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Germanium (Ge)
Germanium (Ge)
Germanium (Ge) is a relatively hard, high-density, IR transmitting material that blocks UV and VIS wavelengths but allows IR from 2µm.
Germanium covers the whole of the 8-14 micron thermal band and is used in lens systems for thermal imaging. Germanium can be AR coated with Diamond producing an extremely tough front optic.
Germanium transmits over 45% between 2-14μm up to 45⁰C but transmission degrades slowly at 100⁰C then more rapidly above 200⁰C. Exposure to higher temperatures can lead to catastrophic failure in the material so Germanium is unsuitable for use in these conditions. Additionally, its relatively high density should be considered where weight is an issue. Germanium has a hardness of HK780, slightly higher than GaAs with which it shares similar mechanical properties.
Typical applications for Germanium include thermal imaging where the material can be used as a front optic while its index of refraction makes Germanium useful for wide-angle lenses and microscopes. Additionally, Germanium components can be used for FLIR (Forward Looking Infrared) and FTIR (Fourier Transformed Infrared) spectroscopy systems, alongside other analytical instruments.
Germanium (Ge) is a high index material that is used to manufacture Attenuated Total Reflection (ATR) prisms for spectroscopy. Its refractive index is such that Germanium makes an effective natural 50% beamsplitter without the need for coatings. Germanium is also used extensively as a substrate for production of optical filters.
Notes
Germanium is grown using the Czochralski technique by a small number of manufacturers in Belgium, USA, China and Russia. The refractive index of Germanium changes rapidly with temperature and the material becomes opaque at all wavelengths a little above 350K as the band gap floods with thermal electrons.
Properties of germanium glass:
| Transmission Range : | 1.8 to 23 μm |
|
Refractive Index : |
4.0026 at 11 μm |
|
Reflection Loss : |
53% at 11 μm (Two surfaces) |
|
Absorption Coefficient : |
<0.027 cm-1 @ 10.6 μm |
|
Reststrahlen Peak : |
n/a |
|
dn/dT : |
396 x 10-6 /°C |
|
dn/dμ = 0 : |
Almost constant |
|
Density : |
5.33 g/cc |
|
Melting Point : |
936 °C |
|
Thermal Conductivity : |
58.61 W m-1 K-1 at 293K |
|
Thermal Expansion : |
6.1 x 10-6/°C at 298K |
|
Hardness : |
Knoop 780 |
|
Specific Heat Capacity : |
310 J Kg-1 K-1 |
|
Dielectric Constant : |
16.6 at 9.37 GHz at 300K |
|
Youngs Modulus (E) : |
102.7 GPa |
|
Shear Modulus (G) : |
67 GPa |
|
Bulk Modulus (K) : |
77.2 GPa |
|
Elastic Coefficients : |
C11=129; C12=48.3; C44=67.1 |
|
Apparent Elastic Limit : |
89.6 MPa (13000 psi) |
|
Poisson Ratio : |
0.28 |
|
Solubility : |
Insoluble in water |
|
Molecular Weight : |
72.59 |
|
Class/Structure : |
Cubic Diamond, Fd3m |
Refractive Index:
No = Ordinary Ray
| µm | No | µm | No | µm | No |
|
2.058 |
4.102 |
2.153 |
4.0919 |
2.313 |
4.0786 |
|
2.437 |
4.0708 |
2.577 |
4.0609 |
2.714 |
4.0562 |
|
2.998 |
4.0452 |
3.303 |
4.0369 |
4.258 |
4.0216 |
|
4.866 |
4.017 |
6.238 |
4.0094 |
8.660 |
4.0043 |
|
9.720 |
4.0034 |
11.04 |
4.0026 |
12.00 |
4.0023 |
|
13.02 |
4.0021 |
