Preparation of Submicron and Micron Size Cr (IV) Optical Composites
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411 Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Research Society
WORK IN PROGRESS Synthesis of the Optical Nano-Particles We have chosen the sol - gel technique for the preparation of the nano and micro-sized particles. A sol is a colloidal suspension of particles in a liquid. A gel is an interconnected, rigid network with pores of sub-micron dimensions and polymeric chains whose average length is greater then a micrometer. The sol - gel process involves a number of steps. 1) The mixing of the colloidal particles or the liquid alkoxide precursors to form a sol. In case of the metal alkoxides, the reactions are, hydrolysis, condensation, and poly-condensation to form the sol. 2) Gelling. With time, the colloid particles and condensed metal species link together to become a three dimensional network. At the gellation point the viscosity increases sharply and a solid or semi-solid "gel" is formed. 3) Aging, drying and densification. Aging (synersis) means keeping the system in the gel state for a period of time to allow the poly-condensation to continue. This increases the strength of the gel. During the drying stage water s removed from the interconnected pore network. Heating the porous, dehydrated, gel at high temperature causes densification to occur. This third step is crucial, because, here is where particle size ad particle size distribution can be controlled. We used the method to prepare "nano and micro particles" of Cr (IV): Ca 2GeO 4, Cr (IV): Li 2CaGeO 4 and Cr (IV): Mg 2 SiO 4. Using a simultaneously recording differential scanning calorimeter and thermo gravimetric analyzer (TA Instruments SDT 2960), we observed the sequence of events occurring during the process. This information is extremely useful for the planning of the third step of the sol - gel process. For instance, we see that formation of the Cr (IV) does not take place below 650 'C. Such thermal scans also allow the quantitative study of the chemistry of the particular process. Comparing the emission spectra of Cr (IV): Ca 2GeO 4 prepared as single crystal and by the sol - gel method indicates that there is no difference between the optical properties of the two materials. Optical microscopy indicated that the size of the powder produced ranged from sub-micron to microns. Control of particle and its distribution during the synthesis stage must take into account two factors: 1) Germanium alkoxide [Ge (OCH 3) 4] hydrolyses extremely fast. 2) The gels must be heat treated at 640 - 700 'C to obtain Cr (IV) luminescence. Due to the extremely fast hydrolysis rate, control of particle size by controlling the pH of the reaction medium is not feasible. We can obtain transparent gels, indicating very small particle size, at low temperatures but the high temperature heat treatment leads to particle size coarsening. There are at least two approaches available to overcome the particle size problem.
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One method is to add a salt (NaCI) solution to the reaction mixture, wash the product from the 700 'C heat treatment with water to remove the s
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