MINISTRY OF EDUCATION
VIET NAM ACADEMY OF
AND TRAINING
SCIENCE AND TECHNOLOGY
GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY
……………..*****…………….
CHU THI ANH XUAN
SYNTHESIS AND STUDY OF MICROWAVE ABSORPTION OF La1.5Sr0.5NiO4 DIELECTRIC/FERRO- FERRIMAGNETIC NANOCOMPOSITE
Specialized: Electronic materials Numerical code: 9.44.01.23
SUMMARY OF DOCTORAL IN MATERIALS SCIENCE
Ha noi, 2018
The work is completed at: INSTITUTE OF MATERIALS SCIENCE - VIET NAM ACADEMY OF SCIENCE AND TECHNOLOGY
Science supervisor:
1. Dr. Dao Nguyen Hoai Nam 2. Prof. Nguyen Xuan Phuc
PhD dissertation reviewer 1:
PhD dissertation reviewer 2:
PhD dissertation reviewer 3:
The thesis will be protected under supervisory board academy level at: Academy at ….. hours….. day …..month ….. 2018 People can find this thesis at: - National library
- Graduate university of Science and Technology library
LIST OF PROJECTS PUBLISHED
Articles in the ISI directory:
1. 1. P.T. Tho, C.T.A. Xuan, D.M. Quang, T.N. Bach, T.D. Thanh, N.T.H. Le, D.H. Manh, N.X. Phuc, D.N.H. Nam, “Microwave absorption properties of dielectric La1.5Sr0.5NiO4 ultrafine particles”, Materials Science and Engineering B, 186 (2014), pp. 101-105.
2. 2. Chu T. A. Xuan, Pham T. Tho, Doan M. Quang, Ta N. Bach, Tran D. Thanh, Ngo T. H. Le, Do H. Manh, Nguyen X. Phuc, and Dao N. H. Nam, “Microwave Absorption in La1.5Sr0.5NiO4/CoFe2O4 Nanocomposites”, IEEE Transactions on Magnetics, Vol. 50, No 6 (2014), pp. 2502804. 3. 3. Xuan T. A. Chu, Bach N. Ta, Le T. H. Ngo, Manh H. Do, Phuc X. Nguyen, and Dao N. H. Nam, “Microwave Absorption Properties of Iron Nanoparticles Prepared by Ball-Milling”, Journal of Electronic Materials, Vol. 45, No. 5 (2016), pp. 2311-2315.
4. 4. T.N. Bach, C.T.A. Xuan, N.T.H. Le, D.H. Manh, D.N.H. Nam, “Microwave absorption properties of (100-x)La1.5Sr0.5NiO4/xNiFe2O4 nanocomposites”, Journal of Alloys and Compounds, 695 (2017), pp. 1658-1662. Articles published in domestic magazines:
5. 5. Chu Thị Anh Xuân, Phạm Trường Thọ, Đoàn Mạnh Quang, Tạ Ngọc Bách, Nguyễn Xuân Phúc, Đào Nguyên Hoài Nam, “Nghiên cứu khả năng hấp thụ sóng vi ba của các hạt nano điện môi La1,5Sr0,5NiO4”, Tạp chí Khoa học Công nghệ, 52 (3B) (2014), tr. 289-297.
6. 6. Chu Thi Anh Xuan, Ta Ngoc Bach, Tran Dang Thanh, Ngo Thi Hong Le, Do Hung Manh, Nguyen Xuan Phuc, Dao Nguyen Hoai Nam, “High- energy ball milling preparation of La0.7Sr0.3MnO3 and (Co,Ni)Fe2O4 nanoparticles for microwave absorption applications”, Vietnam Journal of Chemistry, International Edition, 54(6) (2016), pp. 704-709.
thụ sóng vi ba của tố hợp hạt nano (100
7. 7. Chu Thị Anh Xuân, Tạ Ngọc Bách, Ngô Thị Hồng Lê, Đỗ Hùng Mạnh, Nguyễn Xuân Phúc, Đào Nguyên Hoài Nam, “Chế tạo và nghiên cứu tính chất hấp - x)La1.5Sr0.5NiO4/xNiFe2O4”, Tạp chí Khoa học và Công nghệ - Đại học Thái Nguyên, 157(12/1), tr. 177-181.
8. 8. Chu Thị Anh Xuân, Tạ Ngọc Bách, Đỗ Hùng Mạnh, Ngô Thị Hồng Lê, Nguyễn Xuân Phúc, Đào Nguyên Hoài Nam, “Tính chất hấp thụ sóng điện từ của hệ hạt nano kim loại Fe trong vùng tần số vi ba”, Tạp chí Khoa học – Trường Đại học Sư phạm Hà Nội 2, Số 44 (2016), tr. 16-23.
9. 9. Ta Ngoc Bach, Chu Thi Anh Xuan, Do Hung Manh, Ngo Thi Hong Le, Nguyen Xuan Phuc and Dao Nguyen Hoai Nam, “Microwave absorption properties of La1,5Sr0,5NiO4/La0.7Sr0.3MnO3 nanocomposite with and without metal backing”, Journal of Science of HNUE - Mathematical and Physical Sci., Vol. 61(7) (2016), pp. 128-137.
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Introduction
In recent years, the electromagnetic radiation with the frequency in range of 1-100 GHz has great application in telecommunication, medical treatment, and military. In company with that electromagnetic radiation also brings problems such as: electromagnetic interference, health diseases. Therefore, developing absorbing materials, which has able to absorb electromagnetic radiation, have paid much attention in GHz frequency. Microwave absorption materials (MAM) helps to prevent electromagnetic interference issue, reduce the cross-section reflectivity, and ensure the security of electronic systems. Radar absorption materials (RAM) worked in frequency range of 8-12 GHz is widely used in military systems for stealth technology. Generally, the study on electromagnetic absorption material mainly focuses on three ways: (1) preventing reflectivity signal, (2) enhancing the absorbability of material, and (3) extending frequency range. The increase of loss tangent and absorption efficiency can be obtained if absorbing material can observe both electric and magnetic energy. Moreover, nanotechnology provides the other ways to fabricate absorption material in nanoscale for shielding. MAM with nano-size displays the improvement of absorption ability in comparison with micro-size. Nanotechnology also helps to make the light weight and thin absorbed layer. The microwave absorption ability of material can be determined by relative permeability (r), permittivity (r), and impedance matching between environment and material. The reflection loss (RL) is used to determine the quality of MAM via the formula: RL = 20log|(Z - Z0)/(Z + Z0)|, where Z = Z0(r/r)1/2 is the impedance of material, Z0 is the impedance of air. The maximum reflection loss can be obtained via two mechanisms: (i) the impedance of material equals to impedance of air, |Z| = Z0, which is so called Z matching; (ii) the thickness of absorbing layer satisfies the phase matching or quarter-wavelength condition (d = (2n+1)c/[4f(|r||r|)1/2], n = 0, 1, 2, …). Z matching normally achieves by balancing the permeability and permittivity values, r = r. It can be obtained by fabricating a composite of dielectric and ferrite materials. Recently, there are a lot of publications on MAM based on the nanocomposite of magnetic and dielectric materials in which the RL can be obtained below -50 dB. The RL of nanocomposite is much higher than that of traditional materials such as carbon black-C and carbonyl-Fe. If traditional materials provide the RL below -15 dB, the nanocomposite of ferrite and carbon give very deep RL below -50 dB. For
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instance, a composite of Fe3O4/GCs shows RL around -52 dB at 8.76 GHz, or a composite of BaFe9Mn0.75Co0.75Ti1.5O19/ MWCNTs displays RL ~ -56 dB at 17 GHz. It has been reported that a composite of C/CoFe- CoFe2O4/paraffin is an excellent absorbing material with deep RL below - 71.73 dB at 4.78 GHz. The other core-shell composite Fe/HCNTs and core- shell Co-C in paraffin show the RL about – 50 dB and 62.12 dB at 7.41 GHz and 11.85 dB, respectively. In Vietnam, the study on electromagnetic absorbing material started from 2011 by several group in military. They show ability of nanocomposite of BiFeO3-CoFe2O4 (RL ~ -35.5 dB at 10.2 GHz) in X band. The other nanocomposite Mn0.5Zn0.5Fe2O4 in resin and nano-ferrite Ba-Co have been also studied by them. Besides, the studies on electromagnetic absorption of metamaterial and metamaterial cloaking by a group of Assoc. Profs. Vu Dinh Lam also show prominent results. According to above reason, we propose a project “Synthesis and study of microwave absorption of La1.5Sr0.5NiO4 dielectric/ferro-ferrimagnetic nanocomposite”. This proposal is used to replace the previous name ferro- of microwave “Synthesis ferrimagnetic/dielectric nanocomposite”. We hope that our results contribute to the knowledge on electromagnetic absorbing material and develop the shielding and preventing EMI for electronic device. This dissertation includes four chapter:
absorption study and of
Chapter 1. Microwave absorption phenomena and materials. Chapter 2. Experimental. Chapter 3. Microwave absorption properties of dielectric La1.5Sr0.5NiO4
nanoparticles
Chapter 4. Synthesis and microwave absorption properties of iron
nanoparticle.
Chapter 5. Synthesis and microwave absorption properties of
nanocomposite of dielectric with ferrite and ferromagnetic materials.
The main theme of dissertation:
- Synthesis nanoparticle and nanocomposite of dielectric, ferrites,
ferromagnetic, metal.
- Synthesis nanoparticle and nanocomposite of dielectric,
ferrites, ferromagnetic, metal. Studying the synthesis process and properties of materials.
- Studying the microwave absorption properties and absorption mechanism of
ferromagnetic-dielectric nanocomposite.
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- Finding new material for better absorption performance (RL ~ -40 dB - -60
dB).
The object of thesis:
- Ferromagnetic and ferrites nanoparticle with large µ and Ms, such as
La0.3Sr0.7MnO3, CoFe2O4, NiFe2O4, and Fe. - Colossal dielectric material La1.5Sr0.5NiO4. - Nanocomposite of ferro-ferrite and dielectric materials.
The methodology: This dissertation follows the experimental method. According to the experimental data, we analyse the absorption properties of materials and compare with other reports. Firstly, we synthesize material in nanoscale by high energy ball milling method combined with annealing in furnace at suitable temperature. The crystal structure, morphology, and particle size have been analyzed by X-ray diffraction, scanning electron microscope. The vibrating sample magnetometer (VSM) is used for investigation magentic properties of material. Lastly, the measurement of the reflection and transmission of microwave is done in frequency 4 – 18 GHz by free space method at room temperature. The reflection loss can be calculated by transmission line and NRW method. The experimental results is explained for the absorption properties of material.
The results of dissertation:
The platelet of nanocomposite material with paraffin have been synthesized. The large absorption ability of La1.5Sr0.5NiO4/paraffin has been reported for the first time in frequency 4 – 18 GHz. The RL reaches -36.7 dB, and the absorption efficiency closes to 99.98%
The enhancement of resonance phase matching is observed for measuring
absorption properties by reflection metal-back method.
The contrary behavior on the shifting of
resonance peak of La1.5Sr0.5NiO4/NiFe2O4 and La1.5Sr0.5NiO4/La0.7Sr0.3MnO3 are observed. As NFO and LSMO concentration increases, the absorption peak related to impedance matching tends to high-frequency shift for LSNO/NFO and low- frequency shift for LSNO/LSMO. This different behavior is believed origin from different absorption mechanisms. The composite of LSNO/NFO follows the ferromagnetic resonance of NFO nanoparticle, while LSNO/LSMO relates to the ferromagnetic relaxation of LSMO nanoparticle. In the process of working and writing this thesis, although the author has tried hard but still can not avoid the errors. I wishes to receive the comments, the reviewer of the scientists as well as the people interested in the topic.
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Chapter 1. Microwave absorption phenomena and materials
This chapter presents the researchs and developments of microwave absorption materials. Some basic knowledge relates to the interaction between electromagnetic waves and materials, major absorption mechanisms occurring in absorbers, such as: electromagnetic loss in conductors, dielectric loss and magnetic losses have been presented to support discussions and explain experimental results in the following chapters. This chapter also introduces some of the typical microwave absorption structures and materials, such as resonant absorption layer (Salisbury, Dallenbach), broadband absorption multilayer (Jaumann), inhomogeneous absorber, hybrid microwave absorption materials, magnetic absorbers or metamaterial perfect absorber and some of the specific materials related to the object of thesis (the dielectric material with colossal permittivity-La1.5Sr0.5NiO4, ferrite materials Ni(Co)Fe2O4 and ferromagnetic materials Fe, La0.7Sr0.3MnO3) based on the analysis of previous researched results. This is important for discussing the researched results of thesis.
Chapter 2. Experimental
This chapter presents solid state reaction method combined with a high- energy ball milling technique and proper post-milling thermal annealing processes, allows preparing large amounts of high quality nanopowders required for microwave tranmission/reflection measurements. Structure analysis techniques, elemental determination and magnetic properties measurements of materials have been effectively exploited to assess the quality of the product. Some of electromagnetic parameters techniques of absorbers also introduce. By using free-space transmission techniques, microwave transimission and reflection measurements in the air are carried out in the frequency range of 4-18 GHz. This is the most suitable measurement method for investigating the microwave absorption capability of MAMs that are coated from a mixture of nanoparticles with paraffin on thin plates of mica. The devices, which used in the experimental measurements of this thesis, are modern and high accuracy. Finally, the impedance (Z) and the reflection loss (RL), which are characterized for both weak reflection and high absorption of MAMs, are calculated via the KaleidaGraph data processing software based on transmission line theory and NRW algorithm.
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Chapter 3: Microwave absorption properties of dielectric La1.5Sr0.5NiO4 nanoparticles
3.1. Characteristics of dielectric La1.5Sr0.5NiO4 nanoparticles
3.1.1. Crystal structure and particle size
Figure 3.1. X-ray diffraction pattern of the LSNO powder at 300 K.
Figure 3.2. SEM image of the LSNO powder.
X-ray diffraction data (Fig. 3.1) indicates that La1,5Sr0,5NiO4 particles are single phase of a tetragonal (F4K2Ni-perovskite-type, I4/mmm(139) space group). The nano particle size is about 50 nm. The SEM images (Fig. 3.2) indicates that the particle size is significantly larger than that obtained from the XRD technique, ranging from 100 nm to 300 nm. 3.1.2. Magnetic properties shows 3.3 Figure.
at
Figure 3.3. Magnetic loop, M(H), of room the LSNO material temperature. the magnetization loop, M(μ0H), of LSNO nanoparticles. The result indicates very small magnetic moments with no hysteresis. This proves that the LSNO fabricated nanoparticles exhibit paramagnet- like behavior at room temperature. 3.2. Microwave absorption capability of La1,5Sr0,5NiO4 nanoparticles at different layers thicknesses
The characteristic parameters of La1,5Sr0,5NiO4/paraffin samples with 40/60 vol. percentage, respectively, and different thicknesses, d = 1.5; 2.0; 3.0 and 3.5 mm are summarized in Table 3.1. The RL(f) and |Z|(f) curves are presented in Figures 3.4 (a)-(d).
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Figure 3.4. RL(f) and Z(f) curves of the LSNO/paraffin layers: (a) d = 1.5 mm; (b) d = 2.0 mm; d = 3.0 mm và d = 3.5 mm. Table 3.1. The microwave absorption characteristics for the paraffin- mixed La1,5Sr0,5NiO4 particle layers with different thicknesses.
d (mm) 1.0 1.5 2.0 3.0 3.5
fr (GHz) - 14.7 12.18 9.7 8.2
fz1 (GHz) - 14.3 12.22 9.7 -
fz2 (GHz) - 13.2 - 9.2 -
4.18 13.9 12.7 10.9 10.4 fp (GHz) (n=1)
|Z”|(fz1)(Ω) - 209.5 34.6 18.5 -
- 317.2 - 242 -
|Z”|(fz2)(Ω) RL(fr)(dB) - -24.5 -28.2 -36.7 -9.9
The RL(f) curves of d = 1,5; 2,0 và 3,0 mm samples in fig. 3.4a-c exhibit a deep minimum peak in RL at fr that is close to the fz1 frequency (Tab. 3.1), where the impedance matching condition (|Z| ≈ Z0 = 377 Ω) is satisfied. This suggests that the strong microwave absorption at the minimum absorption notch would be attributed to a resonance caused by impedance matching (Z- matching). However, the resonance could also be caused by a phase
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matching at fp frequency if the phases of the reflected waves from the two sample’s surfaces differ by π:
𝑓𝑝 = (2𝑛 + 1)𝑐/(4𝑑√|𝜀𝑅|. |𝜇𝑅|); n = 0, 1, 2, ... (3.1) It is difficult to determine conclusively which mechanism is responsible for the deep negative RL at fr since both fz1 and fp values are quite close to the fr value. With increasing thickness from 1.5 mm to 3.0 mm, the resonance shifts to lower frequencies while the notch in RL, respectively, becomes deeper (Fig. 3.6). The resonance mechanic that is observed in this samples at minimum absorption peaks is the impedance mathching. The strong absorption is obtained only at fz1 while there is no observable anomaly (except for the t = 1.5 mm sample) in the RL(f) curve at fz2 frequency. The large values of |Z”| (the mismatch at the Z- matching condition) may explain the absence of resonant absorption at the fz2 frequencies.
When the thickness is increased to 3.5 mm (Fig. 3.4(d)), the microwave absorption is strongly suppressed. No absorption notch could be observed at the fp frequencies for all samples (Tab. 3.1). We hope that using a metal backing plate will be drastically reduced the minimum values of RL or can be broadened the resonance frequency region by combining the Z-matching and phase- matching.
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Chapter 4. Synthesis and microwave absorption properties of iron nanopaticles
4.1. The effect of fabricated conditions on the crystal structure, particle size and magnetic properties of Iron metal nanomaterials
listed
Figure 4.1. X-ray diffraction (XRD) of iron samples after from 1-20h milling.
The analysis of crystal structure of Fe samples prepared for 1 to 20 the hours show (Fig. 4.1) appearance of diffraction lines corresponding to the body-centered cubic structure for α-Fe. The average particle size for all samples are in Table 4.1. The magnetization curve, M(H), at temperature of Fe-10h room sample (see the inset of Fig. 4.2) shows a high satutation moment Ms and small coercivity Hc. The satutation magnetization of Fe powder decreased sharply after milling for 10 hours and then decreased slowly for longer milling time (Tab. 4.1; Fig. 4.2).
Figure 4.3. The variability of Ms (Fe- 10h) following presered in air.
Figure 4.2. The dependence of Ms on milling time and the M(H) cuver of Fe-10h sample. Table 4.1. The average particle size D and the satutation magnetization Ms at 10 kOe magnetic field of Fe powder after from 1h to 20h milling. Fe-5h 28 204 Samples D (nm) MS (emu/g) Fe-15h 20 197 Fe-20h 19 194 Fe-10h 21 200 Fe-1h 76 217 Fe-3h 42 209
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168 240 480
Table 4.2. The satutation magnetization value (at 10 kOe) and % volume of Iron oxide shell formation when Fe nano powder is preseved in air in different time. 720 72 24 193 191.2 191 188.6 188.2 188.1 0 200
- 3.0 4.5 6.2 6.5 6.7 6.8 Time (hour) MS (emu/g) Volume of iron oxide (%)
The oxidation of metal powders when milled in the air is an important factor that reduces the total magnetization of the material. The Iron powder was milled in 10h (Fe-10h) with particles size and magnetization begin to stability and to be selected for following study. To verify the continuation of the oxidation in the air of the samples after milling, magnetization measurements were performed and monitored for the transformation of Ms (t) for a long time after grinding. The decrease of Ms saturation magnetization value accoding to the preserved time (Figure 4.3 and Table 4.2) is considered as a result of the natural oxidation of the surface. The reduction rate of Ms after the milling time is almost proportional to the amount of oxide formed (mainly FeO and Fe2O3). The results of the EDX spectral analysis (Fig. 4.3) corresponded to the magnetic properties of Fe in the air. In order to reduce the formation of oxide coatings surrounding Fe metal nanoparticles, the Fe/Paraffin adsorbing layers are spread out within 24 hours of grinding. 4.2. The microwave absorption properties of the iron nanopaticles
4.2.1. The effect of absorbing layer thickness on the microwave absorption properties of Fe/paraffin.
In this study, the absortion layer Fe/paraffin has got diffirent thichnees (d = 1.5; 2; 3 và 3.5 mm) with % volume rate of Fe nano powder is 40% and paraffin is 60%. The microwave absorption properties were measured into two modes: with Al-backed and unbacked sample.
The RL (f) curve of the Fe/Paraffin absorption layers in the 4-18 GHz frequency band is shown in Figure 4.4a-b. Accordingly, the absorption peaks at frequencies near 6 GHz (fr1) and 16 GHz (fr2). The lowest RL of ~-23dB is obtained for d = 3 mm thickness at ~ 15.6 GHz frequency, while the remaining samples present a very weak microwave absorption ability with a value of RL> -9 dB. At low frequencies near 6 GHz, the magnitude of the absorption peaks is approximately and the RL> -7dB represents the weak microwave absorption ability.
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Figure 4.4. RL(f) curves of Fe/paraffin sample with different thicknesses d in the frequency band (a) 4-12 GHz and (b) 14-18GHz. These samples with thickness d = 1.5 mm; 2.0 mm and 3.5 mm (Fig. 4.5) have |Z|/Z0 >2 value at the absorption peaks and thus not satisfy with impedance matching resonance. However, for the sample with thickness d = 3.0 mm, impedance matching resonance |Z|/Z0 = 1 determines the strong microwave absorption at the absorption peaks.
Figure 4.5. The RL(f) và |Z|(f) curves of samples with: (a) d = 1.5 mm; (b) d = 2.0 mm; (c) d = 3.0 mm and (d) d = 3.5 mm.
The value of the phase-matching resonance frequency (fp ~ 5.5 GHz) is very close to the absorbed peak frequency at the low frequency region of 6 GHz (fr1) (Figure 4.4 and Tab. 4.3), showing the resonance effect at the frequency region is determined by phase-matching.
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Table 4.3. The characteristic parameters of Fe/paraffin sheet.
d (mm) fp(n = 2)(GHz) fr1(GHz) fr2(GHz) RL(r1) RL(r2) RL(r1)(GHz) - Al RL(r2)(GHz) - Al 1.5 5.5 5.7 15.6 -6.5 -5.5 -52.7 -9.8 2 5.6 5.6 15.6 -6.4 -6.9 -44.6 -7.7 3 5.4 5.5 15.6 -6 -23 -44.1 -16.8 3.5 5.6 5.6 15.5 -5 -9 -13.2 -13.5
Figue 4.6. The dependence of | S11 | and RL to the frequency of the Fe / Paraffin samples with Al-balcked.
In order to better observe phase-matching in the low frequency region ~ 6 GHz, microwave reflection mesuaments for Al-backed samples to enhance the intensity of the reflected wave from the back of the sample. According to the results shown in Fig. 4.6a, the phase-matching resonance in low frequency region ~ 6 GHz is clearly illustrated by the strong decrease of the signal |S11| to zero and a corresponding absorption peak on the RL curve (f) (Fig. 4.6b). In addition, the results show that the peak absorption move towards the low frequency region as d increases. The microwave reflection measurements on metal backed samples could be used as a simple method to distinguish the phase-matching from the conventional Z-matching resonance. 4.2.2. The effect of Fe and paraffin mass ratios on the microwave absorption properties of Fe/paraffin absorption layers
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exhibited
that Figure 4.7. The RL(f) curves for the the 4- unbacked samples within 18GHz frequency range.
Figures 4.7 and 4.8 show RL(f) curve and the correlation between the RL (f) and Z (f) curves of the Fe/paraffin layers with a thickness d = 3mm and r = mFe / mparaffin = 3/1; 4/1; 4.5/1 and 5/1. With mass ratio r changes from 3/1 to 5/1, the samples weak microwave absorption ability and did not significantly change in the frequency range. The results also is no clear there show evidence of resonance can be observed in the whole range of frequencies. The frequency values fp calculated according to the phase-matching model for the whole samples are listed in Table 4.4.
Figure 4.8. The RL(f) and Z(f) of Fe/paraffin absorption layers with mass ratio r: r = 3/1 (a); r = 4/1; r = 4.5/1 và r = 5/1. Table 4.4. The fp calculation and observed value of all samples with different mass ratios 4/1 5.3 6.1 3/1 r = mFe/mparaffin fp(n = 2)(GHz) (tính toán) 5.3 6.6 fp (GHz) (quan sát) 4.5/1 5/1 5.1 5.2 5.9 5.8
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The results of the microwave reflectance measurement in the 4-18 GHz frequency band for Al-backed samples are shown in Figure 4.9. Figure 4.9a shows a strong decrease of the |S11| value around 6 GHz. This closes to the calculated values for phase-matching frequency fp in Table 4.4, which corresponding to the strong absorption peak on the RL (f) curve (Fig 4.9b). The RL reaches a large negative value of -56.7 dB at 5.4 GHz for r = 4.5/1.
Figure 4.9. The absolute value of reflection coefficient | S11 | (a) and RL (f) (b) of all Fe / Paraffin absorption layers with an Al-plate.
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Chapter 5. Synthesis and microwave absorption properties of nanocomposite of dielectric with ferrite and ferromagnetic materials.
5.1. Synthesis and characteristics of CoFe2O4, NiFe2O4 and La0,7Sr0,3MnO3 materials
) y . t .
v
.
đ ( ộ đ
4 (
2 (
0 4 (
g n ờ ư C
5.1.1. Ferrite CoFe2O4 nanoparticles
X-ray diffraction annealed
2θ (độ)
of CFO
Figure 5.1. pattern nanopowders . Figure 5.2. M(H) loops of the bulk, as-milled, and annealed nanopowder of CFO .
The XRD data (Fig. 5.1) indicate that the nanoparticle powders are single phase; all the diffraction peaks could be indexed to the expected crystal structures of inverse spinel cubic for CoFe2O4.
The magnetization loops, M(H), for our CFO nanoparticles are shown in
typical
comparable Samples
The Table 5.1. The particle size
Fig. 5.2. The CFO nanoparticles show hysteresis characteristics with the saturation magnetization to previously reported data for CFO nanoparticles prepared by other methods. characteristic parameters extracted from these measurements are listed in Table 5.1. 5.1.2. Ferrite NiFe2O4 nanoparticles
Fig. 5.3 presents the XRD patterns of the final NFO nanoparticle powders that were used to prepare for the absorbing plates. The diffraction patterns show that they are single phase with no detectable secondary phase or impurity; all the peaks can be indexed to the expected crystal structures of spinel cubic for NFO.
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) y . t .
v
.
đ ( ộ đ
g n ờ ư C
4 (
2θ (độ)
NFO
Figure 5.3. X-ray diffraction pattern annealed of nanopowders. Figure 5.4. M(H) loops of the bulk, as-milled, annealed and nanopowder of NFO
Fig. 5.4 presents the magnetic hysteresis loops,M(H), of the NFO bulk and nanoparticle powders before and after annealed. The bulk NFO has a saturation magnetization Ms = 49 emu/g and coercivity Hc = 120 Oe. However, due to the contribution from the surface disorder, surface roughness, and shape anisotropy, the as-milled powder has much smaller Ms and larger Hc than those of the bulk sample. The characteristic parameters extracted from these measurements are listed in Table 5.2.
Table 5.2. the crystal particle size
Samples D (nm) HC (Oe)
CFO-MK CFO-MB CFO-M900 42.0 23.2 34.8 MS (emu/g) (Tại H = 10 kOe) 49.0 34.5 45.0 120 967 126
5.1.3. Ferromagnetic La0,7Sr0,3MnO3 nanoparticles
Fig. 5.5 presents the magnetic hysteresis loops, M(H), of LSMO in
Table 5.3. The crystal particle size
Samples
different forms: bulk, as-milled, and annealed nanopowder. The saturation magnetization Ms (at H = 10 kOe) drops from 65.6 emu/g for the bulk to 36.2 emu/g for the nanoparticle powder as a result of damages caused by the milling. This decrease in magnetization the could be detrimental to D (nm) 54.5 LSMO-MK LSMO-MB 32.3 LSMO-M900 38.6 MS (emu/g) 65.6 36.8 53.2 HC (Oe) 5.0 23.0 13.0
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absorbing capability of the nanoparticles. However, an appropriate post- milling heat treatment would largely heal the damages and recover the magnetic characteristics. Fig. 5.6 presents the XRD patterns of LSMO nanoparticle powders. The diffraction patterns show that they are single phase with no detectable secondary phase or possible impurity. The characteristic parameters extracted from these measurements are listed in Table 5.3.
Figure 5.5. Magnetic hysteresis M(H) loops of the La0.7Sr0.3MnO3 samples in different forms Figure 5.6. X-ray diffraction the La0.7Sr0.3MnO3 patterns of nanopowders.
paraffin mixed The (100-x)La1,5Sr0,5NiO4/xCoFe2O4
5.2. Microwave absorption capability of nanocomposites 5.2.1. nanocomposites ( x = 0; 2; 4; 6; 8; 10)
loss and the permittivity of
Figure 5.7. Unbacked samples: RL of (100−x)LSNO/xCFO absorbers within the frequency range of 4–18 GHz . Fig. 5.7 shows the RL(f) curves for all the unbacked samples in frequency range 4–18 GHz. As expected, with doping CFO, the resonance notch in the the RL(f) curves becomes deeper; minimum RL decreases from −12.8dB for x=0 to −31.2 dB for x=8. Further increasing x leads to an abrupt increase of the minimum RL. The decrease of RL with x for x≤8 should be attributed to the the increase of magnetic balancing and permeability caused by the substitution of CFO for LSNO nanoparticles. The
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characteristic parameters such as the resonance frequency fr extracted from these measurements are listed in Table 5.4.
Figure 5.8. Unbacked samples: enlargements of the RL(f) (right axis) and |Z/Z0|(f)(left axis) curves for all the samples in the resonance region near f =14 GHz. (a)x =0, (b)x =2, (c)x =4, (d)x =6, (e)x =8, and (f) x=10. Table 5.4. Summary of the microwave absorption characteristics for the paraffin- mixed (100-x)LSNO/xCFO nanocomposites 10 x (%) 0 2 4 6 8
a. Unbacked
4.2 4.7 4.7 5.0 5.0 4.8 fp (GHz) (n = 0)
fz (GHz) - - - - 12.0- 12.8
fr (GHz)
|Z’|( fr) (Ω) 39.2 - - - -
RL(fr) (dB) 12.0 13.3 12.6 13.6 14.8 12.4 12.5 14.1 290 320 -12.8 -17.8 -24.0 -21.3 -31.2 -10.8
b. Al backed 6.0 6.1 5.7 5.5
fr1 (GHz) fr2 (GHz) RL(fr1) (dB) RL(fr2) (dB) 6.4 6.4 16.2 16.2 16.9 15.5 16.0 16.6 -15.5 -12.5 -54.3 -21.2 -25.5 -6.6 -8.0 -11.2 -53.5 -10.5 -9.7 -6.8
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In Fig. 5.8, the resonance regionshows a zoomed-in view and RL(f) and |Z/Z0| curves for each sample are plotted together for comparison. It is quite clear that the resonance occurs near the minimum of |Z/Z0| that is also close to 1. The closer value of minimum |Z/Z0| to 1 gives a smaller value of RL minimum; the deepest RL notch is observed for x =8 (Fig. 5.8) where |Z/Z0| is found equal to 1. This provides evidence for the main role of the Z- matching mechanism in these resonances. According to (2), when perfect energy absorption or reflection cancelation occurs, Z =Z0 =1 and therefore RL = −∞.
As mentioned above, although the calculations of fp suggest a phase matching of reflected waves at frequencies near 5 GHz, no such resonance is observed. The absence of the predicted resonance at the matching frequency could be caused by the fact that the samples are open circuited. Without a metal backing plate, which is considered as a perfect reflector, the internal reflection wave on the reverse side would be much weaker than that on the incident side of the sample. The cancelation of the two waves is therefore insignificant and phase-matching resonance may not be observed. In order to further prove this assumption, reflection measurements for the corresponding samples with metal backing have been carried out; the results are shown in Fig. 5.9.
Figure 5.9. Al-backed samples. (a) Absolute value of the reflection coefficient, |S11|. (b) RL of all the (100−x)LSNO/xCFO absorbers within the frequency range of 4–18 GHz.
Interestingly, the resonance around 5 GHz is clearly shown by a sharp drop of |S11| [Fig. 5.9(a)] and a corresponding notch in the RL(f)curve (Fig. 5.9(b)). This observation not only proves the existence of the phase- matching resonance near 5 GHz, but also suggests a method to differentiate between the phase-matching and Z-matching resonances, both of them give zero reflection and |Z/Z0|=1 condition.
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paraffin mixed The (100-x)La1,5Sr0,5NiO4/xNiFe2O4
5.2.2. nanocomposite (x = 0; 8; 15; 20; 30; 35)
Figure 5.10. RL(f) curves of paraffin-mixed LSNO and NFO plates.
Fig. 5.10 presents the RL(f) plots of the paraffin-mixed LSNO and NFO nanopowder plates. The NFO sample shows only weak absorption (RL> -5 dB) and no indication of a resonance in the frequency range 4-18 GHz. On the other hand, the LSNO sample shows a clear deep notch in RL at fr ≈ 13.6 GHz that is close to the frequency fZ ≈ 13.7 GHz where the impedance |Z| ≈ Z0≈377Ω (Fig. 5.11a). The deep notch in RL of the LSNO sample can therefore be attributed to an absorbing resonance caused by the effect of impedance matching. The absorption of electromagnetic wave in LSNO is expectedly dominated by dielectric loss when the molecular dipoles are polarized at high frequency and energy of the microwave propagating through the material is dissipated via the dipole rotation. The energy dissipation is maximized at the resonance frequency fr = fZ when the impedance matching condition is satisfied.
Figure 5.11. RL(f) and |Z/Z0|(f) curves of the paraffin mixed (100- x)LSNO/xNFO nanocomposite plates
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fr within
Figure 5.12. The fr1(x) curves of LSNO/NFO and LSNO/LSMO nanocomposites.
Fig. 5.11 shows the RL(f) and |Zr/Z0|(f) curves of the paraffin mixed (100-x)LSNO/xNFO nanocomposite plates in the frequency range of 4- 18 GHz. A summary of the characteristic frequencies extracted from this figure is presented in Table 5.5. All the samples show a sharp absorbing the high resonance at frequency regime of 14-16 GHz. Although the absorption is clearly stronger in the samples containing NFO, the variation of RLwith the NFO content x is not monotonous; RL reaches lowest values of -29.7 dB at x = 8 and -28.5 dB at x = 30, respectively. Since the NFO sample has only very weak absorption as shown in Fig. 5.10, the improvement of absorption in the NFO-added samples would be associated with the dielectric-magnetic balancing rather than the direct contribution of magnetic loss from the added NFO nanoparticles. Noticeably, despite the minimum of RL does not vary monotonously, the resonance frequency fr systematically shifts to higher frequency with increasing the NFO content: fr ≈13.6, 13.9, 14.7, 14.8, 15.3, and 15.9 GHz for x = 0, 8, 15, 20, 30, and 35, respectively. For all the samples, since the fr matches very well to the frequency fZ where |Z/Z0| = 1, these observed resonances must be caused by the Z-matching effect. Although the shift of fr (and fZ) with x could be a direct result of the dielectric-magnetic balancing caused by the doping of NFO, it is worth noting that it could also be caused by other effects, as will be discussed later.
1
𝑒
In the present case, increasing NFO concentration is expected to enhance the interparticle coupling, which slows down the system's response, leading to a decrease in f0. In contrast, both fr and fZ, as shown in Fig. 5.12, increase with x, thus ruling out the role of the relaxation mechanism. One more possible mechanism that can be mentioned here is the well-known ferromagnetic resonance (FMR) absorption with the resonance frequency fFMR is given by:
2𝜋
2𝑚
𝑔 𝑓𝐹𝑀𝑅 = 𝛾𝐻𝑒𝑓𝑓 = (𝐻 + 𝐻𝐴 + 𝜇0𝑀) (5.1)
Although, the FMR-based model could well explain the increase in the resonant frequency of our LSNO/NFO nanocomposites with increasing the NFO concentration, more experiments and analyses are still needed to verify
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this possibility. Based on the fact that fr is always close to fZ for all the samples, we propose that dielectricmagnetic balancing would be the most possible cause for its shifting with x. In addition, the shift of the absorbing resonance toward higher frequency with increasing x would explain the result for NFO when fr and fZ become greater than the upper limit (18 GHz) of the measurements; far below fr and fZ, no significant absorption would be observed.
Table 5.5. Summary of the characteristic frequencies of the LSNO/NFO nanocomposite absorbing plates 15 0 14.7 13.6 14.7 13.7 5.4 5.7 5.7 5.9 30 15.3 15.4 5.2 5.5 8 13.9 14.2 6.1 5.8 20 14.8 14.7 5.5 5.7 35 15.9 15.7 5.6 5.3
x (%) fr (GHz) fz (GHz) fp-QS (GHz) fp-TT (GHz)
Figure 5.13. |S11|(f) (a) and RL(f) of the Al-backed samples. The phase matching effect is much enhanced by metal backing. The results in Fig. 5.11a-f also show that there exists a possible resonance with a notch in the low frequency range of 5-6 GHz. Calculations of the phase matching frequency (Table 5.5), suggest that this notch could be indicative of a resonance caused by the phase matching effect if the phases of the reflected waves from both sides of the sample differ by π. However, this observed phase matching resonance is quite weak, probably due to the incomplete cancellation of the reflected waves. The reflection at the front surface would be much stronger than from the back. To improve the back- surface reflection, the sample was attached on an Al plate and reflection measurement was then carried out. The obtained reflection scattering parameter,|S11|, and reflection loss RL of the Al-backed samples are presented in Fig. 5.13. As a result, the wave cancellation is much improved with the S11 signal drops close to zero and the RL lowers to below -10 dB
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(100-x)La1,5Sr0,5NiO4/xLa0,7Sr0,3MnO3
(-17 dB for x = 30). This result clearly verifies the phase-matching behavior of the resonance. In addition, although the phase matching effect is enhanced by metal backing, due to the strong reflection by the backing plate, the Z- matching resonance near 16 GHz is strongly suppressed. 5.2.3. The paraffin mixed nanocomposite ( x = 0; 4; 8; 10)
Figure 5.14. The RL(f) and |Z/Z0|(f) curves for all the samples in the frequency range of 4–18 GHz, (a) x = 0, (b) x = 4, (c) x = 8 and (d) x = 10. Table 5.6. Summary of the microwave absorption characteristic parameters for paraffin mixed (100-x)La1.5Sr0.5NiO4/xLa0.7Sr0.3MnO3 nanocomposites
x 4 8 10
fp (n=2) fr1(GHz) fr2(GHz) RL(fr1)(dB) RL(fr2)(dB) 5.27 13.5 5.57 -28.5 -2.7 5.26 13.2 5.8 -16.9 -3.3 5.16 13.1 5.53 -14.5 -2.9
fr1(GHz) fr2(GHz) RL(fr1)(dB) RL(fr2)(dB) 0 a. Unbacked samples 5.3 13.6 5.7 -18.2 -2.9 b. Al backed samples - 6.0 - -8.6 15.9 5.4 -17.8 -30.7 15.4 6.3 -8.7 -22 16.6 5.5 -22.5 -53.8
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Figure (100- 5.15. Al-backed x)LSNO/xLSMO absorbers: (a) the reflection coefficient, |S11|, and (b) the corresponding RL.
Figs. 5.14a-d present the RL(f) and Z/Z0(f) curves of all the (100- samples. A x)LSNO/xLSMO the characteristic summary of parameters extracted from these measurements are listed in Table 5.6. It is notable that all the samples exhibit a resonance notch in the RL(f) curves with a large negative value of RL at a frequency fr1 near 13 GHz. Increasing the LSMO substitution enhances the absorbability for x < 4 but degrades it for further substitution. The lowest minimum value of RL of - 28.5 dB is reached at the resonance frequency fr1 ~ 13.6 GHz for the absorption plate containing 4% vol. of LSMO. This result is quite close to those we have observed for (100-x)LSNO/xNFO where the lowest RL = -29.7 dB is found for x = 8. As can be seen in the figures, the matching condition Z = Z0 is satisfied at frequency fZ fr1 for x = 0 and x = 4, indicating that the resonance is associated with the Z-matching phenomenon. For x > 4, the impedance Z gets closest to Z0 at the resonance but the Z-matching condition cannot be satisfied in the whole range of frequency. Figs. 5.15a,b show the frequency dependence of the reflection coefficient |S11| and RL of the Al-backed samples. The influence of metal backing plate in the reflection measurements was also previously studied by Wang et. al. Notably, the |S11| signal drastically drops close to zero (Fig. 5.15a) and correspondingly the resonance develops to a deep notch near 6 GHz in the RL(f) curve. The minimum RL reaches down to −53.8 dB at 5.5 GHz for x = 10. This result convincingly proves the phase-matching nature of the fr2 resonances. Although metal backing could dramatically enhance the phase matching resonance, it suppresses the Z-matching effect by producing a strong reflection from the back side. We therefore propose that using a metal backing plate in the reflection measurement is an effective method not only to prove the existence of the phase-matching, but also to distinguish the phase-matching and Z-matching resonances.
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Conclusions
This dissertation study the microwave absorption of several nanoparticle
and nanocomposite. We have obtained several results listed below:
1. The nanoparticle La1,5Sr0,5NiO4, CoFe2O4, NiFe2O4, La0,7Sr0,3MnO3, and Fe have been synthesized by solid state reaction method and high ball energy milling. The effect of synthesis conditions on the properties of material have been studied.
2. We have developed a method to fabricate platelet composite of material and paraffin. The absorption properties of platelet nanocomposite were measured by free space method to determine the transmission and reflection of microwave through the sample. We use transmission line and NRW methods to analysis absorption data.
3. For the first time, we observed a strong microwave absorption of dielectric nanoparticle of La1,5Sr0,5NiO4. The absorption efficiency is about 99.98%. The large absorption observed in colossal dielectric material is unexpected. Our observation can promote the study on microwave absorption of colossal dielectric material.
4. The effect of thickness and material/paraffin volume ratio on the
absorption properties have been investigated in systematic.
5. We developed the metal-back method for measuring microwave reflection signal. This method allows to distinguish the resonance of phase matching and impedance matching.
6. We introduced a nanocomposite of ferromagnetic nanoparticle and dielectric nanoparticle. This composite can enhance microwave absorption ability of (100-x) La1,5Sr0,5NiO4 + x(CoFe2O4; NiFe2O4; La0,7Sr0,3MnO3). Nanocomposite of ferromagnetic and dielectric material is a good way to enhance the absorption ability of MAM.
7. We have observed a contrary behavior on the shifting of resonance peak of LSNO/NFO and LSNO/LSMO. As NFO and LSMO concentration increases, the absorption peak related to impedance matching tends to high- frequency shift for LSNO/NFO and low-frequency shift for LSNO/LSMO. This different behavior is believed origin from different absorption mechanisms. The composite of LSNO/NFO follows the ferromagnetic resonance of NFO nanoparticle, while LSNO/LSMO relates to the ferromagnetic relaxation of LSMO nanoparticle.
