High-temperature acoustic loss in thickness-shear mode quartz resonators

Sensor and other applications are causing an increased use of quartz resonators at high-temperatures. The Q of a quartz resonator is limited by a number of different mechanisms including the properties of the quartz itself. The Q of all quartz resonators is limited by interaction between the active mode and the thermal phonon bath. Above cryogenic temperatures this intrinsic thermal loss is nearly independent of temperature. Defect and impurities in the quartz also limit the Q of a crystal. At high temperatures alkalis thermally escape from their original interstitial site adjacent to the substitutional aluminum drift randomly along the open Z-axis channel. The release and drift of these ions causes a thermally activated loss which increases rapidly with temperature. In swept quartz the alkalis have been replaced by hydrogen which bonds to an oxygen atom adjacent to the aluminum. The free-ion loss is not present in these swept samples. Instead, the Al-OH center produces a traditional loss peak at about 600 K. We have compared the lithium and sodium free ion-loss and the Al-OH loss in AT-cut, BT-cut, and SC-cut resonator blanks prepared from the same bar of quartz. For Li-compensated samples the thermally-activated free-ion loss is largest in the AT-cut sample and smallest in the SC-cut with the BT intermediate. The AT-cut Na-compensated blank showed the expected free-ion loss that was slightly smaller than that seen in the Li-compensated blank. The Na-compensated SC and BT-cut samples had an unusual loss that increased with T above about 400 K. The 600 K Al-OH peak in the H-compensated SC-cut blank was about 1/3^rd the strength of the peak in the AT-cut blank. The H-compensated BT-cut blanks behaved strangely and further investigation is needed