What are neodymium magnets?

What are neodymium magnets?

Neodymium magnet, also known as NdFeB magnet, is a tetragonal crystal formed by neodymium, iron and boron (Nd2Fe14B). In 1982, Sumitomo’s special metal Sasakawa discovered the neodymium magnet. The magnetic energy product (BHmax) of the magnet was larger than that of SM co magnet, and it was the largest magnetic energy product in the world at that time. Later, Sumitomo special metals successfully developed the powder metallurgy process, and General Motors successfully developed the melt spinning process to prepare NdFeB magnets. This kind of magnet is the permanent magnet which is second only to absolute zero degree holmium magnet, and is also the most commonly used rare earth magnet. NdFeB magnets are widely used in electronic products, such as hard disks, mobile phones, headphones and battery powered tools. In order to avoid corrosion damage, it is necessary to protect the surface of permanent magnet materials, such as electroplating with gold, nickel, zinc and tin, and spraying epoxy resin on the surface.

Classification of NdFeB magnet

NdFeB magnet can be divided into sintered NdFeB magnet and bonded NdFeB magnet. Bonded NdFeB magnet has magnetism in all directions and is corrosion-resistant; sintered NdFeB magnet is easy to be corroded and needs coating on its surface, generally including zinc plating, nickel, environmental protection zinc, environmental protection nickel, nickel copper nickel, environmental protection nickel, etc. And sintered Nd-Fe-B is generally divided into axial magnetization and radial magnetization, depending on the required working face.

Composition of NdFeB magnet

Metal Nd

Neodymium, element symbol nd, atomic number 60, atomic weight 144.24, melting point 920 ℃, boiling point 3127 ℃, density 7.007g/cm3. Neodymium is a metal ingot. The new fracture surface of the ingot is silver gray. It is easy to oxidize in air and combine with oxidation to form neodymium oxide Lavender powder. Generally, it is stored in a closed iron barrel, vacuumized or filled with argon for protection. The anisotropic field of Nd2Fe14B is 5572ka / m.
NdFeB material requires that the raw material of neodymium has good luster, high purity, few impurities and clean surface. The new fracture is silver gray, and the surface is free of visible inclusions and oxidation. The neodymium content is more than 99.5%. Other metal elements, such as CE, La, Gd, SM and ER, are harmful to the magnetic properties of Nd-Fe-B permanent magnets. The content of them in neodymium should be minimized. In the process of transportation and storage, the product should be placed in a dry place, not piled up in the open air, not close to the high temperature, strictly prevent rain and moisture, avoid oxidation.

Metal Fe

The results show that Fe is 2755.5 ℃, the atomic number of iron is 2755.5 ℃, the atomic number of iron is 2755.5 ℃, the atomic number of iron is 1 535.5 ℃, the atomic number of iron is 1 535.5 g, the atomic number of iron is 1 535.5 G. The surface of NdFeB permanent magnet material is required to be clean without oxide layer, interlayer and crack; the transverse structure shall be free of residual shrinkage cavity, interlayer and inclusion; the grade and chemical composition of pure iron shall be in national standard gb-6983-1986. See Table 1-1.
Table 1-1 Chemical composition of pure iron

Grade	Chemical composition
	C	Si	Mn	P	S	Al	Cr	Ni	Cu
DT3	0.04	0.2	0.3	0.02	0.02	0.5	0.1	0.2	0.2
DT3A
DT4	0.025	0.2	0.3	0.02	0.02	0.15至0.50	0.1	0.2	0.2

N, C, s, P, O, V, Si, Mn, Al, Cr in pure iron have influence on the magnetic properties of Nd-Fe-B, which will reduce JS, TC and ha. Therefore, the elements other than iron in raw materials should be as few as possible, and the reasonable selection of raw materials is the key to ensure high magnetic properties.

Metal B

Boron, nonmetal, crystalline boron is grayish black. Boron, element symbol B, atomic number 5, atomic weight 10.811, melting point 2300 ℃, density 2.34g/cm3. Boron has high melting point, high hardness and poor conductivity. In the production of NdFeB permanent magnet, ferroboron alloy is often used. See table 1-2 for the current national standard of ferroboron alloy: GB / t5682-1995.
Table 1-2 national standard product brand and chemical composition of ferroboron alloy

Type	Grade	Chemical composition
		B	C	Si	Al	S	P	Cu
			≤
Low carbon	FeB23C0.05	20.2-25.0	0.05	2	3	0.01	0.015	0.05
	FeB22C0.1	19.0-25.0	0.1	4	3	0.01	0.03	–
	FeB17C0.1	14.0-19.0	0.1	4	6	0.01	0.1	–
	FeB12C0.1	9.0-<14.0	0.1	4	6	0.01	0.1	–

In the production of Nd-Fe-B permanent magnet, it is required that the structure of ferroboron alloy is compact, no pores, high density, less non-metallic inclusions, small gas content, small segregation of ferroboron, low impurity content and low carbon content. The grade and chemical composition of ferroboron alloy meet the current national standard.

Structure of NdFeB magnet

Phase composition of sintered NdFeB magnets

The microstructure of sintered NdFeB permanent magnet is complex and consists of several phases: hard magnetic Nd2Fe14B phase, crystal boundary rich nd phase, grain boundary rich B phase and a small amount of foreign phase. Table 1-3 gives the description of phase composition. Figure 1.1 shows the phase composition diagram of sintered NdFeB materials.
Table 1-3 phase characteristics of sintered NdFeB magnets

Name of phase	Component（Nd:Fe:B)	Characteristics of each phase
Nd2Fe14B phase	2:14:01	Polygon, size and crystal orientation are not the same
Nd rich phase	1:04:04	Large or fine particles precipitate in grain boundaries
B rich phase	Nd: Fe has 1:1.2:1.4, 1:2:2.3, 1:3.5:4.4	Some of them are in the shape of thin layer or isolated particles, which are distributed along the grain boundary or at the junction of grain boundaries
Oxides of Nd	Main component Nd2O3	There are not only large particle precipitation, but also small particle precipitation, mostly located in the grain boundary
Foreign impurity phase	Chloride (NdCl, Nd (OH) Cl) or fe-p-s phase	Granular

Figure 1.1 phase composition diagram of sintered NdFeB materials

Hard magnetic phase of Nd2Fe14B

Nd2Fe14B phase is the only magnetic phase in the sintered NdFeB. The volume fraction of Nd2Fe14B is 80-90%. Nd2Fe14B phase is a dead angle crystal structure, referred to as tetragonal phase, with obvious magnetocrystalline anisotropy. Its crystal structure is shown in Fig. 1.2. It contains four Nd2Fe14B molecules in one cell, with a total of 68 atoms. The number of Nd, Fe and B atoms is 8, 56 and 4 respectively. 68 atoms occupy nine crystal sites, 4f and 4G are occupied by Nd atoms; Fe atoms occupy 16k1, 16k2, 8j1, 8j2, 4C and 4C crystal sites, and 4G crystal sites are occupied by B atom. Because the atomic spacing at different crystal sites is different, it has an important influence on the atomic exchange and Curie temperature in Nd2Fe14B phase. The crystal is composed of six atomic layers. The nd atoms are distributed in the first structural layer, the Fe atoms are distributed in the second, third, fourth, fifth and sixth structural layers, and the B atoms are distributed in the fourth structural layer.

Crystal structure of Nd2Fe14B

Nd rich phase

Nd rich plays an important role in the magnetic properties of sintered NdFeB permanent magnets, mainly distributed in the grain boundaries, but also a few in the Nd2Fe14B grains. According to the distribution and morphology of Nd rich phases, they can be divided into three types:

• (1) the morphology of Nd2Fe14B is thin and uniformly distributed among Nd2Fe14B grains;
• (2) the morphology of Nd2Fe14B is fast and distributed at the grain boundary of Nd2Fe14B;
• (3) the morphology of Nd2Fe14B is granular, which only exists in a few grains. Different nd rich phases have different crystal structures. According to the research of Tang et al., the first kind of thin-layer nd rich phase is face centered cubic structure; the second type of granular nd rich phase is bicubic structure; the third type of granular nd rich phase located in the grain of Nd2Fe14B phase is also face centered cubic structure.

B-rich phase

B-rich phase is a compound of B, and B-rich phase generally has two distribution forms

• (1) the general morphology of B-rich phase is polygonal particles, which are distributed at the junction of grain boundaries or on the grain boundaries of main phases;
• (2) there is also a kind of fine granular B-rich phase, which is often located in a few main phase grains. B-rich phase is a kind of non-magnetic phase. The more B-rich phase is, the smaller the product fraction of the main phase will be and the magnetic properties will be reduced. Therefore, we hope that the smaller the content of B-rich phase in the alloy, the better.

Properties of NdFeB materials

The main technical magnetic properties of permanent magnetic materials are: remanence BR, coercivity HCB, intrinsic coercivity Hcj, maximum magnetic energy product (BH) max, Curie temperature Tc.

Remanence BR

From the saturation state of a magnetic body, the magnetic induction intensity when the magnetic field is monotonically reduced to zero along the saturation hysteresis loop is called remanence, which is represented by the symbol br. The unit is Tesla (T) in Si and Gauss (GS) in CGS. The br of NdFeB permanent magnetic materials can be reached. It is the highest of rare earth permanent magnets.

Coercivity HCB

From the saturation magnetization state of a magnetic body, the magnetic field is monotonically changed along the saturation hysteresis loop, so that the magnetic field strength when the magnetic flux density is zero is called coercivity. It is represented by the symbol HCB. The unit is kiloampere / meter (Ka / M) in the international system of units (SI), and it is Oster in the centimeter gram second system (CGS), abbreviated as the symbol (OE). The HCB of NdFeB permanent magnet is.

Intrinsic coercivity HCJ

From the saturation magnetization state of a magnetic body, the magnetic field changes monotonically along the saturation hysteresis loop, so that the magnetic field intensity decreases to zero is called magnetic polarization coercivity or intrinsic coercivity. It is represented by the symbol Hcj. The unit is kiloampere / meter (Ka / M) in the international system of units (SI) and Oster in the centimeter gram second system (CGS). The intrinsic coercivity Hcj of NdFeB permanent magnet is as high as Ka / M-1. Compared with the first and second generation rare earth permanent magnets, Yuangao is more widely used.

Maximum magnetic energy product (BH) max

At any point of the demagnetization curve of a permanent magnet, the product of the magnetic flux density and the magnetic field intensity, the maximum value of the magnetic energy product obtained on the demagnetization curve is called the maximum magnetic energy product, which is expressed by the symbol (BH) max. The unit is Joule / m3 (J / m3) in Si and goe in CGS. The maximum magnetic energy product (BH) max of NdFeB permanent magnet is as high as kJ / m-3.

Curie temperature TC

Curie temperature refers to the temperature at which ferromagnetic materials change from ferromagnetism to paramagnetism. It is called Curie temperature, expressed by the symbol TC, in centigrade (℃). The Curie temperature of NdFeB permanent magnet is TC / ℃. It is relatively low in permanent magnetic materials, so the application of NdFeB materials is limited.

• The Curie temperature of ferrite is 465 ℃,
• The Curie temperature of NdFeB is 320 ℃ – 460 ° C,
• The Curie temperature of Al Ni Co is 800 ℃,
• The Curie temperature of SmCo is 700-800 ℃,
• The Curie temperature of Fe Cr CO is 680 ℃.

Distinguish

• 1. It is usually placed on a surface that can be adsorbed with a magnet of the same specification, such as: patch, blade, iron door, etc. the size of magnetic force can be identified by hand feeling
• 2. Use electronic scale: a magnet with weak magnetic force is generally related to its density. If the density is small, the magnetic force is relatively small; if the weight is heavy, the magnetic force is relatively large; on the contrary, the weight is light, and the magnetic force is small. If the requirements for the magnet are relatively high, the instrument test is required.

Summary

Each product is divided into several grades according to the maximum magnetic energy product:
The grades of NdFeB magnetic materials are: n35-n52, 35m-50m, 30h-48h, 30sh-45sh, 28uh-35uh, 28eh-35eh.
Brand example: 048021 refers to sintered NdFeB permanent magnet with (BH) max of 366 ~ 398kj/m3 and Hcj of 800kA/m.
The brand of sintered Nd-Fe-B permanent magnet consists of main name and two kinds of magnetic properties. The first part is the main name, which is composed of the chemical symbol nd for Nd, the chemical symbol Fe for iron and the chemical symbol B for boron. The second part is the number before the line, which is the nominal value of the maximum magnetic energy product (BH) max (unit: kJ/m3). The third part is the number after the oblique line, with strong magnetic polarization One tenth of the coercivity value (in Ka / M), and the value is rounded off.
Brand example: ndfeb380 / 800 refers to sintered NdFeB permanent magnet with (BH) max of 366 ~ 398kj/m3 and Hcj of 800kA / Mr.

Surface treatment of NdFeB magnet:

• 1. Phosphating;
• 2. Inorganic salt passivation;
• 3. Electroplating;
• 4. Electrophoresis;
• 5. Vacuum deposition.

Grade	Residual magnetic induction strength	Coercivity	Intrinsic coercivity	Maximum magnetic energy product	Maximum operating temperature
	Br mT	bHc KA/m	iHc KA/m	（BH）max KJ/m3	Tw ℃
	(KG)	(kOe)	(kOe)	(MGOe)
N35	1170-1210	≥868	≥955	263-287	80
	(11.7-12.1)	(≥10.9)	(≥12)	(33-36)
N38	1210-1250	≥899	≥955	287-310	80
	(12.1-12.5)	(≥11.3)	(≥12)	(36-39)
N40	1250-1280	≥923	≥955	318-342	80
	(12.5-12.8)	(≥11.6)	(≥12)	(38-41)
N42	1280-1320	≥923	≥955	318-342	80
	(12.8-13.2)	(≥11.6)	(≥12)	(38-41)
N45	1320-1380	≥876	≥955	342-366	80
	(13.2-13.8)	(≥11.0)	(≥12)	(43-46)
N48	1380-1420	≥835	≥876	366-390	80
	(13.8-14.2)	(≥10.5)	(≥11)	(46-49)
N33H	1130-1170	≥836	≥1353	247-241	120
	(11.3-11.7)	(≥10.5)	(≥17)	(31-34)
N35H	1170-1210	≥868	≥1353	263-287	120
	(11.7-12.1)	(≥10.9)	(≥17)	(33-36)
N38H	1210-1250	≥899	≥1353	287-310	120
	(12.1-12.5)	(≥11.3)	(≥17)	(36-39)
N40H	1240-1280	≥923	≥1353	302-326	120
	(12.4-12.8)	(≥11.6)	(≥17)	(38-41)
N42H	1280-1320	≥955	≥1353	318-342	120
	(12.8-13.2)	(≥12.0)	(≥17)	(40-43)
N45H	1320-1360	≥955	≥1353	342-366	120
	(13.2-13.6)	(≥12.0)	(≥17)	(43-46)
N33SH	1130-1170	≥844	≥1592	247-272	150
	(11.3-11.7)	(≥10.6)	(≥20)	(31-34)
N35SH	1170-1210	≥876	≥1592	263-287	150
	(11.7-12.1)	(≥11.0)	(≥20)	(33-36)
N38SH	1210-1250	≥907	≥1592	287-310	150
	(12.1-12.5)	(≥11.4)	(≥20)	(36-39)
N40SH	1240-1280	≥939	≥1592	302-326	150
	(12.4-12.8)	(≥11.8)	(≥20)	(38-41)

Technological process of NdFeB magnet

• Process flow: batching → melting ingot / strip casting → milling → pressing → sintering and tempering → magnetic testing → grinding → pin cutting processing → electroplating → finished products. The burden is the foundation and sintering tempering is the key.
• Nd-Fe-B magnet blank production tools and performance testing tools: smelting furnace, belt throwing furnace, jaw crusher, air mill, pressing molding machine, vacuum packaging machine, etc., static press, sintering furnace, heat treatment vacuum furnace, magnetic property tester, Gauss meter.
• Nd-Fe-B magnet machining tools: centerless grinding, rounding machine, double end grinding, flat grinding, slicing machine, double-sided grinding, wire cutting, bench drill, special-shaped grinding, etc.

Manufacturing technology of sintered NdFeB magnets

Technological process for manufacturing NdFeB magnets

• Raw materials → batching → smelting → milling → magnetic field forming → sintering and tempering → magnetic property testing → machining → testing → packaging → warehousing

The characteristics of sintered NdFeB permanent magnet are as follows:

• The composition range is wide.
• Due to the serious oxidation, it is necessary to reduce the specific surface area (the powder should not be too fine) without affecting the magnetic properties.
• The sintering temperature range is wide, but attention should be paid to the importance of vacuum sintering.
• The tempering temperature range is wide, and it should be noted that the exposure time of the powder in the atmosphere should be as short as possible.

Another characteristic of sintered Nd-Fe-B permanent magnet process is as follows:

• Permanent magnet alloy or compound with accurate composition can be obtained by melting, because obtaining accurate permanent magnetic alloy is the basis of permanent magnet manufacturing process.
• In order to make full use of the magnetization intensity of the permanent magnet, the axis of easy magnetization of the grain is arranged in the direction of use through near grain orientation.
• In order to make the strong magnetic phase occupy a large volume, it is necessary to eliminate the pores and increase the density. Dense permanent magnets can be obtained by sintering.
• Phase separation, in which the strong magnetic phase is surrounded by the non-magnetic phase or the weak magnetic phase, can be realized by the unstable state during tempering treatment. The phase separation can increase the blocking force of the domain wall displacement, thus increasing the coercivity.

Melting

Smelting is a metallurgical process in which the burden changes in physics and chemistry in a high-temperature furnace to produce coarse metals or metal concentrates and slag.
Purpose of smelting: the purpose of smelting is to change the elemental into alloy. It is to melt the pure metal raw materials {(SM, PR, CO), (SM, Co, Cu, Fe, Zr), (nd, Fe, BFE, Dy, Al) and so on, and ensure that the alloy composition is accurate. All metal raw materials are melted, because the melting point of Fe and metal co is high, try to melt them completely.
The smelting process is as follows: charging → vacuumizing → ar filling → melting → casting → cooling → reclaiming.
Before smelting, raw materials should be washed and dried, and the oxide layer of metal iron should be removed by rust remover.

Milling

Milling is to break the bulk alloy into a certain size of powder, including coarse crushing and grinding into fine powder. In order to obtain high-performance magnets, the oxide layer on the surface of the alloy ingot should be removed first, and then coarse crushing should be carried out.
The purpose of milling is to break the large ingot into powder with certain size.
Mechanical crushing process: ingot (60 ~ 100 mm) → coarse crushing (10 ~ 15 mm) → medium crushing (3 ~ 5 mm) → fine crushing (1 ~ 1.25 mm) → ball milling with screen (0.074 mm).
At present, it is generally used to break with coarse hammer, medium crushing and hammer grinding. The new process saves time and improves efficiency.
Due to the oxidation of rare earth metals and the formation of Nd2O3, the vacuum degree and nitrogen protection should be paid attention to.

Press forming

Forming is an important part of powder metallurgy. Forming is to process loose powder into a certain shape with certain density and strength.
The purpose of pressing is to make the powder into semi-finished products with predetermined geometry, size, density and strength under a certain pressure.
The technological process of pressing is as follows: material weighing → mold assembling → mold closing → orientation → pressing → demoulding → packing.
The quality of the pressing process not only affects the smooth progress of the pressing process and the size of the green body, but also has a great influence on the magnetic properties of the products.

Sintering and tempering

Sinter

Sintering is a process of bonding and migration of powder particles under atomic interaction.
The purpose of sintering is to sinter the pressed blank at a certain temperature, so that it has a certain performance and geometric size of the product, and to achieve the final purpose and required performance.
Sintering process: charging → vacuuming → heating → heat preservation (argon filling) → cooling → discharging.
Sintering is an important link to determine the rate, but also one of the key processes to determine product quality. Therefore, “formula is the foundation, pressing is the condition, sintering is the key”

Tempering

Tempering is a heat treatment process in which the workpiece is heated to the critical temperature, held for a certain time, and then cooled to room temperature.
The main purpose of tempering is to eliminate the stress, refine the grain size, make the nd rich phase uniformly distributed around the main phase, and improve the coercivity.
The tempering treatment is divided into two stages. In the first stage, the tempering temperature is 900 ~ 950 ℃, and the holding time is 3 ~ 4 hours. The second stage is 600 ~ 650 ℃ for 4 ~ 5 hours.

Machining

Processing is to meet the product size requirements and assembly needs, through mechanical processing to achieve the product specified dimensional accuracy and surface roughness.

The processing methods include grinding, slicing, EDM and laser machining.

Advantages of rare earth NdFeB materials

NdFeB is a kind of rare earth material with high coercivity and high magnetic energy product. In recent years, with the continuous improvement of the material, the use temperature is increased and the cost of the material is reduced. The application of Nd-Fe-B rare-earth permanent magnet materials in the development of various motors can significantly reduce the quality of the motor, reduce the size of the motor, and can obtain high-efficiency energy-saving effect and improve the performance of the motor. Combined with the new power electronic technology, the application of rare-earth permanent magnet materials in the motor can realize the mechanical and electrical integration of products It has entered a new stage of development. Rare earth permanent magnet motor is the largest application field of Nd-Fe-B magnet, accounting for about 70% of the total magnet, and the voice coil motor (VCM) supporting computer hard disk accounts for 40% – 50%, so the computer industry is the largest user of permanent magnet motor. The weight of the motor can be obviously reduced by using rare earth permanent magnet motor. For example, the weight of 10kW ordinary generator is 220kg, and that of dilute permanent magnet generator is 92kg. Rare earth permanent magnet motor has high efficiency and energy saving, the average power saving rate is as high as 10%, and the power saving rate of some special motors is as high as 15% – 20%. Compared with the DC motor used in the past, the volume of 1095kw and 230rpm six pole permanent magnet synchronous motor developed by Siemens in Germany is reduced by about 60% and the total loss is reduced by 20%. Because of its high performance to price ratio, NdFeB has become an ideal material for manufacturing high-efficiency, small volume and light weight magnetic functional devices, which is expected to have a revolutionary impact on many application fields. In addition to the wide application of computers, printers, mobile phones, household appliances and medical devices, the application of generators, motors and sound systems in automobiles has become increasingly mature, which will greatly promote the development of NdFeB industry.

China NdFeB permanent magnets manufacturer: www.rizinia.com