The effects of microwave sintering on the sintering behaviour, microstructure and dielectric properties of Bi2O3-doped (Ba0.6Sr0.4)(Ti0.94Cu0.06)O3 (BSTC) ceramics were investigated. The microstructure and dielectric properties of a BSTC ceramic were also studied given different amounts of Bi2O3 doping. Microwave heating with sintering temperatures below 1000°C significantly improves the densification of Bi2O3-doped BSTC ceramics. The BSTC ceramic with 1 wt% Bi2O3 addition sintered at 950°C in air for 30 min exhibited dielectric properties of er = 3756, dielectric loss of tanδ = 7 × 10-3 and bulk density > 96% of theoretical density.
It is well known that barium and strontium titanates (BaTiO3 and SrTiO3) can be formed by the solid solutions because of their identical crystal structures and the comparable ionic radii of Ba2+ and Sr2+ [1-4]. This material is very promising for practical applications, such as phase shifter, delay lines, tunable filters, steerable antennas, etc. [5-7]. However, an ideal BST (BaSrTiO3) system should exhibit the following characteristics: high dielectric constant (er), low dissipation factor (tand), large tunability and a low temperature dependence [5,6,8].
Pure BST ceramics have to be sintered at ~1350˚C, a temperature range much higher than the melting point of conductors such as Ag (961˚C) or Cu (1083˚C). Therefore, only platinum or refractory metals can be used as inner conductors [
Researchers have already reported attempts to decrease the sintering temperature of BST to 900˚C so that it can be used in LTCC (Low temperature cofiring ceramics) applications. Rhim et al. [
Microwave sintering is a method of internal self-heating through the absorption of microwave power. Therefore, internal microwave sintering can be compared with external sintering by thermal conduction or radiation. Microwave sintering is expected to produce a fine grain, a high degree of uniformity and high densification in ceramics, along with improving electrical and mechanical properties [
This experiment is based on (Ba0.6Sr0.4)(Ti0.94Cu0.06)O3 (BSTC) because of its low Tc, high dielectric constant, relatively low loss tangent, and good tunability. In a previous study [
Conventional ceramic fabrication processes were used to prepare the present BSTC samples from commercial powders of BaCO3, SrCO3, CuO and TiO2. The BaCO3, SrCO3, TiO2, and CuO powders were mixed with deionized water for 24 h in a f2 mm zirconia ball-mill. The mixture was dried, calcined at 1100˚C for 6 h in air, and then crushed into a powder. An appropriate amount of Bi2O3 was mixed with the BSTC powders using the same procedure. The powders were mixed with the binder (Polyvinyl alcohol; PVA) additive and then were pressed into disk-shaped specimens. The samples were sintered in air by microwave processing, ramping at 30˚C/min. The temperature of the sample was monitored with a type-R thermocouple shielded with platinum foil and grounded to the inner metallic wall of the microwave furnace. The samples were sintered at various temperatures from 900˚C to 1000˚C, held for 30 min at the peak temperature. Samples were microwave sintered using a single-mode microwave furnace with a cavity of 37 cm × 34.5 cm × 33.5 cm. The microwave sintering experiments were conducted in a 2.4 kW, 2.45 GHz. The samples were encased in a microwave susceptor (SiC) located in a thermal insulation package in the microwave chamber. Sintering temperature is measured using an optical pyrometer (President Honor industries Co., Ltd., Taiwan) focused directly onto the samples. The model of optical pyrometer is SH60 which can be applied at 600˚C - 1200˚C. The pyrometer is directly connected to the controller and does not influence or interfere with the microwave field distribution within the cavity. However, the traditional metal thermocouples can interfere with the microwave field within the cavity preventing accurate measurements from being made. The optical pyrometer was calibrated at several temperature points using a type B PtRh thermocouple placed in contact with the samples.
The crystalline phases of the sintered ceramics were identified by X-ray diffraction pattern analysis (XRD, Bruker D8A, Germany) using Cu-Kα radiation for 2q from 20˚ to 80˚. The diffraction spectra were collected at a scan rate of 2.5˚/min. Microstructural observation of the sintered ceramics was performed using a scanning electron microscope (SEM, JEOL. JEL-6400 Japan) equipped with energy-dispersive spectroscopy (EDS). The bulk density of the sintered pellets was measured using the Archimedes method. Particle size was measured using a particle size analyzer (Malvern, Mastersizer 2000, UK). The capacitance and dissipation factor were measured at 1MHz and 23˚C (HP4278A). The dielectric properties of the samples were measured as a function of temperature using a HP 4284A LCR meter and programmable temperature chamber interfaced to a PC for automated measurements, and samples were measured at temperatures ranging from −55˚C to 125˚C.
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This study aims to find a way to reduce the sintering temperature of BSTC. The influence of Bi2O3 on the microstructural characteristics and dielectric properties was studied. The interaction of Bi2O3 with BSTC after microwave heating was investigated using X-ray diffraction.
stants, low dielectric losses, high tunability [5,8,17] and could be suitable for use in dielectric resonators at microwave frequencies.
SEM micrographs of the BSTC specimens sintered at 950˚C with 0, 0.5, 1, and 3 wt% of Bi2O3 are shown in Figures 4(a)-(d), respectively. The images show that significant densification of BSTC ceramics with Bi2O3 dopant occurred. It is widely accepted that pure BSTC ceramic has to be sintered at 1150˚C for several hours [
relative density of BSTC ceramics is discussed below. The effect of microwave sintering temperature on microstructure was also observed. SEM micrographs of the polished surfaces of the 1 wt% Bi2O3-doped BSTC specimens are shown in Figures 5(a)-(c), for sintering at 900, 950, and 1000˚C, respectively. In
For BSTC with 3 wt% Bi2O3 addition, we observed a microstructure with a significant amount of abnormal grains as shown in Figures 4(d) and 5(c). According to the XRD spectra shown in
As described in the experimental section, the BSTC ceramics with different Bi2O3 additions were sintered in air at temperatures ranging from 900˚C to 1000˚C for 30 min.
It is well known, pure BSTC ceramic has to be sintered at ~1150˚C for several hours, therefore BSTC ceramics with a small amount of Bi2O3 addition using microwave heating can increase the density of BSTC ceramics at 950˚C. Related results reported by other investigators [13,19] show that the Bi2O3 assists in the densification of the BST dielectrics through liquid-phase sintering. It is also interesting to note that the sintered ce-
ramics with the highest bulk density is the ceramics with the greatest amount of Bi2O3 addition at 1000˚C. In fact, the addition of 3 wt% Bi2O3 to the BSTC ceramics that were sintered at 1000˚C has resulted in the highest density among the sintered specimens. The reason for this is that overdoped Bi2O3 did not become volatile or form an appropriate amount of the liquid phase; it remained in the specimens and formed a secondary phase. According to SEM morphology and XRD analysis, the secondary phase of CuBi2O4 increased significantly with greater Bi2O3 additions. Hasegawa et al. [
The dielectric constant (er) of Bi2O3-doped BSTC ceramics as functions of microwave sintering temperatures and amount of Bi2O3 addition was measured at 1 MHz at ambient temperature with the results shown in
Measurement of the electrical properties over a temperature range of −55˚C - 125˚C enabled the Curie peak at the Curie temperature, Tc, to be observed and also gave an idea of the possible effects of temperature variation (i.e. in the region of room temperature which is im-
portant for most applications) [
The effect of the addition of Bi2O3 and microwave sintering on phase evolution, microstructure and dielectric properties of B0.6S0.4(Ti0.94Cu0.06)O3 ceramics has been investigated. It was discovered that adding Bi2O3 to BSTC ceramics by microwave heating can lower the sintering temperature from 1150˚C to 950˚C and in-
creases the bulk density of the sintered ceramics. An Xray diffraction examination of the products indicated that they consist mainly of a Ba0.6Sr0.4TiO3 crystalline phase with a CuBi2O4 as secondary phase, which formed as a result of Bi2O3 addition. A BSTC ceramics formed by adding 0.5 wt% Bi2O3 and microwave heating at 950˚C in air for 2 h gives a permittivity, er = 3756, tanδ = 7 × 10−3 and a bulk density >96% of theoretical.