K. Miller

Theory of magnetic fluid heating with an alternating magnetic field with temperature dependent materials properties for self-regulated heating

Magnetic nanoparticles (MNP) offer promise for local hyperthermia, thermoablative cancer therapy and microwave curing of polymers. Rosensweigs theory predicts that particle size dependence on RF magnetic heating of ferrofluids is chiefly determined by magnetic moment, magnetic anisotropy, and the viscosity of the fluid. Since relaxation times are thermally activated and material parameters can have strong T dependences, heating rates peak at a certain temperature. We extend the model to include the T dependence of the magnetization and anisotropy using mean field theory and literature reported T dependences of selected fluids considered for biomedical applications. We model materials with Curie temperatures near room temperature for which the magnetic properties are strongly T dependent to address the problem of self-regulated heating of ferrofluids.

Controlled oxidation of FeCo magnetic nanoparticles to produce faceted FeCo/ferrite nanocomposites for rf heating applications

We report the oxidation products and qualitative rates for polydisperse FeCo magnetic nanoparticles (MNPs) synthesized using an induction plasma torch. X-ray diffraction (XRD) and TEM showed MNPs to have a thin ferrite shell. Nanopowders were isochronally annealed to promote oxidation and XRD was used to follow the evolution of the FeCo core and the Fe3O4 and FeO oxide shells. Isothermal anneals were used to follow oxidation kinetics at 350 and 500°C. High resolution transmission electron microscopy (HRTEM) revealed faceted morphologies terminated at (100) and (110) FeCo faces with (110)FeCo∥(111)oxide and (100)FeCo∥(100)oxide, and [010]FeCo∥[011]oxide orientation relationships between the FeCo core and oxide shell. We show HRTEM images of MNP chaining and compare the rf heating of samples of aqueous ferrofluids similarly loaded with as synthesized and oxidized MNPs.

Induction heating of FeCo nanoparticles for rapid rf curing of epoxy composites

FeCo magnetic nanoparticles (MNPs) have been investigated for curing polymer epoxy composites through radio-frequency (rf) heating. The rf response of functionalized FeCo MNPs is shown to uniformly cure the epoxy without parasitic heating for potential applications in electronic packaging. The FeCo/(Co,Fe)3O4 MNPs were synthesized using a rf induction plasma torch and were ultrasonicated in diglycidyl ether of bisphenol F epoxy to form stable ferrofluids. Transmission electron microscopy studies reveal as-synthesized MNPs to have a mean diameter of 17.6 nm with a 5.2 nm standard deviation. The mean MNP diameter is reduced to 9 nm after cryomilling and causes particles to form ∼200 nm agglomerates. Ferrofluids of varying MNP concentrations, sizes, and shapes were rf heated using a precision rf coil operated at 20.0 kA/m and 267 kHz frequency. Using a 1.36 vol % ferrofluid, the epoxy composite was effectively rf cured, reaching temperatures >100 °C in ∼70 s. The results suggest that rf heating of FeCo MNPs may provide an effective method to curing epoxy composites.FeCo magnetic nanoparticles (MNPs) have been investigated for curing polymer epoxy composites through radio-frequency (rf) heating. The rf response of functionalized FeCo MNPs is shown to uniformly cure the epoxy without parasitic heating for potential applications in electronic packaging. The FeCo/(Co,Fe)3O4 MNPs were synthesized using a rf induction plasma torch and were ultrasonicated in diglycidyl ether of bisphenol F epoxy to form stable ferrofluids. Transmission electron microscopy studies reveal as-synthesized MNPs to have a mean diameter of 17.6 nm with a 5.2 nm standard deviation. The mean MNP diameter is reduced to 9 nm after cryomilling and causes particles to form ∼200 nm agglomerates. Ferrofluids of varying MNP concentrations, sizes, and shapes were rf heated using a precision rf coil operated at 20.0 kA/m and 267 kHz frequency. Using a 1.36 vol % ferrofluid, the epoxy composite was effectively rf cured, reaching temperatures >100 °C in ∼70 s. The results suggest that rf heating of FeCo MNPs ...

Novel Solder-Magnetic Particle Composites and Their Reflow Using AC Magnetic Fields

Localized heating of solder interconnects in electronic packaging can help alleviate undesirable thermal stresses during the conventional reflow process, where the package is subjected to high temperatures. Localized heating is possible for conductors by electromagnetic induction of eddy currents which results in Joule heating, when subjected to AC magnetic fields. However, for typical solder pastes with fine solder powders dispersed in a flux medium, the eddy current losses, which have strong size dependence, become too small to cause significant heating at reasonable field amplitudes and frequencies. Magnetic materials exhibit losses from hysteretic and relaxation processes, in AC magnetic fields. We investigated the feasibility of solder paste reflow by localized heating of novel solder magnetic particle composites in AC magnetic fields. A solder paste magnetic nanoparticle (MNP) composite was prepared by mechanical mixing of FeCo MNPs with Type III Sn96.5/Ag3.0/Cu0.5 (SAC305) Pb-free solder paste. The pristine solder paste show an insignificant temperature rise when subjected to AC magnetic field of 500 Oe at 280 kHz, whereas the solder-MNP composite samples with particle concentration of 2 wt% or more were able to reflow due to magnetic heating. Here we report first demonstration of a new approach for solder melting in AC magnetic fields using MNPs.

Modeling of temperature profile during magnetic thermotherapy for cancer treatment

Magnetic nanoparticles (MNPs) used as heat sources for cancer thermotherapy have received much recent attention. While the mechanism for power dissipation in MNPs in a rf field is well understood, a challenge in moving to clinical trials is an inadequate understanding of the power dissipation in MNP-impregnated systems and the discrepancy between the predicted and observed heating rates in the same. Here we use the Rosensweig [J. Magn. Magn. Mater. 252, 370 (2002)] model for heat generation in a single MNP, considering immediate heating of the MNPs, and the double spherical-shell heat transfer equations developed by Andra et al. [J. Magn. Magn. Mater. 194, 197 (1999)] to model the heat distribution in and around a ferrofluid sample or a tumor impregnated with MNPs. We model the heat generated at the edge of a 2.15 cm spherical sample of FeCo/(Fe,Co)3O4 agglomerates containing 95 vol % MNPs with mean radius of 9 nm, dispersed at 1.5–1.6 vol % in bisphenol F. We match the model against experimental data for...

High Induction, Low Loss FeCo-Based Nanocomposite Alloys With Reduced Metalloid Content

We present a comparative study of reductions in metalloid and early transition metal content aimed at increasing inductions in FeCo-based nanocomposite alloys. Metalloid B and early transition metal Nb glass formers were replaced with magnetic late transition metals to determine the limits of amorphous and nanocrystalline formation and increase the magnetic flux density. Alloys of composition (Fe65Co35)80+x+yB13-xNb4-ySi2Cu1 (x=0-3, y=0,3) were cast by melt-spinning 50g batches into 2.5 mm wide, 30 μm thick ribbons. Ribbons were nanocrystallized and screened for their magnetic flux densities. The highest magnetic flux density of 1.85 T for quality ribbon was realized for the alloy with x=1, y=3. Magnetization as a function of temperature is reported for amorphous alloys to illustrate the effects of metalloid and early transition metal content on high temperature stability of magnetic properties and crystallization temperatures. Notable alloys were also synthesized by planar flow casting into 2.54 cm wide, 20 μ m ribbons and annealed under a 2.0 T transverse magnetic field. AC magnetic property measurements include permeability, coercive field, and core loss. The x=3, y=0 alloy demonstrated low core loss of 5.1 W/kg under a 0.2 T field and 20 kHz frequency with a high flux density of 1.70 T.

Increased induction in FeCo-based nanocomposite materials with reduced early transition metal growth inhibitors

We report on new high-saturation induction, high-temperature nanocomposite alloys with reduced glass formers. The amounts of the magnetic transition metals and early transition metal growth inhibitors were systematically varied to determine trade-offs between higher inductions and fine microstructures with consequently lower magnetic losses. Alloys of nominal composition (Fe65Co35)79.5+xNb4−xB13Si2Cu1.5 (x=0–4) were cast into a 28 mm wide, 20 μm thick ribbon from which toroidal cores were wound. Inductions and magnetic losses were measured after nanocrystallization and stress relief. We report technical magnetic properties: permeability, maximum induction, remanence ratio, coercive field, and high frequency magnetic losses as a function of composition and annealing temperature for these alloys. Of note is the development of maximum inductions in excess of 1.76 T in cores made of alloys with the x=4 composition and maximum inductions in excess of 1.67 T in alloys with the x=3 composition, which also exhibi...

Magnetism of (Sm,R)2Fe17Ny (R=Y, Tb or mischmetal)

Magnetic properties are reported for nitrides of the formula (Sm1−xRx)2Fe17Ny, where R=Y, Tb, or mischmetal and y=2.5 to 2.8. Substantial replacement of Sm by R is observed for Sm2Fe17−60% by mischmetal and 100% by Y or Tb. In all cases nitrogenation expands the lattice and increases Tc by 300 to 400 K. Magnetization decreases as Sm is replaced by Tb but increases when Y is the dopant. The latter implies antiferromagnetic coupling in Sm2Fe17. The uniaxial anisotropy observed for Sm2Fe17Ny is weakened by replacing Sm with Tb or Y. The weakening is greater in the case of Tb; this follows since the Tb crystal field interaction opposes that of Sm, whereas Y acts essentially as a mere diluent. If dilution were the only effect, HA would fall linearly with composition. Behavior approaching this is observed. Some of the systems appear to be of interest for permanent magnet fabrication.

Metastable γ-FeNi nanostructures with tunable Curie temperature

We report on new metastable γ-FeNi nanoparticles produced by mechanical alloying of melt-spun ribbon using a high energy ball mill followed by a solution annealing treatment in the γ-phase region and water quenching in of the face-centered cubic γ-phase. In the Fe–Ni phase diagram there is a strong compositional dependence of the Curie temperature, Tc, on composition in the γ-phase. This work studies the stabilization of γ-phase nanostructures and the compositional tuning of Tc in Fe–Ni alloys which can have important ramifications on the self-regulated heating of magnetic nanoparticles in temperature ranges of interest for applications in polymer curing and cancer thermotherapies. To date we have achieved Curie temperatures as low as 120 °C by this method.

Fe–Co–Cr nanocomposites for application in self-regulated rf heating

Fe–Co–Cr alloys have been developed with a Curie temperature, Tc, appropriate for ferrofluid cooling and self-regulated heating applications. These alloys have low Curie temperatures, moderate magnetic moments and provide increased heat capacity in a liquid used in a thermal cycle. Amorphous powders have been synthesized by cryo-SPEX milling melt-spun ribbons at 77 K. Transmission electron microscopy reveals cryomilled magnetic nanoparticles (MNPs) with a mean diameter of 4.2 nm to form agglomerates ∼30 nm in size. Vibrating sample magnetometer and superconducting quantum interference device magnetometry of amorphous powders reveal a specific magnetization, σs, of 104 emu/g at 4 K in a 300 mT field and a Tc of 335 K. Nanoparticles were suspended in ferrofluids by ultrasonication with a Pluronic F127 surfactant to stabilize them in aqueous solution. Ferrofluids of varying MNP concentration were rf heated in a 27.2 mT field at 267 kHz. For 1.24 vol % of MNPs in the ferrofluid, a solution reached temperature...