Compression molding of anisotropic NdFeB bonded magnets in a polycarbonate matrix

Abstract

Anisotropic bonded Nd₂Fe₁₄B (NdFeB) magnets in a polycarbonate (PC) binder matrix are fabricated using a compression molding process. The weight fractions (w.f.) of NdFeB in PC on the batch mixer are 20, 50, 75, 85 and 95% compared to the twin screw extruder with 20, 50 and 75% respectively. The density of the 95% batch mixed magnets fabricated was 5.34 g/cm³ and the magnetic properties are, intrinsic coercivity H_ci = 942.99 kA/m, remanence B_r = 0.86 T, and energy product (BH)_max = 120.96 kJ/m³. The measured tensile properties are in the range of 27-59 MPa, comparable to that of polyamide (PA), polyphenylene sulfide (PPS) bonded magnets and demonstrating potential for bonded magnet applications. Scanning electron microscopy showed that the onset of failure occurs in the magnetic particle- matrix interface. This study demonstrates that compression additive molding technique can be used to fabricate high performance NdFeB polycarbonate composite magnets with improved mechanical properties.

Introduction

Permanent magnets are omnipresent in a broad range of industries. They are used in devices like actuators, transducers, sensors, magnetic resonance imaging (MRI) machines as well as consumer goods such as speakers and personal computers [1]. A typical automobile uses permanent magnets in starter motors, seat adjusters, wipers and traction motors in hybrid vehicles in various proportions [2,3]. Neodymium magnets are widely used rare earth magnets and exhibit the highest magnetic strength among all commercially available permanent magnets, up to ten times greater than conventional ferrite magnets [4]. They are comprised of Nd₂Fe₁₄B intermetallic compound as their main phase, which has a unique tetragonal structure with the easy axis parallel to the c-axis [5]. The unique crystal structure contributes to large uniaxial anisotropy and exceptional magnetic properties of the compound having a remanence (B_r) of 1.4 T, intrinsic coercivity (H_ci) of 2000 kA/m and maximum energy product (BH_max) as high as 440 kJ/m³ [3]. General Motors and Sumitomo Special Metals first developed neodymium magnets independently [6,7]. The research was motivated by the high cost of Samarium-cobalt (SmCo), another critical magnet in the first generation of the rare earth family. Since then extensive research has been conducted in improving intrinsic properties and manufacturing techniques of neodymium based permanent magnets [8]. NdFeB magnets are classified into two broad categories of sintered and bonded magnets. Until late 1990’s, sintered magnets were produced through powder metallurgy. More recently, strip casting techniques is dominating higher grade NdFeB magnets [9,10]. This is followed by sintering and heat treatment. The precursor for sintering involves conventional metallurgy powder, melt spun microcrystalline material produced by rapid solidification and crushed casted strips depending upon the performance needed for producing sintered magnets. Precursors (or flakes) are grounded to particles by fine grinding methods like jet milling. Advanced techniques such as atomization and hydrogen decrepitation deabsorbation recombination (HDDR) are used to produce nanocrystalline phased materials with desired morphologies [11]. While sintered magnets retain their full density and magnetic strength, they have issues of brittleness, poor and corrosion.