What are the Important Characteristics of Soft Magnetic Materials?
- Initial and maximum permeability
- High saturation induction
- Low hysteresis energy losses
- Low eddy current loss in alternating flux application
What is "Permeability"?
- Imagine a sponge. Some sponges have big holes while some are more tightly structured. Each absorbs liquid to point of saturation. Magnetic Shielding materials react in much the same way. Depending upon its structure, magnetic shielding material absorbs magnetic interference to a point of saturation. As magnetic interference increases, so too should the permeability of the material
What is a Typical Range of Coercivity for Soft Magnetic Materials?
- Soft magnetic material exhibit coercive force from 5 Oe to as low as 0.002 Oe (Note: 1Oe=0.8 A per cm)
What is the Difference Between Normal S-Shaped Hysteresis Loop and Square Hysteresis Loop
- Ratio of Br/Bs is 0.5 to 0.75 in S-Shaped Hysteresis Loop
- Ratio of Br/Bs is 0.8 to 1.0 in Square Shaped Hysteresis Loop
- Note: Br: Remnant Magnetisation, Bs: Saturation Magnetisation
What are the Approximate Dimensions of Atom, Domain and Grain for Iron Element
- Dimension of an Iron Atom is 2.866A and there are 10^15 atoms in a Domain and 10^6 Domains in a Grain
- 1A is 10^(-10)m
What are the factors influencing domain pinning which makes it difficult to move the domain boundry?
- The presence of impurities (C, S, O, N, Mn, P)
- Plastic Strain (Mechanical Working)
- Inclusions or any crystal imperfections leads to domain pinning
What are the advantages of preferred oriented Fe-Ni sheets over randomly oriented Fe-Ni sheets?
- Preferred oriented Fe-Ni sheets have better initial permeability, less hysteresis loss and higher stauration magnetisation than those of randomly oriented Fe-Ni sheets
How does one achieve preferred oriented Fe-Ni sheets?
- Following combinations will favour preferred orientation:
- Less impurities and inclusion
- Mechanical working like rolling
- Anneal in Hydrogen
What is the effect of order or disorder structure in Ni-Fe sheets (50-80% Ni) on permeability?
- The perfectly ordered structure has relatively low permeability, slow cooling past 500ᵒC favours ordering
- Quenching results higher permeability due to suppressing the order transformation
Why is Heat Treatment Necessary?
- Typical manufacturing methods for magnetic shielding involve bending, forming, welding, cold working, and mechanical finishing. Bending and forming are mechanical operations which can work-harden and/or stress high permeability materials. Welding introduces oxygen to the material, and mechanical finishing and cold working can introduce carbon. Each of these factors contributes to the degradation of shielding performance of high permeability materials.
How Does Heat Treatment Increase The Permeability of Shielding Material?
- When the shielding materials are heated to 2100°F for 1-2 hours, the grain of the material grows, increasing the materials ability to absorb magnetic flux. In addition, the hydrogen atmosphere in the heat chamber produces a chemical reaction with the shielding material, removing impurities such as carbon, Sulfur and oxygen, thereby enhancing permeability
- Finally, rapid and controlled cooling of the part freezes the desired grain of the shield, yielding maximum permeability. The temperature and the time the parts are in the heat chamber are regulated with care as it is crucial that the pats maintain their structural and dimensional integrity
What are the Terms used in Magnetics and what are their meaning:
Why is Magnetic Shielding necessary?
- We are surrounded by magnetic fields (both AC and DC) from the earth’s magnetic field to man-made sources such as magnets, motors and transformers. When a piece of sensitive equipment is being affected by these fields we need to produce a shield. Examples that are affected are cathode ray tubes, photo multiplier tubes, audio transformers, scanning electron microscopes, position sensors
How does a magnetic shield work?
- To be a good magnetic shielding material it must have a high permeability which means that the magnetic field lines are strongly attracted to the shielding material. Mumetal, 48% NiFe are the most common alloys chosen based on the intensity of the magnetic field. If the magnetic field is too high for the material chosen, it will saturate and become ineffective. In this case one can use a multilayer shield with a combination of the above alloys. These alloys also have a very low Remanence to prevent them becoming permanently magnetized
- One cannot stop or block magnetic field lines. They will travel from the N pole of the source to the S pole. What we can do is to alter the path that these magnetic fields lines take on their journey. Magnetic shielding materials “conduct” magnetic field lines better than air (and most other materials). In short, they create a “path of least resistance” in which the magnetic field lines can travel
What are the factors considered for designing a magnetic shield?
- Magnetic Field & Its location:
- Magnetic Field at source
- Distance between the field source and the sensing point
- Magnetic Field required at the sensing point
- Materials and Properties:
- The shield material has to be chosen such that the field generated by the current and the field generated by external sources do not saturate the shield material
- Lower the Ni-grade, higher is the saturation level of the material. For designs where lowest error from the Hysteresis is required, 80% Ni-alloys shows best results and for high saturation fields 48% Ni-alloys are chosen
Shield Dimensions: Different geometries influence the Shielding Factor, Magnetic gain and Saturation level as illustrated below for PCB shields: