Rare earth magnets display exceptional strength of magnetism. Their ability to be made into compact designs allows for their use in various industries. Their brittleness, however, calls for their careful handling to prevent chipping or breaking. Furthermore, their extraction and processing is subject to environmental concern.
What is Rare Earth Magnets?
Rare earth magnets share their name from rare earth elements which are used to make them. Rare earth elements constitute seventeen elements with similar chemical characteristics in the periodic table.
Samarium and neodymium are two of the most used rare earth elements in making rare earth magnets. Magnets made from rare earth elements display great strength of magnetism. Furthermore, they can retain their magnetism for an extended period.
Advantages of Rare Earth Magnet
Rare earth magnets have distinctive qualities which present many advantages. Some of these benefits are discussed below:
Excellent Magnetic Strength
Rare earth magnets, neodymium magnets particularly, offer the highest magnetic strength available. They are therefore utilized in applications requiring strong magnetic fields in more compact designs.
While neo-magnets are susceptible to corrosion, they can be coated with anticorrosive material. Nonetheless, samarium-cobalt magnets are highly resistant to corrosion allowing their use in harsh environments with exposure to moisture and chemicals.
High Magnetic Stability
These magnets can maintain their magnetic properties for extended periods making them reliable in their applications.
Rare earth magnets by virtue of their structure generate strong magnetic fields. As such, even when made in small designs, they maintain their magnetic performance. This allows them to be used where space and weight are significant considerations without sacrificing magnetic performance.
Resistance to Demagnetization
These magnets generate some of the strongest magnetic fields. As such, they can easily retain their magnetism when subjected to external magnetic fields and other demagnetizing factors.
High Curie Temperatur
A magnet loses its magnetic properties beyond the Curie temperature. Rare earth magnets, especially samarium-cobalt magnets, have high Curie temperatures. This allows for their use in applications with elevated temperatures like aerospace-based systems and motors.
Rare Earth Magnets Limitations
Despite the outstanding advantages of rare earth magnets, they are not devoid of a few limitations. As such, you need to carefully assess these limitations and mitigate them for successful implementation. Some of the challenges you will encounter when working with rare earth magnets include:
- Cost: Rare earth magnets are relatively expensive compared to other permanent magnet types. This stems from their rarity and their mining and processing complexities. Furthermore, given that only a few countries control global production, their prices are highly controlled.
- Brittleness: Rare earth magnets can chip or crack when subjected to mechanical impacts or shock. As such, machining them is challenging and they require careful handling.
- Safety Considerations: The strength exhibited by rare earth magnets poses a safety risk if mishandled. Since they exert strong attraction force, the magnets can cause injuries when body parts are caught in the middle for instance.
- Temperature Sensitivity: When you compare the Curie temperatures of samarium cobalt and Neo magnets, the former ranks higher. This makes the latter magnets capable of losing their magnetic properties when operated at higher temperatures above the Curie point.
- Corrosion Vulnerability: Neo rare earth magnets contain iron which makes them susceptible to corrosion. Without the use of coating protection, they can deteriorate in humid and/or corrosive environments ultimately affecting their performance and longevity.
Properties of Rare Earth Magnets
Rare earth magnet properties define the core aspects that describe a magnet’s strength. These properties include:
The tendency of a magnet to resist demagnetization defines its coercivity. It defines a rare earth magnet’s ability to maintain its magnetic field strength when subjected to demagnetizing factors.
Magnets with higher coercivity values indicate greater resistance to demagnetization. The coercivity values of rare earth magnets are high buttressing their magnetic performance. These magnets can easily withstand external magnet fields and temperatures below their Curie point.
When comparing the two rare earth magnets, samarium cobalt magnets have higher coercive values than neodymium magnets. As such, they can retain their magnetism at higher temperatures and in harsher conditions.
Factors like the manufacturing process, material composition, and structure influence rare earth magnets’ coercivity. Nonetheless, rare earth magnets are not entirely immune to demagnetization with their coerciveness having limitations.
In making a magnet, you subject it to a strong external field. When you remove this field, the magnet retains some of that energy. That is remanence. It describes the strength of a magnet in retaining magnetism upon magnetization and removal of the external magnetizing field.
The remanence values of rare earth magnets are high. This makes them display a high magnetic performance. Both NdFeB and SmCo magnets exhibit a high level of remanence.
The remanence of rare earth magnets is influenced by factors. These include its microstructure, the manufacturing process, the material constitution and the magnetizing method. Rare earth magnets having high remanence ensure durability and longevity in applications.
Energy Product (BHmax)
This property measures the maximum energy density a magnet can store. It is an indicator of a magnet’s overall strength and efficiency where a higher energy product indicates a more powerful magnet.
The energy product of rare earth magnets is expressed in mega gauss-oersteds (MGOe) or kilojoules per cubic meter (kJ/m³). When calculated, it is taken as the product of magnetic induction and the magnetizing field strength.
Neo magnets have a very high energy product capable of exceeding 30 MGOe making them the most powerful permanent magnets available. This is due to their strong magnetic induction and high magnetizing field strength.
SmCO magnets can have values ranging between 20 to 32 MGOe, which is also considered high. While slightly lower than NIB magnets’, it compliments their other outstanding features of corrosion resistance and temperature stability.
A rare earth magnet when subjected to elevated temperatures can eventually lose its magnetic capabilities. The temperature at which this occurs is called the Curie temperature. At this temperature, the material’s magnetic moments, usually aligned in a specific direction, become randomized. This causes the material to lose its permanent magnetization.
Neo rare earth magnets have a low Curie temperature beyond which they fail to display their magnetic qualities. SmCo rare earth magnets however have a high Curie temperature making them reliable in high temperature applications.
Rare Earth Magnets Types
There are two primary types of rare earth magnets available globally: neodymium and samarium-cobalt magnets. These two rare earth permanent magnet types have several points of difference like magnetic properties, composition and applications.
Neodymium Magnets (NdFeB)
These are popular globally and known for their incredible magnetic performance. In making these magnets, iron and boron are combined with neodymium. Sometimes small amounts of other elements are added to enhance certain characteristics.
Neo magnets display the highest magnetic strength, making them the strongest permanent magnets available. They also have a high energy product allowing them to generate powerful magnetic fields even in compact sizes.
Furthermore, these magnets exhibit high resistance to demagnetization even though their Curie temperature is relatively low at between 310-400°C. NIB magnets find use in electric motors, generators, sensors, and audio equipment.
Samarium-Cobalt Magnets (SmCo)
SmCo magnets have outstanding resistance to corrosion, high-temperature stability, and excellent magnetic properties. Their elemental constitution includes samarium and cobalt with trace amounts of other elements like iron and copper.
Samarium-cobalt magnets have a higher coercivity compared to NdFeB magnets and can resist demagnetization even at elevated temperatures. While their energy product is lower than Neo magnets, it is still high typically above 20 MGOe.
The higher Curie temperature displayed by samarium-cobalt magnets allows their use where high-temperature stability is desired. The corrosion resistance displayed by these magnets is admirable. This begs for their use in environments susceptible to corrosive factors.
Manufacturing Rare Earth Magnets
The process of manufacturing rare earth permanent magnets consists a few composite steps. The primary method used is powder metallurgy which generally takes the following form:
Step 1: Raw Material Preparation and Alloying
Here, the individual raw materials of alloy constituents are combined. Iron and boron are added to neodymium to make Neo magnet material. On the other hand, cobalt and samarium powders are mixed to make SmCo magnet material.
The powder raw materials are combined in stoichiometric ratios to achieve the desired alloying proportions. The powders are then melted before cooling and grinding into a powder of single extraction. Further crushing and milling processes follow to achieve the desired particle size.
Step 2: Pressing
In this process, a die is used to press the magnet alloy powder at high pressure to form a compact shape. The result is the formation of a green compact that resembles the final magnet shape but is devoid of magnetic properties.
The die or mold used for pressing is made in the desired magnet’s shape and dimensions. These fixtures are fashioned from hardened steel and precisely engineered to match the magnet’s ultimate shape. They may also feature a release agent coating in the inner surface to facilitate the ejection process.
When the mixed rare earth alloy powder is placed into the die, a hydraulic or mechanical press exerts a large pressure. The pressure compacts the powder into the die shape, forming a green compact sharing physical aspects with the final magnet.
Careful control of the pressure ensures a uniform density is achieved preventing defects such as cracks in the compact. Upon compaction, the green compact is ejected carefully from the die to prevent damage.
Step 3: Sintering
Here, the green compacts are transformed into fully functional magnets by activating their magnetic properties. The sintering process involves subjecting the green compacts to high temperatures in a controlled atmosphere utilizing a vacuum or inert gas.
The green compacts are loaded into a sintering furnace capable of withstanding the high temperatures required for the process. A preheating stage is undertaken to prevent defects by removing any moisture and volatile components from the compacts.
During sintering, the temperature is raised to just below the material’s melting point. This initiates a densification process causing the particles in the green compacts to fuse together into a dense, solid structure.
As the temperature rises, it causes the alignment of the magnetic domains within the material. This gives the magnet its magnetic properties. The temperature and duration are carefully controlled to achieve the desired magnetic properties. Otherwise, the material could develop defects.
A cooling process ends the sintering process with the furnace gradually cooled to prevent thermal shock. Carrying out gradual cooling also helps maintain the magnets’ structural integrity and stabilize their magnetic properties.
Step 4: Machining and Shaping
Machining magnets help achieve the desired shape and dimensional tolerances for an application, involving processes like cutting, grinding, and polishing. Computer-aided design software can be used to create detailed plans for machining and magnet shaping.
These designs are executed by specialized machining tools like CNC machines, diamond saws, lathes, and grinders. Surface finishing may also succeed in cutting and grinding to achieve the desired surface quality.
Step 5: Coating
Coating rare earth magnets helps protect them from environmental factors that can cause corrosion and mechanical damage. The coating material used will depend entirely on the specific application and use of the magnet. Common coating materials used include epoxy resin, nickel, and zinc.
Prior to coating application, the magnet’s surface is cleaned. This eliminates any contaminants or residues that may inhibit coating adhesion to the material. Certain coating materials have specific protective properties as follows:
- Zinc: These are primarily used for corrosion protection.
- Nickel: Nickel offers protection against mechanical damage, good corrosion resistance, and durability.
- Epoxy Resin: Popular where moisture resistance is needed and electrical insulation.
- Gold or Silver: Used where resistance to tarnishing and conductivity are required in specialized applications.
- Phosphate Coatings: These are usually used to enhance adhesion prior to applying other coatings.
Various techniques are used to apply the coating such as dip coating, electroplating, and electroless plating. In electroplating, the magnet is immersed in an electrolyte bath before being subjected to an electric current. The current deposits the coating achieving a uniform and controlled thickness.
Electroless plating is similar to electroplating save for the use of an electric current. Coating deposition is instead chemical resulting in even coatings with excellent adhesion. When using dip coating, you immerse the magnet in a coating solution before allowing it to dry.
It is essential to control coating thickness according to the desired specifications. This provides adequate protection without compromising on the performance of the magnet. A drying or curing process follows to harden the coating.
Step 6: Magnetization
This process activates the rare earth materials’ magnetic properties transforming them from non-magnetic materials into powerful permanent magnets. It involves aligning the magnetic domains within the material by subjecting them to a strong external magnetic field.
Magnetizing rare earth magnets in specific directions or orientations tailors them to their intended applications. They could be magnetized axially i.e. along their length or diametrically i.e. across their diameter or by other custom orientation. Most are typically bipolar with a north and south pole.
Rare earth magnets can be magnetized by use of an electromagnet consisting a set of conductive coils. This can be wound over it or in close proximity. When an electricity pulse or direct current is passed through the coils, it results in a magnetic field.
The magnet’s desired magnetic properties will influence the strength of the applied magnetic field. The exposure to the applied magnetic field is for a duration long enough to ensure the magnetic domains align fully. As such, the duration will vary with the magnet type and size.
Factors to Consider When Selecting Rare Earth Magnets
Whenever a choice is to be made between two similar products, the decision is weighed by evaluating common factors. Ensure your choice is capable of meeting the performance expectations while operating reliably in the intended environment.
Some of the key factors to consider when selecting rare earth magnets are captured below:
There are four primary magnetic properties you consider when selecting a rare earth magnet: coercivity, remanence, energy product and Curie temperature. NIB magnets have higher remanence and energy products than SmCO magnets. When it comes to Curie temperature values and coercivity, the come second.
Type of Rare Earth Magnet
When considering rare earth magnets, you can go with samarium-cobalt magnets or neodymium magnets. Depending on your application needs, note that Neo magnets offer high magnetic strength, while SmCo magnets offer corrosion resistance and high-temperature stability.
Shape and Dimensions
Rare earth magnets can come in various shapes and sizes in addition to being customizable. The magnet you select should fit the space and design requirements of the application area. You can have disc magnets, block magnets, and cylinder-shaped magnets to mention a few.
Where strength and other properties are of little concern, cost is a factor you use to make your selection choice. Ideally, NIB magnets are more affordable than samarium-cobalt magnets. The high cost of SmCo magnets is attributed to their expensive manufacturing process.
The temperature limitations of the magnet during use should influence your selection choice. For elevated temperatures, SmCO magnets are more suitable than NdFeB magnets. This is due to their higher Curie temperatures making them resistant to demagnetization.
Direction of Magnetization
Rare earth magnets can be magnetized axially, diametrically or even through their thickness. The application will dictate the direction of magnetization and as such should be considered. The magnet’s magnetization direction should align with the specific system needs.
Each application warrants a minimum magnetic strength value and required field intensity for adequate performance. When choosing your magnet, calculate these values to determine whether it suits your requirements. A key indicator of magnetic strength for instance is the magnet’s energy product.
Some application environments leave the magnet exposed to corrosive substances such as chemicals and moisture. In this instance, you will need a rare earth magnet capable of tolerating such conditions as SmCo or a coated magnet.
Machining Rare Earth Magnets
Machining rare earth magnets is crucial for achieving the ultimate magnet shape and desired dimensions. Machining of rare earth magnets is typically undertaken after the sintering process. The magnets are inspected for any irregularities and visual defects.
In machining efforts, design software is typically used to develop detailed plans capturing final dimensions, tolerances and shape. There are several tools, equipment and machines that can be used in the machining process like grinders, cutters, and lathe machines.
Cutters are used to cut sintered rare earth magnets into the desired shapes. Grinders then refine the dimensions of the magnets by removing material excesses and rough edges. To achieve precise control over the dimensions, CNC machines can be utilized.
Hazards Posed By Rare Earth Magnets
The strong magnetic performance of rare earth magnets has great potential for posing certain hazards if mishandled. Some of these potential hazards are highlighted below:
Pinching and Crushing Hazards
The strong magnetic display of rare earth magnets can exert extremely strong attractive forces over a short distance. As such, two or more such magnets can come into contact with each other or with ferrous materials in a forceful manner. When this happens suddenly encountering parts of the body like fingers or skin, it can cause serious injuries.
Small rare earth magnets like those fashioned as small spheres or balls pose a swallowing hazard, especially by children. Ingesting multiple magnets can initiate attraction to each other even across intestinal walls. This may result in blockages or intestinal damage which can be life-threatening.
Strong magnetic fields are generated by rare earth magnets. When these fields interact with medical devices like pacemakers and other implants, they can interfere with their operation. Such interference can be fatal if not quickly addressed.
Coatings Used on Rare Earth Magnets
When coatings are applied to rare earth magnets, they protect them from mechanical damage, environmental factors, and corrosion. The coating choice essentially depends on the magnet’s operating conditions and application.
Some of the coatings used on rare earth magnets include:
- Epoxy Resin: This coating is especially useful where moisture resistance and electrical insulation are desired. The dip coating method is typically used to apply epoxy resin to excellent results.
- Silver: Similar to gold, silver coating is a precious metal coating that finds use in specialized applications requiring corrosion resistance and conductivity.
- Nickel: As a common coating, it is preferred for its durability and corrosion resistance. It can be used for a wide range of applications and can be applied to various thicknesses. Nickel coating is usually applied by the electroplating method.
- Zinc: The corrosion protection offered by zinc is reliable and cost-effective. The coating provided by zinc can be made into a dull or bright finish depending on the desired appearance.
- PTFE: Some rare earth magnets are coated with polytetrafluoroethylene which offers corrosion resistance and low-friction properties. Such use is found where friction is an undesired occurrence like in mechanical systems.
- Copper: When copper is used as a coating, it offers corrosion resistance and can also allow electrical conductivity. This helps in applications where conductivity is also desired.
Ceramic Magnets vs. Rare Earth Magnets
Ceramic magnets are derived by combining ferric oxide with either barium or strontium carbonate. The resulting magnet while cost-effective, displays outstanding thermal and corrosion performance.
A comparison of ceramic magnets and rare earth magnets is discussed as follows:
The magnetic strength displayed by ceramic magnets is moderate compared to the strong magnetic performance of rare earth magnets. As such rare earth magnets make some of the strongest permanent magnets available.
The corrosion performance of ceramic magnets is excellent allowing their applications in highly corrosive environments without need for coatings. Rare earth magnets, especially neodymium magnets, are highly susceptible to corrosion and require coatings to enhance corrosion performance.
The mechanical strength exhibited by ceramic magnets is formidable making them less brittle and likely to disintegrate on impact. Contrarily, rare earth magnets are brittle and easily crack and break apart on mechanical impact requiring careful handling.
The temperature stability of ceramic magnets is high allowing operation at high temperatures without significant loss of magnetic strength. On the other hand, while SmCo rare earth magnets have an equally excellent temperature stability, NIB magnets have lower temperature stability.
The raw materials used to make ceramic magnets are inexpensive as opposed to rare earth magnets’ making them more cost-effective. As a result, where the magnetic strength performance is secondary, ceramic magnets are a reliable alternative.
You will find ceramic magnets used in applications requiring low to moderate magnetic strength such as loudspeakers and refrigerator magnets. Alternately, rare earth magnets find use in high-performance applications requiring strong magnetic fields like industrial motors and medical equipment.
Applications of Rare Earth Magnets
There are numerous uses of rare earth magnets spanning multiple industries given their outstanding magnetic properties. Their use has allowed for the development of more advanced, compact and performance-efficient devices.
Here are some common applications of rare earth magnets:
i. Electric Motors and Generators: Here, the strong magnetic fields of rare earth magnets help enhance device performance and efficiency. Neodymium magnets used in wind turbine generators help in the conversion of the rotating blades’ kinetic energy into electrical energy.
ii. Automotive: Many automotive functions like power steering and other electronic systems include magnetic components derived from rare earth elements. Electric vehicle motors implement neodymium magnets for a high power-to-weight ratio necessary for an efficient propulsion system.
iii. Consumer Electronics: Cellular devices like smartphones and tablets incorporate these magnets for functions like magnetic latches and vibration motors. Additionally, these magnets are found in high-definition loudspeakers and headphones where they produce high-quality audio in compact designs.
iv. Magnetic Separators: Large-scale application of separators such as in waste management, recycling or manufacturing depends on rare earth magnets. Their high magnetic strength can eliminate the largest ferrous pieces in landfills and the smallest in food particles.
v. Sensors and Switches: Sensors incorporate rare earth magnets due to their high magnetic strength performance in compact form. Such sensors like proximity sensors, reed switches, and Hall-effect sensors find use in electronics applications like robotics.
vi. Medical Devices: Many compact devices used in the medical field like hearing aids use rare earth magnets as primary components. In MRI machines, these magnets generate strong magnetic fields to help produce detailed images of the human anatomy.
vii. Military and Aerospace: High-temperature stability is a requirement for most parts and components featured in military and aerospace hardware. SmCo rare earth magnets meet this performance criteria allowing their use in navigation instruments and missile guidance systems.
Rare earth magnets offer several benefits depending on your particular needs and applications. Where magnetic strength is the primary concern, they are the go-to permanent magnet choice. Nonetheless, they require proper handling given their brittle nature and hazard potential.