Abstract

Ideal piezoelectric material should possess the following four properties: (1) high piezoelectric coupling, (2) low acoustic loss (high Q), (3) temperature compensation and (4) capable to produce large crystals at low cost. To find any crystal which can meet all these requirements is a the challenge. Recent work from Russia, Japan and France shows that LGS and some of its isomorphs (such as LGN and LGT) can satisfy all these requirements and thus stir a lot of interest. Since these crystals can be grown directly from the melt by the conventional Czochralski pulling method, the availability of low cost large size crystals is a possibility. Melt composition control is the most critical parameter to adjust. Moreover, the melt has small selective evaporation and the composition does change with time. It introduces inclusions and limits the crystal yield. Crystal cracking is a more serious problem which, we believe, is primarily due to twinning. Based on the P321 space group symmetry, the crystal is expected to have electrical twins. Using chemical etching, twinning domains are observed. To grow single domain crystal, it needs very tight control of the growth process free from any thermal or mechanical perturbation. Using our newly developed adaptive control system and full power UPS back up, we are able to grow high quality single domain LGS crystals and its isomorphs with material yield better than 80%. Typical crystal size is 50 mm in diameter and 120 mm in length using a small 75 mm diameter crucible. The crystal produced is essentially transparent for all three compositions. This is in contrast to the typical dark brown color crystals available in the market. The growth of large diameter LGS crystals would depend entirely on the ability to minimize the thermal mechanical stress and thus avoid the formation of twinning.

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