Overview of rare earth permanent magnet materials

In a broad sense, all materials that can be magnetized by a magnetic field and use the magnetic properties of the material in practical applications become magnetic materials. It includes hard magnetic materials, soft magnetic materials, semi-hard magnetic materials, magnetostrictive materials, magneto-optical materials, magnetic bubble materials and magnetic refrigeration materials, among which hard magnetic materials and soft magnetic materials are the most used. The main difference between hard magnetic materials and soft magnetic materials is that hard magnetic materials have high anisotropy field, high coercivity, large hysteresis loop area, and large magnetic field required for technical magnetization to saturation. Due to the low coercivity of the soft magnetic material, it is easy to demagnetize after the technical magnetization reaches saturation and the external magnetic field is removed, while the hard magnetic material due to the high coercivity, after the technical magnetization to saturation and the magnetic field is removed, it will remain long-term Very strong magnetism, so hard magnetic materials are also called permanent magnetic materials or constant magnetic materials. In ancient times, people used the natural magnetite in the ore to grind into the required shape to guide or attract iron devices. The compass is one of the four ancient Chinese inventions and has made important contributions to human civilization and social progress. In modern times, the research and application of magnetic materials began after the industrial revolution, and has been rapidly developed in a short period of time. Nowadays, the research and application of magnetic materials are unparalleled in breadth and depth, with various types of high performance. The development and application of magnetic materials, especially rare earth permanent magnet materials, play a huge role in promoting the development of modern industry and high-tech industries.

Performance requirements of permanent magnet material 

The main properties of permanent magnet materials are determined by the following parameters:

• Maximum magnetic energy product: The maximum magnetic energy product is the maximum product of the magnetic induction intensity and the magnetic field intensity on the demagnetization curve. The larger the value, the greater the magnetic energy stored per unit volume and the better the performance of the material.
• Saturation magnetization: It is an extremely important parameter for permanent magnet materials. The higher the saturation magnetization of the permanent magnet material, the higher the maximum magnetic energy product and the upper limit that the residual magnetization of the material may reach.
• Coercive force: After the ferromagnet is magnetized to saturation, the reverse external magnetic field required to reduce its magnetization or magnetic induction to zero is called coercive force. It characterizes the material’s ability to resist demagnetization.
• Remanence: After the ferromagnet is magnetized to saturation and the external magnetic field is removed, the residual magnetization or residual magnetic induction remaining in the magnetization direction is called remanence.
• Curie temperature: The critical temperature at which a strong ferromagnetic body changes from ferromagnetism and ferrimagnetism to paramagnetism is called the Curie temperature or Curie point. The high Curie temperature indicates that the use temperature of permanent magnet materials is also high.

The main types of rare earth permanent magnet materials

So far, rare earth permanent magnet materials have two categories and three generations of products.
The first category is the rare earth-cobalt alloy system (ie RE-Co permanent magnet), which includes two generations of products. In 1996, K. Strant discovered that SmCo5 alloy has a very high number of magnetic anomalies, and produced the first generation of rare earth permanent magnet 1:5 SmCo alloy. Since then, the research and development of rare earth permanent magnet materials began, and they were put into production in 1970; the second generation of rare earth permanent magnet materials, the 2:17 SmCo alloy, was put into production in about 1978. They are all permanent magnet material alloys based on metallic cobalt.
The second category is NdFeB alloy (ie Nd-Fe-B permanent magnet). In 1983, both Japan and the United States discovered the NdFeB alloy, which is called the third-generation permanent magnet material. When Nd atoms and Fe atoms are replaced by different RE atoms and other metal atoms, they can develop into a variety of different compositions and different properties. Nd-Fe-B series permanent magnet material. Its preparation methods mainly include sintering method, reduction diffusion method, melt rapid quenching method, bonding method, casting method, etc., among which sintering method and bonding method are most widely used in production. The following table lists the magnetic properties of different rare earth permanent magnet materials.

Material type	Maximum energy product/Kj.m-3	Residual flux/T	Magnetic coercivity/kA.m-1	Intrinsic coercivity/kA.m-1	Curie temperature/0C
SmCo5 series	100	0.76	550	680	740
SmCo5 series (high Hc)	160	0.90	700	1120	740
Sm2Co17 series	240	1.10	510	530	920
Sm2Co17 series (high Hc)	280	0.95	640	800	920
Sintered Nd-Fe-B series	240-400	1.1-1.4	800-2400	－	310-510
Bonded Nd-Fe-B series	56-160	0.6-1.1	800-2100	－	310
Sm-Fe-N series	56-160	0.6-1.1	600-2000	－	310-600

It can be seen from the above table that the NdFeB permanent magnet material has the best comprehensive magnetic properties, and the sintering method is better than the bonding method. Therefore, the following mainly introduces the production process of sintered NdFeB permanent magnet materials.

Production process of sintered ndfeb magnet

The overall process is as follows:

Rare Earth Ore is discovered and mined→Ore is processed and refined→Refined metal has elements added to create rare earth alloy→Melting (raw material) and strip casting→Hydrogen Decrepitation→Jet Milling→Pressing Under External Magnetic Field→Cold Isostatic Pressing→Sintering→Annealing→Machining and Grinding→Plating/Coating→Magnetizing→Packing and Shipping

Processing steps of neodymium magnets

In the manufacture of high-quality, high-tech neodymium magnets, there are many main production steps and many sub-steps. Each step is very important, and each step is an important part of highly accurate operation.
This is the main step:

Step.1 Rare Earth Ore Mining

First, the rare earth ore is discovered and then mined. Most rare earth mines are open pit, so the ore is removed with large equipment after removing any soil overburden.

Photo Credit : A REE open-pit mine | Ecomerge.blogspot.com

Step.2 Ore Processing and Refining

Next, the Rare Earth Ore is crushed and milled. Then the ore goes through a flotation process where it is mixed with water and special reagents to separate the rare earth elements from the tailings. Depending on the source of the ore, the concentrate may also undergo electrolytic refining. Rare earth metals can be refined and extracted electrochemically, or by distillation, ion exchange or other techniques.The concentrate (refined ore) is then smelted. This means it is heated up to very high temperatures (~1500°C) so the valuable metals can be separated from unusable materials in the ore.
Rare Earth Elements are often found with other valuable metals, such as precious metals and even significant quantities of base metals like copper and nickel, so multiple steps are taken to separate them.
Extraction of rare earths is difficult because many of them have very similar properties, making refinement a challenge. This is one of the cost factors; because the refinement methods require the use of expensive chemicals and time –consuming processes.
For example, it’s not well known, but about 20-30% of the Neodymium in Neodymium magnets is really Praseodymium. In fact, the alloy used to make magnets is called PrNd because these two elements are chemically so similar that not only are they too similar to easily separate, but they are also so similar that it would make only a small difference in the quality of the magnet.

Step.3 Alloying

During the alloying process, small additions of other metals are made to NdFeB alloy to refine and modify the micro structure of the final product, enhancing its magnetic properties and enhancing the effects of other processes.

Step.4 Strip Casting

Alloyed NdFeB is now ready for melting and strip casting. It is heated in a vacuum furnace and a stream of molten metal is forced under pressure onto a cooled drum where it is rapidly cooled at approximately 100,000 degrees-per-second. The high cooling rate produces very small grains of metal that help simplify and enhance the effect of the downstream processing. Also, small grains are an important part of producing high-quality magnets.

Vacuum strip casting furnace rapidly solidifies NdFeB magnet material to create very small grains. From: idealmagnetsolutions.com

Step.5 Hydrogen Decrepitation

While the grains are very small from strip casting, the material from strip casting comes out of the caster in sheets that must be reduced to powder in order to make magnets. The next step after this is Hydrogen Decrepitation –a process that introduces hydrogen to purposely disintegrate the magnet material. The metal is now brittle enough that it can easily be broken into smaller pieces, which is why it is called Hydrogen Decrepitation. In the processing of most metals, processors avoid the introduction of hydrogen into them.
Hydrogen embrittlement can be a major problem for many metals. In this case, the hydrogen is purposely introduced in order to make the material disintegrate. Then it is easy to grind it even smaller in a subsequent operation. The decrepitated material is now ready for the next step.

Hydrogen Decrepitation is a process step used in the production of Neodymium magnets to create extremely small grains in the material.

Step.6 Jet Milling

The Jet Mill uses a high-speed stream of cyclonic inert gas to grind pieces of NdFeB metal into powder. The metal impinges on other pieces of metal powder inside the cyclone.The cyclone automatically classifies the particles by size as they go through the system, so a narrow –and very favorable- particle size distribution is maintained.
The cyclonic air flow naturally separates the particles and prevents the material from having contact with the sides of the pressure vessel due to the gas flow pressure and velocity because different particle sizes have different aerodynamics.

The jet mill is a very clean and effective way to grind NdFeB metal down to powder

Step.7 Pressing Under External Magnetic Field

The powder is kept in an inert gas atmosphere and handled in glove boxes before going to the automated press. The powder enters a mold and is pressed between plates while under a strong magnetic field forming a block of material.The magnetic field orients the grains so that the magnetic domains remain aligned in the designed direction for all subsequent processing steps.

The magnetic field can be oriented two ways:

• 1) in alignment with the block.
• 2) perpendicular to the block. Sintered Neodymium magnets are typically pressed perpendicular to the block in order to achieve the highest anisotropy (strongest north-south magnetization).

Pressing in a perpendicular magnetic field

Step.8 Cold Isostatic Pressing

The block of material is bagged and submerged into a cold isostatic press (CIP) under great pressure. This removes any remaining air gaps in the block, which comes out of this press quite a bit smaller than it was when it went in.

Step.9 Sintering

The pressed block is removed from the bag and sintered. Sintering is a process where the blocks are placed in a furnace at a very high temperature just below the melting point of the metal. At this temperature of >1000^oC, the individual atoms have a lot of motion, which allows the blocks to develop their full magnetic and mechanical properties.

The magnetic domains maintain the same orientation they had before sintering. At this temperature, full density is achieved and the blocks have shrunk to their final size.

Neodymium magnet material achieves full density in the sintering furnace

Step.10 Annealing

After sintering, there are pent-up stresses in the metal from all the movement during sintering, so the blocks are heat-treated again in a step fashion at lower temperatures to reduce the stresses.

The blocks are ramped up to a high holding temperature for a set time and then they are ramped down to a lower holding temperature. Once the holding time is achieved, the now stress-free blocks are slowly cooled to room temperature.

Step.11 Cutting, Machining and Grinding

NdFeB magnets have had a lot of value added by now due to all of the prior steps. Cutting, machining and grinding are performed according to a strict control plan, and waste is minimized by design.

Wire cutting is performed with very fine wire to minimize kerf losses. Machining and grinding are minimized by close controls throughout the previous processes. Waste material is reused and recycled.

Wire cutting machines are used to cut magnets precisely and economically

Step.12 Surface Treatment

Most Neodymium magnets now get a final surface treatment before leaving the plant. The baseline treatment is nickel-copper-nickel electroplate, which protects the magnet from corrosion in most typical use environments.

Some end users specify no coating at all for various reasons. Others specify coatings with greater protection than Ni-Cu-Ni can offer.Aluminum-Zinc offers much greater protection than NiCuNi. IVD aluminum is another choice specified by end users. Epoxy is a very good coating for intense environments and is specified by end users with applications where magnets could be exposed to salt fog.

Rizinia.com applies corrosion-resistant coatings for all types of environments. This is a continuous spray aluminum-zinc coating line.

Step.13 Testing

Testing and evaluation are performed on magnet material at almost every process step, and records of every data point are kept. With such intensive testing requirements, rizinia.com keeps a substantial inventory of test equipment in house to maintain and improve product quality, production efficiency and cost.

Rigorous testing insures only top-quality products are shipped to the customer

Step.14 Magnetizing

One of the last steps is magnetizing. The material is placed inside an electric coil which is energized to produce a very strong magnetic field for a short time. After the coil is de-energized, the magnetic field in the magnet remains.

NdFeB composition and processing differences

High-temperature neodymium magnets usually require the addition of heavy rare earth elements (HREE), such as dysprosium and terbium. Rare earth elements improve the magnet’s resistance to demagnetization in the presence of high temperature and opposite magnetic fields.

The relative scarcity of HREE has led some leading NdFeB companies to develop methods and processes to reduce or eliminate HREE requirements for high-temperature NdFeB magnet grades.

Periodic Table of Elements LREE&HREE

Diffusion of grain boundaries

In recent years, some leading NdFeB magnet manufacturers have created high temperature/high coercivity of NdFeB magnets that do not contain rare earth elements (or greatly reduce rare earth elements) by improving the control of grain size and shape and by using grain boundary diffusion technology. Power rating.
Grain boundary diffusion (GBD) is a method of selectively introducing rare earth elements into the grain boundary phase of a magnet. GBD can produce high coercivity, while greatly reducing the use of rare earth elements such as dysprosium and terbium, thereby alleviating people’s concerns about the use of these rare and expensive rare earth elements.

Crystal shape and size

In many metallurgical systems, the properties of materials are affected by the shape of individual crystals (or grains) in the metal structure and the average shape and size of the grains in the entire microstructure. Strictly controlling the process can improve the magnetic properties at high temperatures while reducing the demand for HREE.
Each manufacturing process must be carefully monitored to verify that each step is executed accurately to achieve quality, performance, and economy.

The production of NdFeB magnets requires a lot of capital investment

These processes require substantial capital equipment investment. For example, just to manufacture magnet blocks, vacuum belt casting machines, hydrogen blasting equipment, jet grinding equipment, magnetic directional presses, cold isostatic presses, and sintering and annealing furnaces are required. These are the main capital expenditure costs.
Very precise cutting, processing and grinding equipment makes the magnet block reach a certain size. Since the magnet material is prepared by powder metallurgy and other processes, it has added considerable value to the parts when the parts are processed and ground.
The cutting plan is very careful. Wire cutting is performed using very thin wires to minimize kerf loss. Grinding is used when necessary, but carefully planned so that material loss is as small as possible.
Electroplating and other coating operations require large amounts of capital to produce high-quality products in an economical and environmentally friendly manner.

Application of neodymium magnet

Neodymium magnets power so many devices that it is easy to forget them. Almost every hybrid and electric vehicle relies on neodymium magnets. Wind turbines, ship propulsion systems, air conditioners, mobile phones, audio equipment and many other applications rely on neodymium magnets to achieve a smooth appearance, thereby creating downstream economic benefits in many new systems.
Industrial motors made of NdFeB magnets can achieve high uptime with an efficiency of over 95%, which can save electricity and natural resources. Neodymium (NdFeB) magnets can create more functions in smaller spaces in more applications than ever before.
Neodymium iron boron magnets provide the highest performance in the smallest material volume, so they are an attractive choice for more and more designers of demanding applications.
For example, if an engineer designs a system based on permanent magnets that has high power requirements and size or space constraints, then the system is likely to use neodymium magnets. The strength of the magnetic field provided by the neodymium magnet is nearly 20 times the strength of the magnetic field per unit volume provided by the ferrite magnet, and the weight is only 1/10 of that of the ferrite magnet. Therefore, the design using the NdFeB magnet may produce a ripple effect, thereby reducing The size of the ferrite magnet.

Source: China Rare Earth Permanent Magnet Manufacturer – www.rizinia.com