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Investigation of dielectric relaxation in BaTiO3 ceramics modified with BiYO3 by impedance spectroscopy
Journal of Alloys and Compounds 653 (2015) 596e603
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Investigation of dielectric relaxation in BaTiO3 ceramics modified with BiYO3 by impedance spectroscopy Yaru Wang, Yongping Pu*, Panpan Zhang School of Materials Science & Engineering, Shaanxi University of Science and Technology, Xi'an 710021, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 July 2015 Received in revised form 31 August 2015 Accepted 1 September 2015 Available online 4 September 2015
The polycrystalline (1 x)BaTiO3 xBiYO3 ((1 x)BT xBY) (x ¼ 0.2, 0.3) ceramics were prepared by a solid state reaction technique. X-ray diffraction (XRD) analysis shows the information of single phase compound with a perovskite structure. Well grain growth is found in SEM range. The dielectric studies shows that the diffusive factor (g) is close to 1.8, which confirms that (1 x)BT xBY exhibits a diffuse phase transition. Moreover, impedance and electric modulus spectroscopy analysis in the frequency of (20 Hze2 MHz) within temperature domain (250e400 C) show that two relaxation processes are involved which are contributed to bulk and grain boundary effects. The plots of Z00 and M00 are dispersed with increase in frequency at different temperatures, which proves the dielectric relaxation. The plots of Z0 and the ac conductivity studies show the NTCR character of the compounds. The experimental value of conduction activation is 0.98 eV, which is near to the conduction activated energy of the electron from second ionization of oxygen vacancies. © 2015 Published by Elsevier B.V.
Keywords: Dielectric relaxation Impedance spectroscopy BaTiO3
1. Introduction Relaxor ferroelectrics have attracted much interest owing to the rich diversity of their structural and physical properties, which are extensively used for the Multilayer Ceramic Capacitor (MLCC) application. BaTiO3 with the perovskite type structure is a ferroelectric compound, which is widely applied for dielectric field. At present, BaTiO3 has been modified to obtain relaxor materials. A crossover to the relaxor state is observed in BaTiO3 by both heterovalent and isovalent ionic substitutions. The relaxor behavior is induced by substitutions either on A- or B- or on both A- and B-sites of the perovskite lattice ABO3, or appears in compounds, such as BaTiO3eBaSnO3 [1]. Promising high temperature dielectric properties have been demonstrated within relaxor complex solid solution in which BiScO3 is an end member: examples include BaTiO3eBiScO3 [2], K0.5Bi0.5TiO3eBiScO3 [3], BaTiO3eBiZn0.5Ti0.5O3eBiScO3 [4], also with Bi3þ as one of the mixed A-site substituents, these include Bi0.5Na0.5TiO3eBaTiO3eK0.5Na0.5NbO3 [5]. Impedance spectroscopy (IS) is a powerful tool to investigate electroceramics [6]. Over the last few decades, it has been successful in studying various electroceramics including BaTiO3 [7]
* Corresponding author. Tel.: þ86 29 86168137. E-mail address: [email protected] (Y. Pu). http://dx.doi.org/10.1016/j.jallcom.2015.09.012 0925-8388/© 2015 Published by Elsevier B.V.
and CaCu3Ti4O12 [8]. IS has been employed to investigate BNT [9,10] and BNT-BKT-KNN [11], with a focus on electrical conductivity at elevated temperature. IS analysis on the dielectric relaxation is critical for better understanding the material system [12]. It enables us to evaluate the relaxation frequency like, bulk like, grain boundaries like and electrode interface effects in the frequency domain of the materials. In this study, BaTiO3 ceramics modified with BiYO3 in order to increase the dielectric properties. Furthermore, the effects of the BiYO3 addition on the relaxor behavior, conduction behavior, impedance, and electric modulus have also been investigated. 2. Experimental procedure (1 x)BaTiO3 xBiYO3 (x ¼ 0.2, 0.3) ceramics, abbreviated as (1 x)BT xBY, were prepared via a conventional solid state reaction technique. The starting materials were high purity (99.9%) BaCO3, TiO2, Bi2O3 and Y2O3. The powders were dried in an oven at 200 C for 24 h and then weighed according to the stoichiometric ratios. All batches were mixed by ball-milling with zirconia grinding media in distilled water for 4 h. After drying, the mixed powders were granulated with polyvinyl alcohol (PVA, 5wt%) and sieved through 80e40 mesh nylon sieve, then pressed into disks with 10 mm in diameter and 1.5 mm in thickness. The samples were heat treated at 600 C for 30 min to eliminate the PVA, then
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sintered at 1300 C for 2 h in air. Phase structure of the ceramics was determined using X-ray diffraction (XRD D/max-2200 PC, RIGAKU, Japan) patterns and Microstructures was studied by scanning electron microscope (JSM-6700, JEOLLtd., Tokyo, Japan). To characterize the electrical properties, the specimens were ground polished to obtain parallel surfaces. In addition, silver paste was applied to opposite parallel faces and coated pellets were fired at 600 C for 10 min. Measurements of dielectric relative permittivity, loss tangent, conduction behavior, impedance, electric modulus as a function of temperature (250e400 C) and frequency (20 Hze2 MHz) were performed using an Agilent 4980A.
The dielectric loss (tand) at low frequencies is high and decreases sharply with frequency as shown in Fig. 2, which can be explained by the following consideration. When an alternating electric field is applied, not only polarization loss but also leakage loss generates. The tand is thus derived from two part, i.e., the polarization loss and the leakage loss. The dielectric loss always can be described by the following formula:
3. Results and discussion
where D denotes the tand; DP denotes the polarization loss; DG denotes the leakage loss. It can be observed that at a certain temperature as frequency u goes to 0, i.e., state electric field, DP goes to 0. That is to say, the tand is almost attributed to the leakage loss at low frequencies. In such a case that the frequency is very low, ut 1, so the tand could be described approximately as below.
3.1. Structural and microstructural analysis Fig. 1 shows the X-ray diffraction patterns of (1 x)BT xBY powders with x ¼ 0.2, 0.3 at room temperature. It is observed that stable perovskite phase is formed in the compositions x ¼ 0.2, 0.3, while the second phase appeared for x ¼ 0.3 compositions. The solubility of BiYO3 in BaTiO3 is found to be smaller than that of yttrium in BaTiO3(12.2 at%) [13], but much higher than bismuth [14]. The XRD pattern confirms that the crystallographic structure of (1 x)BT xBY samples is in cubic symmetry for x ¼ 0.2, 0.3. The SEM micrograph (inside Fig. 1) of 0.7BT-0.3BYceramics indicates that the samples has dense microstructure and the secondary phase are appeared. 3.2. Dielectric study Dielectric constant decreases with the increasing frequency in both cases as shown in Fig. 2. At lower frequency, the permittivities of the compositions are very high and the frequency dispersion of permittivities is stronger with elevation of temperature. High values of ε0 at lower frequencies are due to the different types of polarization (i.e., dipolar, ionic, electronic, and interfacial) in the materials. At high temperature, space charges near grain boundaries and electrode contacts are activated and have their displacement along the field direction. This causes space charge polarization which is highly temperature dependent [15].
D ¼ DP þ DG ¼
Dy
ðεS ε∞ Þut g þ εS þ ε∞ u2 t2 uε0
! 1 ε∞ þ ðεS ε∞ Þ 1 þ u2 t2
g uε0 εS
(1)
(2)
the tand is inversely proportional to frequency, which explains why the tand decreases with frequency at low frequencies. To characterize the dielectric dispersion and diffuseness of BTBY samples, an empirical expression is used [16,17]:
1 1 ðT Tm Þg ; ð1 g 2Þ ¼ ε εm C
(3)
where g and C are modified constants, the diffuseness values g ¼ 1 and 2 are related to a normal ferroelectric and ideal relaxor [18e20], respectively. The experimental data for BT-BY are plotted in Fig. 3. The values of g are found to be 1.738 and 1.788 at 1 kHz for x ¼ 0.2, 0.3, respectively, implying the strong relaxor behavior.
3.3. Impedance and modulus of spectroscopy study Complex impedance spectroscopy (CIS) is an important and powerful technique in studying the electrical properties such as contribution of bulk (grain), grain boundary and electrode polarization of the materials by different equivalent circuits. The complex impedance of the samples can be modeled as a parallel combination of RC (R ¼ resistance and C ¼ capacitance) circuits. The frequency dependence of electrical properties of a materials is represented in terms of complex impedance (Z*) and modulus (M*) [21]. The following are the complex impedance related parameters: complex impedance 00
Z* ¼ Z 0 jZ ¼ Rs j=uCs
(4)
complex modulus 00
M* ¼ 1=ε* ¼ M0 þ jM ¼ juC0 Z
(5)
complex admittance
00 Y* ¼ Y 0 þ jY ¼ 1 Rp þ juCp
(6)
complex permittivity 00
Fig. 1. X-ray diffraction pattern of (1 x)BT xBY (x ¼ 0.2, 0.3) samples and SEM image of 0.7BT-0.3BYceramics.
ε* ¼ ε0 jε
loss tangent
(7)
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Fig. 2. Frequency dependence of the dielectric properties of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
tan d ¼ ε
00
. . . 00 . 00 00 ε0 ¼ M M 0 ¼ Z 0 Z ¼ Y Y 0
(8)
pffiffiffiffiffiffiffi wherej ¼ 1 is the imaginary factor and u ¼ 2pf is the angular frequency. 3.4. Impedance analysis
Fig. 3. ln(1/ε 1/εm)as a function of ln(T Tm) for (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
Complex impedance spectroscopy is well known technique to describe the electrical properties of polycrystalline electroceramics. Grain and grain boundary contributions to the electrical properties of dielectric materials like conductivity, dielectric constant etc. are better analyzed using this technique. The real impedance (Z0 ) variation with frequency is shown in Fig. 4 in a range of frequency of 20 Hze2 MHz within temperature domain 250e400 C for (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics. The magnitude of Z0 decreases with the rise in temperature (NTCR behavior) and their values for all temperatures merge in the higher frequency region. A possibility of increase in the magnitude of Z0 in the samples is observed due to increase in ac conductivity with temperature and
Fig. 4. Variation of real part modulus(Z0 ) with frequency at different temperature of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
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Fig. 5. Variation of imaginary part modulus (Z00 ) with frequency at different temperature of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
frequency. The merging of all the curves at all temperatures in the high frequency region suggests a possible release of space charge and a consequent lowering of the barrier properties in the materials [22e26]. Fig. 5 shows the variation of imaginary part of impedance (Z00 ) with frequency for (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics at different temperatures. With the increase of frequency, imaginary part of impedance (Z00 ) increases initially, attains a peak (Z00 max) and then decreases with frequency at all measured temperatures. The broadening of peak and its shift towards higher frequency side with temperature indicate the presence of temperature dependent electrical relaxation phenomenon, and the relaxation time decreases with increasing temperature [27]. At higher frequency side all the curves are merged which might be due to the reduction in space charge polarization at higher frequency. The plot of Z0 versus Z00 (Nyquist or ColeeCole plots) at different temperatures is shown in Fig. 6. All the semicircles are depression instead of a semicircle centered on the real axis, which indicates non-Debye type relaxation mechanism. Two semicircles can be
observed in the two diagrams, which show that two distinct dielectric relaxation process exist in the samples, and the suppressed semicircular arcs indicate two different contributions from the grain interior and grain boundary. Fig. 7 shows the normalized imaginary parts of the impedance (Z00 /Z00 max) as a function of frequency at the selected temperatures. The peaks are observed with a slight symmetric broadening at each temperature, especially at higher temperature. The asymmetric broadening of the peaks suggests the presence of electrical processes in the material with a spread of relaxation time [28]. 3.5. Modulus analysis The imaginary impedance loss spectra shows a weak grain effect compare to grain boundaries and electrode effect which are sometimes difficult to analyze. As electric modulus spectra highlight the smaller capacitance value, it will magnify the grain effects which will be easier to analyze. So we prepare to electric modulus
Fig. 6. Nyquist plots of Z0 and Z00 (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics at different temperature.
600
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Fig. 7. Normalized imaginary part of the impedance (Z00 /Z00 max)with frequency at different temperature of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
[29]. Fig. 8 represents the real (M0 ) variation against frequency of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics. In the low frequency region, the magnitudes of M0 tend to zero and coincide with each other, which confirms a negligibly small contribution of electrode effect. The value of M0 has followed a continuous dispersal with increase in frequency which may be caused by short range mobility of charge carriers. In M0 plot, all the curves attain to asymptotic form at higher frequencies, because of the stretched exponential character of relaxation time of the materials [30]. . Fig. 9 represents the imaginary (M00 ) variation against frequency of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics. The value of M00 for two samples shows a clear maximum and the peak of M00 max shifts to higher frequencies with elevation of temperature, which indicates a thermally activated dielectric relaxation process in which the hopping mechanism of charge carriers dominates intrinsically [27]. To investigate the relaxation mechanism of the peak of M00 max, the thermally activated parameters during relaxation process are calculated using Arrhenius law:
E t ¼ t0 exp KB T
(9)
Where t0 is the pre-exponential factor (or the relaxation time at infinite temperature), E denotes the activation energy of the relaxation process, T is the absolute temperature, and KB is the Boltzmann constant. The condition uptp ¼ 1 is fulfilled at the peak position, where u ¼ 2pf is the angular frequency of measurement and the subscript p denotes values at peak position. Fig. 10 shows the Arrhenius plots for the peak of M00 max. The relaxation parameter E ¼ 1.03 eV, 0.82 eV, are obtained from the fitting line for x ¼ 0.2, 0.3 respectively. The value of activation energy for x ¼ 0.2 is bigger than that of x ¼ 0.3. The long range charge carriers of x ¼ 0.2 are hard to cross the barrier, so the resistance of x ¼ 0.2 is larger than that of x ¼ 0.3. This result is in accordance with the Fig. 4. Fig. 11 shows the normalized plot of M00 /M00 max at different temperature for (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics. All the peaks indeed collapse into one master curve and almost perfectly
Fig. 8. Variation of real part modulus(M0 ) with frequency at different temperature of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
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Fig. 9. Variation of imaginary part modulus(M00 ) with frequency at different temperature of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
overlap at different temperature. It suggests that the distribution of the relaxation time is temperature independent [31]. These relaxation processes may due to the presence of immobile species at low temperature and defects which become mobile at high temperature [30]. At high frequency, the M00 /M00 max vs log(f/fmax) curve represents the range of frequencies in which the charge carriers are spatially confined to their potential wells, and therefore they can make only localized motions inside the well. The peak indicates the transition from the long-range to the short-range mobility with the evaluation of the frequency. Fig. 12 shows M0 versus M00 graph for (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics. Two semicircles can be observed in the two diagrams, the first semicircle at low frequency regime represents the capacitive grain boundaries effect and the second one at high frequency regime represents the capacitive grain effect. The asymmetric semicircular arc confirms the presence of electrical relaxation phenomenon in the materials [32]. Fig. 13 shows the peak positions of ε00 , M00 and Z00 which uses normalized values to facilitate easy comparison. The combined
plot of M00 and Z00 versus frequency can distinguish whether the short range or long range movement of charge carries is dominant in a relaxation process [31]. The separation of peak positions of M00 and Z00 indicates that the relaxation process is dominated by the short range movement of charge carriers and departs from a ideal Debye type behavior while the frequencies coincidence suggests the long range movement of charge carriers is dominant [33e35]. 3.6. AC conductivity analysis Fig. 14 demonstrates the variation of sac with frequency of (1 x)BT xBY (x ¼ 0.2) ceramics at different temperatures. The sac is calculated using an empirical relaxation sac ¼ uεrε0tand (u ¼ angular frequency, ε0 ¼ vacuum permittivity). The value of sac increases with increase in the whole range of frequency, which is coincide with the change of Fig. 4. The sac indicates a dispersion that shifts to higher frequency with increasing temperature, and it
Fig. 10. Temperature dependence of M00 max for (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
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Fig. 11. Modulus scaling behavior of (M00 /M00 max) vs log(f/fmax) of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
Fig. 12. Nyquist plots of M0 and M00 of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics at different temperature.
monotonously decreases with decreasing frequency and saturates to a constant value at low frequencies. Extrapolating these curves at low frequencies gives the dc conductivity (sdc). The resulting sdc is plotted as the function of reciprocal temperature in Fig. 15 and it well obeys the Arrhenius relation:
s ¼ s0 expðEcond =KB T Þ
(10)
where s0 is the pre-exponential term and Econd is the conduction activation energy. The experimental values of conduction activation, Econd ¼ 0.98 eV, is near to 1 eV which is the conduction activated energy of the electron from second ionization of oxygen vacancies [36]. Therefore, the conducting species in this temperature range is likely to come from the second ionization of oxygen vacancies Vo€ as reported by Li at al [37]. These datas on conductivity give on BT-BY ceramics, the typical relaxor ferroelectric nature can be clearly excluded, and it should be a point defect induced behavior. At higher temperature, the following equations can be happened:
1 OO / O2 þ VO€ þ 2e 2
(11)
It can be imaged that the relaxor ferroelectriclike behavior in the ceramics is attributed to the occurrence of polar cluster due to the point defect ordering. The increasing trend of conductivity may be due to the disordering of cations between the neighboring site and the presence of space charge [38].
4. Conclusions Electrical properties of (1 x)BaTiO3 xBiYO3 ((1 x) BT xBY) (x ¼ 0.2, 0.3) ceramics have been investigated by the conduction behavior, impedance, electric modulus studies. Impedance and electric modulus spectroscopy analysis in the frequency of (20 Hze2 MHz) within temperature domain (250e400 C) shows two relaxation processes. Real and imaginary parts of complex impedance and modulus properties of the materials have followed a continuous dispersal with increase in
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frequency at different temperature, which proves the dielectric relaxation. The plot of the ac conductivity studies shows the NTCR character of the compounds, and the activated energy is not far from the electron from second ionization of oxygen vacancies.
Acknowledgments This research was supported by the National Natural Science Foundation of China (51372144) and the Key Program of Innovative Research Team of Shaanxi Province (2014KCT-06).
References
Fig. 13. Normalized ε00 , M00 and Z00 spectra of 0.8BT-0.2BY ceramics at 400 C.
Fig. 14. Frequency dependence of AC conductivity at different temperatures of 0.8BT0.2BY ceramics.
Fig. 15. Temperature dependence of conductivity for 0.8BT-0.2BY ceramics.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Investigation of dielectric relaxation in BaTiO3 ceramics modified with BiYO3 by impedance spectroscopy Yaru Wang, Yongping Pu*, Panpan Zhang School of Materials Science & Engineering, Shaanxi University of Science and Technology, Xi'an 710021, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 July 2015 Received in revised form 31 August 2015 Accepted 1 September 2015 Available online 4 September 2015
The polycrystalline (1 x)BaTiO3 xBiYO3 ((1 x)BT xBY) (x ¼ 0.2, 0.3) ceramics were prepared by a solid state reaction technique. X-ray diffraction (XRD) analysis shows the information of single phase compound with a perovskite structure. Well grain growth is found in SEM range. The dielectric studies shows that the diffusive factor (g) is close to 1.8, which confirms that (1 x)BT xBY exhibits a diffuse phase transition. Moreover, impedance and electric modulus spectroscopy analysis in the frequency of (20 Hze2 MHz) within temperature domain (250e400 C) show that two relaxation processes are involved which are contributed to bulk and grain boundary effects. The plots of Z00 and M00 are dispersed with increase in frequency at different temperatures, which proves the dielectric relaxation. The plots of Z0 and the ac conductivity studies show the NTCR character of the compounds. The experimental value of conduction activation is 0.98 eV, which is near to the conduction activated energy of the electron from second ionization of oxygen vacancies. © 2015 Published by Elsevier B.V.
Keywords: Dielectric relaxation Impedance spectroscopy BaTiO3
1. Introduction Relaxor ferroelectrics have attracted much interest owing to the rich diversity of their structural and physical properties, which are extensively used for the Multilayer Ceramic Capacitor (MLCC) application. BaTiO3 with the perovskite type structure is a ferroelectric compound, which is widely applied for dielectric field. At present, BaTiO3 has been modified to obtain relaxor materials. A crossover to the relaxor state is observed in BaTiO3 by both heterovalent and isovalent ionic substitutions. The relaxor behavior is induced by substitutions either on A- or B- or on both A- and B-sites of the perovskite lattice ABO3, or appears in compounds, such as BaTiO3eBaSnO3 [1]. Promising high temperature dielectric properties have been demonstrated within relaxor complex solid solution in which BiScO3 is an end member: examples include BaTiO3eBiScO3 [2], K0.5Bi0.5TiO3eBiScO3 [3], BaTiO3eBiZn0.5Ti0.5O3eBiScO3 [4], also with Bi3þ as one of the mixed A-site substituents, these include Bi0.5Na0.5TiO3eBaTiO3eK0.5Na0.5NbO3 [5]. Impedance spectroscopy (IS) is a powerful tool to investigate electroceramics [6]. Over the last few decades, it has been successful in studying various electroceramics including BaTiO3 [7]
* Corresponding author. Tel.: þ86 29 86168137. E-mail address: [email protected] (Y. Pu). http://dx.doi.org/10.1016/j.jallcom.2015.09.012 0925-8388/© 2015 Published by Elsevier B.V.
and CaCu3Ti4O12 [8]. IS has been employed to investigate BNT [9,10] and BNT-BKT-KNN [11], with a focus on electrical conductivity at elevated temperature. IS analysis on the dielectric relaxation is critical for better understanding the material system [12]. It enables us to evaluate the relaxation frequency like, bulk like, grain boundaries like and electrode interface effects in the frequency domain of the materials. In this study, BaTiO3 ceramics modified with BiYO3 in order to increase the dielectric properties. Furthermore, the effects of the BiYO3 addition on the relaxor behavior, conduction behavior, impedance, and electric modulus have also been investigated. 2. Experimental procedure (1 x)BaTiO3 xBiYO3 (x ¼ 0.2, 0.3) ceramics, abbreviated as (1 x)BT xBY, were prepared via a conventional solid state reaction technique. The starting materials were high purity (99.9%) BaCO3, TiO2, Bi2O3 and Y2O3. The powders were dried in an oven at 200 C for 24 h and then weighed according to the stoichiometric ratios. All batches were mixed by ball-milling with zirconia grinding media in distilled water for 4 h. After drying, the mixed powders were granulated with polyvinyl alcohol (PVA, 5wt%) and sieved through 80e40 mesh nylon sieve, then pressed into disks with 10 mm in diameter and 1.5 mm in thickness. The samples were heat treated at 600 C for 30 min to eliminate the PVA, then
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sintered at 1300 C for 2 h in air. Phase structure of the ceramics was determined using X-ray diffraction (XRD D/max-2200 PC, RIGAKU, Japan) patterns and Microstructures was studied by scanning electron microscope (JSM-6700, JEOLLtd., Tokyo, Japan). To characterize the electrical properties, the specimens were ground polished to obtain parallel surfaces. In addition, silver paste was applied to opposite parallel faces and coated pellets were fired at 600 C for 10 min. Measurements of dielectric relative permittivity, loss tangent, conduction behavior, impedance, electric modulus as a function of temperature (250e400 C) and frequency (20 Hze2 MHz) were performed using an Agilent 4980A.
The dielectric loss (tand) at low frequencies is high and decreases sharply with frequency as shown in Fig. 2, which can be explained by the following consideration. When an alternating electric field is applied, not only polarization loss but also leakage loss generates. The tand is thus derived from two part, i.e., the polarization loss and the leakage loss. The dielectric loss always can be described by the following formula:
3. Results and discussion
where D denotes the tand; DP denotes the polarization loss; DG denotes the leakage loss. It can be observed that at a certain temperature as frequency u goes to 0, i.e., state electric field, DP goes to 0. That is to say, the tand is almost attributed to the leakage loss at low frequencies. In such a case that the frequency is very low, ut 1, so the tand could be described approximately as below.
3.1. Structural and microstructural analysis Fig. 1 shows the X-ray diffraction patterns of (1 x)BT xBY powders with x ¼ 0.2, 0.3 at room temperature. It is observed that stable perovskite phase is formed in the compositions x ¼ 0.2, 0.3, while the second phase appeared for x ¼ 0.3 compositions. The solubility of BiYO3 in BaTiO3 is found to be smaller than that of yttrium in BaTiO3(12.2 at%) [13], but much higher than bismuth [14]. The XRD pattern confirms that the crystallographic structure of (1 x)BT xBY samples is in cubic symmetry for x ¼ 0.2, 0.3. The SEM micrograph (inside Fig. 1) of 0.7BT-0.3BYceramics indicates that the samples has dense microstructure and the secondary phase are appeared. 3.2. Dielectric study Dielectric constant decreases with the increasing frequency in both cases as shown in Fig. 2. At lower frequency, the permittivities of the compositions are very high and the frequency dispersion of permittivities is stronger with elevation of temperature. High values of ε0 at lower frequencies are due to the different types of polarization (i.e., dipolar, ionic, electronic, and interfacial) in the materials. At high temperature, space charges near grain boundaries and electrode contacts are activated and have their displacement along the field direction. This causes space charge polarization which is highly temperature dependent [15].
D ¼ DP þ DG ¼
Dy
ðεS ε∞ Þut g þ εS þ ε∞ u2 t2 uε0
! 1 ε∞ þ ðεS ε∞ Þ 1 þ u2 t2
g uε0 εS
(1)
(2)
the tand is inversely proportional to frequency, which explains why the tand decreases with frequency at low frequencies. To characterize the dielectric dispersion and diffuseness of BTBY samples, an empirical expression is used [16,17]:
1 1 ðT Tm Þg ; ð1 g 2Þ ¼ ε εm C
(3)
where g and C are modified constants, the diffuseness values g ¼ 1 and 2 are related to a normal ferroelectric and ideal relaxor [18e20], respectively. The experimental data for BT-BY are plotted in Fig. 3. The values of g are found to be 1.738 and 1.788 at 1 kHz for x ¼ 0.2, 0.3, respectively, implying the strong relaxor behavior.
3.3. Impedance and modulus of spectroscopy study Complex impedance spectroscopy (CIS) is an important and powerful technique in studying the electrical properties such as contribution of bulk (grain), grain boundary and electrode polarization of the materials by different equivalent circuits. The complex impedance of the samples can be modeled as a parallel combination of RC (R ¼ resistance and C ¼ capacitance) circuits. The frequency dependence of electrical properties of a materials is represented in terms of complex impedance (Z*) and modulus (M*) [21]. The following are the complex impedance related parameters: complex impedance 00
Z* ¼ Z 0 jZ ¼ Rs j=uCs
(4)
complex modulus 00
M* ¼ 1=ε* ¼ M0 þ jM ¼ juC0 Z
(5)
complex admittance
00 Y* ¼ Y 0 þ jY ¼ 1 Rp þ juCp
(6)
complex permittivity 00
Fig. 1. X-ray diffraction pattern of (1 x)BT xBY (x ¼ 0.2, 0.3) samples and SEM image of 0.7BT-0.3BYceramics.
ε* ¼ ε0 jε
loss tangent
(7)
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Fig. 2. Frequency dependence of the dielectric properties of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
tan d ¼ ε
00
. . . 00 . 00 00 ε0 ¼ M M 0 ¼ Z 0 Z ¼ Y Y 0
(8)
pffiffiffiffiffiffiffi wherej ¼ 1 is the imaginary factor and u ¼ 2pf is the angular frequency. 3.4. Impedance analysis
Fig. 3. ln(1/ε 1/εm)as a function of ln(T Tm) for (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
Complex impedance spectroscopy is well known technique to describe the electrical properties of polycrystalline electroceramics. Grain and grain boundary contributions to the electrical properties of dielectric materials like conductivity, dielectric constant etc. are better analyzed using this technique. The real impedance (Z0 ) variation with frequency is shown in Fig. 4 in a range of frequency of 20 Hze2 MHz within temperature domain 250e400 C for (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics. The magnitude of Z0 decreases with the rise in temperature (NTCR behavior) and their values for all temperatures merge in the higher frequency region. A possibility of increase in the magnitude of Z0 in the samples is observed due to increase in ac conductivity with temperature and
Fig. 4. Variation of real part modulus(Z0 ) with frequency at different temperature of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
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Fig. 5. Variation of imaginary part modulus (Z00 ) with frequency at different temperature of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
frequency. The merging of all the curves at all temperatures in the high frequency region suggests a possible release of space charge and a consequent lowering of the barrier properties in the materials [22e26]. Fig. 5 shows the variation of imaginary part of impedance (Z00 ) with frequency for (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics at different temperatures. With the increase of frequency, imaginary part of impedance (Z00 ) increases initially, attains a peak (Z00 max) and then decreases with frequency at all measured temperatures. The broadening of peak and its shift towards higher frequency side with temperature indicate the presence of temperature dependent electrical relaxation phenomenon, and the relaxation time decreases with increasing temperature [27]. At higher frequency side all the curves are merged which might be due to the reduction in space charge polarization at higher frequency. The plot of Z0 versus Z00 (Nyquist or ColeeCole plots) at different temperatures is shown in Fig. 6. All the semicircles are depression instead of a semicircle centered on the real axis, which indicates non-Debye type relaxation mechanism. Two semicircles can be
observed in the two diagrams, which show that two distinct dielectric relaxation process exist in the samples, and the suppressed semicircular arcs indicate two different contributions from the grain interior and grain boundary. Fig. 7 shows the normalized imaginary parts of the impedance (Z00 /Z00 max) as a function of frequency at the selected temperatures. The peaks are observed with a slight symmetric broadening at each temperature, especially at higher temperature. The asymmetric broadening of the peaks suggests the presence of electrical processes in the material with a spread of relaxation time [28]. 3.5. Modulus analysis The imaginary impedance loss spectra shows a weak grain effect compare to grain boundaries and electrode effect which are sometimes difficult to analyze. As electric modulus spectra highlight the smaller capacitance value, it will magnify the grain effects which will be easier to analyze. So we prepare to electric modulus
Fig. 6. Nyquist plots of Z0 and Z00 (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics at different temperature.
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Fig. 7. Normalized imaginary part of the impedance (Z00 /Z00 max)with frequency at different temperature of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
[29]. Fig. 8 represents the real (M0 ) variation against frequency of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics. In the low frequency region, the magnitudes of M0 tend to zero and coincide with each other, which confirms a negligibly small contribution of electrode effect. The value of M0 has followed a continuous dispersal with increase in frequency which may be caused by short range mobility of charge carriers. In M0 plot, all the curves attain to asymptotic form at higher frequencies, because of the stretched exponential character of relaxation time of the materials [30]. . Fig. 9 represents the imaginary (M00 ) variation against frequency of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics. The value of M00 for two samples shows a clear maximum and the peak of M00 max shifts to higher frequencies with elevation of temperature, which indicates a thermally activated dielectric relaxation process in which the hopping mechanism of charge carriers dominates intrinsically [27]. To investigate the relaxation mechanism of the peak of M00 max, the thermally activated parameters during relaxation process are calculated using Arrhenius law:
E t ¼ t0 exp KB T
(9)
Where t0 is the pre-exponential factor (or the relaxation time at infinite temperature), E denotes the activation energy of the relaxation process, T is the absolute temperature, and KB is the Boltzmann constant. The condition uptp ¼ 1 is fulfilled at the peak position, where u ¼ 2pf is the angular frequency of measurement and the subscript p denotes values at peak position. Fig. 10 shows the Arrhenius plots for the peak of M00 max. The relaxation parameter E ¼ 1.03 eV, 0.82 eV, are obtained from the fitting line for x ¼ 0.2, 0.3 respectively. The value of activation energy for x ¼ 0.2 is bigger than that of x ¼ 0.3. The long range charge carriers of x ¼ 0.2 are hard to cross the barrier, so the resistance of x ¼ 0.2 is larger than that of x ¼ 0.3. This result is in accordance with the Fig. 4. Fig. 11 shows the normalized plot of M00 /M00 max at different temperature for (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics. All the peaks indeed collapse into one master curve and almost perfectly
Fig. 8. Variation of real part modulus(M0 ) with frequency at different temperature of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
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Fig. 9. Variation of imaginary part modulus(M00 ) with frequency at different temperature of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
overlap at different temperature. It suggests that the distribution of the relaxation time is temperature independent [31]. These relaxation processes may due to the presence of immobile species at low temperature and defects which become mobile at high temperature [30]. At high frequency, the M00 /M00 max vs log(f/fmax) curve represents the range of frequencies in which the charge carriers are spatially confined to their potential wells, and therefore they can make only localized motions inside the well. The peak indicates the transition from the long-range to the short-range mobility with the evaluation of the frequency. Fig. 12 shows M0 versus M00 graph for (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics. Two semicircles can be observed in the two diagrams, the first semicircle at low frequency regime represents the capacitive grain boundaries effect and the second one at high frequency regime represents the capacitive grain effect. The asymmetric semicircular arc confirms the presence of electrical relaxation phenomenon in the materials [32]. Fig. 13 shows the peak positions of ε00 , M00 and Z00 which uses normalized values to facilitate easy comparison. The combined
plot of M00 and Z00 versus frequency can distinguish whether the short range or long range movement of charge carries is dominant in a relaxation process [31]. The separation of peak positions of M00 and Z00 indicates that the relaxation process is dominated by the short range movement of charge carriers and departs from a ideal Debye type behavior while the frequencies coincidence suggests the long range movement of charge carriers is dominant [33e35]. 3.6. AC conductivity analysis Fig. 14 demonstrates the variation of sac with frequency of (1 x)BT xBY (x ¼ 0.2) ceramics at different temperatures. The sac is calculated using an empirical relaxation sac ¼ uεrε0tand (u ¼ angular frequency, ε0 ¼ vacuum permittivity). The value of sac increases with increase in the whole range of frequency, which is coincide with the change of Fig. 4. The sac indicates a dispersion that shifts to higher frequency with increasing temperature, and it
Fig. 10. Temperature dependence of M00 max for (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
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Fig. 11. Modulus scaling behavior of (M00 /M00 max) vs log(f/fmax) of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics.
Fig. 12. Nyquist plots of M0 and M00 of (1 x)BT xBY (x ¼ 0.2, 0.3) ceramics at different temperature.
monotonously decreases with decreasing frequency and saturates to a constant value at low frequencies. Extrapolating these curves at low frequencies gives the dc conductivity (sdc). The resulting sdc is plotted as the function of reciprocal temperature in Fig. 15 and it well obeys the Arrhenius relation:
s ¼ s0 expðEcond =KB T Þ
(10)
where s0 is the pre-exponential term and Econd is the conduction activation energy. The experimental values of conduction activation, Econd ¼ 0.98 eV, is near to 1 eV which is the conduction activated energy of the electron from second ionization of oxygen vacancies [36]. Therefore, the conducting species in this temperature range is likely to come from the second ionization of oxygen vacancies Vo€ as reported by Li at al [37]. These datas on conductivity give on BT-BY ceramics, the typical relaxor ferroelectric nature can be clearly excluded, and it should be a point defect induced behavior. At higher temperature, the following equations can be happened:
1 OO / O2 þ VO€ þ 2e 2
(11)
It can be imaged that the relaxor ferroelectriclike behavior in the ceramics is attributed to the occurrence of polar cluster due to the point defect ordering. The increasing trend of conductivity may be due to the disordering of cations between the neighboring site and the presence of space charge [38].
4. Conclusions Electrical properties of (1 x)BaTiO3 xBiYO3 ((1 x) BT xBY) (x ¼ 0.2, 0.3) ceramics have been investigated by the conduction behavior, impedance, electric modulus studies. Impedance and electric modulus spectroscopy analysis in the frequency of (20 Hze2 MHz) within temperature domain (250e400 C) shows two relaxation processes. Real and imaginary parts of complex impedance and modulus properties of the materials have followed a continuous dispersal with increase in
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frequency at different temperature, which proves the dielectric relaxation. The plot of the ac conductivity studies shows the NTCR character of the compounds, and the activated energy is not far from the electron from second ionization of oxygen vacancies.
Acknowledgments This research was supported by the National Natural Science Foundation of China (51372144) and the Key Program of Innovative Research Team of Shaanxi Province (2014KCT-06).
References
Fig. 13. Normalized ε00 , M00 and Z00 spectra of 0.8BT-0.2BY ceramics at 400 C.
Fig. 14. Frequency dependence of AC conductivity at different temperatures of 0.8BT0.2BY ceramics.
Fig. 15. Temperature dependence of conductivity for 0.8BT-0.2BY ceramics.
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