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July 26th, 2011

Metallic conductivity in high-temperature cement

Published on July 26th, 2011 | By: Eileen De Guire

Crystal structure of Mayenite (12CaO-7Al2O3). Credit: Paul Myers, CERAM Research Ltd. on AZoM

With the Cements Division meeting wrapping up in Nashville, Tenn. this week, an article in the July 1 issue of Science caught my eye. It pulls together an unusual gang: cement, iron smelting slag, crystal chemistry and quantum physics. The work shows how insulating, light-metal oxides can be transformed into electrical conductors at high temperatures, effectively becoming metallic cements.

Since the early 1800s chemists have known that solutions of alkali metals dissolved in polar solvents like water or ammonia have interesting properties. For example, dilute alkali-ammonia solutions are bright blue and exhibit electrolytic conductivity. Concentrated solutions are a striking golden-bronze and exhibit metallic conductivity. In ammonia, the alkali valence electrons are ionized quickly and released into the solution.

Nearly a century ago, Kraus described  (J. Am. Chem. Soc. 36 864 (1914)) the electrons, the charge carriers in the solution, as “the negative electron surrounded with an envelope of solvent molecules,” that is, the electron is surrounded by ammonia molecules. A few years later, Gibson and Argo (Phys. Rev. 7, 33 (1916)) named these surrounded electrons “solvated electrons.” (The subject of solvated ions came up in an earlier post on anomalous supercapacitance observations.)

Metal-amine solutions can be condensed ionic solids, known as electrides, in which the electrons are trapped in the compound’s structural cavities or channels. However, organic-based electrides, such as crown ethers, are not thermally stable.

Kim et. al., based in Japan, wondered whether a thermally stable electride material could be found, and in earlier research, were able to synthesize thermally stable inorganic electrides from calcium aluminate, 12CaO-7Al2O3 (or C12A7). This compound is known by geologists as mayenite and by cement chemists as one of the components in alumina cement. Mayenite is also a constituent of the slag produced by the iron smelting process. The electride compound, designated as C12A7:O2-, traps O2- ions but has no charge carriers in the molten state because CaO and Al2O3 are stable, electrically insulating oxides.

By reacting C12A7:O2- with elemental titanium at high temperatures, an electride can by made that traps an electron instead on an ion (C12A7:e). The question the Kim team sought to answer is whether solvated electrons exist in the molten C12A7:e the same way solvated electrons exist in metal–ammonia solutions. (See Kim, et. al. in Science, Vol. 33, doi: 10.1126/science.1204394)

It turns out they do. During the reaction with titanium, electrons are trapped at the oxygen ion vacancies and coordinated-solvated-by calcium within the cage-like structure. Like the metal–ammonia solvated solutions, the C12A7 melt transforms from a transparent, insulating C12A7:O2- melt to a colorful, electrically conducting C12A7:e melt.

When the concentration of solvated electrons in solution reaches high enough levels (~1021 electrons/cm3), the electrical conductivity becomes metallic. In a Perspectives article in the same issue of Science, Peter Edwards remarks, “This must surely be one of the most unusual and spectacular observations of the transition to the metallic state—turning liquid cement into liquid metal.” The metallic conductivity comes about by extensive delocalization of the solvated wave functions across the melt.

The Kim team took the experiment one step further and studied glasses made from C12A7:e. Using a wide range of tools like Raman spectroscopy optical absorption spectroscopy, electron spin resonance measurements and iodometry, the atomic structure of the glass was established. They found that the solvated electrons are frozen into the glass, but the majority of them adopt a two-electron, spin-paired state. That is, instead of overlapping wave functions, the electrons pair off to form peanut shaped bipolaron structures, and the result is amorphous, semiconductive oxide glass.

As Edwards says, Kim’s work “represents a material showing the ultimate confinement of a quantum particle—an electron “set” in cement.” Both Kim and Edwards suggest that the ability to tune electrical conductivity of melts, slags and glasses should lead to new applications for the light-metal oxide, semiconducting class of materials. Kim et al., expect there may be other inorganic compounds that can be electrides, and that this work will lead also to the study of elemental electride materials under high pressure.

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