Self Propogating High Temperature Synthesis of Magnetic Materials

Background

Using fireworks is probably not the first idea to come to mind for making novel materials. But our team of researchers at University College London is currently harnessing the power of ‘controlled fireworks’ for producing a wide range of ceramics. Far from being Roman candles or Catherine wheels, however, these so-called fireworks are in fact extreme chemical reactions in which temperatures of up to 2000°C are generated in a fraction of a second, allowing virtually single-phase products to be made in an instant. Such processes are called self-propagating high-temperature synthesis (SHS) reactions.

This way of making materials offers many advantages - reaction rate and temperature are easily managed, product microstructure can be accurately controlled and there is significantly less need for post-production treatments. In a unique new twist on the process, we have found that applying external magnetic fields during the combustion reaction can, in many cases, dramatically alter the microstructure of the final product. This discovery could lead to exciting new opportunities in ceramics synthesis, including the production of miniature devices with controlled magnetic structures.

Ferrites

One class of ceramics being made by the new technique is the ferrites. Ferrites are widely used as both hard and soft magnets. They include the M-type ferrite BaFe₁₂0₁₉, which is used for permanent magnets and as the magnetic strips on credit and debit cards, the cubic ferrite LiFe₅O₈ used in microwave and memory core applications, and the normal ferrite MgZnFe₄0₈, which is found in television set focusing systems and in high frequency cellular telephones.

Conventional synthesis Methods vs SHS

Conventional methods of synthesising these materials involve multiple grinding, heating and cooling of suitable precursor compounds. Reactions need extended time periods mainly because interdiffusion in solids is slow, even at high temperatures. By contrast, the fireworks route is rapid and does not need an external heat source to drive the reaction. These are self-propagating reactions that provide the energy to overcome the interdiffusion barrier from within the starting materials, the key being an exothermic chemical reaction.

SHS Reactions

Some SHS reactions have been known and used for many years. The thermite reaction, which is used in welding railway tracks, and the Goldsmidt reaction, which is still used to produce one third of the world's chromium, were discovered more than one hundred years ago. SHS reactions have also been found for the synthesis of many complex intermetallic compounds, metal nitrides, oxides, borides and carbides.

What Happens during a SHS Reaction?

SHS reactions can he thought of as ‘controlled fireworks’ because they proceed through a propagation wave that moves away from the initial source of ignition at a uniform speed. Most of the chemical reaction occurs in the short-lived molten zone created by this propagation wave. These reactions typically feature rapid heating and rapid cooling, and often produce metastable products of unusual or unique phase or composition. An example of such a reaction is shown in figure 1.

The simplest SHS reactions can be carried out in air using free-form powder precursors at ambient pressure, although one drawback of these conditions is that the product may be highly porous. Alternatively, reactions can be performed in a mould, and extrusion technologies can be used to manipulate and consolidate the molten reaction zone to produce complex net shape products. In some cases these products do not require any post-reaction densification, nor do they need any additional post-reaction processing such as machining. In some systems the post-reaction shaped article must be sintered to complete the synthesis of the material, but in many cases the SHS reaction is sufficient in itself to produce the desired final article.

Effect of Magnetic Fields on SHS

Our latest research on SHS technology has yielded an intriguing way to further control the chemical fireworks. We have found that applying a magnetic field to magnetic precursors can influence the temperature and speed of the propagation wave. In ferrites this causes changes in the magnetic properties of the materials, with the bulk coercivity, remanence and saturation magnetisation all varying as a function of the applied field strength. The proportion of ferrite phase in the ‘applied field’ SHS products is higher than when no magnetic field is used. Acicular grains of ferrite form preferentially in the direction of the applied field, which in turn leads to a magnetically anisotropic macrostructure. Perhaps surprisingly, differences in the bulk magnetic properties of the applied field product compared to a ‘zero field’ material remain even after it has been ground, sintered and re-ground. This is clearly shown by hysteresis loop data, such as that in figure 2 for a magnetically soft ferrite, MgZnFe₄0₈.

How does the Magnetic Field Affect the Process?

The magnetic field acts on the pre-combustion mixture of reactants, orientating some or all of the constituent particles along the field lines and so giving rise to a textured powder. In the production of ferrites this applies especially to the iron particles, which are the internal fuel source for the SHS reaction. Thanks to this orientation, the resulting fireworks are faster and hotter than their counterparts in zero magnetic field, and the levels of unreacted impurities in the product are significantly reduced. Both the microstructure and the magnetic domain structure of the product are textured according to the direction of the applied field.

Where does this Lead?

This leads to unprecedented possibilities for manufacturing products with controlled spatial variations in their magnetic properties, which could be specifically tailored for various applications including miniature devices. The applied field SHS production technology for making such devices is extremely flexible, allowing many different sets of conditions to be used. We hope to carry out reactions using both pre-textured and composite powdered precursors in either static or time-varying uniform or inhomogeneous applied magnetic fields. These reactions could run under ambient pressure or consolidation conditions in a static or flowing gaseous environment of air or another selected gas such as oxygen. Products made using our proven SHS technology are likely to be sufficiently dense for practical use without the need for extensive post-reaction processing. The researchers have filed for a patent, and are currently in consultation with industry, looking to exploit our process.