silica composites: High-temperature dielectric properties at X-band

Solid State Communications 154 (2013) 64–68

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Tetra-needle zinc oxide/silica composites: High-temperature dielectric properties at X-band Jie Yuan a,b, Wei-Li Song a, Xiao-Yong Fang a,c, Xiao-Ling Shi a, Zhi-Ling Hou a, Mao-Sheng Cao a,n a

School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, People’s Republic of China School of Information Engineering, Minzu University of China, Beijing 100081, People’s Republic of China c School of Science, Yanshan University, Qinhuangdao 066004, People’s Republic of China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 September 2012 Received in revised form 15 October 2012 Accepted 18 October 2012 by F. Peeters Available online 26 October 2012

Nano-structural ZnO of unique electrical and optical properties is highly attractive in dielectric and microwave applications, specifically considered as ideal fillers for achieving light-weight composites. In this work, tetra-needle ZnO (T-ZnO) by a direct combustion approach was selected as the filler in silica nanopowders for fabricating T-ZnO/SiO2 composites. Temperature effects on dielectric properties of the composites were investigated in the temperature range of 300–700 1C at 8.2–12.4 GHz. The observed imaginary permittivity and loss tangent was found to be temperature dependent, and the mechanism associated with relaxation and electrically conductive loss was discussed. & 2012 Elsevier Ltd. All rights reserved.

Keywords: A. Zinc oxide D. High temperature D. Dielectric properties D. Loss tangent

1. Introduction: ZnO nanostructures, such as nanowires or nanorods [1–3], nanobelts [4], nanospings [5], nanorings [6], nanobows [7], nanocables [8], nano-tetra-needle or nanomulti-needle [9,10], nano-cagelike [11] are well-known for special electrical, optical, piezoelectric and pyroelectric properties [1–17]. Recently, lightweight ZnO-based nanomaterials have been found to offer unique dielectric and microwave absorption properties and their related great potential applications have attracted much attention [18–25]. For instance, Chen et al. [18] fabricated ZnO nanowires and investigated their microwave absorption, which has been proposed to be influenced by micro-antenna radiation and interface polarization. Cao and coworkers [19–21] reported the dielectric and microwave absorption properties of the cage-like ZnO composites, demonstrating impressive microwave attenuation by the combined effects of interface scattering, micro-current attenuation, micro-antenna radiation and dielectric relaxation. In the work by Yan and coworkers [22,23], ZnO nanocombs and nanorods with different morphologies were synthesized, demonstrating strong microwave absorption with a peak up to 12 dB at 11 GHz in the as-prepared ZnO nanocombs. Guo and coworkers

n

Corresponding author. E-mail addresses: [email protected] (J. Yuan), [email protected] (M.-S. Cao). 0038-1098/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2012.10.028

[24] grew radial ZnO nanowires on prism tips with enhanced dielectric constants at a low percolation threshold. Zhang and coworkers [25] fabricated three-dimensional ZnO micro/nanorod networks with dramatic improvement in dielectric properties and microwave absorption. On the other hand, the exploration of ZnObased hetero-structures has provided an important approach for achieving particular performance in dielectric and microwave absorption properties [26–29]. For example, Liu and coworkers [26,27] prepared ZnO-coated iron nanocapsules with strong natural resonance and dipolar polarization. Deng and coworkers [28] synthesized Zn–ZnO core-shell structures with an enhancement in dielectric properties. In the work by Song et al. [29] ZnO nanoparticles immobilized on the carbon nanotube surface were found to produce resonance in the ZnO/carbon nanotube interface, resulting in widening microwave absorption band. Among various influence factors of the dielectric and microwaveabsorption properties, temperature effect has been currently wellmaterialized in some typical dielectric/semi-conductive materials [30–39], such as ferroferric oxide [31], manganese dioxide [32], carbon fibers [33], carbon nanotubes [34–36], boron nitride [37], chromium oxide [38], etc. For example, increasing complex permittivity with growing temperature, also known as positive temperature effect, has been observed in both carbon nanotubes and carbon fibers [33,34], thus offering promising potential applications in certain extreme environment where thermal stability coupled with high performance at high temperature is required. Apparently, implication of the high-temperature dielectric measurement of ZnO (morphology

J. Yuan et al. / Solid State Communications 154 (2013) 64–68

unknown) in the MHz range [39] suggests more room for investigating temperature effects on ZnO-based materials. In the work reported here, tetra-needle ZnO (T-ZnO) powders fabricated by direct combusting approach were processed with silica nanopowders. The resulting composites were applied to dielectric measurement in the temperature range of 300–700 1C at 8.2–12.4 GHz (X band). Temperature effect was observed in the imaginary permittivity of the composites, suggesting enhancement in dielectric loss with increasing temperature. The related temperature-dependent loss tangent with respect to relaxation loss and electrically conductive loss was discussed.

2. Experimental section 2.1. T-ZnO preparation The T-ZnO in this work was prepared by combusting Zn powders at high temperature [9,10,40]. In a typical procedure, a certain amount of Zn powders were firstly uniformly settled in a quartz boat, followed by putting in the center of an pre-heated one-end sealed horizontal tube furnace. The Zn powders were heated to 950 1C and the quartz boat was slowly pulled out of the furnace until black fume disappeared. The as-prepared samples were collected until cooling to room temperature.

65

dimensions of 22.86 mm  10.16 mm  2 mm. The resulting samples were transferred to a furnace for sintering at 700 1C for 1 h before characterization and measurement. 2.3. Measurement Powder X-ray diffraction (XRD) characterization was carried out on an XPert PRO, Cu-Ka powder diffraction system. Scanning electron microscopy (SEM) images were acquired on a Hitachi S4800 fieldemission SEM. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) was performed on a JEM-2100 system. High-temperature dielectric measurement was applied on an Anritsu 37269D vector network analyzer coupled with a specific heating testing chamber [33]. High-temperature dielectric properties of the samples were carried out at 8.2–12.4 GHz (X-band) in the temperature range of 300–700 1C. In a typical high-temperature dielectric measurement, the samples were vertically settled in the testing chamber. For high-temperature testing, an increment of 20 1C/min was set for raising temperature. Until the targeted temperature was achieved, the heating system was stabilized for 10 min to ensure the testing accuracy. Dielectric measurement was subsequently carried out, followed by increasing temperature with a constant increment (20 1C/min) for reaching the next targeted temperature.

3. Results and discussions 2.2. Nanocomposite fabrication The traditional cold-press approach was applied to fabricate the T-ZnO/SiO2 nanocomposites. In a typical experiment, the asprepared T-ZnO powders and SiO2 xerogel nanopowders fabricated by sol–gel method (20 wt% T-ZnO and 80 wt% SiO2) were homogeneously dispersed in an ethanol solvent (20 ml), followed by 1 h sonication and drying in an oven (80–100 1C). A portion of the dried mixture powders were collected and applied to cold press (20 MPa pressure) for obtaining a rectangular sample with

T-ZnO powders were achieved by direct oxidation of commercially Zn powders without any catalyst. A portion of the resulting T-ZnO powders were used for specimen by microscopy and other techniques. Representative SEM images show that the T-ZnO powders appear tetra-needle shape with four needles on the order of 10 mm or longer in length (Fig. 1a, b). The results from TEM imaging (Fig. 2a) on the one needle of the T-ZnO suggest around 50–100 nm in thickness for the T-ZnO needles. Thus, the aspect ratios of T-ZnO needles are on the order of 100 and larger.

Fig. 1. (a) and (b) SEM images of T-ZnO; (c) SEM image of T-ZnO/SiO2 composites and (d) XRD spectra of T-ZnO.

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Fig. 2. (a) TEM image of a T-ZnO needle; (b) high-resolution TEM of T-ZnO; and (c) SAED patterns of T-ZnO.

Powder X-ray diffraction was applied to T-ZnO powders and the resulting patterns (Fig. 1d) of the T-ZnO were typical wurtzite ZnO in the MDI Jade database, with corresponding lattice parameters of a ¼0.3249 nm and c ¼0.5206 nm. Additionally, characterization results from high-resolution TEM (Fig. 2b) exhibits that T-ZnO has single-crystalline structure with a lattice spacing of 0.52 nm, consistent with XRD results. According to the SAED pattern (Fig. 2c), T-ZnO grows along with [0 0 0 2] as the preferential direction (Fig. 2b). For composite fabrication, SiO2 xerogel nanopowders (SiO2) prepared from a sol–gel method were selected as the matrix due to their excellent thermal stability and low thermal expansion at high temperature. In the fabrication of the T-ZnO/SiO2 composites, the homogenous blend of SiO2 and T-ZnO powders was achieved via a combination of sonication and evaporation process, followed by the cold press to form rectangular samples. The cross section of the resulting samples was applied to SEM imaging (Fig. 1c), exhibiting that T-ZnO was homogenously dispersed in SiO2 powders as expectation. The high-temperature dielectric properties in terms of complex permittivity (real permittivity, e0 ; imaginary permittivity, e00 ) of the T-ZnO/SiO2 composites were evaluated by wave-guide measurement at X-band. Shown in Fig. 3 are the dielectric properties of the composites at 300, 500 and 700 1C. Interestingly, temperature was found to be a different factor in real and imaginary permittivity. The plots in Fig. 4 demonstrate the dielectric properties vs. temperature at five selected frequencies (8.2, 9.2, 10.2, 11.2 and 12.2 GHz). The comparison between real and imaginary permittivity suggests that real permittivity is much less sensitive to temperature while imaginary permittivity is monotonically dependent on temperature (Fig. 4). For example, slight change was observed in the real permittivity at 8.2 GHz when temperature increases from 300 to 700 1C (Fig. 4a). However, imaginary permittivity ( 0.03 at 300 1C) enhances to  0.13 at 700 1C. In this case, corresponding loss tangent (ratio of imaginary permittivity-to-real permittivity) found to be temperature-dependent also increases to 0.06, approximately 3-fold enhancement from 300 to 700 1C (Fig. 4b). According to Debye theory, real permittivity and imaginary permittivity associated with relaxation loss and electrically

Fig. 3. (a) Real and imaginary permittivity and (b) loss tangent of T-ZnO/SiO2 composites at (-&-) 300 1C, (-J-) 500 1C and (-B-) 700 1C.

conductive loss is known as

e0 ¼ e1 þ

es e1 1 þ o2 t2

e00 ¼ ðes e1 Þ

ot 1þ o2 t2

ð1Þ

þ

s e0 o

ð2Þ

where s is temperature-dependent electrical conductivity, o the angular frequency, t the polarized relaxation time, es the static permittivity and eN the high-frequency permittivity. Possibly, the relaxation loss with respect to polarization is a reason for the influence of dielectric loss. On the other hand, T-ZnO could be considered as an n-type semiconductor due to the oxygen vacancies in the nanostructure, which is responsible for positive charge migration. The velocity for the charge mobility produced by electronic attracting is as sffiffiffiffiffiffiffiffiffiffiffi 3kB T v¼ ð3Þ mne Here, kB is Boltzmann’s constant, mne the effective mass of an electron, and T the absolute temperature. As the positive charge center, the charge of each oxygen vacancy is suggested to be 2e, and these charges are favorable to form scattering when they are close enough to deviate due to the internal electronic repulsion around oxygen vacancies. Therefore, the scattering possibility of

J. Yuan et al. / Solid State Communications 154 (2013) 64–68

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Scheme 1. Schematic illustration of the micro-current in the ZnO conductive network.

where _oopt is the mean energy of optical phonons, and es and eopt is the static-state and high-frequency permittivity of ZnO, respectively. According to the relationship between scattering possibility and mobility for charge carriers, the scattering possibility of the positive carriers for ZnO is     1 mn 1 1 mn A B ð7Þ ¼ e þ þ _o =k T ¼ e 3=2 m e tion topt e T e opt B 1 The band-gap width of ZnO (Eg/h) is considered to be larger than microwave energy, thus the intrinsic carrier concentration to be  n n 3=4 3=2 me mh ð2pkB T Þ n0 ¼ eEg =2kB T ¼ CT 3=2 eEg =2kB T ð8Þ 3 4p3 _ Here, mnh is the effective mass of a hole. Therefore, the intrinsic carrier concentration increases with growing temperature. The electrical conductivity, on the other hand, could be considered as follows:

s ¼ n0 em

ð9Þ

Entering Eqs. (7) and (8) into Eq. (9), the electrical conductivity can be written as

s¼

e2 CT 3=2 eEg =kB T A mne 3=2 þ _ooptB=kT T e

Fig. 4. (a) Real and imaginary permittivity and (b) loss tangent of T-ZnO/SiO2 composites at (-&-) 8.2 GHz, (-J-) 9.2 GHz, (-D-) 10.2 GHz, (-B-) 11.2 GHz and (-) 12.2 GHz.

the positive charge center for a charge carrier follows [41] 2 !2 3 n 2 1 nion e4 p m e e v s 0 e 5 ¼ ln41 þ 1=3 tion 2pmne e0 es 2 v3 nion e2

ð4Þ

where nion is the concentration of oxygen vacancies. In Eq. (4), due to the slow change of the function, enter Eq. (3) into Eq. (4) and thus the scattering possibility is 1

tion

¼



nion e4

2p mne e0 es

2

u3

¼ AT 3=2

ð5Þ

Thus, scattering possibility decreases with increasing temperature so that directional mobility for the charges increases, which is responsible for the increase of electrical conductivity. For polar compound semiconductors, the charge carriers might be scattered by long optical wave and the corresponding scattering possibility is [41] 1

topt

¼

 1=2   e2 2mne _oopt 1 1 B  ¼ _o =k T   e opt B 1 4pe0 _2 e_oopt =kB T 1 eopt es

ð6Þ

ð10Þ

1

where _oopt is calculated to be 75–100 meV [42]. Therefore, the results from Eqs. (2) and (10) demonstrate that the enhancement of both electrical conductivity and corresponding dielectric loss in T-ZnO is mainly on the basis of improved charge mobility and increased intrinsic carrier concentration when temperature increases. Additionally, no direct influence of DC electrical conductivity was found in real permittivity according to Eq. (1), suggesting real permittivity less sensitive to temperature. Besides the microstructural effects, various influences on bulk composites may also result in the dielectric loss. Typically, electric conductive network (Scheme 1) can be easily formed by such tetra-needle structures with large aspect ratios in the needles [19]. In this regard, the as-established network obviously provides effective electrically conductive loss in electromagnetic field [19–21], specifically due to the enhanced electrical conduction in the high temperature range. As already suggested by the results here, increasing temperature is responsible for the enhanced dielectric loss and imaginary permittivity. 4. Conclusions In summary, ZnO processed by direct combusting Zn powders appears tetra-needle with large aspect ratio in each needle. T-ZnO was dispersed in the SiO2 matrix to give nanocomposites with

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enhanced imaginary permittivity with increasing temperature at 300–700 1C. The influence of oxygen vacancies in regard of relaxation loss and electrically conductive loss was taken into account in the temperature-dependent loss tangent. These results demonstrate temperature effects on the dielectric properties, offering strategies for designing dielectric or microwave devices for high-temperature applications.

Acknowledgment This work was supported by the National Science Foundation of China (Grant Nos. 51132002, 50872159 and 50972014). References [1] L. Guo, Y.L. Ji, H.B. Xu, J. Am. Chem. Soc. 124 (2002) 14864–14865. [2] Q. Wan, K. Yu, T.H. Wang, C.L. Lin, Appl. Phys. Lett. 83 (2003) 2253–2255. [3] Y.W. Zhu, H.Z. Zhang, X.C. Sun, S.Q. Feng, J. Xu, Q. Zhao, B. Xiang, R.M. Wang, D.P. Yu, Appl. Phys. Lett. 83 (2003) 144–146. [4] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947–1949. [5] X.Y. Kong, Z.L. Wang, Appl. Phys. Lett. 84 (2004) 975–977. [6] X.Y. Kong, Y. Ding, R.S. Yang, Z.L. Wang, Science 303 (2004) 1348–1351. [7] W.L. Hughes, Z.L. Wang, J. Am. Chem. Soc. 126 (2004) 6703–6709. [8] J.J. Wu, S.C. Liu, C.T. Wu, K.H. Chen, L.C. Chen, Appl. Phys. Lett. 81 (2002) 1312–1314. [9] Y.N. Zhao, M.S. Cao, J.G. Li, Y.J. Chen, Mater. Res. Bull. 40 (2005) 1745–1750. [10] Y.N. Zhao, M.S. Cao, H.B. Jin, L. Zhang, C.J. Qiu, Scripta Mater. 54 (2006) 2057–2061. [11] Y.N. Zhao, M.S. Cao, H.B. Jin, X.L. Shi, X. Li, S. Agathopoulos, J. Nanosci. Nanotechnol. 6 (2006) 2525–2528. [12] A.H. Macdonald, P. Schiffer, N. Samarth, Nat. Mater. 4 (2005) 195–202. [13] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897–1899. [14] M.J. Lockyear, A.P. Hibbins, J.R. Sambles, P.A. Hobson, C.R. Lawrence, Appl. Phys. Lett. 94 (2009) 041913. [15] J.W. Li, S.Z. Ma, X.J. Liu, Z.F. Zhou, C.Q. Sun, Chem. Rev. 112 (2012) 2833–2852. [16] J. Joo, B.Y. Chow, M. Prakash, E.S. Boyden, J.M. Jacobson, Nat. Mater. 10 (2011) 596–601. [17] K. Chung, C.H. Lee, G.C. Yi, Science 330 (2010) 655–657.

[18] Y.J. Chen, M.S. Cao, T.H. Wang, Q. Wan, Appl. Phys. Lett. 84 (2004) 3367–3369. [19] M.S. Cao, X.L. Shi, X.Y. Fang, H.B. Jin, Z.L. Hou, W. Zhou, Y.J. Chen, Appl. Phys. Lett. 91 (2007) 203110. [20] X.Y. Fang, X.L. Shi, M.S. Cao, J. Yuan, J. Appl. Phys. 104 (2008) 096101. [21] X.Y. Fang, M.S. Cao, X.L. Shi, Z.L. Hou, W.L. Song, J. Yuan, J. Appl. Phys. 107 (2010) 054304. [22] R.F. Zhuo, H.T. Feng, J.T. Chen, D. Yan, J.J. Feng, H.J. Li, B.S. Geng, S. Cheng, X.Y. Xu, P.X. Yan, J. Phys. Chem. C 112 (2008) 11767–11775. [23] R.F. Zhuo, H.T. Feng, Q. Liang, J.Z. Liu, J.T. Chen, J.J. Feng, H.J. Li, S. Cheng, B.S. Geng, X.Y. Xu, J. Wang, Z.G. Wu, P.X. Yan, G.H. Yue, J. Phys. D–Appl. Phys. 41 (2008) 185405. [24] G.S. Wang, Y. Deng, Y. Xiang, L. Guo, Adv. Funct. Mater. 18 (2008) 2584–2592. [25] H.F. Li, Y.H. Huang, G.B. Sun, X.Q. Yan, Y. Yang, J. Wang, Y. Zhang, J. Phys. Chem. C 114 (2010) 10088–10091. [26] X.G. Liu, D.Y. Geng, P.J. Shang, H. Meng, F. Yang, B. Li, D.J. Kang, Z.D. Zhang, J. Phys. D—Appl. Phys. 41 (2008) 175006. [27] X.G. Liu, D.Y. Geng, H. Meng, P.J. Shang, Z.D. Zhang, Appl. Phys. Lett. 92 (2008) 173117. [28] Y. Zhang, Y. Wang, Y. Deng, M. Li, J.B. Bai, ACS Appl. Mater. Inter. 4 (2012) 65–68. [29] W.L. Song, M.S. Cao, B. Wen, Z.L. Hou, J. Cheng, J. Yuan, Mater. Res. Bull. 47 (2012) 1747–1754. [30] M.S. Cao, Z.L. Hou, J. Yuan, L.T. Xiong, X.L. Shi, J. Appl. Phys. 105 (2009) 106102. [31] M. Hotta, M. Hayashi, K. Nagata, ISIJ Int. 51 (2011) 491–497. [32] X.L. Shi, M.S. Cao, X.Y. Fang, J. Yuan, Y.Q. Kang, W.L. Song, Appl. Phys. Lett. 93 (2008) 223112. [33] M.S. Cao, W.L. Song, Z.L. Hou, B. Wen, J. Yuan, Carbon 48 (2010) 788–796. [34] W.L. Song, M.S. Cao, Z.L. Hou, J. Yuan, X.Y. Fang, Scripta Mater. 61 (2009) 201–204. [35] S.E. San, Y. Yerli, M. Okutan, F. Yılmaz, O. Gunaydın, Y. Hamesc, Mater. Sci. Eng. B 138 (2007) 284–288. [36] W.L. Song, M.S. Cao, Z.L. Hou, X.Y. Fang., X.L. Shi, J. Yuan, Appl. Phys. Lett. 94 (2009) 233110. [37] Z.L. Hou, M.S. Cao, J. Yuan, X.Y. Fang, X.L. Shi, J Appl. Phys. 105 (2008) 076103. [38] D.C. Dube, D. Agrawal, S. Agrawal, R. Roy, Appl. Phys. Lett. 90 (2007) 124105. [39] T.A. Baeraky, Egypt. J. Solids 30 (2007) 13–18. [40] J. Grabowska, A. Meaney, K.K. Nanda, J.P. Mosnier, M.O. Henry, J.R. Duclere, E. McGlynn, Phys. Rev. B 71 (2005) 115439. [41] J.J. Wang, X.Y. Fang, G.Y. Feng, W.L. Song, Z.L. Hou, H.B. Jin, J. Yuan, M.S. Cao, Phys. Lett. A 374 (2010) 2286–2289. [42] L.N. Wang, X.Y. Fang, Z.L. Hou, Y.L. Li, K. Wang, J. Yuan, M.S. Cao, Chin. Phys. Lett. 28 (2011) 027101.