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Materials Science at Oregon State University

Creating composites for high frequency applications by aligning magnetic microdisks in a rotating magnetic field

Date: Thursday, Oct 30th
Presenter: Dr. Travis Walker, OSU Chemical Engineering


Fluid flow surrounds us. From the flow of water from a faucet and blood in our veins to the processing of most materials, we are both blessed and frustrated with the flow of material. People have developed a great intuition for fluids that experience a linear strain in the presence of a stress; however, most biological and industrial fluids exhibit a nonlinear response to deformation. This nonlinear response can be a hindrance when trying to get ketchup out of a glass bottle, or it can be a benefit when spreading paint on a vertical wall. These everyday examples illustrate that complex fluids can be difficult to process, yet when exploited, this complexity can also be desirable.

Fundamental interests:
The addition of multiple phases to flow systems drastically increases the complexity of the flow physics. These complexities reveal themselves on both the macro- scopic and microscopic length scales and can involve solids as well as immiscible and miscible fluids. Complexities such as the presence of polymers, surfactants, colloids, and particulates to flow systems create complex fluids or soft materials that respond in a nonlinear way to stress. The interactions of particles with viscoelastic materials are found readily in nature. From the suspension of shavings in drill mud to the adhesion of particulates in the mucus lining of our lungs, the non-Newtonian rheological properties of the interacting fluid can dominate the phys- ical responses of solid particles.

Soft composites for high-frequency inductors:
Composites made from embedding fine metallic magnetic particles in an insulating matrix are promising materials for high frequency inductor and antennae applications. Recently, composite materials consisting of magnetic par- ticles with high aspect ratios (i.e., rod-shaped or disk-shaped particles) are gaining increased attention as they exhibit enhanced high frequency permeability in comparison to composites with spherical particles. Moreover, magnetic alignment of these high aspect ratio particles further increases the high frequency permeability and ferromagnetic resonance frequency. Typically, the alignment is achieved by applying an external magnetic field during curing of the matrix. With rod-shaped particles, this constant field results in a composite material with uniaxial anisotropy. In this study, we show that curing a composite of disk-shaped particles in a rotating magnetic field produces a composite with planar anisotropy. We investigate the dynamics of the align- ment process to determine the conditions for achieving a high degree of alignment while avoiding inhomogeneous distribution of particles due to sedimentation or agglomeration. We use Ni and NiFe microdisks in a UV curable binder as the study system and report the effect of alignment time on the microstructural and magnetic properties of the composite. The physical orientations of the embedded Ni and NiFe microdisks inside the composites are investigated by dark field optical microscopy. The cross sections of microdisk composites with varying alignment time are observed in planes orthogonal and parallel to the axis of rotation. Theoretical models, based on Stokes flow of a single magnetic oblate ellipsoidal particle in a rotating magnetic field, are developed to enable understanding and control of the observed alignment dynamics process. Comparisons of times scales are made to controlled single particle experiments.