High-temperature dielectric properties of (Al, Nb) co-doped SrTiO3 ceramics

Author’s Accepted Manuscript High-temperature dielectric properties of (Al, Nb) co-doped SrTiO3 ceramics L. Tong, D. Zhang, H. Wang, Q.J. Li, Y. Yu, Y.D. Li, S.G. Huang, Y.M. Guo, C.C. Wang www.elsevier.com

PII: DOI: Reference:

S0167-577X(16)30957-0 http://dx.doi.org/10.1016/j.matlet.2016.06.001 MLBLUE21002

To appear in: Materials Letters Received date: 30 April 2016 Revised date: 27 May 2016 Accepted date: 1 June 2016 Cite this article as: L. Tong, D. Zhang, H. Wang, Q.J. Li, Y. Yu, Y.D. Li, S.G. Huang, Y.M. Guo and C.C. Wang, High-temperature dielectric properties of (Al, Nb) co-doped SrTiO3 ceramics, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.06.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

High-temperature dielectric properties of (Al, Nb) co-doped SrTiO3 ceramics L. Tong, D. Zhang, H. Wang, Q.J. Li, Y. Yu, Y.D. Li, S.G. Huang, Y.M. Guo, C.C. Wang* Laboratory of Dielectric Functional Materials, School of Physics & Material Science, Anhui University, Hefei 230601, China *

Corresponding author. [email protected]

Abstract SrTi1-x(Al, Nb)xO3 (x=0, 0.005, 0.05, and 0.1) ceramic samples were prepared using the conventional solid-state reaction method. The dielectric properties of these samples were investigated in temperature range of 50-600 oC and frequency range of 102-106 Hz. The samples show two dielectric relaxations related to grain and grain boundary. The effects of (Al, Nb) co-doping were found to have influence on the grain-boundary relaxation by reducing its activation energy and enhancing the conductivity. Our results show that it is unsuitable to enhance the dielectric properties of SrTiO3 via (Al, Nb) co-doping. Key words: ceramics; dielectric properties; grain; grain boundary; (Al, Nb) co-doping; 1.

Introduction Strontium titanate, SrTiO3 (STO), has been studying for more than 50 years due to its quantum paraelectric

behavior, which leads to rich dielectric properties generally achieved by introducing different dopants [1-3]. The plethora of doping investigations were performed on STO samples doped with single dopant. For example, the A-site substantiation of Sr2+ by dopants such as Ba2+, Bi+, Pb2+, and Ca2+ [1,4-6], as well as the trivalent ions (Nd3+, Dy3+, Pr3+, Sm3+, etc.) [7-10] could widely modify the dielectric behavior of STO. Recently, remarkable improvement of dielectric properties behavior was reported in trivalent-element (In3+) and pentavalent-element (Nb5+) co-doped rutile TiO2 [11,12]. This fact suggests that dual doping is superior to mono doping in tuning the dielectric properties. Since TiO2 is a constituent raw material in STO, it implies trivalent-element and pentavalent-element dual doping might have positive influence on the dielectric properties of STO. To the best of 1

our knowledge, the effects of co-doping on the dielectric properties of STO have not been studied earlier. In this work, we performed investigation on the dielectric properties of (Al, Nb) co-doped STO ceramics. Instead of In3+, Al3+ is used because of its high natural abundance and low cost. Besides, it is the same main group (13) as In3+ and superior dielectric properties have been reported in (Al, Nb) co-doped TiO2 [11,12]. 2.

Experimental details SrTi1-x(Al, Nb)xO3 (x = 0, 0.005, 0.05, and 0.1) ceramics samples were prepared by the standard conventional

solid-state reaction method. The raw materials used in this work are SrCO3 (99.99%)，TiO2 (99.99%), Al2O3 (99.99%), and Nb2O5 (99.99%)，which were stored at 200 oC for 10 h to completely remove any absorbed water. The stoichiometric powders were thoroughly mixed for 2 hours using a mortar and then calcined at 1100 oC for 12 h. The resultant mixture was reground, pressed into pellets with a size of 14 mm in diameter and 1-2 mm in thickness and finally sintered at 1250 oC for 20 h. Phase purity of the sintered pellets was characterized by X-ray powder diffraction (XRD) performed on a MXP18AHF diffractometer (Mac Science Co Ltd, Yokohama, Japan) with CuK radiation. Dielectric properties were measured using a Wayne Kerr 6500B precise impedance analyzer. 3.

Results and discussion Figure 1 shows the XRD patterns of SrTi1-x(Al, Nb)xO3 (x = 0, 0.005, 0.05, and 0.1) ceramic samples recorded

at room temperature. The pattern peaks for all samples can be indexed based on a cubic structure with Pm-3m space group (JCPDS file No. 86-0179) without no unindexable peaks confirming the single phase nature of the compounds. Figure 2

the temperature ( T ) dependence of the dielectric loss tangent ( tan ) and dielectric

constant (  ' , inset) for SrTi1-x(Al, Nb)xO3 (x=0, 0.005, 0.05, and 0.1) ceramics recorded at 100 Hz. The curve of

tan (T ) for the pure sample registers a pronounced peak at ~200 oC followed by a rapidly increase appearing at ~400 oC. The peak corresponds to a dielectric constant kink as observed in

2

 ' (T ) ,

indicating that it presents a

relaxation process. The rapid increase in tan (T ) leading to a strong background is caused by conductivity. A slight (Al, Nb)-doping level of x=0.005 is found to notably depress the peak and shift the increasing background to a lower temperature of ~300 oC. Further increasing the doping level leads to a slight depression and shifting of the peak and background, respectively. The above results evidence that the (Al, Nb)-doping is harmful to the relaxation but beneficial to the conductivity. It was reported that there are two high-temperature relaxations in pure STO ceramics [13]. Actually, one notes that there exists a weak dielectric-constant kink occurring around 450 oC in the

 ' (T )

curve of the pure sample

(inset of Fig.1). This is indicative of another relaxation that might be obscured by the background. Since a relaxation process can be described by different dielectric function [14], to study the observed dielectric relaxations, the dielectric function of electric modulus, M * (  M ' jM "  1/  * , where ,

j   1 ) is used. The merit of this

function is that it can greatly lessen the background and therefore is a powerful function in revealing the relaxation shadowed by the background [15]. Figure 3 shows the imaginary part of the electric modulus M T  as a function of temperature under various frequencies ranging from 100 Hz to 1 MHz for SrTi1-x(Al, Nb)xO3 (x=0, 0.005, 0.05, and 0.1) ceramics. Owing to the absence of the background, a set of thermally activated relaxation can be clearly seen for all samples. The top-right inset of Fig.3 (a) is an enlarged view of the high-temperature range as indicated by the arrow, which clearly shows the high-temperature relaxation. This relaxation becomes more and more distinct with increasing the (Al, Nb)-doping level. The M T  spectra confirms two sets of dielectric relaxations in both pure and doped samples. For brevity, the low- and high-temperature relaxations are labeled as LTR and HTR, respectively. To calculated the relaxation parameters，The top-left insets of Fig. 3 present the measuring frequency

f

versus the reciprocal of the peak temperature 1000 / TP for the corresponding samples according to the Arrhenius law,

3

f  f0exp( Ea / kBTP ) where

(1)

f 0 is the pre-exponential factor, Ea is the activation energy, and kB is the Boltzmann constant. The

calculated values of

f 0 and Ea are given in Table I. From which we can see that the activation energy of TLR

for all samples is very comparable with the typical value (1.0 eV) of the dipolar relaxation caused by oxygen-vacancy diffusion in perovskites or related oxides [16]. Hence, this dielectric relaxation can be reasonably ascribed to be associated with the migration of oxygen vacancies in STO ceramic. To obtain more information about the HTR, the dielectric function of complex impedance, Z *  Z ' jZ " , a powerful function in separating the dielectric contributions from grain and grain-boundary, is used. Figure 4(a) shows the complex impedance spectra plotting by Z " vs Z ' , for the pure STO at selected temperatures. It is seen that when T  320 oC, the spectrum behaves as an semicircular arc indicative of very large resistance. The arc is depressed by increasing temperature indicative of the semiconductive nature of the sample. At T  360 oC, the arc is found to be followed by a small tail at the lowest frequencies. The tail becomes more and more pronounced with increasing temperature. This finding indicates that another dielectric response becomes active when T  360 oC. Theoretically, the low- and high-frequency responses are associated with the grain-boundary and grain, respective. To further convince this point, we conduct ac conductivity analysis. Figure 4(b) shows the ac conductivity as a function of frequency recorded at different temperatures. The noteworthy observation is that the curves can be clarified into two regions (I and II). In region II, the curves behave as plateaus in the low-frequency range followed by linear increases (in log-log scale) in the high-frequency range. This behavior is the typical feature of the universal dielectric response (UDR) [17]:

 (T , f )   dc  A(T ) f s where

(2)

 dc is the dc conductivity, A(T ) and s are temperature-dependent constants. In region I, only the

low-frequency plateaus can be obtained, the high-frequency linear increase goes out of the measuring frequency

4

window. The plateaus in region I and II are reported to be related to the dc conductivity of grains and grain boundaries, respectively [18]. The Arrhenius plots of the dc conductivity deduced from regions I and II are plotted in Fig. 4(c). The activation energies are calculated to be 1.04 and 1.42 eV, which agree well with those of the LTR and HTR. This finding confirms that the LTR and HTR are caused by grains and grain boundaries, respectively. Additionally, the demarking temperature separating regions I and II found to be ~360 oC is in good agreement with that deduced from the impedance results, which further confirms that the grain-boundary relaxation becomes active in the temperature range of T  360 oC. Based on the above points, the present results can be well explained: the observed relaxations are

induced by

the same hopping process of oxygen vacancies, which firstly causes a dipolar relaxation (LTR) inside the grains and then yields a Maxwell-Wagner relaxation (HTR) at grain boundaries therein the vacancies are blocked giving rise to space charge effect at the boundaries. The (Al, Nb) co-doping has a main influence on the grain boundaries. It reduces the energy barrier at the grain boundaries leading to better conductivity of the doped samples. This effect also leads to less oxygen vacancies being blocked by the boundaries, giving rise to smaller capacitance of the boundaries and enhanced electric modulus peak of the HTR in the doped samples. 4.

Conclusions The effects of (Al, Nb) co-doping on the dielectric properties of SrTiO3 were investigated in temperature range

of 50- 600 oC and frequency range of 102-106 Hz. Two relaxations were observed in both pure and doped samples. The low-temperature relaxation is argued to a dipolar relaxation caused by oxygen vacancies hopping motions inside grains. The high-temperature one is the Maxwell-Wagner relaxation caused by oxygen vacancies blocked by and grain boundaries. The (Al, Nb) co-doping has no obvious influence on the grain relaxation, but notably depresses the energy barrier at the grain boundaries. Acknowledgments The authors thank financial support from National Natural Science Foundation of China (Grant Nos. 51572001, 5

11404002 and 11404003). This work was supported in part by the Weak Signal-Detecting Materials and Devices Integration of Anhui University (Grant No. Y01008411). References [1] J.G. Bednorz, K.A. Müller, Sr1−xCaxTiO3: An XY Quantum Ferroelectric with Transition to Randomness, Phys. Rev. Lett. 52 (1984) 2289-2292. [2] C. Ang, Z. Yu, Oxygen-vacancy-related low-frequency dielectric relaxation and electrical conduction in Bi:SrTiO3, Phys. Rev. B 62 (2000) 228-236. [3] A. Tkach, P.M. Vilarinho, A.L. Kholkin, A. Pashkin, S. Veljko, J. Petzelt, Broad-band dielectric spectroscopy analysis of relaxational dynamics in Mn-doped SrTiO3 ceramics, Phys. Rev. B 73 (2006) 104113. [4] A.T. Mitsui, W.B. Westphal, Dielectric and X-Ray Studies of CaxBa1-xTiO3 and CaxSr1-xTiO3, Phys. Rev. 124 (1961) 1354-1359. [5] Y. Zhi, A. Chen, P.M. Vilarinho, P.Q. Manatas, J.L. Baptista, Dielectric Relaxation Behaviour of Bi:SrTiO3: I The Low Temperature Pemittiviy Peak, J. Eur. Ceram. Soc. 18 (1998) 1613-1619. [6] G. Triani, A. Hilton, B. Ricketts, Dielectric energy storage in Pb xSr1-xTiO3 ceramics, J. Mater. Sci. Mater. Electron. 12 (2001) 17–20. [7] Z.Y. Shen, Y.M. Li, Z.M. Wang, Y. Hong, R.H. Liao, Structural characteristics and dielectric properties of Nd-doped SrTiO3 ceramics by introducing Ti vacancies for valence compensation, Adv. Mater. Res. 1435 (2011) 284-286. [8] G. Li, H. Liu, Z. Wang, H. Hao, M. Cao, Z. Yu, Dielectric properties and relaxation behavior of Sm substituted SrTiO3 ceramics, J. Mater. Sci. Mater. Electron. 25 (2014) 4418-4424. [9] Z.Y. Shen, Y.M. Li, Q.G. Hu, W.Q. Luo, Z.M. Wang, Dielectric properties of B–site charge balanced Dy– doped SrTiO3 ceramics for energy storage, J. Electroceram. 34 (2015) 236–240. [10] X.F. Wang, Q.B. Hu, L.B. Li, X.M. Lu, Effect of Pr substitution on structural and dielectric properties of SrTiO3, J. Appl. Phys. 112 (2012) 044106. [11] W.B. Hu, Y. Liu, R.L. Withers, T.J. Frankcombe, L. Norén, A. Snashall, M. Kitchin, P. Smith, B. Gong, H. Chen, J. Schiemer, F. Brink, J. Wong-Leung, Electron-pinned defect-dipoles for high-performance colossal permittivity materials, Nature Mater. 12 (2013) 821-826. [12] X.J. Cheng, Z.W. Li, J.G. Wu, Colossal permittivity in ceramics of TiO2 co-doped with niobium and trivalent cation, J. Mater. Chem. A 3 (2015) 5805-5810. 6

[13] C.C. Wang, C.M. Lei, G.J. Wang, X.H. Xun, T. Li, S.G. Huang, H. wang, and Y.D. Li, Oxygen-vacancy-related dielectric relaxations in SrTiO3 at high temperatures, J. Appl. Phys. 113 (2013) 094103. [14] R. Gerhardt, Impedance and dielectric spectroscopy revised: distinguish localized relaxation from long-range conductivity, J. Phys. Chem. Solids 55 (1994) 1491-1506. [15] C.C. Wang, J. Wang, X.H. Sun, L.N. Liu, J. Zhang, J. Zheng, C. Cheng, Oxygen-vacancy-related dielectric relaxations in Na0.5K0.5NbO3, Solid State Commun. 179 (2014) 29–33. [16] See for example, C.C. Wang, M.N. Zhang, K.B. Xu, G.J. Wang, Origin of high-temperature relaxor-like behavior in CaCu3Ti4O12, J. Appl. Phys. 112 (2012) 034109. [17] A.K. Jonscher, The ‘universal’ dielectric response, Nature 267 (1977) 673-679. [18] L.N. Liu, C.C. Wang, X.H. Sun, G.J. Wang, C.M. Lei, T. Li, oxygen-vacancy-related relaxations of Sr3CuNb2O3 at high temperature, J. Alloys Compd. 552 (2013) 279–282.

TABLE I. Relaxation parameters for the LTR and HTR of SrTi1-x(Al, Nb)xO3 samples LTR Doping level (x)

HTR

E a (eV)

f 0 (Hz)

E a (eV)

f 0 (Hz)

0

0.97

1.10×1013

1.52

2.85×1014

0.005

0.96

5.10×1012

0.98

8.75×1011

0.05

0.91

3.77×1012

1.15

5.40×1013

0.1

0.93

4.04×1012

1.13

4.75×1013

Fig. 1. XRD patterns of the SrTi1-x(Al, Nb)xO3 samples.

7

Fig. 2. Temperature dependence of the loss tangent (main panel) and dielectric permittivity (inset) for SrTi1-x(Al, Nb)xO3 samples recorded at 100 Hz.

Fig. 3 The imaginary part of electrical modulus as a function of temperature for SrTi1-x(Al, Nb)xO3 samples measured at the frequencies of 102, 103, 104, 105, 5105, and 106 Hz (increasing from left to right). The top-left insets in each figures are the Arrhenius plots of the LTR and HTR for corresponding samples. The solid lines the these inset are the linear fitting results. The top-right inset in Fig.3(a) is an enlarged view of the rectangle region as indicated by the arrow.

Fig. 4 The complex impedance plots (a) and frequency dependence of the ac conductivity (b) of the pure STO measured at different temperatures. The hatched area in Fig.4 (b) indicates region II. (c) The Arrhenius plots of the dc conductivity with the data deduced from regions I and II. 8

Highlights



The (Al, Nb) co-doped SrTiO3 ceramics were prepared via solid-state reaction method.



Two sets of relaxations resulting from the grain and grain-boundary, respectively.



The co-doping has no obvious influence on the grain-relaxation.



The co-doping notably reduces the activation energy of the grain-boundary-relaxation.

9