Preparation of coated iron powders by chemical plating

Abstract

Small iron particles can be produced by reduction of γ-Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> and α-FeOOH with hydrogen; they show outstanding magnetic properties and can be used for recording media. The main difficulty concerns the tendency of these particles to oxidize. We have studied a preparation process in two steps: a) reduction of iron oxides or oxy-hydroxides; b) chemical plating in the same furnace. The magnetic properties of the iron powders chiefly depend on the reduction step. The water content in the gas leaving the furnace has been shown to be the main parameter determining the obtainment of powders with high magnetic properties. The iron powders, which were pyrophoric, have been quenched in chemical plating bath of suitable formulation and coated with cobalt or copper. Typical magnetic properties are: saturation magnetization σ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</inf> = 130-165 emu/g, intrinsic coercivity <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</inf> H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> = 350-450 Oe; best magnetic properties: σ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</inf> =155 emu/g, <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</inf> H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> =700 Oe. The coated powders have been submitted to heat treatment in air saturated with water, showing a good resistance to oxidation.