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Fused Silica

Fused silica is nearly 100% pure silicon dioxide which is fused at high temperatures into an amorphous material. It is transparent over a wide spectral range and it has a low coefficient of thermal expansion. The thermal, mechanical and chemical stability due to its purity makes fused silica resistant to scratching and thermal shock. These properties make fused silica an excellent choice for many applications in high quality optical systems. Fused silica offers a number of advantages over glass:

  • Better ultraviolet and infrared transmission
  • Low thermal expansion coefficient
  • Wider thermal operating range 
  • High resistance to radiation darkening form high energy particles
  • Resistance to scratching

Synthetic fused silica shows transmittance variations, particularly in deep ultraviolet and infrared. These variations are related to manufacture and impurity content. In the ultraviolet, these variations have been attributed to uncontrollable fluctuations in metallic impurity content at the parts per billion level. Ultraviolet transmittance is the basis for the classifications UV grade and optical quality. The following figures show external transmittances for UV-grade and optical grade fused silica.

 

Figure 1: External transmittance curve for optical grade fused silica.

 

Figure 2: External transmittance curve for ultraviolet grade fused silica.

The table below shows the properties of a typical synthetic fused silica.

  Wavelength (um)
Refractive Index
Wavelength (um) Refractive Index  
  0.200
1.55051 1.000 1.45042  
  0.220 1.52845 1.064 1.44962  
  0.250
1.50745 1.100 1.44920  
  0.300
1.48779 1.200 1.44805  
  0.320 1.48274 1.300 1.44692  
  0.360
1.47529 1.500 1.44462
 
  0.400
1.47012 1.600 1.44342  
  0.450
1.46557 1.700 1.44217  
  0.488
1.46302 1.800 1.44087  
  0.500
1.46233 1.900 1.43951  
  0.550
1.46008 2.000 1.43809  
  0.588
1.45860 2.200 1.43501  
  0.600 1.45804 2.400 1.43163  
  0.633
1.45702 2.600 1.42789  
  0.650
1.45653 2.800 1.42377  
  0.700
1.45529 3.000 1.41925  
  0.750
1.45424 3.200 1.41427  
  0.800
1.45332 3.370 1.40990  
  0.850
1.45250 3.507 1.40566  
  0.900
1.45175 3.707 1.39936  
  Abbe Constant
67.6
 
  Density (g/cm3)
2.20
 
  Knoop Hardness
570
 
  Poisson's Ratio
0.17
 
  Young's Modulus (GPa)
72.7
 
  Coefficient of Thermal Expansion (m/m·°C)
5.5X10-7 (20°C~320°C)  
  Thermal Conductivity (W/m·°C) 1.4
 
  Softening Point (°C) 1683
 
  Annealing Point (°C) 1215
 
  Strain Point (°C) 1120
 
  Shear Wave Velocity of Sound (m/s)
3.75X103
 
  Longitudinal Wave Velocity of Sound (m/s)
5.90X103  
  Sonic Attenuation (dB/m)
Less than 11 in MHz Frequency Range
 
  Table 1:Fused Silica Properties
 

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