CaF2values describe: CaF₂
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Why it wins
When fused silica stops transmitting: MgF2 and CaF2 open the VUV, ZnSe and Ge open the IR. Laser windows, spectroscopy ports, cryostat optical access.
Why not the alternative
Silica cuts off near 170 nm and past 3.5 um; sapphire covers much but not the VUV edge or the deep IR.
Watch out
Soft, hygroscopic edges (MgF2 less so), thermal shock limits; ZnSe is toxic to machine.
Properties
The values below are candidate: compiled from the sources named, not yet individually validated.
Provisional provenance: Kurt J. Lesker materials notes.
Wet cleaning
| Recipe | dry solvent drag-wipe only, methanol or acetone on lens tissue |
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| Forbidden | water: CaF₂ and especially KBr are soluble; ultrasonics |
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| Limit | solubility, which rules out every aqueous step |
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Vacuum and outgassing
| Outgassing, unbaked (10 h) | 1e-9 mbar·L/s/cm² |
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| Outgassing, baked | 1e-12 mbar·L/s/cm² |
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| Vapour pressure | none |
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Temperature
| Bake, assembled | 150 °C |
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| Vacuum degas | 200 °C |
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| Metallurgical limit | cleaves and cracks under thermal shock |
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Thermal
| CTE | 18.9 ppm/K |
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| Thermal conductivity | 9.7 W/m·K |
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| Specific heat | 854 J/kg·K |
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| Melting / softening | 1418 °C |
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Mechanical
| Strength | 36 flex MPa |
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| Elongation | nil |
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| Young's modulus | 76 GPa |
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| Hardness | 160 HV |
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| Density | 3.18 g/cm3 |
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Electrical and magnetic
| Relative permeability | 1.0 |
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| Resistivity | >1e14 Ω·cm |
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Engineering
| Corrosion | water-soluble to varying degrees by material |
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| Joining | indium or elastomer sealed, never brazed |
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| Process notes | high CTE and low strength make sealing the hard part |
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| Availability and cost | optics suppliers, standard sizes |
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Grades
| MgF2 VUV grade | transmits to 120 nm: excimer and Lyman alpha |
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| CaF2 excimer grade | 193 nm lithography and UV laser duty |
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| ZnSe CVD | CO2 laser and broadband IR |
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| Ge | long-wave IR, opaque visible |
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Sources · 1
- Kurt Lesker viewport materials notes
← The Matter