Fused silica and fused quartz: experience from GW projects

Transcription

1 Fused silica and fused quartz: experience from GW projects K.A. Strain May 2006

2 contents introduction to fused silica/quartz mechanical/thermal properties polishing/flatness/stability optical properties suppliers and cost guidance

3 fused quartz (transparent) fused in electrically heated furnaces selected grains of natural quartz are melted and cooled rapidly enough to form a glass properties that differ from fused silica variable UV transmission due to metallic impurities most abundant are usually Na, K etc. naturally low OH, so good IR transmission cheaper in lowest grades

4 fused silica from combustion of a purified, silicon-containing gas resulting soot is then fused and processed extreme chemical purity possible (except usually OH, which can be up to 1000 ppm) properties that differ from fused quartz reliable UV transmission OH absorption lines in NIR up to 0.1% OH cost of basic material is higher quality control tends to be very good

5 mechanical properties defects the most common defects are small bubbles, especially in the cheapest fused quartz grades low density 2201 kg/m 3 low mechanical loss (surface dominated) not very stiff - Young's modulus 73 Gpa strong, hard, non porous, somewhat brittle at sharp edges

6 thermal/electrical properties low expansion 5.5 x 10-6 /K at room temperature resistant to thermal shock dimensionally stable (retains alignment well) can make mounting in metal holders challenging very good insulator (surface contamination often dominates conduction) very low electrical loss dielectric constant 3.82

7 figuring/stability figure standard quality is typically in the range from 100 nm ~ 1000nm rms over relatively large areas best possible figure is ~1nm rms over ~0.1 m 2 limited by metrology and/or cost but consider the gravitational sag, if flatness matters flatness is most simply obtained with pieces of thickness at least 10% of their diameter (initial figuring gets more expensive for thin pieces)

8 polishing/coating silica is one of the easiest materials to polish standard polishes are down to about ~1nm rms roughness (on mm scales) yields quite low total integrated scatter (<0.1% UV) best superpolishing goes down to <0.1 nm surface roughness, when the scattering is only a few ppm in the UV (but expensive) all the usual optical coating methods work well on silica

9 radiation hardness fused silica has good radiation hardness fused quartz can darken on exposure depends very much on precise balance of impurities, so synthetic material is more reliable in this respect I am not an expert in this area!!!

10 optical properties refractive index ~1.5 (visible) homogeneous (ppm) low birefringence

11 optical properties: transmission pure silica has a very wide transmission band (~180nm to ~3.5 µm) within this band transmission is limited by Rayleigh scattering. This is a few ppm/cm in the red, and scales as λ -4 OH limits the IR transmission (predicable absorption lines at 1.5, 2.2 and 2.7 µm ) metallic impurities limit UV transmission (variable in low cost materials)

12 UV transmission: fused silica there is usually a smooth UV cutoff best UV grades are 90% transmitting for 10mm thickness at 180 nm opaque at 160 nm black ideal transmission blue total transmission

13 UV transmission: fused quartz example herasil: relatively good natural material

14 suppliers - synthetic most commonly used in the GW work: Corning (7980 several grades) Heraeus (Suprasil several grades) both provide very high quality materials, often exceeding specifications others definitely worth considering: Saint Gobain (Spectrosil) GE (type 012)

15 suppliers - natural GE (largest manufacturer?) ingots up to 1.8m dia. (Types 124/144) Heraeus (Herasil several grades, HOQ300 etc.) Saint Gobain (Vitreosil) there are many others check quality very carefully if looking for the very cheapest options

16 synthetic cost guide from ~$1.5k/kg to >$3k/kg, for unfinished pieces cut to size polishing costs are very dependent on size, shape and required quality (superpolishing can be expected to exceed material costs, perhaps by a large margin) coating costs depend very much on required quality and size natural much wider range of cost for bulk materials from about $500/kg to more than the most expensive synthetic materials (e.g. Homosil)

17 reference most manufacturers now have most info on the web except for routine materials we normally approach finishing/polishing contractors and request they obtain materials to our specifications, allowing them to choose the material for special requirements we usually contact local representatives of the manufacturer who are better able to discuss options

18 40kg suprasil mirrors (1nm figure) supported on 4x0.1mm x1mm silica ribbons welded to silica ears bonded on with... hydrated silica gw mirror suspensions