Core-Shell Nanomagnets for High-Temperature Magnetic Exchange Coupling
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Solution Overview
Problem
Current permanent magnets used in applications like electric vehicles and windmill generators degrade at high temperatures and often contain expensive rare-earth or precious metals, limiting their availability and effectiveness.
Innovation Solution
Development of magnetic exchange coupled core-shell nanomagnets with a magnetically hard core surrounded by a magnetically soft shell, allowing for high maximum energy product (BH)max over a wide temperature range without using rare-earth or precious metals, achieved by controlling the shell thickness for efficient magnetic exchange coupling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If rare-earth magnets (Nd2Fe14B, Dy-doped Nd2Fe14B, SmCo, Sm2Fe17N3) are used to achieve high maximum energy product, then (BH)max is improved, but operation temperature is limited and material cost increases
Solution Approach 1:
The patent employs a core-shell composite structure where a magnetically hard core (e.g., MnAl, MnBi, or hexaferrite) is surrounded by a magnetically soft shell (e.g., permalloy, cobalt-iron alloy, or nickel-iron alloy). This composite structure combines the high coercivity of hard magnetic materials with the high saturation magnetization of soft magnetic materials, achieving high (BH)max while maintaining thermal stability up to 500°C without requiring rare-earth elements
Solution Approach 2:
The patent applies local quality by creating distinct magnetic properties in different regions of the nanomagnet particle. The core region is engineered with high coercivity characteristics to provide thermal stability, while the shell region is engineered with high saturation magnetization to enhance the energy product. The shell thickness is precisely controlled (typically 5-50 nm) to optimize the exchange coupling effect between core and shell, allowing each region to contribute its specific magnetic property to the overall performance
2Temperature
If Dy is added to increase operation temperature, then Curie temperature is improved, but magnetization decreases and coercivity increases excessively
Solution Approach 1:
The patent extracts the temperature stabilization function from the magnetic moment carrier (rare-earth elements like Dy) and assigns it to the magnetically hard core material instead. The soft shell material then exclusively provides high saturation magnetization without the detrimental effect of reduced magnetization. This separation of functions allows the magnet to achieve both high Curie temperature and high magnetization independently, avoiding the trade-off imposed by Dy addition
3Power
If magnetically soft shell is added to increase saturation magnetization, then (BH)max is improved, but exchange coupling efficiency decreases with increased shell thickness
Solution Approach 1:
The patent optimizes the shell thickness parameter within a specific range (5-50 nm, preferably 10-30 nm) to balance two competing requirements: sufficient thickness to provide adequate saturation magnetization enhancement, and sufficient thinness to maintain strong exchange coupling between core and shell. The patent also controls the core radius (50-200 nm) and uses specific material compositions to optimize the exchange coupling length, achieving maximum (BH)max at optimized geometric parameters
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 solution provides high temperature stability and cost-effectiveness by maintaining magnetic performance above 150°C without the need for expensive materials, achieving a high maximum energy product through magnetic exchange coupling between the core and shell.
Implementation Method 1
magnetic exchange coupling between the core and shell
Data Source
AI summary
A permanent magnet is fabricated such that it has a magnetically hard core surrounded by a thin magnetically soft shell. The magnetically hard core provides a relatively high intrinsic coercivity (Hci), and the magnetically soft shell provides a relatively high magnetic flux density (B). Due to magnetic exchange coupling between the core and shell, a relatively high maximum energy product (BH)max is achievable over a wide temperature range, including temperatures above 150° C. Further, such effects can be achieved without using rare-earth metals or precious metals helping to keep the manufacturing costs of the magnet low. To allow sufficient exchange magnetic coupling between the core and shell, the width of the shell is less than about 40 nanometers, and the overall dimensions are controlled such that the width of the shell is less than two times the Bloch domain wall thickness of the core.