(510ar) Synthesis and Characterization of Magneto-Dielectric Composites for Radio Frequency Applications

Microwave material design has become an increasingly important area of researc due to its many applications - one of which is radio frequency (RF) materials. RF devices, such as antennas, circulators, transmission lines, etc., will increasingly depend on advances in material design to achieve future performance gains. One area of particular importance to RF designers, for both military and commercial applications, is advances in magneto-dielectric materials. Such materials exhibit an engineered permittivity and permeability (electric and magnetic properties, respectively), while still maintaining very low loss. Newly developed radio frequency materials currently have two hurdles to overcome ? they have narrow bandwidth due to the use of resonant structures, and they have relatively high loss [1]. Note that with conventional materials, meeting these design goals is extremely challenging. Since low loss is critical for many RF design applications, micro- or nano-sized inclusions are of interest since very-small inclusion dimensions will reduce two of the most important loss mechanisms ? eddy currents and multi-domain wall resonance. Unfortunately, uniform particle size and shape is very challenging to achieve. The effect of varying size and shape is particle-to-particle variation in the permeability [2]. Methods currently used to implement the homogenization of the electromagnetic properties of these materials have two underlying assumptions. The first assumption is that the inclusions are small enough to obtain a static polarizability and/or magnetization. Secondly, the particles are sufficiently distant from each other to ignore mutual coupling. The novel aspect of this work is that homogenization and design of materials will use methods that simulate inclusions that do not meet these assumptions. The overall goals of this research can be broken down into two parts: 1) the development of a numerical design tool to simulate a composite with inclusions of varying morphology and predict the magnetic/electric properties and 2) the synthesis of nanocomposites to validate this numerical program. Major objectives include synthesizing test coupons and analyzing the permittivity and permeability as well as dispersion quality and comparing actual properties to that of the numerical model. The focus of this paper is the synthesis and characterization of the composites; later work will highlight the numerical design tool and computational results. For this study, nanocomposites were cured using diglycidyl ether of bisphenol F (DGEBF) with 4,4' diamino diphenyl sulfone (DDS) as the curing agent. Nanoparticles used for this study include iron oxide (Fe₃O₄) nanopowder of spherical morphology. An ultrasonication technique was used for synthesis of the composites with varying weight percent of the inclusions. Dielectric properties were analyzed as a function of both frequency and temperature. Magnetic properties were measured as a function of frequency. A sufficient dielectric constant was achieved (near 6); whereas, the magnetic permeability was still low. The dielectric properties increased with temperature and decreased with extent of cure ? a trend that can be related to the fact that the dipolar groups in the reactants decrease in number during the curing process, while the viscosity increases. Future research will focus on measuring the mechanical properties of the composites to see if the material can maintain good properties while still having high concentrations of iron oxide filler. Moreover, a more detailed look at the effects on cure characteristics will be carried out. Also, different processing techniques will be used ? i.e. microwave processing, compression molding, etc.

[1] Harper, C.A. Electronic Materials and Processes Handbook. 3rd Edition. McGraw-Hill, Chapter 1. (2003).

[2] Mallikarjuna, N.N., et al. ?Novel High Dielectric Constant Nanocomposites of Polyanaline Dispersed with Fe₂O₃ Nanoparticles.? Journal of Applied Polymer Science. 97:1868-1874. (2005).