MODERN GLASS — STOOKEY 333 



Considering the fact that every change in state of matter — con- 

 densation of vapor to a liquid or a solid, crystallization or evapora- 

 tion of a liquid, formation of a new crystal from an old one, every 

 chemical reaction — must be initiated by formation of nuclei of the 

 new phase, it is amazing that so little fundamental research has been 

 done on the subject of nucleation. One reason for the dearth of ex- 

 perimental research may be the difficulty of holding the submicro- 

 scopic nucleus still and preventing its instant alteration to a larger 

 particle before its properties can be studied. If this is so, the new 

 glass-ceramics may become a useful medium for this important re- 

 search because the crystallization process can be initiated, controlled, 

 or halted at will simply by cooling the glass (see pi. 3) . 



By thus being able to examine the nuclei in situ, it becomes possible 

 to make measurements of their number and size — a difficult procedure 

 by condensation methods. This will permit a comparison between 

 various theories of nucleation kinetics, since all such theories include 

 the critical nucleus size and nucleation frequency as fundamental 

 parameters. This is true even of Willard Gibbs's early formulation 

 relating the thermodynamic work required to form the smallest 

 (critical) stable particle of a new phase. 



CAN GLASS BE ELECTRONICALLY CONDUCTIVE? 



All the electrical charge carriers in glass have traditionally been 

 believed to be metal ions. One disadvantage of ionic conduction in 

 glass is that movement of ions through the glass by diffusion alters the 

 glass composition. This means the electrical characteristics of the 

 glass can constantly vary with time. For example, glass conducting 

 direct current by ion exchange can become "polarized" so that the 

 conduction process becomes blocked. 



Much research is being directed toward overcoming such problems. 

 Kesearchers in England (J. E. Stanworth), Holland (H. J. I. Trap 

 and J. M. Stevels), and the U.S.A. (A. David Pearson et al. of 

 Bell Telephone Laboratories) are currently reporting electronic con- 

 ductivity in a variety of glasses. Some of these contain high concen- 

 trations of vanadium pentoxide ; others contain a single element addi- 

 tive in two or more valency states; and still others are low-melting, 

 nonoxide glasses combining arsenic, tellurium, and iodine. 



Undoubtedly there will be controversy as to whether these are truly 

 amorphous glasses or whether the electrons originate in a crystalline 

 component. Here again, the thorny question of molecular structure 

 arises. It may be that an intermediate structure exists that permits 

 electronic conductivity — a structure in which the molecules are more 

 nearly ordered, as they are in crystals. 



