As a result of the second project stage (2024–2025), a technology was developed for fabricating a multilayer structure consisting of a highly anisotropic dielectric plate and a piezoelectric layer with a high electromechanical coupling coefficient. This technology was successfully validated on paratellurite and lithium niobate plates (as well as lithium tantalate and piezoceramics). The fabrication process includes chemical cleaning with drying, plasma cleaning and surface activation, followed by low-temperature bonding in vacuum. The final step involves grinding and polishing of the piezoelectric layer or substrate to achieve the specified surface parameters and layer thicknesses. The obtained layered structure samples based on paratellurite and lithium niobate were characterized using scanning electron microscopy, X-ray diffraction analysis, Raman spectroscopy, energy-dispersive analysis, and atomic force microscopy. The results showed that no new chemical bonds or complex oxides are formed during low-temperature bonding, and no new phases at the interface that are uncharacteristic of the starting materials are observed. Surface adhesion is achieved through van der Waals forces and electrostatic forces arising from the activation of surface charge states during plasma treatment.
The finite element method was used to develop an optimal interdigital transducer topology for acoustoelectronic devices based on the TeO₂/LiNbO₃ layered structure (“dielectric plate – piezoelectric layer”). The modelling reproduced the real experimental situation and made it possible to assess the influence of the transducer geometric parameters on the backward wave characteristics in the studied structures.
Based on the “dielectric plate – piezoelectric layer” (TeO₂/LiNbO₃) structure, a set of experimental acoustoelectronic device samples was fabricated, including both arrays of interdigital transducers on a single plate and individual transducers with various combinations of paratellurite and lithium niobate plates of different orientations. Comparison of the experimental results with theoretical data showed nearly perfect agreement in both the amplitude and the position of the peak for the backward A₁ wave.
An automated experimental setup for studying the effect of temperature on acoustic wave properties was developed and built, with the capability for automatic data recording on a personal computer. The setup is based on a compact low-temperature chamber, a vector network analyzer, and a 16-channel transducer switching unit.
Experimental studies of the effect of temperature on the properties of backward acoustic waves confirmed the theoretical results of the first project stage. As temperature increases, the resonance peak corresponding to the backward A₁ wave shifts monotonically toward lower frequencies. The temperature sensitivity of the backward A₁ wave peak position is almost twice that of the forward SH₁ wave.
Theoretical studies of the influence of liquid conductivity and viscosity showed that the highest sensitivity of attenuation to conductivity was exhibited by the backward wave existing in the lithium niobate (Y-cut) / tellurium (X-cut) structure at a LiNbO₃/Te thickness ratio of 0.03. The highest sensitivity of phase velocity was observed for the same wave at a thickness ratio of 0.02. The highest sensitivity of attenuation to viscosity was shown by the wave in a bare tellurium plate without a film. The highest sensitivity of phase velocity was observed for the wave in a tellurium plate with a lithium niobate film at a film-to-plate thickness ratio of 0.03, for the viscosity range of 0 to 1000 cP. In the viscosity range of 1000 to 1500 cP, the most sensitive was the same wave configuration with a film thickness equal to 0.02 of the plate thickness.