Effects of Oxide Additives on the Phase Structures and Electrical Properties of SrBi₄Ti₄O₁₅ High-Temperature Piezoelectric Ceramics

Abstract

In this work, SrBi₄Ti₄O₁₅ (SBT) high-temperature piezoelectric ceramics with the addition of different oxides (Gd₂O₃, CeO₂, MnO₂ and Cr₂O₃) were fabricated by a conventional solid-state reaction route. The effects of oxide additives on the phase structures and electrical properties of the SBT ceramics were investigated. Firstly, X-ray diffraction analysis revealed that all these oxides-modified SBT ceramics prepared presented a single SrBi₄Ti₄O₁₅ phase with orthorhombic symmetry and space group of Bb21m, the change in cell parameters indicated that these oxide additives had diffused into the crystalline lattice of SBT and formed solid solutions with it. The SBT ceramics with the addition of MnO₂ achieved a high relative density of up to 97%. The temperature dependence of dielectric constant showed that the addition of Gd₂O₃ could increase the T_C of SBT. At a low frequency of 100 Hz, those dielectric loss peaks appearing around 500 °C were attributed to the space-charge relaxation as an extrinsic dielectric response. The synergetic doping of CeO₂ and Cr₂O₃ could reduce the space-charge-induced dielectric relaxation of SBT. The piezoelectricity measurement and electro-mechanical resonance analysis found that Cr₂O₃ can significantly enhance both d₃₃ and k_p of SBT, and produce a higher phase-angle maximum at resonance. Such an enhanced piezoelectricity was attributed to the further increased orthorhombic distortion after Ti⁴⁺ at B-site was substituted by Cr³⁺. Among these compositions, Sr_0.92Gd_0.053Bi₄Ti₄O₁₅ + 0.2 wt% Cr₂O₃ (SGBT-Cr) presented the best electrical properties including T_C = 555 °C, tan δ = 0.4%, k_p = 6.35% and d₃₃ = 28 pC/N, as well as a good thermally-stable piezoelectricity that the value of d₃₃ was decreased by only 3.6% after being annealed at 500 °C for 4 h. Such advantages provided this material with potential applications in the high-stability piezoelectric sensors operated below 500 °C.

1. Introduction

Piezoelectric ceramics, which are a kind of synthetic polycrystalline ferroelectric material, have been used as sensing materials for many electrical devices such as ultrasonic transducers, vibration sensors and multi-layer actuators [1,2,3,4,5]. In recent years, the concern for the environmental pollution and people’s health highlights the main differences between the commercial lead-based piezoelectric ceramics (lead zirconate titanate-PZT) and the developed lead-free piezoelectric ceramics (such as calcium barium titanate-BTO, potassium sodium niobate-KNN, etc.) [6,7,8]. Bismuth-layered structure ferroelectrics (BLSFs, also called Aurivillius phase), with large spontaneous polarization and fatigue-free properties, are promising candidates for ferroelectric random access memories (FRAMs) [9]. The chemical formula of BLSFs can be described as (Bi₂O₂)²⁺ (A_m−1B_mO_3m+1)²⁻, where m delegates to the number of octahedral layers in the perovskite layer between the bismuth oxide layers and values normally from 1 to 5. Such as Bi₂WO₆ (m = 1, T_C = 950 °C) [10], Bi₃TiNbO₉ (m = 2, T_C = 904 °C) [11], Bi₄Ti₃O₁₂ (m = 3, T_C = 675 °C) [12], SrBi₄Ti₄O₁₅ (m = 4, T_C = 520 °C) [13], Sr₂Bi₄Ti₅O₁₈ (m = 5, T_C = 267 °C) [14]. Because of high Curie temperature, BLSFs have received more and more attention due to the urgent need of high-temperature sensor, actuator, and transducer applications in recent years [15].

Among the Aurivillius family, SrBi₄Ti₄O₁₅ (SBT) captures an orthorhombic symmetry with space group A21am at room temperature, including four perovskite-like TiO₆ octahedron units stacked in between (Bi₂O₂)²⁺ layers [16]. However, some disadvantages associated with SBT such as difficulty in polarization, high leakage current, volatilization of the bismuth during sintering, and a low density and undesirable properties caused by the random arrangement of plate-like crystal grains [17], such as a tenuous piezoelectric activity (d₃₃~10 pC/N) [18]. Over the last few decades, a large number of investigations focused on the modification of the electrical properties of SBT piezoceramics through the ionic substitution at A-site [19,20,21] or B-site [22,23,24]. Cao et al. [25] found a large enhancement of piezoelectric properties (d₃₃ = 30 pC/N) in Mn-modified (B-site) SrBi₄Ti₄O₁₅ as well as good thermal stability at elevated temperatures, while its T_C remains almost unchanged at ~530 °C. However, most reports on enhancing the ferroelectric and piezoelectric properties of SBT ceramics concentrate on A-site rather than B-site [26]. For example, the Curie temperature increased by doping SBT with Na⁺ and Pr³⁺ at A-site [27]. The dielectric constant and loss decreased, whereas the Curie temperature increased when Na⁺ and Nd³⁺ were substituted to A-site in SBT [17]. A-site cerium-modified SrBi₄Ti₄O₁₅ ceramics showed a high stability of dielectric properties [28]. On the other hand, some oxide compounds like Cr₂O₃, MnO₂ and U₃O₈ as additives have been also proved to play a notable effect on the physical and electrical properties of BLSF ceramics [29].

Among these modified SBT ceramics, the compositions with both high Curie temperature and high piezoelectric property at the same time were rarely reported, and most of the as-reported works focused on the modification of the SBT ceramics with one kind of element/oxide. In this work, two kinds of oxides (Gd₂O₃, CeO₂, MnO₂ and Cr₂O₃) were co-doped into the SBT ceramics, and after these the oxide-modified SBT piezoceramics were fabricated via a traditional solid-state reaction process; the effect of these oxides on the phase structure and electrical properties of SBT ceramics have been investigated in detail.

2. Materials and Methods

2.1. Sample Preparation

Highly purified metal oxides of SrCO₃ (99%), Bi₂O₃ (99%), TiO₂ (99%), CeO₂ (99.99%) and Gd₂O₃ (99.99%) were weighed according to the stoichiometric formula of three designed compositions: SrBi₄Ti₄O₁₅ (SBT), Sr_0.92Gd_0.053Bi₄Ti₄O₁₅ (SGBT) and Sr_0.96Ce_0.04Bi₄Ti₄O₁₅ (SCBT), respectively. These powders as starting materials were milled for 10 h under the condition of ethanol as a dissolvent and zirconium ball as a milling medium. The dried mixtures were calcined at 850 °C for 4 h, and then the calcined powders of SGBT and SCBT were divided into four equal parts according to their quality, respectively. Secondly, 0.2 wt% of CeO₂ (99.99%), MnO₂ (99%) and Cr₂O₃ (99%) were severally added into three parts of SGBT powders, while 0.2 wt% of Gd₂O₃ (99.99%), MnO₂ (99%) and Cr₂O₃ (99%) were severally added into three parts of SCBT powders. Then, the mixtures of SBT, SGBT, SGBT-Ce, SGBT-Mn, SGBT-Cr, SCBT, SCBT-Gd, SCBT-Mn, SCBT-Cr were milled again in the same requirement. After drying, polyvinyl alcohol (PVA) as binder was added to the uniform mixture to form granules. The granules were pressed into pellets of 10 mm in diameter and 1 mm in thickness. After burning out PVA at 600 °C for 2 h, these pellets were sintered between 1100 °C and 1200 °C for 2 h in a sealed alumina crucible to obtain the ceramics with the maximum density.

2.2. Sample Characterization

The crystallographic structure of all sintered samples was determined by an X-ray diffractometer (DX2700, Dandong, China) employing Cu-Kα radiation (λ = 1.5418 Å). Meanwhile, the relative density of all sintered samples was calculated as the ratio of the apparent density measured by the Archimedes method to the theoretical density obtained from crystallographic structures (XRD). In order to measure the electrical properties, the samples were polished and fired with silver paste as the electrodes at 700 °C for 10 min. The dielectric constant (ε_r) and loss tangent (tan δ) as a function of temperature were recorded using an LCR analyzer (TH2829A, Tonghui, China) attached to a programmable furnace. Samples were poled under a DC field of 6–10 kV/mm for 15 min in a silicone oil bath at 150 °C. The electrical impedance (|Z|) and phase angle (θ) as a function of frequency was measured using an impedance analyzer (TH2829A, Tonghui, China). The planar electromechanical coupling factor (k_p), mechanical quality factor (Q_m) and planar frequency constant (N_p) were calculated by the IEEE standard. Thermal depoling behavior was investigated by annealing the polarized samples at different temperatures for 4 h, and then the piezoelectric charge coefficient (d₃₃) was remeasured using a quasi-static d₃₃ m (ZJ-6AN, IACAS, Beijing, China) when the samples were cooled to room temperature.

3. Results and Discussion

3.1. Phase Structure of Ceramics

The appearances of the oxide-modified SBT piezoceramics are presented in Figure 1. As can be seen from these figures, the pure SBT ceramic seems to be taupe; after it was doped with different oxides, different colors were presented. However, all these oxide-modified SBT piezoceramics were sintered in a uniform color and free of cracks, blotches, striations and holes, at least seen from their surfaces. The change of color also proves that the oxides as additives have dissolved into SBT, leading different color emerging mechanisms to the ceramics.

The XRD patterns of the oxide-modified SBT piezoceramics are shown in Figure 2. It can be seen that these samples display a single SrBi₄Ti₄O₁₅ phase crystallized in the orthorhombic structure with Bb21m (36) space group (JCPDS No: 43-0973). There is no impurity detected from XRD patterns, which indicates that these oxide additives have been incorporated into the crystal lattice of SrBi₄Ti₄O₁₅. The strongest diffraction of all these samples appears at the (1 1 9) peak, stating the fact that SrBi₄Ti₄O₁₅ belongs to the BLSF with the structure of four layer (m = 4) [30]. Some variations observed from the details of XRD patterns can be related with the lattice distortions of SBT caused by doping. By contrasting with the pure SBT, three diffraction peaks of the doped SBT: (0 0 10), (0 0 16) and (0 0 20) are weakened, which indicates that their grains orientating along the c-axis becomes fewer [31]. Inversely, the diffraction peaks of (2 0 0)/(0 2 0) are enhanced, which states that the number of grains oriented along the a-b plane increased.

The lattice parameters of the oxide-modified SBT ceramics are given in

Table 1. Lattice parameters of the oxide-modified SBT ceramics.

Compositions		SBT	SGBT	SGBT-Ce	SGBT-Mn	SGBT-Cr	SCBT	SCBT-Gd	SCBT-Mn	SCBT-Cr
		Orthorhombic, Bb21m
a (Å)		5.43177	5.42744	5.42855	5.42849	5.42483	5.42807	5.42893	5.42653	5.42411
b (Å)		5.43718	5.43507	5.43741	5.43693	5.43767	5.43818	5.43877	5.43772	5.43598
c (Å)		40.95524	40.93938	40.92796	40.88468	40.89918	40.95129	40.9501	40.95103	40.93853
V (Å3)		1209.54	1207.65	1208.08	1206.68	1206.46	1208.83	1209.12	1208.38	1207.85
b/a		1.0010	1.0014	1.0016	1.00155	1.0024	1.0019	1.0018	1.0020	1.0022

. The lattice parameters (a, c and v) of the oxide-modified SBT decrease, whereas the values of orthorhombic distortion (b/a) increase, which may be attributed to the ion-substitution effect caused by the addition of different oxides. The bismuth oxide layer is very strong and bismuth ion in the bismuth oxide layer are difficult to be substituted by other ions [32], meanwhile ions with similar ionic radius and same coordination number are more likely to be mutually substituted [31], consequently Sr²⁺ located at the A-site in the perovskite layers are substituted by Gd³⁺/Ce⁴⁺ with smaller ion radius. Ti⁴⁺ located at the B-site in the perovskite layers would be substituted by Mn³⁺ and Cr³⁺. The lattice distortion caused by ion substitution can result in the change of electrical properties for ferroelectric compounds [33]; the larger b/a value is, more distorted the lattice is. Among these compositions, the unit cell of SGBT-Cr has the largest orthorhombic distortion with a b/a value of 1.0024.

Table 2. Density data of the oxide-modified SBT ceramics.

Compositions	SBT	SGBT	SGBT-Ce	SGBT-Mn	SGBT-Cr	SCBT	SCBT-Gd	SCBT-Mn	SCBT-Cr
ρtheoretic (g/cm3)	7.4456	7.4667	7.4614	7.4724	7.4522	7.441	7.4292	7.446	7.4367
ρactual (g/cm3)	7.0598	6.8822	7.137	7.0692	6.3155	7.1173	7.08528	7.2209	6.5354
ρrelative (%)	94.8	92.2	95.7	95.7	84.7	95.6	95.4	97	87.8

lists the density of the oxide-modified SBT ceramics. The relative density of SBT is measured as 94.8%, which has been changed after the addition of different oxides. According to the results given by Table 2, the addition of CeO₂ and MnO₂ played a positive effect on the densifying process of the SBT ceramic during sintering.

3.2. Dielectric Properties of Ceramics

Figure 3 exhibits the temperature dependence of dielectric constant (ε_r) and loss tangent (tan σ) of the oxide-modified SBT ceramics. As can be seen, all the samples show a dielectric anomaly around 540 °C, which can be related to the ferroelectric-paraelectric phase transition of the ceramics. The peak position is considered as the Curie temperature (T_C). For the pure SBT (T_C = 537 °C, Figure 3a), a sharp rise in the values of ε_r occurred above 350 °C at low frequencies (100 Hz and 500 Hz), which can be attributed to the dielectric response of a large number of space charges to the external electric field. Moreover, its permittivity peaks are broadened and strongly dependent with frequency in terms of strength, and their positions seem to be also dependent with frequency as marked by the slightly oblique arrows. Therefore, this can be considered as a typical relaxed dielectric behavior, which is partially due to the compositional fluctuation in the crystallographic sites. In Figure 3b, SGBT exhibits a higher T_C ~ 557 °C as well as a normal phase transition. This result may be attributed to the lattice distortion of the pseduo-perovskite structure since the bivalent strontium ions were substituted by the trivalent gadolinium ions at the A-site. The tolerance factor (t) which is used for evaluating the stability of ABO₃-type perovskite structure can be calculated by the expression as follows [34]:

$$t=\frac{r_{\mathrm{A}}{+r}_{\mathrm{O}}}{\sqrt{2}{(r}_{\mathrm{B}}{+r}_{\mathrm{O}})}$$

(1)

where r_A, r_B and r_O are the ionic radius of A, B and the oxygen ion, respectively. One-third of bivalent strontium ions (1.44 Å) and two-thirds of bismuth ions (1.30 Å) occupy the A site at the perovskite-like structure of pure SBT ceramics. According to the atomic percentage of the A-/B-site, the average ionic radius for Sr_0.92Gd_0.053Bi₄Ti₄O₁₅ could be reckoned as follows: r_A = $1/3$ (0.92r_Sr2+ + 0.053r_Gd3+) + $2/3$ r_Bi3+ = 1.33 Å (r_Gd3+ = 1.107 Å), r_B = r_Ti4+ = 0.605 Å, r_O2− = 1.40 Å. The tolerance factor of SGBT and SBT were calculated to be 0.96 and 0.97, respectively, according to Equation (1). The reduced tolerance factor indicates that the perovskite structure of SGBT is more stable; in this case, the phase transition from the ferroelectric state to the paraelectric state needs more energy, which corresponds to a higher T_C. As can be seen from Figure 3c, T_C of SCBT (531 °C) is less low than that of SBT, which could be attributed to the reduced stability of oxygen octahedron after adding CeO₂ into SBT, since the coordination number of introduced Ce⁴⁺ is smaller than that of Sr²⁺. The dielectric loss peak appearing around 500 °C at the low frequency of 100 Hz could be attributed to the space-charge relaxation as an extrinsic dielectric response [35]. The similar dielectric anomaly was also observed in cobalt-modified SBT [23]. The defect dipoles which are formed by combining space charges or ions with opposite charges may be slow to follow the external electric field, thereby contributing to the dielectric loss [36]. Therefore, the relaxation phenomenon reflected by the dielectric loss peaks or bumps in the wide temperature sweep can be related to the viscoelastic reorientation of defect dipoles following the external electric field at high temperature [37]. On the other hand, for all the oxide-doped compositions, the characteristic temperatures of permittivity peaks agree with that of loss peaks well. Especially, SCBT-Cr shows the most flat dielectric loss curve at 100 Hz, which indicates that the synergetic doping of CeO₂ and Cr₂O₃ could significantly improve the temperature stability of the dielectric properties of SBT.

3.3. Electro-Mechanical Coupling Property

Figure 4 shows the electro-mechanical resonance spectroscopy of the oxide-modified SBT ceramics. As can be seen, there are no resonance-antiresonance peaks in the pure SBT ceramic at the measured frequency range from 20 Hz to 2 MHz. The resonance-antiresonance peaks of SGBT and SCBT appear, respectively, at 184 kHz and 186 kHz. A high angle indicates the fully poled state of the specimen [38]. The position generated the resonance-antiresonance peak and the maximum phase angle also converts with the introduction of other additives. SCBT obtaind the maximum phase angle value (θ = −24.8°), which indicates its more fully polarized degree.

Table 3. Electro-mechanical properties of the oxide-modified SBT ceramics.

Compositions	SGBT	SGBT-Ce	SGBT-Mn	SGBT-Cr	SCBT	SCBT-Gd	SCBT-Mn	SCBT-Cr
kp (%)	5.72	5.2	4.03	6.35	3.7	4.5	5.8	6.51
Qm	423	465	1240	355	1885	787	443	374
Np (Hz·m)	2655	2745	2711	2481	2751	2740	2694	2613

presents electro-mechanical properties of the oxide-modified SBT ceramics. Clearly, oxide additives also affect the electro-mechanical coupling properties of the SBT ceramic, especially as the addition of Cr₂O₃ has a significant impact on it. SGBT-Cr and SCBT-Cr obtain relatively high k_p, low Q_m and N_p. The oxygen vacancies in piezoceramics usually result in the increase in Q_m and the decrease in k_p for ferroelectric ceramics [39]. A higher k_p achieved by SGBT-Cr and SCBT-Cr can be attributed to the reduced oxygen vacancy concentration caused by the addition of Gd₂O₃ and CeO₂.

3.4. Lower Limiting Frequency

Piezoelectric ceramic materials not only generate charges under the condition of applied stress or strain, but also ensure that the charges must be maintained for a period of time to be monitored by the system in actual engineering applications. The time of the maintained charge is proportional to the RC time constant. The minimum available frequency of sensor is considered to be the lower limiting frequency (f_LL). The relationship between RC time constant and f_LL is as follows:

$$f_{LL}=\frac{1}{2\pi RC}$$

(2)

where C is the capacitance (1 kHz) and R is the insulation resistance. Low values of f_LL allow the dynamic bandwidth to be extended to sonic frequencies [40]. The addition of different oxides decreases the f_LL of SBT as shown in the inset of Figure 5 at room temperature. The result indicates that the addition of oxides could improve the resistivity of SBT ceramics. Due to superfluous electrons generated by higher valence, Gd³⁺ and Ce⁴⁺ substituted lower valence Sr²⁺ can neutralize the oxygen vacancies, which increases the resistivity of SBT. The lower limiting frequency of the oxide-modified SBT ceramics at different temperatures are also compared with each other in Figure 5. The f_LL values of all compositions gradually increase with the rise in temperature, which may be attributed to the decrease in resistivity of the samples with increasing temperature. SCBT shows a lower f_LL value in the measured temperature range as compared to others. High resistivity can prevent applied electrical signals from leaking away in the process of using, only the modified SBT ceramics with high resistivity can be used in high-temperature piezoelectric fields.

3.5. Piezoelectric Properties

The thermal stability of the piezoelectricity of the oxide-modified SBT ceramics is displayed in Figure 6. As can be seen from the insert, before annealing, the piezoelectric properties of the SBT ceramic (d₃₃ = 10 pC/N) can be improved notably by adding only one of CeO₂ and Gd₂O₃, a higher d₃₃ ~ 22 pC/N was achieved in SCBT and SGBT. When considering that the addition of CeO₂ and Gd₂O₃ could reduce the concentration of oxygen vacancies as mentioned above, thus the less pinning of domain walls and the elevated resistivity tend to promote the sufficient orientation of ferroelectric domains along the applied electric field during polarization. It is noteworthy that the addition of Cr₂O₃ can further enhance the piezoelectric properties of the SBT ceramic that d₃₃ up to 28 pC/N was observed for SGBT-Cr and 26 pC/N for SCBT-Cr. As shown in Table 1, a larger orthorhombic distortion is obtained for SGBT-Cr and SCBT-Cr, in which a larger spontaneous polarization is believed to form [33]. Further, the thermal stability of piezoelectricity of the oxide-modified SBT ceramics was investigated by the annealing experiment. In general, the d₃₃ values of all compositions slowly decrease with increasing the annealing temperature from room temperature to 400 °C, and then drastically drop after 400 °C, until they reach zero when the annealing temperature exceeded their T_C. The thermal degradation of piezoelectricity can be attributed to the decoupling of space charges at moderate temperatures and the depolarization of intrinsic dipoles at high temperatures [41]. It should be noted that the d₃₃ values of SGBT-Cr were decreased by only 3.6% after being annealed at 500 °C and by 18% after being annealed at 550 °C (which is approaching T_C). This result indicates the composition with a good thermally stable piezoelectricity.

In final, dielectric and piezoelectric properties of the oxide-modified SBT ceramics were summarized in

Table 4. Comparison of electrical properties between the oxide-modified SBT ceramics and other compositions reported.

Compositions	εr (1 kHz)	tan δ (1 kHz)	Tc (°C)	d33 (pC/N)
SBT	152	0.9	537	10
SGBT	145	0.4	557	22
SGBT-Ce	168	0.4	552	14
SGBT-Mn	156	0.3	551	22
SGBT-Cr	156	0.3	555	28
SCBT	175	0.1	531	22
SCBT-Gd	169	0.2	539	22
SCBT-Mn	176	0.2	539	24
SCBT-Cr	124	0.3	542	26
SBT-Sm [20]	220	2.0	520	20
SBT-3Co [23]	200	0.6	528	28
SBT-4Mn [25]	180	0.8	530	30

. The high piezoelectric constant, low dielectric loss, and high Curie temperature presented by some compositions demonstrated the successful modification on the SBT ceramic applied by the oxides. As compared to the modified SBT ceramics reported by other works [20,23,25], the optimized composition SGBT-Cr also possesses the competitive electrical properties with a combination of high T_C ~ 555 °C and a high d₃₃ ~ 28 pC/N.

4. Conclusions

The effects of oxide additives (Gd₂O₃, CeO₂, MnO₂ and Cr₂O₃) on the phase structures and electrical properties of the SBT ceramics were investigated in this work, some main results were obtained as follows: XRD patterns demonstrated that all the oxide-modified SBT ceramics were a single SrBi₄Ti₄O₁₅ phase. The SBT ceramics with the addition of MnO₂ presented a high relative density up to 97%. The addition of Gd₂O₃ increased the T_C of SBT, which can be related to the larger orthorhombic distortion caused by the substitution of Gd³⁺ with a smaller ionic radius for Sr²⁺ at A-site. In addition, the addition of CeO₂ reduced the T_C of SBT, based on the fact that the stability of oxygen octahedron tends to be weakened by Ce⁴⁺ with higher coordination number substituting for Sr²⁺ at A-site. The synergetic doping of CeO₂ and Cr₂O₃ could significantly improve the temperature stability of the dielectric properties of SBT. Cr₂O₃ can significantly enhance the k_p of SBT, at the same time, the addition of these oxides also reduced the f_LL of SBT at high temperatures. The addition of oxides could improve the piezoelectric property of SBT (d₃₃ = 10 pC/N); in particular, SCBT-Cr and SGBT-Cr obtained a higher d₃₃ of 26 pC/N and 28 pC/N, respectively. Among these compositions, SGBT-Cr (Sr_0.92Gd_0.053Bi₄Ti₄O₁₅ + 0.2 wt% Cr₂O₃) presented the best electrical properties, such as: T_C = 555 °C, tan δ = 0.4%, k_p = 6.35%, d₃₃ = 28 pC/N, as well as a good thermally stable piezoelectricity that the values of d₃₃ was decreased by only 3.6% after being annealed at 500 °C for 4 h and retained 82% after being annealed at the temperature approaching T_C.