Zafer Turgut

Magnetic and Vibrational Properties of High-Entropy Alloys

The magnetic properties of high-entropy alloys based on equimolar FeCoCrNi were investigated using vibrating sample magnetometry to determine their usefulness in high-temperature magnetic applications. Nuclear resonant inelastic x-ray scattering measurements were performed to evaluate the vibrational entropy of the 57Fe atoms and to infer chemical order. The configurational and vibrational entropy of alloying are discussed as they apply to these high-entropy alloys.

Coupled Inductor Characterization for a High Performance Interleaved Boost Converter

Interleaved power converter topologies have received increasing attention in recent years for high power and high performance applications. The advantages of interleaved boost converters include increased efficiency, reduced size, reduced electromagnetic emission, faster transient response, and improved reliability. The front end inductors in an interleaved boost converter are magnetically coupled to improve electrical performance and reduce size and weight. Compared to a direct coupled configuration, inverse coupling provides the advantages of lower inductor ripple current and negligible dc flux levels in the core. In this paper, we explore the possible advantages of core geometry on core losses and converter efficiency. Analysis of FEA simulation and empirical characterization data indicates a potential superiority of a square core, with symmetric 45deg energy storage corner gaps, for providing both ac flux balance and maximum dc flux cancellation when wound in an inverse coupled configuration.

Thermomagnetic analysis of FeCoCrxNi alloys: Magnetic entropy of high-entropy alloys

The equimolar alloy FeCoCrNi, a high-entropy alloy, forms in the face-centered-cubic crystal structure and has a ferromagnetic Curie temperature of 130 K. In this study, we explore the effects of Cr concentration, cold-rolling, and subsequent heat treatments on the magnetic properties of FeCoCrxNi alloys. Cr reductions result in an increase of the Curie temperature, and may be used to tune the TC over a very large temperature range. The magnetic entropy change for a change in applied field of 2T is ΔSm = −0.35 J/(kg K) for cold-rolled FeCoCrNi. Cold-rolling results in a broadening of ΔSm, where subsequent heat treatment at 1073 K sharpens the magnetic entropy curve. In all of the alloys, we find that upon heating (after cold-rolling) there is a re-entrant magnetic moment near 730 K. This feature is much less pronounced in the as-cast samples (without cold-rolling) and in the Cr-rich samples, and is no longer observed after annealing at 1073 K. Possible origins of this behavior are discussed.

Structural investigations and magnetic properties of sol-gel Ni0.5Zn0.5Fe2O4 thin films for microwave heating

Nanocrystalline Ni0.5Zn0.5Fe2O4 thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology, magnetic, and microwave absorption properties of the films calcined in the 673–1073 K range were studied with x-ray diffraction, scanning electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, vibrating sample magnetometry, and evanescent microwave microscopy. All films were uniform without microcracks. Increasing the calcination temperature from 873 to 1073 K and time from 1 to 3 h resulted in an increase of the grain size from 12 to 27 nm. The saturation and remnant magnetization increased with increasing the grain size, while the coercivity demonstrated a maximum near a critical grain size of 21 nm due to the transition from monodomain to multidomain behavior. The complex permittivity of the Ni–Zn ferrite films was measured in the frequency range of 2–15 GHz. The heating behavior was studied in a multimode microwave cavity at 2.4 GHz. The highest microwave heating rate in the temperature range of 315–355 K was observed in the film close to the critical grain size.

Persistent current in coils made out of second generation high temperature superconductor wire.

We report the results of an experimental study of a persistent coil made out of YBa2Cu3O7−δ coated conductors. The magnitude of the persistent current and the rate of decay were investigated. Two distinct modes of relaxation are evident—one is flux creep and the other, which is much faster, is of less obvious origin. Our conclusion is that the persistent current in such a coil can be large enough and decay slowly enough so that coated conductors can be used to make persistent coils for variety of applications.

Effect of aging on magnetic properties of Hiperco® 27, Hiperco® 50, and Hiperco 50 HS® alloys

We started a long-term aging study that will identify the aging related changes in magnetic, mechanical, and electrical properties of three Fe–Co soft magnetic alloys. We performed the aging at a temperature of 773 K and in two different environmental chambers, argon gas and air, in order to determine the oxidation resistance of these alloy laminates as well. Each aging batch includes creep and yield stress test specimens, rings for ac magnetic measurements and specimens for electrical resistivity and microstructural analysis. Here we report the change in total power losses after 2000 h annealing up to frequencies of 2 kHz for the Hiperco® 27, Hiperco® 50, and Hiperco® 50 HS alloys. We also report the temperature dependence of total power losses between 298 and 773 K for these alloys.

Effects of chemical composition and B2 order on phonons in bcc Fe–Co alloys

The phonon density of states (DOS) gives insight into interatomic forces and provides the vibrational entropy, making it a key thermodynamic function for understanding alloy phase transformations. Nuclear resonant inelastic x-ray scattering and inelastic neutron scattering were used to measure the chemical dependence of the DOS of bcc Fe–Co alloys. For the equiatomic alloy, the A2→B2 (chemically disordered→chemically ordered) phase transformation caused measurable changes in the phonon spectrum. The measured change in vibrational entropy upon ordering was −0.02±0.02 k_B/atom, suggesting that vibrational entropy results in a reduction in the order–disorder transition temperature by 60±60 K. The Connolly–Williams cluster inversion method was used to obtain interaction DOS (IDOS) curves that show how point and pair variables altered the phonon DOS of disordered bcc Fe–Co alloys. These IDOS curves accurately captured the change in the phonon DOS and vibrational entropy of the B2 ordering transition.

High strength bulk Fe–Co alloys produced by powder metallurgy

Fe–Co alloys are extensively used in lamination form, but there are certain power generation applications that require Fe–Co rotors in bulk form. Experiencing only a dc magnetic field, these rotors can be as large as 0.5m in diameter, depending on the size of the generator. The forging of such large pieces of Fe–Co has proven to be difficult. The present study investigates powder metallurgy processing of a gas atomized FeCoNbV alloy through hot isostatic pressing (HIP) for manufacturing large size rotors with improved mechanical strength. Gas atomized FeCoNbV alloy powders with and without ball milling were hot isostatic pressed at temperatures between 675 and 850°C at a fixed pressure of 193MPa for up to 6h. Ball milling prior to HIP improved the yield strength. A further improvement in yield strength and in ductility was obtained after a disordering heat treatment at 730°C followed by a rapid quench to room temperature. The optimum HIP and annealing conditions resulted in samples with yield strengths of...

Effects of Zr, Nb, and Cu substitutions on magnetic properties of melt-spun and hot deformed bulk anisotropic nanocomposite SmCo type magnets

Structure and magnetic properties of both melt-spun and hot deformed bulk Sm–Co type nanocomposite magnets have been investigated with various metal additions, including Zr, Cu, and Nb. The Zr and Nb additions play important roles in constraining grain growth, resulting in an increase of coercivity Hc. The Cu addition significantly improves the squareness of BH loops as well as the energy product (BH)max. A typical hot deformed bulk anisotropic nanocomposite SmCo type magnet with Mr(hard)∕Mr(easy)∼0.4, Hc∼9kOe and (BH)max of 13.2MGOe was obtained.

Effect of surfactant molecular weight on particle morphology of SmCo5 prepared by high energy ball milling

Surfactant-assisted high energy ball milling (HEBM) is a widely used technique for producing nanostructured magnetic materials with oleic acid (OA) being the most commonly utilized surfactant reported in literature to date. No conclusive explanation has been presented for the wide use of OA and only a few studies have deviated from its use. OA has a boiling point of 360 °C which presents issues for complete removal of the surfactant after the HEBM process. Exposing the nanostructured materials to the high temperatures required for surfactant removal is known to result in grain growth and oxidation. In other studies, select surfactant systems, such as octanoic acid or oleylamine, have been used, however, a systematic study examining the dependence of surfactant selection on overall particle (flake) morphology has yet to be performed. In this study, we have qualitatively and quantitatively examined the effects of surfactant selection on the morphology and magnetic properties of SmCo5 utilizing surfactants with lower boiling points that are structurally similar to OA. Our results demonstrate that there was little change in the morphological and magnetic properties for the different surfactants. The implication is that lower boiling point surfactants may be used for HEBM, which require less severe conditions for surfactant removal after milling thereby preserving the integrity of the powders.