Preparation and Characterization of Nano-structured Ceramic Powders Synthesized by Emulsion Combustion Method

The emulsion combustion method (ECM), a novel powder production process, was originally developed to synthesize nano-structured metal-oxide powders. Metal ions in the aqueous droplets were rapidly oxidized by the combustion of the surrounding flammable liquid. The ECM achieved a small reaction field and a short reaction period to fabricate the submicron-sized hollow ceramic particles with extremely thin wall and chemically homogeneous ceramic powder. Alumina, zirconia, zirconia–ceria solid solutions and barium titanate were synthesized by the ECM process. Alumina and zirconia powders were characterized to be metastable in crystalline phase and hollow structure. The wall thickness of alumina was about 10nm. The zirconia–ceria powders were found to be single-phase solid solutions for a wide composition range. These powders were characterized as equiaxed-shape, submicron-sized chemically homogeneous materials. The powder formation mechanism was investigated through the synthesis of barium titanate powder with different metal sources.     

BaTiO_3纳米粉体的制备及结构的光谱表征

采用钛酸丁脂和八水氢氧化钡为原料的溶胶-凝胶法制备了纳米BaTiO3粉体。经不同温度烧结,并利用X射线衍射和Raman光谱分析进行结构表征。实验表明,经1073K烧结后粉体粒径为26 2nm,颗粒分布均匀,还有少量BaCO3存在。在这个温度下得到的晶粒主要是立方相,并含有少量的四方相。经1273K烧结后,BaTiO3完全是四方相,粉体粒径为80 0nm,此时BaCO3已完全分解。另外探讨了溶胶-凝胶制备纳米BaTiO3粉体的反应机理,解释了BaTiO3物相的形成。     

Origin of dark holographic scattering patterns in photorefractive crystals

We explain the origin of recently reported dark parametric holographic scattering patterns observed in photorefractive strontium–barium niobate (SBN) and barium–calcium titanate (BCT) crystals. Taking into account the model for parametric four-wave-mixing processes in photorefractive crystals, exponential gain factors are presented for the scattering patterns and compared with the gain for the scattering background. We interpret the dark scattering patterns as a result of counteracting energy-transfer processes. PACS 42.65.Hw; 42.70.Nq     

Theoretical and experimental study on the photoluminescence in BaTiO3 amorphous thin films prepared by the chemical route

A polymeric precursor method was used to synthesize BaTiO₃ amorphous thin film processed at low temperature. The luminescence spectra of BaTiO₃ amorphous thin films at room temperature revealed an intense single-emission band in the visible region. The visible emission band was found to be dependent of the thermal treatment history. Photoluminescence (PL) properties for different annealing temperatures were investigated. It was concluded that the intensity of PL is strongly dependent on both the heat treatment of the films and the presence of an inorganic disordered phase. Experimental optical absorption measurements showed the presence of a tail. These results are interpreted by the nature of these exponential optical edges and tails, associated with defects promoted by the disordered structure of the amorphous material. We discuss the nature of visible PL at room temperature in amorphous barium titanate in the light of the results of recent experimental and quantum mechanical theoretical studies. Our investigation of the electronic structure involved the use of first-principle molecular calculations to simulate the variation of the electronic structure in the barium titanate crystalline phase, which is known to have a direct band gap, and we also made an in-depth examination of amorphous barium titanate.     

Microstructure and blue photoluminescence enhancement of silicon nanoporous pillar array embedded in ferroelectric barium strontium titanate

A silicon nanoporous pillar array (Si-NPA) with micrometer/nanometer hierarchical structure was fabricated by hydrothermal etching, followed by spin-coating barium strontium titanate (BST) on Si-NPA substrate. The photoluminescence (PL) spectra of the Si-NPA and BST/Si-NPA thin film were investigated. The emission band of freshly prepared Si-NPA located at 630 nm, and a blueshift at 425 nm as well as degradation in intensity after annealing at 600 °C for 1 h was observed, which might be explained by a quantum confinement effect model. BST ferroelectric material provided a static-electric field and induced the excited carriers in Si-NPA to migrate toward the opposite direction and recombine in an interfacial oxide layer. Therefore, BST enhanced blue emission of Si-NPA as well as passivated Si-NPA.     

Specific features of the ferroelectric state in two-layer barium strontium titanate-based heterostructures

The structural properties of one- and two-layer heterostructures based on the barium–strontium titanate of various compositions deposited by the Frank–Van der Merve on a magnesium oxide substrate have been studied. The heterostructures have been prepared by the rf sputtering of the stoichiometric ceramic targets in a Plazma 50 SE deposition system. The principal difference of this method of deposition from known analogs is that the growth of single-crystal films occurs from a disperse oxide phase formed in the plasma of a high-current rf discharge during the ceramic target sputtering at the cluster level. The peculiarities of the manifestation of the ferroelectric state in the two-layer heterostructures when changing the sequence order of the films with various compositions of barium–strontium titanate.     

Investigation of the piezoelectric constants of polycrystalline ferroelectrics of the barium titanate type

A study has been made of the piezoelectric constants of polycrystalline ferroelectrics of the barium titanate type and of their dependence on temperature of sintering of the ceramic and on the constant voltage applied. The measurements were made under static and dynamic conditions. It is shown that the piezomoduli of certain solid solutions in the systems Ba(Ti, Zr, Sn)O₃ and Ba(Ti, Si, Sn)O₃ are somewhat greater than those of barium titanate.     

Dielectric Properties of Calcium,Strontium, or Barium Titanate Sintered with 5 mol.% of Lithium and Calcium Fluorides

Calcium, strontium, and barium titanates were previously synthesized then mixed and wet-ground with 5 mol.% of lithium and calcium fluorides. These mixtures were pressed into pellets and air-fired. The ceramics were investigated by X-ray diffraction, scanning electron microscopy, and dielectric measurements. Each sample is a perovskite single phase. The oxyfluoride deriving from calcium titanate exhibits a phase transition around 283 K. For strontium titanate, no phase transition is detected. On the other hand, the ceramic related to barium titanate displays a ferroelectric phase transition around 300 K and a dielectric relaxation at about 10 MHz. These oxyfluorides could be of interest for capacitor manufacturing.     

Effect of titanium alkoxides on the properties of barium-strontium-calcium titanate powders obtained by a sol-gel method

This is a study of the effect of titanium alkoxides on the properties of barium-strontium-calcium titanate powders produced by a sol-gel method. Sols were synthesized using titanium tetraethoxide, titanium tetraisoproxide, and titanium tetrabutoxide. Xerogels of titanium oxide doped with barium, strontium, and calcium in a proportion of 60:30:10 were obtained by drying the sols at a temperature of 70°C. Thermogravimetric analysis, along with differential scanning calorimetry, IR spectroscopy, and x-ray phase analysis, were used to study the changes in the xerogel and the crystallite dimensions in the powder induced by thermal processing. The morphology and specific surface area of the powders were determined by scanning electron microscopy and low-temperature physical adsorption-desorption of nitrogen. These studies lead us to recommend sols based on Ti(OC₃H₇)₄ with a crystallite particle size in the powder of ∼63 to 40 nm for creating homogeneous ceramic materials from barium-strontium-calcium titanate.     

Second Harmonic Generation in Microstructured Barium Titanate

We investigate the second harmonic generation under femtosecond pulse-periodic laser radiation in barium titanate in the form of ceramics, in pores of a globular photonic crystal, and in a water colloidal suspension. We measure the dependence of the second harmonic radiation intensity on the incident laser power. Excitation of the second harmonic was carried out by powerful (10⁸ W) pulses of a solid-state Yb:KGW laser (wavelength, 1,026 nm) operating at 200 kHz. We estimate the efficiency of the second harmonic generation in various microstructured phases of barium titanate and show that the threshold of plasma formation in a suspension of barium titanate microparticles in water is substantially higher than in ceramics and in the ferroelectric photonic crystal. The second-harmonicgeneration power can be significantly increased in a water suspension of barium titanate microparticles.     

Self-Pumped Phase Conjugation in a Ce:BaTiO3 Crystal Pumped with Femtosecond Laser Pulses at 1053 nm Wavelength

Self-pumped phase conjugation in a cerium-doped barium titanate crystal pumped with femtosecond laser pulses at a 1053 nm wavelength was experimentally studied. A reflectivity of a self-pumped phase conjugation as high as 15#x0025; was demonstrated.     

Nanocrystals formation on Ho doped strontium barium niobate glass

The study of two different methods to obtain strontium barium niobate nanocrystals immersed in a glass matrix has been carried out. Ho₂O₃-doped SrO-BaO-Nb₂O₅-B₂O₃ glasses were fabricated using the melt quenching method. Glass ceramic samples were obtained from the precursor glass by thermal treatment in a furnace and by laser irradiation. These glass ceramic samples are formed by a glassy phase and a crystalline phase of strontium barium niobate nanocrystals. This structure was confirmed by X-ray diffraction and Atomic Force Microscope images. The incorporation of Ho³⁺ ions in the strontium barium niobate nanocrystals were corroborated by optical measurements, which produced an increment in the luminescence intensity compared to the precursor glass.     

Single-mode operation of a laser-diode array with frequency-selective phase-conjugate feedback

A new technique for single-mode operation of laser-diode arrays is presented. A gain-guided GaAlAs laser diode array is coupled to an external frequency-selective phase-conjugate feedback system that contains a photorefractive barium titanate crystal, a Fabry-Perot etalon, and a spatial filter. The etalon is the key component, and it forces the array, which has low spatial and spectral coherence when it runs freely, to oscillate in a single spatial and a single longitudinal mode. At a drive current of two times the threshold current, the far-field pattern is reduced to only 1.4 times the diffraction limit, the spectral bandwidth is less than 0.02 nm, and the coherence length is increased by a factor of 70. The technique has general validity and can be applied to various other multimode laser systems.     

Electron microscopy of barium strontium titanate nanostructures in the aluminum oxide matrix

Electron microscopy has revealed the mechanism of formation of barium strontium titanate nanostructures during crystallization in the bulk of pores of the aluminum matrix. The nanostructures have the form of nanotubes or nanocolumns 100–200 nm in diameter and a few microns long, which are formed by barium strontium titanate crystals 3–100 nm in size.     

Nitrogen containing barium titanate: Preparation and characterisation

The purpose of this work is to study the incorporation of nitrogen into barium titanate at oxygen sites by ammonolysis. The nitrogen content of the resulting barium titanate powders strongly depends on the flow rate of ammonia and the ratio Ba/Ti. The amount of resulting oxygen vacancies is lowered by co-doping with fluoride and tantalum.By avoiding an excess of BaO and exchanging 10 mol% Ti by Ta it is possible to produce powders with nitrogen content of more than 1 wt%. Partial substitution of barium oxide by barium fluoride also increases the nitrogen content. The nitrogen containing powders are colored and show an additional absorption at 500 nm measured in the UV/vis reflectance spectrum. The concentrations of nitrogen and fluorine in the samples were determined using ion-sensitive electrodes. X-ray powder diffraction shows the composition of the powders. The thermal stability was investigated by thermogravimetric analysis, and in order to calculate the specific surface area the three-point BET method was used.     

钛酸钡瓷粉样品中多元素的X射线荧光光谱分析

本文采用四硼酸锂和偏硼酸锂混合熔剂熔样，人工合成法制备钛酸钡瓷粉XRF标样，选用Sr和Ca内标元素消除制样和仪器漂移产生的误差，建立了无须进行烧失量校正的X射线荧光光谱分析钛酸钡瓷粉样品的分析方法，分析结果的准确度和精密度符合生产要求，也适用于其它含B元素样品的分析。     

Time domain response and the data analysis of ferroelectric discharge current

The time domain method is more reliable for the study of nonlinear dielectric response compared with frequency domain analysis. A Tikhonov regularization method, which is widely adopted for ill-posed problem, is described for derivation of the relaxation time distribution function, g(τ), from the ferroelectric discharge current in time domain. The new method allows study of the structure variation and the relaxation behavior of ferroelectrics at different temperatures. For barium stannate titanate ceramics (BTS20), g(τ) has been successfully derived; the relaxation peaks move to shorter times with increasing temperature in the range 20–60°C, which may indicate a space charge thermal activation process. However, g(τ) could not be derived from the discharge current by the regularization method for BTS20 at temperatures above 60°C or for lanthanum-doped lead zirconium titanate transparent ceramic (PLZT), since the data do not satisfy the discrete Picard condition, which is a valid criterion for regularization method.     

Preparation and characterization of titania based nanowires

A new method for preparation of titania nanowires with diameter around 10 nm and length up to 2–3 μm is described. The precursor was prepared from sodium titanate by adding ethylene glycole (EG) and heating at temperature of 198°C for 6 h under reflux. The sodium titanate glycolate formed by this way aggregated into 1D nanostructures and was subsequently transformed into titania glycolate during a chemical treatment with 98% sulfuric acid. Titania nanowires with variable amount of anatase and rutile were prepared by heating to temperatures in the range 350–1000°C. The precursor as well as titania based samples were characterized by X-ray diffraction, Infrared spectroscopy, Scanning electron microscopy, High resolution transmission microscopy, Thermogravimetry, Differential thermal analysis, Evolved gas analysis and Emanation thermal analysis. The nitrogen adsorption/desorption was used for surface area and porosity determination. The photoactivity of the prepared titania samples was assessed by the photocatalytic decomposition of 4-chlorophenol in an aqueous slurry under UV irradiation of 365 nm wavelength.     

110 W ceramic Nd<Superscript>3+</Superscript> : Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> laser

A high-power, continuous-wave 0.6% Nd³⁺-doped ceramic Y₃Al₅O₁₂ (Nd:YAG) laser has been developed. 110 W laser output at 1064 nm was obtained, with a slope efficiency of about 41%. The M² factor was found to be around 6. The laser performance of the ceramic laser material was found to compare favorably with that obtained with single crystal Nd:YAG. PACS 42.55.-f; 42.55.Xi; 42.70.Hj     

Dielectric losses in submicrometer barium titanate near the Curie temperature

Low-frequency dielectric losses (tanδ) in barium titanate ceramics with crystallite sizes of ~100 nm near the Curie temperature are studied. It is shown that the observed tanδ maximum is described by a low-frequency fluctuation mechanism of dielectric losses. An analysis of experimental data shows that the critical nucleus size in ultrafine material is smaller than the one in bulk barium titanate.