Abstract
Platinum (Pt) gratings on langasite (LGS) substrates are a widely used structures in high temperature surface acoustic wave (SAW) devices. Multiple modes can be excited in PtLGS SAW devices owing to the heavy weight of the Pt electrode and leaky waves in the LGS substrate. In this work, we report on a detailed mode analysis of PtLGS SAW devices, where three commonly used LGS cuts are considered. A three-dimensional (3D) finite element method (FEM) numerical model was developed, and the simulation and experiment results were compared. The experiment and simulation results showed that there are two modes excited in the PtLGS SAW devices with Euler angle (0, 138.5, 27) and (0, 138.5, 117), which are Rayleigh-type SAW and SH-type leaky wave, respectively. Only the Rayleigh-type mode was observed in the PtLGS SAW devices with Euler angle (0, 138.5, 72). It was found that the acoustic velocities are dependent on the wavelength, which is attributed to the change of wave penetration depth in interdigital transducers (IDTs) and the velocity dispersion can be modulated by the thickness of the Pt electrode. We also demonstrated that addition of an Al2O3 passivation layer has no effect on the wave modes, but can increase the resonant frequencies. This paper provides a better understanding of the acoustic modes of PtLGS SAW devices, as well as useful guidance for device design. It is believed that the Rayleigh-type SAW and SH-type leaky waves are potentially useful for dual-mode sensing applications in harsh environments, to achieve multi-parameter monitoring or temperature-compensation on a single chip.
Highlights
In the last several decades, surface acoustic wave (SAW) devices have been explored extensively for sensing applications in harsh environments with great potential [1,2]
Device up to 800 ◦ C [10,11,12]; Maskay et al developed a Pt-alloy-based LGS SAW device for strain sensing [13]; Thiele et al reported on the gas sensing performance of Pt/LGS SAW devices up to 750 ◦ C [14]
The research by Naumenko et al revealed the effect of anisotropy on SAW behavior and the interaction of resonant modes of Pt/LGS SAW devices, with Euler angles such as (0◦, 22◦, 90◦ ) and (0◦, 138.5◦, 26.7◦ ) [19,20,21]
Summary
In the last several decades, surface acoustic wave (SAW) devices have been explored extensively for sensing applications in harsh environments with great potential [1,2]. Schulz et al presented the temperature performance of Pt/LGS SAW device up to 800 ◦ C [10,11,12]; Maskay et al developed a Pt-alloy-based LGS SAW device for strain sensing [13]; Thiele et al reported on the gas sensing performance of Pt/LGS SAW devices up to 750 ◦ C [14] In hostile environments such as gas turbines [15], high temperature, high pressure and large strain often coexist, and monitoring these parameters is very crucial for machine state monitoring and structure health maintenance. The research by Naumenko et al revealed the effect of anisotropy on SAW behavior and the interaction of resonant modes of Pt/LGS SAW devices, with Euler angles such as (0◦ , 22◦ , 90◦ ) and (0◦ , 138.5◦ , 26.7◦ ) [19,20,21] Their results were based on a numerical technique and were not compared with experimental results, so we are unable to verify the appropriateness of the analysis. LGS SAW devices with three Euler angles [(0◦ , 138.5◦ , 27◦ ), (0◦ , 138.5◦ , 117◦ ) and (0◦ , 138.5◦ , 72◦ )] were considered, that are all very attractive for sensing applications
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