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

• Prepared single crystal Nd2Fe14BHx and Nd2Fe14B particles around 100 nm in size.
• Nd2Fe14B particles embedded in CaO matrix exhibit coercivity of 14.7 kOe.
• Coercivity of free Nd2Fe14BHx particles can be tuned between 1.4 and 4.6 kOe.
• Freeing Nd2Fe14B particles from the matrix costs 70% of the coercivity.

Polydispersed single crystal particles of the Nd2Fe14B and (Nd0.75Dy0.25)2Fe14B 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 Hc 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 Hc 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 Hc 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 Hc can be expected if the 2:14:1 particles are coated with or sintered in the presence of a rare-earth-rich phase.

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