Papers by Keyword: BST

Abstract: The (Ba, Sr)TiO₃ (hereafter BST) ceramics are promising candidate for applying in tunable devices. MgO coated BST-Mg₂TiO₄ (BSTM-MT) composite ceramics were prepared to obtain the low dielectric constant, low dielectric loss, good dielectric constant temperature stability, and high tunability of BST ceramics. The Ba_0.55Sr_0.40Ca_0.05TiO₃ nanoparticles were coated with MgO using the precipitation method and then mixed with Mg₂TiO₄ powders to fabricate BSTM-MT composite ceramics. The morphologies, phases, elements, and dielectric properties of the sintered ceramics were investigated. The core-shell structure of BST powder wrapped with MgO was clearly observed from the TEM image. After sintered at 1100 °C for 2 h, the composite ceramics expressed dense microstructures from SEM images and two main phases BST and Mg2TiO4 were detected in the XRD patterns. The dielectric constant and loss tangent were both reduced after the coating. The reduced dielectric constant and loss tangent of BSTM-MT were 190, 0.0011 (2MHz), respectively. The ceramics exhibited the diffuse phase transition near the Curie temperature and the Curie temperature shifted from 10 °C to 5 °C after the coating. Since the continuous Ti-O bonds were disconnected with the MgO coating, the tunability was reduced to 15.14 % under a DC bias field of 1.1 kV/mm. The optimistic dielectric properties made it useful for the application of tunable capacitors and phase shifters.

Abstract: For energy storage devices such as capacitances, pulsed power applications and other any electronic systems, high permittivity and high breakdown strength (BDS) are necessary for dielectric materials used in high energy storage capacitors. Ba_0.4Sr_0.6TiO₃ nano-powders were directly synthesized from solution and then wrapped by a thin surrounding shell with composition of zinc oxides dopants in aluminum oxides. This core-shell structure was characterized by a conjunction of XRD, and TEM analysis. The core-shell nanoparticles were pressed into pellets with 10 mm diameter and then sintered at different temperature in air. The specimen coated with 2.0wt% compounds of aluminum oxides doped with zinc oxides showed the maximum breakdown strength of 475kV/cm and the energy storage density of 4.7J/cm3 while the tanδ was only 0.002, whereas the uncoated Ba_0.4Sr_0.6TiO₃ was only 86.9kV/cm, 0.23J/cm3 and 0.005, respectively. And the energy storage density and the BDS decreased when the shell content was more than 2.0wt%.

Abstract: In this work, (Ba0.65Sr0.35)TiO3 (BST) and Ba(Zr0.20Ti0.80)O3 (BZT) ceramic targets were prepared using the traditional solid-state reaction technique, which were sintered at 1200 °C for 2 h. The thin films of BST, BZT and sandwich structure BST/BZT/BST were grown on Pt/Ti/SiO2/Si(100) and Si(100) substrates by rf-sputtering at 500 °C, respectively. And all samples crystallied at temperatures 650 °C for 30 min in oxygen atmosphere. The cross-sectional images of the thin films were characterized by scanning electron microscope (SEM). The dielectric constant and dissipation factor tan of the BZT, BST and BST/BZT/BST thin films are 680 and 0.030, 240 and 0.021, 85 and 0.018, respectively at 100 kHz. Compared with BST and BZT films, the BST/BZT/BST film has lower dielectric constant and lower dissipation. The remanent polarization (Pr) of the sandwich structure BST/BZT/BST thin film is up to 9.57 μC/cm2, the Pr value is larger than that BST (0.25μC/cm2) film and BZT (8.45μC/cm2) film. The optical properties (refractive index n and extinction coefficient k) of the BST/BZT/BST film on Si(100) substrate were measured by n & k analyzer 2000.

Abstract: This paper reports a study of Ba_0.9Sr_0.1TiO₃ films deposited on Pt/Ti/SiO₂/Si substrates. The annealing temperatures were 750°C, 850°C and 950°C. An increase of the average size of grains was observed, from 60 nm at 750°C to 110 nm at 950°C, as well as an increase of the dielectric constant, remnant polarization and tunability. When the annealing time was decreased from 1 hour to 15 min, the dielectric constant and remnant polarization values have been increased. The optimized annealing conditions (950°C for 15 min) give the following results: ε_r = 780 and tgδ = 0.01 at 100 kHz, Pr = 13 µC/cm², Ec = 63 kV/cm and a tunability of 55%.

Abstract: Ba_xSr_1-xTiO₃ [(BST), 0£x£1] is a solid solution ferroelectric material exhibiting a large dielectric constant non-linear change with an applied DC electric field. And Ba(Zr_xTi_1-x)O₃ solid solution also has received much attention because of the good tunbality. In this paper, Ba_0.6Sr_0.4(Zr_xTi_1-x)O₃ (x=0.15,0.20,0.25,0.30,) ceramic was obtained by traditional ceramic processing. The structural, surface morphological, dielectric properties, tunable properties of this material as well as the mechanism of their nonlinear dielectric constant under DC electric field were systematically examined. The Zr ions substitution of Ti ions has a strong effect on the grain sizes and the dielectric property. The results show that Ba_0.6Sr_0.4(Zr_xTi_1-x)O₃ (x = 0.2, 0.25,0.3) ceramic is a promising candidate for the DC electric field tunable materials for microwave electronics application, because they exhibit high relative tunability (44%, 48%, 35%, respectively) as the strength of electric field is up to 1.5 kV/mm, and low dielectric loss (0.0026, 0.00044, 0.0015, respectively) at 100 kHz at room temperature. X-ray diffraction analyses indicated that the samples present perovskite phase. Scanning electron microscopy micrograph of sample indicated that the grain size decreased and density increased with Zr concentration increasing.

Abstract: 100(1-y)wt%Ba_0.50Sr_0.50TiO₃—100ywt%Mg₂TiO₄ (BST-MT, y=0.76, 0.8, 0.82, marked as S1, S2, S3) composite ceramics with low permittivity (ε<60) were prepared by a traditional solid reaction method. The microstructure and dielectric properties of BST-MT composite ceramics were investigated. With the content of MT increasing, the dielectric peaks of BST-MT were depressed and broadened, thus the thermal stability was improved. However, the tunability declined with y increasing. In order to offset the temperature sensitivity, we obtained an Electrical field-Temperature (E-T) relationship by changing the dc bias field within a wide temperature range so that we could keep the tunability at a fixed value (3%). Such result explored a new route to compensate the temperature sensitivity of BST system in designing tunable microwave devices

Abstract: A multi-components multi-layer co-fired ceramic varactor with superior performance is here presented. The concept combines a tunable capacitor with optimized properties in a restricted temperature range with PTCR layers to control the overall device temperature. Different prototypes are produced as demonstrator units; one of them is described in detail. The device may be tuned by a primary bias voltage applied to the capacitor and by a secondary voltage applied to the PTCR to set the working temperature. By proper choice of materials, bias voltage and working temperature, the varactor is optimized to the requested figure of merit for a specific application. The prototype here described has shown a tunability >83% @ 2.75 V//m (Patent Pending)

Abstract: We investigated an influence of interface layer on a tunability of parallel plate (Ba, Sr)TiO3 thin film capacitors. BST thin film capacitors with top electrode of Pt, Au and Ag were fabricated. BST films had thickness of 40, 60, 80 and 120nm. The tunability increased with increasing the BST film thickness. Considering the interfaces between BST films and electrodes as Schottky junctions, depletion layers were formed in the interfaces depending on the difference of the work function of metal electrodes. Larger external bias voltages were applied to the depletion layer than interior BST film, because the permittivity in the depletion layer was smaller than that in interior BST film. Therefore, the depletion layer lowered the tunability. Tunability decreased with increasing the thickness of the depletion layers.

Abstract: A series of BST(Ba0.65Sr0.35TiO3) sol with different viscosity were prepared by using Ba(CH3COOH)2, Sr(CH3COOH)2 and Ti(OC4H9)4 as raw materials. The BST thin films were fabricated by sol-gel method and spin-coating process on Si(100) substrates. The effect of sol viscosity on crystallization, microstructure and dielectric properties of BST thin films were analyzed by using X-ray diffractometry, scanning electron microscopy and impedance phase analyzer. The results show that the BST films coated by sol with viscosity of 3.0cp and annealed at 750°C for 1h are basically non-crystalline. The crystallization degree increases with the increase of sol viscosity at the same annealing temperature. The ABO3 perovskite structure is formed when the sol viscosity increases to 3.5cp. With further increasing sol viscosity to 6.0cp, the crystal grains of the film grow well and range very compact, the surface become smooth, and the films have optimal dielectric properties.

Abstract: The structure and property of Ce-doped Ba0.2Sr0.8TiO3 (BST) were investigated as a function of Ce content. The density experiment results confirmed that increasing the Ce doping ratio caused the decrease in shrinkage factor of BST in the sintering procedure. Additionally, both Scanning Electron Microscope (SEM) and X-ray diffraction (XRD) analysis showed that the grain size of Ce-doped BST was dependent on the Ce content. Further more, the dielectric constant and dielectric loss had a curve relationship with increasing Ce content. The improvement of the electrical properties of Ce doping BST may be related to the decrease in the concentration of oxygen vacancies. According to the research, the diameter of grain, the dielectric constant and loss factor of the 1mol% Ce-doped Ba0.2Sr0.8TiO3 were 500nm, 365.8 and 0.0063, respectively.