MnBi Bonded Magnet Coating for Coercivity and Oxidation Control
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Solution Overview
Problem
Existing methods for producing high-purity α-MnBi phase for bonded permanent magnets face challenges in achieving large quantities with controlled grain size and purity, leading to issues with coercivity and remanence, particularly in applications requiring high coercivity and moderate remanence.
Innovation Solution
A method involving the use of high-purity α-MnBi feedstock powder coated with a multi-binder system, comprising an inner polymer binder for inter-grain boundary phase and an outer polymer binder for mechanical strength, to enhance coercivity and stability, while allowing for magnetic alignment and protection against oxidation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If conventional magnet fabrication methods (sintering and hot pressing) are used, then bulk anisotropic magnets can be produced, but the eutectic reaction between liquid bismuth and solid MnBi at 535 K limits the maximum temperature, preventing adequate grain growth and texture development
Solution Approach 1:
A binder phase is introduced as an intermediary material that enables hot pressing at temperatures above the eutectic point (535 K) without direct contact between MnBi powder and the heating environment. The binder acts as a protective medium allowing grain growth and texture development while preventing unwanted eutectic reactions, thus resolving the temperature limitation while achieving proper grain size control
Solution Approach 2:
The invention changes the processing parameters by using hot pressing instead of conventional sintering, allowing temperature to exceed the eutectic point. This parameter change enables adequate grain growth and texture development while the binder composition and processing temperature are optimized to prevent excessive grain growth or unwanted phase formation
2Quantity of substance
If high purity α-MnBi phase is synthesized through arc-melting, melt-spinning, and annealing, then purity over 90% can be achieved, but production of large quantities with controlled grain size (3-10 micron) and monocrystalline structure has not been reported
Solution Approach 1:
The invention performs preliminary action by pre-forming monocrystalline MnBi powder particles with controlled grain size (3-10 micron) through arc-melting and controlled cooling before consolidation. This preliminary crystal structure formation ensures that subsequent hot pressing and bonding operations maintain the monocrystalline nature and desired grain size, enabling large-scale production while preserving material quality
Solution Approach 2:
The invention replaces conventional mechanical comminution methods (ball milling, jet milling) that can contaminate and oxidize fine powders with a controlled solidification process during arc-melting. By controlling the cooling rate and atmosphere during solidification, monocrystalline powder with precise grain size is produced directly, avoiding the need for subsequent size-reduction operations that compromise purity and grain size control
3Manufacturing precision
If grain size is increased to allow proper texture development, then texture quality improves, but other phases form during excessive grain growth at high temperature
Solution Approach 1:
The binder phase serves as an intermediary that enables temperature control during hot pressing. It allows the system to reach temperatures sufficient for texture development while preventing excessive grain growth that would cause unwanted phase formation. The binder creates a controlled thermal environment that maintains phase purity while achieving desired texture quality
4Reliability
If grain size is decreased to prevent phase formation, then phase purity is maintained, but particles become too small and easily oxidized after milling
Solution Approach 1:
The invention performs preliminary action by forming the final monocrystalline powder particles with optimal grain size (3-10 micron) directly during the arc-melting and controlled solidification process, before any milling operations. This preliminary formation ensures particles are large enough to resist oxidation while maintaining phase purity, eliminating the need for subsequent size-reduction operations that would increase oxidation susceptibility
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The bonded MnBi magnets exhibit enhanced coercivity and mechanical strength, suitable for applications like PM-assisted synchronous reluctance motors, despite a slight decrease in remanence and maximum energy product.
Implementation Method 1
The MnBi powder is coated with at least one polymer binder to provide inter-grain boundary phase and thereby retain a high degree of texture and coercivity of the MnBi powder in the bonded bulk magnet
Implementation Method 2
The polymer(s) used in practice of the embodiments herein not only mechanically hold the powders together as a binder, but also function as a boundary phase to isolate powder particles from one another to reduce exchange coupling among the particles and thus retain a higher Hc of the bonded bulk magnet
Implementation Method 3
The anti-oxidation coating on the bonded magnets may or may not be needed, depending on how well the powders are coated with the first layer, the coating materials selected, and how big the MnBi particles are
Data Source
AI summary
The present invention provides a method for making anisotropic or isotropic MnBi bonded bulk permanent magnet wherein starting high purity α-MnBi (LTP) mono-crystalline fine feedstock powder particles or c-axis textured polycrystalline coarse powder particles are coated or covered with a single binder coating or a multi-binder coating system. The processed MnBi powder (which is coated or mixed with one or more polymer(s)) is pressed and/or consolidated to produce a dense anisotropic bonded magnet under a magnetic field or a dense isotropic bonded magnet without a magnetic field, at room temperature or elevated temperature. The polymer(s) used herein serve multiple functions: holding the powders together as a binder, isolating powder particles as a boundary phase to reduce magnetic exchange coupling among the particles and thus preferably retain a higher coercivity Hc close to that of the starting MnBi powder, and protecting the powder from oxidation.