  We present the characterization of 8-12 GHz whispering gallery mode
resonators machined in high-quality sapphire crystals elaborated with different
growth techniques. These microwave resonators are intended to constitute the
reference frequency of ultra-stable Cryogenic Sapphire Oscillators. We
conducted systematic tests near 4 K on these crystals to determine the unloaded
Q-factor and the turnover temperature for whispering gallery modes in the 8-12
GHz frequency range. These characterizations show that high quality sapphire
crystals elaborated with the Heat Exchange or the Kyropoulos growth technique
are both suitable to meet a fractional frequency stability better than 1x10-15
for 1 s to 10.000 s integration times.
