Density Change of K<sub>2</sub>O-Na<sub>2</sub>O-SiO<sub>2</sub> Glasses by Heating

Abstract

Using purified sand and the materials of reagent grade K2O-Na2O-SiO2 glasses were melted in a platinum crusible.The quenched samples were subjected to reheating by holding at a constant temperature in the range of 300°-525°C, and the change of the density was measured by floating method.The data obtained were analysed in the light of the new theory posturated by the author (cf. J. Ceram. Assoc. Japan, 66, 117, 1958).The density change ΔD may be represented byΔD=_??_pie-t/φi=pe-t/φ4+qe-t/φ3+re-t/φ2+se-t/φ1+…where t is the time, φ relaxation time, P, q, …are the constants whose absolute values increase monotonously with decreasing temperature, having the increments falling off below a certain temperature. φ′s have the respective apparent activation energies and increase with decreasing temperature. The apparent activation energies, however, are possible to become more or less smaller with decreasing temperature.The activation energy of the largest time constant, φ4, have the value close to that of viscosity giving φ4≅480 min at the temperature corresponding to 1014.3 pois. Assuming that the part of the density change represented by this term takes place with the flow of the same structural units as in viscous flow it is possible to calculate the size of the units by applying the Eyring's formula with a result that the volume is about 15 Å3 containing 1-2 O2- ions.Around the strain point it may fairly be presumed that the mobility of separate ions would predominate that of molecular groups.The other terms than the first correspond to the elastic after effect in the range of transformation temperature and below, being probably caused by the expansion, contraction, and rotation of silicate chain anions. There is a possibility of giving a wide distribution of comparatively low activation energies which take part in the density change at low temperatures.In the range of composition covered by the present paper the amount of contraction by heating decreased with increasing alkali content, the trend which reversed at the alkali content of 20-25 mol per cent. This decrease of contraction may be interpreted by the obstructing effect of alkaline ions to the change of network structure, and the point of inflection would correspond to the same trend of the composition-density diagrams.It is highly probable that the density change caused by the migration of alkaline ions would be less than 10-3g/cm3. In mixed alkali-glasses, therefore, the influence of thermal history would be highly reservable owing to the small mobility of mixed alkaline ions, and the larger contribution of the place exchange of alkaline ions to the density change would bring about the larger thermal after effect.The author modified his model representing the energy state of a glass structure by an assembly of small cubes of equal volume each being assigned the minimum free energy of the level, L1, L2, …as follows:(1) Li is composed of the levels l11, l12 having the values of free energy close to each other.(2) lij are composed of lij1*, lij2*.And the transformation L↔L corresponds, for example, to the growth and disappearance of microphases which bring about the change of structure, l↔l to the expansion, contraction, and rotation of the chain structure-units, and l*↔l* to the movement of alkalineions.