Ba0.5Sr0.5TiO3 Research Articles

Application of atomic layer deposited Al2O3 as charge injection layer for high-permittivity dielectrics

The current transport through (Ba,Sr)TiO3 (BST)/Al2O3 bilayer structures with Pt electrodes is studied using current–voltage measurements. Due to its low permittivity compared to BST the Al2O3 layer, which is deposited by atomic layer deposition (ALD), enables electron injection by tunneling at low thickness, such that transport becomes limited by the bulk conductivity of BST. A current rectification of up to 106 is observed at an Al2O3 thickness of 1.8 nm, indicating that hole transport does not contribute to the current. The measurements are complemented by the determination of the energy band alignment at the interface using photoelectron spectroscopy. A Fermi level pinning in the Al2O3 layer, which seems to be characteristic for ALD films, leads to a significant modification of the energy band alignment. This pinning does not prohibit electron injection, which relies on the potential drop across the Al2O3 layer.

Preparation of BST nanotube and its infrared absorption properties

Ba0.6Sr0.4TiO3 (BST) nanotubes are fabricated successfully by sol-sel method with the through-hole anodic aluminum oxide (AAO) template for the first time so far as we know. This fabrication method is easy to realize at low cost because the through-hole AAO template and the BST sol can be acquired easily at low cost, so this is very valuable in the fabrication of other similar nanostructures. First, the steady BST sol is prepared and the well aligned through-hole anodic aluminum oxide template is fabricated by a two-step anodization method; second, the BST sol is introduced into the ordered nanohole arrays of the through-hole AAO template by dipping and spinning; and finally, the samples are fired in air at 650℃ for 1 h to get BST nanotubes. X ray diffraction (XRD) patterns reveal that the BST nanotubes are of cubic perovskite structures, and grow mainly along [110] crystal orientation. Scanning electron microscope (SEM) results show that the thickness and pore size of the through-hole AAO template are about 16 μm and 75 nm, respectively. The length, external and inner diameters of the BST nanotubes are about 16 μm, 75 nm and 50 nm, respectively. Measurements of BST nanotubes give results highly matched with that of the through-hole AAO template. Fourier transform infrared spectroscopy (FTIR) results shows that in the 1350-1650 cm-1 waveband, the composite structure of AAO/BST nanotubes has two obvious absorption peaks which are respectively at 1470 and 1550 cm-1, while the BST film does not have; the absorption property of the composite structure is about two times of the pure through-hole AAO membrane. Finally, the possible reasons of this phenomenon about infrared absorption are discussed.

Fabrication of Barium Strontium Titanate Nanophotocatalysts with Gridding Structures and Their Photocatalytic Activities

Ba0.7Sr0.3TiO3 (BST) nanocatalysts were prepared by a Sol-Gel method at pH of 2.4 and 4, respectively, both of which were presented as tetragonal phase. TEM images indicated that the BST sample obtained at pH = 2.4 was nanoparticles with an average size of about 100 nm, while that obtained at pH = 4 was gridding structures composed of nanocubes with a mean size of 100 nm. The photocatalytic behavior of BST in the degradation of Rhodamine B (RhB) was investigated. It was found that the BST catalysts with gridding structures showed much better photocatalytic performance than the BST nanoparticles. This could be attributed to the fact that the photo-generated carriers can migrate easily through the gridding-structured BST sample, which would enhance the photocatalytic activity during the degradation process. It was also found that both BST photocatalysts in this work showed good catalytic stability.

Development and Application of Ba0.5Sr0.5TiO3 (BST) Thin Film as Temperature Sensor for Satellite Technology

Barium Strontium Titanate (BST) thin film has been manufactured by using chemical solution deposition (CSD) method with annealing temperature of 850 ̊C and variations of the holding times of 8, 15, 22 and 29hours. BST thin film was pyroelectric material which was sensitive to temperature. Thin film which has been made will be used for temperature sensors on the satellite. The BST thin film sensitivity has been characterized against the temperature. Before analyzed, BST thin film was connected to the amplifier (op amp) circuit for increasing the sensitivity. This film was put into the furnace with temperature variations of around 30-100 ̊C. Then the electrical conductivity and pyroelectric properties from the resulted thin films were characterized. The results showed that the higher the temperature lead to the lower the resistivity and the higher the conductivity. Based on the data, the BST thin film of 29hours has the highest pyroelectric coefficient of 8x10-5 C.m-2.K-1 among others.

Characterizations of Electrical and Optical Properties on Ferroelectric Photodiode of Barium Strontium Titanate (Ba0.5Sr0.5TiO3) Films Based on the Annealing Time Differences and its Development as Light Sensor on Satellite Technology

Photodiode of Ba0.5Sr0.5TiO3 (BST) film has successfully been created by growing BST on the surface of p-type Si (100) substrate with chemical solution deposition (CSD) method and spin coating technique at a speed of 3000rpm for 30seconds. BST films were made with concentration of 1M and annealing process at the temperature of 850°C for 8, 15, 22, and 29hours. Characterizations of electrical and optical properties of the films were then conducted. I-V characterization was conducted in dark and bright conditions with green, yellow, and red filters. The results showed that the films had photodiode characteristic. Characterizations of optical properties were the absorbance and the reflectance. The obtained absorbance curve showed that the most absorbed and reflected wavelengths were in the range of 400-500nm and 570-600nm respectively. Data of film reflectance can be used to calculate band gap energy (Eg). The electrical and optical properties indicate which the BST film has a prospect to be used as a light sensor on the satellite.

Grain size dependent phase transition and superparaelectric behavior of ferroelectric BST

In this article, we investigate the grain size dependent phase transition and polarization behavior of ferroelectric Ba0.9Sr0.1TiO3 (BST). Starting with nanocrystalline powders (crystalline size≈22nm), various grain sizes were obtained in nano–submicron domain (202–745nm) by regulating the sintering temperatures (Tsin) in the range of 1050–1300°C. All samples were found to possess diffuse phase transition characteristics with frequency independent broad dielectric maxima near transition temperature due to the lattice strain contribution. Dielectric stiffness showed tarnished step-like anomalous behavior in the paraelectric state for Tsin≥1200°C due to the existence of polar (superparaelectric) nano-regions generated by local polarization by off-centered Ti4+ ions exhibiting an exceptionally rare delicate polarization hysteresis loop.

PVDF–BaSrTiO3 nanocomposites for flexible electrical energy storage devices

We have explored the energy storage capability of Ba0·5Sr0 ·5TiO3 (BST)–polyvinylidene fluoride (PVDF) nanocomposites. Morphologically uniform BST colloidal nanocubes were prepared in high yield by a solvothermal method at temperatures as low as 150°C. As-synthesised BST nanocubes were used as fillers (35 vol%) in PVDF polymer matrix. The unique dielectric-polymer films show enhanced dielectric constant (>27) and enhanced electrical breakdown strength (Eb) (2·85 MV/cm). The resulting dielectric energy density for BST–PVDF is 9·7 J/cm3, which is a result of the interplay between dependencies of permittivity and breakdown strength on volume fraction. We propose that the strong nanoparticle–polymer matrix interfacial interaction is the main reason for the observed improved dielectric properties. This wet-chemical-assisted fabrication approach can be readily extended to other combinations of polymers and ceramics with concomitant improvement in properties. Key parameters of various materials (e.g. chemical composition, shape, size and surface reactivity) can be readily controlled in this method, opening up a new pathway to highly flexible macroelectronics.

Effects of Ba0.7Sr0.3TiO3-based buffer layers and La/Mn doping on the crystallization behavior and multiferroic properties of BiFeO3thin films

A detailed study of the BiFeO3 (BFO)-based bilayer films with improved multiferroic properties is reported. The BFO film without any impurity phase was obtained using Ba0.7Sr0.3TiO3 (BSTO) as a buffer layer. And the leakage currents of the bilayer film were found to reduce effectively through La and Mn co-doping of the BFO Layer. Moreover, magnetodielectric response and ferroelectric domains were observed in the Bi0.9La0.1Fe0.95Mn0.05O3 (BLFMO)/BSTO film. We additionally investigated the effects of the BSTO–Ni0.8Zn0.2Fe2O4 (NZFO) buffer layer on the multiferroic performances and surface morphology of the BLFMO film. It was found that the BSTO–NZFO composite film could significantly enhance the magnetic and magnetodielectric properties of the BLFMO film. In particular, the BLFMO/BSTO–NZFO film possessed a higher coercivity value as compared to the BLFMO/BSTO film.

Room-Temperature Negative Capacitance in a Ferroelectric–Dielectric Superlattice Heterostructure

We demonstrate room-temperature negative capacitance in a ferroelectric-dielectric superlattice heterostructure. In epitaxially grown superlattice of ferroelectric BSTO (Ba0.8Sr0.2TiO3) and dielectric LAO (LaAlO3), capacitance was found to be larger compared to the constituent LAO (dielectric) capacitance. This enhancement of capacitance in a series combination of two capacitors indicates that the ferroelectric was stabilized in a state of negative capacitance. Negative capacitance was observed for superlattices grown on three different substrates (SrTiO3 (001), DyScO3 (110), and GdScO3 (110)) covering a large range of substrate strain. This demonstrates the robustness of the effect as well as potential for controlling the negative capacitance effect using epitaxial strain. Room-temperature demonstration of negative capacitance is an important step toward lowering the subthreshold swing in a transistor below the intrinsic thermodynamic limit of 60 mV/decade and thereby improving energy efficiency.

Microfabrication of high aspect ratio BST pillar arrays by epoxy gel casting from aqueous suspensions with added water soluble epoxy resin

The present paper reports on the successful fabrication of micro-sized pillar arrays of Ba0.6Sr0.4TiO3 (BST) electroceramics via cost effective epoxy gel casting (EGC) consolidation method based on the in situ polymerization of aqueous suspensions with added water soluble epoxy resin. The synthesized BST solid solution powder was fine milled and surface protected against hydrolysis to avoid its degradation in the aqueous environment. The micro patterning process was divided in the following steps: (i) production of high quality SU-8 master moulds by photolithography and of the respective negative replicas in PDMS (soft mould); (ii) casting of stable high solid loading BST suspensions in the soft moulds with added setting agents followed by de-moulding, drying and sintering. Patterned array of hexagonal shaped pillars with 320μm of height and side length ∼40μm were obtained with smooth side wall features along height. Various structural, mechanical and electrical properties of BST material after sintering were also measured to demonstrate the feasibility of the proposed aqueous processing route.

Crystal Structure and Effective Dielectric Response of Ba0.5Sr0.5TiO3–MgO Composites Synthesized In Situ Process

(1 − x)Ba0.5Sr0.5TiO3(BST)–xMgO composite ceramics have been prepared in situ by a citrate–nitrate combustion process, and their crystal structure and effective dielectric response have been investigated systematically. Results reveal that MgO particles homogeneously disperse in BST particles. Rietveld refinement of X‐ray diffraction data shows that the amount of the incorporation of MgO into BST lattice is increased with increasing the MgO volume fraction (ƒM). It is identified that the incorporation of Mg2+ into B site gives rise to the formation of F centers in the composites. Related with the incorporation of Mg2+ into BST lattice, the permittivity of BST in the composite is strongly dependent on ƒM, which is defined by the relation . Also, the high percolation threshold of ƒM = 0.60 is observed, which is well in agreement with the theoretical predictions of the modified effective medium approximation coupled with the spherical inclusion model.

Dielectric properties of (Ba,Sr)TiO3 thin films in MHz and THz frequency regions: Quantitative evaluation of the orientational polarization

We have deposited 1.0-µm-thick BaTiO3 (BTO), SrTiO3 (STO), Ba0.5Sr0.5TiO3 (BST) thin films on MgO substrates by pulsed laser deposition technique and measured their dielectric properties by terahertz-time domain spectroscopy (THz-TDS) and an impedance analyzer with interdigital surface electrodes in the temperature range from 20 to 290 K. By comparing the real part of the dielectric constant at 1 MHz and 1 THz, we have estimated the temperature dependence of the orientational polarization. The results suggest that the orientational polarization emerges in the ferroelectric phase of (Ba,Sr)TiO3, and the concentration of orientational polarization components of BST exceeds 90% of the total below 150 K, while it is about 30% at room temperature.

Lanthanum doping effect in barium strontium titanate glass ceramics

Lanthanum doping effect in barium strontium titanate (Ba,Sr)TiO3 (BST) glass ceramics has been investigated by differential scanning calorimeter (DSC), X-ray diffractometry and Raman spectroscopy. Studies were performed on specimens with lanthanum oxide (La2O3) concentrations between 0 and 1.0mol% for BaO–SrO–TiO2–SiO2–Al2O3 glass system. The analysis of the dependence of DSC measurements on La2O3 concentration has shown that the increase of La2O3 concentration promotes the formation of the barium strontium titanate crystalline phase. The X-ray diffractometry results have demonstrated that the amount of barium strontium titanate crystalline phase increases and then decreases with the increase of the La2O3 concentration. The Raman spectrum results, which coincide with the X-ray diffractometry patterns, confirmed that lanthanum ions diffused into the lattice structure and modified the tetragonality of the barium strontium titanate phase. However, the additive of 1.0mol% La2O3 may go beyond the solid solubility limit of lanthanum in the BST lattice.

Energy storage properties in Ba0.4Sr0.6TiO3 ceramics with addition of semi-conductive BaO–B2O3–SiO2–Na2CO3–K2CO3 glass

Semi-conductive BaO–B2O3–SiO2–Na2CO3–K2CO3 glass additives were added to Ba0.4Sr0.6TiO3 (BST) ceramics to investigate its influences on the microstructure, dielectric properties, breakdown strength (BDS), and energy storage density. The sintering temperatures were lowered about 150°C with a high relative density (∼98%). X-ray diffraction patterns of all BST glass ceramics suggest perovskite structures without any detectable secondary phase. Room temperature dielectric constant of BST glass ceramics decreases with increasing Na2CO3 content. BDS and energy storage density of BST glass ceramics increase with increasing Na2CO3 content, and reach their maxima of 280kV/cm and 0.72J/cm3 at the highest Na+ and K+ content. These results suggest that the BST glass ceramics could find their potential applications in energy storage capacitor devices.

Lattice deformation in thin BiFeO3 and Ba0.8Sr0.2TiO3 epitaxial films on (001)MgO single crystalline substrates

Bi0.98Nd0.02FeO3 (BNFO) and Ba0.8Sr0.2TiO3 (BST) epitaxial films produced by rf cathode sputtering on (001)MgO substrates are studied by means of XRD. The thickness dependences of uniform lattice deformations and linear dislocation density, generated due to epitaxial stress relaxation, are plotted.

Ferroelectrics based absorbing layers

We show that ferroelectrics-based periodic structure made of BaSrTiO3 (BST) cubes, arrayed onto a metal plate with a thin dielectric spacer film exhibit a dramatic enhancement of absorbance with value close to unity. The enhancement is found around the Mie magnetic resonance of the Ferroelectrics cubes with the backside metal layer stopping any transmitted waves. It also involves quasi-perfect impedance matching resulting in reflection suppression via simultaneous magnetic and electrical activities. In addition, it was shown numerically the existence of a periodicity optimum, which is explained from surface waves analysis along with trade-off between the resonance damping and the intrinsic loss of ferroelectrics cubes. An experimental verification in a hollow waveguide configuration with a good comparison with full-wave numerical modelling is at last reported by measuring the scattering parameters of single and dual BST cubes schemes pointing out coupling effects for densely packed structures.

Microwave characteristics of sol-gel based Ag-doped (Ba0.6Sr0.4)TiO3 thin films

Dielectric Ba0.6Sr0.4TiO3 (BST) thin films with a different concentration of Ag-dopant of 0.5, 1, 1.5, 2, 3, and 5mol % have been prepared using an alkoxide-based sol-gel method on a Pt(111)/TiO2/SiO2/Si substrate and their surface morphology and crystallinity have been examined using scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis, respectively. An on-chip metal-insulator-metal capacitor has been fabricated with the prepared thin film ferroelectric sample. Concentric coplanar electrodes are used for high frequency electrical characterization with a vector network analyzer and a probe station. The SEM images show that increasing Ag doping concentration leads to a decrease in grain size. XRD reveals that the fabricated films show good BST crystallinity for all the concentration while a doping concentration of 5mol % starts to show an Ag peak, implying a metallic phase. Improved microwave dielectric loss properties of the BST thin films are observed in a low Ag doping level. Especially, BST with an Ag doping concentration of 1mol % shows the best properties with a dielectric constant of 269.3, a quality factor of 48.1, a tunability at the electric field of 100kV/cm of 41.2 %, a leakage-current density of 1.045×10−7A/cm2 at an electric field of 100kV/cm and a figure of merit (defined by tunability (%) divided by tan δ (%)) of 19.59 under a dc bias voltage of 10V at 1GHz.

Improved dielectric performance of barium strontium titanate multilayered capacitor by means of pulsed laser deposition and slow injection sol–gel methods

AbstractA Pt/BST/NiFe/Cu multilayered capacitor was fabricated incorporating a polycrystalline Ba0.5Sr0.5TiO3(BST) film deposited using the pulsed laser deposition technique. Qualitative X-ray diffraction analysis confirmed a perovskite structure for the deposited BST dielectric films which were fired at various temperatures. No intermediate phase was discernable with a post-annealing temperature of 750 °C and highly crystallized thin film was obtained at a post-annealing temperature of 800 °C. The fabricated capacitor with a BST film thickness of 665 nm exhibited respectable electrical performance with a dielectric constant,kof 657, and a dielectric loss, tan δ = 0.0137 at room temperature at an applied frequency of 1 MHz. The recorded charge storage density and leakage current density were 4.6 μC cm−2and 33 nA cm−2, respectively, with ± 5 V bias.

Dielectric properties of La/Zr codoped Ba0.67Sr0.33TiO3 ceramics prepared from citrate–nitrate combustion derived powders

La/Zr codoped Ba0.67Sr0.33TiO3 (BST) ceramics were fabricated via citrate–nitrate combustion derived powders, and the microstructure and dielectric properties of BST ceramics were investigated. All ceramic samples show a pure perovskite structure. The dielectric constant and loss decrease with increasing Zr content. The additions effectively suppress the grain growth of BST ceramics. It is found that the temperature-permittivity characteristics for codoped BST ceramics could be controlled using various doping content.

Nanocrystalline Barium Strontium Titanate Ceramics Synthesized via the “Organosol” Route and Spark Plasma Sintering

Dense nanocrystalline barium strontium titanate Ba0.6Sr0.4TiO3 (BST) ceramics with an average grain size around 40 nm and very small dispersion were obtained by spark plasma sintering at 950°C and 1050°C starting from nonagglomerated nanopowders (~20 nm). The powders were synthesized by a modified “Organosol” process. X‐ray diffraction (XRD) and dielectric measurements in the temperature range 173–313 K were used to investigate the evolution of crystal structure and the ferroelectric to paraelectric phase transformation behavior for the sintered BST ceramics with different grain sizes. The Curie temperature TC decreases, whereas the phase transition becomes diffuse for the particle size decreasing from about 190 to 40 nm with matching XRD and permittivity data. Even the ceramics with an average grain size as small as 40 nm show the transition into the ferroelectric state. The dielectric permittivity ε shows relatively good thermal stability over a wide temperature range. The dielectric losses are smaller than 2%–4% in the frequency range of 100 Hz–1 MHz and temperature interval 160–320 K. A decrease in the dielectric permittivity in nanocrystalline ceramics was observed compared to submicrometer‐sized ceramics.