91Ó°ÊÓ

Cite the two desirable characteristics of glasses.

(a) What is crystallization? (b) Cite two properties that may be improved by crystallization.

For refractory ceramic materials, cite three characteristics that improve with and two characteristics that are adversely affected by increasing porosity.

Compare the manner in which the aggregate particles become bonded together in clay-based mixtures during firing and in cements during setting.

What is the distinction between glass transition temperature and melting temperature?

Compare the temperatures at which sodalime, borosilicate, \(96 \%\) silica, and fused silica may be annealed.

The viscosity \(\eta\) of a glass varies with temperature according to the relationship $$\eta=A \exp \left(\frac{Q_{\mathrm{vis}}}{R T}\right)$$ where \(Q_\text {vis}\) is the energy of activation for viscous flow, \(A\) is a temperature-independent constant, and \(R\) and \(T\) are, respectively, the gas constant and the absolute temperature. A plot of \(\ln \eta\) versus \(1 / T\) should be nearly linear, and with a slope of \(Q_{\text {vis }} / R .\) Using the data in Figure \(13.7,\) (a) make such a plot for the soda-lime glass, and (b) determine the activation energy between temperatures of 900 and \(1600^{\circ} \mathrm{C}\).

(a) Explain why residual thermal stresses are introduced into a glass piece when it is cooled. (b) Are thermal stresses introduced upon heating? Why or why not?

Borosilicate glasses and fused silica are resistant to thermal shock. Why is this so?

In your own words, briefly describe what happens as a glass piece is thermally tempered.