Richard W. Geyer

Amorphous-crystalline transition in thermoelectric NbO2

Density functional theory was employed to design enhanced amorphous NbO2 thermoelectrics. The covalent-ionic nature of Nb–O bonding is identical in amorphous NbO2 and its crystalline counterpart. However, the Anderson localisation occurs in amorphous NbO2, which may affect the transport properties. We calculate a multifold increase in the absolute Seebeck coefficient for the amorphous state. These predictions were critically appraised by measuring the Seebeck coefficient of sputtered amorphous and crystalline NbO2 thin films with the identical short-range order. The first-order phase transition occurs at approximately 550 °C, but amorphous NbO2 possesses enhanced transport properties at all temperatures. Amorphous NbO2, reaching −173 μV K−1, exhibits up to a 29% larger absolute Seebeck coefficient value, thereby validating the predictions.

Thermomechanical response of thermoelectrics

We have theoretically investigated the product of elastic modulus and linear coefficient of thermal expansion for 20 thermoelectrics. The product is inversely proportional to equilibrium volume, which is consistent with the Debye-Gruneisen model. Oxides exhibit larger products, while the products of Te-containing thermoelectrics are considerably smaller. This is likely due to strong bonding in these oxides, which makes them prone to thermal stress, thermal shock, and thermal fatigue. As this product is rarely available in literature and the equilibrium volume is easily measurable, this work provides a quick estimation for the thermomechanical response of thermoelectric phases.

High-throughput exploration of thermoelectric and mechanical properties of amorphous NbO2 with transition metal additions

To increase the thermoelectric efficiency and reduce the thermal fatigue upon cyclic heat loading, alloying of amorphous NbO2 with all 3d and 5d transition metals has systematically been investigated using density functional theory. It was found that Ta fulfills the key design criteria, namely, enhancement of the Seebeck coefficient and positive Cauchy pressure (ductility gauge). These quantum mechanical predictions were validated by assessing the thermoelectric and elastic properties on combinatorial thin films, which is a high-throughput approach. The maximum power factor is 2813 μW m−1 K−2 for the Ta/Nb ratio of 0.25, which is a hundredfold increment compared to pure NbO2 and exceeds many oxide thermoelectrics. Based on the elasticity measurements, the consistency between theory and experiment for the Cauchy pressure was attained within 2%. On the basis of the electronic structure analysis, these configurations can be perceived as metallic, which is consistent with low electrical resistivity and ductil...

Theoretical and experimental study of NbO2 nanoslice formation

We have used reactive magnetron sputtering to form NbO2 nanoslices. These unique nanostructures with the dominating diffractions peaks related to low surface energy NbO2 planes are self-assembled into two different populations: aligned and random nanoslices. To provide an explanation for the formation of these NbO2 nanoslices with respect to competitive growth of nanorods, we employed density functional theory based molecular dynamics. (1 1 0) and (0 0 1) surfaces together with the related nanoslices and nanorods were considered, corresponding to low and high surface energy configurations, respectively. For the (1 1 0) configuration, nanoslices are more stable at all temperatures (0 K–900 K), which is consistent with experimental observations. On the other hand, for the (0 0 1) configuration nanorods are more stable at and above room temperature. We suggest that the texture modulation is the physical origin of NbO2 nanostructure formation.

Modulation of transport properties of RuO2 with 3d transition metals

Using density functional theory, we have demonstrated that alloying of RuO2 (P42/mnm) with 3d transition metals (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) gives rise to a substantial increase in the Seebeck coefficient probably due to quantum confinement. As Fe yields the largest enhancement, it was selected for experimental verification. We synthesized combinatorial Ru–Fe–O thin films and subsequently measured their transport properties at elevated temperatures. The Fe-alloyed samples increase the Seebeck coefficient threefold with respect to the unalloyed RuO2 specimen thereby verifying the theoretical prediction. The here obtained power factor of 274 μW K−2 m−1 is not only the largest reported value for RuO2 based compounds but it also occurs at ~600 °C thus increasing the Carnot efficiency significantly.

Atomistic growth phenomena of reactively sputtered RuO2 and MnO2 thin films

We have synthesized RuO2 and MnO2 thin films under identical growth conditions using reactive DC sputtering. Strikingly different morphologies, namely, the formation of RuO2 nanorods and faceted, nanocrystalline MnO2, are observed. To identify the underlying mechanisms, we have carried out density functional theory based molecular dynamics simulations of the growth of one monolayer. Ru and O2 molecules are preferentially adsorbed at their respective RuO2 ideal surface sites. This is consistent with the close to defect free growth observed experimentally. In contrast, Mn penetrates the MnO2 surface reaching the third subsurface layer and remains at this deep interstitial site 3.10 A below the pristine surface, resulting in atomic scale decomposition of MnO2. Due to this atomic scale decomposition, MnO2 may have to be renucleated during growth, which is consistent with experiments.

Theoretical and experimental study of NbO 2 nanoslice formation

We have used reactive magnetron sputtering to form NbO2 nanoslices. These unique nanostructures with the dominating diffractions peaks related to low surface energy NbO2 planes are self-assembled into two different populations: aligned and random nanoslices. To provide an explanation for the formation of these NbO2 nanoslices with respect to competitive growth of nanorods, we employed density functional theory based molecular dynamics. (1 1 0) and (0 0 1) surfaces together with the related nanoslices and nanorods were considered, corresponding to low and high surface energy configurations, respectively. For the (1 1 0) configuration, nanoslices are more stable at all temperatures (0 K–900 K), which is consistent with experimental observations. On the other hand, for the (0 0 1) configuration nanorods are more stable at and above room temperature. We suggest that the texture modulation is the physical origin of NbO2 nanostructure formation.

Theoretical and experimental study of NbO2nanoslice formation

We have used reactive magnetron sputtering to form NbO2 nanoslices. These unique nanostructures with the dominating diffractions peaks related to low surface energy NbO2 planes are self-assembled into two different populations: aligned and random nanoslices. To provide an explanation for the formation of these NbO2 nanoslices with respect to competitive growth of nanorods, we employed density functional theory based molecular dynamics. (1 1 0) and (0 0 1) surfaces together with the related nanoslices and nanorods were considered, corresponding to low and high surface energy configurations, respectively. For the (1 1 0) configuration, nanoslices are more stable at all temperatures (0 K–900 K), which is consistent with experimental observations. On the other hand, for the (0 0 1) configuration nanorods are more stable at and above room temperature. We suggest that the texture modulation is the physical origin of NbO2 nanostructure formation.