An Alnico magnet is a permanent magnet made by combiming aluminum, nickel, iron, cobalt, and other elements. They come in isotropic, non-directional, or anisotropic, mono-directional, form...
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This article takes an in depth look at ceramic magnets and their use.
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A ceramic magnet, also known as a ferrite magnet, is a permanent magnet made by combining iron oxide and strontium carbonate. They are a man made magnet produced by heating the two elements to over 2000° F, which triggers a chemical reaction that changes the two element mixture into a ferrite material with a magnetic field.
Ceramic magnets are a low cost replacement for natural magnets and are found in everyday devices and industrial applications. They are the most widely used type of magnets in the world, found in 75% of all products that require a magnet. Accompanied with their low cost is their superior corrosion resistance.
In the 1960s, manufacturers were searching for a low cost alternative to metal magnets. It was near the beginning of the rapid development of electronics, and metal magnets significantly increased the cost of producing recording equipment. The discovery of ceramic magnets made them immediately popular due to their low cost, corrosion resistance, and resistance to demagnetization.
Ceramic magnets can be found in DC motors, magnetic separators, magnetic resonance imaging scanners, and various forms of sensors. The base element of ceramic magnets is ferrite, which combines iron oxide and strontium carbonate.
The first step in manufacturing ceramic magnets is the calcining of iron oxide powder and strontium carbonate. Calcining or calcination is an industrial heating process designed to change the chemical and physical properties of solid materials by exposing them to extremely high temperatures. The materials treated in the process are minerals, metals, and various types of ore.
Calcining heats materials to temperatures in excess of 1800° F or 1000° C. In the case of ceramic magnets, the calcining process produces a metallic oxide material, SrO-6 (Fe2O3). Depending on the grade of the ceramic magnet, other elements may be added, such as cobalt or lanthanum, to enhance the final product’s performance.
The purpose of milling is to transform the fine powder into finer particles as small as two micrometers (µ), which is far smaller than human hair. These tiny particles have their own magnetic domain.
A common form of milling used to mill the ferrite powder is a ball mill, a type of grinding machine. Traditional milling includes the use of rotary cutters. To transform ferrite powder into finer ferrite powder, a ball mill is used that uses metal balls to grind the powder.
The structure of a ball mill includes a compartment that can be vertical or horizontal that rotates. Inside the compartment are metal balls that are knocked around with the ferrite material striking the material multiple times reducing the size of the particles.
For the extremely fine powder to move on to the compacting process, it can be slightly liquified to form a slurry. The creation of the slurry changes the powder into a mud like texture that is created by the addition of water. This is very carefully completed to ensure that the mixture has the correct consistency for compacting.
It is during compacting that the shape of the ceramic magnet is achieved. It is a process of pressing, shaping, and molding the wet slurry into the final form of the magnet, which can be a wide and diverse number of sizes and configurations. An external magnetic field is applied during compacting aligning the anisotropic magnetic field.
If the material is not formed into a slurry and dry pressed, when the magnetic field is applied, it will be isotropic with weak magnetic properties.
Sintering, also known as frittage, is a method for forming a solid, hard mass through heat and pressure. The shaped magnets are slowly heated from 482° F or 250° C to 1652° F or 900° C. The amount of time the magnet endures the heating process determines its quality, with high quality ones being heated much longer than low quality ones. Heating can last from 20 to 36 hours.
Though the sintering process uses extremely high temperatures, they are never high enough to melt or liquify the material. It is a method of diffusing the atoms of a material across particle boundaries to form a solid mass.
The purpose of sintering is to add strength and integrity to ceramic magnets. As the heat in sintering rises, the small minute spaces between the particles are decreased, causing the material to squeeze tightly together.
In the diagram below, the round red circles represent the ferrite particles as a powder, mixture, and after sintering.
After sintering, the completed magnets are allowed to cool to room temperature. They are then ground and shaped to the proper dimensions required by their design. Millimeters of material are removed with each pass of the grinding tool until the proper size is achieved. The cutting process is completed using a diamond cutter for increased precision and accuracy.
Coating or plating of ceramic magnets is an extra measure designed to protect the magnet and increase its usefulness. There is a wide range of materials that are used to coat ceramic magnets.
Nickel-copper coating, or nickel coating, is completed in three layers that include a layer of nickel, a layer of copper, and a layer of nickel.
Epoxy is a long lasting coating that protects against corrosion.
Zinc is an inexpensive coating material that protects against corrosion and is a natural water barrier.
PTFE or Teflon improves the impact resistance of ceramic magnets to enhance their resistance to breakage.
Gold is used to protect the skin of users. Normally, 22 carat gold is used. Gold coated ceramic magnets are used for magnetic therapy.
The five coatings listed above are only a few of the choices of coatings that are available. In many cases, the type of coating is dependent on the application for which the ceramic magnet is designed.
Regardless of the many uses for ceramic magnets, they are normally divided into a few simple groups. The manufacturing process for ceramic magnets makes it possible to create a wide assortment of configurations and sizes, from ones that are shaped like tiny blocks to others that resemble hockey pucks. It is this versatility, along with their low price, that has made ceramic magnets so widely used.
The divisions for ceramic magnets are dependent on their magnetic properties and the applications for which they are manufactured. They can easily be divided into five types, which are soft, permanent, spin, moment, and piezomagnetic.
Soft ceramic magnets are ferrimagnetic with a cubic crystal structure. They are easy to magnetize and demagnetize. Soft ceramic magnets have a wide and varied number of applications, are produced in large quantities, and have a high output value. They are used for filters, transformers, radio cores, and tape recording and video heads.
One of the main methods for classifying soft ceramic magnets is by their low coercivity. The coercivity of a magnet is measured by their magnetic hysteresis loop or their magnetization curve.
Permanent ceramic magnets have a uniaxial anisotropic hexagonal structure. They can keep their strong properties for an extended period of time and can be used to generate a magnetic field. Permanent ceramic magnets are hard, which is the reason for their constant and consistent strength. They are used in refrigerators, microphones, automobile applications, and cordless appliances.
Hard ceramic magnetics are difficult to demagnetize due to their high coercivity, making them impossible to change. The strong and permanent magnetic field of hard ceramic magnets makes them ideal for applications that require strength and reliability. Since hard ceramic magnets are durable, they are used in telecommunications equipment that cannot fail and dependable.
Ceramic moment magnets have rectangular hysteresis loops. When they are in the presence of a small magnetic field, they become magnetized and saturated. Once the external magnetic field is removed, the magnet remains magnetized. This type of magnet is made from magnesium manganese ferrite and lithium manganese ferrite. They are an essential part of the memory cores of computers.
Piezomagnetic ceramic magnets have material that is mechanically elongated or shortened in the direction of the magnetic field when magnetized. In piezomagnetic materials, a magnetic field is created when the material is placed under stress or other form of deformation. It is made possible in a material when things are missing from its crystal structure.
The use of Piezomagnetic ceramic magnets can be found in transducers and magneto strictive parts for ultrasounds.
The concept of a spin ceramic magnet is based on rotary magnetism where there are two perpendicular stable magnetic fields and an electromagnetic wave magnetic field. The combination of the various fields causes constant rotation. Though some metal magnets have spin magnetism, they are not sustainable because of their eddy current loss, which has made the use of ceramic magnets necessary.
Ceramic magnets are used in a wide assortment of products, from speakers and recorders to large communication systems. Their low cost and flexibility make them an ideal method for providing magnetism. The main division of ceramic magnets is between soft and hard with hard magnets having high coercivity and soft magnets having low coercivity.
Prior to the development of ceramic magnets in the 1960s, there wasn’t an inexpensive way of providing a magnetic field since most magnets were made of metal. After the introduction of ceramic magnets, the cost of many products radically lowered and became generally available.
In a speaker, when sound is applied, the voice coil moves in and out in response to the signal and creates a magnetic field. A ceramic magnet attached to the voice coil controls the motion of the voice coil and affects the speaker’s sound quality. Ceramic magnet speakers are heavier to achieve the highest quality of sound and tend to have more punch and clarity.
Direct current motors use the force of a magnetic field to create the turning motion of the motor. An armature located in a magnetic field turns by the force of the magnetic field to produce DC current. In the case of a permanent ceramic magnet DC motor, a permanent magnet is used to produce the magnet field.
Permanent ceramic magnet motors are used in automobile starters, windshield wipers, washers, and air conditioners. They are used where there isn’t any need to control the speed of the motor, which is normally done by controlling the magnetic field.
MRI scanners use magnetic fields combined with radio waves to form an image of the human body. The process is used in hospitals to establish a medical diagnosis.
They come in various sizes and shapes, with some models having openings in the side to prevent patience from feeling claustrophobic.
The essential element of an MRI system is the extremely strong magnet that produces a stable magnetic field that runs the length of the MRI tube. The uniform magnetic field provides a clean and high quality image of the insides of a patient's body.
Small motors do not require a battery. They generate power for the spark plugs using a magneto. The voltage from the magneto creates a spark that jumps the spark plug gap that ignites the engine’s fuel. The function of the magneto is to supply current to the ignition system and produce voltage for the spark plugs. For the motor to work properly, the magneto must be precisely placed in relation to the flywheel to generate the correct magnetic field.
The magneto is a combination of distributor and generator, unlike a conventional distributor in that it creates its own spark without external voltage. Rotating ceramic magnets break the electrical field that causes the current in the primary windings, as seen in the image below.
The term ceramic magnet is a general descriptor for 27 different types of magnets. Each type and grade is designed to perform a particular function, from operating an MRI scanner to starting a lawnmower. Their high temperature and corrosion resistance, as well as their low cost, making them an ideal choice.
The most commonly used grades of ceramic magnets are C1, C5, and C8, which have a simple construction because of how they are manufactured. The differences between the grades of ceramic magnets are determined by their ferromagnetic properties.
The “C” labelling system for ceramic magnet grades has been replaced by a “Y” system. The grades of the magnets are labeled with the letter Y followed by a number or a set of numbers and letters. The numbers placed after the “Y” are an indication of the energy of the magnet. Letters that follow the number are descriptors for the varying characteristics of the magnet.
The “Y” system of labeling is slowly becoming the main method for labeling ceramic magnets. It has been accepted in most countries. Since the transition to the new system has been slow, the “C” labeling system is still widely used in the United States.
C1 ceramic magnets are weak and very small magnets that can easily fit into tight spaces. They have an operating temperature of 480° F or 249° C, making them ideal for use in high temperature applications. C1 ceramic magnets are used in speakers, small motors, and reed switches.
C5 ceramic magnets are the most popular of the different ceramic magnets and are magnetized in the direction of their orientation. They can be demagnetized, which restricts their use in some applications.
C8 ceramic magnets are used due to their exceptional energy and resistance to demagnetization. They are commonly used where the length of the magnet may be a problem, or there is the possibility of demagnetization.
Ceramic magnets C1, C5, and C8 are some of the more commonly used ceramic magnets and are found in many of the products on the market. They are only three of the 27 ceramic magnets that are offered by manufacturers. The list below contains a few of the more popular ceramic magnets and their properties and characteristics.
The magnetic field is a tool used to describe the magnetic force of something magnetic. Magnets have two poles that are attraction or repulsion or negative and positive. The vector method of describing a magnetic field uses dots on a grid where the vector points in the direction of a compass, and its length is dependent on the magnetic force. The field lines method depicts the pattern of the magnetic force using smooth curved lines.
There are two parameters that are used to measure a magnetic field, which are strength and direction. Of the two, the direction is the easiest to measure. The measurement of the strength of a magnetic field is more difficult and has only been available for a little over 30 years.
A magnetic field happens when a charge is in motion. The greater the charge, more motion is created, leading to a stronger magnetic field. An electromagnetic force from magnetism and magnetic fields is one of the four fundamental forces of nature.
There are many magnetization directions and are only limited by the size of the magnet. As can be seen, by the images below, there is axial magnetization that goes through the axis of the magnet and diametrical that goes across the length of the magnet. In addition, there are multiple pole versions with multipole axial and multipole diametrical.
The terms axial and diametrical are used to describe the magnetization direction in cylinders, disks, rings, and arc shaped magnets. With axial, the magnetic force is in the end planes, while with diametrical, the field is in the inner and outer arcs.
Magnetic fields are divided into two categories, which are isotropic and anisotropic. With isotropic magnets, magnetization direction is random in several directions, which leads to weaker isotropic magnets. Anisotropic magnets are directional and have a predetermined magnetic direction that gives them greater force.
The difference between isotropic and anisotropic can be easily seen in the image below where isotropic seems to be scattered and random while anisotropic is directional and even.
Since their development, there have been increasing uses discovered for ceramic magnets. They are permanent magnets and are the most durable and strong magnets in existence. The magnet permeability of ceramic magnets is the main reason for their strength.
About 75% of all magnets used in the world are ceramic magnets. There are several reasons for their wide use, the first of which is their low cost.
The first benefit that is mentioned every time ceramic magnets are discussed is their cost, which is significantly lower than other forms of magnets. The process of producing ceramic magnets and the magnetic field they produce is more powerful than magnets that are found in nature. The main reason ceramic magnets are less expensive is because they are formed from nonmetallic materials.
The viability of a magnet’s usefulness is determined by how it retains its magnetism when placed in stressful or harsh conditions. In the case of some magnets, they demagnetize when faced with vibrations or high temperatures. Though all magnets will demagnetize at increased temperatures, ceramic magnets are more durable than most and are capable of withstanding extreme temperatures and still be magnetized.
Ceramic magnets have excellent magnetic permanence when placed in the harshest of conditions. Their ability to withstand electrical exposure is the reason they are used in DC motors.
The mixture of iron oxide and strontium carbonate is naturally corrosion resistant. The properties of the two materials and their strengths are passed on to ceramic magnets, making them naturally corrosion resistant. This particular property is a necessity in industrial and manufacturing environments.
Once a ceramic magnet is magnetized, it is nearly impossible to demagnetize. This is essential in applications that depend on the resilience of magnets.
In addition to the ease with which ceramic magnets can be magnetized, they can also be magnetized in multiple directions, which makes them adaptable to a variety of applications.
Since ceramic magnets are made from compacted powdered material, they can be produced in any shape, size, or configuration that is required. The wide assortment of sizes makes it possible for ceramic magnets to be placed in small toys as well as large MRI scanners.
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