Acoustic loss in cultured quartz

Abstract

The Q of a quartz resonator is limited by the resonator design, the mounting structure and electrodes, the surrounding atmosphere, and the quartz itself. The loss of all quartz resonators is limited by the interaction between the active vibration and the thermal phonon bath. This intrinsic loss limits the Q of 5 MHz overtone AT-cut crystals to about 3 million. Defects cause additional losses. Of these, substitutional aluminum with its charge compensating-ion is the most important. The Al-Li center which dominates in unswept cultured quartz does not contribute any loss. However, the Al-Na center which is present in quartz grown without the addition of lithium to the mineralizer causes a strong loss at 54 K in 5 MHz crystals. The Al-OH center which is present in swept quartz (and after irradiation) has a loss peak near 600 K. Low-temperature loss peaks are associated with the Al-hole center which is produced by irradiation or by vacuum electrolysis. The Al-Na loss in a BT-cut resonator is approximately one-fifth that of an equivalent AT-cut while SC-cuts are intermediate. The Al-Na is also observed in low-frequency bar resonators. These results suggest that the Al-Na defect couples to all modes. At high temperatures a strong rapidly-increasing thermally-activated loss is observed. This high temperature loss is primarily due to alkali ions that have thermally escaped from the Al-site. Other defect centers also cause loss in crystals. The OH-growth defects do not have an associated loss. However, there are alkali-compensated versions of these defects which have loss peaks just above room temperature. Sweeping converts the alkali versions into OH-growth defects and increases the mechanical Q.