Hard Magnetic Materials: Types and Applications | Electrical Engineering

In this article we will discuss about:- 1. Meaning of Hard Magnetic Materials 2. Types of Hard Magnetic Materials 3. Applications.

Meaning of Hard Magnetic Materials:

Materials having the highest possible saturation magnetization, remanance, and coercive force are used as permanent magnets. The usefulness of a permanent magnet is determined by the magnetic energy it can deliver at various flux densities. Since the magnetic potential energy of the magnetized material is approximately equal to BH/2, the available energy may be obtained by plotting (BH) as a function of H along the demagnetization curve as shown in Fig. 4.17; this curve is called an external energy curve.

The maximum value of BH is called the energy product of the magnet and is designated (BH)_max. This (BH)_max, corresponds to the area of the largest B-H rectangle that can be constructed within the second quadrant of the hysteresis curve. The value of the energy product is representative of the energy required to demagnetize a permanent magnet i.e. the larger (BH)_max the harder is the material in terms of the magnetic characteristics. Fig. 4.18 gives the energy products of various materials with their demagnetization curves. In general, the energy available from any magnet is approximately equal to the volume of the magnet times its energy product.

Hard magnetic alloys are classified into three groups, dependent upon the microstructural transformation which gives the high energy product. Transformation hardening alloys undergo a martensitic transformation upon cooling with a resultant fine scale structure of high mechanical hardness and internal stress. High carbon steels, and alloy steels containing, W, Cr, Co or Al, fall into this category.

Precipitation hardening alloys demonstrate a yet finer structure, with a high resistance to domain growth and rotation. In the group are Cunife (Cu-Ni-Fe), Cunico (Cu-Ni-Co), Alnico alloys, and silmanal (Ag-Mn-Al). Although silmanal has a relatively low saturation magnetization, its coercive force is considerably greater than Bismanal. The hardening alloys FePt and CoPt form super lattices upon cooling and again exhibits a high coercive force.

The Alinco type alloys are commercially the most important of the hard magnetic materials. Large magnets are made by special casting techniques and small ones by powder metallurgy. If the cast alloys are directionally solidified to grow columnar grains with parallel (100) directions, the energy product is doubled. After casting the alloy solution is annealed at 1300°C and then given a short (-10 min.) heat treatment at 800°C.

If this heat treatment is carried out in a magnetic field the demagnetization curve and energy product are considerably enhanced as shown in Fig. 4.19. This improvement in direction of the applied field is acquired at the expense of the properties at right angles to the field. After the 800°C heat treatment further improvement is possible by prolonged heat treatment (~14 hours) at ~580°C in the absence of a magnetic field.

Types of Hard Magnetic Materials:

1. Conventional Hard Magnetic Materials:

The conventional hard magnetic materials have (BH)_max values that range between about 2 and 80 kJ/m³ (0.25 and 10 MGOe). These include ferromagnetic materials-magnet steels, cunife (Cu-Ni-Fe) alloys, alnico (Al-Ni-Co) alloys-as well as the hexagonal ferrites (BaO-6Fe₂O₃).

The hard magnet steels are normally alloyed with tungsten and/or chromium. Under the proper heat-treating conditions these two elements readily combine with carbon in the steel to form tungsten and chromium carbide precipitate particles, which are especially effective in obstructing domain wall motion. For the other metal alloys, an appropriate heat-treatment forms extremely small single-domain and strong magnetic iron-cobalt particles within a nonmagnetic matrix phase.

2. High-Energy Hard Magnetic Materials:

Permanent magnetic materials having energy product in excess of about 80 kJ/m³ (10 MGOe) are considered to be of the high energy type. These are recently developed intermetallic compounds that have a variety of compositions; the two that have found commercial exploitation are SmCo₅ and Nd₂Fe₁₄B. Their magnetic properties are also listed in Table 4.10.

The magnetization-demagnetization behaviour of these materials is a function of domain wall mobility, which in turn, is controlled by the final microstructure—that is, the size, shape, and orientation of the crystallites or grains, as well as the nature and distribution of any second-phase particles that are present.

Of course, microstructure will depend on how the material is processed. Two different processing techniques are available for the fabrication of SmCO₅Nd₂Fe₁₄B magnets- powder metallurgy (sintering) and rapid solidification (melt spinning). The synthesis of magnets using powder metallurgy involves various steps like material preparation, dry pressing, sintering, annealing etc.

For rapid solidification, the alloy, in molten form, is quenched very rapidly such that either an amorphous or very fine grained and thin solid ribbon is produced. This ribbon material is then pulverized, compacted into the desired shape, and subsequently heat treated.

3. Fine Particle Magnetic Materials:

Hard magnetic material of high coercive force can alternatively be made by bonding together magnetic particles smaller than one domain wall thickness. Since the particles are single domains and separated from one another by a resin or nonmagnetic metal binder, the magnetization can only be changed by moment rotation. Ferroxdur, the hard magnetic ferrite, is prepared by magnetic pressing, sintering and magnetic annealing of the fine BaO ferrite powder.

Fine particle magnets with energy products approaching that of alnico have been produced from single domain Fe or Fe-Co alloy particles bonded with Pb. The fine particles, precipitated electrolytically from a solution on a liquid mercury cathode, are elongated along the easy direction of magnetization, and less than a domain boundary width (about 150 Å) in thickness.

The high coercive force of these elongated single domain (ESD) magnets, results from three factors; the fine, single domain particle, the shape anisotropy of the particles, and the fact that the particle is elongated in the easy magnetization direction. The material is cooled, the binder hardens, and a material with an energy product of 9 × 10³ joules/m³ results under good conditions.

Fig. 4.18 shows the demagnetization curve for the theoretical ESD, energy product under optimum conditions is 1.6 × 10⁵ joules/m³, but this assumes zero binder volume and completely solid iron, that is, a 100 percent dense powder. If necessary (the alloying elements for alnico are strategic materials), better ESD materials could be made, but at present, ESD material is relatively expensive and seldom used in place of alnico.

Applications of Hard Magnetic Materials:

These high-energy hard magnetic materials are employed in a host of different devices in a variety of technological fields. One common application is in motors. Permanent magnets are far superior to electromagnets in that their magnetic fields are continuously maintained and without the necessity of having to expend electrical power; furthermore, no heat is generated during operation.

Motors using permanent magnets are much smaller than their electromagnet, counterparts and are utilized extensively in fractional horse-power units. Familiar motor applications include the following- in cordless drills and screw drivers; in automobiles (stearing, window winder, and wiper, washer, and fan motors); in audio and video recorders; and clocks.

Other common devices that employ these magnetic materials are speakers in audio systems, light-weight earphones, hearing aids, and on computer peripherals. The permanent magnets are also used in instruments like galvanometer, ammeters, voltmeter, wattmeter, compasses, electron beam focusing and positioning, telephones, loudspeakers, eddy current brakes etc.