Primary and Secondary Phase Separation in Glasses

Abstract

Several years ago, during a study of phase separation in glasses, we detected with the aid of electron microscopy and low-angle x-ray scattering the possibility of formation of some kind of highly disperse structure together with the main phases [1]. Three hypotheses have been put forward on the origin of this “fine” structure: that it is fluctuational in character [2], that a special kind of phase separation, described as “microphase separation,” exists [3], and that it is formed during quenching of the glass to room temperature, as the result of supersaturation of the phases followed by secondary separation in it [4]„ Somewhat later Tran Thach-Lan [5], investigating phase separation in sodium silicate glasses, found that each phase is homogeneous; i.e., he did not detect a “fine” structure. At the present time there are no doubts regarding the existence of this structure or the causes of its formation [6, 7]. It is described as a “background structure” in [7], but in our opinion it is more correct to speak of “secondary separation,” as this stresses its origin.