Margitta Hengst

Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La3Ga5SiO14 (LGS), La3Ga5.5Nb0.5O14 (LGN) and La3Ga5.5Ta0.5O14 (LGT) II. Piezoelectric and elastic properties

Mass densities, relative dielectric constants, and piezoelectric material parameters were determined on single crystals of LGS, LGN, and LGT grown with high structural perfection by the Czochralski technique. Optimized values of the elastic stiffnesses and elastic compliances were derived by measuring and critically comparing the propagation velocities of both bulk (BAW) and surface acoustic waves (SAW).

REECa4O(BO3)3 (REECOB): New Material for High-Temperature piezoelectric applications

Especially in terms of the modern environmental consciousness with demand for more efficient, cleaner and more ecological machinery, processes has to be improved. This is mainly essential for combustion engines, coal fired electrical plants as well as gas heating installations etc. Processes have to be measured and controlled in situ in order to increase efficiency by manipulating parameters. Doing so, sensors which can resist and work under high temperature/high pressure conditions are strongly required. Unfortunately, most piezoelectric sensor materials known today (like quartz, Bi4Ti3O12-and PZT-ceramics) can not proceed at elevated temperatures above 600°C. Others that can, are either expensive and energy-consuming in production (GaPO4) or only available in sufficient size as naturally occurring minerals (tourmaline) with fluctuating properties. In the last few years, the new material REECOB (REECa4O(BO3)3 with REE = rare earth elements: Gd, Y, La, Sm, Nd) emerged to be a promising candidate for high temperature applications, displaying constant piezoelectric properties up to 1,200°C. This material has made a steep career in optical applications (laser host material, nonlinear optics) since the mid-1990s but good properties for sensing applications are only known for a few years (Shimizu et al. 2004; Markiewicz et al. 2006). Investigations on ultra high temperature properties are relatively new (Zhang et al. 2008a, b, c). Unfortunately, thermomechanical data of the material are very rare and partially contradictional.

High temperature piezoelectric single crystals: Sr 3 NbGa 3 Si 2 O 14 , Sr 3 TaGa 3 Si 2 O 14 and GdCa 4 O(BO 3 ) 3

We have successfully grown high quality piezoelectric single crystals of Sr₃NbGa₃Si₂O₁₄ (SNGS), Sr₃TaGa₃Si₂O₁₄ (STGS) and GdCa₄O(BO₃)₃ (GdCOB) by the Czochralski technique. Dielectric, elastic and piezoelectric constants of SNGS and STGS were measured in a temperature range from 25°C to 400°C. Some parameters of SNGS, STGS and GdCOB crystals important for acoustic applications have been investigated also at temperatures up to 940°C. The strong piezoelectric activity of all investigated materials is kept at least up to this high temperature.