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Magnetic Nanocomposite Paste: An Ideal High- m, K and Q Nanomaterial for Miniaturized Antennas and Inductors

T.D. Xiao, P.M. Raj, L. Wan, H. Zhang, X. Ma, D.E. Reisner, Y.D. Zhang, D. Balaraman, S. Bhattacharya, I.R. Abothu, M. Swaminathan and R. Tummala
Inframat Corp., US

magnetic, nanocomposite, paste, nanomaterial, miniaturized antennas, inductors, wireless, RF circuits, modules

Current wireless systems are limited by RF technologies in their size, communication range, efficiency and cost. RF circuits are difficult to miniaturize without compromising performance. Though recent developments in IC and system-level packaging technologies (System-On-Package) can provide smaller size of RF transceivers with embedded passive components, antenna is still considered the largest barrier for RF system because its miniaturization is limited by the physics of electromagnetic radiation. The low radiation efficiency, a major limitation of current low profile patch antennas can be overcome by higher magnetic permeability and lower loss. A high permittivity in combination with permeability dielectric can increase the path of the normal current and hence increase the radiation efficiency even further. These dielectrics also can potentially lead to an order of magnitude reduction in size. At the same time, if the material can be designed to have both high permittivity and permeability, it can have good impedance match with that of free space. The bandwidth of antennas on magnetic substrate are also known to be wider than the antenna of conventional dielectric substrate. Keeping these requirements in mind, a novel magnetic nanocomposite is designed with a high magnetic permeability, similar dielectric constant and lowest loss compared to any existing magnetic material. Reducing the particle size and the separation between neighboring particles down to the nanoscale leads to novel magnetic coupling phenomena resulting in higher permeability and lower magnetic anisotropy. Co- or Fe-based nanocomposites show permeability value much higher than that obtainable from the bulk Co or Fe metal due to the reduced eddy current losses and interparticle exchange coupling effects at very high frequencies. The exchange interaction, which leads to magnetic ordering within a grain, extends out to neighboring environments. The exchange interaction in nanocomposites also leads to a cancellation of magnetic anisotropy of individual particles and the demagnetizing effect, leading to ultra superior soft magnetic properties. By choosing a system with high tunneling excitation energy, a huge increase in the resistivity (from 10 – 6 Ohm-cm in pure metal to 10 9 Ohm-cm in nanocomposite) can be achieved through the nanocompositing technique. Because the metal particle is of nanosized, the eddy current produced within the particle is also negligibly small, leading to much smaller eddy current loss for nanocomposites to that of conventional microsized ferrites and powder materials. Co- and Fe-based magnetic nanoparticles were synthesized via wet chemical synthesis techniques. The wet chemical techniques include a co-precipitation reduction and a microemulsion method. The precipitation synthesis procedure includes (1) preparation of aqueous precursor solutions containing metal and a reducing agent, (2) atomization of the precursor solutions to form a nanoparticle colloidal suspensions with maximal nucleation and minimal growth, (3) refluxing of the colloidal solution under controlled pH and time to form the desired microstructure and phases, (4) washing and filtration, and (5) low temperature calcinations. Using a microemulsion technique, a silane coupling-agent is attached to the surface of the suspended nanoparticles, followed by coating of the magnetic nanoparticles with functionalized polymers. Different polymers and emulsifiers were selected in this study at various synthesis conditions in order to improve the properties of the nanocomposite. TEM images indicated that the resultant functionalized magnetic nanoparticles had a homogeneous particle size distribution with its average magnetic core diameter of 12 nm and polymer coating thickness of 1 - 2 nm. Thick/thin films (10 and 50 microns) were obtained by spin coating/screen-printing technique. Critical parameters for this high quality spin-coated film including paste viscosity and filler content. Conventional multiline TRL techniques are inherently disadvantageous to characterize the properties of nanocomposites with unknown K, m and loss. Hence, a new characterization approach has been applied. To use this new approach, three different length transmission lines will be designed and fabricated. S-parameters of these lines will be obtained for two ABCD matrices. The advantages of this approach are that all the four parameters of the transmission lines C, L, G, and R can be found simultaneously, and then the permittivity and permeability can be obtained through these parameters. Electrical characterization showed show that the Co/SiO2 nanocomposite sample has a permeability _17 with a flat frequency response up to 350 MHz and a permeability of 15 up to 1 GHz frequency range with little core loss. The dielectric constant of this material varied from 10-12. The properties are much superior to that of microsized ferrite and are ideal for miniaturized antennas and inductors. The broader impact of this nanocomposite is improved performance of wireless data retrieval systems with advanced antennas enabled by nanomaterials and manufacturing processes that would be more efficient, provide at least an order of magnitude in size reduction, an order of magnitude in transmitting range, and higher gain. Traditionally, the antenna has always been a separate component, whether it was an internal antenna or external antenna, but this new class of antennas can ultimately be fabricated as an integral part of the RF module with ultimate reduction in size and cost of an RF subsystem. These nanocomposites can also be used as packaging substrates, leading to antenna supported RF modules.

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