Field-structured Composites Research Articles

Field-structured magnetic platelets as a route to improved thermal interface materials

The development of high-performance thermal interface materials (TIMs) is crucial to enabling future generations of microelectronics because the TIM is usually the limiting thermal resistance in the heat removal path. Typical TIMs achieve modest thermal conductivities by including large volume fractions of randomly-dispersed, highly-conductive, spherical particles in a polymer resin. This paper explores field-structured magnetic platelet composites as a new approach to more effective TIMs. The motivation for this approach is rooted in shape functional theory, which shows that when the particle material has a significantly higher thermal conductivity than that of the polymer, the particle shape and orientation are the factors that limit conductivity enhancement. Oriented platelets are highly effective for heat transfer and if these are magnetic, then magnetic fields can be used to both orient and agglomerate these into structures that efficiently direct heat flow. In this paper we show that such field-structured composites have a thermal conductivity anisotropy of ∼3, and at the highest particle loading of 16 vol.% we have achieved a 23-fold conductivity enhancement, which is 3-times larger than that achieved in unstructured platelet composites and 8-times greater than unstructured spherical particle composites.

Field-structured composites for efficient, directed heat transfer

Thermally conductive composites are needed for a variety of applications, especially as thermal interface materials. Unfortunately, the addition of even highly conductive particles to a polymer raises its conductivity only slightly and incurs a commensurate increase in the viscosity of the prepolymer resin. Meaningful increases in the thermal conductivity of the final composite require a filled resin that has the rheology of a typical paste. In this paper, we report on the use of uniaxial, biaxial, and triaxial ac magnetic fields to organize thermally conductive magnetic particles into structures that conduct heat more efficiently. Experimental results are compared to an extension of the Maxwell–Eucken theory that takes into account spatial correlations between particles. Both theory and experiment show that the thermal conductivity of field-structured composites increases essentially linearly with particle loading, and that the enhancement is significantly greater than that of unstructured composites. The measured conductivity enhancements are found to be comparable to the enhancements in magnetic permeability of these composites and are far less than the enhancements in the electrical conductivity. These results indicate the value of field structuring in enhancing the performance of a thermal interface material and show that particle percolation is not a critical factor in achieving high performance.

Giant magnetic susceptibility enhancement in field-structured nanocomposites

We demonstrate through experiment and simulation that when mono-domain Fe nanoparticles are formed into chains by the application of a magnetic field, the susceptibility of the resulting structure is greatly enhanced (11.4-fold) parallel to the particle chains and is much larger than transverse to the chains. Simulations show that this significant enhancement is expected when the susceptibility of the individual particles approaches 5 in MKS units, and is due to the spontaneous magnetization of individual particle chains, which occurs because of the strong dipolar interactions. This large enhancement is only possible with nanoparticles, because demagnetization fields limit the susceptibility of a spherical multi-domain particle to 3 (MKS). Experimental confirmation of the large susceptibility enhancement is presented, and both the enhancement and the susceptibility anisotropy are found to agree with simulation. The specific susceptibility of the nanocomposite is 54 (MKS), which exceeds the highest value we have obtained for field-structured composites of multi-domain particles by a factor of four.

Magnetostriction of field-structured magnetoelastomers

We investigate the magnetostriction of field-structured magnetoelastomers, which are an important class of materials that have great potential as both sensors and actuators. Field-structured magnetoelastomers are synthesized by suspending magnetic particles in a polymeric resin and subjecting these to magnetic structuring fields during polymerization. These structuring fields can consist of as many as three orthogonal ac components, allowing a wide variety of particles structures--chains, sheets, or networks--to be formed. A principal issue is how particle structure and loading affects the magnetostriction of these materials. To investigate magnetostriction in these field-structured composites we have constructed a constant stress, optical cantilever apparatus capable of 1 ppm strain resolution. Magnetoelastomers having a wide range of particle loadings and structures are investigated, and it is shown that the observed deformation depends strongly on composite structure. The best magnetoelastomers exhibit a contractive strain of 10,000 ppm, the worst materials exhibit a negative, tensile response, which we show is due to the dominance of demagnetizing field effects over magnetostriction. Finally, some discussion is given to the surprising finding that magnetostriction is proportional to the sample prestrain. Simulations of a chain of particles in an elastomer show that particle clumping transitions can occur, but this does not account for the dependence of magnetostriction on prestrain.

Controlling percolation in field-structured particle composites: Observations of giant thermoresistance, piezoresistance, and chemiresistance

Composites of conductive particles in an insulating phase are conductive if the particle volume fraction exceeds the percolation threshold. Composites prepared slightly above the percolation threshold have a conductivity that is sensitive to small volume changes, and thus have potential as temperature, pressure, or chemical sensors. In practice it is difficult to prepare composites close to the percolation threshold, and the critical current-carrying path gives a rather low sample conductivity. We find that magnetic-field-structured composites, consisting of gold-coated magnetic particle chains in a polymeric resin, can be reproducibly brought to the percolation threshold, regardless of particle concentration. The low-dimensionality conducting chains form a dense population of critical current paths with extreme sensitivity to composite volume changes. These field-structured composites thus exhibit giant thermoresistance, piezoresistance, and chemiresistance, and should be useful as sensor materials.

Magnetostriction of Field-Structured Composites

ABSTRACTField-structured magnetic particle composites are an important new class of materials that have great potential as both sensors and actuators. These materials are synthesized by suspending magnetic particles in a polymeric resin and subjecting these to magnetic fields while the resin polymerizes. If a simple uniaxial magnetic field is used, the particles will form chains, yielding composites whose magnetic susceptibility is enhanced along a single direction. A biaxial magnetic field, comprised of two orthogonal ac fields, forms particle sheets, yielding composites whose magnetic susceptibility is enhanced along two principal directions. A balanced triaxial magnetic field can be used to enhance the susceptibility in all directions, and biased heterodyned triaxial magnetic fields are especially effective for producing composites with a greatly enhanced susceptibility along a single axis. Magnetostriction is quadratic in the susceptibility, so increasing the composite susceptibility is important to developing actuators that function well at modest fields. To investigate magnetostriction in these field-structured composites we have constructed a sensitive, constant-stress apparatus capable of 1 ppm strain resolution. The sample geometry is designed to minimize demagnetizing field effects. We have demonstrated field-structured composites with nearly 10,000 ppm strain, and have shown that at large magnetic fields a structural phase transition occurs within the composite. These experimental results are compared to microscopic, self-consistent field simulations of magnetostriction in these complex, disordered materials.

Anisotropic magnetism in field-structured composites

Magnetic field-structured composites (FSCs) are made by structuring magnetic particle suspensions in uniaxial or biaxial (e.g., rotating) magnetic fields, while polymerizing the suspending resin. A uniaxial field produces chainlike particle structures, and a biaxial field produces sheetlike particle structures. In either case, these anisotropic structures affect the measured magnetic hysteresis loops, with the magnetic remanence and susceptibility increased significantly along the axis of the structuring field, and decreased slightly orthogonal to the structuring field, relative to the unstructured particle composite. The coercivity is essentially unaffected by structuring. We present data for FSCs of magnetically soft particles, and demonstrate that the altered magnetism can be accounted for by considering the large local fields that occur in FSCs. FSCs of magnetically hard particles show unexpectedly large anisotropies in the remanence, and this is due to the local field effects in combination with the large crystalline anisotropy of this material.

Enhanced dielectric standoff and mechanical failure in field-structured composites

We report dielectric breakdown experiments on electric-field-structured composites of high-dielectric-constant ${\mathrm{BaTiO}}_{3}$ particles in an epoxy resin. These experiments show a significant increase in the dielectric standoff strength perpendicular to the field structuring direction, relative to control samples consisting of randomly dispersed particles. To understand the relation of this observation to microstructure, we apply a simple resistor-short breakdown model to three-dimensional composite structures generated from a dynamical simulation. In this breakdown model the composite material is assumed to conduct primarily through particle contacts, so the simulated structures are mapped onto a resistor network where the center of mass of each particle is a node that is connected to neighboring nodes by resistors of fixed resistance that irreversibly short to perfect conductors when the current reaches a threshold value. This model gives relative breakdown voltages that are in good agreement with experimental results. Finally, we consider a primitive model of the mechanical strength of a field-structured composite material, which is a current-driven, conductor-insulator fuse model. This model leads to a macroscopic fusing behavior and can be related to mechanical failure of the composite.

Electrostriction in field-structured composites: Basis for a fast artificial muscle?

The electrostriction of composites consisting of dielectric particles embedded in a gel or elastomer is discussed. It is shown that when these particles are organized by a uniaxial field before gelation, the resulting field-structured composites are expected to exhibit enhanced electrostriction in a uniform field applied along the same axis as the structuring field. The associated stresses might be large enough to form the basis of a polymer-based fast artificial muscle.

Thermal coarsening of uniaxial and biaxial field-structured composites

When a suspension of colloidal particles is subjected to a strong electric or magnetic field, the induced dipolar interactions will cause the particles to form organized structures, provided a sufficient permittivity or permeability mismatch exists, respectively, between the particles and the suspending liquid. A uniaxial field will produce uniaxial structures, and a biaxial field, such as a rotating field, will produce biaxial structures, and either of these structures can be pinned by polymerizing the continuous phase to produce field-structured composites. We have previously reported on the coarsening of field-structured composites in the absence of thermal effects, i.e., Brownian motion. Athermal simulations are primarily valid in describing the deep quenches that occur when the induced dipolar interactions between particles greatly exceed kBT. However, deep quenches can lead to kinetic structures that are far from equilibrium. By introducing Brownian motion we have shown that structures with significantly greater anisotropy and crystallinity can form. These structures have enhanced material properties, such as the conductivity, permittivity, and optical attenuation. Careful anneals at certain fixed fields, or at continuously increasing fields, should produce more anisotropic structures than the deep quenches we have used to synthesize real materials.