Water evaporation is a ubiquitous natural process that harvests thermal energy from the ambient environment. It has previously been utilized in a number of applications including the synthesis of nanostructures and the creation of energy-harvesting devices. Here, we show that water evaporation from the surface of a variety of nanostructured carbon materials can be used to generate electricity. We find that evaporation from centimetre-sized carbon black sheets can reliably generate sustained voltages of up to 1 V under ambient conditions. The interaction between the water molecules and the carbon layers and moreover evaporation-induced water flow within the porous carbon sheets are thought to be key to the voltage generation. This approach to electricity generation is related to the traditional streaming potential, which relies on driving ionic solutions through narrow gaps, and the recently reported method of moving ionic solutions across graphene surfaces, but as it exploits the natural process of evaporation and uses cheap carbon black it could offer advantages in the development of practical devices.

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We report a novel phase of carbon possessing a monoclinic C2/m structure (8 atoms/cell) identified using an ab initio evolutionary structural search. This polymorph, which we call M-carbon, is related to the (2x1) reconstruction of the (111) surface of diamond and can also be viewed as a distorted (through sliding and buckling of the sheets) form of graphite. It is stable over cold-compressed graphite above 13.4 GPa. The simulated x-ray diffraction pattern and near K-edge spectroscopy are in satisfactory agreement with the experimental data [W. L. Mao, Science 302, 425 (2003)10.1126/science.1089713] on overcompressed graphite. The hardness and bulk modulus of this new carbon polymorph are calculated to be 83.1 and 431.2 GPa, respectively, which are comparable to those of diamond.

Experimental studies established that calcium undergoes several counterintuitive transitions under pressure: fcc → bcc → simple cubic → Ca-IV → Ca-V, and becomes a good superconductor in the simple cubic and higher-pressure phases. Here, using ab initio evolutionary simulations, we explore the behavior of Ca under pressure and find a number of new phases. Our structural sequence differs from the traditional picture for Ca, but is similar to that for Sr. The β-tin (I4 1 ∕amd) structure, rather than simple cubic, is predicted to be the theoretical ground state at 0 K and 33-71 GPa. This structure can be represented as a large distortion of the simple cubic structure, just as the higher-pressure phases stable between 71 and 134 GPa. The structure of Ca-V, stable above 134 GPa, is a complex host-guest structure. According to our calculations, the predicted phases are superconductors with Tc increasing under pressure and reaching approximately 20 K at 120 GPa, in good agreement with experiment.evolutionary algorithms | high pressure | structure prediction | density functional theory | superconductivity C alcium exhibits a nontrivial and somewhat mysterious behavior under pressure. At 19.5 GPa it transforms from the fcc to the body-centered cubic (bcc) structure, and then, at 32 GPa, to the simple cubic (sc) structure (1, 2). Such a sequence of transitions is exactly opposite to normal intuition, as it is accompanied by a decrease of coordination numbers (12 → 8 → 6) and sphere packing efficiency (0.74 → 0.68 → 0.52). Good metal at ambient conditions, fcc-Ca shows increasing electrical resistivity under pressure (3-5). Even more intriguingly, the resistivity of the fcc phase at and just below this maximum has negative temperature derivative, characteristic of the nonmetallic (semiconducting) state, consistent with a small band gap found in ab initio calculations (6) in the same pressure range. Such demetallization under pressure is counterintuitive, because at sufficiently high pressure all materials must become free-electron metals, the expected behavior is an increasing tendency to the free-electron limit under pressure (see ref. 7 for a discussion). Contrary to these expectations, strong departure from the free-electron state under pressure has also been found for sodium (8, 9) and lithium (10-12) at megabar pressures.Ab initio calculations (13) confirmed the fcc → bcc → sc structure sequence and yielded reasonably accurate values for the transition pressures. However, the sc phase encounters problems: It cannot be explained within the Hume-Rothery approach (Fermi surface-Brillouin zone interaction) unless one assumes 4 valence electrons per atom (14), and, even more seriously, lattice dynamics calculations (15, 16) showed that it is dynamically unstable, and although this dynamical instability may be lifted by anharmonic effects (16), other structures (see below) have much lower enthalpies. This seeming contradiction with experiments that initially showed a perfect sc structure (1, 2, 17) is largely resolved by re...

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