Comparative Electrical Studies of Ni/ MWNT Bulk Composites Comparative Electrical Studies of Ni/ MWNT Bulk Composites S. Suárez Vallejo 1 , F. Soldera 1 , J. García 2 & F. Mücklich 1 FUNKTI ONSWERKSTOFFE FUNKTI ONSWERKSTOFFE http:/ / www.uni http:/ / www.uni-saarland.de/ fak8/ fuwe/ index.html saarland.de/ fak8/ fuwe/ index.html Contact : s.suarez@mx.uni-saarland.de 1 Department for Materials Science, Functional Materials, Saarland University, Saarbrücken, Germany 2 Helmholtz-Zentrum Berlin für Materialien und Energie GmbH. Hahn-Meitner-Platz 1, 14109 Berlin, Germany ABSTRACT The objective of this work is to exploit the CNTs properties to enhance the electrical performance of the matrix in MMCs. Nickel matrix composites reinforced with multiwalled carbon nanotubes (Ni/MWNT) were manufactured by spark plasma consolidation, which consists in applying a high pressure to a Ni/MWNT blend and a high current pulse for h d f ff l d d d b h fl h d f f h h l h d h ashort period of time. Different loads were tested in order to observe the influence on the densification of the composites. The samples were characterized with x‐ray diffraction and FIB/SEM dual beam cross sections. Densities were determined by Archimedes’ method. We have observed that the final density was about 92% of the theoretical density estimated by the mixture law. X‐ray diffractograms of the composites show no carbide formation, even though a very high current is used in the processing (approximately 195 kA). This indicates that there was no CNT degradation due to high temperature sparking. The electrical performance of the composites was investigated in a four‐terminal sensing device at room temperature and compared to Ni/MWNT samples, manufactured with different powder metallurgical methods, as well as to a high purity Ni rod (99.9 %). For spark plasma consolidated samples, results have shown an improvement in the electrical conductivity up to 6.5 times compared to measurements in pure Ni under the same conditions. This enhancement can be justified by the presence of a network of multiple quasi‐ballistic conduction paths that increasing the composite’s effective conductance [1]. Also, as the density increases, the interface between the matrix and reinforcement is improved facilitating the electron transport through the material [2‐4]. The low electrical resistance showed by these rapid manufactured composites could find their application field as electrical contact materials in low voltage switching devices. MICROSTRUCTURAL ANALYSIS The Ni/MWNT blends were manufactured based on a colloidal mixing procedure which consists on the dispersion of the MWNT agglomerates in N,N Dimethylformamide (DMF) and the MANUFACTURING Al2O3 Al2O3 Graphite Ni + MWNT Blend Steel Ni + MWNT Blend Ni + MWNT Blend Graphite Cu plates Ni/MWNT HUP 1%wt. Ni/MWNT HUP 1%wt. (A) FIB cross section. The observable porosity is in the nanometer range and the CNT clusters are A B Ni/MWNT CP+S 1%wt. Ni/MWNT CP+S 1%wt. (A) FIB cross section. CNT clusters and porosity can be appreciated. (B) Magnification of a A B A B Ni/MWNT SPC 1%wt. Ni/MWNT SPC 1%wt. (A) FIB cross section. A good cohesion between clusters and matrix is obsevable (B) Magnification of a subsequent mixture with Ni dendritic powder in the solvent. After drying the blend, green pellets were pressed in a steel mould and densified by three different routes. • Pressure‐less sintering (CP+S) • Hot uniaxial pressing (HUP) • Spark plasma consolidation (SPC) After densification, we have observed: • Random reinforcement distribution through matrix  Very good final densities (up to 92%) G i b d di t ib ti h i l d t f d i SEM/FIB examination • FIB/SEM x‐sections were made on the samples to observe the distribution of the individual CNTs as well as the agglomerated clusters. Also, the pore size and distribution was examined In all the samples the XRD analysis • Phase analysis was made on the different samples after the manufacturing. This was made in order to evaluate the presence of CNTs degradation due to overheating and/or mechanical damage. Although the carbides formed by nickel are metastable, it is known that the formation of these smaller (B) Magnification of a region where individual CNTs and small clusters are placed at the grain border segment where it can be seen that the densification was not fulfilled and the position of the clusters at the nickel particles’ edge. region where clusters of CNTs and porosity can be identified.  Grain border distribution enhancing load transfer and carriage 4 Point Probe measurements ELECTRICAL CONDUCTIVITY Sample Sintering pressure [MPa] Sintering temperature [°C] Theoretical density [g.cm ‐3 ] Measured density [g.cm ‐3 ] Relative density [%] CP+S 990 (pellet) 950 8,82 7,51 85,1 HUP 264 750 8,82 7,64 86,6 SPC_A 377 ‐‐ 8,82 7,74 87,8 SPC_B 377 ‐‐ 8,82 8,09 91,8 Table 1 ‐ Samples‘ density measured with the Archimedes method The tendency shows that the conductivity increases with the density in a non‐linear form. All the samples was examined. In all the samples the CNTs were found to be placed at the grain borders. Also, in the CP+S and SPC the pore size was larger and the densification was less regular than in HUP samples. phases could be due to, for example, mechanical alloying or the reaction of Ni with amorphous carbon [5]. Despite the extreme conditions to which the CNTs are subjected, the diffractograms didn’t show any traces of either Ni 3 C or NiC. That means that there was no degradation of the nanotubes and further reaction to the nickel in the matrix. CPS HUP SPC The electrical resistivity measurements were made with a 4 point probe device at 298K. The orientation of the CNTs in the matrix was observed with FIB/SEM cross section, finding that they are placed perpendicular to the pressing direction. The conductivity measurements were made in this direction in samples produced by the three different methods (SPC, HUP and CP+S) as well as for a pure Ni rod. Sample % of Pure Ni resistivity Density CP+S 75 6 85 1 show an improvement compared to pure nickel measured under the same conditions. The observed enhancement can be justified by the connectivity of the nanotubes throughout the matrix reducing the composite effective resistance. This connectivity was verified by tomographies made on the samples. Another important feature to be considered for the transfer of properties is the wettability and adherence of the matrix material to the reinforcement. A direct measurement of this is the final density. Also, comparing the CP+S and HUP samples to the SPC, the increase in conductivity is of about 4 times. As seen on the FIB cross sections, the CONCLUDING REMARKS 4‐point measurement device CNT reinforced Ni composites were manufactured by three different powder metallurgy methods. In the microscopical analysis we have seen a good dispersion and distribution of the CNTs in the matrix regardless of the method. Furthermore, despite the extreme conditions that the blends are exposed to, no carbide formation is observable and therefore we can affirm that there is no CNTs degradation due to sparking or overheating. With the spark plasma consolidation we achieved a good densification and the best conductivity. This is due to the improved reinforcement‐matrix interface that enhances the conduction transfer and a lower amount of porosity. We achieved a great improvement of electrical CP+S 75,6 85,1 HUP 58,2 86,6 SPC_A 15,7 87,8 SPC_B 15,6 91,8 Electrical conductivity obtained with the four point probe method for the manufactured composites amount of CNT clusters is higher, thus handicapping the electrical transport due to the decay in the properties and the densification hindering [6]. conductivity up to 6,5 times compared to pure Ni with low power consumption and lower process times than the traditional methods. Acknowledgements All authors wish to acknowledge the European Union for the funding through the project “NanoCom Network” (FP7‐People‐ 2009‐IRSES) Pr. N°: 247524. S.S.V. wants to thank the German Academic Exchange Service (Deutsche Akademische Austausch Dienst – DAAD) for the financial support. J.G. thanks the financial support of the joint research group "Microstructural Analysis" (Hemholtz‐Zentrum Berlin / Ruhr Universität Bochum). References [1] M. Stadermann et al., Nanoscale study of conduction through carbon nanotube networks. Phys. Rev. B, Vol. 69, 201402 (2004) [2] R. Sanjinés et al., Electrical properties and applications of carbon based nanocomposite materials: An overview. Surf. Coat. Technol. (2011) doi:10.1016/j.surfcoat.2011.01.025 (In Press) [3] K. Yan et al., The interface effect of the effective electrical conductivity of carbon nanotube composites. Nanotechnology, Vol. 18, 255705 (2007) [4] S. R. Bakshi et al., Carbon nanotube reinforced metal matrix composites – a review. Int. Mat. Reviews, Vol. 55, N°1, 41-64 (2010) [5] Yue L. et al., Magnetic properties of disordered Ni3C. Phys. Rev. B, Vol. 62 N°13, 8969 (2000) [6] Stahl H. et al., Intertube coupling in ropes of single walled carbon nanotubes. Phys. Rev. Letters, Vol. 85, N°24. pp 5186 (2000)