Low dielectric dissipation and enhanced tunability of Ba0.6Sr0.4TiO3 thin films by the modified composition and multilayer structure

Ba_0.6Sr_0.4Ti_1+yO₃ (BST, y?=?0.1, 0.15, 0.2, 0.25, 0.3) thin films were fabricated on Pt-coated silicon substrates by modified sol-gel techniques. It was found that the tunability of BST thin films and dissipation factor decreased with the increase of Ti content. The multilayer structure of Ba_0.6Sr_0.4Ti_1+yO₃(200 nm)/Ba_0.6Sr_0.4TiO₃(100 nm)/Ba_0.6Sr_0.4Ti_1+yO₃ (200 nm; y?=?0.1, 0.2, 0.25) was designed to enhance the tunability. Our results indicated that the modified composition and multilayer structure were beneficial to lowering the dielectric dissipation and enhancing the tunability simultaneously. The tunability of 26.7% and dielectric dissipation of 0.013 were achieved for modified BST thin films.     

ENHANCED TUNABILITY AND RELAXOR CHARACTERISTICS IN CALCIUM SUBSTITUTED BARIUM ZIRCONIUM TITANATE THIN FILMS

ABSTRACT

Thin films of BaZr_0.25Ti_0.75O₃ in which Ba^{2 +} is partially substituted by Ca^{2 +} are prepared by a sol-gel method. Phases and dielectric properties of the films are studied.The crystal structure of BaZr_0.25Ti_0.75O₃ is cubic and does not change with Ca substitution upto x = 0.25 in Ba_1-xCa_xZr_0.25Ti_0.75O₃. A change from a normal to relaxor behavior is observed for x = 0.20 as indicated by a change in the frequency of dielectric maxima with temperature and other parameters.The dielectric constant and transition temperatures are found to increase with increasing Ca substitution. A tunabilty of 71%, much higher than reported in literature for this material, is observed for x = 0.1 i.e for Ba_0.9Ca_0.1Zr_0.25Ti_0.75O₃.     

Dielectric tunability and microwave properties of Ba0.5Sr0.5TiO3-BaMg6Ti6O19 composite ceramics for tunable microwave device applications

Ba_0.5Sr_0.5TiO₃-BaMg₆Ti₆O₁₉ microwave composite ceramics with low dielectric constant and relatively high tunability were fabricated via the solid-state reaction method. The microstructures and microwave dielectric properties of the composite ceramics have been investigated. BaMg₆Ti₆O₁₉ and Ba_0.5Sr_0.5TiO₃ can be friendly coexistent in the composite material system without obvious chemical reactions. With increasing content of BaMg₆Ti₆O₁₉ from 10 wt.% to 60 wt.%, the dielectric anomalous peaks of ferroelectric-paraelectric phase transition for the composite ceramics are suppressed and broadened. The dielectric constant can be effectively tailored from 2035 to 129. For composite ceramics with 60 wt.% content of BaMg₆Ti₆O₁₉, the dielectric loss still keeps around 0.002 and the tunability is 13.4% measured at a dc-applied electric field of 30 kV/cm. The Q value of composite ceramics with 20 wt.% content BaMg₆Ti₆O₁₉ is 367 and the dielectric constant is cut down to 665 at the microwave band of 1.579 GHz.     

Redox behavior and electrical properties of Sr and Zr substituted BaTiO3

The oxidation and reduction behavior of Sr and Zr substituted BaTiO₃ has been studied by measuring equilibrium electrical conductivities as a function of oxygen activity. The substitution of Sr⁺² and Zr⁺⁴ for Ba²⁺ and Ti⁴⁺ in BaTiO₃ shifted the conductivity profiles to lower oxygen partial pressures. The replacement of Ba with smaller Sr ions suppresses the oxygen dissociation from the lattice, consequently increasing the enthalpy for reduction (ΔH _n ), whereas the Sr substitution requires less energy for oxidation (ΔH _p ). The increase in reduction enthalpy at lower oxygen activities suppresses the formation of oxygen vacancies, the materials becoming resistant to reduction process. Such a reduction resistant behavior gave rise to greater electrical resistivities for (Ba_0.9Sr_0.1)(Ti_0.9Zr_0.1)O₃ than for Ba(Ti_0.9Zr_0.1)O₃ when the samples were sintered at lower P(O₂).     

Synthesis and characterization of (BaxSr1−x)Ti1+yO3+z thin films and integration into microwave varactors and phase shifters

Abstract

Precise control of composition and microstructure is critical for the production of (Ba_xSr_1?x)Ti_1+yO_3+z (BST) dielectric thin films with the large dependence of permittivity on electric field, low losses, and high electrical breakdown fields that are required for successful integration of BST into tunable high frequency devices. Here we review recent results on composition-microstructure-electrical property relationships of polycrystalline BST films produced by magnetron sputter deposition, that are appropriate for microwave devices such as phase shifters. Films with controlled compositions were grown from a stoichiometric Ba_0.5Sr_0.5TiO₃ target by control of the background processing gas pressure. It was determined that the (Ba+Sr)/Ti ratios of these BST films could be adjusted from 0.73 to 0.98 by changing the total (Ar+O₂) process pressure, while the O₂/Ar ratio did not strongly affect the metal ion composition. Film crystalline structure and dielectric properties as a function of the (Ba+Sr)/Ti ratio are discussed. Optimized BST layers yielded capacitors with low dielectric losses (0.0047), among the best reported for sputtered BST, while still maintaining tunabili-ties suitable for device applications. These BST films were used to produce distributed-cir-cuit phase-shifters, using a discrete periodic loading of a coplanar waveguide with integrated BST varactors on high-resistivity silicon. Phase shifters yielding 30 degrees of phase shift per dB of insertion loss were demonstrated at 20GHz.     

Characterization of metal (Fe,Co, Ni,Cu) and fluorine codoped barium strontium titanate thick-films for microwave applications

The influence of codoping of Fe, Co, Ni, Cu as acceptors and F as donor on the dielectric properties of screen-printed Ba_0.6Sr_0.4TiO₃ ceramic thick-films has been investigated. The undoped and codoped Ba_0.6Sr_0.4TiO₃ powders were synthesized through a sol-gel route. The thermal gravimetric analysis revealed the difference of the thermal decomposition behaviour between the undoped and codoped precursors. The ceramic powders were characterized with x-ray diffraction, scanning electron microscopy and BET measurements. Larger crystallite sizes of the codoped powders were observed. The densification behaviour of the powders was recorded by dilatometry and indicated that codoping influences the sintering mechanism. The permittivity, dielectric loss and tunability of the undoped and codoped thick-films were characterized with coplanar waveguide structures up to 30 GHz.     

Synthesis of (Ba<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>)(Ti<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> ceramics and effect of Sr content on room temperature dielectric properties

The compositions in the system (Ba_1−x Sr _x )(Ti_0.5Zr_0.5)O₃ with different Sr (x) content, were synthesized through solid oxide reaction route. The phase formation behaviors in the system were investigated by XRD. The room temperature dielectric properties of the compositions were investigated in the frequency range 10 Hz to 13 MHz. The solid solution system Ba_1−x Sr _x Ti_0.5Zr_0.5O₃ remains as cubic perovskite up to x < 0.6 and transforms into the tetragonal structure above x > 0.6. Composition with x = 0.6 contains a mixture of cubic and tetragonal phases with broadened diffraction pattern. It is observed that the increasing of Sr substitution results in the decreasing of bulk density, average grain size and dielectric constants etc. in the composition system. The AC dielectric conductivity of the ceramics also decreases with the increase in Sr-substitution due to decrease in loss as well as grain size with that substitution.     

Synthesis of (Ba0.5Sr0.5)(Ti1 − x Zr x )O3 ceramics: Effect of Zr content on room temperature electrical properties

The phase formation behavior and room temperature dielectric properties of bulk perovskite solid solution composition (Ba_0.5Sr_0.5)(Ti_1 − x Zr _x )O₃ have been investigated. The samples with different Zr-content were prepared through solid state reaction. The XRD investigation showed that Zr⁺⁴ is systematically dissolved in Ba_0.5Sr_0.5TiO₃ lattice up to about 60 atm.% substitution, having cubic Pm3m structure. Eighty atom percent Zr substituted composition showed to contain a cubic phase similar to that of x = 0.6 composition and a tetragonal (I4/mcm) phase. That is the solid solution breaks around at 80 atm.% Zr substitutions. Ba_0.5Sr_0.5ZrO₃ was having orthorhombic Imma structure. Decrease in grain sizes were observed with increase in Zr content. The permittivity of the ceramics decreased with the increase in Zr substitution. The frequency dependency of dielectric loss in the frequency range 10 Hz to 10 MHz, were improved with Zr substitution in the ceramics. The room temperature ac and dc conductivity also decreased significantly with the increase in Zr-content.     

Excellent dielectric constants observed in heterogeneous conduction Ba(Zr<Subscript>0.25</Subscript>Ti<Subscript>0.75</Subscript>)O<Subscript>3</Subscript> ceramics doped with Sr(Fe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>)O<Subscript>3</Subscript>

The (1-x)Ba(Zr_0.25Ti_0.75)O₃-xSr(Fe_0.5Nb_0.5)O₃ or (1-x)BZT-xSFN ceramics have been fabricated via a solid-state reaction technique. All ceramics exhibit a pure phase perovskite with cubic symmetry. The addition of a small amount of SFN (x?=?0.1) produces an obvious change in dielectric behavior. Very high dielectric constants (ε_r?>?164,000 at 1 kHz and temperature?>?150°C) are observed and the value is obviously higher than dielectric constants for Ba(Zr_0.25Ti_0.75)O₃ and Sr(Fe_0.5Nb_0.5)O₃ ceramics. The ferroelectric measurement data suggests that the unmodified sample exhibited a ferroelectric behavior. However, a transformation from a ferroelectric to a relaxor-like behavior is noted with increasing x concentration. Impedance Spectroscopy (IS) analysis indicates that the presence of excellent dielectric constants is due to the heterogeneous conduction in the ceramics after adding SFN, which can be explained in terms of the Maxwell-Wagner polarization mechanism.     

Deposition and characterization of low-loss epitaxial non-linear dielectric thin films for microwave devices

Abstract

Oxide ferroelectric thin films for frequency-tunable microwave devices, in which the dielectric constant of the non-linear dielectric is varied by application of electric fields, have been deposited using PLD. We have fabricated single phase epitaxial Ba_0.6Sr_0.4TiO₃ and KTaO₃ thin film capacitors for applications at 300K and 77K, respectively. Single phase KTaO₃ films were obtained only with excess potassium source in the target along with KTaO₃ perovskite phase. The films have been characterized for structure and morphology by X-ray diffraction and AFM. The dielectric properties were measured in the low frequency range from 100 kHz to 10 MHz, using interdigitated capacitors. Low loss tangents (0.002 at 300K) were observed for highly oriented Ba_0.6Sr_0.4TiO₃ films. The importance of low losses for various devices is discussed and the dielectric constants, loss tangents and tunability of these films are reported in this paper.     

Co-Modification of Ba5NdTi3Ta7O30 Dielectric Ceramics by Substitution and Introducing Secondary Phase

Co-modification of Ba₅NdTi₃Ta₇O₃₀ dielectrics ceramics was investigated through Pb substitution for Ba and introducing Bi₄Ti₃O₁₂ secondary phase. The dielectric constant increased from 150 to 283, the temperature coefficient of the dielectric constant decreased from –2500 ppm/°C to –1279 ppm/°C, and the dielectric loss decreased to 0.0007 at 1 MHz. Meanwhile, the bi-phase ceramics were investigated to achieve temperature stable ceramics with high dielectric constant and low dielectric loss. As the composition x varied from 0.4 to 0.7 for (1 – x)(Ba_0.8Pb_0.2)₅NdTi₃Ta₇O₃₀/xBi₄Ti₃O₁₂, the temperature coefficient of the dielectric constant changed from negative to zero to positive.     

IN-PLANE DIELECTRIC PROPERTIES OF EPITAXIAL Ba(Zr0.3Ti0.7)O3 THIN FILM GROWN ON LSAT (001) SINGLE CRYSTAL SUBSTRATE

ABSTRACT

Ba(Zr_0.3Ti_0.7)O₃ (BZT) thin film was deposited on (LaAlO₃)0.3(Sr₂AlTaO₆)_0.35 [LSAT] (001) single crystal substrate using pulsed laser deposition. The X-ray diffraction pattern reveals an epitaxial growth of the film with a pure perovskite phase. The in-plane dielectric properties of the Ba(Zr_0.3Ti_0.7)O₃ thin film was characterized as a function of frequency (1 kHz–500 MHz), temperature (125 K–373 K) and dc electric field (0–13.3 V/μm) using gold interdigital electrodes. The relative permittivity of the film exhibits strong dependence of the dc bias field over the whole frequency range. The relative permittivity has a high tunability of 73%–50% at room temperature in the frequency range of 1 kHz to 500 MHz, showing the potential of our Ba(Zr_0.3Ti_0.7)O₃ thin film to be used in microwave devices.     

Dielectric and relaxor ferroelectric properties of Ba-doped Pb(Zr,Ti)O3 ceramics

(Pb,Ba)(Zr,Ti)O₃ is a relaxor ferroelectric material. Dielectric and ferroelectric properties of (Pb_1-x Ba _x )(Zr_0.70Ti_0.30)O₃ ceramics have been investigated for compositions varying in the range of 0.20?≤?x?≤?0.30. Reagent grade PbO, ZrO₂, TiO₂ and BaCO₃ raw powders were used, ceramics were fabricated by convenient solid state reaction. The experimental results show that the substitution of Ba for Pb can enhance the ferroelectric relaxor characteristics. With the Ba content increasing, the electric hysteresis was narrowed and the polarization was reduced. Meanwhile the temperature T _m that corresponding to the maximal dielectric constant was decreased. It has also been found that the hydrostatic pressure may cause the phase transition more diffuse and move T _m to higher temperature.     

Dielectric Aging of Cr-Doped PZT Ceramics

The aging of dielectric properties of Cr-Doped Pb(Zr_0.52Ti_0.48)O₃ were investigated. The composition of Pb(Zr_0.52Ti_0.48)O₃ with 0.1, 0.2, 0.4, 0.6, 1.0 and 3.0 mol% of Cr₂O₃ addition were prepared by conventional mixed oxide technique. The aging has been measured at room temperature after quenching from high temperatures above the Curie temperature. The dielectric aging of all compositions revealed relaxor aging behavior. The aging rate of stable stage obeyed a logarithmic time dependence, while the aging rate decreased with increasing Cr₂O₃ at low concentration (0.1–0.6). Further increase in Cr₂O₃ concentration, the aging rate turned to increase.     

Structure,Ferroelectric, Dielectric and Energy Storage Studies of Ba0.70Ca0.30TiO3, Ba(Zr0.20Ti0.80)O3 Ceramic Capacitors

Ba_0.70Ca_0.30TiO₃-(BCT),Ba(Zr_0.2Ti_0.8)O₃-(BZT) ceramics were fabricated by conventional mixed oxide route to develop inorganic dielectric materials suitable for use as an insulator with high dielectric constant and low energy loss for capacitor applications. The structural phase transition, ferroelectric, dielectric and energy storage properties of BCT, BZT ceramic capacitors were investigated. Room temperature X-ray diffraction (XRD) patterns revealed prominent peaks corresponding to tetragonal perovskite crystal structure for both BCT, BZT solid solutions. Slim ferroelectric hysteresis (P-E) loops were observed for BCT, BZT solid solutions. Temperature dependent dielectric property measurements of BCT, BZT solid solutions have shown a high dielectric constant and low dielectric loss. Room temperature (300K) breakdown field strength and energy densities were obtained from the integral area of P-E loops. For the BCT ceramics, the largest recoverable energy (unreleased energy) density is 1.41 J/cm³ with dielectric breakdown strength as high as 150 kV/cm. For the BZT ceramics, the largest recoverable energy (unreleased energy) density is 0.71 J/cm³ with dielectric breakdown strength as high as 150 kV/cm. Bulk BCT, BZT ceramics have shown interesting energy densities; these might be the strong candidate materials for capacitor applications.     

Dielectric properties and tunability of BST and BZT thick films for microwave applications

Abstract

Dielectric properties and tunability of Ba_0.60Sr_0.40TiO₃ (BST) and BaZr_0.25Ti_0.75O₃ (BZT) thick films and bulk ceramics have been investigated as a function of temperature (90 K - 320 K) in the kHz region. Thick films show compared to bulk ceramics low permittivity and a very broad ferroelectric phase transition which leads to low temperature dependence of tunability. Tunability of 30% can be achieved with an external field of 2 kV/mm. Measurements in the microwave region adumbrate that the dielectric losses of BZT are about three times higher than that of BST thick films.     

Dielectric properties of (Ba1-2x Sr x Ca x )TiO3 ferroelectric ceramics

Ferroelectric ceramics of (Ba_1?2x Sr _x Ca _x )TiO₃ (0?≤?x?≤?0.30) were prepared by a routine solid-state reaction technique. Co-substitution of Sr²⁺ and Ca²⁺ for Ba²⁺ with equal mole in BaTiO₃ restrain the maximal dielectric constant K _m strongly when 0.2?≥?x?>?0. However, composition (Ba_0.4Sr_0.3Ca_0.3)TiO₃ shows the higher maximal dielectric constant. With increasing Sr and Ca content x, temperature T _m of the maximal dielectric constant K _m shifts to low temperature and all selected compositions for study exhibit a broad phase transition temperature range. The composition (Ba_0.4Sr_0.3Ca_0.3)TiO₃ presents characteristics of ferroelectric relaxor, value of K _m decreases and temperature T _m increases with increasing frequency.     

Microwave frequency conversion in coplanar waveguide on bilayered substrate with (Ba,Sr)TiO3 film

Abstract

Experimental and simulated spectra of microwave (30GHz) signal at the output of the coplanar waveguide (CPW) based on Ba_xSr_1-xTiO₃ film are presented and discussed. The modulation of Ba_xSr_1-xTiO₃ film dielectric constant by the application of high frequency (f ≈ 20MHz) control signal results in the generation of side frequency components in the output signal spectrum. The analysis of experimental data is performed on the basis of considering the CPW section with tunable effective dielectric constant as a phase modulator     

The tunability and dielectric properties of the compositionally graded Ba(Zr x Ti1−x )O3 thin films

The compositionally graded Ba(Zr _x Ti_1?x )O₃ films with a compositional gradient from BaTiO₃ to BaZr_0.35Ti_0.65O₃ were fabricated on LNO-buffered Pt/Ti/SiO₂/Si substrates by a sol-gel deposition method. In order to confirm the compositional gradient, a combination of X-ray Photoelectron Spectroscopy (XPS) and Ar ion etching was employed to produce the composition depth profile. Dielectric constant peaks, common to a ferroelectric transition, were not observed in the temperature range from ?50 to 100 °C, within which the dielectric constant showed negligible temperature dependence. The compositionally graded Ba(Zr _x Ti_1?x )O₃ thin films with weak temperature dependence of tunability could be attractive materials for situations in which precise control of temperature would be either impossible or too expensive.     

Composition dependent electrocaloric behavior of (SrxBa1-x)Nb2O6 ceramics

This article presents electrocaloric effect in (Sr_xBa_1-x)Nb₂O₆ ceramics (where x = 0.25, 0.50 and 0.75) using an indirect approach based on Maxwell's relations. Here, we have calculated various parameters of electrocaloric effect like temperature change (ΔT), entropy change (ΔS) and heat carrying capacity (ΔQ) of material due to the change in polarization under two different electric fields of 30 kV/cm and 20 kV/cm. (Sr_xBa_1-x)Nb₂O₆ ceramics is well known pyroelectric material, where by increasing Sr/Ba ratio the ferroelectric behavior turns towards relaxor behavior. While in terms of electrocaloric effect performance (temperature change ΔT) is increase as the Sr/Ba ratio increases. Additionally, maximum ΔT (0.30 K) was found for (Sr_xBa_1-x)Nb₂O₆ ceramic having x=0.75 molar concentration under the electric fields of 30 kV/cm.