ULTRASONIC CHARACTERIZATION OF LANGATATE

Langatate (LGT, La₃Ga_5.5Ta_0.5O₁₄) is a synthetic piezoelectric material that has piezolectric coupling coefficients 4-5x those of quartz and has no crystalline phase change up to its melting point >1400 °C.  Because of these attractive features, LGT has been targeted as an alternative piezoelectric material in applications such as RF filters for wireless communications and as a sensor material in high temperature environments.

This work, part of my PhD research, focused on determining the elastic and piezoelectric properties of LGT as well as their respective temperature coefficients.  These properties, which are related to the acoustic wave phase velocity by the Christoffel Equation, were determined through complementary velocity measurement techniques: pulse-echo overlap (a time-of-flight technique) and thickness-mode resonance measurements of thin (0.2-0.4 mm) plates.  Due to the anisotropy of the material, phase velocities of longitudinal and shear waves along five different crystal orientations were measured to fully determine the elastic and piezoelectricity tensors.  The determined elastic and piezoelectric properties were then validated through measurements of the surface acoustic wave (SAW) phase velocity along seven different orientations.  SAWs were used for validation because the accurate prediction of any given SAW phase velocity requires knowledge of multiple elastic and piezoelectric constants.

More information about the material property extraction can be found in these publications:

• Elastic and piezoelectric constant determination
• Elastic and piezoelectric constant validation
• Accurately determining time-of-flight in pulse-echo measurements

Side view of a large LGT boule (single crystal).	Top view of a large LGT boule (single crystal).
Experimental setup for a pulse-echo measurement.  A buffer rod, below the sample, is used to separate the echoes in time.	Example pulse-echo measurement showing reflections off of the buffer rod as well as the sample.  The objective is to accurately determine the delay between successive reflections.
Examples of SAW devices deposited on an LGT wafer. The arrows between transducers denote propagation direction.	Micrograph of interdigital transducers used to excite SAW.  The fingers are 4 μm wide.
SAW phase velocities measured for material property characterization. Points represent measurements while the solid trace shows the predicted velocity as a function of azimuthal angle.