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Enhanced dielectric properties in Nb-doped BT-BMT ceramics
Ceramics International 42 (2016) 19413–19419
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Enhanced dielectric properties in Nb-doped BT-BMT ceramics Raz Muhammad a,n, Yaseen Iqbal b a b
Department of Physics, Abdul Wali Khan University Mardan, KP, Pakistan Department of Physics, University of Peshawar, KP 25120, Pakistan
art ic l e i nf o
a b s t r a c t
Article history: Received 1 July 2016 Received in revised form 23 August 2016 Accepted 24 August 2016 Available online 25 August 2016
The dielectric properties of lead-free solid solution, 0.5BaTiO3–0.5Bi(Mg1/2 þ x/6Ti(1 x)/2Nbx/3)O3 (x ¼0.01– 0.25), synthesized via a conventional solid state route were investigated. The thermal stability range of relative permittivity remarkably increased with an increase in x from 0.01 to 0.25. 0.5BaTiO3–0.5Bi(Mg1/ 2 þ x/6Ti(1 x)/2Nbx/3)O3 (x ¼ 0.20) composition showed high temperature stable relative permittivity (1100), revealing a change of less than 715% across 55–500 °C and loss tangent ( r0.025) over the temperature range 102–425 °C. In comparison with other known materials for high temperature applications, the excellent temperature stability and high electrical resistivity of the x ¼0.2 composition suggested that this material can be a promising base material for fabrication of high temperature ceramic capacitors. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Perovskites BaTiO3 High temperature dielectrics
1. Introduction A number of dielectric materials have been investigated for their possible use in fabrication of high temperature ( 4200 °C) ceramic capacitors. These capacitors are used in space exploration, automotive vehicles, down-hole drilling, military equipment, mining and several other applications [1]. The temperature in geothermal wells can exceed 400 °C; however, due to the relatively limited market for these MLCC's, the primary efforts were focused upon automotive applications. The automotive sector requires operating temperatures up to 500 °C for exhaust sensing [2]. The main problem in this area has been the achievement of thermal stability of relative permittivity (Δεr o15%) and low dielectric loss (tan δ r0.025). BaTiO3–based (BT) materials have been the focus of intensive research for these applications due to their interesting dielectric properties. Most of these ceramics have relaxor-like characteristics. Relaxors have a broad frequency dependent relative permittivity peak as a function of temperature and strong frequency dispersion in the loss tangent. The relaxorlike behavior in these materials is associated with the formation of nano-polar regions (PNRs) due to the mixed valence cations at both the A- and B-sites of the ABO3 unit cell of the host lattice. These relaxors are of interest for high temperature dielectrics due to their stable relative permittivity at elevated temperatures; however, in some cases, the thermal stability arises due to the core-shell segregation [3–6]. n
Corresponding author. E-mail address: [email protected] (R. Muhammad).
http://dx.doi.org/10.1016/j.ceramint.2016.08.152 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Bi-based solid solutions are of special interest for high temperature applications such as 0.7Ba0.8Ca0.2TiO3–0.3Bi(Zn1/2Ti1/2)O3 (BCT-BZT) [7], La2O3-doped 0.4Ba0.8Ca0.2TiO3–0.6Bi(Mg1/2Ti1/2)O3 (BCT-BMTþ L) [8], 0.9BaTiO3–0.1BiMg2/3Ta1/3O3 (BT-BMTa) [9], 0.03Bi(Zn2/3Nb1/3)O3–0.97(K1/2Na1/2)NbO3 (BZN-KNN) [10], 2 mol% Ba deficient 0.5BaTiO3–0.25Bi(Zn1/2Ti1/2)O3–0.25BiScO3 (BT-BZTBS-deficient) [11], Ba0.8Ca0.2TiO3–Bi(Mg1/2Zr1/2)O3 (BCT-BMZ) [12], (Li, Ta, Sb)-modified (K, Na)NbO3 (KNLNTS) [13] and 2 mol% Ba deficient 0.45BaTiO3–0.275Bi(Mg1/2Ti1/2)O3–0.275BiScO3 (BT-BMTBS-deficient) [14] which were reported to exhibit interesting dielectric properties. Among these, BaTiO3–BiMg1/2Ti1/2O3 (BT-BMT) ceramics have been extensively investigated. BT has a tetragonal structure and exhibits a sharp Curie point near 125 °C [15], corresponding to the ferro- to para-electric phase transition. BMT has a rhombohedral symmetry at room temperature, with Tc 430 °C [16,17]. 0.5BT–0.5BMT has been reported to have a relative permittivity ¼2248 at temperatures ranging from 167 to 400 °C and still researchers are working on the optimization of its properties because of its high relative permittivity [18–20]. Unfortunately, BT-BMT exhibits a short temperature stability range and high dielectric losses which are not beneficial for high temperature MLCCs applications. Most recently, Muhammad et al. [21] investigated Bi(Mg2/3Nb1/3)O3-modified BT ceramics which were reported to exhibit high temperature stable εr (¼ 940 710%) in the temperature range 40–487 °C. Donor doping such as Nb5 þ for Tisite is useful to fill up the oxygen vacancies and suppress oxide ion conduction [22]. Therefore, in the present study, the effect of Nbdoping on the dielectric characteristics of BT-BMT ceramics was investigated which showed thermally stable dielectric properties
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heating at 800 °C for 2 h. The temperature dependence of relative permittivity and loss tangent were measured from room temperature to 500 °C, using an HP 4284A precision LCR Meter over a frequency range 1 kHz–1 MHz. Impedance spectroscopy of these samples was carried out using an E4980A (Agilent) impedance analyzer at 20 Hz–2 MHz.
3. Results and discussion
Fig. 1. Room temperature X-ray diffraction patterns of BT–BMTN ceramics.
Table 1 Lattice parameters, unit cell volume and density of BT-BMTN ceramics. x
Lattice parameter
Cell volume Experimental density
Theoretical density
Relative density
0.01 0.05 0.10 0.15 0.20 0.25
3.9990 (20) 3.9993 (24) 4.0086 (5) 4.0125 (5) 4.0234 (5) 4.0279 (16)
63.95 (6) 63.97 (7) 64.41 (15) 64.60 (14) 65.13 (15) 65.35 (5)
6.839 6.831 6.799 6.786 6.750 6.722
95.3 94.6 97.0 95.2 94.8 96.8
6.52 6.46 6.60 6.46 6.40 6.51
over a wide range of temperature along with the high DC resistivity and time constant.
2. Experimental procedure A number of compounds in the 0.5BaTiO3–0.5Bi(Mg1/2 þ x/ (BT-BMN) (x ¼0.01–0.25) composition series were prepared using high purity (499%) BaCO3, Bi2O3, TiO2, MgO and Nb2O5 (Sigma Aldrich). To ensure correct stoichiometry, the carbonates were dried overnight at 180 °C and oxides at 800 °C prior to weighing. The batch compositions were mixedmilled in disposable polyethylene mill jars for 12 h, using isopropanol as a lubricant and Y-toughened zirconia balls as grinding media. The slurries were dried in an oven at 90 °C overnight and sieved in order to dissociate agglomerates (if any). The resulting powders were calcined at 950 °C for 4 h in covered alumina crucibles to prevent the volatilization of bismuth oxide. Calcined powders were mixed-milled using an agate mortar and then pressed into 10 mm diameter pellets at 50 MPa. In order to minimize the volatilization of bismuth, green pellets were buried in the calcined powders of the same composition and sintered in the temperature range 1100–1175 °C for 2 h at a heating/cooling rate of 5 °C/min. Density measurements of the sintered pellets were carried out using a high precision MDs-300 electronic densitometer, based on Archimedes principle. Phase analysis of crushed sintered pellets was carried out using a D-5000 X-ray diffractometer (Siemens, Germany) with Cu-Kα radiation (0.15406 nm) operating at 40 kV. The microstructure of the fractured surfaces of sintered samples was examined using a JEOL JSM-5910 scanning electron microscope (Japan). For electrical characterization, the sintered pellets were polished to achieve parallel smooth faces. Electrodes were formed via pasting a gold paste on the opposite faces, followed by 6Ti(1 x)/2Nbx/3)O3
X-ray diffraction (XRD) patterns of BT–BMTN (x ¼0.01, 0.05, 0.10, 0.15, 0.20 and 0.25) samples sintered at 1150 °C for 2 h are shown in Fig. 1. All the samples crystallized into a cubic perovskite structure and were indexed accordingly. There was no evidence of second phase formation within the detection limit of the in-house XRD facility which indicated the solubility of Nb in the BT-BMT lattice. The observed shifting of XRD peaks towards relatively lower 2θ angles suggested an increase in lattice parameters with an increase in x and hence an increase in the unit cell volume, Table 1. The observed expansion may be associated with the replacement of smaller Ti4 þ cations by larger B-site cations such as Mg2 þ and Nb2 þ in the present case [9]. Fig. 2 shows secondary electron SEM images (SEIs) of fractured surfaces of BT–BMT–BMN (x ¼0.01, 0.05, 0.10, 0.15, 0.20 and 0.25) samples sintered at 1150 °C for 4 h. The average grain size observed in the microstructure of x¼ 0.01 sample was 3 mm which decreased to 1.8 mm with an increase in x to 0.05. Upon further increase in x to Z0.15, the average grain size increased to 3 mm. The microstructure of all the samples investigated in the present study comprised densely packed grains, consistent with the measured relative densities (Table 1). The grain morphology of the x ¼0.15 sample appeared different (i.e. spheroidal with slightly deformed edges) from the other samples which may be due to the whole grains unaffected by fracturing. Similar grains can be seen, for example, in the void of the x¼ 0.01 sample marked as ‘A’ in Fig. 2. This type of morphology has been previously reported for BaTiO3-BiAlO3 ceramics, [23]. The observed variation in dielectric properties (εr and tan δ) with temperature ( 20–500 °C) at 1 kHz–1 MHz for the current samples is shown in Fig. 3. These samples showed strong dispersion below as well as above the temperature of maximum relative permittivity (Tm), indicative of “relaxor-like” behavior which may arise due to the occupancy of multiple cations with different valence at the same site. In the case of BT–BMTN, Bi3 þ and Ba2 þ occupy the A-site while Ti4 þ , Mg2 þ and Nb5 þ occupy the B-site. Their different ionic radii and valence may lead to the formation of local fields which hinder long-range dipole alignment, giving rise to polar nano-regions (PNRs), resulting in a relaxor-like behavior [10]. The change in relative permittivity above Tm for BT–BMTN system is less than that of the classical relaxor dielectrics, known as the temperature stable relaxor dielectrics. The relaxor behavior is described via a modified Curie-Weiss law, given by Eq. (1)
⎛ T − Tm ⎞γ 1 1 − =⎜ ⎟ εr εr (max) ⎝ C ⎠
(1)
where C and γ are the Curie constant and diffusion constant, respectively. For a normal ferroelectric, γ ¼1 while for an ideal relaxor γ ¼2. The values of γ for BT–BMTN ceramics were calculated from the slopes of the log(1/ε 1/εm) versus log(T Tm) plots shown in Fig. 4. In order to decrease the space charge contribution to relative permittivity, the data recorded at 1 MHz was used. The observed variation in γ from 1.58 to 1.67 indicated a relaxor-like behavior for the samples investigated in the present study.
R. Muhammad, Y. Iqbal / Ceramics International 42 (2016) 19413–19419
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Fig. 2. SEM micrographs of the fractured surfaces of BT–BMTN (x¼ 0.01–0.25) ceramics.
Tm was observed to decrease from 180 to 110 °C with an increase in Nb content from 0.01 to 0.25 which may be due to the increased compositional disorder in the polar nano-regions [24]. εr also decreased with an increase in x, possibly due to the replacement of highly polarizable Ti-ions by Mg- and Nb-ions. The larger Nb ions at the Ti-site are located at the center of BO6 octahedra and got “stuck” because Ti ions are relatively smaller, which are displaced from the center causing extra polarization [25]. Therefore, relatively larger cations (Nb5 þ ) may have decreased spontaneous polarization and hence relative permittivity. The change in relative permittivity as a function of temperature is shown in Fig. 5. The flattening of the dielectric peak increased with an increase in x from 0.01 to 0.25 and the optimum dielectric properties were observed for the x¼ 0.2 composition with a relatively broader operating range. A comparison of the temperature dependence of
the dielectric properties of x¼0.2 samples and other materials are given in Table 2. For this composition, the relative permittivity was 1100 715% over the temperature range 55–500 °C and tan δ was r0.025 at 102–425 °C which suggested that this composition can be a potential candidate material for high temperature capacitors applications. For x ¼0.25, thermal stability was unaffected, however; relative permittivity decreased and dielectric losses increased at low as well as high temperatures. The relative permittivity (1020 7 15%) for x ¼0.25 was stable in the temperature range 60–500 °C while dielectric losses were o0.025 over the temperature range 105–390 °C. One of the most important properties of a capacitor material for such applications is the temperature coefficient of relative permittivity (TCεr) in the operating temperature regime which should be close to zero for optimum performance. TCεr of the
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Fig. 3. Temperature dependent εr and tan δ of BT–BMTN ceramics at 1 kHz–1 MHz.
samples under-study was calculated using Eq. (2) using the relative permittivity data measured at 1 kHz. TCεr of the x ¼0.2 sample was 380 ppm/°C in the temperature range 100–500 °C, indicating its thermal stability, and hence suitability for the development of capacitors for applications in harsh environments.
TCεr =
1 ⎛⎜ Δεr ⎞⎟ εr (mid) ⎝ ΔT ⎠
(2)
Similarly, the x ¼0.2 sample appeared to be a good insulator with high electrical resistivity (4 GΩ-m). Moreover, the time constant (RC) of the sample was 59 s which is in agreement with the previous studies [26]. The high temperature complex impedance spectroscopy of the BT-BTMN samples was carried out to study the electrical characteristics of these samples. The typical impedance results are shown in Fig. 6. The cole-cole plots (Z″ versus Z′) for these sample showed distorted semicircles; however, the spectroscopic plots of Z″ and M″/εo versus log(f) for x ¼0.01 and 0.05 samples showed relaxation peaks at slightly different frequencies,
indicative of more than one conduction mechanisms across the sample. This can be explained on the basis of relaxation time (τ ¼RC). If the time constant differs by at least two orders, then these effects separately appear at different frequencies and if the different is smaller. The capacitance calculated from Z″ and M″/εo peaks at 650 °C were very close to each other (Table 3), which may be attributed to the core-shell effect which is generally reported for BT-based compounds [27] but is worthy of further investigation. For samples with x 40.05, single Debye-like peaks were observed at the same frequency, suggesting a single electroactive region, corresponding to the bulk because of the observed capacitance. The presence of the single Debye-like peak at the same frequency indicated similar chemical composition across the sample [28,29]. Arrhenius plot of conductivity for the BT-BTMN samples is shown in Fig. 7 and the activation energies obtained from these plots are given in Table 3. The band gaps of pure BT and BMT are 2.94 and 3.5 eV, respectively. The addition of Nb5 þ and increasing concentration of Mg2 þ is expected to increase the band
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gap when the tolerance factor is taken into account [30,31]. The substitution of Nb5 þ ions modify the electron energy levels of BMT and hence band gap. Thus, an increase in the Nb5 þ concentration may result in an increase in the magnitude of energy gap between the Fermi level and the conduction band, and hence, a decrease in the number of conduction band electrons [32]. The activation energy is approximately equal to half of the optical band gap (Eg 2Ea). Therefore, the activation energy of x ¼0.01 sample suggests that ionic conduction may be responsible for the leakage conductivity which is due to migration of oxygen vacancies, generally present in Bi-based perovskites [25]. The higher activation energies of the samples with x 40.01 suggested a predominantly intrinsic conduction mechanism [33]. This trend coincides with the result obtained from the temperature dependent dielectric properties which shows a decrease in dielectric losses at higher temperatures. Considering the scope of the present study, the observed thermal stability of BT–BMTN (x¼ 0.2) makes it a potential candidate material for fabricating capacitors for applications in harsh environments.
4. Conclusions Fig. 4. Plots of log(1/ε 1/εm) versus log(T Tm) for BT–BMTN ceramics at 1 MHz.
0.5BaTiO3–0.5Bi(Mg1/2 þ x/6Ti(1 x)/2Nbx/3)O3 (x¼0.01–0.25) ceramics were prepared via a conventional solid state sintering route. The sample with x¼0.2 showed a high temperature stable relative permittivity (1100) across 55–500 °C, loss tangent (r0.025) at 102– 425 °C and temperature coefficient of relative permittivity 380 ppm/°C at 100–500 °C. The complex impedance spectroscopy of the x¼0.2 sample revealed a homogeneous electrical microstructure. Similarly, the DC resistivity and time constant of the x¼0.2 sample was found to be 4 GΩ-m and 59 s at 300 °C, respectively. The dielectric properties of 0.5BaTiO3–0.5Bi(Mg1/2 þ x/ 6Ti(1 x)/2Nbx/3)O3 (x¼0.2) makes this material a promising candidate material for the fabrication of high temperature MLCC's.
Acknowledgment
Fig. 5. Change in relative permittivity of BT-BMTN ceramics as a function of temperature.
The author (R. Muhammad) acknowledges the financial support (1-8/HEC/HRD/2013/2504) extended by the Higher Education Commission of Pakistan for the research fellowship at the University of Sheffield, United Kingdom. The cooperation of Prof. I M Reaney is specially acknowledged for allowing us to work in the Electroceramics Lab, Department of Materials Science & Engineering, University of Sheffield and discussions during the fellowship.
Table 2 A comparison of dielectric properties of BT–BMTN ceramics with literature. Sample
Tm (°C) (1 kHz) εr
0.20 BT-BMTA BCT-BMT-La BT-BMN (0.5) BT–BZT–BS-deficient BT-BMT BCT-BMZ BT-BMT-BS-deficient KNLNTS 0.03BZN–0.97KNN BCT-BZT
138 – 135 96 100 277 27 200 225/310 – 90
(max)
1140 1278 940 974 – 2285 700 1550 2100 – 1180
(1 kHz) εr
(mid) 250
1100 – 865 940 1100 2248 – 1591 2000 2000 1030
(1 kHz) T-range (°C) εr 7 x% (1 kHz) 55–500 ( 7 15) 30–150 ( 7 15) 50–485 ( 7 10) 40–487 ( 7 10) 80–500 ( 7 10) 167–400 ( 7 15) 27–308 ( 7 15) 105–500 ( 7 15) 120–335 ( 7 15) 100–400 ( 715) 25–425 ( 715)
T-range (°C) tan δ (1 kHz)
TCer ppm/°C (Trange)
Ref.
102–425 30–198 100–400 74–455 100–450 (0.02) 238–400 (0.02) 27–300 (0.02) 200–400 (0.05) 120–335 (0.04) 100–400 (0.05) 110–420 (0.01)
380 (100–500) – 390 (135–485) – 182 (100–500) – – 75 (200–400) – – –
Present work [9] [8] [21] [11] [16] [12] [14] [13] [10] [7]
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Fig. 6. Cole-cole and spectroscopic plot of Z″ and M″/εo versus logf for BT–BMTN ceramics.
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Table 3 Activation energies and capacitance of BT-BTMN ceramics. Sample
0.01
0.05
0.10
0.15
0.20
0.25
EZ″ (eV) EM″ (eV) CZ″ (pF) at 650 °C CM″ (pF) at 650 °C
1.50 1.81 181.6 125.7
1.88 1.80 118.8 106.5
2.09 2.08 82.5 98.7
2.02 1.92 98.4 91.0
2.08 2.10 81.8 73.1
– – – –
Fig. 7. Arrhenius plot of bulk conductivity versus temperature for BT–BMTN ceramics extracted from (a) Z″ and (b) M″/εo plots.
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Short communication
Enhanced dielectric properties in Nb-doped BT-BMT ceramics Raz Muhammad a,n, Yaseen Iqbal b a b
Department of Physics, Abdul Wali Khan University Mardan, KP, Pakistan Department of Physics, University of Peshawar, KP 25120, Pakistan
art ic l e i nf o
a b s t r a c t
Article history: Received 1 July 2016 Received in revised form 23 August 2016 Accepted 24 August 2016 Available online 25 August 2016
The dielectric properties of lead-free solid solution, 0.5BaTiO3–0.5Bi(Mg1/2 þ x/6Ti(1 x)/2Nbx/3)O3 (x ¼0.01– 0.25), synthesized via a conventional solid state route were investigated. The thermal stability range of relative permittivity remarkably increased with an increase in x from 0.01 to 0.25. 0.5BaTiO3–0.5Bi(Mg1/ 2 þ x/6Ti(1 x)/2Nbx/3)O3 (x ¼ 0.20) composition showed high temperature stable relative permittivity (1100), revealing a change of less than 715% across 55–500 °C and loss tangent ( r0.025) over the temperature range 102–425 °C. In comparison with other known materials for high temperature applications, the excellent temperature stability and high electrical resistivity of the x ¼0.2 composition suggested that this material can be a promising base material for fabrication of high temperature ceramic capacitors. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Perovskites BaTiO3 High temperature dielectrics
1. Introduction A number of dielectric materials have been investigated for their possible use in fabrication of high temperature ( 4200 °C) ceramic capacitors. These capacitors are used in space exploration, automotive vehicles, down-hole drilling, military equipment, mining and several other applications [1]. The temperature in geothermal wells can exceed 400 °C; however, due to the relatively limited market for these MLCC's, the primary efforts were focused upon automotive applications. The automotive sector requires operating temperatures up to 500 °C for exhaust sensing [2]. The main problem in this area has been the achievement of thermal stability of relative permittivity (Δεr o15%) and low dielectric loss (tan δ r0.025). BaTiO3–based (BT) materials have been the focus of intensive research for these applications due to their interesting dielectric properties. Most of these ceramics have relaxor-like characteristics. Relaxors have a broad frequency dependent relative permittivity peak as a function of temperature and strong frequency dispersion in the loss tangent. The relaxorlike behavior in these materials is associated with the formation of nano-polar regions (PNRs) due to the mixed valence cations at both the A- and B-sites of the ABO3 unit cell of the host lattice. These relaxors are of interest for high temperature dielectrics due to their stable relative permittivity at elevated temperatures; however, in some cases, the thermal stability arises due to the core-shell segregation [3–6]. n
Corresponding author. E-mail address: [email protected] (R. Muhammad).
http://dx.doi.org/10.1016/j.ceramint.2016.08.152 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Bi-based solid solutions are of special interest for high temperature applications such as 0.7Ba0.8Ca0.2TiO3–0.3Bi(Zn1/2Ti1/2)O3 (BCT-BZT) [7], La2O3-doped 0.4Ba0.8Ca0.2TiO3–0.6Bi(Mg1/2Ti1/2)O3 (BCT-BMTþ L) [8], 0.9BaTiO3–0.1BiMg2/3Ta1/3O3 (BT-BMTa) [9], 0.03Bi(Zn2/3Nb1/3)O3–0.97(K1/2Na1/2)NbO3 (BZN-KNN) [10], 2 mol% Ba deficient 0.5BaTiO3–0.25Bi(Zn1/2Ti1/2)O3–0.25BiScO3 (BT-BZTBS-deficient) [11], Ba0.8Ca0.2TiO3–Bi(Mg1/2Zr1/2)O3 (BCT-BMZ) [12], (Li, Ta, Sb)-modified (K, Na)NbO3 (KNLNTS) [13] and 2 mol% Ba deficient 0.45BaTiO3–0.275Bi(Mg1/2Ti1/2)O3–0.275BiScO3 (BT-BMTBS-deficient) [14] which were reported to exhibit interesting dielectric properties. Among these, BaTiO3–BiMg1/2Ti1/2O3 (BT-BMT) ceramics have been extensively investigated. BT has a tetragonal structure and exhibits a sharp Curie point near 125 °C [15], corresponding to the ferro- to para-electric phase transition. BMT has a rhombohedral symmetry at room temperature, with Tc 430 °C [16,17]. 0.5BT–0.5BMT has been reported to have a relative permittivity ¼2248 at temperatures ranging from 167 to 400 °C and still researchers are working on the optimization of its properties because of its high relative permittivity [18–20]. Unfortunately, BT-BMT exhibits a short temperature stability range and high dielectric losses which are not beneficial for high temperature MLCCs applications. Most recently, Muhammad et al. [21] investigated Bi(Mg2/3Nb1/3)O3-modified BT ceramics which were reported to exhibit high temperature stable εr (¼ 940 710%) in the temperature range 40–487 °C. Donor doping such as Nb5 þ for Tisite is useful to fill up the oxygen vacancies and suppress oxide ion conduction [22]. Therefore, in the present study, the effect of Nbdoping on the dielectric characteristics of BT-BMT ceramics was investigated which showed thermally stable dielectric properties
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heating at 800 °C for 2 h. The temperature dependence of relative permittivity and loss tangent were measured from room temperature to 500 °C, using an HP 4284A precision LCR Meter over a frequency range 1 kHz–1 MHz. Impedance spectroscopy of these samples was carried out using an E4980A (Agilent) impedance analyzer at 20 Hz–2 MHz.
3. Results and discussion
Fig. 1. Room temperature X-ray diffraction patterns of BT–BMTN ceramics.
Table 1 Lattice parameters, unit cell volume and density of BT-BMTN ceramics. x
Lattice parameter
Cell volume Experimental density
Theoretical density
Relative density
0.01 0.05 0.10 0.15 0.20 0.25
3.9990 (20) 3.9993 (24) 4.0086 (5) 4.0125 (5) 4.0234 (5) 4.0279 (16)
63.95 (6) 63.97 (7) 64.41 (15) 64.60 (14) 65.13 (15) 65.35 (5)
6.839 6.831 6.799 6.786 6.750 6.722
95.3 94.6 97.0 95.2 94.8 96.8
6.52 6.46 6.60 6.46 6.40 6.51
over a wide range of temperature along with the high DC resistivity and time constant.
2. Experimental procedure A number of compounds in the 0.5BaTiO3–0.5Bi(Mg1/2 þ x/ (BT-BMN) (x ¼0.01–0.25) composition series were prepared using high purity (499%) BaCO3, Bi2O3, TiO2, MgO and Nb2O5 (Sigma Aldrich). To ensure correct stoichiometry, the carbonates were dried overnight at 180 °C and oxides at 800 °C prior to weighing. The batch compositions were mixedmilled in disposable polyethylene mill jars for 12 h, using isopropanol as a lubricant and Y-toughened zirconia balls as grinding media. The slurries were dried in an oven at 90 °C overnight and sieved in order to dissociate agglomerates (if any). The resulting powders were calcined at 950 °C for 4 h in covered alumina crucibles to prevent the volatilization of bismuth oxide. Calcined powders were mixed-milled using an agate mortar and then pressed into 10 mm diameter pellets at 50 MPa. In order to minimize the volatilization of bismuth, green pellets were buried in the calcined powders of the same composition and sintered in the temperature range 1100–1175 °C for 2 h at a heating/cooling rate of 5 °C/min. Density measurements of the sintered pellets were carried out using a high precision MDs-300 electronic densitometer, based on Archimedes principle. Phase analysis of crushed sintered pellets was carried out using a D-5000 X-ray diffractometer (Siemens, Germany) with Cu-Kα radiation (0.15406 nm) operating at 40 kV. The microstructure of the fractured surfaces of sintered samples was examined using a JEOL JSM-5910 scanning electron microscope (Japan). For electrical characterization, the sintered pellets were polished to achieve parallel smooth faces. Electrodes were formed via pasting a gold paste on the opposite faces, followed by 6Ti(1 x)/2Nbx/3)O3
X-ray diffraction (XRD) patterns of BT–BMTN (x ¼0.01, 0.05, 0.10, 0.15, 0.20 and 0.25) samples sintered at 1150 °C for 2 h are shown in Fig. 1. All the samples crystallized into a cubic perovskite structure and were indexed accordingly. There was no evidence of second phase formation within the detection limit of the in-house XRD facility which indicated the solubility of Nb in the BT-BMT lattice. The observed shifting of XRD peaks towards relatively lower 2θ angles suggested an increase in lattice parameters with an increase in x and hence an increase in the unit cell volume, Table 1. The observed expansion may be associated with the replacement of smaller Ti4 þ cations by larger B-site cations such as Mg2 þ and Nb2 þ in the present case [9]. Fig. 2 shows secondary electron SEM images (SEIs) of fractured surfaces of BT–BMT–BMN (x ¼0.01, 0.05, 0.10, 0.15, 0.20 and 0.25) samples sintered at 1150 °C for 4 h. The average grain size observed in the microstructure of x¼ 0.01 sample was 3 mm which decreased to 1.8 mm with an increase in x to 0.05. Upon further increase in x to Z0.15, the average grain size increased to 3 mm. The microstructure of all the samples investigated in the present study comprised densely packed grains, consistent with the measured relative densities (Table 1). The grain morphology of the x ¼0.15 sample appeared different (i.e. spheroidal with slightly deformed edges) from the other samples which may be due to the whole grains unaffected by fracturing. Similar grains can be seen, for example, in the void of the x¼ 0.01 sample marked as ‘A’ in Fig. 2. This type of morphology has been previously reported for BaTiO3-BiAlO3 ceramics, [23]. The observed variation in dielectric properties (εr and tan δ) with temperature ( 20–500 °C) at 1 kHz–1 MHz for the current samples is shown in Fig. 3. These samples showed strong dispersion below as well as above the temperature of maximum relative permittivity (Tm), indicative of “relaxor-like” behavior which may arise due to the occupancy of multiple cations with different valence at the same site. In the case of BT–BMTN, Bi3 þ and Ba2 þ occupy the A-site while Ti4 þ , Mg2 þ and Nb5 þ occupy the B-site. Their different ionic radii and valence may lead to the formation of local fields which hinder long-range dipole alignment, giving rise to polar nano-regions (PNRs), resulting in a relaxor-like behavior [10]. The change in relative permittivity above Tm for BT–BMTN system is less than that of the classical relaxor dielectrics, known as the temperature stable relaxor dielectrics. The relaxor behavior is described via a modified Curie-Weiss law, given by Eq. (1)
⎛ T − Tm ⎞γ 1 1 − =⎜ ⎟ εr εr (max) ⎝ C ⎠
(1)
where C and γ are the Curie constant and diffusion constant, respectively. For a normal ferroelectric, γ ¼1 while for an ideal relaxor γ ¼2. The values of γ for BT–BMTN ceramics were calculated from the slopes of the log(1/ε 1/εm) versus log(T Tm) plots shown in Fig. 4. In order to decrease the space charge contribution to relative permittivity, the data recorded at 1 MHz was used. The observed variation in γ from 1.58 to 1.67 indicated a relaxor-like behavior for the samples investigated in the present study.
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Fig. 2. SEM micrographs of the fractured surfaces of BT–BMTN (x¼ 0.01–0.25) ceramics.
Tm was observed to decrease from 180 to 110 °C with an increase in Nb content from 0.01 to 0.25 which may be due to the increased compositional disorder in the polar nano-regions [24]. εr also decreased with an increase in x, possibly due to the replacement of highly polarizable Ti-ions by Mg- and Nb-ions. The larger Nb ions at the Ti-site are located at the center of BO6 octahedra and got “stuck” because Ti ions are relatively smaller, which are displaced from the center causing extra polarization [25]. Therefore, relatively larger cations (Nb5 þ ) may have decreased spontaneous polarization and hence relative permittivity. The change in relative permittivity as a function of temperature is shown in Fig. 5. The flattening of the dielectric peak increased with an increase in x from 0.01 to 0.25 and the optimum dielectric properties were observed for the x¼ 0.2 composition with a relatively broader operating range. A comparison of the temperature dependence of
the dielectric properties of x¼0.2 samples and other materials are given in Table 2. For this composition, the relative permittivity was 1100 715% over the temperature range 55–500 °C and tan δ was r0.025 at 102–425 °C which suggested that this composition can be a potential candidate material for high temperature capacitors applications. For x ¼0.25, thermal stability was unaffected, however; relative permittivity decreased and dielectric losses increased at low as well as high temperatures. The relative permittivity (1020 7 15%) for x ¼0.25 was stable in the temperature range 60–500 °C while dielectric losses were o0.025 over the temperature range 105–390 °C. One of the most important properties of a capacitor material for such applications is the temperature coefficient of relative permittivity (TCεr) in the operating temperature regime which should be close to zero for optimum performance. TCεr of the
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Fig. 3. Temperature dependent εr and tan δ of BT–BMTN ceramics at 1 kHz–1 MHz.
samples under-study was calculated using Eq. (2) using the relative permittivity data measured at 1 kHz. TCεr of the x ¼0.2 sample was 380 ppm/°C in the temperature range 100–500 °C, indicating its thermal stability, and hence suitability for the development of capacitors for applications in harsh environments.
TCεr =
1 ⎛⎜ Δεr ⎞⎟ εr (mid) ⎝ ΔT ⎠
(2)
Similarly, the x ¼0.2 sample appeared to be a good insulator with high electrical resistivity (4 GΩ-m). Moreover, the time constant (RC) of the sample was 59 s which is in agreement with the previous studies [26]. The high temperature complex impedance spectroscopy of the BT-BTMN samples was carried out to study the electrical characteristics of these samples. The typical impedance results are shown in Fig. 6. The cole-cole plots (Z″ versus Z′) for these sample showed distorted semicircles; however, the spectroscopic plots of Z″ and M″/εo versus log(f) for x ¼0.01 and 0.05 samples showed relaxation peaks at slightly different frequencies,
indicative of more than one conduction mechanisms across the sample. This can be explained on the basis of relaxation time (τ ¼RC). If the time constant differs by at least two orders, then these effects separately appear at different frequencies and if the different is smaller. The capacitance calculated from Z″ and M″/εo peaks at 650 °C were very close to each other (Table 3), which may be attributed to the core-shell effect which is generally reported for BT-based compounds [27] but is worthy of further investigation. For samples with x 40.05, single Debye-like peaks were observed at the same frequency, suggesting a single electroactive region, corresponding to the bulk because of the observed capacitance. The presence of the single Debye-like peak at the same frequency indicated similar chemical composition across the sample [28,29]. Arrhenius plot of conductivity for the BT-BTMN samples is shown in Fig. 7 and the activation energies obtained from these plots are given in Table 3. The band gaps of pure BT and BMT are 2.94 and 3.5 eV, respectively. The addition of Nb5 þ and increasing concentration of Mg2 þ is expected to increase the band
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gap when the tolerance factor is taken into account [30,31]. The substitution of Nb5 þ ions modify the electron energy levels of BMT and hence band gap. Thus, an increase in the Nb5 þ concentration may result in an increase in the magnitude of energy gap between the Fermi level and the conduction band, and hence, a decrease in the number of conduction band electrons [32]. The activation energy is approximately equal to half of the optical band gap (Eg 2Ea). Therefore, the activation energy of x ¼0.01 sample suggests that ionic conduction may be responsible for the leakage conductivity which is due to migration of oxygen vacancies, generally present in Bi-based perovskites [25]. The higher activation energies of the samples with x 40.01 suggested a predominantly intrinsic conduction mechanism [33]. This trend coincides with the result obtained from the temperature dependent dielectric properties which shows a decrease in dielectric losses at higher temperatures. Considering the scope of the present study, the observed thermal stability of BT–BMTN (x¼ 0.2) makes it a potential candidate material for fabricating capacitors for applications in harsh environments.
4. Conclusions Fig. 4. Plots of log(1/ε 1/εm) versus log(T Tm) for BT–BMTN ceramics at 1 MHz.
0.5BaTiO3–0.5Bi(Mg1/2 þ x/6Ti(1 x)/2Nbx/3)O3 (x¼0.01–0.25) ceramics were prepared via a conventional solid state sintering route. The sample with x¼0.2 showed a high temperature stable relative permittivity (1100) across 55–500 °C, loss tangent (r0.025) at 102– 425 °C and temperature coefficient of relative permittivity 380 ppm/°C at 100–500 °C. The complex impedance spectroscopy of the x¼0.2 sample revealed a homogeneous electrical microstructure. Similarly, the DC resistivity and time constant of the x¼0.2 sample was found to be 4 GΩ-m and 59 s at 300 °C, respectively. The dielectric properties of 0.5BaTiO3–0.5Bi(Mg1/2 þ x/ 6Ti(1 x)/2Nbx/3)O3 (x¼0.2) makes this material a promising candidate material for the fabrication of high temperature MLCC's.
Acknowledgment
Fig. 5. Change in relative permittivity of BT-BMTN ceramics as a function of temperature.
The author (R. Muhammad) acknowledges the financial support (1-8/HEC/HRD/2013/2504) extended by the Higher Education Commission of Pakistan for the research fellowship at the University of Sheffield, United Kingdom. The cooperation of Prof. I M Reaney is specially acknowledged for allowing us to work in the Electroceramics Lab, Department of Materials Science & Engineering, University of Sheffield and discussions during the fellowship.
Table 2 A comparison of dielectric properties of BT–BMTN ceramics with literature. Sample
Tm (°C) (1 kHz) εr
0.20 BT-BMTA BCT-BMT-La BT-BMN (0.5) BT–BZT–BS-deficient BT-BMT BCT-BMZ BT-BMT-BS-deficient KNLNTS 0.03BZN–0.97KNN BCT-BZT
138 – 135 96 100 277 27 200 225/310 – 90
(max)
1140 1278 940 974 – 2285 700 1550 2100 – 1180
(1 kHz) εr
(mid) 250
1100 – 865 940 1100 2248 – 1591 2000 2000 1030
(1 kHz) T-range (°C) εr 7 x% (1 kHz) 55–500 ( 7 15) 30–150 ( 7 15) 50–485 ( 7 10) 40–487 ( 7 10) 80–500 ( 7 10) 167–400 ( 7 15) 27–308 ( 7 15) 105–500 ( 7 15) 120–335 ( 7 15) 100–400 ( 715) 25–425 ( 715)
T-range (°C) tan δ (1 kHz)
TCer ppm/°C (Trange)
Ref.
102–425 30–198 100–400 74–455 100–450 (0.02) 238–400 (0.02) 27–300 (0.02) 200–400 (0.05) 120–335 (0.04) 100–400 (0.05) 110–420 (0.01)
380 (100–500) – 390 (135–485) – 182 (100–500) – – 75 (200–400) – – –
Present work [9] [8] [21] [11] [16] [12] [14] [13] [10] [7]
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Fig. 6. Cole-cole and spectroscopic plot of Z″ and M″/εo versus logf for BT–BMTN ceramics.
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Table 3 Activation energies and capacitance of BT-BTMN ceramics. Sample
0.01
0.05
0.10
0.15
0.20
0.25
EZ″ (eV) EM″ (eV) CZ″ (pF) at 650 °C CM″ (pF) at 650 °C
1.50 1.81 181.6 125.7
1.88 1.80 118.8 106.5
2.09 2.08 82.5 98.7
2.02 1.92 98.4 91.0
2.08 2.10 81.8 73.1
– – – –
Fig. 7. Arrhenius plot of bulk conductivity versus temperature for BT–BMTN ceramics extracted from (a) Z″ and (b) M″/εo plots.
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