Key Engineering Materials Vols. 336-338

Abstract: Na0.5Bi0.5TiO3 (NBT) is considered to be an excellent candidate for lead-free piezoelectric ceramics. In this study, we propose a hydrothermal method for the preparation of single phase NBT powder at relatively low treatment-temperature. The particle size and morphology of the synthesized powders were examined by SEM. The powders were further pressed into disk and sintered at 1120°C/2h in air, and its properties and microstructure were compared with traditionally prepared samples.

Abstract: SrBi2(Ta1-xNbx)2O9(SBTN) ferroelectric ceramics with typical bismuth layered perovskite structure were synthesized with a reactive hot-press sintering process at 1000°C for 2h. The as-sintered SBTN (x=0.2, 0.4, 0.6, 0.8) materials had a maximum relative density of 95.97%. Bi-layered perovskite structure was obtained. The grain sizes increased with increasing Nb content. In 1MHz frequency, remanent polarization (Pr) of SBTN (x=0.2, 0.4, 0.6, 0.8) were 10.0, 3.0, 1.7 and 1.0, while coercive field (Ec) of SBTN (x=0.2, 0.4, 0.6, 0.8) were 50, 32, 33 and 33kV/cm, respectively.

Abstract: Nd-doped bismuth titanate Bi4-xNdxTi3O12 (BNT) thin films were fabricated on Pt/Ti/SiO2/Si substrates by sol-gel method and spin-coating technique. The structures and the ferroelectric properties of the films were investigated. Nd doping leads to a marked improvement in the remanent polarization (Pr) and the coercive field (Ec). At the applied electric field of 260 kV/cm, Pr and Ec of the BNT film with x=0.5 annealed at 650oC are 19 μC/cm2 and 135 kV/cm, respectively. Moreover, the BNT film with x=0.5 showed a fatiguefree behavior up to 3×1010 read/write cycles.

Abstract: The electrical properties of Gd-doped bismuth titanates (Bi3.25Gd0.75Ti3O12, BGT) prepared by a conventional ceramic technique have been investigated. At applied d.c. field below 200V/mm, the current-voltage curve of the BGT ceramic exhibits a negative differential resistance behavior. The impedance spectrum indicates that the sample consists of semiconducting grain and moderately insulating grain boundary regions. XRD, SEM and EPMA analyses reveal crystalline phase characterized by a Bi-layered perovskite structure of Bi4Ti3O12 (BIT) and the distribution of every element is uniform. The BGT sample exhibits randomly oriented and plate-like morphology.

Abstract: Nd-doped bismuth titanate and random oriented Bi4-xNdxTi3O12 (BNT) thin films were fabricated on Pt/Ti/SiO2/Si substrates by rf magnetron sputtering technique. These samples had polycrystalline Bi-layered perovskite structure without preferred orientation, and consisted of well developed rod-like grains with random orientation. The remanent polarization (Pr) and coercive field (Ec) of the BNT film with x = 0.5 were above 19 μC/cm2 and 50 KV/cm, respectively. Nd doping into BIT caused a shift of the Curie temperature (TC) of the BIT from 675°C to 660, 520, 410 and 256oC for the films with x = 0.25, 0.5, 0.75 and 1.0, respectively. The experimental results indicated that Nd doping into BIT result in a remarkable improvement in ferroelectric and dielectric properties.

Abstract: Pr6O11-doped bismuth titanate and random oriented BixPryTi3O12 ( y = 0.3, 0.6, 0.9, 1.2) thin films were fabricated on Pt/Ti/SiO2/Si substrates by rf magnetron sputtering technique. These samples had polycrystalline Bi-layered perovskite structure without preferred orientation, and consisted of well developed rod-like grains with random orientation. Pr doping into BIT caused a shift of the Curie temperature (TC) of the BIT from 675°C to 578 , 517, 398 , and 315oC for the films with y = 0.3, 0.6, 0.9, and 1.2, respectively. The experimental results indicated that Pr doping into BIT result in a remarkable improvement in dielectric properties. Raman analysis shows that Pr3+ and Pr4+ ions substitution only appears in A-sit.

Abstract: Dielectric characteristics of strontium titanate based ceramics doped with samarium were investigated within temperature range from –60oC to 150oC. XRD was used to identify the phase compositions of the ceramics and the microstructures were characterized by SEM. Good temperature coefficient of capacitance (TCC) was observed: TCC = +18% ~ −11% with high dielectric constant of 18000 and relative low dielectric loss. After further modification, the present ceramics might be promising candidates for the multilayer ceramic capacitors applications at high temperature.

Abstract: Bismuth titanate nanopowders were prepared by the hydrothermal method from Bi(NO3)3·5H2O, TiCl4 and KOH aqueous solution. The influences of reaction temperature and KOH concentration on the crystalline phase, morphology, and grain size of the prepared Bi4Ti3O12 powders were investigated. The results showed that well-crystallized, dispersive Bi4Ti3O12 nanopowders could be prepared by the hydrothermal treatment at 220–260
°C for 6 h, in the case of [Ti]= 0.1 M, Bi:Ti = 4:3 and [KOH]=1.0–2.0 M. The prepared Bi4Ti3O12 powders of the orthorhombic structure were observed to be rectangle slice-shaped, and the grain sizes were about 30 nm in width and above100 nm in length. It was also confirmed that the desired concentration of KOH decreased with increasing of the treatment temperature.

Abstract: Ferroelectric thick films of Bi3NbTiO9 (BNT) with Aurivillius type structure were fabricated using the airflow deposition method at room temperature. The precursor powder for deposition was prepared by a conventional mixed oxide route. The BNT films obtained from airflow deposition had thickness between 25 and 90 μm. Subsequent sintering showed that the films were sinterable at lower temperatures than bulk samples prepared from the same powder using isostatic cold pressing technique. Thick films sintered at 900 0C displayed higher density and a higher electrical breakdown field compared with bulk samples. SEM microscopy showed similar grain size for the thick films sintered at 900 0C and corresponding bulk samples sintered at 1150 0C. A pronounced preferential grain orientation of 70% along the c-axis was observed for thick films with thickness of 252m sintered at 9000C. While the grain orientation in bulk samples was random the grains in the thick films were aligned along the substrate plane. This significant microstructural difference between thick film and bulk samples is believed to be associated with directional nature of the deposition process. The novel method allowed anisotropic film formation without applying pressure during sintering. Low sintering temperature allows reduction in bismuth losses and better grain growth control, which also contributes to the advantages of the method.

Abstract: The (1-x)PST-xPZT(PSTZT) complex perovskite relaxor ferroelectric ceramics were prepared by wolframite precursor process(named two-step-sintering method, TSSM) and conventional oxides mixing method (named one-step-sintering method, OSSM), respectively. The experimental results demonstrated that pure perovskite PSTZT ceramics could be acquired by OSSM or TSSM. The temperature dependence of permittivity and dielectric loss of PSTZT ceramics indicated that PSTZT ceramics are relaxor ferroelectric ceramics with a complete diffusive phase transition. The pyroelectric properties of PSTZT ceramics prepared by OSSM and TSSM were also investigated via quiet-state method. It was found that the larger the x value of (1-x)PST-xPZT is, the lower the pyroelectric coefficient of PSTZT ceramics is. The pyroelectric coefficient of 0.9PST-0.1PZT ceramics is about (8~25) × 10-8C/cm2·K at room temperature. The pyroelectric figure of merit of PSTZT ceramics was also discussed.