Pyroelectricity is a very promising phenomenon in three- and two-dimensional materials, but first-principles calculations have not so far been used to elucidate the underlying mechanisms. Here we report density-functional theory (DFT) calculations based on the Born-Szigeti theory of pyroelectricity, by combining fundamental thermodynamics and the modern theory of polarization. We find satisfactory agreement with experimental data in the case of bulk benchmark materials, showing that the so-called electron-phonon renormalization, whose contribution has been traditionally viewed as negligible, is important. We predict out-of-plane pyroelectricity in the recently synthesized Janus MoSSe monolayer and in-plane pyroelectricity in the group-IV monochalcogenide GeS monolayer. It is notable that the so-called secondary pyroelectricity is found to be dominant in GeS monolayer. The present work opens a theoretical route to study the pyroelectric effect using DFT and provides a valuable tool in the search for new candidates for pyroelectric applications.
First-principles calculations are made for the primary pyroelectric coefficients of wurtzite GaN and ZnO. The pyroelectricity is attributed to the quasiharmonic thermal shifts of internal strains (internal displacements of cations and anions carrying their Born effective charges). The primary (zero-external-strain) pyroelectricity dominates at low temperatures, while the secondary pyroelectricity (the correction from external thermal strains) becomes comparable with the primary pyroelectricity at high temperatures. Contributions from the acoustic and the optical phonon modes to the primary pyroelectric coefficient are only moderately well described by the corresponding Debye function and Einstein function respectively.
The underlying wettability alteration mechanism responsible for enhanced oil recovery (EOR) in carbonate reservoir is a long-standing issue for which no consensus has been reached yet. In this paper, we report extensive quantum molecular dynamics simulations to reveal the roles of wettability modifiers Na + , Cl-, Ca 2+ , Mg 2+ , and SO 4 2ions in EOR. Characterizing wettability by contact angle using the work-of-adhesion approach, we find that the calcite surface is strongly hydrophilic, with the first two wetting layers hindering all ions from reaching the surface, i.e. ions are only "proximally adsorbed". Na + and Clions settle closer to the surface and actually disturb the interfacial water structure of the first two wetting layers, which renders the surface less water-wet and thus inhibits oil recovery, as observed. Ca 2+ , Mg 2+ and SO 4 2ions, on the other hand, settle farther from the surface and retain the interfacial water structure, but render the surface more water-wet by modifying the effective charge on the surface, which enhances oil recovery, as observed. The impact of the ions is more pronounced at high temperatures as proximal adsorption is enhanced. In addition to theory, we report new core flooding measurements that corroborate the theoretical results. The present study brings new insights into the wettability alteration mechanism in EOR at the atomic scale.
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