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TRANSFORMATION REGION OF GLASS *
Author(s) -
Winter Aniuta
Publication year - 1943
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1943.tb15210.x
Subject(s) - refractive index , thermal expansion , borosilicate glass , birefringence , atmospheric temperature range , materials science , prism , thermodynamics , limiting , optics , thermal , mineralogy , transformation (genetics) , range (aeronautics) , glass transition , chemistry , composite material , physics , polymer , gene , engineering , mechanical engineering , biochemistry
A bstract Evidence for the existence of a “transformation region” rather than a “transformation point” in the 10 14 poise range of glasses has been produced by a variety of experiments, both static and dynamic. Refractive index was measured (1) on small specimens quenched in air and (2) directly in the furnace by the single‐prism and three‐prism methods. Quantitative agreement was obtained, and the method of quenched samples is recommended as the simplest of those tested. Thermal expansion coefficients were also determined on the quenched samples. Disannealing lowered the refractive index of glasses; for a borosilicate crown, by an order of 0.006. The change resulting from strains was comparatively small and in the opposite direction so that birefringence was found to be an untrustworthy indicator of the state of a glass. For a series of optical glasses, the difference between the refractive‐index extremes is a function of the lowest temperature at which the minimum index can be realized. Thermal expansion coefficients may also vary with heat‐treatment. Two limiting states of glass are postulated to explain the observations. In between these states lies the region of transformation, including an intermediate state for each temperature between the two limiting temperatures. For all of the glasses e‐xamined, the extent of the transformation region was about 100°C. The rate of transformation into the low‐temperature state may be studied from “equilibrium curves” by plotting stable refractive index as a function ‐of temperature. Whereas the conversion to the high‐temperature state is instantaneous at the upper temperature of the transformation region, the conversion to the low‐temperature condition demands a minimum time at each temperature, which explains why annealing is such a time‐consuming process. The annealing time, however, may be drastically reduced by following the equilibrium curves in a stepwise fashion. With insufficient annealing, higher temperature states are frozen in and transformation continues even at room temperature as proved by measurements extending over sixteen months of study.