Tunable microwave devices using low-sintering-temperature screen-printed barium strontium titanate (BST) thick films

A tunable microwave phase shifter composed of a low-sintering-temperature, screen-printable barium strontium titanate (BST) film and silver metallization was fabricated on an alumina substrate and co-fired at 900 °C for 3 h. The dielectric properties of the films were characterized in a frequency range of 0.8–8 GHz using scattering parameter measurements and a quasi-static coplanar waveguide transmission line model. The temperature dependency of the films was measured through capacitance measurements in a frequency range of 0.5–2.5 GHz. The figure of merit (phase shift/dB of insertion loss) of the phase shifter was found to be 14.6 at 3 GHz with an applied bias field strength of 2.5 V/μm. The performance of the phase shifter is briefly discussed and compared with other phase shifters fabricated by direct screen-printing of BST films.     

Screen printed barium strontium titanate films for microwave applications

Powders to be used in inks for screen printing of Ba_0.5Sr_0.5TiO₃ thick films have been prepared by two different routes — calcination of BaCO₃, SrCO₃ and TiO₂ by a conventional solid-state reaction, and thorough mixing of uncalcined nano-sized BaTiO₃ and SrTiO₃. The effect of these two different production routes on the microstructural properties in terms of grain size and density has been assessed using SEM. Electrical measurements at 1 kHz indicate that both permittivity and dielectric loss are affected by the density of the films. Permittivity also appears to be affected by the grain size of the films, which increases dramatically above a particular sintering temperature. The Curie temperature also decreases as sintering temperature increases and this has been linked to observed Ba diffusion from the film into the alumina substrate.     

Dielectric and pyroelectric properties of barium strontium calcium titanate ceramics

(Ba_0.6Sr_0.3Ca_0.1)TiO₃ powders were prepared by the sol-gel method using a solution of Ba, Sr and Ca acetate and Ti isopropoxide, and the specimens doped with MnCO₃ (0.1 mol%) and Y₂O₃ (0.5 mol%) were fabricated by the cold isostatic press method. The urethane pot and zirconia ball were used in the mixing and grinding process, and the green pellets were sintered at 1450 °C for 2 h in the alumina crucible. The specimen exhibited a dense and void-free grain structure with grain size of about 3 μm. The dielectric constant and the dielectric loss at Curie temperature were 16,600 and 1.2%, respectively. The specimen under a 4 kV cm⁻¹ DC bias field showed the maximum pyroelectric coefficient of 550×10⁻⁹ C cm⁻² K^-1 at Curie temperature. The figure of merit F.M.D¹ for specific detectivity (D¹) of the specimen, applied with DC 8 kV cm⁻¹ bias field, was the highest value of 17.6×10⁻⁹ C cm J⁻¹ at Curie temperature.     

Effects of glass additions on the microstructure and dielectric properties of barium strontium titanate (BST) ceramics

Commercial glass frits (lead borosilicate glasses) were employed as the sintering aids to reduce the sintering temperatures of BST ceramics. The effects of the glass content and the sintering temperature on the microstructures, dielectric properties and tunabilities of BST ceramics have been investigated. Densification of BST ceramics of 5 wt% glass content becomes significant from sintering temperature of 1000 °C. The glass content shows a strong influence on the Curie temperature T_c, permittivity and the diffuse transition. X-ray results show all BST ceramics exhibit a perovskite structure and also the formation of a secondary phase, Ba₂TiSi₂O₈. The shift of BST diffraction peaks towards higher angle with increasing the glass content indicates the substitution of Pb²⁺ in Ba²⁺ site, which mainly accounts for the diffuse transition observed in these BST ceramics. BST ceramics with 10 wt% glass additives possess the highest tunability at all four sintering temperatures. A tunability of 12.2% at a bias field of 1 kV/mm was achieved for BST ceramics with 10 wt% glass content sintered at 900 °C.     

Influence of sputter deposition parameters on the properties of tunable barium strontium titanate thin films for microwave applications

Barium strontium titanate (BST) thin films are studied with respect to their application as tunable dielectric at microwave frequencies. BST thin films are deposited by means of radio-frequency magnetron sputtering on platinized Si substrates. The substrate to target distance during sputter deposition is varied and the effect on structure, topology, composition and electronic properties is monitored using X-ray diffraction, atomic force microscopy, Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy. These findings are related to the dielectric measurements, which are carried out at 1 MHz and in the microwave range up to 8 GHz using metal-insulator-metal structures with Pt electrodes. For further device evaluation, leakage current measurements are carried out. Changing the process parameter strongly affects the composition of the films. The results emphasize the possibility for enhancing the microwave properties by fine-tuning of the chosen process parameter.     

Microstructure and microwave properties of inkjet printed barium strontium titanate thick-films for tunable microwave devices

Barium strontium titanate (BST) is a promising material for passive tunable microwave devices such as phase shifters, tunable filters or tunable matching networks. This publication covers the preparation of BST thick-films for microwave applications through inkjet printing. A Ba_0.6Sr_0.4TiO₃ ink was prepared, printed on alumina substrates and sintered at temperatures between 1100 °C and 1200 °C. The microstructure of the thick-films reveals the evolution of grain growth and porosity with increasing sintering temperature. Furthermore, a reaction with the substrate was observed for T ≥ 1175 °C. A maximum tunability of 36% was observed at temperatures right below the onset of the substrate reactions. This process conditions were used for the preparation of a loaded line phase shifter, which successfully shows the capability of the inkjet printing process for future microwave device fabrication.     

Energy storage properties of low concentration Fe-doped barium strontium titanate thin films

Ba_0.7Sr_0.3Fe_xTi_1?xO₃ (x = 0, 0.004, 0.008, 0.01, 0.015) ceramic thin films were prepared on platinized silicon wafers via sol-gel deposition method. Significant enhancement of grain size and dielectric constant were observed when x<0.008, accounting for effectively enhanced polarization, and beneficial for the energy storage applications. Excessive iron doping resulted in reduced grain size and deteriorated energy storage properties. The optimal energy storage behavior was achieved in samples with x=0.008, exhibiting a high discharged energy density of 7.6 J/cm³.     

Predicting dielectric tunability of compositionally-graded barium strontium titanate thin films on metal substrates by thermodynamic modeling

Dielectric properties of compositionally-graded barium strontium titanate (BST) thin films respectively on Ti and stainless steel (SS) plates were computed using a modified thermodynamic model. Calculated results predict that higher dielectric constant and tunability can be obtained when the films are onto Ti substrates, which have smaller thermal expansion coefficient (TEC). For Ti substrates, “up-graded” films (Ba/Sr ratio increases from substrate to surface) exhibit higher tunability than “down-graded” films; whereas “up-graded” films on SS plates have relatively lower tunability. The received larger dielectric constant and tunability are because of smaller total out-of-plane polarization, which is resulting from smaller total compressive strain. The calculated dielectric constant and tunability of such graded films are in accordance with those of the films fabricated via sol-gel approaches, indicating that the dielectric behavior of ferroelectric films can be adjusted by designing compositional gradient.     

Synthesis of barium strontium titanate nanopowders by microwave hydrothermal method

Barium strontium titanate (BST) powders of high purity, good crystallinity and well dispersed with a diameter of 50–90?nm are prepared by microwave hydrothermal method using tetrabutyl titanate, barium nitrate and strontium nitrate as the main raw materials. The phase composition, element composition and microstructure of the powders are characterised by X-ray diffraction, Fourier transform infrared spectroscopy, energy dispersive spectroscopy and transmission electron microscopy. The results show that Ba_0.6Sr_0.4TiO₃ powders could be synthesised under the conditions that the reaction temperature is 70°C, the reaction time is 10?min, and the value of pH is 14, which indicates that the reaction temperature, time and the value of pH have a great effect on the crystallinity of nanopowders, and the dispersant (OP-10) plays a role in the dispersion of the nanopowders.     

The influence of processing on the microstructure and the microwave properties of Co-F-codoped barium strontium titanate thick-films

The influence of processing on the microstructure and the dielectric properties of Co-F-codoped Ba_0.6Sr_0.4TiO₃ (BST) thick-films has been investigated. BST powders with different particle sizes were prepared and applied on alumina substrates by screen-printing. The resulting thick-films were sintered at different holding times and characterized with respect to their microstructure and microwave properties. The microstructure of the thick-films shows a clear dependency on sintering time and initial particle size. In addition to grain growth, the formation of a secondary phase is observed at the interface between substrate and BST with increasing sintering time. The dielectric characterization at microwave frequencies shows an increase of tunability with larger grain size while the dielectric loss is even lowered. This shows the strong influence of the microstructure on the material properties and the possibility of tailoring the material through specific processing.     

Highly porous barium strontium titanate (BST) ceramic foams with low dielectric constant from particle‐stabilized foams

A novel method for fabrication of highly porous barium strontium titanate (BST) ceramic foams based on particle‐stabilized foaming method was developed for the first time, in which propyl gallate (PG) was employed as BST particle modifier. The results showed that the stability of wet BST foams closely depends on the pH value and PG concentration, which could be explained by the adsorption behavior of PG on BST particle surface. BST ceramic foams with dense, uniform, and closed pore and defect‐free wall were obtained. The pore size and porosity can be well controlled by adjusting solid loading and sintering temperature. It was revealed that not only sintering temperature but also solid loading significantly influenced the growth of BST grain. The BST ceramic foams exhibited high porosity in the range of 81%‐95%, low dielectric constant in the range of 47‐150, and low dielectric loss below 0.0025. The BST ceramic foams with higher porosity presented a tendency of lower dielectric constant and the fitting results indicated that the natural logarithm of dielectric constant was linear correlated with porosity.     

Preparation and dielectric tunability of bismuth-based pyrochlore dielectric thick films on alumina substrates

Metal–insulator–metal capacitors structures, employing the cubic pyrochlore Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) thick films, were fabricated by screen-printing techniques on alumina substrates. Chemical compatibility and microstructure between layers was studied by EDS/SEM analysis. The dielectric properties of BZN thick films have been investigated as a function of temperature and frequency. The films exhibited the dielectric constant up to 130, and the dielectric loss less than 0.005 at 1 MHz. The dielectric tunabilities of the films have been compared at different temperature. The BZN thick film showed low tunability of about 1.8% at 10 kHz under 100 kV/cm dc bias voltage in temperature from −150 to 180 °C. The temperature stability of dielectric properties for BZN thick film should be useful for designing temperature stable frequency agile.     

PZT thick films on LTCC substrates with an interposed alumina barrier layer

PZT thick films (PbZr_0.53Ti_0.47O₃ with the addition of 6% PbO and 2% Pb₅Ge₃O₁₁) with a low sintering temperature were printed and fired on LTCC substrates (951, Du Pont), covered with an alumina barrier layer. The electrical characteristics (remanent polarisation, coercive field, dielectric constant and dielectric loss) of these PZT thick films, together with sets prepared on “unprotected” LTCC substrates and on alumina substrates were compared. Whereas the electrical characteristics of the films on LTCC substrates deteriorated significantly due to interactions between the LTCC substrates and the PZT layers the values obtained for the LTCC/alumina barrier structures were comparable with those on ceramic alumina substrates.     

Synthesis,characterisation and dielectric properties of low-loss Zr-doped barium strontium titanate materials

ABSTRACT

A series of materials of the composition Ba_0.7Sr_0.3Zr_xTi_(1???x)O₃ (BSZT), where x?=?0.00, 0.01, 0.02 and 0.03, were synthesised through the sol–gel method whereupon the manifestations of partial substitution of Ti⁴⁺ by Zr⁴⁺ on the structural and dielectric behaviour of Ba_0.7Sr_0.3TiO₃ (BST) were studied. X-ray diffraction patterns of all samples showed the development of a single-phase crystalline perovskite structure. The dielectric behaviour of the ceramic samples has been studied in detail in the frequency range 10 Hz to 1 MHz at room temperature, and as a function of increase in temperature. The sample with 2% Zr recorded the highest dielectric constant. The dielectric loss values for all the BSZT compositions are remarkably low, tan?δ?=?0.021～0.026 at 1 MHz, in contrast to the literature reports. It is observed that small amount of Zr doping on BST, where 0?<?x?≤?0.03 results in the enhancement of dielectric constant while decreasing the bulk density and dielectric loss.     

Dielectric properties of barium titanate ceramics with different materials powder size

The dielectric properties of barium titanium ceramics fabricated with nano-size fine powders (about 40 nm) are compared with that fabricated with micro-size coarse powders (about 2 μm). Three kinds of ceramics were fabricated; one using pure nano-size fine powders, the other using pure micro-size coarse powders, and the third using the combination of both. The sintering temperature of the ceramics with pure nano-size fine powders is 150 °C lower than that with pure micro-size coarse powders. For the same sintering conditions, the relative density of the ceramics is increases with the amount of nano-size fine powders. The grain size of the ceramics body with pure micro-size coarse powder is about 5 μm, but that of pure nano-size fine powder is about 1 μm. The room temperature dielectric constant of the ceramics increases with the increasing of the amount of nano-size fine powder. For pure nano-size fine powders, the room temperature dielectric constant is about 5000, and that of micro-size coarse powders is about 2200.     

Effects of cerium doping on dielectric properties and defect mechanism of barium strontium titanate glass-ceramics

The effect of cerium content on phase evolution, dielectric properties and defect mechanism has been investigated in (Ba,Sr)TiO₃ glass-ceramics. Cerium mainly acts as an isovalent dopant in the B-site of ABO₃ perovskite structure at low content (1 mol%) and then cerium substitution gradually occurs in the A-site with increasing cerium content. A compensation mechanism related to variation in oxygen vacancy concentration has been identified. When cerium content increased to 2 mol%, the maximum values of dielectric constant and energy storage density were simultaneously achieved. The impedance spectra revealed the highest conductivity. It is due to the increase in the concentration of charge carriers accompanied by the decrease in the activation energy of oxygen vacancy migration. With a further addition of cerium to 3 mol%, the opposite trend was observed. The result is related to the presence of more cation vacancies, which, in turn, limits the diffusion rate of oxygen vacancy.     

Effects of MnO2 concentration on dielectric properties of barium strontium titanate glass ceramics

The phase development, microstructural evolution and dielectric properties of manganese-doped barium strontium titanate glass ceramics have been studied. The specimens with (Ba,Sr)TiO₃ (BST) as the major crystalline phase were prepared by bulk crystallization process. The results show that the dielectric constant and the dielectric loss measured at room temperature pass through a maximum with increasing MnO₂ concentration. This MnO₂ concentration dependence of dielectric properties was also investigated by impedance analyses. The evidence of impedance spectroscopy indicates that the activation energy values of grain and grain boundary coincide with the change in dielectric properties.     

Effects of phase constitution and microstructure on energy storage properties of barium strontium titanate ceramics

Barium strontium titanate (Ba_0.3Sr_0.7TiO₃, BST) ceramics have been prepared by conventional sintering (CS) and spark plasma sintering (SPS). The effects of phase constitution and microstructure on dielectric properties, electrical breakdown process and energy storage properties of the BST ceramics were investigated. The X-ray diffraction analysis and dielectric properties measurements showed that the cubic and tetragonal phase coexisted in the SPS sample while the CS sample contained only tetragonal phase. Much smaller grain size, lower porosity, fewer defects and dislocation were observed in SPS samples, which greatly improved the electrical breakdown strength of the Ba_0.3Sr_0.7TiO₃ ceramics. The enhanced breakdown strength of the SPS samples resulted in an improved maximum electrical energy storage density of 1.13 J/cm³ which was twice as large as that of the CS sample (0.57 J/cm³). Meanwhile, the energy storage efficiency was improved from 69.3% to 86.8% by using spark plasma sintering.     

Structural and mechanical properties of Sr-doped barium niobate thick films

Polycrystalline strontium barium niobates Sr _x Ba_{1 ? x} Nb₂O₆ (SBN, 0.40 ≤ x ≤ 0.75) were prepared by standard solid state ceramic method at relatively at low temperatures. Thick SBN films prepared by simple, low-cost, screen printing route were characterized by XRD. Preliminary structural analysis exhibits the formation of tetragonal tungsten bronze crystal structure at room temperature. Texture coefficient [TC(hkl)], dislocation density (ρ_D), density of crystallites per unit surface area (Ψ), mechanical properties, and microwave behavior of synthesized materials are reported.