Improved longitudinal EMAT transducer for elastic constant extraction

Abstract

Elastic constants are extracted by measuring the shear and longitudinal bulk acoustic wave (BAW) velocities along several crystal orientations. Rapid, precise measurement of crystal elastic properties are fundamental in the evaluation of new materials for the fabrication of acoustic wave based filters and resonators. The extraction of elastic constants can be done by the use of external piezoelectric transducers in pulse echo or sweep frequency time domain reflection techniques. Recently introduced resonant ultrasound techniques can also be used for elastic constants extraction. This paper discusses a novel, highly efficient design for a longitudinal electromagnetic acoustic transducer (L-EMAT) to be used as an alternative in crystal elastic constant extraction. EMATs are used to launch bulk acoustic waves (BAWs) in samples by way of Lorentz's force due to the induction of surface eddy currents in the presence of a static magnetic field. The EMAT technology is a powerful, non-contact technique used for nondestructive evaluation (NDE) of materials. For NDE applications, acoustic wave mode selection is not critical, provided a dominant mode is identified to detect potentially catastrophic microfractures and metal fatigue in structures such as airplane wings and railroad tracks. Recently, this technology has been extended to the extraction of elastic constants of crystals. For precise elastic constant extraction, plate resonators are employed. It is desirable to avoid the excitation of various longitudinal and shear modes by the same EMAT transducer, for two main reasons: (a) the resonating frequency peaks may overlap, reducing the accuracy of the measurement; (b) the efficiency of the transducer in generating the desired mode increases, if only one mode is excited by the EMAT. The longitudinal EMAT design and fabrication originally reported in this paper explores mode selectivity and improved efficiency by: i) increasing the static magnetic field at the surface of the sample; ii) maximizing the current density in the sample's metallic layer; and iii) orienting the current and static magnetic field such that they are orthogonal. The fabricated longitudinal EMAT was used to measure the longitudinal BAW phase velocity values of X-cut and Z-cut quartz samples, and X-cut langatate (LGT, La3Ga5.5Ta0.5O14) samples. Details of the longitudinal EMAT design, its impedance response, the resonant peaks in the frequency spectra of the measured quartz and LGT plate resonators are reported in the paper. The measured results using a RITEC SNAP system showed better than 0.1% accuracy on the quartz and LGT samples when compared to published constants. The designed longitudinal EMAT has shown to be an accurate and simple alternative in determining crystal elastic constants