Thermal shock can damage materials due to uneven expansion caused by sudden temperature changes. Glass and ceramics are vulnerable, but can be improved with stronger materials or reduced expansion. Pyrex uses boron to prevent shock. Testing is done in thermal shock chambers.
Thermal shock describes how certain materials are subject to damage when exposed to a sudden change in temperature. Glass and some other materials are vulnerable to this process, in part because they don’t conduct heat energy very well. This is easily observed when a hot glass is exposed to ice water – the result is cracked, cracked or even shattered glass.
The damage is a reaction to a rapid and extreme temperature fluctuation, but the process is a little more complicated than that. Shock is the result of a thermal gradient, which refers to the fact that the temperature change occurs unevenly. The change in temperature causes the molecular structure of an object to expand, due to the weakening of the bonds that hold the molecules together. The existence of the thermal gradient means that this expansion occurs unevenly and glass in particular is very vulnerable to this process.
In the hot glass example, this means that the rapid temperature change causes some parts of the glass to quickly become much hotter than other parts. This, in turn, causes uneven expansion, which puts stress on the molecular structure. If the stress becomes great enough, the material’s strength is exceeded and the glass breaks.
Ceramics and glass are both vulnerable to this process, in part because they are not good conductors of thermal energy, and also because they do not have high tensile strength. Even so, these materials are often used for applications where temperature extremes are common, because they have very high melting points. The problem then becomes how to prevent thermal shock while maintaining the extreme temperatures required by the process.
The impact resistance of glass and ceramics can be improved by improving the strength of the material or by reducing its tendency for uneven expansion. An example of success in this sector is Pyrex®, the brand name of a type of glass best known to consumers as cookware, but also used for the production of laboratory glassware. The type of glass traditionally used to make Pyrex® is called borosilicate glass, due to the addition of boron, which prevents shock by reducing the glass’s tendency to expand.
When materials need to be tested for their ability to withstand extreme temperatures, they are tested inside a thermal shock chamber. Inside the chamber, they are exposed to rapid cycling of extremely hot and cold temperatures, to determine the temperatures at which the material’s tensile strength is exceeded. This type of testing is used in a wide variety of industries, including the development of land, air and space vehicles, as well as industrial manufacturing.
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