Synthesis-phase–composition relationship and high electric-field-induced electromechanical behavior of samarium-modified BiFeO3ceramics

Acta Materialia, Volume 83, 15 January 2015, Pages 149–159.

Julian Walker1,3, Peter Bryant2, Valsala Kurusingal2, Charles Sorrell1, Danjela Kuscer3, Goran Drazic4, Andreja Bencan3, Valanoor Nagarajan1, Tadej Rojac3

 

  1. The School of Materials Science and Engineering, University of New South Wales, Sydney, Australia and
  2. Thales Australia, Rydalmere, Australia and
  3. Electronic Ceramics Department, Jozef Stefan Institute, Jamova Cesta, 39, Ljubljana SI-1000, Slovenia and
  4. Laboratory for Materials Chemistry, National Institute of Chemistry, Ljubljana, Slovenia.

 

Abstract

Solid-state (non-activated) and mechanochemical activation (activated) synthesis methods were used to produce Sm-modified BiFeO3 ceramics of composition Bi0.88Sm0.12FeO3. The first part shows that the formation of Bi0.88Sm0.12FeO3 using the two synthesis methods followed a different reaction pathway on annealing the powders. The non-activated ceramics reacted by forming two intermediate phases, isostructural to BiFeO3 and SmFeO3, and then inter-diffusing, forming the final Bi0.88Sm0.12FeO3 solid solution. Unlike the non-activated samples, the activated ceramic powders formed Bi0.88Sm0.12FeO3phase on annealing the powders, without apparent intermediate phases. As revealed by transmission electron microscopy, the non-activated reaction pathway caused the Pbam phase to form as chemical inhomogeneous (Sm-rich) isolated nano-sized grain inclusions in the final ceramics. Conversely, the activated reaction pathway caused the Pbam phase to form chemically homogeneous nano-regions within the R3c phase grains. The results demonstrate the important role of processing in the appearance of the frequently discussed anti-polar Pbam phase in this system. In the second part, the high electric-field-induced polarization and strain behaviors of these ceramics were studied by means of polarization–electric (P–E) and strain–electric field (S–E) hysteresis loops, and the S–E loops were compared with those of unmodified BiFeO3. Bipolar S–E loops of Bi0.88Sm0.12FeO3 had a distinctive butterfly shape with less frequency dependence relative to BiFeO3 at driving-field frequencies of 0.1–100 Hz. BiFeO3 ceramics exhibite strong driving electric-field-frequency-dependent domain switching, the origins of which were previously attributed to a domain-wall pinning mechanism and “hardening” behavior. This study shows that Sm-modification induces a “hardening–softening” transition in BiFeO3 ceramics.

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Significance statement

Bismuth ferrite (BiFeO3 or BFO) based piezoelectric ceramics have proven to be particularly challenging from a synthesis perspective, with a high sensitivity to impurity elements and both narrow composition and temperature ranges for stabilization of the desired piezoelectric perovskite phase. While many studies probing the influence of both temperature and impurity elements on the ceramic phase composition exist, there is little discussion in the literature regarding the critical importance of the synthesis methodology and the impact of specific processing methods on the ceramic´s properties. This work provides a detailed case study of two different synthesis methods, one conventional and another non-conventional, and the resulting ceramics, providing much needed insight into the specific links between the synthesis method, the reaction process and the final ceramic properties.

Here we present a systematic comparison of two synthesis methods for samarium (Sm)-modified BFO ceramics (BSFO); solid state synthesis and mechanochemical activation assisted synthesis. The results demonstrate that mechanochemical activation used in conjunction with a reaction sintering technique for BSFO can improve the phase purity and the chemical homogeneity of the ceramics by reducing the formation of secondary phases at low temperatures and reducing the reaction dependence on long distance diffusion in order to reduce chemical gradients. This is achieved by the reduction of crystallite size, the amorphization of crystal structures and the homogeneous mixing of oxide powders during the mechanochemical activation process. These improvements represent key areas that influence the reproducibility of ceramics and thus are of pivotal importance to industrial scale processing.

In addition to the key insights provided regarding processing, the work addresses the influence of Sm addition on the macroscopic strain-electric field hysteresis loop behavior, relative to unmodified BFO ceramics. Interestingly, Sm appears to induce a kind of ˝hardening-softening˝ transition. The observation of such ˝softening˝ behavior in BSFO is particularly interesting as the Sm3+-Bi3+ substitution is isovalent. Similar softening behavior observed in the widely used Pb(Zr,Ti)O3 (PZT) system by doping with Nb2O3, is believed to be related to defect concentration which is altered by the aliovalent substitution with Nb2O3. Thus, while this study does not divulge the precise mechanisms for the ˝softening˝ behavior in BSFO ceramics, the data does suggest that a different mechanism, relative to that proposed for the PZT system, is likely to underpin the fundamental change in the behavior.

Synthesis-phase-composition relationship and high electric-field-induced electromechanical behavior of samarium-modified BiFeO3 ceramics

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