Atomistic modelling of adsorption and segregation at inorganic solid interfaces

The impact of tilt grain boundaries on the thermal transport in perovskite SrTiO3 layered nanostructures. A computational study

Thermal management at solid interfaces presents a technological challenge for modern thermoelectric power generation. Here, we define a computational protocol to identify nanoscale structural features that can facilitate thermal transport in technologically important nanostructured materials. We consider the highly promising thermoelectric material, SrTiO3, where tilt grain boundaries lower thermal conductivity. The magnitude of the reduction is shown to depend on compositional and structural arrangements at the solid interface. Quantitative analysis indicates that layered nanostructures less than 10 nm will be required to significantly reduce the thermal conductivity below the bulk value, and it will be virtually independent of temperature for films less than 2 nm depending on the orientation with a reduction of thermal transport up to 75%. At the nanoscale, the vibrational response of nanostructures shows concerted vibrations between the grain boundary and inter-boundary regions. As the grain boundary acts markedly as a phonon quencher, we predict that any manipulation of nanostructures to further reduce thermal conductivity will be more beneficial if applied to the inter-boundary region. Our findings may be applied more widely to benefit other technological applications where efficient thermal transport is important.

Crystal structure and thermoelectric properties of Sr-Mo substituted CaMnO3: a combined experimental and computational study

A combination of experimental and computational techniques has been employed to study doping effects in perovskite CaMnO₃. High quality Sr-Mo co-substituted CaMnO₃ ceramics were prepared by the conventional mixed oxide route. Crystallographic data from X-ray and electron diffraction showed an orthorhombic to tetragonal symmetry change on increasing the Sr content, suggesting that Sr widens the transition temperature in CaMnO₃ preventing phase transformation-cracking on cooling after sintering, enabling the fabrication of high density ceramics. Atomically resolved imaging and analysis showed a random distribution of Sr in the A-site of the perovskite structure and revealed a boundary structure of 90° rotational twin boundaries across {101}_orthorhombic; the latter are predominant phonon scattering sources to lower the thermal conductivity as suggested by molecular dynamics calculations. The effect of doping on the thermoelectric properties was evaluated. Increasing Sr substitution reduces the Seebeck coefficient but the power factor remains high due to improved densification by Sr substitution. Mo doping generates additional charge carriers due to the presence of Mn³⁺ in the Mn⁴⁺ matrix, reducing electrical resistivity. The major impact of Sr on thermoelectric behaviour is the reduction of the thermal conductivity as shown experimentally and by modelling. Strontium containing ceramics showed thermoelectric figure of merit (ZT) values higher than 0.1 at temperatures above 850 K. Ca_0.7Sr_0.3Mn_0.96Mo_0.04O₃ ceramics exhibit enhanced properties with S_1000K = -180 μV K^-1, ρ_1000K = 5 × 10^-5 Ωm, k_1000K = 1.8 W m^-1 K^-1 and ZT ≈ 0.11 at 1000 K.

The structure and dynamics of hydrated and hydroxylated magnesium oxide nanoparticles

An understanding of the structure of water on metal oxide nanoparticles is important due to its involvement in a number of surface processes, such as in the modification of transport near surfaces and the resulting impact on crystal growth and dissolution. However, as direct experimental measurements probing the metal oxide-water interface of nanoparticles are not easily performed, we use atomistic simulations using experimentally derived potential parameters to determine the structure and dynamics of the interface between magnesium oxide nanoparticles and water. We use a simple strategy to generate mineral nanoparticles, which can be applied to any shape, size, or composition. Molecular dynamics simulations were then used to examine the structure of water around the nanoparticles, and highly ordered layers of water were found at the interface. The structure of water is strongly influenced by the crystal structure and morphology of the mineral and the extent of hydroxylation of the surface. Comparison of the structure and dynamics of water around the nanoparticles with their two-dimensional flat surface counterparts revealed that the size, shape, and surface composition also affects properties such as water residence times and coordination number.