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Dielectric behavior of spark plasma sintered BaTi0.7Zr0.3O3 relaxor ferroelectrics
Results in Physics 15 (2019) 102799
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Results in Physics journal homepage: www.elsevier.com/locate/rinp
Dielectric behavior of spark plasma sintered BaTi0.7Zr0.3O3 relaxor ferroelectrics
T
⁎
Mohamad M. Ahmada,b, , Latifah Alismaila, Adil Alshoaibia, Abdullah Aljaafaria, H. Mahfoz Kotba,c, Reda Hassaniend a
Department of Physics, College of Science, King Faisal University, Al-Ahsaa 31982, Saudi Arabia Department of Physics, Faculty of Science, The New Valley University, El-Kharga 72511, Egypt c Department of Physics, Faculty of Science, Assiut University, Assiut 71516, Egypt d Department of Chemistry, Faculty of Science, The New Valley University, El-Kharga 72511, Egypt b
A R T I C LE I N FO
A B S T R A C T
Keywords: Barium titanate zirconate Relaxor ferroelectrics Spark plasma sintering Dielectric properties
In the present work we have studied the transport and dielectric behavior of BaTi0.7Zr0.3O3 (BTZ30) relaxor ferroelectrics prepared by spark plasma sintering. Nanocrystalline powder of BTZ30 was obtained by solid state reaction followed by mechanical milling. Fine and coarse-grained ceramics of BTZ30 were prepared by spark plasma sintering at 1100 and 1400 °C, respectively. The obtained materials were characterized by X-ray diffraction and scanning electron microscopy. The grain size of the BTZ30 ceramics increased from 200 nm to 3–5 μm as the sintering temperature was increased from 1100 °C to 1400 °C. The electrical and dielectric properties of BTZ30 ceramics were studied by impedance spectroscopy over −150 to +200 °C temperature range. The dielectric constant exhibits very large values > 105 for the spark plasma sintered ceramics associated with very high dielectric loss and electrical conductivity. The insulating nature of the BTZ30 ceramics could be retained by re-oxidation of the samples. The dielectric data of the annealed have been analyzed by the modified Curie-Weiss law and the relaxation process in the materials was fitted by Vogel-Fulcher model.
Introduction Point defects, such as oxygen vacancies, play prominent role in tailoring the electrical and dielectric properties of ferroelectric materials [1–3]. The presence of oxygen vacancies leads to resistance degradation and aging phenomena in ferroelectric materials, Therefore, the origin of oxygen vacancies and their relaxation properties need to be explored. The crystal structure and the dielectric and electrical properties of BaTiO3 depend strongly on the type of impurities [4–9]. In this regard, barium titanate has been studied extensively after modified with various additives. Zr+4 substitution in the Ti+4 sites in barium titanate has attained considerable interest. Zr+4 ions are more stable than Ti+4 and have a larger ionic radius, leading to lattice expansion. Moreover, the substitution of Ti+4 by Zr+4 is expected to reduce the electrical conduction by electronic hopping between Ti+4 and Ti+3. Moreover, the Zr+4 substitutions at Ti-sites can lead to different features such as, shifting of the ferroelectric transition temperature, broadening or merging of the dielectric peaks. Consequently, diffuse or relaxor behavior occurs [10–12]. BaTi1−xZrxO3 (BZT) solid solutions have attracted considerable
⁎
attention due to their potential applications in various electronic devices. Zr+4 substitutions into Ti+4 sites in BaTiO3 exhibit interesting changes in the dielectric behavior of the materials. For low Zr content < 10%, the orthorhombic to tetragonal and the rhombohedral to orthorhombic phase transition temperatures corresponding to pure BaTiO3 increase. In contrast, the tetragonal to cubic phase transition temperature decreases. With increasing Zr content to ~15%, the three phase transitions are merged or pinched into a single diffuse phase transition [13]. With further increase in Zr contents > 15%, the pinched transition temperature decreases and a diffuse dielectric anomaly is observed. Above 25% of Zr content the BTZ system exhibits typical relaxor-like behavior. BaTi1−xZrxO3 ceramics are usually prepared by conventional sintering at high temperatures of about 1550 °C for several hours [10–13]. The high temperature sintering process leads to the formation of coarsegrained ceramics with substantial grain growth. Meanwhile, spark plasma sintering (SPS) is an innovative sintering technology that is becoming increasingly important in the processing of numerous materials, such as nanostructured materials, composite materials and functionally graded materials [14,15]. SPS is based on the spark plasma
Corresponding author at: Department of Physics, College of Science, King Faisal University, Al-Ahsaa 31982, Saudi Arabia. E-mail address: [email protected] (M.M. Ahmad).
https://doi.org/10.1016/j.rinp.2019.102799 Received 19 May 2019; Received in revised form 12 October 2019; Accepted 7 November 2019 Available online 11 November 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 15 (2019) 102799
M.M. Ahmad, et al.
Fig. 1. XRD of SPS-BTZ30 ceramics sintered by spark plasma sintering at 1100 °C and 1400 °C.
effect due to electrical spark discharge phenomenon wherein a highenergy, low-voltage pulse current momentarily generates spark plasma at high temperatures (many thousands of °C) in fine local areas between particles. Spark plasma sintering temperatures are commonly 200–500 °C lower than conventional sintering techniques, classifying SPS as a lower-temperature sintering technology [14,15]. Moreover, material processing cycle (pressure and temperature rise and hold time) is completed in short periods of approximately 5 to 25 min. The relatively low temperatures combined with fast processing times ensure tight control over the microstructure with very little or negligible grain growth. In the current work we will study the electrical and dielectric properties of spark plasma sintered BaTi0.7Zr0.3O3 (SPS-BTZ30) ceramics. We have prepared BTZ30 solid solutions by employing different techniques of solid state reaction, mechanical milling and spark plasma sintering. The spark plasma sintering of BTZ30 materials was performed at different temperatures ranging from 1100 °C to 1400 °C, which are considerably lower than the conventional sintering temperature of 1550 °C. However, we show in this paper the results of the samples sintered at 1100 °C and 1400 °C.
Experiment The powder of BTZ30 samples was prepared by solid state reaction and mechanical milling. The start materials are BaCO3, TiO2 and ZrO2. Stoichiometric amounts of the starting materials have been ball milled in 2-propanol for 12 h with a speed of 350 rpm. The dried powder was calcined at 1200 °C for 12 h. The calcined powder was crushed and ball milled again with a speed of 550 rpm for 6 h. Dense fine-grained BTZ30 ceramics were prepared by SPS under vacuum. In the SPS experiment we have fixed the applied pressure to 80 MPa. The samples were sintered at different temperatures of 1100 °C (SPS-BTZ30-1100) and 1400 °C (SPS-BTZ30-1400), with a heating rate of 150 K/min and a dwelling time of 10 min. The samples were characterized by x-ray powder diffraction (XRD) and scanning electron microscopy (SEM) techniques. XRD data were collected over the 0 ≤ 2θ ≤ 80 range using Stadi-P Image Plate, IP (Stoe and Cie GmbH, Darmstadt) with monochromatic radiation (λ = 1.5406 Å). The morphology and the grain size of the obtained ceramics have been investigated by field-emission scanning electron microscope (FE-SEM). For electrical measurement, gold electrodes were sputtered on the both sides of the pellets. The electrical and dielectric properties were studied by impedance spectroscopy (IS) measurements using Novocontrol concept 50 system in the −120 to 200 °C temperature range as described elsewhere [16].
Fig. 2. SEM micrographs of BTZ30 powder and SPS-BTZ30 ceramics sintered at 1100 and 1400 °C.
Results and discussion XRD patterns of SPS-BTZ1100 and SPS-BTZ1400 ceramics are shown in Fig. 1. Clearly, the two samples exhibit pure cubic structure similar to BaTiO3, with no impurity phases were detected. The morphology of BTZ30 powder and SPS-BTZ30 ceramics is shown in Fig. 2. 2
Results in Physics 15 (2019) 102799
M.M. Ahmad, et al.
Fig. 3. The temperature dependence of ε′ for as-prepared SPS-BTZ30 ceramics at selected frequencies, (a) SPS-BTZ30-1100 and (b) SPS-BTZ30-1400 ceramics.
Fig. 4. Temperature dependence of the dielectric loss of as-prepared (a) SPS-BTZ30-1100 and (b) SPS-BTZ30-1400.
behavior of ε′ of the spark plasma sintered ceramics is quite different from that of the ceramics sintered by conventional technique [10–13]. At low temperatures, there is a step increase of ε′ followed by a region where ε′ is almost independent of temperature. The dielectric constant attains giant values of ε′ > 105, especially at low frequencies. With increasing frequency the value of ε′ decreases to about 103 for the data measured at 10 MHz frequency. No ferroelectric or relaxor peaks could be detected in the dielectric behavior of the as prepared SPS-BTZ30 ceramics. Most probably the ferroelectric phases have been screened by the huge values of ε′. With increasing the SPS temperature, ε′ becomes less dependent on frequency. The observed dielectric behavior is associated with very large value of the dielectric loss as shown in Fig. 4. Clearly, the observed giant dielectric behavior and the very large dielectric loss cannot be attributed to dipolar polarization only. Most probably, these features are correlated with highly conductive properties of the materials. The frequency variation of the ac conductivity at different temperatures is shown in Fig. 5. Two plateaus are observed at low and high frequency regions, respectively. The low frequency plateau could be assigned to the grain boundary conduction, whereas the
We notice that the powder sample of BTZ30 has crystallite size in the 20–100 nm range. The obtained nano-sized BTZ30 powder is due to the ball milling process during the synthesis of the materials. For spark plasma sintered BTZ30 ceramics we notice that dense ceramics are obtained after sintering at 1400 °C. The grain size of SPS-BTZ30-1100 ceramics is about 200 nm. However, the SPS-BTZ30-1100 ceramics are relatively porous due to the low SPS temperature. With increasing the SPS temperature to 1400 °C, the grain size increases considerably to 3–5 μm as shown in Fig. 2 for SPS-BTZ30-1400 ceramics. Moreover, the relative density of SPS-BTZ30 ceramics increased from about 95% for SPS-BTZ30-1100 ceramics to about 98% for SPS-BTZ30-1400 ceramics. The increased density with increasing the SPS temperature agrees well with the SEM micrographs (Fig. 2), where SPS-BTZ30-1100 ceramics exhibit relatively porous morphology compared to SPS-BTZ3-1400 ceramics. We have performed the impedance spectroscopy measurements before and after annealing the SPS-BTZ30 ceramics. The temperature dependence of the dielectric constant of the as prepared SPS-BTZ30 ceramics sintered at 1100 °C and 1400 °C is shown in Fig. 3. The 3
Results in Physics 15 (2019) 102799
M.M. Ahmad, et al.
Fig. 5. The frequency dependence of σ ' at selected temperatures of as prepared SPS-BTZ30 ceramics sintered at (a) 1100 °C and (b) 1400 °C.
Fig. 6. Temperature dependence of the dielectric constant of annealed (a) BTZ30-1100 and (b) BTZ30-1400.
high frequency plateau is assigned to the grain conduction. For the asprepared SPS-BTZ30-1100 ceramics, the electrical conductivity of the grains is very high in the range of 10−5–10−2 S/cm, whereas the grain boundary conductivity is in the range of 10−6–10−3 S/cm. For SPSBTZ30-1400 ceramics, the conductivity is even higher as shown in Fig. 5(b). These results indicate that the grains and grain boundaries of the as-prepared SPS-BTZ30 ceramics are highly conductive compared to the insulating nature of conventionally sintered BTZ30 ceramics. Similar behavior has been reported for spark plasma sintered BaTiO3 ceramics [17–19]. The above results of the dielectric constant and the electrical conductivity of SPS-BTZ30 ceramics indicate that very large concentration of free charge carriers are introduced into the materials during the SPS process. The SPS is performed under vacuum conditions, which are reducing atmosphere. Therefore, SPS of oxide materials in reducing atmosphere will lead to the formation of oxygen vacancies and the release of free electrons according to the following relation [17–19];
Oox ⇒
1 O2 Vo·· + 2e− 2
(1)
The formation of oxygen vacancies will also lead to the reduction of Ti+4 into Ti+3 according to the relation;
Ti4 + + e− ⇒ Ti3 +
(2)
The density of charge carriers such as oxygen vacancies, free electrons and Ti+3 is expected to increase with increasing the SPS temperature. Therefore, the observed behavior of the dielectric properties in as-prepared SPS-BTZ30 ceramics is attributed to the high density of free charge carriers in the materials. In this case, besides the dipolar polarization mechanism that is usually exist in ferroelectric materials; two more polarization mechanism can contribute to the giant dielectric behavior of as-sintered SPS-BTZ ceramics. The two mechanisms are the interfacial polarization and the polarization due to the polaron hopping. In order to recover the insulating nature of SPS-BTZ30 ceramics we 4
Results in Physics 15 (2019) 102799
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Fig. 7. The temperature dependence of the dielectric loss tan δ measured at various frequencies of annealed SPS-BTZ30 ceramics sintered at (a) 1100 °C, (b) 1400 °C.
Fig. 8. The frequency dependence of the electrical conductivity of the annealed SPS-BTZ30 ceramics sintered at (a) 1100 °C, (b) 1400 °C.
conductivity of the annealed samples varies linearly with the frequency, where the conductivity has very low values in the range 10−12–10−9 S/ cm as shown in Fig. 8. We have analyzed the dielectric data of the annealed SPS-BTZ301400 ceramics by the modified form of the Curie-Weiss law [20]:
have annealed the spark plasma sintered ceramics in air at 1100 °C for a period of 2 h. This annealing process is expected to re-oxidize the materials and reduce the density of oxygen vacancies. The dielectric data of the annealed SPS-BTZ30-1100 and SPS-BTZ30-1400 ceramics show typical relaxaor ferroelectric behavior (Fig. 6). The frequency dispersion of both the maximum value of the dielectric constant ε′m and its temperature Tm is observed. Additionally, it is noticed that the dielectric constant improved with increasing the SPS temperature from 1100 °C to 1400 °C. ε′ has a value of about 3300 at 10 kHz for SPSBTZ30-1100 ceramics with Tm value of about −27 °C. With increasing the SPS temperature to 1400 °C, ε′ increases considerably to 16,200 at 10 kHz and Tm also shifted to −47.7 °C. The temperature dependence of tan δ of the annealed SPS-BTZ30 ceramics is shown in Fig. 7 at selected frequencies. For SPS-BTZ30-1100 ceramics, tan δ decreases with increasing temperature and it has very low values at temperature above Tm, whereas for SPS-BTZ30-1400 ceramics tan δ shows a relaxation peak that increases with increasing frequency. Moreover, the electrical
1/ ε′ − 1/ εm′ = (T − Tm )γ / C1
(3)
where γ and C1 are constants. The parameter γ gets the value 1 ≤ γ ≤ 2, where γ = 1 reduces Eq. (3) to the normal Curie-Weiss law, whereas the value of γ = 2 represents the quadratic dependence that is valid for ideal ferroelectric relaxors [20]. Therefore, the value of γ will give an indication on the relaxor behavior of the materials. The plots of ln (1/ε′ − 1/ε′m) versus ln (T − Tm) for the annealed SPS-BTZ30-1400 ceramics are shown in Fig. 9. A value of γ = 1.84 was estimated, which agrees well with different ferroelectric relaxors [11,21–25]. In relaxor ferroelectric materials the frequency dispersion of the maxima of the dielectric constant has been attributed to the distribution 5
Results in Physics 15 (2019) 102799
M.M. Ahmad, et al.
τ = τ0 exp [Ea/ kB (Tm − Tf )]
(4)
where kB is the Boltzmann constant, Ea the activation energy, τ0 the pre-exponential factor (the inverse of the attempt frequency f0) and Tf is the Vogel-Fulcher (freezing) temperature. We notice in Fig. 10 that the relaxation time increases with decreasing temperature and τ becomes extremely large at the freezing temperature Tf, at which the fluctuation of polarization gets frozen. Similar behavior was observed in other systems such as spin-glass materials [26]. We have fitted our experimental data to the V-F model (Eq. (4)), and excellent fitting is obtained as shown in Fig. 10, and the fitting parameters are summarized in Table 1. Conclusions Nanocrystalline powder of BaTi0.7Zr0.3O3 (BTZ30) relaxor ferroelectric material was prepared by the solid state reaction and mechanical milling techniques with a crystallite size of 20–100 nm. BTZ30 ceramics were successfully obtained by spark plasma sintering at low temperatures of 1100 and 1400 °C compared to the conventional sintering that is usually performed at 1550 °C. SPS-BTZ30-1100 ceramics have fine-grain size of 200 nm, however the grain size increased to 3–5 μm for SPS-BTZ30-1400 sample. The dielectric constant of as prepared SPS-BTZ30-1100 and SPS-BTZ30-1400 ceramics is very large with a value > 105, which is almost independent of temperature. The as prepared SPS-BTZ30 ceramics are highly conductive, with the grain (bulk) conductivity as high as 0.1 S/cm. These features are due to the creation of large density of charge carriers such as oxygen vacancies, free electrons and the reduction of Ti+4 to Ti+3. The insulating nature of BTZ30 ceramics was recovered by annealing the samples in air at 1100 °C for 2 h, leading to re-oxidation of the materials. The annealed BTZ30 ceramics exhibit common features of relaxor ferroelectric materials. The current work suggests that BTZ30 ceramics could be prepared by SPS at low temperatures of 1100 °C, however spark plasma sintering at 1400 °C is necessary to obtain high density BTZ30 ceramics with proper dielectric properties.
Fig. 9. The variations of ln (1/ε′ − 8 1/ε′m) versus ln (T − Tm) for annealed SPS-BTZ30-1400 ceramics at 10 kHz. The solid lines are the straight-line fits of the modified Curie – Weiss law (Eq. (3)).
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. Acknowledgements Fig. 10. Relaxation time τ versus temperature for the annealed SPS-BTZ301400 ceramics. The solid curve is the non-linear fitting to the Vogel-Fulcher relation (Eq. (4)).
The authors are grateful for the financial support from the Deanship of Scientific Research, King Faisal University, Saudi Arabia under the Research Group project #17122003.
Table 1 The fitting parameters of the modified Curie-Weiss law and V-F model of annealed SPS-BTZ30-1400 ceramics.
SPS-BTZ30
Tm (°C)
γ
τ0 (s)
Ea (e.V)
Tf (°C)
−47.7
1.84
3.56 × 10−10
0.077
–56.8
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Robels U, Arlt G. J Appl Phys 1993;73:3454. Zhang L, Ren X. Phys Rev B 2006;73:094121. Ahmad MM, Yamada K, Meuffels P, Waser R. Appl Phys Lett 2007;90:112902. Makovec D, Samardžija Z, Drofenik M. J Am Ceram Soc 2005;87:1324. Bouwma J, De Vries K, Burggraaf A. Phys Status Solidi A 1976;35:281. Kumar PM, Rao KS. Ferroelectrics 1989;94:299. Maier J, Schwitzgebel G, Hagemann HJ. J Solid State Chem 1985;58:1. Uematsu K, Sakurai O, Mizutani N, Kato M. J Mater Sci 1984;19:3671. Johnson D, Cross L, Hummel F. J Appl Phys 1970;41:2828. Dobal PS, Dixit A, Katiyar RS, Yu Z, Guo R, Bhalla AS. J Appl Phys 2001;89:8085. Yu Z, Ang C, Guo R, Bhalla AS. J Appl Phys 2002;92:2655. Tang XG, Chew KH, Chan HLW. Acta Mater 2004;52:5177. Yu Z, Ang C, Guo R, Bhalla AS. Appl Phys Lett 2002;81:1285. Trzaska Z, Couret A, Monchoux JP. Acta Mater 2016;118:100. Zhang ZH, Liu ZF, Lu JF, Shen XB, Wang FC, Wang YD. Scripta Mater 2014;81:56. Ahmad MM, Al-Quaimi M. Phys Chem Chem Phys 2015;17:16007. Guillemet-Fritsch S, Valdez-Nava Z, Tenailleau C, Lebey T, Durand B, Chane-Ching JY. Adv Mater 2008;20:551. [18] Han H, Ghosh D, Jones JL, Nino J. JC. J Am Ceram Soc 2013;96:485. [19] Han H, Voisin C, Guillemet-Fritsch S, Dufour P, Tenailleau C, Turner C, et al. J Appl
of relaxation times, which originate from the formation of polar nanoregions (PNR) [11,21–25]. In the present work we have calculated the relaxation time, τ, of the annealed SPS-BTZ30-1400 ceramics from the frequency dependence of the maximum temperature Tm of the dielectric constant (Fig. 6(b)). The variations of ln τ versus temperature for the annealed SPS-BTZ30-1400 ceramics are shown in Fig. 10. The experimental data could not be analyzed by the normal Arrhenious relation. Therefore, we have analyzed the relaxation time by the empirical Vogel-Fulcher (V-F) model that is expressed as:
6
Results in Physics 15 (2019) 102799
M.M. Ahmad, et al.
[23] [24] [25] [26]
Phys 2013;113:024102. [20] Uchino K, Nomura S. Ferroelectrics 1982;44:55. [21] Ang C, Jing Z, Yu Z. J Phys: Condens Matter 2002;14:8901. [22] Maiti T, Guo R, Bhalla AS. J Appl Phys 2006;100:114109.
7
Jiang XP, Zeng M, Chan HLW, Choy CL. Mater Sci Eng A 2006;438–440:198. Anwar S, Sagdeo PR, Lalla NP. J Phys: Condens Matter 2006;18:3455. Wang Y, Pu Y, Zheng H, Jin Q, Cao Z. RSC Adv 2016;6:4296. Levi LP, Ogieliski AT. Phys Rev Lett 1986;57:2288.
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Results in Physics journal homepage: www.elsevier.com/locate/rinp
Dielectric behavior of spark plasma sintered BaTi0.7Zr0.3O3 relaxor ferroelectrics
T
⁎
Mohamad M. Ahmada,b, , Latifah Alismaila, Adil Alshoaibia, Abdullah Aljaafaria, H. Mahfoz Kotba,c, Reda Hassaniend a
Department of Physics, College of Science, King Faisal University, Al-Ahsaa 31982, Saudi Arabia Department of Physics, Faculty of Science, The New Valley University, El-Kharga 72511, Egypt c Department of Physics, Faculty of Science, Assiut University, Assiut 71516, Egypt d Department of Chemistry, Faculty of Science, The New Valley University, El-Kharga 72511, Egypt b
A R T I C LE I N FO
A B S T R A C T
Keywords: Barium titanate zirconate Relaxor ferroelectrics Spark plasma sintering Dielectric properties
In the present work we have studied the transport and dielectric behavior of BaTi0.7Zr0.3O3 (BTZ30) relaxor ferroelectrics prepared by spark plasma sintering. Nanocrystalline powder of BTZ30 was obtained by solid state reaction followed by mechanical milling. Fine and coarse-grained ceramics of BTZ30 were prepared by spark plasma sintering at 1100 and 1400 °C, respectively. The obtained materials were characterized by X-ray diffraction and scanning electron microscopy. The grain size of the BTZ30 ceramics increased from 200 nm to 3–5 μm as the sintering temperature was increased from 1100 °C to 1400 °C. The electrical and dielectric properties of BTZ30 ceramics were studied by impedance spectroscopy over −150 to +200 °C temperature range. The dielectric constant exhibits very large values > 105 for the spark plasma sintered ceramics associated with very high dielectric loss and electrical conductivity. The insulating nature of the BTZ30 ceramics could be retained by re-oxidation of the samples. The dielectric data of the annealed have been analyzed by the modified Curie-Weiss law and the relaxation process in the materials was fitted by Vogel-Fulcher model.
Introduction Point defects, such as oxygen vacancies, play prominent role in tailoring the electrical and dielectric properties of ferroelectric materials [1–3]. The presence of oxygen vacancies leads to resistance degradation and aging phenomena in ferroelectric materials, Therefore, the origin of oxygen vacancies and their relaxation properties need to be explored. The crystal structure and the dielectric and electrical properties of BaTiO3 depend strongly on the type of impurities [4–9]. In this regard, barium titanate has been studied extensively after modified with various additives. Zr+4 substitution in the Ti+4 sites in barium titanate has attained considerable interest. Zr+4 ions are more stable than Ti+4 and have a larger ionic radius, leading to lattice expansion. Moreover, the substitution of Ti+4 by Zr+4 is expected to reduce the electrical conduction by electronic hopping between Ti+4 and Ti+3. Moreover, the Zr+4 substitutions at Ti-sites can lead to different features such as, shifting of the ferroelectric transition temperature, broadening or merging of the dielectric peaks. Consequently, diffuse or relaxor behavior occurs [10–12]. BaTi1−xZrxO3 (BZT) solid solutions have attracted considerable
⁎
attention due to their potential applications in various electronic devices. Zr+4 substitutions into Ti+4 sites in BaTiO3 exhibit interesting changes in the dielectric behavior of the materials. For low Zr content < 10%, the orthorhombic to tetragonal and the rhombohedral to orthorhombic phase transition temperatures corresponding to pure BaTiO3 increase. In contrast, the tetragonal to cubic phase transition temperature decreases. With increasing Zr content to ~15%, the three phase transitions are merged or pinched into a single diffuse phase transition [13]. With further increase in Zr contents > 15%, the pinched transition temperature decreases and a diffuse dielectric anomaly is observed. Above 25% of Zr content the BTZ system exhibits typical relaxor-like behavior. BaTi1−xZrxO3 ceramics are usually prepared by conventional sintering at high temperatures of about 1550 °C for several hours [10–13]. The high temperature sintering process leads to the formation of coarsegrained ceramics with substantial grain growth. Meanwhile, spark plasma sintering (SPS) is an innovative sintering technology that is becoming increasingly important in the processing of numerous materials, such as nanostructured materials, composite materials and functionally graded materials [14,15]. SPS is based on the spark plasma
Corresponding author at: Department of Physics, College of Science, King Faisal University, Al-Ahsaa 31982, Saudi Arabia. E-mail address: [email protected] (M.M. Ahmad).
https://doi.org/10.1016/j.rinp.2019.102799 Received 19 May 2019; Received in revised form 12 October 2019; Accepted 7 November 2019 Available online 11 November 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 15 (2019) 102799
M.M. Ahmad, et al.
Fig. 1. XRD of SPS-BTZ30 ceramics sintered by spark plasma sintering at 1100 °C and 1400 °C.
effect due to electrical spark discharge phenomenon wherein a highenergy, low-voltage pulse current momentarily generates spark plasma at high temperatures (many thousands of °C) in fine local areas between particles. Spark plasma sintering temperatures are commonly 200–500 °C lower than conventional sintering techniques, classifying SPS as a lower-temperature sintering technology [14,15]. Moreover, material processing cycle (pressure and temperature rise and hold time) is completed in short periods of approximately 5 to 25 min. The relatively low temperatures combined with fast processing times ensure tight control over the microstructure with very little or negligible grain growth. In the current work we will study the electrical and dielectric properties of spark plasma sintered BaTi0.7Zr0.3O3 (SPS-BTZ30) ceramics. We have prepared BTZ30 solid solutions by employing different techniques of solid state reaction, mechanical milling and spark plasma sintering. The spark plasma sintering of BTZ30 materials was performed at different temperatures ranging from 1100 °C to 1400 °C, which are considerably lower than the conventional sintering temperature of 1550 °C. However, we show in this paper the results of the samples sintered at 1100 °C and 1400 °C.
Experiment The powder of BTZ30 samples was prepared by solid state reaction and mechanical milling. The start materials are BaCO3, TiO2 and ZrO2. Stoichiometric amounts of the starting materials have been ball milled in 2-propanol for 12 h with a speed of 350 rpm. The dried powder was calcined at 1200 °C for 12 h. The calcined powder was crushed and ball milled again with a speed of 550 rpm for 6 h. Dense fine-grained BTZ30 ceramics were prepared by SPS under vacuum. In the SPS experiment we have fixed the applied pressure to 80 MPa. The samples were sintered at different temperatures of 1100 °C (SPS-BTZ30-1100) and 1400 °C (SPS-BTZ30-1400), with a heating rate of 150 K/min and a dwelling time of 10 min. The samples were characterized by x-ray powder diffraction (XRD) and scanning electron microscopy (SEM) techniques. XRD data were collected over the 0 ≤ 2θ ≤ 80 range using Stadi-P Image Plate, IP (Stoe and Cie GmbH, Darmstadt) with monochromatic radiation (λ = 1.5406 Å). The morphology and the grain size of the obtained ceramics have been investigated by field-emission scanning electron microscope (FE-SEM). For electrical measurement, gold electrodes were sputtered on the both sides of the pellets. The electrical and dielectric properties were studied by impedance spectroscopy (IS) measurements using Novocontrol concept 50 system in the −120 to 200 °C temperature range as described elsewhere [16].
Fig. 2. SEM micrographs of BTZ30 powder and SPS-BTZ30 ceramics sintered at 1100 and 1400 °C.
Results and discussion XRD patterns of SPS-BTZ1100 and SPS-BTZ1400 ceramics are shown in Fig. 1. Clearly, the two samples exhibit pure cubic structure similar to BaTiO3, with no impurity phases were detected. The morphology of BTZ30 powder and SPS-BTZ30 ceramics is shown in Fig. 2. 2
Results in Physics 15 (2019) 102799
M.M. Ahmad, et al.
Fig. 3. The temperature dependence of ε′ for as-prepared SPS-BTZ30 ceramics at selected frequencies, (a) SPS-BTZ30-1100 and (b) SPS-BTZ30-1400 ceramics.
Fig. 4. Temperature dependence of the dielectric loss of as-prepared (a) SPS-BTZ30-1100 and (b) SPS-BTZ30-1400.
behavior of ε′ of the spark plasma sintered ceramics is quite different from that of the ceramics sintered by conventional technique [10–13]. At low temperatures, there is a step increase of ε′ followed by a region where ε′ is almost independent of temperature. The dielectric constant attains giant values of ε′ > 105, especially at low frequencies. With increasing frequency the value of ε′ decreases to about 103 for the data measured at 10 MHz frequency. No ferroelectric or relaxor peaks could be detected in the dielectric behavior of the as prepared SPS-BTZ30 ceramics. Most probably the ferroelectric phases have been screened by the huge values of ε′. With increasing the SPS temperature, ε′ becomes less dependent on frequency. The observed dielectric behavior is associated with very large value of the dielectric loss as shown in Fig. 4. Clearly, the observed giant dielectric behavior and the very large dielectric loss cannot be attributed to dipolar polarization only. Most probably, these features are correlated with highly conductive properties of the materials. The frequency variation of the ac conductivity at different temperatures is shown in Fig. 5. Two plateaus are observed at low and high frequency regions, respectively. The low frequency plateau could be assigned to the grain boundary conduction, whereas the
We notice that the powder sample of BTZ30 has crystallite size in the 20–100 nm range. The obtained nano-sized BTZ30 powder is due to the ball milling process during the synthesis of the materials. For spark plasma sintered BTZ30 ceramics we notice that dense ceramics are obtained after sintering at 1400 °C. The grain size of SPS-BTZ30-1100 ceramics is about 200 nm. However, the SPS-BTZ30-1100 ceramics are relatively porous due to the low SPS temperature. With increasing the SPS temperature to 1400 °C, the grain size increases considerably to 3–5 μm as shown in Fig. 2 for SPS-BTZ30-1400 ceramics. Moreover, the relative density of SPS-BTZ30 ceramics increased from about 95% for SPS-BTZ30-1100 ceramics to about 98% for SPS-BTZ30-1400 ceramics. The increased density with increasing the SPS temperature agrees well with the SEM micrographs (Fig. 2), where SPS-BTZ30-1100 ceramics exhibit relatively porous morphology compared to SPS-BTZ3-1400 ceramics. We have performed the impedance spectroscopy measurements before and after annealing the SPS-BTZ30 ceramics. The temperature dependence of the dielectric constant of the as prepared SPS-BTZ30 ceramics sintered at 1100 °C and 1400 °C is shown in Fig. 3. The 3
Results in Physics 15 (2019) 102799
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Fig. 5. The frequency dependence of σ ' at selected temperatures of as prepared SPS-BTZ30 ceramics sintered at (a) 1100 °C and (b) 1400 °C.
Fig. 6. Temperature dependence of the dielectric constant of annealed (a) BTZ30-1100 and (b) BTZ30-1400.
high frequency plateau is assigned to the grain conduction. For the asprepared SPS-BTZ30-1100 ceramics, the electrical conductivity of the grains is very high in the range of 10−5–10−2 S/cm, whereas the grain boundary conductivity is in the range of 10−6–10−3 S/cm. For SPSBTZ30-1400 ceramics, the conductivity is even higher as shown in Fig. 5(b). These results indicate that the grains and grain boundaries of the as-prepared SPS-BTZ30 ceramics are highly conductive compared to the insulating nature of conventionally sintered BTZ30 ceramics. Similar behavior has been reported for spark plasma sintered BaTiO3 ceramics [17–19]. The above results of the dielectric constant and the electrical conductivity of SPS-BTZ30 ceramics indicate that very large concentration of free charge carriers are introduced into the materials during the SPS process. The SPS is performed under vacuum conditions, which are reducing atmosphere. Therefore, SPS of oxide materials in reducing atmosphere will lead to the formation of oxygen vacancies and the release of free electrons according to the following relation [17–19];
Oox ⇒
1 O2 Vo·· + 2e− 2
(1)
The formation of oxygen vacancies will also lead to the reduction of Ti+4 into Ti+3 according to the relation;
Ti4 + + e− ⇒ Ti3 +
(2)
The density of charge carriers such as oxygen vacancies, free electrons and Ti+3 is expected to increase with increasing the SPS temperature. Therefore, the observed behavior of the dielectric properties in as-prepared SPS-BTZ30 ceramics is attributed to the high density of free charge carriers in the materials. In this case, besides the dipolar polarization mechanism that is usually exist in ferroelectric materials; two more polarization mechanism can contribute to the giant dielectric behavior of as-sintered SPS-BTZ ceramics. The two mechanisms are the interfacial polarization and the polarization due to the polaron hopping. In order to recover the insulating nature of SPS-BTZ30 ceramics we 4
Results in Physics 15 (2019) 102799
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Fig. 7. The temperature dependence of the dielectric loss tan δ measured at various frequencies of annealed SPS-BTZ30 ceramics sintered at (a) 1100 °C, (b) 1400 °C.
Fig. 8. The frequency dependence of the electrical conductivity of the annealed SPS-BTZ30 ceramics sintered at (a) 1100 °C, (b) 1400 °C.
conductivity of the annealed samples varies linearly with the frequency, where the conductivity has very low values in the range 10−12–10−9 S/ cm as shown in Fig. 8. We have analyzed the dielectric data of the annealed SPS-BTZ301400 ceramics by the modified form of the Curie-Weiss law [20]:
have annealed the spark plasma sintered ceramics in air at 1100 °C for a period of 2 h. This annealing process is expected to re-oxidize the materials and reduce the density of oxygen vacancies. The dielectric data of the annealed SPS-BTZ30-1100 and SPS-BTZ30-1400 ceramics show typical relaxaor ferroelectric behavior (Fig. 6). The frequency dispersion of both the maximum value of the dielectric constant ε′m and its temperature Tm is observed. Additionally, it is noticed that the dielectric constant improved with increasing the SPS temperature from 1100 °C to 1400 °C. ε′ has a value of about 3300 at 10 kHz for SPSBTZ30-1100 ceramics with Tm value of about −27 °C. With increasing the SPS temperature to 1400 °C, ε′ increases considerably to 16,200 at 10 kHz and Tm also shifted to −47.7 °C. The temperature dependence of tan δ of the annealed SPS-BTZ30 ceramics is shown in Fig. 7 at selected frequencies. For SPS-BTZ30-1100 ceramics, tan δ decreases with increasing temperature and it has very low values at temperature above Tm, whereas for SPS-BTZ30-1400 ceramics tan δ shows a relaxation peak that increases with increasing frequency. Moreover, the electrical
1/ ε′ − 1/ εm′ = (T − Tm )γ / C1
(3)
where γ and C1 are constants. The parameter γ gets the value 1 ≤ γ ≤ 2, where γ = 1 reduces Eq. (3) to the normal Curie-Weiss law, whereas the value of γ = 2 represents the quadratic dependence that is valid for ideal ferroelectric relaxors [20]. Therefore, the value of γ will give an indication on the relaxor behavior of the materials. The plots of ln (1/ε′ − 1/ε′m) versus ln (T − Tm) for the annealed SPS-BTZ30-1400 ceramics are shown in Fig. 9. A value of γ = 1.84 was estimated, which agrees well with different ferroelectric relaxors [11,21–25]. In relaxor ferroelectric materials the frequency dispersion of the maxima of the dielectric constant has been attributed to the distribution 5
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τ = τ0 exp [Ea/ kB (Tm − Tf )]
(4)
where kB is the Boltzmann constant, Ea the activation energy, τ0 the pre-exponential factor (the inverse of the attempt frequency f0) and Tf is the Vogel-Fulcher (freezing) temperature. We notice in Fig. 10 that the relaxation time increases with decreasing temperature and τ becomes extremely large at the freezing temperature Tf, at which the fluctuation of polarization gets frozen. Similar behavior was observed in other systems such as spin-glass materials [26]. We have fitted our experimental data to the V-F model (Eq. (4)), and excellent fitting is obtained as shown in Fig. 10, and the fitting parameters are summarized in Table 1. Conclusions Nanocrystalline powder of BaTi0.7Zr0.3O3 (BTZ30) relaxor ferroelectric material was prepared by the solid state reaction and mechanical milling techniques with a crystallite size of 20–100 nm. BTZ30 ceramics were successfully obtained by spark plasma sintering at low temperatures of 1100 and 1400 °C compared to the conventional sintering that is usually performed at 1550 °C. SPS-BTZ30-1100 ceramics have fine-grain size of 200 nm, however the grain size increased to 3–5 μm for SPS-BTZ30-1400 sample. The dielectric constant of as prepared SPS-BTZ30-1100 and SPS-BTZ30-1400 ceramics is very large with a value > 105, which is almost independent of temperature. The as prepared SPS-BTZ30 ceramics are highly conductive, with the grain (bulk) conductivity as high as 0.1 S/cm. These features are due to the creation of large density of charge carriers such as oxygen vacancies, free electrons and the reduction of Ti+4 to Ti+3. The insulating nature of BTZ30 ceramics was recovered by annealing the samples in air at 1100 °C for 2 h, leading to re-oxidation of the materials. The annealed BTZ30 ceramics exhibit common features of relaxor ferroelectric materials. The current work suggests that BTZ30 ceramics could be prepared by SPS at low temperatures of 1100 °C, however spark plasma sintering at 1400 °C is necessary to obtain high density BTZ30 ceramics with proper dielectric properties.
Fig. 9. The variations of ln (1/ε′ − 8 1/ε′m) versus ln (T − Tm) for annealed SPS-BTZ30-1400 ceramics at 10 kHz. The solid lines are the straight-line fits of the modified Curie – Weiss law (Eq. (3)).
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. Acknowledgements Fig. 10. Relaxation time τ versus temperature for the annealed SPS-BTZ301400 ceramics. The solid curve is the non-linear fitting to the Vogel-Fulcher relation (Eq. (4)).
The authors are grateful for the financial support from the Deanship of Scientific Research, King Faisal University, Saudi Arabia under the Research Group project #17122003.
Table 1 The fitting parameters of the modified Curie-Weiss law and V-F model of annealed SPS-BTZ30-1400 ceramics.
SPS-BTZ30
Tm (°C)
γ
τ0 (s)
Ea (e.V)
Tf (°C)
−47.7
1.84
3.56 × 10−10
0.077
–56.8
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of relaxation times, which originate from the formation of polar nanoregions (PNR) [11,21–25]. In the present work we have calculated the relaxation time, τ, of the annealed SPS-BTZ30-1400 ceramics from the frequency dependence of the maximum temperature Tm of the dielectric constant (Fig. 6(b)). The variations of ln τ versus temperature for the annealed SPS-BTZ30-1400 ceramics are shown in Fig. 10. The experimental data could not be analyzed by the normal Arrhenious relation. Therefore, we have analyzed the relaxation time by the empirical Vogel-Fulcher (V-F) model that is expressed as:
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