光学ガラス研磨面上に生ずる‘ヤケ’の実験的研究

V. 青ヤケ層の屈折率.浸蝕液中における青ヤケの成長

抄録

The first half of the paper deals with the measurement of the refractive indices of the AOYAKE layers artificially produced on test pieces of three kinds of optical glasses, SF 3, SK 5 and LaK 13; and the second half with the growth rates of the above mentioned AOYAKE layers.
The AOYAKE layers were produced by attacking the test pieces with the following corrosives: for SF 3, 1N and 1/10N HNO₃ at 40°, 50° and 60°C; for SK 5, 1/100N and 1/1000N HNO₃ at 40°, 50° and 60°C; and for LaK 13, pure water at 40°, 50° and 60°C.
The refractive index n_ƒ of the AOYAKE layer was measured by Abeles' method. Within the range of the conditions of corrosion studied, n_ƒ was found to be scarcely dependent on the concentration and temperature of the corrosive and the duration of treatment. The mean n_ƒ's. for SF 3, SK 5 and LaK 13 were 1.484, 1.477 and 1.500 respectively.
It was noticed that n_ƒ decreases with the lapse of time if the test piece taken out from the corrosive is left exposed to the atmosphere, until it reaches a final value which probably is determined by the humidity of the atmosphere. This change in n_f is remarkable in LaK 13. For this glass, it was also found that n_ƒ decreases if the AOYAKE layer is desiccated by slightly heating with the aid of an electric lamp or by evacuation, and again increases if it is left exposed to the atmosphere.
Heat-treatment increases n_ƒ. In the case of SF 3 and SK 5, this change is accompanied by a, decrease in thickness of the AOYAKE layer.
This is not the case with LaK 13.
The growth rate, i.e, the time rate of increase of the thickness d, of the AOYAKE layer was studied by observing the interference colour. This gives n_ƒd, from which we can calculate d with the aid of the knowledge of n_ƒ. In the growth of the AOYAKE layer on SK 5 attacked by 1/100N HNO₃ at 60°C, the d-t curve is linear, suggesting that the rate-determining process is the reaction at the interface between the layer and the mother glass. In all of the other cases studied, however, the curves show a general trend that the growth rate deacreases with time. These curves are accompanied by curious deflection points which seemingly indicate that the growth takes place in some step-wise manner. It is believed, however, that these deflections are deceptive, because they occur at those (optical) thicknesses which correspond to the sensitive colours. It would be more reasonable to look for the cause of the deflections in optics rather than in the mechanism of the growth of AOYAKE layers. If we neglect the deflections for this reason and smooth out the d-t curves, we find that d is roughly proportional to t^0.4. This suggests that the leading rate-determining process would be the diffusion of ions through the layer (if the growth-rate is determined exclusively, by this factor, d is expected to be proportional to t^0.5).
From the smoothed d-t curves we can calculate the activation energies for the growth of the AOYAKE layers. The activation energies at d=100mμ are estimated at 18.9, 14.3 and 9.7 kcal/mol for SF 3, SK 5 and LaK 13 respectively.