Imaging with lithium niobate/epoxy composites

Lithium niobate, LiNbO3, is a very promising material for high temperature applications in non-destructive testing because of its high Curie temperature. However, for commercial applications LiNbO3 has often been neglected because of its low electromechanical coupling coefficients. This paper explores the potential of LiNbO3 composites, by means of room temperature characterization and measurements, for further use in operation in high temperature transducers. LiNbO3 composites of 1-3 connectivity in an epoxy matrix with volume fractions of LiNbO3 of 33% and 54% were fabricated. The composites were characterized by electrical impedance measurements and the results were compared with modelled impedance characteristics. Many parameters were predicted accurately, including an improvement of more than 75% in thickness mode electromechanical coupling coefficient, from kT=0.17 for bulk LiNbO3 to kT=0.32 for composite material. Some large discrepancies between simulation and experiment were also identified when a conventional one-dimensional model was used to calculate equivalent composite material parameters; however, finite element modelling was more accurate. After characterization, the composite was configured for use as a linear array. Functional measurements were conducted on steel blocks with a side drilled hole to represent a crack tip. This was detected by the array and visualized in time-of-flight diffraction B-scan images.