Influence of Structural Phase Transitions on the Mechanical Behavior of Lead-Free Potassium Sodium Niobate Piezoceramics
Final Report Abstract
The focus of this project was understanding the role of stress on the electromechanical properties and stable phase of KNN-based materials, especially the role of stress on materials with chemical substitutions used to tune the R-O, O-T, and T-C phase boundaries. These materials were selected as previous work has shown a significant enhancement near the lower temperature interferroelectric phase boundary, which was suggested to be due to the formation of broad mixed phase region and the eventual formation of a R-T boundary. Importantly, stress-modulated hysteretic remanent processes, such as domain and phase boundary motion, are critical for applications that applied external mechanical fields as well as the mechanical reliability of ferroelectrics. Here, the fracture behavior strongly impacts the mechanical stability. As such, studies in this project on the influence of mechanical properties of KNN-based ceramics were concentrated into three major areas: (i) stress-dependent macroscopic small and large-signal properties, (ii) in situ stress-dependent structural investigations, and (iii) fracture behavior. One of the major unforeseen limitations encountered during this project was the difficulty scaling up the sample volume of KNN-BLT-BZ. Here, numerous batches were produced in China and sent to FAU, where the persistent issue of porosity and microcracking limited the maximum stress and electric field that could be applied. Chipping, fracture, and sample failure were routinely observed, even at moderate stresses. In order to address the primary scientific questions of the project, Li-doped KNN was also investigated in collaboration with Prof. Kakimoto, which did not display these issues. Despite this, however, all of the major goals of the project were met or exceeded as the influence of stress on the phase boundaries in KNN, in particular the interferroelectric phase transitions, could be directly investigated. Temperature- and stress-dependent small signal piezoelectric properties revealed a maximum response near the polymorphic phase boundary that displayed a decay with increasing temperature and eventual loss of piezoelectric response at the depolarization temperature. With increasing stress, however, the 푑33 in this region was observed to be highly stress-sensitive due to enhanced extrinsic contributions. In contrast, temperature-dependent ferroelastic measurements of KNN-BLT-BZ did not reveal any specific anomaly indicating variations in the extrinsic contributions, thereby suggesting that the observed changes were from destabilized domain structures. In order to address this, in situ stress-dependent synchrotron and Raman spectroscopy were performed to directly observe the influence of stress on the stable crystal phase. During the synchrotron studies, both KNN-BLT-BZ and LKNN ceramics were investigated, although due to the mechanical stability of the KNN-BLT-BZ samples that limited the maximum stress to approximately -100 MPa, the initial analysis was based on LKNN with the data on the stress-dependent KNN-BLT-BZ crystal structure currently being prepared for publication. Importantly, however, LKNN ceramics also show an interferroelectric phase boundary near room temperature, which allowed us to complete the primary goals of the project. As part of this study, for the first time, a compressive stress induced structural phase transformation was observed in KNN. This is an important finding, as the observed ferroelastic hysteresis loop, which has been until now understood to be solely due to ferroelasticity, is actually a combination of both effects and that the resulting material after loading has a different phase content than in the unloaded state. This is expected to affect numerous factors, such as electromechanical behavior, internal residual stresses, and the frequency response. In addition, to extend these measurements beyond what was proposed in the project description, researchers at FAU developed a heating arrangement for the compact loading frame to be used to directly characterize the stressdependent Raman spectra of KNN-based materials up to 175 °C. Here, the stress-modulated phase transformation found in XRD data was also observed using Raman spectroscopy for the first time. With increasing temperature and corresponding distance from the interferroelectric phase transition, it was found that stress was no longer able to induce a structural phase transformation. And lastly, the crack resistance of unpoled and electrically poled KNN-BLT-6BZ was investigated using three techniques to determine the crack tip toughness, short crack and long crack resistance, and the non-linear stress-strain behavior. Their relative magnitudes were found to be consistent with each other and could be rationalized when considering their temperature dependence and texture evolution during electric field poling. Experimental values from compression and electric field poling tests were used in a predictive model that was able to rationalize the obtained temperatureand texture-dependence. KNN-BLT-6BZ was found to have fracture toughness values comparable to soft PZT. It has been verified to be tougher than BCT-BZT, but considerably less tough than NBT-BT. In summary, despite the initial issues with mechanical stability, the project met all of the primary research goals related to investigating the influence of stress on the interferroelectric phase boundary. In addition to advancing the state-of-the-art in understanding stress-dependent electromechanical response of KNN-based materials, two novel experimental arrangements were developed, that are highly useful for future studies of functional ceramics and of potential commercial importance.
Publications
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“Temperature dependent fracture toughness of KNN-based lead-free piezoelectric ceramics”, Acta Mater 174, 369 (2019)
Li YW, Liu YX, Ochsner PE, Isaia D, Zhang YC, Wang K, Webber KG, Li JF, Rodel J
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“Electric-field-induced strain of (Li,Na,K)NbO 3-based multilayered piezoceramics under electromechanical loading”, J Appl Phys 128 (2020)
Nishiyama H, Martin A, Hatano K, Kishimoto S, Sasaki N, Khansur NH, Webber KG, Kakimoto KI
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“Fracture behavior in electrically poled alkaline bismuth- and potassium-based lead-free piezoceramics using Vickers indentation”, Scripta Mater, 194, 113647 (2020)
Wang X, Lalitha KV, Tan C and Li YW
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“High temperature piezoelectric response of polycrystalline Li-doped (K,Na)NbO3 ceramics under compressive stress”, J Appl Phys 127 (2020)
Martin A, Khansur NH, Eckstein U, Riess K, Kakimoto K, Webber KG
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“Stress-modulated optimization of polymorphic phase transition in Li-doped (K,Na)NbO3”, Appl Phys Lett 117 (2020)
Khansur NH, Martin A, Riess K, Nishiyama H, Hatano K, Wang K, Li JF, Kakimoto K, Webber KG
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“Alkali volatilization of (Li,Na,K)NbO3-based piezoceramics and large-field electrical and mechanical properties”, J Ceram Soc Jpn 129, 127 (2021)
Nishiyama H, Martin A, Hatano K, Kishimoto S, Sasaki N, Webber KG, Kakimoto K
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“Deformation and bending strength of highperformance lead-free piezoceramics”, J Am Ceram Soc 105, 3128 (2022)
Lalitha KV, Isaia D, Gong W, Wu C, Li YW, Rödel J
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“In situ combined stress- and temperature-dependent Raman spectroscopy of Li-doped (Na,K)NbO3”, J Am Ceram Soc 105, 2735 (2022)
Khansur NH, Eckstein U, Bergler M, Martin A, Wang K, Li JF, Cicconi MR, Hatano K, Kakimoto K, de Ligny D, Webber KG
