Pyrex glass thermal properties

Thermal expansion is a most important property in practice. It is used in most everyday thermometers. The shattering of ordinary glass on being cooled rapidly is due to the thermal contraction of the outer layers, and is prevented in special glasses such as Pyrex glass or fused silica, which have low thermal expansion. The thermal expansion of components in electronic devices is important, and the difference in thermal expansion of materials in a construction can lead to grave difficulties. The coincidence of the thermal expansion of steel and concrete at normal temperatures allows the use of steel-reinforced concrete in buildings. 

Of the three commonly available types of glasses, borosilicate (Pyrex-type) glasses appear to be the most satisfactory. They combine the properties of good electrical resistance, resistance to thermal shock, and mechanical... 

In borosilicate glasses, BjOj acts as a flux for silica, and when combined with alkali oxides, the thermal expansion can be controlled to match various materials. Alkali borosil-icates tend to phase-separate. Alumina helps to reduce this tendency. A typical composition (alkali borosilicate) is SlSiOj ISBjOj 4Na20 - 2AI2O3 (wt%). This is close to that of Corning code 7740, used for Pyrex brand products. Its approximate properties are listed in Table 6.2. 

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