Autor: Ivan P. Parkin,a Quentin A. Pankhurst,b Louise Afeck,a Marco D. Aguasa

Self-propagating high temperature synthesis of BaFe12O19,

Mg0.5Zn0.5Fe2O4 and Li0.5Fe2.5O4

Barium ferrite (BaFe12O19), lithium ferrite (Li0.5Fe2.5O4) and magnesium zinc ferrite (Mg0.5Zn0.5Fe2O4) have been prepared by self-propagating high-temperature synthesis (SHS) reactions from iron, iron(III) oxide and metal oxides and peroxides. The driving force for the reactions is the oxidation of iron powder. Reactions were

carried out under an oxygen and with the addition of sodium perchlorate as an internal oxidising agent.

The reactions were studied by time. A white beam of X-rays was used in combination with three energy sensitive detectors. Spectra were acquired with scan intervals of 100±250 ms and an effective 2h range of 10± 60?. Transformation of reactants to products occurred on the order of 200 ms in all of the systems studied with the exception of the applied barium ferrite synthesis where an intermediate of Fe3O4 was observed. Lattice expansion effects during SHS enabled the rates of cooling in the system to be investigated. All of the materials synthesised by SHS were examined both before and after annealing. Solid state reactions require prolonged heating even at elevated temperatures because diffusion within solids is slow. Conventional synthesis, also known as the «ceramic method», involves many grinding, heating and cooling steps, in order to overcome the solid state diffusion barrier. Many synthetic routes have been developed to improve upon the ceramic method. These techniques aim to reduce the solid state diffusion length by mixing the components on an atomic scale. Self-propagating high-temperature synthesis (SHS) is an alternative solid state route that makes use of the exothermicity of a chemical reaction. Self-propagating high-temperature synthesis was investigated in the late 1960s by Merzhanov and co-workers and has become a separate branch of scient. SHS enables the preparation of a wide variety of materials, such as intermetallic compounds, metal carbides, oxides, borides and nitrides. The nature of SHS also allows novel applications, for example, coating the inside of pipes, or containment of radioactive material. SHS reactions are highly exothermic.

Enough heat is released in the reaction to ignite successive layers of reaction mixture; hence no external heating is required. The reactions are characterised by rapid heating and cooling and may be ignited in two ways–at a point or in bulk. From point initiation the reaction proceeds via a propagation (also known as synthesis and combustion) wave which moves through the reaction mixture. Temperatures in excess of 1000 0 C are reached. The products require minimal treatment to produce single-phase materials.

We are interested in the formation of ferrite materials by SHS, particularly BaFe12O19, Mg0.5Zn0.5Fe2O4 and Li0.5Fe2.5O4. Ferrites are magnetic oxides that many uses in commercial applications. M-Type hexagonal barium ferrite, BaFe12O19, was ?rst studied in an industrial context by the Philips Laboratories in 1952. Mg0.5Zn0.5Fe2O4 has applications immobile phones and television set focussing systems due to its high saturation magnetisation. Li0.5Fe2.5O4 has the cubic inverse spinel structure, occupying octahedral sites. It is a soft ferrite with a very high Curie temperature of 638 0C. These features mean it applications in microwave devices. Two crystallographic forms of Li0.5Fe2.5O4 have been identi?ed: a superstructured form in which the lithium and iron atoms are ordered, and a disordered form in which lithium and iron have a random statistical distribution over all the octahedral positions. This is a novel area of work; the only previously published results were for increased temperatures and reaction rates for SHS of titanium carbide and strontium ferrite. Much work has been published on SHS combustion theory by Merzhanov. Novikov found the empirical relation that the adiabatic combustion temperature must exceed 1500 0C for a reaction to propagate. In particular, it would be interesting to discover information about the reaction pathways taken during SHS, estimate reaction temperatures from thermal expansion effects. Time resolved X-ray diffraction (TRXRD) is a technique that is able to probe these questions. TRXRD experiments on SHS reactions have been performed on just a handful of occasions using synchrotron. A monochromatic X-ray beam and detectors were used, typically with a range of 2h and 30±1000 ms resolution.

Experimental. All chemicals were obtained from Aldrich Chemical Co. and used. Manipulations, weighing and grindings were performed in a Saffron glove box under a nitrogen atmosphere. Sintering was carried out on ground powders in a Carbolite rapid heating furnace with heating and cooling rates of 20 0C min21. Samples were ground after the SHS reaction and also after sintering; all measurements were recorded on powder samples. The powders were analysed by X-ray diffraction. Scanning electron micrographs and electron microprobe data were obtained on both a Hitachi S-4000. Spectra were folded to remove baseline curvature and were calibrated relative to a-iron at room temperature. FT-IR spectra were obtained as KBr pellets on a Nicolet 205. Synthesis wave temperatures were determined by FLIR thermal imaging camera. BaO2 ( 1.40 g, 8.3 mmol), Fe ( 2.50 g, 44.7 mmol) and Fe2O3 ( 3.57 g, 22.4 mmol) were ground together in a pestle and mortar. The green mixture was placed on a ceramic tile and ignited at one end with a hot nichrome wire (800 0C). This induced an orange propagation wave (920 0C) which moved across the mixture. The post-SHS products were ground.

Magnesium zinc ferrite. MgO ( 0.36 g, 8.95 mmol), ZnO ( 0.73 g, 8.95 mmol), Fe ( 1.00 g, 17.9 mmol) and Fe2O3 ( 1.43 g, 8.95 mmol) were ground together in a pestle and mortar. NaClO4 ( 0.82 g, 6.72 mmol) was added and lightly ground together with the rest of the mixture. The reactions were performed in the same way as for BaFe12O19 and a bright yellow synthesis (1280 0C) wave with velocity 7 mm/s was observed. The post-SHS products were ground and washed with y1 l of distilled water to remove the NaCl. They were then dried.

Lithium ferrite. Li2O2 ( 0.205 g, 4.5 mmol), Fe ( 1.00 g, 17.9 mmol) and Fe2O3 ( 2.145 g, 13.4 mmol) were ground together in a pestle. The reactions were performed in the same way as for BaFe12O19 and an orange propagation wave (850 0C), ca. 1± 2 mm s21, was observed.

Conclusions

Self-propagating high temperature synthesis provides an easy route to a range of ferrite magnets. The reactions proceed by a synthesis wave of temperature 800 ± 1300 0C and velocity 2± 10mm/s, dependent on the system. In some cases, such as lithium ferrite and magnesium zinc ferrite, the desired ferrite is formed directly from the SHS, whilst for hexagonal barium ferrite the product requires annealing at 1150 0 С . This reveals that the SHS process initially cools at a rate of ca. 1000 0C for all three systems studied. Further studies are planned for the use of TRXRD in fast chemical systems

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