- Email: [email protected]
Preparation and microwave absorbing properties of the core-nanoshell composite absorbers with the magnetic fly-ash hollow cenosphere as core
Journal of Environmental Sciences 2011, 23(Supplement) S74–S77
Preparation and microwave absorbing properties of the core-nanoshell composite absorbers with the magnetic fly-ash hollow cenosphere as core Ruxin Che ∗, Chunxia Wang, Yingjuan Ni, Bing Yu College of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian 116028, China. E-mail: [email protected]
Abstract Electromagnetic (EM) wave pollution has become the chief physical pollution for environment. The core-nanoshell composite absorbers with magnetic fly-ash hollow cenosphere as core and nanocrystalline magnetic material as shell were prepared by high-energy ball milling . The results of X-ray diffraction analysis (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and vector network analyzer (VNA) analysis indicated that perfect-crystalline nanomagnetic material coating was gotten with a particle size of 12 nm, being dried at 60°C for 2 hr and calcined at 400°C for 1 hr after ball milling. The exchange-coupling interaction happened between ferrite of cenosphere and soft magnet γ-Fe2 O3 coating, it enhances magnetic loss of composite absorbers. In the frequency between 1 MHz and 1 GHz, the absorbing effectiveness of the core-nanoshell composite absorbers can achieve –30 dB, it is better than single material and is consistent with requirements of the microwave absorbing material at the low-frequency absorption. Key words: magnetic fly-ash hollow cenosphere; core-nanoshell composite absorbers; high-energy ball milling; absorbing effectiveness
Introduction Microwave absorbing material plays a great role in electromagnetic (EM) pollution controlling, electromagnetic interference shielding and stealth technology, etc. (Petrov and Gagulin, 2001). An ideal microwave absorbing material owns such advantages as tiny thickness, low density, wide bandwidth and flexibility simultaneously. Therefore, it is eager to develop the high-performance microwave absorbing materials. The magnetic fly-ash hollow cenosphere is a byproduct of thermal power plant. This is turning waste into treasure for the application of cenosphere. When a coal-burning boilers works, the majority of iron minerals in the coal form Fe2 O3 , Fe3 O4 with the carbon, carbon monoxide acting, they combine with the new silicon, aluminum, calcium cenosphere material. A number of experiments (Jia et al., 2006; Quan et al., 2003) show that the cenosphere has the electromagnetic properties in low-frequency. The cause is that they have the magnetic loss, dielectric loss and empty effect, in which microwave can be absorbed time after time. The magnetic fly ash hollow cenosphere is a kind of microwave absorbent, due to the conductive behavior of Fe2 O3 , Fe3 O4 and SiO2 , high dielectric can be obtained in microwave frequencies. Raw material powders are ground in a high-energy ballmill, and the powders are squeezed, deformed, fractured, and reformed repeatedly. As a result, a large number of defects are generated in the particles, and the powders * Corresponding author. E-mail: [email protected]
tend to be amorphous, which causes the powder activity to be increased greatly due to the high energy of the system. The high-energy ball milling has seldom been used in core-nanoshell composite absorbers research up to now. In this study, highly active products of milling were obtained by the high-energy ball milling, which were markedly decreased the calcining temperature of the powders. The core-nanoshell composite absorbers were obtained after calcining, which the magnetic fly-ash hollow cenosphere is core and nanocrystalline magnetic material is shell. The exchange-coupling interaction happens between ferrite of hollow cenosphere and nanocrystalline magnetic material coating. It can get the complex phases system and enhance magnetic loss of composite absorbers in low-frequency absorption.
1 Materials and methods 1.1 Pre-treatment of magnetic fly-ash hollow cenosphere The magnetic fly-ash hollow cenosphere was screened under 5000 and 7000 head, and then carried out classification for each magnetic particle size, the magnetic fields were 0.05, 0.098 and 0.2 T. Ultrasonic cleaning technology was used in pretreatment of cenosphere, which solved the problem of corrosion during removing impurities and greasy dirt with acid or alkali liquor. Meanwhile it removed the left liquid in crevices of cenosphere, which increased the surface
Preparation and microwave absorbing properties of the core-nanoshell composite absorbers······
S75
a
SEM images of magnetic fly-ash hollow cenosphere. (a) before pre-treatment; (b) after pre-treatment.
activity of the particles, breaked the unity between the particles, improved the quality of coating processes (Zheng et al., 2008). SEM images of cenosphere before and after pre-treatment are shown in Fig. 1. 1.2 Synthesis The core-nanoshell composite absorbers with magnetic fly-ash hollow cenosphere as core were prepared through high-energy ball milling method, using Fe(NO3 )3 ·9H2 O (analytical grade, Shenyang No. 5 Reagent Factory, China) and citric acid (analytical grade, Shenyang No. 5 Reagent Factory, China) as starting chemicals. First, Stoichiometric amounts of Fe(NO3 )3 , citric acid and magnetic fly-ash hollow cenosphere were weighed precisely, mixed and put into a ball mill with 240 revolutions per min for 2 hr at room temperature, in which the ZrO2 milling balls of the XDQM changeable rate planetary ball grinder were weighed accurately according to a milling balls/starting chemicals mass ratio of 6:l, with the small balls (3 mm)/big balls (6 mm) with a mass ratio of l:1. A kind of viscid substance was got. The viscid substance was dried at 60°C for 2 hr. Finally, the samples were calcined at 400°C in air for 1 hr. The final product was core-nanoshell composite powders.
the first endothermic peaks is at 166.2°C accompanied, the decomposition of citric acid may occur, because the citric acid begin to decompose between 160–180°C according to the different purity of the citric acid. There is a great exothermic peak at about 200–270°C. The biggest exothermic peak appears at 253.4°C, in which the strong solid-state reaction occurs with yielding H2 O, CO and CO2 . This pyrolysis process was confirmed by the TGDTA curves. A stable temperature is lower than 400°C, which attained for the core-nanoshell composite absorbers. Figure 3 shows the XRD patterns of samples. After calcined, the sample contained the magnetic fly-ash hollow cenosphere and γ-Fe2 O3 . In addition, from the XRD peaks, the average grain size (D) is calculated using the 180
2 Results and discussion
0
140 120
-2
TG
100 80
-4
60 40
-6
20
1.3 Characterization
0 166.2ć
-20 100
200
300
Fig. 2
400
500
600
700
180
Magnetic fly-ash hollow cenosphere γ-Fe2O3
160
The pyrolysis processes of the sample were investigated by TG-DTA curve (Fig. 2). Two main regions occur at 130–180°C and 200–270°C. The DTA curve shows that
100
120
80
100
60
80
40
60
20
20 20
180 160 140 120
140
40
2.1 TG-DTA curves of sample analysis
-8 800
Tempreture (ć) TG-DTA curve of the sample.
200
Intensity (a.u.)
The experimental techniques employed in our investigations include: the thermal analysis was carried out by thermogravimetry and differential thermal analysis (TGDTA, 449e, Linseis, Germany) with a heating rate of 10°C min in the air; the crystal structure of sample was examined by X-ray diffractometer (XRD, D5005 Bruker, Seifert-SPM, Germany) with Cu Kα radiation; the microstructure was observed by scanning electronic microscope (SEM, JSM-6360LV, JEOL, Japan) and transmission electronic microscope (TEM, Hitachi-800, JEOL, Japan); and microwave absorbing property analysis was conducted by Vector network analyzer (VNA, ZVA40, Rohde & Schwarz, Taiwan, China).
253.4ć DTA
160
ΔT (ć)
Fig. 1
b
Mass lost (mg)
Supplement
30
40
50 60 2θ (degree) Fig. 3 XRD patterns of the samples.
0 -20 70
Journal of Environmental Sciences 2011, 23(Supplement) S74–S77 / Ruxin Che et al.
S76
2.2 TEM and SEM observation
“Scherrer” equation:
D(hkl) =
Vol. 23
kλ βcosθ
(1)
where, λ is the X-ray wavelength employed, θ (degree) is the diffraction angle of the most intense peak, and β is defined as β2 = β2m − β2s , βm and βs are the experimental full width at half maximum (FWHM) and the FWHM of a standard silicon sample, respectively. The as-obtained D is 12 nm for γ-Fe2 O3 particle. The D of composite phases cannot be calculated directly by this method due to overlapping of the two sets of diffraction peaks. When calcined temperature is lower than 400°C , materials mostly consist of soft magnet γ-Fe2 O3 , which is metastable state. The exchange coupling happens between γ-Fe2 O3 and ferrite of the magnetic fly-ash hollow cenosphere. Exchange coupling is close quarters action, its range of effective exchange coupling equal to the thickness of magnetic domain wall (about 10 nm). When grain size is about 10 nm, the effect of remanence increasing is notable, this shows that controlling the calcined temperature is favourable to the magnetic loss.
It is observed from the transmission electron microscope (TEM) and scanning electron microscope (SEM) morphologies as shown in Figs. 4 and 5, that the core-nanoshell composite particles are spherical with welldefined core/shell structures. The core is magnetic fly-ash hollow cenosphere, and the shell is the exchange-coupling ferrite which was composed by ferrite of the fly-ash hollow cenosphere and γ-Fe2 O3 nanoparticles. In view of an EM wave absorbent, the “core-nanoshell” structure of particle would be perfect to absorb EM wave energy by the magnetic/dielectric losses and their appropriately matching (Panagopoulos and Georgiou, 2009). 2.3 Microwave absorbing properties of the corenanoshell composite particles Table 1 is the sample serial number. The relationship of absorbing effectiveness and frequency on samples has been showed in Fig. 6. It shows the changes of absorbing effectiveness with frequency on samples. The absorbing effectiveness of samples have a difference at the same frequency because the component of absorbent is different. The absorbing effectiveness of the single material has some changes with frequency rise, but the composite particles have obvious changes. Microwave absorbing
b a 0.5 μm 0.5 μm Fig. 4 The TEM image of cenospheres before and after being coated. (a) before coated; (b) after coated.
0.5 μm
a
0.5 μm
b
Fig. 5 The SEM image of cenospheres before and after being coated. (a) before coated; (b) after coated.
Supplement
Preparation and microwave absorbing properties of the core-nanoshell composite absorbers······ Table 1
Sample discription
Sample serial number
Absorbent name
Sample 1 Sample 2 Sample 3 Sample 4
Fly-ash hollow cenosphere Magnetic fly-ash hollow cenosphere γ-Fe2 O3 Magnetic fly-ash hollow cenosphere (had not been pre-treated)+ γ-Fe2 O3 Magnetic fly-ash hollow cenosphere (had been pre-treated) + γ-Fe2 O3
Sample 5
Absorbing effectiveness (dB)
-5 -10 -15 Sample 1 Sample 2 Sample 3 Sample 4 Sample 5
-25 -30 -35
0
200
dipole polarization is the dominant at 1 MHz and 1 GHz frequency and the weak space charge polarization mainly works at higher frequency. Sample 4 had not been pre-treated, so it contain more impurities and the coating is unevenly. Its microwave absorbing properties is poorer than sample 5.
3 Conclusions The core-nanoshell composite absorbers with magnetic fly-ash hollow cenosphere as nuclear were prepared by high-energy ball milling method. The pre-treatment is beneficial to coat evenly and improve the surface activity. The diameter of γ-Fe2 O3 is 12 nm, in which the exchange-coupling interaction happens between them. The exchange-coupling interaction enhance magnetic loss of composite absorbers, so the microwave absorptivity of the core-nanoshell composite absorbers is better than single material. In the frequency between 1 MHz and 1 GHz, the absorbing effectiveness can achieve –30 dB, and it is consistent with requirements of the microwave absorbing material at the low-frequency absorption.
0
-20
S77
400
600
800
1000
Incident wave frequency (MHz) Fig. 6 Relationship of absorbing effectiveness and frequency on samples.
properties of the core-nanoshell composite particles is much stronger than the single material, which possibly implies the existence of exchange coupling action between ferrite of cenosphere and γ-Fe2 O3 nanoparticles. In the core-nanoshell composite particles, two kinds of mechanisms could be used to explain the stronger microwave absorbing properties. The first mechanism is the exchange coupling between γ-Fe2 O3 and ferrite of the magnetic fly-ash hollow cenosphere, its range of effective exchange coupling equal to the thickness of magnetic domain wall (about 10 nm). When grain size is about 10 nm, the effect of remanence increase is notable, so the magnetic loss can be increased, and it is better to improve absorbing properties. The second mechanism is the dipole polarization. The nano-composites is a such perfect system, because the polarized cores can play the role of dipoles, especially at the low-frequency, as demonstrated in the γ-Fe2 O3 nanoparticles which coated the magnetic fly-ash hollow cenosphere. Considering the nano-particles on core/shell-type microstructure, it is reasonable that the
Acknowledgments This work was supported by the Science Research Plan Project of Higher Education Department of Liaoning Province (No. 2008086). The authors would like to thank Dr. Zhihua Zhang for TEM analytical support, Dr. Zhiqiang Li for SEM analysis, and Dr. Zhiyong Jia for VNA measurements.
References Jia Z Y, Wang Q, Zhou M L, 2006. Electromagnetic properties of magnetic fly-ash hollow cenosphere. Journal of Functional Materials, 6(37): 877–879. Petrov V M, Gagulin V V, 2001. Microwave absorbing materials. Inorganic Materials, 37(2): 93–98. Panagopoulos C N, Georgiou E P, 2009. Surface mechanical behaviour of composite Ni-P-fly ash/zincate coated aluminium alloy. Applied Surface Science, 255(13): 6499– 6503. Quan B P, Xu H, Gu H C, Sun T, Zhao H, 2003. Progress in research and application of fly ash particles. Industrial Minerals and Processing, 45(11): 31–33. Zheng H, Shao Q, Leng S W, Sun T, 2008. Palladium-free activation electroless nickel plating on cenosphere surface. Surface Technology, 37(1): 56–58.
Preparation and microwave absorbing properties of the core-nanoshell composite absorbers with the magnetic fly-ash hollow cenosphere as core Ruxin Che ∗, Chunxia Wang, Yingjuan Ni, Bing Yu College of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian 116028, China. E-mail: [email protected]
Abstract Electromagnetic (EM) wave pollution has become the chief physical pollution for environment. The core-nanoshell composite absorbers with magnetic fly-ash hollow cenosphere as core and nanocrystalline magnetic material as shell were prepared by high-energy ball milling . The results of X-ray diffraction analysis (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and vector network analyzer (VNA) analysis indicated that perfect-crystalline nanomagnetic material coating was gotten with a particle size of 12 nm, being dried at 60°C for 2 hr and calcined at 400°C for 1 hr after ball milling. The exchange-coupling interaction happened between ferrite of cenosphere and soft magnet γ-Fe2 O3 coating, it enhances magnetic loss of composite absorbers. In the frequency between 1 MHz and 1 GHz, the absorbing effectiveness of the core-nanoshell composite absorbers can achieve –30 dB, it is better than single material and is consistent with requirements of the microwave absorbing material at the low-frequency absorption. Key words: magnetic fly-ash hollow cenosphere; core-nanoshell composite absorbers; high-energy ball milling; absorbing effectiveness
Introduction Microwave absorbing material plays a great role in electromagnetic (EM) pollution controlling, electromagnetic interference shielding and stealth technology, etc. (Petrov and Gagulin, 2001). An ideal microwave absorbing material owns such advantages as tiny thickness, low density, wide bandwidth and flexibility simultaneously. Therefore, it is eager to develop the high-performance microwave absorbing materials. The magnetic fly-ash hollow cenosphere is a byproduct of thermal power plant. This is turning waste into treasure for the application of cenosphere. When a coal-burning boilers works, the majority of iron minerals in the coal form Fe2 O3 , Fe3 O4 with the carbon, carbon monoxide acting, they combine with the new silicon, aluminum, calcium cenosphere material. A number of experiments (Jia et al., 2006; Quan et al., 2003) show that the cenosphere has the electromagnetic properties in low-frequency. The cause is that they have the magnetic loss, dielectric loss and empty effect, in which microwave can be absorbed time after time. The magnetic fly ash hollow cenosphere is a kind of microwave absorbent, due to the conductive behavior of Fe2 O3 , Fe3 O4 and SiO2 , high dielectric can be obtained in microwave frequencies. Raw material powders are ground in a high-energy ballmill, and the powders are squeezed, deformed, fractured, and reformed repeatedly. As a result, a large number of defects are generated in the particles, and the powders * Corresponding author. E-mail: [email protected]
tend to be amorphous, which causes the powder activity to be increased greatly due to the high energy of the system. The high-energy ball milling has seldom been used in core-nanoshell composite absorbers research up to now. In this study, highly active products of milling were obtained by the high-energy ball milling, which were markedly decreased the calcining temperature of the powders. The core-nanoshell composite absorbers were obtained after calcining, which the magnetic fly-ash hollow cenosphere is core and nanocrystalline magnetic material is shell. The exchange-coupling interaction happens between ferrite of hollow cenosphere and nanocrystalline magnetic material coating. It can get the complex phases system and enhance magnetic loss of composite absorbers in low-frequency absorption.
1 Materials and methods 1.1 Pre-treatment of magnetic fly-ash hollow cenosphere The magnetic fly-ash hollow cenosphere was screened under 5000 and 7000 head, and then carried out classification for each magnetic particle size, the magnetic fields were 0.05, 0.098 and 0.2 T. Ultrasonic cleaning technology was used in pretreatment of cenosphere, which solved the problem of corrosion during removing impurities and greasy dirt with acid or alkali liquor. Meanwhile it removed the left liquid in crevices of cenosphere, which increased the surface
Preparation and microwave absorbing properties of the core-nanoshell composite absorbers······
S75
a
SEM images of magnetic fly-ash hollow cenosphere. (a) before pre-treatment; (b) after pre-treatment.
activity of the particles, breaked the unity between the particles, improved the quality of coating processes (Zheng et al., 2008). SEM images of cenosphere before and after pre-treatment are shown in Fig. 1. 1.2 Synthesis The core-nanoshell composite absorbers with magnetic fly-ash hollow cenosphere as core were prepared through high-energy ball milling method, using Fe(NO3 )3 ·9H2 O (analytical grade, Shenyang No. 5 Reagent Factory, China) and citric acid (analytical grade, Shenyang No. 5 Reagent Factory, China) as starting chemicals. First, Stoichiometric amounts of Fe(NO3 )3 , citric acid and magnetic fly-ash hollow cenosphere were weighed precisely, mixed and put into a ball mill with 240 revolutions per min for 2 hr at room temperature, in which the ZrO2 milling balls of the XDQM changeable rate planetary ball grinder were weighed accurately according to a milling balls/starting chemicals mass ratio of 6:l, with the small balls (3 mm)/big balls (6 mm) with a mass ratio of l:1. A kind of viscid substance was got. The viscid substance was dried at 60°C for 2 hr. Finally, the samples were calcined at 400°C in air for 1 hr. The final product was core-nanoshell composite powders.
the first endothermic peaks is at 166.2°C accompanied, the decomposition of citric acid may occur, because the citric acid begin to decompose between 160–180°C according to the different purity of the citric acid. There is a great exothermic peak at about 200–270°C. The biggest exothermic peak appears at 253.4°C, in which the strong solid-state reaction occurs with yielding H2 O, CO and CO2 . This pyrolysis process was confirmed by the TGDTA curves. A stable temperature is lower than 400°C, which attained for the core-nanoshell composite absorbers. Figure 3 shows the XRD patterns of samples. After calcined, the sample contained the magnetic fly-ash hollow cenosphere and γ-Fe2 O3 . In addition, from the XRD peaks, the average grain size (D) is calculated using the 180
2 Results and discussion
0
140 120
-2
TG
100 80
-4
60 40
-6
20
1.3 Characterization
0 166.2ć
-20 100
200
300
Fig. 2
400
500
600
700
180
Magnetic fly-ash hollow cenosphere γ-Fe2O3
160
The pyrolysis processes of the sample were investigated by TG-DTA curve (Fig. 2). Two main regions occur at 130–180°C and 200–270°C. The DTA curve shows that
100
120
80
100
60
80
40
60
20
20 20
180 160 140 120
140
40
2.1 TG-DTA curves of sample analysis
-8 800
Tempreture (ć) TG-DTA curve of the sample.
200
Intensity (a.u.)
The experimental techniques employed in our investigations include: the thermal analysis was carried out by thermogravimetry and differential thermal analysis (TGDTA, 449e, Linseis, Germany) with a heating rate of 10°C min in the air; the crystal structure of sample was examined by X-ray diffractometer (XRD, D5005 Bruker, Seifert-SPM, Germany) with Cu Kα radiation; the microstructure was observed by scanning electronic microscope (SEM, JSM-6360LV, JEOL, Japan) and transmission electronic microscope (TEM, Hitachi-800, JEOL, Japan); and microwave absorbing property analysis was conducted by Vector network analyzer (VNA, ZVA40, Rohde & Schwarz, Taiwan, China).
253.4ć DTA
160
ΔT (ć)
Fig. 1
b
Mass lost (mg)
Supplement
30
40
50 60 2θ (degree) Fig. 3 XRD patterns of the samples.
0 -20 70
Journal of Environmental Sciences 2011, 23(Supplement) S74–S77 / Ruxin Che et al.
S76
2.2 TEM and SEM observation
“Scherrer” equation:
D(hkl) =
Vol. 23
kλ βcosθ
(1)
where, λ is the X-ray wavelength employed, θ (degree) is the diffraction angle of the most intense peak, and β is defined as β2 = β2m − β2s , βm and βs are the experimental full width at half maximum (FWHM) and the FWHM of a standard silicon sample, respectively. The as-obtained D is 12 nm for γ-Fe2 O3 particle. The D of composite phases cannot be calculated directly by this method due to overlapping of the two sets of diffraction peaks. When calcined temperature is lower than 400°C , materials mostly consist of soft magnet γ-Fe2 O3 , which is metastable state. The exchange coupling happens between γ-Fe2 O3 and ferrite of the magnetic fly-ash hollow cenosphere. Exchange coupling is close quarters action, its range of effective exchange coupling equal to the thickness of magnetic domain wall (about 10 nm). When grain size is about 10 nm, the effect of remanence increasing is notable, this shows that controlling the calcined temperature is favourable to the magnetic loss.
It is observed from the transmission electron microscope (TEM) and scanning electron microscope (SEM) morphologies as shown in Figs. 4 and 5, that the core-nanoshell composite particles are spherical with welldefined core/shell structures. The core is magnetic fly-ash hollow cenosphere, and the shell is the exchange-coupling ferrite which was composed by ferrite of the fly-ash hollow cenosphere and γ-Fe2 O3 nanoparticles. In view of an EM wave absorbent, the “core-nanoshell” structure of particle would be perfect to absorb EM wave energy by the magnetic/dielectric losses and their appropriately matching (Panagopoulos and Georgiou, 2009). 2.3 Microwave absorbing properties of the corenanoshell composite particles Table 1 is the sample serial number. The relationship of absorbing effectiveness and frequency on samples has been showed in Fig. 6. It shows the changes of absorbing effectiveness with frequency on samples. The absorbing effectiveness of samples have a difference at the same frequency because the component of absorbent is different. The absorbing effectiveness of the single material has some changes with frequency rise, but the composite particles have obvious changes. Microwave absorbing
b a 0.5 μm 0.5 μm Fig. 4 The TEM image of cenospheres before and after being coated. (a) before coated; (b) after coated.
0.5 μm
a
0.5 μm
b
Fig. 5 The SEM image of cenospheres before and after being coated. (a) before coated; (b) after coated.
Supplement
Preparation and microwave absorbing properties of the core-nanoshell composite absorbers······ Table 1
Sample discription
Sample serial number
Absorbent name
Sample 1 Sample 2 Sample 3 Sample 4
Fly-ash hollow cenosphere Magnetic fly-ash hollow cenosphere γ-Fe2 O3 Magnetic fly-ash hollow cenosphere (had not been pre-treated)+ γ-Fe2 O3 Magnetic fly-ash hollow cenosphere (had been pre-treated) + γ-Fe2 O3
Sample 5
Absorbing effectiveness (dB)
-5 -10 -15 Sample 1 Sample 2 Sample 3 Sample 4 Sample 5
-25 -30 -35
0
200
dipole polarization is the dominant at 1 MHz and 1 GHz frequency and the weak space charge polarization mainly works at higher frequency. Sample 4 had not been pre-treated, so it contain more impurities and the coating is unevenly. Its microwave absorbing properties is poorer than sample 5.
3 Conclusions The core-nanoshell composite absorbers with magnetic fly-ash hollow cenosphere as nuclear were prepared by high-energy ball milling method. The pre-treatment is beneficial to coat evenly and improve the surface activity. The diameter of γ-Fe2 O3 is 12 nm, in which the exchange-coupling interaction happens between them. The exchange-coupling interaction enhance magnetic loss of composite absorbers, so the microwave absorptivity of the core-nanoshell composite absorbers is better than single material. In the frequency between 1 MHz and 1 GHz, the absorbing effectiveness can achieve –30 dB, and it is consistent with requirements of the microwave absorbing material at the low-frequency absorption.
0
-20
S77
400
600
800
1000
Incident wave frequency (MHz) Fig. 6 Relationship of absorbing effectiveness and frequency on samples.
properties of the core-nanoshell composite particles is much stronger than the single material, which possibly implies the existence of exchange coupling action between ferrite of cenosphere and γ-Fe2 O3 nanoparticles. In the core-nanoshell composite particles, two kinds of mechanisms could be used to explain the stronger microwave absorbing properties. The first mechanism is the exchange coupling between γ-Fe2 O3 and ferrite of the magnetic fly-ash hollow cenosphere, its range of effective exchange coupling equal to the thickness of magnetic domain wall (about 10 nm). When grain size is about 10 nm, the effect of remanence increase is notable, so the magnetic loss can be increased, and it is better to improve absorbing properties. The second mechanism is the dipole polarization. The nano-composites is a such perfect system, because the polarized cores can play the role of dipoles, especially at the low-frequency, as demonstrated in the γ-Fe2 O3 nanoparticles which coated the magnetic fly-ash hollow cenosphere. Considering the nano-particles on core/shell-type microstructure, it is reasonable that the
Acknowledgments This work was supported by the Science Research Plan Project of Higher Education Department of Liaoning Province (No. 2008086). The authors would like to thank Dr. Zhihua Zhang for TEM analytical support, Dr. Zhiqiang Li for SEM analysis, and Dr. Zhiyong Jia for VNA measurements.
References Jia Z Y, Wang Q, Zhou M L, 2006. Electromagnetic properties of magnetic fly-ash hollow cenosphere. Journal of Functional Materials, 6(37): 877–879. Petrov V M, Gagulin V V, 2001. Microwave absorbing materials. Inorganic Materials, 37(2): 93–98. Panagopoulos C N, Georgiou E P, 2009. Surface mechanical behaviour of composite Ni-P-fly ash/zincate coated aluminium alloy. Applied Surface Science, 255(13): 6499– 6503. Quan B P, Xu H, Gu H C, Sun T, Zhao H, 2003. Progress in research and application of fly ash particles. Industrial Minerals and Processing, 45(11): 31–33. Zheng H, Shao Q, Leng S W, Sun T, 2008. Palladium-free activation electroless nickel plating on cenosphere surface. Surface Technology, 37(1): 56–58.