3 rd ILIAS-GW Annual General Meeting – Oct 2006

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

2. Investigations into hydroxy-catalysis bonding 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.

3. 10-21 10-22 10-23 10-24 h Investigations into hydroxy-catalysis bonding eg. LIGO AdvLIGO 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. Wire loop arm length: 4 km Quadruple stagesilica ribbons/fibres h Design sensitivity curves for the LIGO and AdvLIGO detectors. Design sensitivity curves for the Virgo and Advanced Virgo detectors.

4. 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 SC mount SC mirror Picture of silicon carbide mirrors bonded to a silicon carbide base mount for GAIA

5. The processes involved in hydroxy-catalysis bonding 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.

6. The processes involved in hydroxy-catalysis bonding 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.

7. 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. 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. 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 • 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. Plot of pH (calculated pH = 14 − log10[OH]) against settling time, using NaOH.

10. Hydroxy-catalysis bonding dependence on temperature • 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: Picture of temperature controlled environment constructed for carrying out hydroxy-catalysis bonding. Schematic diagram of temperature controlled environment constructed for carrying out hydroxy-catalysis bonding.

11. Hydroxy-catalysis bonding dependence on temperature • 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. • settling time increased approx 4.5 when cooled ~ 20o. • Large variation in settling time at each temperature, up to 50%. • A better controlled measurement of settling time is desirable. 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.

12. 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 ~ ± 500mm) decreased due to the friction in the bonding solution increasing until the final “grab” point. Picture of shaking and readout scheme used to measure the settling time for hydroxy-catalysis bonds. Schematic of the sample and the mechanical oscillation used for measuring the settling time for hydroxy-catalysis bonds.

13. Hydroxy-catalysis bonding dependence on temperature 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

14. Hydroxy-catalysis bonding dependence on temperature • Interpretation: • The Arrhenius equation (1st order): • We can say that the reaction rateis 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 Ea/kB

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

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

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

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: 0.4ml cm-2, 0.2ml cm-2 and 0.1ml 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 = 25.4mm 12.7mm

19. 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) SiO2 Si Silicon-silica hydroxy-catalysis bond