Effect of thermal depolarization on the poling‐induced domain texture and piezoelectric properties in Mg‐doped NBT‐6BT

Recently, poled Na₀.₅₀Bi₀.₅₀TiO₃‐BaTiO₃ (NBT‐BT)‐based polycrystalline materials have been characterized as possessing a high degree of poling‐induced domain texture in their remanent state. This finding is suggested to be the reason for their stable mechanical quality factor at high‐vibration velocity, making them promising candidates for high‐power applications. The materials in consideration are prone to self‐heating and thermal run‐away, particularly at slightly elevated temperatures. Therefore, this paper evaluates the temperature dependence of the poling‐induced domain texture of (Na₀.₄₇Bi₀.₄₇Ba₀.₀₆)TiO₃ (NBT‐6BT) doped with 0.5 mol% of Mg as compared to undoped NBT‐6BT. Its influence on small‐signal, large‐signal, and high‐power properties was investigated. To obtain a fundamental understanding of crystal structure, in‐situ synchrotron measurements were conducted as function of temperature to establish a relationship between structure and piezoelectric properties of both Mg‐doped and undoped NBT‐6BT materials

Designing properties of (Na1/2Bix) TiO3-based materials through A-site non-stoichiometry

Point defects largely determine the properties of functional oxides. So far, limited knowledge exists on the impact of cation vacancies on electroceramics, especially in (Na1/2Bi1/2)TiO3 (NBT)-based materials. Here, we report on the drastic effect of A-site non-stoichiometry on the cation diffusion and functional properties in the representative ferroelectric (Na1/2Bi1/2)TiO3–SrTiO3 (NBT–ST). Experiments on NBT/ST bilayers and NBT–ST with Bi non-stoichiometry reveal that Sr2+-diffusion is enhanced by up to six orders of magnitude along the grain boundaries in Bi-deficient material as compared to Bi-excess material with values of grain boundary diffusion B108 cm2 s 1 and B1013 cm2 s 1 in the bulk. This also means a nine orders of magnitude higher diffusion coefficient compared to reports from other Sr-diffusion coefficients in ceramics. Bi-excess leads to the formation of a material with a core–shell microstructure. This results in 38% higher strain and one order of magnitude lower remanent polarization. In contrast, Bi-deficiency leads to a ceramic with a grain size six times larger than in the Bi-excess material and homogeneous distribution of compounds. Thus, the work sheds light on the rich opportunities that A-site stoichiometry offers to tailor NBT-based materials microstructure, transport properties, and electromechanical properties.T. F., A. A., and K. G. W. gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft under WE 4972/2 and FR 3718/1-1. T. F. thanks Dr Edvinas Navickas for his help with the ToF-SIMS measurements. M. A. acknowledges the support of the Feodor Lynen Research Fellowship Program of the Alexander von Humboldt Foundation. M. D. and L. M.-L. acknowledge financial support from the Hessen State Ministry of Higher Education, Research and the Arts via LOEWE RESPONSE. L. M.-L. acknowledges financial support from DFG Grant MO 3010/3-1

Na1/2Bi1/2TiO3-Based Piezoceramics for High-Power Applications

In addition to the need for the efficient use of resources, the toxicity of lead for humans and the environment has led to a broad research field for lead-free ferroelectric materials and triggered the development of new concepts and technologies. The use of lead-based ceramics is regulated by the Restriction of Hazardous Substances (RoHS) in the European Union (EU). PZT has an exception from the RoHS directive until a suitable replacement is found. Despite the efforts made in the past few decades, PZT remains the most widely used material system, and it seems there is no single material system that can completely replace it. Individual solutions for specific applications continue to emerge, and some material systems even outperform PZT in certain applications. Therefore, there is a need for further research to develop and optimize alternative lead-free materials with desirable piezoelectric properties while addressing the environmental and health concerns associated with the use of lead-based ceramics. This work addresses the challenges associated with (Na1/2Bi1/2)TiO3-based (NBT) materials and establishes their potential as replacements for PZT in high-power applications (high-power refers to electrical excitation and operation in mechanical resonance at high vibration velocity). NBT-based materials, such as (Na1/2Bi1/2)TiO3-xBaTiO3 (NBT-xBT) and (Na1/2Bi1/2)TiO3-x(K1/2Bi1/1)TiO3 (NBT-xKBT), have been identified as promising candidates due to their stable mechanical losses with increasing output power, in contrast to PZT, which exhibits strongly increasing mechanical losses under the same conditions. Despite these advantages, there remain several challenges associated with NBT-based materials, including a lack of mechanistic understanding and unknown issues regarding the transfer from laboratory to real-world applications. Therefore, the following questions are addressed: • What properties are crucial for the use in high-power applications, and why? • How do the material properties change when measured under application-like conditions? Systematic chemical modifications of the NBT-xBT and NBT-xKBT systems are discussed regarding their electromechanical properties, such as the piezoelectric coefficient, coupling factor, and different losses, including dielectric and electromechanical losses. This discussion includes their evaluation regarding temperature stability. A general doping strategy is established for the NBT-xBT and NBT-xKBT systems, enabling for a mechanistic discussion and comparison with doping in systems such as PZT and BaTiO3 (BT). In addition to the optimization and mechanistic discussion, the NBT-based materials are evaluated and classified for use in high-power applications. This process involves fatigue measurements and comparison with a current PZT standard material. Finally, a theory is developed and proposed to explain the underlying mechanism of why NBT-based materials have low and stable extrinsic contributions with increasing output power. Therefore, the following questions have not been adequately answered up to now and are a major part of this work: • Is there a general guideline for designing the properties of NBT-based materials for high-power applications, and what are the underlying physical mechanisms it is based on? • How do the long-term performances of NBT-based alternatives compare with currently used standard materials? • What is the origin of the exceptional stable extrinsic contribution to the strain against the increasing vibration velocity of NBT-based materials

Influence of Zn2+ doping on the morphotropic phase boundary in lead-free piezoelectric (1 – x)Na1/2Bi1/2TiO3-xBaTiO3

A series of morphotropic phase boundary (MPB) compositions of (1–x) Na1/2Bi1/2TiO3-xBaTiO3 (x = 0.05, 0.055, 0.06, 0.065, 0.07), with and without 0.5 mol% Zn-doping was synthesized using the solid-state route. The samples were characterized using X-ray diffraction, dielectric analysis, and electromechanical measurements (piezoelectric d33 coefficient, coupling factor kp, mechanical quality factor Qm, and internal bias field Ebias). The increase in the ferroelectric-relaxor transition temperature upon Zn-doping was accompanied by a shift of the MPB toward the Na1/2Bi1/2TiO3-rich side of the phase diagram. Higher tetragonal phase fraction and increased tetragonal distortion were noted for Zn-doped (1 – x)Na1/2Bi1/2TiO3-xBaTiO3. In addition, ferroelectric hardening and the presence of an internal bias field (Ebias) were observed for all doped compositions. The piezoelectric constant d33 and the coupling coefficient kp decreased by up to ∼30%, while a 4- to 6-fold increase in Qm was observed for the doped compositions. Apart from establishing a structure–property correlation, these results highlight the chemically induced shift of the phase diagram upon doping, which is a crucial factor in material selection for optimal performance and commercialization

Effect of thermal depolarization on the poling-induced domain texture and piezoelectric properties in Mg-doped NBT-6BT

Recently, poled Na0.50Bi0.50TiO3-BaTiO3 (NBT-BT)-based polycrystalline materials have been characterized as possessing a high degree of poling-induced domain texture in their remanent state. This finding is suggested to be the reason for their stable mechanical quality factor at high-vibration velocity, making them promising candidates for high-power applications. The materials in consideration are prone to self-heating and thermal run-away, particularly at slightly elevated temperatures. Therefore, this paper evaluates the temperature dependence of the poling-induced domain texture of (Na0.47Bi0.47Ba0.06)TiO3 (NBT-6BT) doped with 0.5 mol% of Mg as compared to undoped NBT-6BT. Its influence on small-signal, large-signal, and high-power properties was investigated. To obtain a fundamental understanding of crystal structure, in-situ synchrotron measurements were conducted as function of temperature to establish a relationship between structure and piezoelectric properties of both Mg-doped and undoped NBT-6BT materials

Texture-based ferroelectric hardening in Na1/2_{1/2}Bi1/2_{1/2}TiO3_3 -based piezoceramics

Na$_{1/2}$Bi$_{1/2}$TiO$_3$-based (NBT-based) ceramics offer a viable option to replace lead-based materials for high-power applications as they are characterized by a stable mechanical quality factor with increasing vibration velocity in comparison to lead-based piezoceramics. Recently, the minor and stable extrinsic contributions were revealed as the origin for the stability of the mechanical quality factor with increasing vibration velocity. This work identifies the very unusual high poling degree as cause for the small extrinsic contributions. To this end, complete pole figure densities have been quantified and correlated to the piezoelectric coefficient and electromechanical quality factor. This hypothesis is further strengthened by correlating the piezoelectric constant (sum of intrinsic and extrinsic contributions) with the remanent polarization (correlates to remanent texturing degree). In order to assess a full picture of NBT-based piezoceramics, 0.94Na$_{1/2}$Bi$_{1/2}$TiO$_{3−0.06}$BaTiO$_3$ has been considered with and without Zn doping and with quenching. It is compared to 0.79Na$_{1/2}$Bi$_{1/2}$TiO3−0.21K$_{1/2}$Bi$_{1/2}$TiO$_3$ with and without Mg doping. Finally, a contrast to soft Pb(Zr$_{1/2}$Ti$_{1/2}$)O$_3$ (PZT) flushes out the impact of domain wall motion on the piezoelectric coefficient and the electromechanical quality factor. Whereas a PZT-based reference material exhibits a linear increase in the piezoelectric constant with increasing remanent polarization, the NBT-based materials deviate from the linear trend, indicating a decrease in extrinsic contributions

Designing properties of (Na1/2Bix)TiO3-based materials through A-site non-stoichiometry

Point defects largely determine the properties of functional oxides. So far, limited knowledge exists on the impact of cation vacancies on electroceramics, especially in (Na1/2Bi1/2)TiO3 (NBT)-based materials. Here, we report on the drastic effect of A-site non-stoichiometry on the cation diffusion and functional properties in the representative ferroelectric (Na1/2Bi1/2)TiO3–SrTiO3 (NBT–ST). Experiments on NBT/ST bilayers and NBT–ST with Bi non-stoichiometry reveal that Sr2+-diffusion is enhanced by up to six orders of magnitude along the grain boundaries in Bi-deficient material as compared to Bi-excess material with values of grain boundary diffusion ∼10−8 cm2 s−1 and ∼10−13 cm2 s−1 in the bulk. This also means a nine orders of magnitude higher diffusion coefficient compared to reports from other Sr-diffusion coefficients in ceramics. Bi-excess leads to the formation of a material with a core–shell microstructure. This results in 38% higher strain and one order of magnitude lower remanent polarization. In contrast, Bi-deficiency leads to a ceramic with a grain size six times larger than in the Bi-excess material and homogeneous distribution of compounds. Thus, the work sheds light on the rich opportunities that A-site stoichiometry offers to tailor NBT-based materials microstructure, transport properties, and electromechanical propertie