X-Ability (Japan)

Effective management of reusable waste heat is one of the most important technologies for reducing energy consumption and promoting energy conservation. In particular, chemical heat storage is a practical solution to the spatial and temporal mismatches of energy use. Among various chemical reactions, the reversible hydration process of periclase (MgO) to brucite [Mg(OH)2] is a practical candidate for the future applications in various social systems. In this article, in order to clarify the atomistic reaction mechanism of thermochemical heat storage in the MgO hydration reaction, we calculated potential energy profiles to understand the complex thermochemical reaction on the solid surface. By assuming a series of elementary reaction processes in the early stage of the surface reaction, we have practically estimated the reaction energetics for the hydration reaction in both the MgO(001) terrace and step geometries. First, we calculate two important thermodynamic parameters that characterize the thermochemical reaction (the reaction heat and the transition temperature). Our computational results show good agreement with the available thermodynamic data. Then, to clarify the nature of the thermochemical heat storage process, we introduce the 2D potential energy diagram, which is determined by the dislocation distance of the Mg atoms and the coordination (solvation) number of the Mg atoms. This 2D potential energy diagram clearly shows that the activation energy is highly dependent on the surface geometry of MgO, while an atomistic process in this rate-determining step (an extraction of Mg atoms from the inner layer to the surface aqueous layer) is exactly the same in both the terrace and step surface. These computational results lead to a simple guideline for the design of thermochemical heat storage systems.

We perform large-scale density-functional calculations using the real-space finite-difference scheme endorsed by the Gordon Bell prize in 2011 that reveal detailed atomic and electronic structures of atomic steps on silicon carbide (SiC) polar surfaces for the first time. The accurate structural optimization elucidates characteristic atomic reconstruction among the upper and lower edge atoms, which is peculiar to compound semiconductors having both covalent and ionic nature. The calculated formation energies of all the possible atomic steps lead us to unequivocally identify the abundant atomic steps on the Si-face SiC polar surfaces. The energetics thus obtained for the atomic steps provides a natural and persuasive microscopic reason for the difference in the step morphology observed experimentally, i.e., the meandering and straight step edges depending on the inclined direction on the polar vicinal SiC surfaces. Electron states caused by those atomic steps are also calculated, which assists in the identification of the atomic steps by future experiments.

ABSTRACT Data mining from published papers can generate large experimental datasets that have been overlooked in computational materials informatics. We developed an open web system Starrydata2 to accelerate a comprehensive digitization of data of materials from as-reported plot images in published papers, without sample selection based on performance. By plotting results obtained from our dataset on experimental thermoelectric properties of 434 samples of rock-salt-type (PbTe-type) thermoelectric materials, we revealed differences in electronic structure of parent compounds PbTe, PbSe, PbS, and SnTe from just experimental data. We observed that the calculated Seebeck coefficients were fairly consistent with experimental data for n-type PbTe but not for p-type PbTe, indicating possible modifications in its valence-band electronic structure. We evaluated the electron relaxation time τ el from 207 reported samples of n-type PbTe by combining calculations and experimental data. We found that τ el is not a constant but varies by at least two orders of magnitude. Achieving long τ el was suggested to be critical in increasing the thermoelectric figure of merit ZT.