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If you’re working with magnets, it’s essential to understand the concept of Curie temperature.
In this post, we’ll dive into what Curie temperature means for magnets.
And we’ll explore how it affects their performance in various applications.
Let’s get started!
What is Curie Temperature?
Definition
The Curie temperature is the maximum temperature at which a magnetic material retains its permanent magnetic properties.
- When a magnet is heated above its Curie point, it loses its ferromagnetism.
- And it becomes paramagnetic.
At this stage, the magnet’s domains become disordered.
- And it can no longer maintain a permanent magnetic field.
Importance
It’s important to note that magnets will start losing strength as they approach their Curie point.
- While the Curie temperature represents the absolute maximum temperature limit.
- That’s why the maximum operating temperature is the more critical practical limit to consider.
- It is lower than the Curie temp.
You should consider it when selecting a magnet for a specific application.
Common Curie Temperatures
Alnico (Aluminum-Nickel-Cobalt)
Alnico magnets have the highest maximum operating temperatures.
- Maximum operating temperature: 450-900°C
- Curie temperature: 700-860°C / 1,292-1,580°F
Samarium Cobalt (SmCo)
SmCo rare earth magnets have the second highest operating temperatures.
- Max operating temperature: 250-350°C
- Curie temperature: 700-800°C / 1,292-1,472°F
Ceramic/Ferrite
Ceramic or ferrite magnets are the most widely used magnet material.
- Maximum operating temperature: 250-300°C
- Curie temperature: 450-460°C / 842-860°F
Neodymium (NdFeB)
Adding elements like cobalt, terbium and dysprosium to NdFeB can increase the Curie point.
- Neodymium Curie temperatures range from 310-400°C.
Standard neodymium has a lower maximum operating temperature around 80°C.
Special high-temperature grades of neodymium rare earth magnets can operate at 140-200°C.
- Maximum operating temperature: 80-200°C
- Curie temperature: 310-400°C / 590-752°F
Manganese compounds
Several manganese-based magnetic compounds are listed with relatively high Curie points:
- Such as MnBi (357°C), MnSb (314°C), MnAs (45°C).
However, these are less common in practical applications.
So, in summary:
- For the most demanding high-temperature applications, alnico and samarium cobalt magnets are the primary choices,
- Followed by ceramic and specialized high-temp neodymium for intermediate temperatures
Curie Temperature & Magnet Performance
Now that we know what Curie temperature is, let’s explore how it impacts magnet performance.
We’ll focus on neodymium magnets as an example:
Irreversible loss above max operating temp
When a neodymium magnet is heated to:
- above its maximum operating temperature (80-200°C)
- but below the Curie point
- it experiences an irreversible loss of magnetic strength.
Even if cooled back down, the magnet’s performance will be permanently weaker than before heating.
This happens because the elevated temperature reverses the magnetization of some individual magnetic domains.
Reversible loss below max operating temp
Even below the maximum operating temperature, neodymium magnets will experience a small reversible loss of strength.
- It is around 0.08-0.12% for every °C temperature rise.
Luckily, this lost strength is regained when the magnet cools back down.
Increased strength at very low temperatures
At extremely low temperatures down to around -150°C, neodymium magnets actually experience a slight increase in field strength.
- However, below -150°C down to near absolute zero, the strength drops significantly (by up to ~13%).
This is due to a change in the preferred magnetization direction of the magnet’s structure.
Complete demagnetization above Curie temp
When heated above their Curie temperature (310-400°C), neodymium magnets completely lose their ferromagnetic properties.
And they become paramagnetic.
- The magnet can no longer maintain a permanent magnetic field.
- So, to avoid permanently damaging your neodymium magnets
It’s crucial to keep them below their maximum operating temperature.
Which is well below the Curie point.
Production Challenges
The main challenges in manufacturing high-Curie-temperature magnets include:
Material composition & phase stability
Achieving the desired magnetic properties requires precise control over the material composition and phase stability.
- For example, in MnAl magnets, the ferromagnetic τ-phase is metastable.
- It tends to decompose into non-magnetic phases at temperatures above 535 K.
It is a key challenge to maintain phase stability at high temperatures.
Microstructure control
The microstructure significantly impacts the magnetic properties and thermal stability of high-Curie-temperature magnets.
- Including grain size, grain boundary composition, and defects like twins and anti-phase boundaries.
Optimizing the microstructure through processing techniques is critical but challenging.
Intrinsic brittleness
Many high-Curie-temperature magnets are based on intermetallic compounds like Fe-Co alloys.
And these alloys are inherently brittle.
So, it is difficult to overcoming this brittleness to achieve sufficient mechanical strength.
Diffusion & chemical partitioning
At high temperatures, diffusion processes and chemical partitioning among phases can degrade the magnetic properties.
It is a major hurdle to suppressing these detrimental high-temperature processes.
Oxidation & corrosion
Many high-performance magnetic alloys are susceptible to oxidation and corrosion at high temperatures.
This issue can deteriorate their properties.
It is critical to protect the magnets from environmental degradation.
Benefited Industries
Magnets with high Curie temperatures are invaluable in industries.
- They require magnetic components to operate reliably at elevated temperatures.
Let’s look at some key sectors:
Automotive
High-temp magnets like SmCo and alnico are used in sensors, electric motors, and under-the-hood components.
They must withstand extreme heat.
Aerospace
SmCo magnets (Curie point: 800°C) are used in aerospace motors, generators, and actuators.
They experience extreme temperature conditions.
Industrial Motors
Alnico (450-900°C) and SmCo (700-800°C) magnets enable high-temperature electric motors and generators to function.
Without the risk of demagnetization.
Sensors
You can deploy magnetic sensors and switches using high-Curie-point magnets in high-temperature industrial or aerospace sensing applications.
Microwave Devices
Certain microwave tubes and devices requiring strong magnetic fields at high operating temperatures employ magnets.
They have Curie points well above the operating temperatures for stable performance.
Improving Efficiency
High-Curie-temperature magnets can significantly improve efficiency in industrial applications.
There are several mechanisms.
Reduced cooling requirements
Applications, like electric motors and generators, operate at high temperatures.
- Using magnets with high Curie points eliminates the need for additional cooling systems.
This improves overall energy efficiency by reducing the power consumed by cooling.
Reliable operation
High-temperature magnets – like samarium cobalt (SmCo) and alnico – maintain their magnetic properties in extreme environments.
Particularly, in aerospace, automotive, and industrial applications.
- Their ability to withstand heat, dust, and moisture ensures consistent performance and efficiency.
High-temperature motor designs
Using magnets with Curie points well above the motor’s operating temperature allows for more compact and efficient motor designs.
- The magnets can be placed closer to heat-generating components without risk of demagnetization.
Efficiency in energy generation
Applications like wind turbines and hydroelectric generators exposed to high temperatures.
- SmCo magnets with Curie points around 800°C enable reliable and efficient operation.
- The magnets maintain a strong field without consuming additional energy for cooling.
Longevity & reduced maintenance
High-temperature magnets are less prone to demagnetization over time.
Even if you expose them to heat in industrial applications.
- This reduces downtime and maintenance requirements associated with magnet replacement.
- Therefore, improves the efficiency of the overall system.
Enabling advanced technologies
High-Curie-point magnets are critical in cutting-edge applications like microwave devices and aerospace components.
- These things require strong, stable magnetic fields at elevated temperatures.
- They enable these systems to operate efficiently in demanding thermal conditions.
In summary, high-Curie-temperature magnets can boost the performance and efficiency of a wide range of industrial applications.
Extreme Working Environments
High-Curie-temperature magnets perform exceptionally well in extreme environmental conditions:
Stability at high temperatures
We specifically engineer high-temperature magnets to operate under heat without suffering thermal demagnetization.
Their magnetic capabilities remain stable even in hot conditions.
These extreme conditions would demagnetize ordinary magnets.
Tolerance to harsh conditions
In addition to heat, high-Curie-temperature magnets are made from robust materials.
They can boost tolerance against harsh conditions like dust and moisture.
- For example, we use alnico magnets in car motors to avoid inefficiencies and premature damage from heat and dust.
Reliable operation in demanding applications
High-tech industries leverage high-temperature magnets for reliable operation in extreme conditions.
- SmCo magnets maintain magnetic properties up to 350°C.
- We use them in aerospace motors, generators and actuators.
- Alnico magnets operate up to 525°C.
- We use them in high-temperature industrial equipment.
Resistance to demagnetization
Below their Curie points, high-temperature magnets are very resistant to demagnetization.
The causes can be external magnetic fields or physical shock compared to regular magnets.
- For example, SmCo magnets have higher coercivity (resistance to demagnetization) than neodymium magnets at high working temperatures.
Performance in cryogenic conditions
Some high-Curie-point magnets like samarium cobalt also perform well at extremely low temperatures.
- SmCo maintains stable performance down to cryogenic temperatures.
- Meanwhile, neodymium magnets can experience permanent losses below -138°C.
In summary, these features make them uniquely suited for demanding applications in extreme conditions.
Wrapping Up
Understanding Curie temperature is key to selecting the right magnets for your application.
Hope this post has given you a comprehensive understanding.
If you need high-temp magnets, send us email anytime!