Key Engineering Materials Vols. 336-338

Abstract: Epitaxial MgO thin films were deposited on Si(100) substrate by atmospheric-pressure metalorganic chemical vapor deposition for using as buffer layers. Ba0.6Sr0.4TiO3 thin films were prepared on MgO/Si(100) substrate by sol-gel technique. The independence of crystallinity on annealing temperature was investigated. The Ba0.6Sr0.4TiO3 (BST) thin films are crystallized in preferential (100) orientation after post-deposition annealing at 850°C and 950°C for 2h in air, respectively. Rutherford backscattering spectroscopy analysis confirmed that both the BST and MgO films have stoichiometric composition.

Abstract: In this study, ferroelectric thin films of Ba(Zr0.1Ti0.9)O3 were successfully deposited on Pt/Ti/SiO2/Si substrate under the optimal rf magnetron sputtering parameters, and their electrical and ferroelectric characteristics were investigated. The MFMIS structure of Al/Ba(Zr0.1Ti0.9)O3/ Pt/Ti/SiO2/Si was proposed in order to be applied as NDRO FRAM applications. From the experimental results obtained, the dielectric constant and the leakage current density of BZT films were about 200 and 1×10-9A/cm2, respectively, under the electrical field of 1 MV/cm. Besides, the saturation polarization and coercive field of Ba(Zr0.1Ti0.9)O3 films were found to be 4 μC/cm2 and 25 kV/cm, respectively, as the frequency of 103 Hz was applied. The variations of saturation polarization and coercive filed of films under various frequencies ranging form 102 to 106 Hz were also discussed.

Abstract: The nano-sized Ba(Zr0.2Ti0.8)O3 powder was prepared by citrate sol-gel method. The crystal structure and the particle size of the powder were characterized by XRD. The morphology of calcined powder and the sintered ceramics was observed by TEM and SEM, respectively. The particle size of the powders calcined at 600oC is about 10 nm. The dense ceramics can be obtained at sintering temperature of 1220 oC, which is much lower than that of the powder prepared by the conventional solid method. The effects of Al2O3 additive on the sintering behavior, microstructure and dielectric properties of Ba(Zr0.2Ti0.8)O3 ceramics were also investigated.

Abstract: Ba0.65Sr0.35TiO3 (BST) thin films on p-silicon substrates were deposited by radio frequency magnetron sputtering. The effects of the deposition parameters on the crystallization and microstructure of BST thin films were investigated by X-ray diffraction and filed emission electron microscopy, respectively. The crystallization behavior of these films was apparently affected by the substrate temperature, annealing temperature and sputtering pressure. The improved crystallization can be observed for BST thin films that deposited at higher temperature. The dominant X-ray diffraction peaks became sharper and more intense as the annealing temperature increased. BST thin films deposited at high sputtering pressure of 3.9 Pa exhibited the (110) + (200) preferred orientation. Possible correlations of the crystallization with the sputtering pressure were discussed. The SEM morphology indicated the film was small grains and smooth.

Abstract: The barium titanate based X7R ceramics, which are doped with rare earth oxide, Nb2O5, Co3O4 and ZnO-B2O3, could be sintered at a rather low temperature to satisfy the X7R requirements with good permittivity and dielectric loss. In this paper, different doping methods of sintering aids have been used to prepare X7R BaTiO3 ceramics. Samples were sintered at a low temperature and the relationship of doping methods and sintering properties has been concluded.

Abstract: Barium titanate ferroelectric films on Pt substrates were fabricated by electrophoretic deposition (EPD) technique. The barium titanate films sintered at different temperature (600°C, 900°C, 1050°C, 1150°C, 1200°C) for 2h, which were prepared via repeated EPD-sintering route 2~3 times were sufficiently dense and had uniform microstructure. The XRD pattern and SEM microphotography have been measured in order to investigate the effects of the final annealing temperature on the phase compositions and microstructure of the film. The experiment results showed that barium titanate films were crystallized into the tetragonal phase at 1050°C and above. The room temperature dielectric constant (ε) and loss tangent (tanδ) at 1kHz were respectively determined for different samples sintered at 600°C, 900°C, 1050°C, 1150°C and 1200°C. Both ε and tanδ showed anomaly peaks at 125°C. The results showed that typical BaTiO3 films sintered at 1050°C having higher dielectric constant (ε=2300) and lower dielectric loss (tanδ=0.02). The room temperature remanant polarization (Pr) and coercive field (EC) were found to be 1.9μC/cm2 and 35kV/cm, respectively.

Abstract: The Nb and Co doped barium titanate was synthesized by sol-gel auto-ignition synthesis process (abbreviated SAS) and compared with the doped powder of the same composition prepared by conventional ball milling process. The phase structure, morphology of the two as-obtained powders and correlative dielectric properties of sintered BaTiO3-based ceramics were measured. The XRD analysis demonstrated that the SAS powder was the Ba(M0.047Ti0.953)O3(M=Nb, Co) solid solution based on BaTiO3; it further suggested that Nb and Co cations could replace the Ti ions and reach reciprocal balance of acceptors and donors during the decomposition step of the organic fuel by igniting the dried gel. TEM observation showed that the Ba(M0.047Ti0.953)O3(M=Nb, Co) particles were spherical with the size ranged from 30 to 110nm. Furthermore, it was found that the value of the Curie temperature of both the doped powders was being lowered in comparison with pure BaTiO3 (Tc≈128°C ); and the dielectric constant at room temperature of the SAS powder was 5840, which was much higher than that of the conventional ball milling doped powder(3013). It was attributed to the maximum homogeneous distribution of dopants in Ba-Ti initial solution at atomic level via the SAS process.

Abstract: Barium strontium titanate (BST) ferroelectric thin films have been successfully prepared on the single crystal silicon substrates using hydrothermal crystallization of sol-gel precursor, which combined the conventional sol-gel process and the hydrothermal method. The precursor film was deposited by the dip-coating method. The multiplayer BST gel films were hydrothermally treated in a Teflon vessel at low temperature of 160-180 °C for 1-3.5 h. The influences of the reaction conditions on the structure and crystallizing degree of BST thin films were discussed. We find that the temperature of as-dried gel films and post hydrothermal treatment are the critical parameters determining the quality of formation of BST thin films. Under optimized conditions, we can obtain well-developed perovskite BST films with smooth and pinhole-free surfaces. These results reveal that this technique is a promising low temperature process for the fabricating of thin films.

Abstract: In this work, a sub-micron grained sized (<500nm) ceramic (> 92% of theoretical) has been obtained using nano-sized BaTiO3 powders synthesized through a high-gravity reactive precipitation (HGRP) process. Permittivity of the prepared ceramic is greater than 2500. Also, low change ratio of temperature in the range of −55°C~ 125°C
i.e.< ±15% renders the ceramic to meet the EIA X7R characteristics. In the ceramic compositions, Dy2O3, BaCO3, CaCO3, MnCO3 and MgO were used as dopants. After being mixed and dry-pressed, the above mentioned compositions in reducing atmosphere (N2 and 5% H2) were sintered at 1210°C−1290°C for 2h, respectively. Experimental results show that HGRP BaTiO3 powders have good dielectric properties. The dielectric properties as well as the microstructure of the ceramic are both sensitive to the change of sintering temperature.

Abstract: Infrared-to-visible upconversion phosphors Y2O3: Yb, Er and Gd2O3:Yb, Er with narrow size distribution were synthesized by using a stable microemulsion system, which included triton X-100, n-hexanol, cyclohexane and water. Appearance-controllable nanocrystals were achieved by adjusting the ratios of assistant surfactant and surfactant. The X-ray diffraction patterns of the samples showed that the powders were cubic phases of Y2O3 and Gd2O3, respectively. After calcination with different temperature settings in air atmosphere, the particles were spherical through the observation by scanning electronic microscopy. Red and green upconversion luminescence was obtained under 980 nm infrared excitation. Several contributions to the intensity of upconversion emission were also investigated. These upconversion phosphors may be used as fluorescent labels for the sensitive detection of biomolecule, immunoassay or DNA assays.