What's called oxidation? Carbonization, magnetization ``that expert help!

/WLX/neirong/pages/nr/nr6201.htm

I. Magnetization and permeability

Definition: experiments have shown that, if the magnetic medium is isotropic, in the case of the external magnetic field is not too strong, the magnetization intensity at any point and the magnetic field strength H has the following relationship:,where for the magnetization of the magnetic medium, which is only and the nature of the magnetic medium and the magnetic medium of the pure number.

Substituting can be obtained:

This is a more direct expression of the relationship between B and H at any point in the magnetic medium, where:, is the relative permeability of the magnetic medium: is called the magnetic permeability of the magnetic medium.

Classification of magnetic media: If you can conduct current in the same magnetic field, successively measured in a vacuum and filled with a certain magnetic medium when the magnetic susceptibility and B, then their ratio is the relative permeability of the magnetic medium, that is:

By the value of the different, magnetic media are divided into three categories:

(1): paramagnetic, such as oxygen, aluminum, tungsten, platinum, chromium, etc..

(2): antimagnetic quality, such as nitrogen, water, copper, silver, gold, bismuth and so on. Indicates complete antimagnetism, such as superconductors are ideal antimagnetizers.

(3): ferromagnetic substances, such as iron, cobalt, nickel, etc.

II. Ferromagnetism

I. Properties of ferromagnetism: The most important characteristic of ferromagnetism is that the magnetic permeability is very high, in the same magnetic field strength, compared with the vacuum or weakly magnetic materials, ferromagnetism in the magnetic susceptibility of hundreds or even tens of thousands of times greater. Ferromagnetic material also has some characteristics different from the weak magnetic material: ferromagnetic magnetic susceptibility B and magnetic field strength H is a nonlinear relationship between the magnetic permeability of ferromagnetic material is not a constant quantity, will change with the change of the magnetic field strength H, and ferromagnetic magnetization process is irreversible, with the phenomenon of hysteresis, generally described by hysteresis return line.

II. The law of magnetization of ferromagnetic material: the ferromagnetic material to be measured for the core made of solenoidal ring, when the coil with a current I, the intensity of the magnetic field inside the ring H = nI, by measuring the current I, we know the ferromagnetic core magnetization of the magnetic field strength of H . A small opening is cut in the core of the solenoidal ring, and since the normal component of the magnetic induction is continuous in the cut and in the core, B measured at the opening with a small coil is the magnetic induction in the ring. According to :, the magnetization can be measured. Since the relationship B to H of ferromagnetic material is not linear, the ferromagnetic material's is not a constant, it varies with H.

The magnetization curve shows the relationship between the magnetic field strength H and the magnetic induction strength B. At the beginning of the experiment I = 0, the unmagnetized iron core H = 0, B = 0, this state is equivalent to the origin of the B ~ H graph O, gradually increase the current in the coil I, accordingly H = nI proportionally increased, the beginning (i.e., oa section) B increased slowly, then (i.e., ab section) B increased very quickly, but after the point of b, the B increase slows down, after the point of c, and then increase the H, B almost no longer increase, this time! The core magnetization is saturated. From O to saturation state c this section of the B ~ H curve is called the core of the initial magnetization curve. When the applied magnetic field from strong and gradually weakened to H = 0, the ferromagnetic material in the B is not zero, but B = Br, Br is called the residual magnetic induction strength, referred to as remanent magnetism. To eliminate the remanent magnetism and return the B in the ferromagnetic mass to zero, it is necessary to add the reverse magnetic field strength Hc, which is called the coercive force. If the reverse current is allowed to continue to increase in order to increase the reverse magnetic field strength H, the magnetization reaches the saturation state f point in the reverse direction. If the current is changed back to the original direction, the magnetization curve forms a closed curve. This is the hysteresis loop of the ferromagnetic material. In the process of magnetization of ferromagnetic material, the change of the magnetization state of ferromagnetic material always lags behind the change of the applied magnetic field. This is the phenomenon of magnetic hysteresis.

III. Mechanism of magnetization of ferromagnetic substances: ferromagnetism mainly originates from the spin magnetic moment of electrons. Neighboring atoms of electrons between the existence of a strong "exchange effect", which is a quantum effect. It prompts the spin moment tends to lower energy parallel arrangement state, the formation of magnetic domain (magnetic domain). Magnetic domains are regions of spontaneous magnetization. The volume of the magnetic domain is about 10-12 - 10-8 meters 3, which contains about 1017 - 1021 molecules . Magnetic domains can be observed with a full-phase microscope. In the absence of an external magnetic field the magnetic domains are oriented to cancel out evenly, have the lowest energy, and are not magnetic. When the external magnetic field is weak, the spontaneous magnetization direction and the direction of the external magnetic field is the same or similar to those domains gradually increase (domain wall displacement), when the external magnetic field is stronger, the direction of spontaneous magnetization of the magnetic domains as a whole, with varying degrees of the direction of the external magnetic field. When all the magnetic domains are along the direction of the external magnetic field, the magnetization of the ferromagnetic material reaches saturation. The saturation magnetization strength is equal to the original magnetization strength in each magnetic domain, and the value is very large, which is the reason why the ferromagnetic material has strong magnetism.

Hysteresis phenomenon is due to doping and internal stresses, etc., when the withdrawal of the external magnetic field when the domain wall of the magnetic domain is difficult to return to its original shape, and manifested. Magnetostriction is caused by the orientation of the magnetic domains in the external magnetic field, which changes the lattice spacing. When the temperature rises, thermal motion disintegrates the regular arrangement of magnetic moments within the domains; at the critical temperature (Curie point), the ferromagnetic material becomes completely paramagnetic.

IV. Classification of ferromagnetic mass: According to the magnitude of coercivity Hc of ferromagnetic mass, ferromagnetic materials are classified into soft, hard and moment magnetic materials.

(1) Soft magnetic materials: magnetic pure iron, silicon steel pomo alloy (Fe, Ni), ferrite and so on. Easy to magnetize, easy to demagnetize. Saturated magnetic induction strength, coercivity (Hc) is small, hysteresis line is elongated, in the alternating magnetic field, residual magnetism is easy to be removed, suitable for relays, motors, and a variety of high-frequency electromagnetic components of the core, magnetic rod.

(2) Hard magnetic materials: tungsten steel, carbon steel, alnico alloy. Hysteresis line is wide and fat, magnetization can be maintained for a long time after a very strong magnetism, suitable for making magnetoelectric meter in the permanent magnet, headphones in the permanent magnet, permanent magnet speakers.

(3) Momentary magnetic materials: manganese-magnesium ferrite, lithium-manganese ferrite and so on. Hysteresis back to the line is rectangular, in both directions of the residual magnetism can be used to represent the computer binary "0" and "1", so it is suitable for making "memory" components. In addition, the use of ferromagnetic magnetostrictive effect, can be used to do the transducer, in ultrasound and detection technology is very useful.