Synthesis of various magnetic nanostructures and the microwave characterizations

SYNTHESIS OF VARIOUS MAGNETIC NANOSTRUCTURES AND THE MICROWAVE CHARACTERIZATIONS YANG YANG (B. Eng.), DJTU A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MATERIALS SCIENCE & ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2013 Acknowledgements As an important period in my life, a 4-year PhD study (2008 ~ 2012) has passed. At this moment when I start to write my thesis, a lot of memories come to the fore. So many people involved in my research career, and hereby I would like to express my sincere thanks to them for their invaluable support and encouragement. First and foremost, I would like to offer my sincerest gratitude to my supervisor Prof. Ding Jun in the Department of Materials Science & Engineering of National University of Singapore, for his immense patience and warm encouragement throughout the years of undertaking my research work.Under his professional guidance, I have learned how to conduct my experimental workand improved myself on technicalwriting skills and presentation skills. Also, I would like to give my earnest appreciation to Dr Yi Jiabao for his kind assistance and pertinent suggestions on my research project. His expertise in many aspects of scientific research is worthy of the utmost admiration and respect. His hardworking spirit influences me deeply. Furthermore, I would like to give my heartfelt appreciation to Dr Li Ling, who guided me with his experienced research knowledge when I was a beginner and impressed me by his intelligence and passion in researches, as well as his outgoing and optimistic character. i In addition, I would like to express my special thanks to all my group members for their willingness to help and friendships, which encourage me greatly during the PhD study; special thanks to the lab officers in the Department of Materials Science & Engineering for their understanding and technical support; special thanks to National University of Singapore to provide me the financial support. Last but not least, I would like toexpress my deep gratitude to my family in China for their everlasting love and constant support. Yang Yang ii Publications during PhD study 1. Ling Li, Yang Yang, Jun Ding, JunminXue, Synthesis of Magnetite Nanooctahedra and Their Magnetic Field-Induced Two-/Three-Dimensional Superstructure, Chem. Mater. 2010 (22) 3183 -3191. 2. Chye Pho Neo, Yang Yang, Jun Ding, Calculation of complex permeability of magnetic composite materials using ferromagnetic resonance model, J. Appl. Phys., 2010 (107) 083906. 3. Yang Yang, Jiabao Yi, Xuelian Huang, JunminXue, Jun Ding, High-coercivity in -Fe2O3 formed after annealing from Fe3O4 nanoparticles, IEEE Trans. Magn. 2011, 47 (10): 3340-3342. 4. Yang Yang, Jun Ding, Microwave property of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes, J. Alloys Compd. 2012 (528) 58-62. 5. Xuelian Huang, Yang Yang, Jun Ding, Epitaxial growth of γ-Fe2O3 thin films on MgO substrates by pulsed laser deposition and their properties, Actamaterialia 2013 (61) 548-557. 6. Xuelian Huang, Yang Yang, Jun Ding, Structureandmagneticpropertiesof Fe3O4 thin films on different substrates by Pulsed Laser Deposition, J. Korean Phys. Soc. 2013 (accepted). 7. Yang Yang, Xiaoli Liu, Jun Ding,Synthesis of -Fe2O3 templates via hydrothermal route and Fe3O4 particles through subsequent chemical reduction (accepted by Science of Advanced Materials) 8. Yang Yang, Xiaoli Liu, Yang Yong, Wen Xiao, Zhiwei Li, DeshengXue, Fashen Li,Jun Ding, Synthesis of nonstoichiometric zinc ferrite nanoparticles with extraordinary room temperature magnetism and their manifold applications (accepted by Journal of Materials Chemistry ) 9. Yang Yang, Zhengwen Li, Chye Pho Neo, Jun Ding, Model Design on Calculations of Microwave Permeability and Permittivity of Fe/SiO2 particles iii with core/shell structure, Journal of Physics and Chemistry of Solids (in the revision) 10. Haitao Zhang, YangYang, Nina Bao, Jun Ding, A Scalable Route to Mesoporous Iron Oxides by the High-energy Ball Milling Technique (submitted to Journal of Physical Chemistry) iv Table of Contents Acknowledgements…………………………………………………… i Publications during PhD study……………………………………….iii Table of contents……………………………………………………….v Summary……………………………………………………………….x List of figures………………………………………………………… xii List of tables………………………………………………………… .xix Chapter Introduction…………………………………………………1 1.1 Fundamentals for microwave absorption……………………………………… .1 1.1.1 Description of microwave absorption ability………………………………2 1.1.2 Calculation of microwave absorption ability………………………………3 1.1.3 Snoek’s law…………………………………………………………………4 1.1.4 Skin effect………………………………………………………………… 1.2 Magnetic materials for microwave absorption………………………………… .8 1.2.1 Metallic magnetic materials……………………………………………… 1.2.2 Ferrites…………………………………………………………………… .9 1.2.2.1 Hexagonal ferrites…………………………………………………… 1.2.2.2 Spinel ferrites…………………………………………………………10 1.3 Brief review of size-controlled synthesis technology………………………… .12 1.3.1 Ceramic sintering method…………………………………………………13 1.3.2 Ball milling method……………………………………………………….14 1.3.3 Wet chemical method…………………………………………………… 15 1.4 Motivations and objectives…………………………………………………… .19 v 1.5 References……………………………………………………………………….22 Chapter Experimental techniques………………………………….27 2.1 Materials synthesis…………………………………………………………… .27 2.1.1 Preparation of Fe/SiO2 core-shell particles by Stöber process………… 27 2.1.2 Preparation of Fe/Al flakes by ball milling and jet milling…………… 27 2.1.3 Synthesis of Fe3O4 nanoparticles by thermal decomposition method… .29 2.1.4 Synthesis of Zn-ferrite nanoparticles by thermal decomposition method……………………………………………………………………30 2.1.5 Synthesis of Fe3O4 nanoparticles via chemical reduction of α-Fe2O3 template………………………………………………………………… 31 2.1.5.1 Synthesis of α-Fe2O3 nanoparticles with various shapes by hydrothermal route………………………………………………… 31 2.1.5.2 Synthesis of Fe3O4 nanoparticles via chemical reduction method using α-Fe2O3 nanoparticles as templates………………………………….32 2.2 Materials characterizations…………………………………………………… 34 2.2.1 Structural and microstructural analysis………………………………… 34 2.2.1.1 X-ray Diffraction (XRD)…………………………………………… 34 2.2.1.2 Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS)………………………………………………….36 2.2.1.3 Transmission Electron Microscopy (TEM)………………………… 37 2.2.1.4 X-ray Photoelectron Spectroscopy (XPS)……………………………39 2.2.2 Magnetic properties characterizations…………………………………….41 2.2.2.1 Vibrating Sample Magnetometer (VSM)…………………………… 41 2.2.2.2 Mössbauer spectroscopy…………………………………………… .42 2.2.2.3 Vector network analyzer (VNA)……… …………………………….44 2.3 References……………………………………………………………………….45 vi Chapter Synthesis and microwave absorbing properties of Fe/SiO2 particles with core/shell structure…………………………………….46 3.1 Introduction…………………………………………………………………… .46 3.2 Experimental results………………………………………………………… 48 3.2.1 Characterizations on the Fe/SiO2 core/shell structure ………………… 48 3.2.2 Investigations on the electromagnetic parameters…………………… 49 3.2.3 The comparison between the measured electromagnetic performance and the calculated results………………………………………………………51 3.2.4 Evaluationon the microwave absorbing performance……………………52 3.3 Summary……………………………………………………………………… .54 3.4 References……………………………………………………………………….54 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes…………….56 4.1 Introduction…………………………………………………………………… 56 4.2 Experimental results…………….…………………………………………… .58 4.2.1 Effect of jet milling on the morphology of different materials……….… 58 4.2.2 Fabrication and characterizations of micron and submicron Fe90Al10 flakes…………………………………………………………………… .64 4.2.3 Microwave absorption property of as-prepared Fe90Al10 flakes with different sizes…………………………………………………………… 68 4.3 Summary……………………………………………………………………… 71 4.4 References………………………………………………………………………72 Chapter 5Size controllable Fe3O4nanoparticles and the synthesis of octahedral microwave absorbing properties…………….73 5.1 Introduction…………………………………………………………………… .73 vii 5.2 Experimental results………………………………………………………… 75 5.2.1 Reaction kinetics study………………………………………………… 75 5.2.2 Effect of experimental parameters on the formation of octahedral Fe3O4 nanoparticles………………………………………………………………77 5.2.2.1 Effect from the molar ratio of precursors to surfactant………………77 5.2.2.2 Effect from the concentration of surfactant………………………….78 5.2.3 Synthesis and characterizations of octahedral Fe3O4 nanoparticles with various sizes……………………………………………………………….79 5.3 Investigations on microwave absorption performance………………………… 87 5.4 Summary……………………………………………………………………… .88 5.5 References……………………………………………………………………….89 Chapter 6Synthesis of Zn-ferrite nanoparticles with high saturation magnetization and theirmicrowave absorption property………… 91 6.1 Introduction…………………………………………………………………… .91 6.2 Experimental results………… .……………………………………………… .95 6.2.1 Synthesis and characterizations on large size Zn-ferrite nanoparticles… .95 6.2.1.1 Effect of the molar ratio of Zn precursor to Fe precursor on the composition and morphology……………………………………… .95 6.2.1.2 Investigation on the mechanism for large room temperature magnetization of as-synthesized Zn-ferrite nanoparticles………… 100 6.2.2 Study on the size control over as-syntheized Zn-ferrite nanoparticles… 103 6.2.3 Microwave absorption performance of high 𝐌𝐬 Zn-ferrite nanoparticles…………………………………………………………… 106 6.3 Summary……………………………………………………………………….108 6.4 References…………………………………………………………………… .110 Chapter Synthesis of -Fe2O3 templates via hydrothermal route viii and Fe3O4 particles through subsequent chemical reduction…… .113 7.1 Introduction…………………………………………………………………….113 7.2 Experimental results……………………………………………………………116 7.2.1 Synthesis of-Fe2O3 with various shapes by hydrothermal treatment…116 7.2.1.1 Mechanism on the formation of -Fe2O3 nanoparticles with different morphology…………………………………………………………116 7.2.1.2 Shape controllable synthesis of -Fe2O3 nanoparticles………….…118 7.2.1.3 Size controllable synthesis of -Fe2O3 rods……………………… 120 7.2.2 Chemical reduction of -Fe2O3 to Fe3O4 nanoparticles……………… .122 7.2.2.1 Effect of reducing agent (oleic acid) on the reduction process…….122 7.2.2.2 Effect of 5%H2 /95%Ar protection gas on the reduction process….126 7.2.2.3 Characterizations on the as-reduced Fe3O4 nanoparticles………….128 7.2.3 Microwave characterizations on as-reduced Fe3O4 particles……………131 7.3 Summary……………………………………………………………………….135 7.4 References…………………………………………………………………… .136 Chapter Conclusions and future works………………………… .139 8.1 Conclusions………………………………………………………………….…139 8.2 Future works………………………………………………………………… .142 ix Summary Magnetic materials have a wide range of uses in fundamental science and in technological applications. The nanostructure impacts greatly on the properties of magnetic materials. In this work, various magnetic nanostructures were achieved and their microwave absorbing performance was investigated. The contributions of this work are summarized below:  Fe/SiO2 core/shell structure was prepared via Stöber process. The results show a lower permittivity value of Fe/SiO2 than Fe particles, which may attribute to the suppression of skin effect by the insulating coating layer. The improvement on the microwave absorption was observed at relatively high frequency (above 12 GHz). Besides, the optimal thickness of Fe/SiO2 composite was 2.2 mm, smaller than that of Fe composite (3 mm), which makes it suitable for lighter microwave absorber.  Jet Pulverizer (so called jet mill) was introduced to refine the particles size of metallic alloys. By combining the jet mill with the ball mill process, Fe/Al flakes with lateral sizes ranging from 100 μm to 0.5 μm were successfully fabricated. Subsequently, size-dependent magnetic property and electromagnetic performance of Fe/Al flakes were investigated. The microwave absorption performance of Fe/Al alloys was improved by shaping the particles into flakes. The resonance frequency of Fe/Al flakes shifts to higher band when the size of flakes decreases.  Both Fe3O4 and Zn-ferrite nanoparticles have been synthesized by thermal decomposition method. Our special design ofthe synthesis was using only one capping ligand, i.e. oleic acid. This design allowed us to easily control the particle x size by modifying the precursor-to-capping ligand ratio. Very high saturation magnetization was observed in Zn-ferrite particles with size above 100 nm. The nonstoichiometric structure and the Zn substitution of Fe atoms at tetrahedral sites may account for the high magnetization. The electromagnetic spectra show that Zn-ferrite particles exhibit very high permeability 1.4 at the frequency of 3.25 GHz. The resultant reflection loss reaches -38 dB.This result makes as-synthesized Zn-ferrite outstanding as microwave absorbing material.  Fe3O4 nanoparticles were also synthesized by using a developed chemical reduction route. Prior to the chemical reduction process, hematite (α-Fe2O3) template was prepared by employing hydrothermal route. Various shapes of α-Fe2O3 nanoparticles, including rings, tubes and rods, were used as templates. The reduction process was assisted by the surfactant (oleic acid) and the protective gas (5%H2+95%Ar). This method was proved to be versatile for reducing various α-Fe2O3 nanoparticles without changing the initial morphology. Investigations on the electromagnetic performance indicated that the nanostructures (rings, tubes and rods) could enhance the resonance frequency of Fe3O4 particles. The resonance peak of 70 nm tubes and 98 nm rods were shifted to 4.46 GHz and 4.82 GHz. In the view of microwave absorption, the relative low reflection loss (-28 dB) was observed in 154 nm rings, which displayed moderate resonance frequency and permeability value. The results indicated that the tradeoff between high resonance frequency and high permeability value is necessary when design an effective microwave absorber. xi List of Figures Fig. 1.1 A Schematic diagram for the definition of microwave absorption ability. Fig. 1.2 A schematic diagram of the skin effect. Fig. 1.3 Schematic illustration of normal spinel structure, i.e. A2+B3+2O4. A2+ is located at tetrahedral sites (bubbles in green tetrahedron); B3+ is located at octahedral sites (yellow bubbles). Fig. 2.1 Photo image of the jet miller systems including air compressor, air drier as well as jet milling machine. Fig. 2.2 Schematic diagram of X-ray diffraction by a crystal. Fig. 2.3 Schematic illustration of different working modes of transmission electron microscopy: (a) diffraction mode and (b) imaging mode. Fig. 2.4 Typical diffraction patterns for (a) single crystalline structure and (b) polycrystalline structure. Fig. 2.5 Schematic illustration of a VSM system. Fig. 2.6 The effects of (a) the isomer shift; (b) the quadrupole splitting and (c) the magnetic splitting on the nuclear energy levels of 57 Fe. The Mössbauer absorptions and the resulting spectra are also shown. δrepresents isomer shift and Δ represents the quadrupole splitting. Fig. 3.1 An inhomogeneous sphere with a spherical core and a spherical layer. Fig. 3.2 SEM images of (a) Fe particles and (b) Fe/SiO2 particles; (c) TEM images of Fe/SiO2 particles; (d) EDS spectrum of Fe/SiO2 particles; (e1) ~ (e4) are elemental mapping corresponding to C, O, Fe and Si, respectively; (f) Magnetic hysteresis loops of Fe Fe/SiO2 particles. The scale bars in (a), (b) and (c) stand for 1μm. Fig. 3.3 Comparison of experimental results: (a) the complex permittivity and (b) complex permeability of Fe/Epoxy and Fe/SiO2/Epoxy composites. The volume concentration of magnetic filler is 8.6%. Fig. 3.4 Comparison of experimental results: (a) the complex permittivity and (b) xii complex permeability of Fe/Epoxy and Fe/SiO2/Epoxy composites. The volume concentration of magnetic filler is 15.5%. Fig. 3.5 Comparison of experimental results: (a) the complex permittivity and (b) complex permeability of Fe/Epoxy and Fe/SiO2/Epoxy composites. The volume concentration of magnetic filler is 26.3%. Fig. 3.6 Comparisons between calculated and experimental results of the effectiveelectromagnetic parametersfor Fe/SiO2composites. The filler concentration is labeled on top of figures. Fig. 3.7 The calculated reflection loss curves for (a) Fe/Epoxy composite and (b) Fe/SiO2/Epoxy composite.The thicknesses of the microwave absorbers are assumed to be 2.2 mm, 2.6 mm, mm and 3.4 mm for calculation. Fig. 4.1 Schematic diagram of the major part of jet mill. Fig. 4.2 The SEM images of iron particles:(a) commercial iron particles with sizes ranging from μm to 5μm; (b) and (c) are jet milled iron particles at bars, which are collected from the bottom and top part of the receiver, respectively. The scale bar for μm is for all three images. Fig. 4.3 The SEM images of iron particles: (a) commercial iron particles with sizes larger than 20 μm; (b) and (c) are jet milled iron particles at bars, which are collected from the bottom and top part of the receiver, respectively. Fig. 4.4 (a) X-ray diffraction patterns and (b) M-H loops of Fe-based solid solutions; the inset is for a clear observation of saturation magnetization. Fig. 4.5 The SEM images of Fe90Si10: (a) ball milled particles, which are further pulverized by jet milling; (c) and (d) jet milled products, which are collected from the bottom and top part of the receiver, respectively; (b) the EDS spectrum. Fig. 4.6 The SEM images of Fe90Co10: (a) ball milled particles, which are further pulverized by jet milling; (c) and (d) jet milled products, which are collected from the bottom and top part of the receiver, respectively; (b) the EDS spectrum. Fig. 4.7 The SEM images of Fe90Al10: (a) ball milled particles, which are further pulverized by jet milling; (c) and (d) jet milled products, which are collected from the xiii bottom and top part of the receiver, respectively; (b) the EDS spectrum. Fig. 4.8 TGA plots and the derivative curves for (a) commercial iron with particles size ranging from μm to μm; (b) jet milled Fe90Co10 flakes and (c) jet milled Fe90Al10 flakes. Fig. 4.9 SEM images of as-prepared Fe90Al10 samples: (a) Sample S0 after high energy ball milling for 12 h; (b) Sample S4 - spherical particles (large particles at the bottom part of the receiver) after jet milling; (c) Sample S1 obtained after wet ball milling for 0.5 h; (d) Sample S2 is obtained after wet ball milling for 5h; (e) Sample S3 - submicron flakes (small particles on the top part of the receiver) after jet milling; (f) a typical EDS spectrum. Fig. 4.10 (a) XRD patterns and (b) M-H loops of as-prepared Fe90Al10 flakes. Fig. 4.11 The relationship of electromagnetic parameters and frequency in the range of 0.1~18 GHz:(a) and (b) are the real and imaginary part of measured complex permittivity, respectively; (c) and (d) are the real and imaginary part of measured complex permeability, respectively. Fig. 4.12 The calculated theoretical reflection loss in the frequency range of 0.1 GHz to 18 GHz for the as-prepared Fe90Al10 flakes. 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 and 35 are indicated. Fig. 5.2 The morphology observed in SEM images of reaction solution sampled at 280 ℃ with different reaction time: min, min, 35 and 60 min. The scale bars in all the images stand for 500 nm. Fig. 5.3 XRD pattern (A) and typical SEM image (B) of the hematite nanoparticles synthesized by using 40 mmol of Fe(acac)3. The inset picture shows the red color of the hematite nanoparticle dispersion. Fig. 5.4 TEM image of Fe3O4nanocubes magnetically separated from the product synthesized by using 40 mmol of Fe(acac)3. Fig. 5.5 TEM images of the as-obtained Fe3O4 nanoparticles by reducing the xiv concentration of oleic acid: A) 0.35, B) 0.19, C) 0.10 and D-E) 0.07 M. Fig. 5.6 (A) Typical TEM image of 53 nm Fe3O4nanooctahedra. (B) Schematic 3D model of one octahedron-shaped nanoparticle. TEM and HRTEM images of 53 nm Fe3O4nanooctahedra with different projection shapes: (C, F) hexagonal (zone axis: ), (D, G) rectangle (zone axis: , and (E, H) parallelogram (zone axis: ). Scale bars: (A) 100 nm; (C - E) 20 nm; and (F - H) nm. Fig. 5.7 Illustration of particle size measurement. As the orientation of the octahedron-shaped particles can easily lead to the wrong estimation of particle size, the length between two opposite vertices for parallelogram-shaped projection was taken as the particle size, as indicated by the red lines. Fig. 5.8 SEM (A, B) and TEM (C - E) images of the as-synthesized Fe3O4nanooctahedra with different average sizes by adjusting the concentration of precursor [Fe(acac)3]: (A) ~ 430 nm (inset: SEM image of one single octahedron-shaped particle); (B) 114 nm; (C) 21 nm; (D) 18 nm; and (E) nm. (F) TEM image of nm-sized spherical nanoparticles. Scale bars: (A) μm (inset: 100 nm); (B) μm; and (C - F) 50 nm. Fig. 5.9 Size Distribution histograms for different sized nanooctahedra: (A) 114.3±12.2 nm; (B) 53.3±3.0 nm; (C) 21.3 ±1.1 nm; (D) 17.5±0.9 nm; (E) 7.5±0.6 nm and (F) 5.7±0.5 nm. Fig. 5.10 (A) X-ray diffraction patterns of the as-synthesized Fe3O4nanooctahedra with different average sizes, and (B) the change of the (311) peak intensity of each class of nanoparticles. All samples were deposited on glass substrates from their hexane dispersions. Fig. 5.11 (A) Magnetization as a function of applied field for the powder samples of the as-synthesized Fe3O4nanooctahedra with different sizes at room temperature and (B) the magnetization-field curves at low applied field. Fig. 5.12 Linear relationship between Ms1/3 and 1/r . (Remark: the average particle size r ∗ used in calculation has been converted to equivalent radius for spherical nanoparticles). xv Fig. 5.13 Size-dependent coercivities of the as-synthesized Fe3O4nanooctahedra. (Remark: the average particle size r ∗ used in calculation has been converted to equivalent radius of spherical particles, less than 10 nm: superparamagnetic; 18 to 25 nm: quasi-superparamagnetic; above 25 nm: ferrimagnetic). Fig. 5.14 Photographs of liquid suspension (in hexane) of Fe3O4 nanoparticles: nm (left) and 53 nm (right). Fig. 5.15 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the calculated frequency dependent reflection loss plots of 114 nm Fe3O4 nanoparticles. The measurement is performed in the frequency range of 0.1 to 18 GHz. Fig. 6.1 Schematic illustration of inverse spinel structure of Fe3O4, in the case, half of Fe atoms at tetrahedral sites are replaced by M atoms. Fig. 6.2 (a-f) SEM images of as-synthesized samples. All the scale bars stand for 200 nm. Fig. 6.3 SEM image of sample ZF7, which are synthesized with using 12 mmol of Fe precursors and 15 mmol of Zn precursors. Fig. 6.4 Schematic drawings for different shapes of Zn ferrite nanoparticles. (A) Octahedron formed at low precursor/surfactant ratio (0.42-0.58); (B) polyhedron (truncated octahedron) formed at medium ratio (0.58-0.72) and (C)cube formed at high ratio (0.72-0.78). When the ratio is over 0.78, irregular particles will be obtained. The precursors include Fe(acac)3 and Zn(acac)2. Fig. 6.5 (a) Typical XRD patterns and (b) magnetic hysteresis loops of Zn ferrite samples. Fig. 6.6 (a) The Mössbauer spectra of Zn ferrite samples and fitted curves; (b) Variation of area ratio of FeA to FeB versus Zn dopant concentration d. The experimental values are evaluated from the fitting results of Mössbauer spectra, while the red dash line follows FeA/FeB = (1-d)/2. FeA refers to the Fe atom at A sites. Fig. 6.7 (a-c) SEM images of samples with different sizes, which are shown in corresponding histograms. The error bar means particle size deviation from the average value. Inset TEM image in (c) is for 13.4 nm Zn ferrite nanoparticles. xvi Fig. 6.8 Magnetic hysteresis loops for 102.2 nm, 26.5 nm and 13.4 nm Zn ferrite nanoparticles. The inset shows the coercivity. Fig. 6.9 SEM image of Zn ferrite particles (ZF7) synthesized with using 16 mmol of Fe precursors and mmol of Zn precursors. Fig. 6.10 (a) and(b) are the permittivity (ɛ', ɛ") and permeability (μ', μ") spectra; (c) and (d) are the calculated frequency dependent reflection loss plots. Fig. 7.1 SEM images of (a) -Fe2O3 disks (10 h at 220℃); (b) mixed product of disk and rings (20 h at 220℃) and (c) -Fe2O3 rings (48h at 220℃). The scale bars for all the images stand for 200 nm. Fig. 7.2 SEM images of -Fe2O3 with different shapes: (a) 117 nm -Fe2O3 balls; (b) 74 nm -Fe2O3 rings; (c) 70 nm -Fe2O3 tubes and (d) 98 nm -Fe2O3 rod. The scale bars for all the images stand for 200 nm. Fig. 7.3 (a) SEM images of 120 nm -Fe2O3 rods and (b) TEM images of capsules and broken ones involved in as-prepared 120 nm -Fe2O3 rods. Fig. 7.4 SEM images of (a) 61 nm Fe3O4 rods and (b) 55 nm Fe3O4 rods. The scale bars on these images stand for 200 nm. (c) The HRTEM image for as-synthesized -Fe2O3rod and the corresponding SAED pattern (inset). Fig. 7.5 SEM images of (a) 74 nm -Fe2O3 rings, i.e. sample A; and as reduced samples: (b) sample B; (c) sample C; (d) sample D. The scale bars on these images stand for 200 nm. (e) The photo image of two samples, the one labeled with letter ‘A’ is for sample A dispersed in TOA, the other one with ‘T’ is for transparent solution obtained after reduction process when the ratio of oleic acid to -Fe2O3 rings is adjusted to be 29:1. The colour of sample B, C and D seems the same, as shown by the inset photo in figure (c). (f) The XRD patterns for different samples. Fig. 7.6 The M-H loops of as-reduced samples B, C and D. Fig. 7.7 SEM images of as-reduced samples: (a) sample E; (b) sample F; (c) sample G. The scale bars on these images stand for 200 nm. Photo images of sample B and sample C dispersed in TOA solution are also displayed by insets. (d) The XRD patterns for different samples. xvii Fig. 7.8 The M-H loops of as-reduced samples E, F and G. Fig. 7.9 SEM images of as-reduced samples: (a) 154 nm rings; (b) 70 nm tubes; (c) 98 nm rods. The scale bars on these images stand for 500 nm. All the scale bars stand for 500 nm. (d) The M-H loops for as reduced samples. Fig. 7.10 SEM images of as reduced sample: (a) 61 nm Fe3O4 rods; (b) 55 nm Fe3O4 rods. The scale bars on these images stand for 200 nm. (c) The HRTEM image for as-synthesized Fe3O4 rod and the corresponding SAED pattern (inset). (d) The M-H loops of as-reduced Fe3O4 rods with different sizes. Fig. 7.11 (a) The X-ray patterns and (b) XPS spectra of -Fe2O3 and Fe3O4 samples. Fig. 7.12 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the calculated frequency dependent reflection loss plots for 74 nm-Fe3O4 rings. Fig. 7.13 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the calculated frequency dependent reflection loss plots for 154 nm-Fe3O4 rings. Fig. 7.14 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the calculated frequency dependent reflection loss plots for 70 nm-Fe3O4tubes. Fig. 7.15 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the calculated frequency dependent reflection loss plots for 98 nm-Fe3O4rods. xviii List of Tables Table 2.1 The amounts of starting materials for synthesis of Fe3O4 nanoparticles with different sizes. Table 2.2 The amounts of starting material for the synthesis of Zn-ferrite nanoparticles with various compositions and sizes. Table 2.3 Synthesis conditions for -Fe2O3 nanoparticles with various shapes and sizes. Table 4.1 Summarized properties of the as-prepared samples. Table 7.1 Reduction conditions for phase transformation from -Fe2O3 to Fe3O4nanorings. Table 7.2 Characterized parameters for various magnetic structures summarized from the measured electromagnetic spectra and the calculated reflection loss curves. xix [...]... particles on the top part of the receiver) after jet milling; (f) a typical EDS spectrum Fig 4.10 (a) XRD patterns and (b) M-H loops of as-prepared Fe90Al10 flakes Fig 4.11 The relationship of electromagnetic parameters and frequency in the range of 0.1~18 GHz:(a) and (b) are the real and imaginary part of measured complex permittivity, respectively; (c) and (d) are the real and imaginary part of measured... min and 60 min The scale bars in all the images stand for 500 nm Fig 5.3 XRD pattern (A) and typical SEM image (B) of the hematite nanoparticles synthesized by using 40 mmol of Fe(acac)3 The inset picture shows the red color of the hematite nanoparticle dispersion Fig 5.4 TEM image of Fe3O4nanocubes magnetically separated from the product synthesized by using 40 mmol of Fe(acac)3 Fig 5.5 TEM images of. .. O, Fe and Si, respectively; (f) Magnetic hysteresis loops of Fe Fe/SiO2 particles The scale bars in (a), (b) and (c) stand for 1μm Fig 3.3 Comparison of experimental results: (a) the complex permittivity and (b) complex permeability of Fe/Epoxy and Fe/SiO2/Epoxy composites The volume concentration of magnetic filler is 8.6% Fig 3.4 Comparison of experimental results: (a) the complex permittivity and. .. permeability of Fe/Epoxy and Fe/SiO2/Epoxy composites The volume concentration of magnetic filler is 15.5% Fig 3.5 Comparison of experimental results: (a) the complex permittivity and (b) complex permeability of Fe/Epoxy and Fe/SiO2/Epoxy composites The volume concentration of magnetic filler is 26.3% Fig 3.6 Comparisons between calculated and experimental results of the effectiveelectromagnetic parametersfor... performance of Fe/Al alloys was improved by shaping the particles into flakes The resonance frequency of Fe/Al flakes shifts to higher band when the size of flakes decreases  Both Fe3O4 and Zn-ferrite nanoparticles have been synthesized by thermal decomposition method Our special design ofthe synthesis was using only one capping ligand, i.e oleic acid This design allowed us to easily control the particle... permeability, respectively Fig 4.12 The calculated theoretical reflection loss in the frequency range of 0.1 GHz to 18 GHz for the as-prepared Fe90Al10 flakes 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 min and 35 min are indicated Fig 5.2 The morphology observed in SEM images of reaction solution sampled at... further pulverized by jet milling; (c) and (d) jet milled products, which are collected from the bottom and top part of the receiver, respectively; (b) the EDS spectrum Fig 4.7 The SEM images of Fe90Al10: (a) ball milled particles, which are further pulverized by jet milling; (c) and (d) jet milled products, which are collected from the xiii bottom and top part of the receiver, respectively; (b) the. .. 7.5±0.6 nm and (F) 5.7±0.5 nm Fig 5.10 (A) X-ray diffraction patterns of the as-synthesized Fe3O4nanooctahedra with different average sizes, and (B) the change of the (311) peak intensity of each class of nanoparticles All samples were deposited on glass substrates from their hexane dispersions Fig 5.11 (A) Magnetization as a function of applied field for the powder samples of the as-synthesized Fe3O4nanooctahedra... diffraction patterns and (b) M-H loops of Fe-based solid solutions; the inset is for a clear observation of saturation magnetization Fig 4.5 The SEM images of Fe90Si10: (a) ball milled particles, which are further pulverized by jet milling; (c) and (d) jet milled products, which are collected from the bottom and top part of the receiver, respectively; (b) the EDS spectrum Fig 4.6 The SEM images of Fe90Co10:...Summary Magnetic materials have a wide range of uses in fundamental science and in technological applications The nanostructure impacts greatly on the properties of magnetic materials In this work, various magnetic nanostructures were achieved and their microwave absorbing performance was investigated The contributions of this work are summarized below:  Fe/SiO2 . SYNTHESIS OF VARIOUS MAGNETIC NANOSTRUCTURES AND THE MICROWAVE CHARACTERIZATIONS YANG YANG (B. Eng.), DJTU A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY. illustration of a VSM system. Fig. 2.6 The effects of (a) the isomer shift; (b) the quadrupole splitting and (c) the magnetic splitting on the nuclear energy levels of 57 Fe. The Mössbauer. electromagnetic parameters and frequency in the range of 0.1~18 GHz:(a) and (b) are the real and imaginary part of measured complex permittivity, respectively; (c) and (d) are the real and imaginary