Research On The Preparation Of Tungstate Micro-Crystallites With Scheelite Structure By Precicitation Method

Scheelite-type tungstates belong to self-activated phosphor, whose luminescence originated from WO42-complexing ions, and they can present high efficient fluorescence excited under UV, X-ray and CR. They have broad and potential applications at scintillator media, solid-state optoelectronic devices, Raman, optical fiber, magnetic materials, catalysis and laser matrix.In this article, by using organic surfactants CTAB and / or PVA as the template, combined with ultrasonic vibration technology, SrWO4, BaWO4 and CaWO4 micro-crystallites with different morphologies were prepared under a simple and moderate condition. The influences of treatment conditions, such as concentration of reactants, concentration of CTAB or PVA, pH value, temperature, solvent and mixed condition of the reactants on the morphylogies and photoluminescence of micro-crystallites were investigated. The surface morphology, compositon and photoluminescence properties were investigated by means of mordern analyses techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectrometer (FT-IR spectra) and fluorescence spectrometer (PL). The main conclusions acquired from the present work as follows:(1) The SrWO4, BaWO4 and CaWO4 micro-crystallites with different morphologies were prepared by the use of precipitation techniques.(2) The effect of different treatment conditions on the morphology and luminescence properties of SrWO4, BaWO4 and CaWO4 micro-crystallites have been studied in detail. The results indicate that: ①With the reactants concentration increasing, SrWO4 micro-crystallites transform from the octahedron to flower-shaped, rod-like and spherical morphologies, and the size of BaWO4 micro-crystallites has a tend to grow bigger.②With the pH value increasing, the olive-like, octahedron-shaped and branch-like SrWO4 micro-crystallites were obtained,respectively, whereas the particle size of BaWO4 octahedron decrease.③With the CTAB concentration increasing, CaWO4 micro-crystallites tend to grow agglomerate-like morphology, SrWO4 and BaWO4 micro-crystallites tend to grow tinterspersed. With the increase in PVA concentration, the coexistence of flower- and olive-like, olive-like and flower-like CaWO4 were obtained, respectively. But the products of CaWO4 prepared in the mixed solution of CTAB and PVA have the similar morphology prepared in PVA solution, both of them were olive-shaped.④The CaWO4 micro-crystallites prepared at a higher temperature have a stronger XRD diffraction peak and luminescence intensity. With the temperature increasing, the number of SrWO4 octahedrons decrease and more and more smaller and non-regularity SrWO4 particles appear in the products, and SrWO4 particles have a tend to aggregate together. But it is strange that many needle-like SrWO4 micro-crystallites appeared in the products when the temperature is 130℃.⑤The CaWO4 micro-crystallites were prepared by magnetic stirring and ultrasonic shocking,respectively. It shows that ultrasonic vibration is conducive to reducing the product size and changing the morphology of the micro-crystallites.⑥Different morphologies of CaWO4, SrWO4 and BaWO4 micro-crystallites were prepared by adding different ratio of solvent in aqueous solution, such as ethanol, ethylene glycol and triethanolamine. It shows that the solvent have an impact on the morphology of the products.(3) The micro-crystallites prepared under different conditions have different emission intensity. The CaWO4 and SrWO4 micro-crystallites show single luminescence peaks and the peaks are smooth, while the emission peak of BaWO4 micro-crystallites are sharper.