Synthesis of various magnetic nanostructures and the microwave characterizations 5

Chapter Size controllable synthesis of octahedral Fe3O4 nanoparticles and the microwave absorbing properties Chapter Size controllable synthesis of octahedral Fe3O4 nanoparticles and the microwave absorbing properties 5.1 Introduction For Fe-based alloys, the flake-like structure is effective to improve the microwave absorption performance. The most commonly used method to synthesize Fe-based alloys particles is the high energy ball milling process, however, only flake-like structure could be obtained via this method. To investigate more on the effect of particle shape of magnetic materials on the microwave absorption performance, we further choose spinel ferrites as our studying materials. Compared with Fe-based alloys, spinel ferrite possesses relative high resistivity and moderate saturation magnetization. When look back to the Snoek’s law as following (μi − 1)ƒr = γ4πMs (1.10 in Chapter 1) we could learn that the moderate saturation magnetization of spinel ferrite will lead to a much lower Snoek’s limitation, which means that the resonance frequency of spinel ferrites is very low, usually below GHz.[1] To enhance its resonance frequency for microwave applications, we should enhance its saturation magnetization or induce shape anisotropic field into the particles. As a typical member in spinel family, Fe3O4 was firstly synthesized in this work for the study on microwave absorption 72 Chapter Size controllable synthesis of octahedral Fe3O4 nanoparticles and the microwave absorbing properties performance. One of the challenges for modern chemists and materials scientists is to control and manipulate the shapes of materials in the nanometer scale, as different shapes of the nanostructures can introduce novel electronic, optical, or magnetic properties, compared with their spherical counterparts.[2-4] Substantial progress has been made on the shape-controlled synthesis of semiconductor nanocrystals. Several typical nonspherical examples such as triangles, rods, cubes, arrows and tetrapods[5-8] have been reported. Magnetic iron oxide nanoparticles (e. g., magnetite and maghemite) comprise another important class of nanostructured materials, which have widespread applications as diverse as environmental remediation, magnetic recording and bimolecular tagging, imaging, and sensing.[9-12] Compared to semiconductor and metallic nanocrystals, magnetic nanoparticles with nonspherical shapes demonstrate more appealing anisotropic magnetic properties.[13] Apart from the most common spherical shape, magnetic iron oxide nanoparticles with cubic, tetra-pod, tubular, triangular, ring/tube-like, octahedral and pyramidal shapes have been fabricated.[14-17] Despite the efforts described above to enrich the library of the shapes of iron oxide nanostructures, the size range of the products obtained through any single route is usually limited. To the best of our knowledge, there were few reports on synthesizing nonspherical iron oxide particles with sizes ranging from sub-10-nm up to several 73 Chapter Size controllable synthesis of octahedral Fe3O4 nanoparticles and the microwave absorbing properties hundred nanometers through a single route. It is known that the superparamagnetic limit for magnetite is ∼20 nm.[18,19] Most organic-phase synthesis methods reported previously have mainly focused on the size control of to 20 nm from the thermal decomposition processes of iron acetylacetonate or iron oleate.[20] In this chapter, we are going to investigate a facile synthesis of single crystalline octahedron-shaped magnetite (Fe3O4) nanoparticles bound with {111} planes through a thermal decomposition route. The particle size can be readily tuned from 6nm to ∼ 430 nm with narrow size distributions (σ [...]... regimes The surfactant/precursor ratio and the concentration of surfactant are the two crucial parameters for synthesizing the octahedron-shaped nanoparticles with uniform sizes Further experimental and theoretical study is needed to fully understand the formation thermodynamics and kinetics We have also investigated the microwave absorption performance of the as-synthesized Fe3O4 nanocrystals with size of. .. range of 0.1 GHz to 18 GHz, the real part and the imaginary part of complex permittivity are almost constant Compared with the commonly used NiZn-ferrite and Mn-ferrite,[33- 35] of which the resonance peak is no more than several hundreds of megahertz, the Fe3O4 nanocrystals show an enhancement in the resonance frequency The reflection loss curves in Fig 5. 15b are calculated by using Fig 5. 15 (a) are the. .. than 50 nm readily aggregate and settle at the bottom of solutions, as shown by the photo images in Fig 5. 14 Our method offers a Fig 5. 14 Photographs of liquid suspension (in hexane) of nanoparticles: 6 nm (left) and 53 nm (right) simple route to synthesize magnetite with different sizes that are either superparamagnetic or single- or multi- domain ferrimagnetic, depending on their sizes The as-synthesized... of 114 nm The resonance peak shift to higher frequency compared with the NiZn- and MnZn-ferrite The reflection loss can reach -16 .5 dB at the thickness of 6 mm, which is going to be used as references for 88 Chapter 5 Size controllable synthesis of octahedral Fe3O4 nanoparticles and the microwave absorbing properties comparison with the microwave absorption performance of other materials in the following... ɛ") and permeability (μ', μ") spectra and (b) are the calculated frequency dependent reflection loss plots The measurement is performed in the frequency range of 0.1 to 18 GHz 87 Chapter 5 Size controllable synthesis of octahedral Fe3O4 nanoparticles and the microwave absorbing properties the measured electromagnetic parameters A set of reflection loss curves are obtained by adjusting the thickness of. .. nm and (F) 5. 7± nm 0 .5 The crystallographic information of the as-synthesized nanooctahedra with different sizes was studied using X-ray diffraction (XRD), as summarized in Fig 5. 10 The positions and relative intensities of all diffraction peaks match well with the standard magnetite diffraction data (JCPDS no 19-0629) The crystalline sizes calculated from 82 Chapter 5 Size controllable synthesis of. .. quasi-superparamagnetic behaviors with small coercivities when the particle sizes ranged from 18 nm to 25nm The coercivity then increases with size, reaches a maximum, and starts to decrease again The decrease in coercivity is associated with the transition from single-domain to multidomain.[17, 25] The single domain particle 85 Chapter 5 Size controllable synthesis of octahedral Fe3O4 nanoparticles and the microwave. .. adjusting the thickness of the absorber composed of Fe3O4/paraffin As seen from the curves, the lowest reflection loss of -16 .5 dB is acquired when the thickness is 6 mm The observed microwave absorption performance of as-synthesized octahedral Fe3O4 is further used for comparison with that of Fe3O4 particles with some other nanostructures or Zn-ferrite particles in the following chapters 5. 4 Summary In this... compared with bulk 83 Chapter 5 Size controllable synthesis of octahedral Fe3O4 nanoparticles and the microwave absorbing properties Fig 5. 11 (A) Magnetization as a function of applied field for the powder samples of the as-synthesized Fe3O4 nanooctahedra with different sizes at room temperature and (B) the magnetization-field curves at low applied field magnetic materials, ms for the particles with finite... Fig 5. 12 By fitting the curve, the values of Ms and d estimated to be 96 30 emu/g Fig 5. 12 Linear relationship between 𝐦 𝐬 𝟏/𝟑 and 𝟏/𝐫 (Remark: the average particle size 𝐫 ∗ used in calculation has been converted to equivalent radius for spherical nanoparticles) 84 Chapter 5 Size controllable synthesis of octahedral Fe3O4 nanoparticles and the microwave absorbing properties and 1.03 nm are close to other . properties 75 of the reaction solution during the synthesis process. To trace the progress of the reaction, aliquots of the solution were sampled using a syringe. As shown in Fig. 5. 1, the magnetic. reaction time: 0 min, 5 min, 35 min and 60 min. The scale bars in all the images stand for 50 0 nm. Chapter 5 Size controllable synthesis of octahedral Fe3O4 nanoparticles and the microwave absorbing. moment of reaction solution. With the reaction Fig. 5. 1 The change of the magnetic moment of the reaction solution versus reaction time at 280 ℃ . Two steps of increases in magnetic moment at 5