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Evaluation of electric, morphological and thermal properties of thermally conductive polymer composites
Accepted Manuscript Evaluation of electric, morphological and thermal properties of thermally conductive polymer composites Sergey M. Lebedev, Olga S. Gefle PII:
S1359-4311(15)00842-X
DOI:
10.1016/j.applthermaleng.2015.08.046
Reference:
ATE 6931
To appear in:
Applied Thermal Engineering
Received Date: 24 November 2014 Revised Date:
25 July 2015
Accepted Date: 2 August 2015
Please cite this article as: S.M. Lebedev, O.S. Gefle, Evaluation of electric, morphological and thermal properties of thermally conductive polymer composites, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.08.046. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Evaluation of electric, morphological and thermal properties of thermally conductive polymer composites ∗
Sergey M. Lebedev , Olga S. Gefle Tomsk Polytechnic University, 2A Lenin Avenue, Tomsk, Russia, 634028
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Abstract Natural graphite powder was introduced into a linear low density polyethylene (LLDPE) to improve the thermal conductivity. The thermal conductivity of LLDPE/graphite composites is increased by more than seventeen times with increase in filler content up to 30 vol.% as compared to that of LLDPE matrix. The percolation threshold for these composites is equal to 10 vol.%. LLDPE/graphite composites can be used as cost effective thermally dissipative materials for electrical engineering and electronic devices due to the high thermal conductivity and thermal diffusivity.
Thermally conductive polymeric materials with high thermal conductivity were developed. Thermal conductivity, thermal diffusivity and electrical properties of new composites were tested. Thermal analysis of new materials was carried out. Heat sink made of new polymer/graphite composites for LED-lamps were produced and studied.
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Highlights
1. Introduction
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The ability of heat dissipation by heat-releasing nonmetallic materials is becoming more and more important problem for semiconductor devices and micro-electronic equipment (for example, circuit boards, computer equipment, and LED light sources) because the local temperature rise limits their reliability and durability. Historically this problem has been solved by means of metals, their alloys, oxides, nitrides and ceramics with high thermal conductivity. With the recent development of filled polymer composites, especially nanocomposites, thermally conductive polymer materials (TCPMs) used as heat-releasing materials due to their low cost, lightweight and processability are becoming an alternative for the use of metals and ceramics. Thermal conductivity of polymer materials can be improved by modifying polymer matrices using micro-, submicro- or nano-sized fillers with high thermal conductivity. Graphite, carbon black, carbon fibers and carbon nanotubes, zinc oxide (ZnO), aluminum nitride (AlN), and various metals are frequently used as fillers for this purpose. Metallic fillers possess a very high thermal conductivity, for example, 200–230 Wm–1K–1 for aluminum, 370–480 Wm– 1 –1 K for copper, 400–450 Wm–1K–1 for silver, etc. Fu et al. [1] studied the thermal conductivity of composites based on the epoxy resin filled with Cu, Al, and Ag. However, the thermal conductivity of the developed composites did not exceed 1.2 Wm–1K–1 even at filler content about 70 wt.%. Zeng et al. [2] improved the thermal conductivity of organic phase change material by the addition of Ag nanowires. The value of the thermal conductivity of composites did not exceed 1.5 Wm–1K–1 at 62 wt.% of Ag nanowires. Furthermore, metallic fillers have several disadvantages which include high weight, incompatibility with some types of polymers and a high susceptibility to oxidation. Other type of fillers for thermally conductive polymer composites is ceramics such as BN (30–600 Wm–1K–1), AlN (180–220 Wm–1K–1), Al2O3 (20–30 Wm–1K–1), ZnO (about 55 Wm–1K–1), etc. However, the thermal conductivity of composites filled with ceramics does not exceed 0.59 Wm–1K–1 [1] for epoxy/BN composite at the filler content about 35.5 wt.% and 1.54 Wm–1K–1 for polyimide/ZnO composites with 27 vol.% [3] while the cost of some ceramics types is also very high. Zhou and coauthors [4] studied HDPE composites filled with BN and Al2O3 using two processing methods: powder mixing and hot pressing methods. They found that for 35 vol.% of BN, Al2O3 or BN+Al2O3 the thermal conductivity of composites did not exceed 1.5 Wm–1K–1. That is, to develop polymer/ceramics thermally conductive composites with high thermal conductivity, the filler content should be more than 50 vol.%. In turn, very high filler content results in a deterioration of rheological properties of such composites, for instance, to reduction of a melt flow index and impossibility of processing composites by extrusion or injection molding. Better results have been attributed to the polymer composites filled with both single-walled and multi-walled carbon nanotubes (SWNT and MWNT) due to their superior thermal, electrical, and mechanical properties. It has been shown that application of carbon nanotube composites allows the lowest electrical percolation threshold to be reached [5,6,7]. Andrews et al. [5] and Alam et al. [6] have found that the electrical percolation threshold for polypropylene/MWCNT and PMMA/MWCNT composites is equal to 0.05 vol.%. Liu and Grunlan [7] and Haggenmuller with co-authors [8] have reported a much lower percolation threshold values of 0.01 vol.% for ∗
Corresponding author E-mail address: [email protected] (S.M. Lebedev)
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SWCNT/epoxy nanocomposites and 0.003 vol.% for SWCNT/LDPE nanocomposites respectively. However these values are characteristic only for an electrical percolation threshold. Indeed, the thermal conductivity of polymer/CNT composites even with adding Ag-nanoparticles does not exceed 1.0 Wm–1K–1 [9,10]. The highest value of the thermal conductivity for LDPE/SWCNT composite reaches 1.8 Wm–1K–1, while the HDPE/SWCNT composites possess the thermal conductivity of 3.5 Wm–1K–1 [8,11] at filler content about 20 vol.%. The absence of significant increase in thermal conductivity for polymer/CNT composites is attributed to the large interfacial thermal resistance between polymer matrix and CNTs [8,12]. Carbon nanotube polymer composites are not so widely applied in industry of thermally conductive materials due to high cost of SWCNT and MWCNT, which limits their practical use. Besides, processing of carbon nanotube polymer composites by means of conventional processing technique may result in destruction of nanotubes due to a high shear deformation of composite materials in extruders or mixers. Furthermore, the outer walls of nanotubes can not be easily wetted by the polymer matrix melt adversely affecting the resulting composite due to poor interfacial interaction. To remedy this problem, the surface of CNTs should be additionally functionalized [9]. In turn, this procedure complicates the technology and increases the final cost of composite materials. Graphite is a versatile material widely used in industry due to its unique physical, electrical and chemical properties [13,14]. The extremely high thermal conductivity of graphite is used in the fabrication of thermally conductive composites [1,15,16]. Its main advantages are low density and cost compared to other materials used for this purpose. Graphite also has an advantage in that it is inert and compatible with most polymers. Various polymers such as epoxy resin [1], silicon rubber, HDPE, LDPE, etc. [17-20] are used as polymer matrices for the thermally conductive materials filled with graphite. Fu and co-authors [1] have reported that the highest thermal conductivity of epoxy/graphite adhesive is equal to 1.68 Wm–1K–1 at 44.3 wt.% of expanded (exfoliated) graphite, which may be attributed to the layered structure of graphite. In turn, it can result in the formation of both numerous contact points between the filler particles and the heat pathways in the matrix [14]. The same result was obtained by Ye et al. [17] for HDPE/graphite composites at 22 vol.% of graphite. Authors of [18] have reported that the maximum thermal conductivity of HDPE/graphite and LDPE/graphite composites were about 2.3 Wm–1K–1 and 2.0 Wm–1K–1 at the filler content of 37 vol.% respectively. Debelak and Lafdi [14] have prepared silicone rubber/exfoliated graphite composites with different size of graphite flakes (graphite flakes were separated using 50, 100, and 150 mesh sieves) the thermal conductivity of which was about 4.3 Wm–1K–1 at 20 wt.% of filler content. The highest values of the thermal conductivity about 8 Wm–1K–1 [19] were obtained for parafin/graphite phase change materials with special core-shell microstructure, and 11 Wm–1K–1 [20] for LDPE/low-temperature expandable graphite composites, respectively. With consideration of the cost of composites filled with expanded (exfoliated) graphite, the processing complexity (an additional chemical treatment of graphite), the chemical reactivity (with likely residual acid in the graphite nanoparticles), the natural graphite is a much more attractive choice than expanded nano-sized graphite as a filler for thermally conductive polymer composites. Moreover, nanoparticles of the expanded graphite can be destructed during processing by the conventional methods such as extrusion or injection molding resulting in the reduction of the thermal conductivity [14]. Therefore development and study of commercially available polymer composites with a high thermal conductivity suitable for extrusion processing or injection molding was the aim of this paper.
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2. Experimental technique and samples
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Measurements of the real part of admittance γa and phase angle between voltage and current ϕ were done under AC voltage 3V in the frequency range from 10–1 Hz to 104 Hz using the Solartron Instrument (Impedance/Gain-Phase Analyzer Solartron 1260 + Dielectric Interface Solartron 1296) [21]. From five to ten measurements per decade over the frequency range were carried out for all samples. At least five samples for each composite were tested. Temperatures and temperature intervals of melting and decomposition of nanocomposites, heat flow and weight loss of the samples were measured in the temperature range from 25 to 600°C at a heating rate of 10°C/min in an argon atmosphere with a combined DSC-DTA-TGA analyzer Q600 “TA Instruments”. From three to five samples of each composite were tested for all test conditions. The study of thermal conductivity of TCPMs was done by the transient hot bridge method (the modified method of a hot wire [22]) using the THB-100 analyzer (Linseis, Germany) [23]. The measuring sensor including a heater and temperature sensor was located between two identical 5 mm thick samples made of the same TCPM. Thermal diffusivity of TCPMs was measured by the flash method [24] with the XFA 500 analyzer (Linseis, Germany). IR detector of XFA analyzer was cooled by liquid nitrogen. Measurements were carried out under the vacuum approximately 2.0⋅10–5 bar. Temperature changes on the surface of the TCPM samples were monitored by IR-camera. Scheme of experiment is presented in Figure 1. A brass rod 7.5 mm in diameter with heater was used as a heating source. The temperature of heating source during this experiment was equal to 50.0 ± 2.0°C. Measuring the temperature redistribution due to the heat transfer at the surface of TCPM sample was beginning from the moment of its placing at the surface of heat source through each 30 s for 20 min. Minimum and maximum temperatures were measured for all samples. The minimum value corresponds to temperature of the sample periphery, and maximum one to the surface temperature at the sample center above the heat source.
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Fig 1. Experimental setup: sample (1), heating source (2), temperature controller (3), IR-camera (4).
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The value of melt-flow index (MFI) was measured by using a versatile melt flow tester (Instron CEAST MF20, Italy) according to ISO 1133 and ASTM D1238. A Philips 515 scanning electron microscope (SEM) was used to investigate the dispersion of filler in the polymer matrix. The samples were fractured in liquid nitrogen and then coated with silver. Linear low density polyethylene (LLDPE Hanwha 3305, Korea) with a melt flow rate 1.9 g/10min was used as a polymer matrix in this study. The density of LLDPE is 0.922 g/cm3. Powder of natural graphite (GE-3, the Russian standard GOST 17022-81) with average particle size of 65 µm was used as filler as received (Figure 2). Filler content (C) in polymer composites was changed from 5 vol.% to 30 vol.%. To improve the interface compatibility of the LLDPE matrix with graphite powder, 2 wt.% of LLDPE-g-MAH (LLDPE grafted with maleic anhydrite) was added into the LLDPE matrix.
Fig. 2. Micrograph of graphite powder.
All TCPMs were prepared by melt mixing in the measuring mixer type 50 EHT (Brabender, Germany) with a bowl volume approximately 55 cm3, maximum operating temperature 500°C, and maximum torque 200 N⋅m. Graphite powder was gradually introduced into the polymer melt up to the required volume fraction, while mixing until all graphite powder was evenly distributed in the matrix. Mixing time was changed from 10 to 25 min. Afterwards, all composites were granulated into pellets by the granulator (Brabender, Germany). To prepare samples, compression molds filled with TCPMs were press at 12.5 MPa for 20 min. The compression molds were slowly cooled at the cooling rate of 4°C/min up to ambient temperature under pressure in air. Samples studied were prepared as plates 85×65×6 mm and discs 12.7 mm in diameter and thicknesses of 0.8 mm placed into a vacuum oven heated up to 190°C for 3 h. After that, the compression molds were pressed in a hydraulic. 3. Experimental results and discussion Figure 3 shows frequency dependencies of the real part of admittance γa = ωε0ε′tanδ (where ω = 2πf is the circular
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frequency, ε0 = 8.854⋅10–12 F/m is the vacuum permittivity, ε′ is the real part of complex permittivity, and tanδ is the loss factor) and phase angle ϕ between voltage and current for all TCPMs. It can be seen that γa for TCPMs in the frequency range from 0.1 Hz to 104 Hz is increased by 2.5 orders of magnitude with increase in filler content from 5 vol.% to 30 vol.%. Measurement of γa for TCPMs with filler content C ≥ 20 vol.% in the frequency range from 0.1 Hz to 30 Hz becomes impossible by means of the Solartron instrument because of sharp increase in the active component of a current. Figure 3b is evident illustration of this statement. For TCPMs at C ≤ 10 vol.%, the angle ϕ is close to 90° (the angle of dielectric loss δ = 90° – ϕ << 1°) and practically does not depend on the frequency. That is, these TCPMs are dielectric materials over the studied frequency range. TCPMs with С ≥ 15 vol.% at frequency more than 1 Hz become semi-conductive materials. TCPMs at C ≥ 20 vol.% possess electrically conductive properties over the studied frequency range (see Figure 3b) and can be used for manufacturing electrically conductive or thermally conductive elements of various electrical engineering and electronic devices.
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b) Fig 3. Frequency dependencies of γa (a) and ϕ (b) for TCPMs with different filler content: 1, 5 vol.%; 2, 10 vol.%; 3, 15 vol.%; 4, 20 vol.%; 5, 30 vol.%.
To estimate the influence of the filler content on thermal properties of TCPMs, the thermal conductivity (λ) and thermal diffusivity (α) of matrix used in the experiment have been measured. The values of λ and α for the LLDPE matrix at room temperature are listed in Table. Table Thermal conductivity and thermal diffusivity of LLDPE matrix. Material LLDPE
Thermal conductivity λ, Wm–1K–1 0.36
Thermal diffusivity α⋅107, m2/s 1.85
Figure 4 shows semi-logarithmic dependencies of the thermal conductivity and thermal diffusivity for TCPMs at room temperature as functions of the filler content (C). It is obvious that values of λ and α for composite with 30 vol.% of filler are increased more than 17 and 5 times as compared to those for the LLDPE matrix. It can be seen that dependencies λ = f(С) and α = f(С) are non-monotonic in character and at C ≥ 10 vol.% a sharp increase in the values of
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λ and α is observed. It can be related to the percolation effect [25-27] and appearance of direct contacts between the filler particles which results in the formation of three-dimensional network [28,29] at such rather low filler content.
Fig. 4. Thermal conductivity and thermal diffusivity as functions of the filler content for LLDPE/graphite composites.
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The percolation effect can be defined as a phase transition when a dramatic growth of the current through material occurs with filler content increase. In the case of filled polymer composites, the electrical conductivity (or thermal conductivity) sharply changes at a precise filler content. The filler content that marks this phase transition is usually referred to as the percolation threshold. The percolation effect can be observed when a continuous network of filler particles (a segregated network) forms in the polymer composite [29-31]. Kusy [32] has introduced into practice the “segregated network” concept in the context of electrically conductive mixtures of polymer and metal powder to explain the formation of electrically conductive network: “...The term “segregated” denotes that microscopic dispersed particles are being restricted into the boundaries of much larger primary particles. From the macroscopic viewpoint this distribution is random too, since both phases are relatively small...”. The segregated network can be formed in semicrystalline polymers such as polyethylene in one of the phases or at the interface of amorphous and crystalline phases [33,34]. Figure 5 shows the SEM micrograph of the fracture surface of LLDPE/graphite composite with 30 vol.% of the graphite. Figure 5 confirms quasi-uniform dispersion of graphite particles in the polymer matrix. It can be seen that graphite particles are randomly dispersed in the polymer matrix. With increasing the filler content, filler particles contact each other forming continuous pathways or segregated networks through the polymer.
Fig. 5. SEM micrograph of the fracture surface of LLDPE/graphite composite with 30 vol.% of graphite.
However for TCPMs studied, substantial anisotropy of electrical and thermal properties is practically absent in contrast to polymer composites filled with carbon fibers, graphite flakes, and carbon nanotubes [35-37]. It is caused by both a stochastic form of filler particles and absence of the so-called aspect ratio effect [14,36,37]. The thermal conductivity of TCPMs investigated can be risen with increase in the filler content because the melt flow index for TCPM with 30 vol.% of graphite is equal to 0.6 g/10min that is sufficient for its processing by the injection molding. The thermal conductivity of LLDPE/graphite composites can be also increased by adding small fraction of carbon
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black (CB) with the average particle size of 50 nm. Indeed, the addition of 3 vol.% CB into the LLDPE/graphite composite with C = 30 vol.% increases the value of λ by 20 % other things being equal. It can be related to formation of a segregated network of filler particles due to different particle sizes of graphite and carbon black [7,31,38]. To estimate an opportunity of heat dissipation by means of TCPMs, the study of thermovision images of heat transfer on the sample surface was performed with IR-camera. Typical thermograms for samples made of polymer matrix and TCPM with 30 vol.% of graphite are shown in Figure 6.
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Fig. 6. Thermovision images of the heat source (a) and heat transfer on the sample surface for TCPM with 30 vol.% of graphite (b-f) and for the LLDPE matrix (g, h) at different time: b, 30 s; c, 120 s; d, 300 s; e, 600 s; f, 900 s; g, 180 s; and h, 900 s.
It can be seen that the maximum temperature on the TCPM sample’s surface does not exceed 40°C after 900 seconds due to high value of λ, heat transfer from a center to periphery of the sample, and convective heat dissipation from the sample surface. Indeed, temperature at the edge of TCPM sample is changed from 26.6°C (30 seconds, Figure 6b) approximately to 32°C (900 seconds, Figure 6f). In contrast, the maximum temperature at the center of the LLDPE
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sample without filler is practically equal to the heat source’s temperature after 900 seconds (Figures 6g and 6h). Temperature at the LLDPE sample edge is increased from room temperature only up to 28.1°C after 900 seconds (Figure 6h). For comparison, the temporal dependencies of temperature at the samples surface for TCPM and LLDPE are shown in Fig. 7. It is obvious that the difference between the maximum temperatures on the surface 900 s after switching of the heat source for the samples made of TCPM with 30 vol.% of graphite and LLDPE is more than 11°C. Moreover, both the maximum temperature and the temperature at the edge of the sample made of the TCPM tend to the saturation with time, and the quasi-steady state thermal regime is observed after 900 s. Besides, the difference between the maximum temperature and the temperature at the sample edge for the LLDPE is 22.3°C while this difference for the sample made of the TCPM is only 7.2°C. This is attributed to the lower thermal conductivity and heat transfer of LLDPE as compared to those for TCPM. When the elements of the cooling system (e.g. heat sink of LED-lamp) will be made of LLDPE, it can result in the overheating of LEDs.
Fig. 7. Temporal dependencies of temperature on the samples surface: 1,2 for the TCPM with 30 vol.% of graphite; 3,4 for the LLDPE sample: 1,3 the maximum temperature; 2,4 temperature at the sample edge.
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Temperature changes on the samples surface are caused by only the thermal conductivity, heat transfer and convective heat dissipation because other phase transitions in the given temperature range have not been observed both for polymeric matrix and TCPMs. Typical DSC-curve for the LLDPE matrix in Fig. 8 well demonstrates this statement. It can be seen that only two endothermic peaks are observed over the temperature range studied. The first of them approximately at (125÷130)°C is conditioned by the melting process, and the latter at (485÷490)°C by the decomposition process of the polymer composite. Similar results were obtained for the LLDPE/graphite composites, not presented here for brevity.
Fig. 8. Thermogram for LLDPE matrix.
LLDPE/graphite composites were used as inexpensive thermally dissipative materials for electrical engineering and electronic devices due to the high thermal conductivity and thermal diffusivity. In particular, LLDPE/graphite composites were used to manufacture cooling elements of LED-lamps (Figure 9).
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Fig. 9. Heat sinks (a) and LED-lamps (b).
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Cooling elements (heat sinks) for LED-lamps were manufactured by injection molding method using all-electric injection molding machine Battenfeld EcoPower 55. Polymer heat sinks made of TCPM with 30 vol.% of graphite have shown a good efficiency in cooling systems for LED-lamps (Figure 10). The study of temperature field of LED-lamps with heat sinks made of TCPMs was performed by the thermovision device during 2 hours after switching on of the voltage. Two types of LED-lamps with heat sinks made of TCPMs with 20 vol.% and 30 vol.% of graphite were mounted in the special setup at some distance from each other for elimination of mutual thermal influence. Both LEDlamps were switched on simultaneously.
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a) b) Fig. 10. IR images of LED-lamps with different material of heat sinks: a, TCPM with 20 vol.% of graphite; b, TCPM with 30 vol.% of graphite.
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It can be seen that the metal base of LED-lamp with the heat sink made of TCPM with 30 vol.% of filler (Figure 10b) is more heated due to the high thermal conductivity of TCPM and best heat transfer compared to other lamp (Figure 10a). Besides, temperature on the surface of heat sink (Figure 10b) is distributed more homogeneously compared to other LED-lamp. When estimating efficiency of heat transfer by polymer heat sink, the main criteria is the minimum temperature difference between the maximum temperature Tmax (at the top part of the heat sink in the vicinity of printed circuit board with LEDs) and the minimum temperature Tcen (at the bottom part of the heat sink) on the surface of heat sink: ∆T = Tmax – Tcen The value of ∆T for TCPM with 30 vol.% of graphite is equal to 11.0°C. The minimum temperature difference on the heat sink surface is conditioned by both the high value of the thermal conductivity of TCPM with 30 vol.% of graphite and maximum speed of the heat transfer from a hot part to a cold one. 4. Conclusions Adding the natural graphite powder into LLDPE matrix allows the thermal conductivity to be increased by a factor of 17.8 compared to that for the LLDPE matrix at the filler content 30 vol.%. The thermal conductivity of developed LLDPE/graphite composites can be increased up to 6.5 Wm–1K–1 at 30 vol.% of graphite. These LLDPE/graphite composites can be processed by the injection molding. The dielectric spectroscopy showed that LLDPE/graphite composites possess dielectric properties up to 10 vol.% of the filler content, while the addition more than 15 vol.% of graphite increases the value of the admittance not less than one-two order of magnitude. IR-study results demonstrate suitable temperature redistribution on the TCPM surface. The results of thermovision study demonstrate high efficiency
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of polymer/graphite heat sinks due to the high speed of the heat transfer. Future study will focus on the effects of compatibilizers and inert fillers in LLDPE/graphite composites that may exhibit better dispersion of fillers in the polymer matrix and reduction of the percolation threshold. References Y.-X. Fu, Z.-X. He, D.-C. Mo, S.-S. Lu, Thermal conductivity enhancement with different fillers for epoxy resin adhesives, Appl. Therm. Eng. 66 (2014) 493–498. [2] J.L. Zeng, Z. Cao, D.W. Yang, L.X. Sun, L. Zhang, Thermal conductivity enhancement of Ag nanowires on an organic phase change material, J. Therm. Anal. Calorim. 101 (2010) 385–389. [3] D. Yorifuji and S. Ando, Enhanced thermal conductivity over percolation threshold in polyimide blend films containing ZnO nano-pyramidal particles: advantage of vertical double percolation structure, J. Mater. Chem. 21 (2011) 4402–4407. [4] W. Zhou, S. Qi, Q. An, H. Zhao, N. Liu, Thermal conductivity of boron nitride reinforced polyethylene composites, Mat. Res. Bull. 42 (2007) 1863–1873. [5] M.S. Alam, K.K. Verma, Investigation of thermal and hardness properties of MWCNTs filled PMMA polymer nanocomposites, Amer. J. Mater., Sci. Appl., 2 (2014) 31–34. [6] R. Andrews, D. Jacques, M. Minot, T. Rantell, Fabrication of carbon multiwall nanotube/polymer composites by shear mixing, Macromol. Mater. Eng. 287 (2002) 395–403. [7] L. Liu, J.C. Grunlan, Clay assisted dispersion of carbon nanotubes in conductive epoxy nanocomposites, Adv. Funct. Mater. 17 (2007) 2343–2348. [8] R. Haggenmueller, C. Guthy, J.R. Lukes, J.E. Fischer, and K.I. Winey, Single wall carbon nanotube/polyethylene nanocomposites: Thermal and electrical conductivity, Macromol. 40 (2007) 2417–2421. [9] P.-C. Ma, N.A Siddiqui, G. Maron, J.-K. Kim, Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: Review, Composites: Part A, 41 (2010) 1345–1367. [10] M. Jouni, A. Boudenne, G. Boiteux, V. Massardier, and B. Garnier, Significant enhancement of electrical and thermal conductivities of polyethylene carbon nanotube composites by the addition of a low amount of silver nanoparticles, Polym. Adv. Technol. 25 (2014) 1054–1059. [11] C. Kingston, R. Zepp, A. Andrady, et al., Release characteristics of selected carbon nanotube polymer composites, Carbon, 68 (2014) 33–57. [12] J. Han, A. Fina, Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review, Prog. Polym. Sci. 36 (2011) 914–944. [13] J.-F. Zou, Z.-Z. Yu, Y.-X. Pan, X.-P. Fang, Y.-C. Ou, Conductive mechanism of polymer/graphite conducting composites with low percolation threshold, J. Polym. Sci. B: Polym. Phys. 40 (2002) 954–963. [14] B. Debelak, K. Lafdi, Use of exfoliated graphite filler to enhance polymer physical properties, Carbon 45 (2007) 1727–1734. [15] C. Lin, D.D.I. Chung, Graphite nanoplatelet pastes vs. carbon black pastes as thermal interface materials, Carbon 47 (2009) 295–305. [16] J. Li, J-K. Kim, Percolation threshold of conducting polymer composites containing 3D randomly distributed graphite nanoplatelets, Compos. Sci. Technol. 67 (2007) 2114–2120. [17] C.-M. Ye, B.-Q. Shentu, Z.-X. Weng, Thermal conductivity of high density polyethylene filled with graphite, J. Appl. Polym. Sci. 101 (2006) 3806–3810. [18] I. Krupa, I. Novak, I. Chodak, Electrically and thermally conductive polyethylene/graphite composites and their mechanical properties, Synthetic Metalls, 145 (2004) 245–252. [19] N. Vitorino, J.C.C. Abrantes, J.R. Frade, Highly conducting core-shell phase change materials for thermal regulation, Appl. Therm. Eng. 66 (2014) 131–139. [20] H. Wu, C. Lu, W. Zhang, X. Zhang, Preparation of low-density polyethylene/low-temperature expandable graphite composites with high thermal conductivity by an in situ expansion melt blending process, Mater. Design 52 (2013) 621–629. [21] Impedance/Gain-Phase Analyzer 1260 and Dielectric Interface 1296, User Guide; www.solartronanalytical.com. (2001). [22] G.C. Glatzmaier and W. F. Ramirez, Simultaneous measurement of the thermal conductivity and thermal diffusivity of unconsolidated materials by the transient hot wire method, Rev. Sci. Instrum. 56 (1985) 1394–1398. [23] User’s manual of Transient Hot Bridge Analyzer THB-100; www.linseis.com. (2013). [24] W.J. Parker, R.J. Jenkins, C.P. Butler, and G.L. Abbott, A Flash Method of Determining Thermal Diffusivity, Heat Capacity, and Thermal Conductivity, J. Appl. Phys. 32 (1961) 1679–1684. [25] E.V. Kharitonov, Dielectric materials with inhomogeneous structure, Moscow: Radio and Communications, (1983) (in Russian). [26] D. Stauffer, A. Aharony, Introduction to percolation theory, London: Taylor and Francis, 2nd Edition, (1992). [27] R. Zhang, A. Dowden, H. Deng, M. Baxendale, T. Peijs, Conductive network formation in the melt of carbon nanotube/thermoplastic polyurethane composite, Compos. Sci. Technol. 69 (2009) 1499–1504. [28] T.J. Moon, J.H. Kim, and C.H. Choi, Physical properties and adhesion of the polymer/metal composites, Polymer (Korea) 7 (1983) 380–391. [29] J. Bouchet, C. Carrot, J. Guillet, G. Boiteux, G. Seytre, M. Pineri, Conductive composites of UHMWPE and ceramics based on the segregated network concept, Polym. Eng. Sci. 40 (2000) 36–45. [30] Y. Xi, A. Yamanaka, Y. Bin, M. Matsuo, Electrical properties of segregated ultrahigh molecular weight polyethylene/multiwalled carbon nanotube composites, J. Appl. Polym. Sci. 105 (2007) 2868–2876. [31] S.M. Miriyala, Y.S. Kim, L. Liu, J.C. Grunlan, Segregated network of carbon black in poly(vinyl acetate) latex: Influence of clay on the electrical and mechanical behavior, Marcomol. Chem. Phys. 209 (2008) 2399–2409. [32] R.P. Kusy, Influence of particle size ratio on the continuity of aggregates, J. Appl. Phys. 48 (1977) 5301–5305. [33] J.G. Mallette, A. Marquez, O. Manero, R. Castro-Rodriguez, Carbon black filled PET/PMMA blends: Electrical and morphological studies, Polym. Eng. Sci. 40 (2000) 2273–2278. [34] C. Calberg, S. Blacher, F. Gubbels, F. Brouers, R. Deltour, R. Jerome, Electrical and dielectric properties of carbon black filled co-continuous two-phase polymer blends, J. Phys. D: Appl. Phys. 32 (1999) 1517–1525.
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S.M. Lebedev, L.I. Leschenko, O.S. Gefle, Elastomeric materials with a non-linear current-voltage characteristic for highvoltage cables, Elect. Technol. 4 (1994) 63–68. K. Mitsuishi, S. Kodama, H. Kawasaki, Effect of filler properties on the modulus of oriented polypropylene filled with flakelike fillers, J. Macromol. Sci. B: Phys. 26 (1987) 479–494. T.S. Chow, The effect of particle shape on the mechanical properties of filled polymers, J. Mater. Sci. 15 (1980) 1873–1888. A. Malliaris, D.T. Turner, Influence of particle size on the electrical resistivity of compacted mixtures of polymeric and metallic powders, J. Appl. Phys. 42 (1971) 614–618.
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Highlights
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Thermally conductive polymeric materials with high thermal conductivity were developed. Thermal conductivity, thermal diffusivity and electrical properties of new composites were tested. Thermal analysis of new materials was carried out. Heat sink made of new polymer/graphite composites for LED-lamps were produced and studied.
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S1359-4311(15)00842-X
DOI:
10.1016/j.applthermaleng.2015.08.046
Reference:
ATE 6931
To appear in:
Applied Thermal Engineering
Received Date: 24 November 2014 Revised Date:
25 July 2015
Accepted Date: 2 August 2015
Please cite this article as: S.M. Lebedev, O.S. Gefle, Evaluation of electric, morphological and thermal properties of thermally conductive polymer composites, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.08.046. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Evaluation of electric, morphological and thermal properties of thermally conductive polymer composites ∗
Sergey M. Lebedev , Olga S. Gefle Tomsk Polytechnic University, 2A Lenin Avenue, Tomsk, Russia, 634028
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Abstract Natural graphite powder was introduced into a linear low density polyethylene (LLDPE) to improve the thermal conductivity. The thermal conductivity of LLDPE/graphite composites is increased by more than seventeen times with increase in filler content up to 30 vol.% as compared to that of LLDPE matrix. The percolation threshold for these composites is equal to 10 vol.%. LLDPE/graphite composites can be used as cost effective thermally dissipative materials for electrical engineering and electronic devices due to the high thermal conductivity and thermal diffusivity.
Thermally conductive polymeric materials with high thermal conductivity were developed. Thermal conductivity, thermal diffusivity and electrical properties of new composites were tested. Thermal analysis of new materials was carried out. Heat sink made of new polymer/graphite composites for LED-lamps were produced and studied.
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1. Introduction
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The ability of heat dissipation by heat-releasing nonmetallic materials is becoming more and more important problem for semiconductor devices and micro-electronic equipment (for example, circuit boards, computer equipment, and LED light sources) because the local temperature rise limits their reliability and durability. Historically this problem has been solved by means of metals, their alloys, oxides, nitrides and ceramics with high thermal conductivity. With the recent development of filled polymer composites, especially nanocomposites, thermally conductive polymer materials (TCPMs) used as heat-releasing materials due to their low cost, lightweight and processability are becoming an alternative for the use of metals and ceramics. Thermal conductivity of polymer materials can be improved by modifying polymer matrices using micro-, submicro- or nano-sized fillers with high thermal conductivity. Graphite, carbon black, carbon fibers and carbon nanotubes, zinc oxide (ZnO), aluminum nitride (AlN), and various metals are frequently used as fillers for this purpose. Metallic fillers possess a very high thermal conductivity, for example, 200–230 Wm–1K–1 for aluminum, 370–480 Wm– 1 –1 K for copper, 400–450 Wm–1K–1 for silver, etc. Fu et al. [1] studied the thermal conductivity of composites based on the epoxy resin filled with Cu, Al, and Ag. However, the thermal conductivity of the developed composites did not exceed 1.2 Wm–1K–1 even at filler content about 70 wt.%. Zeng et al. [2] improved the thermal conductivity of organic phase change material by the addition of Ag nanowires. The value of the thermal conductivity of composites did not exceed 1.5 Wm–1K–1 at 62 wt.% of Ag nanowires. Furthermore, metallic fillers have several disadvantages which include high weight, incompatibility with some types of polymers and a high susceptibility to oxidation. Other type of fillers for thermally conductive polymer composites is ceramics such as BN (30–600 Wm–1K–1), AlN (180–220 Wm–1K–1), Al2O3 (20–30 Wm–1K–1), ZnO (about 55 Wm–1K–1), etc. However, the thermal conductivity of composites filled with ceramics does not exceed 0.59 Wm–1K–1 [1] for epoxy/BN composite at the filler content about 35.5 wt.% and 1.54 Wm–1K–1 for polyimide/ZnO composites with 27 vol.% [3] while the cost of some ceramics types is also very high. Zhou and coauthors [4] studied HDPE composites filled with BN and Al2O3 using two processing methods: powder mixing and hot pressing methods. They found that for 35 vol.% of BN, Al2O3 or BN+Al2O3 the thermal conductivity of composites did not exceed 1.5 Wm–1K–1. That is, to develop polymer/ceramics thermally conductive composites with high thermal conductivity, the filler content should be more than 50 vol.%. In turn, very high filler content results in a deterioration of rheological properties of such composites, for instance, to reduction of a melt flow index and impossibility of processing composites by extrusion or injection molding. Better results have been attributed to the polymer composites filled with both single-walled and multi-walled carbon nanotubes (SWNT and MWNT) due to their superior thermal, electrical, and mechanical properties. It has been shown that application of carbon nanotube composites allows the lowest electrical percolation threshold to be reached [5,6,7]. Andrews et al. [5] and Alam et al. [6] have found that the electrical percolation threshold for polypropylene/MWCNT and PMMA/MWCNT composites is equal to 0.05 vol.%. Liu and Grunlan [7] and Haggenmuller with co-authors [8] have reported a much lower percolation threshold values of 0.01 vol.% for ∗
Corresponding author E-mail address: [email protected] (S.M. Lebedev)
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SWCNT/epoxy nanocomposites and 0.003 vol.% for SWCNT/LDPE nanocomposites respectively. However these values are characteristic only for an electrical percolation threshold. Indeed, the thermal conductivity of polymer/CNT composites even with adding Ag-nanoparticles does not exceed 1.0 Wm–1K–1 [9,10]. The highest value of the thermal conductivity for LDPE/SWCNT composite reaches 1.8 Wm–1K–1, while the HDPE/SWCNT composites possess the thermal conductivity of 3.5 Wm–1K–1 [8,11] at filler content about 20 vol.%. The absence of significant increase in thermal conductivity for polymer/CNT composites is attributed to the large interfacial thermal resistance between polymer matrix and CNTs [8,12]. Carbon nanotube polymer composites are not so widely applied in industry of thermally conductive materials due to high cost of SWCNT and MWCNT, which limits their practical use. Besides, processing of carbon nanotube polymer composites by means of conventional processing technique may result in destruction of nanotubes due to a high shear deformation of composite materials in extruders or mixers. Furthermore, the outer walls of nanotubes can not be easily wetted by the polymer matrix melt adversely affecting the resulting composite due to poor interfacial interaction. To remedy this problem, the surface of CNTs should be additionally functionalized [9]. In turn, this procedure complicates the technology and increases the final cost of composite materials. Graphite is a versatile material widely used in industry due to its unique physical, electrical and chemical properties [13,14]. The extremely high thermal conductivity of graphite is used in the fabrication of thermally conductive composites [1,15,16]. Its main advantages are low density and cost compared to other materials used for this purpose. Graphite also has an advantage in that it is inert and compatible with most polymers. Various polymers such as epoxy resin [1], silicon rubber, HDPE, LDPE, etc. [17-20] are used as polymer matrices for the thermally conductive materials filled with graphite. Fu and co-authors [1] have reported that the highest thermal conductivity of epoxy/graphite adhesive is equal to 1.68 Wm–1K–1 at 44.3 wt.% of expanded (exfoliated) graphite, which may be attributed to the layered structure of graphite. In turn, it can result in the formation of both numerous contact points between the filler particles and the heat pathways in the matrix [14]. The same result was obtained by Ye et al. [17] for HDPE/graphite composites at 22 vol.% of graphite. Authors of [18] have reported that the maximum thermal conductivity of HDPE/graphite and LDPE/graphite composites were about 2.3 Wm–1K–1 and 2.0 Wm–1K–1 at the filler content of 37 vol.% respectively. Debelak and Lafdi [14] have prepared silicone rubber/exfoliated graphite composites with different size of graphite flakes (graphite flakes were separated using 50, 100, and 150 mesh sieves) the thermal conductivity of which was about 4.3 Wm–1K–1 at 20 wt.% of filler content. The highest values of the thermal conductivity about 8 Wm–1K–1 [19] were obtained for parafin/graphite phase change materials with special core-shell microstructure, and 11 Wm–1K–1 [20] for LDPE/low-temperature expandable graphite composites, respectively. With consideration of the cost of composites filled with expanded (exfoliated) graphite, the processing complexity (an additional chemical treatment of graphite), the chemical reactivity (with likely residual acid in the graphite nanoparticles), the natural graphite is a much more attractive choice than expanded nano-sized graphite as a filler for thermally conductive polymer composites. Moreover, nanoparticles of the expanded graphite can be destructed during processing by the conventional methods such as extrusion or injection molding resulting in the reduction of the thermal conductivity [14]. Therefore development and study of commercially available polymer composites with a high thermal conductivity suitable for extrusion processing or injection molding was the aim of this paper.
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2. Experimental technique and samples
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Measurements of the real part of admittance γa and phase angle between voltage and current ϕ were done under AC voltage 3V in the frequency range from 10–1 Hz to 104 Hz using the Solartron Instrument (Impedance/Gain-Phase Analyzer Solartron 1260 + Dielectric Interface Solartron 1296) [21]. From five to ten measurements per decade over the frequency range were carried out for all samples. At least five samples for each composite were tested. Temperatures and temperature intervals of melting and decomposition of nanocomposites, heat flow and weight loss of the samples were measured in the temperature range from 25 to 600°C at a heating rate of 10°C/min in an argon atmosphere with a combined DSC-DTA-TGA analyzer Q600 “TA Instruments”. From three to five samples of each composite were tested for all test conditions. The study of thermal conductivity of TCPMs was done by the transient hot bridge method (the modified method of a hot wire [22]) using the THB-100 analyzer (Linseis, Germany) [23]. The measuring sensor including a heater and temperature sensor was located between two identical 5 mm thick samples made of the same TCPM. Thermal diffusivity of TCPMs was measured by the flash method [24] with the XFA 500 analyzer (Linseis, Germany). IR detector of XFA analyzer was cooled by liquid nitrogen. Measurements were carried out under the vacuum approximately 2.0⋅10–5 bar. Temperature changes on the surface of the TCPM samples were monitored by IR-camera. Scheme of experiment is presented in Figure 1. A brass rod 7.5 mm in diameter with heater was used as a heating source. The temperature of heating source during this experiment was equal to 50.0 ± 2.0°C. Measuring the temperature redistribution due to the heat transfer at the surface of TCPM sample was beginning from the moment of its placing at the surface of heat source through each 30 s for 20 min. Minimum and maximum temperatures were measured for all samples. The minimum value corresponds to temperature of the sample periphery, and maximum one to the surface temperature at the sample center above the heat source.
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Fig 1. Experimental setup: sample (1), heating source (2), temperature controller (3), IR-camera (4).
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The value of melt-flow index (MFI) was measured by using a versatile melt flow tester (Instron CEAST MF20, Italy) according to ISO 1133 and ASTM D1238. A Philips 515 scanning electron microscope (SEM) was used to investigate the dispersion of filler in the polymer matrix. The samples were fractured in liquid nitrogen and then coated with silver. Linear low density polyethylene (LLDPE Hanwha 3305, Korea) with a melt flow rate 1.9 g/10min was used as a polymer matrix in this study. The density of LLDPE is 0.922 g/cm3. Powder of natural graphite (GE-3, the Russian standard GOST 17022-81) with average particle size of 65 µm was used as filler as received (Figure 2). Filler content (C) in polymer composites was changed from 5 vol.% to 30 vol.%. To improve the interface compatibility of the LLDPE matrix with graphite powder, 2 wt.% of LLDPE-g-MAH (LLDPE grafted with maleic anhydrite) was added into the LLDPE matrix.
Fig. 2. Micrograph of graphite powder.
All TCPMs were prepared by melt mixing in the measuring mixer type 50 EHT (Brabender, Germany) with a bowl volume approximately 55 cm3, maximum operating temperature 500°C, and maximum torque 200 N⋅m. Graphite powder was gradually introduced into the polymer melt up to the required volume fraction, while mixing until all graphite powder was evenly distributed in the matrix. Mixing time was changed from 10 to 25 min. Afterwards, all composites were granulated into pellets by the granulator (Brabender, Germany). To prepare samples, compression molds filled with TCPMs were press at 12.5 MPa for 20 min. The compression molds were slowly cooled at the cooling rate of 4°C/min up to ambient temperature under pressure in air. Samples studied were prepared as plates 85×65×6 mm and discs 12.7 mm in diameter and thicknesses of 0.8 mm placed into a vacuum oven heated up to 190°C for 3 h. After that, the compression molds were pressed in a hydraulic. 3. Experimental results and discussion Figure 3 shows frequency dependencies of the real part of admittance γa = ωε0ε′tanδ (where ω = 2πf is the circular
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frequency, ε0 = 8.854⋅10–12 F/m is the vacuum permittivity, ε′ is the real part of complex permittivity, and tanδ is the loss factor) and phase angle ϕ between voltage and current for all TCPMs. It can be seen that γa for TCPMs in the frequency range from 0.1 Hz to 104 Hz is increased by 2.5 orders of magnitude with increase in filler content from 5 vol.% to 30 vol.%. Measurement of γa for TCPMs with filler content C ≥ 20 vol.% in the frequency range from 0.1 Hz to 30 Hz becomes impossible by means of the Solartron instrument because of sharp increase in the active component of a current. Figure 3b is evident illustration of this statement. For TCPMs at C ≤ 10 vol.%, the angle ϕ is close to 90° (the angle of dielectric loss δ = 90° – ϕ << 1°) and practically does not depend on the frequency. That is, these TCPMs are dielectric materials over the studied frequency range. TCPMs with С ≥ 15 vol.% at frequency more than 1 Hz become semi-conductive materials. TCPMs at C ≥ 20 vol.% possess electrically conductive properties over the studied frequency range (see Figure 3b) and can be used for manufacturing electrically conductive or thermally conductive elements of various electrical engineering and electronic devices.
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b) Fig 3. Frequency dependencies of γa (a) and ϕ (b) for TCPMs with different filler content: 1, 5 vol.%; 2, 10 vol.%; 3, 15 vol.%; 4, 20 vol.%; 5, 30 vol.%.
To estimate the influence of the filler content on thermal properties of TCPMs, the thermal conductivity (λ) and thermal diffusivity (α) of matrix used in the experiment have been measured. The values of λ and α for the LLDPE matrix at room temperature are listed in Table. Table Thermal conductivity and thermal diffusivity of LLDPE matrix. Material LLDPE
Thermal conductivity λ, Wm–1K–1 0.36
Thermal diffusivity α⋅107, m2/s 1.85
Figure 4 shows semi-logarithmic dependencies of the thermal conductivity and thermal diffusivity for TCPMs at room temperature as functions of the filler content (C). It is obvious that values of λ and α for composite with 30 vol.% of filler are increased more than 17 and 5 times as compared to those for the LLDPE matrix. It can be seen that dependencies λ = f(С) and α = f(С) are non-monotonic in character and at C ≥ 10 vol.% a sharp increase in the values of
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λ and α is observed. It can be related to the percolation effect [25-27] and appearance of direct contacts between the filler particles which results in the formation of three-dimensional network [28,29] at such rather low filler content.
Fig. 4. Thermal conductivity and thermal diffusivity as functions of the filler content for LLDPE/graphite composites.
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The percolation effect can be defined as a phase transition when a dramatic growth of the current through material occurs with filler content increase. In the case of filled polymer composites, the electrical conductivity (or thermal conductivity) sharply changes at a precise filler content. The filler content that marks this phase transition is usually referred to as the percolation threshold. The percolation effect can be observed when a continuous network of filler particles (a segregated network) forms in the polymer composite [29-31]. Kusy [32] has introduced into practice the “segregated network” concept in the context of electrically conductive mixtures of polymer and metal powder to explain the formation of electrically conductive network: “...The term “segregated” denotes that microscopic dispersed particles are being restricted into the boundaries of much larger primary particles. From the macroscopic viewpoint this distribution is random too, since both phases are relatively small...”. The segregated network can be formed in semicrystalline polymers such as polyethylene in one of the phases or at the interface of amorphous and crystalline phases [33,34]. Figure 5 shows the SEM micrograph of the fracture surface of LLDPE/graphite composite with 30 vol.% of the graphite. Figure 5 confirms quasi-uniform dispersion of graphite particles in the polymer matrix. It can be seen that graphite particles are randomly dispersed in the polymer matrix. With increasing the filler content, filler particles contact each other forming continuous pathways or segregated networks through the polymer.
Fig. 5. SEM micrograph of the fracture surface of LLDPE/graphite composite with 30 vol.% of graphite.
However for TCPMs studied, substantial anisotropy of electrical and thermal properties is practically absent in contrast to polymer composites filled with carbon fibers, graphite flakes, and carbon nanotubes [35-37]. It is caused by both a stochastic form of filler particles and absence of the so-called aspect ratio effect [14,36,37]. The thermal conductivity of TCPMs investigated can be risen with increase in the filler content because the melt flow index for TCPM with 30 vol.% of graphite is equal to 0.6 g/10min that is sufficient for its processing by the injection molding. The thermal conductivity of LLDPE/graphite composites can be also increased by adding small fraction of carbon
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black (CB) with the average particle size of 50 nm. Indeed, the addition of 3 vol.% CB into the LLDPE/graphite composite with C = 30 vol.% increases the value of λ by 20 % other things being equal. It can be related to formation of a segregated network of filler particles due to different particle sizes of graphite and carbon black [7,31,38]. To estimate an opportunity of heat dissipation by means of TCPMs, the study of thermovision images of heat transfer on the sample surface was performed with IR-camera. Typical thermograms for samples made of polymer matrix and TCPM with 30 vol.% of graphite are shown in Figure 6.
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Fig. 6. Thermovision images of the heat source (a) and heat transfer on the sample surface for TCPM with 30 vol.% of graphite (b-f) and for the LLDPE matrix (g, h) at different time: b, 30 s; c, 120 s; d, 300 s; e, 600 s; f, 900 s; g, 180 s; and h, 900 s.
It can be seen that the maximum temperature on the TCPM sample’s surface does not exceed 40°C after 900 seconds due to high value of λ, heat transfer from a center to periphery of the sample, and convective heat dissipation from the sample surface. Indeed, temperature at the edge of TCPM sample is changed from 26.6°C (30 seconds, Figure 6b) approximately to 32°C (900 seconds, Figure 6f). In contrast, the maximum temperature at the center of the LLDPE
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sample without filler is practically equal to the heat source’s temperature after 900 seconds (Figures 6g and 6h). Temperature at the LLDPE sample edge is increased from room temperature only up to 28.1°C after 900 seconds (Figure 6h). For comparison, the temporal dependencies of temperature at the samples surface for TCPM and LLDPE are shown in Fig. 7. It is obvious that the difference between the maximum temperatures on the surface 900 s after switching of the heat source for the samples made of TCPM with 30 vol.% of graphite and LLDPE is more than 11°C. Moreover, both the maximum temperature and the temperature at the edge of the sample made of the TCPM tend to the saturation with time, and the quasi-steady state thermal regime is observed after 900 s. Besides, the difference between the maximum temperature and the temperature at the sample edge for the LLDPE is 22.3°C while this difference for the sample made of the TCPM is only 7.2°C. This is attributed to the lower thermal conductivity and heat transfer of LLDPE as compared to those for TCPM. When the elements of the cooling system (e.g. heat sink of LED-lamp) will be made of LLDPE, it can result in the overheating of LEDs.
Fig. 7. Temporal dependencies of temperature on the samples surface: 1,2 for the TCPM with 30 vol.% of graphite; 3,4 for the LLDPE sample: 1,3 the maximum temperature; 2,4 temperature at the sample edge.
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Temperature changes on the samples surface are caused by only the thermal conductivity, heat transfer and convective heat dissipation because other phase transitions in the given temperature range have not been observed both for polymeric matrix and TCPMs. Typical DSC-curve for the LLDPE matrix in Fig. 8 well demonstrates this statement. It can be seen that only two endothermic peaks are observed over the temperature range studied. The first of them approximately at (125÷130)°C is conditioned by the melting process, and the latter at (485÷490)°C by the decomposition process of the polymer composite. Similar results were obtained for the LLDPE/graphite composites, not presented here for brevity.
Fig. 8. Thermogram for LLDPE matrix.
LLDPE/graphite composites were used as inexpensive thermally dissipative materials for electrical engineering and electronic devices due to the high thermal conductivity and thermal diffusivity. In particular, LLDPE/graphite composites were used to manufacture cooling elements of LED-lamps (Figure 9).
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Fig. 9. Heat sinks (a) and LED-lamps (b).
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Cooling elements (heat sinks) for LED-lamps were manufactured by injection molding method using all-electric injection molding machine Battenfeld EcoPower 55. Polymer heat sinks made of TCPM with 30 vol.% of graphite have shown a good efficiency in cooling systems for LED-lamps (Figure 10). The study of temperature field of LED-lamps with heat sinks made of TCPMs was performed by the thermovision device during 2 hours after switching on of the voltage. Two types of LED-lamps with heat sinks made of TCPMs with 20 vol.% and 30 vol.% of graphite were mounted in the special setup at some distance from each other for elimination of mutual thermal influence. Both LEDlamps were switched on simultaneously.
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a) b) Fig. 10. IR images of LED-lamps with different material of heat sinks: a, TCPM with 20 vol.% of graphite; b, TCPM with 30 vol.% of graphite.
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It can be seen that the metal base of LED-lamp with the heat sink made of TCPM with 30 vol.% of filler (Figure 10b) is more heated due to the high thermal conductivity of TCPM and best heat transfer compared to other lamp (Figure 10a). Besides, temperature on the surface of heat sink (Figure 10b) is distributed more homogeneously compared to other LED-lamp. When estimating efficiency of heat transfer by polymer heat sink, the main criteria is the minimum temperature difference between the maximum temperature Tmax (at the top part of the heat sink in the vicinity of printed circuit board with LEDs) and the minimum temperature Tcen (at the bottom part of the heat sink) on the surface of heat sink: ∆T = Tmax – Tcen The value of ∆T for TCPM with 30 vol.% of graphite is equal to 11.0°C. The minimum temperature difference on the heat sink surface is conditioned by both the high value of the thermal conductivity of TCPM with 30 vol.% of graphite and maximum speed of the heat transfer from a hot part to a cold one. 4. Conclusions Adding the natural graphite powder into LLDPE matrix allows the thermal conductivity to be increased by a factor of 17.8 compared to that for the LLDPE matrix at the filler content 30 vol.%. The thermal conductivity of developed LLDPE/graphite composites can be increased up to 6.5 Wm–1K–1 at 30 vol.% of graphite. These LLDPE/graphite composites can be processed by the injection molding. The dielectric spectroscopy showed that LLDPE/graphite composites possess dielectric properties up to 10 vol.% of the filler content, while the addition more than 15 vol.% of graphite increases the value of the admittance not less than one-two order of magnitude. IR-study results demonstrate suitable temperature redistribution on the TCPM surface. The results of thermovision study demonstrate high efficiency
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of polymer/graphite heat sinks due to the high speed of the heat transfer. Future study will focus on the effects of compatibilizers and inert fillers in LLDPE/graphite composites that may exhibit better dispersion of fillers in the polymer matrix and reduction of the percolation threshold. References Y.-X. Fu, Z.-X. He, D.-C. Mo, S.-S. Lu, Thermal conductivity enhancement with different fillers for epoxy resin adhesives, Appl. Therm. Eng. 66 (2014) 493–498. [2] J.L. Zeng, Z. Cao, D.W. Yang, L.X. Sun, L. Zhang, Thermal conductivity enhancement of Ag nanowires on an organic phase change material, J. Therm. Anal. Calorim. 101 (2010) 385–389. [3] D. Yorifuji and S. Ando, Enhanced thermal conductivity over percolation threshold in polyimide blend films containing ZnO nano-pyramidal particles: advantage of vertical double percolation structure, J. Mater. Chem. 21 (2011) 4402–4407. [4] W. Zhou, S. Qi, Q. An, H. Zhao, N. Liu, Thermal conductivity of boron nitride reinforced polyethylene composites, Mat. Res. Bull. 42 (2007) 1863–1873. [5] M.S. Alam, K.K. Verma, Investigation of thermal and hardness properties of MWCNTs filled PMMA polymer nanocomposites, Amer. J. Mater., Sci. Appl., 2 (2014) 31–34. [6] R. Andrews, D. Jacques, M. Minot, T. Rantell, Fabrication of carbon multiwall nanotube/polymer composites by shear mixing, Macromol. Mater. Eng. 287 (2002) 395–403. [7] L. Liu, J.C. Grunlan, Clay assisted dispersion of carbon nanotubes in conductive epoxy nanocomposites, Adv. Funct. Mater. 17 (2007) 2343–2348. [8] R. Haggenmueller, C. Guthy, J.R. Lukes, J.E. Fischer, and K.I. Winey, Single wall carbon nanotube/polyethylene nanocomposites: Thermal and electrical conductivity, Macromol. 40 (2007) 2417–2421. [9] P.-C. Ma, N.A Siddiqui, G. Maron, J.-K. Kim, Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: Review, Composites: Part A, 41 (2010) 1345–1367. [10] M. Jouni, A. Boudenne, G. Boiteux, V. Massardier, and B. Garnier, Significant enhancement of electrical and thermal conductivities of polyethylene carbon nanotube composites by the addition of a low amount of silver nanoparticles, Polym. Adv. Technol. 25 (2014) 1054–1059. [11] C. Kingston, R. Zepp, A. Andrady, et al., Release characteristics of selected carbon nanotube polymer composites, Carbon, 68 (2014) 33–57. [12] J. Han, A. Fina, Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review, Prog. Polym. Sci. 36 (2011) 914–944. [13] J.-F. Zou, Z.-Z. Yu, Y.-X. Pan, X.-P. Fang, Y.-C. Ou, Conductive mechanism of polymer/graphite conducting composites with low percolation threshold, J. Polym. Sci. B: Polym. Phys. 40 (2002) 954–963. [14] B. Debelak, K. Lafdi, Use of exfoliated graphite filler to enhance polymer physical properties, Carbon 45 (2007) 1727–1734. [15] C. Lin, D.D.I. Chung, Graphite nanoplatelet pastes vs. carbon black pastes as thermal interface materials, Carbon 47 (2009) 295–305. [16] J. Li, J-K. Kim, Percolation threshold of conducting polymer composites containing 3D randomly distributed graphite nanoplatelets, Compos. Sci. Technol. 67 (2007) 2114–2120. [17] C.-M. Ye, B.-Q. Shentu, Z.-X. Weng, Thermal conductivity of high density polyethylene filled with graphite, J. Appl. Polym. Sci. 101 (2006) 3806–3810. [18] I. Krupa, I. Novak, I. Chodak, Electrically and thermally conductive polyethylene/graphite composites and their mechanical properties, Synthetic Metalls, 145 (2004) 245–252. [19] N. Vitorino, J.C.C. Abrantes, J.R. Frade, Highly conducting core-shell phase change materials for thermal regulation, Appl. Therm. Eng. 66 (2014) 131–139. [20] H. Wu, C. Lu, W. Zhang, X. Zhang, Preparation of low-density polyethylene/low-temperature expandable graphite composites with high thermal conductivity by an in situ expansion melt blending process, Mater. Design 52 (2013) 621–629. [21] Impedance/Gain-Phase Analyzer 1260 and Dielectric Interface 1296, User Guide; www.solartronanalytical.com. (2001). [22] G.C. Glatzmaier and W. F. Ramirez, Simultaneous measurement of the thermal conductivity and thermal diffusivity of unconsolidated materials by the transient hot wire method, Rev. Sci. Instrum. 56 (1985) 1394–1398. [23] User’s manual of Transient Hot Bridge Analyzer THB-100; www.linseis.com. (2013). [24] W.J. Parker, R.J. Jenkins, C.P. Butler, and G.L. Abbott, A Flash Method of Determining Thermal Diffusivity, Heat Capacity, and Thermal Conductivity, J. Appl. Phys. 32 (1961) 1679–1684. [25] E.V. Kharitonov, Dielectric materials with inhomogeneous structure, Moscow: Radio and Communications, (1983) (in Russian). [26] D. Stauffer, A. Aharony, Introduction to percolation theory, London: Taylor and Francis, 2nd Edition, (1992). [27] R. Zhang, A. Dowden, H. Deng, M. Baxendale, T. Peijs, Conductive network formation in the melt of carbon nanotube/thermoplastic polyurethane composite, Compos. Sci. Technol. 69 (2009) 1499–1504. [28] T.J. Moon, J.H. Kim, and C.H. Choi, Physical properties and adhesion of the polymer/metal composites, Polymer (Korea) 7 (1983) 380–391. [29] J. Bouchet, C. Carrot, J. Guillet, G. Boiteux, G. Seytre, M. Pineri, Conductive composites of UHMWPE and ceramics based on the segregated network concept, Polym. Eng. Sci. 40 (2000) 36–45. [30] Y. Xi, A. Yamanaka, Y. Bin, M. Matsuo, Electrical properties of segregated ultrahigh molecular weight polyethylene/multiwalled carbon nanotube composites, J. Appl. Polym. Sci. 105 (2007) 2868–2876. [31] S.M. Miriyala, Y.S. Kim, L. Liu, J.C. Grunlan, Segregated network of carbon black in poly(vinyl acetate) latex: Influence of clay on the electrical and mechanical behavior, Marcomol. Chem. Phys. 209 (2008) 2399–2409. [32] R.P. Kusy, Influence of particle size ratio on the continuity of aggregates, J. Appl. Phys. 48 (1977) 5301–5305. [33] J.G. Mallette, A. Marquez, O. Manero, R. Castro-Rodriguez, Carbon black filled PET/PMMA blends: Electrical and morphological studies, Polym. Eng. Sci. 40 (2000) 2273–2278. [34] C. Calberg, S. Blacher, F. Gubbels, F. Brouers, R. Deltour, R. Jerome, Electrical and dielectric properties of carbon black filled co-continuous two-phase polymer blends, J. Phys. D: Appl. Phys. 32 (1999) 1517–1525.
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S.M. Lebedev, L.I. Leschenko, O.S. Gefle, Elastomeric materials with a non-linear current-voltage characteristic for highvoltage cables, Elect. Technol. 4 (1994) 63–68. K. Mitsuishi, S. Kodama, H. Kawasaki, Effect of filler properties on the modulus of oriented polypropylene filled with flakelike fillers, J. Macromol. Sci. B: Phys. 26 (1987) 479–494. T.S. Chow, The effect of particle shape on the mechanical properties of filled polymers, J. Mater. Sci. 15 (1980) 1873–1888. A. Malliaris, D.T. Turner, Influence of particle size on the electrical resistivity of compacted mixtures of polymeric and metallic powders, J. Appl. Phys. 42 (1971) 614–618.
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Thermally conductive polymeric materials with high thermal conductivity were developed. Thermal conductivity, thermal diffusivity and electrical properties of new composites were tested. Thermal analysis of new materials was carried out. Heat sink made of new polymer/graphite composites for LED-lamps were produced and studied.
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