Effect of Polyethylene Glycol Content on the Optical Properties of Ba0.9Sr0.1TiO3 Multilayers

Quasiperiodic Ba_0.9Sr_0.1TiO₃ (BST) multilayers have been fabricated on SrTiO₃ substrates by using chemical solutions containing polyethylene glycol (PEG), and effect of PEG content on optical properties of the BST multilayers have been investigated. Each multilayer with a single perovskite phase displays a layered structure consisting of dense and porous BST layers. It is found that the central wavelength of the reflection-band for BST multilayers shifts to longer wavelength with increasing the PEG concentration within a given polymer amount range. With the same numbers of period and processing condition, the BST multilayers derived from chemical solutions containing PEG additives with a relative molar amount of 0.5, exhibit the highest optical reflectivities.     

Effects of Individual Layer Thickness on the Structure and Electrical Properties of Sol–Gel-Derived Ba0.8Sr0.2TiO3 Thin Films

Ba_0.8Sr_0.2TiO₃ thin films were prepared with various individual layer thicknesses using a sol–gel process. The individual layer thickness strongly affected the structure, ferroelectricity, and dielectric properties of the films. The films prepared with an individual layer thickness of 60 nm showed small equiaxed grains, cubic structure, temperature-independent dielectric constant, and no ferroelectricity. The films prepared with an individual layer thickness of 8 nm showed columnar grains, tetragonal structure, good ferroelectricity, and two dielectric peaks in the dielectric constant–temperature curve. The individual layer thickness for layer-by-layer homoepitaxy growth that resulted in columnar grains was <20 nm.     

Electric Field-Dependent Dielectric Properties and High Tunability of Porous Ba0.5Sr0.5TiO3 Ceramics

Porous Ba_0.5Sr_0.5TiO₃ (BST) ceramics were fabricated by the traditional solid-state reaction process, and their structural, microstructural, dielectric, and tunability properties were systemically investigated. Compared with the fully dense BST samples, porous samples exhibit smaller grain sizes, a more uniform microstructure, and much lower dielectric constants, while at the same time, exhibiting little increase in tunability, which is beneficial to the development of microwave-tunable applications. At a frequency of 10 kHz and a temperature of 18°C, as porosity increased from 0% to 28.8%, the dielectric constant of the BST ceramics (under zero bias field) decreased from ɛ_r(0)∼1690 to ɛ_r(0)∼990, while the dielectric losses were still less than 0.2%, and the tunability increased from 17.6% to 19.6% (2.6 kV/mm).     

Defect Structure and Electrical Properties of Single-Crystal Ba0.03Sr0.97TiO3

The electrical conductivity and thermoelectric power of single-crystalline Ba_0.03Sr_0.97TiO₃ were measured over a wide temperature (800° to 1100°C) and oxygen partial pressure (10⁵ to 10^-15 Pa) range. Our experimental data, like those of previous workers on nominally undoped BaTiO₃ or SrTiO₃, support a defect model based on doubly ionized oxygen vacancies, electrons, holes, and accidental acceptor impurities. The simultaneous measurement of electrical conductivity and thermoelectric power, together with precise experimental data obtained with an advanced thermoectric power measurement technique, enabled us to determine for the first time reliable values for the preexponential factors and the activation energies which characterize the defect equilibrium constants. These calculated values, together with the defect model, were found to give an excellent fit to the experimental data, and were used to generate the boundaries, in P _o2-1/ T space, of the various defect regimes.     

Microwave Dielectric Properties and Low-Temperature Sintering of Ba0.60Sr0.40TiO3 Ceramics

In the present work, the sintering behaviors and dielectric properties of Ba_0.60Sr_0.40TiO₃ (BST) ceramics with the addition of BaCu(B₂O₅) were investigated in detail. The results indicated that the addition reduced the sintering temperature of BST by about 500°C. It was suggested that a liquid phase BaCu(B₂O₅) assisted the densification of BST ceramics at lower temperatures. For a low-level BaCu(B₂O₅) addition (2.0 mol%), the BST sample sintered at 950°C for 5 h displayed good dielectric properties, with a moderate dielectric constant (ɛ=2553) and a low dielectric loss (tan δ=0.00305) at room temperature and at 10 kHz. The sample showed 45.9% tunability at 10 kHz under a dc electric field of 30 kV/cm. At the frequency of 0.984 GHz, BST-added 2.0 mol% BaCu(B₂O₅) possessed a dielectric constant of 2204 and a Q value of 146.7.     

Structural and Optical Properties of Sol–Gel-Derived Au/BaTiO3 Nanocomposite Thin Films

Au/BaTiO₃ nanocomposite thin films with different Au concentrations were prepared by a sol–gel process. The films were characterized using X-ray diffraction, thermal analyses, X-ray absorption spectroscopy, UV-vis spectroscopy, and transmission electron microscopy. The effects of Au concentration and annealing temperature on the structural and optical properties of composite films were investigated. Gold doping lowered the crystallization temperature of as-synthesized amorphous BaTiO₃ and enhanced its crystallinity in post-deposition annealing. The Au–BaTiO₃ interface was also investigated and no alloying occurred between Au and BaTiO₃. The evolution of Au surface plasmon resonance spectra with increasing annealing temperature was observed in the 10 mol% Au/BaTiO₃ thin films. The variations of band-gap energy for Au/BaTiO₃ films were also discussed.     

Recrystallization of Oxygen Ion Implanted Ba0.7Sr0.3TiO3 Thin Films

The crystallization behavior of chemical-solution-deposited and amorphous Ba_0.7Sr_0.3TiO₃ (BST) thin films was analyzed with respect to the evolution of the structural and dielectric properties of the films as a function of the annealing temperature. The amorphous films were produced by oxygen ion implantation into crystalline BST thin films. In the amorphous thin films, the crystallization to the perovskite phase occurred at T = 550°C, whereas the as-deposited CSD films showed the first crystalline XRD-reflex only after annealing at T = 650°C. Here a carbon-rich intermediate phase delayed the crystallization process to higher temperatures.     

Effects of CuO Doping on the Microstructural and Dielectric Properties of Ba0.6Sr0.4TiO3 Ceramics

This study investigates the effect of CuO on the sintering behavior, dielectric properties, and microstructures of Ba_0.6Sr_0.4TiO₃ (BST) ceramics. The ceramics were sintered in air at temperatures ranging from 1000° to 1230°C. It is found that a small amount of added CuO (0.6 mol%) can significantly increase the density and improve the dielectric properties of BST ceramics. Doped BST ceramics can be sintered to a density >95% of the theoretical density at 1150°C. scanning electron microscopic observations show that the BST grain sizes increase with increasing amounts of CuO. No secondary phases in the BST ceramics are observed using X-ray diffraction pattern for CuO additions up to 0.9 mol%. However, compositional analysis using transmission electron microscopy-EDX for the BST ceramics with 0.9 mol% CuO sintered at 1150°C showed that a small level of secondary phase formation is present. On the other hand, large dislocations are observed for BST with 0.6 mol% CuO addition as a result of lattice distortion, which creates the vacancy condensation because of the atomic mismatch in the solid solutions. Optimal CuO doping concentrations can reduce the loss tangents of BST that can also ensure a high dielectric constant. When the doping concentration of CuO is 0.6 mol% and the ceramic is sintered at 1150°C, the BST ceramic has the following properties at 1 MHz: dielectric constant=4094, tan δ=0.55%.     

Construction of PbZr0.4Ti0.6O3- and Ba0.9Sr0.1TiO3-Based Optical Microcavities by Chemical Solution Deposition

Herein we report on a simple, low cost, and feasible route for the construction of PbZr_0.4Ti_0.6O₃ (PZT)- or Ba_0.9Sr_0.1TiO₃ (BST)-based optical microcavities using a single chemical solution containing polymer polyvinylpyrrolidone. The obtained multilayer systems not only exhibit good ferroelectric performance, but also display well-defined resonant modes with a quality factor of no <66. Compared with PZT microcavities, the optical properties of the BST microcavities appear to be superior.     

Fabrication and Properties of Sol–Gel-Derived BiScO3–PbTiO3 Thin Films

BiScO₃–PbTiO₃ (BSPT) thin films near the morphotropic phase boundary were successfully fabricated on Pt(111)/Ti/SiO₂/Si substrates via an aqueous sol–gel method. The thin films exhibited good crystalline quality and dense, uniform microstructures with an average grain size of 50 nm. The dielectric, ferroelectric, and piezoelectric properties of the sol–gel-derived BSPT thin films were investigated. A remanent polarization of 74 μC/cm² and a coercive field of 177 kV/cm were obtained. The local effective piezoelectric coefficient d ^*₃₃ was 23 pC/N at 2 V, measured by a scanning probe microscopy system. The dielectric peak appeared at 435°C, which was 80°C higher than that of Pb(Ti, Zr)O₃ thin films.     

Synthesis and Processing Characteristics of Ba0.65Sr0.35TiO3 Powders from Catecholate Precursors

Powders of composition Ba_0.65Sr_0.35TiO₃ were prepared from catecholate precursor phases, BaTi(C₆H₄O₂)₃ and SrTi (C₆H₄O₂)₃. The physical and chemical properties of the base powders, and those doped with 0.2 wt% manganese, are reported in detail. The dimensions of the primary particles in the starting powders were of the order of 20–50 nm, but the occurrence of abnormal grain growth during sintering promoted grain sizes in the ceramic of up to ∼100 μm. In some microstructures, coarse grains coexisted with a ∼1-μm fraction to produce a characteristic bimodal grain size distribution. By contrast, under comparable sintering conditions, namely 1350° or 1400°C for 1 h, grain growth in Mn-doped samples was suppressed, leading to uniform microstructures with a grain size of only a few micrometers. The pellet densities were nevertheless similar, 97% of theoretical in both doped and undoped samples. No significant difference was observed in the dielectric permittivity of the two compositions: the peak relative permittivity occurred at ∼20°C, with a maximum value of ∼22 000.     

Effects of B2O3 Addition on the Dielectric and Ferroelectric Properties of Ba0.7Sr0.3TiO3 Ceramics

The effects of B₂O₃ addition on the sintering behavior and the dielectric and ferroelectric properties of Ba_0.7Sr_0.3TiO₃ (BST) ceramics were investigated. The dielectric and ferroelectric properties of a BST sample with 0.5 wt% B₂O₃ sintered at <1150°C were as good as those of undoped BST sintered at 1350°C, and the dielectric loss was better. When >1.0 wt% B₂O₃ was added to BST, the overdoped B₂O₃ did not form a liquid phase or volatilize; it remained in the samples and formed a secondary phase that lowered the sintering behavior and the dielectric and ferroelectric properties of the BST.     

The Structure and Dielectric Tunable Properties of Fine-Grained Ba0.6Sr0.4TiO3 Ceramics Prepared by Spark Plasma Sintering

Barium strontium titanate is a promising material for microwave-phased array applications. ^1,2 In this study, highly dense and fine-grained Ba_0.6Sr_0.4TiO₃ ceramics were prepared using the spark plasma sintering (SPS) technique. The structure and dielectric tunable properties of the samples were investigated. The "distorted nano-region" emerged in the interior of the grains of SPS samples, and resulted in the deterioration of the dielectric tunable properties of Ba _x Sr_{1− x} TiO₃. This phenomenon indirectly testified to the assumption of the "polar nano-region" mechanism. After the SPS samples were annealed, the "distorted nano-region" disappeared and better dielectric tunable properties were obtained. The dielectric constant was decreased to 1048, and the K value (Commutation Quality Factor) reached 7089.     

Surface Chemical and Leakage Current Density Characteristics of Nanocrystalline Ag–Ba0.5Sr0.5TiO3 Thin Films

Nanocrystalline x Ag–(1− x )Ba_0.5Sr_0.5TiO₃ (Ag–BST, 0≤ x ≤0.1, where x is the mole fraction of Ag) thin films have been deposited on Pt/Ti/SiO₂/Si substrates by a sol–gel method. The films have been characterized by X-ray diffraction (XRD), scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). The core-level XPS of oxygen (O1 s ) of the Ag–BST films indicate that an optimum amount of Ag ( x =0.02 or 2 mol%) enhances the binding energy of oxygen, possibly through a mechanism in which the electrophilic oxygen dissociates from the Ag surface and fills the oxygen vacancies. Similarly, the binding energy of Ag (Ag3 d ) shows a shift toward a higher value with increasing Ag up to 4 mol%, probably because of the chemical shift of Ag in BST along the surface layers, surface relaxation, or changes in the Fermi level of small Ag particles in the solid solution range of Ag in BST films ( x ≤0.04). The leakage current density of 2 mol% Ag-added BST (∼10⁻⁶ A/cm²) is less by about an order of magnitude than pure BST at an electric field of 200 kV/cm. A defect model is proposed to explain the observed leakage current density of Ag–BST films satisfactorily.     

Fabrication and Characterization of Pyroelectric Ba0.8Sr0.2TiO3 Thin Films by a Sol-Gel Process

A sol-gel process was used to prepare pyroelectric Ba_0.8Sr_0.2TiO₃ thin films with large columnar grains (100–200 nm in diameter) on Pt/Ti/SiO₂/Si substrates, via using a 0.05 M solution precursor. The relationship between dielectric constant and temperature (ɛ_r- T ) showed two distinctive phase transitions in the Ba_0.8Sr_0.2TiO₃ thin films. Both the remnant polarization and the coercive field decreased as the temperature increased from −73°C to 40°C. Its low dissipation factor (tan δ= 2.6%) and high pyroelectric coefficient ( p = 4.6 × 10⁻⁴ C·(m²·K)⁻¹ at 33°C), together with its good insulating properties, made the prepared Ba_0.8Sr_0.2TiO₃ thin films promising for use in uncooled infrared detectors and thermal imaging applications.     

Decomposition and Crystallization of a Sol–Gel-Derived PbTiO3 Precursor

A lead titanate (PbTiO₃) precursor, prepared by the Pechini method, has been heat treated to study the transformation from amorphous to crystalline PbTiO₃. Nucleation of PbTiO₃ in the temperature interval 400°–475°C occurred before completion of the thermal decomposition of the polymeric precursor, resulting in nanocrystalline PbTiO₃ with an unexpectedly high tetragonality ( c/a ratio). Annealing and crystallite growth at 600°C resulted in an increasing c/a ratio with annealing time in line with the expected finite size effect of PbTiO₃. The unusually high c/a ratios observed in PbTiO₃ nucleated at 400°–475°C are discussed in relation to partial reduction and point defects in PbTiO₃.     

Orientation Control in Sol–Gel-Derived BiScO3–PbTiO3 Thin Films

BiScO₃–PbTiO₃ (BSPT) thin films were fabricated via a sol–gel method on Pt(111)/Ti/SiO₂/Si(111) substrates. The effects of different factors on the orientation of the sol–gel-derived BSPT thin films were investigated. The results showed that a higher lead excess concentration, longer drying time, higher pyrolysis temperature, longer pyrolysis time, higher crystallization temperature, and longer crystallization time could enhance the (100) orientation of the BSPT thin films. Based on the experimental results, a mechanism for the orientation evolution in the sol–gel-derived BSPT thin films was proposed. The production of the (100) orientation was attributed to the (100)-oriented PbO nanocrystals forming during the pyrolysis process due to the lattice match.     

Microstructure–Dielectric Properties Relationship in Ba0.6Sr0.4TiO3–Mg2SiO4–Al2O3 Composite Ceramics

0.60Ba_0.6Sr_0.4TiO₃(BST)–(0.40− x )Mg₂SiO₄(MS)– x Al₂O₃ ( x =0, 0.5, 3, 5wt%) composite ceramics exhibit excellent characteristics suitable for tunable device applications. With increasing amount of Al, the dielectric peak can be quantitatively broadened and suppressed; the "phase transition temperature" T _c or ( T _m) shifts to a lower temperature. Meanwhile, the tunability is still high in a wider temperature range. Far from T _c, pyroelectric effects are observed by using the Byer and Roundy technology and Slim polarization hysteresis loops are observed under high ac dielectric field at 10Hz. These proved the existence of spontaneous polarization in certain possible orientations in a broad temperature range above T _c in the paraelectric medium and reveal why 0.60BST–(0.40− x )MS– x Al₂O₃ have such remarkable dielectric nonlinearity.     

Low-Temperature Sintering of (Ba0.6Sr0.4)TiO3

An addition of just 0.4 wt% Li₂O to (Ba_0.6Sr_0.4)TiO₃ powder was able to reduce the sintering temperature to ≤900°C and produce ceramics with a relative density of 97%. Small amounts of two secondary phases were formed during this process: Li₂TiO₃ and Ba₂TiO₄. The addition of Li₂O depresses the ferroelectric character of the (Ba_0.6Sr_0.4)TiO₃ and, as a result, reduces the permittivity, improves the temperature coefficient of permittivity, and reduces the dielectric losses. The tunability shows no significant variation with Li₂O concentration and remains between 16.5% and 13.5%. A low-temperature sintering mechanism was proposed. The mechanism involves the intermediate formation of BaCO₃, its melting and the incorporation of Li⁺ into the BST. The sintering mechanism can be characterized as reactive liquid-phase sintering.     

Transmission Electron Microscopy Investigation of Pt/Ba0.7Sr0.3TiO3/Pt Capacitors with Different Annealing Processes

The effects of postannealing in oxygen ambient and forming gas atmosphere (FGA) on the microstructure and chemistry of Pt/Ba_0.7Sr_0.3TiO₃ (BST)/Pt capacitors prepared by chemical solution deposition have been investigated by means of transmission electron microscopy. The as-deposited film layers of the Pt/BST/Pt capacitors show a columnar structure. The postannealing in O₂ leads to a smoothness of the top film-electrode interface. The additional FGA treatment applied to the postannealing capacitors introduces disordered or amorphous regions at both top and bottom interfaces. In these regions, deviation in composition from the stoichiometry occurs with a higher Ti/(Ba + Sr) ratio. These amorphous regions are believed to be responsible for the increase of the leakage current obtained in the FGA-treated capacitors. The thickness of this amorphous interfacial layer can be reduced significantly by a recovery annealing process in air at low temperatures.