Mechanochemical synthesis of rare earth 3D transition metal submicron particles with the R2Fe14B structure
Date
2018
Authors
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Publisher
University of Delaware
Abstract
The desire for more energy efficient devices with smaller size and volume, drives many researchers to explore the limits of functional materials in the smaller size. The effect of such a drive on permanent magnet materials has led to research on the development of exchange-coupled nanocomposite magnets consisting of a fine mixture of hard and soft phase particles. Particularly, the production of hard-magnetic nanoparticles in the single domain critical size has attracted much attention since the hard magnetic properties of such particles are the highest in this size. In this thesis work, we employed a bottom-up fabrication method, mechanochemical synthesis, for the synthesis of hard magnetic R2Fe14B particles with R=Nd, Pr, Dy. ☐ Mechanochemistry is defined as a branch of chemistry where mechanical energy is utilized to perform chemical and physiochemical transformations on the material. The widely used tool for mechanochemistry is high energy ball milling without using any solvents. The fabrication of R2Fe14B particles is done by mechanochemical synthesis starting with the precursor oxides (R-O, Fe2O3, B2O3) and using metallic calcium as the reduction agent in an inert gas environment. CaO is used as the dispersant material and the synthesis of the particles is completed by a heat treatment at temperatures in the range of 800-900 °C. The average size of the particles is 100-210 nm for Nd2Fe14B, 200-380 nm for Pr2Fe14B and 70-150 nm for Dy2Fe14B particles, depending on the amount of reduction agent, the synthesis temperature and duration of synthesis. ☐ The coercivities of the as-synthesized powders are 12.3 kOe for Nd2Fe14B, 13.6 kOe for Pr2Fe14B, and 41.6 kOe for Dy2Fe14B samples. Following the growth of the particles by the heat treatment, the dispersant is removed via a multistep washing procedure. Due to the use of water, the magnetically collected particles are saturated with interstitial hydrogen; the resulting R2Fe14BHx phase has expanded lattice parameters as verified by the Rietveld analysis of the XRD patterns. The hydrogen diffusion to the R2Fe14B crystal structure results in an almost total loss of the hard magnetic properties for the Nd2Fe14BHx and Pr2Fe14BHx samples (H_c<"1 kOe" ) because of the low anisotropy of the hydrogenated 2:14:1 phase. On the other hand, the coercivity decrease due to the interstitial hydrogen in the Dy2Fe14BHx samples is not as drastic. The coercivity values are partially recovered by a dehydrogenation step which involves annealing of the particles under continuous vacuum at temperatures 300-500 °C. The dehydrogenated samples have H_c= 5.9 kOe and 5.7 kOe for Nd2Fe14B and Pr2Fe14B particles, respectively. ☐ The magnetic properties of the dehydrogenated samples are still lower than the corresponding bulk materials; and this may be due to the oxygen rich amorphous layer on the particle surface as seen in HRTEM images. In order to revive the coercivity, infiltration is conducted with a rare earth rich eutectic alloy Pr3Co0.75Cu0.25 on the Nd2Fe14B and Pr2Fe14B particles. The results indicated that the H_c has increased to about 14 kOe and 16.5 kOe, respectively, indicating that the oxide layer can be removed by infiltration. ☐ 57Fe Mössbauer spectroscopy measurements of the Pr2Fe14B samples are conducted on the powders from different stages of the synthesis. The results have shown that the six different iron sites in the R2Fe14B crystal unit cell have the expected order in magnitude suggested by the Wigner-Seitz cell analysis. However, the hyperfine splitting values are lower than those of ingots due to finite size and surface effects. The weighted averages are 324.3 kOe, 330.6 kOe, and 324.7 kOe for the as-synthesized, dehydrogenated, and infiltrated Pr2Fe14B samples. ☐ High field magnetization measurements of the samples enabled the calculation of anisotropy constants and saturation magnetizations M_s. The anisotropy constants and M_s of the samples are still lower than the values of the bulk materials. The anisotropy constant of bulk Nd2Fe14B is about 5.0 ×〖10〗^7 "erg/" 〖"cm" 〗^"3" . However, the following values were obtained in the nanoparticles: K_1=4.86 ×〖10〗^7 "erg/" 〖"cm" 〗^"3" for as-synthesized, 1.36 ×〖10〗^7 "erg/" 〖"cm" 〗^"3" for washed, 4.45 ×〖10〗^7 "erg/" 〖"cm" 〗^"3" for dehydrogenated, and 4.67 ×〖10〗^7 "erg/" 〖"cm" 〗^"3" for infiltrated Nd2Fe14B powders. The even lower K_1 for the washed particles is expected because of interstitial hydrogen. Even when using the values of determined for M_s and K on the nanoparticles and the Stoner Wohlfarth model of coherent rotation, the calculated coercivities are much higher than those measured indicating that surface defects and disorder are responsible for the observed low coercivities.
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Keywords
Pure sciences, Applied sciences, Magnetic particles, Mechanochemistry, Rare earth metals