Ferrite World

Vol. 13 Ferrite is a material with unlimited potential

This articles republished the past articles have been reorganized and rewritten. It includes the past technologies and products information not currently handled by TDK.

”Tree of Ferrite” grown large from the “Soil of Techniques”

A ferrite is an innovative magnetic material invented in Japan in the 1930s. Initially, it had a very limited applications such as core materials of antenna coils in wireless communication devices and radio receivers. With the flowering of the electronics industry after the 1950s, however, it has extended the range of its applications to, for example, cores for intermediate-frequency transformers in super radios, cores for deflection yokes and flyback transformers in television’s CRTs, magnetic powders for magnetic tapes (semi-hard magnetic ferrites), and magnets for loudspeakers and motors (hard magnetic ferrites, or hard ferrites), and its production volume has rapidly increased. As a ceramic magnetic material that is on par with metallic magnetic materials, ferrites have fundamentally supported the development of the electronics industry.
“Tree of Ferrite” shown below is a tree diagram that outlines various application fields of ferrites. Among these applications, we will introduce some rare examples.
In “Tree of Ferrite,” you can see applications like a memory core and bubble memory in the computer field. In Japan in the 1950s, when a domestic computer named the parametron was developed, ferrites were used as the memory device of the computer. A memory core is a memory device that fine ring ferrite cores are arrayed in a grid structure and through which signal wires for reading and writing are passed. When a current flows through the signal wire, the ring core becomes magnetized due to the current’s magnetic action, but because the orientation of the magnetization reverses depending on the current direction, it can be used as zero or one digital information. Since the magnetization remains semi-permanent, memory cores were used, as highly reliable non-volatile memories, for memory devices of computers installed in space shuttles.
A bubble memory uses a special ferrite thin film. Digital information is stored based on whether magnetization of the cylindrical domain generated vertically from the film surface is upward or downward. For the magnetic thin film for bubble memories, a garnet-type ferrite is used, instead of a spinel-type ferrite (soft ferrite for transformer cores etc.) and a magnetoplumbite-type ferrite (hard ferrite for ferrite magnets, etc.). Garnet-type ferrites are also used in circuit components such as circulators and isolators, which transmit a radio wave in one direction without backflow in communication equipment. 

Ferrites also found in copiers and electrolysis system

Ferrites are also used in an interesting way in copiers based on the xerographic principle (plain paper copiers, or PPCs) such as laser printers. The xerography works when an electrically charged photoconductive drum is exposed to the light in the shape of letters on documents or images emitted from a laser beam, and the areas on the drum surface that have not been exposed to the light remain charged. Then, toner powders are attracted to the drum surface due to the electrostatic action, and transferred and fused to the paper. This is the xerographic principle. Fine ferrite particles called carriers play a role in conveying the toner powders. Because a ferrite is a magnetic material, it is attached to a magnet. Taking advantage of this property, ferrite carriers having toner powders are attached to the magnet roll and then transferred to the photoconductive drum due to the electrostatic action, thereby enabling the images to be developed.
The ferrite carrier is manufactured by a spray drying method. In spray drying, the slurry (muddy) material is atomized and then dried at low temperature with a spray dryer, thereby obtaining spherical ferrite carriers with the uniform particle size.
There are a variety of shaping methods to manufacture ferrite products with many different shapes. Most ferrite magnets are manufactured by wet-type shaping in which the slurry material is injected into the mold and then pressurized and dehydrated while a magnetic field is applied, whereas most transformer cores are manufactured by dry-type shaping in which the particles of the ferrite material are put into the mold and then pressurized. In order to shape complex-shaped transformer cores, high-level knowhow for pressurizing is required.
Extrusion is also used for rod-like or pipe-like ferrite products such as ferrite electrodes. The ferrite electrode is an electrode used in applications such as electrolysis in the electrochemical industry. Because ferrites, a type of ceramic, are superior in corrosion resistance and strength, they have been increasingly used, replacing other materials such as expensive platinum-plated titanium.
Ferrite electrodes are also used for electric protection. Iron structures at a seaport such as steel pipe piles and steel sheet piles cause the local cell action in the seawater, which is the same phenomenon seen in voltaic cells, and the iron dissolves while releasing its ions and corrodes. Electric protection is a method to prevent this corrosion by applying a small current in the opposite direction of a current from the local cell in the iron structure. In addition, ferrite electrodes have the advantage that there is no concern about elution of harmful substances or secondary pollution since their major component is iron oxide. Ferrite electrodes are also used in household appliances such as electric water heaters and alkali ion water purifiers.

Ferrites contribute to environmental preservation

Ferrites are also used in applications in the environmental preservation field such as wastewater treatment. To prevent pollution, there are strict regulations on heavy metal ions contained in the wastewater from plants and other facilities. Depending on types of heavy metal ions, however, some ions cannot be sufficiently removed by typical chemical treatment. To overcome this issue, a wastewater treatment technology called an air oxidation ferrite formation method takes full advantage of the manufacturing process of ferrites.
The air oxidation ferrite formation method employed a coprecipitation method, which was originally developed to obtain high-performance ferrite materials for high frequencies. Among ferrite material manufacturing methods, there is a liquid phase method in which ferrites are deposited in the solution, in addition to a dry method in which material powders are mixed and sintered. The coprecipitation method is representative of the liquid phase method. First, a water solution of metallic salt constituting a ferrite is prepared and stirred in an ion order . As the salt precipitates as hydroxide and others when a strong alkali is added to this water solution, homogeneous spinel-type ferrite powders can be obtained through heating treatment of the precipitation. This is a coprecipitation method. With this technique, if heavy metal ions are contained in a water solution, the ions are displaced by iron ions and caught in the ferrite. The ferrite formation treatment method takes advantage of this technique. As the ferrites formed are a magnetic material, they can be separated easily using a magnetic separator with magnets; thus, this technique is used not only at plants but also at research facilities and universities as a treatment method for environmentally harmful heavy metal ions.
Spinel-type ferrites have a feature that they form a variety of solid solutions by incorporating many types of metallic ions. Ferrites with various properties can be obtained by using a small amount of additives because their iron ions can easily be displaced by other metallic ions.
Spinel-type ferrites are used for transformer cores but there are other various types of ferrites, including magnetoplumbite-type and garnet-type ones. They are used in various fields where the advantage of each can be utilized.
If intelligent life exists on a distant planet in the universe, it might have invented ferrites as we did and come up with extraordinary applications using them. It is because ferrites are electronic materials with infinite possibilities, although they use common iron oxide as a major component. 

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