One type of cage structure around the spin-density of one electron in a model of cement-based semiconducting metallic glass (gray=Al; green=Ca; and red=O). Credit: Akola et al.; Argonne National Lab.
Normally in the creation of a metallic glass, the process begins with a metal that is transformed to a glassy state. Now, however, a US-Finland-Germany-Japan effort has demonstrated a novel way to make metallic glass—beginning with a type of cement that is heated with a laser while floating in a container—which leads to the creation a material that surprisingly behaves as a semiconductor at room temperatures. This material, subjected to a variety of experiments (X-ray and neutron diffraction, extended X-ray absorption) was successfully matched with models that help explain this nonintuitive behavior and point the way to the discovery of other unexpected semiconductors.
Back in 2011, Eileen described how a Japanese group found they could convert mayenite, 12CaO · 7Al2O3 (or C12A7), into a semiconductive oxide glass by reacting it with elemental titanium at high temperatures. They described the material then as an electride that traps electrons. While the Japanese group has gone on to focus on developing 2D applications (e.g., thin films) of the material, the US-Finland-Germany-Japan group wanted to focus on understanding the underlying basic science underlying the properties and and how they correspond to the structure of C12A7 and other glasses in the CaO–Al2O3 system.
The researchers focused on several glasses in the system including 64 mol % CaO (64CaO) glass—comparable to the C12A7— and 50CaO glass. One route they took was to combined several computational techniques (e.g., density functional theory and reverse Monte Carlo simulations) to consider what is going on in the new glass, and developed structural models using a supercomputer at Germany’s Forschungszentrum Jülich.
The other route they took was to analyze actual samples of the glass using different X-ray techniques at SPring-8 in Japan combined with earlier measurements at the Intense Pulsed Neutron Source and the Advanced Photon Source. To obtain samples, the group used a fairly ingenious and simple “bulk” (not in the mass production sense, but producing a large enough sample for experimentation) processing technique to convert the cement. They used a tool called an “aerodynamic levitator” in which they heated the cement with a laser to 2,000°C. According to an Argonne National Lab news release, ANL researchers led by Chris Benmore, developed the levitator as part of their work at the lab’s Advaned Proton Source, for in situ measurements, but in this recent work the levitator also prevented the melting cement from touching the sides of the heating vessel. The point of using the levitator with the C12A7 is that it suppressed crystal growth and allowed the material to cool as a glass.
They then compared predictions based on the structural models with the analytical results and found close correlation.
The researchers say what they learned from all this is that as the material cooled, free electrons are trapped via “efficient elemental mixing” in the cage-like structures first described by the Japanese group in 2011. These trapped electrons are at the root of the unusual conductive behavior in the glass:
“[i]t is concluded that the formation of extended cage structures in the C12A7 electride glass, induced by chemical composition and comparable atomic charges for Al and Ca through efficient elemental mixing, is the structural origin of solvated electrons hosted by the cavity sites (cages) … The analysis shows that the lowest unoccupied molecular orbitals occurs in cavity sites, suggesting that the C12A7 electride glass synthesized from a strongly reduced high-temperature melt can host solvated electrons and bipolarons. Calculations of 64CaO glass structures with few subtracted oxygen atoms (additional electrons) confirm this observation. The comparable atomic charges and coordination of the cations promote more efficient elemental mixing, and this is the origin of the extended cage structure and hosted solvated (trapped) electrons in the C12A7 glass.”
“This phenomenon of trapping electrons and turning liquid cement into liquid metal was found recently, but not explained in detail until now,” Benmore says in the ANL release. “Now that we know the conditions needed to create trapped electrons in materials we can develop and test other materials to find out if we can make them conduct electricity in this way.”
The appeal of this type of metallic glass could be very strong because its properties (not brittle, corrosion resistant, easily processed and molded) would provide new nonmetal options for engineers and designers. Benmore is clearly excited and one application he predicts is as thin-film resistors used in liquid-crystal displays. Although the international group had the benefit of the earlier Japanese group’s work to begin their work with mayenite, they also suspected that other materials could go through a similar transition. Indeed, the researchers now say insights and modeling expertise gained in this round of experiments should lead to other novel semiconductors. Benmore says, “Now that we know the conditions needed to create trapped electrons in materials we can develop and test other materials.”
Materials Development Inc.‘s Rick Weber—another member of the research group—is excited by the work that’s been done so far. He says that effort united novel processing, advanced analytical techniques, rigorous modeling and supercomputing to deliver “good, fundamental science.” Weber, who was a speaker at last year’s International Ceramics Congress in Chicago (ICC4), went on to tell me, “It was great that our modeling matched our experimental data, but this was a very nonintuitive result. It is a real-life example of how researchers have to be open to new ideas and how we can use this array of tools to open up new areas of research.”
Besides the methods and data in their research, Weber says with a laugh that the big takeaway for the research group is the need “to be prepared to be surprised!”
Along with investigators from ANL, Forschungszentrum Jülich, SPing-8, and Materials Development Inc., members of the group included staff and students from Tampere University of Technologyand Aalto University (Finland); and the universities of Tokyo, Yamagata and Osaka Prefecture (Japan).
Their work is reported in an open-access paper published in the Proceedings of the National Academy of Sciences in the article “Network topology for the formation of solvated electrons in binary CaO-Al2O3 composition glasses” (doi:10.1073/pnas.1300908110).
Finally, to see one form of aerodynamic levitation/laser heating in action, check out this brief video from Institut Laue-Langevin: