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Preparation and microwave absorption of porous hollow ZnO by CO2 soft-template
Advanced Powder Technology 25 (2014) 1761–1766
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Preparation and microwave absorption of porous hollow ZnO by CO2 soft-template Biao Zhao a, Gang Shao a, Bingbing Fan a, Yajun Xie a, Bing Sun a, Rui Zhang a,b,⇑ a b
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, Henan 450001, PR China Zhengzhou Aeronautical Institute of Industry Management, Zhengzhou, Henan 450046, PR China
a r t i c l e
i n f o
Article history: Received 27 March 2014 Received in revised form 16 June 2014 Accepted 4 July 2014 Available online 19 July 2014 Keywords: ZnCO3 precursor Porous hollow ZnO CO2 soft template Dielectric loss Microwave absorption
a b s t r a c t The porous hollow ZnO samples were prepared by calcination of ZnCO3 precursor at 450 °C. The structural properties were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), thermogravimetric analysis and differential thermal analysis (TG-DTA). A possible mechanism for the formation of porous hollow microstructure was proposed. The microwave absorption properties of the porous hollow structural ZnO have been investigated. The reflection loss (RL) of the ZnO was calculated based on the relative complex permeability and permittivity. A minimum reflection loss of the wax-composite with 25 wt% porous hollow ZnO is 36.3 dB at 12.8 GHz with a thickness of 4.0 mm. The results indicate that porous hollow structural ZnO can be used as a desirable material for the microwave absorption. Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction In recent years, electromagnetic wave absorbing materials have aroused great interest because of more and more applications in civil, commercial and military [1–3]. Conventional microwave absorbing materials [4–6] were widely investigated and used, but the high weight of such materials severely inhibits their future applications. The microwave absorbing materials with relatively light weight, flexible preparation and strong absorption in a wide band range, are in a high demand nowadays [7]. Therefore, it is urged to seek new types of microwave absorbing materials to satisfy the high demand of the new developments on microwave science and technology. As an important semiconductor with a wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature, Zinic oxide has stimulated a wide range of research interest. ZnO is a versatile functional material, which has wide applications, such as room-temperature ultraviolet (UV) lasers [8], field-effect transistors [9], photodetectors [10], solar cells [11], gas sensors [12] and so forth. A few recent studies showed that Zinic oxide could
⇑ Corresponding author at: School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, Henan 450001, PR China. Tel.: +86 371 60632007; fax: +86 371 60632600. E-mail address: [email protected] (R. Zhang).
also be used as a microwave absorption material. Zhou has studied microwave absorption property of tetra-needle-like ZnO whisker and its polyurethane composites coating. It was shown that microwave absorption was affected by the electric resistivity, the aspect ratio and the contents of the whiskers [13]. Cao et al. found that cage-like ZnO nanostructures exhibit relative strong microwave absorption in the X band, compared with ZnO nanoparticles [14]. The microwave absorption properties of the ZnO nanowirepolyester composites were also reported [15]. Zhang and co-workers have successfully synthesized the crossed ZnO netlike micro/ nanostructures through the direct evaporation of metal zinc on a Si (0 0 1) substrate, the value of minimum reflection loss for the composite with 50 vol% ZnO netlike micro/nanostructures is 37 dB at 6.2 GHz [16]. Some researches about microwave absorption of ZnO/magnetic material composites with different morphologies have also been reported [17–20]. However, to the best of our knowledge, the microwave absorption of porous hollow structural ZnO was not investigated. In this work, a simple, mild self-assembly approach containing subsequent thermal calcination to prepare porous hollow ZnO architectures is reported. Firstly, zinc carbonate ZnCO3 is prepared by a hydrothermal reaction. Then, the precursor ZnCO3 can be easily converted to the porous hollow ZnO by the calcination process. The electromagnetic wave absorption of porous hollow ZnO is also investigated.
http://dx.doi.org/10.1016/j.apt.2014.07.006 0921-8831/Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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2. Experimental section 2.1. Preparation of porous hollow ZnO All reagents were analytically pure and used without further purification. In the typical procedure, 0.01 mol Zn(CH3COO)2 and 0.03 mol NH4HCO3 were dissolved in 60 ml distilled water, respectively, stirred for several minutes to form the homogeneous solution. The pH value of the mixture has been kept in the range of 7–8. Then the mixture was transferred to a Teflon-lined stainless steel autoclave and heated at 110 °C for 8 h. The obtained suspension was allowed to cool naturally to ambient temperature. The final white precipitates were collected, washed with distilled water and ethanol several times, and dried at 80 °C for 5 h. The precursor was calcined in air at 450 °C for 2 h with a heating rate of 5 °C/min to obtain final ZnO product. 2.2. Characterization The crystalline phases of the samples were characterized using the X-ray diffraction ((XD-3, Beijing Purkinje General Instrument Co, Ltd.) technique with a Cu Ka radiation of wavelength k = 1.54056 Å. The morphologies of the products were observed by field emission scanning electron microscopy (FE-SEM, JEOL JSM7001F) operated at 15 kV. The element composition was carried out by an energy dispersive X-ray spectroscopy (EDS, Oxford Instruments) associated with FESEM. The thermogravimetric (TG) and differential thermal analysis (DTA) were carried out on a STA 409/PC thermal analyzer (Netzsch, Germany) with a heating rate of 10 °C/ min in flowing air. The transmission/reflection method was applied to measure the relative complex permeability and permittivity of the sample/wax composites through an Agilent N5244 vector network analyzer system over the frequency range of 2–18 GHz. The cylindrical toroidal samples, with a 3.04 mm inner diameter, 7.00 mm outer diameter, were fabricated by uniformly mixing the absorbents with wax in various mass ratios (e.g. 1:3, 1:1, 3:1) and then pressed into cylindrical compacts. To disclose the microwave absorption properties of the as-synthesized product, the reflection loss (RL) values of porous hollow ZnO was calculated using relative complex permeability and permittivity at a given frequency and thickness layer based on the transmit line theory, which was summarized as the following equations [21]
rffiffiffiffiffi Z in ¼ Z 0
lr 2pfd pffiffiffiffiffiffiffiffiffi lr er tan h j c er
Z in Z 0 RL ¼ 20 log10 Z in þ Z 0 where er ¼ e0 je00 ; lr ¼ l0 jl00 , c is the velocity of light in free space, d is the thickness of the absorber, f is the microwave frequency, Z0 is the impedance of air, and Zin is the input impedance of the absorber. 3. Results and discussion Fig. 1a shows the X-ray diffraction (XRD) pattern of the as-prepared precursor. According to JCPDS card no. 08-0449, the precursor product is a hexagonal zinc carbonate phase ZnCO3, No characteristic peaks were observed for other impurities. Fig. 1b shows the FESEM image of precursor ZnCO3. The hollow structural precursors with the diameter of 8–15 lm were clearly observed. It can be found that the hollow microstructures were constructed by many irregular particles.
To determine the appropriate temperature of the calcination of precursor, the thermal behavior of precursor in air has been investigated. The curves of thermogravimetric analysis (TG) and differential thermal analysis (DTA) are shown in Fig. 2a. It can be seen that there is an obvious endothermic peak on the DTA curve and a corresponding sharp mass loss around 420 °C, indicate that the decomposition temperature of the precursor is around 420 °C. In addition, the decrease percentage of precursor is 36.58%, which is nearly consistent with the theoretical value of 35.10% calculated from the Eq. (4). According to the DTA and TG curves, we choose a temperature of 450 °C for thermal treatment of the precursor to ensure it was decomposed completely. From Fig. 2b, after calcination of the precursor, all diffraction peaks were in agreement with JCPDS card no. 36-1451, which have hexagonal wurtzite phase with space group P63mc and lattice parameters of a = 3.25 Å and c = 5.21 Å. The ZnO phase was well crystallized. No other diffraction peaks have been detected, which indicate that no impurity existed and the precursor has completely transformed into the ZnO phase. The morphologies of the ZnO samples were revealed in Fig. 3 (a–c). It can be observed form Fig. 3a and b that the as-obtained products are mainly composed of hollow microspheres with the similar diameter of precursor ZnCO3. The high-magnification FESEM images (Fig. 3c and inset) show that the hollow ZnO possesses porous surfaces, which are generated from the calcination process. From spectra of Fig. 3d, EDS pattern shows that the porous hollow microstructure consist of Zn and O elements, and the C element is ascribed to the rubberized fabric, which is used for pasting the samples, Pt peaks are also observed in the spectrum because the SEM sample was prepared by sputtering of platinum onto the sample. It can be calculated that the molar ratio of Zn and O is 1:1.087, which is nearly consistent with 1:1. Thus it can be concluded that ZnO is the unique product by the calcination of precursor. The gas bubble as the soft template to prepare hollow structures is a widely application method. Some groups [22–24] have reported that hollow CaCO3, CoOOH and ZnS could be successfully prepared by using CO2, O2 and H2S bubble as soft templates. Under the conditions of our experiments, the following reactions are thought to be occurred in the process of porous hollow ZnO
NH4 HCO3 ! NHþ4 þ HCO3
ð1Þ
ZnðCH3 COOÞ2 ! Zn2þ þ 2CH3 COO
ð2Þ
Zn2þ þ 2HCO3 ! ZnCO3 þ CO2 þ H2 O
ð3Þ
ZnCO3 ! ZnO þ CO2
ð4Þ
From the above reactions, CO2 could be generated along with preparing the precursor ZnCO3. The hollow structure could be obtained using the bubble CO2 as soft-template. The formation mechanism of hollow ZnCO3 was shown in Fig. 4, which is similar to previous paper [25]. According to the above equation, ZnCO3 precursors and CO2 bubble were generated in the mixture solution. Meanwhile, the core-shell structure was formed by assembling of ZnCO3 on the surface of bubble. Then, the CO2 bubble was released on the procedure or drying. Thus, the hollow microstructure was obtained. Finally, porous hollow ZnO can be obtained by the release of CO2 along with the calcination (Eq. (4)). ZnO is a typical dielectric loss material for microwave. To the best of our knowledge, the microwave absorption properties of the porous hollow ZnO have not been studied. Here, the microwave absorption properties of the novel porous hollow ZnO were investigated. Fig. 5a and b shows the frequency dependence of the real part (e0 ) and the imaginary part (e00 ) of the mixture of porous hollow ZnO and paraffin wax with different weight fractions. When the
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Fig. 1. (a) XRD pattern and (b) FESEM image of the precursor ZnCO3.
Fig. 2. (a) TG and DTA curves of the precursor ZnCO3 and (b) XRD pattern of target porous hollow ZnO.
Fig. 3. (a–c) Different magnification SEM of the porous hollow ZnO and (d) the EDS pattern of the ZnO. The inset show the enlarge image.
porous hollow ZnO weight fraction increased from 25% to 75%, e0 increased in the frequency range of 2–18 GHz. The multi-dips with increasing frequency appearing in e0 and e00 for the 25 wt% ZnO waxsamples could be observed. The multi-dips in e00 indicate the high dielectric loss in the the 25 wt% ZnO wax-samples. It is known that the resonance peaks (multi-dips) are usually attributed to polarization [26]. According to dielectric physics, the complex permittivity is derived from electronic polarization, ionic polarization, intrinsic
electric dipole polarization and interfacial polarization. The variation of the permittivity with frequency is mainly related to intrinsic electric dipole polarization and interfacial polarization [27]. At the 25 wt% porous hollow ZnO wax-composite, porous hollow ZnO was distributed uniformly in the paraffin matrix, which can lead to interfacial polarization, space charge accumulation on the surface and relaxation polarization [28]. In EM wave absorbers, currents can be induced through electric dipole polarization and interfacial
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Fig. 4. Schematic illustrating the formation mechanism of porous hollow microstructure.
Fig. 5. Frequency dependence of (a) the real part e0 , (b) imaginary e00 of the complex permittivity and (c) the dielectric loss tangent (tan de) of the wax-composites with various mass ratios of porous hollow powders to the wax.
polarization through the interaction between incident EM waves, the electric field, and mobile electrons. The induced current can be reduced by finite resistivity, which causes heat considered as dielectric loss [29]. As shown in Fig. 5c, the dielectric loss tangent (tande) shows the similar trend with the imaginary part e00 . The 25 wt% porous hollow ZnO presents higher dielectric loss than other samples. The characteristic feature of the ZnO is dielectric materials; the dominant dipolar polarization and the associated relaxation phenomena constitute the loss mechanisms [19]. When the frequency was varied, the dielectric loss owing to the lags of polarization between the interfaces was also considered. The reflection loss (RL) of mixtures of porous hollow ZnO and paraffin wax with different weight fractions were calculated at different thickness, and the results are shown in Fig. 6. The porous hollow ZnO/paraffin wax composite shows a minimum reflection loss of 36.3 dB (99.95% absorption) at 12.8 GHz with a thickness of 4.0 mm for a weight fraction of 25%, and the frequency bandwidth at less than 10 dB is from 12.2 GHz to 14.3 GHz (Fig. 6a). The other samples show weak absorption (Fig. 6b and c). In comparison with ZnO wax-composite, the electromagnetic parameters of the pure wax were also measured. The calculated RL of the pure with the thickness of 1.0–4.0 mm is shown in Fig. 6d. It can be found that the microwave absorption properties of pure wax were
weak and the minimum reflection loss value is only 1.1 dB. However, the minimum reflection loss of other samples is also achieved around the frequency of 12–13 GHz. Through analysis, the dip found at 12–13 GHz corresponds to a matching resonance frequency condition. As shown in Fig. 5, the proper permittivity (impedance match) tuned by mass ratio is necessary to make the composites be a good candidate for EM absorbing coatings. According to the results shown above, incorporation of a relatively small amount of the porous hollow ZnO in wax produces excellent microwave absorption properties. ZnO was randomly dispersed in the wax matrix, which could not connect with each other in the composites. Therefore, the incontinuous local microcurrent network in the composites was formed [14]. Each porous hollow ZnO will generate strong conductive loss, which leads to the attenuation of microwave energy. More importantly, the interfacial electric polarization should be considered. The multi-interfaces between porous hollow ZnO, paraffin matrix, and air bubbles can be beneficial for the microwave absorption because of the interactions of electromagnetic radiation with charge multipoles at the interfaces [16,30]. When the sample was placed under the radiation of electromagnetic wave, the microwave penetrated ZnO shell into the inner hollow space (Fig. 7). It can lead to multiple reflection and diffuse scattering of the incident microwaves, which results in the
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Fig. 6. Frequency dependence of the reflection loss (RL) for the porous hollow ZnO powders/wax composites with the different coating thicknesses in various mass ratios of porous hollow powders to wax of (a) 1:3, (b) 1:1 and (c) 3:1; (d) frequency dependence of the reflection loss (RL) for the pure wax.
hollow ZnO, indicating that there is an appropriate space inside ZnO that allows for multiple reflections of microwaves [31], which increases the propagation paths of the microwaves in the absorbent (Fig. 7).
4. Conclusion In summary, the porous hollow structural ZnO was prepared with a simple, economical method by the calcination of the hollow ZnCO3, which was synthesized using bubble CO2 as soft-template. The formation mechanism of porous hollow microstructure was also discussed. The microwave absorption performances were first found in such porous hollow ZnO/paraffin composites. The maximum reflection loss reaches 36.3 dB at 12.8 GHz with the absorber thickness of 4.0 mm, and the frequency bandwidth at below 10 dB is from 12.2 GHz to 14.3 GHz. It is believed that the excellent microwave absorption from the porous hollow ZnO is due to good impedance match, a combination of the dielectric-type absorption and the interference of multi-reflected microwaves.
References
Fig. 7. A possible mechanism of the electromagnetic wave absorption in porous hollow ZnO.
attenuation of electromagnetic (EM) energy. Otherwise, the porous hollow ZnO possess pores with different sizes, which could be considered to be ‘‘defects’’. The presence of many pores in hollow ZnO could provide another additional pathway for the absorption of electromagnetic wave. The presence of many pores in porous
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Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Preparation and microwave absorption of porous hollow ZnO by CO2 soft-template Biao Zhao a, Gang Shao a, Bingbing Fan a, Yajun Xie a, Bing Sun a, Rui Zhang a,b,⇑ a b
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, Henan 450001, PR China Zhengzhou Aeronautical Institute of Industry Management, Zhengzhou, Henan 450046, PR China
a r t i c l e
i n f o
Article history: Received 27 March 2014 Received in revised form 16 June 2014 Accepted 4 July 2014 Available online 19 July 2014 Keywords: ZnCO3 precursor Porous hollow ZnO CO2 soft template Dielectric loss Microwave absorption
a b s t r a c t The porous hollow ZnO samples were prepared by calcination of ZnCO3 precursor at 450 °C. The structural properties were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), thermogravimetric analysis and differential thermal analysis (TG-DTA). A possible mechanism for the formation of porous hollow microstructure was proposed. The microwave absorption properties of the porous hollow structural ZnO have been investigated. The reflection loss (RL) of the ZnO was calculated based on the relative complex permeability and permittivity. A minimum reflection loss of the wax-composite with 25 wt% porous hollow ZnO is 36.3 dB at 12.8 GHz with a thickness of 4.0 mm. The results indicate that porous hollow structural ZnO can be used as a desirable material for the microwave absorption. Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction In recent years, electromagnetic wave absorbing materials have aroused great interest because of more and more applications in civil, commercial and military [1–3]. Conventional microwave absorbing materials [4–6] were widely investigated and used, but the high weight of such materials severely inhibits their future applications. The microwave absorbing materials with relatively light weight, flexible preparation and strong absorption in a wide band range, are in a high demand nowadays [7]. Therefore, it is urged to seek new types of microwave absorbing materials to satisfy the high demand of the new developments on microwave science and technology. As an important semiconductor with a wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature, Zinic oxide has stimulated a wide range of research interest. ZnO is a versatile functional material, which has wide applications, such as room-temperature ultraviolet (UV) lasers [8], field-effect transistors [9], photodetectors [10], solar cells [11], gas sensors [12] and so forth. A few recent studies showed that Zinic oxide could
⇑ Corresponding author at: School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, Henan 450001, PR China. Tel.: +86 371 60632007; fax: +86 371 60632600. E-mail address: [email protected] (R. Zhang).
also be used as a microwave absorption material. Zhou has studied microwave absorption property of tetra-needle-like ZnO whisker and its polyurethane composites coating. It was shown that microwave absorption was affected by the electric resistivity, the aspect ratio and the contents of the whiskers [13]. Cao et al. found that cage-like ZnO nanostructures exhibit relative strong microwave absorption in the X band, compared with ZnO nanoparticles [14]. The microwave absorption properties of the ZnO nanowirepolyester composites were also reported [15]. Zhang and co-workers have successfully synthesized the crossed ZnO netlike micro/ nanostructures through the direct evaporation of metal zinc on a Si (0 0 1) substrate, the value of minimum reflection loss for the composite with 50 vol% ZnO netlike micro/nanostructures is 37 dB at 6.2 GHz [16]. Some researches about microwave absorption of ZnO/magnetic material composites with different morphologies have also been reported [17–20]. However, to the best of our knowledge, the microwave absorption of porous hollow structural ZnO was not investigated. In this work, a simple, mild self-assembly approach containing subsequent thermal calcination to prepare porous hollow ZnO architectures is reported. Firstly, zinc carbonate ZnCO3 is prepared by a hydrothermal reaction. Then, the precursor ZnCO3 can be easily converted to the porous hollow ZnO by the calcination process. The electromagnetic wave absorption of porous hollow ZnO is also investigated.
http://dx.doi.org/10.1016/j.apt.2014.07.006 0921-8831/Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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B. Zhao et al. / Advanced Powder Technology 25 (2014) 1761–1766
2. Experimental section 2.1. Preparation of porous hollow ZnO All reagents were analytically pure and used without further purification. In the typical procedure, 0.01 mol Zn(CH3COO)2 and 0.03 mol NH4HCO3 were dissolved in 60 ml distilled water, respectively, stirred for several minutes to form the homogeneous solution. The pH value of the mixture has been kept in the range of 7–8. Then the mixture was transferred to a Teflon-lined stainless steel autoclave and heated at 110 °C for 8 h. The obtained suspension was allowed to cool naturally to ambient temperature. The final white precipitates were collected, washed with distilled water and ethanol several times, and dried at 80 °C for 5 h. The precursor was calcined in air at 450 °C for 2 h with a heating rate of 5 °C/min to obtain final ZnO product. 2.2. Characterization The crystalline phases of the samples were characterized using the X-ray diffraction ((XD-3, Beijing Purkinje General Instrument Co, Ltd.) technique with a Cu Ka radiation of wavelength k = 1.54056 Å. The morphologies of the products were observed by field emission scanning electron microscopy (FE-SEM, JEOL JSM7001F) operated at 15 kV. The element composition was carried out by an energy dispersive X-ray spectroscopy (EDS, Oxford Instruments) associated with FESEM. The thermogravimetric (TG) and differential thermal analysis (DTA) were carried out on a STA 409/PC thermal analyzer (Netzsch, Germany) with a heating rate of 10 °C/ min in flowing air. The transmission/reflection method was applied to measure the relative complex permeability and permittivity of the sample/wax composites through an Agilent N5244 vector network analyzer system over the frequency range of 2–18 GHz. The cylindrical toroidal samples, with a 3.04 mm inner diameter, 7.00 mm outer diameter, were fabricated by uniformly mixing the absorbents with wax in various mass ratios (e.g. 1:3, 1:1, 3:1) and then pressed into cylindrical compacts. To disclose the microwave absorption properties of the as-synthesized product, the reflection loss (RL) values of porous hollow ZnO was calculated using relative complex permeability and permittivity at a given frequency and thickness layer based on the transmit line theory, which was summarized as the following equations [21]
rffiffiffiffiffi Z in ¼ Z 0
lr 2pfd pffiffiffiffiffiffiffiffiffi lr er tan h j c er
Z in Z 0 RL ¼ 20 log10 Z in þ Z 0 where er ¼ e0 je00 ; lr ¼ l0 jl00 , c is the velocity of light in free space, d is the thickness of the absorber, f is the microwave frequency, Z0 is the impedance of air, and Zin is the input impedance of the absorber. 3. Results and discussion Fig. 1a shows the X-ray diffraction (XRD) pattern of the as-prepared precursor. According to JCPDS card no. 08-0449, the precursor product is a hexagonal zinc carbonate phase ZnCO3, No characteristic peaks were observed for other impurities. Fig. 1b shows the FESEM image of precursor ZnCO3. The hollow structural precursors with the diameter of 8–15 lm were clearly observed. It can be found that the hollow microstructures were constructed by many irregular particles.
To determine the appropriate temperature of the calcination of precursor, the thermal behavior of precursor in air has been investigated. The curves of thermogravimetric analysis (TG) and differential thermal analysis (DTA) are shown in Fig. 2a. It can be seen that there is an obvious endothermic peak on the DTA curve and a corresponding sharp mass loss around 420 °C, indicate that the decomposition temperature of the precursor is around 420 °C. In addition, the decrease percentage of precursor is 36.58%, which is nearly consistent with the theoretical value of 35.10% calculated from the Eq. (4). According to the DTA and TG curves, we choose a temperature of 450 °C for thermal treatment of the precursor to ensure it was decomposed completely. From Fig. 2b, after calcination of the precursor, all diffraction peaks were in agreement with JCPDS card no. 36-1451, which have hexagonal wurtzite phase with space group P63mc and lattice parameters of a = 3.25 Å and c = 5.21 Å. The ZnO phase was well crystallized. No other diffraction peaks have been detected, which indicate that no impurity existed and the precursor has completely transformed into the ZnO phase. The morphologies of the ZnO samples were revealed in Fig. 3 (a–c). It can be observed form Fig. 3a and b that the as-obtained products are mainly composed of hollow microspheres with the similar diameter of precursor ZnCO3. The high-magnification FESEM images (Fig. 3c and inset) show that the hollow ZnO possesses porous surfaces, which are generated from the calcination process. From spectra of Fig. 3d, EDS pattern shows that the porous hollow microstructure consist of Zn and O elements, and the C element is ascribed to the rubberized fabric, which is used for pasting the samples, Pt peaks are also observed in the spectrum because the SEM sample was prepared by sputtering of platinum onto the sample. It can be calculated that the molar ratio of Zn and O is 1:1.087, which is nearly consistent with 1:1. Thus it can be concluded that ZnO is the unique product by the calcination of precursor. The gas bubble as the soft template to prepare hollow structures is a widely application method. Some groups [22–24] have reported that hollow CaCO3, CoOOH and ZnS could be successfully prepared by using CO2, O2 and H2S bubble as soft templates. Under the conditions of our experiments, the following reactions are thought to be occurred in the process of porous hollow ZnO
NH4 HCO3 ! NHþ4 þ HCO3
ð1Þ
ZnðCH3 COOÞ2 ! Zn2þ þ 2CH3 COO
ð2Þ
Zn2þ þ 2HCO3 ! ZnCO3 þ CO2 þ H2 O
ð3Þ
ZnCO3 ! ZnO þ CO2
ð4Þ
From the above reactions, CO2 could be generated along with preparing the precursor ZnCO3. The hollow structure could be obtained using the bubble CO2 as soft-template. The formation mechanism of hollow ZnCO3 was shown in Fig. 4, which is similar to previous paper [25]. According to the above equation, ZnCO3 precursors and CO2 bubble were generated in the mixture solution. Meanwhile, the core-shell structure was formed by assembling of ZnCO3 on the surface of bubble. Then, the CO2 bubble was released on the procedure or drying. Thus, the hollow microstructure was obtained. Finally, porous hollow ZnO can be obtained by the release of CO2 along with the calcination (Eq. (4)). ZnO is a typical dielectric loss material for microwave. To the best of our knowledge, the microwave absorption properties of the porous hollow ZnO have not been studied. Here, the microwave absorption properties of the novel porous hollow ZnO were investigated. Fig. 5a and b shows the frequency dependence of the real part (e0 ) and the imaginary part (e00 ) of the mixture of porous hollow ZnO and paraffin wax with different weight fractions. When the
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Fig. 1. (a) XRD pattern and (b) FESEM image of the precursor ZnCO3.
Fig. 2. (a) TG and DTA curves of the precursor ZnCO3 and (b) XRD pattern of target porous hollow ZnO.
Fig. 3. (a–c) Different magnification SEM of the porous hollow ZnO and (d) the EDS pattern of the ZnO. The inset show the enlarge image.
porous hollow ZnO weight fraction increased from 25% to 75%, e0 increased in the frequency range of 2–18 GHz. The multi-dips with increasing frequency appearing in e0 and e00 for the 25 wt% ZnO waxsamples could be observed. The multi-dips in e00 indicate the high dielectric loss in the the 25 wt% ZnO wax-samples. It is known that the resonance peaks (multi-dips) are usually attributed to polarization [26]. According to dielectric physics, the complex permittivity is derived from electronic polarization, ionic polarization, intrinsic
electric dipole polarization and interfacial polarization. The variation of the permittivity with frequency is mainly related to intrinsic electric dipole polarization and interfacial polarization [27]. At the 25 wt% porous hollow ZnO wax-composite, porous hollow ZnO was distributed uniformly in the paraffin matrix, which can lead to interfacial polarization, space charge accumulation on the surface and relaxation polarization [28]. In EM wave absorbers, currents can be induced through electric dipole polarization and interfacial
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Fig. 4. Schematic illustrating the formation mechanism of porous hollow microstructure.
Fig. 5. Frequency dependence of (a) the real part e0 , (b) imaginary e00 of the complex permittivity and (c) the dielectric loss tangent (tan de) of the wax-composites with various mass ratios of porous hollow powders to the wax.
polarization through the interaction between incident EM waves, the electric field, and mobile electrons. The induced current can be reduced by finite resistivity, which causes heat considered as dielectric loss [29]. As shown in Fig. 5c, the dielectric loss tangent (tande) shows the similar trend with the imaginary part e00 . The 25 wt% porous hollow ZnO presents higher dielectric loss than other samples. The characteristic feature of the ZnO is dielectric materials; the dominant dipolar polarization and the associated relaxation phenomena constitute the loss mechanisms [19]. When the frequency was varied, the dielectric loss owing to the lags of polarization between the interfaces was also considered. The reflection loss (RL) of mixtures of porous hollow ZnO and paraffin wax with different weight fractions were calculated at different thickness, and the results are shown in Fig. 6. The porous hollow ZnO/paraffin wax composite shows a minimum reflection loss of 36.3 dB (99.95% absorption) at 12.8 GHz with a thickness of 4.0 mm for a weight fraction of 25%, and the frequency bandwidth at less than 10 dB is from 12.2 GHz to 14.3 GHz (Fig. 6a). The other samples show weak absorption (Fig. 6b and c). In comparison with ZnO wax-composite, the electromagnetic parameters of the pure wax were also measured. The calculated RL of the pure with the thickness of 1.0–4.0 mm is shown in Fig. 6d. It can be found that the microwave absorption properties of pure wax were
weak and the minimum reflection loss value is only 1.1 dB. However, the minimum reflection loss of other samples is also achieved around the frequency of 12–13 GHz. Through analysis, the dip found at 12–13 GHz corresponds to a matching resonance frequency condition. As shown in Fig. 5, the proper permittivity (impedance match) tuned by mass ratio is necessary to make the composites be a good candidate for EM absorbing coatings. According to the results shown above, incorporation of a relatively small amount of the porous hollow ZnO in wax produces excellent microwave absorption properties. ZnO was randomly dispersed in the wax matrix, which could not connect with each other in the composites. Therefore, the incontinuous local microcurrent network in the composites was formed [14]. Each porous hollow ZnO will generate strong conductive loss, which leads to the attenuation of microwave energy. More importantly, the interfacial electric polarization should be considered. The multi-interfaces between porous hollow ZnO, paraffin matrix, and air bubbles can be beneficial for the microwave absorption because of the interactions of electromagnetic radiation with charge multipoles at the interfaces [16,30]. When the sample was placed under the radiation of electromagnetic wave, the microwave penetrated ZnO shell into the inner hollow space (Fig. 7). It can lead to multiple reflection and diffuse scattering of the incident microwaves, which results in the
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Fig. 6. Frequency dependence of the reflection loss (RL) for the porous hollow ZnO powders/wax composites with the different coating thicknesses in various mass ratios of porous hollow powders to wax of (a) 1:3, (b) 1:1 and (c) 3:1; (d) frequency dependence of the reflection loss (RL) for the pure wax.
hollow ZnO, indicating that there is an appropriate space inside ZnO that allows for multiple reflections of microwaves [31], which increases the propagation paths of the microwaves in the absorbent (Fig. 7).
4. Conclusion In summary, the porous hollow structural ZnO was prepared with a simple, economical method by the calcination of the hollow ZnCO3, which was synthesized using bubble CO2 as soft-template. The formation mechanism of porous hollow microstructure was also discussed. The microwave absorption performances were first found in such porous hollow ZnO/paraffin composites. The maximum reflection loss reaches 36.3 dB at 12.8 GHz with the absorber thickness of 4.0 mm, and the frequency bandwidth at below 10 dB is from 12.2 GHz to 14.3 GHz. It is believed that the excellent microwave absorption from the porous hollow ZnO is due to good impedance match, a combination of the dielectric-type absorption and the interference of multi-reflected microwaves.
References
Fig. 7. A possible mechanism of the electromagnetic wave absorption in porous hollow ZnO.
attenuation of electromagnetic (EM) energy. Otherwise, the porous hollow ZnO possess pores with different sizes, which could be considered to be ‘‘defects’’. The presence of many pores in hollow ZnO could provide another additional pathway for the absorption of electromagnetic wave. The presence of many pores in porous
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