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The dielectric properties and the relaxation phase transition of copper substituted SrBi2Nb2O9 ferroelectric ceramics
Solid State Communications 149 (2009) 2074–2077
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
The dielectric properties and the relaxation phase transition of copper substituted SrBi2 Nb2 O9 ferroelectric ceramics Pinyang Fang, Huiqing Fan ∗ , Jin Li, Xiaohua Jia, Fujun Liang State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China
article
info
Article history: Received 30 March 2009 Received in revised form 8 August 2009 Accepted 10 August 2009 by Y.E. Lozovik Available online 15 August 2009 PACS: 77.80.-e Keywords: A: Ferroelectrics A: SBN D: Relaxation phase transition
abstract The dielectric properties and the relaxation phase transition of Sr1−x Cux Bi2 Nb2 O9 (SCBN) ferroelectric ceramics prepared by a solid-state process, with x ranging from 0.0 to 0.6, are reported. The microstructure of SrBi2 Nb2 O9 (SBN) was clearly influenced by Cu2+ substituting the Sr2+ of SBN, as observed by X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). An obvious enhancement in dielectric constant was observed and the frequency stability was enhanced for copper substituted SBN compared to SBN. At the same time, the sintering temperature of SBN was reduced significantly by approximately 80 ◦ C because of the copper ions being introduced. The maximum dielectric constant peak broadened gradually with increasing copper content, indicating that the phase transition from a normal ferroelectric to a relaxor ferroelectric occurs in SBN. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction The Bi-layer structure perovskites have recently received considerable interest because of their potential application as nonvolatile random access memory and in high temperature piezoelectric sensor applications [1–3]. This is because they have large remanent polarization, low processing temperature, low fatigue rate, low leakage current and perfect high temperature piezoelectric properties. The general formula of a bismuth layer structure ferroelectric is (Bi2 O2 )2+ (Am−1 Bm O3m+1 )2− , since the phases are built up by the regular intergrowth of (Bi2 O2 )2+ layers and perovskite (Am−1 Bm O3m+1 )2− slabs, where A is a mono-, di-, and tri-valent ion or a combination of them, allowing dodecahedral coordination, B is a combination of cations well suited to octahedral coordination, and m is an integer usually lying in the range 1–5 [4,5]. The crystal structure and chemical compositions of these layered perovskites were studied systematically by Aurivillius [6] in the 1950’s. Among the layered perovskites, SrBi2 Nb2 O9 (SBN), SrBi2 Ta2 O9 (SBT) and their solutions SrBi2 Tax Nb1−x O9 (SBNT) are the most promising candidates, because they possess a reasonable spontaneous polarization which is one of the key parameters for information storage applications. However, they suffer from relatively high processing temperature, relatively low spontaneous polarization, and relatively high dielectric loss [7,8].
∗
Corresponding author. Tel.: +86 29 88494463; fax: +86 29 88492642. E-mail address: [email protected] (H. Fan).
0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.08.016
Much research has been reported in the open literature aimed at improving the dielectric and ferroelectric properties of SBTN materials [9,10]. In particular, doping with various metal oxides has been demonstrated to be one of the effective approaches to improve the properties [11,12]. For example, Bi3+ doping has resulted in an appreciable enhancement of dielectric properties [11]. Wu and Cao [13,14] have substituted Nb5+ with vanadium and found a significant enhancement in dielectric and ferroelectric properties. In addition, it was found that the sintering temperature was reduced significantly, by approximately 200 ◦ C. Forbess et al. [15] and Shrivastava et al. [16– 18] reported the influence of La3+ , Ca2+ and Pb2+ substituted Sr2+ on the structure and dielectric properties of SBN ferroelectric ceramics; insignificant changes were observed in the properties. In the present work, the solid solutions Sr1−x Cux Bi2 Nb2 O9 (SCBN) were fabricated by solid-state reaction. The influence of the Cu2+ substituting Sr2+ of SBN on the sintering temperature, microstructure and dielectric properties is discussed. 2. Experimental procedure The Sr1−x Cux Bi2 Nb2 O9 samples (with x equal to 0.0, 0.2, 0.4 and 0.6, abbreviated as SBN, SCBN2, SCBN4 and SCBN6, respectively) were prepared by the standard solid-state reaction method. Reagent-grade oxide, carbonate and nitrate powders of Bi2 O3 , Nb2 O5 , SrCO3 , and Cu(NO3 )2 · 3H2 O were used as the starting materials. The powders of these raw materials were mixed and ball milled for 12 h using acetone as a medium; this was
P. Fang et al. / Solid State Communications 149 (2009) 2074–2077
SCBN0-Undoped SBN SCBN2-SBN-Cu-0.2 SCBN4-SBN-Cu-0.4 SCBN6-SBN-Cu-0.6 CuO
2075
a
b
5 µm
c
5 µm
d
Fig. 1. XRD patterns of SBN sintered at 1100 ◦ C and copper substituted specimens sintered at 1020 ◦ C; inset, XRD analysis in the 2θ range 28◦ –30◦ .
followed by calcining at 800 ◦ C for 4 h, and the calcined powder was milled again for 24 h. The obtained powder was pressed into pellets of 15 mm in diameter and ∼1 mm in thickness by the cold isostatic pressing method (∼100 MPa). The final sintering was performed at temperatures of 1020 ◦ C for SCBN and 1100 ◦ C for SBN for 1 h, with the samples covered by an alumina crucible, followed by furnace cooling. The crystal structure of each specimen was characterized by X-ray diffraction (XRD) using an automated diffractometer (X’Pert PRO MPD, Philips, Eindhoven, Netherlands) with CuKα 1 radiation. The morphology was characterized by scanning electron microscopy (SEM, S-450, Hitachi). The pellets were polished to a thickness of about 1 mm, and then silver paste was daubed on each sample face and fired as the electrodes at 850 ◦ C for 30 min before dielectric property measurements were obtained. The electrical properties were measured with an applied voltage of 500 mV using an Agilent 4294A impedance analyzer over the frequency range 100 Hz–100 MHz. 3. Results and discussion The XRD patterns of SBN sintered at 1100 ◦ C and copper substituted SBN sintered at 1020 ◦ C are shown in Fig. 1. Further XRD analysis was performed in the 2θ range from 28◦ to 30◦ . The position of the dominant diffraction peaks of SCBN matches closely, which indicates that a solid solution with Bi-layered perovskite phase was formed. The sintering temperature was reduced insignificantly, by approximately 80 ◦ C. Then, some obvious changes could be detected. The intensities of the dominant diffraction peaks decreased gradually and some secondary phases were observed. Moreover, the intensities of the diffraction peaks of the secondary phase (CuO) were enhanced, which indicates that the amounts of the secondary phase CuO were enhanced gradually with increasing copper content. The characteristic (115) diffraction peak broadened gradually then split with increasing copper content, which also indicated that the amounts of the secondary phase CuO were enhanced (shown in the inset of Fig. 1). The SEM images of SBN sintered at 1100 ◦ C and copper substituted SBN sintered at 1020 ◦ C are shown in Fig. 2. The micrograph of SBN shows the classic platelet grain form, and all the grains of copper substituted SBN look plumper than pure SBN. Furthermore, some obvious secondary phase (CuO) could be observed in the grain boundaries for SCBN6 specimen, which is accord with the results of Fig. 1. The lower sintering temperature for copper substituted SBN indicated that the copper ions entering into the SBN lattice may promote the formation of a liquid state at
5 µm
5 µm
Fig. 2. SEM micrographs of the surfaces of the specimens: (a) SBN; (b) SCBN2; (c) SCBN4; (d) SCBN6.
the grain boundaries during sintering, which would increase the diffusion velocity effectively between the grains and result in the secondary phase CuO being enriched in the grain boundaries. The frequency dependence of the dielectric constant and dielectric loss tangent of the specimens, as a function of frequency, which ranged from 100 Hz to 1 MHz, is shown in Fig. 3. It is seen that the dielectric constant was significantly improved by the Cu2+ substituting Sr2+ of SBN throughout the frequency range, which was attributed to the Cu-rich phase being separated at the grain boundaries. The special microstructure (SCBN grain–amorphous/CuO nanograin–SCBN grain junction structure) should be responsible for the enhanced electric properties of these SCBN specimens. The special microstructure can be often be modeled on an ideal equivalent circuit consisting of two parallel RC elements connected in series, one RC element for the bulk response and the other for the grain boundary response (shown in Fig. 3 inset) [19,20]. The enhanced dielectric properties of SCBN specimens may be attributed to the enhancement in the grain boundary capacitance Cgb with increasing copper content, which is in agreement with the results observed in CaCu3 Ti4 O12 ceramic [21].The maximum value of dielectric constant for SCBN6 at lower frequency 100 Hz is about 650, which is nearly 5 times that of SBN at the same frequency. Moreover, the dielectric constant was enhanced, but not particularly influenced by the driving frequency for the SCBN2 specimen, which indicated that the frequency stability of SBN was enhanced by the slight substitution of Cu2+ for Sr2+ in SBN. Otherwise, the dielectric constant of SCBN4 and SCBN6 show a rapid decrease up to about 100 kHz and a slow decrease at higher frequency. The steep decrease in dielectric constant values with increase in frequency up to 100 kHz is attributed to relaxation and resonance absorption [18,22]. The dielectric loss tangent factor was also influenced by the Cu2+ substituting Sr2+ of SBN throughout the frequency range. SBN shows a relatively smaller dielectric loss: the maximum value at lower frequency is about 0.05 and it decreases gradually with the driving frequency increasing. Among the substituted specimens, the dielectric loss tangent of SCBN2 is almost unchanged as the driving frequency increases, and it shows a smaller dielectric loss
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P. Fang et al. / Solid State Communications 149 (2009) 2074–2077
Undoped SBN SBN-Cu-0.2 SBN-Cu-0.4 SBN-Cu-0.6
Grain
Grain boundaries
Rgb
Rg
Cgb
Cg
Fig. 3. Dielectric constant and dielectric loss tangent of SCBN specimens as a function of frequency.
throughout the frequency range compared to the other substituted specimens. It is seen that the frequency stability was enhanced by slight Cu2+ substitution for Sr2+ in SBN. SCBN4 and SCBN6 show a dielectric loss behavior similar to that of their dielectric constant. The temperature dependence of the dielectric constant and dielectric loss tangent for SCBN at 100 kHz between 100 ◦ C and 700 ◦ C is shown in Fig. 4. It is seen that only one dielectric peak is detected in all specimens during the heating process. These specimens are known to have single-phase ferroelectric to paraelectric phase transitions corresponding to a Curie temperature (Tc ) where the dielectric constant has a maximum value [23,24]. In addition, the dielectric peak is broadened as the copper content increases, which is known to be due to either defects being induced [25] or a relaxor-type phase transition [26] where the formation of micropolar regions occurs and each of such regions has its own transition temperature. At the same time, the Tc of SCBN specimens significantly increases as the copper content increases during the heating process. The Tc of SBN is detected to be about 445 ◦ C, which is in accord with the previous results [27]. Furthermore, the Tc of SCBN6 is detected to be about 616 ◦ C, which is consistent with Subbarao’s suggestion that the Curie point of Aurivillius phase materials increases with the decrease of the A-site cation size [28]. The dielectric loss tangents of all specimens were not changed significantly below 450 ◦ C, and then increased rapidly beyond 450 ◦ C. At the same time, an inconspicuous dielectric loss peak could be detected for SCBN2. However, the obvious peaks were not observed around the transition temperature of all specimens. The dielectric loss tangent continues to increase at higher temperature, which might be attributed to the higher conductivity at higher temperatures. The oxygen vacancy is a dominant defect in layered structure perovskite materials, so the formation of a higher concentration of charge carriers, oxygen vacancies and other defects, might all deteriorate the conductivity [29]. For relaxor ferroelectrics, the reciprocal of the dielectric constant and temperature follows the Uchino and Nomura function, a modified Curie–Weiss (CW) law [30], 1/ε − 1/εm = (T − Tm )γ /C
(1)
where C is the Curie constant and the value of γ (ranging from one to two) is an expression of the degree of dielectric relaxation in a ferroelectric. When γ = 1, Eq. (1) expresses the CW behavior of normal ferroelectrics, while γ = 2 reduces to the quadratic dependence [31], which is valid for a canonical relaxor ferroelectric experientially. In order to further confirm the effect of the copper content on the dielectric relaxation behavior of SCBN specimens, the plots of log (1/ε − 1/εm ) as a function of log (T − Tm ) at
Fig. 4. Dielectric constant and dielectric loss tangent as a function of temperature for SCBN specimens at different frequencies.
100 kHz for specimens SCBN4 and SCBN6 are shown in Fig. 5. A linear relationship is observed in all specimens. The slope of the fitting curves is used to determine the γ value. It is found that the γ value increases from 1.35 to 1.94 with the copper content increasing, which indicates that the transformation from a normal ferroelectric to a relaxation ferroelectric has occurred because of the copper being introduced. The relaxor behavior can be explained by the theory modes such as composition fluctuation theory, superparaelectricity theory, the merging of micropolar into macropolar regions, and the random-field model [32–34]. In the SCBN solid solution, the smaller ionic radius Cu2+ substituting the Sr2+ and the secondary phase CuO enrichment in the grain boundaries could result in local order–disorder of the crystal structure which gives rise to polar clusters of nanometric size or PNRs, which are embedded in the disordered matrix. The dielectric properties of the relaxor are believed to result from the complex response of all the PNRs and the matrix [35]. 4. Conclusions The dielectric properties and the relaxation phase transition in copper substituted SBN ferroelectric ceramics prepared by a solidstate process are reported. An obvious enhancement in dielectric constant was detected for copper substituted SBN compared to SBN, which is attributed to the special microstructure of SCBN grain–amorphous/CuO nanograin–SCBN grain junction. At the same time, the frequency stability was enhanced significantly. The sintering temperature of SBN was reduced significantly by approximately 80 ◦ C because of the copper ions being introduced. The maximum dielectric constant peak was broadened gradually as the copper content increased, which indicated that the phase transition from ferroelectric to relaxation ferroelectric occurs. The dielectric properties of the relaxors are believed to result from the complex response of all the PNRs and the matrix. Acknowledgments This work has been supported by the National Nature Science Foundation (50672075), the NCET and 111 Program (B08040) of MOE, and Xi’an Science & Technology Foundation (CXY08006) and the Fundamental Research Foundation (NPU-FFR-200703) of NPU of China.
P. Fang et al. / Solid State Communications 149 (2009) 2074–2077 [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Fig. 5. Log (1/ε−1/εm ) as a function of log (T −Tm ) at 100 kHz for SCBN4 and SCBN6 ceramics (the symbols: experimental data; the solid line: fitting to the modified Curie–Weiss law).
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
The dielectric properties and the relaxation phase transition of copper substituted SrBi2 Nb2 O9 ferroelectric ceramics Pinyang Fang, Huiqing Fan ∗ , Jin Li, Xiaohua Jia, Fujun Liang State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China
article
info
Article history: Received 30 March 2009 Received in revised form 8 August 2009 Accepted 10 August 2009 by Y.E. Lozovik Available online 15 August 2009 PACS: 77.80.-e Keywords: A: Ferroelectrics A: SBN D: Relaxation phase transition
abstract The dielectric properties and the relaxation phase transition of Sr1−x Cux Bi2 Nb2 O9 (SCBN) ferroelectric ceramics prepared by a solid-state process, with x ranging from 0.0 to 0.6, are reported. The microstructure of SrBi2 Nb2 O9 (SBN) was clearly influenced by Cu2+ substituting the Sr2+ of SBN, as observed by X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). An obvious enhancement in dielectric constant was observed and the frequency stability was enhanced for copper substituted SBN compared to SBN. At the same time, the sintering temperature of SBN was reduced significantly by approximately 80 ◦ C because of the copper ions being introduced. The maximum dielectric constant peak broadened gradually with increasing copper content, indicating that the phase transition from a normal ferroelectric to a relaxor ferroelectric occurs in SBN. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction The Bi-layer structure perovskites have recently received considerable interest because of their potential application as nonvolatile random access memory and in high temperature piezoelectric sensor applications [1–3]. This is because they have large remanent polarization, low processing temperature, low fatigue rate, low leakage current and perfect high temperature piezoelectric properties. The general formula of a bismuth layer structure ferroelectric is (Bi2 O2 )2+ (Am−1 Bm O3m+1 )2− , since the phases are built up by the regular intergrowth of (Bi2 O2 )2+ layers and perovskite (Am−1 Bm O3m+1 )2− slabs, where A is a mono-, di-, and tri-valent ion or a combination of them, allowing dodecahedral coordination, B is a combination of cations well suited to octahedral coordination, and m is an integer usually lying in the range 1–5 [4,5]. The crystal structure and chemical compositions of these layered perovskites were studied systematically by Aurivillius [6] in the 1950’s. Among the layered perovskites, SrBi2 Nb2 O9 (SBN), SrBi2 Ta2 O9 (SBT) and their solutions SrBi2 Tax Nb1−x O9 (SBNT) are the most promising candidates, because they possess a reasonable spontaneous polarization which is one of the key parameters for information storage applications. However, they suffer from relatively high processing temperature, relatively low spontaneous polarization, and relatively high dielectric loss [7,8].
∗
Corresponding author. Tel.: +86 29 88494463; fax: +86 29 88492642. E-mail address: [email protected] (H. Fan).
0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.08.016
Much research has been reported in the open literature aimed at improving the dielectric and ferroelectric properties of SBTN materials [9,10]. In particular, doping with various metal oxides has been demonstrated to be one of the effective approaches to improve the properties [11,12]. For example, Bi3+ doping has resulted in an appreciable enhancement of dielectric properties [11]. Wu and Cao [13,14] have substituted Nb5+ with vanadium and found a significant enhancement in dielectric and ferroelectric properties. In addition, it was found that the sintering temperature was reduced significantly, by approximately 200 ◦ C. Forbess et al. [15] and Shrivastava et al. [16– 18] reported the influence of La3+ , Ca2+ and Pb2+ substituted Sr2+ on the structure and dielectric properties of SBN ferroelectric ceramics; insignificant changes were observed in the properties. In the present work, the solid solutions Sr1−x Cux Bi2 Nb2 O9 (SCBN) were fabricated by solid-state reaction. The influence of the Cu2+ substituting Sr2+ of SBN on the sintering temperature, microstructure and dielectric properties is discussed. 2. Experimental procedure The Sr1−x Cux Bi2 Nb2 O9 samples (with x equal to 0.0, 0.2, 0.4 and 0.6, abbreviated as SBN, SCBN2, SCBN4 and SCBN6, respectively) were prepared by the standard solid-state reaction method. Reagent-grade oxide, carbonate and nitrate powders of Bi2 O3 , Nb2 O5 , SrCO3 , and Cu(NO3 )2 · 3H2 O were used as the starting materials. The powders of these raw materials were mixed and ball milled for 12 h using acetone as a medium; this was
P. Fang et al. / Solid State Communications 149 (2009) 2074–2077
SCBN0-Undoped SBN SCBN2-SBN-Cu-0.2 SCBN4-SBN-Cu-0.4 SCBN6-SBN-Cu-0.6 CuO
2075
a
b
5 µm
c
5 µm
d
Fig. 1. XRD patterns of SBN sintered at 1100 ◦ C and copper substituted specimens sintered at 1020 ◦ C; inset, XRD analysis in the 2θ range 28◦ –30◦ .
followed by calcining at 800 ◦ C for 4 h, and the calcined powder was milled again for 24 h. The obtained powder was pressed into pellets of 15 mm in diameter and ∼1 mm in thickness by the cold isostatic pressing method (∼100 MPa). The final sintering was performed at temperatures of 1020 ◦ C for SCBN and 1100 ◦ C for SBN for 1 h, with the samples covered by an alumina crucible, followed by furnace cooling. The crystal structure of each specimen was characterized by X-ray diffraction (XRD) using an automated diffractometer (X’Pert PRO MPD, Philips, Eindhoven, Netherlands) with CuKα 1 radiation. The morphology was characterized by scanning electron microscopy (SEM, S-450, Hitachi). The pellets were polished to a thickness of about 1 mm, and then silver paste was daubed on each sample face and fired as the electrodes at 850 ◦ C for 30 min before dielectric property measurements were obtained. The electrical properties were measured with an applied voltage of 500 mV using an Agilent 4294A impedance analyzer over the frequency range 100 Hz–100 MHz. 3. Results and discussion The XRD patterns of SBN sintered at 1100 ◦ C and copper substituted SBN sintered at 1020 ◦ C are shown in Fig. 1. Further XRD analysis was performed in the 2θ range from 28◦ to 30◦ . The position of the dominant diffraction peaks of SCBN matches closely, which indicates that a solid solution with Bi-layered perovskite phase was formed. The sintering temperature was reduced insignificantly, by approximately 80 ◦ C. Then, some obvious changes could be detected. The intensities of the dominant diffraction peaks decreased gradually and some secondary phases were observed. Moreover, the intensities of the diffraction peaks of the secondary phase (CuO) were enhanced, which indicates that the amounts of the secondary phase CuO were enhanced gradually with increasing copper content. The characteristic (115) diffraction peak broadened gradually then split with increasing copper content, which also indicated that the amounts of the secondary phase CuO were enhanced (shown in the inset of Fig. 1). The SEM images of SBN sintered at 1100 ◦ C and copper substituted SBN sintered at 1020 ◦ C are shown in Fig. 2. The micrograph of SBN shows the classic platelet grain form, and all the grains of copper substituted SBN look plumper than pure SBN. Furthermore, some obvious secondary phase (CuO) could be observed in the grain boundaries for SCBN6 specimen, which is accord with the results of Fig. 1. The lower sintering temperature for copper substituted SBN indicated that the copper ions entering into the SBN lattice may promote the formation of a liquid state at
5 µm
5 µm
Fig. 2. SEM micrographs of the surfaces of the specimens: (a) SBN; (b) SCBN2; (c) SCBN4; (d) SCBN6.
the grain boundaries during sintering, which would increase the diffusion velocity effectively between the grains and result in the secondary phase CuO being enriched in the grain boundaries. The frequency dependence of the dielectric constant and dielectric loss tangent of the specimens, as a function of frequency, which ranged from 100 Hz to 1 MHz, is shown in Fig. 3. It is seen that the dielectric constant was significantly improved by the Cu2+ substituting Sr2+ of SBN throughout the frequency range, which was attributed to the Cu-rich phase being separated at the grain boundaries. The special microstructure (SCBN grain–amorphous/CuO nanograin–SCBN grain junction structure) should be responsible for the enhanced electric properties of these SCBN specimens. The special microstructure can be often be modeled on an ideal equivalent circuit consisting of two parallel RC elements connected in series, one RC element for the bulk response and the other for the grain boundary response (shown in Fig. 3 inset) [19,20]. The enhanced dielectric properties of SCBN specimens may be attributed to the enhancement in the grain boundary capacitance Cgb with increasing copper content, which is in agreement with the results observed in CaCu3 Ti4 O12 ceramic [21].The maximum value of dielectric constant for SCBN6 at lower frequency 100 Hz is about 650, which is nearly 5 times that of SBN at the same frequency. Moreover, the dielectric constant was enhanced, but not particularly influenced by the driving frequency for the SCBN2 specimen, which indicated that the frequency stability of SBN was enhanced by the slight substitution of Cu2+ for Sr2+ in SBN. Otherwise, the dielectric constant of SCBN4 and SCBN6 show a rapid decrease up to about 100 kHz and a slow decrease at higher frequency. The steep decrease in dielectric constant values with increase in frequency up to 100 kHz is attributed to relaxation and resonance absorption [18,22]. The dielectric loss tangent factor was also influenced by the Cu2+ substituting Sr2+ of SBN throughout the frequency range. SBN shows a relatively smaller dielectric loss: the maximum value at lower frequency is about 0.05 and it decreases gradually with the driving frequency increasing. Among the substituted specimens, the dielectric loss tangent of SCBN2 is almost unchanged as the driving frequency increases, and it shows a smaller dielectric loss
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P. Fang et al. / Solid State Communications 149 (2009) 2074–2077
Undoped SBN SBN-Cu-0.2 SBN-Cu-0.4 SBN-Cu-0.6
Grain
Grain boundaries
Rgb
Rg
Cgb
Cg
Fig. 3. Dielectric constant and dielectric loss tangent of SCBN specimens as a function of frequency.
throughout the frequency range compared to the other substituted specimens. It is seen that the frequency stability was enhanced by slight Cu2+ substitution for Sr2+ in SBN. SCBN4 and SCBN6 show a dielectric loss behavior similar to that of their dielectric constant. The temperature dependence of the dielectric constant and dielectric loss tangent for SCBN at 100 kHz between 100 ◦ C and 700 ◦ C is shown in Fig. 4. It is seen that only one dielectric peak is detected in all specimens during the heating process. These specimens are known to have single-phase ferroelectric to paraelectric phase transitions corresponding to a Curie temperature (Tc ) where the dielectric constant has a maximum value [23,24]. In addition, the dielectric peak is broadened as the copper content increases, which is known to be due to either defects being induced [25] or a relaxor-type phase transition [26] where the formation of micropolar regions occurs and each of such regions has its own transition temperature. At the same time, the Tc of SCBN specimens significantly increases as the copper content increases during the heating process. The Tc of SBN is detected to be about 445 ◦ C, which is in accord with the previous results [27]. Furthermore, the Tc of SCBN6 is detected to be about 616 ◦ C, which is consistent with Subbarao’s suggestion that the Curie point of Aurivillius phase materials increases with the decrease of the A-site cation size [28]. The dielectric loss tangents of all specimens were not changed significantly below 450 ◦ C, and then increased rapidly beyond 450 ◦ C. At the same time, an inconspicuous dielectric loss peak could be detected for SCBN2. However, the obvious peaks were not observed around the transition temperature of all specimens. The dielectric loss tangent continues to increase at higher temperature, which might be attributed to the higher conductivity at higher temperatures. The oxygen vacancy is a dominant defect in layered structure perovskite materials, so the formation of a higher concentration of charge carriers, oxygen vacancies and other defects, might all deteriorate the conductivity [29]. For relaxor ferroelectrics, the reciprocal of the dielectric constant and temperature follows the Uchino and Nomura function, a modified Curie–Weiss (CW) law [30], 1/ε − 1/εm = (T − Tm )γ /C
(1)
where C is the Curie constant and the value of γ (ranging from one to two) is an expression of the degree of dielectric relaxation in a ferroelectric. When γ = 1, Eq. (1) expresses the CW behavior of normal ferroelectrics, while γ = 2 reduces to the quadratic dependence [31], which is valid for a canonical relaxor ferroelectric experientially. In order to further confirm the effect of the copper content on the dielectric relaxation behavior of SCBN specimens, the plots of log (1/ε − 1/εm ) as a function of log (T − Tm ) at
Fig. 4. Dielectric constant and dielectric loss tangent as a function of temperature for SCBN specimens at different frequencies.
100 kHz for specimens SCBN4 and SCBN6 are shown in Fig. 5. A linear relationship is observed in all specimens. The slope of the fitting curves is used to determine the γ value. It is found that the γ value increases from 1.35 to 1.94 with the copper content increasing, which indicates that the transformation from a normal ferroelectric to a relaxation ferroelectric has occurred because of the copper being introduced. The relaxor behavior can be explained by the theory modes such as composition fluctuation theory, superparaelectricity theory, the merging of micropolar into macropolar regions, and the random-field model [32–34]. In the SCBN solid solution, the smaller ionic radius Cu2+ substituting the Sr2+ and the secondary phase CuO enrichment in the grain boundaries could result in local order–disorder of the crystal structure which gives rise to polar clusters of nanometric size or PNRs, which are embedded in the disordered matrix. The dielectric properties of the relaxor are believed to result from the complex response of all the PNRs and the matrix [35]. 4. Conclusions The dielectric properties and the relaxation phase transition in copper substituted SBN ferroelectric ceramics prepared by a solidstate process are reported. An obvious enhancement in dielectric constant was detected for copper substituted SBN compared to SBN, which is attributed to the special microstructure of SCBN grain–amorphous/CuO nanograin–SCBN grain junction. At the same time, the frequency stability was enhanced significantly. The sintering temperature of SBN was reduced significantly by approximately 80 ◦ C because of the copper ions being introduced. The maximum dielectric constant peak was broadened gradually as the copper content increased, which indicated that the phase transition from ferroelectric to relaxation ferroelectric occurs. The dielectric properties of the relaxors are believed to result from the complex response of all the PNRs and the matrix. Acknowledgments This work has been supported by the National Nature Science Foundation (50672075), the NCET and 111 Program (B08040) of MOE, and Xi’an Science & Technology Foundation (CXY08006) and the Fundamental Research Foundation (NPU-FFR-200703) of NPU of China.
P. Fang et al. / Solid State Communications 149 (2009) 2074–2077 [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Fig. 5. Log (1/ε−1/εm ) as a function of log (T −Tm ) at 100 kHz for SCBN4 and SCBN6 ceramics (the symbols: experimental data; the solid line: fitting to the modified Curie–Weiss law).
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