researchers at the massachusetts institute of technology (mit) in cambridge, mass. have developed a new, container-less electrochemical method for studying the thermodynamic properties of materials such as aluminum oxide that have ultra-high melting points.

a molten hanging droplet, or pendant (at center), forms from an aluminum oxide rod under intense heat and light from xenon lamps. (melody m. wang)

according to an article on the mit website, this new method demonstrated that the rules of electrochemistry are followed in refractory melts like aluminum oxide, which melt at temperatures above 2,000°c. researchers noted that the melts are “very stable at high temperature.”

the article explained, “adapting a thermal imaging (or arc imaging) furnace more commonly used for floating zone crystal growth, mit graduate student brad nakanishi melted an alumina (aluminum oxide) rod and contacted the liquid pendant droplet that it formed with electrodes, creating an electrochemical cell that allowed decomposition of pure, alumina electrolyte to oxygen gas and aluminum alloy by electrolysis for the first time.”

using decomposition voltage measurements, researchers were able to measure chemical potential, knowns as gibbs energy. the change in gibbs energy is entropy, which is a key to understanding the processes involved in high-temperature melts.

“using this technique, four reflected xenon lamps hone in on the tip of the sample, melting a liquid droplet, which is held to the rod by surface tension and quickly solidifies after the lights are turned off,” the article described. “while the droplet is liquefied, the electrodes are raised into the droplet to complete an electrical circuit, with the liquid alumina itself functioning as the electrolyte.”

although this method seems daunting, the researchers insist it is straightforward and provides a stable droplet and electrode contact. video of the process shows oxygen gas bubbles forming as alumina decomposes into aluminum at the cathode and pure oxygen at the iridium anode.

this method has a number of potential applications in industries, such as measuring how hot a turbine engine can run. the research adds quantitative measurements to previous simulations.

“future work,” the article continued, “will focus on applying these high-temperature electrochemical techniques to investigate the potential for selectively separating the rare earth oxides. though required in only relatively small quantities usually, the individual rare earth elements are essential for high-tech applications, including cell phones and electric vehicles.”

the research was recently published in journal of the electrochemical society. the abstract stated:

“limited knowledge of the thermodynamic and transport properties of refractory materials in the liquid state remains a key challenge limiting their application. using alternating current (ac) and direct current (dc) techniques, the electrochemical kinetics of oxygen evolution and metal deposition was investigated in a pendant droplet of molten alumina (al₂o₃) with three iridium (ir) electrodes in a thermal imaging furnace.

“for the first time, the direct electrolytic decomposition of molten al₂o₃ to oxygen gas and aluminum (al) metal (alloyed with ir) was observed, confirming the ionic nature of molten al₂o₃. the decomposition potential of molten al₂o₃ was measured with high precision using ac voltammetry, and the results demonstrated remarkable sensitivity to variation in temperature enabling measurement of chemical potential and entropy of al at the ir-rich solid-liquid phase boundary for the first time.

“the results were in remarkably close agreement with the most recent thermodynamic assessment of the al-ir system.”

watch the video below to see electrochemical cell as alumina decomposes: