Pure perovskite BiFeO3–BaTiO3 ceramics prepared by reaction flash sintering of Bi2O3–Fe2O3–BaTiO3 mixed powders

Ceramics International 47 (2021) 26947–26954 Available online 17 June 2021 0272-8842/© 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Pure perovskite BiFeO 3 –BaTiO 3 ceramics prepared by reaction flash sintering of Bi 2 O 3 –Fe 2 O 3 –BaTiO 3 mixed powders Ahmed Taibi a , b , Salem Chaguetmi a , Pedro E. S´ anchez-Jim´ enez b , c , ** , Antonio Perej´ on b , c , * , Jos´ e Eduardo García d , Hamid Satha a , Luis A. P´ erez-Maqueda b a Laboratoire LSPN, Universit´ e 8 Mai 1945 Guelma, BP 401, Guelma, 24000, Algeria b Instituto de Ciencia de Materiales de Sevilla, C. S. I. C.-Universidad de Sevilla, C. Am´ erico Vespucio No 49, 41092, Sevilla, Spain c Departamento de Química Inorg´ anica, Facultad de Química, 41012, Universidad de Sevilla, Sevilla, Spain d Department of Physics, Universitat Polit` ecnica de Catalunya, 08034, Barcelona, Spain ARTICLE INFO Keywords: 0.67BiFeO 3 -0.33BaTiO 3 Reaction flash-sintering Current limit Flash sintering ABSTRACT In this work, the 0.67BiFeO 3 -0.33BaTiO 3 ferroelectric ceramic was prepared by Reaction Flash Sintering (RFS). This preparation technique combines synthesis and sintering in a single Flash experiment. The starting oxides reacted during the flash to produce a stoichiometric well-sintered solid solution at a temperature of 858 ◦C by applying a modest field of 35 V cm −1 . The process takes place in a matter of seconds, which allows obtaining a pure perovskite structure without secondary phases. X-ray diffraction (XRD) results show the mixture of rhombohedral and pseudocubic phases expected for a composition that lies within a morphotropic phase boundary (MPB) region, since a significant splitting is observed in the reflections at 2θ values of 39◦and 56.5◦. The microstructure exhibit a peculiar bimodal grain size distribution that determines the electrical properties. As compared with previous results, flash-prepared 0.67BiFeO 3 -0.33BaTiO 3 evidences smaller grain size, as well as slightly lower remanent polarization (P r ) and smaller coercive field (E c ) under similar electric fields. It is also demonstrated that the preparation by RFS provides benefits regarding electrical energy consumption. 1. Introduction Bismuth iron oxide, BiFeO 3 (BFO), is a lead free multiferroic material that has received extensive attention due to its high Curie temperature (Tc ~ 830 ◦C) and its attractive polarization (Ps ~ 100 μ C/cm 2 ) [1–3]. However, the main problem of the single-phase BFO system is its high leakage current and low piezoelectric performance [4,5], which limits its practical applications in devices. It is well known that the combination between BFO and other ABO 3 perovskite systems significantly improves the piezoelectric properties [6–8]. ABO 3 -modified BFO ceramics, such as the (1−x)BiFeO 3 -xBaTiO 3 (BFO-BT) system near the morphotropic phase boundary (MPB) have attracted extensive attention as a new generation of lead-free piezoelectric ceramics because of its promising piezoelectric performance, high Tc and reduced leakage current (T C >400C) [6,9,10]. Basically, dense BiFeO 3 –BaTiO 3 ceramics are synthesized in two steps, including preparation and sintering of BiFeO 3 –BaTiO 3 powders. Both steps need high temperatures and long treatment times, which are high energy consuming. Under such conditions, it is difficult to prepare dense and stoichiometric BiFeO 3 –BaTiO 3 ceramics due to high Bi volatilization and Fe 3+ reduction that lead to the formation of impurities such as Bi 2 Fe 4 O 9 , Bi 25 FeO 39 , or Bi 2 O 3 [11–14]. Thus, other techniques to prepare dense and stoichiometric BiFeO 3 –BaTiO 3 ceramics are required. Since it was first reported in 2010 [15], flash sintering has attracted the attention of many researchers for several reasons, including its lower onset sintering furnace temperatures and its short time densification as compared to conventional preparation techniques [16–22]. Also, it is a promising processing method to prevent the high volatility of certain elements during the preparation of materials, such as potassium [23], bismuth [24–27] and lithium [28]. Moreover, it has been reported that it is possible to prepare single-phase oxides in a single step by flash sintering of its precursors [29]. This methodology has been called reaction flash sintering (RFS) and has been used to prepare other materials, such as lead zirconate titanate (PZT), sodium potassium niobate (KNN) and other multi-phase oxide ceramics [23,30–33]. To the best of our knowledge, there are no report on the preparation of BiFeO 3 –BaTiO 3 * Corresponding author. Instituto de Ciencia de Materiales de Sevilla, C. S. I. C.-Universidad de Sevilla, C. Am´ erico Vespucio no 49, 41092, Sevilla, Spain. ** Corresponding author. Departamento de Química Inorg´ anica, Facultad de Química, 41012, Universidad de Sevilla, Sevilla, Spain. E-mail addresses: [email protected] (P.E. S´ anchez-Jim´ enez), [email protected] (A. Perej´ on). Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint https://doi.org/10.1016/j.ceramint.2021.06.108 Received 26 April 2021; Received in revised form 11 June 2021; Accepted 14 June 2021 Ceramics International 47 (2021) 26947–26954 26948 ceramics using reaction flash sintering. In this work, the preparation of 0.67BiFeO 3 -0.33BaTiO 3 (abbreviated as BFO-33BT) is investigated using the reaction flash sintering method. In BFO-33BT, the BaTiO 3 molar fraction (33%) was chosen since it is located in the morphotropic phase boundary (MPB) range [34]. Compositions where x is in the range 0.25–0.35 lie near a morphotropic phase boundary (MPB), exhibiting rhombohedral and pseudocubic phases simultaneously [3,14,35]. The starting powders reacts during the flash to produce a well-sintered stoichiometric ceramic, with a peculiar microstructure. 2. Materials and methods High purity oxides, Bi 2 O 3 (Sigma-Aldrich, 99.9%), Fe 2 O 3 (Sigma- Aldrich, 99%) and BaTiO 3 (Sigma-Aldrich, 99%), were used in the stoichiometric ratio for the preparation of 0.67BiFeO 3 -0.33BaTiO 3 (BFO-33BT) by RFS method. Stoichiometric amounts of the starting oxides were ball milled for 15 min in a planetary ball-mill with the objective of obtaining a homogeneous mixture. Approximately 900 mg of the powders mixture were pressed into dog-bone shaped specimens under a pressure of 500 MPa. In order to provide good contact sampleelectrodes, platinum paste was added into the holes of the handles of the green-body dog-bones. The distance between the electrodes was 2 cm. The electrode wires were connected to a 1500 W DC power supply (EAPSI 9750–06 DT). Current and voltage input signals were measured using a Keithley 2110 multimeter. The flash event was held for 30 s and the adjustment of the experimental flash conditions (applied electric field (E) and current density limit (I)) was carried out according to the protocol described in a previous work [36]. The XRD measurements were performed in a Rigaku MiniFlex diffractometer, using Cu K α radiation with a scan rate of 0.02◦/s in the 15◦–70◦2θ range. Raman spectra was measured at room temperature using a Horiva Jobin Yvon LabRam HR800 microscope with a green laser excitation of 532 nm, and the fitting of the spectra was carried out using a Gaussian function. The density of the BFO-33BT flashed samples was determined using the Archimedes method. Morphological and compositional characterization was investigated by scanning electron microscopy (Hitachi S-4800), connected with an energy dispersive X-ray spectrometer (EDX). Silver electrodes were painted on parallel polished faces of the samples for electrical measurements. The impedance measurements were performed in a 360–440 ◦C temperature range and over the frequency range 100 Hz-1 MHz, using a Newtons4th Ltd Impedance analysis interface (PSM1735 NumetriQ). The dielectric characterization was performed in disc-shaped unpoled samples using an Agilent E4980A Precision LCR Meter, the real ε ′and imaginary ε ’’ parts of the permittivity being measured at several frequencies between 100 Hz and 1 MHz. Samples were placed in a programmable tubular oven for measurement in a temperature range from room temperature to 600 ◦C. Electric fieldinduced polarization (P-E) hysteresis loops were measured in a typical Sawyer-Tower configuration by applying a sinusoidal electric field of amplitude of 60 kV cm -1 at a frequency of 1 Hz, using a Hewlett-Packard 33120A signal generator and a Trek 663 signal amplifier, at room temperature. The electric field-induced macroscopic strain was measured by adding a WayCon inductive position transducer conditioning with a Solartron OD5 Module to the P-E setup. Samples were then poled in a silicone bath at 80 ◦C under a dc electric field of 60 kV cm -1 for 30 min, and subsequently aged for two days before measurements were taken in order to prevent the influence of aging processes. The static longitudinal direct piezoelectric constant was obtained by using a KFC Technologies PM3500 d 33 -m. 3. Results and discussion 3.1. Preparation of BFO-33BT by reaction flash sintering Series of experiments for the preparation of 0.67BiFeO 3 -0.33BaTiO 3 by reaction flash sintering were performed with the aim of optimizing the experimental conditions, i.e., applied electric field and selected current density limit. The results obtained from these experiments are displayed in Fig. 1. In this figure, Fig. 1(a), two large regions where reaction is either incomplete or heterogeneous (due to localization of the electrical current) can be clearly distinguished. Thus, only in a very narrow range of experimental conditions (electric field of 35 V cm −1 and current density of 60 or 70 mA mm −2 ), within the boundary between incomplete and heterogeneous regions, the reaction is optimal, yielding a homogeneous material. The corresponding captured images of resulting flash sintered samples corresponding to incomplete, heterogeneous and optimal reactions are also included in Fig. 1(b–d). Samples prepared within the optimal experimental conditions range yielded an identical density of 6.90 g cm −3 , corresponding to a relative density of 91%, as determined by the Archimedes method. Therefore, an electric field of 35 V cm −1 and a current density of 60 mA mm −2 can be considered as the best conditions for the preparation of BFO-33BT by reaction flash sintering, as a lower power density (0.210 W mm −3 ) is required as compared to the other sample obtained with a current intensity of 70 mA mm −2 (0.245 W mm −3 ). Fig. 2 shows the evolution of electric field, current intensity and dissipated power as a function of the furnace temperature for the reaction flash sintering experiment at an electric field of 35 V cm −1 and a current density limit of 60 mA mm −2 . The three stages of the flash sintering experiment are clearly observed in these figures. Thus, after a first stage, where the electric field is maintained constant by the power supply at 35 V cm −1 , the current density (Fig. 2(a)) rise sharply in a second stage to the preset maximum value of 60 mA mm −2 (corresponding to a power dissipation of 210 mW mm −3 , Fig. 2(b)). At this point, the power supply shifts from constant voltage (CV) to current controlled (CC) regime. In the third stage, the sample is maintained in a flash-activated state under controlled current (CC). The onset of flash is reached at a furnace temperature of 858 ◦C. The temperature of the sample at stage III of the flash can be estimated by using the blackbody radiation model [37]: T=[T4 0+W A εσ ] 1 / 4 (1) where T and T 0 respectively reflect the real sample temperature and the furnace temperature, W is the power dissipation, A is the surface area of Fig. 1. (a) Experimental conditions used for the reaction flash sintering of BFO- 33BT. Dog bone samples for (b) incomplete reaction (40 V cm −1 , 40 mA mm -2 ), (c) optimum reaction conditions (35 V cm −1 , 60 mA mm -2 ) and (d) heterogeneous reaction (due to localization, 50 V cm −1 , 60 mA mm -2 ). A. Taibi et al. Ceramics International 47 (2021) 26947–26954 26949 the sample (m 2 ), ε is the emissivity (0.9) and σ is Stefan’s constant (5.67 ×10 −8 W m −2 K −4 ). Thus, the estimated real temperature of the sample was found to be approximately 1050 ◦C. Field-assisted processing of materials is commonly analyzed in terms of power dissipation density. The Arrhenius plot of the power dissipation is shown in Fig. 2(c). At low temperatures, the power dissipation exhibits a linear-like type behavior. This behavior is interrupted at 50 mW mm −3 when the flash event takes place, followed by a sharp increase in the 90–210 mW mm −3 power dissipation range that signals the onset of flash. It is remarkable that the inflexion of the curve that signals the flash event occurs within the same narrow range (10–50 mW mm −3 ) described in literature for flash sintering of numerous materials, despite the fact that, in this case, there is reaction and sintering taking place at the same time [38]. Fig. 3 shows the room temperature XRD pattern of the BFO-33BT sample before and after the flash (at 35 V cm −1 and 60 mA mm −2 ). The JCPDS (Joint Compounds Powder Diffraction Standards) cards of BiFeO 3 and BaTiO 3 are also listed in Fig. 3. The XRD peaks of the sample before the flash are broad due to the small crystallites size obtained after ball milling during 15 min and the mixture is completely composed of the starting precursors: Bi 2 O 3 (JCPDS #71–0465), Fe 2 O 3 (JCPDS # 24–0072) and BaTiO 3 (JCPDS #31–0174). After the flash, the XRD pattern of the sample evidences that the composition BFO-0.33BT is within the MPB region of the BFO-BT system. Thus, a significant splitting is observed in the reflections at 2θ values of 39◦and 56.5◦(see inset in Fig. 3), which has been associated with the coexistence of rhombohedral (R3c) and pseudocubic (PC) phases [14,39,40]. The ratio between the R3c and PC phases was estimated by deconvolution of these reflections, using a pseudo-Voigt function, and the results indicate that a mixture of 57% R3c and 43% PC phases is obtained. Thus, it can be concluded that the Bi 2 O 3 –Fe 2 O 3 –BaTiO 3 mixed powders are transformed to a mixture of R3c and PC phases of 0.67BiFeO3-0.33BaTiO3 after the reaction flash sintering. The presented synthesis route using reaction flash sintering led to a significant reduction of the furnace reaction temperature (Ts ~ 858 ◦C) and time (t =30s) for 0.67BiFeO 3 -0.33BaTiO 3 , compared to conventional solid-state method [6,9,41], which requires at least 980 ◦C and 2–3 h of dwell time. The Raman spectrum for the BFO-33BT sample obtained under optimal conditions (35 V cm −1 , 60 mA mm −2 ) is shown in Fig. 4. The spectrum obtained clearly presents 8 mode peaks, three of them belonging to A 1 modes at 177, 250 and 476 cm −1 , and five to E modes at 307, 522, 566, 663 and 750 cm −1 . These values are consistent with previous reports of BiFeO 3 –BaTiO 3 based ceramics. Thus, Bouzidi et al. [42], Qi et al. [43] and Li et al. [44] observed, for conventionally prepared BiFeO 3 –BaTiO 3 , bands at approximately the same values as for the A 1 and E modes. They were able to distinguish two main regions in the Raman spectrum: (i) low frequency vibrational modes below 400 cm −1 , which are essentially associated to the vibrations of B–O band. In particular, the most intense vibration mode, at around 250 cm −1 , has been assigned to Fe/Ti–O. (ii) High frequency vibrational modes above 400 cm −1 reflect to the vibrational frequencies of BO 6 octahedral. Our results are also in good agreement with those obtained by Pakalniˇ skis et al. [35] for BiFeO 3 –BaTiO 3 samples prepared by sol gel method. On the other hand, the appearance of the vibration mode at around 750 cm −1 indicates the existence of the rhombohedral (R3c) symmetry [42], which confirms the mixture of rhombohedral and pseudocubic symmetry expected at the MPB region. Fig. 2. (a) Evolution of electric field and current density, and (b) power dissipation versus furnace temperature for the reaction flash sintering experiment performed at an electric field of 35 V cm −1 and a current density limit of 60 mA mm -2 . (c) Arrhenius plot for the BFO-33BT flashed sample. Fig. 3. XRD patterns of BFO-33BT sample before and after the reaction flash process. A. Taibi et al. Ceramics International 47 (2021) 26947–26954 26950 The surface morphology of the fracture of BFO-33BT reaction flash sintered sample, as observed by SEM, is shown in Fig. 5(a). It corresponds to a well sintered material with a small amount of pores. Fig. 5(b) shows a high-magnification SEM image of the fractured surface. It shows a peculiar microstructure with a bimodal grain size distribution, where small grains are located in the grain boundaries between larger grains. The mean value corresponding to the large grains is 3.4 ±1 μ m, while the mean value corresponding to the smaller grains is 0.4 ±0.1 μ m. The EDS mapping shows that Bi, Fe, Ba and Ti are uniformly dispersed within the flashed sample, both small and large grains show identical compositional ratios, confirming the chemical homogeneity of the BFO-33BT ceramic sample (Fig. 5(c–g)). 3.2. Dielectric and impedance spectroscopy of BFO-33BT flashed sample The dielectric properties of the BFO-33BT flashed sample were studied as a function of temperature at selected frequencies and plotted in Fig. 6. Both the dielectric permittivity (Fig. 6(a)) and dielectric loss (Fig. 6(b)) appeared to increase clearly at temperatures above 200 ◦C, especially when measured at low frequencies. These effects are due to the increase of the electrical conductivity of the BFO-33BT flashed sample and to the associated space-charge polarization [45,46]. Thus, an extrinsic contribution to the dielectric permittivity due to the high dielectric loss is expected. A frequency-dependent dielectric anomaly appears slightly over 400 ◦C that can be attributed to the ferroelectric-paraelectric transition. The dielectric permittivity decreases and the peak shifts to higher temperatures with increasing frequencies. The strong dependence of the dielectric permittivity ( ε r ) on frequency and the diffuse phase transition (Tc ~ 440 ◦C), suggests a relaxation phase transition behavior. This confirms previous finding by Singh et al. [47] having observed a relaxor-type ferroelectric behavior Fig. 4. Raman spectrum for the BFO-33BT sample obtained under optimal conditions (Fig. 3(c)). Fig. 5. SEM micrographs of BFO-33BT obtained under optimal conditions (a) surface morphology of the fracture surface, (b) SEM image at high magnification. EDS elemental mapping of (c) Bi, (d) Fe, (e) Ba, (f) Ti and (g) EDS layered image. A. Taibi et al. Ceramics International 47 (2021) 26947–26954 26951 for samples prepared using BaTiO 3 as starting precursor. The dielectric loss significantly shifts to higher temperatures at higher frequencies, thereby suggesting mobile ions or vacancies might constitute the main conduction mechanism. Overall, the observed properties are consistent with previous reports [3,14,34,48]. In order to check the electrical homogeneity of the BFO-33BT flashed sample, impedance spectroscopy measurements were performed. Impedance data for BFO-33BT in air are displayed in Fig. 7. The Nyquist plot of BFO-33BT flashed sample is presented in Fig. 7(a). The flashed sample exhibits typical single semicircular arcs at all temperatures. As the temperature increases, the intercept of the semicircles on the Z’ axis is smaller, indicating a decrease in the resistivity of the BFO-33BT sample. Fig. 7(b) shows the Arrhenius plots of the conductivity for BFO–33BT. The average activation energy value, Ea, obtained from the plot slope, is found to be equal to 1.10 eV. This Ea value is quite similar to other results found in the literature for BiFeO 3 –BaTiO 3 based ceramics [49–51]. Such value is consistent with reported activation energies of oxygen vacancies (V O ⋅⋅ ), suggesting the conduction mechanism relies on oxygen vacancies that dominate the conductivity as elsewhere reported [14,52]. This mechanism can be further promoted by the synthesis method as it has been shown how ceramics prepared by flash sintering exhibit a significant amount of defects and ion vacancies [38,53]. To verify the existence of short-range and long-range ordering in BFO-33BT sample, spectroscopic plots of imaginary components of impedance, Z′′, and modulus, M′′ , are plotted at 380 ◦C (Fig. 7(c)). As seen from the graph, both peaks are almost in the same position, indicating good electrical homogeneity of BFO-33BT sample. The capacitance increases with increasing temperature (Fig. 7(d)), and it is also noted that at all temperatures the capacitance shows a single capacitance plateau, with an approximate value range of 0.20–0.60 nF cm -1 . This indicates the presence of single electrical responses. 3.3. Ferroelectric and piezoelectric properties of BFO-33BT flashed sample The room-temperature polarization hysteresis loops, measured under 60 kV cm -1 at 1 Hz are shown in Fig. 8. The loops exhibit poor saturation what can be attributed to the bimodal microstructure with relatively small grains obtained by reaction flash sintering. The large density of grain boundaries and defects as well as the consequently smaller ferroelectric domains hamper domain switching and limits maximum polarization [54]. The measured P r , Ec and Smax are included in Table 1. Fig. 8 also displays the electric-field induced strain curves at 60 kV cm -1 . The sample exhibits the symmetric butterfly-shaped loops characteristic of ferroelectric materials. A small maximum strain of 0.034% is measured. All remnant and maximum polarization, as well as maximum strain appear slightly smaller than reported elsewhere, most probably due to the large number of small grains [14,55]. These measured properties are consistent with values reported in previous works [3]. Table 1 also include the values of d 33, ε ′and ε ’’ at room temperature. The measured piezoelectric constant d 33 is 30 pC/N. The small value is a consequence of the low P r , what hampers the piezoelectric properties. 3.4. Comparison between electrical energy consumption in RFS and conventional preparation A comparison of the electrical energy consumption during RFS and conventional preparation method (CM) of the material was performed (Fig. 9). A power analyzer PPA 1500 (Newtons4th Ltd. (N4L)) was used to measure the power supplied by the temperature controller to the furnace resistance during the entire experiments (heating rates of 5 ◦C min -1 ). In order to compare the energy consumption in RFS experiment with that of the conventional method, the electrical energy consumed in both processing methods was calculated. It was considered that the preparation of BFO-33BT by the conventional preparation method takes place in two stages, including synthesis (~800 ◦C for 2 h) and sintering (~1000 ◦C for 3 h) [8,12–14]. Therefore, the total electric energy consumed in the conventional preparation of BFO-33BT is the quantity of energy consumed by the furnace during synthesis and sintering. Regarding the RFS, the energy consumed includes the energy required to heat up the furnace to the onset flash temperature (858 ◦C) and the energy delivered by the power supply during the flash (see Fig. 2(b)). Although at a laboratory scale, the results provide a proof of concept according to which the preparation by reaction flash sintering provides benefits regarding the electrical energy consumption. The total value of 788 Wh of electrical energy consumption in case of the experiment performed by RFS shows a decrease of about 82% compared to the electrical energy consumption of 4512 Wh for conventional preparation (Fig. 9). The experiments also confirmed the basic principles of flash sintering. Thus, energy is transferred directly to the materials, which provide rapid densification (<60 s) at moderate furnace temperatures. During the conventional preparation method, where energy is generated by external heating elements and transferred to the samples via convection, conduction and radiation, 67% of the total electrical energy is consumed in the sintering stage and the rest of 33% is used during the synthesis stage of BFO-33BT. 4. Conclusions In this work, 0.67BiFeO 3 -0.33BaTiO 3 ceramic specimens have been synthesized and sintered in a single step by means of Reaction Flash Sintering technique. The starting powders, previously mechanically Fig. 6. Temperature dependent (a) permittivity and (b) loss for BFO–33BT obtained by reaction flash sintering. A. Taibi et al. Ceramics International 47 (2021) 26947–26954 26952 mixed to ensure homogeneous distribution, reacted during the flash to produce the stoichiometric solid solution and sintered to near full density in a matter of seconds at a temperature of 858 ◦C under a modest field of 35 V cm −1 . Conversely, conventional solid-state preparation procedures typically require two thermal treatments at temperatures around 1000 ◦C during hours, what often leads to sample inhomogeneity due to Bi volatilization. Reducing the temperature and length of the preparation method by RFS allows achieving pure perovskite structure without secondary phases nor impurities. With a 33% content in BaTiO 3 , the structure lies in a MPB region where a rhombohedral R3c phase coexist with a pseudocubic phase. The features of the RFS synthesis method, which greatly reduce the time the material is subjected to high temperatures, lead to a peculiar microstructure of the flashed ceramics. A bimodal grain size distribution with large 3 μ m grains coexisting with small 0.4 μ m grains is obtained. The electrical properties are largely in line with previous results, what validates RFS as energy and time saving technique for the synthesis and sintering of complex ceramics. Moreover, a comparison of the electrical energy consumption during the RFS and conventional preparation method was performed. It is demonstrated that the energy consumption is almost six times reduced, as compared with the conventional method, when RFS is used for the preparation of the 0.67BiFeO 3 -0.33BaTiO 3 ceramic. Fig. 7. Impedance data for BFO-33BT obtained by reaction flash sintering: (a) Nyquist plot, (b) Arrhenius plot of total electrical conductivity, (c) spectroscopic plots of imaginary components of impedance, Z ′′, and modulus, M′′ at 380 ◦C and (d) capacitance versus frequency graph. Fig. 8. P–E hysteresis loop and strain curve of BFO-33BT sample prepared by RFS. Table 1 Remnant polarization (P r ), coercive field (E c ) and strain max (S max ) for BFO- 33BT prepared by reaction flash sintering. The piezoelectric coefficient (d 33 ), dielectric constant ( ε ′) and loss ( ε ’’) at room temperature are also included. Sample P r ( μ C/cm 2 ) E c (kV/cm) S max (%) d 33 (pC/N) ε ′ ε ′′ BFO- 33BT 3.0 24 0.034 30 620 0.07 A. Taibi et al. Ceramics International 47 (2021) 26947–26954 26953 Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work has been supported by the Spanish Government Agency Ministerio de Economıa y Competitividad and FEDER (project CTQ2017–83602-C2–1-R) and by Junta de Andalucıa-Consejerıa de Economıa, Conocimiento, Empresas y Universidad and FEDER (projects P18-FR-1087 and US-1262507). Financial support from project 201960E092 (INTRAMURAL-CSIC) is also acknowledged. A. Taibi would like also to thank the Direction G´ en´ erale de la Recherche Scientifique et du D´ eveloppement Technologique (DGRSDT) of the Algerian Ministry of Higher Education (MESRS) for his PhD scholarship. References [1] V.V. 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