Z. Turgut

Neutron Powder Diffraction of Carbon-Coated FeCo Alloy Nanoparticles

Neutron powder diffraction is used to study the order–disorder transformation in carbon-coated FexCo1−x nanoparticles produced using a radio frequency plasma torch. The nanoparticles, nominally Fe50Co50, are produced from alloy powder and acetylene precursors by gas-phase nucleation from the plasma. The resulting nanoparticles undergo an order–disorder transformation near 730u200a°C, passing from an ordered B2 (CsCl) structure to a disordered A1 (body-centered-cubic) structure upon heating, similar to the transformation seen in bulk equiatomic FeCo. Although it is very difficult to quench the disordered state in bulk samples, the extreme cooling rates present in the plasma reactor produce metastable disordered nanoparticles. Neutron powder diffractograms acquired during a heating–cooling cycle at 27, 500, 710, 800, 710, and 400u200a°C indicate the particles relax to their equilibrium ordered state upon heating first, disorder as they pass through the transformation temperature, and reorder upon cooling.

Nanocrystalline materials for high temperature soft magnetic applications: A current prospectus

Conventional physical metallurgy approaches to improve soft ferromagnetic properties involve tailoring chemistry and optimizing microstructure. Alloy design involves consideration of induction and Curie temperatures. Significant in the tailoring of microstructure is the recognition that the coercivity, (Hc) is roughly inversely proportional to the grain size (Dg) for grain sizes exceeding ∼0·1−1 µm (where the grain size exceeds the Bloch wall thickness,δ). In such cases grain boundaries act as impediments to domain wall motion, and thus fine-grained materials are usually harder than large-grained materials. Significant recent development in the understanding of magnetic coercivity mechanisms have led to the realization that for very small grain sizesDg<∼100 nm,Hc decreases sharply with decreasing grain size. This can be rationalized by the extension of random anisotropy models that were first suggested to explain the magnetic softness of transition-metal-based amorphous alloys. This important concept suggests that nanocrystalline and amorphous alloys have significant potential as soft magnetic materials. In this paper we have discussed routes to produce interesting nanocrystalline magnets. These include plasma (arc) production followed by compaction and primary crystallization of metallic glasses. A new class of nanocrystalline magnetic materials, HITPERM, having high permeabilities at high temperatures have also been discussed.

Fully dense anisotropic nanocomposite Sm(Co,Fe,Zr,Cu,B)z (z=7.5–12) magnets

Fully dense anisotropic nanocomposite Sm(Co0.58Fe0.31Zr0.05Cu0.04B0.02)z (z=7.5–12) magnets have been synthesized via rapid hot pressing and hot deformation processes. The highest (BH)max∼10.6MGOe was observed for a magnet with z=10. X-ray diffraction and M-H measurements indicated that the easy magnetization direction of magnets prefers to be in the hot pressing direction. Transmission electron microscopy investigation confirmed that plastic deformation is an important route for forming magnetic anisotropy in the Sm–Co-type nanocomposite magnets. Some stripe and/or platelike patterns have been observed inside the nanograins (50–200nm), which may present as twins, and stacking faults. The (0001) twins have been observed in the 2:17R phase.Fully dense anisotropic nanocomposite Sm(Co0.58Fe0.31Zr0.05Cu0.04B0.02)z (z=7.5–12) magnets have been synthesized via rapid hot pressing and hot deformation processes. The highest (BH)max∼10.6MGOe was observed for a magnet with z=10. X-ray diffraction and M-H measurements indicated that the easy magnetization direction of magnets prefers to be in the hot pressing direction. Transmission electron microscopy investigation confirmed that plastic deformation is an important route for forming magnetic anisotropy in the Sm–Co-type nanocomposite magnets. Some stripe and/or platelike patterns have been observed inside the nanograins (50–200nm), which may present as twins, and stacking faults. The (0001) twins have been observed in the 2:17R phase.

Microstructural and magnetic observations of compacted FECOV nanoparticles

One attempt to improve core loss of electromagnetic machines is to utilize nanocrystalline alloys, which are predicted to have extremely soft magnetic properties. High magnetic saturation values and high Curie temperatures make iron-cobalt alloys attractive for such applications. In this work iron-cobalt-vanadium nanopowders were synthesized and compacted. The coated particles were characterized and examined in both the powder and compacted states. Higher than expected coercivities are reported and may be due to the inhomogeneous microstructure resulting from interactions with the vanadium.

Crystallization and Nanocrystallization Kinetics of Fe-Based Amorphous Alloys

In this work we describe crystallization kinetics as inferred from time-dependent magnetization studies and thermal analysis for an Allied Signal amorphous Fe-based METGLAS® 2605SA-1 alloy and a NANOPERM (Fe 88 Zr 7 B 4 Cu 1 ) alloy. We illustrate and contrast several phenomena important to understanding crystallization kinetics in particular to the NANOPERM alloy system. In METGLAS® 2605SA-1 primary and secondary crystallization events are observed in differential scanning calorimetry data (DSC) at temperatures of 504 °C and 549 °C, respectively for data taken at a 10 °C/min scan rate. Both temperatures are greater than the Curie temperature of the amorphous alloy. For the NANOPERM alloy primary crystallization (as determined from differential thermal analysis (DTA)) occurs at 500 °C and secondary crystallization at 730 °C and M(t) at temperatures near the primary crystallization temperature is dominated (at short times

Effect of high-temperature aging on electrical properties of Hiperco® 27, Hiperco® 50, and Hiperco® 50 HS alloys

Some more electric aircraft concepts require soft magnetic FeCo materials to be stable at temperatures as high as 773 K for long periods of time. At this high operating temperature, aging related processes may occur that result in changes in material properties. The material supplier typically specifies only room-temperature properties, and only limited reports are available on properties at elevated temperatures. The change in properties as a function of time at 773 K will give information on the lifetime of the material to assist designers when selecting materials for high-temperature applications. We have conducted a study on the effects of long-term aging on the magnetic, mechanical, and electrical properties of Hiperco® 27, Hiperco® 50, and Hiperco® 50 HS FeCo soft magnetic alloys. Samples of each material were aged in argon for 100, 1000, 2000, and 5000 h at 773 K. Here, we report the changes in electrical resistivity after aging. Of the three alloys, high-temperature aging has the greatest effect o...

Fe–Co–V alloy with improved magnetic properties and high-temperature creep resistance

Advanced power systems require soft magnetic materials with a combination of high saturation magnetization and high creep resistance. When the Fe–Co–V alloy laminate is used in a rotor at high temperatures (500–600u200a°C) coupled with very high rpm, significant creep occurs, which destroys the device integrity. Since grain boundary slide is predominantly responsible for creep deformation in the Fe–Co–V alloy at temperature higher than ∼430u200a°C, the approach in this study was to reduce the volume fraction of the grain boundaries by making a Fe–Co–V alloy with very large grains. Very large grains, up to mm range, were readily obtained after small cold deformation of ∼3% followed by a normal recrystallization anneal. Fe–Co–V alloy with large grains displays lower coercivity and higher permeability than the commercial Fe–Co–V alloy. Even though its yield strength at 600u200a°C is lower than the commercial Fe–Co–V, the creep strains of the Fe–Co–V alloy with large grains are only 1/10–1/2 of that for the commercial al...

Carbon Coated Nanoparticle Composites Synthesized in an RF Plasma Torch

FeCo alloy nanoparticles are synthesized in an RF plasma torch reactor and characterized using X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). Bare, uncoated particles exhibit a chain-like agglomeration morphology marked by large ring- and bridge-like structures surrounding open voids. Acetylene was used to generate large numbers of carbon-coated nanoparticles similar to those produced in carbon arc reactors. Conventional TEM of this powder revealed numerous particles below 50 nm in diameter embedded in a carbonaceous matrix. These results establish RF plasma torch processing as a well-characterized, scalable alternative to carbon arc synthesis of encapsulated nanoparticles.

Magnetic properties of FeCo laminates subjected to axial loading

While manufacturing the stator/rotor assemblies of the aircraft power components, increased power losses are typically pronounced due to an imposed axial load onto the stack of magnetic laminates to increase the rigidity of the stack. An axial loader to enable core loss measurements to be made while the laminate stack is subjected to an axial load has been designed. The apparatus and multilayered slotted interface plates, allowing for the toroidal windings and simultaneous load application were used to evaluate the effect of compression up to 27.5 MPa on magnetic properties of commercially available Fe–Co based Hiperco® 50, Hiperco® 50 HS, and Hiperco® 27 alloys. For each composition, we tested two sets of samples: (1) completely insulated and (2) uninsulated. We report the increased losses due to an axial load and discuss the origin of these increased losses in terms of constant size anisotropy for completely insulated laminates.

Structure and magnetic properties of Sm(Co/sub bal/Fe/sub x/Zr/sub 0.05/Cu/sub 0.08/Ga/sub y/B/sub z/)/sub 12/ alloys

3d-rich alloys and their melt-spun materials with nominal compositions of Sm(Co/sub bal/Fe/sub x/Zr/sub 0.05/Cu/sub 0.08/Ga/sub y/B/sub z/)/sub 12/ (x=0.1-0.41, y=0-0.01, z=0.01-0.02) were synthesized and characterized in the temperature range of 10-1473 K and at fields up to 5 T. The main phase of the as-cast alloys was formed in a Th/sub 2/Ni/sub 17/ type structure, exhibiting a strong uniaxial anisotropy. As a result, encouraging hard magnetic properties with T/sub c/=995-1086 K, H/sub a/=40-115 kOe, M/sub s/=10-12.8 kG at 300 K, and H/sub a/=60-180 kOe, M/sub s/=10.4-13.6 kG at 10 K were observed in the as-cast alloys. The melt-spun materials are nano-structured in nature and magnetically hard, even in the as-spun state. The following hard magnetic properties were observed: H/sub ci/=7-10 kOe, 4/spl pi/M/sub s/=9.4-11.7 kG, at 300 K, and H/sub ci/=17-27 kOe, 4/spl pi/M/sub s/=9.6-12.1 kG, at 10 K. The highest (BH)/sub max//spl sim/9.8 MGOe at 300 K was obtained from the Sm(Co/sub bal/Fe/sub 0.31/Zr/sub 0.05/Cu/sub 0.08/B/sub 0.02/)/sub 12/ ribbon material. A Henkel plot analysis indicates the existence of a strong exchange-coupling interaction between the magnetically hard and soft phases in these ribbon materials. The effects of adding Ga, B, and Fe on the magnetic properties will be discussed.