Original Research Paper Synthesis of cobalt nanoparticles to enhance magnetic permeability of metal–polymer composites Pirjo Koskela a,⇑ , Merja Teirikangas b , Ari Alastalo a , Johanna Forsman a , Jari Juuti b , Unto Tapper a , Ari Auvinen a , Heikki Seppä a , Heli Jantunen b , Jorma Jokiniemi a,c a VTT Technical Research Centre of Finland, P.O. Box 1000, 02044 VTT, Espoo, Finland b Microelectronics and Materials Physics Laboratories, University of Oulu, P.O. Box 4500, FI-90014 Oulu, Finland c Department of Environmental Science, Fine Particle and Aerosol Technology Laboratory, University of Eastern Finland, P.O. Box 1627, FI-70211 Kuopio, Finland article info Article history: Received 24 June 2010 Received in revised form 10 September 2010 Accepted 17 September 2010 Available online 29 September 2010 Keywords: Magnetic nanoparticles Cobalt Polymer-matrix composite (PMC) Magnetic properties Modelling abstract Metallic cobalt nanoparticles were synthesized by hydrogen reduction method. Particles were coated in situ with carbon by adding ethene to reaction flow. Particles were characterized by transmission elec- tron microscopy, energy dispersive X-ray emission, X-ray diffraction, X-ray fluorescence and BET method. The observed cobalt particle size distributions in different cobalt batches produced with unvarying reac- tion parameters was reproducible: The mean diameter of primary cobalt particle varied only 5% from the mean value of 76 nm in different batches. Increased carbon precursor concentration decreased mean diameter of cobalt particles to 17 nm. The produced nanoparticles were used as filler material in 0–3 type metal–polymer composites. Composite samples with varying filler loading were fabricated with mixing extrusion and injection moulding techniques. The magnetic properties of the fabricated composites were measured up to 1 GHz. In order to analyse the particle distribution in composite matrix and its effect on magnetic properties the microstructure was studied. Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. 1. Introduction Magnetic nanoparticles are studied widely because of their inter- esting and versatile properties that are desirable in many applica- tions. For example, magnetic nanoparticles can be used to enhance the permeability of an antenna substrate, which helps in miniaturi- zation of antenna without sacrificing its performance. Magnetic nanoparticles have also been utilized as markers of biosensors. A controlled magnetic structure that is difficult to counterfeit can also be an identification marker of originality for a product. In addition, magnetic nanocomposites are good candidates for different electro- magnetic wave and radiation absorption applications [1–4]. When magnetic metal, such as cobalt, is used to enhance the permeability of an insulator layer, it is essential to have the metal- lic material as isolated particles. Otherwise the insulating proper- ties of the dielectric are compromised. Furthermore, utilizing nanoparticles with size between 20 and 100 nm prevents inducing lossy circulating currents into the particles at frequencies of inter- est in communication applications [5]. Isolation of the particles can be done by carbon capsulation. Hydrocarbon gases have been used as sources for carbonous encapsulating material. Methane decom- position to carbon has been studied over a nickel catalyst [6]. In gas phase graphitic carbon coated nickel particles have been formed in situ from liquid Fe(CO) 5 with a mixture of acetylene, H 2 and CO, but also MWNTs were formed [7]. Commercially available micron- and nano-sized nickel particles have been encapsulated by carburisation using ethylene–oxygen reactant flow [8]. With flame spray synthesis a coating method for cobalt powders has been developed where organic cobalt containing precursor disper- sion is ignited with mixture of oxygen and acetylene [9]. We have previously developed a synthesis method for cobalt nanoparticles by hydrogen reduction of cobalt chloride vapour [10]. In this work a method for in situ carbon coating of the metallic nanoparticles is presented. In the hydrogen reduction technique low cost precursor materials, neutral waste and high production rate are obtained simultaneously. It is shown that in situ carbon coating stabilises the particles in air and significantly influences on the size distribu- tion of the produced powder. Additionally, as compared with silicon technology, use of high throughput printing techniques enable direct patterning resulting in reduced material and processing costs. Radio-frequency identifi- cation (RFID), security codes, organic light-emitting diodes (OLED) and organic photovoltaics (OPV) are expected to be among the first commercially viable applications of printed electronics. A general trend in hand-held communication devices using high frequency 0921-8831/$ - see front matter Ó 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2010.09.010 ⇑ Corresponding author. E-mail address: pirjo.koskela@vtt.fi (P. Koskela). Advanced Powder Technology 22 (2011) 649–656 Contents lists available at ScienceDirect Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt