Electrical features of ferroelectric (Ba0.83Ca0.17)TiO3 ceramics with diffused phase transition under pressure

https://doi.org/10.1016/j.jallcom.2020.158216Get rights and content

Highlights

  • (Ba0.83Ca0.17)TiO3 ceramics exhibited chemical and structural local disorder.

  • Local disorder induced the diffusion of ferroelectric phase transition in BCT.

  • Hydrostatic pressure influenced on ferroelectric phase transition.

  • BCT showed high permittivity and piezoelectric features.

  • BCT enables energy storage for application.

Abstract

Ferroelectric (Ba0.83Ca0.17)TiO3 ceramics was sintered by the solid state reaction method. The material was sintered in three steps that allowed remarkably modifying structure and electrical features important from an application point of view. Major tetragonal phase of the material was detected using X-ray diffraction study. The scanning electron microscope test showed fluctuation in chemical composition. The ferroelectric to paraelectric phase transition was diffused and markedly shifted to lower temperature, TC = 339 K, respectively to barium titanate. Moreover, it showed the pressure sensitivity dTC/dp = −37 K/GPa. These effects were attributed to local disorder both in Ba/Ca and Ti sublattices. The high magnitude dielectric permittivity, ε′~1000–2000, was observed in 200–400 K temperature range. The net polarization, which was estimated from the pyroelectric effect, decreased from ~3 µC/cm2 to zero in ~100–400 K range. The ceramics displayed the piezoelectric coefficient, d33 = 58 pC/N, the spontaneous polarization, PS = 11–14 µC/cm2, the remnant polarization, PR~2 µC/cm2, the coercive field, EC~1.5–3 kV/cm, and the energy density storage efficiency factor, ξ ≈ 33%, at room temperature.

Introduction

Ferroelectric (FE) materials have attracted considerable attention as possible candidates for a wide variety of applications in microelectronics, energy harvesting, microwave and memory devices. The Pb-based FE materials belonged to a large class of technologically important materials showing the perovskite ABO3 structure [1], [2], [3]. However, despite of their outstanding FE and/or piezoelectric properties, these materials are not to meet requirements in the face of global environmental problems [4], [5], [6], [7], [8], [9], [10]. Thus, there is an urgent need to develop Pb-free materials with the same properties to overcome this problem. One of the aims is enhancing the dielectric permittivity magnitude in large temperature range around room temperature by modifying features of the FE-paraelectric (PE) phase transition. This task can be gained by doping in the A and B sites. However, a breakdown of the long-range ferroelectric order may appear in doped ferroelectric materials. Such an effect would correspond to local changes in the crystal lattice symmetry. Consequently, either transition from FE to PE phase diffuses or relaxor state could be observed. It is worth mention that local strain-stress effects are expected and they also contribute the phase transition behaviour. The diffused phase transitions (DPT) from FE to PE phase, which corresponds to broadening of the dielectric permittivity peak, is observed for Pb-free solid solutions and disordered (A′A″)BO3 structures [1], [2], [11], [12], [13].

As a classic perovskite-type FE material, barium titanate, BaTiO3 (BT), was extensively investigated so far. This compound was studied because of its attractive dielectric, ferroelectric, and piezoelectric features [2], [3], [14], [15], [16]. We would like to point that isovalently doped BT compounds also showed promising and remarkable features. For instance, effects of Ca2+ ions substitution in BT crystal lattice were reported in many works [17], [18], [19], [20], [21], [22], [23], [24]. Ca2+ ions provided more stable tetragonal structure in wider temperature range and enhanced FE features, in comparison to pure BT properties [17], [25]. Therefore, physical properties of (Ba1-xCax)TiO3 (BCT) would find applications in many industrial and engineering fields, e.g., multilayer capacitors, piezoelectric actuators, and high density energy storage devices [3], [6], [17], [25], [26].

BCT was produced in form of crystals [25], thin films [27], [28], and ceramics [17], [18], [19], [20], [21], [22], [23], [24], which can exhibit different properties. The addition of Ca2+ ions to BT influenced its phase transitions. Good correspondence between experimentally determined sequence of phase transition and the prediction of the Landau-Ginzburg-Devonshire phenomenological model was obtained [29], [30]. The transition from PE cubic Pm3¯m to FE tetragonal P4mm, next to orthorhombic Amm2, and then to rhombohedral R3m phase occurred in BCT solid solution when x < 0.25 [17]. The transition between PE and FE phase, which occurred at TC ≈ 400 K, was affected by the Ca2+ ions. However, different intriguing tendencies were determined, since TC shifted either downward [17], [25], [31] or upward [32], [33]. More precisely, these controversies about the TC shift were addressed to Ca substitution site in crystal lattice [31]. In case of Ca doping in the Ba sublattice, TC slightly increased, reached maximum, and then decreased with doping concentration [19]. In contrary, the Ca doping in Ti sublattice resulted in marked drop of TC [34]. In order to relate the shift of TC with disorder in the Ba/Ca sublattice, a model considering the ionic radii was proposed [35]. Moreover, the temperature of transitions between low temperature phases, tetragonal to orthorhombic (FE-FE) and orthorhombic to rhombohedral markedly decreased with increasing Ca content [17], [25]. Occurrence of the additional phase showing different symmetry was reported for the high Ca content, x > 0.14, beyond of solubility limit [17], [23], [26].

The effect of DPT, deduced from dielectric permittivity studies of BCT ceramics, was related to non-stoichiometry and/or local structural disorder [17], [20], [24], [33], [36], [37]. It was shown that the occurrence of oxygen vacancies affected the stability of the BCT crystal lattice. This effect corresponded to site occupancy of Ca2+ ions. Isovalent substitution of Ca2+ ions to Ba2+ ions sublattice, CaBa, dominated. However, minor substitution of Ca2+ ions to Ti4+ ions sublattice, CaTi, which required charge compensation via oxygen vacancies, VO, also was determined [21], [22], [31]. Residual strain-stress effects also were considered because of large difference between ionic radii of Ba2+ and Ca2+ ions. For instance, influence of stress could manifest itself at interfaces between Ba- and Ca-rich areas. Such micro-heterogeneity would be responsible for the detected relaxor features attributed to dipolar glass [18], [20], [24], [28], [38], [39]. Moreover, it should be mentioned that the cationic ratio, α = (Ba + Ca)/Ti markedly affected diffusivity and occurrence of the FE-PE phase transition shifted downward by several dozens of centigrade [36].

In literature for BCT ceramics, the relationship between grain morphology and dielectric permittivity was scarcely described. Addition of Ca2+ ions to BT influenced shape of the grains and decreased its size [17], [36]. Moreover, a role of internal stress appearing at boundaries of fine grain (average size of ~0.2 µm) of the Ca-doped BT, was proposed as a mechanism responsible for the dielectric permittivity and resistance magnitude change [40]. The grain size influenced not only magnitude of dielectric permittivity but also the shift of the FE-PE phase transition [41], [42], [43]. Moreover, the FE-PE transition in BT ceramics shifted toward lower temperature when hydrostatic pressure was applied in 0–2 GPa range, that is, the TC shifted with rate −40.0 and −34.3 K/GPa when grain size decreased from 5000 to 60 nm [41]. It can be presumed that similar combined influence of the grain size and pressure induced stress-strain effects would be observed for the BCT ceramics.

As already mentioned, electro-mechanical features of BCT materials were mainly studied from application point of view, particularly due to the possibility of their using for miniaturization of electronic components in electronic devices. We would like to point that FE properties of BCT, in comparison to the reference BT, were improved by the Ca ions doping. The piezoelectric d33 coefficient was detected using local piezoresponse force microscopy test [19]. Low dielectric losses, tanδ ~0.01, which were determined for BCT [17], [33] corresponded to high resistivity in vicinity of room temperature, typical for the FE insulators. However, BCT exhibited the thermally generated electrical conductivity in high temperature range, presumably related to the oxygen non-stoichiometry. The activation energy of electrical conductivity, Ea, which lay in the range from ~0.9–0.5 eV was estimated for 340–700 K temperature range, for concentration x = 0.23 [33], [44]. The high magnitude of real part of dielectric permittivity, ε', which varied in 1000–5000 range in vicinity of room temperature and reached ~10000 at phase transition, that is crucial for capacitor construction. The non-stoichiometric BCT samples showed lower ε' magnitude of ~2000–3000 in 250–450 K range with broad maximum in vicinity of 320–350 K [36]. These features dependence on sintering and annealing conditions, actual composition, and measuring field frequency was reported [9], [17], [18], [19], [20], [23], [25], [36].

In this work, we carried out studies of perovskite ceramic (Ba0.83Ca0.17)TiO3 sintered by the solid state reaction method. We chose this material because its high magnitude of real part of complex dielectric permittivity and occurrence of the optimum FE features. Consequently, we tried obtain dielectric, piezoelectric, and FE properties to be accessible in the convenient for applications temperature range, i.e., 200–400 K. Therefore, we modified the technology of preparation of the BCT ceramics in aim to obtain specific electrical properties [17], [24], [36], [45], [46]. However, when a ferroelectric material like BT is doped with Ca atoms, a breakdown of the long-range ferroelectric order may appear due to local disorder and chemical composition non-homogeneity. Therefore, a crossover from ferroelectric phase to relaxor state would occur for high substitution level that is the case of the (Ba0.83Ca0.17)TiO3 studied herein. Indeed, the behaviour of permittivity in vicinity of the low temperature tetragonal to orthorhombic phase transition in the BCT (in 150–250 K range) was attributed to the FE relaxor properties [47]. After that, a manifestation of relaxor features, ascribed to random distribution of electrical and strain fields caused by the Ca doping, was reported for the FE-PE transition [20]. On the other hand, it would be noted that homo-valence substitution at the Ba site did not result in the appearance of a relaxor state even for large Sr concentrations in BaTiO3 [48]. It should be noticed that although appearance of the relaxor state was predominantly ascribed to electrical charge imbalance, e.g., induced by hetero-valence doping, the mismatch of ionic radii causing deformation of the octahedral, also can be taken into consideration [49].

In our work, we determined whether, and to what degree, the FE properties could be maintained in BCT, material, which showed markedly diffused dielectric anomaly related to local non-stoichiometry Electrical polarization, piezoelectric property and pyroelectric effect, and P-E polarization loop features studies were conducted to confirm occurrence of the ferroelectric features. We checked if performed technology would induce such features of the FE-PE transition, which also could show inclination to the ferroelectric relaxor features appearance. Moreover, the sensitivity of its electrical features on applied hydrostatic pressure would be interesting from electro-mechanical application point of view. We presumed relation between structural features and stability of FE ordering of crystal lattice. Such interrelated effects were verified using X-ray diffraction, scanning electron microscopy, and dielectric spectroscopy methods. Moreover, the sensitivity of BCT ceramics on applied pressure, the strain-stress effect, and the possibility of energy storage were studied and discussed.

Section snippets

Ceramics preparation

The (Ba0.83Ca0.17)TiO3 ceramics was obtained via high temperature solid state reaction method from of oxide and carbonates (Aldrich Sigma, purity ≥ 99%), at ambient air conditions, modified in respect to literature reports [17], [24], [36], [45], [46]. However, the material was sintered in three steps instead of conventional calcination followed by one sintering. A Mixing Mill (Retsch MM200) was applied for blending powders of BaCO3, CaCO3, and TiO2. The raw material was dry milled at 11.5 Hz

X-ray diffraction

The studied ceramics was almost one-phase. The XRD pattern was used to identify the actual phases. The ceramics consisted of a BCT main phase (~98%) and small amounts of two other phases (~2%). The main phase showed the tetragonal P4mm symmetry at room temperature (indices in Fig. 1a). In accordance to the Rietveld fitting performed we noted that actual composition, (Ba0.82Ca0.18)TiO3, differed slightly from the nominal (Ba0.83Ca0.17)TiO3 one. The refined crystal lattice parameters determined

Summary and conclusion

In this work, we have studied the sintered BCT ceramic compound, with Ca content of 17%, received by the solid state reaction procedure, which was modified in comparison to literature data [17], [18], [19], [20], [21], [22], [23]. XRD test confirmed that almost one-phase composition (~98%) was formed in the studied ceramics. It should be noticed that performed sintering conditions resulted in occurrence of local non-stoichiometry or fluctuation in chemical composition detected by using SEM

CRediT authorship contribution statement

Andrzej Molak: Conceptualization, Dielectric spectroscopy: Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Data curation, Visualization. Antoni Winiarski: XRD: Methodology, Formal analysis, Investigation, Writing - review & editing. Anna Z. Szeremeta: Dielectric spectroscopy under pressure: Methodology, Formal analysis, Investigation, Writing - review & editing, Data curation, Visualization, Project administration. Dev Kumar Mahato:

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

The authors (A. Z. Szeremeta, S. Pawlus) are grateful for the financial support provided by the National Science Centre, Poland, within the framework of the Opus13 project (Grant No. DEC-2017/25/B/ST3/02321).

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