Effect of Bi 2 O 3 additive on dielectric properties of BaTiO 3 -based ceramics for improving energy density

Introduction

The pulse power capacitors are used in many applications, such as laser weapons, high-power microwave systems and other power systems [1-6], which need release all the stored energy in microseconds. The key problem is how to achieve high permittivity and high breakdown strength (BDS). The BaTiO₃-based ceramics have been studied for decade years in the pulse power capacitors applications, especially BaTiO₃–BiM'O _x (M’ = Sc [79], Al [10], Y [11], Gd [12], Yb [13], Nb [14], etc.) ceramics, due to its large saturated polarisation (P_s), small remnant polarisation (P_r) and moderate BDS. Bi₂O₃ doped BT could decrease the dielectric loss and sintering temperature [15]. Besides, the effect of liquid phase Bi₂O₃ could reduce the sintering temperature and enhance the BDS sharply [16]. The substitution of M’ could decrease the Curie temperature (T_c). Based on the predecessors’ results, BT–Nb₂O₅ system is well known as the high permittivity and temperature stability. Nb is used to adjust the Curie temperature near the room temperature and a few added acceptors for compensation, such as Co, Ni and Mn [1719]. Most researchers focus on controlling the permittivity change to meet the EIA X7R specifications (△C/C_25°C less than ±15%) over the temperature range from −55 to 125°C. However, little paid attention to the mechanism of effect of Bi₂O₃ on the energy storage properties.

In this paper, the properties of BaTiO₃ with Bi₂O₃ and Nb₂O₅ co-doped were investigated systematically. The idea was that BiNbO₄ doping could balance the defect dipoles, form the second phase, decrease Curie temperature, reduce the P_r and suppress the nonlinearity of the BT–BN ceramics for improving energy density [7-9,1113]. The mechanism of effect of Bi₂O₃ on the energy storage properties was investigated systematically.

Experiment procedure

The ceramics of 0.95BaTiO₃–Bi _x Nb_0.05O₄ were prepared by traditional solid-state sintering. BaCO₃ (99.5%, Kelong Co., Ltd., Chengdu, Sichuan, China), TiO₂ (99.99%, Xuancheng Jinghua Co., Ltd., Xuancheng, Anhui, China), Bi₂O₃ (99%, Kelong Co., Ltd), Nb₂O₅ (99%, Kelong Co., Ltd) were used as raw materials. The powders were mixed as proportions of 0.95BaTiO₃–Bi _x Nb_0.05O₄, (x = 0, 0.03, 0.05, 0.07) and ball-milled with deionised water for 24 h. The slurry was dried in drying oven at 100°C for 24 h. The dried powders were pre-sintered in oven at 850°C for 2 h. The temperature of pre-calcinations was confirmed at 850°C, to suppress the volatilisation of Bi₂O₃, since melting point of Bi₂O₃ is 813°C. The calcined powders were ball-milled with deionised water a second time for 24 h. Then, dry the slurry, granulate the dried owders with 5% PVA and press the powders into pellets with diameter of 12 mm and thickness of 0.3 mm. The pellets were sintered from 1100 to 1200°C for 2 h. However, the density of the pellets was not enough when the sintering temperature was 1100°C, the pellets melt in the crucible. The suitable sintering temperature of the samples was confirmed at 1150°C for 2 h.

The density values of the samples were measure the Archimedes density. The crystalline phases and the change of the lattice constant of the samples were measured by the X-ray diffractions (XRD, Bede QC200). The scan rate used for recording XRD pattern was 2° min⁻¹. The surface topographies of the samples were measured by scanning electron microscope (FEI Inspect-F, Holland). The permittivity and loss of samples were measured from 100 Hz to 1 MHz and from 25 to 200°C by using Wayne Kerr LCR Meter. The linearity under high electric field was measured by the polarisation–electric field (P–E) hysteresis loops (RADIANT Precision LC, Albuquerque, NM, U.S.A.) at 1 kHz at room temperature. The changes of the capacitances with the electric field were measured by using Agilent B1505A (Agilent Technologies, Palo Alto, California, U.S.A.) at 1 kHz at room temperature.

Discussion

Structural property

The structure characterisations of 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics were detected by XRD, as shown in Figure 1. When x = 0, the stable phase was tetragonal phase. However, the only stable phase was cubic perovskite phase in Bi/Nb co-doping ceramics instead, in Figure 1(a). The tiny as shown in Figure 1(b), weak second phase peak formed between 29.5 and 32°, when Bi/Nb co-doped. The lattice parameters a, b and c were calculated from XRD patterns, as shown in Table 1. When x = 0.05, a, b, c and the unit cell volume showed minimum values. Generally, the coordination bond of the A-sites was 12 in a stable perovskite ABO₃ structure. Bi₂O₃ was believed to act as the donor, because the ionic radius of Bi³⁺ (103 pm) was nearly comparable to the value of Ba²⁺ (161 pm) [20,21]. The decrease of the unit cell volume was attributed to the substitution of smaller Bi³⁺ for Ba²⁺ based on 12-coordination bond [22], while the increase in the unit cell volume was ascribed to the substitution of larger size B-site cations Nb⁵⁺(0.64Å) for Ti⁴⁺(0.605Å) based on B-O6 octahedra [23]. The decreased unit cell volume proved that the Bi mainly substituted for A-site, forming defects dipoles system, when x ≤ 0.05. The increased unit cell volume was attributed to the second phase forming, when x > 0.05. 0.95BaTiO₃–Bi_0.05Nb_0.05O₄ showed the most compact structure characterisations. Therefore, the defects dipoles achieved a balance, when x = 0.05. Figure 2 showed the typical surface topography of BT–BN system ceramics. It was clear that all the BT–BN ceramics were sintered enough densely and the grain size distributed homogeneous. The grain sizes increased gradually with increased Bi₂O₃ concentration. It could be seen that a few amorous states attached the grain surface in S3 and S4, which fit the XRD result very well. The density values of the samples were measure by the Archimedes method, such as ρ₀ = 5.826, ρ_0.03 = 5.891, ρ_0.05 = 5.972 and ρ_0.07 = 5.951, respectively.

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Figure 1

(a) X-ray diffraction profile of 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics with different Bi doping and (b) the enlarged pattern from 29.5 to 32° of 0.95BaTiO₃–Bi _x Nb_0.07O₄ ceramics.

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Figure 2

Scanning electron microscopic images of thermal etched surface of 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics with different Bi doping, S1: x = 0, S2: x = 0.03, S3: x = 0.05 and S4: x = 0.07.

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Table 1

The crystal parameter of the 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics with different Bi doping.

Samples	Crystal phase	a (Å)	b (Å)	c (Å)	V (Å3)
x = 0	Tetragonal	7.33152	7.33152	7.37043	396.17
x = 0.03	Cubic	5.7818	5.7818	5.7818	193.28
x = 0.05	Cubic	5.7614	5.7614	5.7614	192.71
x = 0.07	Cubic	5.78804	5.78804	5.78804	193.91

Dielectric property

The dielectric properties of 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics were showed in Figure 3. Permittivity and loss decreased with the Bi³⁺ concentration increasing, due to the formed defects dipoles . The created electron traps compensated for intrinsic oxygen vacancy in BaTiO₃, when x ≤ 0.05, which can express by Equations (1) and (2). Excessive Bi³⁺ doping led to an obviously increase of loss, due to the second phase forming, which interrupted the balance. It indicated that system achieved a balance with the minimum loss, just when x = 0.05. The results fit the law of changed lattice parameters very well. (1) (2)

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Figure 3

The dielectric constant and loss of 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics with different Bi doping.

The temperature dependence of 95BaTiO₃–Bi _x Nb_0.05O₄ was shown in Figure 4. It indicated that the permittivity of all the samples decreased with the increased temperature from 25 to 300°C. The Curie peaks of all the samples were below the room temperature because of the Nb substitution for Ti-site. The 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics exhibited nonlinearities in the dielectric constant as a function of the high applied electric field, which was known as dielectric nonlinearity. The nonlinearity was measured under the C-V mode at 1 kHz at room temperature, as shown in Figure 5. It illustrated that the electric field dependence of the permittivity, loss and permittivity coefficient (FPC) for the 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics. The nonlinearity was suppressed gradually as a function of the high applied electric field, with the increased Bi content. The ferroelectricity and nonlinearity were attributed to the tiny movement of Ti⁴⁺ in a local region. The reasons of the weakened nonlinearity were followed: (1) BiNbO₄ doping decreased the Curie temperature, exhibiting the linearity of paraelectric phase above room temperature; (2) the larger cations ionic radius substitution for B-site restricted the movement of Ti⁴⁺, which led the polarisation weakened gradually. Furthermore, a few Bi₂O₃ existed as amorphous state around the grain boundary and formed second phase could improve the BDS for energy density.

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Figure 4

Temperature dependence of dielectric constant and loss tangent of 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics measured from 100Hz to 100 kHz (a) x = 0, (b) x = 0.03, (c) x = 0.05 and (d) x = 0.07.

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Figure 5

Dielectric constant (a), dielectric loss (b) and capacitance change (c) versus the DC electric field for the 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics with different Bi doping at 1 kHz at room temperature.

P–E hysteresis loops of BT–BN ceramics were detected at 1 kHz at room temperature, as showed in Figure 6. It revealed that the remnant polarisation (P_r) decreased with the Bi content increasing, when x ≤ 0.05. However, the P_r increased again, when x = 0.07, due to the unbalanced defect dipoles in 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics. The balance of the created dipole was very helpful for energy storage density, which had the minimum value of P_r. Furthermore, the shapes of P–E hysteresis loops changed from the ferroelectric characteristic to nearly linear characteristic and from fat to slim, which meant the P_r of ceramics decreased. At first, the Bi substituted the Ba-site. Then, the excessive Bi, beyond the solid solubility, existed around the grain boundary as amorphous state.

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Figure 6

P–E field relationship of 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics with different Bi doping.

The properties of storage energy density (W₁) and the energy efficiency (η) were calculated using Equations (3) and (4), respectively: (3) (4) where E is electric field, P_r is remnant polarisation, P_max is the maximum polarisation and W₂ was the energy loss density. W₁ was calculated by integrating the area between the polarisation axis and the discharge curve, and W₂ was obtained by integrating the area between the charge and the discharge curve [13]. The data of P_s and P_r, storage energy density and energy efficiency were showed in Table 2. When x = 0.05, the P_r and efficiency reached the minimum. Considering comprehensively the permittivity, the loss, P_r and P_s, when x = 0.05, the defect dipoles reached balance in 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics, and the energy density achieved the maximum of 0.68 J cm⁻³ with the energy efficiency of 91.5%. Although, the energy density of the buck ceramics was not high enough (<1 J cm⁻³), it is expected that dielectrics can achieve higher energy densities and energy efficiencies by decreasing the grain size and increasing the bulk density, among other methods. If the single layer thickness was less than 5 μm in capacitors, with refined homogeneous grain and decreased grain size, it was expected to achieve higher energy density (>6 J cm⁻³) [8,24,25].

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Table 2

The properties of polarisation, energy density and energy efficiency of 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics with different Bi doping.

	X = 0	X = 3%	X = 5%	X = 7%
Pm (μC cm−2)	15.016	11.363	11.734	12.847
Pr (μC cm−2)	1.73	0.719	0.504	0.641
W1 (J cm−3)	0.467	0.63	0.6814	0.708
W2 (J cm−3)	0.123	0.079	0.063	0.24
η (%)	79.1	88.9	91.5	74.7

Conclusions

BaTiO₃ with Bi₂O₃ and Nb₂O₅ co-doped ceramics were prepared via conventional solid-state reaction method. The structure characteristics, dielectric properties and energy density properties were investigated. The most stable structure characterisations of 0.95BaTiO₃–Bi _x Nb_0.05O₄ ceramics reached, when x = 5%. At the same time, 0.95BaTiO₃–Bi_0.05Nb_0.05O₄ ceramic possessed the minimum P_r of 0.504 μC cm⁻² and the maximum energy efficiency of 91.5%. Furthermore, due to the co-doping of Bi³⁺ and Nb⁵⁺, the dielectric properties revealed a gradual change from typical ferroelectric behaviour finally change to the linear behaviour. Because of the balanced defect dipoles , the formed second phase and the decreased Curie temperature, the nonlinearity of the BT–BN ceramics was suppressed obviously with the Bi₂O₃ content increasing. Comprehensive consideration the permittivity, the loss, P_r and P_s, the maximum energy density of 0.68 J cm⁻³ with the minimum P_r 0.504 μC cm⁻² and the maximum energy efficiency of 91.5% were achieved in 0.95BaTiO₃–Bi _x Nb_0.05O₄. In summary, it was clear that larger polarisation, smaller P_r, relative lower nonlinearity and higher energy efficiency, as well as low Curie temperature, were the determining factors in lead-free relaxor materials for the energy storage applications.