Preparation and characterization of rubidium-beta-alumina by the gel-to-crystallite conversion method

Preparation of Rb-beta -alumina was realized by the gel-to-crystallite conversion method. Reaction of hydrated aluminum hydroxide gel with RbOH in ethanol medium gave rise to the Rb+-inserted pseudoboehmite precursor under wet chemical conditions. The thermal decomposition of the precursor yielded Rb-beta -alumina. The Rb2O:Al2O3 ratio of monophasic Rb-beta -alumina ranged from 1:10 to 1:22. The extended stability in the compositional range is due to the fact that the conduction planes containing Rb+ and O2- ions can have lower occupancy of Rb+ ions for larger sized alkali ions, permitting the steric separation of the adjoining spinel blocks. High-resolution electron microscopy revealed that the decreasing occupancy of alkali ions in the conduction plane is balanced by changing widths of spinel blocks arising from the shift of tetrahedral Al3+ ions to octahedral sites and an accompanying increase in stacking defects. (C) 2000 Elsevier Science Ltd. All rights reserved.

Preparation of the layered double hydroxide (LDH) LiAl2(OH)(7)center dot 2H(2)O, by gel to crystallite conversion and a hydrothermal method, and its conversion to lithium aluminates

A layered double hydroxide (LDH) with chemical composition LiAl2(OH)(7) . 2H(2)O was prepared via a wet chemical route of gel to crystallite (G-C) conversion at 80 degrees C involving the reaction of hydrated alumina gel, Al2O3.yH(2)O (80 < y < 120) with LiOH (Li2O/Al2O3 greater than or equal to 0.5) in presence of hydrophilic solvents such as ethanol under refluxing conditions. The hydrothermal synthesis was carried out using the same reactants by heating to less than or equal to 140 degrees C in a Teflon-lined autoclave under autogenerated pressure (less than or equal to 20 MPa). Transmission electron microscopy showed needle-shaped aggregates of size 0.04-0.1 mu m for the gel to crystallite conversion product, whereas the hydrothermal products consisted of individual lamellar crystallites of size 0.2-0.5 mu m with hexagonal morphology. The LDH prepared through the gel to crystallite conversion could be converted into LiAl(OH)(4) . H2O or LiAl(OH)(3)NO3 . H2O by imbibition of LiOH or LiNO3, respectively, under hydrothermal conditions. Thermal decomposition of LDH above 1400 degrees C gave rise to LiAl5O8 accompanied by the evaporation of Li2O. LiAl(OH)(4) . H2O and LiAl(OH)(3)NO3 . H2O decomposed in the temperature range 400-1000 degrees C to alpha- or beta-LiAlO2. The compositional dependence of the product, the intermediate phases formed during the heat treatment and the possible reactions involved are described in detail.

Direct precipitation of lead zirconate titanate by the hydrothermal method

Direct precipitation of fine powders of lead zirconate titanate (PZT) in the complete range of solid solution, is investigated under hydrothermal conditions, starting from lead oxide and titania/zirconia mixed gels. The perovskite phase is formed in the temperature range of 165 – 340°C. Sequence of the hydrothermal reactions is studied by identifying the intermediate phases. The initial formation of PbO: TiO2 solid solution is followed by the reaction of the same with the remaining mixed gels giving rise to X-ray amorphous PZT phase. Further, through crystallite growth, the X-ray crystalline PZT is formed. This method can be extended for the preparation of PLZT powder as well. The resulting powders are sinterable to high density ceramics.

New nondestructive method for three-dimensional photoelasticity

A new set of equations describing completely the optical phenomena in a model involving continuous rotation of secondary axes and secondary principal-stress differences are obtained. These are solved by Peano-Baker method using experimentally determined characteristic parameters for several wavelengths of light. Experimental verifications are obtained for a rectangular bar subjected to combined torsion and tension. Paper was presented at Third SESA International Congress on Experimental Mechanics held in Los Angeles, CA on May 13–18, 1973.

A new nondestructive method for three-dimensional photoelasticity

A new set of equations describing completely the optical phenomena in a model involving continuous rotation of secondary axes and secondary principal-stress differences are obtained. These are solved by Peano-Baker method using experimentally determined characteristic parameters for several wavelengths of light. Experimental verifications are obtained for a rectangular bar subjected to combined torsion and tension.

Novel composite coded pattern for small angle measurement using imaging method -art.no.62891D

A novel approach for measurement of small rotation angles using imaging method is proposed and demonstrated. A plane mirror placed on a precision rotating table is used for imaging the newly designed composite coded pattern. The imaged patterns are captured with the help of a CCD camera. The angular rotation of the plane mirror is determined from a pair of the images of the pattern, captured once before and once after affecting the tilt of the mirror. Both simulation and experimental results suggest that the proposed approach not only retains the advantages of the original imaging method but also contributes significantly to the enhancement of its measuring range (+/- 4.13 degrees with accuracy of the order of 1 arcsec).

Synthesis, characterization and photoluminescence properties of Gd2O3:Eu3+ nanophosphors prepared by solution combustion method

Gd2O3:Eu3+ (0.5-8.0 mol%) nanophosphors have been prepared by low temperature solution combustion method using metal nitrates as oxidizers and oxalyl dihydrazide (ODH) as a fuel. The phosphors are well characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and photoluminescence (PL) techniques. PXRD patterns of as-formed and calcined (800 degrees C, 3 h) Gd2O3 powders exhibit monoclinic phase with mean crystallite sizes ranging from 20 to 50 nm. Eu3+ doping changes the structure from monoclinic to mixed phase of monoclinic and cubic. SEM micrographs shows the products are foamy, agglomerated and fluffy in nature due to the large amount of gases liberated during combustion reaction. Upon 254 nm excitation the photoluminescence of the Gd2O3:Eu3+ particles show red emission at 611 nm corresponding to D-5(0)-> F-7(2) transition. It is observed that PL intensity increases with calcination temperature. This might be attributed to better crystallization and eliminates the defects, which serve as centers of non-radiative relaxation for nanomaterials. It is observed that the optical energy gap (E-g) is widened with increase Eu3+ content. (C) 2010 Elsevier B.V. All rights reserved.

A hybrid electrochemical-thermal method for the preparation of large ZnO nanoparticles

A simple and efficient two-step hybrid electrochemical-thermal route was developed for the synthesis of large quantity of ZnO nanoparticles using aqueous sodium bicarbonate electrolyte and sacrificial Zn anode and cathode in an undivided cell under galvanostatic mode at room temperature. The bath concentration and current density were varied from 30 to 120 mmol and 0.05 to 1.5 A/dm(2). The electrochemically generated precursor was calcined for an hour at different range of temperature from 140 to 600 A degrees C. The calcined samples were characterized by XRD, SEM/EDX, TEM, TG-DTA, FT-IR, and UV-Vis spectral methods. Rietveld refinement of X-ray data indicates that the calcined compound exhibits hexagonal (Wurtzite) structure with space group of P63mc (No. 186). The crystallite sizes were in the range of 22-75 nm based on Debye-Scherrer equation. The TEM results reveal that the particle sizes were in the order of 30-40 nm. The blue shift was noticed in UV-Vis absorption spectra, the band gaps were found to be 5.40-5.11 eV. Scanning electron micrographs suggest that all the samples were randomly oriented granular morphology.

Role of spatial coherence on the rotation sensitivity of Lau fringes: an experimental study

A simple technique involving the use of a rotating and a stationary diffuser has been developed to vary the spatial coherence of light from a He-Ne laser. Using this technique an experimental investigation of the dependence of rotation sensitivity of Lau fringes on the spatial coherence of the illuminating wavefield has been carried out. It is observed that (i) the rotation sensitivity of Lau fringes varies in a well-defined manner as a function of the spatial coherence of the light used; (ii) the extremely good rotation sensitivity of Lau fringes can be used to great advantage (compared to the conventional double slit method) in the measurement of the spatial coherence of a wavefield; (iii) Lau fringes are formed at various levels of spatial coherence and as such it appears that the Lau effect need not be associated with an incoherent optical field

A parametric method 3 (PM3) study of trans‐stilbene

We report a comparative modified neglect of diatomic overlap (MNDO), Austin method one (AM1), and parametric method 3 (PM3) study of trans‐stilbene (tS) in its ground, excited (singlet and triplet), and ionic (positive and negative polarons and bipolarons) states. We have also calculated the barrier for ring rotation about the backbone single bond. Our results show that PM3 geometries are superior to MNDO and AM1, at least for tS. PM3 predicts, in contrast with MNDO, AM1 and even ab initio 3‐21G, a coplanar structure for tS, in accordance with recent experimental data. Singlet and triplet energies obtained from heats of formation are in surprisingly good agreement with experimental data.

Microwave Irradiation Assisted Method for the Rapid Synthesis of fine Particles of alpha Al2O3 And alpha (Al1-XCrx)(2)O-3 And Their Coatings on Si(100)

Chromium substituted beta diketonate complexes of aluminium have been synthesized and employed as precursors for a novel soft chemistry process wherein microwave irradiation of a solution of the complex yields within minutes well crystallized needles of alpha (Al1 XCrx)(2)O-3 measuring 20 30 nm in diameter and 50 nm long By varying the microwave irradiation parameters and using a surfactant such as polyvinyl pyrrolidone the crystallite size and shape can be controlled and their agglomeration prevented These microstructural parameters as well as the polymorph of the Cr substituted Al2O3 formed may also be controlled by employing a different complex Samples of alpha (Al1 XCrx)(2)O-3 have been characterized by XRD FTIR and TEM The technique results in material of homogeneous metal composition, as shown by EDAX and can be adjusted as desired The technique has been extended to obtain coatings of alpha (Al1 XCrx)(2)O-3 on Si(100)

A note on the slow rotation of a disc in a bounded fluid with a surfactant surface layer

A method involving eigenfunction expansion and collocation is employed to solve the axisymmetric problem of a slowly and steadily rotating circular disc in a fluid of finite extent whose surface is covered with a surfactant film. The present method (originally described by Wang (Acta Mech. 94, 97, 1992)) is observed to produce results of practical importance associated with the problem more quickly and more easily than the one used earlier by Shail and Gooden (Int. J. Multiphase Flow 7, 245, 1992). (C) 1994 Academic Press, Inc.

Wet chemical syntheses of ultrafine multicomponent ceramic powders through gel to crystallite conversion

Coarse (BOn/2)-O-n+/xH(2)O (10crystallites are often in metastable phases, such as cubic BaTiO3 at room temperature or superparamagnetic hexaferrites. Through the same route, luminescent phosphors of aluminates doped with rare-earth metals could be prepared. The present results indicate the general features of the gel-crystallite (G-C) conversion involving the instability of the metal hydroxide gel brought about by the disruption of the ionic pressure in the gel as a result of the faster diffusion of A(2+) ions through the solvent cavities within the gel frame work. This is accompanied by the splitting of the bridging groups like B-(OH)-B or B-O-B, leading to the breakdown of the gel into crystallites. G-C conversion has advantages as a method of synthesis of ceramics in terms of operational cost and procedural simplicity.

Nanoparticles of SrTiO3 prepared by gel to crystallite conversion and their photocatalytic activity in the mineralization of phenol

Nanometre-sized powders of SrTiO3 were prepared at 70-100 degrees C by the wet-chemical method of gel to crystallite (G-C) conversion. The crystallite sizes obtained were in the range 5-13 nm, as estimated by transmission electron microscopy (TEM) studies. The photocatalytic activities of these powders in the mineralization of phenol were evaluated in comparison with Degussa P25 (TiO2). The maximum photocatalytic activity was observed for powders annealed in the range 1100-1300 degrees C. The optical spectra of the particle suspensions in water showed broadened absorption around the band gap region, together with the appearance of an absorption maximum in the UV region. The effect of inorganic oxidizing species as electron scavengers on the rate of the photocatalytic degradation of phenol was studied. The influence of bulk and surface defects, which participate in the charge transfer process during photocatalysis, was investigated systematically.

Wet chemical formation of nanoparticles of binary perovskites through isothermal gel to crystallite conversion

Coarse BO2·xH2O (2 < x < 80) gels, free of anion contaminants react with A(OH)2 under refluxing conditions at 70�100°C giving rise to crystallites of single phased, nanometer size powders of ABO3 perovskites (A = Ba, Sr, Ca, Mg, Pb; B = Zr, Ti, Sn). Solid solutions of perovskites could be prepared from compositionally modified gels or mixtures of A(OH)2. Donor doped perovskites could also be prepared from the same method so that the products after processing are often semiconducting. Faster interfacial diffusion of A2+ ions into the gel generates the crystalline regions whose composition is controllable by the A/B ratio as well as the A(OH)2 concentration.