High-temperature electrical conductivity in piezoelectric lithium niobate

Abstract

Lithium niobate is a promising candidate for use in high-temperature piezoelectric devices due to its high Curie temperature ([Formula: see text]1483 K) and strong piezoelectric properties. However, the piezoelectric behavior has, in practice, been found to degrade at various temperatures as low as 573 K, with no satisfactory explanation available in the literature. We, therefore, studied the electrical conductivity of congruent lithium niobate single crystals in the temperature range of 293–1273 K with an 500 mV excitation at frequencies between 20 Hz and 20 MHz. An analytical model that generalizes the universal dielectric relaxation law with the Arrhenius equation was found to describe the experimental temperature and frequency dependence and helped discriminate between conduction mechanisms. Electronic conduction was found to dominate at low temperatures, leading to low overall electrical conductivity. However, at high temperatures, the overall electrical conductivity increases significantly due to ionic conduction, primarily with lithium ions (Li+) as charge carriers. This increase in electrical conductivity can, therefore, cause an internal short in the lithium niobate crystal, thereby reducing observable piezoelectricity. Interestingly, the temperature above which ionic conductivity dominates depends greatly on the excitation frequency: at a sufficiently high frequency, lithium niobate does not exhibit appreciable ionic conductivity at high temperature, helping explain the conflicting observations reported in the literature. These findings enable an appropriate implementation of lithium niobate to realize previously elusive high-temperature piezoelectric applications.

Characterization of the Elastic, Piezoelectric, and Dielectric Properties of Lithium Niobate from 25 °C to 900 °C Using Electrochemical Impedance Spectroscopy Resonance Method

“…Using the data from the Y-cut and Z-cut samples with the analytical model gives the complete set of elastic, piezoelectric, and dielectric coefficients in Table 3. The gradual increase of electrical conductivity at higher temperatures, particularly above about 500 °C, is consistent with the previously reported observations confirming the gradual appearance of ionic conduction caused by Li+ ion motion [14]. As is expected for any real material in the absence of a phase transformation or chemical reaction, the elastic stiffness coefficients are found to decrease with temperature.…”

“…Of these candidates, lithium niobate is one of the most promising, because it combines a high Curie temperature of about 1210 °C with large piezoelectric coefficients [5][6][7] and is compatible with existing ultrasound technology [8,9]. Nonetheless, the experimental observation of piezoelectricity in LiNbO 3 has been found to often degrade well below the Curie temperature [10][11][12][13]; this has recently been confirmed to be caused by internal shorting of the crystal from the apparition of electrical conductivity at elevated temperatures dominated by Li+ ion motion [14]. Although this phenomenon can be mitigated through the implementation of appropriate design strategies, practical application of LiNbO 3 remains complicated by the poorly characterized material properties at elevated temperatures.…”

Ultrasonic Transducers for In-Service Inspection and Continuous Monitoring in High-Temperature Environments

“…These observations were recently confirmed to be caused by internal shorting of the crystal due to electrical conductivity at elevated temperatures dominated by Li+ ion motion and the composition of the LiNbO element. However, it was shown that it may be overcome by operating at a high frequency (>MHz) and choosing a monocrystal or a stoichiometric element [ 16 ]. The complete set of elastic, piezoelectric and dielectric coefficients has been evaluated from room temperature to 900 C [ 17 , 18 ].…”