Dittmer, Robert (2013)
Lead-Free Piezoceramics – Ergodic and Nonergodic Relaxor Ferroelectrics Based on Bismuth Sodium Titanate.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung
Kurzbeschreibung (Abstract)
Numerous questions regarding the nature of BNT-derived relaxor ferroelectrics are addressed on the basis of a broad experimental foundation including electrical characterization, diffraction, piezoresponse force microscopy, and mechanical measurements. In order to check the general validity of the approach two materials were investigated: the model system (1-y)(0.94Bi1/2Na1/2TiO3-0.06BaTiO3)-yK0.5Na0.5NbO3 (BNT-6BT-100yKNN) and the newly developed (1-y)((1-x)Bi1/2Na1/2TiO3-xBi1/2K1/2TiO3)-yBiZn1/2Ti1/2O3 (BNT-100xBKT-100yBZT).
The field-dependent polarization and strain are demonstrated to bear phenomenological similarities to well-known relaxor ferroelectrics like PLZT or PMN. Compositions with zero y are hypothesized to be nonergodic relaxors featuring partially correlated polar nano-regions (PNRs) in a nominally pseudocubic matrix. These PNRs are the consequence of random fields due to a mixed A-site of the perovskite lattice, i.e., Bi3+ and Na+ in addition to Ba2+ or K+. This compositional disorder is associated with augmented random electric fields caused by charge disorder and random strain fields generated by different ion sizes. Upon application of a sufficiently high electric field the PNRs coalesce into ferroelectric domains that percolate the sample. Eventually, the poled material is macroscopically almost indistinguishable from conventional ferroelectrics, exhibiting a butterfly-shaped strain loop and rectangular polarization loop.
With y>0, additional heterovalent ions are incorporated into the A- and/or B-site, giving rise to enhanced random fields that increase the required threshold field necessary to induce long-range order. At a critical value of y stable long-range order cannot be induced and the material is eventually ergodic. High electric fields, however, still cause the growth of PNRs, which results in high electrostrictive coefficients as well as high maximum polarization and strain. There may still be a threshold field where a reversible formation of micron-sized domains occurs. This transition is reflected by a clear bend in P(E) loops, indicating a change in mechanism. The establishment of an ’unstable’ long-range order at high electric fields is observed for BNT-6BT-3KNN, but not for BNT-20BKT-4BZT for fields <6 kV•mm-1. While S(E) and P(E) saturate in the former case, the latter material may exhibit rapidly increasing large-field parameters even beyond 6 kV•mm-1. For y higher than the critical value, Pmax and Smax decay, which is attributed to the diminished volume fraction of electrically active polar regions. Consequently, a higher electric field is required to achieve the same polarization and strain.
The field-dependent small-signal parameters of piezoelectric constant d33(E) and permittivity εr;33(E) support this image. In nonergodic relaxor compositions, a stable piezoelectricity arises with the emergence of ferroelectric domains and the concurrent alignment thereof with respect to the electric field. At the same time, zero-field permittivity decreases, which is attributed to a diminished domain wall density. Compositions with elevated y, also referred to as ’incipient piezoceramics’, feature a sizable piezoelectric constant only under strong electric fields. Eventually, for a high concentration of heterovalent ions, d33 depends almost linearly on the electric field and permittivity is virtually field-independent.
Further insight into the relaxor properties is provided by diffraction. The initial structure is seemingly cubic on the average, which is ascribed to the small size of the polar regions below the coherence length of the XRD or NRD experiment. Consequently, PNRs do not contribute to the Bragg reflections in the diffraction pattern and the average symmetry appears non-polar, i.e., metrically cubic. For nonergodic relaxor compositions, the field-induced establishment of longrange order is demonstrated by a peak broadening and the emergence of notable non-cubicity. In the case of BNT-20BKT, the application of an electric field >3.7 kV•mm-1 induces tetragonal and rhombohedral distortions. A small addition (y=0.02) of BZT increases the threshold field, indicated by the reflection broadening setting in at 4.6 kV•mm-1. The ergodic BNT-20BKT-4BZT, on the other hand, remains pseudocubic, suggesting that the PNRs may grow but remain too small for detection. Therefore, diffraction is in line with large-signal measurements that likewise suggest the lack of ferroelectric domains.
The observation of pseudocubicity in the initial, unbiased state implies the absence of domains, which is confirmed by PFM measurements. It is concluded that PNRs have a size below the lateral resolution limit of the PFM, i.e., well below 10 nm. Also, the establishment of long-range order as suggested by in situ diffraction is directly observed after application of a DC tip bias. Inversion of the polarity of the DC voltage proved the capability for polarization reversal. As suggested by macroscopic measurements, the tip bias required for the formation and switching of domains is strongly affected by y, here representing the KNN content. Higher y results in a higher threshold and switching voltage. Interestingly, local switching loops can be obtained even for strongly ergodic relaxors such as BNT-6BT-18KNN, albeit only at highest electric fields. The polarization relaxation is furthermore reflected in a temporal decay of piezoresponse that obeys a stretched exponential function, confirming a broad distribution of relaxation times. This distribution of relaxation times is manifested in the frequency dependence of large-signal properties.
The pseudocubic structure is maintained up to high temperatures as demonstrated by temperatureinvariant X-ray and neutron diffraction patterns, which only feature an increase in lattice spacing due to thermal expansion. Nonlinearities in the temperature-dependent Young’s modulus Y(T) indicate that structural changes take place on a limited, local scale. In contrast to phase transitions as in PZT, where Y strongly increases within a narrow temperature range, the variations in Y(T) for the investigated lead-free materials are small and spread out across a broad temperature range of several hundreds of degrees centigrade. It is, therefore, concluded that only a fraction of the volume is affected. The PNRs transform into a high-temperature cubic phase, which displays a higher Young’s modulus. Owing to the random fields, the stability of the PNRs varies, which eventually gives rise to the observed wide temperature range of non-linearly varying Y . At the same time, the permittivity exhibits an intricate thermal evolution. A low-temperature frequency-dispersive shoulder in εr;33(T) indicates the slowing down of dipolar motion, associated with the distribution of PNR correlation length and accordingly distributed relaxation times. A peak at higher temperatures indicates a slight frequency dependence albeit inverted, i.e., higher frequencies cause a decrease in tanδ. Approaches to rationalize the εr;33(T) curve include aging, space charge relaxation or a frustrated domain state, where two PNR species are present and consequently two relaxation temperature ranges exist. None of these hypothesis can presently be rejected and it is likely that all three effects contribute to some extent to the overall dielectric response as a function of temperature.
The thermally induced depoling process is elucidated by contrasting in situ d33(T) measurements with high-temperature second-harmonic generation. Almost all piezoelectricity in poled nonergodic BNT-20BKT has vanished at 140 °C, while SHG measurements still yield non-zero intensity, proving the existence of residual polar volume. This finding suggests that the depoling process consists of two simultaneous contributions. On the one hand, a randomization of polarization vectors takes place, resulting in the disappearance of net polarization and piezoelectricity. On the other hand, domains break up into PNRs, which shrink upon further heating. This means that the polar volume is reduced with increasing temperature. Consequently, it is demonstrated by means of non-zero SHG intensity that polar volume in the form of PNRs exists not only far beyond depolarization temperature, but also in the unpoled and the ergodic relaxor state.
Both lead-free BNT-based material systems are demonstrated to excel in certain applications. Under high electric fields, the strain ratio Smax•Emax-1 surpasses even soft PZT. Moreover, the achievable maximum stress, termed blocking stress, can be up to 60 % higher. Both the large strain and the high blocking stress are beneficial for actuator applications. Moreover, the large and almost temperature-insensitive permittivity for BNT-6BT-100yKNN with high y depicts an attractive starting point for the development of high-capacity, high-temperature capacitor materials. Such a capacitor is, for example, required for automotive applications, where power converters require charge storage for power conditioning at high temperatures. These specialized, tailored solutions may not only reduce the amount of hazardous substances in consumer products but also broaden the horizons of today’s technology.
Typ des Eintrags: | Dissertation | ||||
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Erschienen: | 2013 | ||||
Autor(en): | Dittmer, Robert | ||||
Art des Eintrags: | Erstveröffentlichung | ||||
Titel: | Lead-Free Piezoceramics – Ergodic and Nonergodic Relaxor Ferroelectrics Based on Bismuth Sodium Titanate | ||||
Sprache: | Englisch | ||||
Referenten: | Rödel, Prof. Dr. Jürgen ; Albe, Prof. Dr. Karsten | ||||
Publikationsjahr: | 2013 | ||||
Ort: | Darmstadt | ||||
Datum der mündlichen Prüfung: | 19 Juni 2013 | ||||
URL / URN: | http://tuprints.ulb.tu-darmstadt.de/3621 | ||||
Kurzbeschreibung (Abstract): | Numerous questions regarding the nature of BNT-derived relaxor ferroelectrics are addressed on the basis of a broad experimental foundation including electrical characterization, diffraction, piezoresponse force microscopy, and mechanical measurements. In order to check the general validity of the approach two materials were investigated: the model system (1-y)(0.94Bi1/2Na1/2TiO3-0.06BaTiO3)-yK0.5Na0.5NbO3 (BNT-6BT-100yKNN) and the newly developed (1-y)((1-x)Bi1/2Na1/2TiO3-xBi1/2K1/2TiO3)-yBiZn1/2Ti1/2O3 (BNT-100xBKT-100yBZT). The field-dependent polarization and strain are demonstrated to bear phenomenological similarities to well-known relaxor ferroelectrics like PLZT or PMN. Compositions with zero y are hypothesized to be nonergodic relaxors featuring partially correlated polar nano-regions (PNRs) in a nominally pseudocubic matrix. These PNRs are the consequence of random fields due to a mixed A-site of the perovskite lattice, i.e., Bi3+ and Na+ in addition to Ba2+ or K+. This compositional disorder is associated with augmented random electric fields caused by charge disorder and random strain fields generated by different ion sizes. Upon application of a sufficiently high electric field the PNRs coalesce into ferroelectric domains that percolate the sample. Eventually, the poled material is macroscopically almost indistinguishable from conventional ferroelectrics, exhibiting a butterfly-shaped strain loop and rectangular polarization loop. With y>0, additional heterovalent ions are incorporated into the A- and/or B-site, giving rise to enhanced random fields that increase the required threshold field necessary to induce long-range order. At a critical value of y stable long-range order cannot be induced and the material is eventually ergodic. High electric fields, however, still cause the growth of PNRs, which results in high electrostrictive coefficients as well as high maximum polarization and strain. There may still be a threshold field where a reversible formation of micron-sized domains occurs. This transition is reflected by a clear bend in P(E) loops, indicating a change in mechanism. The establishment of an ’unstable’ long-range order at high electric fields is observed for BNT-6BT-3KNN, but not for BNT-20BKT-4BZT for fields <6 kV•mm-1. While S(E) and P(E) saturate in the former case, the latter material may exhibit rapidly increasing large-field parameters even beyond 6 kV•mm-1. For y higher than the critical value, Pmax and Smax decay, which is attributed to the diminished volume fraction of electrically active polar regions. Consequently, a higher electric field is required to achieve the same polarization and strain. The field-dependent small-signal parameters of piezoelectric constant d33(E) and permittivity εr;33(E) support this image. In nonergodic relaxor compositions, a stable piezoelectricity arises with the emergence of ferroelectric domains and the concurrent alignment thereof with respect to the electric field. At the same time, zero-field permittivity decreases, which is attributed to a diminished domain wall density. Compositions with elevated y, also referred to as ’incipient piezoceramics’, feature a sizable piezoelectric constant only under strong electric fields. Eventually, for a high concentration of heterovalent ions, d33 depends almost linearly on the electric field and permittivity is virtually field-independent. Further insight into the relaxor properties is provided by diffraction. The initial structure is seemingly cubic on the average, which is ascribed to the small size of the polar regions below the coherence length of the XRD or NRD experiment. Consequently, PNRs do not contribute to the Bragg reflections in the diffraction pattern and the average symmetry appears non-polar, i.e., metrically cubic. For nonergodic relaxor compositions, the field-induced establishment of longrange order is demonstrated by a peak broadening and the emergence of notable non-cubicity. In the case of BNT-20BKT, the application of an electric field >3.7 kV•mm-1 induces tetragonal and rhombohedral distortions. A small addition (y=0.02) of BZT increases the threshold field, indicated by the reflection broadening setting in at 4.6 kV•mm-1. The ergodic BNT-20BKT-4BZT, on the other hand, remains pseudocubic, suggesting that the PNRs may grow but remain too small for detection. Therefore, diffraction is in line with large-signal measurements that likewise suggest the lack of ferroelectric domains. The observation of pseudocubicity in the initial, unbiased state implies the absence of domains, which is confirmed by PFM measurements. It is concluded that PNRs have a size below the lateral resolution limit of the PFM, i.e., well below 10 nm. Also, the establishment of long-range order as suggested by in situ diffraction is directly observed after application of a DC tip bias. Inversion of the polarity of the DC voltage proved the capability for polarization reversal. As suggested by macroscopic measurements, the tip bias required for the formation and switching of domains is strongly affected by y, here representing the KNN content. Higher y results in a higher threshold and switching voltage. Interestingly, local switching loops can be obtained even for strongly ergodic relaxors such as BNT-6BT-18KNN, albeit only at highest electric fields. The polarization relaxation is furthermore reflected in a temporal decay of piezoresponse that obeys a stretched exponential function, confirming a broad distribution of relaxation times. This distribution of relaxation times is manifested in the frequency dependence of large-signal properties. The pseudocubic structure is maintained up to high temperatures as demonstrated by temperatureinvariant X-ray and neutron diffraction patterns, which only feature an increase in lattice spacing due to thermal expansion. Nonlinearities in the temperature-dependent Young’s modulus Y(T) indicate that structural changes take place on a limited, local scale. In contrast to phase transitions as in PZT, where Y strongly increases within a narrow temperature range, the variations in Y(T) for the investigated lead-free materials are small and spread out across a broad temperature range of several hundreds of degrees centigrade. It is, therefore, concluded that only a fraction of the volume is affected. The PNRs transform into a high-temperature cubic phase, which displays a higher Young’s modulus. Owing to the random fields, the stability of the PNRs varies, which eventually gives rise to the observed wide temperature range of non-linearly varying Y . At the same time, the permittivity exhibits an intricate thermal evolution. A low-temperature frequency-dispersive shoulder in εr;33(T) indicates the slowing down of dipolar motion, associated with the distribution of PNR correlation length and accordingly distributed relaxation times. A peak at higher temperatures indicates a slight frequency dependence albeit inverted, i.e., higher frequencies cause a decrease in tanδ. Approaches to rationalize the εr;33(T) curve include aging, space charge relaxation or a frustrated domain state, where two PNR species are present and consequently two relaxation temperature ranges exist. None of these hypothesis can presently be rejected and it is likely that all three effects contribute to some extent to the overall dielectric response as a function of temperature. The thermally induced depoling process is elucidated by contrasting in situ d33(T) measurements with high-temperature second-harmonic generation. Almost all piezoelectricity in poled nonergodic BNT-20BKT has vanished at 140 °C, while SHG measurements still yield non-zero intensity, proving the existence of residual polar volume. This finding suggests that the depoling process consists of two simultaneous contributions. On the one hand, a randomization of polarization vectors takes place, resulting in the disappearance of net polarization and piezoelectricity. On the other hand, domains break up into PNRs, which shrink upon further heating. This means that the polar volume is reduced with increasing temperature. Consequently, it is demonstrated by means of non-zero SHG intensity that polar volume in the form of PNRs exists not only far beyond depolarization temperature, but also in the unpoled and the ergodic relaxor state. Both lead-free BNT-based material systems are demonstrated to excel in certain applications. Under high electric fields, the strain ratio Smax•Emax-1 surpasses even soft PZT. Moreover, the achievable maximum stress, termed blocking stress, can be up to 60 % higher. Both the large strain and the high blocking stress are beneficial for actuator applications. Moreover, the large and almost temperature-insensitive permittivity for BNT-6BT-100yKNN with high y depicts an attractive starting point for the development of high-capacity, high-temperature capacitor materials. Such a capacitor is, for example, required for automotive applications, where power converters require charge storage for power conditioning at high temperatures. These specialized, tailored solutions may not only reduce the amount of hazardous substances in consumer products but also broaden the horizons of today’s technology. |
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Alternatives oder übersetztes Abstract: |
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Freie Schlagworte: | Ceramics, piezoceramics, relaxors, ferroelectrics, materials science, electrical properties, mechanical properties, structure, polar nano regions, large strain, polarization, high-temperature dielectrics | ||||
URN: | urn:nbn:de:tuda-tuprints-36212 | ||||
Zusätzliche Informationen: | This PhD research project was conducted from 2009 to 2013 in the group 'Nichtmetallisch-Anorganische Werkstoffe' (NAW) of Prof. Dr. Jürgen Rödel. |
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Sachgruppe der Dewey Dezimalklassifikatin (DDC): | 500 Naturwissenschaften und Mathematik > 500 Naturwissenschaften 500 Naturwissenschaften und Mathematik > 530 Physik 500 Naturwissenschaften und Mathematik > 540 Chemie 600 Technik, Medizin, angewandte Wissenschaften > 620 Ingenieurwissenschaften und Maschinenbau |
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Fachbereich(e)/-gebiet(e): | 11 Fachbereich Material- und Geowissenschaften | ||||
Hinterlegungsdatum: | 06 Okt 2013 19:55 | ||||
Letzte Änderung: | 06 Okt 2013 19:55 | ||||
PPN: | |||||
Referenten: | Rödel, Prof. Dr. Jürgen ; Albe, Prof. Dr. Karsten | ||||
Datum der mündlichen Prüfung / Verteidigung / mdl. Prüfung: | 19 Juni 2013 | ||||
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