Elsevier

Advanced Powder Technology

Volume 25, Issue 6, November 2014, Pages 1761-1766
Advanced Powder Technology

Original Research Paper
Preparation and microwave absorption of porous hollow ZnO by CO2 soft-template

https://doi.org/10.1016/j.apt.2014.07.006Get rights and content

Highlights

  • The porous hollow ZnO architectures were prepared by a two-step method.

  • The porous hollow ZnO/paraffin wax composite shows excellent microwave absorption.

  • The minimum reflection loss of porous hollow ZnO/paraffin wax was −36.3 dB at 12.8 GHz.

  • The excellent properties were attributed to impedance match and interference of multi-reflected microwaves.

Abstract

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.

Graphical abstract

The porous hollow ZnO was prepared by calcination of ZnCO3 precursor at 450 °C. The porous hollow ZnO wax-composites show excellent microwave absorption with a minimum reflection loss of −36.3 dB at 12.8 GHz for a weight fraction of 25%.

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Introduction

In recent years, electromagnetic wave absorbing materials have aroused great interest because of more and more applications in civil, commercial and military [1], [2], [3]. Conventional microwave absorbing materials [4], [5], [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 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 nanowire-polyester 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], [18], [19], [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.

Section snippets

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

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 μm were clearly observed. It can be found that the hollow microstructures were constructed by many irregular particles.

To determine the appropriate

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 

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