Journal of the American Ceramic Society

Huey, Bryan D.; Nath Premnath, Ramesh; Lee, Sungjun; Polomoff, Nicholas A.; Green, D. J.

doi: 10.1111/j.1551-2916.2012.05099.xpmid: N/A

A high speed variation of Scanning Probe Microscopy with continuous image rates on the order of 1 frame per second is applied to investigate the nucleation and growth of individual ferroelectric domains. Movies of consecutive images directly identify nascent domains and their nucleation times, while tracking their development with time and voltage reveals linear domain growth at lateral velocities near 1 mm/s, even faster for nascent domains. Nanoscale maps of nucleation times and growth velocities indicate that domain nucleation and growth are uncorrelated, varying extensively with position. Domain switching dynamics do strongly couple to film defects; for instance, grain boundaries can profoundly pin domain walls, and polarization reversal kinetics are influenced by strain fields near microcracks or in asymmetric specimens. The influence of the onset of switching fatigue is observed as well. These results highlight the importance of updating classical interpretations of ferroelectric switching for truly rigorous models of polarization dynamics. Coupling high speed SPM imaging with in situ activation by voltage or other parameters therefore provides an important methodology to research dynamic surface properties with nanoscale resolution, extendable to a range of materials such as photovoltaics, thermoelectrics, batteries, fuel cells, multiferroics, phase change systems, etc.

Chen, Xuefeng; Cao, Fei; Zhang, Hongling; Yu, Gang; Wang, Genshui; Dong, Xianlin; Gu, Yan; He, Hongliang; Liu, Yusheng; Brennecka, G. L.

doi: 10.1111/j.1551-2916.2012.05070.xpmid: N/A

The evolution of dynamic hysteresis of lead zirconate titanate stannate antiferroelectric bulk ceramics at various frequencies (f) and field amplitudes (E0) has been investigated systematically. A series of asymmetrical minor loops were observed when the working fields were slightly lower than the forward switching field EAF. As E0 increased above EAF, symmetrical double loops appeared and their energy densities w against f and E0 were found to be fit the scaling behavior as w ∝∝∝∝ f−0.02(E0 − Ec)0.08, where Ec representing the threshold field is designated as EAF. The study is helpful to application of antiferroelectrics as energy‐storage capacitors.

Qi, Peng; Fan, Yuancheng; Li, Hongqiang; Zhang, Qiwei; Zhai, Jiwei; French, R. H.

doi: 10.1111/j.1551-2916.2012.05072.xpmid: N/A

A metamaterial device with resonance frequency at THz was reported in this article. The device was composed of split‐ring resonators (SRR), ferroelectric Ba0.7Sr0.3TiO3 (BST70) film, and alumina substrates. The SRR of 140 nm thick Au/Ti was fabricated on BST70 film with periodicity of 50 μm, width of 10 μm, and gap of 8 μm. The BST70 thin film was deposited on alumina substrates using a sol–gel technique. Terahertz time domain spectroscopy was used to characterize electromagnetic response of the metamaterial at various temperatures. Finite difference time domain was used to simulate the transmission spectra of the metamaterial at THz frequencies. Dielectric constant of the BST70 film at THz, in the temperature range from 0°C to 100°C, was derived by comparing the simulated resonance frequency with the experimental curves.

Kang, Huajun; Chen, Jun; Liu, Laijun; Hu, Changzheng; Fang, Liang; Xing, Xianran; Takenaka, T.

doi: 10.1111/j.1551-2916.2012.05078.xpmid: N/A

A piezoelectric system of (1−x)Bi(Ni1/2Ti1/2)O3–x(Pb(1−y)SryTiO3) (BNT–PST) is developed to have good high temperature piezoelectric properties with much improved resistivity. The crystal structure shows that the phase transformation from tetragonal phase to rhombohedral one is gradually shifted to the composition with high content of PT by the substitution of strontium. The problem of leakage current for the Bi(Ni,Ti)O3–PbTiO3 can be well resolved by introducing the dopant of strontium. Both dielectric loss and DC resistivity can be much reduced not only at room temperature but also high temperature. An optimum composition of 0.53Bi(Ni1/2Ti1/2)O3–0.47(Pb0.95Sr0.05)TiO3 is obtained to have a good potential application of high temperature piezoelectric (d33 = 205 pC/N, EC = 3.32 kV/mm, Pr = 41.2 μC/cm2). Strontium could be a useful dopant for the studies on the high temperature piezoelectric ceramics in future.

Wee, Sung Hun; Cantoni, Claudia; Zuev, Yuri L.; Specht, Eliot D.; Goyal, Amit; Wong‐Ng, W. K.

doi: 10.1111/j.1551-2916.2012.05084.xpmid: N/A

Phase stability of cubic‐pyrochlore‐structured RE3TaO7 (RTO, where RE = rare earth elements) pinning additives in YBa2Cu3O7−δ (YBCO) superconductor and the pinning properties influenced by RTO addition into YBCO films were investigated. RTO completely reacts with YBCO and is converted to cubic‐double‐perovskite‐structured Ba2RETaO6 (BRETO), a more thermodynamically stable tantalate phase within YBCO. In RTO‐doped YBCO films, BRETO self‐assembled nanocolumns align along the c‐axis of the film and play a major role in the improvement of flux pinning and Jc performance over wide magnetic field and angular ranges.

Guo, Hai; Li, Fang; Wei, RongFei; Zhang, Hao; Ma, ChongGeng; Ballato, J.

doi: 10.1111/j.1551-2916.2012.05097.xpmid: N/A

Eu/Tb single‐doped and co‐doped oxyfluoride aluminosilicate glasses and glass ceramics containing GdPO4 nanocrystals were elaborated, and their structural and luminescent properties were investigated by X‐ray diffraction, Transmission electron microscopy, excitation and emission spectra, as well as decay lifetime measurement. Excited by 350 nm light, a combination of blue (5d→4f of Eu2+), green (5D4→7F5 of Tb3+) and red (5D0→7F2 of Eu3+) emissions emerges to achieve white emission in Eu/Tb co‐doped precursor glasses and glass ceramics. Our research shows that Eu/Tb co‐doped precursor glasses and GdPO4‐based glass ceramics may be applied in lighting and luminescent aspects.

Wang, Hong‐Qiang; Dai, Ye‐Jing; Zhang, Xiao‐Wen; Damjanovic, D.

doi: 10.1111/j.1551-2916.2012.05098.xpmid: N/A

In this study, we used conventional ceramics techniques to prepare lead‐free ceramics (K0.5Na0.5)NbO3‐x mol CuO (KNN‐ xCu) sintered in air atmosphere and argon atmosphere, respectively, and investigated the corresponding hardening characteristics. The microstructure, P–E hysteresis loops, and X‐ray photoelectron spectroscopy results indicate that the Cu2+ ions can play different roles on the hardening mechanism in air atmosphere and argon atmosphere. The Cu2+ ions are considered to replace the Nb5+ ions for forming O2− vacancies, and act as a hardener when sintered in air. However, when sintered in argon atmosphere, a large portion of Cu2+ ions could be reduced into Cu+ ions and then replaced by Na+.

Tran, Tien B.; Hayun, Shmuel; Navrotsky, Alexandra; Castro, Ricardo H. R.; Chaim, R.

doi: 10.1111/j.1551-2916.2012.05103.xpmid: N/A

Fully dense nanocrystalline MgO‐based ceramics were consolidated by field and pressure assisted sintering (also known as Spark Plasma Sintering) using anhydrous nanosized powders (~5 nm) prepared by laser ablation and handled in a controlled, water‐free environment prior to sintering. The high reactivity of the powders promoted excellent sinterability at relatively low temperatures. Highly transparent specimens were produced by sintering at 0.23–0.28Tm (650°C–800°C) for 15–30 min with good control over the final nanometric grain size. To preserve the nanocrystalline nature of the samples, the high temperature exposure time during sintering was reduced by the application of 300 MPa uniaxial pressure; higher pressure or two‐step procedures did not provide extra benefits to densification or microstructural control.

Roy, Biplab Kumar; Cho, Junghyun; Karatchevtseva, I.

doi: 10.1111/j.1551-2916.2012.05104.xpmid: N/A

We have developed a “two‐step” solution‐based processing to deposit high‐k ceramic thin films. Electrochemically pre‐coated amorphous titania layers were “hydrothermally” or “hydrothermal‐electrochemically” converted to crystalline BaTiO3 films at ≤90°C. In hydrothermal process, an in situ electrochemical protection (ECP) mechanism enables utilization of common metallic substrates (e.g., Cu), which otherwise suffer corrosion from the precursor. Microstructural evolution of BaTiO3 films along with the effectiveness of ECP is discussed in this article. Derived BaTiO3 shows a dielectric constant of ~80 at 100 kHz, which makes it a promising candidate for high‐density microelectronic capacitor applications.

Liu, Wen‐Hui; Gai, Wei‐Zhuo; Deng, Zhen‐Yan; Tang, Junwang; Dreizin, E.

doi: 10.1111/j.1551-2916.2012.05105.xpmid: N/A

Previous work indicated that γ‐Al2O3 modified Al powder could react with water and generate hydrogen under ambient conditions. In this work, an ultrasonic dispersion procedure was used to disperse and mix the Al and γ‐Al2O3 powders. It was found that γ‐Al2O3 modified Al powder prepared by the ultrasonic procedure has a higher hydrogen‐generation speed than that prepared by a traditional ball‐milling method. Microstructure analyses showed that the ultrasonic procedure greatly eliminated the agglomeration of Al and γ‐Al2O3 powders so that γ‐Al2O3 grains covered Al particle surfaces more uniformly. This implies that a favorable processing procedure is the key to obtain high performance modified Al powder.