This paper is a brief review of the physical and chemical methods of strengthening glass, in particular that due to the large for small ion exchange resulting from treatment in molten salt at temperatures below the annealing range for t h e glass. Abrasion of treated glass is shown to reduce markedly the strength of experimental alkali-lime-silica glasses as well as commercial glasses of this and other types. Treated alkalialumina-silica a n d alkali-zirconia-silica glasses, however, are less affected by abrasion. Strength after abrasion increases with alumina or zirconia content, reaching 117,000 psi for cane of a 35% alumina glass. The explanation for the aluminum or zirconium effect may involve a n unusual ionic environment caused by their presence in the lattice.

samples were also magnetized to saturation with no resultant change in impedance. IV. ConclusionsThe study of the electrical properties of high-density irondeficient nickel-zinc ferrites has led to the conclusions that:(1) The dc resistivity for iron-deficient nickel-zinc ferrites sintered to 99.9% of theoretical density is similar for compositions between Nf.~Zllo.wFel.oo04-and Nio.40Zllo.51Fel.oo04-.(2) For dense Nio.a,Zllo.oaFel.soOa-: (a) Room temperature resistivity varies with sintering temperature from 3 X 10' n-cm at llOO°C to 2x10' n-cm at 1220°C; ( b ) temperature dependence of resistivity is equivalent in air and in oxygen from 25" to 700°C for the test conditions described and is insensitive to changes in grain size between 6 and 15 pm; (c) activation energy for conduction is approximately 0.57 eV; and ( d ) densification proceeds rapidly to 2 99% of the theoretical density for sintering temperatures 2 1170°C; the room temperature resistivity for these specimens increases to an equilibrium value with sintering time by bulk cation diffusion. Samples sintered at lower temperatures require long sintering periods to attain equivalent densification, and the room temperature resistivity is invariant with sintering time and corresponding porosity.( 3 ) The Seebeck coefficient for the ferrites studied is 550 pV/"C from 200" to 700°C.(4) The dielectric constant, E', for the p-type ferrites varies from 17.3 at 0.5 MHz to 16.4 at 15 MHz. AcknowledgmentsThe writers thank M. Schwartz of IBM's Components Division and T. Pohl and A. Mullaly of San Jose Systems Development Division for technical assistance.Diffusion and stress profiles were determined in an Na-for-Li exchanged LiZ0-AI,O,-Si0, glass as a function of the time of reheating in air. Comparison of the two sets of data by a thermal-stress analogy gave a fair correlation, although the depth of the compressive layer was overestimated by about 16%. Stress data alone were used to obtain an interdiffusion coefficient at 400°C which agreed well with the diffusivity obtained directly. The stress and concentration fractions were related graphically; the relation suggests that there is a slight accommodation of the larger ion by the glass structure without stress buildup during the early stages of exchange. These results led to a method of predicting the modulus of rupture of an exchanged glass after it is reheated in air.

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