334 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



STRONG GLASS IS FLEXIBLE 



We all think of glass as being rigid and brittle. In reality it is 

 more elastic and flexible than spring steel. The reason that we have 

 not been able to use this valuable property, except in glass fiber, is 

 again the low strength. When we have mastered the strength prob- 

 lem, we have automatically gained the important bonus of flexibility 

 and elasticity. Glass springs, flexible windows, and other manifesta- 

 tions of these properties are within sight. Already, tempered glass 

 springs have been flexed for millions of cycles without the work- 

 hardening or fatigue that occurs in metals. 



Wliile glass is elastic, it is completely nonmalleable and nonductile 

 except at high temperatures. Ductility implies an irreversible yield 

 or flow under stress, lead metal being a good example of ductile mate- 

 rial. If we could give glass a little ductility, it would help a great deal 

 in decreasing the complete, "catastrophic" nature of the break, as well 

 as preventing very high local stresses from building up without relief 

 by flow. 



So far, there seems to be no practical solution to this problem. It 

 may be that current work by J. A. Pask at the Univei-sity of California 

 and others on the ductility of single crystals of the alkali halides and 

 magnesium oxide will shed some light on this problem, particularly 

 with respect to the glass ceramics. However, the step from single 

 crystals to polycrystalline materials is sure to be great. 



OLDEST GLASS GETS NEW RESEARCH 



Fused silica is a one-component glass, simply silicon dioxide, and 

 as such is one of the simplest and oldest glasses in existence. Tradi- 

 tionally, it has been made by melting quartz sand, but many modern 

 applications for this material require a purity and homogeneity 

 beyond that provided by this simple method. 



High-purity silica can now be made by high-temperature hydrolysis 

 of silicon tetrachloride. The products of this reaction are pure silicon 

 dioxide and hydrochloric acid. The silica condenses out continuously 

 as a growing disk of optical-quality, amorphous glass. The HCl 

 passes off as vapor. 



Wliere is this improved material utilized? Recent research has 

 shown that the small gas molecules of helium and hydrogen, normally 

 blocked by glass, diffuse readily through high silica glass. Hence, 

 serious thought is being given to use of such glass as molecular sieves 

 for separation of helium from natural gas. Hydrogen diffuses more 

 slowly than its molecular size would suggest, the reason being that it 

 reacts chemically with the glass. 



High-purity silica crucibles are employed in manufacture of pure 

 single-crystal silicon for transistors. The same high-purity material 

 is the heart of acoustical delay lines used in computers, because its 



