SrTiO3 Powders Research Articles

Structural conditions that leads to photoluminescence emission in SrTiO3: An experimental and theoretical approach

Complex cluster [TiO5⋅VOz] and [SrO11⋅VOz] (where VOz=VOX, VO•, VO••) vacancies were identified in disordered SrTiO3 powders prepared by the polymeric precursor method, based on experimental measurements by x-ray absorption near edge structure spectroscopy. The paramagnetic complex states of [TiO5⋅VO•] and [SrO11⋅VO•] with unpaired electrons were confirmed by electron paramagnetic resonance spectroscopy. The disordered powders showed strong photoluminescence at room temperature. Structural defects of disordered powders, in terms of band diagram, density of states, and electronic charges, were interpreted using high-level quantum mechanical calculations in the density functional framework. The four periodic models used here were consistent with the experimental data and explained the presence of photoluminescence.

TWO-STEP SINTERING AND SPARK PLASMA SINTERING OF Al2O3, ZrO2 AND SrTiO3 CERAMICS

ABSTRACT The influence of Two Step Sintering and Spark Plasma Sintering processes on the microstructure of three kinds of oxide ceramics was studied. Al2O3 (with initial particle size of 240 nm), ZrO2 (60 nm, stabilized with 3 mol% Y2O3) and SrTiO3 (50 nm) powders were cold isostatically pressed and pressureless sintered with different heating regimes. It was found that the method of TSS does not bring about significant reduction of final grain size in Al2O3 and ZrO2 ceramics, but does reduce the grain size in perovskite SrTiO3 ceramics. Pressureless sintering of SrTiO3 leads only to 95%TD dense ceramics with the minimum grain size of 400 nm. In comparison, by SPS, with an applied uniaxial pressure of 200 MPa, SrTiO3 ceramic of 98%TD dense with grain size of 80 nm was prepared.

Dielectric Constant Evaluation of Dielectric Powders by Complex Impedance Analysis with Model of RC–RC Parallel Circuits Connected in Series

A new evaluation method for the high dielectric constants of SrTiO3 and BaTiO3 powders has been developed using the complex impedance measurement analyzed by a model of RC–RC parallel circuits that are individually connected in series. The model properly represented the various dispersions of the powders in the slurry specimens. This evaluation method was experimentally examined using SrTiO3 powder with an average particle size of 300 nm and BaTiO3 powder, 500 nm. The dielectric constants were obtained by (1) fitting the complex impedance of the models of RC parallel circuits connected in series to the measured complex impedance of the slurry specimens and (2) applying simultaneous equations to calculate each parameter of the model of RC–RC parallel circuits connected in series. The obtained dielectric constants of the SrTiO3 and BaTiO3 powders were estimated to be 232±3 and 2,610±190, respectively. They were in good agreement with the dielectric constant of the paraelectric SrTiO3 single crystal and the average value of the ferroelectric BaTiO3 single crystal.

A Study on Dielectric Constants of ${\rm Epoxy/SrTiO}_{3}$ Composite for Embedded Capacitor Films (ECFs)

Epoxy/BaTiO3 composites have been of great interest as embedded capacitor materials, mainly due to high dielectric constant of ceramic powders, and good process compatibility of epoxy with printed circuit boards (PCBs). However, one of the potential problems of epoxy/BaTiO3 composite is the dielectric relaxation at gigahertz range. In this paper, epoxy/SrTiO3 composite embedded capacitor films (ECFs) were fabricated for high-frequency applications. Dielectric constants of epoxy/SrTiO3 composite embedded capacitor films with various SrTiO3 particles loading for three different commercial SrTiO3 powders were measured, and experimental data were fitted to the Lichtenecker equation for the prediction of the effective dielectric constant of SrTiO3 powders in epoxy/ SrTiO3 composite ECFs. Using a rectangular cavity resonator method, dielectric constants of epoxy/SrTiO3 composite ECFs were measured at gigahertz range (1-10 GHz). At gigahertz frequencies, dielectric constants of epoxy/SrTiO3 composite ECFs were nearly constant. In contrast, epoxy/BaTiO3 composite ECFs showed decrease of dielectric constants above 5 GHz. Therefore, it is concluded that epoxy/SrTiO3 composite ECFs can be more effectively to be applied for high-frequency applications.

Influence of processing conditions on the thermal decomposition of SrTiO3 precursors

The present work investigates the influence of milling and calcination atmosphere on the thermal decomposition of SrTiO3 powder precursors. Both pure and neodymium-modified SrTiO3 samples were studied. Milling did not significantly influence numerical mass loss value, but reduced the number of decomposition steps, modifying the profiles of the TG and DTA curves. On the other hand, milling increases the amount of energy liberated by the system upon combustion of organic matter. It was also observed that the milling process, associated to the calcination in an oxygen atmosphere, considerably decreases the amount of organic matter and increases the final mass loss temperature.

Effect of CaTiO<sub>3</sub> Content on the Dielectric Properties of Ba<sub>0.5</sub>Sr<sub>0.5</sub>TiO<sub>3</sub>

In this paper, a microwave material (Ba0.85Ca0.15)0.5Sr0.5TiO3 with 5 wt% MgO additive was prepared by traditional solid-state reactions, i.e., mixing BaTiO3 and SrTiO3 powders and then doping CaTiO3 and MgO. It was found that, with the increasing CaTiO3 content, the dielectric constant of the resultant material decreases and the tunability and dielectric loss increase.

Preparation and Characterization of Semi-Conducting (Ba,Sr)TiO<sub>3</sub> Powders by a Wet Chemical Route

Ultrafine (Ba,Sr)TiO3-based powders used for making positive temperature coefficient of resistance (PTCR) were synthesized by two chemical steps: polyacrylamide gel process and liquid phase coating process. First, donor doped (Ba,Sr)TiO3 powders were synthesized by polyacrylamide gel process at rather low temperature, and the physical and chemical characteristics of the gels and the precursorpowders were investigated by DSC, TG, XRD and TEM. Secondly, acceptor and sintering aid were coated by an aqueous solution containing acceptor (Mn) and additive (Si). The characterization and sintering behavior of the semi-conducting powders and the PTC effect of ceramic samples prepared by the as-obtained powders were investigated.

Effect of High-Q Ba4Ti13O30 Materials on The Dielectric Properties of (Bax,Sr1−x)Tio3 Films For Microwave Communication

Nano-sized BaTiO3, SrTiO3 and TiO2 powders have been used to synthesize (Ba x ,Sr1−x )TiO3 materials via mixed oxide process. Then (Ba x ,Sr1−x )TiO3 (BST) (x = 0.5 and 0.6) with addition of high-Q (quality factor) Ba4Ti13O30 (BT) materials were screen printed on Al2O3 substrates. The prepared thick films of BST/BT with three layered and five layered composite materials were sintered at 1275°C (4 h). The microwave dielectric properties of the bulk ceramics and the thick films were measured by resonant cavity method at higher microwave frequencies (9.5 GHz). The three layered thick films have dielectric constant of 13.4 for x = 0.5 and 15.5 for x = 0.6. The five layered thick films possess dielectric constant of 12.7 for x = 0. 5 and 15.8 for x = 0.6. The frequency × quality factor (f × Q) increases markedly with a larger magnitude of 1600 for the three layered thick films than the five layered ones. The results imply that the BST/BT composite thick films have potential application in tunable microwave devices.

Effect of excess TiO2 on the phase evolution and densification of sol-gel derived (Ba,Sr)TiO3 powders

Nanoscaled (Ba 2/3 Sr 1/3 )Ti 1+x O 3 powders have been prepared by sol-gel technique. Their phase evolution and densification behaviors have been studied by differential thermal analysis (DTA) and high temperature dilatometer, respectively. It is found the addition of 2 mol%-excess amount of TiO 2 lowers the activation energy required for the formation of the perovskite phase by about 130 kJ/mol and thus lowers the crystallization temperature or BST powders. However, the excess amount of TiO 2 makes the nano-powder difficult to sinter. Transmission electron microscopy reveals that a metastable nano-porous layer has formed on the surface of TiO 2 -excess nanopowder and this may account for the slow densification rate.

Phase and morphology investigation of BaxSr1−xTiO3 (x=0.6, 0.7 and 0.8) powders

This study aims to understand thoroughly the effects of calcining and sintering conditions on the relative permittivity and sintering behaviors of the solid solution method of preparing BSTs. BST (BaxSr1−xTiO3, x = 0.6, 0.7 and 0.8) was synthesized using an established solid-state reaction method [R.K. Roeder, E.B. Slamovich, Stoichiometry control and phase selection in hydrothermally derived BaxSr1−xTiO3 powders, J. Am. Ceram. Soc. 82 (7) (1999) 1655, R. Ganesh, E. Goo, Microstructure and dielectric characteristics of PbxBa0.5−xSr0.5TiO3 ceramics, J. Am. Ceram. Soc. 79 (1996) 225, J.W. Liou, B.S. Chiou, Effect of direct-current biasing on the dielectric properties of barium strontium titanate, J. Am. Ceram. Soc. 80 (12) (1997) 3093, T. Noh, S. Kim, C. Lee, Chemical preparation of barium–strontium titanate, Bull. Korean. Chem. Soc. 16 (1995) 1180]. BaTiO3 and SrTiO3 powders were used as starting materials. Crystallinity and lattice parameter of the calcined powders was improved by increasing the calcining temperature, as indicated by the increase in intensity of the X-ray diffraction peak. The particle size distribution also increasing at higher calcining temperature. The composition investigated here display dielectric properties characteristic of normal ferroelectrics.

Preparation of (Ba,Sr)TiO3 Thick Film on ZrO2 Substrates by Inkjet Printing

The thick films were prepared by the inkjet method with ink containing BaTiO3 and SrTiO3 powders. The ink droplets from a piezoelectric-type inkjet head were observed. The velocity of ink droplets was increased with increasing applied voltage of the inkjet head. The dot and line patterns were formed by inkjet printing on a ZrO2 substrate with a Pt electrode. The lines printed on the substrate have 220 µm width. The thickness profiles were measured for the single dot and the 6.5×6.5 mm2 square pattern. The thickness was uniform over the printed thick film. The thick films printed by the inkjet method were fired in the temperature range from 900 to 1200°C. The grain growth advanced with increasing firing temperature. Ba0.7Sr0.3TiO3 was identified by X-ray diffraction patterns at firing temperatures of 1100 and 1200°C. The dielectric constant and tan δ of Ba0.7Sr0.3TiO3 thick films fired at 1200°C without DC bias voltage at 10 MHz were 350 and 11%, respectively. The dielectric constant decreased slightly with increasing frequency. The dielectric constant decreased with increasing DC bias voltage. It is expected that the inkjet method with ink containing several ceramic powders is applicable to the fabrication of thick film patterns.

Preparation and Curie Temperatures of Nanosized (Pb,Sr)TiO3 Powders

Nanosized perovskite lead strontium titanate (PbxSr1-x)TiO3 (PST) powders were successfully prepared by a simple coprecipitation method. A lead-strontium titanyl oxalate (PSTO) precursor was first synthesized at room temperature and the precursor was then calcined at various temperatures to produce the PST powders ranging from 10 to 200 nm in size. The effects of PST particle size on Curie temperature and dielectric constant have been investigated. High-temperature X-ray diffraction results showed that the Curie temperature of Pb0.7Sr0.3TiO3 powder with a size of 10 nm decreased from 290 to 120–130°C.

Synthesis and characterization of chromium doped SrTiO3 photocatalyst

SrTiO3 and chromium doped SrTiO3 (SrTi(1−x)CrxO3) photocatalysts were synthesized by the polymeric precursor method using citric acid and ethylene glycol. Single phase SrTiO3 and SrTi(1−x)CrxO3 nano-size powder were produced at 450 °C. A new band gap in the visible light range was formed by chromium doping in SrTiO3. The absorption of visible light is of practical importance for the application of SrTiO3 as a photocatalyst.

Effect of pore-forming agent on PTCR characteristics of n-BaTiO3

Porous n-BaTiO3 ceramics are synthesized by the addition of pore-forming agent into the (Ba,Sr)TiO3 powder. From the DTA-TGA analysis for samples containing the PEG and corn-starch, it was found that an exotherm occurred at 262 and 315°C, respectively, weight loss commenced at 165 and 252°C, and was virtually complete by 265 and 472°C, respectively. The porosity of n-BaTiO3 ceramics increased and the grain size decreased with increasing the pore-forming agent. From the XRD analysis at high angles, all the samples with and without the pore-forming agent at room-temperature exhibit the tetragonal structure. PTCR jump of the samples containing pore-forming agent is 1-2 orders higher than that of sample without the pore-forming agent. It is also found that the development of PTCR characteristic in the porous n-BaTiO3 ceramics containing pore-forming agent is related to grain boundaries, which basically equals that in ordinary BaTiO3 without pore-forming agent, from the complex impedance results.

The Preparation and Electrical Properties of SrTiO3-Based Capacitor-Varistor Double-Function Ceramics

SrTiO3 powders were prepared using a sol–gel process. The electrical properties and the microstructure of doped SrTiO3-based capacitor-varistor double functions ceramics prepared by the sol–gel process were compared with those of ceramics prepared via the solid-state reaction method using X-ray diffraction, scanning electron microscopy, and measurements of dielectric constants and current-voltage characteristics. The sol–gel based materials exhibited enhanced capacitor-varistor properties compared to those prepared by a conventional solid-state method. These enhanced properties are attributed to the smaller, more uniform grain structure obtained in the sol–gel-derived material, which promotes the formation of an effective core-shell structure in which an acceptor-doped layer encapsulate the donor-doped SrTiO3 grains. The effect of such microstructures on electrical properties is discussed.

Combinatorial Synthesis of Oxide Powders with an Autopipetting System.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Determination of Diffusion Kinetic Parameters and Potential Barrier Height at the Grain Boundary of the Semiconducting SrTiO<sub>3</sub> Ceramics

The chemical and electrical characteristics of the grain boundary of the polycrystalline semiconducting SrTiO3 ceramics, which were synthesized by hot-press sintering Na-coated semiconducting SrTiO3 powders, were investigated in terms of process parameters such as sintering temperature. Diffusion kinetic parameters such as diffusion coefficients and activation energy of diffusing ions at particular heat-treatment conditions were obtained by fitting experimental data with computer-simulated results as well as electrical features near the grain boundary of the ceramics.

Sol-Gel Processed Strontium Titanate Ceramics

Ferroelectric strontium titanate (SrTiO3) ceramics have been prepared by sol-gel processing technique. Strontium acetate [Sr(CH3COO)2] and titanium (IV) isopropoxide [Ti(OC3H7]4 were used as the precursors for the fabrication of the samples. The stock solution for the preparation of strontium titanate powder was prepared by dissolving strontium acetate in ethanol and acetic acid solvents and mixing the resultant solution with titanium (IV) isopropoxide in equimolecular amounts. The as fired powder was found to be amorphous. Annealing the amorphous powder at 700°C for one hour in air yields cubic phase crystalline SrTiO3 powder. Ceramic samples were prepared by pressing and sintering the crystalline powder. The effect of sintering temperature on the structural and dielectric properties were investigated and analyzed.

Photocatalytic Activity of SrTiO3 Codoped with Nitrogen and Lanthanum under Visible Light Illumination

Yellow SrTiO3 powders codoped with nitrogen and lanthanum (STO:N,La) were studied as visible light photocatalysts. The crystal phase of STO:N,La exhibited a pure perovskite phase, and O and Sr sites atoms were substitutionally doped with N and La atoms, respectively. The first principle calculation of STO:N,La indicated that the edge of the N(2p) band is situated above the valence band, which consisted of O(2p) orbitals, and the La orbitals did not exist in the band gap of SrTiO3. STO:N,La exhibited a higher oxidation activity of gaseous 2-propanol under vis illumination than SrTiO3 doped only with nitrogen (STO:N). The high activity of STO:N,La was due to the decrease in the oxygen vacancies, which acted as electron-hole recombination centers, because codoping with La3+ and N3- ions maintained the charge balance. The optimum doping density of N and La for visible light activity was 0.5%, and STO:N,La(0.5%) had an activity under UV illumination similar to pure SrTiO3.

Mechanochemical Synthesis of Fluorine‐Doped SrTiO3 and Its Photo‐oxidation Properties.

AbstractFor Abstract see ChemInform Abstract in Full Text.