RSF project No. 23-79-10079 “Reverse acoustic waves in piezoelectric multilayer structures and the development of a new type of sensors based on them”
The head: Ph.D. Smirnov A.V.
As a result of the implementation of the 1st stage of the project by the transmission matrix method, a theoretical analysis of the propagation of acoustic waves in dielectric plates characterized by strong anisotropy (tellurium, barium titanate, lithium iodate, paratellurite) was carried out. Dispersion dependences of acoustic waves of various types (antisymmetric and symmetric Lamb waves, waves with transverse horizontal polarization) and orders (zero and higher orders) for various crystallographic orientations of the studied materials are constructed. It was found that branches corresponding to reverse waves are present in paratellurite TeO2, tellurium Te and lithium iodate LiIO3. However, no such waves were detected in the studied frequency range in barium titanate. It was found that the frequency range of the existence of such waves is smaller than in materials such as lithium niobate and potassium niobate. Apparently, this is due to the low piezoactivity of these materials. The transmission matrix method has been used to theoretically analyze the features of the existence of inverse acoustic waves in layered structures of a substrate made of a material with strong anisotropy (h2)/piezoelectric film (h1). Tellurium, paratellurite and lithium iodate were used as substrates, and lithium niobate (LiNbO3), potassium niobate (KNbO3), and piezoceramic materials pzt and cts19 were used as films. It is shown that as the film thickness increases, the phase velocity of the reverse wave at a fixed frequency increases and the point with zero group velocity shifts to the region of lower frequencies. For most of the considered layered structures, an increase in the thickness of the film leads to a decrease in the frequency range of the existence of an inverse acoustic wave. However, using the example of layered structures with a tellurium substrate and films of lithium niobate and potassium niobate, the possibility of increasing the frequency range of the existence of an inverse acoustic wave with increasing film thickness in the h1/h2 range from 0 to 0.04 was shown. When using cts19 and pzt films, a stronger shift of the zero group velocity point to the low frequency region is observed than when using lithium niobate and potassium niobate films. This can be explained by the higher density of piezoceramic materials. The theoretical analysis was carried out by the finite element method and the dispersion dependences of the inverse acoustic wave A1 were obtained, provided that an ideal conductive screen was approached to one of the surfaces of the piezoelectric plate. It is shown that when the hf parameter is fixed, the approximation of the metal screen leads to an increase in the phase velocity of the reverse branch of the A1 wave. Based on the data obtained, a topology of electrode structures of counter-pin converters and an experimental sample was created – a set of counter-pin converters with different spatial periods (wavelength 1.0-2.0 mm, pitch 0.1 mm) formed on the surface of a lithium niobate plate with a thickness of 350 microns, using projection photolithography and magnetron sputtering at direct current. An experimental setup has been developed and created to determine the value of the electromechanical coupling coefficient of reverse acoustic waves in piezoelectric plates with a large electromechanical coupling coefficient. The main elements of the installation are a vector circuit analyzer, a precision micro screw with a non-rotating spindle and a lithium niobate plate with a set of counter-pin converters created on its surface. The effect of the distance from the back side of the piezoelectric plate (the reverse side of the arrangement of the electrode structures of the resonators) to the conductive screen (in the range of 0-500 microns and an error of ±0.5 microns) on the frequency dependences S11 of the sample parameters was experimentally studied. It is shown that the approach of a metal screen to the surface of a piezoelectric leads to a shift in the resonant frequency of the reverse wave towards smaller ones (h is the thickness of the plate, f is the frequency of the wave, and consequently to an increase in the phase velocity of the reverse acoustic wave). For the first time, the experimentally confirmed effect of increasing the phase velocity of reverse acoustic waves when a metal screen approaches the surface of a piezoelectric can be explained as follows. Apparently, the electric field accompanying the reverse wave penetrates deeply into the piezoelectric plate. The approach of the metal screen leads to the localization of the electric field near one side of the plate, which in turn leads to a tightening of the elastic modulus and, accordingly, to an increase in the phase velocity. A similar effect was observed for Gulyaev-Bluestein waves with a change in the conductivity of the layer on the surface of the piezoelectric. However, in the case of Gulyaev-Bluestein waves, the moment of maximum localization of the electric field near the surface came and then its phase velocity began to fall due to the increasing influence of the shorting effect of tangential electric fields. In this case, the electric field does not reach maximum localization and therefore there is no effect of reducing the phase velocity. It makes sense to conduct additional research on the direct metallization of the surface by forming an ideally conductive film on the surface of a piezoelectric. This will be done at the next stage of the project. The transfer matrix method was used for the first time to calculate the phase and group velocities, the angle of energy flow and polarization for the detected reverse waves with a temperature change of 1 degree. The dependences of the temperature coefficient of velocity and the temperature coefficient of delay for the reverse wave in the range hf=1.7-1.9 in the lithium iodate plate are obtained. It was found that this parameter is an order of magnitude higher for reverse waves than for forward waves in piezoelectric plates. This, apparently, is due to the proximity of these waves to the cutoff frequency, which leads to their strong dependence on changes in the parameters of the propagation medium. This conclusion is confirmed by the fact that the tcv value has the greatest value at the beginning of the frequency range of the existence of the reverse wave, has a minimum in the middle at a value of the hf parameter of about 1750 m/s, and increases with a further increase in frequency. This fact is of interest for the development of very sensitive temperature sensors, but requires additional research. The dependences of the temperature coefficient of velocity and the temperature coefficient of delay for the previously detected reverse waves in the structures of a lithium iodate plate or paratellurite piezoelectric film of lithium niobate or zinc oxide are calculated using the transmission matrix method. It was found that even in this case, the temperature coefficient of the velocity of the reverse wave remains very large. The results obtained require experimental verification.