Synthesis of various magnetic nanostructures and the microwave characterizations 8

Chapter Conclusions and future works Chapter Conclusions and future works 8.1 Conclusions Various magnetic structures, including Fe/SiO2 core/shell, micron- and submicronFe/Al flakes, Fe3O4 rings, Fe3O4 tubes, Fe3O4 rods, Fe3O4 and Zn-ferrite nanocrystals, were successfully prepared for microwave applications in this work. Large scale synthesis methods were developed for the microwave measurements. The achievement on the synthesis and the findings in the microwave absorption performance of as-synthesized magnetic nanostructures are concluded below:  Fe90Al10 flakes with micron- and submicron-sizes were fabricated via different milling routes. Traditional wet ball-milling process with pure ethanol as additive was performed to prepare micro-sized Fe/Al flakes. With increasing wet-milling time, the flake size increases. For Fe/Al submicron-flakes, Micron-Master Jet Pulverizer (so called jet mill) was employed. The results reveal that Fe90Al10 submicron-flakes with an average lateral size of 500 nm could be obtained through the combination of ball milling and jet milling. Microwave performance of the obtained Fe90Al10 flakes with different sizes was further studied.  Monosized octahedron-shaped magnetite (Fe3O4) nanoparticles with average sizes ranging from nm to ∼ 430 nm were synthesized via a thermal decomposition route. The novelty of the method is that only one kind of surfactant (oleic acid) is used in the synthesis process. Therefore, the size of 139 Chapter Conclusions and future works Fe3O4 nanoparticles is easily tuned by adjusting the molar ratio of surfactant to Fe precursor. In other words, we are able to adjust the magnetic properties of the as-synthesized nanoparticles from superparamagnetic to single-domain to multidomain regimes.  Nonstoichiometric zinc ferrite (ZndFe3-dO4) nanoparticles with d ranging from to 0.5 were synthesized via thermal decomposition route employing oleic acid as surfactant. The zinc dopant concentration was controlled by the ratio of Zn/Fe precursors. High room temperature saturation magnetization of 110 emu/g was obtained for large Zn-ferrite particles (more than 100 nm) with nominal composition of Zn0.468Fe2.532O4. The origin of the extraordinary magnetic property was revealed as the Zn substitution of Fe atoms at the tetrahedral site (A site) in the spinel magnetite phase. It has been found, that the precursor/surfactant ratio was an important parameter for the control over the shape and size of as-synthesized Zn ferrite particles. Hence the developed thermal decomposition method is not only suitable for the synthesis of high quality Fe3O4 nanoparticles, also suitable for the synthesis of Zn-ferrite nanoparticles with novel properties.  Fe3O4 particles with some novel structures (rings, tubes and rods) were synthesized via chemical reduction route by employing -Fe2O3 particles as templates. In the reduction process, both of surfactant (oleic acid) and protective gas (5%H2/95%Ar gas mixture) were employed to complete the phase 140 Chapter Conclusions and future works transformation. The effect of oleic acid and H2 gas on the reduction process of -Fe2O3 particles was studied through a series of experiments. The results indicated that the chemical reduction method was convenient and feasible to reduce various hematite particles to pure magnetite phase.  The insulating SiO2 shell layer is effectively to suppress the skin effect of Fe particles, which makes the microwave permeate into the microwave absorber more easily. As a result, the resonance peak corresponding to the lowest reflection loss shifts to higher frequency band. Compared to the spherical Fe/Al alloys, the Fe/Al flakes show two advantages on the microwave absorption. The one is that the thickness of the flakes is rather smaller than the skin depth, thus the incident microwave can permeate into the particles. The other one is that the introduction of shape anisotropy could enhance the Snoek’s law limitation, resulting in a higher resonance frequency. Therefore a great improvement has been made on the microwave absorption by shaping the metallic alloy into flakes. The frequency peak shifts to higher band with decreasing the lateral size of flakes. By studying the microwave absorption performance of various Fe3O4 nanostructures, we also found that the shaped structures (ring, tube and rod) could greatly enhance the resonance frequency to higher band. Especially for the Fe3O4 nanorods, the resonance peak shifts to 4.82 GHz. As for effective microwave absorption, a balance between the high resonance peak and the high permeability should be 141 Chapter Conclusions and future works found. Fe3O4 rings with an outer diameter of 154 nm show a moderate resonance frequency (4.01 GHz) but a relative low reflection loss (-28 dB) compared to Fe3O4 particles with other structures. As investigated, Zn-ferrite particles with very high saturation magnetization (104 emu/g) show a very high permeability value of 1.4 at the resonance frequency 3.45 GHz, resulting in the lowest reflection loss of -38 GHz in this work. As predicted by the Snoek’s law, the high saturation magnetization is very necessary for effective microwave application. 8.2 Future works (1) This study introduces the effect of jet milling process on the large alloys particles. The use of jet mill may allow us to reduce the particles size to less than μm. Although the reported ball milled particles are of nanocrystalline structure, the irregular shape and broad size distribution of ball milled products are still problems in practical applications. For example, the application as microwave absorber requires the magnetic particles dispersing in the medium. The non-homogenous dispersion of magnetic particles in medium may come from the broad size distribution, which is bad for the microwave absorbing performance. Thus the jet milling machine is useful to overcome this problem. In future, the use of jet milling for the synthesis of submicron Fe/Al flakes may be tried on other materials. (2) The thermal decomposition method is adopted to synthesize monosized Fe3O4 and 142 Chapter Conclusions and future works Zn-ferrite particles. The particles size could be tuned from hundreds of micrometers to several nanometers. Fe3O4 and Zn-ferrite particles with large size (above 100nm) have been applied as microwave absorber. The mechanism on the multi microwave absorption peaks of as-synthesized Zn-ferrite particles is still unclear. In future, some simulation work could be performed to make it clear. Besides, we have also presented some small Fe3O4 and Zn-ferrite particles, which are superparmagnetic. It is expecting to apply these small nanoparticles into biomedical applications, such as hyperthermia for cancer therapy. Besides Zn-ferrite, the other members in spinel family, such as Mn-, Ni-ferrite as well as Mn/Zn-,Ni/Zn-ferrite can also be synthesized by using the developed thermal decomposition method. They are highly demanded as soft magnetic materials. It would also be very interesting to study the properties of other kinds of spinel ferrites synthesized by the developed thermal decomposition method. (3) The novelty of as-developed chemical reduction method lies in its simplicity as well as its capabilities on reducing various α-Fe2O3 particles. It can be used to produce Fe3O4 particles with more novel structures in future. The applications of as-reduced Fe3O4 particles are very promising. According to the simulation work based on Landau-Lifshits-Gilbert (LLG) equation done by our group member, the anisotropic Fe3O4 particles such as tubes and rods could exhibit very high permeability values when they are aligned parallel to the incident microwave. 143 Chapter Conclusions and future works Hence the technique on the sample preparation for microwave measurement could be improved to build aligned structure. One of most interesting nanostructures is the Fe3O4 discs, which has been produced via the developed chemical reduction. The aligned structure of Fe3O4 disks and its microwave absorption performance is under studied currently. Future works are expected to be related with the magnetic and electrical properties of as-obtained Fe3O4 particles with various structures as well as their applications, such as battery, biomedical and magnetic applications. 144 . synthesized via a thermal decomposition route. The novelty of the method is that only one kind of surfactant (oleic acid) is used in the synthesis process. Therefore, the size of Chapter 8. advantages on the microwave absorption. The one is that the thickness of the flakes is rather smaller than the skin depth, thus the incident microwave can permeate into the particles. The other one. achievement on the synthesis and the findings in the microwave absorption performance of as-synthesized magnetic nanostructures are concluded below:  Fe 90 Al 10 flakes with micron- and submicron-sizes