Recently, Huazhong University of Science and Technology has gained significant attention. A doctoral student from the School of Materials Science at the university synthesized the LK-99 that can achieve magnetic levitation. As many have seen, when a magnet approaches the material from below, the material stands upright due to repulsion. Even after changing the magnetic pole and approaching the material, it still stands upright due to repulsion.
This “small black dot” continuously falls or stands up as the neodymium magnet approaches or moves away, regardless of whether it’s the South pole or North pole, demonstrating its diamagnetic properties.
Setting aside whether LK-99 is truly a superconductor, the credit for its levitation goes to neodymium magnets.
Speaking of neodymium magnets, one cannot overlook the Tesla Model S. Elon Musk is truly audacious. During the launch event of Tesla’s first car, the Model S, they hadn’t even completed its assembly. The chassis was based on the Mercedes-Benz CLS, and the body aluminum panels and engine cover were attached to the steel frame using neodymium magnets.
When Tesla was manufacturing its first two full-size car models, they used induction motors to provide power. These motors were based on Nikola Tesla’s original motor design, an impressive invention that predates the discovery of rare earth magnets by nearly 100 years.
Induction motors generate their own magnetism through electricity and propel rotor movement, operating without any type of permanent magnets.
While the design of induction motors is effective, there’s a reason why Tesla switched to permanent magnet motors for the Model 3 in 2017: the Model 3 is a smaller car that requires a smaller motor while still delivering ample power. Hence, starting from the Model 3, Tesla began using neodymium motors because they are space-saving, lighter, and can generate greater force.
In fact, magnets are not only applied in cars but also widely used in mobile phone speakers, headphones, vibration motors, electromagnets, hairdryers, fans, refrigerators, washing machines, and more.
▲ The proportion of magnets used in motors, sensors, speakers, and other applications
▲ Magnets applied in consumer electronic products
Besides neodymium magnets, what are the other types of magnets? How is the production process? Let’s discuss it in this article.
Maximum Energy Product of Magnets
Broadly speaking, there are three types of magnets: permanent magnets, temporary magnets, and electromagnets. This article will focus primarily on permanent magnets.
▲ Classification of Magnets and a Comparison between Permanent Magnets and Electromagnets
Permanent magnets generate a magnetic field that can be sustained even in the presence of opposing magnetic fields. Motors using permanent magnets are more efficient than those without. Currently, all-known strong magnets contain rare-earth elements, which are crucial components of electric vehicles and wind turbines. Due to growing demand and limited supply, elements like Nd and Dy have become critical materials.
The uniqueness of permanent magnets is that once they are produced, they provide magnetic flux without requiring energy input, resulting in zero operating costs. In contrast, electromagnets require continuous current to generate a magnetic field.
A significant characteristic of permanent magnets is their ability to maintain a magnetic field even in the presence of an opposing external magnetic field. However, if the strength of the opposing magnetic field is sufficiently high, the internal magnetic domains of the permanent magnet will align with the opposing field, causing demagnetization.
Permanent magnets essentially function as energy storage devices. The energy is injected during the initial magnetization, and if manufactured and handled properly, it will remain in the magnet indefinitely. Unlike batteries, magnets’ energy doesn’t deplete, and they remain usable. This is because magnets don’t exert a net effect on their surrounding environment. Instead, they use their energy to attract or repel oher magnetic objects, aiding in the conversion between electrical and mechanical energy.
The energy of a magnetic field is proportional to the product of B and H. When the product of BH is maximized, denoted as (BH)max, the smallest volume of magnet is required to produce a given magnetic field within a certain gap. The higher the (BH)max, the smaller the magnet’s volume needed to generate a given magnetic flux density. (BH)max can be considered the static magnetic energy per unit volume of magnet material. BH is measured in Mega Gauss Oersteds (MGOe) or kJ/m³.
In the permanent magnet industry, the maximum energy product represents the magnetic energy density of the permanent magnet and is the most commonly used parameter for characterizing permanent magnet performance.
Classification of Permanent Magnets
Conventionally, permanent magnets can be divided into four types: neodymium iron boron (NdFeB), samarium cobalt (SmCo), aluminum nickel cobalt (AlNiCo), and ceramic or ferrite magnets.
▲ Photo Credit: Dura Magnetics, Inc
Among these, samarium cobalt magnets (SmCo) and neodymium magnets (NdFeB) are referred to as rare-earth magnets because samarium and neodymium are part of the rare-earth elements in the periodic table.
Based on the processing method, mag nets can be further categorized into cast, sintered, and bonded magnets.
▲ Photo Credit: Manufacture of Mordern Permanent Magnet Materials，Arnold Magnetic Technologies Corp.
Neodymium Magnets
Neodymium magnets (NdFeB) are one of the most widely used permanent magnet materials in commercial applications, known for their high energy product and magnetic strength.
Neodymium magnets are the strongest and most controversial magnets, falling under the category of rare-earth magnets due to their composition of neodymium, iron, and boron elements.
Neodymium iron boron magnets are prone to oxidation due to their iron content and have poor corrosion resistance, often requiring coatings like nickel plating, epoxy resin coating, or poly-p-xylylene coating.
However, they are high-energy-density products (up to 55 MGOe) with high coercivity, and their use allows for smaller sizes of hard disk drives, motors, and audio devices.
Neodymium magnets operate within a temperature range of 80°C to 200°C. However, high-quality neodymium materials that can work above 120°C can become quite expensive.
Considering cost-effectiveness, neodymium magnets are often the preferred choice.
Fully-dense neodymium magnets are typically manufactured through powder metallurgy. Micron-sized neodymium iron boron powder is produced in an inert gas atmosphere, then compacted in rigid steel molds or rubber molds. Rubber molds are compacted from all sides with a fluid, known as isostatic pressing. Steel molds create shapes similar to the final product, while rubber molds produce large blocks of neodymium magnet alloy.
▲ The various Elements of Neodymium Magnets
The magnetic properties of neodymium alloys are induced by applying a magnetic field before or during the pressing process, which imparts preferred magnetization direction to the neodymium magnet alloy. The arrangement of particles creates an anisotropic alloy, significantly improving the remanence (Br) and other magnetic characteristics of the finished rare-earth magnet.
After pressing, neodymium magnets go through sintering and heat treatment processes until they reach a fully dense state. Molded magnets are ground to their final dimensions, but magnets produced using the rubber mold method are typically square and need to be cut into their final geometries using a grinder.
The production process of neodymium magnets can be summarized as follows:
- Discovery and extraction of rare-earth ore.
- Processing and refining the ore.
- Adding rare-earth alloy elements to refined metals.
- Melting (raw materials) and strip casting.
- Hydrogen decrepitation.
- Jet milling.
- Pressing under an external magnetic field.
- Cold isostatic pressing.
- Sintering.
- Annealing.
- Machining and grinding.
- Plating/coating.
- Magnetizing.
- Packaging and transportation.
▲ Production Process of Neodymium Magnets
▲ The Production Process of Neodymium Magnets
Samarium Cobalt Magnets
Samarium Cobalt (SmCo) is another high-performance permanent magnet material primarily composed of cobalt and samarium, and it is the most expensive magnetic material to manufacture. The majority of the cost arises from the high cobalt content and the brittleness of samarium alloy.
▲ The Elements of Samarium Cobalt Magnets.
This type of permanent magnet material possesses high corrosion resistance and can withstand high temperatures up to 350°C and sometimes even up to 500 degrees. Compared to other less temperature-resistant permanent magnet types, this provides them with a significant advantage. Similar to neodymium magnets, samarium cobalt magnets also require coatings to prevent corrosion.
▲ Photo Credit: Arnold Magnetic Technologies
The drawback of this magnet type is its low mechanical strength. Samarium cobalt magnets are prone to becoming brittle and fracturing. However, when temperature and corrosion resistance are required, samarium cobalt magnets might be the most suitable choice.
NdFeB performs well at lower temperatures, while SmCo excels at higher temperatures. Neodymium magnets are the strongest permanent magnets at room temperature and up to around 180 degrees Celsius, measured by their residual magnetization (Br). However, their strength rapidly decreases as the temperature rises. As the operating temperature approaches 180 degrees Celsius, the performance of samarium cobalt magnets begins to surpass that of NdFeB magnets.
Samarium cobalt is the second strongest magnetic material, possessing excellent resistance to demagnetization. It is widely used in the aerospace industry or in industrial sectors prioritizing performance over cost.
Samarium cobalt magnets developed in the 1970s are stronger than ceramic and aluminum-nickel-cobalt magnets but not as strong as neodymium magnets. Samarium cobalt magnets are primarily categorized into two groups based on their energy range. The first group, Sm1Co5 (also known as 1-5), has an energy product range between 15-22 MGOe. The second group, Sm2Co17 (2-17), has a range of 22-32 MGOe.
Both samarium cobalt and neodymium magnet alloys are powdered metals. They are compacted under the presence of a strong magnetic field and then sintered.
Neodymium magnets are highly reactive to environmental conditions, while samarium cobalt rare-earth magnets possess strong corrosion resistance. Samarium cobalt rare-earth magnets can endure high temperatures without demagnetizing, whereas caution must be exercised when using neodymium in conditions exceeding room temperature. Neodymium magnets are less brittle than samarium cobalt magnets and are easier to process and integrate into magnetic assemblies. Both alloys require diamond tools, electrical discharge machining, or grinding during processing. Reference website: https://www.duramag.com/materials/rare-earth-magnets/
▲ Photo Credit: Magnet Guide & Tutorial, www.Allianceorg.com
Aluminum Nickel Cobalt Magnets
Aluminum nickel cobalt magnets (AlNiCo) are traditional permanent magnet materials primarily composed of aluminum, nickel, and cobalt.
They are among the oldest modern commercial permanent magnets, with T. Mishima founding them in Japan in the early 20th century.
Although they have higher remanence, their relatively low coercivity results in a lower energy product (BH)max compared to other types of magnets.
Aluminum nickel cobalt magnets can be produced through sintering (similar to ceramic magnets) or casting.
Cast aluminum nickel cobalt can be made into intricate shapes, while sintered aluminum nickel cobalt has slightly lower magnetic properties but better mechanical properties due to its fine grain structure, resulting in uniform flux distribution and mechanical strength.
Sintering aluminum nickel cobalt involves induction melting, followed by grinding into fine particles, pressing, sintering, testing, coating, and magnetizing.
Different production methods impact magnet performance. Sintering provides improved mechanical performance, while casting offers higher energy density.
The grades of sintered aluminum nickel cobalt magnets range from 1.5 to 5.25 MGOe, while cast magnets range from 5.0 to 9.0 MGOe. Anisotropic aluminum nickel cobalt magnets allow customization of the magnetization direction, which is useful.
Aluminum nickel cobalt alloys have high maximum operating temperatures and excellent corrosion resistance. Certain grades of aluminum nickel cobalt can work at temperatures above 500°C. They are widely used in applications such as microphones, speakers, electric guitar pickups, electric motors, traveling wave tubes, Hall sensors, and more.
Ceramic Ferrite Magnets
Ceramic magnets, also known as ferrite magnets, consist of sintered iron oxide and materials like barium carbonate or strontium carbonate.
Ferrite magnets are known for their low cost, good corrosion resistance, and high-temperature stability up to 250°C.
While their magnetic performance is far lower than that of NdFeB magnets, their low cost makes them suitable for large-scale production. This is due to the inexpensive, abundant, and non-strategic materials used to manufacture this alloy.
Ceramic magnets can be isotropic, meaning they possess the same magnetic properties in all directions, or anisotropic, which means they are magnetized in the direction they are pressed. The strongest ceramic magnets reach up to 3.8 MGOe, making them the weakest type of permanent magnet. Despite their lower magnetism, they exhibit better resistance to demagnetization compared to other magnets.
▲ Comparison of Magnetic Strength between Ferrite and Neodymium Magnets.
Due to the commodity nature of the alloy and high tooling costs in the West, most of the world’s ceramic magnetic materials come from China.
Ceramic magnets have low energy products, good corrosion resistance, and are often used in components containing mild steel, suitable for working at moderate temperatures.
Manufacturing ceramic magnets involves pressing and sintering. Since they can be brittle, diamond wheels should be used if grinding is required.
Overall, ceramic magnets strike a balance between magnetic strength and cost-effectiveness, with their brittleness offset by their excellent corrosion resistance. They also possess strong coercivity and demagnetization resistance, making them an economical choice for common applications such as toys, crafts, motors, and more.
Rare-earth magnets greatly improve weight or size, while ferrite becomes a better choice for applications that don’t require high energy density, such as electric windows, seats, switches, fans, blowers in appliances, some power tools, and audio devices.
▲ Photo Credit: http://www.magnet-sdm.com/2017/06/23/1186/
Bonded Magnets
Bonded magnet materials combine ceramic, neodymium iron boron, or samarium cobalt powders with various plastic binders.
They can be formed into complex magnet shapes with finished dimensions through injection molding or compression bonding.
▲ Bonded Magnet
Common binders include polyester, thermosetting epoxy resins, and nylon for rigid bonded magnets, and vinyl-based and nitrile rubber for flexible bonded magnets.
The use of bonded magnets reduces the need for secondary processing, lowering production costs. However, due to the introduction of binder materials, bonded magnets have relatively lower maximum operating temperatures and poorer thermal stability.
Bonded magnets are commonly used in automotive parts due to their suitability for mass production and cost-effective production of complex shapes.
Neodymium iron boron-cobalt composite magnets (NdFeB/SmCo Composite) combine neodymium iron boron magnets and cobalt magnets to balance the advantages of both. They can offer higher performance under high-temperature and high-magnetic field conditions.
In summary, the article covers the classification, production processes, and basic applications of permanent magnets.
▲ The Production Process of Bonded Magnets.
NOTE:The article is from WeChat official accounts “Luo Luo’s Diary”.
