Mechanochemical synthesis of fine R2Fe14BHx and R2Fe14B powders with R = Nd or Nd-Dy

Polydispersed single crystal particles of the Nd2Fe14B and (Nd0.75Dy0.25)(2)Fe14B compounds with a short axes in the 60-140 nm range were prepared by high-energy ball milling and subsequent annealing of rare-earth oxides, iron oxide and boron oxide in the presence of a calcium reducing agent and a calcium oxide dispersant. The particles embedded in the CaO/Ca matrix in the presence of excess rare earth exhibit coercivity H-c of 14.7 and 18.9 kOe. Removal of the matrix and excess rare earth is accompanied by interstitial modification of the 2:14:1 compounds with hydrogen; the H-c of the Dy-free and Dy-containing particles declines with the washing to 1.4 and 3.2 kOe, respectively. After desorption of the hydrogen through vacuum annealing, the H-c increases, but only to 4.6 and 6.2 kOe. This incomplete recovery is tentatively attributed to local anisotropy defects caused by loss of the rare earth atoms from the particle surfaces. If correct, this model implies that a much higher H-c can be expected if the 2:14:1 particles are coated with or sintered in the presence of a rare-earth-rich phase. (C) 2013 Elsevier B.V. All rights reserved.