Compression molding of anisotropic NdFeB bonded magnets in a polycarbonate matrix
Graphical abstract
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 (Br) of 1.4 T, intrinsic coercivity (Hci) of 2000 kA/m and maximum energy product (BHmax) 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.
Section snippets
Polymer bonded magnets
Polymer bonded magnets (PBM's) offer advantages such as intricate geometry manufacturing, enhanced thermal stability, corrosion resistance and high mechanical properties [12]. Bonded magnets require the use of a polymer binder system. Typically, bonded magnets have an intermediate energy product (79.58-143.24 kJ/m3) and lower density [13,14]. The strength of bonded magnets depends on the volume fraction of the magnetic compound by a squared proportionality, given by the following relation [15]:
Materials and methods
Two types of compounding equipment- a low volume (100 – 200 g) lab scale batch mixer (Brabender Plasticorder W50) and a high throughput (10 kg/hr) twin screw extruder (Berstorff Z25) were employed for melt processing and compounding. Compounded extrudates were compression molded into flat plates using the Carver 30 Ton Model #3895 hydraulic press. Fig. S1 (supplemental material) illustrates a representative flow of the manufacturing steps. Extrusion grade polycarbonate (PC) Lexan resin with
Thermal analysis
Fig. S4 show the DSC results for neat PC, analyzed using a heat-cool cycle. It was found from the heat cycle that the melting point of PC was in the range of 145 – 150 °C and the recrystallization starts at 155 °C. The melting point represents the lower limit needed for processing the resin [31]. It was found that barrel temperatures ranging from 180 – 220 °C were optimal for processing the resin with magnetic compound. It was found that although 180°C was sufficient barrel temperature to
Conclusions
PC-NdFeB bonded magnets have been manufactured using melt mixing and compression molding techniques. The mechanical properties, magnetic properties and microstructure has been methodically examined. The bonded magnets demonstrated competitive tensile strength as compared to injection molded nylon- and PPS-bonded permanent magnets [24,25]. The melt mixed and compression molded 95 wt. % PC-NdFeB bonded magnet exhibited ultimate tensile strength of 44 MPa for NdFeB loaded magnets, showing
Disclaimer
“The information, data, or work presented herein was funded in part by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgements
This research was supported by the Tate Technology Inc. Part of the magnetic characterization research was supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. We also gratefully acknowledge partial graduate and undergraduate students support of this work and use of assets enabled by Institute of Advanced Composites Manufacturing Innovation (IACMI) under
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