Anisotropic Nd$_{2}$Fe$_{14}$B Submicron Flakes by Non-Surfactant-Assisted High Energy Ball Milling

Abstract

This paper reports on the formation mechanism, structure, crystallographic anisotropy, magnetic properties and postannealing of single-crystal and [001] textured poly-nanocrystalline Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> B flakes with a micron, submicron or nanosize thickness prepared by a non-surfactant-assisted high energy ball milling (HEBM) technique. Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15.5</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">78.5</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> ingot micro-powders were the precursor while ethanol was the only milling medium. Similar to the structural evolution process in the surfactant-assisted HEBM, single-crystal flakes with a micron then submicron thickness were first formed. With further milling, [001] textured poly-nanocrystalline flakes with a submicron then nanosize thickness were formed. The formation of anisotropic flakes was mainly related to the polar nature of ethanol which would form a thin coating layer on the flakes through carboxylate bonds to prevent cold welding and agglomeration during the HEBM. The Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> B flakes prepared by the HEBM in ethanol for 5 h have a [001]-in-plane texture, a thickness in the range of 100-450 nm and a width in the range of 0.7-18 μ m, a coercivity <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 2.3 kOe, and a I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">006</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">105</sub> value of Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> B phase of 5.3. Postannealing at 400-550 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C for 0.5 h had little effect on the flake dimensions while resulting in a reduced [001] texture and reduced remanence values besides coarser grain sizes. With increasing annealing temperature, <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> first increased, reached its maximum value of 2.8 kOe at 450 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, then decreased. An anisotropic magnetic behavior was found in all of the as-milled and annealed flakes.