New materials for the next generation of criogenic gravitational wave detectors

The research work developed in the present PhD thesis deals with the measurement of mechanical dissipations at low temperature of silicon and silicon carbide, as candidate materials in the carrying out the sensitive mass of the next generation gravitational wave detector DUAL. Measurements are performed at the ultra-cryogenic Test Facility located in the same building that houses the gravitational waves detector AURIGA, at Legnaro National Laboratories (LNL), INFN, Padova, Italy. Few ultra-cryogenic measurements are performed at Low Temperature Laboratories, Physics Department, University of Trento, Italy, because in the meantime the LNL dilution refrigerator was under upgrading for the next (2008) ultra-cryogenic run. Early the Ultra- Cryogenic Transducer Test Facility was designed in order to measure the thermal noise of transduction and amplifcation chains for acoustic detectors of gravitational waves; it allowed to test the whole capacitive readout of the AURIGA detector (i.e. capacitive transducer + double stage SQUID amplifer + resonating LC matching line) in the same environment as it will be operating in the main detector. The writer participated actively to the upgrading phase in order to rearrange this apparatus for mechanical dissipation measurements of interesting materials in samples of circular (disks) or rectangular (cantilevers) shape, as function of cryogenic and ultra-cryogenic temperature. TF has been provided for all the sensors required to perform these measurements, i.e. a piezoelectric actuator driven by high voltage amplifer, four readout and as much as charge lines for capacitive displacement sensors, a readout line for displacement measures to be performed by the optical lever method, four lines for thermometers sensitive also at low temperature, pressure sensors. Low losses clamping systems has been developed to be employed on samples of circular shapes (wafers or disks). In particular, a clamping system so called \nodal" has been developed. With this nodal suspension a thin disk is clamped on its centre by two spheres. Since for most normal modes the centre is at rest, nodal suspension should contribute in neglecting amount to total loss. This type of nodal suspension has been experimented at cryogenic temperatures in the two versions with and without glue on contact points. Such nodal suspension has been employed in loss angle measurements on inltrated and mono-crystalline ( polytypes 4H and 6H) silicon carbide disks. In particular, on mono-crystalline silicon, employing this suspension, very low loss angles has been measured (of the order of 10-8) and it was pointed out that above 40 K thermoelastic dissipation dominates. Mechanical loss measurements are performed by the so called \ring down" method (excitation and decay acquisition of the normal modes of the sample under investigation). A special capacitive displacement sensor has been designed and carried out in few prototypes for samples of dielectric material (not conductors). This sensor was carried out in two models, large and small area. It has been employed in loss angle measurements on silicon and mono-crystalline silicon carbide, reported in the present thesis. For instance, employing this sensor, the thermoelastic dissipation curve has been measured as function of temperature for a thin silicon wafer, as reported in the thesis. This sensor is still employed successfully for cryogenic measurements of mechanical dissipation. It could be evaluated the possibility that, if suitably optimized, this sensor could have a displacement sensitivity high enough to allow the measurement of thermal noise. This could be very useful, for instance in the case that DUAL detector should be carried out in a non conductor material. As concerning silicon carbide, mechanical dissipation measurements have been performed on different samples of different fabrication methods. Silicon carbide was investigated as sintered, inoltrated and 4H and 6H mono-crystalline polytypes. In the best case (6H-SiC) the measured loss angle results of the order of 10-7, unfortunately still too high to t the requirements of DUAL detector. However an improvement is awaited from thermal post processing on samples. Finally, in order to estimate if the whole mass of DUAL detector could be machined bonding together smaller silicon parts, the first measurement of mechanical dissipation as a function of temperature has been performed on a silicon wafer that was made by direct bonding technique starting from three thinner wafers. The result of these measurements is that the measured loss angle due to the (two) bonding layers is about 510-3, not far from the DUAL requirement (loss angle of the order of 10-3.