3. Permanent Magnets as energy materials Flashcards
What are the “soft” and “hard” magnetic materials? Give an example (schematic drawing) of typical M(H) curves for these type of magnets.
Hc = intrinsic coercivity
Soft magnets have low Hc, that means it has low resistance to demagnetisation. Like Iron
Hard magnets has high Hc, high resistance to demagnetisation. Like Permanent Magnets, they stay magnetized for a long time.
Name state of the art magnetic materials with the highest energy product |BH|_max. Analyse what limits their wider application as energy materials.
Neodymium Iron Boride (highest energy product and dominates the market for Permanent Magnets.
The highest energy product is found in compounds containing the rare earth element neodymium and dysprosium. For example the so called NEOMAX containing neodymium, iron and boron has a very high energy product.
The main concern for their wider application is that both Nd and Dy are not highly abundant, very rare. They are even regarded as critical, because of the high demand and the low availability. So very costly.
What are the straightforward approaches to predict magnetic properties? How does calculating the resulting spin moment help in the development of new magnets?
Predict magnetic properties by calculating the spin moment.
We can calculate the spin moment by using a simple formula, µ = g*sqrt(S(S+1)).
S: sum of spin quantum numbers of the individual unpaired electrons
g: gyromagnetic ratio = 2 ish
In some cases where you have heavy metal ions in the nucleus (in lanthanoids), this can give rise to an orbital moment. Then the equation looks like this: µ = sqrt(4S(S+1) + L(L+1)) where S is the spin quantum number and L is the orbital angular momentum quantum number of the ion.
The resulting spin moment can then tell us something about the strength of the magnets. So a higher resulting spin momentum, will give a understanding if it can be a permanent magnet (if high resulting spin momentum).
Explain what the driving forces responsible for the formation of the ferromagnetic domains are.
Driving force: divide into ferromagnetic domains can reduce the magnetostatic energy.
A magnetic domain: A magnetic domain is a region within a magnetic material in which the magnetization is in a uniform direction.
The atoms individual magnetic moment is aligned with another and point in the same direction. When cooled below a temperature called the Curie temperature, the magnetization of a piece of ferromagnetic material spontaneously divides into many small regions called magnetic domains.
Analyze why the “Neo-Dy”-magnets play the dominant role in e.g. wind generators.
For generators, the resistance to demagnetization is critical. By introducing dysprosium in neodymium, the intrinsic coercivity is increased, and such magnets are therefore favorable.
Why are permanent magnets advantageous in wind power production?
Because they can be operated efficiently at low revolution speeds, which are necessary for windmills to run at.
Give some advantages of permanent magnet motors over induction motor.
It is more efficient, weighs less and is smaller in size.
What is the physicist’s way of describing magnetism?
In terms of circulating currents in materials, that is motion of electrons.
What is the engineer’s way of describing magnetism?
In terms of magnetic poles.
Give a definition of a permanent magnet.
An inorganic solid that exhibit magnetic effects other than diamagnetism. In these materials there are some unpaired electrons in their outer valence shells. These electrons are usually located on metal cations, and can have both spin and orbital motion, which together generate a magnetic moment associated with the electrons.
What classes of materials are mainly exhibiting permanent magnetic properties?
Compounds of transition metals and lanthanoids. Most of these elements contains unpaired d and f electrons.
These unpaired electrons can generate a magnetic momentum, because of the electron spin and orbital angular momentum
What is the definition of the magnetic induction, B? What is the unit it is measured in?
The magnetic induction is a material’s response to a magnetic field, H. [B] = T
What is the definition of magnetization, M. What is the unit it is measured in?
Magnetization is the total magnetic moment per unit volume. It is measured in oersted ([M] = Oe).
What is the definition of the magnetic susceptibility, X?
The susceptibility is the ratio of M to H.
What is the permeability?
The permeability is the ratio of B to H.
