Nd₂Fe₁₄C-based magnet with better permanent magnetic properties prepared by a simple mechanochemical method

Highlights: • HEBM in heptane, Nd-Fe alloy disproportionates into NdH_2+δ, α-Fe and Fe₃C. • Annealed to 850 °C in vacuum, NdH_2+δ decomposes and Nd₂Fe₁₄C phase is formed. • The H and C atoms come from the decomposition of heptane. • Adding B to Nd-Fe promotes the formation of Nd₂Fe₁₄C and suppresses Nd-C and α-Fe. - Abstract: Nd₂Fe₁₄C-based magnet is prepared by a mechanochemical method, namely high-energy ball-milling Nd₂Fe₁₁B_x (x = 0–0.15) alloy in heptane (C₇H₁₆), followed by annealing to 850 °C in vacuum. Under the action of high-energy ball-milling, Nd₂Fe₁₁B_x react with heptane to form NdH_2+δ, Fe-(CB), C, etc. H₂ is released and Nd₂Fe₁₇, Nd₂Fe₁₇C_x (x = 0–3), Nd₂Fe₁₄C, Nd carbides and α-Fe are formed in the subsequent annealing. C amount depends on ball-milling time t. Long time ball milling or high C content suppresses the formation of 2:17 phase and favors the formation of 2:14:1 phase in the final products. Excessive ball-milling results in the quick increase of α-Fe. The maximum of magnetically hard Nd₂Fe₁₄C is obtained at t = 4 h. For Nd₂Fe₁₁ samples, there exists considerable quantity of Nd carbides and α-Fe phase appears earlier and increases rapidly with extending the ball-milling time t. The addition of B element shortens the ball-milling time of the formation of maximum Nd₂Fe₁₄C and prominently suppresses the formation of Nd carbide and α-Fe. The optimum magnetic properties, coercivity _iH_c of 1193.7 kA/m, remanence M_r of 580.9 kA/m, maximum magnetic energy product (BH)_max of 91.7 kJ/m³ is approaching to its theoretic value of 99.2 kJ/m³ for isotropic Nd₂Fe₁₄C magnet, are obtained in Nd₂Fe₁₁B_0.06 alloy ball milled for 3.5 h.