1Research Laboratories, JFE MINERAL COMPANY, LTD., Chiba, Japan
In recent years, piezoelectric single crystals based on Pb(Mg1/3Nb2/3)TiO3 (PMN-PT) have been used for ultrasonic medical transducers to provide better image quality. Commercially PMN-PT single crystals are grown by conventional Bridgman technique. A remaining issue in the crystal growth is the composition variation along the growth direction due to the segregation phenomena. As the variation results in large fluctuations of the dielectric / piezoelectric properties, the crystal portion with the desired properties is only 30% of the crystal length. The low yield in the crystal growth directly leads to high cost for PMN-PT single crystals. Our objective is to improve the composition uniformity and extend the portion which has desired properties.
Theoretically, continuous feed growth is a promising technique to eliminate the composition variation. Its main feature is to feed raw material into the melt during crystal growth. If the composition change due to segregation can be compensated by fed raw material, the melt composition should stay the same during growth and the grown crystal should have uniform composition. In this study a continuous feed mechanism was installed to a vertical Bridgman furnace and PMN-PT single crystals were grown by the continuous feed technique. The composition and the properties were evaluated.
PMN-PT single crystals of 80 mm in diameter and 200 mm in length have been successfully grown by the continuous feed technique. The crystals show homogeneous composition distribution along the growth direction. TiO2 concentration variation was only 6% in 80% of the crystal length as shown in fig. 1. As a result of homogeneous composition, the crystal shows uniform and desired properties. The dielectric constants (Kr) and the piezoelectric constants (d33) were 5200 - 6000 and 1300 - 1700 pC/N, respectively.
PMN-PT single crystals with uniform properties were successfully grown by the continuous feed technique. The uniform property portion was 80% of the crystal length and much larger than that typically produced using the conventional Bridgman growth method. So the continuous feed technique can deliver more uniform quality and improve the cost-effectiveness of PMN-PT single crystals.
