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Enhanced the dielectric relaxation characteristics of BaTiO3 ceramic doped by BiFeO3 and synthesized by the microwave sintering method
Materials Chemistry and Physics 250 (2020) 123034
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Enhanced the dielectric relaxation characteristics of BaTiO3 ceramic doped by BiFeO3 and synthesized by the microwave sintering method Tao Fan a, Cong Ji a, Gang Chen a, b, *, Wei Cai a, b, Rongli Gao a, b, Xiaoling Deng a, b, Zhenhua Wang a, b, Chunlin Fu a, b a b
School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing, 401331, PR China Chongqing Key Laboratory of Nano Micro Composite Materials and Devices, Chongqing, 401331, PR China
H I G H L I G H T S
� BFO dopant and MWS method both inhibit the grains growth. � BFO dopant effectively improves the relaxation behavior of BT-based ceramics. � MWS technique enhances frequency dispersion and phase diffusion behavior. � BFO dopant and MWS method could obtain slim P-E hysteresis loops. A R T I C L E I N F O
A B S T R A C T
Keywords: Dielectric relaxation behavior Ferroelectric properties Conventional sintering Microwave sintering BaTiO3–BiFeO3 ceramic
(1-x) BaTiO3-x BiFeO3 ((1-x) BT-x BFO) (x ¼ 0, 0.01, 0.03, 0.06, 0.1) ceramics with 0.15 wt% SiO2 additive were synthesized by microwave sintering (MWS) and conventional sintering (CS). The crystal structure, micromor phology, dielectric relaxation behavior and ferroelectric properties of (1-x) BT-x BFO-0.15 wt% SiO2 ceramics were investigated in detail. The results show that a phase structure transforms from a tetragonal phase to a pseudo-cubic phase for all samples and MWS method contributes to refine the grain and make the grain size more uniform. Moreover, a transition undergoes gradually from normal ferroelectric to relaxor ferroelectric is observed. The diffuseness coefficient γ of (1-x) BT-x BFO-0.15 wt% SiO2 ceramics increases with increasing BFO content and sintered by MWS when x � 0.06. The maximum diffuseness coefficient (γ ¼ 1.69) can be obtained at x ¼ 0.06 for MWS ceramic. Polarization-electric (P-E) hysteresis loops are gradually slimmer as x increasing, and a slimmer P-E hysteresis loops is obtained by MWS method. The remnant polarization and coercive field reduce using BFO dopant and MWS method, which also suggests that the dielectric relaxation behavior is strengthened. These results indicate that BFO dopant and MWS technique are both effective methods to improve the relaxation behavior of BT-based ceramics.
1. Introduction BaTiO3 (BT) is one of lead-free ferroelectric materials, which has been most widely applied in the last years. It is a typical ferroelectric perovskite (ABO3) structure and exhibits high dielectric constant (ε), well ferroelectric and piezoelectric properties, and large remnant po larization. However, it suffers from large remnant polarization, poor temperature stability and low breakdown field strength, which can hinder the application in energy storage. Nanostructures [1–4], ion doping and solid solution are commonly used to improve the properties of materials. Relaxor ferroelectric with slim P-E hysteresis loops and the
good temperature stabilization of the relative permittivity receive the extensive concern in the field of energy storage. BT and relaxation fer roelectrics form a solid solution, which is a considerable way to enhance it dielectric relaxation behavior. It is well known that Bi-based perovskite ferroelectric material pos sesses a high Curie temperature (TC) and a small tolerance factor t. Hence, BT can show a relaxation behavior by being incorporated Bibased perovskites [5]. Many relaxor ferroelectrics are formed solid so lutions of BT with BiScO3 (BS) [6], BiAlO3 (BA) [7,8], BiYbO3 (BY) [9–11] and BiFeO3 (BFO) [12]. Wei et al. investigated the BaTiO3-BiMO3 (M ¼ Al, In, Y, Sm, La) series and found that 0.9BaTiO3-0.1BiInO3
* Corresponding author. School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing, 401331, PR China. E-mail address: [email protected] (G. Chen). https://doi.org/10.1016/j.matchemphys.2020.123034 Received 16 January 2020; Received in revised form 2 April 2020; Accepted 3 April 2020 Available online 8 April 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. XRD patterns of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics sintered by CS at 1300 � C for 2 h: (a) 20� ~80� ; (b) 44� ~46� .
Fig. 2. XRD patterns of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics sintered by MWS at 1300 � C for 0.5 h: (a) 20� ~80� ; (b) 44� ~46� .
exhibited the largest diffuseness exponent γ of 2.1511, the lowest remnant polarization (Pr) of 0.454 μC/cm2 and a slim hysteresis loops, which showed strong relaxation behavior [5]. Among Bi-based perov skites, BiFeO3 possesses a large spontaneous polarization (exceeding 100 μC/cm2) and a small t (0.84) [13]. The combined substitution of Ba2þ with Bi3þ and Ti4þ with Fe3þ in BT ceramics may increase the ferroelectric properties and the diffusive phase transition. This leads to a broaden temperature region of the relative dielectric constant, and an improvement of its temperature stability. Wang et al. [14] found that (1-x) BT-x BFO (0� x �0.08) ceramics fabricated by CS transformed from a ferroelectric phase to a relaxor ferroelectric phase with the increment of BFO, and the relaxation ferroelectric behavior occurred at the ceramics with x ¼ 0.02, 0.06, 0.08. All the P-E hysteresis loops of BT-BFO ceramics are slim with small Pr and large saturated polarization (Ps). It demonstrates that BT-BFO
ceramics can apply in energy storage as a considerable lead-free relaxor ferroelectric material. However, it is inevitable that the volatilization of Bismuth and variation of Iron ion valence can produce oxygen vacancies during the sintering process of BFO, resulting in the reduction of the remnant po larization (Pr) caused by the domain wall pinning effect. It is well known that the preparation method also has an effect on the properties of samples [15,16]. Microwave sintering (MWS) method can reduce the volatilization of Biþ and prepare the small particles, due to its fast heating speed and short holding time [17]. Accordingly, MWS can reduce the oxygen vacancy and enhance the relaxation behavior of ferroelectric materials. As a sintering aid, SiO2 dopant can increase the density, enhance the breakdown strength and improve the sintering behavior of BT ceramic due to the formation of a liquid phase [18–20]. Therefore, the addition 2
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ceramics were fabricated by using a traditional solid-state reaction method. TiO2 (99.8%), BaCO3 (99.8%), Bi2O3(99%), Fe2O3(99%), BaTiO3(99.5%) and SiO2(99.99%) were applied to this experiment as raw materials. These materials were weighted according to the stoi chiometric proportion with 6 mol% excess of Bi2O3, then all materials mixed by ball-milling with agate balls in distilled water for 8 h at 400 r/ min using a planetary ball mill. The slurries were dried and calcined at 1100 � C for 5 h. The obtained powders were ball milled and dried again. Polyvinyl alcohol (PVA) was put into the mixed powders as a binder, and these powders were pressed into pellets with 10 mm diameter and 1 mm thick at 15 MPa. Green pellets were divided into two parts after burning off the PVA in all pellets. Some pellets were sintered at 1300 � C for 2 h in the traditional chamber furnace with the heating rate 3 � C/min in air. Others were sintered by microwave at 1300 � C for 0.5 h in a microwave muffle furnace (HAMiLab-M1500, Synotherm, China). The phase composition and crystal structure of sintered pellets were detected by X-ray diffraction (XRD, Japan, SmartLab-9, Rigaku) with Cu-Kα radiation (λ ¼ 1.541 Å), 0.02� scan step in the diffraction angle (2θ) from 20� to 80� . The field-emission scanning electron microscope (SEM, JSM-7800F, JEOL, Japan) was used to observe the microstructure of these samples. The pellets painted silver slurry on both sides as electrodes were sintered at 850 � C for 0.5 h. The dielectric properties of BT-BFO ceramics were characterized by a LCR analyzer (HP4980A, Agilent, USA) in the temperature range from room temperature to 300 � C at six different frequencies (100 Hz, 1 kHz, 10 kHz, 100 kHz, 1 MHz, 2 MHz, respec tively) and in the frequency range from 20 Hz to 2 MHz at room tem perature. The relative permittivity (ε) can be calculated according to the following formula:
ε¼
Cd
ε0
¼
4Ct
ε0 π d 2
¼
14:4Ct d2
(1)
where C is the capacitance (pF), d is the diameter (cm), t is the thickness (cm), ε0 is the permittivity of free space (8.85 � 10 12 F/m). A ferro electric test system (TF2000, air-ACCT Inc, Germany) was employed to determine ferroelectric hysteresis hoops in the frequency of 100 Hz at room temperature. 3. Results and discussions 3.1. Crystal structure Figs. 1 and 2 show the XRD patterns of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics sintered by CS and MWS, respectively. The patterns indicate that all component ceramics are mainly composed of perovskite struc ture and a few second phases of Ba2TiSi2O8 are observed, which is due to the SiO2 dopant. Compared with CS, MWS method increases the internal molecular or ionic kinetic energy, and improve the densification. In addition, MWS can shorten the sintering time and accelerate grain boundary diffusion [21,22]. However, different materials have different absorption coupling degree to microwave, resulting in different diffu sion degree and different second phase. As shown in Fig. 2, it can be found that a SiO2 phase appears in the XRD of the MWS sample with x ¼ 0.1 around 36� owing to the reduction of the solid solubility for SiO2 in MWS sample by the fast heating rate of MWS. The enlarged XRD patterns of CS and MWS ceramics range of 2θ from 44� to 46� are presented in Figs. 1 (b) and Fig.2 (b), respectively. As shown in Figs. 1 (b) and Fig.2 (b), the samples with x ¼ 0, 0.01, 0.03 display an evident splitting of the (002)/(200) peaks, and the two diffraction peaks merge when x > 0.03. It is found that the crystal structure of samples transforms gradually from a tetragonal phase to a pseudo-cubic phase with the increase of x. Moreover, a morphotropic phase boundary (MPB) with the tetragonal phase and pseudo-cubic phase coexists in 0.94BT-0.06 BFO ceramic, which is consistent with Guo [23] and Chen’s reported results [24]. Diffraction peaks of CS and MWS ceramics all shift to a higher angle
Fig. 3. SEM images of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics: x ¼ 0, (a) CS, (b) MWS; x ¼ 0.01, (c) CS, (d) MWS; x ¼ 0.03, (e) CS, (f) MWS; x ¼ 0.06, (g) CS, (h) MWS; x ¼ 0.1, (i) CS, (j) MWS.
of SiO2 into BT-BFO ceramics may enhance the densification, which can improve the breakdown strength of BT-BFO ceramics. However, the influences of SiO2 dopant on the microstructure, the ferroelectric properties and dielectric relaxation behaviors of BT-BFO ceramics syn thesized by MWS have not been reported until now. In the present work, (1-x) BT-x BFO (x ¼ 0, 0.01, 0.03, 0.06, 0.1) ceramics with 0.15 wt% SiO2 were prepared by different sintering methods of conventional sintering (CS) and MWS, respectively. The crystal structure, dielectric relaxation and ferroelectric properties of BTBFO ceramics were studied, and their dielectric relaxation mechanism was also discovered. 2. Experimental procedure A series of (1-x) BT-x BFO (x ¼ 0, 0.01, 0.03, 0.06, 0.1 mol%) 3
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Fig. 4. The relative permittivity and dielectric loss as function of temperature for (1-x) BT-x BFO (x ¼ 0–0.1) ceramics of CS (a, c, e, g, i) and MWS (b, d, f, h, j) obtained at different frequencies from 1 kHz to 2 MHz.
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Fig. 5. The relative permittivity and dielectric loss as function of temperature for (1-x) BT-x BFO (x ¼ 0–0.1) ceramics of CS and MWS measured at 1 kHz: (a) CS; (b) MWS.
Fig. 6. The curves of ln (1/ε-1/εm) versus ln (T-Tm), and inset temperature as function of the inverse relative permittivity for (1-x) BT-x BFO (x ¼ 0–0.06) ceramics by CS and MWS at 1 kHz: (a) x ¼ 0; (b) x ¼ 0.01; (c) x ¼ 0.03; (d) x ¼ 0.06.
3.2. Surface morphology
Table 1 The temperature for the maximum relative permittivity (Tm), the Curie-Weiss constant (C), the starting temperature of deviation from Curie-Weiss Law (TCW), the Curie-Weiss temperature (T0), the degree of deviation from CurieWeiss Law (ΔTm) for (1-x) BT-x BFO (x ¼ 0–0.06) ceramics. Samples CS
MWS
C ( � 105K) T0 (� C) Tm (� C) TCW (� C) Δ Tm (� C) C ( � 105 K) T0 (� C) Tm (� C) TCW (� C) Δ Tm (� C)
0
0.01
0.03
0.06
0.817 130.8 148 161 13 0.926 118.5 148 161 22
1.478 111.6 125.1 140 17.9 1.556 101.2 126 150 24
1.563 66.2 97.9 118 20.1 1.129 69.2 111 136 25
2.008 2.5 59 93 34 1.895 32 58.2 103 45
The microstructures of BT-BFO ceramics prepared by CS and MWS are given in Fig. 3, and exhibit more uniform grains. The large grains are presented in Fig. 3 (a) and 3 (c), which belongs to BT and 0.99BT0.01BFO CS ceramics respectively. The average grain sizes of CS and MWS ceramics are 9.56, 8.56, 4.27, 1.84, 1.12 μm and 0.74, 3.42, 3.22, 1.03, 0.73 μm, respectively. Finer grains of MWS ceramics were ob tained compared with CS ceramics, which indicates that the MWS method contributes to smaller grains because of its fast heating [17]. All samples exhibit smaller grains and fewer pores with increasing x, indi cating that the BFO dopant possibly inhibits grains growth. Defect di poles were formed with the addition of BFO, restricting barium vacancies and weakening the diffusion mechanism of grains growth [14, 24]. It is confirmed that grain size tends to reduce with the increasing BFO content. The substitution of Ba2þ by Bi3þand Ti4þ by Fe3þ can introduce many defects due to the different valences of substituted ions. This can be expressed by reactions as following:
from x ¼ 0 to x ¼ 0.1 shown in Figs. 1 and 2. This results from the substitution of large ionic radii Ba2þ (0.135 nm) and Fe3þ(0.064 nm) by Bi3þ (0.103 nm) and Ti4þ (0.0603 nm), respectively.
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constant peak with a decrease of TC as x increases for x ¼ 0.1 sample, which means a strong frequency diffusion. All peaks width of MWS samples are broader than those of CS samples as shown in Fig. 4, which is due to the smaller grains obtained by MWS method. MWS can accel erate ion diffusion due to its rapid heating, resulting in more BFO diffusing into BT and wider dielectric dispersion peaks. Moreover, the finer grains obtained by MWS can also make the dielectric dispersion peaks broaden. The TC moves to the low-temperature region as x in creases seen in Fig. 5, which is related to the disruption of Ti–O bounding and the reduction of the tetragonal phase. The doping of Fe3þ, different from Ti4þ in covalent bonding character and ionic radius, could disrupt Ti–O bounding. The distance between Ti atoms reduces by dis rupted, resulting in the appearance of thermal rearrangement within a larger temperature range [25]. While the bond strength of Fe–O bond is larger than that of Ti–O bond, thus the cubic phase occurs at low tem perature, which weakens the phase transition temperature of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics. TC shifts to low-temperature region, indi cating that BT possesses well temperature stability of the dielectric properties. The value of εm increases first and then decreases for CS ceramics with increasing x, and εm always decreases for MWS ceramics with increasing x. The grain size makes an effect on the dielectric con stant. The large grains lead to the reduction of the internal stress. The 90� domain size decreases and then the 90� domain boundary increases with the grain size decreasing, resulting in enhancement of the dielectric constant. However, the ultra-fine grain cannot produce 90� domain which restrains the internal stress, so it has abnormal dielectric constant [26–29]. As a conclusion, the dielectric constant first increases and then decreases with the decrease of grain size for BT ceramics. To describe the degree of the dielectric relaxation of (1-x) BT-x BFO (x ¼ 0–0.1) samples, the modified Curie-Weiss law is given as following [30]:
Fig. 7. The diffuseness coefficient (γ) dependence of BFO content (x) for (1-x) BT-x BFO (x ¼ 0–0.06) ceramics.
1 ’’ Bi2 O3 → 2Bi:Ba þ VBa þ 2Oo þ O2 2 TiO2
2Fe3 O4 ���!6Fe’Ti þ 3VO:: þ 8Oo
(2) (3)
’’ Where VBa is the vacancy with two negative charges at the Ba2þ position, :: VO is the vacancy with two positive charges at the O2 position, Bi:Ba is the substation of Ba2þ by Bi3þ with a positive charge, Fe’Ti is the sub station of Fe3þ by Ti4þ with a negative charge. The block phase circled by a yellow square in Fig. 3 (a), it might be the second phase Ba2TiSi2O8, which is consistent with the results of the XRD patterns.
1
εr
¼
T
TCW C
(4) Tm Þγ C
1
1
3.3. Dielectric properties
ε
εm
The temperature dependence of the relative permittivity and dielectric loss for (1-x) BT-x BFO (x ¼ 0–0.1) ceramics sintered by CS and MWS within the frequency range from 1 kHz to 2000 kHz are illustrated in Fig. 4. There is a different that the BT ceramic exhibits a slightly sharp peak caused by the addition of SiO2. With increasing BFO content and frequency, those Curie peaks become broader, which dis plays a dispersion phase transition and frequency diffusion. This sug gests that the addition of BFO and SiO2 can strengthen the dielectric relaxation behavior of BT ceramic. This phenomenon can be explained by the fact that the substitution of Fe3þ for Ti4þ disrupts the long-range of Ti-displacements. Meanwhile, it is related to the formation of polar nanoclusters due to the substitution of Bi3þ for Ba2þ and Fe3þ for Ti4þ on A-site and B-site, respectively [25]. For x ¼ 0.03, 0.06 ceramics, a typical relaxor-like behavior was observed. A rather broad and flat dielectric
Where the diffuseness exponent γ is between 1 and 2, γ ¼ 1 suggests a normal ferroelectric and γ ¼ 2 is an ideal relaxor ferroelectric. Tm is the temperature of εm, C is the Curie-Weiss constant, ε is the relative permittivity, TCW is temperature at which the dielectric constant begins to follow the Curie-Weiss law. Fig. 6 inset presents an inverse dielectric constant of (1-x) BT-x BFO (x ¼ 0–0.06) ceramics versus the temperature. ΔTm, the degree of de viation from Curie-Weiss Law, could be calculated by ΔTm ¼ TCW-T0, where TCW is the Curie-Weiss Law starting temperature, T0 is the CurieWeiss temperature. The starting temperature TCW decreases with the addition of BFO. ΔTm largen with x increasing as illustrated in Table 1, indicating the addition of BFO broadens the deviation from Curie-Weiss Law, and improve relaxation behavior. ΔTm of CS ceramics are larger than that of MWS ceramics, which means MWS enlarges the deviation
¼
ðT
(5)
Fig. 8. The dielectric constant and loss as function of frequency (20 Hz-2 MHz) for (1-x) BT-x BFO ceramics measured at room temperature: (a) CS; (b) MWS. 6
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Fig. 9. P-E hysteresis loops of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics prepared by CS (a, c, e, g, i) and MWS (b, d, f, h, j) measured at 100 Hz under different electric fields.
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Fig. 10. P-E hysteresis loops of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics sintered by using CS and MWS measured at 100 Hz at room temperature: (a) CS; (b) MWS.
Fig. 11. Composition dependences of EC and Pr for the (1-x) BT-x BFO (x ¼ 0–0.1) ceramics: (a) CS; (b) MWS.
from Curie-Weiss Law and enhance relaxation behavior more effectively in compare to the addition of BFO. The greater deviation, the stronger frequency dispersion and phase diffusion demonstrate that the relaxa tion behavior strengthens. The tendency of ΔTm more accurately quantifies the degree of relaxation, which is further consistent with the result of the temperature dependence of the relative permittivity. The diffuseness coefficient γ is a fitting data to show the degree of relaxation behavior. For BT-BFO ceramics at 1 kHz, it calculated by the slopes of ln (1/ε-1/εm)-ln (T-Tm) plots as shown in Fig. 6. They are 1.38, 1.45, 1.48, 1.62 for CS samples and 1.48, 1.64, 1.65, 1.69 for MWS samples, respectively. The γ values as a function of x for (1-x) BT-x BFO (x ¼ 0–0.06) ceramics shown in Fig. 7 γ of CS samples gradually in creases with the addition of BFO, and that of MWS samples is similar to CS samples. And MWS ceramics show a larger γ. Those results reveal that both the doping of BFO and MWS method can strengthen the relaxation behavior, which is consistent with the results of Fig. 4. Frequency dependence of the dielectric constant and loss of (1-x) BTx BFO (x ¼ 0–0.1) ceramics at room temperature is shown in Fig. 8. The dispersion of samples occurs in the low-frequency region, which is mainly caused by the BFO content. With an increase of the substitution of Ba2þ by Bi3þ and Ti4þ by Fe3þ, the permittivity decreases when the long rang order is broken. At high-frequency region, the variation of dielectric constants is small. The values of εr in low-frequency range are larger than that in high frequency (10 kHz-2 MHz), which is related to the different polarization mechanism. At low frequencies, the larger relative permittivity is due to the co-contribution of space charge po larization, displacement polarization and dipole orientation polariza tion. However, only space charge polarization and displacement polarization contribute to polarization at high frequencies, resulting in the reduction of εr. The εr of x ¼ 0 composition has an obviously decrease with increasing frequency in Fig. 8 (a). However, the other compositions have a platform among the frequency of 1 kHz–2 MHz, indicating the independence between frequency and permittivity in these samples. In Fig. 8 (b), the compositions of x ¼ 0, 0.01 have an obviously decline with the increase of frequency, and the remaining compositions appear a slow
plateau in the frequency range of 1 kHz–2 MHz. It can be concluded that the incorporation of BFO and microwave sintering can improve the frequency stability of the dielectric properties for BT ceramic. At lowfrequency region, the value of the dielectric loss is high and gradually decreases with an increase of frequency until up to a constant value. 3.4. Ferroelectric properties The P-E hysteresis loops of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics prepared by CS and MWS at room temperature are shown in Fig. 9. As the electric field (E) increases, hysteresis loops become fatter and more saturated, which can explain by the larger E which makes domain switch easily. From Fig. 9 (a, c, e, g), the addition of BFO make the P-E hys teresis loops become slim, indicating the enhancement of relaxation behavior. P-E hysteresis loops of 0.97BT-0.03BFO ceramics, a contrac tion area appears in Fig. 5 (b), which is possibly due to the leakage current. And more holes might be one of the reasons for the leakage current in the SEM of 0.97BT-0.03BFO ceramics. As shown in Fig. 9, the breakdown electric field of MWS ceramics is larger than that of CS ce ramics, which is related to the compactness of samples sintered by mi crowave. The breakdown electric field of 0.9BT-0.1BFO ceramic of MWS is 90 kV/cm at a frequency of 100 Hz. Fig. 10 shows the relationship of Pr, coercive field EC versus x for (1x) BT-x BFO (x ¼ 0–0.1) ceramics with different compositions. From Fig. 10 (a), the P-E hysteresis loops become slim as x increases, indi cating the BFO can weaken the ferroelectricity and strengthen the relaxation behavior. Similarly, the P-E hysteresis loops of MWS ceramics become slim gradually with the increase of BFO in Fig. 10 (b). Mean while, there is a clear reduction in 2Pr (6.942, 3.477, 1.8716, 2.203 and 1.2376 μC/cm2) of CS samples with the increase of BFO as shown in Fig. 11 (a). With the increase of BFO, grains become small. Small grains, which possess small domain size and more domain walls, make domain inversion difficult. Thus, the reduction of 2Pr is relate to the small grains. The 2Pr (4.007, 6.961, 1.7552, 0.8532 and 1.0432 μC/cm2) of MWS samples is presented in Fig. 11 (b). An abnormal data appears in x ¼ 0.01 8
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MWS samples, which may be caused by large grains in SEM. The mini mum value of 2Ec is about 4 kV/cm for 0.94BT-0.06BFO MWS samples. The decrease of Ec leads to the reduction of energy of domain reor ientation. The value of Ec is related to grain sizes and defects.
[2]
[3]
4. Conclusions The crystal structure, microstructure, dielectric and ferroelectric properties of (1-x) BT-x BFO-0.15 wt% SiO2 ceramics prepared by CS and MWS were characterized, and the dispersion degree was fitting. The result show that the crystal structure of CS and MWS ceramics trans forms from the tetragonal phase to the cubic phase with an increase of BFO. Due to the addition of SiO2, the second phase appears in the XRD images. And the average grain sizes of BFO doping components are finer. Furthermore, MWS ceramics possess a smaller average grain size. With an increase of x, Curie peaks continuously widened, and phase transition diffusion and frequency dispersion phenomena were observed. TC shifts toward the low-temperature region, which related to the Ti–O bond destruction caused by Fe3þ replacing Ti4þ. The γ of MWS samples is obviously larger than that of CS, and it can achieve 1.69 at x ¼ 0.06, which indicates that MWS is helpful to enhance the relaxation behavior of BaTiO3-based ceramics. With the doping of BFO and MWS method, PE hysteresis loops also become slimmer. It was proved that microwave sintering and BFO enhance the relaxation behavior of BT-based ceramics.
[4]
[5] [6] [7] [8] [9] [10] [11]
Declaration of competing interest
[12]
The authors declared that we have no financial and personal re lationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
[13] [14] [15]
CRediT authorship contribution statement [16]
Tao Fan: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Cong Ji: Validation, Formal analysis, Visuali zation, Software. Gang Chen: Validation, Formal analysis, Visualiza tion, Writing - review & editing. Wei Cai: Resources, Writing - review & editing, Supervision, Data curation. Rongli Gao: Resources, Writing review & editing, Supervision, Data curation. Xiaoling Deng: Writing review & editing. Zhenhua Wang: Writing - review & editing. Chunlin Fu: Writing - review & editing.
[17] [18] [19] [20]
Acknowledgments
[21]
This work was supported by the undergraduate science and tech nology innovation training program project of Chongqing university of science and technology (YKJCX1820213, YKJCX1920214), the 9th Chongqing KeHui graduate student innovation and entrepreneurship competition (09311167), Chongqing Research Program of Basic Research and Frontier Technology (CSTC2019jcyj-msxmX0071, cstc2018jcyjAX0416), the program for innovation teams in University of Chongqing, China (CXTDX201601032), the Innovation Program for Chongqing’s Overseas Returnees(cx2019159), Research foundation of education bureau of Chongqing, China (KJQN201801509), the leading talents of scientific and technological innovation in Chongqing (CSTCCXLJRC201919) and the excellent talent project in university of Chongqing (2017–35).
[22]
[23] [24] [25] [26] [27]
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Pezeshkpour, Polyvinyl alcohol/Fe3O4@carbon nanotubes nanocomposite: electrochemicalassisted synthesis, physicochemical characterization, optical properties, cytotoxicity effects and ultrasound-assisted treatment of aqueous based organic compound, J. Ind. Eng. Chem. 65 (2018) 349–362. M. Wei, J. Zhang, K. Wu, H.G. Chen, C.R. Yang, Effect of BiMO3 (M¼Al, In, Y, Sm, Nd, and La) doping on the dielectric properties of BaTiO3 ceramics, Ceram. Int. 43 (2017) 9593–9599. N. Kumar, D.P. Can, Tailoring transport properties through nonstoichiometry in BaTiO3-BiScO3 and SrTiO3-Bi (Zn1/2Ti1/2) O3 for capacitor applications, J. Mater. Sci. 51 (2016) 9404–9414. H.C. Yu, Z.G. Ye, Dielectric properties and relaxor behavior of a new (1-x) BaTiO3xBiAlO3 solid solution, J. Appl. Phys. 103 (2008), 034114. S.Y. Zheng, O. Erika, L.J. Liu, D.P. Shi, Y.M. Huang, L.L. Fan, J. Chen, L. Fang, Electrostrictive and relaxor ferroelectric behavior in BiAlO3-modifed BaTiO3 leadfree ceramics, E, J. Appl. Phys. 113 (2013) 305–320. T. Strathdee, L. Luisman, A. Feteira, K. Reichmann, Ferroelectric-to-relaxor crossover in (1-x) BaTiO3-x BiYbO3 (0 � x � 0.08) ceramics, J. Am. Ceram. Soc. 94 (2011) 2292–2295. Z.B. Shen, X.H. Wang, B.C. Luo, L.T. Li, BaTiO3-BiYbO3 perovskite materials for energy storage application, J. Mater. Chem. A. 3 (2015) 18146–18153. G. Chen, X.D. Peng, C.L. Fu, W. Cai, R.L. Gao, P.G. Fan, X.Y. Zhang, X. Yi, C. Ji, H. Q. Yang, H.L. Yong, Microstructure, dielectric and ferroelectric properties of (1-x) BaTiO3-xBiYbO3 ceramics fabricated by conventional and microwave sintering methods, J. Mater. Sci. Mater. Electron. 29 (2018) 20017–20032. Y.R. Wang, Y.P. Pu, X. Li, H.Y. Zheng, Z.Y. Gao, Evolution from ferroelectric to diffused ferroelectric, and relaxor ferroelectric in BaTiO3-BiFeO3 solid solutions, Mater. Chem. Phys. 183 (2016) 247–253. D. Lebeugle, D. Colson, A. Forget, M. Viret, Very Large Spontaneous electric polarization in BiFeO3 single crystals at room temperature and its evolution under cycling fields, Appl. Phys. Lett. 91 (2007), 022907. Y.J. Wu, Y.H. Huang, N. Wang, J. Li, M.S. Fu, X.M. Chen, Effects of phase constitution and microstructure on energy storage properties of barium strontium titanate ceramics, J. Eur. Ceram. Soc. 37 (2017) 2099–2104. A. Asfaram, M. Ghaedi, H. Javadian, A. Goudarzi, Cu- and S- @SnO 2 nanoparticles loaded on activated carbon for efficient ultrasound assisted dispersive μSPEspectrophotometric detection of quercetin in Nasturtium officinale extract and fruit juice samples: CCD-RSM design, Ultrason. Sonochem. 47 (2018) 1–9. H. Abidi, M. Ghaedi, A. Rafiei, A. Jelowdar, A. Salimi, A. Asfaram, A. Ostovan, Magnetic solid lipid nanoparticles co-loaded with albendazole as an anti-parasitic drug: sonochemical preparation, characterization, and in vitro drug release, J. Mol. Liq. 268 (2018) 11–18. Y.P. Pu, L. Zhang, M.T. 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Yong, Effects of sintering method and BiFeO3 dopant on the dielectric and ferroelectric properties of BaTiO3-BiYbO3 based solid solution ceramics, Ceram. Int. 44 (2018) 16880–16889. X.Z. Guo, Y.G. Wu, Y.N. Zou, Z.Y. Wang, Effects of addition of BiFeO3 on phase transition and dielectric properties of BaTiO3 ceramics, J. Mater. Sci. Mater. Electron. 23 (2012) 1072–1076. X.L. Chen, J. Chen, D.D. Ma, L. Fang, H.F. Zhou, High relative permittivity, low dielectric loss and good thermal stability of BaTiO3-Bi(Mg0.5Zr0.5)O3 solid solution, Ceram. Int. 41 (2015) 2081–2088. G. Schileo, L. Luisman, A. Feteira, M. Deluca, K. Reichmsnn, Structure–property relationships in BaTiO3-BiFeO3-BiYbO3 ceramics, J. Eur. Ceram. Soc. 33 (2013) 1457–1468. W.R. Buessem, L.E. Cross, A.K. Goswami, Phenomenological theory of high permittivity in fine-grained barium titanate, J. Am. Ceram. Soc. 49 (1966) 33–36. Z. Zhao, V. Buscaglia, M. Viviani, M. Viviani, M.T. Buscaglia, L. Mitoseriu, A. Testino, M. Nygren, M. 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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Enhanced the dielectric relaxation characteristics of BaTiO3 ceramic doped by BiFeO3 and synthesized by the microwave sintering method Tao Fan a, Cong Ji a, Gang Chen a, b, *, Wei Cai a, b, Rongli Gao a, b, Xiaoling Deng a, b, Zhenhua Wang a, b, Chunlin Fu a, b a b
School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing, 401331, PR China Chongqing Key Laboratory of Nano Micro Composite Materials and Devices, Chongqing, 401331, PR China
H I G H L I G H T S
� BFO dopant and MWS method both inhibit the grains growth. � BFO dopant effectively improves the relaxation behavior of BT-based ceramics. � MWS technique enhances frequency dispersion and phase diffusion behavior. � BFO dopant and MWS method could obtain slim P-E hysteresis loops. A R T I C L E I N F O
A B S T R A C T
Keywords: Dielectric relaxation behavior Ferroelectric properties Conventional sintering Microwave sintering BaTiO3–BiFeO3 ceramic
(1-x) BaTiO3-x BiFeO3 ((1-x) BT-x BFO) (x ¼ 0, 0.01, 0.03, 0.06, 0.1) ceramics with 0.15 wt% SiO2 additive were synthesized by microwave sintering (MWS) and conventional sintering (CS). The crystal structure, micromor phology, dielectric relaxation behavior and ferroelectric properties of (1-x) BT-x BFO-0.15 wt% SiO2 ceramics were investigated in detail. The results show that a phase structure transforms from a tetragonal phase to a pseudo-cubic phase for all samples and MWS method contributes to refine the grain and make the grain size more uniform. Moreover, a transition undergoes gradually from normal ferroelectric to relaxor ferroelectric is observed. The diffuseness coefficient γ of (1-x) BT-x BFO-0.15 wt% SiO2 ceramics increases with increasing BFO content and sintered by MWS when x � 0.06. The maximum diffuseness coefficient (γ ¼ 1.69) can be obtained at x ¼ 0.06 for MWS ceramic. Polarization-electric (P-E) hysteresis loops are gradually slimmer as x increasing, and a slimmer P-E hysteresis loops is obtained by MWS method. The remnant polarization and coercive field reduce using BFO dopant and MWS method, which also suggests that the dielectric relaxation behavior is strengthened. These results indicate that BFO dopant and MWS technique are both effective methods to improve the relaxation behavior of BT-based ceramics.
1. Introduction BaTiO3 (BT) is one of lead-free ferroelectric materials, which has been most widely applied in the last years. It is a typical ferroelectric perovskite (ABO3) structure and exhibits high dielectric constant (ε), well ferroelectric and piezoelectric properties, and large remnant po larization. However, it suffers from large remnant polarization, poor temperature stability and low breakdown field strength, which can hinder the application in energy storage. Nanostructures [1–4], ion doping and solid solution are commonly used to improve the properties of materials. Relaxor ferroelectric with slim P-E hysteresis loops and the
good temperature stabilization of the relative permittivity receive the extensive concern in the field of energy storage. BT and relaxation fer roelectrics form a solid solution, which is a considerable way to enhance it dielectric relaxation behavior. It is well known that Bi-based perovskite ferroelectric material pos sesses a high Curie temperature (TC) and a small tolerance factor t. Hence, BT can show a relaxation behavior by being incorporated Bibased perovskites [5]. Many relaxor ferroelectrics are formed solid so lutions of BT with BiScO3 (BS) [6], BiAlO3 (BA) [7,8], BiYbO3 (BY) [9–11] and BiFeO3 (BFO) [12]. Wei et al. investigated the BaTiO3-BiMO3 (M ¼ Al, In, Y, Sm, La) series and found that 0.9BaTiO3-0.1BiInO3
* Corresponding author. School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing, 401331, PR China. E-mail address: [email protected] (G. Chen). https://doi.org/10.1016/j.matchemphys.2020.123034 Received 16 January 2020; Received in revised form 2 April 2020; Accepted 3 April 2020 Available online 8 April 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. XRD patterns of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics sintered by CS at 1300 � C for 2 h: (a) 20� ~80� ; (b) 44� ~46� .
Fig. 2. XRD patterns of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics sintered by MWS at 1300 � C for 0.5 h: (a) 20� ~80� ; (b) 44� ~46� .
exhibited the largest diffuseness exponent γ of 2.1511, the lowest remnant polarization (Pr) of 0.454 μC/cm2 and a slim hysteresis loops, which showed strong relaxation behavior [5]. Among Bi-based perov skites, BiFeO3 possesses a large spontaneous polarization (exceeding 100 μC/cm2) and a small t (0.84) [13]. The combined substitution of Ba2þ with Bi3þ and Ti4þ with Fe3þ in BT ceramics may increase the ferroelectric properties and the diffusive phase transition. This leads to a broaden temperature region of the relative dielectric constant, and an improvement of its temperature stability. Wang et al. [14] found that (1-x) BT-x BFO (0� x �0.08) ceramics fabricated by CS transformed from a ferroelectric phase to a relaxor ferroelectric phase with the increment of BFO, and the relaxation ferroelectric behavior occurred at the ceramics with x ¼ 0.02, 0.06, 0.08. All the P-E hysteresis loops of BT-BFO ceramics are slim with small Pr and large saturated polarization (Ps). It demonstrates that BT-BFO
ceramics can apply in energy storage as a considerable lead-free relaxor ferroelectric material. However, it is inevitable that the volatilization of Bismuth and variation of Iron ion valence can produce oxygen vacancies during the sintering process of BFO, resulting in the reduction of the remnant po larization (Pr) caused by the domain wall pinning effect. It is well known that the preparation method also has an effect on the properties of samples [15,16]. Microwave sintering (MWS) method can reduce the volatilization of Biþ and prepare the small particles, due to its fast heating speed and short holding time [17]. Accordingly, MWS can reduce the oxygen vacancy and enhance the relaxation behavior of ferroelectric materials. As a sintering aid, SiO2 dopant can increase the density, enhance the breakdown strength and improve the sintering behavior of BT ceramic due to the formation of a liquid phase [18–20]. Therefore, the addition 2
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ceramics were fabricated by using a traditional solid-state reaction method. TiO2 (99.8%), BaCO3 (99.8%), Bi2O3(99%), Fe2O3(99%), BaTiO3(99.5%) and SiO2(99.99%) were applied to this experiment as raw materials. These materials were weighted according to the stoi chiometric proportion with 6 mol% excess of Bi2O3, then all materials mixed by ball-milling with agate balls in distilled water for 8 h at 400 r/ min using a planetary ball mill. The slurries were dried and calcined at 1100 � C for 5 h. The obtained powders were ball milled and dried again. Polyvinyl alcohol (PVA) was put into the mixed powders as a binder, and these powders were pressed into pellets with 10 mm diameter and 1 mm thick at 15 MPa. Green pellets were divided into two parts after burning off the PVA in all pellets. Some pellets were sintered at 1300 � C for 2 h in the traditional chamber furnace with the heating rate 3 � C/min in air. Others were sintered by microwave at 1300 � C for 0.5 h in a microwave muffle furnace (HAMiLab-M1500, Synotherm, China). The phase composition and crystal structure of sintered pellets were detected by X-ray diffraction (XRD, Japan, SmartLab-9, Rigaku) with Cu-Kα radiation (λ ¼ 1.541 Å), 0.02� scan step in the diffraction angle (2θ) from 20� to 80� . The field-emission scanning electron microscope (SEM, JSM-7800F, JEOL, Japan) was used to observe the microstructure of these samples. The pellets painted silver slurry on both sides as electrodes were sintered at 850 � C for 0.5 h. The dielectric properties of BT-BFO ceramics were characterized by a LCR analyzer (HP4980A, Agilent, USA) in the temperature range from room temperature to 300 � C at six different frequencies (100 Hz, 1 kHz, 10 kHz, 100 kHz, 1 MHz, 2 MHz, respec tively) and in the frequency range from 20 Hz to 2 MHz at room tem perature. The relative permittivity (ε) can be calculated according to the following formula:
ε¼
Cd
ε0
¼
4Ct
ε0 π d 2
¼
14:4Ct d2
(1)
where C is the capacitance (pF), d is the diameter (cm), t is the thickness (cm), ε0 is the permittivity of free space (8.85 � 10 12 F/m). A ferro electric test system (TF2000, air-ACCT Inc, Germany) was employed to determine ferroelectric hysteresis hoops in the frequency of 100 Hz at room temperature. 3. Results and discussions 3.1. Crystal structure Figs. 1 and 2 show the XRD patterns of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics sintered by CS and MWS, respectively. The patterns indicate that all component ceramics are mainly composed of perovskite struc ture and a few second phases of Ba2TiSi2O8 are observed, which is due to the SiO2 dopant. Compared with CS, MWS method increases the internal molecular or ionic kinetic energy, and improve the densification. In addition, MWS can shorten the sintering time and accelerate grain boundary diffusion [21,22]. However, different materials have different absorption coupling degree to microwave, resulting in different diffu sion degree and different second phase. As shown in Fig. 2, it can be found that a SiO2 phase appears in the XRD of the MWS sample with x ¼ 0.1 around 36� owing to the reduction of the solid solubility for SiO2 in MWS sample by the fast heating rate of MWS. The enlarged XRD patterns of CS and MWS ceramics range of 2θ from 44� to 46� are presented in Figs. 1 (b) and Fig.2 (b), respectively. As shown in Figs. 1 (b) and Fig.2 (b), the samples with x ¼ 0, 0.01, 0.03 display an evident splitting of the (002)/(200) peaks, and the two diffraction peaks merge when x > 0.03. It is found that the crystal structure of samples transforms gradually from a tetragonal phase to a pseudo-cubic phase with the increase of x. Moreover, a morphotropic phase boundary (MPB) with the tetragonal phase and pseudo-cubic phase coexists in 0.94BT-0.06 BFO ceramic, which is consistent with Guo [23] and Chen’s reported results [24]. Diffraction peaks of CS and MWS ceramics all shift to a higher angle
Fig. 3. SEM images of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics: x ¼ 0, (a) CS, (b) MWS; x ¼ 0.01, (c) CS, (d) MWS; x ¼ 0.03, (e) CS, (f) MWS; x ¼ 0.06, (g) CS, (h) MWS; x ¼ 0.1, (i) CS, (j) MWS.
of SiO2 into BT-BFO ceramics may enhance the densification, which can improve the breakdown strength of BT-BFO ceramics. However, the influences of SiO2 dopant on the microstructure, the ferroelectric properties and dielectric relaxation behaviors of BT-BFO ceramics syn thesized by MWS have not been reported until now. In the present work, (1-x) BT-x BFO (x ¼ 0, 0.01, 0.03, 0.06, 0.1) ceramics with 0.15 wt% SiO2 were prepared by different sintering methods of conventional sintering (CS) and MWS, respectively. The crystal structure, dielectric relaxation and ferroelectric properties of BTBFO ceramics were studied, and their dielectric relaxation mechanism was also discovered. 2. Experimental procedure A series of (1-x) BT-x BFO (x ¼ 0, 0.01, 0.03, 0.06, 0.1 mol%) 3
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Fig. 4. The relative permittivity and dielectric loss as function of temperature for (1-x) BT-x BFO (x ¼ 0–0.1) ceramics of CS (a, c, e, g, i) and MWS (b, d, f, h, j) obtained at different frequencies from 1 kHz to 2 MHz.
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Fig. 5. The relative permittivity and dielectric loss as function of temperature for (1-x) BT-x BFO (x ¼ 0–0.1) ceramics of CS and MWS measured at 1 kHz: (a) CS; (b) MWS.
Fig. 6. The curves of ln (1/ε-1/εm) versus ln (T-Tm), and inset temperature as function of the inverse relative permittivity for (1-x) BT-x BFO (x ¼ 0–0.06) ceramics by CS and MWS at 1 kHz: (a) x ¼ 0; (b) x ¼ 0.01; (c) x ¼ 0.03; (d) x ¼ 0.06.
3.2. Surface morphology
Table 1 The temperature for the maximum relative permittivity (Tm), the Curie-Weiss constant (C), the starting temperature of deviation from Curie-Weiss Law (TCW), the Curie-Weiss temperature (T0), the degree of deviation from CurieWeiss Law (ΔTm) for (1-x) BT-x BFO (x ¼ 0–0.06) ceramics. Samples CS
MWS
C ( � 105K) T0 (� C) Tm (� C) TCW (� C) Δ Tm (� C) C ( � 105 K) T0 (� C) Tm (� C) TCW (� C) Δ Tm (� C)
0
0.01
0.03
0.06
0.817 130.8 148 161 13 0.926 118.5 148 161 22
1.478 111.6 125.1 140 17.9 1.556 101.2 126 150 24
1.563 66.2 97.9 118 20.1 1.129 69.2 111 136 25
2.008 2.5 59 93 34 1.895 32 58.2 103 45
The microstructures of BT-BFO ceramics prepared by CS and MWS are given in Fig. 3, and exhibit more uniform grains. The large grains are presented in Fig. 3 (a) and 3 (c), which belongs to BT and 0.99BT0.01BFO CS ceramics respectively. The average grain sizes of CS and MWS ceramics are 9.56, 8.56, 4.27, 1.84, 1.12 μm and 0.74, 3.42, 3.22, 1.03, 0.73 μm, respectively. Finer grains of MWS ceramics were ob tained compared with CS ceramics, which indicates that the MWS method contributes to smaller grains because of its fast heating [17]. All samples exhibit smaller grains and fewer pores with increasing x, indi cating that the BFO dopant possibly inhibits grains growth. Defect di poles were formed with the addition of BFO, restricting barium vacancies and weakening the diffusion mechanism of grains growth [14, 24]. It is confirmed that grain size tends to reduce with the increasing BFO content. The substitution of Ba2þ by Bi3þand Ti4þ by Fe3þ can introduce many defects due to the different valences of substituted ions. This can be expressed by reactions as following:
from x ¼ 0 to x ¼ 0.1 shown in Figs. 1 and 2. This results from the substitution of large ionic radii Ba2þ (0.135 nm) and Fe3þ(0.064 nm) by Bi3þ (0.103 nm) and Ti4þ (0.0603 nm), respectively.
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constant peak with a decrease of TC as x increases for x ¼ 0.1 sample, which means a strong frequency diffusion. All peaks width of MWS samples are broader than those of CS samples as shown in Fig. 4, which is due to the smaller grains obtained by MWS method. MWS can accel erate ion diffusion due to its rapid heating, resulting in more BFO diffusing into BT and wider dielectric dispersion peaks. Moreover, the finer grains obtained by MWS can also make the dielectric dispersion peaks broaden. The TC moves to the low-temperature region as x in creases seen in Fig. 5, which is related to the disruption of Ti–O bounding and the reduction of the tetragonal phase. The doping of Fe3þ, different from Ti4þ in covalent bonding character and ionic radius, could disrupt Ti–O bounding. The distance between Ti atoms reduces by dis rupted, resulting in the appearance of thermal rearrangement within a larger temperature range [25]. While the bond strength of Fe–O bond is larger than that of Ti–O bond, thus the cubic phase occurs at low tem perature, which weakens the phase transition temperature of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics. TC shifts to low-temperature region, indi cating that BT possesses well temperature stability of the dielectric properties. The value of εm increases first and then decreases for CS ceramics with increasing x, and εm always decreases for MWS ceramics with increasing x. The grain size makes an effect on the dielectric con stant. The large grains lead to the reduction of the internal stress. The 90� domain size decreases and then the 90� domain boundary increases with the grain size decreasing, resulting in enhancement of the dielectric constant. However, the ultra-fine grain cannot produce 90� domain which restrains the internal stress, so it has abnormal dielectric constant [26–29]. As a conclusion, the dielectric constant first increases and then decreases with the decrease of grain size for BT ceramics. To describe the degree of the dielectric relaxation of (1-x) BT-x BFO (x ¼ 0–0.1) samples, the modified Curie-Weiss law is given as following [30]:
Fig. 7. The diffuseness coefficient (γ) dependence of BFO content (x) for (1-x) BT-x BFO (x ¼ 0–0.06) ceramics.
1 ’’ Bi2 O3 → 2Bi:Ba þ VBa þ 2Oo þ O2 2 TiO2
2Fe3 O4 ���!6Fe’Ti þ 3VO:: þ 8Oo
(2) (3)
’’ Where VBa is the vacancy with two negative charges at the Ba2þ position, :: VO is the vacancy with two positive charges at the O2 position, Bi:Ba is the substation of Ba2þ by Bi3þ with a positive charge, Fe’Ti is the sub station of Fe3þ by Ti4þ with a negative charge. The block phase circled by a yellow square in Fig. 3 (a), it might be the second phase Ba2TiSi2O8, which is consistent with the results of the XRD patterns.
1
εr
¼
T
TCW C
(4) Tm Þγ C
1
1
3.3. Dielectric properties
ε
εm
The temperature dependence of the relative permittivity and dielectric loss for (1-x) BT-x BFO (x ¼ 0–0.1) ceramics sintered by CS and MWS within the frequency range from 1 kHz to 2000 kHz are illustrated in Fig. 4. There is a different that the BT ceramic exhibits a slightly sharp peak caused by the addition of SiO2. With increasing BFO content and frequency, those Curie peaks become broader, which dis plays a dispersion phase transition and frequency diffusion. This sug gests that the addition of BFO and SiO2 can strengthen the dielectric relaxation behavior of BT ceramic. This phenomenon can be explained by the fact that the substitution of Fe3þ for Ti4þ disrupts the long-range of Ti-displacements. Meanwhile, it is related to the formation of polar nanoclusters due to the substitution of Bi3þ for Ba2þ and Fe3þ for Ti4þ on A-site and B-site, respectively [25]. For x ¼ 0.03, 0.06 ceramics, a typical relaxor-like behavior was observed. A rather broad and flat dielectric
Where the diffuseness exponent γ is between 1 and 2, γ ¼ 1 suggests a normal ferroelectric and γ ¼ 2 is an ideal relaxor ferroelectric. Tm is the temperature of εm, C is the Curie-Weiss constant, ε is the relative permittivity, TCW is temperature at which the dielectric constant begins to follow the Curie-Weiss law. Fig. 6 inset presents an inverse dielectric constant of (1-x) BT-x BFO (x ¼ 0–0.06) ceramics versus the temperature. ΔTm, the degree of de viation from Curie-Weiss Law, could be calculated by ΔTm ¼ TCW-T0, where TCW is the Curie-Weiss Law starting temperature, T0 is the CurieWeiss temperature. The starting temperature TCW decreases with the addition of BFO. ΔTm largen with x increasing as illustrated in Table 1, indicating the addition of BFO broadens the deviation from Curie-Weiss Law, and improve relaxation behavior. ΔTm of CS ceramics are larger than that of MWS ceramics, which means MWS enlarges the deviation
¼
ðT
(5)
Fig. 8. The dielectric constant and loss as function of frequency (20 Hz-2 MHz) for (1-x) BT-x BFO ceramics measured at room temperature: (a) CS; (b) MWS. 6
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Fig. 9. P-E hysteresis loops of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics prepared by CS (a, c, e, g, i) and MWS (b, d, f, h, j) measured at 100 Hz under different electric fields.
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Fig. 10. P-E hysteresis loops of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics sintered by using CS and MWS measured at 100 Hz at room temperature: (a) CS; (b) MWS.
Fig. 11. Composition dependences of EC and Pr for the (1-x) BT-x BFO (x ¼ 0–0.1) ceramics: (a) CS; (b) MWS.
from Curie-Weiss Law and enhance relaxation behavior more effectively in compare to the addition of BFO. The greater deviation, the stronger frequency dispersion and phase diffusion demonstrate that the relaxa tion behavior strengthens. The tendency of ΔTm more accurately quantifies the degree of relaxation, which is further consistent with the result of the temperature dependence of the relative permittivity. The diffuseness coefficient γ is a fitting data to show the degree of relaxation behavior. For BT-BFO ceramics at 1 kHz, it calculated by the slopes of ln (1/ε-1/εm)-ln (T-Tm) plots as shown in Fig. 6. They are 1.38, 1.45, 1.48, 1.62 for CS samples and 1.48, 1.64, 1.65, 1.69 for MWS samples, respectively. The γ values as a function of x for (1-x) BT-x BFO (x ¼ 0–0.06) ceramics shown in Fig. 7 γ of CS samples gradually in creases with the addition of BFO, and that of MWS samples is similar to CS samples. And MWS ceramics show a larger γ. Those results reveal that both the doping of BFO and MWS method can strengthen the relaxation behavior, which is consistent with the results of Fig. 4. Frequency dependence of the dielectric constant and loss of (1-x) BTx BFO (x ¼ 0–0.1) ceramics at room temperature is shown in Fig. 8. The dispersion of samples occurs in the low-frequency region, which is mainly caused by the BFO content. With an increase of the substitution of Ba2þ by Bi3þ and Ti4þ by Fe3þ, the permittivity decreases when the long rang order is broken. At high-frequency region, the variation of dielectric constants is small. The values of εr in low-frequency range are larger than that in high frequency (10 kHz-2 MHz), which is related to the different polarization mechanism. At low frequencies, the larger relative permittivity is due to the co-contribution of space charge po larization, displacement polarization and dipole orientation polariza tion. However, only space charge polarization and displacement polarization contribute to polarization at high frequencies, resulting in the reduction of εr. The εr of x ¼ 0 composition has an obviously decrease with increasing frequency in Fig. 8 (a). However, the other compositions have a platform among the frequency of 1 kHz–2 MHz, indicating the independence between frequency and permittivity in these samples. In Fig. 8 (b), the compositions of x ¼ 0, 0.01 have an obviously decline with the increase of frequency, and the remaining compositions appear a slow
plateau in the frequency range of 1 kHz–2 MHz. It can be concluded that the incorporation of BFO and microwave sintering can improve the frequency stability of the dielectric properties for BT ceramic. At lowfrequency region, the value of the dielectric loss is high and gradually decreases with an increase of frequency until up to a constant value. 3.4. Ferroelectric properties The P-E hysteresis loops of (1-x) BT-x BFO (x ¼ 0–0.1) ceramics prepared by CS and MWS at room temperature are shown in Fig. 9. As the electric field (E) increases, hysteresis loops become fatter and more saturated, which can explain by the larger E which makes domain switch easily. From Fig. 9 (a, c, e, g), the addition of BFO make the P-E hys teresis loops become slim, indicating the enhancement of relaxation behavior. P-E hysteresis loops of 0.97BT-0.03BFO ceramics, a contrac tion area appears in Fig. 5 (b), which is possibly due to the leakage current. And more holes might be one of the reasons for the leakage current in the SEM of 0.97BT-0.03BFO ceramics. As shown in Fig. 9, the breakdown electric field of MWS ceramics is larger than that of CS ce ramics, which is related to the compactness of samples sintered by mi crowave. The breakdown electric field of 0.9BT-0.1BFO ceramic of MWS is 90 kV/cm at a frequency of 100 Hz. Fig. 10 shows the relationship of Pr, coercive field EC versus x for (1x) BT-x BFO (x ¼ 0–0.1) ceramics with different compositions. From Fig. 10 (a), the P-E hysteresis loops become slim as x increases, indi cating the BFO can weaken the ferroelectricity and strengthen the relaxation behavior. Similarly, the P-E hysteresis loops of MWS ceramics become slim gradually with the increase of BFO in Fig. 10 (b). Mean while, there is a clear reduction in 2Pr (6.942, 3.477, 1.8716, 2.203 and 1.2376 μC/cm2) of CS samples with the increase of BFO as shown in Fig. 11 (a). With the increase of BFO, grains become small. Small grains, which possess small domain size and more domain walls, make domain inversion difficult. Thus, the reduction of 2Pr is relate to the small grains. The 2Pr (4.007, 6.961, 1.7552, 0.8532 and 1.0432 μC/cm2) of MWS samples is presented in Fig. 11 (b). An abnormal data appears in x ¼ 0.01 8
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MWS samples, which may be caused by large grains in SEM. The mini mum value of 2Ec is about 4 kV/cm for 0.94BT-0.06BFO MWS samples. The decrease of Ec leads to the reduction of energy of domain reor ientation. The value of Ec is related to grain sizes and defects.
[2]
[3]
4. Conclusions The crystal structure, microstructure, dielectric and ferroelectric properties of (1-x) BT-x BFO-0.15 wt% SiO2 ceramics prepared by CS and MWS were characterized, and the dispersion degree was fitting. The result show that the crystal structure of CS and MWS ceramics trans forms from the tetragonal phase to the cubic phase with an increase of BFO. Due to the addition of SiO2, the second phase appears in the XRD images. And the average grain sizes of BFO doping components are finer. Furthermore, MWS ceramics possess a smaller average grain size. With an increase of x, Curie peaks continuously widened, and phase transition diffusion and frequency dispersion phenomena were observed. TC shifts toward the low-temperature region, which related to the Ti–O bond destruction caused by Fe3þ replacing Ti4þ. The γ of MWS samples is obviously larger than that of CS, and it can achieve 1.69 at x ¼ 0.06, which indicates that MWS is helpful to enhance the relaxation behavior of BaTiO3-based ceramics. With the doping of BFO and MWS method, PE hysteresis loops also become slimmer. It was proved that microwave sintering and BFO enhance the relaxation behavior of BT-based ceramics.
[4]
[5] [6] [7] [8] [9] [10] [11]
Declaration of competing interest
[12]
The authors declared that we have no financial and personal re lationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
[13] [14] [15]
CRediT authorship contribution statement [16]
Tao Fan: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Cong Ji: Validation, Formal analysis, Visuali zation, Software. Gang Chen: Validation, Formal analysis, Visualiza tion, Writing - review & editing. Wei Cai: Resources, Writing - review & editing, Supervision, Data curation. Rongli Gao: Resources, Writing review & editing, Supervision, Data curation. Xiaoling Deng: Writing review & editing. Zhenhua Wang: Writing - review & editing. Chunlin Fu: Writing - review & editing.
[17] [18] [19] [20]
Acknowledgments
[21]
This work was supported by the undergraduate science and tech nology innovation training program project of Chongqing university of science and technology (YKJCX1820213, YKJCX1920214), the 9th Chongqing KeHui graduate student innovation and entrepreneurship competition (09311167), Chongqing Research Program of Basic Research and Frontier Technology (CSTC2019jcyj-msxmX0071, cstc2018jcyjAX0416), the program for innovation teams in University of Chongqing, China (CXTDX201601032), the Innovation Program for Chongqing’s Overseas Returnees(cx2019159), Research foundation of education bureau of Chongqing, China (KJQN201801509), the leading talents of scientific and technological innovation in Chongqing (CSTCCXLJRC201919) and the excellent talent project in university of Chongqing (2017–35).
[22]
[23] [24] [25] [26] [27]
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