TVIP 2015, 18. 20. 3. 2015, HUSTOPEČE - HOTEL CENTRO Electrical, thermal and surface properties of nickel-based composites Ing. Igor Novák, PhD., Ing. Ondrej Žigo, PhD., Ing. Marian Valentin, PhD., Mgr. Jozef Prachár Ústav polymérov SAV, Dúbravská cesta 9, 845 41 Bratislava, Slovensko E-mail: igor.novak@savba.sk Summary Electrically and thermally conductive composites made using high density polyethylene (HDPE) matrix blended with a special grade of branch-structured nickel particles were studied. Composites with high filler content were highly electrically and thermally conductive. The electrical conductivity of composites reached a value of 8.3 x10 3 S m -1 when filled with 30 vol. % of the filler, and the thermal conductivity obtained using this filler content was found to be 1.99 W.m -1. K -1. The percolation concentration of the filler within the HDPE matrix, which was determined from electrical conductivity measurements, was determined to be 8 vol. %. Key words: epoxy adhesive, oak wood, surface properties, radio-frequency plasma, modification, adhesive bonding. Introduction Thermally and electrically conductive materials are designed by blending polymeric matrices with the convenient fillers. A filler with high electrical conductivity often also possesses high thermal conductivity and vice versa. Such fillers include graphite (1, 2), exfoliated graphite and graphene (3, 4), metals (5) and metalized fillers (6). However, the oxidation of certain metals (e.g., aluminum, copper, iron) causes them to become electrically insulating, although they still maintain high levels of thermal conductivity. Sometimes, this effect can be advantageous; for instance, particularly in electronic devices, high thermal conductivities are required to facilitate heat release, but for safety reasons, it is desirable that electrical conductivities be kept low. This paper presents results pertaining to the preparation and characterization of electrically and thermally conductive composites, the production of which is based on an high-density polyethylene (HDPE) matrix and nickel powder. The surface and adhesive properties of these composites are also discussed. Experimental Materials and preparation of composites High-density polyethylene (HDPE BP 5740 3VA, British Petrol, UK, melting temperature = 129.3 o C, melting enthalpy = 199 J/g) was used as the matrix, while a special grade of fine nickel particles with the trade name Inco Type 210 (Novamet Ltd. Sins, England) was used as the filler. Composites were prepared by mixing both components in the 50-ml mixing chamber of a Brabender Plasticorder PLE 331 (Germany) at 180 o C for 10 min at a mixing speed of 35 rpm. Electrical conductivity measurements The electrical conductivities of composites were determined at room temperature using a twopoint method (insulating/semiconductive samples) or a four-point method (conductive samples
above the percolation threshold) in a van der Pauw arrangement using a Keithley 237 High- Voltage Source Measurement Unit and a Keithley 2010 Multimeter equipped with a 2000-SCAN 10 Channel Scanner Card. Circular gold electrodes were deposited on both sides of the measured samples. Each measurement was repeated at least 2 3 times. Thermophysical measurement For thermal conductivity measurements, specimens with dimensions of 4x4x0.5 cm were compressed and molded at 170 o C for 3 min using a laboratory press (Fontijne 200, The Netherlands). Surface properties measurements The polarities of composites were characterized simply by measuring the contact angles of redistilled water droplets placed on the surface of the HDPE/nickel composites. The contact angles were measured using a Surface Energy Evaluation System (SEE) equipped with a CCD camera (Masaryk University, Czech Republic). Six drops of re-distilled water (with a volume of 3 µl) were placed on a cleaned composite surface. At least six contact-angle measurements were obtained and averaged. Results and Discussion Thermal conductivity The dependence of the thermal conductivities of composites (kc) upon volume filler content is shown in Fig. 1. A non-linear increase of kc with increasing filler content was observed in the whole concentration region. This increase is a common behavior of composites filled with thermally conductive fillers. In describing the thermal conductivity of a heterogeneous material, we must take into account the influence of various parameters, including the geometry and orientation of filler particles in the matrix, the filler concentration and the ratio between the filler s thermal conductivity and the thermal conductivity of the matrix. Based on these factors, many different models have already been developed, but none of these has general validity. Most previously published models were established for polymers filled with spherical particles and partly for fibers, flakes and irregularly shaped particles. Usually, these models also consider uniform particle-size distribution and even dispergation of the filler within the matrix. The nickel particles used in this work exhibited far more complicated behavior than that predicted by the simple morphology for which the thermal conductivity models were commonly developed. At the submicron level, the filler is formed by roughly spherical particles; however, these particles link to one another and form branched, chain-like morphologies. For this reason, it is difficult to select any one model that correctly describes the experimental results. The Hashin Shtrikman model [6] is considered one of the best for estimating the lower bound when no information about particle distribution in the matrix is available. As can be seen in Fig. 1, the Hashin Shtrikman model describes the experimental data only up to 5 vol.%, far below the common predictions for cases where up to 10-12 vol.% fillers are employed. For these higher concentrations, the experimental data are significantly higher than those calculated from the model, indicating that nickel particles cannot be considered as individual particles dispersed within a matrix.
Figure 1 Thermal conductivities of the HDPE/nickel composites. Electrical conductivity The dependence of the electrical conductivities of HDPE/nickel composites on filler volume content is shown in Fig. 2. Electrically conductive composites composed of an insulating polymeric matrix and an electrically conductive filler demonstrate typical sigmoidal behavior, as shown in Fig. 2. The percolation effect is experimentally observed in the dependence of conductivity versus filler content and manifests itself as a dramatic increase in conductivity (by several orders of magnitude) in a rather narrow filler concentration range within the area of the percolation threshold. In general, the percolation effect is a well-known phenomenon observed in filler-matrix systems as abrupt extreme changes in certain physical properties within rather narrow concentration ranges of heterogeneity. The effect is explained as the formation of conductive paths (through the matrix) in such a way that the conductive particles are in close contact at a filler concentration corresponding to the percolation threshold. The percolation threshold is a mathematical term related to percolation theory, which is the formation of longrange connectivity in random systems. In engineering, percolation is the slow flow of fluids through porous media or current flow though a heterogeneous conductor. However, in mathematics and physics, percolation generally refers to simplified lattice models of random systems and the nature of the connectivity within them. An important task is to find the so-called percolation threshold, that is, the critical value of the occupation probability such that infinite connectivity (percolation) first occurs. For the composites investigated in this study, the conductive network begins to develop at a filler content of 3.5 vol.%, and the network is developed at 14.6 vol.%. The percolation threshold was arbitrarily determined as 8 vol.%. Surface properties The dependence of the contact angle on the volume filler content is shown in Fig. 3. The presence of nickel in the HDPE matrix leads to a decrease in contact angles, indicating that the hydrophilicity of the surface increases. The contact angle of water on the neat HDPE (93 o )
Figure 2 Electrical conductivities of the HDPE/nickel composites. decreases to 80 o as the nickel is increased by filling with 60 wt.% of the filler (13 vol.%). The next incremental increase in filler content causes no further change in the contact angle. This behavior corresponds to the development of a filler network, as discussed above. When this filler network has developed, further increases in the filler content do not lead to further changes in either the electrical conductivity or the contact angle. Figure 3 Surface properties of the HDPE/nickel composites.
Conclusions A new type of electrically and thermally conductive composites based on an HDPE matrix and nickel particles was prepared and investigated. The percolation concentration of the filler within the HDPE matrix, as determined from electrical conductivity measurements, was found to be 8 vol.%. Composites with high filler content were highly electrically and thermally conductive; the electrical conductivity of composites reached 8.3 x10 3 S.m -1 when the composite was filled with 30 vol.% of the filler. In this case, the thermal conductivity of the composite was found to be four times higher than the thermal conductivity of neat matrix. The presence of nickel in the HDPE matrix leads to a decrease in the contact angles, indicating that the hydrophilicity of the surface increases. The contact angle of water on neat HDPE (93 o ) decreased to 80 o as the nickel was increased via filling with 60 wt.% of the filler (13 vol.%). Further increases in the filler content did not lead to further changes in the contact angle. Acknowledgements This contribution was supported by Ministry of Education of Slovak Republik and Slovak Academy of Sciences, project VEGA, No. 2/0199/14. References 1. Bigg, D.M. Thermal and electrical conductivity of polymer materials. Adv. Polym. Sci 1995;119:1-30. 2. Zhang, Z, Fang, X. Study on paraffin/expanded graphite composite phase change thermal energy storage material. Energy Convers. Manage 2006;47(3):303-10. 3. Stappers, L., Yuan, Y., Fransaer, J. Novel composite coatings for heat sink applications. J Electrochem. Soc. 2005;152(7):C 457-61. 4. Klason, C., McQueen, D.H., Kubat, J. Electrical properties of filled polymers and some examples of their applications. Macromol. Symp. 1996; 108: 247-60. 5. Guľ, V,E. Structure and properties of conducting polymer composites. Utrecht: VSB BV; 1996. 6. Chang-Ming, Y., Bao-Qing, S., Shi-Xue, W. Thermal conductivity of high density polyethylene filled with graphite. J. Appl. Polym. Sci. 2006; 101: 3806-10. 7. Novák, I., Krupa, I., Chodák, I. Investigation of the correlation between electrical conductivity and elongation at break in polyurethane-based adhesives. Synth. Metals 2002;131:93-8.