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

Coarse (BOn/2)-O-n+/xH(2)O (10tridymite structure and chromates. The nanosized crystallites 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.

Role of B2O3 on the phase stability and long phosphorescence of SrAl2O4 : Eu, Dy

The role of B2O3 addition on the long phosphorescence of SrAl2O4:Eu2+, Dy3+ has been investigated. B2O3 is just not an inert high temperature solvent (flux) to accelerate grain growth, according to SEM results. B2O3 has a substitutional effect, even at low concentrations. by way of incorporation of BO4 in the corner-shared AlO4 framework of the distorted 'stuffed' tridymite structure of SrAl2O4. which is discernible from the IR and solid-state MAS NMR spectral data. With increasing concentrations, B2O3 reacts with SrAl2O4 to form Sr4Al4O25 together with Sr-borate (SrB2O4) as the glassy phase, as evidenced by XRD and SEM studies. At high B2O3 contents, Sr4Al14O25 converts to SrAl2B2O7 (cubic and hexagonal), SrAl12O19 and Sr-borate (SrB4O7) glass. Sr4Al14O25:Eu2+, Dy3+ has also been independently synthesized to realize the blue emitting (lambda(em)approximate to490 nm) phosphor. The afterglow decay as well as thermoluminescence studies reveal that Sr4Al14O25:Eu, Dy exhibits equally long phosphorescence as that of SrAl2O4:Eu2+, Dy3+. In both cases, long phosphorescence is noticed only when BO4 is present along with Dy3+ and Eu2+. Here Dy3+ because of its higher charge density than Eu2+ prefers to occupy the Sr sites in the neighbourhood of BO4, as the effective charge on borate is more negative than that of AlO4. Thus. Dy3+ forms a substitutional defect complex with borate and acts as an acceptor-type defect center. These defects Eu2+ ions and the subsequent thermal release of hole at room temperature followed by the trap the hole generated by the excitation of recombination with electron resulting in the long persistent phosphorescence. (C) 2003 Elsevier Science B.V. All rights reserved.