Beveridge, Nicola Louise
Characterisation of silicon-silicon hydroxide catalysis bonds for future gravitational wave detectors.
PhD thesis, University of Glasgow.
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The first generation of gravitational wave detectors are currently undergoing significant upgrades to increase their sensitivity by a factor of ten. These upgrades include the installation of quasi-monolithic silica suspensions in an attempt to reduce the thermal noise of the test masses and their suspensions. Fused silica fibres are welded to fused silica interface pieces, called ‘ears’, which provide suitable welding points onto the sides of the mirror when bonded to the mirror using the high strength chemical jointing technique of hydroxide-catalysis bonding.
Plans are developing for the design of potential ‘future generation’ gravitational wave detectors. These detectors may operate at cryogenic temperatures to further reduce thermal noise. Silicon is a prime candidate material for use in the test masses and their suspensions because of its desirable thermo-mechanical properties in the cryogenic regime. With some adaptation, hydroxide catalysis bonding may also be a viable technique for use in third generation detectors; however, to evaluate its suitability it is essential to quantify both the strength of silicon-silicon bonds at cryogenic temperatures and the mechanical loss of such bonds, as this has a direct effect on the contributions of the bond to the overall thermal noise of a bonded suspension.
To make bonding of silicon components possible, the bonding surfaces must ideally have a thin coating of SiO2, with which the hydroxide can react to form the bond. In Chapters 3 and 4, the strength of hydroxide catalysis bonds between silicon blocks at room and cryogenic temperatures is investigated. Chapter 3 investigates the minimum required thickness of SiO2 necessary for a successful bond. The bond strength, tested using a 4-point bend strength test, is found to reduce significantly with oxide layer thicknesses below 50 nm at cryogenic temperature. A Weibull analysis of the results showed a characteristic strength of approximately 41MPa at 77K and 35MPa at room temperature for samples with a minimum oxide layer of 50 nm. In chapter 4 the effect on the oxide layer deposition method and the purity of the silicon ingot on the strength of the bond are studied. Bend strength tests were performed on hydroxide-catalysis bonds formed between silicon samples of different crystallographic orientation and purity that had been oxidised using a range of methods. The three methods used were; dry thermal oxidation, ion beam sputtering and e-beam deposition. It was found that the method used influenced the strength of the resulting bond, with the e-beam deposited layers producing the weakest samples. It is postulated that the reason for the lower strength of the e-beam samples is correlated with the lower density of this type of coating compared with other coating methods.
The mechanical loss of the bond between silicon cantilevers between 10K and 250K was measured in Chapter 5. The experimental set up is described, the results are presented and then analysed to establish an upper limit of 0.12 for the second bending mode below 100K. The lowest loss measured was 0.06 at 12K.
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