Electrical properties and charge compensation mechanisms of Cr-doped rutile, TiO2.

Cr-doped rutile, Ti1-xCrxO2-x/2-δ, powders and ceramics with 0 ≤ x ≤ 0.05 were prepared by solid state reaction and sintered at 1350 °C. Cr distribution is homogeneous with no evidence of either segregation or crystallographic shear plane formation. For high x compositions, >∼0.01, Cr substitution is charge-compensated ionically by oxygen vacancies with two Cr3+ ions for each vacancy and the materials are electronically insulating. For low x compositions, the materials are semiconducting. This is attributed to a new charge compensation mechanism involving Ti3+ ions created in response to the local electroneutrality requirement for two trivalent cations to be in close proximity to each oxygen vacancy. At very low dopant concentrations, ≪0.01, the dopants are well-separated and instead, some Ti3+ ions act as a second dopant to preserve local electroneutrality. For intermediate x compositions, a core-shell structure is proposed consisting of semiconducting grain interiors containing Ti3+ ions surrounded by a more insulating shell with Cr3+ ions as the only acceptor dopant. Lattice parameters show unusual, non-linear Vegard's law behaviour characterised by a maximum in cell volume at intermediate x ∼ 0.005, that is attributed to the composition-dependent presence of Ti3+ ions.

Deposition pressure-induced microstructure control and plasmonic property tuning in hybrid ZnO-Ag x Au1-x thin films.

Self-assembled oxide-metallic alloy nanopillars as hybrid plasmonic metamaterials (e.g., ZnO-Ag x Au1-x ) in a thin film form have been grown using a pulsed laser deposition method. The hybrid films were demonstrated to be highly tunable via systematic tuning of the oxygen background pressure during deposition. The pressure effects on morphology and optical properties have been investigated and found to be critical to the overall properties of the hybrid films. Specifically, low background pressure results in the vertically aligned nanocomposite (VAN) form while the high-pressure results in more lateral growth of the nanoalloys. Strong surface plasmon resonance was observed in the UV-vis region and a hyperbolic dielectric function was achieved due to the anisotropic morphology. The oxide-nanoalloy hybrid material grown in this work presents a highly effective approach for tuning the binary nanoalloy morphology and properties through systematic parametric changes, important for their potential applications in integrated photonics and plasmonics such as sensors, energy harvesting devices, and beyond.

Field-assisted growth of one-dimensional ZnO nanostructures with high defect density.

One-dimensional ZnO nanostructures have shown great potential in electronics, optoelectronics and electromechanical devices owing to their unique physical and chemical properties. Most of these nanostructures were grown by equilibrium processes where the defects density is controlled by thermodynamic equilibrium. In this work, flash sintering, a non-equilibrium field-assisted processing method, has been used to synthesize ZnO nanostructures. By applying a high electric field and limiting a low current flow, ZnO nanorods grew uniformly by a vapor-liquid-solid mechanism due to the extreme temperatures achieved near the hot spot. High density basal stacking faults in the nanorods along with ultraviolet excitonic emission and a red emission under room temperature demonstrate the potential of defect engineering in nanostructures via the field-assisted growth method.

Ceramic Material Processing Towards Future Space Habitat: Electric Current-Assisted Sintering of Lunar Regolith Simulant.

In situ utilization of available resources in space is necessary for future space habitation. However, direct sintering of the lunar regolith on the Moon as structural and functional components is considered to be challenging due to the sintering conditions. To address this issue, we demonstrate the use of electric current-assisted sintering (ECAS) as a single-step method of compacting and densifying lunar regolith simulant JSC-1A. The sintering temperature and pressure required to achieve a relative density of 97% and microhardness of 6 GPa are 700 °C and 50 MPa, which are significantly lower than for the conventional sintering technique. The sintered samples also demonstrated ferroelectric and ferromagnetic behavior at room temperature. This study presents the feasibility of using ECAS to sinter lunar regolith for future space resource utilization and habitation.

Nanoscale stacking fault-assisted room temperature plasticity in flash-sintered TiO2.

Ceramic materials have been widely used for structural applications. However, most ceramics have rather limited plasticity at low temperatures and fracture well before the onset of plastic yielding. The brittle nature of ceramics arises from the lack of dislocation activity and the need for high stress to nucleate dislocations. Here, we have investigated the deformability of TiO2 prepared by a flash-sintering technique. Our in situ studies show that the flash-sintered TiO2 can be compressed to ~10% strain under room temperature without noticeable crack formation. The room temperature plasticity in flash-sintered TiO2 is attributed to the formation of nanoscale stacking faults and nanotwins, which may be assisted by the high-density preexisting defects and oxygen vacancies introduced by the flash-sintering process. Distinct deformation behaviors have been observed in flash-sintered TiO2 deformed at different testing temperatures, ranging from room temperature to 600°C. Potential mechanisms that may render ductile ceramic materials are discussed.