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Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry
Journal of Alloys and Compounds xxx (xxxx) xxx
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Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry Xu Guo, Yongping Pu*, Wen Wang, Jiamin Ji, Mengdie Yang, Ruike Shi, Jingwei Li School of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science & 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 10 September 2019 Received in revised form 28 October 2019 Accepted 29 October 2019 Available online xxx
The environmentally friendly and high effective colossal permittivity (CP, > 104) ceramic materials play an indispensable role in the development of electronic field. In this research, SrTi1-xTaxO3 (0.000 x 0.014) (STTO) ceramic samples were fabricated by sintering in nitrogen atmosphere. It can be found that the CP (22790@1 kHz, 22063@1 MHz), low dielectric loss (0.012@1 kHz, 0.025@1 MHz), good stability of frequency (20e106 Hz) and temperature (25e350 C) were achieved simultaneously for x ¼ 0.010 ceramic samples. Meanwhile, the mechanism of achieving CP and low dielectric loss was deeply discussed based on the sintering of different atmospheres and the analyses of SEM and XPS. Through ’ ’ ,, experimental comparison, it can be detected that modest Ti’Ti V,, O TiTi defect dipoles and VO 3TiTi , TaTi defect clusters were main reasons for CP and dielectric loss, which can hinder the long-range transition of electrons and bring about electrons movement within narrow areas. Consequently, the local polarization of electrons increased the permittivity and suppressed the long range motion of electrons, which resulted in low dielectric loss in a wide range of frequency. © 2019 Elsevier B.V. All rights reserved.
Keywords: Colossal permittivity Low dielectric loss SrTiO3 Defect chemistry
1. Introduction The environmentally friendly CP (>104) materials have progressively received attention in virtue of their potentiality that can be used not only to make capacitors, but also as electromechanical, thermoelectric and photoelectric transducers [1e6]. Howbeit CP materials are often accompanied by high dielectric loss (>0.1) and strong dependence on frequency, which impedes their applications in practice and production, such as BaTiO3 [7e11], CaCu3Ti4O12 (CCTO) [12e14], NiO [15e18], A (Fe1/2M1/2)O3 [19], ZnO [20] and TiO2 [21e24]. Hence, the materials with CP and low dielectric loss in a wide frequency range can broaden field of application and boost developments of CP materials. Since the model of electron pinning defect dipole (EPDD) has been proposed in Nb þ In co-doped ceramic materials [25], numbers of TiO2-based ceramic materials are deeply discussed [26e32]. It is undeniable that several modified ceramic materials can indeed achieve higher permittivity and lower dielectric loss than materials in other systems based on the theory of EPDD, but there is still a problem that dielectric loss cannot satisfy the
* Corresponding author. E-mail address: [email protected] (Y. Pu).
requirements of the market. Then some researchers designed the second phase to reduce the dielectric loss by increasing the activation energy of grain boundary [3,33]. Unfortunately, this way can reduce permittivity in general and the dielectric loss still retains an unsatisfactory lever at high frequency, which is a problem that all CP materials need to solve. It is known that the selection of matrix materials is of great importance to generation of different performance. Additionally, the design of defect chemistry has always been an effective method to improve the properties of materials and appropriate defect concentration can often bring excellent properties [34e36]. SrTiO3 (ST) is a favourable dielectric material, carrying low dielectric loss (<0.01), wide band gap of Eg~3.2 eV and high insulation resistance [37e42]. Moreover, because the quantum fluctuation inhibits the appearance of ferroelectric phase in ST ceramics, paraelectric phase structure still maintains when the temperature is close to 0 K. There is no abrupt change in structure or dielectric properties, which is different from ferroelectrics. Most importantly, ST ceramics have better structural stability and dielectric stability than TiO2 ceramics. Recently, a number of Re ions (Re¼ Er3þ, La3þ,Gd3þ, Nb5þ,Ce3þ, Sm3þ, Y3þ, Zr4þ) modified ST ceramic materials are extensively researched [43e51]. It’s easy to find that the modified ST ceramic materials sintered in nitrogen atmosphere
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Please cite this article as: X. Guo et al., Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152866
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have higher permittivity, revealing that the dielectric properties of the materials can be impacted by the concentration of oxygen vacancies while the internal cause is unclear. The Maxwell-Wagner polarization initiates the CP behavior of Y modified ST. The main factors resulting in CP and low dielectric loss are different defectdipoles contacted with O vacancies, Ti vacancies and Sr vacancies for Sm-, Zr- and Ho-doped ST ceramics. This makes it meaningful to explore the generation of fine dielectric property for ST-based ceramics. In addition, it was reported that the doping modification of Ta ions can obtain excellent dielectric properties [52e54], which is principally due to the fact that Ta ions can bring electronic compensation as donor doping. Thus, the excellent CP behavior is expected for Ta-doped ST ceramics. In this work, the crystal structure, micromorphology and dielectric properties of STTO ceramics have been studied comprehensively. The relation among doping content of Ta ions, point defects and CP behavior are deeply discussed. The mechanism of CP behavior for Ta-doped ST ceramics sintered in different atmospheres is proposed through the analyses of XPS and so on. 2. Experimental procedure High-purity raw materials consisted of TiO2 (99.9%), SrCO3 (99.9%) and Ta2O5 (99.8%) were utilized to fabricate STTO ceramic samples through conventional solid state reaction method. The well-distributed STTO slurry could be acquired by wet ball milling for 8 h with zirconia as medium, and they were dried at 80 C for 24 h firstly and then calcined at 1150 C for 2.5 h under air atmosphere. Some small disks designed thickness of 1.0 mm and diameter of 12 mm could be achieved by adopting a cold isostatical method after repeating ball milling with 2 wt% SiO2 and drying. SiO2 is a typical sintering aid, which can reduce the sintering temperature and save energy. The Ta-doped ST samples were achieved under this condition which is the flow of 65 mL/min with high purity nitrogen at 1490 C for 3h and unmodified pellets were sintered at 1440 C for 3h with the same flow conditions in the same atmosphere. Furthermore, a handful of Ta-doped and undoped ST pellets were sintered at air atmosphere and oxygen atmosphere with the same process severally. Ultimately, a series of tests were used to characterize the properties of ceramic samples after coating silver electrode and annealing at 780 C for 20 min. The diffraction meter of X-ray (XRD D/max-2200PC, RIGAKU, Japan) was applied to analyze the change of the crystal structure and the photoemission spectroscopy of X-ray (XPS, VG Multilab 2000) was used to testify some point defects existed in ceramic samples. Utilizing a field emission scanning electron microscope (FE-SEM) (S-4800, Rigaku Co, Japan) observed the morphology of ceramics which was polished first and then heat treated under 50 C below sintering temperature for 25 min and coated a layer of Au. The relation of dielectric property and frequency (20e2 106 Hz) or temperature (30e450 C) were studied by an impedance analyzer (Agilent 4980A) respectively. 3. Results and discussion Fig. 1(a) describes the XRD patterns of STTO ceramic samples sintered at 1490 C in nitrogen for 3 h. It can be clearly seen that all STTO ceramic samples are pure phase perovskite crystal structure. Although SiO2 is added to the ceramic samples as a sintering aid, it cannot be observed in XRD patterns which is mainly because traces of SiO2 does not affect the crystal structure of STTO ceramic samples. It is easy to detect from Fig. 1(b) that the position of (110) diffraction peak is offset to low angle, which is resulted from the substitution of Ta5þ for Ti4þ, based on the fact that the ionic radius of Ta5þ (0.64 Å) is more closer to Ti4þ (0.61 Å) than Sr2þ (1.44 Å)
Fig. 1. (a) The XRD patterns of STTO ceramic samples sintered in nitrogen atmosphere at 1490 C, (b) the enlarged drawing of (110) diffraction peak.
[53]. This is consistent with the change of lattice parameters (see Table 1) and reflects that Ta5þ have successfully entered the ST crystal structure. All STTO ceramic samples maintain cubic phases at the same time. Fig. 2 displays SEM diagrams of STTO ceramic samples sintered at 1490 C in nitrogen for 3 h. It is unambiguous that the grain size gradually increases with Ta5þ increasing, which may be affected by donor doping or nitrogen atmosphere. It can be found from Fig. 2(f) that all STTO ceramic samples exhibit good relative density (>90%) through the test of Archimedes drainage method, suggesting that STTO ceramic samples have appropriate sintering process. Frequency dependences of dielectric loss and permittivity are clearly depicted in Fig. 3(a). It’s obvious that the larger the Ta ions doping amount is, the higher the permittivity is, which is accompanied with CP behavior when 0.006 x 0.014, manifesting Ta ions play an essential role. Nevertheless, the pattern of permittivity is different from dielectric loss which achieves minimum value of 0.012 (1 KHz) when x ¼ 0.010. According to the researched reports, the Maxwell-Wagner polarization can cause colossal permittivity, but it can also lead to the increase of the dielectric loss at the same time. Furthermore, it leaves at high frequency and results in the decrease of permittivity [14,55]. However, it can be seen that the permittivity for 0.006 x 0.014 ceramic samples hasn’t changed much, implying the Maxwell-Wagner polarization is not a contributor to CP of STTO ceramic samples. It is worth considering that there is a dielectric relaxation peak occurring at high frequency, which is caused by dipole polarization. This indicates the defect dipoles have a vital influence on CP behavior. Besides, it can be found that x ¼ 0.010 ceramic samples hold lower dielectric loss than x ¼ 0.002 and 0.014 ceramic samples, revealing that the concentration of dipoles plays a dominant role in permittivity and dielectric loss within wide frequency. In addition, it cannot be ignored that the doping of Ta5þ can lead to electronic compensation and increase permittivity. Fig. 3(b) illuminates the relation between temperature and dielectric property of STTO. In contrast with x ¼ 0.006 and 0.014 ceramic samples, x ¼ 0.010 ceramic samples maintain steady from room temperature to 400 C and lower dielectric loss, which illustrates that the concentration of defect dipoles has a great influence on temperature stability of ceramic samples. In order to explore the origin of CP of STTO, for x ¼ 0.010 ceramic samples, the connection of temperature, permittivity and dielectric loss at different frequency are demonstrated in Fig. 4(a) and (b) respectively. As the temperature increases, there is a relaxation peak (peak 1) which shifts towards high frequency.
Please cite this article as: X. Guo et al., Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152866
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Table 1 Structure type, relative density, lattice parameter, the permittivity and dielectric loss (1 kHz) for STTO ceramic samples. x
Type
Relative density (%)
Lattice Parameters (Å)
Permittivity
Dielectric loss
0.000 0.002 0.006 0.010 0.014
cubic cubic cubic cubic cubic
96.7 95.9 91.8 94.2 93.5
3.904 3.905 3.907 3.908 3.911
392 7096 12773 22790 25846
0.001 0.055 0.028 0.012 0.036
Fig. 2. (aee) SEM micrographs of surface for STTO ceramic samples sintered in nitrogen atmosphere. (f) Average grain sizes and relative density as a function of x.
Fig. 3. (a) Frequency and (b) temperature dependence of permittivity and dielectric loss for STTO ceramic samples sintered in nitrogen atmospheres.
Arrhenius law is adopted to fit activation energy of relaxation peak, as follows [43,56]:
f ¼ f0 expð Ea = kB TÞ
(1)
where f denotes frequency values at the peak position, kB, f0 and Ea Please cite this article as: X. Guo et al., Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152866
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Fig. 4. Temperature dependences of (a) permittivity and (b) dielectric loss for x ¼ 0.010 ceramic samples at different frequencies, (c) Fitting of activation energy of peak 1.
are the Boltzmann constant, a constant and activation energy severally. It can be observed from Fig. 4(c) that the value of activation energy for peak 1 is 0.77 eV, which is quite close to the energy (0.7 eV) of V,, O secondary ionization [57]. Combined with low dielectric loss, it can be inferred that the oxygen vacancy can form defect dipoles with other point defects to limit the motion of electrons. According to design of the doping mechanism and the sintering system, the following defect equations may exist in ceramic samples.
1 Ta2 O5 þ 2SrO/2Ta,Ti þ 2e’ þ 6O O þ 2SrSr þ O2 [ 2
(2)
1 ,, ’ O O / VO þ 2e þ O2 [ 2
(3)
Ti4þ þ e’ /Ti3þ
(4) 3þ
The variations of the oxygen vacancy and Ti
can be found from
Fig. 5 that display XPS spectrum of (a - c) Ti 2p, (d - f) O 1s for x ¼ 0.002, 0.010, 0.014 ceramic samples sintered in nitrogen. It can be spotted that Ti 2p3/2 binding energy has positions of ~457.3eV/ 458.2eV for Ti3þ/Ti4þ severally. According to some researched works, the peaks coordinated in the position of ~530.49 eV, ~531.60 eV and ~529.41 eV denote oxygen vacancies or eOH, the adsorbed H2O in the surface and TieO bonds respectively [24]. It can be observed that the oxygen vacancy and Ti3þ gradually increase with raising Ta ions, which can be reflected by the change of their fitting area. This proves that Ta ions can urge the formation of Ti3þ and the oxygen vacancy. Besides, combined with dielectric properties analysis, oxygen vacancy and Ti3þ are essential for CP and low dielectric loss, suggesting that there are electronic pinning defect dipoles among oxygen vacancy, Ta,Ti and Ti’Ti . They may be ’ ’ ’ , , , Ti’Ti V,, O TiTi , TiTi TaTi and VO 3TiTi TaTi . The x ¼ 0.010 ceramic samples are selected to study the relationship between dielectric properties and sintering atmosphere because of good dielectric properties. Fig. 6(a) and (b) show the permittivity and dielectric loss for x ¼ 0.010 ceramic samples
Fig. 5. XPS spectrum of (aec) Ti 2p, (def) O 1s for x ¼ 0.002, 0.010, 0.014 ceramic samples sintered in nitrogen atmospheres.
Please cite this article as: X. Guo et al., Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152866
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indicating a thermally activated dielectric relaxation process. Fig. 7(aec) display SEM images of x ¼ 0.010 ceramic samples sintered in nitrogen, air and oxygen atmospheres respectively. Although the grain size of ceramics sintered in air and oxygen is smaller than that sintered in nitrogen, the variation range is finite. This indicates that the doping of Ta ions is a more important factor affecting the increase of the grain size associated with the analysis of Fig. 2. It can be found from Table 2 that there is a great difference in the dielectric properties by changing sintering atmospheres, actually which can be understood that oxygen vacancy is critical to CP behavior. The ceramic samples of x ¼ 0.010 sintered in oxygen atmosphere exhibit lower permittivity and higher dielectric loss and poor frequency stability, which is opposite when sintering atmosphere is nitrogen. Moreover, the permittivity of samples sintered in oxygen has also been improved obviously, which is due to electronic compensation produced by Ta ions doping. The departure of oxygen produces electrons which are subsequently obtained by Ti4þ to become Ti3þ, revealing that the defect dipoles related to the oxygen vacancy and Ti3þ are the main reason to CP and ’ dielectric loss. Hence, Ti’Ti V,, defect dipoles and O TiTi ’ ,, , VO 3TiTi TaTi defect clusters are the reason for CP and low dielectric loss. XPS spectrum of O 1s and Ti 2p for x ¼ 0.010 ceramic samples sintered in different atmospheres are shown in Fig. 8. It is distinct that the amount of Ti3þ and oxygen vacancies raises from nitrogen to oxygen, verifying the correctness of Table 2 analysis. Mechanism for achieving fine dielectric properties is exhibited in Fig. 9. For x ¼ 0.002 ceramic samples sintered in nitrogen, defect dipoles cannot be effectually formed to limit the movement of electrons, resulting to a higher level of dielectric loss than x ¼ 0.010 and 0.014 ceramic samples. While x ¼ 0.010, excellent dielectric ’ properties are created by the moderated Ti’Ti V,, O TiTi defect ’ ,, , dipoles and VO 3TiTi TaTi defect clusters which can localize electrons. When the doping content increases, the point defects concentration increases, which will lead to the expansion of the localized area and the increase of dielectric loss. Eventually, the deterioration of dielectric properties is triggered.
4. Conclusions Fig. 6. Frequency dependence of (a) permittivity and (b) dielectric loss for x ¼ 0.010 ceramic samples sintered in nitrogen atmosphere at different temperatures.
sintered in nitrogen atmosphere as a function of frequency at different temperatures respectively. The permittivity values of x ¼ 0.010 ceramic samples decline at low frequencies (20e104 Hz) with the increase of frequency in the temperature range from 240 to 360 C, and become nearly independent on temperature at high frequencies (104e106 Hz). Additionally, the permittivity values increase rapidly with increasing temperature at low frequencies, which can be attributed to the space polarization. As shown in Fig. 6(b), it can be seen that one dielectric relaxation peak was obtained. When the temperature is higher, the relaxation peaks move towards higher frequencies with increasing temperature,
In this study, high quality and lead free STTO ceramic samples are prepared by sintering in nitrogen atmosphere. It can be perceived that good dielectric properties, as well as good frequency (20e106 Hz) and temperature (25e350 C) stability are achieved at
Table 2 Permittivity and dielectric loss (1 kHz and 1 MHz) of 0.010 ceramic samples in different sintering atmospheres. x ¼ 0.010
N2
Air
O2
Permittivity (1 kHz) Permittivity (1 MHz) Dielectric loss (1 kHz) Dielectric loss (1 MHz)
22790 22063 0.012 0.025
14217 10353 0.029 0.158
6258 2952 0.047 0.558
Fig. 7. SEM images of x ¼ 0.010 ceramic samples in different sintering atmospheres.
Please cite this article as: X. Guo et al., Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152866
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Fig. 8. XPS spectrum of (aec) Ti 2p, (def) O 1s for x ¼ 0.010 ceramic samples sintered in different atmospheres.
Fig. 9. Mechanism of obtaining excellent dielectric properties.
Please cite this article as: X. Guo et al., Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152866
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the same time when x ¼ 0.010 and 0.014. It exhibits the CP of 22790 (1 kHz), 22063 (1 MHz) and dielectric loss of 0.012 (1 kHz), 0.025 (1 MHz) when x ¼ 0.010. Within the test frequency range, the permittivity is more than 20000 and the dielectric loss is less than 0.03, showing excellent frequency stability. Moreover, we deeply discuss the mechanism of achieving the CP and low dielectric loss based on the sintering of different atmospheres, XRD, SEM and XPS. Through experimental comparison, it can be found that Ti’Ti V,, O ’ , Ti’Ti defect dipoles and V,, O 3TiTi TaTi defect clusters are main providers for CP and low dielectric loss, which can hinder the longrange motion of electrons and bring about electrons movement in tiny areas. On the one hand, the local polarization of electrons increases the permittivity. On the other hand, the long range motion of electrons is suppressed, resulting in low dielectric loss. Furthermore, the appropriate defect concentration is also an important factor to the origin of CP and low dielectric loss of STTO. When the concentration of defect dipoles is high, the dielectric loss will rise due to the fact that the range of electronic movement is wide. Consequently, this study achieves excellent dielectric properties in a wide range of frequency and can promote applications and developments of CP materials.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements This work was financed by the National Natural Science Foundation of China (51872175), the International Cooperation Projects of Shaanxi Province (2018KW-027).
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Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry Xu Guo, Yongping Pu*, Wen Wang, Jiamin Ji, Mengdie Yang, Ruike Shi, Jingwei Li School of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science & 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 10 September 2019 Received in revised form 28 October 2019 Accepted 29 October 2019 Available online xxx
The environmentally friendly and high effective colossal permittivity (CP, > 104) ceramic materials play an indispensable role in the development of electronic field. In this research, SrTi1-xTaxO3 (0.000 x 0.014) (STTO) ceramic samples were fabricated by sintering in nitrogen atmosphere. It can be found that the CP (22790@1 kHz, 22063@1 MHz), low dielectric loss (0.012@1 kHz, 0.025@1 MHz), good stability of frequency (20e106 Hz) and temperature (25e350 C) were achieved simultaneously for x ¼ 0.010 ceramic samples. Meanwhile, the mechanism of achieving CP and low dielectric loss was deeply discussed based on the sintering of different atmospheres and the analyses of SEM and XPS. Through ’ ’ ,, experimental comparison, it can be detected that modest Ti’Ti V,, O TiTi defect dipoles and VO 3TiTi , TaTi defect clusters were main reasons for CP and dielectric loss, which can hinder the long-range transition of electrons and bring about electrons movement within narrow areas. Consequently, the local polarization of electrons increased the permittivity and suppressed the long range motion of electrons, which resulted in low dielectric loss in a wide range of frequency. © 2019 Elsevier B.V. All rights reserved.
Keywords: Colossal permittivity Low dielectric loss SrTiO3 Defect chemistry
1. Introduction The environmentally friendly CP (>104) materials have progressively received attention in virtue of their potentiality that can be used not only to make capacitors, but also as electromechanical, thermoelectric and photoelectric transducers [1e6]. Howbeit CP materials are often accompanied by high dielectric loss (>0.1) and strong dependence on frequency, which impedes their applications in practice and production, such as BaTiO3 [7e11], CaCu3Ti4O12 (CCTO) [12e14], NiO [15e18], A (Fe1/2M1/2)O3 [19], ZnO [20] and TiO2 [21e24]. Hence, the materials with CP and low dielectric loss in a wide frequency range can broaden field of application and boost developments of CP materials. Since the model of electron pinning defect dipole (EPDD) has been proposed in Nb þ In co-doped ceramic materials [25], numbers of TiO2-based ceramic materials are deeply discussed [26e32]. It is undeniable that several modified ceramic materials can indeed achieve higher permittivity and lower dielectric loss than materials in other systems based on the theory of EPDD, but there is still a problem that dielectric loss cannot satisfy the
* Corresponding author. E-mail address: [email protected] (Y. Pu).
requirements of the market. Then some researchers designed the second phase to reduce the dielectric loss by increasing the activation energy of grain boundary [3,33]. Unfortunately, this way can reduce permittivity in general and the dielectric loss still retains an unsatisfactory lever at high frequency, which is a problem that all CP materials need to solve. It is known that the selection of matrix materials is of great importance to generation of different performance. Additionally, the design of defect chemistry has always been an effective method to improve the properties of materials and appropriate defect concentration can often bring excellent properties [34e36]. SrTiO3 (ST) is a favourable dielectric material, carrying low dielectric loss (<0.01), wide band gap of Eg~3.2 eV and high insulation resistance [37e42]. Moreover, because the quantum fluctuation inhibits the appearance of ferroelectric phase in ST ceramics, paraelectric phase structure still maintains when the temperature is close to 0 K. There is no abrupt change in structure or dielectric properties, which is different from ferroelectrics. Most importantly, ST ceramics have better structural stability and dielectric stability than TiO2 ceramics. Recently, a number of Re ions (Re¼ Er3þ, La3þ,Gd3þ, Nb5þ,Ce3þ, Sm3þ, Y3þ, Zr4þ) modified ST ceramic materials are extensively researched [43e51]. It’s easy to find that the modified ST ceramic materials sintered in nitrogen atmosphere
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Please cite this article as: X. Guo et al., Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152866
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have higher permittivity, revealing that the dielectric properties of the materials can be impacted by the concentration of oxygen vacancies while the internal cause is unclear. The Maxwell-Wagner polarization initiates the CP behavior of Y modified ST. The main factors resulting in CP and low dielectric loss are different defectdipoles contacted with O vacancies, Ti vacancies and Sr vacancies for Sm-, Zr- and Ho-doped ST ceramics. This makes it meaningful to explore the generation of fine dielectric property for ST-based ceramics. In addition, it was reported that the doping modification of Ta ions can obtain excellent dielectric properties [52e54], which is principally due to the fact that Ta ions can bring electronic compensation as donor doping. Thus, the excellent CP behavior is expected for Ta-doped ST ceramics. In this work, the crystal structure, micromorphology and dielectric properties of STTO ceramics have been studied comprehensively. The relation among doping content of Ta ions, point defects and CP behavior are deeply discussed. The mechanism of CP behavior for Ta-doped ST ceramics sintered in different atmospheres is proposed through the analyses of XPS and so on. 2. Experimental procedure High-purity raw materials consisted of TiO2 (99.9%), SrCO3 (99.9%) and Ta2O5 (99.8%) were utilized to fabricate STTO ceramic samples through conventional solid state reaction method. The well-distributed STTO slurry could be acquired by wet ball milling for 8 h with zirconia as medium, and they were dried at 80 C for 24 h firstly and then calcined at 1150 C for 2.5 h under air atmosphere. Some small disks designed thickness of 1.0 mm and diameter of 12 mm could be achieved by adopting a cold isostatical method after repeating ball milling with 2 wt% SiO2 and drying. SiO2 is a typical sintering aid, which can reduce the sintering temperature and save energy. The Ta-doped ST samples were achieved under this condition which is the flow of 65 mL/min with high purity nitrogen at 1490 C for 3h and unmodified pellets were sintered at 1440 C for 3h with the same flow conditions in the same atmosphere. Furthermore, a handful of Ta-doped and undoped ST pellets were sintered at air atmosphere and oxygen atmosphere with the same process severally. Ultimately, a series of tests were used to characterize the properties of ceramic samples after coating silver electrode and annealing at 780 C for 20 min. The diffraction meter of X-ray (XRD D/max-2200PC, RIGAKU, Japan) was applied to analyze the change of the crystal structure and the photoemission spectroscopy of X-ray (XPS, VG Multilab 2000) was used to testify some point defects existed in ceramic samples. Utilizing a field emission scanning electron microscope (FE-SEM) (S-4800, Rigaku Co, Japan) observed the morphology of ceramics which was polished first and then heat treated under 50 C below sintering temperature for 25 min and coated a layer of Au. The relation of dielectric property and frequency (20e2 106 Hz) or temperature (30e450 C) were studied by an impedance analyzer (Agilent 4980A) respectively. 3. Results and discussion Fig. 1(a) describes the XRD patterns of STTO ceramic samples sintered at 1490 C in nitrogen for 3 h. It can be clearly seen that all STTO ceramic samples are pure phase perovskite crystal structure. Although SiO2 is added to the ceramic samples as a sintering aid, it cannot be observed in XRD patterns which is mainly because traces of SiO2 does not affect the crystal structure of STTO ceramic samples. It is easy to detect from Fig. 1(b) that the position of (110) diffraction peak is offset to low angle, which is resulted from the substitution of Ta5þ for Ti4þ, based on the fact that the ionic radius of Ta5þ (0.64 Å) is more closer to Ti4þ (0.61 Å) than Sr2þ (1.44 Å)
Fig. 1. (a) The XRD patterns of STTO ceramic samples sintered in nitrogen atmosphere at 1490 C, (b) the enlarged drawing of (110) diffraction peak.
[53]. This is consistent with the change of lattice parameters (see Table 1) and reflects that Ta5þ have successfully entered the ST crystal structure. All STTO ceramic samples maintain cubic phases at the same time. Fig. 2 displays SEM diagrams of STTO ceramic samples sintered at 1490 C in nitrogen for 3 h. It is unambiguous that the grain size gradually increases with Ta5þ increasing, which may be affected by donor doping or nitrogen atmosphere. It can be found from Fig. 2(f) that all STTO ceramic samples exhibit good relative density (>90%) through the test of Archimedes drainage method, suggesting that STTO ceramic samples have appropriate sintering process. Frequency dependences of dielectric loss and permittivity are clearly depicted in Fig. 3(a). It’s obvious that the larger the Ta ions doping amount is, the higher the permittivity is, which is accompanied with CP behavior when 0.006 x 0.014, manifesting Ta ions play an essential role. Nevertheless, the pattern of permittivity is different from dielectric loss which achieves minimum value of 0.012 (1 KHz) when x ¼ 0.010. According to the researched reports, the Maxwell-Wagner polarization can cause colossal permittivity, but it can also lead to the increase of the dielectric loss at the same time. Furthermore, it leaves at high frequency and results in the decrease of permittivity [14,55]. However, it can be seen that the permittivity for 0.006 x 0.014 ceramic samples hasn’t changed much, implying the Maxwell-Wagner polarization is not a contributor to CP of STTO ceramic samples. It is worth considering that there is a dielectric relaxation peak occurring at high frequency, which is caused by dipole polarization. This indicates the defect dipoles have a vital influence on CP behavior. Besides, it can be found that x ¼ 0.010 ceramic samples hold lower dielectric loss than x ¼ 0.002 and 0.014 ceramic samples, revealing that the concentration of dipoles plays a dominant role in permittivity and dielectric loss within wide frequency. In addition, it cannot be ignored that the doping of Ta5þ can lead to electronic compensation and increase permittivity. Fig. 3(b) illuminates the relation between temperature and dielectric property of STTO. In contrast with x ¼ 0.006 and 0.014 ceramic samples, x ¼ 0.010 ceramic samples maintain steady from room temperature to 400 C and lower dielectric loss, which illustrates that the concentration of defect dipoles has a great influence on temperature stability of ceramic samples. In order to explore the origin of CP of STTO, for x ¼ 0.010 ceramic samples, the connection of temperature, permittivity and dielectric loss at different frequency are demonstrated in Fig. 4(a) and (b) respectively. As the temperature increases, there is a relaxation peak (peak 1) which shifts towards high frequency.
Please cite this article as: X. Guo et al., Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152866
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Table 1 Structure type, relative density, lattice parameter, the permittivity and dielectric loss (1 kHz) for STTO ceramic samples. x
Type
Relative density (%)
Lattice Parameters (Å)
Permittivity
Dielectric loss
0.000 0.002 0.006 0.010 0.014
cubic cubic cubic cubic cubic
96.7 95.9 91.8 94.2 93.5
3.904 3.905 3.907 3.908 3.911
392 7096 12773 22790 25846
0.001 0.055 0.028 0.012 0.036
Fig. 2. (aee) SEM micrographs of surface for STTO ceramic samples sintered in nitrogen atmosphere. (f) Average grain sizes and relative density as a function of x.
Fig. 3. (a) Frequency and (b) temperature dependence of permittivity and dielectric loss for STTO ceramic samples sintered in nitrogen atmospheres.
Arrhenius law is adopted to fit activation energy of relaxation peak, as follows [43,56]:
f ¼ f0 expð Ea = kB TÞ
(1)
where f denotes frequency values at the peak position, kB, f0 and Ea Please cite this article as: X. Guo et al., Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152866
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Fig. 4. Temperature dependences of (a) permittivity and (b) dielectric loss for x ¼ 0.010 ceramic samples at different frequencies, (c) Fitting of activation energy of peak 1.
are the Boltzmann constant, a constant and activation energy severally. It can be observed from Fig. 4(c) that the value of activation energy for peak 1 is 0.77 eV, which is quite close to the energy (0.7 eV) of V,, O secondary ionization [57]. Combined with low dielectric loss, it can be inferred that the oxygen vacancy can form defect dipoles with other point defects to limit the motion of electrons. According to design of the doping mechanism and the sintering system, the following defect equations may exist in ceramic samples.
1 Ta2 O5 þ 2SrO/2Ta,Ti þ 2e’ þ 6O O þ 2SrSr þ O2 [ 2
(2)
1 ,, ’ O O / VO þ 2e þ O2 [ 2
(3)
Ti4þ þ e’ /Ti3þ
(4) 3þ
The variations of the oxygen vacancy and Ti
can be found from
Fig. 5 that display XPS spectrum of (a - c) Ti 2p, (d - f) O 1s for x ¼ 0.002, 0.010, 0.014 ceramic samples sintered in nitrogen. It can be spotted that Ti 2p3/2 binding energy has positions of ~457.3eV/ 458.2eV for Ti3þ/Ti4þ severally. According to some researched works, the peaks coordinated in the position of ~530.49 eV, ~531.60 eV and ~529.41 eV denote oxygen vacancies or eOH, the adsorbed H2O in the surface and TieO bonds respectively [24]. It can be observed that the oxygen vacancy and Ti3þ gradually increase with raising Ta ions, which can be reflected by the change of their fitting area. This proves that Ta ions can urge the formation of Ti3þ and the oxygen vacancy. Besides, combined with dielectric properties analysis, oxygen vacancy and Ti3þ are essential for CP and low dielectric loss, suggesting that there are electronic pinning defect dipoles among oxygen vacancy, Ta,Ti and Ti’Ti . They may be ’ ’ ’ , , , Ti’Ti V,, O TiTi , TiTi TaTi and VO 3TiTi TaTi . The x ¼ 0.010 ceramic samples are selected to study the relationship between dielectric properties and sintering atmosphere because of good dielectric properties. Fig. 6(a) and (b) show the permittivity and dielectric loss for x ¼ 0.010 ceramic samples
Fig. 5. XPS spectrum of (aec) Ti 2p, (def) O 1s for x ¼ 0.002, 0.010, 0.014 ceramic samples sintered in nitrogen atmospheres.
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indicating a thermally activated dielectric relaxation process. Fig. 7(aec) display SEM images of x ¼ 0.010 ceramic samples sintered in nitrogen, air and oxygen atmospheres respectively. Although the grain size of ceramics sintered in air and oxygen is smaller than that sintered in nitrogen, the variation range is finite. This indicates that the doping of Ta ions is a more important factor affecting the increase of the grain size associated with the analysis of Fig. 2. It can be found from Table 2 that there is a great difference in the dielectric properties by changing sintering atmospheres, actually which can be understood that oxygen vacancy is critical to CP behavior. The ceramic samples of x ¼ 0.010 sintered in oxygen atmosphere exhibit lower permittivity and higher dielectric loss and poor frequency stability, which is opposite when sintering atmosphere is nitrogen. Moreover, the permittivity of samples sintered in oxygen has also been improved obviously, which is due to electronic compensation produced by Ta ions doping. The departure of oxygen produces electrons which are subsequently obtained by Ti4þ to become Ti3þ, revealing that the defect dipoles related to the oxygen vacancy and Ti3þ are the main reason to CP and ’ dielectric loss. Hence, Ti’Ti V,, defect dipoles and O TiTi ’ ,, , VO 3TiTi TaTi defect clusters are the reason for CP and low dielectric loss. XPS spectrum of O 1s and Ti 2p for x ¼ 0.010 ceramic samples sintered in different atmospheres are shown in Fig. 8. It is distinct that the amount of Ti3þ and oxygen vacancies raises from nitrogen to oxygen, verifying the correctness of Table 2 analysis. Mechanism for achieving fine dielectric properties is exhibited in Fig. 9. For x ¼ 0.002 ceramic samples sintered in nitrogen, defect dipoles cannot be effectually formed to limit the movement of electrons, resulting to a higher level of dielectric loss than x ¼ 0.010 and 0.014 ceramic samples. While x ¼ 0.010, excellent dielectric ’ properties are created by the moderated Ti’Ti V,, O TiTi defect ’ ,, , dipoles and VO 3TiTi TaTi defect clusters which can localize electrons. When the doping content increases, the point defects concentration increases, which will lead to the expansion of the localized area and the increase of dielectric loss. Eventually, the deterioration of dielectric properties is triggered.
4. Conclusions Fig. 6. Frequency dependence of (a) permittivity and (b) dielectric loss for x ¼ 0.010 ceramic samples sintered in nitrogen atmosphere at different temperatures.
sintered in nitrogen atmosphere as a function of frequency at different temperatures respectively. The permittivity values of x ¼ 0.010 ceramic samples decline at low frequencies (20e104 Hz) with the increase of frequency in the temperature range from 240 to 360 C, and become nearly independent on temperature at high frequencies (104e106 Hz). Additionally, the permittivity values increase rapidly with increasing temperature at low frequencies, which can be attributed to the space polarization. As shown in Fig. 6(b), it can be seen that one dielectric relaxation peak was obtained. When the temperature is higher, the relaxation peaks move towards higher frequencies with increasing temperature,
In this study, high quality and lead free STTO ceramic samples are prepared by sintering in nitrogen atmosphere. It can be perceived that good dielectric properties, as well as good frequency (20e106 Hz) and temperature (25e350 C) stability are achieved at
Table 2 Permittivity and dielectric loss (1 kHz and 1 MHz) of 0.010 ceramic samples in different sintering atmospheres. x ¼ 0.010
N2
Air
O2
Permittivity (1 kHz) Permittivity (1 MHz) Dielectric loss (1 kHz) Dielectric loss (1 MHz)
22790 22063 0.012 0.025
14217 10353 0.029 0.158
6258 2952 0.047 0.558
Fig. 7. SEM images of x ¼ 0.010 ceramic samples in different sintering atmospheres.
Please cite this article as: X. Guo et al., Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152866
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Fig. 8. XPS spectrum of (aec) Ti 2p, (def) O 1s for x ¼ 0.010 ceramic samples sintered in different atmospheres.
Fig. 9. Mechanism of obtaining excellent dielectric properties.
Please cite this article as: X. Guo et al., Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152866
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the same time when x ¼ 0.010 and 0.014. It exhibits the CP of 22790 (1 kHz), 22063 (1 MHz) and dielectric loss of 0.012 (1 kHz), 0.025 (1 MHz) when x ¼ 0.010. Within the test frequency range, the permittivity is more than 20000 and the dielectric loss is less than 0.03, showing excellent frequency stability. Moreover, we deeply discuss the mechanism of achieving the CP and low dielectric loss based on the sintering of different atmospheres, XRD, SEM and XPS. Through experimental comparison, it can be found that Ti’Ti V,, O ’ , Ti’Ti defect dipoles and V,, O 3TiTi TaTi defect clusters are main providers for CP and low dielectric loss, which can hinder the longrange motion of electrons and bring about electrons movement in tiny areas. On the one hand, the local polarization of electrons increases the permittivity. On the other hand, the long range motion of electrons is suppressed, resulting in low dielectric loss. Furthermore, the appropriate defect concentration is also an important factor to the origin of CP and low dielectric loss of STTO. When the concentration of defect dipoles is high, the dielectric loss will rise due to the fact that the range of electronic movement is wide. Consequently, this study achieves excellent dielectric properties in a wide range of frequency and can promote applications and developments of CP materials.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements This work was financed by the National Natural Science Foundation of China (51872175), the International Cooperation Projects of Shaanxi Province (2018KW-027).
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Please cite this article as: X. Guo et al., Colossal permittivity and low dielectric loss in Ta doped strontium titanate ceramics by designing defect chemistry, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152866