Mechanism Of Crystal Phase Evolution Of GeO₂in Acid-induced Liquid Phase Deposition Process

The properties and applications of GeO₂were introduced in this thesis. GeO₂solidphases were grown through an acid-induced liquid phase deposition （LPD） process. TheLPD growth of GeO₂soild phase involves dissolution of hexagonal GeO₂powders in anaqueous alkaline solution and precipitation of GeO₂crystal phase from the solution withaddition of acid. The reaction and growth mechanisms were studied. Aqueous germanateion solutions containing different amounts of GeO₂were synthesized to investigate thechemical reactions and molecular structures of the precursor solutions. The morphologiesand crystal structures of GeO₂solid phases precipitated out of the aqueous germanate ionsolutions under various conditions （different concentration, deposition time, etc） werestudied. Polyvinylpyrrolidone （PVP） was added to the precursor solution to disclose theinfluence of surfactant on the morpholog modification of the GeO₂solid phase. Thecrystal growth mechanism is discussed based on above experimental results.Transparent and stable aqueous germanate ion solutions can be obtained byintereaction between hexagonal GeO₂powders and aqueous ammonia within a certainconcentration range. The reactions involving in the germanate ion aqueous solutions withdifferent GeO₂concentrations were determined by TG-DSC and FTIR analyses.Germanate ions were decomposed after adding acid. The decomposition behavior causesgradual increase in concentration of GeO₂solutes. As a result, GeO₂crystl phasesprecipitate out and keep growing up into large particles. A longer precipitation timeallows to form a GeO₂ceramic film on the substrate surface. The rod-like tetragonalGeO₂crystals precipitated firstly from the solution, following by precipitation out ofcubic hexagonal GeO₂phases. Actually, the rod-like tetragonal solid phases undergoesre-dissolution in the mother solution as the cubic hexagonal GeO₂phases appear. Onlyhexagonal GeO₂phases exist with progression of the LPD process. The tetragonal GeO₂ crystals precipitate out firstly because its saturated solubility is much lower than that ofhexagonal GeO₂phases. Our calculations based on thermodynamics show that the Gibbsfree energy decreases as tetragonal GeO₂transformed into hexagonal GeO₂. In otherwords, the transformation can proceed spontaneously. The tetragonal GeO₂re-dissolvesin the mother solution to provide more GeO₂solutes for the growth of hexagonal GeO₂phases. On the basis of Blavy law, the crystal plane （111） of tetragonal GeO₂phase growsfaster compared with other planes, which leads to formation of a tetragonal GeO₂phasewith a rod-like shape. BFDH model and HP model significantly affects the shapeevolution of hexagonal GeO₂phases, resulting in developing a truncated cube-like orperfect cubic-like shapes. The spindle-like GeO₂particles can be grown from theprecursor solution containing a PVP surfactant. PVP has a special molecular structure,which makes it possible for GeO₂planes with higher adsorption energy to adsorb PVPmolecular chains. The growth rates of these planes reduce. These planes may retain in thesurface of the crystal solid phase. And then, the spindle-like particles can be formed.