High-temperature acoustic loss in. AT-cut, BT-cut and SC-cut quartz resonators

The Q of a crystal is limited by several different mechanisms including the properties of the quartz itself. In as-grown quartz at high temperatures the alkali ions associated with the substitutional aluminum thermally escape from their interstitial site and drift in the Z-axis channel. These free ions cause a thermally activated loss. In swept quartz the alkali ions have been replaced with hydrogen which bonds to one of the oxygen atoms adjacent to the aluminum. The resulting Al-OH center causes a traditional loss peak at about 600 K in 5 MHz crystals. We are in the process of comparing the Li⁺ and Na ⁺ free ionloss and the Al-OH loss in AT-cut, BT-cut, and SC-cut resonators. The comparison is being made between 5 MHz 3rd blanks taken from a single bar of Toyo SQ-grade quartz that has 8-10 ppm aluminum. AT-cut and BT-cut 10 MHz 3rd crystals from single bar of Motorola quartz with 10-12 ppm aluminum are also included in the study. The original blanks were electrodiffused with the Li⁺, Na⁺ or with hydrogen to ensure presence only. one species of ion. The free-ion loss due to Li⁺ is consistently greater than that due to Na+. In the 5 MHz crystals the free-ion loss is greater for the AT-cuts, than for the BT-cuts, then for the SC-cuts (c-mode) for both Li⁺ and Na⁺. The expected 50% frequency-related reduction in the free-ion loss was observed for the 10 MHz BT-cut crystals. However, the reduction was significantly larger for the AT-cut 10 MHz crystals. SC-cut crystals are normally operated on the slow c-mode but can be run on the faster b-mode. Both the Li⁺ and Na⁺ free-ion loss for the b-mode appears to be nearly the same as for the c-mode. The AT-cut blank showed the largest 600 K Al-OH loss peak, the SC-cut (c-mode) was the next largest, followed by the BT-cut, and the SC-cut (b-mode) was the smallest