Dielectric breakdown of BaO–B2O3–ZnO–[(BaZr0.2Ti0.80)O3]0.85 [(Ba0.70Ca0.30)TiO3]0.15 glass-ceramic composites

Journal of Non-Crystalline Solids 358 (2012) 3510–3516

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Dielectric breakdown of BaO–B2O3–ZnO–[(BaZr0.2Ti0.80)O3]0.85 [(Ba0.70Ca0.30)TiO3]0.15 glass-ceramic composites Venkata Sreenivas Puli a,⁎, Ashok Kumar a, R.S. Katiyar a,⁎, X. Su b, C.M. Busta b, D.B. Chrisey b, M.Tomozawa b a b

Department of Physics and Institute for Functional Nano Materials, University of Puerto Rico, San Juan, Puerto Rico, PR-00936, USA Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY-12180, USA

a r t i c l e

i n f o

Article history: Received 6 December 2011 Received in revised form 7 May 2012 Available online 2 June 2012 Keywords: Alkali free glass; Electro ceramics; Dielectric properties; Electric breakdown

a b s t r a c t We have successfully synthesized and characterized the alkali-free glass 0.3 BaO + 0.6B2O3 + 0.1ZnO (BBZ) and electro ceramics [(BaZr0.2Ti0.80)O3]0.85–[(Ba0.70Ca0.30)TiO3]0.15 (BZT–BCT) composite for high energy density storage capacitor applications. First single phase BZT–BCT ceramic powders were prepared by conventional solid state reaction technique. X-ray diffraction studies of the sintered pellets revealed the pure perovskite phase with tetragonal to pseudocubic (rhombohedral phase). Raman spectroscopy results also confirmed the perovskite phase with tetragonal structure. These powders were mixed with 10–50 weight percentage of glass powder and were ground using low energy ball milling. The pellets of glass-ceramic composites with different amounts of glass were tested for ferroelectric at fixed frequency (50 Hz), dielectric, and breakdown field properties under a wide range of frequency. Pure ceramic BZT–BCT sample exhibits the well saturated hysteresis behavior, and as we increase the amount of glass composition with pure BZT–BCT, glass mixed ceramic hysteresis loops turn into linear behavior. Compositional enhancement in glass-ceramic composites indicates decrease in the room temperature permittivity and enhancement in breakdown field. The glass‐ceramic composites have shown better dielectric breakdown field but low energy density compared to parent ceramics. © 2012 Elsevier B.V. All rights reserved.

1. Introduction High energy density capacitors can store more than ten times energy per unit volume than the common capacitor (device for storing electric charge), and the accumulated energy in dielectrics is determined by dielectric permittivity and breakdown strength, which depends linearly on the dielectric permittivity and quadratically on the electric field [1]. Polymers and ceramics are the primary dielectrics for solid-state capacitors; ceramic capacitors are more useful due to high operating temperatures when compared to polymer films. Glass and glass-ceramics are also two of the new promising materials for high temperature capacitor applications [2]. Lead-based glass (PbO–Na2O–Nb2O5–SiO2) materials are the most commonly used high permittivity materials in commercial dielectric components. However, with increasing concerns about lead toxicity which is harmful to health, research attention turned toward lead-free materials. Lead-free glasses: BaO–B2O3–ZnO (BBZ), Bi2O3–B2O3–ZnO– BaO–SiO2 are potential replacements for lead based (PbO–B2O3– SiO2–ZnO, PbO–Na2O–Nb2O5–SiO2) glass frits. BaO–B2O3–ZnO (BBZ) is one of the notable alkali free glass system with dielectric constants of 14–18 and a coefficient of thermal expansion of 8–9 × 10 − 6/K [3,4].

⁎ Corresponding authors. E-mail addresses: [email protected] (V.S. Puli), [email protected] (R.S. Katiyar). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.05.018

These lead-free glass powders are widely used as dielectric layers in various types of plasma display panels (PDPs), ceramic capacitors and as well as in sealing glasses and enamels [3,5]. Glass-ceramic composites are first produced by obtaining a glass matrix using melt casting method [3]. Depending upon the sample dimensions and test protocol, glass synthesized by a bulk process can exhibit dielectric breakdown strengths in the range of 4–9 MV/cm [6]. Conventional ceramics show low breakdown strength of 10 kV/mm, despite having high dielectric permittivity values, during solid-state reaction process due to residual pores formed [7,22]. A glass-ceramic material is a composite created through controlled crystallization of an appropriate glass composition with pore-free structure and fine grains [8,9]. Glass-ceramics composites have the potential to serve as highenergy density capacitor materials for portable electronic or pulsed power applications and are also particularly useful in heart defibrillators and hybrid automotive vehicles, to name a few [10]. Higher electrical breakdown fields can be achieved in ceramics by optimizing, extrinsic material properties such as defect chemistry, microstructural development, reduced sample thickness, grain size, and electrode configuration, and these parameters play a crucial role in making high energy storage capacitors. Ceramic materials usually have large permittivity values, they are limited by their relative small breakdown strength and surprisingly maximum energy storage is not obtained in high dielectric constant materials but in those materials which display intermediate dielectric constant and highest

V.S. Puli et al. / Journal of Non-Crystalline Solids 358 (2012) 3510–3516

(a)

30

40

50

60

(113)

(2 2 0) (3 0 0) (10 3)

(2 1 1)

(2 1 0)

(10 0) 20

70

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(113)

(10 3)

(300)

(220)

(211)

(210)

(002) (200)

(111)

(100)

(b)

BZCT50

Intensity (arb.unit)

Stoichiometric ratio of BaCO3, CaCO3,TiO2, Zr2O3 powder were mixed for 2 h by adding isopropanol as milling media with zirconium ball in a low energy ball miller. Powders dried overnight were calcined at 1250 °C for 10 h. The alkali-free (0.3BaO + 0.6 B2O3 + 0.1 ZnO) glass is prepared from precursor powders of BaCO3, B2O3, and ZnO. The powder is melted in a platinum crucible at 1200 °C and allowed to fine for 1 h stirring every 10 min. The melt is then poured into a graphite mold heated to 500 °C. The mold is placed into a furnace at 500 °C for 1 h and subsequently furnace cooled. Phase formation of the powder was checked by X-ray diffraction (XRD) technique, and later, crystallized powder was mixed with alkali-free 0.3BaO + 0.6 B2O3 + 0.1 ZnO glass powder (10–50 weight percentage of glass powder), and was again mixed for 2 h by adding Isopropanol as milling media in a low energy ball miller. Glass ceramic composite powder was mixed with 4% PVA (polyvinyl alcohol) as an organic binder to increase the strength of green pellets and was formed into a pellet with13 mm diameter and 0.5 mm thickness. Pellets were kept at 500 °C for 30 min for binder removal and then sintered at 900 °C for 4 h in a Carbolite furnace. The structural and surface morphology of the sintered pellets were analyzed by XRD using CuKα = 1.54 Å radiation, Raman spectroscopy and scanning electron microscopy (SEM), respectively. Ferroelectric measurements were done with a Radiant Technologies (RT 6000 HVA-4000 V) amplifier by connecting copper cables on either side of the disks with silver paint. To measure dielectric properties, sintered disks were painted with silver on either side of the surfaces and were dried at 350 °C for 1 h for electrode formation. Temperature-dependent dielectric properties were carried out with an Alfa impedance analyzer with fully computer interfaced Novocontrol thermal stage in the temperature range of 273–400 K in a frequency limit of 100 Hz–10 MHz. Electrical breakdown voltage of the ten samples of each composition was measured at room temperature using Trek high voltage amplifier.

Intensity (arb.unit)

2. Experimental

( 0 0 2) (2 0 0)

(101) (110) (111)

[(BaZr0.2Ti0.80)O3]0.85 [(Ba0.70Ca0.30)TiO3]0.15

(101) (110)

ultimate breakdown voltage [11]. In the present work, we present dielectric breakdown measurements and the dielectric properties for composite mixtures of alkali-free glass and ceramics.

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BZCT40

BZCT30 BZCT20 BZCT10 20

30

40

50

60

70

80

2θ (degree) Fig. 1. Room temperature XRD patterns of (a) (Ba0.955Ca0.045) (Zr0.17TiO0.83)O3 ceramics sintered at 1500 °C and (b) glass mixed ceramics (BZCTG10–BZCTG50) sintered at 900 °C, with 2θ angle ranging from 10 to 80°.

3. Results 3.1. X-ray and scanning electron microscopy (SEM) characterization The XRD patterns of the samples with pure BZT–BCT and different amounts (10–50 wt.%) of alkali free glass (0.3BaO+ 0.6 B2O3 + 0.1 ZnO) mixed BZT–BCT ceramics sintered at 900 °C for 2 h are shown in Fig. 1. Fig. 2 shows the SEM micrographs of BZT–BCT ceramics sintered at 1500 °C for 4 h and glass mixed BZT–BCT (BZCTG10–BZCTG50) sintered at 900 °C for 4 h. SEM micrographs of disks revealed porefree and dense surfaces. The average grain size of the sintered BZT– BCT ceramic pellets is between ~200 and 250 μm. Whereas the average grain size of the glass mixed (BZCTG10–BZCTG50) ceramic pellets is between 5 and 10 μm. 3.2. Raman spectroscopy Raman spectroscopy results can help to detect molecular vibrations directly and are very sensitive to non-uniform distortions of the crystal lattice in short-range ordering [12]. Raman spectra of the BZT–BCT ceramics and glass mixed ceramic BZT–BCT are shown in Fig. 3, in order to explore the phase structural transformation of the sintered pellets at room temperature. The shape of the Raman spectra resembles that of lead-free BZT–BCT ceramics [13,22]. The BZT–BCT has a basic matrix of BaTiO3, with an ABO3 type perovskite structure. For present BZT–BCT and glass mixed ceramics, the faint E(TO) mode

is observed at around ~37–40 cm − 1. The A1(TO1) anti-symmetry mode detected at ~ 115–169 cm − 1 shifted toward higher frequencies. The anti-symmetry mode A1(TO2) between ~230–249 cm − 1 is also shifted toward the higher frequency, while the A1 (TO3)/B1 mode at around ~504–517 cm − 1 is due to O–Ti–O symmetric stretching vibrations [16]. A1(LO3)/E(LO) observed at around 708–721 cm − 1 and for pure BZT–BCT, A1g breathing mode is witnessed at ~ 792, where it is not found in glass mixed ceramics. 3.3. Ferroelectric and dielectric studies P–E hysteresis loops for [(BaZr0.2Ti0.80)O3]0.85 [(Ba0.70Ca0.30) TiO3]0.15–(BZT–BCT) ceramics and glass ceramics composite (BZCTG10– BZCTG50) were presented in Fig. 4; well-saturated ferroelectric hysteresis P–E loop acquired under the maximum electric field before the breakdown is observed for BZT–BCT ceramic composition with polarization maxima 14 μC/cm2, remanent Polarization (Pr)—5.40 μC/cm2, coercive Field, (Ec)—1.71 kV/cm, respectively. Non-saturated ferroelectric hysteresis loops were recorded for BZCTG10–BZTCG30 and linear hysteresis behavior is observed both in BZCTG40 and BZCTG50 glass mixed ceramic compositions. As the glass composition increased from 10 wt.% to 40 wt.% remanent polarization (Pr) decreased from ~ 2.2 μC/cm 2 to 0.025 μC/cm 2 and the coercive field values also decreased from ~ 9.29 kV/cm to 3.06 kV/cm.

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Fig. 2. SEM micrographs of (Ba0.955Ca0.045) (Zr0.17TiO0.83)O3 ceramics sintered at 1500 °C and glass mixed ceramics (BZCTG10–BZCTG50) sintered at 900 °C.

A1 (TO3)

100

200

300

400

500

600

A1 (LO3)/E (LO4)

A 1(TO2)

A1 (TO1)

B1/E (TO3)

Intensity (arb.unit)

E(TO1)

The temperature dependent-dielectric permittivity and dielectric loss curves for the pure BZT–BCT ceramics and the alkali free glass mixed BZT–BCT ceramics (BZCTG10, BZCTG20, BZCTG30, BZCTG40,

700

BZT-BCT BZCTG10 BZCTG20 BZCTG30 BZCTG40 BZCTG50

A1g

800

Wave number (cm-1) Fig. 3. Room temperature Raman spectra of (Ba0.955Ca0.045) (Zr0.17TiO0.83) O3 ceramics sintered at 1500 °C and glass mixed ceramics (BZCTG10–BZCTG50) sintered at 900 °C.

BZCTG50) in the frequency region 100 Hz–1 MHz were shown in Figs. 5 (a–f) and 6 (a–f). Room temperature dielectric permittivity values were shown in Table 1, pure BZT–BCT ceramics composition showed a maximum room temperature dielectric permittivity (ε ~ 7135); whereas the overall room temperature dielectric permittivity values for glass mixed ceramics were drastically reduced to that of the BZT–BCT ceramic materials. As the weight percentage of glass mixed ceramics increased from 10% to 20%, dielectric permittivity increased from ε ~ 147 (BZCTG10) to ε ~ 194 (BZCTG20) and then decreased drastically for BZCTG50 composition. Pure BZT–BCT and glass mixed ceramics have shown low dielectric loss values, as we increase the amount of glass compositions from 10% to 50% showing low dielectric loss values. At a low frequency region (100 Hz) glass mixed ceramics (BZCTG30, BZCTG40) exhibit high noise behavior, which were not shown in Fig. 6, as mentioned earlier there is no obvious reason for inconsistent dielectric loss behavior for glass mixed ceramics. 3.4. Dielectric breakdown studies There are three commonly accepted breakdown mechanisms for solids: intrinsic breakdown, thermal breakdown and ionization breakdown [24]. Material parameters such as porosity, grain and external

V.S. Puli et al. / Journal of Non-Crystalline Solids 358 (2012) 3510–3516 [(BaZr Ti

)O ]

[(Ba

Ca

)TiO ]

5

P(μC/cm2)

P(μC/cm2)

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0 -5 -10 -15 -15 -10 -5

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E(kV/cm)

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P(μC/cm2)

2

-4

E(kV/cm) 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8

BZCTG20

BZCTG10

P(μC/cm2)

15

3513

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0.05 0.00 -0.05 -0.10

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-20

0

20

40

-0.15 -40

-20

E(kV/cm)

0

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E(kV/cm)

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E(kV/cm)

Fig. 4. P–E hysteresis loops of (Ba0.955Ca0.045) (Zr0.17TiO0.83)O3 ceramics sintered at 1500 °C and glass mixed ceramics (BZCTG10–BZCTG50) sintered at 900 °C.

parameters like frequency and ramp rate of applied voltage influence dielectric breakdown strength and pores were one of the main causes of electrical breakdown, as the porosity increases, electrical breakdown strength decreases [24]. In general the intrinsic breakdown of a dielectric is defined as the maximum or the highest value of the breakdown strength obtained after eliminating all the known secondary effects which may influence the dielectric breakdown voltage. It is extremely difficult to ascertain whether, observed breakdown was intrinsic or not, but its combination of both intrinsic breakdown and extrinsic breakdown. For more informative discussion about breakdown strength and related effects, reader requested to refer the reference [24] and references therein. Glass mixed ceramics (BZCTG10–BZCTG50) were completely crystallized at 900 °C, and crystallized pellets with 0.5 mm thickness, were immersed in HT-200 oil to test the dielectric breakdown voltage. Room temperature dielectric breakdown voltage and energy storage density values were presented in Table 1 and Fig. 7. The dielectric energy storage density values were calculated using the formula, Ed = (ε0 εr Eb2) / 2, where Ed is the energy storage density (J/cm3), ε0 is the permittivity of free space (8.85 × 10− 14 F/cm), εr is the relative permittivity. Pure BZT–BCT ceramics prepared through solid-state sintering route exhibit, dielectric breakdown strength of 153 kV/cm and a high energy storage density of ~7.48 J/cm 3, where the real breakdown voltage occurred and the calculated value using above formula and the measured energy density for pure ferroelectric BZT–BCT ~2.10 J/cm 3 from P–E hysteresis [22]. 50 wt.% of glass mixed ceramic BZT–BCT (BZCTG50) composition exhibited a maximum of 600 kV/cm dielectric breakdown field strength even though the dielectric breakdown field strengths for glass mixed ceramics are moderately high, whose energy storage density values calculated are considerably low (~0.14 J/cm3, 0.15 J/cm3, 0.04 J/cm3, 0.02 J/cm3, 0.12 J/cm 3), which were due to the low room temperature dielectric permittivity values. Weibull plots are commonly used to interpret the dielectric breakdown strength data using a variety of factors: intrinsic material factors like composition and bulk structure as well as extrinsic material factors like sample thickness, temperature, surface condition, ambient atmosphere [6]. The statistical distribution probability for electrical breakdown failure was estimated by two parameter Weibull function: ln(ESi) along X-axis and ln(− ln(1 − i / (n+ 1))), along Y-axis where (ESi) is the specific dielectric breakdown strength of the each glass mixed ceramic sample and n is the sum of the samples and i is the serial number of the sample [7] and Weibull plots were shown in Fig. 8, for the

glass mixed ceramics (BZCTG10–BZCTG50). The two Weibull function should be linear and the slope parameter of the straight line (β) which determines the range of dielectric breakdown strength and the X-axis intercept is ln(α), where α is the scale of parameter, which relates to the magnitude of the dielectric breakdown strength. The calculated slope (β) and intercepts for the glass mixed ceramics (BZCTG10– BZCTG50) were shown in Table 2 with standard errors. In comparison to other compositions, BZCTG30 has shown highest β with a standard error of 1.77719. To achieve the higher values of energy storage density values, it is necessary, to optimize few parameters such as high dielectric permittivity, higher values of breakdown field strengths, using micro- or nanometers of sample thickness, etc. 4. Discussion The XRD patterns for BZT–BCT ceramics were identified as pure perovskite phases with tetragonal crystal structure as shown in Fig. 1(a). Fig. 1(b) shows the glass mixed ceramics with two main phases: the tetragonal BZT–BCT and the glass-ceramic phase. The intensity of the glass-ceramic phase at around 2θ = 29° increased with increased weight percentage of glass. SEM micrographs (Fig. 2) revealed that, both pure BZT–BCT and glass mixed ceramics exhibit crystalline behavior. Especially grain growth enhancement in pure BZT–BCT ceramics is attributed to higher sintering temperature and is an effective way to obtain dense microstructures with large grains and which also minimized the grain boundary contribution in dielectric and electric properties. Higher sintering temperature of BZT–BCT might be the reason for pore-free ceramics with dense microstructures. Pore-free nature of the BZT–BCT composition, might be the reason for higher dielectric permittivity and energy density values of the BZT–BCT ceramics. Initial compositions of glass mixed ceramics revealed few pores, and when the glass composition increased pore-free micrographs were observed as shown in Fig. 2. The observed pores might be the reason for low breakdown field strength of glass compositions for initial glass mixed ceramics and pore-free nature might be the possible reason for moderately high breakdown voltages for higher glass mixed compositions. Raman spectroscopy is a powerful technique for the study of ferroelectric materials because of the close relationship between ferroelectricity and lattice dynamics [23]. As shown in Fig. 3, the weak E(TO) mode was observed at around 37–40 cm − 1, which was not observed

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V.S. Puli et al. / Journal of Non-Crystalline Solids 358 (2012) 3510–3516 100kHz 50kHz 10kHz 1kHz 100Hz

8000 7000

(a)

6000

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ε

ε

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4000

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[(BaZr0.2Ti0.80)O3]0.85 [(Ba0.70Ca0.30)TiO3]0.15

1000 250

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BZCTG10

250

300

T(K)

400

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T(K) 100Hz 1kHz 10kHz 100kHz 1MHz

210

(c)

100Hz 1kHz 10kHz 100kHz 1MHz

65

(d) 60

ε

200

ε

55

190

50 180

BZCTG20 250

300

350

400

BZCTG30 45 250

450

300

T(K)

400

450

T(K) 100Hz 1kHz 10kHz 100kHz 1MHz

14.5 14.0

350

(e)

100Hz 1kHz 10kHz 100kHz 1MHz

10.0

(f) 9.5

13.5

ε

ε

9.0 13.0

8.5

12.5 12.0

8.0

BZCTG40

11.5 250

300

350

400

450

T(K)

BZCTG50 7.5 250

300

350

400

450

T(K)

Fig. 5. (a–f) The temperature dependence of the dielectric permittivity of (Ba0.955Ca0.045) (Zr0.17TiO0.83)O3 ceramics sintered at 1500 °C and glass mixed ceramics (BZCTG10– BZCTG50) sintered at 900 °C at 100–1 MHz.

in the pure BaTiO3 Raman spectra due to over damping effect in all temperature ranges coupled with acoustic modes [14,15]. The anti symmetric modes A1 (TO3)/B1 correspond to higher concentrations of polar [TiO6] octahedral, tetragonally distorted clusters in an overall cubic matrix of Ba(Zr0.25Ti0.75)O3; the E(TO + LO) mode is particularly the symptom of the existence of polar [TiO6] clusters in a perovskite structure [17]. E(TO3)/B1 mode is also one of the characteristic tetragonal symmetry modes for pure BaTiO3 at ~ 305 cm − 1, whereas the broad and asymmetric E(TO3)/B1 modes were observed at around 288–305 cm − 1 for present BZT–BCT and glass mixed BZT–BCT ceramics. E(TO3)/B1 mode for pure BZT–BCT is shifted to 288 cm − 1 whereas this mode is slightly shifted to lower frequency region at initial wt.% of glass compositions, as the glass composition increased, the asymmetric E(TO3)/B1 shifted toward higher frequency region which also confirms the tetragonal phase existence in both pure BZT–BCT and glass mixed BZT–BCT. The peak positions can be affected due to hydrostatic pressure effect that originates from clamping grains by their neighbors while crossing the transition from a cubic to a tetragonal distorted phase [18]. Polar [ZrO6] and especially [TiO6] clusters

are responsible for the mode A1(LO3)/E(LO) observed at around 708–721 cm − 1 [19]. Mode E (TO), A1(TO), at respective Raman frequency positions also confirms the presence of the tetragonal phase formation. For pure BZT–BCT, A1g breathing mode is witnessed at ~792 cm − 1, which is not observed in the glass mixed BZT–BCT ceramics. There is no evidence of the A1g octahedral breathing mode at ~800 cm − 1 in the characteristic spectra of BaTiO3 [20], whereas the broad mode which is independent of temperature appeared in the Zr doped BaTiO3 [21], which is Raman inactive in simple perovskite structured materials, since the mode is symmetrical and does not result in a change in polarization, however for complex perovskite materials and solid solutions with two or more B-site species, A1g mode becomes Raman active, due to the presence of dissimilar ions in the center of the octahedra create asymmetry in the breathing like mode and it is certainly due to the chemical nature, but not the structural distortion of the lattice [21]. Combining both Raman and XRD patterns, we can conclude that all the samples synthesized through solid state sintering process are all composed of the tetragonal structure with glass mixed phases.

V.S. Puli et al. / Journal of Non-Crystalline Solids 358 (2012) 3510–3516

(a)[(BaZr

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Ti0.80 )O 3]0.85 [(Ba 0.70 Ca 0.30 )TiO 3]0.15 100kHz 50kHz 10kHz 1kHz 100Hz

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Fig. 6. (a–f) The temperature dependence of the dielectric loss of (Ba0.955Ca0.045) (Zr0.17TiO0.83)O3 ceramics sintered at 1500 °C and glass mixed ceramics (BZCTG10–BZCTG50) sintered at 900 °C at 100–1 MHz.

Sample

Dielectric permittivity (20 °C)

Breakdown field (kV/cm)

Energy density (J/cm3)

BZT–BCT BZCTG10 BZCTG20 BZCTG30 BZCTG40 BZCTG50

7135 147 194 51 12 8

153 148 136 140 227 600

7.48 0.14 0.15 0.04 0.02 0.12

Breakdown Strength Energy density 0.14

600

0.12 500 0.10 400

0.08 0.06

300

0.04 200

0.02

Energy density (J/cm3)

Table 1 Dielectric permittivity, dielectric loss, breakdown field strength, and energy storage density of (Ba0.955Ca0.045) (Zr0.17TiO0.83)O3 ceramics sintered at 1500 °C and glass mixed ceramics (BZCTG10–BZCTG50) sintered at 900 °C.

Considerable increase in the dielectric permittivity values in the initial BZCTG10, BZCTG20 composition when compared to that of their counterparts might be due to the higher content of polarizable ions. BZCTG20 composition showed an increased room temperature dielectric permittivity compared to all other compositions and there

Breakdown Field (kV/cm)

As mentioned earlier, pure BZT–BCT ceramics exhibit well saturated hysteresis behavior, and as the glass composition increased from 10 wt.% to 50 wt.% P–E hysteresis loop saturation completely disappeared as shown in Fig. 4 and higher amount of glass mixed ceramics compositions adopted complete linear behavior. Due to linear behavior BZCTG50 composition exhibits negligible remanence polarization and coercive field values. Non-saturation hysteresis behavior of the compositions might be possibly because of the high coercive field values which resulted in electrical breakdown before complete saturation occurs. Another possible reason for non-saturation in P–E hysteresis loops might be existence of secondary residual glass, glass-ceramic mixed phases and also the stress applied by the surrounding rigid glass matrix.

0.00

100 0.1

0.2

0.3

0.4

0.5

Weight % of Glass ceramic composite Fig. 7. Composition dependence of the energy density and the dielectric breakdown as a function of mole fraction of glass mixed ceramics (BZCTG10–BZCTG50) sintered at 900 °C.

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1.5 1.0 0.5

ln(-ln(1-i/(n+1)))

Table 2 Weibull parameters ln(α) and ln(β) with standard errors for glass mixed ceramics (BZCTG10–BZCTG50) sintered at 900 °C.

BZCTG10 BZCTG20 BZCTG30 BZCTG40 BZCTG50 Liner Fit

0.0 -0.5 -1.0

Sample

ln(α) ± standard error

ln(β) ± standard error

BZCTG10 BZCTG20 BZCTG30 BZCTG40 BZCTG50

− 26.48832 ± 1.24897 − 27.17943 ± 1.16043 − 75.08043 ± 8.68068 − 71.83439 ± 5.66892 − 28.89923 ± 1.17952

5.43197 ± 0.26094 5.6955 ± 0.24762 15.26854 ± 1.77719 13.56301 ± 1.07785 4.40481 ± 0.18999

-1.5 -2.0

Acknowledgments

-2.5 -3.0

4.5

5.0

5.5

6.0

6.5

ln(ESi) Fig. 8. Weibull plots of the dielectric breakdown strength data of glass mixed ceramics (BZCTG10–BZCTG50) sintered at 900 °C and the straight line is a representation of linear fitted function (all the lines are drawn as guides to the eyes).

is no particular reason found for this increase. The overall decrease in dielectric permittivity values can be explained by the formation of more of the glass mixed ceramic phase. Pure BZT–BCT ceramics exhibited a maximum energy density of ~7.48 J/cm3 (calculated using the formula Ed = (ε0 εr Eb2) / 2) and the measured value is ~2.10 J/cm3 from P–E hysteresis [23], when compared to the glass mixed ceramic compositions. As shown in Fig. 8 the BDS data of all compositions of the glass mixed ceramic samples sintered at 900 °C for 4 h follow the two-parameter Weibull distribution and all the plots show a relatively good linearity. As mentioned earlier, glass mixed ceramic could not achieve high energy densities due to low room temperature dielectric permittivity values as shown in the table.

5. Conclusions Ferroelectric polarization measurements revealed that as the glass composition increased, glass mixed ceramics P–E hysteresis loops become almost linear with increased glass weight percentage. As the glass wt.% of the composition increased from 10 to 50, the dielectric break down field strength is increased (~ 600 kV/cm) and dielectric permittivity increased initially and then decreased for later compositions due to higher amount of glass. Even though the present samples exhibited good break down strengths, their energy storage densities are comparably low due to low dielectric permittivity values compared to that of the pure BZT–BCT ceramics. Higher breakdown strength and low loss might be due to the presence of alkali free glass and low loss dielectric BZT–BCT compositions.

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