Ferrite

Ferrites are ferromagnetic ceramic materials, compounds of iron, boron and barium or strontium or molybdenium. Ferrites have a high magnetic permeability, which allows them to store stronger magnetic fields than iron. Ferrites are often produced as powder, which can be sintered into solid cores. Ferrite cores are used in electronic inductors and electromagnets. Early computer memories stored data in the magnetic fields of ferrite cores, which were assembled into arrays of core memory. Ferrite powders are used in the coatings of magnetic recording tapes. One such type of material is iron (III) oxide.

Ferrite is a class of ceramic material with useful electromagnetic properties and an interesting history. Ferrite is rigid and brittle. Like other ceramics, ferrite can chip and break if handled roughly. Luckily it is not as fragile as porcelain and often such chips and cracks will be merely cosmetic. Ferrite varies from silver gray to black in color. The electromagnetic properties of ferrite materials can be affected by operating conditions such as temperature, pressure, field strength, frequency and time.

There are basically two varieties of ferrite: soft and hard. This is not a tactile quality but rather a magnetic characteristic. 'Soft ferrite' does not retain significant magnetization whereas 'hard ferrite' magnetization is considered permanent. Fair-Rite ferrite materials are of the 'soft' variety.

Ferrite has a cubic crystalline structure with the chemical formula MO.Fe2O3 where Fe2O3 is iron oxide and MO refers to a combination of two or more divalent metal (i.e: zinc, nickel, manganese and copper) oxides. The addition of such metal oxides in various amounts allows the creation of many different materials whose properties can be tailored for a variety of uses.

The oxide ceramics have the general composition MIIFeIII2O4 or MIIO ? Fe2O3, which comprise permanent magnetic dipoles.

Ferrite components are pressed from a powdered precursor and then sintered (fired) in a kiln. The mechanical and electromagnetic properties of the ferrite are heavily affected by the sintering process which is time-temperature-atmosphere dependent.

A short history of Ferrites

The history of ferrites (magnetic oxides) began centuries before the birth of Christ with the discovery of stones that would attract iron. The most plentiful deposits of these stones were found in the district of Magnesia in Asia Minor, hence the mineral's name became magnetite (Fe3O4).

Much later, the first application of magnetite was as 'Lodestones' used by early navigators to locate magnetic North. In 1600 William Gilbert published De Magnete, the first scientific study of magnetism. In 1819 Hans Christian Oersted observed that an electric current in a wire affected a magnetic compass needle. With further contributions by Faraday, Maxwell, Hertz and many others, the new science of electromagnetism developed.

Naturally occurring magnetite is a weak 'hard' ferrite. 'Hard' ferrites possess a magnetism which is essentially permanent. In time, man-made 'hard' ferrites with superior properties were developed but producing an analogous 'soft' magnetic material in the laboratory proved elusive.

During the 1930's research on 'soft' ferrites continued, primarily in Japan and the Netherlands. However, it was not until 1945 that J. L. Snoek of the Phillips Research Laboratories in the Netherlands succeeded in producing a 'soft' ferrite for commercial applications. Originally manufactured in a few select shapes and sizes, primarily for inductor and antenna applications, 'soft' ferrite has proliferated into countless sizes and shapes for a multitude of uses. Ferrites are used predominately in three areas of electronics: low level applications, power applications, and Electro-Magnetic Interference (EMI) suppression.

The breadth of application of ferrites in electronic circuitry continues to grow. The wide range of possible geometries, the continuing improvements in material characteristics and their relative cost-effectiveness make ferrite components the choice for both conventional and innovative applications.

Ferrite shrinks when sintered. Depending on the specific ferrite, this shrinkage can range from 10% to 17% in each dimension. Thus the unfired component's volume may be as much as 60% larger than the sintered value. Maintaining correct dimensional tolerances as well as the prevention of cracking and warpage related to this shrinkage are fundemental concerns of the manufacturing process.