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IEC TR 62518 – Rare Earth Sintered Magnets: Stability of Magnetic Properties at Elevated Temperatures
Rare earth sintered magnets — particularly Nd-Fe-B and Sm2Co17 — deliver the highest energy products of any permanent magnet material, making them indispensable for high-performance motors, generators, and magnetic systems. However, their magnetic properties are strongly temperature-dependent. IEC TR 62518, published in 2009, provides a systematic technical analysis of flux stability in rare earth sintered magnets at elevated temperatures, classifying flux losses and establishing experimental methodologies for long-term aging prediction.
Key Scope: This Technical Report covers classification of magnetic flux loss due to temperature (reversible, irreversible, and permanent), long-term aging behavior, experimental procedures, and the influence of material properties such as HcJ on thermal stability.
Classification of Flux Loss and Aging Mechanisms
IEC TR 62518 establishes three distinct categories of flux loss in permanent magnets operating at elevated temperatures:
- Reversible flux loss — a temporary, temperature-dependent reduction in flux that is fully recovered when the magnet returns to its original temperature. Characterized by the reversible temperature coefficient of Br (typically -0.10 to -0.13%/K for Nd-Fe-B and -0.03 to -0.05%/K for Sm2Co17)
- Irreversible flux loss — a permanent reduction in flux at the application temperature that remains when the magnet is returned to room temperature. This is caused by thermal activation overcoming local coercivity barriers in lower-coercivity grain regions
- Permanent flux loss — long-term structural or chemical changes in the magnet microstructure due to prolonged thermal exposure, including grain boundary diffusion and oxidation at elevated temperatures
| Loss Type | Recovery on Cooling | Time Dependence | Primary Cause |
|---|---|---|---|
| Reversible | Fully recovered | Instantaneous | Temperature-dependent magnetization |
| Irreversible | Partially/not recovered | Logarithmic (fast initial, slow later) | Domain unpinning in weak grains |
| Permanent | Not recovered | Power law (slow, progressive) | Microstructural degradation |
Engineering Insight: The distinction between reversible and irreversible losses is critical for motor design. A surface permanent magnet motor operating at 120°C will experience reversible loss of approximately 12% from Nd-Fe-B magnets (which is acceptable if accounted for in the design). However, if the magnet’s HcJ is too low for the operating temperature, irreversible losses can exceed 30%, permanently degrading motor torque. Always select a magnet grade with sufficient HcJ for the maximum operating temperature plus a safety margin.
Experimental Methodology and Long-Term Aging Prediction
The technical report specifies experimental procedures for evaluating temperature stability. Test samples are subjected to controlled thermal cycles with precise temperature profiling, and flux measurements are taken at room temperature after each cycle to separate reversible from irreversible losses. Key experimental parameters include: temperature range (typically 20°C to 200°C for Nd-Fe-B, up to 350°C for Sm2Co17), dwell time at each temperature, number of thermal cycles, and the L/D ratio of the test specimens.
A particularly valuable contribution of IEC TR 62518 is the methodology for predicting long-term irreversible flux loss using accelerated aging tests. The irreversible flux loss follows a logarithmic time dependence described by the relationship: Flux Loss (%) = A + B * log(t), where A and B are material-specific coefficients determined from short-term tests (typically 100-1000 hours), and t is time. This allows engineers to predict flux loss over years of operation from test data spanning only weeks.
Practical Application: For Nd-Fe-B magnets operating at 120°C with HcJ = 1200 kA/m, the standard’s data shows approximately 8% irreversible loss after 1 hour, increasing to about 12% after 10,000 hours (1.14 years). Using the logarithmic prediction model, engineers can estimate that after 20 years of operation, the irreversible loss will reach approximately 15% — information essential for designing with adequate safety margins.
Influence of HcJ and Material Selection
The standard provides comprehensive data on how intrinsic coercivity (HcJ) affects irreversible flux loss for both Sm2Co17 and Nd-Fe-B magnet families. For Nd-Fe-B magnets, HcJ is the single most important parameter determining high-temperature stability. Magnets with HcJ values below 800 kA/m exhibit severe irreversible losses above 80°C, while grades with HcJ > 2000 kA/m can operate reliably at temperatures up to 180°C. For Sm2Co17 magnets, the relationship between HcJ and temperature stability is less critical because of their fundamentally higher Curie temperatures and stronger intrinsic anisotropy.
Frequently Asked Questions
Q: What is the maximum operating temperature for Nd-Fe-B magnets according to IEC TR 62518?
A: There is no single maximum temperature — it depends on the magnet grade’s HcJ value. Standard grades (HcJ ~ 800-1000 kA/m) are limited to approximately 80°C. High-temperature grades (HcJ ~ 2000 kA/m) can operate up to 180°C. For each application, the acceptable irreversible loss over the product lifetime must be defined, and the operating temperature selected accordingly.
A: There is no single maximum temperature — it depends on the magnet grade’s HcJ value. Standard grades (HcJ ~ 800-1000 kA/m) are limited to approximately 80°C. High-temperature grades (HcJ ~ 2000 kA/m) can operate up to 180°C. For each application, the acceptable irreversible loss over the product lifetime must be defined, and the operating temperature selected accordingly.
Q: Are Sm2Co17 magnets always more thermally stable than Nd-Fe-B?
A: Yes, Sm2Co17 magnets have significantly better thermal stability with reversible temperature coefficients of -0.03%/K (vs -0.12%/K for Nd-Fe-B) and can operate at temperatures up to 350°C. However, they are more expensive and have lower remanence (Br). The choice between the two depends on the operating temperature, cost constraints, and required magnetic performance.
A: Yes, Sm2Co17 magnets have significantly better thermal stability with reversible temperature coefficients of -0.03%/K (vs -0.12%/K for Nd-Fe-B) and can operate at temperatures up to 350°C. However, they are more expensive and have lower remanence (Br). The choice between the two depends on the operating temperature, cost constraints, and required magnetic performance.
Q: How can I minimize irreversible flux loss in my magnetic circuit design?
A: Key strategies include: (1) select a magnet grade with HcJ at least 2x the maximum demagnetizing field the magnet will experience at the operating temperature, (2) maintain an adequate L/D ratio (length/diameter) to reduce self-demagnetization, (3) avoid thermal cycling near the material’s knee point on the BH curve, and (4) apply a stabilization treatment (controlled partial demagnetization) after assembly to precondition the magnet.
A: Key strategies include: (1) select a magnet grade with HcJ at least 2x the maximum demagnetizing field the magnet will experience at the operating temperature, (2) maintain an adequate L/D ratio (length/diameter) to reduce self-demagnetization, (3) avoid thermal cycling near the material’s knee point on the BH curve, and (4) apply a stabilization treatment (controlled partial demagnetization) after assembly to precondition the magnet.
