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Crystran's Guide to thermal shock in optical materials

01st Nov 2020

Read our Novemeber 2020 newsletter. 

 

 
Crystran's Guide to thermal shock in optical materials
Crystalline optical materials vary widely in how they may be treated thermally, particularly in regards to allowable rates of heating / cooling to avoid the onset of cleaving. In addition to material type, the size and shape of the component plays a large factor in this assessment.
For very large pieces >1kg of CaF2 (Grade 2) we would use a rate of 10ºC/hour
Smaller pieces may be heated and cooled at higher rates. Given differences in lattice strength, there is only a limited correlation between allowable temperature gradient and thermal conductivity / expansion coefficients. This assessment is based purely on our practical experience of how these different materials behave when we cut and polish them in our dedicated factory.
Grade 1. Natural cooling rate on removal from oven, ~ 5ºC/min No particular precautions.
Grade 2. Slightly Sensitive to thermal shock. ~ 2ºC/min (Cool over 1 to 2 hours from 150ºC)
Grade 3. Sensitive to thermal shock. ~ 0.75ºC/min (Cool over 3 to 4 hours from 150ºC)
Grade 4. Very sensitive to thermal shock. ~ 0.3ºC/min (Cool overnight. 7 to 8 hours from 150ºC)
As a general guide, the following recommendations are based on circular windows, 50mm diameter and 6mm thick.
MaterialSymbolThermal conductivity
W m-1 K-1
Thermal Expansion
x 10-6 K-1

Max Temp °C
Crystran Shock Grade
Barium Fluoride BaF2 11.7 18 300 IV
Calcium Fluoride CaF2 9.71 19 500 III
Lithium Fluoride LiF 11.3 37 400 IV
Magnesium Fluoride MgF­2 21/33 13.7/8.9 500 II
Strontium Fluoride SrF2 8.3 18 500 III
           
Sodium Chloride NaCl 6.49 44 400 II
Sodium Fluoride NaF 3.75 36 400 III
Potassium Bromide KBr 4.82 43 300 III
Potassium Chloride KCl 6.53 36 400 II
           
Cesium Bromide CsBr 0.94 48 400 I
Cesium Iodide CsI 1.1 48 200 I
           
Silver Bromide AgBr 1.21 30 200 I
Silver Chloride AgCl 1.15 31 200 I
           
Thallium Bromo-Iodide KRS-5 0.54 58 200 I
           
Magnesium Oxide MgO 42 10.8 2000 I
Sapphire Al2O3 27 5.6 1700 I
Crystal Quartz SiO2 10.7/6.2 7.1/13.2 1000 I
           
Zinc Sulfide (Multispectral) ZnS 27.2 6.5 250 I
Zinc Selenide ZnSe 18 7.1 250 I
           
Diamond C 2600 1 700 I
Silicon Si 163 2.6 150 I
Germanium Ge 58 6.1 80 I
Gallium Arsenide GaAs 48 5.7 200 II
Maximum working temperatures are Crystran recommendations only and not related to melting points.
 
 
 This data is for advice only. Crystran Ltd cannot be responsible for any damage caused as a result of using this data sheet.
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