Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Fernández Sánchez, E.

López Andrés, Sol

López-Acevedo Cornejo, Victoria

Prieto Colorado, Carmelo

Docta Complutense

- 83-TMICRORAMAN ANAL YSIS OF CORUNDUM CRYSTALLIZEDBY CRYOLITE FLUXPRIETO COLORADO, A. C.*I, FERNANDEZ SANCHEZ, E. 1, LOPEZ-ACEVEDO,V. 2 and LOPEZ ANDRES, S. 21 Física de la Materia Condensada, Cristalografia y Mineralogía. Facultad de Ciencias.Universidad de Valladolid. 47005-Valladolid(Spain)2 Cristalografia y Mineralogía Facultad de Ciencias Geológicas. UniversidadComplutensede Madrid.28040-Madrid(Spain). .Corundum is crystallized by differentmethods, among them Vemeuil, Czochralski andflux growth give the crystals with the better quality. The Vemueil method allos theobtention of crystals with size and quality optimal for jewelry applications. Czochralskicorundums give high quality crystals for technological applications. The flux methodproduces crystals very similar to natural corundum.The choice of the growth methods depends on several factors. Our choice of the fluxmethod was due to the availability of different phases formed with the componentsinvolved in the corundum growth by this procedure.Usually the fluxes used to crystallize corundum are lead, bismuth and boron oxidesand/or fluorides (PbO, PbF2,Bi203' B203)' the proportions ofwhich can be varied. Alsoother mixtures can be used, the components of which are sodium carbonates, sulphates,fluorides and aluminates and cryolite (N~AlFe). The high solubility of the Al203 inmelted cryolite is the principIe of the Hall-Heroult process for the fabrication ofaluminum. This process takes place at temperatures well below the melting point of purealumina, this is the reason why the cryolite is an ideal solvent to obtain corundumcrystals with the flux method.A mixture of commercial alumi& and cryolite with Cr203 as dopant and dye (in a01.7:15:0:00.5 g. proportion) has been used in the present work. A platinun crucible (h =4 cm and 0 = 3,6 cm) covered with a glass fiber cap was used. The fussion wasperformed in a mufllekiln at 12000C. The mixture was allowed to cool down to roomtemperature in 24 hours. The identification and preliminary characterisation of theproducts were performed by optical trasmissionand reflection optical microscopy (OM),scannig electron microscopy (SEM), x-ray dispersed energy qualitative chemical analysis- 84-.¡ ,(SEM+XDE), and x-Ray diffiaction (XRD). This has been coinplemented with a detailedmicroRaman study of the differents identifiedphases ohf the melt.Micro-Raman spectra were recorded on a DILOR X-y raman spectrometer attachedto a metallographic microscope. The excitation was done with either 488.0 or 514.5 nmlines of an Ar+ laser, focuse by the microscope objective. The scattered light was alsocolleced by the microscope objective. The Raman signal spectrally resolved with thespectrograph and the detection was done with a liquid nitrogen cooled CCD: The Ramanspectra were obtained in backscattering with the laser perpendicular to the sample planeoThe samples were two thin layers cut along planes parallel and perpendicular to thecrucible axis.Corundum plates were identified on the crucible walls near the edge. These plateswere pink colored; they can be isolated or formingbunches with small rose shape up to 5mm diameter with micrometric thickness and (0001) crystal orientation. At the opticalmicroscope they show helicoidal dislocations and small crystals spared over the surface,which were identified as sodium heptaluminun eleventhoxide(NaAl7011)by microRamanand DRX. The residual melt exhibits an intenseyellow color (dark green in some specificpoints). It is formed of fibers with radial distribution and optically isotrope round grainsin between the fibers. The morphology of these grains can be associated with a vitreousphase, with a poor crystallinity, the Raman spectrum reveals the presence of corundumbeneath these grains. Higher magnificationof the fibers reveals that they are formed of atangle of aligned thin plates. The space in between plates is filled with a granular materialwith vitreousaspect. This materiall has been identified as a cryolite, the characteristicRaman modes of the Corundum are also observed, though very weak. One observed aswell the presence of octaedric structures on the crucible walls, which were associatedwith villiaumite (NaF). Also, some thick plates with hexagonal shape, identified ascorundum and small hexagonal prisms, probably related to a-sodium oxifluoride. TheJdark green colored material was cryolite, eskolaite (Cr203)and NasAlO4.In conclusion, a full identificat~ of the different solid phases resulting of the cryoliteflux grown corundum has been achieved using microRamanspectroscopy.AcknowledgementsThis work has been funded by the Spanish Commission for Science and Technology(DGICYT n° PB96-0619)

English

Microraman analysis of corundum crystallized by cryolite flux

http://eprints.ucm.es/15371/1/1999_Microraman_Analysis_Corundum_Crystallized_Cryolite_Flux_%28_Georaman%2799%29.pdf

Microraman analysis of corundum crystallized by cryolite flux

Abstract

Similar works

Full text

Available Versions

Docta Complutense