1. 3rd ILIAS-GW Annual General Meeting – Oct 2006 Investigations into hydroxy-catalysis bonding S. Reid

2. 3rd ILIAS-GW Annual General Meeting – Oct 2006 Current applications Originally developed for NASA’s Gravity Probe B mission, launched April 2004. (Gwo et al.) GEO600 currently operates with quasi- monolithic fused silica suspensions and mirrors. This technology allows improved thermal noise in the suspension systems. Construction of the ultra-rigid, ultra- stable optical benches for the LISA Pathfinder mission. Picture of a GEO600 sized silica test mass in Glasgow with silica ears jointed using hydroxy-catalysis bonding Silica fibres are welded to the ears in the completion of the lower-stage of the GEO600 mirror suspension. Investigations into hydroxy-catalysis bonding

3. 3rd ILIAS-GW Annual General Meeting – Oct 2006 Investigations into hydroxy-catalysis bonding Planned applications The planned upgrades for AdvLIGO and Advanced VIRGO plan to incorporate the GEO600 technology for significantly improved thermal noise performance (in addition to other improvements, e.g higher power lasers). Construction of the ultra-rigid, ultra-stable optical benches for LISA. Design sensitivity curves for the LIGO and AdvLIGO detectors. 10 -21 10 -22 10 -23 10 -24 h Wire loop arm length: 4 km Quadruple stage silica ribbons/fibres eg. LIGO AdvLIGO Design sensitivity curves for the Virgo and Advanced Virgo detectors. h

4. 3rd ILIAS-GW Annual General Meeting – Oct 2006 Catalysis bonding settling time dependence Motivation Many optical systems layouts have stringent requirements for strength, rigidity, stability and alignment. Hydroxy-catalysis bonding fulfills all these requirement. One possible disadvantage of this technique is that the time taken for a typical bond to “set” at room temperature is in the region of a few tens of seconds. This only allows a short period of time in which to align the various components on the optical bench. Therefore need investigate ways to extend the bonding time to better aid the precise pre-alignment of the optical components Picture of silicon carbide mirrors bonded to a silicon carbide base mount for GAIA SC mirror SC mount

5. 3rd ILIAS-GW Annual General Meeting – Oct 2006 Step 1 of 3 – Hydration of silica surfaces to be bonded The surface of silica is hydrophilic and will attract OH− ions to fill any open bonds from the silica (hydration). A simplified, 2D schematic of the surface of silica when hydrated. The processes involved in hydroxy-catalysis bonding

6. 3rd ILIAS-GW Annual General Meeting – Oct 2006 Step 2 of 3 – Etching of the silica surfaces to be bonded Placing a solution with a high concentration of OH− ions on the surface of silica causes etching to take place. Free OH − ions form weak bonds with silicon atoms on the substrate surface causing the original lattice bonds to weaken It becomes possible for the silicate molecule to break away from the bulk structure, producing Si(OH) − 5 molecules in solution. The processes involved in hydroxy-catalysis bonding

7. 3rd ILIAS-GW Annual General Meeting – Oct 2006 The processes involved in hydroxy-catalysis bonding Step 3 of 3 – Polymerisation of silicate in solution However, below pH 11, the silicate ion hydrolyses to soluble Si(OH) 4 and OH −. When the concentration of Si(OH) 4 molecules reaches 1  2%, the solution polymerises and becomes “rigid” (R.K. Iler, 1979, The Chemistry of Silica). Si(OH) 4 is a monomer which likes to form a polymer arrangement: < pH 11 > pH 11

8. 3rd ILIAS-GW Annual General Meeting – Oct 2006 A definition of settling time Polymerisation – a definition for settling time If the rate at which the conversion of Si(OH) − 5  Si(OH) 4 and the resultant polymerisation is fast compared to the rate of etching, then the settling time can be approximately defined as the time taken for the pH of the bonding solution to drop to pH=11. Perhaps counter intuitively, the settling time will increase as the concentration of OH− ions increases.  The greater the concentration of OH− ions in solution, the longer the etching process continues.

9. 3rd ILIAS-GW Annual General Meeting – Oct 2006 Catalysis bonding dependence on hydroxide concentration Using the Rate Law for chemical reactions, where k is the rate constant, [X] is the concentration of substance X and  is known as the order of the reaction. The hydroxy-catalysis reaction can be approximated a first order reaction (  =1), the reaction is dependant only on the OH− ion availability/concentration Plot of pH (calculated pH = 14 − log 10 [OH]) against settling time, using NaOH. Two bonds taken at each concentration. The “grab” point of the bonding was monitored manually and by using a stop-clock. Linearity over the pH range ~ 12.5  14. Intercept (polymerisation point) at pH  12.4 Strong evidence from initial results that, for given concentrations of hydroxide, the settling time may be defined by the time taken to reach a given pH value. Therefore, settling time may be extended by increasing [OH] concentration.

10. 3rd ILIAS-GW Annual General Meeting – Oct 2006 A different way to increase the settling time is to lower temperature at which bonding occurs. Chemical reaction tend to follow the Arrhenius equation, exhibiting an exponential dependence on the temperature of the reactants: Schematic diagram of temperature controlled environment constructed for carrying out hydroxy-catalysis bonding. Picture of temperature controlled environment constructed for carrying out hydroxy-catalysis bonding. Hydroxy-catalysis bonding dependence on temperature

11. 3rd ILIAS-GW Annual General Meeting – Oct 2006 A preliminary set of measurements of the settling time for bonding measured by continually moving the bonding silica samples manually. Two bonds were made at each temperature. Plot of settling time as a function of temperature for the hydroxy-catalysis bonding technique when using 0.23 mol l -1 NaOH solution (9.2g NaOH per litre of water, a typical concentration used by Gwo et al.) with exponential fit. settling time increased approx 4.5  when cooled ~ 20 o. Large variation in settling time at each temperature, up to 50%. A better controlled measurement of settling time is desirable. Hydroxy-catalysis bonding dependence on temperature

12. 3rd ILIAS-GW Annual General Meeting – Oct 2006 Hydroxy-catalysis bonding dependence on temperature The readout scheme - the upper piece of silica was held and an oscillating force applied to it by means of a loud speaker and elastic band. This upper sample was then brought into contact with the lower silica sample, which was rigidly clamped. The bonding solution was carefully place between the surfaces as they were brought into contact. During bonding, the amplitude of the upper silica sample’s motion (initially ~ ± 500  m) decreased due to the friction in the bonding solution increasing until the final “grab” point. Schematic of the sample and the mechanical oscillation used for measuring the settling time for hydroxy-catalysis bonds. Picture of shaking and readout scheme used to measure the settling time for hydroxy-catalysis bonds.

13. 3rd ILIAS-GW Annual General Meeting – Oct 2006 Results: An average of 8  10 bonds performed at each temperature point (a total of 69 bonds). However, sometimes the bonding solution did not apply correctly to the silica surfaces (due to confined space and restricted visibility of samples within the setup used) and ~25% of the results were null/excluded. Plot of settling time as a function of temperature for the hydroxy-catalysis bonding technique when using 0.1 mol l -1 KOH solution Hydroxy-catalysis bonding dependence on temperature

14. 3rd ILIAS-GW Annual General Meeting – Oct 2006 Interpretation: The Arrhenius equation (1 st order): We can say that the reaction rate is inversely proportional to the settling time: Rearrange to give: And taking the natural logarithm: Therefore a plot of ln[settling time] versus 1/T should yield a straight line with gradient E a /k B Hydroxy-catalysis bonding dependence on temperature

15. 3rd ILIAS-GW Annual General Meeting – Oct 2006 Interpretation contd: The results obtained using the controlled motion and readout scheme (KOH results) follows closely to the previous analysis. The room temperature (25 o C) point differs. This temperature point was taken outside of the built enclosure, perhaps indicating that some unintentional change occurred in the experimental setup. The natural log of the settling time plotted as a function of 1/T when using 0.23 mol l -1 NaOH solution. The natural log of the settling time plotted as a function of 1/T when using 0.1 mol l -1 KOH solution. 25 o C E a = 0.597 eV per molecule of OH − E a = 0.545 eV per molecule of OH − Hydroxy-catalysis bonding dependence on temperature

16. Silicate bonding of silicon. cleaned silicon samples placed in furnace at 1000°C after ~1hr, 50 to 100nm oxide growth Preparation of silicon samples to be bonded: surface flatness ≤ /10, surfaces thoroughly cleaned from contaminants prior to bonding and sufficient oxidisation for the silicate bonding.

17. Strength testing of Si-Si silicate bonds Si-Si bonded sample for testing Si-Si sample placed in clamp 40Kg load suspendedSi-Si sample under load 40Kg clamped sample sample clamp wire loop rubber ring lightly clamped

18. Thermal conductivity of Si-Si silicate bonds The first set of silicate bonded silicon-silicon have been fabricated with varying volumes of 1:6 sodium silicate solution at Glasgow. Samples sent to Florence for thermal conductivity measurements. Volumes of bonding solution: 0.4  l cm -2, 0.2  l cm -2 and 0.1  l cm -2. (Advanced LIGO specification) Mechanical strength at low temperature to be carried out between Glasgow and the Rutherford Appleton Lab in the UK. diameter = 25.4mm12.7mm

19. 3rd ILIAS-GW Annual General Meeting – Oct 2006 Investigations into hydroxy-catalysis bonding Future work includes: Further investigation of bonding samples with a ground finish. - align optics without danger of optical contacting - no time constraint on achieving alignment - apply bonding solution with pieces in situ Silicon-silicon bonded samples for use in monolithic silicon suspensions. - mechanical strength (already passed initial T cycling to 77K) - thermal conductivity (& at low T) - mechanical loss (& at low T) Silicon-silica hydroxy-catalysis bond SiO 2 Si