Wook Jo

Temperature-dependent dielectric permittivity of 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3 (BNT-6BT) lead-free piezoceramics was studied to disentangle the existing unclear issues over the crystallographic aspects and phase stability of the system. Application of existing phenomenological relaxor models enabled the relaxor contribution to the entire dielectric permittivity spectra to be deconvoluted. The deconvoluted data in comparison with the temperature-dependent dielectric permittivity of a classical

The mechanism of the giant unipolar strain recently observed in a lead-free piezoceramic, 0.92(Bi0.5Na0.5)TiO3−0.06BaTiO3−0.02(K0.5Na0.5)NbO3 [S.-T. Zhang, A. B. Kounga, E. Aulbach, H. Ehrenberg, and J. Rödel, 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

The correlation between structure and electrical properties of lead-free (1−x)(Bi1/2Na1/2)TiO3–xBaTiO3 (BNT-100xBT) polycrystalline piezoceramics was investigated systematically by in situ synchrotron diffraction technique, combined with electrical property characterization. It was found that the morphotropic phase boundary (MPB) between a rhombohedral and a tetragonal phase evolved into a morphotropic phase region with electric field. In the unpoled material, the MPB was positioned at the trans

Lead-free piezoceramics aiming at replacing the market-dominant lead-based ones have been extensively searched for more than a decade worldwide. Some noteworthy outcomes such as the advent of commercial products for certain applications have been reported, but the goal, i.e., the invention of a lead-free piezocermic, the performance of which is equivalent or even superior to that of PZT-based piezoceramics, does not seem to be fulfilled yet. Nevertheless, the academic effort already seems to be

Ferroelectric and piezoelectric properties of SrTiO 3 –modified (0, 3, and 5 mol%) 0.8 Bi 1/2 Na 1/2 TiO 3 –0.2 Bi 1/2 K 1/2 TiO 3 lead‐free piezoceramics were investigated as a model system in an attempt to lay a guideline for developing lead‐free piezoelectric materials with large strains. Two guidelines, one for the choice of base composition and the other for the choice of chemical modifiers, were assumed from our current understanding of the mechanism involved. Dielectric permittivity of bo

The effect of (K 0.5 Na 0.5 )NbO 3 (KNN) addition on the ferroelectric behavior of (Bi 1/2 Na 1/2 )TiO 3 –(Bi 1/2 K 1/2 )TiO 3 (BNT–BKT) lead‐free piezoceramics was investigated. Polarization and strain hysteresis loops indicate that the ferroelectric order is disrupted significantly with the addition of KNN as a replacement for BNT and the destabilization of the ferroelectric order is accompanied by an enhancement of the unipolar strain, which peaks at a value of ∼0.48% (corresponding to a larg

The stability of electrically induced long-range ferroelectric order in a relaxor 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3 ceramic material has been investigated by temperature-dependent X-ray diffraction and electrical property measurements. The depolarization and ferroelectric-to-relaxor transition are identified as separate and discrete processes. It is observed that the induced ferroelectric domains first lose their ferroelectric/ferroelastic texture coincident with a peak signal in the thermally ind

Recently developed lead‐free incipient piezoceramics are promising candidates for off‐resonance actuator applications with their exceptionally large electromechanical strains. Their commercialization currently faces two major challenges: high electric field required for activating the large strains and large strain hysteresis. It is demonstrated that design of a relaxor/ferroelectric composite provides a highly effective way to resolve both challenges. Experimental results in conjunction with nu

The electrical and dielectric properties of (1 − x )(0.94Bi 1/2 Na 1/2 TiO 3 –0.06BaTiO 3 )– x (K 0.5 Na 0.5 NbO 3 ) with x = 0, 0.03, 0.09, 0.18 have been investigated by impedance spectroscopy over a wide temperature range. The dc conductivity of the ceramics follows the Arrhenius law with an activation energy ranging from ~1.20 to 1.50 eV. Measurements under different atmospheres show the materials exhibit n ‐type semiconducting behavior at elevated temperatures. The presence of a highly pola

The dielectric and electromechanical properties of 0.75 Bi 1/2 Na 1/2 TiO 3 –0.25 SrTiO 3 (25 ST ) as a function of temperature and frequency were studied. It is shown that the 25 ST is a relaxor ferroelectric as evidenced by the temperature‐dependent dielectric relaxations with an incipient piezoelectricity featured by the presence of a reversible electric‐field‐induced phase transformation at room temperature. The transition occurs on a broad electric field strength range depending on field am

The interface atomic structure was proposed to have a critical effect on microstructure evolution during sintering of ceramic materials. In liquid‐phase sintering, spherical grains show normal grain growth behavior without exception, while angular grains often grow abnormally. The coarsening process of spherical grains with a disordered or rough interface atomic structure is diffusion‐controlled, because there is little energy barrier for atomic attachments. On the other hand, kink‐generating so

Phase stability of (1 − x) (Bi1/2Na1/2)TiO3-x BaTiO3 (0 ≤ x ≤ 0.15) under electric field was investigated by measuring volume changes during a bipolar poling cycle. The unique nature of field-dependent phase stability with three distinctive regions is revealed by comparative studies using commercial soft PZT and relaxor PLZT. For x ≤ 0.06 and x ≥ 0.13, similarly with PZT and PLZT, the axial strain expands with the contracting radial strain, but the former results in a remanent volume demonstrati

A high‐temperature dielectric, (1– x )(0.6 Bi 1/2 Na 1/2 TiO 3 –0.4 Bi 1/2 K 1/2 TiO 3 )– x K 0.5 Na 0.5 NbO 3 , off the morphotropic phase boundary of the parent matrix 0.8 Bi 1/2 Na 1/2 TiO 3 –0.2 Bi 1/2 K 1/2 TiO 3 , has been developed for application as a high‐temperature capacitor. In addition to temperature‐dependent permittivity and dielectric loss, DC conductivity and field‐dependent permittivity are reported. These properties are correlated with temperature‐dependent structure data meas