and Fang studied the effect of the EG addition on the thermal properties of the paraffin (m.p.: 48–50 1C)/EG composite prepared as formstable PCM, and they reported that the latent heat capacity of the PCM decreased with increase of the mass fraction of the graphite 137 [8,14]. This study aimed to prepare the composites of paraffin (ndocosane, m.p.: 42–44 1C)/xGnPTM with low mass fraction of xGnP to obtain a form-stable composite PCM and to investigate the effect of xGnP addition on thermal conductivity and melting time, melting temperature, and latent heat capacity of the paraffin, especially to keep latent heat of the paraffin as xGnP contents is the main purpose of this research. 2. Experimental 2.1. Materials xGnPTM are prepared from sulfuric acid-intercalated expandable graphite (3772) obtained from Asbury Graphite Mills, Inc., NJ, USA by applying a cost- and time-effective exfoliation process initially proposed by Drzal’s group [21,26]. The sulfuric acid-based GIC was fabricated from natural graphite through chemical oxidation in the presence of concentrated sulfuric acid. It is composed of layered, but compactly fastened nanoplates of graphite shown in Fig. 1a. Fig. 1b shows a worm- or accordionlike expanded structure of GIC which was exfoliated up to 300–500 times in their initial volume by rapid heating in a microwave environment. Multi-pores structure is observed from high magnification ( 350) of EG shown in Fig. 1c. Pulverization using an ultrasonic processor is employed to break down the worm-like structure and to reduce its size, resulting in individual graphite nanoplatelets that are o10 nm thick and have an average diameter of 15 mm as shown in Fig. 1d. This xGnP is denoted as Fig. 1. Scanning electron microphotographs of (a) acid-intercalated graphite, (b) expended graphite by microwave EG ( 50), (c) expended graphite by microwave EG ( 350), and (d) exfoliated graphite; xGnP15. ARTICLE IN PRESS 138 S. Kim, L.T. Drzal / Solar Energy Materials & Solar Cells 93 (2009) 136–142 xGnP-15 and was used in this study as a reinforcement in paraffin matrix for PCM. The Brunauer–Emmet–Teller (BET) surface area of the xGnP was measured using an auto N2 absorption instrument (ASAP 2010, Micrometrics, USA). The measurement results showed that the BET surface area of the EG was around 30 m2/g. Paraffin wax (n-docosane) with melting temperature of 53–57 1C was purchased from Sigma-Aldrich company. frequency range of 0.1–1,00,000 Hz and converted to conductivity by taking into account the sample dimensions. Eq. (1) can be used to calculate the resistivity of the sample. R ¼ I S=T (1) where I is the impedance value at 1 Hz, R is the resistivity, S is the intercept surface area, T is the thickness of the sample. 2.2. Preparation of paraffin/xGnP composite PCM To establish the relationship between thermal conductivity of the composite PCM and the mass fraction of xGnP and determine the minimum mass fraction of xGnP that is adequate to obtain paraffin/xGnP composite as form-stable PCM, the composite PCMs were prepared by stirring of xGnP in liquid paraffin with mass fraction of 1%, 2%, 3%, 5% and 7%. The paraffin was melted by heating it at 75 1C, and then, the xGnP was mixed into the liquid paraffin. After being filtered and dried, the paraffin/EG composite PCM was obtained. To check the availability of PCMs as continuous PCMs, the samples were remelted for measuring electrical and thermal conductivity. 2.3. Characterization techniques 2.3.1. Scanning electron microscopy (SEM) The morphology of intercalated graphite, exfoliated graphite, xGnP and paraffin/xGnP composite PCMs were observed by SEM at room temperature. A JEOL (model JSM-6400) SEM with accelerating voltage of 12 kV and working distance of 15 mm was used to collect SEM images. To compare images by gold coating, non-coating and gold coating samples were prepared. A gold coating of a few nanometers in thickness was coated on samples. 2.3.2. Electrical property measurement The resistivity of paraffin/xGnP composite PCMs was measured, with a Gamry instrument under FAS2TM Femtostat plug system and potentiostatic mode, along the flow direction, in case of the injection-molded samples, using impedance spectroscopy by applying the two-probe method at room temperature. Samples with dimensions of 5 3 12 mm3 were cut from the middle portion of flexural bars, and the resistivity was measured along the thickness direction (5 mm). The two surfaces that were connected to the electrodes were first treated with O2 plasma (14 min, 550 W) in order to remove the top surface layers which are rich in polymer, to ensure good contact of the sample surface with the electrodes. The resistance of sample was measured in the 2.3.3. Thermal conductivity measurement The thermal conductivity of paraffin wax and paraffin/xGnP composite PCMs were measured using a UNITHERMTM machineUNITHERMTM Model 2022 (Anter Corporation, Pittsburgh, PA). The tests were performed according to ASTM E1530 (Standard test method for evaluating the resistance to thermal transmission of materials by the guarded heat flow meter method technique). Specimens of 1 in diameter were prepared with stainless mold as shown in Fig. 2. Hot liquid sample was put into the mold and cooled down by liquid nitrogen. In order to ensure that the sample thickness was within the recommended range, 3–5 discs were stacked-up for the composites with higher xGnP loading. The samples were tested at 20 1C under an applied load of 30 psi. Reported results represent the average of three measurements for each xGnP loading. 2.3.4. Differential scanning calorimeter (DSC) The melting and heat storage behaviors of the paraffin/xGnP composite PCMs were studied using a TA Instruments 2920 DSC equipped with a cooling attachment, under a nitrogen atmosphere. The data were collected with a scan rate of 10 1C min–1 over a temperature range of 50–110 1C. The measurement was made using a 5–10 mg sample on a DSC sample cell after the sample was quickly cooled to 50 1C from the melt of the first scan. 2.3.5. Thermogravimetric analysis (TGA) TGA was conducted with a TA Instruments TGA 2950 that was fitted to a nitrogen purge gas from 30 to 600 1C. This unit has the ability to decrease the ramp rate when an increased weight loss is detected in order to obtain better temperature resolution of a decomposition event. The general ramp rate was 4 1C/min with a weight loss detection sensitivity set to 4.0 in the furnace control software. Approximately 5–15 mg of cut samples was used to determine the decomposition temperatures. Fig. 2. Mold for paraffin/xGnP composite PCM and thermal conductivity test sample: (a) mold for paraffin/xGNP composite PCM and (b) mold and sample for thermal conductivity. ARTICLE IN PRESS S. Kim, L.T. Drzal / Solar Energy Materials & Solar Cells 93 (2009) 136–142 139 3. Results and discussion 3.2. Resistivity of paraffin/xGnP composite PCMs 3.1. Morphology of paraffin/xGnP composite PCMs This high electrical conductivity was detected by the resistivity of paraffin/xGnP composite PCMs as xGnP loading content as shown in Fig. 5. The incorporation of xGnP can greatly decrease the resistivity of composites with a sharp transition from an electrical insulator to an electrical conductor. For the purpose of finding a percolation threshold for the resistivity 1, 2, 3, 5 and 7 wt% xGnP loaded samples were measured. The percolation threshold of xGnP-LLDPE nanocomposite by solution mixing and injection molded was between 1 and 2 wt%. This percolation threshold is extremely low compared to the Author’s result [26] of xGnP dispersed into low linear density poly propylene. It was between 12 and 15 wt%. Resistivity of 1 wt% of xGnP loaded was high around 109 O cm, even second melted sample. However, resistivity was down to 104 O cm from 2 wt% of xGnP. The percolation threshold for the resistivity depends very much on the geometry of the conducting fillers. Fillers with elongated geometry such as sheets can be used to achieve very low percolation threshold value, due to the fact that sheets with higher aspect ratios have great advantage over spherical or elliptical fillers in forming conducting networks in polymer matrix. As we check the morphology in Figs. 3 and 4, xGnP were connected to each other to make electric conductivity. As continuous PCMs, resistivity of second melted sample was measured. Resistivity of second melted samples showed similar behavior with first melted samples. Reproducibility of paraffin/xGnP composite PCMs as continuous PCMs was manifested (Fig. 5). The cryogenically fractured surface of the paraffin/xGnP composite PCMs was studied by SEM. Fig. 3 shows the SEM photographs of the paraffin/xGnP composite PCMs of 2% and 5% xGnP loading contents with gold coating and non-coating (magnification of 2000). It is observed from Fig. 3a and c that the dispersions of the xGnP in the paraffin wax are uniform. xGnP was well-dispersed in paraffin. Actually, it looks like paraffin covered slightly on the xGnP surface. It is a different morphology compared to the Author’s result in which xGnP is dispersed in LLDPE polymer matrix [26]. We can easily recognize the existence of xGnP by its uniform shape. As shown in Fig. 3b and d the dispersion of xGnP in paraffin is indicated by the clear white plate phase even it was not coated by gold. From this morphology of non-coating samples, it can be expected that these 2% and 5% xGnP-loaded PCM composites are electrically conductive because SEM can detect only electrically conductive materials by the electric beam. The dispersion of xGnPs covered by paraffin is more significantly indicated in high magnification ( 5000) as shown in Fig. 7. Furthermore, the uniform xGnP particle size is indicated with these figures. Although xGnP loading contents were low, 2 and 5 wt%, xGnP are well embedded and dispersed enough to show their existence. Fig. 3. Scanning electron microphotographs of 5 and 2 wt% of paraffin/xGnP composite PCMs by coating condition for SEM (low magnification, 2000): (a) paraffin/xGNP 5%—gold coating, (b) paraffin/xGNP 5%—non-coating, (c) paraffin/xGNP 2%—gold coating, and (d) paraffin/xGNP 2%—non-coating. ARTICLE IN PRESS 140 S. Kim, L.T. Drzal / Solar Energy Materials & Solar Cells 93 (2009) 136–142 Fig. 4. Scanning electron microphotographs of 5 and 2 wt% of paraffin/xGnP composite PCMs by coating condition for SEM (high magnification, 5000): (a) paraffin/xGNP 5%—gold coating, (b) paraffin/xGNP 5%—non-coating, (c) paraffin/xGNP 2%—gold coating, and (d) paraffin/xGNP 2%—non-coating. 1010 paraffin/xGnP PCMs 2nd melted 1st melted 109 Resistivity (ohm∗cm) 108 107 106 105 104 ductivity of the composite PCM including mass 7 wt% xGnP is found to be 0.8 W/mK. These results are comparable to Zhang’s report [29]. Ten percent of graphite mass fraction in the shapestabilized PCM showed 0.229 W/mK, while 20% of cases were 0.482 W/mK in this report. Theoretically, the thermal conductivities will increase continually with increasing additive quantity of exfoliate graphite. The thermal conductivity of paraffin/xGnP composite PCMs was increased as xGnP loading contents. Reproducibility of paraffin/xGnP composite PCMs as continuous PCMs for thermal conductivity also reappeared like electric conductivity, even second samples were little higher than the first samples. 103 3.4. Thermal storage performance and thermal stability of paraffin/ xGnP composite PCMs 102 101 1 2 3 4 5 6 xGnP loading content (wt%) 7 Fig. 5. Resistivity of paraffin/xGnP composite PCMs by melting times. 3.3. Thermal conductivity of paraffin/xGnP composite PCMs The thermal conductivity results of pure paraffin and the paraffin/xGnP composite PCMs are shown in Fig. 6. It can be found that the thermal conductivities of the composite PCMs improve evidently compared to that of pure paraffin. When the thermal conductivity of pure paraffin is 0.26 W/mK, the thermal con- The heating and freezing curves by DSC measurements of the paraffin and the paraffin/xGnP composite PCMs are presented in Fig. 7. It can be seen from the heating curve in Fig. 7(a) that the paraffin has two phase change peaks. The first phase change peak at about 35.3 1C is lower and corresponds to the solid–solid phase transition of the paraffin, and the second peak is very high at around 55.2 1C, corresponding to the solid–liquid phase change. These peaks are matched with pure paraffin peaks in the previous report [30]. The DSC curve of the paraffin/xGnP composite PCMs is shown in Fig. 7b–d. There are also two peaks around 35 and 55 1C in the DSC curve of the paraffin/xGnP composite PCMs, and the thermal characteristics of the composite PCM are very close to ARTICLE IN PRESS S. Kim, L.T. Drzal / Solar Energy Materials & Solar Cells 93 (2009) 136–142 0.9 Thermal Conductivity (W/mK) 0.8 2nd melted Y=0.080X + 0.31 R2=0.99 0.7 0.6 0.5 1st melted Y=0.085X + 0.26 R2=0.99 0.4 0.3 0.2 0 1 2 3 4 5 xGnP loading content (wt%) 6 7 Fig. 6. Thermal conductivity of paraffin/xGnP composite PCMs by melting times. 128.8J/g 26.2J/g 29.2J/g 128.1J/g 35.3°C 51.0°C paraffin/xGnP1% 32.9°C Heat flow (w/g) 32.9°C Heat flow (w/g) those of the pure paraffin. This is because there is no chemical reaction between the paraffin and the EG in the preparation of the composite PCM [8]. The latent heat of the paraffin is obtained as the total area under the peaks of the solid–solid and solid–liquid transitions of the paraffin in the composite by numerical integration. From the Fig. 8, it can be seen that the latent heat of the paraffin/xGnP composite PCMs approach those of the pure paraffin. The latent heat of paraffin/xGnP composite PCMs did not decrease as loading xGnP contents to paraffin. There is no significant difference of the latent heat between paraffin and paraffin/xGnP composite PCMs. Previous results showed a decrease of latent heat as graphite loading contents increased [8,30]. In these results, due to graphite and EG, although thermal conductivity of paraffin/graphite composite PCM was increased, the latent heat of PCM was decreased as graphite loading contents. They explained the reason that the three-dimensional net structure confines the molecular heat movement of the paraffin in the PCM composites. However, in the case of xGnP, there was no problem because of good dispersion of xGnP in paraffin with high surface area. 51.1°C paraffin only 141 134.0J/g 26.8J/g 27.8J/g 131.0J/g 35.1°C 55.1°C 55.2°C 20 30 40 50 Temperature (°C) 70 10 51.4°C paraffin/xGnP3% 33.0°C Heat flow (w/g) 60 28.1J/g 127.6J/g 34.9°C 30 40 50 Temperature (°C) paraffin/xGnP5% 132.4J/g 27.8J/g 20 27.4J/g 20 30 40 50 Temperature (°C) 60 70 131.5J/g 27.4J/g 35.4°C 130.0J/g 54.9°C 55.1°C 10 60 50.8°C 32.9°C Heat flow (w/g) 10 70 10 20 30 40 50 Temperature (°C) Fig. 7. The heating and freezing curves by DSC of paraffin/xGnP composite PCMs. 60 70 ARTICLE IN PRESS 142 S. Kim, L.T. Drzal / Solar Energy Materials & Solar Cells 93 (2009) 136–142 surface of the paraffin/xGnP composite PCMs, xGnP was welldispersed into paraffin wax, and it led to high electric conductivity and thermal conductivity. As increasing xGnP loading contents, electric conductivity and thermal conductivity were increased. The results clearly indicated an almost linear relationship between thermal conductivity and mass fraction of xGnP in composite PCM. The percolation threshold of Paraffin/xGnP composite PCMs on resistivity was between 1 and 2 wt%. This low percolation threshold was caused by well dispersion of high aspect ratio of xGnP. On the other hand, latent heat was not decreased as xGnP loading contents. xGnP of uniform high surface area showed improved thermal storage performance. As a result, xGnP can be considered as an effective heat diffusion promoter to improve thermal conductivity of PCMs without reducing its latent heat storage capacity. 140 Latent heat capacity (J/g) 130 120 Heaing, Phase transition Cooling, Phase transition Heaing, Phase change Cooling, Phase change 110 40 30 20 0 1 2 3 4 xGnP loading content (wt%) 5 Acknowledgement Fig. 8. Latent heat storage performance of paraffin/xGnP composite PCMs at phase transition and phase change by DSC. 100 paraffin only paraffin/1% xGnP PCMs paraffin/3% xGnP PCMs paraffin/5% xGnP PCMs Weight change (%) 80 References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] 60 40 20 0 0 100 200 300 400 Temperature (°C) 500 This work (Sumin Kim) was supported by the Soongsil University Research Fund. 600 Fig. 9. Thermal decomposition behavior of paraffin/xGnP composite PCMs by TGA. The mass loss of the paraffin/xGnP composite PCMs is shown in Fig. 9. As the xGnP loading increased, the thermal stability of the composites did not significantly differ until 3 wt%, while at 5 wt% it slightly increased. After thermal decomposition, after 300 1C, we can check the xGnP contents of each composite with weight percent of remaining materials. xGnP of 1, 3 and 5 wt% was exactly loaded in the paraffin/xGnP composite PCMs. [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] 4. Conclusion Paraffin/xGnP composite PCMs were prepared for high electric conductivity, thermal conductivity and latent heat storage. The paraffin/xGnP composite PCMs can be easily prepared by stirring of xGnP in liquid paraffin. From the cryogenically fractured [26] [27] [28] [29] [30] D. Feldman, M.M. Shapiro, D. Banu, Sol. Energy Mater. 13 (1986) 1. C. Alkan, Thermochim. Acta 451 (2006) 126. A. Karaipekli, A. Sarı, K. Kaygusuz, Renew. Energy 32 (2007) 2201. B. Zalba, J.M. Marin, L.F. Cabeza, H. Mehling, Appl. Therm. Eng. 23 (2003) 251. S.D. Sharma, K. Sagara, Int. J. Green Energy 2 (2005) 1. M. Xiao, B. Feng, K. Gong, Energy Convers. Manage. 43 (2002) 103. B. He, F. Setterwall, Energy Convers. Manage. 43 (2002) 1709. Z. Zhang, X. Fang, Energy Convers. Manage. 47 (2006) 303. A. Abhat, Sol. Energy 30 (1983) 313. B. Zalba, J.M. Marin, L.F. Cabeza, H. Mehling, Appl. Therm. Eng. 23 (2003) 251. S. Himran, A. Suwono, G.A. Mansoori, Energy Sources 16 (1994) 117. E.M. Alawadhi, C.H. Amon, IEEE Trans. Components Packag. Technol. 26 (2003) 116. H. Yin, X. Gao, J. Ding, Z. Zhang, Energy Convers. Manage. 49 (2008) 1740. A. Sarı, A. Karaipekli, Appl. Therm. Eng. 27 (2007) 1271. R. Velraj, R.V. Seeniraj, B. Hafner, C. Faber, K. Schwarzer, Sol. Energy 65 (1999) 171. Z. Liu, X. Sun, C. Ma, Energy Convers. Manage. 46 (2005) 971. B. Tryba, A.W. Morawski, K. Kalucki, J. Phys. Chem. Solids 65 (2004) 165. B. Tryba, A.W. Morawski, M. Inagaki, Carbon 43 (2005) 2417. Y.F. Zhao, M. Xiao, S.J. Wang, X.C. Ge, Y.Z. Meng, Compos. Sci. Technol. 67 (2007) 2528. F. Kang, Y. Leng, T.-Y. Zhang, J. Phys. Chem. Solids 57 (1996) 889. H. Fukushima, Graphite nanoreinforcements in polymer nanocomposites, Ph.D. Thesis, Michigan State University, East Lansing, MI, USA, 2003. K. Kalaitzidou, Exfoliated graphite nanoplatelets as reinforcement for multifunctional polypropylene nanocomposites, Ph.D. Thesis, Michigan State University, East Lansing, MI, USA, 2006. K. Kalaitzidou, H. Fukushima, L.T. Drzal, Carbon 45 (2007) 1446. H. Park, K. Kalaitzidou, H. Fukushima, L.T. Drzal, In: Proceedings of Society of Plastics Engineers, Automotive Composites Conference & Exhibition, MI, USA, 2007. W. Liu, I. Do, H. Fukushima, L.T. Lawrence Drzal, in: Proceedings of Society of Plastics Engineers, Automotive Composites Conference & Exhibition, Troy, MI, USA, 2007. S. Kim, I. Do, L.T. Drzal, Polym. Test. 2008, submitted for publication. A. Mills, M. Farid, J.R. Selman, S. Al-Hallaj, Appl. Therm. Eng. 26 (2006) 1652. S. Pincemin, R. Olives, X. Py, M. Christ, Sol. Energy Mater. Sol. C 92 (2008) 603. Y. Zhang, J. Ding, X. Wang, R. Yang, K. Lin, Sol. Energy Mater. Sol. C 90 (2006) 1692. Y. Cai, Q. Wei, F. Huang, W. Gao, Appl. Energy 85 (2008) 765.
© Copyright 2024