Synthesis and characterizations of microwave sintered ferrite powders and their composite films for practical applications S.R. Shannigrahi n , K.P. Pramoda, F.A.A. Nugroho Institute of Materials Research and Engineering, A n STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore article info Article history: Received 18 May 2011 Accepted 26 July 2011 Available online 7 August 2011 Keywords: Ferrite Microwave sintering EMI shielding abstract Phase pure single phase ferrite powders of (Ni x R 1x ) 0.5 Zn 0.5 Fe 2 O 4 (R ¼Mn, Co, Cu; x ¼0, 0.5) were manufactured using microwave sintering at 930 1C for 10 min in air atmosphere. The powders were characterized for their structure, microstructure, thermal, and magnetic properties. Selected powders were used as fillers to prepare their composite films using polymethyl methacrylate polymers as matrix. The composite films were prepared using the melt blending approach and were tested for their microstructure, thermal, and magnetic hysteresis loop as well as 3D magnetic field space mappings using an electromagnetic compatibility scanner. Among the studied ferrites, cobalt doped ferrites and their composites showed the best electromagnetic interference (EMI) shielding effectiveness value and have potential for practical EMI shielding applications. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Ferrites are technologically very important materials because of their high magnetic permeability, low core loss, and soft magnetic nature, which make them suitable for different applica- tions like electrical components, memory devices, and magnetos- trictive devices as well as electronic components such as transformers, choke coils, noise filters, recording heads, etc [1–7]. Thus ferrites have been extensively studied by several researchers with different endeavors [5–9]. A number of research- ers have endeavored to prepare ferrite materials in different forms [10–12]. Ferrites prepared by the conventional ceramic method involve high temperature, which can result in the loss of their fine particle size. The bulk properties of the similar type of ferrites differ among compositions. In the recent years many works have been focused to establish possible cost effective manufacturing–processing scheme to optimize technologically important parameters [13–18]. Verma et al. have reported the development of a new ferrite with low power loss based on manganese nickel zinc ferrite composition for switch mode power supplies [19]. Several other works have been reported on sinter- ing behavior, including densification and grain growth of NiCuZn ferrites and their modified forms using microwave (MW) heat treatment [20–28]. Bhaskar et al. have reported low power loss MgCuZn ferrites using the MW sintering method [29]. Yadoji et al. have reported the comparative property studies of Ni 1 x Zn x Fe 2 O 4 prepared using conventional and MW techniques with the set sintering recipe of 1275 1C for 30 min [30]. It is reported that the properties of base NiZnFe 2 O 4 can be further improved by intro- ducing suitable modifiers like Mn, Cu, Co, Cu etc. So, the search for new modified ferrites as well as the cost effective processing recepies is very much active in current ferrite materials research. To the best of our knowledge, no report has been published on a systematic investigation of MW sintering of (Ni x R 1x ) 0.5 Zn 0.5 Fe 2 O 4 (NRZF); (R ¼ Mn, Co, Cu; x ¼ 0, 0.5). In view of the importance of these materials and the utilization of MW sintering, in this work we have prepared NRZF ferrites through a one stage sintering process using MW and prepared their composite films, and studied their structure, microstructure, thermal, and mag- netic properties. 2. Experimental procedure 2.1. Ferrite powder synthesis The ferrite powders of (Ni x R 1 x ) 0.5 Zn 0.5 Fe 2 O 4 (NRZF) [R ¼ Mn, Co, Cu; x ¼ 0, 0.5] were prepared by a one stage heat treatment process using a multi-mode MW tube furnace with the magne- tron frequency of 2.45 GHz (MW-T0316V, Syno-Therm) the max- imum operating temperature up to 1400 1C, and 0.5–3 kW (Fig. 1a). An adjustable electrical control system was used to control the energy to be delivered to the sample at a programmed rate. The heating chamber was a double walled, stainless steel, water cooled tubular cavity that stayed cool to touch, even when processing temperature was  1400 1C. Inside the chamber was a high purity quartz crystal cylinder where samples were loaded for processing. The temperature was recorded using a high precision optical pyrometer. Stoichiometry mixtures of the individual Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.07.050 n Corresponding author. Tel.: þ6568748299; fax: þ6568720785. E-mail address: santi-s@imre.a-star.edu.sg (S.R. Shannigrahi). Journal of Magnetism and Magnetic Materials 324 (2012) 140–145