Synthesis of various magnetic nanostructures and the microwave characterizations

SYNTHESIS OF VARIOUS MAGNETIC NANOSTRUCTURES AND THE MICROWAVE CHARACTERIZATIONS YANG YANG B.. Chye Pho Neo, Yang Yang, Jun Ding, Calculation of complex permeability of magnetic comp

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

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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

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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

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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

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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)

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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 1 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……… 6

1.2 Magnetic materials for microwave absorption……… 8

1.2.1 Metallic magnetic materials……… 8

1.2.2 Ferrites……… 9

1.2.2.1 Hexagonal ferrites……… 9

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

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1.5 References……….22

Chapter 2 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

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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 4 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 synthesis of octahedral Fe3O4nanoparticles and the microwave absorbing properties……….73

5.1 Introduction……… 73

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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 7 Synthesis of  -Fe2O3 templates via hydrothermal route

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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 8 Conclusions and future works……… 139

8.1 Conclusions……….…139

8.2 Future works……… 142

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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

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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

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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 57Fe 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)