Magnets

Chapter 1: What is the principle of magnets?

This chapter will cover the fundamentals of magnets, including their manufacturing processes and how they function.

What is a Magnet?

Magnets are materials that exert a noticeable force on other materials without direct contact. This force, known as magnetic force, can either attract or repel. While most materials exhibit some degree of magnetic force, only certain materials exhibit a strong enough force to be classified as magnets. The Earth itself acts as a massive magnet.

All magnets have two points where the magnetic force is strongest, known as the poles. On a rectangular bar magnet, these poles are located directly across from each other. They are referred to as the North Pole (or north-seeking pole) and the South Pole (or south-seeking pole).

How Magnetism Works

Magnetism arises from the behavior of very small particles called electrons. All matter is composed of atoms, which in turn consist of electrons, neutrons, and protons. Electrons spin around the atom's nucleus, generating tiny magnetic forces. In some materials, many electrons align their spins in the same direction, resulting in a cumulative magnetic effect that manifests as a single large magnet.

A magnet can be created by rubbing a piece of metal with an existing magnet, ensuring that the metal is rubbed continuously in one direction. This process aligns the electrons in the metal, causing them to spin in the same direction. Electric current can also generate magnets. As electricity is the flow of electrons, the movement of these electrons in a wire creates a magnetic field, similar to the effect of electrons spinning around an atomic nucleus. This type of magnet is known as an electromagnet.

Metals like nickel, cobalt, iron, and steel are excellent for making permanent magnets due to the arrangement of their electrons. Once magnetized, these metals can retain their magnetic properties indefinitely and are referred to as hard magnets. Conversely, some metals can exhibit magnetic properties temporarily when exposed to or brought near a hard magnet, earning them the designation of soft magnets.

Most materials, such as wood, water, and air, exhibit very weak magnetic properties. Magnets, however, strongly attract objects containing metals like iron, nickel, or cobalt. They also attract or repel other hard magnets when brought close to each other. This behavior occurs because each magnet has two opposite poles: the north pole and the south pole. North poles attract south poles of other magnets, while like poles repel each other.

Manufacturing Magnets

The most common method for manufacturing magnets is powder metallurgy. As magnets are made from various materials, their manufacturing processes vary. For example, electromagnets are produced using metal casting techniques, while flexible permanent magnets are made through plastic extrusion. In this process, raw materials are heated, mixed, and then forced through an opening under high pressure. The following outlines the general process of magnet manufacturing.

Preparing the Powder

Iron, boron, and neodymium are heated to a molten state in a vacuum or induction melting furnace with inert gas to prevent chemical reactions with air. Once the molten alloy cools, it is broken into small metal strips, which are then pulverized into a fine powder with a diameter ranging from 3 to 7 microns. This powder is highly reactive and can ignite in air, so it must be kept away from exposure to oxygen.

Isostatic Compaction

The process of isostatic compaction, also known as pressing, involves placing powdered metal into a mold, or die. To align the powder particles, a magnetic force is applied during the compaction process. Hydraulic rams then compress the material to within 0.125 inches (0.32 cm) of its intended thickness. Typically, high pressures ranging from 10,000 psi to 15,000 psi (70 MPa to 100 MPa) are used. For creating other designs and shapes, the powder is placed in an airtight evacuated container before being shaped by gas pressure.

Heating of the Material

After isostatic compaction, the slug of powdered metal is removed from the die and placed in an oven for sintering. Sintering is the process of applying heat to compressed powdered metals to transform them into solid, fused metal pieces.

The sintering process consists of three main stages. In the first stage, the material is heated to low temperatures to remove moisture and any contaminants trapped during compaction. In the second stage, the temperature is increased to about 70-90% of the alloy's melting point and maintained for several hours or days, allowing the particles to bond and fuse together. The final stage involves slowly cooling the material in controlled temperature increments to complete the sintering process.

Annealing of the Material

After the heating process comes the process of annealing. This is when the sintered material undergoes another step by step controlled heating and cooling process in order to discard any or all residual stresses that are left within the material and make it stronger.

Magnet Finishing

The sintered magnets typically undergo various machining processes, such as grinding to achieve smooth, parallel surfaces or cutting into smaller parts from block magnets. Due to the hardness and brittleness of the material (Rockwell C 57 to 61), diamond wheels are used for slicing and abrasive grinding. Precision slicing often eliminates the need for further grinding, but both processes must be executed with care to minimize chipping and cracking.

In some cases, the final magnet shape is well-suited for processing with a shaped diamond grinding wheel, similar to shaping bread loaves. This grinding wheel provides precise dimensions and closely approximates the desired final shape, a condition referred to as "near net shape." Any remaining excess material is removed in a final machining step, resulting in a smooth surface where needed. The finished magnet is then coated with a protective layer to seal its surface.

Magnetizing Process

Magnetizing occurs after the finishing process. Once manufacturing is complete, the magnet must be charged to produce an external magnetic field. This is achieved using a solenoid—a hollow cylinder designed to accommodate various magnet sizes and shapes. The solenoid can be customized with fixtures to impart specific magnetic patterns or designs. To avoid handling and assembling powerful magnets while they are magnetized, large assemblies can be magnetized as a whole. It is important to consider the substantial requirements for the magnetizing field during this process.

All critical aspects of magnet selection should be discussed with both engineering and production teams. Up to this point in the manufacturing process, the material is simply a piece of compressed metal. While the isostatic pressing process aligns the powder particles, it does not impart any magnetic effect. To achieve magnetization, the material is placed between the poles of a strong electromagnet and oriented in the desired direction for magnetization. Once the electromagnet is energized, its magnetic field aligns the magnetic domains within the material, transforming it into a strong permanent magnet.

Magnetic Separation

The process of magnetic separation involves using a magnet to attract and separate magnetic materials from a mixture. This technique is particularly effective for separating ferromagnetic minerals, such as those containing cobalt, iron, and nickel. Metals like silver, aluminum, and gold are not magnetic and thus are not separated by this method. Magnetic separation typically employs various mechanical methods to isolate magnetic materials. In this process, magnets are arranged inside separator drums containing liquids. As the drums rotate, the magnetic particles are drawn to the magnets, creating a magnetic concentrate, such as an ore concentrate.

The process of magnetic separation is also employed in electromagnetic cranes to extract magnetic materials from unwanted substances, highlighting its importance in waste management and shipping. This method efficiently separates unnecessary metals from goods, ensuring material purity. It is widely used in recycling facilities to remove components from recyclable materials, separate metals, and purify ores. Historically, magnetic pulleys, overhead magnets, and magnetic drums were commonly used for recycling in the industry.

Magnetic separation is particularly valuable in mining iron due to iron's strong attraction to magnets. It is also applied in processing industries to remove metal contaminants from products. This method plays a crucial role in pharmaceutical and food industries, where it helps monitor and control pollution and process chemicals. Additionally, weak magnetic separation is used to produce high-quality, iron-rich products with minimal contaminants, enhancing their reusability.

Chapter 2: What are the different types of magnets?

Different types of magnets are categorized as follows:

Alnico Magnets

Alnico magnets come in cast, sintered, and bonded forms, with cast alnico magnets being the most common. They are an essential category of permanent magnet alloys composed of nickel (Ni), aluminum (Al), iron (Fe), and cobalt (Co), with minor additions of titanium (Ti) and copper (Cu). Alnico magnets exhibit relatively high coercivities due to the shape anisotropy of the iron (Fe) or cobalt (Co) particles, which are precipitated within a weakly ferromagnetic or non-ferromagnetic nickel-aluminum matrix. After cooling, isotropic alnico magnets are tempered at high temperatures for several hours.

Spinodal decomposition is a phase separation process where the final sizes and shapes of particles are determined early in the process. Alnico magnets are known for their excellent temperature coefficients, meaning they experience minimal changes in field output with temperature variations. These magnets can operate at the highest temperatures of any magnet type.

Demagnetization of the alnicos can be reduced if the working point is improved, such as for making use of a longer magnet than before in order to increase the length to diameter ratio which is a good rule of thumb guide for the Alnico magnets. All external demagnetizing factors must be taken into consideration however. A huge length to diameter ratio and a good magnetic circuit may also be required.

Bar Magnets

Bar magnets are rectangular objects made from steel, iron, or other ferromagnetic materials with strong magnetic properties. They have two poles: a north pole and a south pole.

When a bar magnet is suspended freely, it aligns itself such that its north pole points towards the Earth's magnetic north pole.

There are two main types of bar magnets. Cylindrical bar magnets, also known as rod magnets, have a high thickness in diameter, which enhances their magnetic properties. The second type is rectangular bar magnets, which are commonly used in manufacturing and engineering applications due to their greater magnetic strength and field compared to other magnets.

When a bar magnet is broken in half, each piece will still have both a north and a south pole, even if broken into smaller parts multiple times. The magnetic force of a bar magnet is most intense at its poles. When two bar magnets are brought near each other, opposite poles attract, while like poles repel. Bar magnets are effective at attracting ferromagnetic materials like cobalt, nickel, and iron.

Bonded Magnets

Bonded magnets consist of two primary components: a non-magnetic polymer and hard magnetic powder. The powder can be derived from various magnetic materials such as alnico, ferrite, neodymium, cobalt, and iron. Additionally, multiple types of magnetic powders can be combined to create hybrid mixtures. The properties of these powders are meticulously optimized through precise chemical processes and step-by-step manufacturing, ensuring that bonded magnets perform effectively regardless of the materials used.

Bonded magnets have numerous advantages in that the near net shape manufacturing requires no or low finishing operations when compared to other metallurgical processes. Therefore value added assemblies can be made economically in one operation. These magnets are a highly versatile material and they consist of multiple processing options. Some advantages of bonded magnets are that they have excellent mechanical properties and great electrical resistivity when compared with sintered materials. These magnets are also available in different complex sizes and shapes. They have good geometric tolerances with very low secondary operations. They are also available with multipole magnetization.

Ceramic Magnets

Ceramic magnets, also known as ferrite magnets, are a type of permanent magnet and are the most cost-effective compared to other magnet types. Made from iron oxide and strontium carbonate, these magnets offer medium magnetic strength and can operate at high temperatures. Their key advantages include corrosion resistance and ease of magnetization, making them a popular choice for a wide range of applications, including industrial, technical, and commercial uses. Ceramic magnets come in various grades, with Grade 5 being commonly used, and they are available in different shapes such as blocks and rings. They can also be custom-manufactured to meet specific customer requirements.

Ferrite magnets can operate at high temperatures, although their magnetic properties decrease as the temperature rises. They require specialized machining skills for precision shaping. An advantage of ceramic magnets is their resistance to surface rust, thanks to a protective film of magnetic powder on their surface. They are typically bonded to products using superglue. However, ceramic magnets are very brittle and can easily break if dropped or subjected to impact, so they require careful handling to avoid damage.

Electromagnets

Electromagnets are magnets whose magnetic field is generated by an electric current. Typically, they consist of a wire wound into a coil. When current flows through the wire, it creates a magnetic field. This field vanishes when the current is turned off. Electromagnets are usually composed of wire turns wound around a magnetic core made from ferromagnetic material. The core concentrates the magnetic flux, resulting in a more powerful magnet.

Electromagnets offer the advantage of quickly adjusting the magnetic field by regulating the electric current in the winding, unlike permanent magnets. However, they require a continuous current supply to maintain their magnetic field. Electromagnets also have some drawbacks: they heat up rapidly, consume significant amounts of energy, and can discharge large amounts of energy if there is an interruption in the current. Despite these challenges, they are commonly used in various electrical devices such as generators, relays, electro-mechanical solenoids, motors, loudspeakers, and magnetic separation equipment. Additionally, they are useful in industry for moving heavy objects and picking up iron and steel scrap. Electromagnets attract ferromagnetic materials like nickel, cobalt, and iron. Like most magnets, similar poles repel each other, while opposite poles attract.

Flexible Magnets

Flexible magnets are designed to bend and flex without breaking or sustaining damage. Unlike rigid magnets, these magnets can be deformed, rolled, or bent without losing their magnetic properties. The flexible magnet shown in Figure 2:6 illustrates this capability. Typically, flexible magnets consist of a synthetic substrate, such as vinyl, coated with a thin layer of ferromagnetic powder. The substrate is made from a very pliable material, and when the ferromagnetic powder is applied, it imparts magnetic properties to the entire surface.

Various production methods are used to manufacture flexible magnets, but most involve applying ferromagnetic powder to a synthetic substrate. The ferromagnetic powder is mixed with an adhesive binding agent to ensure it adheres to the substrate. Flexible magnets come in various forms, including sheets of different designs, shapes, and sizes, as well as strips that are thinner and longer than the sheets. These magnets are commonly used in applications such as motor vehicles, doors, metal cabinets, and buildings.

On the market they are usually sold and packaged in rolls. Flexible magnets are versatile with their bendable properties and they can wrap around machines so easily as well as other surfaces and components. A flexible magnet is supported even with surfaces which are not perfectly smooth or flat. Flexible magnets can be cut and shaped into desired shapes and sizes. Most of them can be cut even with a traditional cutting tool. Flexible magnets are not affected by drilling, they will not crack but they will form holes without damaging the surrounding magnetic material.

Industrial Magnets

An industrial magnet is a powerful magnet used across various sectors in the industrial field. These magnets come in various shapes and sizes and are known for their diverse grades and qualities, which help retain residual magnetism. Industrial permanent magnets can be made from materials like alnico, rare earth, or ceramic. They are crafted from ferromagnetic substances that are magnetized by an external magnetic field and can maintain their magnetized state over an extended period. Industrial magnets hold their magnetic properties without external support and feature two poles where the magnetic intensity is highest.

Samarium-cobalt industrial magnets are known for their ability to withstand high temperatures up to250 °C.. These magnets are highly resistant to corrosion due to their lack of iron content. However, their production is costly because of the high price of cobalt. Despite the expense, samarium-cobalt magnets are valued for their ability to produce very strong magnetic fields and are commonly used in applications involving high operating temperatures, such as in motors, sensors, and generators.

Alnico industrial magnets are made from a combination of aluminum, cobalt, and nickel, with possible additions of copper, iron, and titanium. Compared to samarium-cobalt magnets, alnico magnets can endure even higher temperatures, up to 525 °C, making them highly heat-resistant. They are also more susceptible to demagnetization due to their sensitivity. Industrial electromagnets, on the other hand, are adjustable and can be turned on and off as needed.

Industrial magnets have various applications, such as:

They are used to lift sheet steel, iron castings, and iron plates. These powerful magnets are commonly employed in manufacturing industries to facilitate the handling and transfer of heavy objects. By placing the magnet on top of the object and activating it, the magnet securely holds the item, allowing for efficient relocation to the desired location. One of the key advantages of industrial lifting magnets is the significant reduction in the risk of musculoskeletal issues among workers.

Utilizing industrial magnets helps protect manufacturing workers from injuries by eliminating the need for manual lifting of heavy materials. This enhances productivity in manufacturing settings, as manual handling of heavy objects is both time-consuming and physically demanding, impacting overall efficiency. Industrial magnets streamline these processes, contributing to improved workplace safety and productivity.

Neodymium Magnets

Rare earth magnets, such as neodymium magnets, are permanent magnets known for their exceptionally strong magnetic fields, producing strengths of over 1.4 teslas. These powerful magnets have a wide range of applications. They are commonly used in hard disk drives, where they help magnetize the tracks and segments to store data. Neodymium magnets are also integral to the function of loudspeakers, headphones, microphones, and earphones, enhancing audio quality with their strong magnetic properties.

The current-carrying coils that are found in these devices are used together with permanent magnets to alter electricity into mechanical energy. Another application is that the small sized neodymium magnets are mostly used to place dentures perfectly in place. These magnets are used in residential and commercial buildings on the doors for safety reasons and total security. Another practical use of these magnets is in making therapy jewelry, necklace, and jewelry. Neodymium magnets are greatly used as anti-lock brake sensors, these anti-lock brakes are installed in cars and numerous vehicles.

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Chapter 3: What are some common applications of magnets?

This chapter will explore various applications of magnets across different industries and technologies.

Applications of Magnets

Magnets are employed in a wide range of applications across various contexts and purposes. They come in sizes ranging from tiny components to large structures. In everyday life, magnets are integral to many devices; for instance, hard disks in computers use magnetic elements to store and retrieve data. Additionally, magnets are found in televisions, radios, and speakers, playing a crucial role in their operation.

Magnetic Stripe

Magnetic stripe technology enables data storage on plastic cards by encoding information into tiny magnetic bits along a stripe on the card. This technology underpins credit and debit cards, significantly reducing cash transactions globally. Known as magstripe technology, it has led to the creation of highly durable cards with robust data integrity. Thanks to magnetic stripe cards, financial institutions and banks can efficiently handle a wide range of card-based transactions and processes.

Magnetic stripes are integral to countless transactions daily and are widely used in various identification cards. Card readers can quickly extract details from these magnetic cards, which are then sent to banks for authorization. However, in recent years, a new technology has increasingly rivaled magnetic card transactions: contactless payment systems. This modern method uses signals from a small chip to transfer transaction details, eliminating the need for a magnetic stripe. Apple Inc. has been a pioneer in advancing contactless payment systems.

In speakers, a small coil of wire and a magnet work together to convert electronic signals into sound vibrations. Similarly, generators utilize magnets to transform mechanical energy into electrical energy. Magnets are also crucial in various mechanical and electrical motors, where they convert electrical energy into mechanical motion.

Magnets play a key role in lifting large metal objects that are too heavy for manual handling, such as in crane operations. They are also used in the separation and filtering of metallic ores from crushed rocks, and in the food processing industry to remove small metal contaminants from grains. These are just a few examples of the diverse applications of magnets.

Drawbacks of Magnets

Several significant drawbacks are associated with different types of magnets. Ferrite magnets, for instance, are produced through molding and sintering, making them challenging to machine. As a result, ferrite magnets typically have simple shapes and large dimensional tolerances. Samarium Cobalt magnets are quite brittle, which complicates the processing of small-sized products. Additionally, many magnets suffer from degradation at very high temperatures. Neodymium magnets, on the other hand, are prone to corrosion and thus require protective coatings to maintain their durability.

Conclusion

Magnets come in different forms, from simple bar magnets to very large permanent industrial magnets. Every type of magnet has two poles and even if they are cut in half, they will still have these two poles. Magnets are of great importance to the human community, but however they can be demagnetized at excessive temperatures and pressure.

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