1,720,969 research outputs found
GLASS TRANSITION TEMPERATURE AND CRYSTALLISATION OF GLASSES IN THE LI2O 4GeO2-K2O 4GeO2 COMPOSITION RANGE
No full textThe effect of replacing Li2O by K2O on the glass transition temperature and on devitrification behaviour in a series of glasses in the lithium tetragermanate-potassium tetragermanate composition range has been studied by differential thermal analysis, X-ray diffraction and Fourier-transform infrared spectra. All glasses studied exhibit internal crystal nucleation. The progressive replacing of Li2O by K2O causes the value of the glass transition temperature to go through a minimum at molar ratio Li2O/K2O = 1. The crystallising phases have been identified and related to the devitrification mechanism. The effect of the specific surface of the glass samples on devitrification processes has also been pointed ou
BULK NUCLEATION AND CRYSTAL GROWTH IN CALCIUM TETRA AND DIGERMANATE GLASSES
No full textCalcium tetragermanate and calcium digermanate glasses were prepared and studied with the aid of differential thermal analysis, X-ray diffraction and FTIR spectroscopy. The molar ratio of GeO6/GeO4 groups increases with the CaO/GeO2 molar ratio. A two steps crystallisation process was found: initially microcrystals are formed that at higher temperatures are transformed in well shaped crystals. An unidentified phase crystallises in both glasses. CaGe2O5 crystals also grow in calcium digermanate glass. The influence of the specific surface area of the glass sample and of a heat treatment of nucleation was pointed out. The activation energy for crystal growth in each glass was evaluated by a non isothermal metho
THE NON-ISOTHERMAL DEVITRIFICATION OF Li2O TiO2 6GeO2 GLASS
No full textThe effect of a partial replacing of GeO2 by TiO2 structure and non-isothermal devitrification of lithium heptagermanate glass, examined by Fourier transform infrared spectra, differential thermal analysis and X-ray diffraction, is reported and discussed. The structure and the glass-transition temperature are not affected by the substitution of GeO2 by TiO2 but the investigated glass, unlike lithium heptagermanate glass, devitrifies only from the surface into a crystalline phase isomorphous with Li2Ge7O15 crystals. The validity of the Arrhenius-type relationship between the rate constant of crystal growth and the absolute temperature is also discussed
THE NON-ISOTHERMAL DEVITRIFICATION OF GLASSES IN THE SrO 4GeO2-BaO 4GeO2 COMPOSITION RANGE
No full textThe effect of replacing SrO by BaO on the glass transition temperature and on devitrification behaviour in a series of glasses in the strontium tetragermanate - barium tetragermanate composition range has been studied by differential thermal analysis, X-ray diffraction and Fourier-transform infrared spectra. All glasses studied exhibit internal crystal nucleation. The progressive replacing of SrO by BaO causes the decrease of the glass transition temperature. Solid solutions between SrGe4O9 and BaGe4O9 were found to crystallize in glass containing both SrO and BaO. The effect of the specific surface of the glass samples on devitrification processes has been also pointed ou
GEL SYNTHESIS AND CRYSTALLIZATION OF POTASSIUM HEPTAGERMANATE GLASS POWDERS
No full textA potassium heptagermanate (K2O7GeO2) gel was prepared and its crystallization behavior was studied with the aid of X-ray diffraction and differential thermal analysis. The gel preparation involves hydrolysis and polycondensation of germanium ethoxide with potassium ethoxide. The gel prepared is an amorphous solid containing residues that are lost on heating. The temperatures for the gel to glass transformation are below the glass transition temperature of the glass. During the heat treatment required for the gel to glass conversion, the gel is kinetically stable to crystallization. It is then possible to convert the porous particulate gel-glass into a monolithic dense glass by sintering or hot pressing techniques without devitrification
GEL SYNTHESIS AND CRYSTALLISATION OF MIXED SODIUM POTASSIUM TETRAGERMANATE GLASS POWDERS
No full textA series of glass powders in the sodium tetragermanate-potassium tetragermanate composition range was synthesised by the sol-gel method. The effect of replacing Na2O by K2O on the glass transition temperature and on devitrification behaviour has been studied by Fourier transform infrared spectra, differential thermal analysis and x-ray diffraction. Progressive replacing of Na2O by K2O causes the value of the glass transition temperature to go through a minimum at molar ratio Na2O/K2O=1. The crystallising phases in the mixed alkali members of the series have been identified as a solid solution between Na4Ge9O20 and K4Ge9O20 crystals
GEL SYNTHESIS AND CRYSTALLIZATION OF POTASSIUM TETRAGERMANATE GLASS POWDERS
No full textPotassium tetragermanate gel (K2O 4GeO2) was synthesized by hydrolytic polycondensation of germanium ethoxide with potassium ethoxide in alcoholic medium. The crystallization behavior of the gel, examined by differential thermal analysis and X-ray diffraction, is reported and discussed. Potassium tetragermanate gel crystallizes in two steps. At about 593°C, metastable K4Ge9O20 crystals are formed as the major crystalline phase with a small amount of K2Ge4O9 crystals. At higher temperatures, metastable K4Ge9O20 are converted to the thermodynamically stable K2Ge4O9 phase. These results are compared with those of conventionally prepared glass of the same composition
NON-ISOTHERMAL DEVITRIFICATION OF STRONTIUM GERMANATE GLASSES
No full textThe devitrification behavior of strontium germanate glasses; examined by differential thermal analysis, Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), is reported. The glass compositions are expressed by the general formula: xSrO(1-x)GeO2 with x = 0.050, 0.125, 0.200 and 0.250. The glasses with x = 0.050 and 0.125 exhibit two glass transition temperatures due to a phase separation. The phases crystallizing during DTA runs were identified and devitrification mechanisms proposed. In all glasses SrGe4O9 crystals grow from bulk nuclei. The effect on devitrification processes of the specific surface area of the samples was also pointed out
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