One of the most important thermal properties of fused quartz is its extremely low coefficient of thermal expansion: 5.5 x 10-7 /°C (20-320 °C). Its coefficient is 1/34 that of copper and only 1/7 of borosilicate glass. This makes the material particularly useful for optical flats, mirrors, furnace windows and critical optical applications which require minimum sensitivity to thermal changes.
A related property is its unusually high thermal shock resistance. For example, thin sections can be heated rapidly to above 1500 °C and then plunged into water without cracking.
Effects of Temperature
Fused quartz is a solid material at room temperature, but at high temperatures, it behaves like all glasses. It does not experience a distinct melting point as crystalline materials do, but softens over a fairly broad temperature range. This transition from a solid to a plastic-like behavior, called the transformation range, is distinguished by a continuous change in viscosity with temperature.
Viscosity is the measure of the resistance to flow of a material when exposed to a shear stress. Since the range in “flowability” is extremely wide, the viscosity scale is generally expressed logarithmically. Common glass terms for expressing viscosity include: strain point, annealing point, and softening point, which are defined as:
- Strain Point: The temperature at which the internal stress is substantially relieved in four hours. This corresponds to a viscosity of 1014.5 poise, where poise = dynes/cm2 sec.
- Annealing Point: The temperature at which the internal stress is substantially relieved in 15 minutes, a viscosity of 1013.2 poise.
- Softening Point: The temperature at which glass will deform under its own weight, a viscosity of approximately 107.6 poise. The softening point of fused quartz has been variously reported from 1500 °C to 1670 °C, the range resulting from differing conditions of measurement.
Devitrification and particle generation are limiting factors in the high temperature performance of fused quartz. This is a two step process of nucleation and growth. In general, the devitrification rate of fused quartz is slow for two reasons: the nucleation of the cristobalite phase is possible only at the free surface, and the growth rate of the crystalline phase is low.
Nucleation in fused quartz materials is generally initiated by surface contamination from alkali elements and other metals. This heterogeneous nucleation is slower in non stoichiometric fused quartz, such as Momentive Performance Materials quartz, than in stoichiometric quartz materials.
The growth rate of cristobalite from the nucleation site depends on certain environmental factors and material characteristics. Temperature and quartz viscosity are the most significant factors, but oxygen and water vapor partial pressures also impact the crystal growth rate. Consequently, the rate of devitrification of fused quartz increases with increasing hydroxyl (OH)- content, decreasing viscosity and increasing temperature. High viscosity, low hydroxyl fused quartz materials produced by Momentive Performance Materials, therefore, provide an advantage in devitrification resistance.
The phase transformation to Beta-cristobalite generally does not occur below 1000 °C. This transformation can be detrimental to the structural integrity of fused quartz if it is thermally cycled through the crystallographic inversion temperature range (around 270 °C). This inversion is accompanied by a large change in density and can result in spalling and possible mechanical failure.
Resistance To Sag
The most significant chemical factor effecting the sag resistance of fused quartz is the hydroxyl (OH)- content. Momentive Performance Materials controls the (OH)- content in its quartz to meet the specific needs of its customers. To maximize the performance of tubes used in high temperature semiconductor processes, it is important to understand the impact of changes in diameter and wall thickness. In one study using 214 LD fused quartz tubing, it was found that the sag rate decreases as the wall thickness of the tube is increased. Generally, as the wall thickness doubles, the sag rate decreases by a factor of approximately 3. Also, it was shown that with a fixed wall thickness, the sag rate decreases as the tube diameter decreases.