An inexpensive and efficient method for the synthesis of BTO and STO at temperatures lower than 200 °C

During the past decade the reduction in size of functional architectures has been a dominating trend in many fields of science and technology. The search for electronic materials that can be cheaply solution-processed into nanopowders, while simultaneously providing quality device characteristics, represents a major challenge for material scientists. Solvothermal process is used in order to obtain fine nanoparticles of BaTiO₃ and SrTiO₃ at low temperatures by using an inorganic, ionic precursor. Rietveld refinement proves the presence of a mixture of 65% tetragonal and 35% cubic nanoparticles in the barium titanate powder with an average size of 73 nm and 67 nm, respectively. FTIR shows that an acid treatment allows the elimination of carbonate impurities.     

Synthesis of BaTiO<Subscript>3</Subscript> powder from barium titanyl oxalate (BTO) precursor employing microwave heating technique

Cubic barium titanate (BaTiO₃) powder was synthesized by heating barium titanyl oxalate hydrate, BaTiO(C₂O₄)₂·4H₂O (BTO) precursor in microwave heating system in air at 500°C. Heating BTO in micro-wave above 600°C yielded tetragonal form of BaTiO₃. Experiments repeated in silicon carbide furnace showed that BaTiO₃ was formed only above 700°C. The product obtained was cubic.     

Structural change of hydrothermal BaTiO3 powder

Formation rate of tetragonal BaTiO₃ powder by hydrothermal synthesis and its dielectric property were studied. Submicron tetragonal BaTiO₃ powders were prepared hydrothermally, using Ba(OH)₂ · 8H₂O, TiO₂ (anatase) and KOH as starting chemicals. Characterization via X-ray diffractometry, Field Emission Scanning Electron Microscopy confirmed that increasing calcination temperature (from 1100 to 1300°C) promotes the formation of tetragonal BaTiO₃. Tetragonal BaTiO₃ ceramics obtained from optimum condition (Tetragonal BaTiO₃ powders calcined at 1200°C for 3 h after hydrothermal synthesized at 200°C for 168 h) exhibited submicron size of 0.5–0.7 m, monodispersed type and high relative permittivity.     

Characteristics of La0.7Ca0.3MnO3 Powders Prepared by the Solution Combustion and Solid State Reaction Methods for Colossal Magnetoresistance Applications

La_0.7Ca_0.3MnO₃ powders were prepared by both the solution combustion method and the solid state reaction method and were calcinated at various calcination temperatures and time intervals in air atmosphere. In the solid state reaction method, single-phase La_0.7Ca_0.3MnO₃ was obtained after heat treatment of the powder at 1000°C for 24 hr. In the solution combustion method, however, single-phase La_0.7Ca_0.3MnO₃ powder could be obtained easily when the powder was heat-treated at 650°C for only 30 min. Polycrystalline La_0.7Ca_0.3MnO₃ powder, using the solution combustion method, showed good powder characteristics, such as an average grain size of 50 nm and a specific surface area of 92 m²/g. The resistance as a function of temperature and the magnetoresistance ratio in La_0.7Ca_0.3MnO₃ thin films were attempted to examine the colossal magnetoresistance characteristics. These thin films also showed excellent colossal magnetoresistance properties in that 96% of the maximum magnetoresistance ratio was obtained at 97K.     

Structural and gas sensing behavior of nanocrystalline BaTiO3 based liquid petroleum gas sensors

In this paper nanosized BaTiO₃ thick films based gas sensor has been fabricated for liquid petroleum gas (LPG). Doping with different concentrations of metal oxides influenced the sensitivity of BaTiO₃ thick films for LPG sensor. We present the characterization of both their structural properties by means of X-ray powder diffraction (XRD) and the electrical characteristics by using gas-sensing properties. X-ray powder diffraction analyses revealed the persistence of cubic phase with grain growth 65 nm. The LPG-sensing properties of BaTiO₃ thick films doped with CuO and CdO are investigated. It was found that 10 wt.% CuO: 10 wt.% CdO doped BaTiO₃ based LPG sensor shows better sensitivity and selectivity at an operating temperature 250 °C which is an important commercial range for LPG alarm development. Incorporation of 0.3 wt.% Pd doped CuO:CdO:BaTiO₃ element shows maximum sensitivity with high cross selectivity to the other gases including CO, H₂ and H₂S at an operating temperature 225 °C for low concentrations of LPG sensor.     

Synthesis and anomalous magnetic properties of LaFeO3 nanoparticles by hot soap method

Nanocrystalline LaFeO₃ particles were synthesized at low temperatures by using hot soap technique. The synthesis was based on the thermal decomposition of organometallic compounds precipitated in a hot coordinating solvent. Moderate heat treatment at low temperature far below the combustion point of organic compounds produced spherical LaFeO₃ nanoparticles with average diameter of about 15 nm. The crystalline phase, structure and particle size of obtained products were characterized by X-ray diffraction, infrared spectroscopy and transmission electron microscopy observations. In spite of the antiferromagnetic nature of bulk LaFeO₃, the obtained nanoparticles exhibited anomalous large magnetization. Superparamagnetic behavior with a blocking temperature of about 30 K was observed in both magnetization and Mössbauer spectroscopic analyses.     

Effect of temperature and time on solvothermal synthesis of tetragonal BaTiO<Subscript>3</Subscript>

Tetragonal BaTiO₃ nanoparticles are synthesized via solvothermal route in an ethanol water mixture. Ba(OH)₂·8H₂O is used as Ba precursor and TiO₂ (P25 Degussa ∼25 nm, 30% anatase, 70% rutile) is used as Ti precursor in the Ba : Ti molar ratio 2 : 1. Effect of temperature and time study on solvothermal synthesis of BaTiO₃ revealed that a moderate reaction temperature i.e. 185°C and longer reaction time favour tetragonal phase stabilization. Dissolution–precipitation appears to be the transformation mechanism for the crystallization of BaTiO₃ from particulate TiO₂ precursor.     

Hydrothermal precipitation and characterization of nanocrystalline BaTiO3 particles

Crystalline BaTiO₃ powders were precipitated by reacting fine TiO₂ particles with a strongly alkaline solution of Ba(OH)₂ under hydrothermal conditions at 80°C to 240°C. The characteristics of the powders were investigated by X-ray diffraction, transmission electron microscopy, thermal analysis and atomic emission spectroscopy. For a fixed reaction time of 24 hours, the average particle size of BaTiO₃ increased from 50 nm at 90°C to 100 nm at 240°C. At synthesis temperatures below 150°C, the BaTiO₃ particles had a narrow size distribution and were predominantly cubic in structure. Higher synthesis temperatures produced a mixture of the cubic and tetragonal phases in which the concentration of the tetragonal phase increased with increasing temperature. A bimodal distribution of sizes developed for long reaction times (96 h) at the highest synthesis temperature (240°C). Thermal analysis revealed little weight loss on heating the powders to temperatures up to 700°C. The influence of particle size and processing-related hydroxyl defects on the crystal structure of the BaTiO₃ powder is discussed.     

Preparation and electrical properties of nanoporous BaTiO3

Nanoporous barium titanate with high specific surface area was prepared from co-gel precursors through solvothermal method followed by supercritical drying. The samples were accumulated by BaTiO₃ nanoparticles with excellent crystallinity. The BaTiO₃ obtained at 60 °C exhibited a high BET surface area of 117 m²/g. The porosity reduced with the increasing solvothermal temperature. Raman spectra indicated that the solvothermal-synthesized BaTiO₃ was composed by both cubic phase and tetragonal phase. The relations between dielectric properties and the porosity of the samples were also investigated. The introduction of pores reduced the dielectric constant obviously. The dielectric constant of the obtained sample increased with the decrease of the porosity.     

Microstructure and ferroelectric properties of epitaxial BaTiO3/SrRuO3/SrTiO3 (001) films grown by pulsed laser deposition under high oxygen pressure

BaTiO₃ films were epitaxially grown on SrTiO₃ (001) substrates buffered with SrRuO₃ films as bottom electrode by pulsed laser deposition under high oxygen pressure of 30 Pa. The quality of the BaTiO₃/SrRuO₃/SrTiO₃ multilayer films was analyzed by means of X-ray diffraction, atomic force microscopy and transmission electron microscopy. BaTiO₃ films were found to be highly c-axis-oriented tetragonal phase with c/a = 1.002. The dielectric constant first increased with increasing temperature, and showed a peak at the Curie temperature of about 356 K. The films had well-saturated hysteresis loops with a remnant polarization of 7.3 μC/cm² and a coercive field of 29.5 kV/cm at room temperature.     

Dielectric properties,thermal expansion and heat capacity of (1 − x)Na0.5Bi0.5TiO3–xBaTiO3 single crystals (x = 0, 0.02, 0.025, 0.0325 and 0.05)

High-quality and large-size lead-free (1 − x)Na_0.5Bi_0.5TiO₃–xBaTiO₃ single crystals (x = 0, 0.025, 0.0325 and 0.05) were grown using Czochralski method. The obtained samples were of pure perovskite structure with rhombohedral symmetry at room temperature. Thermal expansion, heat capacity, ferroelectric and dielectric properties were measured in a wide temperature range. The broad anomalies observed in thermal expansion and heat capacity were corresponded to structural, ferroelectric and dielectric anomalies, related to temperature features of polar regions and formation of a long-range order ferroelectric phase. The Burns temperature was found to increase with increasing BaTiO₃ content. At low-frequency (100 Hz–100 kHz) the samples showed diffuse phase transitions. The obtained results were discussed in terms of local electric and strain fields caused by a difference in ionic radii between (Na,Bi) and Ba ions.     

Characterization of tetragonal BaTiO3 nanopowders prepared with a new soft chemistry route

Preparation of BaTiO₃ nanopowders (37-42 nm) is carried out by a controlled reconstructive thermal decomposition and crystallization from an amorphous polymeric precursor with polyvinyl alcohol (PVA) and sucrose at 400-600 °C in air. The Rietveld refinement of the XRD profile, processed at 600 °C in 2 h, infers the P4mm tetragonal crystal structure (95% of tetragonality) of the as prepared BaTiO₃ nanopowders, with a = 0.3994 nm and c = 0.4024 nm. A cubic symmetry (Pm3m) of 5% in amount with a = 4.0057 is also detected in addition with tetragonal symmetry. The characteristic tetragonal splitting of 002/200 XRD peaks also supports the tetragonal symmetry (c / a = 1.0075) of the as prepared BaTiO₃ nanopowders. The average particle size (D) of the BaTiO₃ powders, estimated with the help of the specific surface area, measured by BET method, is 39.91 nm. Average D value, calculated by Δ2θ_1 / 2 in the XRD peaks with the Debye Scherrer relation is ∼ 40 nm. TEM study measures the particle size of the BaTiO₃ powders with an average diameter of 37 to 42 nm.     

A novel approach to prepare tetragonal BaTiO3 nanopowders

BaTiO₃ nanopowders were attempted to synthesize by using a novel straight-forward, solvent free reactions under autogenic pressure at elevated temperature (RAPET) approach. The as-prepared BaTiO₃ nanoparticles were characterized by X-ray diffraction, transmission electron microscopy, high-resolution TEM, and convergent-beam electron diffraction. It was found that Ba(OH)₂·8H₂O and Ti(OBu)₄ could be appropriate starting materials to synthesize BaTiO₃ at 973 K for 2 h by using RAPET approach. Pure tetragonal BaTiO₃ nanopowders could be obtained by exceeding moderate amount of Ba(OH)₂·8H₂O in the starting materials. The obtained BaTiO₃ nanoparticles had well dispersion and crystallinity, possessed a tetragonal perovskite structure at room temperature and relatively narrow particle size distribution.     

Effects of Bi2O3 and Cr2Ti3O9 Co-doping on dielectric properties in BaTiO3-based ceramics

A microwave ceramic with general composition (1-x-y) BaTiO₃ + x Cr₂Ti₃O₉ + y Bi₂O₃ has been prepared by solid state synthesis at 1300-1400 °C. The phase composition, perovskite structural parameters and dielectric properties have been obtained by X-ray diffraction and dielectric measurements as a function of chemical composition and temperature. At low doping levels the formation of BaTiO₃-based solid solution has been found. The precipitation of BaCrO₃ has been detected at x = y = 2.0 mol%. A model of the incorporation of Cr³⁺ and Bi³⁺ ions into BaTiO₃-based crystal lattice has been proposed. Diffused phase transition in the temperature range 100-140 °C have been revealed by dielectric measurements for different ceramic composition. As high dielectric constant as 7311 and as low dielectric loss as 0.02 have been found for the composition of 0.98BaTiO₃-0.01Cr₂Ti₃O₉-0.01Bi₂O₃.     

Morphologies and crystal structures of nano-sized Ba1−xSrxTiO3 primary particles prepared by flame spray pyrolysis

Nano-sized Ba_1−xSr_xTiO₃ (BST) powder was prepared by flame spray pyrolysis using “CA-assisted” spray solution. The effects of the mole ratios of Ba to Sr components on the mean sizes, morphologies, and crystal structures of the BST powder prepared by flame spray pyrolysis were investigated. The precursor powders obtained by flame spray pyrolysis had large size, fractured and hollow structures irrespective of the mole ratios of Ba to Sr components. The post-treated BST powders had slightly aggregated morphology of the primary particles with nanometer sizes. The slightly aggregated BST powders turned to nano-sized primary particles by a simple milling process. The milled BaTiO₃ particles post-treated at temperature of 1000 °C had spherical-like shape. On the other hand, the milled Ba_0.5Sr_0.5TiO₃ and SrTiO₃ particles had square shape. The mean sizes of the milled BaTiO₃, Ba_0.5Sr_0.5TiO₃ and SrTiO₃ particles were each 110, 32, and 48 nm. Phase pure BST powder was obtained at a post-treatment temperature of 1000 °C irrespective of the mole ratios of Ba to Sr components. The BaTiO₃ powder had tetragonal crystal structure. On the other hand, the BST except for the BaTiO₃ composition had cubic crystal structures at post-treatment temperature of 1000 °C. The mean crystallite sizes of the milled Ba_1−xSr_xTiO₃ primary particles were changed from 29 to 37 nm according to the mole ratios of Ba to Sr components.     

Diffuse phase transformations in neodymium-doped BaTiO3 ceramics

Phase transitions in BaTiO₃ doped with neodymium have been studied using temperature-capacitance measurements, differential scanning calorimetry, X-ray powder diffraction and electron paramagnetic resonance spectroscopy. The coexistence of a para-electric cubic phase with the ferroelectric tetragonal and orthorhombic phases is noticed within the temperature range corresponding to the stability region of the latter phases in undoped BaTiO₃. The broad maximum in the effective dielectric constant shifts to lower temperature with increase in neodymium content. The heat of transformation measured at the Curie point tends to zero above 3.5at%Nd and the phase transition gradually approaches the second order. These are characteristics of a diffuse phase transformation. Using the quantitative results from the above studies, a T-x topological diagram is constructed for the BaTiO₃-xNd system wherex < 5.5at%. The phase contents vary with processing parameters as well as grain size, and the very existence of more than one phase in a given area of the T-x diagram indicates the metastable thermodynamic equilibrium prevailing in BaTiO₃-Nd ceramics. The inhomogeneous distribution of lattice defects may be the major cause for such a behaviour.     

Effect of BaTiO<Subscript>3</Subscript> heat treatment on the microstructure and dielectric properties of BaTiO<Subscript>3</Subscript>-based ceramics

We have studied the effect of heat treatment of the starting BaTiO₃ powder on the dielectric properties and microstructure of X7R-type BaTiO₃-based ceramics. The results demonstrate that annealing of BaTiO₃ stabilizes the degree of tetragonality in the crystal lattice of the ceramics. Microstructural analysis shows that the annealing temperature has no effect on the average grain size of the ceramics. Increasing the BaTiO₃ annealing temperature increases the dielectric permittivity of the core phase and reduces the temperature coefficient of capacitance (TCC). We obtained an X7R-type BaTiO₃-based ceramic material (BaTiO₃ annealing temperature, 1150°C; firing temperature, 1160°C) with the following properties: ɛ_25°C = 2230, TCC = ±12% (−55 to 125°C), and tanδ_25°C = 0.013.     

A modified method for barium titanate nanoparticles synthesis

In this research, a modified, cost effective sol-gel procedure applied to synthesize BaTiO₃ nanoparticles. XRD and electron microscopy (SEM and TEM) applied for microstructural characterization of powders. The obtained results showed that the type of precursors, their ratio and the hydrolysis conditions had a great effect on time, temperature and therefore the costs of the synthesis process. By selection, utilization of optimized precursor's type, hydrolysis conditions, fine cubic BaTiO₃ nanoparticles were synthesized at low temperature and in short time span (1 h calcination at 800 °C). The proposed procedure seems to be more preferable for mass production.The result indicated that the polymorphic transformation to tetragonal (ferroelectric characteristic) occurred at 900 °C, which might be an indication of being nanosized.     

Phase transition and dielectric properties of BaTiO3 ceramics containing 10 mol% BaGeO3

Dielectric properties and the cubic ? tetragonal phase transition temperature of dense BaTiO₃ ceramics containing 10 mol% BaGeO₃, sintered between 840 and 1350 °C, have been investigated. The ceramic bodies were prepared from a nano-sized BaTi_0.9/Ge_0.1O₃ powder consisting of both BaTiO₃ and BaGeO₃ phases. The addition of BaGeO₃ leads to a reduction and broadening of the permittivity maximum, and to a small downshift of the paraelectric ? ferroelectric phase transition temperature, compared to a pure BaTiO₃ ceramic. Lower sintering temperatures and thus small grain sizes of the ceramics cause an additional reduction of the maximum permittivity down to 2800. Both DTA and dilatometric measurements reveal also a downshifting of the phase transition temperature, as well as a decrease of the latent heat.     

Synthesis of boron carbide powder from polyvinyl borate precursor

Polyvinyl borate (PVBO) was prepared by the condensation of poly(vinyl alcohol) (PVA) and boric acid, and used as a precursor for boron carbide. Boron carbide powder was synthesized by the pyrolysis of the PVBO precursor in air at 600 °C for 2 h, followed by heat treatment in Ar flow at 1300 °C for 5 h, which is a relatively low temperature compared with conventional carbothermal methods. Pyrolysis of the PVBO precursor resulted in submicron-size particles of B₂O₃ dispersed in a carbon matrix. In addition, the pyrolysis temperature governed the carbon content in the pyrolyzed product of the PVBO precursor, which led to the synthesis of crystalline boron carbide powder with little free carbon.