Magnetic engineering is a highly complex and wide-ranging field. The versatility of this technology opens up numerous possibilities for us as a manufacturer - and fascinates us every day anew. Whether in material development, process engineering or the production of products, when working with magnets fall many technical terms, which we like to explain here.
The operating point is defined as the intersection of the operating load line and the B curve. It is the point on the demagnetisation curve whose coordinates are the magnet’s magnetic flux density and the magnetic field strength in an operating state. The operating point of a permanent magnet must always be in the linear region of the demagnetisation curve, taking the effects of temperature (temperature coefficients of Br and HCJ) and external opposing fields into account. If the operating point comes into the nonlinear region near the knee, the magnet will become partially demagnetised (irreversible losses).
For some decades, axial pressing has been one of the dominant shaping technologies employed in the fields of technical ceramics and powder metallurgy. It becomes especially cost-effective when large quantities and high component quality standards are involved. The process readily lends itself to automation, which in turn boosts productivity levels. A distinction with axial pressing is made between wet pressing and dry pressing. In dry pressing applications, high levels of compression and component strength are achieved in blank component condition (green blanks), and press masses without clearly defined visual properties can also be shaped. The green blanks manifest very good practical usage properties, e.g. exceptional dimensional integrity, high strength and geometric complexity. In the magnet production process, a magnetic field is very frequently set up during the pressing process to align individual particles of powder axially or transversally, thereby achieving a high level of anisotropy in the component. The dry pressing process delivers great cost-effectiveness and productivity while also achieving compliance with stringent quality requirements, even with maximum quantities with relatively low levels of scrap.
In our figures there are auxiliary lines for determining the operating load line of a magnet without surrounding iron. To construct the operating load line, connect the origin of the graph with factor h : D. The factor h . D describes the ratio of the operating load line varies within a magnet; our figures show average values. For a very small h : D ratio (< 0.3) one should take into account that the operating point in the centre of the magnet is much lower than the average value.
Irreversible losses occur when the operating point is not on the linear part of the demganetisation curve.
Irreversible losses are also possible wherever the actual demagnetisation curve differs from the theoretical, linear behaviour. Some irreversible losses are inevitable with rises in temperature and in external fields. By a one-time stabilisation, magnets can be set to a constant value. The disadvantage is that the induction is lowered.
The magnetic permeability (µ), sometimes referred to as „conductivity“, is defined as the ratio of the magnetic induction B to the magnetic field H. The permeability in vacuum is a constant:
µ0 = 1.256 mT / kA / m. The ratio in matter ist he substance's characteristic absolute permeability µ= µr⋅µ0. (µr=relative permeability).
Substances may be diamagnetic (µr < 1), paramagnetic (µr > 1), or ferromagnetic (µr >>1), with values ranging from 1 to over 100,000.
The Remanence Br [mT] is the remaining magnetisation in a magnetic material at field strength H = 0 kA/m, after it has been magnetised to saturation in a closed path.
⇒ See hysteresis loop (Point 5)
Remanence, also known as residual magnetism, is one of the most important parameters for a magnet. It expresses the magnetic flux that flows through a body continuously once it has become separated from its original magnetic field source.
The preferred direction is the direction in which magnetic grains are oriented in a magnet. The requirements for this vary with the type of magnet.
The preferred direction is that direction in which the magnet reaches its highest values. For ring an circular magnets the preferred direction is axial or diametric, for rectangular magnets it is determined by the height h, for segment magnets it is diametric or radial. The preferred direction of magnets is rached by pressing of the powder in a magnetic field (vertical or parallel to the pressing direction).
The operating load line describes the properties of the magnetic loop. Is tangle depends on the magnet geometry and the magnetically soft pole pieces used.
If a permanent magnet has no surrounding iron, the angle of the operating load line depends only on the magnet geometry. In systems with magnetically soft pole pieces, the angle of the operating load line depends on the relationship of the air gap to the magnet´s length. When an external magnetic field (H ≠ O) is applied, the new operating load line is displaced parallel to the former one.
Maximum temperature at which a magnet with length-to-diameter ratio h:D ≥ 0.5 may be in operation, under normal ambient conditions. The maximum operating temperature is reduced for smaller length-to-diameter ratios and/or when opposing magnetic fields are present. Our Technical Applications Department is happy to support you with maximum operating temperature calculations.
Electrical charge is a specific instance of the general notion of physical charge and is one of the fundamental concepts of physics. Forces acting between these charges have a multiplicity of impacts on the human environment. Virtually all visible physical processes can be traced back in some shape or form to the action of electrical charges.
The coercivity HCB is the magnetic field strength required to bring the polarisation back to 0 in a ferromagnetic material which has been magnetised to saturation.
⇒ see hysteresis loop (Point 6)
The coercivity HcJ is the magnetic field strength required to bring the polarisation back to 0 in a ferromagnetic material which has been magnetised to saturation.
⇒ see hysteresis loop (Point 7)
Magnetism is associated with moving electrical charge. Magnetic moments are generated by the motion of electrons around the nucleus in atoms, and also from the internal spin of the electrons. Together they make up the magnetic moment of the atom, adding vectorially. If they sum to zero, the material is called diamagnetic. For paramagnetic, ferromagnetic, antiferromagnetic and ferromagnetic materials, the moments sum to a quantity different from zero.
Passivating protective coatings such as a zinc, chrome and aluminium are electrochemically more active than the metal they cover. When new, they bear the corrosive attack alone, acting as sacrificial anodes. As long as they are intact, the covered metal has cathodic corrosion protection, and the component remains fully functional. If small defects and small holes are in the coating, the surrounding sacrificial coating provides protection. Once larger areas of the coating are abraded, the covered metal will corrode.
Temperature coefficients describe the temperaturedependent behaviour of permanent magnets. The remanence temperature coefficient of hard ferrit magnets is about -0.19%/K. That is, a temperature rise of 1 Kelvin reduces the remanence by 0.19%. Sm2Co17 magnets have the lowest temperature coefficients, at -0.03%/K.
If an application requires special protection, virtually all magnet materials can be coated without difficulty. The magnet´s material and application determine which coating is suitable for it. An universal coating equally suitable for all possible uses has not yet been developed.
Note that a nickel coating always causes a „magneitc short“. This reduces the magnetic properties (remanence BI, the energy product (B⋅H)max, and the coercivity HCB) by 5 – 7%.
Metallic layers, usually applied by electroplating, protect the relatively brittle material frome edge demage. Good plated coatings have multiple layers; they protect better than single layer systems.
Suitable for SmCo and for NdFeB magnets, they offer very good protection from moisture and steam. However, to protect from corrosive media, plastic coatings are usually superior to metal. Small components are usually coated in bulk, without contact points, because, except for passivating coatings, damage to the metallic coat could accelerate corrosion.
Organic coatings are characterised by their coating technology and coating material. In contrast to metallic coatings, they are suitable for all magnet types.
The disadvantages of plastic, as compared to metal, are lower application temperatures and the lower resistance to steam. An advantage of plastic ist he better resistance to corrosive media.
The most resistant and even coating is obtained with electro dipping (E-coat). However, it can be used only for conducting magnets, and the process leaves small contact spots on the magnet.
Wet painting and passivating coatings can be applied to nearly all magnets and offer very good corrosion protection. The process does, however, require larger layer thickness tolerances. For special corrosion requirements, two-coat systems of passivating paint and top coat are possible.
A teflon coating is more a niche product because of the higher process temperature, about 300°C , and the higher price; but it offers the highest corrosion protection, extensive chemical resistance, and high temperature resistance.
Parylene, a relatively soft plastic deposited from a gaseous state, is suitable to coat all magnet materials; it forms a waterlight, contourfaithful, closed covering. It is safe for good, guarantees effective protection against moisture, and makes magnets highly resistant to chemicals. The only shortcoming is that it cannot be exposed to mechanical loads.
Ferrite magnets belong to the oxide ceramics material group. They pose virtually no health risk.The barium content of some magnets should be noted. Under some conditions, for example with acids, traces of barium can become dissolved. Since barium is a heavy metal, it is preferable for some applications to use strontium ferrite magnets instead.
The toxicity of rare earth metals and their compounds is not well understood. They were long considered completely harmless, and some have even been used therapeutically for medical purposes. In mechanical processing, it has been recognised that breathing the magnetic dust, especially together with cobalt, is a hazard. Breathing the soluble saltsin air-borne dust results in a small elevation of blood levels. Resorption of traces by ingestion is, by contrast, considered harmless. There are no limits for cobalt in the drinking water ordinances. Investigations showed that SmCo magnets have good chemical resistance in neutral and alkaline media. Of course, these metal compounds have no acid resistance. Natural samarium, a main component of SmCo magnets, has about 15% abundance of the isotope147Sm. Despite this, external contact is completely harmless. The consituents of NdFeB magnets are not hazardous. Nonetheless, one should avoid intake of dust and dissolved material.
Radiation effects on permanent magnets
Exposure of permanent magnets to radiation can cause structural defects. Structure-dependent properties such as coercivity, induction, and remanence are directly affected, and intrinsic properties such as saturation magnetisation and curie temperature are indirectly affected. Detecable magnetic changes are seen only above a threshold irradiation level that varies between materials. Currently no reliable irradiation level limits have been established. However, deterioration or change has been observed in a small number of eperiments at high irradiation levels. For example, irradiation by 5.4 ⋅ 1018 thermal neutrons per square centimetre and 1.2 ⋅ 1017 fast neutrons per square centimetre at 50°C resulted in a 3% decrease in the saturation magnetisation of Fe2O3, the essential material in hard ferrites. NdFeB magnets lose more than 50% of their magnetisation at a proton doese of 4 ⋅ 106 rad and practically all of their magnetisation at 4.5 ⋅ 107 rad. SmCo magnets begin to exhibit significant demagnetisation at higher doeses, around 109 to 1010 rad. Sm2Co17 is less sensetive than SmCo5.
