CERAMIC CAPACITORS

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NON MAGNETIC CAPACITORS

ULTRA-LOW ESR, RF & MICROWAVE SYSTEMS

In today’s world of medical systems, there is a trend in MRI equipment to increase the

magnetic field – mostly from 1.5T to 3.0T. The higher signal strength obtained can then

be translated into higher spatial resolution, enabling doctors to see finer details on the

images. Whence the importance of non magnetic properties in the electronic compo-

nents used in such systems.

At present, components with a significant magnetic response create parasitic black

dots on the images, which may result in inaccurate or more difficult diagnosis – for

instance the electrolytic capacitors aluminum or tantalum-based (paramagnetic). Not

only this, but magnetic losses will overheat the system and reduce the reliability of the

electronic components. Problems such as these - system temperature and component

reliability - due to a significant magnetic response can moreover occur in any electronic

equipment, though usually with a lower level of criticality.

To further improve reliability in such systems:

The electronic components used in MR systems, like the multilayer ceramic capaci-

tors from EXXELIA, must have a very low magnetic response (diamagnetic);

A classification is needed for R&D engineers designing such systems, to quantify

the magnetic response. This way, any component used in new developments – ir-

respective of its configuration, with wires or ribbons, etc. - would be guaranteed for

MR applications;

Non magnetic components should also be proposed for non medical applications

involving high RF power, so as to minimize losses and thereby improve the overall

system performance.

The comprehensive magnetic study described below was conducted by the I.C.M.C.B.

(the Bordeaux Institute of Chemistry of condensed materials), a laboratory under the

aegis of the C.N.R.S. (French National Center for Scientific Research).

http://www.cnrs.fr/index.html

I. MAGNETIC FIELD NOTIONS

I.1. Magnetic Permeability

This is the degree of magnetization of a material that responds linearly to an applied

magnetic field. The magnetic permeability (µ) of a given material is related to the

permeability of vacuum (µ0, in Henries per meter) times its relative permeability

(µR, no unit):

µ = µ0 x µR

µ0 is a universal constant, the magnetic constant, and has the value.

4∏ x 10–7 H/m

µR is related to the material under test.

In vacuum, air, gases, ... µR is equal to 1. These materials do not modify magnetic field

lines. There are three types of materials:

Diamagnetic (silver, copper, gold, lead, ...) in which µR ≤ 1 and close to 1

Paramagnetic (platinum, aluminum, magnesium, ...) where µR ≥ 1 and close to 1

Ferromagnetic (nickel, cobalt, iron, ...) with µR >> 1

I.2. Paramagnetism

Paramagnetism is a form of magnetism which occurs only in the presence of an exter-

nally applied magnetic field. Paramagnetic materials are attracted to magnetic fields,

and hence have a relative magnetic permeability µR greater than one - or, equivalently,

positive magnetic susceptibility. However, unlike ferromagnets, which are also attract-

ed to magnetic fields, paramagnets do not retain any magnetization in the absence of

an externally applied magnetic field.

I.3. Diamagnetism

Diamagnetism is a weak repulsion from a magnetic field. It is form of magnetism that is

exhibited by a substance only in the presence of an externally applied magnetic field.

All materials show a diamagnetic response in an applied magnetic field but for materi-

als which show some other form of magnetism (such as ferromagnetism or paramag-

netism), the diamagnetism is completely overpowered.

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