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The relationship between Temperature & Magnet
publisherGordon
time2014/03/05
- Not at any temperature, magnetic materials are magnetic. They have a critical temperature Tc, which is Curie temperature.
What's Curie Temperature?
Curie point (Curie
point) is also called Curie temperature (Curie temperature, Tc) or magnetic
transition point. Refers to the temperature at which the spontaneous
magnetization in magnetic materials drops to zero, and is the critical point
where ferromagnetic or ferrimagnetic materials transform into paramagnetic
materials. When the temperature is lower than the Curie point, the substance
becomes a ferromagnet. At this time, the magnetic field related to the material
is difficult to change. When the temperature is higher than the Curie point,
the substance becomes a paramagnet, and the magnetic field of the magnet easily
changes with the surrounding magnetic field. The magnetic sensitivity at this
time is about 10-6. The Curie point is determined by the chemical composition
and crystal structure of the substance.
|
Material |
Curie Temp.
|
|
NdFeB |
310~380℃
|
|
SmCo |
750~820℃
|
|
AlNiCo |
750~850℃
|
Hard Ferrite | ≥450℃ |
Why high temperature would lead to demagnetization?
As the temperature
rises, the increased thermal motion of the metal lattice will affect the
ordered arrangement of the magnetic moments of the magnetic domains. When the
temperature reaches a sufficient arrangement to destroy the magnetic moments of
the magnetic domains, the magnetic domains are disintegrated and the average
magnetic moment becomes zero , The magnetic disappearance of the ferromagnetic
substance becomes a paramagnetic substance, and a series of ferromagnetic
properties (such as high permeability, hysteresis loop, magnetostriction, etc.)
associated with the magnetic domain all disappear, and the corresponding
ferromagnetic substance The permeability is converted to the permeability of
paramagnetic substances.
Application
Using this
feature, people have developed many control elements. For example, the working
principle of rice cooker which is using the characteristics of the curie point
of magnetic materials. A permanent magnet and a soft magnet with a curie point
of 103 ° C are installed in the center of the bottom of the rice cooker. When
the water in the pan is dry, the temperature of the food will rise from 100 °
C. When the temperature reaches about 103 ° C, the soft magnet loses its
attraction. At this time, the spring between the permanent magnet and soft
magnet will separate them and drive the power to be cut, stop heating.