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We couple the spark plasma sintering (SPS) technique with the ball mill process to fabricate a new electro-chemo-mechanical functional carbon-metal oxide hybrid material. Graphene oxide (GO) or carbon nanotubes (CNT) carbon allotropes are mechanically mixed with gadolinium-doped ceria (CGO). SPS allows consolidating dense carbon-metal oxide hybrids with uniform morphology and high crystallinity. The carbon allotropes and SPS lead to highly reduced CGO. The metal oxide presents a chemically tuned grain boundary with highly oxygen defect concentration at the near-grain boundary region. The hybrid interface can hinder chemical oxidation at high-temperature depending on the carbon allotropy. As a result, the hybrid materials with CNT display high ionic conductivity and activation energy, reducing the space charge layer and charge distribution. This feature enhances the electromechanical properties at room temperature. We especially disclose electrostriction in the CNT-based hybrids superior to the pure CGO and follow a non- Newnham relation. Notably, integrating carbon materials with metal oxides using the SPS can generally be applied to synthesising a range of functional metal oxide composite.

Oxygen defective ceria, e.g. Gd-doped ceria, shows giant electromechanical properties related to a complex local rearrangement of its lattice. Although they are not entirely identified, the electroactive mechanisms arise from cations and oxygen vacancies (VO) pairs (Ce-VO), and the local structural elastic distortion in their surroundings. Here, we study the geometry and behaviour of Ce-Vo pairs in a grain boundaries-free bulk Ce0.9Gd0.1O1.95 single crystal under an AC electric field of ca. 11 kV/cm. The analysis was carried out through X-ray absorption spectroscopy (XAS) techniques at Ce L-III edge. Using Density Functional Theory (DFT) calculations, we investigated the effects of the strain on states' density and orbitals at the valence band edge. Our research indicates that the electrostriction increases at low temperatures. The electromechanical strain has a structural nature and can rise of one order of magnitude, i.e., from 5·10-4 at room temperature to 5·10-3 at -193 °C, due to an increase in the population of the electrically active pairs. At a constant VO concentration, the material can thus configure heterogeneous pairs and elastic nanodomains that are either mechanically responsive or not.

Some oxygen defective fluorites are non-Newnham electrostrictors, i.e., the electromechanical response does not depend on their dielectric properties. Here, we show gigantic electrostriction in nanocrystalline 25 mol% praseodymium doped ceria (PCO) bulk ceramics. The material was fabricated with a field-assisted spark plasma sintering (SPS) process from high-purity nanoscale PCO powders (<20 nm). The SPS process consolidates the powders into a single-phase, highly dense material with a homogeneous microstructure and large grain boundary extension. Various thermally and chemically stable ionic defects are incorporated into the nanostructure, leading to superior electrical conductivity. The material shows an electrostriction strain coefficient (M33) of ∼10−16 m2 V−2 at frequencies below 100 Hz at room temperature. Such performance is comparable and even superior to Newnham's electrostrictors, such as ferroelectric ceramics and polymeric actuators. Comparative analysis with polycrystals suggests that nanostructured PCO possesses electromechanically active nanodomains of Pr3+–VO pairs. Such results are unexpected and open novel insights on non-Newnham electrostrictors.

Oxygen defective cerium oxides CeO2-δ exhibits a non-classical giant electromechanical response that is superior to lead-based electrostrictors. In this work, we report the key-role of acceptor dopants, with different size and valence (Mg2+, Sc3+, Gd3+, and La3+), on polycrystalline bulk ceria. Different dopants tune the electrostrictive properties by changing the electrosteric dopant-defect interactions. We find two distinct electromechanical behaviors: when the interaction is weak (dopant-vacancy binding energy ≤ 0.3 eV), electrostriction displays high coefficient (M_33), up to 10−17 (m/V)2, with strongly time-dependent effects. In contrast, we observe no time-dependent effects when the interaction becomes strong (≥ 0.6 eV).

Cerium oxide is rendered as a key-material for electrochemical devices e.g. electrolytes, anodes, and other solid-state electrochemical cells. The ionic conductivity of these oxides is controlled by the dopant type/charge, more specifically with the microstructure and dopant configuration at the blocking barrier. These are mainly associated with solid-state mass diffusion phenomena that control the material assembling at the atomic scale. In this study, we explore the mechanisms constructing the ionic conductivity blocking barrier effects as a function of dopant type/concentration. These are the result of mass diffusion effects involved in the microstructural consolidation mechanisms from nanoscaled particles to continuum ceramic dense layers. Our experimental results show that ionic conductivity blocking effects are the net result of a combination of defectcations configurations controlled by a multifold overlapping of dopant concentration, cation sizes, electrostatic interactions and density of cation diffusion migration paths. We observe that conductivity can be maximized by tailoring the ceria solid-solutions with strong electrostatic-elastic interaction, allowing local enrichment of solute content. Such effect also leads to steady electromechanical properties at low temperatures, highlighting the impact of designing of microstructural features in tuning characteristic properties in electroceramics.

In recent times, rare-earth-doped cerium oxides, e.g. Gd-doped ceria have shown giant electrostriction that are orders of magnitude higher than known inorganic electrostrictors. This non-classical electrostriction is fundamentally independent of Newnham's scaling law and is functioned with the existence of oxygen vacancies (Vo) in the host lattice, associated with Ce-Vo pairs. Here, electro-chemo-mechanical properties of highly defective calcium doped ceria (CDC) ceramics, with variously doping levels (Ce0.9CaxO2-x,x = 0.025-0.15) are investigated. As expected, CDC samples display rather different electrochemical properties depending on a vacancy concentration and microstructure. They reveal primary creep at room temperature, emphasizing an unexpected anelasticity in ceria. The electrostrictive effect describes being independent of either the nominal vacancy concentration or the related blocking effects at the grain boundary, exhibiting a high electrostriction coefficient (M33) in order of ≈ 10-18 m2V-2 in a wide range of frequency. Remarkably, electrostriction show neither strain saturation nor relaxation with field amplitude and frequencies, respectively. These key findings unveil a new form of electromechanical response in ceria that is independent of composition fluctuations and dominated by strong columbic and elastic energy of Ca2+- Vo pairs.

Recent studies demonstrate that highly defective cerium oxide, e.g. Gd-doped ceria, is capable of generating a giant electromechanical effect that is orders of magnitude larger than other ceramic based electrostrictors, e.g. lead-based piezoelectrics. This is an unconventional response as it does not obey the Newnham’s empirical law and it is atomistically explained by the lability of Ce- pairs. In this report, we investigate the electrostrictive properties of defective ceria with 0.25−3.75% oxygen vacancies introduced by a co-doping (Sm, Nd) concept. The codoping strategy leads to a different oxygen vacancy configuration in the lattice than for the single dopant case, delineated by a relatively lower vacancy-dopant association energy. This results in electrostrictive response with high strain coefficient (M33 in the 0.1-10 10-17 m2/V2) as well as in marked, strain saturation effects to the applied field from 0 to 120 kV/m and in a relaxation behavior by increasing the frequency of the applied field (0.1 to 1 kHz). Remarkably, M33 follows a strict dependency on oxygen vacancy configuration rather than their nominal concentration. Such outcomes define the hypothesis that electrostriction does occur due to oxygen vacancy concentration but it is firmly tuned by oxygen vacancy

In recent times, non-classical giant electromechanical properties are reported in both defective cerium oxide (Ce1M1−xO2−δ) thin films and bulks, which are even superior to current leadbased electrostrictors e.g. lead magnesium niobate (PMN). The electrostrictive properties of such material mainly functioned by oxygen vacancy concentration, with strict dependency on their configuration at the blocking barriers. In this work, we investigate the effect of oxygen defects configuration on electro-chemo-mechanical properties of bulk ceria. This is accomplished by tuning the oxygen defects blocking barrier density in following methodologies:(1) Changing blocking barrier density by consolidating nano-metric gadolinium doped ceria (GDC) powders of fixed doping concentration in different conditions, including field assisted spark plasma sintering (SPS), fast firing and conventional method at hightemperatures.(2) Changing blocking barrier properties by consolidating calcium doped ceria (CDC) powders with different dopant concentration in conventional method at high temperatures.By correlating both results with electro-chemo-mechanical properties, we finally demonstrate that solute drag effect drives the oxygen vacancy distribution in the materials, which plays a key role in ceria electrostriction, overcoming expected contributions from grain size and nominal dopant concentration.

Mass diffusion controls material structuring from the atomic to the macro-scale defining properties and functionalities. We show here that surface energy in Ce0.9Gd0.1O3-d shaped nanocrystals, i.e. nanocubes, nanorods and spherical nanoparticles, can control solid state diffusion mechanisms over a long range, leading to extreme microstructural diversity.