Nilay G. Akdogan

Rare earth-cobalt hard magnetic nanoparticles and nanoflakes by high-energy milling

High-energy ball milling has been shown to be a promising method for large-scale fabrication of rare earth-transition metal nanoparticles. In this work, we report crystallographically anisotropic SmCo(5), PrCo(5) and Sm(2)(Co, Fe)(17) nanoparticles (particle size smaller than 10 nm) obtained by surfactant-assisted ball milling and study their size and properties as a function of the milling conditions. By milling nanocrystalline precursor alloys, we obtained SmCo(5) platelets (flakes) approximately 100 nm thick with an aspect ratio as high as 10(2)-10(3). The unusual shape evolution of this brittle material is attributed to its increased plasticity in the nanocrystalline state. The nanoflakes are susceptible to re-crystallization annealing and exhibit a room-temperature coercivity of up to 19 kOe. The successful fabrication of rare earth-cobalt nanoparticles and ultra-thin flakes provides hope for the development of nanocomposite permanent magnets with an enhanced energy product.

Novel Nd2Fe14B nanoflakes and nanoparticles for the development of high energy nanocomposite magnets

High energy ball milling has been shown to be a promising method for large-scale fabrication of rare earth-transition metal nanoparticles. In this work, magnetically hard Nd-Fe-B nanopowders with a coercivity in the range of 1.2-4 kOe have been produced by surfactant-assisted ball milling of nanocrystalline precursor alloys. The nanopowders consisted of Nd(2)Fe(14)B flakes with a thickness below 100 nm and an aspect ratio as high as 10(2)-10(3) and anisotropic square nanoparticles with a size of 11 nm. Both the nanoparticles and nanoflakes showed a strong [001] out-of-plane texture. The nanoparticles showed a spin reorientation temperature which is lower (117 K) than the bulk value (135 K). The successful fabrication of Nd-Fe-B nano-thin flakes and anisotropic nanoparticles provides hope for the development of nanocomposite permanent magnets with an enhanced energy product.

Anisotropic Sm-(Co,Fe) nanoparticles by surfactant-assisted ball milling

Magnetically hard Sm2(Co0.8Fe0.2)17 and SmCo5 nanoparticles have been produced by using surfactant-assisted low- and high-energy ball milling techniques. Surfactants prevent the rewelding of the crashed particles during the milling process. Heptane was used as the milling medium and oleic acid as the surfactant. High-energy ball milling experiments took place in a milling vial with carbon steel balls by using an SPEX 8000M high-energy ball milling machine. The coercivity was found to increase with milling time with values of 2.3 kOe for Sm2(Co0.8Fe0.2)17 and 18.6 kOe for SmCo5 after 4 h of milling. Transmission electron microscopy data showed that the milled powders consisted of nanoparticles with an average size of 5–6 nm and a narrow size distribution. Samples deposited on copper coated carbon grid showed self-assembled nanoparticles which could be further aligned when subjected to a magnetic field.

Anisotropic PrCo

Magnetically hard PrCo5 nanoparticles have been produced by using surfactant-assisted high-energy ball milling techniques. The use of surfactant leads to the dispersion of the crashed particles and prevents them from rewelding during the milling process. Heptane was used as the milling medium, and oleic acid as the surfactant. High-energy ball milling experiments took place in a milling vial with steel balls by using a Spex 8000 M high-energy ball milling machine. The coercivity was found to increase with milling time with a value of 6.8 kOe after 12 h of milling. TEM data showed that the milled powders consist of nanoparticles with an average size of 8-10 nm.

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Anisotropic Nd2Fe14B nanoparticles and nanoflakes have been produced by surfactant-assisted high-energy ball milling (SA-HEBM) of precursor nanocrystalline alloys. A two-stage high-energy ball milling was performed to obtain the nanopowders and nanoparticles; first the coarse powders were subjected to a wet milling followed by a wet surfactant-assisted milling. Different shaped nanoparticles have been obtained by varying the time of the first stage of the milling process and then separated by sonication. For a surfactant-free wet milling of 4 h, followed by the SA-HEBM, the nanopowders consisted of a mixture of Nd2Fe14B flakes with a thickness below 200 nm and an aspect ratio as high as 102–103, and anisotropic square nanoparticles with a size of 10 nm. However, for a shorter wet milling, nearly spherical nanoparticles with a size of 2.7 nm were obtained. Low-temperature coercivities have been obtained with maximum values of 4 kOe for square nanoparticles and 2.5 kOe for the nearly spherical nanoparticles.

Nanoparticles by Surfactant-Assisted Ball Milling

The effects of powder preparation and conditions of hot pressing on the structure and density of consolidated Sm2Fe17Nx and Sm2Fe17Nx∕Fe–Co magnets have been studied. Densities achieved in the case of Sm2Fe17Nx powders prepared through a low-energy milling were higher than after a high-energy milling. The difference is mostly caused by the different particle size, and it can be eliminated by an additional low-energy milling of the coarse high-energy milled powder. The presence of a ductile soft magnetic phase greatly facilitates densification, leading to considerably higher absolute and relative densities in hot-pressed Sm2Fe17Nx∕Fe0.65Co0.35 composites. We have found that during hot pressing, nitrogenization of the Sm2Fe17 phase may occur in situ if pressed together with Fe–Co–N powders. Because Fe–Co–N releases nitrogen below the decomposition temperature of Sm2Fe17Nx, we were able to fabricate the Sm2Fe17Nx∕Fe0.65Co0.35 composite with the density up to 7.6g∕cm3 by hot-pressing mixtures of Sm2Fe17 and (...