Synthesis of various magnetic nanostructures and the microwave characterizations 4

We tried commercial iron particles with different sizes on the jet mill.. When iron particles with size larger than 20 μm were used, the morphology of iron particles changed a lot after

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Chapter 4 Microwave properties of micron and sub-micron Fe90Al10

flakes fabricated via ball milling and jet milling routes

4.1 Introduction

Ferromagnetic metal-based materials display high saturation magnetizations which make them of interest for microwave applications, namely higher working frequencies and a broader working frequency band than bulk ferromagnetic oxides.[1] However, the skin effect is always a problem of metal-based materials, such as Fe, especially in GHz applications Chapter 3 introduced a core/shell structure, of which an insulating SiO2 shell layer was coated on the surface of Fe particles, to reduce the skin effect An optional route to reduce the skin effect is to shape Fe particles into some special shapes to make sure that the particle size in a certain dimensional is comparative with the skin depth Therefore, the microwave absorption performance could be improved

In recent literatures, by virtue of the optimized particle shape and microwave permeability performance, ferromagnetic flakes are reported as promising candidates for electromagnetic wave absorption in GHz.[2-6] Theoretically, the metallic thin flakes could provide relative high permeability in gigahertz frequencies compared with the materials constrained by the traditional Snoek’s limit With taking the shape anisotropy into consideration, the Snoek’s law becomes the following equation[7]

(μi− 1)ƒr = 1

Hea)1/2 (Eq 1.11 in Chapter 1)

Where μi, ƒr, γ and Ms are the initial permeability, resonance frequency, gyromagnetic ratio and saturation magnetization of the material, Hea represents the in-plane anisotropy field, and Hha represents the out-of-plane anisotropy field It could be found that the product of μi and ƒr for flake-shaped particles can be much higher than that of isotropic particles because the out-of-plane anisotropy Hha of flake-like particles is much larger than the in-plane anisotropy Hea (Hha ≫ Hea), while γ and Ms remain unchanged compared to isotropic particles.[8] Therefore the resonance frequency of flake-like magnetic materials could exceed the traditional

Snoek’s law limitation ( (μi− 1)ƒr =2

3γ4πMs) and achieve a higher resonance frequency Moreover, the skin effect could be effectively suppressed by the flakes because the thickness of as-prepared flakes could be comparative with the skin depth

As a consequence, higher permeability can be obtained Through the studies on carbonyl iron particles with different morphology, Wen et al found that higher value

of magnetic permeability and permittivity could be obtained in the composites for thin flake carbonyl iron than spherical powders.[9] Zhou et al.[10] reported a comparison work on flake-like and spherical Fe3Co2 particles, and found that flake-like Fe3Co2

particles owned higher permeability value corresponding to a higher resonance frequency All of these works show the improvement of the flake-shape on the electromagnetic properties of particles

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As seen from the above works, most of them have adopted the mechanical milling method to fabricate the flake-like particles Mechanical milling turns to be an effective way to fabricate metallic alloys in a large quantity, and the morphology of as-milled metallic particles is much dependent on the milling time.[11-13] However, the product after mechanical milling is usually with a broad size distribution To make

an improvement on the morphology of as-milled products, jet milling was further used in the current study The principle of jet milling will be briefly introduced in the third part of this work According to our best knowledge, nanocrystalline alloy flakes have not been fabricated by using jet milling

The poor chemical stability of Fe particles also limits the applications when the particle is small In our work, FeM alloys were studied instead of pure Fe FeM flakes with micron- and submicron-sizes were prepared by two steps milling, i.e high energy ball milling and jet milling Among the FeM alloys, Fe90Al10 was mainly studied due to its relative high saturation magnetization and its relative good chemical stability The results show that when Fe90Al10 flake size reaches submicron-scale, the resonance frequency shifts to higher frequency band Through the calculation of the reflection loss by using electromagnetic wave transmission line theory, the as-prepared submicron-scale flakes are found to be very promising to make lightweight absorber with effective absorbing property at high frequency band

4.2 Experimental results

4.2.1 Effect of jet milling on the morphology of different materials

Fig 4.1 is the schematic diagram of the major working part of jet mill The milling process is started with the tangential feed of the powder particles from the feed funnel into the flat circular grinding chamber by the compressed feed air The powder particles are then accelerated in a spiral movement inside the grinding chamber The pulverization of raw material takes place due to the collision between particles and the

collision between particles and the wall of the chamber The larger particles of the product get retained at the periphery of the chamber by centrifugal force and the smaller particles exit from the central port of the chamber

After jet milling, the products could be collected from a receiver Based on the weight difference, the milled particles accumulate at the bottom or the top part of the receiver

We tried commercial iron particles with different sizes on the jet mill As seen from the SEM images in Fig 4.2, the initial iron particle sizes range from 1 μm to 5 μm

Feed funnel Compressed feed

Compressed grind

air

Grinding chamber

Fig 4.1 Schematic diagram of the major part of jet mill

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After jet milling, the particles changes little The products collected from the bottom part and the top part of the receiver showed few difference When iron particles with size larger than 20 μm were used, the morphology of iron particles changed a lot after jet milling process As seen from Fig 4.3b, all the particle sizes were reduced to less than 5 μm Fig 4.3c showed some more small pieces of particles Hence we can know that jet mill is more effective on the pulverization of large size particles (˃ 5 μm)

We further tried to some Fe-based alloys on the jet mill The alloys were iron solid solutions (Fe90M10; M=Si, Co and Al) prepared by high energy ball milling The

Fig 4.2 The SEM images of iron particles: (a) commercial iron particles with sizes ranging from 1 μm to 5μm; (b) and (c) are jet milled iron particles at 4 bars, which are collected from the bottom and top part of the receiver, respectively The scale bar for 5 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 4 bars, which are collected from the bottom and top part of the receiver, respectively

formation of solid solutions was proved through XRD, VSM and EDS measurement While the XRD patterns in Fig 4.4a demonstrated that there was not any peak brought by the solute elements but only BCC Fe phase (JCPDS no 87-0722) was observed in these solid solutions The existence of elements Si, Co and Al in different solid solutions was confirmed by the EDS spectra, as shown in Fig 4.5 to Fig 4.7 As revealed by the magnetic hysteresis loops (Fig 4.4b and its inset), the saturation of Fe

is around 220 emu/g The addition of the solute into iron would reduce its magnetism

in some extent The Fe90Co10 solid solution still processes a very high saturation

magnetization of 212 emu/g, while the saturation magnetization values of Fe90Si10 and

Fe90Co10 are reduced to 185 emu/g and 184 emu/g Before jet milling, all of the solid solutions showed irregular shapes and nonuniform sizes The SEM images in Fig 4.5

to Fig 4.7 also show the morphology modification of three kinds of Fe-based alloys after jet milling In our studies, we fixed the parameters during jet milling process, such as the feed speed of raw materials and the grind air pressure Hence the differences in the fineness of the milling products are probably due to the different

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

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Fig 4.6 The SEM images of Fe 90 Co 10 : (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 Fe 90 Al 10 : (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 4.5 The SEM images of Fe 90 Si 10 : (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.

mechanical properties of the feed material, such as the hardness As reported,[14] materials which have relatively low hardness show the higher rate of breakage in particles Jet milling is possible to reduce the size of ball milled products to less than

5 μm for all the alloys under study If particles in smaller sizes are desired, we should collect the light-weight product on the top layer of the receiver These products are much less than those products accumulated at the bottom part of the receiver As we can see from Fig 4.6d and Fig 4.7d, the Fe/Co flakes and Fe/Al flakes in submicron scale could be produced by jet milling This finding aroused us great interesting in the fabrication of Fe-based alloy flakes with different sizes The chemical stability is always a problem for alloys Thermogravimetric analysis (TGA) was used to check the stability of the produced Fe solid solutions at elevated temperature The TGA

measurement was performed with the temperatures ranging from room temperature to

Fig 4.8 TGA plots and the derivative curves for (a) commercial iron with particles size ranging from 1 μm to 5 μm; (b) jet milled Fe 90 Co 10 flakes and (c) jet milled

Fe 90 Al 10 flakes

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1000 ℃ Although nitrogen gas with purity of 99.9% was used as the protection gas, when the temperature rose to 350 ℃, the weight of commercial Fe (size: 1 μm to 5 μm) began to increase rapidly, as displayed by the relationship of the weight loss against the temperature in Fig 4.8a The only reason could be the very tiny amount of oxygen impurity in nitrogen gas At elevated temperatures, Fe is found to be much more sensitive to the atmosphere.[15] In this work, the temperature at which the weight of the sample starts to increase is used to evaluate the stability The higher the temperature, the better the stability For jet milled samples, the stability of Fe90Al10

and Fe90Co10 submicron flakes were studied The weight increase in percentage against temperature was plotted and shown in Fig 4.8b&c The derivative of mass was also plotted to show the mass change rate Fe(Al) flakes showed a better stability than the commercial iron particles While Fe(Co) flakes showed the worst stability Therefore, Fe90Al10 was selected for further investigations on the size-controllable synthesis and the microwave absorption performance

4.2.2 Fabrication and characterizations of micron and submicron Fe/Al flakes

Sample S0 was obtained after mechanical milling of the mixture of Fe and Al metallic powders for 12 h using a Spex mixer (high energy ball mill machine) From the SEM micrographs (Fig 4.9a), the particles of sample S0 have irregular and isotropic shapes with a broad size distribution from 1 m to 10 m After the first step of dry-milling process, the powder (Sample S0) was subsequently mixed with anhydrous ethanol and

milled for 0.5 h and 5 h, respectively The process is so called wet-milling The corresponding products after wet-milling were named as Sample S1 (milled for 0.5 h) and Sample S2 (milled for 5 h) As shown in Figs 3.9c and d, the particles after wet milling process are flakes with average lateral size around 50 m for S1 and 100 m for S2, respectively The thicknesses for the two samples are similar, about 0.5 m Furthermore, the jet milling was employed to reduce the particle size For jet mill, the pressure of the compressed grind air is a vital parameter which could affect the size and shape of resultant products.[16] In this work, the pressure, depending on the attached air compressor system, was set at 4 bar After the jet-milling, two sorts of particles could be obtained, as they were separated based on different masses (different particle sizes) The final product of small particles (Sample S3) showed a

Fig 4.9 SEM images of as-prepared Fe 90 Al 10 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

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flake-like structure in submicron-scale with average lateral size less than 500 nm and

a thickness of 50 nm, as shown in Fig 4.9e The bigger particles were named as Sample S4 Compared with Sample S0, Sample S4 (shown in Fig 4.9b) had quasi-spherical shape with particle sizes ranking in the range of 1 m to 5 m A typical EDS spectrum shown in Fig 4.9f illustrates that the atomic ratio of Fe to Al for as-prepared flakes is almost 9:1 for all samples obtained, and the composition changes little with different sizes of the flakes

As it is well known, fracture and welding are the two major processes during high-energy ball-milling If anhydrous ethanol is added, the milling energy of the wet-milling is strongly reduced If we use the isotropic particles as the starting material, the initial isotropic particles (with a particle size ranking form 1 to 10 m) were flattened during the first wet-milling After 0.5 h, flakes with a lateral size of 50

m and a thickness of 50 nm were formed A prolonged wet-milling, the flakes were welded together and form larger flakes with a lateral size of 100m, while the thickness was kept unchanged (50 nm)

To investigate the phase of the as-prepared samples after milling process, XRD patterns were taken (as shown in Fig 4.10a), only the peaks for bcc-Fe could be observed for all the four samples By using the Scherrer equation [Eq (2.2)], the estimated value of grain size of as-milled particles is about 6 nm, which indicates the formation of Fe(Al) solid solution phase with a nanocrystalline structure after milling