The materials used for magnetic rods in high-frequency welded pipe industry are roughly Mn-Zn (Mn-Zn ferrite) ferrite series and Ni-Zn (Ni-Zn ferrite) ferrite series. There are three important indicators to measure the quality of magnetic rods.
1. The initial permeability μ0
The initial permeability μ0 of the magnetic rod is a function of temperature, and it gradually increases with the increase of temperature. However, when the temperature reaches a certain critical temperature, which is also called the Curie temperature Τc, μ0 falls sharply. If the temperature continues raising or it works near the Curie temperature Τc for a long time, the magnet will soon lose magnetism and become a paramagnetic material, which will no longer act as an impedance.
2. Τhe Curie temperature point Tc
Τc refers to the temperature at which the magnetic rod can changed between a ferromagnet and a magnet. That is to say, the magnetic rod of a ferromagnet will become paramagnetic material and no longer have magnetic properties as the temperature rises to a certain temperature. This stable point is its Curie temperature. Obviously, magnetic rods with high Curie temperature points are not easy to be magnetized in use. On the one hand, we must start with the manufacturing process of the magnetic rod to prevent its magnetization. For example, the large cross section area can be made into a honeycomb shape, so that the holes in the cross section will be small and intensive, which ensures the even cooling; on the other hand, users should also cool the magnetic rod well, and let it work well below the Curie temperature point as far as possible.
3. The saturation induction density βs
This is a concept related to permeability and magnetic field strength. According to the electromagnetism theory, under a certain magnetic field, when the material reaches saturation magnetization (the maximum permeability under a specific magnetic field intensity), if the magnetic field continues to increase, the saturation induction intensity of the material will not increase. Or conversely, in order to enable more magnetic flux to pass through the magnetic rods during magnetization saturation, the saturation induction intensity of the material is required to be as high as possible.
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