Sand Transport Studies in Monterey Bay, California: Annual Report, Part 5, 1973

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Lead-Free Piezoceramics – Ergodic and Nonergodic Relaxor Ferroelectrics Based on Bismuth Sodium Titanate

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

Peculiar Bi-ion dynamics in Na1/2Bi1/2TiO3 from terahertz and microwave dielectric spectroscopy

Dynamics of the main dielectric anomaly in Na1/2Bi1/2TiO3 (NBT) was studied by time-domain THz and microwave spectroscopy, using also previously published data and their new overall fits. Above the dielectric maximum temperature Tm ~ 600 K, the response consists of coupled sub-THz oscillator and a relaxation mode, assigned to strongly anharmonic Bi-ion vibrations and hopping, whose slowing down explains the paraelectric-like permittivity increase to Tm. Below Tm, the main relaxation continues slowing down and additional relaxation, assigned to quasi-Debye losses, appears in the 10^11 Hz range. The oscillator hardens on cooling and takes over the whole oscillator strength. The permittivity decrease below Tm is caused by the reduced strength of the relaxations due to dominance of the rhombohedral phase within the coexistence region with the tetragonal phase. The anharmonic dynamics of Bi is supported by previous structural studies. NBT represents a hybrid between standard and relaxor ferroelectric behaviour

Nanoscale phase quantification in lead-free (Bi1/2Na1/2)TiO3-BaTiO3 relaxor ferroelectrics by means of Na 23 NMR

We address the unsolved question on the structure of relaxor ferroelectrics at the atomic level by characterizing lead-free piezoceramic solid solutions (100-x)(Bi1/2Na1/2)TiO3-xBaTiO3 (BNT-xBT) (for x=1,4,6, and 15). Based on the relative intensity between spectral components in quadrupolar perturbed Na23 nuclear magnetic resonance, we present direct evidence of the coexistence of cubic and polar local symmetries in these relaxor ferroelectrics. In addition, we demonstrate how the cubic phase vanishes whenever a ferroelectric state is induced, either by field cooling or changing the dopant amount, supporting the relation between this cubic phase and the relaxor state.open0

Electric-field-induced strain mechanisms in lead-free 94%(Bi1/2Na1/2)TiO3-6%BaTiO3

High resolution neutron diffraction has been used to investigate the structural origin of the large electric-field-induced remanent strain in 94(Bi1/2Na1/2)TiO3-6BaTiO(3) ceramics. The virgin material was found to be a mixture of near-cubic phases with slight tetragonal and rhombohedral distortions of a(0)a(0)c(+) and a(-)a(-)a(-) octahedral tilt type, respectively. Application of an electric field of 4.57 kV/mm transformed the sample to a predominantly rhombohedral a(-)a(-)a(-) modification with a significantly higher degree of structural distortion and a pronounced preferred orientation of the c-axis along the field direction. These electric field-induced structural effects contribute significantly to the macroscopic strain and polarization of this system.open40

Lead-free high-temperature dielectrics with wide operational range

The dielectric, electrical and structural properties of (1-x) (0.94Bi(1/2)Na(1/2)TiO(3)-0.06BaTiO(3))-xK(0.5)Na(0.5)NbO(3) (BNT-BT-xKNN) with x=0.09, 0.12, 0.15, and 0.18 were investigated as potential candidates for high-temperature capacitors with a working temperature far beyond 200 degrees C. Temperature dependent dielectric permittivity (epsilon) showed two local broad maxima that at the optimal composition of KNN (x=0.18) are combined to form a plateau. This then results in a highly temperature-insensitive permittivity up to similar to 300 degrees C at the expense of a small reduction in absolute permittivity values. High-temperature in situ x-ray diffraction study showed pseudocubic symmetry without obvious structural changes, which implies that the dielectric anomalies observed could only be a consequence of a slight change in space group. BNT-BT-0.18KNN showed a permittivity of similar to 2150 at the frequency of 1 kHz at 150 degrees C with a normalized permittivity epsilon/epsilon(150 degrees C) varying no more than +/- 10% from 43 to 319 degrees C. With very good electrical properties persisting up to 300 degrees C, i.e., a resistivity on the order of magnitude of 10(8) Omega m and the RC constant of about 1 s, the examined BNT-BT-xKNN compositions present a good starting point for the development of high-temperature capacitor materials.open343

Ergodicity reflected in macroscopic and microscopic field-dependent behavior of BNT-based relaxors

The effect of heterovalent B-site doping on ergodicity of relaxor ferroelectrics is studied using (1 - y)(0.81Bi(1/2)Na(1/2)TiO(3)-0.19Bi(1/2)K(1/2)TiO(3))-yBiZn(1/2)Ti(1/2)O(3) (BNT-BKT-BZT) with y - {0.02;0.03;0.04} as a model system. Both the large- and small-signal parameters are studied as a function of electric field. The crystal structure is assessed by means of neutron diffraction in the initial state and after exposure to a high electric field. In order to measure ferroelastic domain textures, diffraction patterns of the poled samples are collected as a function of sample rotation angle. Piezoresponse force microscopy (PFM) is employed to probe the microstructure for polar regions at a nanoscopic scale. For low electric fields E < 2 kV.mm(-1), large- and small-signal constitutive behavior do not change with composition. At high electric fields, however, drastic differences are observed due to a field-induced phase transition into a long-range ordered state. It is hypothesized that increasing BZT content decreases the degree of non-ergodicity; thus, the formation of long-range order is impeded. It is suggested that frozen and dynamic polar nano regions exist to a different degree, depending on the BZT content. This image is supported by PFM measurements. Moreover, PFM measurements suggest that the relaxation mechanism after removal of the bias field is influenced by surface chargesopen2

Multiple drugs compete for transport via the Plasmodium falciparum chloroquine resistance transporter at distinct but interdependent sites

Mutations in the "chloroquine resistance transporter" (PfCRT) are a major determinant of drug resistance in the malaria parasite Plasmodium falciparum. We have previously shown that mutant PfCRT transports the antimalarial drug chloroquine away from its target, whereas the wild-type form of PfCRT does not. However, little is understood about the transport of other drugs via PfCRT or the mechanism by which PfCRT recognizes different substrates. Here we show that mutant PfCRT also transports quinine, quinidine, and verapamil, indicating that the protein behaves as a multidrug resistance carrier. Detailed kinetic analyses revealed that chloroquine and quinine compete for transport via PfCRT in a manner that is consistent with mixed-type inhibition. Moreover, our analyses suggest that PfCRT accepts chloroquine and quinine at distinct but antagonistically interacting sites. We also found verapamil to be a partial mixed-type inhibitor of chloroquine transport via PfCRT, further supporting the idea that PfCRT possesses multiple substratebinding sites. Our findings provide new mechanistic insights into the workings of PfCRT, which could be exploited to design potent inhibitors of this key mediator of drug resistance

Local structure change evidenced by temperature-dependent elastic measurements: Case study on Bi1/2Na1/2TiO3-based lead-free relaxor piezoceramics

The temperature-dependent Young's modulus Y(T) of the lead-free piezoceramics of 0.8Bi(1/2)Na(1/2)TiO(3)-0.2Bi(1/2)K(1/2)TiO(3) (20BKT) and 0.96(0.8Bi(1/2)Na(1/2)TiO(3)-0.2Bi(1/2)K(1/2)TiO(3))-0.04 BiZn1/2Ti1/2O3 (4BZT) is measured with the impulse excitation technique and contrasted with corresponding dielectric and structural data. While the dielectric properties suggest a phase transition, the high resolution XRD patterns remain virtually unchanged from room temperature up to high temperatures, confirming no change in their long-range order. In contrast, the elastic properties indicate a broad and diffuse ferroelastic transition denoted by a minimum in Y(T). By analogy to the elastic and dielectric data of PbZrxTi1-xO3 and PLZT, it is concluded that 20BKT and 4BZT are relaxors with polar nanoregions embedded in a metrically cubic matrix. Interestingly, no indication for the freezing temperature was reflected in any of the employed measurement techniques. From the saturation of Y(T), it is suggested that the Burns temperature may be approximated as 700 degrees C. Moreover, it is found that the modification with the ternary end-member BiZn1/2Ti1/2O3 results in an increase in Young's modulus. A comparison with the Bi1/2Na1/2TiO3-BaTiO3-K0.5Na0.5NbO3 yields the same results.open0

Investigation of the depolarisation transition in Bi-based relaxor ferroelectrics

The loss of macroscopic polarisation in relaxor ferroelectric (Na0.8K0.2)(1/2)Bi1/2TiO3 ceramics doped with BiZn1/2Ti1/2O3 has been studied by electrical and structural methods. These indicate that the phenomena that are coupled in a displacive phase transition are not necessarily coupled in the depolarisation of Na1/2Bi1/2TiO3-based relaxors and a concept of correlated and uncorrelated switching of dipoles within adjacent unit cells is used to explain this. Second harmonic generation performed on poled ceramics during heating yields values of the freezing temperature and shows a broad temperature range of similar to 100 degrees C across which the structure changes from field-induced ferroelectric to an equilibrium-state ergodic relaxor. Electrical poling at room temperature causes poled regions to increase in size by similar to 2 orders of magnitude. A model illustrating the main steps in thermal depolarisation is described that does not require a phase transition to take place on a unit cell level.open1