What is Physical Characteristics of Borosilicate Glass
The common type of borosilicate glass used for laboratory glassware has a very low thermal expansion coefficient (3.3 × 10−6 K−1),about one-third that of ordinary soda-lime glass. This reduces material stresses caused by temperature gradients, which makes borosilicate a more suitable type of glass for certain applications (see below). Fused quartzware is even better in this respect (having one-fifteenth the thermal expansion of soda-lime glass); however, the difficulty of working with fused quartz makes quartzware much more expensive, and borosilicate glass is a low-cost compromise. While more resistant to thermal shock than other types of glass, borosilicate glass can still crack or shatter when subjected to rapid or uneven temperature variations.
Among the characteristic properties of this glass family are:
· Different borosilicate glasses cover a wide range of different thermal expansions, enabling direct seals with various metals and alloys like [molybdum] glass with a CTE of 4,6, tungsten with a CTE around 4,0 and Kovar with a CTE around 5,0 because of the matched CTE with the sealing partner
· Allowing high maximum temperatures of typically about 500 °C (932 °F)
· Showing an extremely high chemical resistance in corrosive environments. Norm tests for example for acid resistance create extreme conditions and reveal very low impacts on glass
The softening point (temperature at which viscosity is approximately 107.6 poise) of type 7740 Pyrex is 820 °C (1,510 °F).
Borosilicate glass is less dense (about 2.23 g/cm3) than typical soda-lime glass due to the low atomic mass of boron.
The temperature differential that borosilicate glass can withstand before fracturing is about 165 °C (329 °F). This compares well with soda lime glass, which can withstand only a 37 °C (99 °F) change in temperature and is why typical kitchenware made from traditional soda-lime glass will shatter if a vessel containing boiling water is placed on ice, but Pyrex or other borosilicate laboratory glass will not.
Optically, borosilicate glasses are crown glasses with low dispersion (Abbe numbers around 65) and relatively low refractive indices (1.51–1.54 across the visible range).
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