Investigation on SAW properties of LGS and optimal cuts for high-temperature applications.

A promising perspective for surface acoustic wave (SAW) device applications at high temperature has been opened by langasite (LGS). The SAW properties of LGS in singly and doubly rotated cuts at 250 degrees C are investigated. Three noticeable regions for SAW-cut orientations and propagation directions at high temperature are put forward and are defined by Euler angles [0 degrees, 20 degrees --> 50 degrees, 35 degrees --> 45 degrees], [0 degrees, 85 degrees --> 110 degrees, 0 degrees --> 5 degrees], and [0 degrees, 138 degrees --> 145 degrees, 20 degrees --> 23 degrees], respectively. The first region includes zero or comparatively reduced temperature coefficient of delay (TCD) (<2 ppm) and smaller electromechanical coupling factor (K2) (0.2%-0.35%); the second one exhibits higher K2 (0.35%-0.45%) and moderate TCD (<5 ppm); and the highest K2 (>0.45%) and larger TCD (25-30 ppm) characterize the last region. For some typical orientations within the above-mentioned three regions, the temperature dependency of SAW characteristics (up to 1000 degrees C) is discussed. The second region [0 degrees, 85 degrees --> 110 degrees, 0 degrees --> 5 degrees], especially the orientation [0 degrees, 90 degrees, 0 degrees], has better comprehensive characteristics of SAW and is more suitable for high-temperature applications. Therefore, we should give a top priority to the orientation [0 degrees, 90 degrees, 0 degrees] in the design of SAW devices operating at high temperature. Comparison between published experimental results and numerical predictions based on LGS constants and their temperature coefficients available in the literature reveals that the theoretical results of TCD under 250 degrees C are in agreement with the experimental ones (the relative error of TCD is within 10%).

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