As part of the project, a comprehensive theoretical and experimental study of backward acoustic waves in piezoelectric plates was carried out. These unique waves are characterized by oppositely directed phase and group velocities, as well as the existence of a zero group velocity point. The features of their excitation and detection in various plates and layered structures were established. Using the finite element method, electrode structure topologies were developed that efficiently excite the backward wave. The possibility of broadening the frequency range of backward wave existence was demonstrated by creating layered structures containing films of strong piezoelectrics (lithium niobate, piezoceramics) and plates of highly anisotropic materials (paratellurite, lithium iodate, tellurium). It was shown that increasing the film density shifts the zero group velocity point toward lower frequencies, and that for the “tellurium plate – lithium niobate film” structure, a 2.5-fold increase in the backward wave existence range is achievable. A technological setup for forming layered structures by low-temperature bonding was created. A fabrication technology for multilayer “anisotropic dielectric – piezoelectric layer” structures was developed and validated. Experimental samples of layered structures were produced and subjected to structural and phase analysis.
For the first time, an increase in the phase velocity of backward acoustic waves upon approach of a metallic screen to the surface of a piezoelectric at a fixed frequency was theoretically predicted and experimentally confirmed. This effect was found to be associated with the anomalous dispersion of backward waves.
For the first time, the temperature coefficients of velocity and delay were calculated for backward acoustic waves in lithium iodate and paratellurite plates, as well as in structures based on them in the presence of piezoelectric films of lithium niobate and zinc oxide. It was found that these coefficients are more than 20 times greater than those for forward waves in the same material at a fixed frequency. This is attributed to the anomalous dispersion of these waves and their existence in the region between the cutoff frequency and the thickness resonance frequency. The results of the theoretical analysis were experimentally confirmed on fabricated acoustoelectronic devices based on a lithium niobate plate and a LiNbO₃/TeO₂ layered structure. The obtained results open up prospects for the development of ultra-sensitive temperature sensors.
For the first time, the influence of liquids with varying viscosity and conductivity on the properties of backward acoustic waves was studied both theoretically and experimentally. Using a YX-LiNbO₃ plate as an example, it was shown that the backward A₁ wave is sensitive to liquid conductivity across the entire studied range, whereas the forward SH₁ wave exhibits saturation at low conductivity values. To enhance the sensitivity of these waves to liquid conductivity, a combination of a strong piezoelectric film with a highly anisotropic material plate is recommended, and to reduce insertion losses, the use of a buffer layer with low acoustic impedance is advised.
The feasibility of developing acoustoelectronic sensors for the electrical and mechanical properties of liquids based on backward acoustic waves was demonstrated. To this end, the influence of the liquid’s permittivity and acoustic impedance on the resonance characteristics of a single-port acoustoelectronic device was studied experimentally, numerically, and analytically. It was shown that the resonance frequency shift of the backward wave in the frequency dependence of the S₁₁ parameter is governed primarily by permittivity, while changes in Q-factor and peak dip depth are determined mainly by acoustic impedance. The miniaturization challenge for the developed devices was solved, enabling the transition from a set of interdigital transducers with different spatial periods to a single transducer.
Based on the studies conducted, recommendations were formulated for the design of sensing devices based on backward waves in plates, covering potential application areas and fabrication specifics of acoustoelectronic devices.
All project objectives were completed in full. The results obtained are of high scientific novelty and practical significance for the development of acoustoelectronic sensors for temperature and for the electrical and mechanical properties of liquids based on backward acoustic waves. The results have been published in high-ranking journals: Ultrasonics (Q1), International Journal of Engineering Science (Q1), and Wave Motion (Q2). A manuscript has been submitted to the International Journal of Mechanical Sciences (Q1).
However, to build a complete picture that would allow a full assessment of the fundamental properties of backward acoustic waves, additional theoretical and experimental studies are needed on the effects of external electric and magnetic fields on their properties — such as energy flow direction, phase and group velocity, frequency range of existence, and cutoff frequency. To fully exploit the unique features of these waves, devices based on direct measurement of their group velocity need to be experimentally realized. Furthermore, additional studies already carried out have demonstrated the feasibility of using sensing coatings in the implementation of acoustoelectronic devices based on backward waves, opening up prospects for the development of selective gas sensors on their basis.