Non-classical electrostrictive response in bulk ceria: tailoring by microstructure and defect chemistry

Electromechanically active materials, including piezoelectrics and electrostrictors, have a broad range of actuating and sensing applications from daily-life electronics to biomedical devices. The state-of-the-art ceramic materials of this kind contain the hazardous element, i.e., lead (Pb), whose usage is restricted by the European Union RoHS directive and to be banned in the near future.

Thus, it is of utmost interest to seek out substitutive materials with comparable performances. Most recently, a large electromechanical response has been discovered in highly oxygen defective Gd-doped cerium oxides (GDC) thin films. This compound is biocompatible, environment-friendly, economically cheap, and even more abundant in nature than lead.

The observed electromechanical strain is in gigantic character, with electrostrictive strain coefficient M33 ~10−17-10−18 (m/V)2 several orders of magnitudes higher than the best-performing commercial relaxor electrostrictors. Somewhat surprising, the material develops a compressive strain parallel to the field direction, which is opposite to classical behavior.

The atomistic mechanism of such action is understood to be governed by the lability of local atomic bonds, as directed by the presence of oxygen vacancies. This concept is fundamentally unique compared to the existing classical theory of electrostriction. Owing to this peculiar behavior, the electromechanical response in cerium oxide does not obey Newnham's empirical relationship for the classical electrostrictors.

Against these backgrounds, ceria based compound can be considered as a new family of next-generation electromechanical material. Up to the present time, the non-classical electrostriction (ES) for the ceria-based compound is mostly reported in pure ceria and GDC thin films. Experimental outcomes show that electrostriction indeed requires a certain amount of oxygen vacancies.

Moreover, the correlation between the electrostriction strain coefficient (M33) with oxygen vacancy concentration is found to be very complex. Taking into account that the thin-film is constrained to a substrate, thus subjected to a different level of stress depending on various factors, which could affect/alter the electrostrictive response of the material.

Thin films are tiny by definition (below 1 μm). Consequently, their actuation and processing require more complicated, time-intensive, and financially expensive steps than for traditional ceramic processing technology. In this context, it would be rational to investigate ES in force-free conditions, i.e., macroscale bulk materials, and shed some light on the current status of the ES properties in cerium oxides.

Anyhow, for technical and industrial-scale applications, ES response ought to be introduced in bulk components. Considering such facts, the thesis is concentrated to achieve several objectives: i. To disclose the relationship in the electro-chemo-mechanical coupling, comprising defects chemistry, ionic migration, mechanical properties, and electromechanical strain. ii.

To figure out the role of the different types of dopants and their associated oxygen vacancy on the ES response. iii. Investigate the effect of microstructure on the ES properties. iv. To verify whether the frequency-related electrostriction response in Gd-doped ceria represents the entire doped ceria system.

Firstly, the thesis work focuses on investigating the model material Ce1−xGdxO2−δ (GDC). Among all ceria compounds, GDC is enriched with more literature data, including ionic conductivity, defects association, local crystal structure, mechanical as well as electromechanical properties. To investigate the role of microstructure, high-density bulk Gd-doped ceria (GDC) with a fixed Gd concentration are fabricated by different thermal treatments, including field-assisted spark plasma sintering (SPS), cold sintering process (CSP), fast-firing and conventional sintering.

Such methods lead to developing different microstructures from nanocrystalline to microcrystalline grains with tuned oxygen vacancy configurations at the ion-blocking barriers. In common with thin films, the electrostrictive response in GDC bulk illustrates a non-Debye type frequency-dependent strain relaxation.

The most striking finding is that electrostriction at low-frequency regime follows neither grain size nor bulk conductivity dependency. Instead, it shows a strict relation with the oxygen vacancy configuration. Whereas, the high-frequency region is mostly unaffected by any governing factors. To verify such a result, microcrystalline Gd-doped ceria and Sm/Nd co-doped ceria with various dopant concentrations from low-to-high are also explored.

Even though it shows a dependency on the nominal vacancy concentration, the configuration effect at the ion-blocking barrier holds a similar conclusion. The material with a high ion-blocking factor presents a large M33 value at the low-frequency regime, whereas the high-frequency, is shown to be almost untunable for these dopants.

Interestingly, it is found that the electrode material can also influence electromechanical strain, particularly at the lower applied frequencies. The frequency-dependent electrostriction relaxation disappears when the dopant is too different than of host Ce4+. For a divalent dopant such as Ca2+, a steady electromechanical strain with little fluctuation is noticed up to 1 kHz.

No dramatic effect on dopant concentration, configuration, and microstructure is observed. Undersized dopant such as Sc3+ and Mg2+ also demonstrates an analogous response like calcium-doped ceria. Such implications highlight that enhanced dopant-defect interaction in the lattice as induced by the combination of electrostatic and elastic interaction plays an emerging impact on the frequency-controlled strain relaxation mechanism.

In addition to that, the mechanical properties, comprising elastic modulus, hardness, creep, are examined by the nanoindentation method at room temperatures. No strong correlation between such properties and the electrostriction coefficient is identified. The choice of dopant, concentration variation, and sintering protocol were selected in such a way that direct comparison between measured and literature available data is possible.