1.4.1 Physical properties related to RF heating
Since the magnetism of the material has a large utility in different areas like medicine and physics, properties of the magnetic nanoparticles and the key factors affecting their magnetism have been widely explored. To apply magnetic particles to induction heating in chemistry, their properties should be optimized, namely saturation magnetisation, coercivity, Curie temperature, thermal stability and corrosion resistance. Some of these properties will be discussed in this section. The heating rates of a material under RF field can be estimated from its magnetization M-H curves. The magnetization curves of ferromagnetic and paramagnetic material are quite different. For the ferromagnetic particles, when an external magnetic field is applied, the magnetism of the particles will rise until getting to the saturation magnetisation (Ms). However, with the fading of the external field, part of the
magnetism will retain in the particles (Mr) and another magnetic field on the
opposite direction called coercivity (Hc) is needed to pull the magnetism back
to zero. The M-H curves describing the relationship between magnetism of the material and external field strength is also called hysteresis loop (Figure 1.5 (a)). However, for paramagnetic material, the magnetism is induced by the external magnetic field, having no hysteresis phenomenon (Figure 1.5 (b)). For soft magnetic material which has a low coercivity field around several hundred A/m, the heat generated was found out to be proportional to the area of hysteresis loop [53, 54]. The ferromagnetic material obviously shows much better heating property than paramagnetic material.
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(a) (b)
Figure 1.5 Schematic view of magnetization curves of (a) ferromagnetic material (b) paramagnetic material
The Curie temperature is the point where particles transit from ferromagnetism to paramagnetism. Magnetic particles loss the heating ability under RF heating once Curie temperature is reached [55]. In this way, the existence of Curie point limits the highest possible temperature the reactions can be performed under RF heating. Another aspect needed to be considered is the corrosion resistance of the magnetic material. Since the magnetic material is immerged in the reactant mixture, its composition or crystal type may change during the reaction process when oxygen, high temperature, strong acid or basic reactants are used, leading to the loss of the heating property. Thus necessary stability of the material in the chemical environment is required. The physical characteristics mentioned above vary with the composition of the magnetic particles.
1.4.2 Type of the magnetic material
The components of magnetic nanoparticles are mainly metal, metal oxide or alloy of the iron-group elements.
Metals and their alloys
Pure Fe, Co and Ni and their alloys demonstrate ferromagnetism. For the pure metals, their Curie temperatures rose with the increasing size of the particles in the nanometre range. The values of the Curie temperature for Fe, Co, Ni at 3 nm are 680, 1040 and 320 ºC, respectively [56]. Without a
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protective coating, they are very easy to get oxidized in air or by oxidants [57], resulting in the mechanical and magnetic change. Iron powder was found out could be heated to a certain extent in an RF field below 25 kHz, which was attributed to the instability of the iron itself [58]. Co and Ni nanoparticles showed better resistance and under the same AC magnetic field, the Ni particles showed higher specific absorption rate (SAR) than that of Co particles [59].
Iron oxides
The γ-Fe2O3 and Fe3O4 are the common magnetic materials to be chosen for
induction heating. The Curie temperature of 25 nm Fe3O4 nanoparticles is
849 ºC [60]. Nevertheless, the mixed iron oxides, Fe3O4 can be oxidized into
iron (III) oxide, γ-Fe2O3 if heated over 150-170 ºC [61-63]. On the other side,
the ferromagnetic γ-Fe2O3 transforms to the paramagnetic α-Fe2O3 above
500 ºC [64], limiting the maximum allowed temperature in a catalytic process. Compared to pure Fe nanoparticles, the SARs of iron oxides were found to be higher, and the SAR of γ-Fe2O3 was higher than that of Fe3O4. Also, the
strength of magnetism varied with the particle size as the SAR of 25 nm Fe2O3 nanoparticles is much higher than that of 9 nm (Figure 1.6)[65].
Figure 1.6 The SAR of different magnetic nanoparticles as a function of frequency of magnetic field [65].
Metal ferrites
Ferrites are mixed metal oxides of Mn, Ni, Zn, Cu and Co. They are often obtained to regulate magnetic properties of ferromagnetic materials. Different from simple iron oxides, their Curie temperatures and relative magnetic
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properties can be easily tuned by the control of their chemical composition, phase structure and crystal size [66-68]. For instance, an addition of Zn to nickel ferrite which forms Ni0.5Zn0.5Fe2O4 increases the specific surface area
while decreases both the Curie temperature and SARs [54, 55]. Even though the properties of metal ferrites are varied, the Curie temperature of common metal ferrites like MnFe2O4, NiFe2O4,CoFe2O4 are located above 550 ºC [60,
69, 70]. The SAR of metal ferrite is normally lower than that of Fe3O4 [71],
but they showed an improved magnetic stability at high temperatures and in high frequency RF field [72]. In particular the magnetism of nickel ferrite has a good thermal stability which can tolerate temperature up to 600 ºC [73, 74]. Also, the heating rate shows only a slight decrease even at high frequency [72].