Suchergebnisse

The Earth's solid inner core is a highly attenuating medium. It consists mainly of iron. The high attenuation of sound wave propagation in the inner core is at odds with the widely accepted paradigm of hexagonal close-packed phase stability under inner core conditions, because sound waves propagate through the hexagonal iron without energy dissipation. Here we show by first-principles molecular dynamics that the body-centered cubic phase of iron, recently demonstrated to be thermodynamically stable under the inner core conditions, is considerably less elastic than the hexagonal phase. Being a crystalline phase, the body-centered cubic phase of iron possesses the viscosity close to that of a liquid iron. The high attenuation of sound in the inner core is due to the unique diffusion characteristic of the body-centered cubic phase. The low viscosity of iron in the inner core enables the convection and resolves a number of controversies. ; Computations were performed using the facilities at the Swedish National Infrastructure for Computing (SNIC) located at the National Supercomputing Center in Linköping. The authors also wish to thank the Swedish Research Council (VR) for financial support (Grants 2013-5767, 2014-4750, and 2017-03744) and National Natural Science Foundation of China (Grant No. 11804175). A.B.B. and T.B. acknowledge support from Olle Engkvist Byggmästare Foundation. S.I.S. acknowledges the support from the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-MatLiU No. 2009 00971). M.M. acknowledges financial support by the Spanish Ministry of Economy and Competitiveness (CGL2013- 41860-P and CGL2017-86070-R).

CaMnNbO (x = 0, 0.5, 0.6, 0.7 and 0.10) thin films have been grown by a two-step sputtering/annealing method. First, rock-salt-structured (Ca,Mn,Nb)O thin films were deposited on 1100 sapphire using reactive RF magnetron co-sputtering from elemental targets of Ca, Mn and Nb. The CaMnNbO films were then obtained by thermally induced phase transformation from rock-salt-structured (Ca,MnNb)O to orthorhombic during post-deposition annealing at 700 °C for 3 h in oxygen flow. The X-ray diffraction patterns of pure CaMnO showed mixed orientation, while Nb-containing films were epitaxially grown in [101] out of-plane-direction. Scanning transmission electron microscopy showed a Ruddlesden-Popper (R-P) secondary phase in the films, which results in reduction of the electrical and thermal conductivity of CaMnNbO. The electrical resistivity and Seebeck coefficient of the pure CaMnO film were measured to 2.7 Ω cm and -270 μV K at room temperature, respectively. The electrical resistivity and Seebeck coefficient were reduced by alloying with Nb and was measured to 0.09 Ω cm and -145 μV K for x = 0.05. Yielding a power factor of 21.5 μW K m near room temperature, nearly eight times higher than for pure CaMnO (2.8 μW K m). The power factors for alloyed samples are low compared to other studies on phase-pure material. This is due to high electrical resistivity originating from the secondary R-P phase. The thermal conductivity of the CaMnNbO films is low for all samples and is the lowest for x = 0.07 and 0.10, determined to 1.6 W m K. The low thermal conductivity is attributed to grain boundary scattering and the secondary R-P phase. ; The authors acknowledge the funding from the Swedish Foundation for Strategic Research (SSF) through the Future Research Leaders 5 program, the Swedish Research Council (VR) under Project No. 2016-03365 and 2014-4750, the Knut and Alice Wallenberg Foundation through the Wallenberg Academy Fellows program and through support of the Electron Microscopy Laboratory at Linköping University, the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971), the Swedish Energy Agency under project 46519-1, and the Spanish Ministry through the project MAT2017-86450-C4-3-R.