Low temperature sintering properties of Y-doped BaTiO3 ceramics by BaB2O4 sintering aid

The sintering process of semiconducting Y-doped BaTiO₃ ceramics added with BaB₂O₄ as low temperature sintering aid were investigated. When the low temperature sintering aid BaB₂O₄ added Y-doped BaTiO₃ ceramics prepared by Sol-Gel method, the sintering temperature of BaTiO₃-based ceramics would be greatly decreased, and also widen sintering range. Y-doped BaTiO₃ ceramics with BaB₂O₄ addition can be obtained at 1050 °C. Ceramics samples with room temperature resistivity 60-80 Ω cm, ratio of the maximum resistivity to minimum resistance (Rmax/Rmin) 10⁴ and temperature coefficient of resistivity (α) 10%/°C were obtained.     

Resistance reduction effect of potassium bismuth titanate doping in yttrium–manganese co-doped medium Curie temperature barium titanate lead free ceramics through improved synthesis process

Through improved synthesis process, resistance reduction effect of (K_0.5Bi_0.5)TiO₃ (KBT) doping in Y–Mn co-doped BaTiO₃ (BT) lead free ceramics was investigated. By different doping methods (doping K₂O, Bi₂O₃ and TiO₂ or synthesized KBT), medium Curie temperature (around 130 °C) lead free BT ceramics were obtained with ultra-low resistivity (13.84 Ωcm) with a temperature maintaining process at 700 °C. In this contribution, effect of sintering process and doping methods is discussed in detail.     

Characterization of the texturing of YBa2Cu3O7−δ in the presence of liquid phase additives

Grain growth and texturing of YBa₂Cu₃O_7−δ is greatly influenced by the presence of liquid phase additives during sintering. Oxides such as TiO₂, SiO₂, Bi₂O₃, and Pr₆O₁₁ were incorporated into the liquid phase during the sintering of YBa₂Cu₃O_7−δ (123) by use of grain boundary diffusion couples and the microstructure was analyzed using scanning electron microscopy/electron dispersive spectroscopy. Exaggerated grain growth and domain formation was observed in bulk specimens. Differential thermal analysis and real time dynamic x-ray diffraction were used to determine reaction sequencing. The ability and extent of domain formation was determined for 123 samples coupled with impurity oxides to be a function of sintering temperature (940–980°) and oxygen partial pressure. Enhanced texturing was observed at low PO₂ atmospheres. The addition of Bi₂O₃ and TiO₂ was shown to degrade dc magnetic susceptibility of 123 whereas SiO₂ and Pr₆O₁₁ enhanced it. The domain formation and texturing takes place in the bulk for 123 at temperatures of 980°C or below (i.e. well below the peritectic decomposition temperature) by the interaction of an impurity doped liquid phase followed by a precipitation and exaggerated grain growth.     

Energetics of Donor‐Doping,Metal Vacancies,and Oxygen‐Loss in A‐Site Rare‐Earth‐Doped BaTiO3

The energetics of La‐doping in BaTiO₃ are reported for both (electronic) donor‐doping with the creation of Ti³⁺ cations and ionic doping with the creation of Ti vacancies. The experiments (for samples prepared in air) and simulations demonstrate that ionic doping is the preferred mechanism for all concentrations of La‐doping. The apparent disagreement with electrical conduction of these ionic doped samples is explained by subsequent oxygen‐loss, which leads to the creation of Ti³⁺ cations. Simulations show that oxygen‐loss is much more favorable in the ionic‐doped system than undoped BaTiO₃ due to the unique local structure created around the defect site. These findings resolve the so‐called “donor‐doping” anomaly in BaTiO₃ and explain the source of semiconductivity in positive temperature coefficient of resistance (PTCR) BaTiO₃ thermistors.     

High‐Rate Continuous Synthesis of Nanocrystalline Perovskites and Metal Oxides in a Colliding Vapor Stream of Microdroplets

A high‐rate, continuous synthesis of functional nanomaterials using a home engineered reactor is reported. The reactor is able to produce low‐cost, kilogram‐scale BaTiO₃ nanopowders with a nanocrystalline particle size less than 30 nm at mild temperatures (<100 °C) and ambient pressure. Nebulization and collision of warm microdroplets (60–80 °C) of Ba(OH)₂ and Ti(O‐nBu)₄ very quickly result in total hydrolysis and subsequent conversion to BaTiO₃, yielding 1.3 kg/day of high purity, highly crystalline nanoparticles (25–30 nm). This synthesis procedure also enables high‐rate production of TiO₂ anatase (2.9 kg/day). It therefore provides a general platform for processing and scaling up of functional inorganic nanomaterials under very mild conditions.     

Li-doped (Ba,Sr)TiO3 thick film interdigital capacitors for microwave applications

Microwave properties of Li-doped (Ba,Sr)TiO₃ thick film interdigital capacitors have been investigated. According to the reported papers, BaSrTiO₃ materials, paraelectric state at the room temperature, have high dielectric permittivity (>500 @ 1 MHz) and low loss tangent (<0.01 @ 1 MHz) in epitaxial thin film form; however, the sintering temperature of BaSrTiO₃ is over 1350 °C. In order to reduce the sintering temperature, Li (3 wt%) was added to the BaSrTiO₃ materials, and 10 μm thick Li-doped (Ba,Sr)TiO₃ films were screen printed on the alumina (Al₂O₃) substrate and sintered at 900 °C. Interdigital capacitor patterns with five fingers of 200 μm gap and 250 μm length were also designed and fabricated by employing the screen printing method with Ag electrode. The structural feature was analyzed with X-ray diffraction method. Frequency and temperature-dependent dielectric properties were characterized from 1 kHz to 1 MHz and 303-403 K, respectively. Also, current-voltage characteristics were investigated with an elevated temperature. Microwave transmission and reflectance properties of thick film interdigital capacitors will be discussed, and frequency dispersion of dielectric properties will be presented. Specially, designed Au/Li-doped (Ba,Sr)TiO₃/Ag-Pd/Al₂O₃ vertical structure was prepared to measure the tunability. In this sandwich type structure, Li-doped (Ba,Sr)TiO₃ films showed tenability of 7.15% at a bias electric field of 20 kV/cm.     

New Strategy for Polysulfide Protection Based on Atomic Layer Deposition of TiO2 onto Ferroelectric‐Encapsulated Cathode: Toward Ultrastable Free‐Standing Room Temperature Sodium–Sulfur Batteries

The room temperature (RT) sodium–sulfur batteries (Na–S) hold great promise for practical applications including energy storage and conversion due to high energy density, long lifespan, and low cost, as well based on the abundant reserves of both sodium metal and sulfur. Herein, freestanding (C/S/BaTiO₃)@TiO₂ (CSB@TiO₂) electrode with only ≈3 wt% of BaTiO₃ additive and ≈4 nm thickness of amorphous TiO₂ atomic layer deposition protective layer is rational designed, and first used for RT Na–S batteries. Results show that such cathode material exhibits high rate capability and excellent durability compared with pure C/S and C/S/BaTiO₃ electrodes. Notably, this CSB@TiO₂ electrode performs a discharge capacity of 524.8 and 382 mA h g^?1 after 1400 cycles at 1 A g^?1 and 3000 cycles at 2 A g^?1, respectively. Such superior electrochemical performance is mainly attributed from the “BaTiO₃‐C‐TiO₂” synergetic structure within the matrix, which enables effectively inhibiting the shuttle effect, restraining the volumetric variation and stabilizing the ionic transport interface.     

Positive temperature coefficient of resistivity in thin films of barium titanate

One of the perspective directions in using positive temperature coefficient (PTC) thermistors is their application in surface heater in various electronic devices and integrated circuits. The special interest in this connection makes possible fabrication of thin film structures on the basis of materials with PTCR. In the given work a result of the investigation of Y–BaTiO₃ thin films has been presented. Thin films have been deposited by RF-sputtering technique. Films had thickness of 1.5–2 μm. Subsequent annealing was carried out to obtain polycrystalline films of a single phase. The temperature dependence of resistance of films has been investigated. The increase of the resistance were observed in the temperature range 50–80°C. The jump of the resistance in field of phase transition was 70%.     

Template‐Based Growth of Various Oxide Nanorods by Sol–Gel Electrophoresis

The ability to form oxide nanorods is of great interest in a number of areas. In this paper, we report the template‐based growth of nanorods of several oxide ceramics, formed by means of a combination of sol–gel processing and electrophoretic deposition. Both single metal oxides (TiO₂, SiO₂) and complex oxides (BaTiO₃, Sr₂Nb₂O₇, and Pb(Zr_0.52Ti_0.48)O₃) have been grown by this method. Uniformly sized nanorods of about 125–200 nm in diameter and 10 μm in length were grown over large areas with near unidirectional alignment. Desired stoichiometric chemical composition and crystal structure of the oxide nanorods was readily achieved by an appropriate procedure of sol preparation, with a heat treatment (700 °C for 15 min) for crystallization and densification.     

Structural Transformations during Formation of Quasi‐Amorphous BaTiO3

A model of structural transformations of amorphous into quasi‐amorphous BaTiO₃ is suggested. The model is based on previously published data and on X‐ray photoelectron spectroscopy data presented in the current report. Both amorphous and quasi‐amorphous phases of BaTiO₃ are made up of a network of slightly distorted TiO₆ octahedra connected in three different ways: by apices (akin to perovskite), edges, and faces. Ba ions in these phases are located in the voids between the octahedra, which is a nonperovskite environment. These data also suggest that Ba ions compensate electrical‐charge imbalance incurred by randomly connected octahedra and, thereby, stabilize the TiO₆ network. Upon heating, the edge‐to‐edge and face‐to‐face connections between TiO₆ octahedra are severed and then reconnected via apices. Severing the connections between TiO₆ octahedra requires a volume increase, suppression of which keeps some of the edge‐to‐edge and face‐to‐face connections intact. Transformation of the amorphous thin films into the quasi‐amorphous phase occurs during pulling through a steep temperature gradient. During this process, the volume increase is inhomogeneous and causes both highly anisotropic strain and a strain gradient. The strain gradient favors breaking those connections, which aligns the distorted TiO₆ octahedra along the direction of the gradient. As a result, the structure becomes not only anisotropic and non‐centrosymmetric, but also acquires macroscopic polarization. Other compounds may also form a quasi‐amorphous phase, providing that they satisfy the set of conditions derived from the suggested model.     

Perovskite‐Induced Ultrasensitive and Highly Stable Humidity Sensor Systems Prepared by Aerosol Deposition at Room Temperature

A new capacitive‐type humidity sensor is proposed using novel materials and fabrication process for practical applications in sensitive environments and cost‐effective functional devices that require ultrasensing performances. Metal halide perovskites (CsPbBr₃ and CsPb₂Br₅) combined with diverse ceramics (Al₂O₃, TiO₂, and BaTiO₃) are selected as sensing materials for the first time, and nanocomposite powders are deposited by aerosol deposition (AD) process. A state‐of‐the‐art CsPb₂Br₅/BaTiO₃ nanocomposite humidity sensor prepared by AD process exhibits a significant increase in humidity sensing compared with CsPbBr₃/Al₂O₃ and CsPbBr₃/TiO₂ sensors. An outstanding humidity sensitivity (21426 pF RH%^?1) with superior linearity (0.991), fast response/recovery time (5 s), low hysteresis of 1.7%, and excellent stability in a wide range of relative humidity is obtained owing to a highly porous structure, effective charge separation, and water‐resistant characteristics of CsPb₂Br₅. Notably, this unprecedented result is obtained via a simple one‐step AD process within a few minutes at room temperature without any auxiliary treatment. The synergetic combination of AD technique and perovskite‐based nanocomposite can be potentially applied toward the development of multifunctional sensing devices.     

Preparation and characterization of Si-doped barium titanate nanopowders and ceramics

Si-doped barium titanate nanopowders and ceramics were prepared through the sol-gel process. The powders and ceramics were characterized by methods of XRD, SEM and TEM. The dielectric properties of the ceramics were also determined. The results indicated that the powders were nanopowders and they were all cubic BaTiO₃ phase with the concentration of Si ?5.0 mol%. When the concentration of Si increased to 10.0 mol%, another phase of Ba₂TiSi₂O₈ appeared. After sintering, the cubic BaTiO₃ phase was transformed into tetrahedron BaTiO₃ phase. Si doping with low concentration resulted in improving grain growth and reduced dielectric loss. As the sintering temperature increased, the dielectric properties of the ceramics decreased. BaTiO₃ ceramics doped with Si all had a small peak at room temperature in ε−T cures. Specially, the ceramic doped with 0.3 mol% Si had evident double peaks, the maximum room temperature permittivity (4081) and the minimum dielectric loss (0.004).     

Low temperature formation of insulating layers on silicides by anodic oxidation

The anodic oxidation of the silicides CoSi, CoSi₂, CrSi₂, Ni₂Si, NiSi, NiSi₂, Pd₂Si, PtSi, TiSi₂ and ZrSi₂ was studied by using Rutherford backscattering of 2MeV alpha particles. The room temperature oxidation was carried out at a constant current density of 8.9 mA cm⁻² using n-methylacetamide (2% H₂0 and 1% KNO₃) as electrolyte. No oxidation of Pd₂Si and PtSi was detected. Pure SiO₂ layers were grown on CoSi, CoSi₂, Ni₂Si, NiSi and NiSi₂ at a much lower rate than on Si〈100〉 and with a thickness increase per volt of 0.6 ± 0.03 nm V⁻¹. Mixed layers of SiO₂/metal oxide were grown on CrSi₂, TiSi₂ and CrSi₂. All oxidations occurred at the expense of the silicide layer. It is also shown how the purity of the SiO₂ layer formed can be predicted from thermodynamic considerations.     

Epitaxial TiOx Surface in Ferroelectric BaTiO3: Native Structure and Dynamic Patterning at the Atomic Scale

Surfaces and interfaces of ferroelectric oxides exhibit enhanced functionality, and therefore serve as a platform for novel nano and quantum technologies. Experimental and theoretical challenges associated with examining the subtle electro‐chemo‐mechanical balance at metal‐oxide surfaces have hindered the understanding and control of their structure and behavior. Here, combined are advanced electron‐microscopy and first‐principles thermodynamics methods to reveal the atomic‐scale chemical and crystallographic structure of the surface of the seminal ferroelectric BaTiO₃. It is shown that the surface is composed of a native <2 nm thick TiO_x rock‐salt layer in epitaxial registry with the BaTiO₃. Using electron‐beam irradiation, artificial TiO_x sites with sub‐nanometer resolution are successfully patterned, by inducing Ba escape. Therefore, this work offers electro‐chemo‐mechanical insights into ferroelectric surface behavior in addition to a method for scalable high‐resolution beam‐induced chemical lithography for selectively driving surface phase transitions, and thereby functionalizing metal‐oxide surfaces.     

Effects of Annealing Temperature on the Electric Properties of 0.94(Na0.5Bi0.5)TiO3–0.06BaTiO3 Ferroelectric Thin Film

0.94(Na_0.5Bi_0.5)TiO₃–0.06BaTiO₃ (NBT–BT6) ferroelectric thin films have been fabricated on Pt–Ti–SiO₂–Si(100) substrate by metal–organic decomposition. The effects of annealing temperature (650–800°C) on the microstructure, and the piezoelectric, ferroelectric, and dielectric properties of the thin films were studied in detail. The residual stress was evaluated by the orientation average method to clarify its dependence on annealing temperature and grain size, and it was correlated with the electric properties to understand the mechanism of piezoelectric enhancement. Among the thin films, NBT–BT6 thin film annealed at 750°C has the largest effective piezoelectric coefficient, 95.1 pm/V, remnant polarization, 49.7 μC/cm², spontaneous polarization, 105.2 μC/cm², and dielectric constant, 504, and the lowest dielectric loss, 0.05, and tensile residual stress, 24.5 MPa. For the NBT–BT6 thin film annealed at 750°C, a wide temperature range, 183–210°C, around the phase transition temperature (T _m) was observed in the dielectric temperature plots, and the diffusion coefficients (γ) were quantitatively assessed as 1.6, 1.78, and 1.6. Piezoelectric performance is discussed on the basis of the dispersion phase transition and residual stress.     

Solid-state synthesis of modified (Bi0.5Na0.5)TiO3 piezoelectric nanocrystalline ceramics and evaluating their properties

The piezoelectric nanocrystalline ceramics of (Bi_0.5Na_0.5) TiO₃, 0.94(Bi_0.5Na_0.5) TiO₃–0.06BaTiO₃, 0.82(Bi_0.5Na_0.5) TiO₃–0.18(Bi_0.5K_0.5) TiO₃ and 0.85(Bi_0.5Na_0.5) TiO₃–0.144(Bi_0.5K_0.5)TiO₃–0.006BaTiO₃ (abbreviated as BNT, BNBT6, BNKT18 and BNT–BT–BKT, respectively) have been synthesized by a modified solid state approach using high-energy planetary ball-milling. The crystal structures of ceramics were determined using X-ray diffraction (XRD) method and that the microstructures as well as the morphology of the sintered ceramic specimens were observed using scanning-electron microscopy (SEM). The dielectric coefficient was also calculated based on its relation with a constant capacitance measured by an electrical circuit on the basis of the Wetston–Bridge and the piezoelectric coefficient (d₃₃) measured with a d₃₃-meter. On the calcination of powders the XRD results showed that the perovskite phase was formed perfectly and the crystallite sizes of BNT, BNBT6, BNKT18 and BNT–BT–BKT were estimated at about >100, 55, 36 and 63 nm, respectively. Also, the crystallite sizes of the calcinated BNT powders over the course of 5, 10, 20, 30 and 40 h of ball-milling were estimated at about 86, 82, 72, 53, 81 nm, respectively. Moreover, the results of XRD and SEM analysis of the sintered powders at 750–1150 °C confirmed the positive effect of nanocrystalline formation during ball-milling in decreasing the sintering temperature and increasing the density of the sintered samples. Furthermore, electrical calculations such as dielectric and piezoelectric coefficients showed that the modified BNKT18 nanocrystalline ceramic sintered at 1150 °C was to have the best values of dielectric (ε_r=792 at 1 kHz) and piezoelectric coefficients (d₃₃=85.9 pC/N) in comparison with the other synthesized piezoelectric ceramics.     

Microstructural and electrical property evolution in an acceptor-dopant free positive temperature coefficient thermistor

The effect of varying sintering temperature in the range 1270–1430 °C on the resistivity–temperature characteristics of semiconducting BaTiO₃ based positive temperature coefficient of resistance thermistors containing a donor-dopant, but without acceptor doping, was investigated by impedance spectroscopy. As the sintering temperature was increased the specimen resistivity around the Curie temperature decreased, while the peak resistivity, obtained above the Curie temperature, remained approximately constant. The change in PTC behaviour with increasing sintering temperature is inconsistent with the standard double Schottky barrier model, but is explained in terms of grain size variations coupled with a, sintering temperature independent, grain boundary barrier layer thickness of 0.50±0.04 μm.     

Ultrathin Amorphous Silica Membrane Enhances Proton Transfer across Solid‐to‐Solid Interfaces of Stacked Metal Oxide Nanolayers while Blocking Oxygen

A large jump of proton transfer rates across solid‐to‐solid interfaces by inserting an ultrathin amorphous silica layer into stacked metal oxide nanolayers is discovered using electrochemical impedance spectroscopy and Fourier‐transform infrared reflection absorption spectroscopy (FT‐IRRAS). The triple stacked nanolayers of Co₃O₄, SiO₂, and TiO₂ prepared by atomic layer deposition (ALD) enable a proton flux of 2400 ± 60 s^?1 nm^?2 (pH 4, room temperature), while a single TiO₂ (5 nm) layer exhibits a threefold lower flux of 830 s^?1 nm^?2. Based on FT‐IRRAS measurements, this remarkable enhancement is proposed to originate from the sandwiched silica layer forming interfacial SiOTi and SiOCo linkages to TiO₂ and Co₃O₄ nanolayers, respectively, with the O bridges providing fast H⁺ hopping pathways across the solid‐to‐solid interfaces. Together with the complete O₂ impermeability of a 2 nm ALD‐grown SiO₂ layer, the high flux for proton transport across multi‐stack metal oxide layers opens up the integration of incompatible catalytic environments to form functional nanoscale assemblies such as artificial photosystems for CO₂ reduction by H₂O.     

Influence of polarization electric field on the dielectric properties of BaTiO3-based ceramics

The titanate of barium (BaTiO₃) is a ferroelectric material with perovskite structure. The electric properties (ferroelectricity, positive temperature coefficient, piezoelectricity, etc.) of BaTiO₃ were largely studied. Usually, addition of small quantities of impurity can modify the dielectric properties of BaTiO₃ and widen its number of applications. In this investigation different additions of sodium niobate (NaNbO₃), according to the composition, (1−x) BaTiO₃+x NaNbO₃ with x=1, 3, 5, 7 and 10 mol%, were considered. Additionally, the effect of simultaneous addition of 3 mol% NaNbO₃ and 3 mol% SiO₂ on the dielectric properties we studied as function of temperature, frequency and the applied bias. The obtained results reveal a net evolution of permittivity with addition. Furthermore, the obtained results show that the permittivity is independent of the applied electric field at temperature lower than Curie point (T_c), but presents relatively high values at low frequencies. Beyond this temperature, the permittivity considerably increases (about 25×10³) and largely depends both on frequency and applied voltage.     

The microwave properties of Li doped 0.7(Ba,Sr)TiO3-0.3MgO thick film interdigital capacitors on the alumina substrates

The crystalline and electrical properties of Li doped 0.7(Ba,Sr)TiO₃-0.3MgO thick film interdigital capacitors have been investigated. Screen printing method was employed to fabricate Li doped 0.7(Ba,Sr)TiO₃-0.3MgO thick films on the alumina substrates. (Ba,Sr)TiO₃ materials have high dielectric permittivity (>500 @ 1 MHz) and low loss tangent (0.01 @ 1 MHz) in the epitaxial thin film form. To improve dielectric properties and reduce sintering temperature, MgO and Li were added, respectively. 10 μm thick films were screen printed on the alumina substrates and then interdigital capacitors with seven fingers of 200 μm finger gap were patterned with Ag electrode. Current-voltage characteristics were analyzed with elevated temperature range. Up to 50 °C, the thick films showed positive temperature coefficient of resistivity (dρ/dT) of 6.11 × 10⁸ Ω cm/°C, then film showed negative temperature coefficient of resistivity (dρ/dT) of −1.74 × 10⁸ Ω cm/°C. From the microwave measurement, the relative dielectric permittivity of Li doped 0.7(Ba,Sr)TiO₃-0.3MgO thick films interdigital capacitors were between 313 at 1 GHz and 265 at 7 GHz.