*2.2. Temperature Coefficient of Delay (TCD)*

The temperature dependence of SAW device is expressed in terms of TCD given by [1,26,27]

$$TCD = TCD\_o - \frac{V\_{35} - V\_{15}}{20 \times V\_{25}} \,\text{}\tag{4}$$

where *TCD<sup>o</sup>* is the coefficient of thermal expansion of base layer, and *V*35, *V*25, *and V*<sup>15</sup> are the SAW velocities of the layered structure at respective temperatures. The TCD is measured in the units of ppm ◦C −1 .

Using the material parameters listed in Table 1, the authors obtained the SAW phase velocity for the multi-layered structure through the software. The coupling coefficient and TCD were consequently calculated using the SAW velocity so obtained in Equations (3) and (4).
