La dégradation du niobate de lithium à haute température

Abstract

The objective of this thesis is to identify the phenomena responsible for the degradation of the piezoelectric behavior at high temperature of lithium niobate (LiNbO3). Indeed, lithium niobate, unlike other piezoelectric materials, possesses both a high Curie temperature (1210 °C or 1483K) and piezoelectric coefficients high enough for applications as transducer. This material is therefore a promising candidate for high temperature piezoelectric applications. However, the literature reports a degradation of lithium niobate starting at temperatures as low as 570K. Nevertheless, the mechanisms at the origin of this degradation are uncertain. One hypothesis could explain this degradation of the piezoelectric properties of lithium niobate at high temperature : the appearance of an ionic conduction at high temperature. We have therefore studied the electrical conductivity of congruent single crystals of lithium niobate in a temperature interval running from 20 °C (293K) to 1000 °C (1273K) over a frequency range from 20 Hz to 20 MHz. The evolution of the electrical conductivity of lithium niobate is described through an analytical model. This model generalizes the universal dielectric relaxation law with the Arrhenius equation to take simultaneously into account the variations of the temperature and the excitation frequency of the crystal. The conduction mechanisms are identified as electronic at low temperature and ionic, dominated by the motion of lithium ions (Li+), at high temperature. This ionic conductivity can therefore cause an internal short circuit in the LiNb3 crystal, reducing the observable piezoelectricity. Moreover, the excitation frequency of the crystal greatly influences the temperature above which the ionic conduction is dominant : at high temperature the ionic conduction can be greatly decreased with a sufficiently high excitation frequency, thus clarifying the confusion present in the literature. These results clarify and identify the high-temperature mechanisms involved in lithium niobate, thus allowing an appropriate implementation for high-temperature piezoelectric applications.

Date	29 Jul 2021
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Ricardo J. Zednik (Supervisor) & Pierre Bélanger (Co-supervisor)