Powder metallurgy and permanent magnets

Powder metallurgy, PM, is a route of manufacturing based on the compaction of powders that are afterwards sintered to create a solid product. This technique has been used in the production of permanent magnets since the 18th century and most of high performance permanent magnets have been fabricated this way. Magnetic properties are so dependent on the starting material, microstructure, magnetic alignment and heat treatment process, therefore powder metallurgy represents an ideal route to control most of these factors.

What is powder metallurgy?

Although powder metallurgy encompasses many steps an operation before and after, the basic procedure in the manufacture of PM parts is comprised by the following stages. For deeper explanations and more information about each one, it is worth to take a look to this Introduction to Powder Metallurgy.

Powder metallurgy 2Figure 1 – Schematic diagram of the powder metallurgy process. Source: Ames

Mixing

The object of mixing is to provide a homogeneous mixture and to incorporate the lubricant. The main function of the lubricant is to reduce the friction between the powder mass and the surfaces of the tools along which the powder must slide during compaction, thus assisting the achievement of the desired uniformity of density from top to bottom of the compact. Of equal importance is the fact that the reduction of friction also makes it easier to eject the compact and so minimises the tendency to form cracks.

Shaping

The mixed powders are pressed to shape using either a press, which could be uniaxial, or isostatic; or an injection system under pressures depending on the characteristics of the starting material. At this stage, the compacts maintain their shape by virtue of cold-welding of the powder grains within the mass. The compacts must be sufficiently strong to be handled safely and avoid its breakage. This is a critical operation in the process, since the final shape and several properties are essentially determined by the level and uniformity of the as-pressed density.

Sintering

The thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles.

Suffice to say that atomic diffusion takes place and is followed by recrystallization and grain growth. Consequently, the pores tend to become rounded and the total porosity, as a percentage of the whole volume, tends to decrease.

The operation is almost invariably carried out under a protective atmosphere, because of the large surface areas involved, and at temperatures between 60 and 90% of the melting-point of the main constituent. For powder mixtures, however, the sintering temperature may be above the melting-point of the lower-melting constituent, so that sintering in all these cases takes place in the presence of a liquid phase, hence the term liquid phase sintering.

Why powder metallurgy in permanent magnets?

Powder metallurgy techniques have been found to offer advantages in the fabrication of permanent magnets. It seems that the most of the best magnetic properties are obtained by employing powder metallurgy routes rather than casting processes. Careful control of particle size and particle orientation, using a magnetic field to align the starting particles, are among two of the greatest advantages of the powder metallurgy process.

Powder metallurgy is a near net shape manufacturing technique which means that the final product is almost finished due to the compaction was done in a shape as close as possible to the final one. So from this point of view the powder metallurgy technique is very advantageous as the machining stage can be whether dispensed or minimised, thus decreasing costs and saving materials.

Summarizing, all these factors mentioned above play a vital role in controlling and enhancing the magnetic properties of permanent magnets and soft magnets as well.

How this relationship started?

As explained in a previous post (link of my previous post about History of permanent magnets), Gowin Knight, an English entrepreneur, made a fortune by manufacturing magnets but he didn’t publish his methods in life. After his death in the 1770’s his method was published and consisted of stirring a slurry of iron fillings to obtain a suspension of finely divided iron oxide. This was mixed with linseed oil to create a paste that was moulded into shape and baked. The resulting block was magnetized with an energy product of 2 kJ/m3, which was really high by that time.

However, it is not the strength of the magnets what we are focusing at today, but the technique used by Gowin Knight. It means that the first time powder metallurgy was used in the production of permanent magnets was more than 250 years ago.

Gowin KnightFigure 2 – Portrait of Gowin Knight. Source: Science & Society Picture Library Prints

During the 19th century, casting techniques were developed and magnetic steels were industrially processed thus replacing powder metallurgy as the dominant permanent magnet manufacturing method.

In the 1950s Philips Company discovered ferrites immediately after the Second World War and due it is not a metal but a ceramic, they decided to choose powder metallurgy as the production technique. This decision was helped by the massive development of powder metallurgy as a consequence of the manufacture of tungsten filaments for bulbs as well as the development of cement carbides in 1909 and 1922 respectively.

References

Dowson, G., Whittaker, D., 2008. Introduction to Powder Metallurgy. The process and its products. European Powder Metallurgy Association, EPMA.

Moosa, I.S., 2014. History and Development of Permanent Magnets. International Journal for Research & Development in Technology, 2, p18-26.

Ramakrishnan, P., 1983. History of powder metallurgy. Indian Journal of History of Science, 18, p109-114.

REE in Cigarette Lighters Flint: Ferrocerium

In today’s world, we all know what a cigarette lighter is and how easy it is to start a fire with a flick of fingers. We can have lighters with easy access and with vast amount of different types, shapes and sizes. They can be quite inexpensive and disposable or luxurious and commemorable depending on your choice. However, this wasn’t the case from the beginning and, as for almost all industrial products, the invention and the development of the lighters has an interesting past. You can find a very enlightening timeline of lighters as well as matches in here [1] and as a fun fact; lighters were invented before the matches by the way. We all know that the invention of fire (or more precisely controlled use of fire) by Homo erectus during Early Stone Age was a big deal for humankind. Somehow, our early ancestors found their way to primitively create fire either by percussion or friction principle just as viewed from the figure below. Either of these principles allows you to ignite some sparks that are needed to be immediately directed onto a rather easily flammable material such as tinder, black char cloth, etc. With a little bit of patience and blowing air, congratulations, you got your fire!

Percussion and friction principles

Percussion and friction principles

Although materials have changed significantly over centuries, campers and some modern lighters still use methods based on the percussion principle. As a simple and widely known example, let me give you the flint-and-steel method. Here, one strikes a hard rock such as natural flint (a form of quartz mineral) onto another one with iron content such as pyrite. The natural flint doesn’t create sparks itself but instead, on striking, it creates a fresh surface of iron on the other rock which aggressively reacts with oxygen in air [2]. Another fun fact: the sparks here are actually bundles of flying metal oxide particles formed by the oxidation of metal with air and that have very high temperatures. Only by contacting with a suitable “fuel”, these sparks can ignite and grow an actual fire before cool down to the ambient temperature.

Döbereiner’s lamp

Döbereiner’s lamp

Setting aside this ancient but still in use method, the first lighter invented by Johann Wolfgang Döbereiner in 1823, utilized a chemical process between zinc and sulfuric acid to produce the quite flammable hydrogen gas that bypasses a platinum metal catalysts giving a great amount of heat and light after ignition with oxygen. Unfortunately, as can be seen in the figure left, this so-called Döbereiner’s lamp was large, quite dangerous as it could unexpectedly explode thereby requiring a safer and more practical lighter. Invented and patented by Carl Auer von Welsbach in 1903, ferrocerium solved this problem and made it possible for today’s lighters to exist [3]. This intermetallic alloy is technically the replacement of the pyrite-like, iron-containing rock in flint-and-steel method and is still in use. Furthermore, ferrocerium is a man-made flint and, unlike natural flint, on scratching with a rough surface like ridged steel, it can easily produce brighter sparks. The sparking occurs due to low ignition temperature of Cerium metal (150-180 °C) where the temperature of sparks can be as high as 1650 °C. The original ferrocerium invented by von Welsbach was created by 30% Fe that was added into purified Ce giving the name ferrocerium or Auermetall. The compositions of the later lighter flints included lanthanum as well as other heavy metals to control the processing, brightness and ignition characteristics. Today’s lighter flints are derived from 75% mischmetal and 20% iron. The mischmetal contains around 50% cerium, 25% lanthanum and minor amounts of neodymium and praseodymium [4]. In order to ease ignition small amounts of magnesium is also added to the composition. A typical composition of modern ferrocerium flints is given in the table below whereas the image below shows a pack of them from the market.

A pack of ferrocerium lighter flints

A pack of ferrocerium lighter flints

Element

Fe Ce La Nd Pr

Mg

Wt.% 19 38 22 4 4

4

However, as mentioned above, sparking only is not enough for creating a fire unless you have a suitable fuel. Typically, naphtha (very similar to gasoline) or butane is used as the fuel both of which are volatile liquids. Naphtha-based lighters employ a saturated cloth wick and fiber packing to absorb the fluid and prevent it from leaking. They employ an enclosed top to prevent the volatile liquid from evaporating, and to conveniently extinguish the flame. Butane lighters have a valved orifice that meters the butane gas as it escapes [5]. In the figure below you can see the skeleton of a typical lighter and if you are a fan of Zippo lighter please visit [3, 6]. If you are more than that, that is, a truly devoted fan of lighters then I can suggest you “The Legend of the Lighter” authored by Ad van Wert and many other books with beautiful pictures of historical lighters in [7]. And for the future of the lighters… No need to worry as these handy “fire-creators” will still be around for a very long while although they are mostly and rightfully associated with smoking. That is actually why lighter manufacturers have widely fled to Europe and Asia from the U.S where smoking is less popular than the other two. Hence, perhaps in the future, we might also consider the recycling of the flints from disposed lighters for their REE contents even though the flint is a very small piece of the lighter requiring an economical analysis.

A typical disposable lighter

A typical disposable lighter

References:

[1] http://www.toledo-bend.com/VCL/articles/index.asp?request=lighterHistory

[2] https://en.wikipedia.org/wiki/Lighter

[3] http://www.madehow.com/Volume-7/Lighter.html

[4] https://en.wikipedia.org/wiki/Ferrocerium

[5] http://lighters.askdefine.com/

[6] http://www.ehow.com/how-does_4896757_zippo-lighter-work.html

[7] http://www.vintagelighterbook.com/outline_of_books.pdf

A brief history of permanent magnets

Magnetic materials are essential in every human being daily life. They played a vital role in the development of modern technology up to the point of being part of the massive generation of electricity in the 19th century. Therefore, it is interesting to take a look to the past and observe the history of such an important group of materials nowadays.

Manisa, Turkey

Manisa, TurkeyModern Manisa, Turkey

The history of permanent magnets start in Magnesia ad Sypilum, modern Manisa, in the west of Turkey. Magnesia ad Sypilum was an ancient Greek city in Ionia, which was named after the Magnetes, an ancient Greek tribe of that region who discovered mysterious stones that could attract or repel each other more than 3500 years ago. After reading the name of the tribe who made the discovery is easy to guess where magnetism, magnetite or magnet word came from, isn’t it? Those lodestones were indeed magnetite, Fe3O4, with an energy product of only 1 kJ/m3.

Magnetic steels

Although much had been discovered about magnetism, it wasn’t up to the 18th century when there was a change in the materials that magnets where used to be produced. Gowin Knight, an English entrepreneur, made a fortune by manufacturing magnets but he didn’t publish his methods in life. After his death in the 1770’s his method was published and consisted of stirring a slurry of iron fillings to obtain a suspension of finely divided iron oxide. This was mixed with linseed oil to create a paste that was moulded into shape and baked. The resulting block was magnetized with an energy product of 2 kJ/m3.

During the 19th these magnetic steels were improved by adding tungsten and this combined with chromium, thus giving an energy product of 2.5 kJ/m3. But it was in 1917 when two Japanese scientist, K. Honda and T. Takai added cobalt to tungsten steel to increase the coercive force of permanent magnets improving industrial applications with an energy product of 7.6 kJ/m3.

Al-Ni-Fe & Alnico

In 1930, a new alloy of nickel, aluminium and iron was developed by Mishima in England with an energy product of 10 kJ/m3. After this discovery, a new generation of permanent magnets was developed based on Al-Ni-Fe by the addition of copper, cobalt, niobium and titanium, getting the name of Alnico and achieving energy product values up to 72 kJ/m3.

All this new alloys were cheaper than cobalt steels, presented higher magnetic properties and didn’t required as much operations as the steel did. Therefore, Alnico rapidly displaced small electromagnets in motors, transformers, and loudspeakers, lowering the cost and simplifying the construction. Consequently, for the next twenty years Alnicos were extensively researched throughout the world and many companies competed to maximize its properties.

Ceramic magnets: ferrites

During the 1950’s, as a result of their work with soft ferrites, the Philips organisation discovered that hard magnetic ferrites could be produced. In general, these ferrites are based on barium (BaFe12O19) or strontium (SrFe12O19), and iron oxide, achieving energy products about 28 kJ/m3.

Although the magnetic properties of the ferrites are lower than Alnico magnets, the great advantage of ferrite magnets is their cost due to they have the lowest price per unit of energy product. Additionally, it is possible to produce flexible rubber magnets and plastic magnets by mixing ferrite powder with the rubber or plastic prior to its manufacture. These flexible ferrites have many applications such as fridge magnet, loudspeakers or magnetic recording tapes.

Rare earth magnets

In spite of rare earth magnets, a Nd-Fe alloy, were reported in 1935, even before than ceramic magnets, they were not developed until the 1960s, when a concerted research effort to identify new permanent magnets based on alloys of rare earth elements was carried out.

In 1967, Strnat et al. investigated phases of the type RCo5, where R means one rare earth element of the following: yttrium, cerium, praseodymium or samarium. This first generation of rare earth permanent magnets were produced by liquid phase sintering of magnetically aligned powders, being SmCo5 the reference amongst them with energy products about 190 kJ/m3. Immediately after their discovery, this magnets found a place in space and military industry due to their high energy product compared with those of previous materials.

After the development of SmCo5, a second generation of rare earth magnets emerged in the early 1970’s containing copper, cobalt and rare earth elements, and led to the development of the high energy product alloy Sm2Co17. These magnets evolved by the addition of iron to become Sm2(Co,Fe)17, which opened a door to the addition of more alloying elements such as niobium, vanadium or zirconium, that helped to increase the magnetic properties of the material, thus giving an energy product of about 240 kJ/m3.

The third generation of rare earth permanent magnets began in 1984 when General Motors in the US and Sumitomo in Japan simultaneously developed a new magnet, Nd2Fe14B. As it was explained in a previous post, Sumitomo magnets were made by sintering aligned powders and General Motors’ ones were produced by hot pressing of melt spun ribbons, with an energy product of 290 and 114 kJ/m3 respectively. This new generation of magnets rapidly displaced others in a number of applications such as electric motor, hard disk drives or loudspeakers. Currently, NdFeB-based magnets produced are able to give an energy product over 470 kJ/m3.

Other permanent magnets

Cu-Ni-Fe & Cu-Ni-Co

The Cu-Ni-Fe and Cu-Ni-Co alloys were developed in the 1930s and were used due to their high ductility, thus easing the manufacturing process. They offered an energy product of 11 kJ/m3, which was higher than the one given by Al-Ni-Fe but it was surpassed when the Alnico’s were developed.

Fe-Cr-Co

A new kind of magnets was developed in the 1970’s, the FeCrCo magnets. They were a replacement for CuNiFe due to those are not commercially available anymore. In spite of giving an energy product of just 55 kJ/m3, comparable with to those of Alnico magnets, they are characterised by its ductility and high working temperature and they are used in tachometers, micro relays and nautical instruments amongst others.

Pt-Co

The equiatomic Pt-Co alloy was developed in the 1950’s and was the most expensive permanent magnet in commercial production. With an energy product of 76 kJ/m3, they were better than Alnico alloys and due to their corrosion resistance they were used in biomedical applications. However, with the arrival of rare-earth-based permanent magnets that properties were achievable and they were inevitably superseded.

Sm-Fe-Nitride

In the 1990’s a new magnetic compound was discovered by Coey and Sun consisting of Sm2Fe17N with an energy product about 400 kJ/m3. The development of this alloy is still ongoing and it is a promising new candidate for permanent magnet applications, although it is commercially available on its bonded form with an energy product of 112 kJ/m3; the highest up to date on this kind of magnets.

 

As it can be seen, permanent magnet materials are important components of consumer, transport, industrial, military and aerospace systems. There has been an increasing use of permanent magnet materials as the properties of these materials have been improving, which means how relevant are they in nowadays’ world.

References

Overshott, K.J., 1991. Magnetism: It is Permanent. IEE Proceedings A,138, p22-30.

Moosa, I.S., 2014. History and Development of Permanent Magnets. International Journal for Research & Development in Technology, 2, p18-26.