Preparation of nanocrystalline lithium niobate powders at low temperature

A facile route to prepare lithium niobate (LiNbO₃) powders was proposed by an alternative solid‐state method. Stoichiometric Li₂C₂O₄ and ammonium niobium oxalate were mixed with small amounts of water and then dried at room temperature. It was demonstrated that Li[NbO(C₂O₄)₂]·n H₂O intermediate was produced by an ion‐exchange reaction. Pure LiNbO₃ powders were successfully synthesized by heating the intermediate at 500, 600 and 700 °C for 3 h. X‐ray diffraction (XRD), scanning electron microscopy (SEM), Fourier‐transform infrared (FTIR) spectroscopy, UV‐Vis diffuse reflectance (UV‐Vis) spectroscopy and thermogravimetric (TG) analysis were used to characterize the precursor compound and as‐prepared samples. XRD results reveal that all the products are identified as hexagonal structure with high relative crystallinity (>87%). The particle size is found to be about 40 nm for the mixture calcined at 500 °C according to XRD data, which is in good agreement with SEM data. The as‐prepared LiNbO₃ powders by this method are high quality according to FTIR spectra. (Li_0.996Nb_0.005)Nb_0.999O₃ phase was formed when the calcination temperature was raised to 800 °C. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Some peculiarities of Ti in‐diffusion in lithium niobate

The process of one‐ and two‐dimensional Ti in‐diffusion in lithium niobate, LiNbO₃ (LN), single crystals of 〈x〉 and 〈z〉 orientations at 950‐1060°C in oxygen/water vapor medium had been studied. In the case of one‐dimensional diffusion, a flat diffusion front consisting of Ti solid solution in LN is formed. The process kinetics is described by Fick's equation for the case of a permanent source. In the case of two‐dimensional Ti diffusion, the diffusion zone contour acquires the shape close to elliptical. The diffusion rate in the tangential direction is about an order of magnitude higher than in the normal direction. A model qualitatively describing such nontrivial character of the diffusion process is suggested. The model is based on: a) incongruent lithium evaporation out of LN at high temperatures; b) low thermodynamic activity of Li in LiTiO₃ – product of Ti interaction with LN. These factors produce a significant nonlinearity of the process, since a strong chemical bond between Li and Ti in this phase leads to a mutual increase in their diffusive mobility. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Direct preparation of K0.5Na0.5NbO3 powders

K_0.5Na_0.5NbO₃ powders have been directly synthesized by an alternative solid–state method. Stoichimometric mixture of ammonium niobium oxalate and C₄H₄O₆KNa·4H₂O were calcined in temperature range from 500 to 800 °C for 3 h. The precursor and calcination products were characterized with respect to stoichiometry, purity, crystalline structure, particle size and powder morphology using X–ray diffraction (XRD), X‐ray fluorescence (XRF) spectrometer, scanning electron microscope (SEM), Fourier transform infrared (FTIR) spectra, thermogravimetric (TG) analysis, differential scanning calorimetry (DSC) and UV–Vis diffuse reflectance (UV–Vis) spectroscopy. XRD and XRF results reveal that stoichiometric K_0.5Na_0.5NbO₃ powders could be synthesized by the method. The particle size is about 68 nm for the precursor calcined at 500 °C according to XRD data, which is in good agreement with SEM data. The average band gap energy is estimated to be 3.18 eV by UV–vis diffuse reflectance spectra. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low‐temperature preparation of bismuth ferrite microcrystals by a sol‐gel‐hydrothermal method

Well‐crystallized pure perovskite bismuth ferrite (BiFeO₃) powders with various morphologies have been synthesized by a novel sol‐gel‐hydrothermal route for the first time, which combined the conventional sol‐gel process and the hydrothermal method. The as‐prepared samples were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA) and ferroelectric test system. The results revealed that the compositions, morphological and dimensional changes in bismuth ferrite samples synthesized by sol–gel–hydrothermal method strongly depend on the concentrations of mineralizer. Ferroelectric hysteresis loops are displayed in the BiFeO₃ samples. The bismuth ferrites were hydrothermally synthesized at as low a temperature as 180 °C, which is comparatively lower than that synthesized by the normal sol–gel route. The formation mechanism of the bismuth ferrite crystalline was also discussed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of stoichiometric LiNbO3 nanopowder through a wet chemical method

Stoichiometric lithium niobate powder which are used as feeding material in near stoichometric LiNbO₃ crystal growth have been successfully prepared from commercial niobium hydroxide [Nb(OH)₅] and nontoxic DL‐malic acid by a wet chemical method. The synthesis temperature was pre‐determined by the results from thermogravimetric and differential thermal analysis. The structure and morphology of the as‐prepared samples were observed by using the infrared spectroscopy and the scanning electron microscopy. The X‐ray diffraction experiment showed that lithium niobate powder had an ilmenite structure, and its unit cell parameters were calculated to be a = b = 0.5140 nm, c = 1.3738 nm, and V = 0.3144 nm³. The melting point of the synthesized powder is 1239 °C and Curie temperature T_c is 1122 °C. This synthesis method would be helpful to grow the near‐stoichiometric LiNbO₃ crystal with double crucible techniques. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sintering behavior of ZnO:Al ceramics fabricated by sol‐gel derived nanocrystalline powders

ZnO:Al ceramics (Zn:Al, 0.95:0.05) were prepared by using sol‐gel derived nanocrystalline powders. XRD patterns of the doped ceramics revealed the existence of both zincite (JCPDS 36‐1451) and gahnite (JCPDS 5‐0669) phases. Gahnite phase (ZnAl₂O₄) was segregated along the ZnO grain boundaries. At the sintering temperature of 1200 °C, relative density of the undoped and Al doped ceramics were measured as 0.695 and 0.628, respectively. Both grain size and relative density of the ceramics decreased with Al doping. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nd(III) and Yb(III) ions incorporated in Y4Al2O9 obtained by sol‐gel method: synthesis,structure, crystals and luminescence

The nanocrystalline powders of Y₄Al₂O₉ (YAM) pure and doped by Nd, Yb and codoped by Nd and Yb were obtained via modified sol‐gel method. These powders were characterized by X‐ray diffraction method, scanning electron microscopy and high resolution scanning electron microscopy, luminescence spectroscopy and differential thermal analysis (DTA). We obtained single phase powders of crystalline structure with average size 70 nm exhibiting interesting luminescent properties. Efficient non‐radiative energy transfer between Nd and Yb was found. DTA confirmed the phase transition at about 1400 °C. From these nanocrystalline powders, the crystals YAM:Yb, YAM:Yb,Nd were grown by micro‐pulling down technique. They were cracking during cooling owing to the phase transition. Luminescent properties of YAM:Nd,Yb crystals were identical with properties of corresponding nanopowders within experimental incertitude. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Heat‐treatment influence on the microstructure and magnetic properties of rare‐earth substituted SrFe12O19

Rare‐earth substituted strontium ferrite nanopowders SrFe_12‐xR_xO₁₉ (R = La, Gd and Er; x = 0.2, 0.5 and 1) were prepared by sol‐gel‐autocombustion method and subsequent heat treatments. Structural and magnetic properties of SrFe_12‐xR_xO₁₉ powders heat treated at 800, 900 and 1000⁰C, for various times, were characterized with an X‐ray diffractometer, a vibrating sample magnetometer and a scanning electron microscope. The results of X‐ray diffraction measurements showed the M‐type hexagonal structure formation by heat treatments. Magnetic properties, such as specific saturation magnetization σ_s, specific remanent magnetization σ_r and coercivity H_c, as well as microstructure depend on the heat treatment conditions (temperature and time). The coercivity H_c exhibits a great increase after a critical heat treatment time. When the heat treatment time increases, one obtains an increase in H_c after a shorter heat treatment time. This jump of H_c was explained by a transition from the superparamagnetic state to normal state of the single domain nanoparticles. The occurrence of an agglomerated structure composed of magnetically interacting ultrafine crystallites also contributes to the increase of H_c.The heat treatment determines a reduced grain growth due to the internal stress generated by R ions. With increasing R content the σ_s and σ_r decrease due to the dissolution of R ions into the hexaferrite lattice. We believe that by selecting the time and temperature of the heat treatment, microstructure and magnetic properties suitable for magnetic recording media application can be obtained. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low‐temperature urea‐assisted hydrothermal synthesis of Bi2S3 nanostructures with different morphologies

Different morphologies of single‐crystalline orthorhombic phase bismuth sulfide (Bi₂S₃) nanostructures, including sub‐microtubes, nanoflowers and nanorods were synthesized by a urea‐assisted hydrothermal method at a low temperature below 120 °C for 12 h. The as‐synthesized powders were characterized by X‐ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high‐resolution transmission electron microscopy (HRTEM) and UV‐vis spectrophotometry. The experimental results showed that the sulfur sources had a great effect on the morphology and size of the resulting powders. The formation mechanism of the Bi₂S₃ nanostructures with different morphologies was discussed. All Bi₂S₃ nanostructures showed an appearance of blue shift relative to the bulk orthorhombic Bi₂S₃, which might be ascribed to the quantum size effect of the final products. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On‐line monitoring of lithium carbonate dissolution

Dissolution of lithium carbonate (Li₂CO₃) in aqueous solution was investigated using three on‐line apparatuses: the concentration of Li₂CO₃ was measured by electrical conductivity equipment; CLD (Chord Length Distribution) was monitored by FBRM (Focused Beam Reflectance Measurement); crystal image was observed by PVM (Particle Video Microscope). Results show dissolution rate goes up with a decrease of particle size, and with an increase in temperature; stirring speed causes little impact on dissolution; ultrasound facilitates dissolution obviously. The CLD evolution and crystal images of Li₂CO₃ powders in stirred fluid were observed detailedly by FBRM and PVM during dissolution. Experimental data were fitted to Avrami model, through which the activation energy was found to be 34.35 kJ/mol. PBE (Population Balance Equation) and moment transform were introduced to calculate dissolution kinetics, obtaining correlation equations of particle size decreasing rate as a function of temperature and undersaturation. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Particle size of powders under hydrothermal conditions

Various non‐oxide (CuI, AgI, AgCl, PbS, CuS and ZnS) and oxide (ZnO, TiO₂, SnO₂, CeO₂ and ZrO₂) powders were prepared under hydrothermal conditions to investigate the effects of temperature, pH and precursors on the particle size of powders. It was found that the particle sizes of PbS, CuS and ZnS powders were much smaller than that of CuI, AgI and AgCl powders prepared under the same conditions. The particle sizes of TiO₂, SnO₂, CeO₂ and ZrO₂ powders are much smaller than that of ZnO powders prepared under the same conditions. It is concluded that the solution conditions have a certain effect on the particle size of powders under the hydrothermal conditions. The particle size of powders increased with the rising of temperature. Additional factors affecting the particle size were uncovered through studying the nucleation mechanism. The particle size was mainly related to the Madelung constant and the electric charge number of ions. Powders with smaller particle size resulted from systems that possessed the larger Madelung constant and ionic charge number. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis by precipitation method and investigation of SnO2 nanoparticles

In this paper the synthesis of SnO₂ nanoparticles with average particle size up to about 70 nm using SnCl₂2H₂O and NH₄OH in 1‐botanol solution by the precipitation method is reported and the inhibition of sodium dodecyl sulphate (SDS) on the SnO₂ particle growth is investigated by soaking SnO₂precursor in the SDS solution for 24 h. The as‐prepared SnO₂and SDS modified‐SnO₂ powders, then, were calcined at different temperatures and the X‐ray powder diffraction (XRD) and Fourier transform infrared spectroscopy (FT‐IR) were used to characterize the output samples. The XRD results reveal that the structure of tin‐dioxide is tetragonal rutile and the as‐prepared SnO₂ nanoparticles grow with increasing the annealing temperature, while the SDS treatment prevents the particle growth under the same condition. Furthermore, the FT‐IR results indicate the formation of tin‐hydroxyl group which are then converted into tin‐dioxide with heat treatment. Further characterization of the samples by the transmission electron microscopy (TEM) and the photoluminescence (PL) spectroscopy was carried out. The room temperature PL spectra of SnO₂exhibits broad and strong peak attributed to the surface defects such as oxygen vacancies and intensity of which decreases with the increase in particle size. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterization of doped lithium aluminate nanocrystalline particles by sol‐gel method

Nanocrystalline particles of Co²⁺doped lithium aluminate (Co²⁺:LAO) and Ni²⁺‐doped lithium aluminate (Ni²⁺:LAO) were synthesized by sol–gel method. The crystalline nature and particle size of the samples were characterized by X‐ray diffraction analysis (XRD). The morphology and the presence Co²⁺ and Ni²⁺ in the synthesized samples were analyzed by scanning electron microscope (SEM) and energy dispersive X‐ray analysis (EDAX). The presences of functional groups in the samples were analyzed using FT‐IR analysis. The optical absorbance of the synthesized samples were observed using UV absorption spectral analysis. The frequency dependent dielectric behaviour of the synthesized nano materials was analyzed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and crystal growth of sillenite phases in the Bi2O3 ‐ TiO2 ‐ Nb2O5 system

The synthesis of Bi₂O₃‐Nb₂O₅ sillenite phase (BNbO) and the solubility of this phase with Bi₁₂TiO₂₀ was investigated by solid‐state reaction synthesis and niobium doped Bi₁₂TiO₂₀ (BTO:Nb) crystals were grown by the Top Seeded Solution Growth (TSSG) technique. The structures of polycrystalline compounds were checked by X‐ray powder diffraction method at room temperature. The correct composition of the sillenite phase stabilized with niobium was determined as Bi₁₂[Nb_0.17Bi_0.83]O_19.7 (BNbO) with unit cell parameter a = 10.261(2) Å. The system BTO‐BNbO is poorly soluble, but niobium doped BTO crystals were grown from the liquid composition 10Bi₂O₃ : xTiO₂ : (1‐x)/2 Nb₂O₅, with x = 0.95 and 0.90. A niobium concentration limit in the liquid phase is established in order to grow BTO:Nb with good crystalline quality. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of barium oxalate crystals in agar–agar gel and their characterization

Single crystals of barium oxalate have been grown by gel method using agar‐agar gel as media of growth at ambient temperature. The optimum conditions were established by varying various parameters such as concentration of gel, concentration of reactant, gel setting time etc. Prismatic platy shaped transparent crystals were obtained. The grown crystals were characterized through powder X‐ray diffraction (XRD), Fourier transform infrared (FT‐IR) studies, Thermogravimetric (TGA) and Differential thermal analysis (DTA). The compound crystallizes under monoclinic structure with lattice parameters a = 6.6562 Å, b = 8.0464 Å, c = 2.8090 Å, β= 96.832°, and V = 149.38 ų. The FT‐IR spectrum indicates OH and carbonyl group along with the presence of metal‐oxygen bond. The TGA indicates 17.75% weight loss at 550°C from which the decomposition pattern is formulated. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reduction of domain wall width in VTE treated near stoichiometric LiNbO3 crystals

LiNbO₃ is a ferroelectric crystal and grows with multi domains. Different domains are separated by boundaries which are known as domain boundaries. Domain walls for congruent and VTE (Vapor Transport Equilibration) treated near stoichiometric lithium niobate samples were visualised in different crystallographic directions using chemical etching technique. The sample is etched in the mixture of HF and HNO₃ (in 1:2 volume ratios) for 10 minutes at boiling temperature. Measured domain wall width was found approximately 15‐20 µm for congruent (CLN) and it reduces to 1‐3 µm for VTE treated near stoichiometric (SLN). Activation energies were also measured by two‐probe method and found to be increasing in stoichiometric sample. This activation energy is related to defect density in the crystals. Activation energy is higher for less defective crystals. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterization of magnesium doped lead titanate

Magnesium (Mg) doped lead titanate powders (PT: Mg) ceramics, with various Mg contents (0, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20, and 30 mole %), are prepared by the sol‐gel method and characterized by X‐Ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. XRD results show that the magnesium diminishes the lattice tetragonality of PT:Mg. The average particle size and morphology were studied by SEM. The investigations done by Raman spectroscopy allow the analyzis of the substitution mechanism behavior related to the Mg incorporation into PT structure. A concentration threshold appears around 10% of Mg corresponding in a change of the compensation process with Mg²⁺ ions replacing Pb²⁺ ions below and both Pb²⁺ and Ti⁴⁺ above. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Large‐scale production of well‐dispersed submicro ZrB2 and ZrC powders

ZrB₂ and ZrC powders were synthesized via metallothermic reduction route using ZrO₂, B/C, and Mg as raw materials. Low packing density of the green mixture and high heating temperature of the furnace are crucial for the formation of well‐dispersed submicro powders. Optimum reaction time helped achieve good crystallization and high purity. The as‐prepared samples were characterized by XRD, FESEM, TEM, and particle size analyzer. Results showed that well‐dispersed ZrB₂ powders with mean particle size of 0.534 μm and ZrC powders with mean particle size of 0.376 μm can be obtained. Oxygen content can be controlled lower than 1.0 wt%. The bench‐scale output is about 10 kg/d.     

Study of the effect of the base,the silica and the niobium sources on the [Nb]-MCM-41 synthesized at room temperature

[Nb]-MCM-41 was synthesized at room temperature by varying the base (tetramethylammonium hydroxide or ammonium hydroxide), the silica source (tetraethyl orthosilicate or tetramethyl orthosilicate), the niobium source (ammonium niobium oxalate, NH₄[NbO(C₂O₄)₂(H₂O)₂] · 3H₂O, or potassium niobate, K₈Nb₆O₁₉) and the order of addition of the niobium source (before or after the silica source). These variations were determinant in the amount of niobium incorporated into the framework and in the structural order of the [Nb]-MCM-41 obtained. Only one method led to the formation of [Nb]-MCM-41 with the desired characteristics and active in the epoxidation of cis-cyclooctene with tert-butyl hydroperoxide leading to 19% conversion and 95% selectivity for cyclooctene oxide after 10 h.     

Growth and characterization of Nd:CLNGG crystal

The disordered laser crystal neodymium-doped calcium lithium niobium gallium garnet (Nd:CLNGG) was successfully grown by the Czochralski method. Its thermal properties, including the average linear thermal expansion coefficient, thermal diffusion coefficient, specific heat, and thermal conductivity were measured, and continuous-wave (CW) laser performance at 1.06 μm was demonstrated. The maximum power of 1.48 W was achieved with corresponding optical conversion efficiency of 12.4% and slope efficiency of 16.2%.