Dielectric Properties and Low‐Temperature Sintering of the Ba0.6Sr0.4TiO3 Ceramics with B2O3/CuO Additions

The sintering behaviors and dielectric properties of Ba_0.6Sr_0.4TiO₃ ceramics were investigated as a function of B₂O₃ and CuO content. The addition of both B₂O₃ and CuO reduced the sintering temperature of Ba_0.6Sr_0.4TiO₃ about 500°C. It was suggested that a liquid phase BaCu(B₂O₅) was formed and assisted the densification of Ba_0.6Sr_0.4TiO₃ ceramics. Ba_0.6Sr_0.4TiO₃ ceramics co‐doped with 3.0 mol% B₂O₃, and 2.0 mol% CuO, sintered at 950°C for 5 h, had a dense microstructure and showed good microwave dielectric properties of a moderate dielectric constant (ε = 1048), low dielectric loss (0.0090) and high tunability (42.2%) at dc electric field of 30 kV/cm.     

Effect of La doping on tunable behavior of sol–gel-derived PST thin film

Pb_xSr_1−xTiO₃ and (Pb_0.4Sr_0.6)_1−xLa_2x/3TiO₃ thin films were prepared on ITO/glass substrate by sol–gel technique. The crystalline phase structures of thin films were investigated by XRD. The dielectric properties were measured by LCZ meter. Results show that the perovskite phase was stable in Pb_xSr_1−xTiO₃ thin film. Its lattice constant was found to increase with the increase of x. When Pb/Sr ratio was about 50/50, the transformation of the perovskite phase between cubic and tetragonal took place at room temperature. To ensure better tunability, Pb_0.4Sr_0.6TiO₃ thin film was selected in both paraelectric state (cubic structure) and near the phase transformation point between cubic and tetragonal. The tunability of more than 80% and figure of merit (FOM) of 14.17 were obtained. (Pb_0.4Sr_0.6)_1−xLa_2x/3TiO₃ was also prepared as La-doped thin film. Its dielectric constant was decreased below x = 0.4 and then increased above x = 0.4 with the increase in x. The inharmonic coefficient depicting ability of tunability of (Pb_0.4Sr_0.6)_1−xLa_2x/3TiO₃ thin film kept constant due to the changeable shape of the crystal cell under electric field at a fixed temperature below x = 0.4, which is in good agreement with Johnson's theory. And the coefficient reveals a distinctive variable because of easily changeable shape under different bias above x = 0.4. The tunability of about 70% and FOM of about 7 were obtained in the (Pb_0.4Sr_0.6)_1−xLa_2x/3TiO₃ thin film.     

Reduced leakage current,enhanced energy storage and dielectric properties in (Ce,Mn)-codoped Ba0.6Sr0.4TiO3 thin film

Ba_0.6Sr_0.4TiO₃, Ce-doped Ba_0.6Sr_0.4TiO₃, Mn-doped Ba_0.6Sr_0.4TiO₃, (Ce,Mn) co-doped Ba_0.6Sr_0.4TiO₃ (abbreviated as BST, BSTCe, BSTMn, BSTCeMn) thin films were deposited on LaNiO₃(LNO)/Si substrates. The effects of ion doping on the microstructure and electrical properties of BST-based thin film have been researched and discussed. The X-ray diffraction pattern shows that each sample has pure perovskite phase structure with high (l00) peaks. The microstructure of each film is quite dense with uniform size. Compared with pure BST, improved insulating properties can be found in ion-doped BST thin films. For all the films, Ohmic conduction, space charge limited conduction and interface-limited Fowler-Nordheim tunneling should be the main conduction mechanisms within different electric field regions. For the case of BSTCeMn thin film, it possesses enhanced energy storage performance with a recoverable energy storage density (18.01?J/cm³) and a energy storage efficiency (75.1%) under 2000?kV/cm. This can be closely related to the small remanent polarization value (P_r=?1.89 μC/cm²), large maximum polarization value (P_max=?28.08?μC/cm²) as well as big maximum electric field (2000?kV/cm). Also, it exhibits a large dielectric constant of 405 and a small dissipation factor of 0.075 at 500?kHz.     

Achieving high breakdown strength and figure of merit of Ba0.6Sr0.4TiO3 films through coating a Y3Fe5O12 layer

Low tunability and figure of merit significantly limited the application of Ba_0.6Sr_0.4TiO₃ (BST) ferroelectric film, which originates from the low electric breakdown strength and high dielectric loss of BST layer. Garnet structured Y₃Fe₅O₁₂ (YIG) exhibits the merits of good microwave dielectric property and a much high resistivity, which are helpful for enhancing the breakdown strength and suppressing the dielectric loss. In this work, Y₃Fe₅O₁₂/Ba_0.6Sr_0.4TiO₃ (YIG/BST) composite films were fabricated via chemical solution deposition method. The composite films exhibited a low dielectric loss (0.006) and an almost frequency independent dielectric constant in a frequency range from 10 kHz to 1 MHz. The electric breakdown strength was significantly enhanced from less than 400 kV/cm to around 800 kV/cm through coating a YIG layer, causing an excellent tunability of 72.84% and an ultra-high figure of merit (FOM=118) at 800 kV/cm in YIG/BST film. It is physically clarified that the conduction loss plays an important role in BST film while the intrinsic loss is the dominate factor for the YIG/BST composite films.     

Property optimization of Ba0.4Sr0.6TiO3–BaMoO4 composite ceramics for tunable microwave applications

(1 ? x)Ba_0.4Sr_0.6TiO₃–xBaMoO₄ ceramics with x = 5, 10, 20, 30, 40 and 60 wt% were prepared by traditional solid-state reaction method. Two crystalline phases, a cubic perovskite structure Ba_0.4Sr_0.6TiO₃ (BST) and a tetragonal scheelite structure BaMoO₄ (BM) were obtained by XRD analysis. The microwave dielectric properties of Ba_0.4Sr_0.6TiO₃–BaMoO₄ composite ceramics were investigated systematically. The results show that the composite ceramics exhibited promising microwave properties. The dielectric constant can be adjusted in the range from 900 to 78, while maintaining relatively high tunability from 27.3% to 12.8% under a direct current electric field of 60 kV/cm and Q values from 619 to 67 in the gigahertz frequency region.     

Investigation on Tunable Performance of BMN/BST Multilayer and BMN–BST Composite Thin Films

The BMN/BST multilayer and BMN–BST composite thin films have been fabricated by pulsed laser deposition on Au/TiO_x/SiO₂/Si substrate. The multilayer thin films comprising one, two, and four periodic compositional Bi_1.5MgNb_1.5O₇/Ba_0.6Sr_0.4TiO₃ (BMN/BST) layers (PCBLs) have been elaborated with the final same thickness. The four PCBLs show the largest dielectric constant of ~168 and tunability of 40.6% at a maximum applied bias field of 0.67 MV/cm and the lowest loss tangent of ~0.006, whereas the figure of merit (FOM) is 72. The BMN–BST composite thin films exhibit medium dielectric constant of ~238, low loss tangent of ~0.0053, and superior tunable dielectric properties at room temperature. Calculations of tunability and FOM display a maximum value of 49.3% at 670 kV/cm and ~88, respectively.     

Effects of raw materials on nonhydrolytic sol-gel synthesis of Ba0.6Sr0.4TiO3

The effects of raw materials on the nonhydrolytic sol-gel synthesis of Ba_0.6Sr_0.4TiO₃ were systematically studied in this work. The samples were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM). Ba(CH₃COO)₂, Sr(CH₃COO)₂, BaCO₃ and SrCO₃ can undergo deesterification substitution reaction with Ti(OC₄H₉)₄ to synthesize Ba_0.6Sr_0.4TiO₃ phase. Sample taking Ba(CH₃COO)₂ and Sr(CH₃COO)₂ as barium and strontium source has better Ba_0.6Sr_0.4TiO₃ synthesis results than that of samples using BaCO₃ or SrCO₃. Ba_0.6Sr_0.4TiO₃ phase can not be synthesized at low temperature taking BaCl₂, SrCl₂ and TiOSO₄ as barium, strontium or titanium source due to their strong ionic bonding. TiCl₄ undergoes an alcoholysis reaction with (oxygen donor) alcohol to form a multimolecular association structure and solvated complex chlorotitanium alkoxide, which reduces the possibility of a deesterification polycondensation reaction. The Ba_0.6Sr_0.4TiO₃ phase cannot be synthesized at low temperatures when CH₃CH₂OH or C₃H₇OH is used as the solvent. The synthesis of Ba_0.6Sr_0.4TiO₃ is insufficient because of the coordination ability and solvation effect when CH₃COOH is used as the solvent. The synthesis of Ba_0.6Sr_0.4TiO₃ is the best when Ba(CH₃COO)₂, Sr(CH₃COO)₂, Ti(OC₄H₉)₄ and glycerol (C₃H₈O₃) are used as the barium source, strontium source, titanium source and solvent, respectively. Pure Ba_0.6Sr_0.4TiO₃ phase with a particle size of 17–43 nm and high dispersion is synthesized when the molar ratio of barium-strontium-titanium is 0.6:0.4:1.2.     

Ba0.4Sr0.6TiO3/MgO Composites with Enhanced Energy Storage Density and Low Dielectric Loss for Solid-State Pulse-Forming Line

(100−x) wt% Ba_0.4Sr_0.6TiO₃−x wt% MgO composites (10≤x≤30) were prepared using Ba_0.4Sr_0.6TiO₃ powder and nanosized MgO powder (∼60 nm) by a solid-state reaction. The energy storage density and dielectric loss were investigated for the purpose of a potential application in solid-state pulse-forming line. The results show that Ba_0.4Sr_0.6TiO₃/MgO composites exhibit a notably enhanced energy density and low dielectric loss, compared with pure Ba_0.4Sr_0.6TiO₃. The enhancement of the energy density is attributed to the notable increase in breakdown strength of the composites and the improvement of dielectric constant stability with regard to electric field. In the case of x=30, the samples exhibited a breakdown strength of 33.1 kV/mm, an energy density of 1.14 J/cm³, a moderate dielectric constant of 270, and a low dielectric loss of 4 × 10⁻⁴.     

Thickness effects on microwave properties of (Ba,Sr)TiO3 films for frequency agile technologies

We fabricated (Ba_0.6Sr_0.4)TiO₃ (BST) thin films of various thicknesses on sapphire (−1 1 2 0) substrates using metal-organic decomposition method. These films showed grain growth from 160 to 650 nm with an increase in the thickness from 90 to 1050 nm. At microwave frequencies, the measured capacitances of the planar capacitors decreased with the film thickness because the electro-magnetic field propagates across high permittivity BST films to the low permittivity sapphire substrate. However, we found that the BST-thin film permittivity remained large up to 90 nm thick, based on electro-magnetic field analysis using the finite element method. On the other hand, the BST thin film tunability decreased with the film thickness.     

Microstructures and dielectric properties of spark plasma sintered Ba0.4Sr0.6TiO3/CaCu3Ti4O12 composite ceramics

(1 − x)Ba_0.4Sr_0.6TiO₃/xCaCu₃Ti₄O₁₂ composite ceramics were prepared by spark plasma sintering. Sintering behavior, microstructures and dielectric properties of the composite ceramics were investigated by XRD, SEM, EDS and dielectric spectrometer. Dense composite ceramics consisting of Ba_0.4Sr_0.6TiO₃ phase and CaCu₃Ti₄O₁₂ phase were prepared at 800 °C for 0 min. The dielectric loss of the composite ceramic decreased with increasing amount of Ba_0.4Sr_0.6TiO₃, and the high dielectric constant were retained. Moreover, the better temperature stability of dielectric constant was obtained. These improvements of dielectric characteristics have great scientific significance for potential application.     

Low Dielectric Loss and Good Dielectric Thermal Stability of xNd(Zn1/2Ti1/2)O3–(1−x)Ba0.6Sr0.4TiO3 Thin Films Fabricated by Sol–Gel Method

xNd(Zn_1/2Ti_1/2)O₃–(1?x)Ba_0.6Sr_0.4TiO₃ (xNZT–BST) thin films were fabricated on Pt/Ti/SiO₂/Si substrates by sol–gel method with x = 0, 3%, 6%, and 10%. The structures, surface morphology, dielectric and ferroelectric properties, and thermal stability of xNZT–BST thin films were investigated as a function of NZT content. It was observed that the introduction of NZT into BST decreased grain size, dielectric constant, ferroelectricity, tunability, and significantly improved dielectric loss and dielectric thermal stability. The corresponding reasons were discussed. The 10%NZT–BST thin film exhibited the least dielectric loss of 0.005 and the lowest temperature coefficient of permittivity (TCP) of 3.2 × 10^?3/°C. In addition, the figure of merit (FOM) of xNZT–BST (x = 3%, 6%, and 10%) films was higher than that of pure BST film. Our results showed that the introduction of appropriate NZT into BST could modify the dielectric quality of BST thin films with good thermal stability. Especially for the 3%NZT–BST thin film, it showed the highest FOM of 33.58 for its appropriate tunability of 32.87% and low dielectric loss of 0.0098.     

Growth process approaches for improved properties of tunable ferroelectric thin films

Two approaches are shown for property control of tunable Ba_xSr_1−xTiO₃ (BST) films: (i) two-step growth and (ii) self-selective epitaxial growth. Both processes are based on the deposition of an amorphous ultra thin BST layer prior to the growth of an epitaxial BST film. In the first approach, the mechanical strain in the epitaxial films is controlled by employing a two-step growth process, where the very thin amorphous layer deposited first acts as an absorption layer for the misfit strain through its crystallization. Two-step grown films on LaAlO₃ substrates showed effective relaxation of the compressive misfit strain, which resulted in higher in-plane permittivity and tunability of the film. In the second approach, self-selective epitaxial growth, epitaxial/amorphous BST composite structures are achieved where the two phases are electrically inter-connected in parallel. Having a low permittivity, the amorphous component efficiently suppressed the effective permittivity of the composite capacitor, while the epitaxial component helps keep the tunability as large as that of a purely epitaxial BST film. The resultant composite film demonstrated a low permittivity having a high tunability, which is the contradictory phenomenon for a pure tunable ferroelectric.     

Enhanced Electrocaloric Effects in Spark Plasma‐Sintered Ba0.65Sr0.35TiO3‐Based Ceramics at Room Temperature

Improvement of electrocaloric effect was investigated in the lead‐free undoped and Mn‐doped Ba_0.65Sr_0.35TiO₃ ceramics prepared by spark plasma sintering process. Owing to the merit of spark plasma sintering process, a fully dense undoped and Mn‐doped Ba_0.65Sr_0.35TiO₃ ceramics with fine grain sizes could be obtained. The electrocaloric (EC) effect can be significantly enhanced from 0.83 K for conventional sintered Ba_0.65Sr_0.35TiO₃ to 3.08 K for spark plasma‐sintered Mn‐doped Ba_0.65Sr_0.35TiO₃ ceramics since the dielectric strength was dramatically increased. This work indicated an effective way to achieve the significantly enhanced EC effect in a lead‐free system at room temperature.     

Polar nano-clusters in nominally paraelectric ceramics demonstrating high microwave tunability for wireless communication

Dielectric materials, with high tunability at microwave frequencies, are key components in the design of microwave communication systems. Dense Ba_0.6Sr_0.4TiO₃ (BST) ceramics, with different grain sizes, were prepared in order to optimise the dielectric tunability via polar nano cluster effects. Dielectric permittivity and loss measurements were performed at both high and low frequencies and were supported by results from X-ray powder diffraction, scanning and transmission electron microscopies, Raman spectroscopy and piezoresponse force microscopy. The concentration of polar nano clusters, whose sizes are found to be in the range 20–50 nm, and the dielectric tunability increase with increasing grain size. A novel method for measurement of the microwave tunability in bulk dielectrics is presented. The highest tunability of 32 % is achieved in ceramics with an average grain size of 10 μm. The tunability of BST ceramics with applied DC field is demonstrated in a prototype small resonant antenna.     

Grain size-driven effect on the functional properties in Ba0.6Sr0.4TiO3 ceramics consolidated by spark plasma sintering

Dense fine-grained Ba_0.6Sr_0.4TiO₃ ceramics with submicronic grains sizes (GS) have been prepared using nanopowders synthesized via sol-gel route and consolidated by Spark Plasma Sintering (SPS). By changing SPS parameters, the GS was reduced from 214 nm to 74 nm. Diffuse ferroelectric-paraelectric phase transitions and low values of dielectric permittivity (<1000) at the Curie temperature (T_C ∼280 K) were revealed by Impedance Spectroscopy in all sintered ceramics. The GS reduction from submicron to nanoscale range reflects in a gradually diminishment of dielectric constant, tunability, polarisation and storage energy properties. Raman spectroscopy investigations pointed out the presence of polar nanoclusters above the T_C. The short-range polar order is affected by the GS decrease, but becomes more thermally stable. The observed properties of Ba_0.6Sr_0.4TiO₃ nanostructured ceramics are interpreted by considering the interplay between the GS reduction, the role of low-permittivity grain boundaries and the diffuse character of the ferroelectric-to-paraelectric transformation.     

Improvement of dielectric loss of (Ba,Sr)(Ti,Zr)O3 ferroelectrics for tunable devices

(Ba_0.6Sr_0.4)(Ti_1−xZr_x)O₃ (0.05 ≤ x ≤ 0.3) ferroelectric materials have cubic perovskite structure and show paraelectric properties at room temperature. Curie point shifted to a negative value as increasing Zr content in (Ba_0.6Sr_0.4)(Ti_1−xZr_x)O₃ system. When Zr substituted 0.1 mol, the dielectric constant, dielectric loss, tunability, Curie point and FOM were 4500, 0.0005, 63%, −1.6 °C and 1260, respectively. This composition shows excellent microwave dielectric properties than those of (Ba_0.6Sr_0.4)TiO₃ ferroelectrics, which are limelight materials for tunable devices such as varactors, phase shifters and frequency agile filters, etc.     

Microstructure and dielectric properties of Ba0.6Sr0.4TiO3-MgAl2O4 composite ceramics

(1 − x)Ba_0.6Sr_0.4TiO₃-xMgAl₂O₄(x = 25, 30, 35 and 40 wt%) composite ceramics were prepared by conventional solid-state reaction method. The microstructures, dielectric properties and tunability of the composites have been investigated. The XRD patterns analysis reveals two crystalline phases, a cubic perovskite structure Ba_0.6Sr_0.4TiO₃ (BST) and a spinel structure MgAl₂O₄ (MA). SEM observations show that the BST grains slightly dwindle and agglomerate with increasing amounts of MA. A dielectric peak with very strong frequency dispersion is observed at higher MA content, and the Curie temperature shifts to a higher temperature with increasing MA content. The ceramic sample with 30 wt% MA has the optimized properties: the dielectric constant is 1503, the dielectric loss is 0.003 at 10 kHz and 25 °C, the tunability is 23.63% under a dc electric field of 1.0 kV/mm, which is suitable for ferroelectric phase shifter.     

Influence of low concentration MgCo2(VO4)2 addition on microwave dielectric properties of Ba0.6Sr0.4TiO3 ceramics

Ba_0.6Sr_0.4TiO_3?x MgCo₂(VO₄)₂ ceramics with x = 0, 0.5, 1.0, 2.0, and 5.0 wt% was fabricated via conventional solid-state reaction process. The effects of such additives on the structure, dielectric and tunability properties were systemically investigated. A small number of secondary phase identified as Ba₃(VO₄)₂ appeared in Ba_0.6Sr_0.4TiO_3?x MgCo₂(VO₄)₂ ceramics when x is more than 5.0 wt%. With increasing of MgCo₂(VO₄)₂ content, the peak values of permittivity gradually decreased and shifted to low temperature. The Ba_0.60Sr_0.40TiO₃ added with 0.5 wt% MgCo₂(VO₄)₂ possesses a dielectric constant of 2763, Q value of 267 at ～1 GHz and tunability of 35.9% under dc electric field of 30 kV/cm at 10 kHz.     

The Impact of Composite Effect on Dielectric Constant and Tunability in Ferroelectric–Dielectric System

A theoretical study was carried out to analyze the impact of composite effect on dielectric constant and tunability in the ferroelectric–dielectric system. Models of Ba_0.5Sr_0.5TiO₃–Mg₂TiO₄, Ba_0.45Sr_0.55TiO₃–MgO and Ba_0.5Sr_0.5TiO₃–MgO–Mg₂SiO₄ system were constructed. The corresponding dielectric constant, tunability, and electric field distribution were obtained from the finite element analysis and the connections between these parameters were analyzed. The effects of the relative relationship between the dielectric constant of ferroelectric and dielectric on the whole dielectric properties were also analyzed by constructing a series of models with different dielectrics.