Le niobate de lithium à haute température pour les applications ultrasons

Abstract

The aim of this thesis is to find a piezoelectric material suitable for high temperature ultrasound applications. Indeed, their operating temperature is actually limited to 300°C due to the piezoelectric element inside. Extend this limitation would allow non destructive testing (NDT) by ultrasound on high temperature structure in order to do structural health monitoring (SHM) or in-service inspections (ISI). With good electromechanical properties and a high Curie temperature (1200°C), lithium niobate (LiNbO3) is a good candidate. But some studies declared that chemical processes like ionic conductivity or chemical decomposition prevent this material from ultrasound transducers applications above 600°C. However, recent studies showed that LiNbO3 is able to produce ultrasounds up to 1000°C and that conductivity was not visible. This lead to a hypothesis: an ionic conductivity is present in lithium niobate at high temperature (>500°C) but has a limited impact on its properties at high frequency (>100kHz). A high temperature characterization of lithium niobate is thus necessary to verify this hypothesis. To do so, the resonance method was employed. It leads to a characterization of most of the electromechanical coefficients from a single electrochemical impedance spectroscopy and an explicit model making the link between properties coefficients and the impedance spectrum. Coefficients are deduced from the best fit of the model over experimental data. An experimental setup was done in order to well control the temperature of the sample and to measure its electrochemical impedance spectrum. Unfortunately, actual models used for resonance method are inaccurate when coupling modes occur. This means to have samples with different shapes in order to isolate each main vibration mode. Moreover, these models usually do not take into account harmonics and shear modes. This is why a new wide frequency analytical model was developed in order to take into account coupling modes, shear modes and harmonics. Although this complex model is the most accurate to date, there are still resonances and coupled modes that are not modeled. The Y-cut sample characterization was done up to 750°C and confirmed that lithium niobate is hopeful. Piezoelectric coefficients are stable in function of temperature and stiffness and permittivity have the expected behavior. A thermoelectric effect which has a similar impact than ionic conductivity on the impedance spectrum was observed. Consequently, the ionic conductivity was not quantified. Although additional studies are needed, the resonances level at 750°C seems point out that lithium niobate can be used for ultrasound applications at high frequencies (>100kHz).

Date	31 Jan 2017
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Ricardo J. Zednik (Supervisor) & Pierre Bélanger (Co-supervisor)