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Register a new userThermal and Magnetic Properties of Nanostructured Ferrimagnetic Composites with Graphene - Graphite Fillers
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We report the results of an experimental study of thermal and magnetic properties of nanostructured ferrimagnetic iron oxide composites with graphene and graphite fillers synthesized via the current activated pressure assisted densification. The thermal conductivity was measured using the laser-flash and transient plane source techniques. It was demonstrated that addition of 5 wt. % of equal mixture of graphene and graphite flakes to the composite results in a factor of x2.6 enhancement of the thermal conductivity without significant degradation of the saturation magnetization. The microscopy and spectroscopic characterization reveal that sp2 carbon fillers preserve their crystal structure and morphology during the composite processing. The strong increase in the thermal conductivity was attributed to the excellent phonon heat conduction properties of graphene and graphite. The results are important for energy and electronic applications of the nanostructured permanent magnets.
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We investigated thermal properties of the epoxy-based composites with a high loading fraction - up to f=45 vol.% - of the randomly oriented electrically conductive graphene fillers and electrically insulating boron nitride fillers. It was found that both types of the composites revealed a distinctive thermal percolation threshold at the loading fraction f>20 vol.%. The graphene loading required for achieving the thermal percolation was substantially higher than the loading for the electrical percolation. Graphene fillers outperformed boron nitride fillers in the thermal conductivity enhancement. It was established that thermal transport in composites with the high filler loading, above the thermal percolation threshold, is dominated by heat conduction via the network of percolating fillers. Unexpectedly, we determined that the thermal transport properties of the high loading composites were influenced strongly by the cross-plane thermal conductivity of the quasi-two-dimensional fillers. The obtained results shed light on the debated mechanism of the thermal percolation, and facilitate the development of the next generation of the efficient thermal interface materials for electronic applications.
The authors proposed a simple model for the lattice thermal conductivity of graphene in the framework of Klemens approximation. The Gruneisen parameters were introduced separately for the longitudinal and transverse phonon branches through averaging over phonon modes obtained from the first-principles. The calculations show that Umklapp-limited thermal conductivity of graphene grows with the increasing linear dimensions of graphene flakes and can exceed that of the basal planes of bulk graphite when the flake size is on the order of few micrometers. The obtained results are in agreement with experimental data and reflect the two-dimensional nature of phonon transport in graphene.
We report on the preparation of flexible polymer composite films with aligned metallic fillers comprised of atomic chain bundles of the quasi-one-dimensional (1D) van der Waals material tantalum triselenide, TaSe3. The material functionality, embedded at the nanoscale level, is achieved by mimicking the design of an electromagnetic aperture grid antenna. The processed composites employ chemically exfoliated TaSe3 nanowires as the grid building blocks incorporated within the thin film. Filler alignment is achieved using the blade coating method. Measurements conducted in the X-band frequency range demonstrate that the electromagnetic transmission through such films can be varied significantly by changing the relative orientations of the quasi-1D fillers and the polarization of the electromagnetic wave. We argue that such polarization-sensitive polymer films with quasi-1D fillers are applicable to advanced electromagnetic interference shielding in future communication systems.
We report on heat conduction properties of thermal interface materials with self-aligning magnetic grapheme fillers. Graphene enhanced nano-composites were synthesized by an inexpensive and scalable technique based on liquid-phase exfoliation. Functionalization of graphene and few-layer-graphene flakes with Fe3O4 nanoparticles allowed us to align the fillers in an external magnetic field during dispersion of the thermal paste to the connecting surfaces. The filler alignment results in a strong increase of the apparent thermal conductivity and thermal diffusivity through the layer of nano-composite inserted between two metallic surfaces. The self-aligning magnetic grapheme fillers improve heat conduction in composites with both curing and non-curing matrix materials. The thermal conductivity enhancement with the oriented fillers is a factor of two larger than that with the random fillers even at the low ~1 wt. % of graphene loading. The real-life testing with computer chips demonstrated the temperature rise decrease by as much as 10oC with use of the non-curing thermal interface material with ~1 wt. % of the oriented fillers. Our proof-of-concept experiments suggest that the thermal interface materials with functionalized graphene and few-layer-graphene fillers, which can be oriented during the composite application to the surfaces, can lead to a new method of thermal management of advanced electronics.
The results of measurements of XPS spectra and magnetic properties of graphene/Co composites prepared by adding of CoCl$_2$x6H$_2$O diluted in ethyl alcohol to highly-splitted graphite are presented. XPS Co 2p measurements show two sets of 2p$_{3/2,1/2}$-lines belonging to oxidized and metallic Co-atoms. This means that metal in composite is partly oxidized. This conclusion is confirmed by magnetic measurements which show the presence of the main (from the metal) and shifted (from the oxide) hysteresis loops. The presence of oxide layer on the metal surface prevents the metal from the full oxidation and (as shown by magnetic measurements) provides the preservation of ferromagnetic properties after long exposure in ambient air which enables the use of graphene/metal composites in spintronics devices.
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