Dwight Viehland

Role of coexisting tetragonal regions in the rhombohedral phase of Na0.5Bi0.5TiO3-xat.%BaTiO3 crystals on enhanced piezoelectric properties on approaching the morphotropic phase boundary

The ferroelectric domain and local structures of Na0.5Bi0.5TiO3-xat.%BaTiO3 (NBT-x%BT) crystals for xu2009=u20090, 4.5, and 5.5 have been investigated by transmission electron microscopy. The results show that the size of polar nano-regions was refined with increasing xat. %BT. The tetragonal phase volume fraction, as identified by in-phase octahedral tilting, was found to be increased with BT. The findings indicate that the large electric field induced strains in morphotropic phase boundary compositions of NBT-x%BT originate not only from polarization rotation but also polarization extension.

Mn-Modified Pb(Mg1/3Nb2/3)O3–PbTiO3 Ceramics: Improved Mechanical Quality Factors for High-Power Transducer Applications

Piezoelectric ceramics are potential high-power electroacoustic sources, and have been studied for many years. However, when these devices are driven under high-level vibration, the electromechanical characteristics depart significantly from the conventional linear piezoelectric relationships due to the loss and nonlinear behavior in terms of elastic and dielectric properties. In this paper, we present results concerning the development of modified Pb(Mg1/3Nb2/3)O3?PbTiO3 (PMN?PT) ceramics with improved mechanical quality factors for high-power application. We have focused efforts on base PMN?PT compositions close to the morphotropic phase boundary. Evidence will be presented which indicates that the improved mechanical quality factor results from the introduction of hard characteristics into PMN?PT by the substituents. Investigations of the nonlinearity of the electromechanical properties on the vibrational amplitude were performed using a constant displacement method. The degradation of the mechanical quality factor with increasing drive amplitude also presents thermal stability problems for the usage of these materials in high-power transducer applications.

Relaxational polarization dynamics in soft ferroelectrics

Investigations of the frequency dependence of the P–E (polarization versus field) and e–E (strain versus field) behavior have been performed on soft 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 polycrystalline ferroelectrics at various maximum field strengths below and above that of the coercive field (Ec). For E<Ec, a strong relaxational polarization was observed. At low frequencies, in this field range, P–E curves indicative of polarization switching were observed with an apparent remanence, however at higher frequencies near-linear P–E behavior was found.

Substituent effects on the mechanical quality factor of Pb(Mg1/3Nb2/3)O3–PbTiO3 and Pb(Sc1/2Nb1/2)O3–PbTiO3 ceramics

The mechanical quality factor of Pb(Mg1/3Nb2/3)O3–PbTiO3(PMN–PT) and Pb(Sc1/2Nb1/2)O3–PbTiO3(PSN–PT) ceramics have been investigated for compositions close to the morphotropic phase boundary. Investigations were performed for various types of lower valent substituents on the B-site cation. These investigations have shown significant differences between the modified PMN–PT and PSN–PT crystalline solutions. In PMN–PT, lower valent substitutents had little effect upon the mechanical quality factor; however in PSN–PT it was found to be significantly increased. A crystal chemical approach is then used to explain these differences.

Direct evidence of correlations between relaxor behavior and polar nano-regions in relaxor ferroelectrics: A case study of lead-free piezoelectrics Na0.5Bi0.5TiO3-x%BaTiO3

Diffuse scattering and relaxor behavior in Na0.5Bi0.5TiO3 (NBT) and NBT-5.6 at.u2009%BaTiO3 (NBT-5.6%BT) were investigated. X-ray diffraction revealed two types of diffuse scattering in NBT: (i) broad and (ii) asymmetric L-shaped. After modification with 5.6%BT, the broad diffuse scattering patterns became narrow, and the asymmetric L-shaped ones were replaced by symmetric ones. The symmetric diffuse scattering in NBT-5.6%BT disappeared with increasing dc electric field (E) for Eu2009≥u20099.5u2009kV/cm where the frequency dispersion in the dielectric constant disappeared. These results demonstrate that the relaxor characteristics are directly correlated with the diffuse scattering and the presence polar nano-regions.

Substituent Effects in 0.65Pb(Mg1/3Nb2/3O30.35PbTiO3 Piezoelectric Ceramics

Pb(Mg1/3Nb2/3O3–-PbTiO3 (PMN-PT) ceramics with base compositions close to the morphotropic phase boundary are potential materials for many applications such as transducers and actuators due to their high dielectric constants and electromechanical coupling factors. However, their dielectrical and mechanical losses are too high for high-power applications. In this paper, the dielectric and electromechanical properties of piezoelectric PMN-PT ceramics were investigated in specimens containing various A-site and B-site substituents with the goal of developing lower loss materials for wider applications. Emphasis was placed on various transition metal cation substituents of both lower and higher valences. Mn substituent was found to be the most promising substituent investigated for developing high power low loss piezoelectric PMN-PT ceramics.

Mechanical aging behavior of oriented Pb(Mg1/3Nb2/3)O3–PbTiO3 and Pb(Zn1/3Nb2/3)O3–PbTiO3 single crystals

The aging characteristics of 〈001〉-oriented Pb(Zn1/3Nb2/3)O3–PbTiO3u200a(PZN-PT) and Pb(Mg1/3Nb2/3)O3–PbTiO3u200a(PMN-PT) single crystals have been studied under electrical and mechanical drive. A significant increase in hysteretic loss and decrease in piezoelectric constant were observed under continuous excitation. Systematic investigations of piezoelectric aging were then performed using a novel mechanical fatigue method. Piezocrystals were placed between two magnetostrictive plates and clamped. Periodic longitudinal compressive stresses were generated by applying magnetic fields. At both short and long times, the piezoelectric constant was found to decrease slowly. However, in an intermediate range, a pronounced decrease was observed over a narrow time range. The dynamics of aging have been shown to scale to a hierarchical relaxation process, typical of disordered systems such as random field and glassy states.

Alternating current field effect on the freezing temperature of relaxor ferroelectrics

Alternating current-field effects on the freezing temperature of La-modified lead magnesium niobate–lead titanate crystalline solutions for compositions near to the morphotropic phase boundary have been studied by dielectric spectroscopy. Charge imbalance due to La substitution (0–10 at.u200a%) in [Pb(Mg1/3Nb2/3)O3]0.65(PbTiO3)0.35 was compensated by changing the B-site cation ratio. The frequency dispersion in the dielectric constant was found to increase with increasing La substitution. Relaxor-like dielectric characteristics were induced for La concentrations greater than 5 at.u200a%. It was also found that the dielectric constant maximum (emax′) increased and the temperature of emax′ decreased with increasing ac field. In addition, a threshold electric field was observed above which nonlinearities became evident in the dielectric response and the freezing temperature was reduced.

Dependence of dielectric relaxation on ac drive in [Pb(Mg1/3Nb2/3)O3](1−x)–(PbTiO3)x single crystals

Dielectric measurements under different ac drive voltages were performed on single crystals of (PbMg1/3Nb2/3O3)(1−x)(PbTiO3)xu2002[(PMN)(1−x)(PT)x] for 0⩽x⩽0.3. A significant increase in permittivity was observed with increasing ac drive for all compositions. The magnitude of increase depended on temperature and measurement frequency. Alternating current drive effects were found to be a low frequency phenomenon, primarily below 106 Hz. Three distinct ac-drive amplitude ranges are believed to exist in which various types of contributions dominate in the dielectric relaxation: (i) a glassy mechanism is dominant at low drives and gives rise to the typical relaxor ferroelectric characteristics in the linear dielectric response, (ii) a kink or boundary vibration type which is dominant at intermediate drives and gives rise to the nonlinearities in the dielectric response of relaxors under moderate ac drives, and (iii) a domain wall motion type which is dominant at higher drives and results in polarization reversal.

Stability of glassy and ferroelectric states in the relaxors PbMg1/3Nb2/3O3 and PbMg1/3Nb2/3O3-12% PbTiO3

The stability of the disordered glassy phase in the relaxors PbMg1/3Nb2/3O3 and (PbMg1/3Nb2/3O3)0.88(PbTiO3)0.12, called PMN and PMN-PT, was investigated by preparing partially polarized samples and allowing them to age at zero field in the temperature range for which the phase is history-dependent. The PMN-PT polarization would spontaneously increase until long-range order formed, first appearing as giant polarization noise. Thus the thermodynamically stable phase in PMN-PT appears to be ferroelectric. In contrast, a PMN sample lacking the sharp first-order field-driven transition found in some other samples spontaneously depolarized, consistent with its glassy state being thermodynamically stable. Detailed thermal depolarization results in PMN show two distinct broad peaks as well as a small fraction of material with a distribution of abrupt melting transitions.