Electro‐chemomechanical Contribution to Mechanical Actuation in Gd‐Doped Ceria Membranes

Abstract

Gd‐doped ceria (CGO), one of the most extensively studied oxygen ion conductors, is a low dielectric constant/low mechanical compliance material exhibiting large nonclassical electrostriction. The electromechanical response of the micro‐electromechanical devices with CGO films as an active material described previously can not be attributed exclusively to electrostriction. Here it is shown that, below 1 Hz, in addition to electrostriction (second‐harmonic response), there is a strong contribution of the electro‐chemomechanical effect (ECM, first harmonic response). ECM is the change in mechanical dimensions of ionic and mixed ionic‐electronic conductors as a result of a change in chemical composition induced by an electric field. In batteries, the presence of ECM is highly detrimental. In ceria at room temperature, it was considered to be negligible because of slow oxygen diffusion. This work demonstrates ECM actuation at ambient temperature and moderate electric field (<5 V µm−1). ECM‐induced strain is attributed to reversible oxidation/ reduction of TiO2 layers at the Ti‐CGO interface. At 25 °C, the ECM bending strain is 1.2 × 10−6, increasing exponentially with temperature. These data suggest that with a proper choice of materials, ECM‐type response can be a viable mechanism for mechanical actuation at ambient and also at slightly elevated temperatures.

All‐Solid‐State Electro‐Chemo‐Mechanical Actuator Operating at Room Temperature

“…We have applied SPM to investigate the variation in displacement across the central region of the membrane actuator in response to application of 8 V, 50 mHz ( Figure a). In contrast to the electrostrictive response, where this entire region moves in unison due to homogenous expansion of the membrane, [ 15,16 ] here the amplitude | Δh | is maximum ( ∆h max = 0.43 ± 0.02 µm) at the center of that region, while the response at the edge diminishes to ≈25% of maximum. The distribution of displacement amplitude is cylindrically symmetric about the membrane center and all positions sampled move in phase.…”

“…Recently, an ECM contribution to the room temperature electromechanical response of an electrostrictive 20 mol% Gd‐doped ceria (Gd 0.2 Ce 0.8 O 1.9 , henceforth 20GDC) tethered membrane, sandwiched between two Ti thin film metal contacts, was reported. [ 15 ] The ECM response was attributed to in situ oxidation/reduction reactions producing several‐nm thick layers of TiO 2−δ [ 16 ] at the upper and lower interface between the 20GDC electrolyte and the Ti metal contacts according to the reversible reaction: Ti + 2O − ⇌ TiO 2−δ , where TiO 2 may take either the rutile or anatase phase. The Ti molar volume increases markedly upon oxidation; [ 15 ] however, once formed, the TiO 2−δ interfacial layers impede further ion diffusion into the metal contact, permitting only very limited response.…”

“…[ 15 ] The ECM response was attributed to in situ oxidation/reduction reactions producing several‐nm thick layers of TiO 2−δ [ 16 ] at the upper and lower interface between the 20GDC electrolyte and the Ti metal contacts according to the reversible reaction: Ti + 2O − ⇌ TiO 2−δ , where TiO 2 may take either the rutile or anatase phase. The Ti molar volume increases markedly upon oxidation; [ 15 ] however, once formed, the TiO 2−δ interfacial layers impede further ion diffusion into the metal contact, permitting only very limited response. Here, we bring to realization a practical application of the ECM concept in a prototype of a Si‐integrated, ECM‐based membrane actuator specifically designed to provide micron‐size displacements, and long‐term stability.…”

Dopant Concentration Controls Quasi-Static Electrostrictive Strain Response of Ceria Ceramics

“…When metals support oxygen ion exchange with aliovalent cation-doped ceria thin films, then the electro-chemomechanical effect (material expansion due to electric-field-induced oxygen diffusion) becomes dominant at low frequencies. 30 Therefore, in earlier work, 21 aluminum electrodes were generally used. In the present report, we have also tested the effects of the addition of sputtered TiN films, as conductive but nonmetallic contacts, as well as sputtered Ta 2 O 5 (20 nm)/Ta films.…”

Surface Piezoelectricity and Pyroelectricity in Centrosymmetric Materials: A Case of α-Glycine

“…The displacement (in response to the applied voltage) was measured with a single-beam Michelson–Morley interferometer [ 14 , 15 , 16 ] equipped with a SR830 lock-in amplifier (Stanford Research Systems Inc., Sunnyvale, CA, USA). A proportional-integral-derivative (PID) feedback system consisting of P-841.01 piezo actuator and E-709.SRG piezo controller (Physik Instrumente, Karlsruhe, Germany) was used for the working point stabilization against the slow changes of the optical path length.…”