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High dielectric constant in Al-doped ZnO ceramics using high-pressure treated powders
Journal of Alloys and Compounds 657 (2016) 90e94
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
High dielectric constant in Al-doped ZnO ceramics using highpressure treated powders Xuhai Li, Xiuxia Cao, Liang Xu, Lixin Liu, Yuan Wang, Chuanmin Meng*, Zhigang Wang** National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, Chinese Academy of Engineering Physics, Mianyang 621900, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2015 Received in revised form 18 September 2015 Accepted 8 October 2015 Available online xxx
Pure and Al-doped ZnO ceramics were prepared by conventional sintering high pressure treated nano powders at 1150 C for 2 h, and the effect of Al-doping on the microstructure and dielectric properties were mainly investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results revealed that Al reacted with ZnO and formed ZnAl2O4 nano-precipitates on the grain boundaries. Small amount of ZnAl2O4 nano-precipitates (i.e. < 1 mol%) enhanced the grain growth of ZnO, while larger amount suppressed the grain growth. AC impedance analysis indicated that Al-doping decreases the resistivity of ZnO ceramics. Dielectric studies revealed that the incorporation of Al in ZnO can significantly enhance the dielectric constant (1.39 104 at 1 kHz) of ZnO ceramics. And the dielectric behavior can be well-explained by MaxwelleWagner relaxation. © 2015 Elsevier B.V. All rights reserved.
Keywords: Ceramics Sintering Dielectric response Microstructure High pressure
1. Introduction It is known that the dielectric constant of a material will ultimately decide the level of miniaturization in electronic devices based on capacitive components [1]. In the past decades, ZnO has attracted considerable attention because it can be used for a wide range of applications including dielectrics, owning to its wide direct band gap (3.37 eV) and large exciton binding energy (60 meV). Although the dielectric properties of ZnO nano-structures, films, and ceramics have been investigated by many researchers, further improvements are still required before them become viable for commercial applications in devices [2e11]. Increasing the conductivity of ceramics by introducing conductive particles has been proved to be an effective approach to enhance the dielectric constant of ceramics [12]. Doping with group-III elements can significantly improve the conductivity of ZnO ceramics [13,14]. Among the group-III doped ceramics, Aldoped ZnO emerged as the most promising candidate due to its high temperature stability and the fact that Al is abundant [15]. Recently, Wang et al. [6] investigated the effect of Al doping concentration on the microwave dielectric properties of ZnO powders,
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Meng), [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.jallcom.2015.10.079 0925-8388/© 2015 Elsevier B.V. All rights reserved.
and found that the real permittivity of the prepared powders have been improved in the frequency range of 8.2e12.4 GHz due to the generation of AlZn defects. Zamiri et al. [7] also found that doping with 2 mol% Al could significantly enhance the dielectric constant of ZnO nanostructures. However, none of the previous studies offers the effect of Al doping on the dielectric properties of ZnO ceramics, and the progress in developing high dielectric constant ZnO is still lacking. Our previous studies have confirmed that giant dielectric constant attributed to MaxwelleWagner relaxation can be achieved in high pressure treated ZnO porous ceramics [16,17]. In this work, the aim is to investigate the effect of Al doping on the microstructure and dielectric properties of ZnO ceramics. Giant dielectric constant was found in the doped ceramics compared with literature reported previously [7]. 2. Experimental Pure and Al-doped ZnO ceramics were prepared by conventional solid state sintering high pressure treated powders, as reported previously [16,17]. High purity ZnO (99.9%, Aladdin) and Al (99.9%, Aladdin) were used as the starting materials. The pressed pellets were finally sintered at temperatures of 1150 C for 2 h in air. The bulk density was obtained using the Archimedes method. The crystalline phase identification was determined using X-ray diffractometry
X. Li et al. / Journal of Alloys and Compounds 657 (2016) 90e94
(X'pert Pro, PANalytical, the Netherlands) with Cu Ka radiation. The microstructures of the ZnO ceramic pieces were characterized by scanning electron microscopy (SEM, FEI Quanta 250, USA). The chemical bonding states were analyzed by X-ray photoelectron spectroscopy (XPS) equipped with an Al Ka X-ray source. After polishing and cleaning, silver paste was coated on both sides of the ZnO pieces, and the electrode contacts were cured at 100 C for 3 h. Finally, after aging for 24 h, the dielectric response and AC impedance spectra of the specimens were measured using an Agilent 4294A Precision LCR Meter (Agilent Technologies Inc. USA) over a frequency range of 40e10 M Hz. The AC conductivity was calculated with the formula sAC ¼ u$ε0$ε0 $tand (u is the angular frequency). 3. Results and discussion Fig. 1 shows the XRD patterns of pure and Al-doped ZnO ceramics sintered at 1150 C. All of the diffraction peaks of pure sample can be exactly indexed to the hexagonal wurtzite structure of ZnO (JCPDF #89-7102), no characteristic peaks of other phases were observed. Except the ZnO main phase, extra diffraction peaks corresponding to the ZnAl2O4 (JCPDF #82-1043) spinel phase were observed in the XRD pattern of doped samples, and no alumina or aluminum was detected in the sintered samples. A detectable slight shift toward the larger angle was observed in the Al doped sample, as shown in the inset of Fig. 1. This confirms that Al3þ (0.54 Å) has been substituted for Zn2þ (0.74 Å) in the unit cell, leading to a decreased lattice parameter and cell volume. For the solubility limit of Al in ZnO is reported to be as low as 0.3 mol% [18], only minute of the doped Al atoms were expected to be dissolved in ZnO and the rest were expected to react with ZnO and thus form spinel phase. The intensity of XRD peaks from the spinel phase increases with the increasing content of Al dopant. This indicates that with the increase of Al-doping amount more ZnAl2O4 precipitates have formed due to the reaction of ZnO and Al. The ZnAl2O4 spinel impurity has been generated by the following reactions:
4Al þ 3O2 /2Al2 O3
(1)
Al2 O3 þ ZnO/ZnAl2 O4
(2)
Fig. 2(a)-(c) show the fracture SEM images of the pure and doped ZnO samples sintered at 1150 C. The result shows that a porous sample with significant grain growth can be achieved by conventional sintering HPT powders. For the doped samples, some
Fig. 1. XRD patterns of pure and Al-doped ZnO ceramics. The inset is high magnification of the (002) peak.
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fine particles with sizes varying from 100 nm to 300 nm were observed to precipitate mostly along the gain boundaries. It reveals that Al reacted with ZnO to form the ZnAl2O4 phase at grain boundaries due to the limited solubility [18], and the amount of ZnAl2O4 increases with increasing content of Al dopant which is in accordance with the results of XRD, as shown in Fig. 2(b) and (c). Both grain size and density of prepared samples are found to increase firstly and then decrease with the increase of Al content. The average grain sizes of the sintered samples were estimated from the SEM images using the linear intercept method [19]. The average grain sizes of pure, 1 mol% Al-doped, and 3 mol% Al-doped ZnO samples are ~2.04, ~3.92 and ~1.58 mm, respectively. And the measured densities for pure, 1 mol% doped and 3 mol% doped are 4.83, 5.19, 4.81 g/cm3, corresponding to a relative density of 85.0%, 91.4% and 84.7%, respectively. The grain growth mechanism for pure ZnO was considered to correlate with the diffusion of zinc ions in the ZnO structure, possibly by an interstitial reaction. The densification process was significantly depressed as demonstrated by high porosity in the microstructures and the Al content seems to have huge effect on the densification process at the same sintering temperature. As the melting point of ZnAl2O4 is as low as 1000 C, a low content of ZnAl2O4 (i.e. 1 mol%) may cause enormous grain growth of ZnO (Fig. 2(b)), while the formation of large amount of ZnAl2O4 secondary phase may contribute to the prohibition of grain growth by blocking mass transportation and pinning the grain boundaries of the matrix materials [20]. Fig. 3 shows the XPS spectra of pure and 3 mol% Al-doped ZnO ceramics: (a) survey, (b) O 1s, (c) Zn 2p and (d) Zn 2p3/2. The binding energies obtained in the XPS analysis are standardized by using C 1s at 284.6 eV as the reference. From the XPS survey spectra, the presence of ZnO is clearly manifested from its various orbital levels, no peaks of other elements except Zn, O and C are observed in the spectrum. The binding energy value of Al 2p is not detected due to the low resolution. Fig. 3(b) shows the binding state of O 1s spectra for the pure and 3 mol% Al-doped ZnO ceramics. The O 1s spectra can be resolved into three Gaussian components by XPSPEAK41 soft, which centered around 530.05 eV, 530.8 eV and 532.16 eV for pure ZnO sample, and 529.8 eV, 530.6 eV and 531.85 eV for 3 mol% Al doped sample, respectively. The low binding energy peak of O 1s can be ascribed to O2 ions on wurtzite structure of a hexagonal Zn2þ ion array. The medium binding energy peak of the O 1s peak is associated with O2 ions that are in oxygen-deficient regions within the ZnO matrix. Therefore, the variation in relative area under this peak indicates the variation of O vacancies in the samples. It is observed that the medium peak of 3 mol% Al sample is weaker than that of undoped one, implying that Al-doping can introduced more oxygen vacancies. The high binding energy component is probably attributed to either the presence of loosely bound oxygens on the surface of the ZnO thin films belonging to a specific species, e.g., eCO3, or corresponds to hydroxides [21]. Fig. 3(c) and 3(d) give the XPS data of Zn 2p and Zn 2p3/2 for pure and 3 mol% Al-doped ZnO ceramics, respectively. The Zn 2p3/2 spectra for both the pure and the Aldoped ZnO ceramics include two components corresponding to metallic Zn (ZneZn) and Zn in the oxidized state (ZneO) [21]. Moreover, the peaks of Zn 2p3/2 for Al-doped sample were shifted to a lower binding energy, revealing that more oxygen vacancies are produced by the introduction of Al. The AC impedance spectra of pure and doped ZnO ceramics on a 00 0 Nyquist plot (Z vs Z ) interpreted using equivalent circuit model are shown in Fig. 4. It is obvious that all specimens show only one incomplete and depressed semi-circle which is derived from the grain boundary barrier, suggesting the appearance of non-Debye type relaxation resulting from the heterogeneity of the barriers of the specimens [22]. The incomplete semicircle reveals that the
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Fig. 2. Fractured surface SEM images of (a) pure, (b) 1% Al-doped, and (c) 3% Al-doped ZnO ceramics. (d), (e) and (f) are the corresponding enlarged images.
relaxation frequency decreases to lower frequency which could be attributed to the development of porosity (Fig. 2) in the ceramics [23], and the measurement of this low frequency was not possible
in this experiments. Moreover, the resistance of grain boundary (Rgb) decreased sharply with the increase concentration of Al dopant.
Fig. 3. XPS spectra (a) survey, (b) O 1s, (c) Zn 2p, and (d) Zn 2p3/2 of pure and 3 mol% Al-doped ZnO ceramics.
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It is seen from Fig. 5(c) that the AC conductivity decreases with decreasing frequency and becomes independent of frequency after a certain value. It has been vastly reported in the literature that the AC conductivity gradually increases with increasing frequency of applied AC field is mainly attributed to the enhanced migration of electron. According to Jonscher, the bulk conductivity can be described by the universal dielectric response [24].
s ¼ sDC þ s0 f n
Fig. 4. AC impedance spectra of pure and Al-doped ZnO ceramics. The inset is the enlarged spectra. The symbols represent the experimental data, whereas the continuous line represents the modeling curve.
The frequency dependence of dielectric properties for the doped and undoped ZnO ceramics is plotted in Fig. 5(a) and (b). It can be found that the relative permittivity ε0 for all the specimens decreased slightly, from 40 to 100 kHz, with a sharper drop evident above 1 MHz, which is due to dielectric relaxation mainly associated with oxygen vacancies. ε0 for 1 mol% Al-doped specimen drops at higher frequency which could be attributed to the dipolar polarization induced by Al-doping. Also, the results revealed that the value of ε0 obtained for Al doped sample was much higher than that of pure ZnO ceramics, and it increases with the increase of Al content. As shown in Fig. 5(b), at frequency below 103 Hz, the dielectric loss is very high and it decreases sharply with the increase of frequency, it remains at low values (~0.4) in 103~105 Hz, and then increases greatly with the further increase of frequency. On the other hand, it is also noticeable that dielectric loss increases slightly when Al is incorporated into ZnO lattice.
(3)
where sDC is the DC bulk conductivity, s0 is a pre power factor, f is frequency, and n is the power of the applied frequency, which should be between 0 and 1. As seen, the conductivity is agreeable with Eq. (3) at high frequencies above the transition range. It is also found that a slightly decreased AC conductivity was obtained for the 1 mol% Al-doped sample, while further increase of Al dopant concentration to 3 mol% leads to an increased AC conductivity. For the ionic radii of Al3þ (0.51 Å) is smaller than that of Zn2þ (0.74 Å), Al3þ can either enter interstitial sites or substitute Zn2þ of the ZnO lattice. The interstitial substitution of Al3þ can occurs either directly or by substitution of Zn atoms on its regular site at first and then moving to the interstitial position [25].
1 3 Al O /Al0i þ ho þ O2 [ 2 2 3 4
(4)
where Al0i is an Al atom at interstitial site and ho is a hole with positive charge. In this case, Al behaves as an acceptor and decreases the conductivity. On the other side, the substitution of Zn2þ with Al3þ would supply electron to the conduction band, hence reducing the resistivity of the ZnO grains: [26]
1 ZnO Al2 O3 !2AlZn þ 2e0 þ 2OX O þ O2 [ 2
(5)
The decrease of AC conductivity for 1 mol% Al-doped sample is mainly due to the acceptor effect of Al dopant and the precipitation of high-resistivity ZnAl2O4 nanoparticles on the grain boundaries. While the increase of AC conductivity for 3 mol% Al-doped sample is mainly due to the donor effect of Al dopant. Al ions produce defects such as oxygen vacancies and zinc interstitials in the ZnO host system, and these defects tend to segregate at the grain boundaries. Therefore, doping increases the defect ions which facilitate the formation of grain boundary defect barrier leading to blockage to the flow of charge carriers. And the high surface areas of pores supplies opportunities for impurities to be distributed, the additives are mostly positioned at grain boundaries or crystal surfaces in crystal materials [27]. Some impurity atoms congregated near the grain boundary also, which may cause impurity conduction [4]. Interfacial conduction and impurities play an important role in increasing the tand of the Al-doped ZnO ceramics. The accumulation of charge at interfaces induced the interfacial polarization, MaxwelleWagner effects [28]. These lead to a greatly enhanced dielectric constant in 3 mol% Al-doped ZnO ceramics, e.g. ε0 ~1.39 104 at 1 kHz, which is ~225% higher than that of pure ZnO ceramics. This work may provide a new pathway for the dielectric enhancement in ZnO ceramics. 4. Conclusions
Fig. 5. Frequency-dependent of (a) dielectric constant, (b) dielectric loss, and (c) AC conductivity for pure and Al-doped ZnO ceramics at room temperature.
Pure and Al-doped ZnO ceramics have been successfully synthesized by conventional sintering high pressure treated nano powders. Results reveal that Al3þ has been completely incorporated in the ZnO matrix. The dielectric study of the prepared samples showed an enhancement in the dielectric constant (~1.39 104 at 1 kHz) at room temperature due to Al-doping. The conductivity of
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ZnO ceramics was also increased through incorporation of Al ions in the ZnO lattice due to the increased number of charge carriers. Therefore, the simple and low cost fabrication of Al-doped ZnO ceramics and its giant dielectric properties make this material very interesting for high dielectric applications. Acknowledgments The authors thank Prof. W.J. Zhu and Prof. Q. Wu at the Institute of Fluid Physics for helpful discussions, and thank Foundation of National Key Laboratory of Shock Wave and Detonation Physics for financial support (Grant No. 9140C670101140C67280). References [1] C.C. Homes, T. Vogt, S.M. Shapiro, S. Wakimoto, A.P. Ramirez, Optical response of high-dielectric-constant perovskite-related oxide, Science 293 (2001) 673e676. [2] R. Tripathi, A. Kumar, C. Bharti, T.P. Sinha, Dielectric relaxation of ZnO nanostructure synthesized by soft chemical method, Curr. Appl. Phys. 10 (2010) 676e681. [3] M.K. Gupta, B. Kumar, Enhanced ferroelectric, dielectric and optical behavior in Li-doped ZnO nanorods, J. Alloys Compd. 509 (2011) L208eL212. [4] X.L. Su, Y. Jia, X.Q. Liu, J.B. Wang, J. Xu, X.H. He, C. Fu, S.T. Liu, Preparation, dielectric property and infrared emissivity of Fe-doped ZnO powder by coprecipitation method at various reaction time, Ceram. Int. 40 (2014) 5307e5311. rid, Investigations on electrical con[5] A. Tabib, N. Sdiri, H. Elhouichet, M. Fe ductivity and dielectric properties of Na doped ZnO synthesized from sol gel method, J. Alloys Compd. 622 (2015) 687e694. [6] Y. Wang, F. Luo, L. Zhang, D.M. Zhu, W.C. Zhou, Microwave dielectric properties of Al-doped ZnO powders synthesized by coprecipitation method, Ceram. Int. 39 (2013) 8723e8727. [7] R. Zamiri, B. Singh, M.S. Belsley, J.M.F. Ferreira, Structural and dielectric properties of Al-doped ZnO nanostructures, Ceram. Int. 40 (2014) 6031e6036. ski, [8] V. Kapustianyk, Y. Eliyashevskyy, B. Turko, Z. Czapla, S. Dacko, B. Barwin Influence of technological factors on conductivity and dielectric dispersion in ZnO nanocrystalline thin films, J. Alloys Compd. 531 (2012) 64e69. [9] C.L. Cheng, J. Hu, J.L. He, Characterization of dielectric behavior in ZnO electroceramic: superior grain boundary, inferior grain boundary and grain, Mater. Lett. 132 (2014) 240e242. [10] M. Arshad, A.S. Ahmed, A. Azam, A.H. Naqvi, Exploring the dielectric behavior of Co doped ZnO nanoparticles synthesized by wet chemical route using impedance spectroscopy, J. Alloys Compd. 577 (2013) 469e474. [11] M.E. Abrishami, A. Kompany, S.M. Hosseini, Varistor behavior of Mn doped ZnO ceramics prepared from nanosized precursors, J. Electroceram. 29 (2012)
125e132. [12] J.Q. Huang, H. Zheng, Z.H. Chen, Q. Gao, N. Ma, P.Y. Du, Percolative ceramic composites with giant dielectric constants and low dielectric losses, J. Mater. Chem. 19 (2009) 3909e3913. [13] L. Han, N.V. Nong, L.T. Hung, T. Holgate, N. Pryds, M. Ohtaki, S. Linderoth, The influence of a- and g-Al2O3 phases on the thermoelectric properties of Aldoped ZnO, J. Alloys Compd. 555 (2013) 291e296. [14] P. Jood, R.J. Mehta, Y.L. Zhang, G. Peleckis, X.L. Wang, R.W. Siegel, T. BorcaTasciuc, S.X. Dou, G. Ramanath, Al-doped zinc oxide nanocomposites with enhanced thermoelectric properties, Nano Lett. 11 (2011) 4337e4342. [15] J. T-Thienprasert, S. Rujirawat, W. Klysubun, J.N. Duenow, T.J. Coutts, S.B. Zhang, D.C. Look, S. Limpijumnong, Compensation in Al-doped ZnO by Alrelated acceptor complexes: synchrotron X-ray absorption spectroscopy and theory, Phys. Rev. Lett. 110 (2013), 055502e1e055502e5. [16] X.H. Li, L. Xu, L.X. Liu, Y. Wang, X.X. Cao, Y.J. Huang, C.M. Meng, Z.G. Wang, High pressure treated ZnO ceramics towards giant dielectric constants, J. Mater. Chem. A 2 (2014) 16740e16745. [17] X.H. Li, Y.J. Huang, L. Xu, L.X. Liu, Y. Wang, X.X. Cao, C.M. Meng, Z.G. Wang, Effect of powder size on the microstructure and dielectric properties of ZnO ceramics, Mater. Res. Bull. 68 (2015) 87e91. [18] H. Serier, M. Gaudon, M. Menetrier, Al-doped ZnO powdered materials: Al solubility limit and IR absorption properties, Solid State Sci. 11 (2009) 1192e1197. [19] J.C. Wurst, J.A. Nelson, Lineal intercept technique for measuring grain size in two-phase polycrystalline ceramics, J. Am. Ceram. Soc. 55 (1972), 109e109. [20] Y.H. Zhang, J. Han, Microstructure and temperature coefficient of resistivity for ZnO ceramics doped with Al2O3, Mater. Lett. 60 (2006) 2522e2525. [21] J.J. Ding, M.Q. Wang, X.Y. Zhang, C.X. Ran, Field emission mechanism insights of graphene decorated with ZnO nanoparticles, RSC Adv. 3 (2013) 14073e14079. [22] J. Wu, T. Qi, T.T. Li, Q.W. Qin, G.Q. Li, B.L. Zhu, Z.D. Xiang, C.S. Xie, The AC impedance and variable temperature dielectric spectroscopic analysis of MnO2 doped and un-doped ZnO-V2O5 ceramics, J. Mater. Sci. Mater. Electron. 23 (2012) 1143e1150. [23] N. Al-Hardan, M.J. Abdullah, A.A. Aziz, Impedance spectroscopy of undoped and Cr-doped ZnO gas sensors under different oxygen concentrations, Appl. Surf. Sci. 257 (2011) 8993e8997. [24] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectrics Press, London, 1983. [25] M. Houabes, S. Bernik, C. Talhi, A. Bui, The effect of aluminum oxide on the residual voltage of ZnO varistors, Ceram. Int. 29 (2005) 783e789. [26] A. Sedky, A.A. Sawalha, A.M. Yassin, Enhancement of electrical conductivity by Al-doped ZnO ceramic varistors, Phys. B 404 (2009) 3519e3524. [27] E. Mouchon, P. Colomban, Microwave absorbent: preparation, mechanical properties and r.f.-microwave conductivity of SiC (and/or mullite) fiber reinforced Nasicon matrix composites, J. Mater. Sci. 31 (1996) 323e334. [28] P. Lunkenheimer, V. Bobnar, A.V. Pronin, A.I. Ritus, A.A. Volkov, A. Loidl, Origin of apparent colossal dielectric constants, Phys. Rev. B 66 (2002), 052105-1052105-4.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
High dielectric constant in Al-doped ZnO ceramics using highpressure treated powders Xuhai Li, Xiuxia Cao, Liang Xu, Lixin Liu, Yuan Wang, Chuanmin Meng*, Zhigang Wang** National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, Chinese Academy of Engineering Physics, Mianyang 621900, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2015 Received in revised form 18 September 2015 Accepted 8 October 2015 Available online xxx
Pure and Al-doped ZnO ceramics were prepared by conventional sintering high pressure treated nano powders at 1150 C for 2 h, and the effect of Al-doping on the microstructure and dielectric properties were mainly investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results revealed that Al reacted with ZnO and formed ZnAl2O4 nano-precipitates on the grain boundaries. Small amount of ZnAl2O4 nano-precipitates (i.e. < 1 mol%) enhanced the grain growth of ZnO, while larger amount suppressed the grain growth. AC impedance analysis indicated that Al-doping decreases the resistivity of ZnO ceramics. Dielectric studies revealed that the incorporation of Al in ZnO can significantly enhance the dielectric constant (1.39 104 at 1 kHz) of ZnO ceramics. And the dielectric behavior can be well-explained by MaxwelleWagner relaxation. © 2015 Elsevier B.V. All rights reserved.
Keywords: Ceramics Sintering Dielectric response Microstructure High pressure
1. Introduction It is known that the dielectric constant of a material will ultimately decide the level of miniaturization in electronic devices based on capacitive components [1]. In the past decades, ZnO has attracted considerable attention because it can be used for a wide range of applications including dielectrics, owning to its wide direct band gap (3.37 eV) and large exciton binding energy (60 meV). Although the dielectric properties of ZnO nano-structures, films, and ceramics have been investigated by many researchers, further improvements are still required before them become viable for commercial applications in devices [2e11]. Increasing the conductivity of ceramics by introducing conductive particles has been proved to be an effective approach to enhance the dielectric constant of ceramics [12]. Doping with group-III elements can significantly improve the conductivity of ZnO ceramics [13,14]. Among the group-III doped ceramics, Aldoped ZnO emerged as the most promising candidate due to its high temperature stability and the fact that Al is abundant [15]. Recently, Wang et al. [6] investigated the effect of Al doping concentration on the microwave dielectric properties of ZnO powders,
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Meng), [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.jallcom.2015.10.079 0925-8388/© 2015 Elsevier B.V. All rights reserved.
and found that the real permittivity of the prepared powders have been improved in the frequency range of 8.2e12.4 GHz due to the generation of AlZn defects. Zamiri et al. [7] also found that doping with 2 mol% Al could significantly enhance the dielectric constant of ZnO nanostructures. However, none of the previous studies offers the effect of Al doping on the dielectric properties of ZnO ceramics, and the progress in developing high dielectric constant ZnO is still lacking. Our previous studies have confirmed that giant dielectric constant attributed to MaxwelleWagner relaxation can be achieved in high pressure treated ZnO porous ceramics [16,17]. In this work, the aim is to investigate the effect of Al doping on the microstructure and dielectric properties of ZnO ceramics. Giant dielectric constant was found in the doped ceramics compared with literature reported previously [7]. 2. Experimental Pure and Al-doped ZnO ceramics were prepared by conventional solid state sintering high pressure treated powders, as reported previously [16,17]. High purity ZnO (99.9%, Aladdin) and Al (99.9%, Aladdin) were used as the starting materials. The pressed pellets were finally sintered at temperatures of 1150 C for 2 h in air. The bulk density was obtained using the Archimedes method. The crystalline phase identification was determined using X-ray diffractometry
X. Li et al. / Journal of Alloys and Compounds 657 (2016) 90e94
(X'pert Pro, PANalytical, the Netherlands) with Cu Ka radiation. The microstructures of the ZnO ceramic pieces were characterized by scanning electron microscopy (SEM, FEI Quanta 250, USA). The chemical bonding states were analyzed by X-ray photoelectron spectroscopy (XPS) equipped with an Al Ka X-ray source. After polishing and cleaning, silver paste was coated on both sides of the ZnO pieces, and the electrode contacts were cured at 100 C for 3 h. Finally, after aging for 24 h, the dielectric response and AC impedance spectra of the specimens were measured using an Agilent 4294A Precision LCR Meter (Agilent Technologies Inc. USA) over a frequency range of 40e10 M Hz. The AC conductivity was calculated with the formula sAC ¼ u$ε0$ε0 $tand (u is the angular frequency). 3. Results and discussion Fig. 1 shows the XRD patterns of pure and Al-doped ZnO ceramics sintered at 1150 C. All of the diffraction peaks of pure sample can be exactly indexed to the hexagonal wurtzite structure of ZnO (JCPDF #89-7102), no characteristic peaks of other phases were observed. Except the ZnO main phase, extra diffraction peaks corresponding to the ZnAl2O4 (JCPDF #82-1043) spinel phase were observed in the XRD pattern of doped samples, and no alumina or aluminum was detected in the sintered samples. A detectable slight shift toward the larger angle was observed in the Al doped sample, as shown in the inset of Fig. 1. This confirms that Al3þ (0.54 Å) has been substituted for Zn2þ (0.74 Å) in the unit cell, leading to a decreased lattice parameter and cell volume. For the solubility limit of Al in ZnO is reported to be as low as 0.3 mol% [18], only minute of the doped Al atoms were expected to be dissolved in ZnO and the rest were expected to react with ZnO and thus form spinel phase. The intensity of XRD peaks from the spinel phase increases with the increasing content of Al dopant. This indicates that with the increase of Al-doping amount more ZnAl2O4 precipitates have formed due to the reaction of ZnO and Al. The ZnAl2O4 spinel impurity has been generated by the following reactions:
4Al þ 3O2 /2Al2 O3
(1)
Al2 O3 þ ZnO/ZnAl2 O4
(2)
Fig. 2(a)-(c) show the fracture SEM images of the pure and doped ZnO samples sintered at 1150 C. The result shows that a porous sample with significant grain growth can be achieved by conventional sintering HPT powders. For the doped samples, some
Fig. 1. XRD patterns of pure and Al-doped ZnO ceramics. The inset is high magnification of the (002) peak.
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fine particles with sizes varying from 100 nm to 300 nm were observed to precipitate mostly along the gain boundaries. It reveals that Al reacted with ZnO to form the ZnAl2O4 phase at grain boundaries due to the limited solubility [18], and the amount of ZnAl2O4 increases with increasing content of Al dopant which is in accordance with the results of XRD, as shown in Fig. 2(b) and (c). Both grain size and density of prepared samples are found to increase firstly and then decrease with the increase of Al content. The average grain sizes of the sintered samples were estimated from the SEM images using the linear intercept method [19]. The average grain sizes of pure, 1 mol% Al-doped, and 3 mol% Al-doped ZnO samples are ~2.04, ~3.92 and ~1.58 mm, respectively. And the measured densities for pure, 1 mol% doped and 3 mol% doped are 4.83, 5.19, 4.81 g/cm3, corresponding to a relative density of 85.0%, 91.4% and 84.7%, respectively. The grain growth mechanism for pure ZnO was considered to correlate with the diffusion of zinc ions in the ZnO structure, possibly by an interstitial reaction. The densification process was significantly depressed as demonstrated by high porosity in the microstructures and the Al content seems to have huge effect on the densification process at the same sintering temperature. As the melting point of ZnAl2O4 is as low as 1000 C, a low content of ZnAl2O4 (i.e. 1 mol%) may cause enormous grain growth of ZnO (Fig. 2(b)), while the formation of large amount of ZnAl2O4 secondary phase may contribute to the prohibition of grain growth by blocking mass transportation and pinning the grain boundaries of the matrix materials [20]. Fig. 3 shows the XPS spectra of pure and 3 mol% Al-doped ZnO ceramics: (a) survey, (b) O 1s, (c) Zn 2p and (d) Zn 2p3/2. The binding energies obtained in the XPS analysis are standardized by using C 1s at 284.6 eV as the reference. From the XPS survey spectra, the presence of ZnO is clearly manifested from its various orbital levels, no peaks of other elements except Zn, O and C are observed in the spectrum. The binding energy value of Al 2p is not detected due to the low resolution. Fig. 3(b) shows the binding state of O 1s spectra for the pure and 3 mol% Al-doped ZnO ceramics. The O 1s spectra can be resolved into three Gaussian components by XPSPEAK41 soft, which centered around 530.05 eV, 530.8 eV and 532.16 eV for pure ZnO sample, and 529.8 eV, 530.6 eV and 531.85 eV for 3 mol% Al doped sample, respectively. The low binding energy peak of O 1s can be ascribed to O2 ions on wurtzite structure of a hexagonal Zn2þ ion array. The medium binding energy peak of the O 1s peak is associated with O2 ions that are in oxygen-deficient regions within the ZnO matrix. Therefore, the variation in relative area under this peak indicates the variation of O vacancies in the samples. It is observed that the medium peak of 3 mol% Al sample is weaker than that of undoped one, implying that Al-doping can introduced more oxygen vacancies. The high binding energy component is probably attributed to either the presence of loosely bound oxygens on the surface of the ZnO thin films belonging to a specific species, e.g., eCO3, or corresponds to hydroxides [21]. Fig. 3(c) and 3(d) give the XPS data of Zn 2p and Zn 2p3/2 for pure and 3 mol% Al-doped ZnO ceramics, respectively. The Zn 2p3/2 spectra for both the pure and the Aldoped ZnO ceramics include two components corresponding to metallic Zn (ZneZn) and Zn in the oxidized state (ZneO) [21]. Moreover, the peaks of Zn 2p3/2 for Al-doped sample were shifted to a lower binding energy, revealing that more oxygen vacancies are produced by the introduction of Al. The AC impedance spectra of pure and doped ZnO ceramics on a 00 0 Nyquist plot (Z vs Z ) interpreted using equivalent circuit model are shown in Fig. 4. It is obvious that all specimens show only one incomplete and depressed semi-circle which is derived from the grain boundary barrier, suggesting the appearance of non-Debye type relaxation resulting from the heterogeneity of the barriers of the specimens [22]. The incomplete semicircle reveals that the
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Fig. 2. Fractured surface SEM images of (a) pure, (b) 1% Al-doped, and (c) 3% Al-doped ZnO ceramics. (d), (e) and (f) are the corresponding enlarged images.
relaxation frequency decreases to lower frequency which could be attributed to the development of porosity (Fig. 2) in the ceramics [23], and the measurement of this low frequency was not possible
in this experiments. Moreover, the resistance of grain boundary (Rgb) decreased sharply with the increase concentration of Al dopant.
Fig. 3. XPS spectra (a) survey, (b) O 1s, (c) Zn 2p, and (d) Zn 2p3/2 of pure and 3 mol% Al-doped ZnO ceramics.
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It is seen from Fig. 5(c) that the AC conductivity decreases with decreasing frequency and becomes independent of frequency after a certain value. It has been vastly reported in the literature that the AC conductivity gradually increases with increasing frequency of applied AC field is mainly attributed to the enhanced migration of electron. According to Jonscher, the bulk conductivity can be described by the universal dielectric response [24].
s ¼ sDC þ s0 f n
Fig. 4. AC impedance spectra of pure and Al-doped ZnO ceramics. The inset is the enlarged spectra. The symbols represent the experimental data, whereas the continuous line represents the modeling curve.
The frequency dependence of dielectric properties for the doped and undoped ZnO ceramics is plotted in Fig. 5(a) and (b). It can be found that the relative permittivity ε0 for all the specimens decreased slightly, from 40 to 100 kHz, with a sharper drop evident above 1 MHz, which is due to dielectric relaxation mainly associated with oxygen vacancies. ε0 for 1 mol% Al-doped specimen drops at higher frequency which could be attributed to the dipolar polarization induced by Al-doping. Also, the results revealed that the value of ε0 obtained for Al doped sample was much higher than that of pure ZnO ceramics, and it increases with the increase of Al content. As shown in Fig. 5(b), at frequency below 103 Hz, the dielectric loss is very high and it decreases sharply with the increase of frequency, it remains at low values (~0.4) in 103~105 Hz, and then increases greatly with the further increase of frequency. On the other hand, it is also noticeable that dielectric loss increases slightly when Al is incorporated into ZnO lattice.
(3)
where sDC is the DC bulk conductivity, s0 is a pre power factor, f is frequency, and n is the power of the applied frequency, which should be between 0 and 1. As seen, the conductivity is agreeable with Eq. (3) at high frequencies above the transition range. It is also found that a slightly decreased AC conductivity was obtained for the 1 mol% Al-doped sample, while further increase of Al dopant concentration to 3 mol% leads to an increased AC conductivity. For the ionic radii of Al3þ (0.51 Å) is smaller than that of Zn2þ (0.74 Å), Al3þ can either enter interstitial sites or substitute Zn2þ of the ZnO lattice. The interstitial substitution of Al3þ can occurs either directly or by substitution of Zn atoms on its regular site at first and then moving to the interstitial position [25].
1 3 Al O /Al0i þ ho þ O2 [ 2 2 3 4
(4)
where Al0i is an Al atom at interstitial site and ho is a hole with positive charge. In this case, Al behaves as an acceptor and decreases the conductivity. On the other side, the substitution of Zn2þ with Al3þ would supply electron to the conduction band, hence reducing the resistivity of the ZnO grains: [26]
1 ZnO Al2 O3 !2AlZn þ 2e0 þ 2OX O þ O2 [ 2
(5)
The decrease of AC conductivity for 1 mol% Al-doped sample is mainly due to the acceptor effect of Al dopant and the precipitation of high-resistivity ZnAl2O4 nanoparticles on the grain boundaries. While the increase of AC conductivity for 3 mol% Al-doped sample is mainly due to the donor effect of Al dopant. Al ions produce defects such as oxygen vacancies and zinc interstitials in the ZnO host system, and these defects tend to segregate at the grain boundaries. Therefore, doping increases the defect ions which facilitate the formation of grain boundary defect barrier leading to blockage to the flow of charge carriers. And the high surface areas of pores supplies opportunities for impurities to be distributed, the additives are mostly positioned at grain boundaries or crystal surfaces in crystal materials [27]. Some impurity atoms congregated near the grain boundary also, which may cause impurity conduction [4]. Interfacial conduction and impurities play an important role in increasing the tand of the Al-doped ZnO ceramics. The accumulation of charge at interfaces induced the interfacial polarization, MaxwelleWagner effects [28]. These lead to a greatly enhanced dielectric constant in 3 mol% Al-doped ZnO ceramics, e.g. ε0 ~1.39 104 at 1 kHz, which is ~225% higher than that of pure ZnO ceramics. This work may provide a new pathway for the dielectric enhancement in ZnO ceramics. 4. Conclusions
Fig. 5. Frequency-dependent of (a) dielectric constant, (b) dielectric loss, and (c) AC conductivity for pure and Al-doped ZnO ceramics at room temperature.
Pure and Al-doped ZnO ceramics have been successfully synthesized by conventional sintering high pressure treated nano powders. Results reveal that Al3þ has been completely incorporated in the ZnO matrix. The dielectric study of the prepared samples showed an enhancement in the dielectric constant (~1.39 104 at 1 kHz) at room temperature due to Al-doping. The conductivity of
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ZnO ceramics was also increased through incorporation of Al ions in the ZnO lattice due to the increased number of charge carriers. Therefore, the simple and low cost fabrication of Al-doped ZnO ceramics and its giant dielectric properties make this material very interesting for high dielectric applications. Acknowledgments The authors thank Prof. W.J. Zhu and Prof. Q. Wu at the Institute of Fluid Physics for helpful discussions, and thank Foundation of National Key Laboratory of Shock Wave and Detonation Physics for financial support (Grant No. 9140C670101140C67280). References [1] C.C. Homes, T. Vogt, S.M. Shapiro, S. Wakimoto, A.P. Ramirez, Optical response of high-dielectric-constant perovskite-related oxide, Science 293 (2001) 673e676. [2] R. Tripathi, A. Kumar, C. Bharti, T.P. Sinha, Dielectric relaxation of ZnO nanostructure synthesized by soft chemical method, Curr. Appl. Phys. 10 (2010) 676e681. [3] M.K. Gupta, B. Kumar, Enhanced ferroelectric, dielectric and optical behavior in Li-doped ZnO nanorods, J. Alloys Compd. 509 (2011) L208eL212. [4] X.L. Su, Y. Jia, X.Q. Liu, J.B. Wang, J. Xu, X.H. He, C. Fu, S.T. Liu, Preparation, dielectric property and infrared emissivity of Fe-doped ZnO powder by coprecipitation method at various reaction time, Ceram. Int. 40 (2014) 5307e5311. rid, Investigations on electrical con[5] A. Tabib, N. Sdiri, H. Elhouichet, M. Fe ductivity and dielectric properties of Na doped ZnO synthesized from sol gel method, J. Alloys Compd. 622 (2015) 687e694. [6] Y. Wang, F. Luo, L. Zhang, D.M. Zhu, W.C. Zhou, Microwave dielectric properties of Al-doped ZnO powders synthesized by coprecipitation method, Ceram. Int. 39 (2013) 8723e8727. [7] R. Zamiri, B. Singh, M.S. Belsley, J.M.F. Ferreira, Structural and dielectric properties of Al-doped ZnO nanostructures, Ceram. Int. 40 (2014) 6031e6036. ski, [8] V. Kapustianyk, Y. Eliyashevskyy, B. Turko, Z. Czapla, S. Dacko, B. Barwin Influence of technological factors on conductivity and dielectric dispersion in ZnO nanocrystalline thin films, J. Alloys Compd. 531 (2012) 64e69. [9] C.L. Cheng, J. Hu, J.L. He, Characterization of dielectric behavior in ZnO electroceramic: superior grain boundary, inferior grain boundary and grain, Mater. Lett. 132 (2014) 240e242. [10] M. Arshad, A.S. Ahmed, A. Azam, A.H. Naqvi, Exploring the dielectric behavior of Co doped ZnO nanoparticles synthesized by wet chemical route using impedance spectroscopy, J. Alloys Compd. 577 (2013) 469e474. [11] M.E. Abrishami, A. Kompany, S.M. Hosseini, Varistor behavior of Mn doped ZnO ceramics prepared from nanosized precursors, J. Electroceram. 29 (2012)
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