Do low and high temperatures have the same effect on magnets?
It’s interesting to see such a popular video on the Internet.
After heating, the magnets that are absorbed fall to the ground one after another. Why is that? We saw this experiment a long time ago.
Hold a few needles with a thread, close to the magnet, and we’ll see that the magnet pulls the needle. At this point, we use the burning candle to approach the aspirated needle:
After a while, something magical happened and the needle fell off. This shows that high temperature can be degaussed, and what we have to do is to keep magnetic objects away from high-temperature fire sources. Let’s take a look at this video:
The reason is: the magnet has magnetism, because of its internal free electron movement has certain directionality. According to the remanence temperature coefficient, the magnetism of the magnetic material decreases with the increase of temperature. If the temperature rises to the Curie temperature of the material, the movement direction of the internal electrons will become disordered and lose the directionality. For example, when the temperature of NdFeB magnet material is 310-400 ℃, the microstructure of the magnet material changes and the magnetism completely disappears. Coins have two sides. If there is a spear, there is a shield. Therefore, we often see a kind of magnet called high temperature magnetic magnet. Seeing this name, our first question may be: can a high temperature resistant magnet not degausse at high temperature? In fact, high-temperature magnets do not mean that they will not demagnetize at high temperatures. No matter what kind of magnets are made of, they will have a limit temperature, either high or low. When the working temperature exceeds the limit temperature, the magnetic force will slowly decrease, but the magnetic force at this time is reversible. When the temperature returns to the normal state, use the magnetizer to magnetize, and immediately restore the magnetic force. This is what we see in the video, when the candle goes out, people call again… Here we will also insert a small experiment. Someone has done such an experiment. The neodymium magnet is put into a high-temperature furnace together with some metals it adsorbs.
After burning for a period of time, take out:
At this time, it was found that the magnet was like a stone, which was pushed down and lost its magnetism.
Here’s a concept: Curie temperature. Curie temperature means that when the magnet rises from the limit temperature or exceeds the Curie temperature, the magnetic force disappears to zero. At this time, the magnetic force is irreversible, even if the magnetizing machine is used again, it will not help. Seeing is believing. The following video shows heating a neodymium magnet to about 1000 ℃:
Therefore, we must have a high temperature degaussing string in our work and life. Generally speaking, if the working temperature exceeds 200 ℃, it is a high-temperature magnet. The limit operating temperatures of various magnetic materials are listed below:
- NdFeB: Curie temperature 312 ℃, according to the brand, the limit working temperature is 80 ~ 240 ℃;
- Ferrite: Curie temperature 450 ℃, limit working temperature 250 ℃;
- 1: The Curie temperature is 750 ℃ and the limit working temperature is 250 ℃;
- 2: The Curie temperature is 825 ℃ and the limit working temperature is 300 ℃;
- Al Ni Co: Curie temperature 860 ℃, limit working temperature 450 ℃.
High temperature degaussing, we should be so far, intuitive mechanical readers should be clear, then the question comes, what will happen to magnets at low temperature? Some people have studied it. Put the metal fruit knife on the magnet, you can see that it can stand still because of its strong magnetism:
Put the magnet into the plastic box. At this time, liquid nitrogen is added to the small box. After a while, a layer of ice forms on the magnet:
Take out the magnet, and then put the metal knife on the magnet. Suddenly, it is found that the knife is firmly attached, and it is difficult to take it off:
What’s the principle?
Low temperature (cold) slows down the particles inside the permanent magnet, so in some cases, the magnetic force even increases. However, the respective maximum values must also be kept in mind. If we compare the magnetic properties of ferrite and nd magnets in cold climate, this time, the Nd (strong magnetic Nd-Fe-B) magnet works better and can withstand – 196 ° C, while the ferrite magnet will lose its magnetism at – 40 ° C.