Synthesis of various magnetic nanostructures and the microwave characterizations 1

Chapter Introduction Chapter Introduction This work is developed based on the synthesis of various magnetic nanostructures for the application in microwave absorption. This chapter gives an overview on the fundamental knowledge on the microwave absorption, the development on magnetic materials for microwave application and large scale synthesis technologies for the magnetic materials. After introducing the background information, the motivations of my research work are listed. 1.1 Fundamentals for microwave absorption Microwave is the electromagnetic waves with wavelengths ranging from cm to m. The corresponding frequency range is between 300MHz (0.3GHz) to 300GHz.[1] Microwave technology has been mainly the exclusive domain of the defense industry. With the rapid development of communication systems for applications, such as cellular mobile telephony, broadband wireless access, wireless local area networks and satellite base cellular communications, microwave technology is closely bound up with our daily life because these communication systems are employed everywhere, including corporate offices, private homes and public recreation places. Although we have benefited from these modern communication systems, the microwave radiation emitted by these systems have severe consequences on our body.[2] Another problem caused by the unprecedented growth of communication systems is the severe Chapter Introduction electromagnetic interference among the electronic devices.[3] Therefore, microwave absorbing materials are required to alleviate these problems by absorbing the unwanted microwave. 1.1.1 Description of microwave absorption ability A schematic diagram for the measurement of microwave absorption ability is shown in Fig. 1.1. Ideally, the perpendicular incident microwave is converted into two parts: Incident wave Reflected wave Metal Absorber Fig. 1.1 A Schematic diagram for the definition of microwave absorption ability. the reflected and the absorbed microwaves. If Pin is incident power density at a measuring point, and Pr is reflected power density at the same measuring point, Pab is absorbed power by the composite. Their relation can be expressed as Pin = Pr + Pab P R = 10 log (P r ) in P A = 10 log ( Pab ) in (1.1) (1.2) (1.3) where R and A are reflection loss (RL) and absorption loss in decibels (dB), Chapter Introduction respectively. Thus, the microwave efficiency can be evaluated from R using Eq. (1.3). Larger absolute value of R means more effective wave-absorbing ability of the absorbers.[4] 1.1.2 Calculation of microwave absorption ability The specular method was commonly used as a theoretical approach in explaining the propagation characteristics of a transverse electromagnetic (TM) wave in a single-layer absorber backed by a perfect conductor. For incident wave perpendicular to the surface of a single-layer absorber backed by a perfect conductor, the input impedance (Zin) at the air-material interface is given by: [5,6] Zin = Z0 (μ∗r /ϵ∗r )2 tanh(γ ∙ t) where Z0 = √(μ0 /ϵ0 ) = 377Ω (1.4) is intrinsic impedance of free space, γ = [jω(μ∗r ε∗r )2 ] /c is propagation factor in the material, ω is angular frequency, c is the speed of light and t is thickness of the sample. The complex permittivity εr∗ and magnetic permeability μ∗r can be measured experimentally. They could be given as ε∗r = εr ε0 = (ε′r − jε′′ r ) ∙ ε0 (1.5) μ∗r = μr μ0 = (μ′r − jμ′′ r ) ∙ μ0 (1.6) The reflection coefficient (Г) is defined as Г = (Zin − Z0 )/(Zin + Z0 ) (1.7) Substitute Eq. (1.4) into Eq. (1.7), and then the value of Гcan be obtained. Finally, R L Chapter Introduction in decibels (dB) can be written as R L = 20lg|Г| (1.8) When the reflection coefficient (Г) reaches its minimal value zero, which means Zin = Z0 , so called impedance match, the lowest reflection loss can be obtained. Usually, the microwave absorbing characteristics is evaluated by the resonance frequency ƒr and the reflection loss R L as well as the thickness t of the absorber. Effective microwave absorption refers to a low R L value at a high ƒr position, and a small t for the design of lightweight absorber. The initial expression of Zin is defined as Zin = √ μ∗r ε∗r =√ μ r μ0 εr ε0 = Z0 = √ μ0 ε0 (1.9) Hence, materials with the property of μr = εr are ideal for microwave absorption. Unfortunately, for the existing materials, the relative permeability generally does not approach the magnitude of the relative permittivity at microwave frequency band. The difference between these two parameters can be reduced when the permeability of the magnetic materials is high. 1.1.3 Snoek’s law For soft magnetic bulk materials, the product of the intrinsic permeability μi and the resonance frequency ƒr is constrained by a constant related to the saturation Chapter Introduction magnetization 4πMs , as shown by the Snoek’s law[7]: (μi − 1)ƒr = γ4πMs (1.10) where ƒr is resonance frequency and γ ≈ 3MHz/Oe gyromagnetic factor. The expression in Eq. (1.10) is valid for the polycrystalline bulk materials with uniaxial or cubic magnetocrystalline anisotropy field. It shows that there exist trade-offs between high permeability levels and operation at high frequency band. Hence an effective way to attain is using magnetic materials with high Ms can be used to obtain high permeability μi at microwave frequency band. There is another expression of Snoek’s law for planar materials, which is given as[8] 1/2 H (μi − 1)fr = γ4πMs ( ) H ea (1.11) where 𝐻ℎ𝑎 and 𝐻𝑒𝑎 are the out-of-plane and in-plane anisotropy fields, respectively. Other parameters in Eq. (1.11) have the same meaning as those in Eq. (1.10). When compared these two equations, the difference lies in the right side could be found. A smaller 𝐻𝑒𝑎 with larger 𝐻ℎ𝑎 is good combination for the materials with higher permeability and higher working frequency, which can be induced by an artificial or anintrinsic bianisotropy system.[9] Hence, the shape construction of magnetic particles is another effective way to attain high permeability μi at elevated frequency range. In a way, the Snoek’ law could be seen as a criterion for magnetic materials used as microwave absorber. The calculated value from the right side of equations for the Chapter Introduction Snoek’s law is used to evaluate whether a magnetic material could be an effective microwave absorber. Researchers usually seek for a large calculated value when design an absorber. For this part, we could know that both of the enhancement in the saturation magnetization and the induce shape anisotropic field into the particles can extend the Snoek’s limitation. 1.1.4 Skin effect In the view of the Snoek’s law, high saturation magnetization is required to obtain effective microwave absorption. As such, metallic magnetic materials are commonly employed as microwave absorbing materials. While for metallic materials, skin effect is nontrivial due to their relative low resistivity. When a microwave wave penetrates into a conductive material, mobile charges on the surface are made to oscillate back and forth in the same frequency as the impinging fields. An alternating electric current will be brought by the movement of these charges. The current density is greatest at Field Strength X Z d Fig. 1.2 A schematic diagram of the skin effect. Chapter Introduction the conductor’s surface. The decrease in current density with depth is so called skin effect,[10] as displayed by the scheme in Fig. 1.2. When the depth is d, the corresponding electric field reduces from initial = ⁄ to E, as below: (1.12) The skin depth δ is the distance over which the current falls to 1/e of its original value, which is dependent on the electrical conductivity of the materials as well as the incident magnetic field. The relationship is described as δ=√ 2ρ (1.13) ωμ where ω is the angular frequency of current, ω = 2π × frequency , μ is the absolute magnetic permeability, μ = μ0 ∙ μr . Hence it can be written as δ=√ 2ρ 2πƒμ0 μr = √πμ0 ∙√ ρ μr ƒ (1.14) Where ƒ is frequency of the wave in Hz, ρ is resistivity of the medium in Ω·m, μ0 = 4π × 10 H/m, μr is relative permeability of the material. The skin effect can dissipate the incident microwave by inducing the eddy current on the surface of the material but hinder the microwave to penetrate into the inner part of the materials. Thus, the particle size should not significantly exceed the skin depth for the sake of high microwave absorbing efficiency. For example, the resistivity of iron Chapter Introduction is ρ = × 10 Ω·m, when the frequency is ƒ = GHz, its relative permeability is around 10.[11] As a result, the skin depth is around μm. Wu et al.[12] have proven that the 1μm carbonyl iron particles show better microwave absorbing performance than 10 μm carbonyl iron particles through experimental results and theoretical calculations. The difference is well explained by size-dependent skin effect using the calculation model derived from Landau-Lifshitz-Gilbert (LLG) equation. It is necessary to reduce the skin effect of metal-based materials to improve their microwave absorbing ability. 1.2 Magnetic materials for microwave absorption Based on the Snoek’s law and skin effect, the option of magnetic materials used for microwave absorbers should have high saturation magnetization, high anisotropy field and high resistivity. 1.2.1 Metallic magnetic materials Metallic materials (Fe, Co, Ni and their alloys) are commonly used for microwave absorbers due to their high saturation magnetizations.[13-15] The problem of the metallic materials is the skin effect due to the high conductivity, resulting in a very small skin depth. Hence the metallic particles used for magnetic filler of microwave absorbers should be pulverized into small particles with a size comparable to the skin depth. Non-metal elements, such as boron and silicone,[16-18] are doped into these Chapter Introduction alloys to enhance the resistivity. Besides, the skin effect could be reduced by structured metallic magnetic particles, such as the Fe nanowires, Co and Fe/Co nanoplatelets, FeSiB flakes and Ni fibers, as well as hollow structures.[19-23] For these structured particles, either the radius size or the thickness of magnetic particles is less than or comparable with the skin depth. Hence the skin effect could be effectively suppressed. 1.2.2 Ferrites Ferrites are usually non-conductive ferromagnetic ceramic compounds derived from iron oxides such as hematite (α-Fe2O3) or magnetite (Fe3O4). In the big family of ferrites, two groups of ferrites, hexagonal ferrites and spinel ferrites, are commonly used for microwave absorbers. They are named according to their crystal structures. 1.2.2.1 Hexagonal ferrites There are several types of hexagonal ferrites named as M, W, Y and Z phases.[24-26] The hexagonal ferrites have attracted intensively attentions due to their very strong magnetic anisotropy. At room temperature, Co2Z barium ferrite (Ba3Co2Fe24O41) shows a c-plane anisotropy with a large out-of-plane anisotropy field of 12 kOe and a small in-plane anisotropy field of about 0.120 kOe.[27] M-type barium ferrite (BaFe12O19) exhibits ferromagnetic resonance around 50 GHz due to its very high magnetocrystalline anisotropy induced by the anisotropic structure.[28] The disadvantage of hexagonal ferrites is the relative low saturation magnetization when Chapter Introduction compared with metallic magnetic materials or spinel ferrites, resulting in an insufficient high permeability. To overcome this problem, some divalent metal cations (Zn2+, Co2+, Ni2+ etc.) and trivalent cations (Al3+, Cr3+ etc.) are doped to substitute part of Ba2+ or Fe3+ cations.[29-31] The function of the doped element is to change the quantity of spin down or spin up moments, resulting in an enhancement of the saturation magnetization.[32] In some cases, a metallic magnetic layer is coated on the surface of hexagonal ferrites nanoparticles to increase the magnetization.[33] 1.2.2.2 Spinel ferrites The crystal structure of spinel ferrite is much simpler than that of hexagonal ferrites. Normal spinel structures are usually cubic closed-packed oxides with one octahedral and two tetrahedral sites per oxide. The tetrahedral points are smaller than the 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). 10 Chapter Introduction octahedral points. B3+ ions occupy the octahedral holes because of a charge factor, but can only occupy half of the octahedral holes. A2+ ions occupy 1/8th of the tetrahedral holes. Fig. 1.3 shows the schematic illustration of spinel structure with A2+ at tetrahedral sites (bubbles in the green tetrahedron) and B3+ at octahedral sites (yellow bubbles). Inverse spinel structures however are slightly different in that one must take into account the crystal field stabilization energies (CFSE) of the transition metals present. Some ions may have a distinct preference on the octahedral site which is dependent on the d-electron count. If the A2+ ions have a strong preference for the octahedral site, they will force their way into it and displace half of the B3+ ions from the octahedral sites. If the B3+ ions have a low or zero octahedral site stability energy (OSSE), then they have no preference and will adopt the tetrahedral site. A typical example of an inverse spine structure is Fe3O4. In which, Fe2+ ions and half of the Fe3+ ions occupy octahedral sites, while the other half of the Fe3+ ions occupy tetrahedral sites. So the electron transfer between Fe3+ and Fe2+ gives rise to ion jumps and relaxation in the Fe3O4 particles and contributes a particular dielectric loss. It is well known that, MnZn-ferrite (MnaZn1-aFe2O4) and NiZn-ferrite (NiaZn1-aFe2O4) are two typical spinel ferrites suitable for microwave application due to their high permeability. Many researches have doped some magnetic or nonmagnetic elements,[34-37] such as Cr, Cu, Co and rare earth elements into the MnZn- and 11 Chapter Introduction NiZn-ferrites to enhance the resistivity and further improve the microwave absorbing property. As seen from the overviews, no obvious improvement has been made on increasing the saturation magnetization. The Snoek’s law limitation still exists even for the doped spinel ferrites, resulting in the resonance frequency positioning at megahertz. Inspired by hexagonal ferrites, we have known that the Snoek’s law could be extended by magnetically anisotropic materials. For spinel ferrites, the magnetic anisotropy can be induced by shaping the materials into anisotropic structures. Fe3O4 is selected for our works due to its relatively high saturation magnetization (~ 90 emu/g), as well as the well-developed synthesis technologies for Fe3O4. 1.3 Brief review of size-controlled synthesis technology In the design of microwave absorber, magnetic materials have to be used as fine particles dispersed in an insulating matrix. Besides the intrinsic properties of magnetic materials, the microstructure and morphology are also important to the microwave absorption performance of the composite. The uniformity of magnetic particles in size and shape is very necessary to form homogeneous composite. For materials with morphological features on the nanoscale, size-dependent properties become more important. For magnetic materials, the particle size is of great importance to the physical properties, such as magnetization, coercivity, domain structure and some critical temperature points (the Curie temperature, the Néel temperature and the blocking temperature). The magnetic recording density is remarkably enhanced while 12 Chapter Introduction the saturation magnetization is reduced as the particle size decreases.[38] The superparamagnetism of small ferro- or ferri-magnetic nanoparticles opens up the road to biological applications, such as magnetic resonance imaging, magnetic field guided drug delivery carriers, bioseparation and hyperthermia agent in cancer treatment.[39] referring to bioapplications, the dispersiblity and the toxicity of nanoparticles are also relevant to the particle size.[40,41] Hence the control over the particles size is significant to various applications. The synthetic route is a key factor that determined the particle sizes and the development on the synthetic routes regarding to size control is briefly introduced as following: 1.3.1 Ceramic sintering method Traditional ceramic sintering is a kind of solid state reaction method for preparation of polycrystalline solids using a mixture of solid starting materials. The solid materials not react with each other at room temperature and high temperatures (such as 1000℃ [42]) are required to form a desired phase. This method has been used to prepare various solid oxides including transitional metal oxides [43, 44] and ferrites.[45,46] The morphology of resultant product is dependent on several factors, such as the reaction temperature, the heating rate, the surface area of the solids materials and their reactivity and thermodynamic free energy change along with the reaction, and so on. Experimentally, the temperature and heating rate are more controllable factors compared with others. The simultaneous control over the particle size, the 13 Chapter Introduction composition as well as the microstructure of resultant powders are realized by using different reaction temperatures.[47-49] In the work done by Gharagozlou,[50] the size of Co-ferrite particles showed an increase trend with calcination temperatures. However, the agglomeration led by calcination is unavoidable in the solid reaction process, which makes it difficult to distinguish each particle. 1.3.2 Ball milling method Ball milling is also a kind of solid state reaction which happens in a sealed vial. This technique is well established for the preparation of non-equilibrium phases, alloys with extended solubility limits and nanocrystalline materials based on a mechanical alloying process.[51] The starting materials as well as a number of balls are transferred into the vial before the operation, and then ground at a provided speed. The resulting particles are products of the repeated and cold welding caused by the ball impacts.[52] The controllable parameters involved in the reaction process are the operation speed, the ball-to-powder ratio and the grinding period, as investigated by Serra et al. on the ball-milled Fe77Nb7B15Cu1.[53] The product of ball milling is usually irregular powders in micron scale, and prolonged milling leads to narrower distributed powders due to the balance is reached between the fracture and welding process.[54] Gheisari et al.[55] reveal that the higher grinding speed is helpful to reach such balance, but resulting in relative larger particle size. In general, the morphology is not controllable because it varies easily with the materials and the 14 Chapter Introduction experimental conditions. To make an improvement on the ball milling method, medium-assistant milling process (so called wet-milling, relative to dry-milling without using medium) has been introduced. The medium could be organic chemicals, such as ethyl alcohol, methyl alcohol, heptane and stearic acid and so on.[56-59] The medium is added into the vial together with the starting powders. Unlike the irregular particles obtained after dry milling process, it is easy to obtain micron-size flaky particles by wet-milling method. It has been intensively used to produce iron-based flaky alloys, such as FeAl, FeSiB, FeCo, FeNdB and FeNi.[60-64] So far, the milled flaky products are all in micron scale and with a broad size distribution. 1.3.3 Wet Chemical methods Wet chemical methods are commonly used to synthesize a wide variety of nanomaterials. There are several solution strategies to obtain size-controllable nanoparticles. (1) One is the template-direct synthesis, in which various kinds of templates are used to guide the formation of nanocrystals with specific shape and size. Ion-track etched membranes and alumina membranes are developed for nanowires, nanotubes and nanorods.[65,66] These templates are commercially available with various pore sizes which afford direct control over the diameter of one dimensional products, while the length increased with the processing time. Nucleotides/nucleic acids are attractive biomineralization templates for the growth of inorganic nanoparticles.[67] The ability 15 Chapter Introduction to tune the size of nanoparticles depends on the control over the nucleic acid structure, composition and sequence.[68] the After template-directed synthesis procedure, the after-treatment process is needed to remove the template, such as calcination or solvent extraction.[69] Therefore, in most cases, template-free methods are preferred due to their simpler operation process. (2) A second one is the seed-mediate growth. The strategy based on the temporal separation of nucleation and growth processes is considered to be very efficient to control the nanoparticles size and shape precisely. Bastús et al.[70] succeeded to synthesize uniform quasi-spherical gold nanoparticles up to ~200 nm by using a kinetically controlled seeded growth strategy via the reduction of gold precursor by sodium citrate. Jana et al.[71] used nm silver nanoparticles as seeds for the preparation of 42 nm silver rods and μm silver wires. Sun et al.[72] applied this method to the synthesis of Fe3O4 nanoparticles. nm Fe3O4 nanoparticles were prepared in advance and used as seeds to prepare larger particles in a following step. The resultant particles size could be precisely tuned by adjusting the quantity of used seeds. Li et al.[73] also used Fe3O4 seeds to prepare size-controlled Fe3O4/Ag heterodimer nanocrystals. As seen from the previous works,[74-76] the seed-mediate growth method was intensively applied to the synthesis of ferrite oxides magnetic nanoparticles with size less than 20 nm. This method usually leads to very narrow size distribution without any size-selection procedure. 16 Chapter Introduction (3) A third strategy is the so called one-step synthesis route, which means that the nucleation and growth processes take place in a continuous process. Typical methods based on this strategy like coprecipation, hydrothermal and thermal decomposition have been investigated for a long period. Coprecipitation process involves two stages:[77] a short burst of nucleation when the concentration of the species reaches critical supersaturation, and a slow growth of the nuclei by diffusion of the solutes to the surface of the crystal. The size and shape of the coprecipitated nanoparticles can be tailored by pH adjustment, ionic strength, nature of the salts (chlorides, sulfates or nitrates). For the coprecipitation of Fe3O4 nanoparticles, the Fe(Ⅱ)/Fe(Ⅲ) concentration ratio is a relative important factor which affects the particle size.[78] Until now, the coprecipitation method is not suitable for precise control over the particle size. A size sorting process is needed to narrow down the size distribution. The poor crystallinity is another problem,[79] especially for magnetic particles, because it reduces the magnetism in some extent. This problem is usually overcome by a further calcination process. Like coprecipitation process, hydrothermal treatment is also performed in aqueous media but under high pressure and high temperature. It is commonly used to prepare metal oxides by employing different precursors.[80,81] During the hydrothermal synthesis, the grain growth mainly origins from a faster deposition rate than the dissolution rate of the precursor on the crystal surface. The rates are sensitive to the 17 Chapter Introduction reaction temperature, the pH value of the solution. For a better control over the crystal growth, surfactants such as poly(vinylpyrrolidone),[82] cetyltrimethyam-monium bromide [83] and polyisobutylene bis-succinimide [84] are used. Even though, it is difficult to control the particle size of magnetic materials synthesized by hydrothermal route.[85,86] This may be induced by the aggregation of magnetic particles. While the size control over nonmagnetic nanoparticles, such as hematite,[87,88] is much easier. Thermal decomposition method refers to the decomposition of organometallic precursors in high boiling point organic solvent. It is a very promising way to synthesize the monodisperse semiconductor,[89] bimetallic[90] and oxides[91] nanocrystals with uniform and controllable size and shape. The study on the reaction kinetics by Kwon et al.,[92] the formation of high-quality nanoparticles attributes to the burst of nucleation followed by high growth rate. Two-dimensional and three-dimensional assemblies[93,94] have proven the uniformity of nanoparticles synthesized by thermal decomposition method. During the synthesis process, the concentration of surfactant as well the molar ratio of surfactant to precursors is found important to the size and shape of final products.[95, 96] The choice of surfactant, therefore, is a key factor. For the synthesis of Fe3O4 nanoparticles, the commonly used surfactants are oleic acid (OA), oleylamine (OL), 1,2-hexadecanediol and oleates.[97,98] Besides, the reaction temperature profile should be properly set for the synthesis of nanoparticles with desired shape and size.[99] So far, this method has 18 Chapter Introduction been mainly explored to control the size of magnetite nanoparticles within the range of 4-20 nm, due to the superparamagnetic limit for magnetite is around 20 nm.[100] It is expected to exploit the method for the synthesis of larger size nanocrystals (above 100 nm). 1.4 Motivations and objectives Based on the above introduction, it is necessary to improve the electric and magnetic properties by structuring the magnetic materials for microwave applications. In this work, we focus on the synthesis of various magnetic nanostructures, including core/shell structure, flaky structure, tube- and rod-like structures, as well as nanocrystals with large sizes. Although these magnetic nanostructures have morphological features on nanoscale, they actually possess as high magnetizations as bulk materials. This superiority renders them suitable for the application in microwave absorbers. The effect of various magnetic nanostructures on the microwave absorption ability is investigated in this work. To achieve various magnetic nanostructures, some novel methods are exploited in this work. The high energy ball milling is a traditional way to prepare alloy materials; however, it is the first time that jet milling is employed to narrow down the broad size distribution of ball-milling product. Up to now, most of the chemical syntheses of magnetic particles have focused on small particles (several ten nanometers or smaller). The synthesis of uniform large size magnetic nanocrystals with large size (above 100 nm) is still a 19 Chapter Introduction challenge. The chemical synthesis of Fe3O4 with controllable size has been developed not only for its multifold applications, but also for its similarity with other spinel ferrites. In other words, the successful control over Fe3O4 particles may open up a way to the synthesis of other kind of spinel ferrite. The thermal decomposition method shows its advantages in size control of Fe3O4 nanoparticles with cubic, octahedral, polyhedral or spherical shapes, but it is unsuited to synthesize the magnetic particles with anisotropic shapes. So far it is still a challenge to control the size of magnetic particles with some anisotropic structures like disk, tube as well as rod and so on. Unlike Fe3O4, great progress has been made on the size controllable synthesis of α-Fe2O3 nanoparticles with various novel shapes. Hydrothermal method has been used to synthesize α-Fe2O3 nanoparticles with controllable sizes and shapes. Fe3O4 phase could be further obtained by annealing α-Fe2O3 in reductive atmosphere. However, the annealing conditions are critical for the formation of Fe3O4. Precisely control on the temperature is required to avoid the impurity of FeO phase or iron phase during the annealing process. Hence more suitable methods should be developed to convert α-Fe2O3 to Fe3O4, while preserving the initial morphology. The major objectives of this work are to employ different structures to overcome the limitations of Fe-based alloy and spinel ferrites when used as microwave absorbing materials. The major objectives are shown as following: 20 Chapter Introduction 1) Fe/SiO2 particles with core/shell structure were synthesized. The insulating SiO2 shell layer was used to reduce the skin effect of Fe particles. 2) Fe/Al flakes in micron and submicron scale were fabricated. The flake-like structures were used to reduce the skin effect and to extend the Snoek’s limitation of Fe-based alloys. 3) Fe3O4 nanoparticles with different structures were synthesized. 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[10 0] It is expected to exploit the method for the synthesis of larger size nanocrystals (above 10 0 nm) 1. 4 Motivations and. .. 27, 11 098 -11 105 (2 011 ) s, [ 71] N R Jana, L Gearheart, C J Murphy, Chem Commun., 617 - 618 (20 01) [72] S H Sun, H Zeng, J Am Chem Soc., 12 4, 8204-8205 (2002) [73] X M Li, H L Si, J Z Niu, H B Shen, C H Zhou, H Yuan, H Z Wang, L Ma, L S Li, Dalton Trans., 39, 10 984 -10 989 (2 010 ) [74] J Mohapatra, A Mitra, D Bahadur, M Aslam, CrystEngComm, 15 , 524-532 (2 013 ) [75] Y W Oh, J P Liu, J Magn., 11 (3), 12 3 -12 5... strategy like coprecipation, hydrothermal and thermal decomposition have been investigated for a long period Coprecipitation process involves two stages:[77] a short burst of nucleation when the concentration of the species reaches critical supersaturation, and a slow growth of the nuclei by diffusion of the solutes to the surface of the crystal The size and shape of the coprecipitated nanoparticles... is the first time that jet milling is employed to narrow down the broad size distribution of ball-milling product Up to now, most of the chemical syntheses of magnetic particles have focused on small particles (several ten nanometers or smaller) The synthesis of uniform large size magnetic nanocrystals with large size (above 10 0 nm) is still a 19 Chapter 1 Introduction challenge The chemical synthesis. .. process, the concentration of surfactant as well the molar ratio of surfactant to precursors is found important to the size and shape of final products.[95, 96] The choice of surfactant, therefore, is a key factor For the synthesis of Fe3O4 nanoparticles, the commonly used surfactants are oleic acid (OA), oleylamine (OL), 1, 2-hexadecanediol and oleates.[97,98] Besides, the reaction temperature profile... Lisjak, M Drofenik, Cryst Growth Des., 12 , 517 4- 517 9 (2 012 ) [87] S Yang, Y Y Xu, Y Q Sun, G Y Zhang, D Z Gao, CrystEngComm, 14 , 7 915 -79 21 (2 012 ) 24 Chapter 1 Introduction [88] L J Wan, S C Yan, X Y Wang, Z S Li, Z G Zou, CrystEngComm, 13 , 2727-2733 (2 011 ) [89] H B Li, D Chen, L L Li, F Q Tang, L Zhang, J Ren, CrystEngComm, 12 , 11 27 -11 33 (2 010 ) [90] S H Sun, S Anders, T Thomson, J E E Baglin, M F Toney,... objectives Based on the above introduction, it is necessary to improve the electric and magnetic properties by structuring the magnetic materials for microwave applications In this work, we focus on the synthesis of various magnetic nanostructures, including core/shell structure, flaky structure, tube- and rod-like structures, as well as nanocrystals with large sizes Although these magnetic nanostructures. .. bimetallic[90] and oxides[ 91] nanocrystals with uniform and controllable size and shape The study on the reaction kinetics by Kwon et al.,[92] the formation of high-quality nanoparticles attributes to the burst of nucleation followed by high growth rate Two-dimensional and three-dimensional assemblies[93,94] have proven the uniformity of nanoparticles synthesized by thermal decomposition method During the synthesis. .. mechanical alloying process.[ 51] The starting materials as well as a number of balls are transferred into the vial before the operation, and then ground at a provided speed The resulting particles are products of the repeated and cold welding caused by the ball impacts.[52] The controllable parameters involved in the reaction process are the operation speed, the ball-to-powder ratio and the grinding period,... Sci., 253, 2 611 -2 617 (2006) [77] A P Jadhav, C W Kim, H G Cha, A U Pawar, N A Jadhav, U Pal, Y S Kang, J Phys Chem C, 11 3, 13 600 -13 604 (2009) [78] R R Qiao, C H Yang, M Y Gao, J Mater Chem., 19 , 6274-6293 (2009) [79] J Lodhia, G Mandarano, N J Ferris, P Eu, S F Cowell, Biomed Imaging Interv J., 6(2), e12 (2 010 ) [80] F Zhou, X M Zhao, H Xu, C G Yuan, J Phys Chem C, 11 1, 16 51- 1657 (2007) [ 81] Y Li, H Liao, . band. The difference between these two parameters can be reduced when the permeability of the magnetic materials is high. 1. 1.3 Snoek’s law For soft magnetic bulk materials, the product of the. the unwanted microwave. 1. 1 .1 Description of microwave absorption ability A schematic diagram for the measurement of microwave absorption ability is shown in Fig. 1. 1. Ideally, the perpendicular. Chapter 1 Introduction 1 Chapter 1 Introduction This work is developed based on the synthesis of various magnetic nanostructures for the application in microwave absorption.