Origin of the large strain response in (K0.5Na0.5)NbO3-modified (Bi0.5Na0.5)TiO3-BaTiO3 lead-free piezoceramics

The mechanism of the giant unipolar strain recently observed in a lead-free piezoceramic, 0.92(Bi0.5Na0.5)TiO3-0.06BaTiO(3)-0.02(K0.5Na0.5)NbO3 [S.-T. Zhang, A. B. Kounga, E. Aulbach, H. Ehrenberg, and J. Rodel, Appl. Phys. Lett. 91, 112906 (2007) was investigated. The validity of the previously proposed mechanism that the high strain comes both from a significant volume change during the field-induced phase transition, from an antiferroelectric to a ferroelectric phase and the domain contribution from the induced ferroelectric phase was examined. Monitoring the volume changes from the simultaneously measured longitudinal and transverse strains on disk-shaped samples showed that the phase transition in this specific material does not involve any notable volume change, which indicates that there is little contribution from a volume change due to the phase transition to the total strain response. Temperature dependent hysteresis measurements on unpoled samples of a nearby ferroelectric composition, 0.93(Bi0.5Na0.5)TiO3-0.06BaTiO(3) -0.01(K0.5Na0.5)NbO3 demonstrated that the origin of the large strain is due to the presence of a nonpolar phase that brings the system back to its unpoled state once the applied electric field is removed, which leads to a large unipolar strain.open1539

Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3-BaTiO3-K0.5Na0.5NbO3. II. Temperature dependent properties

The temperature dependence of the dielectric and ferroelectric properties of lead-free piezoceramics of the composition (1-x-y)Bi0.5Na0.5TiO3-xBaTiO(3)-yK(0.5)Na(0.5)NbO(3) (0.05 <= x <= 0.07, 0.01 <= y <= 0.03) was investigated. Measurements of the polarization and strain hystereses indicate a transition to predominantly antiferroelectric order when heating from room temperature to 150 degrees C, while for 150 < T < 200 degrees C both remnant polarization and coercive field increase. Frequency-dependent susceptibility measurements show that the transition is relaxorlike. For some samples, the transition temperature T-d is high enough to allow mostly ferroelectric ordering at room temperature. These samples show a drastic increase of the usable strain under an external electric field just after the transition into the antiferroelectric state at high temperatures. For the other samples, T-d is so low that they display significant antiferroelectric ordering already at room temperature. In these samples, the usable strain is relatively stable over a wide temperature range. In contrast to T-d, the temperature T-m of the transition into the paraelectric high-temperature phase depends far less on the sample composition. These results confirm that the high strain in this lead-free system is due to a field-induced antiferroelectric-ferroelectric phase transition and that this effect can be utilized in a wide temperature range.open8

Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3-BaTiO3-K0.5Na0.5NbO3. I. Structure and room temperature properties

Lead-free piezoelectric ceramics, (1-x-y)Bi0.5Na0.5TiO3-xBaTiO(3)-yK(0.5)Na(0.5)NbO(3) (0.05 <= x <= 0.07 and 0.01 <= y <= 0.03), have been synthesized by a conventional solid state sintering method. The room temperature ferroelectric and piezoelectric properties of these ceramics were studied. Based on the measured properties, the ceramics were categorized into two groups: group I compositions having dominant ferroelectric order and group II compositions displaying mixed ferroelectric and antiferroelectric properties at room temperature. A composition from group II near the boundary between these two groups exhibited a strain as large as similar to 0.45% at an electric field of 8 kV/mm. Polarization in this composition was not stable in that the piezoelectric coefficient d(33) at zero electric field was only about 30 pm/V. The converse piezoelectric response becomes weaker when the composition deviated from the boundary between the groups toward either the ferroelectric or antiferroelectric compositions. These results were rationalized based on a field induced antiferroelectric-ferroelectric phase transition.open12510

Frequency-dependence of large-signal properties in lead-free piezoceramics

The dependence of large signal properties of (1-x)(0.81Bi 1/2Na 1/2TiO 3-0.19Bi 1/2K 1/2TiO 3)-xBi(Zn 1/2Ti 1/2)O 3 with x 0.02, 0.03, and 0.04 on the measurement frequency was investigated for a wide range of frequencies from 0.1 Hz to 100 Hz. A significant frequency dispersion in the characteristic parameters representatively maximum and coercive values was denoted. On extension with the temperature dependent dielectric permittivity measurement, it was shown that the observed frequency dependence is primarily correlated with the dynamics of field-induced phase transition from a relaxor state to a long-range ferroelectric state. Increasing the substitutional disorder introduced by Bi(Zn 1/2Ti 1/2)O 3 addition was demonstrated to pronounce the frequency dependence. It was proposed that the change be due to the increase in random fields and consequent dominance of ergodicity, based on the frequency-dependent hysteresis measurements at an elevated temperature above so-called induced-ferroelectric-to-relaxor transition temperature.open8

Wetting and pressureless infiltration in the CuTi/Al2O3 system under poor vacuum

Work is aimed at preparation of Cu/Al2O3 composite by pressureless metal infiltration of porous alumina with simple vacuum equipment. Improvement of wetting by addition of Ti to Cu is investigated in sessile drop tests with various geometrical configurations in vacuum of 1–10 Pa. The results showed that a pre-alloyed Cu did not wet alumina substrate while stacked Cu and Ti blocks did, even though the latter had the same nominal alloy content (Ti: 10 wt%). This result had been proved by a following pressureless infiltration test, i.e. a CuTi/Al2O3 composite was formed with stacked Cu and Ti blocks at 1300 °C. Wettability, interfacial chemistry and microstructure were discussed with the aid of SEM and EDAX analysis in the paper. It was suggested that the initial reactant, which benefits the wettability, could be evolved to the one with a higher O:Ti ratio due to poor vacuum condition, higher temperature and longer holding time

Experimental determination of sintering stresses and sintering viscosities

A loading dilatometer assisted by two high-resolution lasers was applied for the accurate measurement of radial and axial strains during uniaxial load assisted sintering. An improved hot forging technique was employed for the first time for the experimental determination of sintering stress and sintering viscosity by hot forging samples under various loads which were pre-sintered to different densities. The technique of discontinuous hot forging was discussed in detail and the effect of the developing anisotropy and different grain growth were analysed. The sintering stress and uniaxial viscosity were both obtained as functions of density ranging from 65 to 96% and compared with theoretical models. Alumina powder with a grain size of about 150 nm was used in this study

Laser-Assisted High-Resolution Loading Dilatometer and Applications

new hot forging apparatus has been successfully developed and applied for the experimental determination of continuum mechanical sintering parameters and theoretical modeling of thin-film sintering. A precise automatic loading system and high-resolution laser measurement system enable accurate measurement of radial and axial strains of cylindrical specimens under constant or intermittent uniaxial loads of 0-5000 N during high-temperature densification. Preliminary experimental results are demonstrated and the key features of technological significance in the design introduced in detail