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NDFEB MAGNET

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Nd-Fe-B permanent magnet, tetragonal system crystal, is made up of Nd, Fe, B. So far, it is the most powerful permanent magnetic material called “ Magnet King” with the most practical value and it can be put in mass production. Classified by the different production technologies, Nd-Fe-B can be sorted as sintered magnet, bonded magnet and heat pressed magnet. Nd-Fe-B Sintered magnet is a kind of anisotropic compact magnet fabricated by powder metallurgy method; Nd-Fe-B bonded magnet is a kind of non compact isotropic magnet bonded with Nd-Fe-B powder and polymer. The Nd-Fe-B heat pressed magnet is a kind of anisotropic compact magnet manufactured by hot pressing and thermal deformation. Among them, Nd-Fe-B sintered magnet is of the highest performance and largest use amount; Nd-Fe-B bonded magnets can be magnetized through arbitrary direction. It can be manufactured as permanent magnet with complex shape and high-precision dimension by compression molding and injection molding. Nd-Fe-B heat pressed is seldom used and it is just put in mass production in Japanese big company. Its main application field focuses on EPS system.PS
Main magnetic performances include remanence(Br), magnetic induction coercivity(bHc), intrinsic coercivity(jHc), and maximum energy product (BH)Max. Except those, there are several other performances: Curie Temperature(Tc), Working Temperature(Tw), the temperature coefficient of remanence(α), temperature coefficient of intrinsic coercivity( β), permeability recovery of rec(μrec) and demagnetization curve rectangularity(Hk/jHc).
In the year of 1820, scientist H.C.Oersted in Denmark found that needle near the wire that is with current deflect, which reveals the basic relationship between electricity and magnetism, then, Electromagnetics was born. Practice shows that the strength of the magnetic field and current with current the infinite wire generated around it is proportional to the size, and is inversely proportional to the distance from the wire. In SI unit system, the definition of carrying 1 amperes of current infinite wire at a distance of 1/ wire (2 pi) magnetic field strength meters distance is 1A/m (an / M); to commemorate Oersted's contribution to electromagnetism, in unit of CGS system, the definition of carrying 1 amperes of current infinite conductor in the magnetic field strength of 0.2 wire distance the distance is 1Oe cm (Oster), 1/ (1Oe = 4 PI) *103A/m, and magnetic field strength is usually expressed in H.
Modern magnetic studies show that all magnetic phenomena originate from the current, which is called a magnetic dipole.The maximum torque of the magnetic field in vacuum is the magnetic dipole moment Pm per unit external magnetic field, and the magnetic dipole moment per unit volume of the material is J, and the SI unit is T (Tesla). The vector of the magnetic moment per unit volume of material is M, and the magnetic moment is Pm/ μ0 , and the SI unit is A/m (M / m). Therefore, the relationship between M and J: J =μ0M, μ0 is for vacuum permeability, in SI unit, μ0 = 4π * 10-7H/m (H / m).
When a magnetic field is applied to any medium H, the magnetic field intensity in the medium is not equal to H, but the magnetic intensity of H plus the magnetic medium J. Because the strength of the magnetic field inside the material is showed by magnetic field H through the medium of induction. To different with H, we call it the magnetic induction medium, denoted as B: B= μ0H+J (SI unit) B=H+4πM (CGS units)
The unit of magnetic induction intensity B is T, and CGS unit is Gs (1T=10Gs). Magnetic phenomenon can be vividly represented by the magnetic field lines, and magnetic induction B can also be defined as magnetic flux density. Magnetic induction B and magnetic flux density B can be universally used in concept.
Magnet magnetic field magnetization to saturation after the withdrawal of the external magnetic field in the closed state, the magnet magnetic polarization J and internal magnetic induction B and will not disappear because of the disappearance of the H and the external magnetic field, and will maintain a certain size value. This value is called the residual magnetic induction magnet, referred to as the remanence Br, SI unit is T, CGS unit is Gs (1T=10⁴Gs). The demagnetization curve of the permanent magnet, when the reverse magnetic field H increases to a value of bHc, the magnetic induction intensity of B magnet was 0, called the H value of the reverse magnetic material magnetic coercivity of bHc; in the reverse magnetic field H = bHc, does not show the ability of external magnet flux, the coercivity of bHc characterization of permanent magnetic material to resist external reverse magnetic field or other demagnetization effect. Coercivity bHc is one of the important parameters of magnetic circuit design. When the reverse magnetic field H = bHc, although the magnet does not show the magnetic flux, but the magnetic intensity of the magnet J remains a large value in the original direction. Therefore, the intrinsic magnetic properties of bHc are not sufficient to characterize the magnet. When the reverse magnetic field H increases to jHc, the vector micro magnetic dipole magnet internal is 0. The reverse magnetic field value is called the intrinsic coercivity of jHc. Coercivity jHc is a very important physical parameter of permanent magnetic material, and it is the characterization of permanent magnetic material to resist external reverse magnetic field or other demagnetization effect, to maintain an important index of its original magnetization ability.
In the B-H curve of demagnetization of permanent magnetic materials(on second quadrant), different point corresponding magnets are at different working conditions. The B-H demagnetization curve of a certain point on the Bm and Hm (horizontal and vertical coordinates) represents the size of the magnet and the magnetic induction intensity and the magnetic field of the state. The ability of BM and HM of the absolute value of the product Bm*Hm is on behalf of the state of magnet external work, which is equivalent to magnetic energy stored in the magnet, called BHmax. The magnet in a state of maximum value (BmHm) represents the magnet external work ability, called the maximum energy product of the magnet, or energy product, denoted as (BH)m. BHmax unit in the SI system is J/m3 (joules / m3), and the CGS system for MGOe , 1MGOe = 10²/4π kJ/m3.
The Curie temperature is the temperature at which the magnetization of the magnetic material is reduced to zero, and is the critical point for the conversion of ferromagnetic or ferrimagnetic materials into para-magnetic materials. The Curie temperature Tc is only related to the composition of the material and has no relation to the micro-structure of the material. At a certain temperature, the magnetic properties of permanent magnetic materials can be reduced by a specified range compared with that at room temperature. The temperature is called the working temperature of the magnet Tw. The magnitude of the magnetic energy reduction depends on the application of the magnet, is an undetermined value, the same permanent magnet in different applications have different working temperature Tw. The Curie temperature of Tc magnetic material represents the theory of the operating temperature limit of the material. It is worth noting that the working Tw of any permanent magnet is not only related to the Tc, but also related to the magnetic properties of the magnet, such as jHc, and the working state of the magnet in the magnetic circuit.
The definition of demagnetization curve of B-H magnet working point D reciprocating change track line back magnet dynamic, the slope of the line for the return permeability μrec. Obviously, the return permeability μrec characterizes the stability of the magnet under dynamic operating conditions. It is the squareness of the permanent magnet B-H demagnetization curve, and is one of the important magnetic properties of permanent magnets. For sintered Nd-Fe-B magnets, μrec = 1.02-1.10, the smaller the μrec is, the better the stability of the magnet under dynamic operating conditions.
The magnetic circuit is referred to a specific field in the air gap, which is combined by one or a plurality of permanent magnets, the current carrying wire, iron according to a certain shape and size. Iron can be pure iron, low carbon steel, Ni-Fe, Ni-Co alloy with high permeability materials. Soft iron, also known as yoke, it plays a flux control flow, increase local magnetic induction intensity, prevent or reduce the magnetic leakage, and increase the mechanical strength of the components of the role in the magnetic circuit. The magnetic state of a single magnet is usually referred to as an open state when the soft iron is absent; when the magnet is in a flux circuit formed with soft iron, the magnet is said to be in a closed circuit state.
Usually the production process of sintered Nd-Fe-B magnet production process is: smelting alloy castings → jet milling → magnetism orienting and pressuring → sintering → tempering → magnetic properties testing → rough finishing → cutting → grinding → semi-finished products inspection → electroplating → finished inspection → packaging and storing.

The mechanical properties of sintered Nd-Fe-B magnets:  

Bending Strength /MPa Compression Strength /MPa Hardness /Hv Yong Modulus /kN/mm2 Elongation/%
250-450 1000-1200 600-620 150-160 0

It can be seen that the sintered Nd-Fe-B magnet is a typical brittle material. During the process of machining, assembling and using of magnets, it is necessary to pay attention to prevent the magnet from being subjected to severe impact, collision, and excessive tensile stress, so as to avoid the cracking or collapsing of the magnet. It is noteworthy that the magnetic force of sintered Nd-Fe-B magnets is very strong in magnetized state, people should take care of their personal safety while operating, to prevent fingers climbing by strong suction force.

The factors that affect the precision of the sintered Nd-Fe-B magnet are he processing equipment, tools and processing technology, and the technical level of the operator, etc. In addition, the micro-structure of the material has a great influence on the machining precision of the magnet. For example, the magnet with main phase coarse grain, surface prone to have pitting at machining state; magnet abnormal grain growth, surface machining state is prone to have ant pit; the density, composition and orientation is uneven, the chamfer size will be uneven; magnet with higher oxygen content is brittle, and prone to chipping off angle during the machining process; the magnet main phase of coarse grains and Nd rich phase distribution is not uniform, uniform plating adhesion with the substrate, the coating thickness uniformity, and the corrosion resistance of the coating will be more than the main phase of fine grain and uniform distribution of Nd rich phase difference magnetic body. In order to obtain high precision sintered Nd-Fe-B magnet products, the material manufacturing engineer, machining engineer and the user should fully communicate and cooperate with each other.