What is ferrite?
A natural mineral form of ferrite called magnetite has existed since the Earth's early beginnings, and was the "lodestone" used by ancient navigators in primitive compasses. Commercially used ferrites are all manufactured products, and were developed independently by scientists in Japan and Holland during the 1940's. There are two main types of ferrite, which are
referred to as "hard" and "soft" ferrites. These terms do not refer to
physical hardness, but rather to their abilities to retain magnetism.
Soft ferrites retain very little permanent magnetism, and are the
types used in impeders.
All ferrites are ceramic crystal structures formed from ions of iron,
manganese, nickel, chromium, zinc and oxygen. They are not
alloys or mixtures of metals. Physically they are hard, dense and
brittle, and can only be machined using diamond or cubic boron
nitride saws and grinders.
Ferrites have similar but superior magnetic properties to the iron
or steel laminations used in low frequency transformers, with the
additional advantage that they have much higher electrical
resistivity than metals, so eddy current heating is reduced. They
can also be easily formed in a variety of intricate shapes.
Ferrites are relatively expensive because of the complex and critical manufacturing processes involved, although the raw materials are plentiful and cheap. A simplified overview of the manufacturing process is as follows:
Final grinding and polishing may be applied if necessary to meet critical dimensional requirements, however this is an expensive process that is not normally applied to ferrite used in impeders. Impeder ferrite has fairly wide tolerances on length, diameter & straightness & these must be accommodated in the design of the impeder.Ferrite specifications
A theoretically perfect ferrite for impeders would be able to support an infinite amount of magnetic flux, and would have zero losses, requiring no cooling. Such a material has yet to be developed, however there are vast differences in the magnetic & electrical properties of commercially manufactured ferrites, which permit them to be tailored to the application in which they are used. Most of the ferrite produced in the form of rods or tubes is designed for antenna rods in radio receivers, where it operates at high frequencies, but at minuscule power levels. Impeder ferrite operates at very high power levels, but relatively low frequencies of 200 - 500kHz.
High purity oxide powders of iron, zinc, manganese, cobalt and nickel are mixed and blended.
The mixture is then calcined - heated in a kiln at 900 ° C to 1100 ° C - to start the process of forming the ferrite crystal lattice.
The calcined material is ground to a fine powder in a ball mill, then dried, mixed with a binder and formed into shapes either by extrusion or pressing.
The shaped pieces are dried, then sintered in a controlled atmosphere furnace at 1200 ° to 1400° to form the spinal crystal structure. During this phase, the material shrinks by as much as 40%.
Choosing the wrong grade of ferrite for use in impeders can have a disastrous effect on the efficiency of the welding process, and the life of the impeder.
All ferromagnetic materials are limited in the amount of magnetic flux that they can support. This is known as the saturation flux density, and it is normally expressed in Gauss or Teslas (10,000 Gauss = 1 Tesla). At high power densities, modern HF welders can establish very high flux densities with an impeder, so it is important to choose a grade of ferrite that will support this flux without saturation.
Ferrite used in impeders also requires cooling because electrical & magnetic losses in the material cause waste heat to be generated. The lower the losses in the ferrite, the less cooling required. Ferrite that operates at a lower temperature & which undergoes less thermal cycling will last longer & require less frequent replacement. Ferrite losses are expressed in Kilowatts/kg., or watts per cubic centimeter at a specified frequency & flux density. For impeder use, these losses should be as low as possible. The chart below shows comparative losses of various ferrite grades commonly used in impeders.
Saturation flux density & magnetic permeability both decrease at higher temperatures, so keeping the ferrite cool greatly improves its performance.
The ferrite that we stock is made to our specifications by leading manufacturers around the world, and it has been specifically formulated for high saturation flux density, high permeability & the lowest possible losses. There are other grades of ferrite available, often at lower cost, that will not perform as well, however using these in impeders will result in a loss of welding efficiency that costs many times more than the apparent savings in the cost of the ferrite!