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Electro-Infiltrated Nickel/Iron-Oxide and Permalloy/Iron-Oxide Nanocomposites for Integrated Power Inductors

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Electro-Infiltrated Nickel/Iron-Oxide and Permalloy/Iron-Oxide Nanocomposites for Integrated Power Inductors Journal of Magnetism and Magnetic Materials 493 (2020) 165718 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Research articles Electro-infiltrated nickel/iron-oxide and permalloy/iron-oxide nanocomposites for integrated power inductors T ⁎ Connor S. Smitha, , Shehaab Savliwalab, Sara C. Millsc, Jennifer S. Andrewc, Carlos Rinaldib,d, David P. Arnolda a Department of Electrical and Computer Engineering, Gainesville, FL 32611, USA b Department of Chemical Engineering, Gainesville, FL 32611, USA c Department of Material Science and Engineering, Gainesville, FL 32611, USA d J. Crayton Pruitt Family Department of Biomedical Engineering, Gainesville, FL 32611, USA ARTICLE INFO ABSTRACT Keywords: Magnetic nanocomposite materials are of interest for applications including power inductors and transformers, Soft magnetic materials where a combined large bandwidth, low loss, and relatively high permeability are desired. This work demon- Electroplating strates the fabrication of nickel/iron-oxide nanocomposites, up to ~3 μm thick, and permalloy/iron-oxide na- Magnetic nanocomposites nocomposites, up to ~1 μm thick, using an electro-infiltration process, whereby the voids in an iron-oxide na- Magnetic nanoparticles noparticle film are filled with electroplated metal. Measurements show that the magnetic nanocomposites Power electronics exhibit hybrid magnetic properties: modestly high permeability and saturation attributed to the metal matrix phase, but with an increased bandwidth attributed to the iron-oxide inclusion phase. At 10 MHz, the nickel/iron- oxide nanocomposite material exhibits a relative permeability of ~23, with a loss tangent around 0.1; the permalloy/iron-oxide nanocomposite exhibits a relative permeability of ~136, with a loss tangent around 0.15. 1. Introduction multilayers (2–2 connectivity) are commonly fabricated using sput- tering or other thin-film deposition techniques [17,16,15,18,5,19,35]. Magnetic nanocomposites have great potential for improving the More complex composites with 0–3or1–3 connectivities are more power density and efficiency of compact power electronic devices by difficult to microfabricate. However, magnetic nanocomposites com- providing a relatively high permeability at high frequencies with low posed of a nanoparticle phase and a metal matrix phase have been made losses [24,25]. Specifically, the need for high permeability is critical for via cluster deposition [1,12] and magnetic composite electroplating devices such as power microinductors, which see an increase in in- [7,8]. These techniques are plagued by either expensive deposition ductance with an increase in permeability of their core materials. Op- tools (as seen in the cluster deposition method), or low particle fill erating such inductors at higher frequencies is also desirable, as their ratios (< 5%) (as seen in the composite electroplating method), re- power handling increases with frequency, as seen by sulting in nanocomposites that are less attractive for applications. 2 Pmax= VfBμ core sw s /2 , where Pmax is the maximum power handling However, the method of electro-infiltration, developed by Wen et al. capacity of the inductor, Vcore is the volume of the core, fsw is the and Hayashi et al., has shown promise as a path to creating magnetic switching frequency, Bs is the saturation magnetization, and μ is the nanocomposites in a cost-effective manner and with much higher fill magnetic permeability [21]. However, traditional microfabricated ratios (~60%) [10,33,32,31]. magnetic core materials usually see both a decrease in permeability and In this work, electro-infiltration was used to fabricate two different high losses at higher frequencies, so an extension of the operating magnetic nanocomposite materials: one composed of iron-oxide nano- bandwidth of magnetic core materials is of great interest [9]. particles (IONs) embedded in a nickel matrix, and the other composed A magnetic nanocomposite comprises two different material phases of IONs embedded in a Ni-Fe matrix (the target composition was where at least one of the phases has a characteristic length scale of Ni80Fe20, and hereafter is referred to as “NiFe” or “permalloy”). The ~100 nm. The typical goal of a magnetic nanocomposite is to achieve a structure of these composites was examined using scanning electron combination of desirable electromagnetic properties that are otherwise microscopy (SEM) and energy dispersive spectroscopy (EDS), which unachievable using single-phase materials. Simple nanocomposite help confirm that the IONs have been properly embedded in the ⁎ Corresponding author. E-mail address: connor.smith@ufl.edu (C.S. Smith). https://doi.org/10.1016/j.jmmm.2019.165718 Received 19 April 2019; Received in revised form 15 July 2019; Accepted 17 August 2019 Available online 24 August 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved. C.S. Smith, et al. Journal of Magnetism and Magnetic Materials 493 (2020) 165718 electroplated metals. Vibrating sample magnetometry (VSM) was used same. The particles were then suspended in toluene and equilibrium to measure the dc magnetic characteristics to determine if the compo- magnetization measurements were obtained at 300 K on a SQUID site acts as a hybrid of its components by showing a magnetic saturation MPMS 3 (Quantum Design) between −7 T to 7 T. The magnetization and coercivity between those of the components. Finally, an Agilent data was fitted using a Langevin-Chantrell to obtain a volume-weighted E4991A Material Analyzer was used to measure the complex perme- distribution of magnetic diameters for the particle suspension ability spectra of the composite and its constituent phases to investigate [4,14,26]. Igor Pro 6.3.7 was the program used to perform the regres- permeability and loss characteristics for potential applications in power sion. electronics. X-ray diffraction patterns were collected using a Panalytical X’pert powder diffractometer with a Cu anode (Kα radiation) and scintillation 2. Materials and methods detector (45 kV, 40 mA). The iron oxide nanoparticle sample was scanned with a step size of 0.008°. The phase of the particles was then 2.1. Iron-oxide nanoparticles determined via comparison to database patterns from the International Center for Diffraction Data (ICDD), with the following ID number for Iron oxide nanoparticles were synthesized using a two-step semi- magnetite – 98-004-4525. Scherrer’s formula, τkλβ= /cosθ, was then batch thermal decomposition method developed by Vreeland et al. used to calculate the crystallite size of the particles. Here, τ is the mean [29]. First, a stable, stoichiometrically defined precursor iron (III) crystallite size, k is the shape factor (assumed to be 0.9 for spherical oleate was synthesized, via a high temperature (320 °C) displacement particles), λ is the X-ray wavelength (1.54 Å), β is the full width half reaction between iron acetylacetonate and a stoichiometric excess of maximum value in radians, and θ is the Bragg angle. oleic acid (1:5 Fe: oleic acid molar ratio). For this, 20.53 g of Fe(acac)3 and 80.2 g of oleic acid were charged to a 500 ml round bottom reactor 2.2. Magnetic nanocomposites with continuous stirring at 350 rpm. A condenser was used to set up reflux in the system, as well as control the rate of off-gassing of the Electro-infiltration was used to microfabricate the nanocomposite system. The reactor was immersed in a molten metal bath and heated to structures, as developed and described by Wen et al. [33]. As seen in 320 °C using an underdamped PID control profile to prevent overshoot. Fig. 1, magnetic nanoparticles are drop-cast into molds formed on a Reaction endpoint was determined in previous experiments via FTIR silicon substrate, which are then dried in air at room temperature to analysis to be 35 mins after the reactor temperature crossed 300 °C. At form a porous particle film layer. Then, these molds are placed into an this point, the reactor was cooled by raising it out of the molten metal electroplating bath to infiltrate and fill the porous particle films with a and product was cooled under inert (argon) flow. The iron (III) oleate metal matrix and form a composite material. Compared to previous with 35–37% free oleic acid, and bridging coordination, was stored in work [33], the main modification to the electro-infiltration technique sealed vials at room temperature. here is the addition of a retaining magnet held near the backside of the Next, we used a semi-batch thermal decomposition reaction to particle mold sample during electroplating, which is believed to help synthesize a monodisperse population of iron oxide nanoparticles hold the magnetic nanoparticles in place as opposed to falling off into [28,29]. Briefly, an iron oleate with 35–37% free oleic acid and brid- the bath. In some samples, the particles will stay in place in the mold ging coordination was diluted in 1-octadecene to 0.22 M, and added at during plating, but more consistently, without the retaining magnet in a continuous flowrate of 12 ml/hr to a mixture of 10.03 g docosane and place, particles are visually seen to fall off in small (about < 0.5 mm) 6.23 g oleic acid (20 ml of initial reaction volume) in a round bottom pieces. Finally, the molds on the silicon substrate are removed, leaving reactor. The mixture was stirred continuously (via overhead stirring) at a composite magnetic material behind. Further details of this process 350 rpm, while immersed in a molten metal bath soaking at 350 °C. The are explained below. precursor drip was stopped at 5 h of reaction time, and the reaction was The modified electro-infiltration process begins with (100) p-type stopped by raising the reactor out of the molten metal. The product was silicon substrate that was sputtered with a 20 nm titanium adhesion recovered once cooled down. layer and a 100 nm gold layer, which acts as an electrode for the The nanoparticles were purified from the excess reactants using electroplating processing and has a low propensity to oxidize.
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