A group of researchers led by recent NPRE PhD Michael A. Jaworski have combined strong magnetic fields with non-uniform temperatures to produce a self-stirring liquid metal system.
The effect holds promise for several areas of science and technology, including metallurgy and fusion energy that use liquid metals. Physical Review Letters has accepted a paper on the work for publication.
Jaworski, who earned his PhD in October 2009, said the phenomenon surprised him and his group.
“The thermoelectric magneto-hydrodynamic (TEMHD) flow was not expected to be the dominant flow driver when we started the project,” he said. “We expected a more ordinary and typical driver to be operating in the experiment based on surface tension effects. TEMHD had never been directly observed before, and I doubted it would be significant even if it were a ‘real effect.’
“It was definitely a “wow, that’s really weird” moment when we were able to watch the flow swirl about the target in a way that wasn’t consistent with the surface tension effect. After several tests, we confirmed the origin of the flow to be TEMHD.”
Jaworski and his collaborators used lithium, a liquid metal that has strong thermoelectric properties and creates currents in non-uniform temperature fields. In the experiments, the thermoelectric currents interacted with the magnetic field and pushed the fluid, resulting in a swirling motion. The resulting velocity was significant, with speeds greater than a foot per second in some cases, and matched a theoretical prediction for the flow.
The technique could lead to a more efficient, cost-effective means of stirring liquid metals.
Many metallurgical processing systems use rotating external magnetic fields to stir molten metals, but power losses result from passing the rotating magnetic fields through conducting walls. Combining a steady magnetic field with a temperature-controlled insert in a material would encourage TEMHD stirring without power losses.
“Since every single metal product one uses passes through a metallurgical process, there is a large potential impact,” Jaworski said.
He also believes the effect could have a global impact on the use of fusion energy.
Theoretically, fusion could provide a non-carbon emitting energy source that would not be prone to solar and wind energy’s intermittency problems. However, fusion is not cost-effective now because science has not yet produced materials to build machines able to withstand fusion’s intense power fluxes.
“One alternative to solid materials are liquid metal plasma-facing-components(PFCs), of which lithium is a lead candidate,” Jaworski said. “If it can be made to work and be controlled, liquid metal PFCs have the potential to address several outstanding issues that are hampering our ability to create power plants based on fusion. TEMHD pumping can be used in the PFCs as well as other areas of the machine.”
Jaworski’s doctoral thesis, “Thermoelectric magnetohydrodynamic and thermocapillary driven flows of liquid conductors in magnetic fields,” was based on this work.
As a master’s degree student in NPRE, Jaworski worked on what are now state-of-the-art light sources for semiconductor chip manufacturers. For his PhD, he chose to bring new direction to previous liquid metal experiments. Upon observing significant results on the CDX-U machine at Princeton’s Plasma Physics Laboratory, Jaworski decided to build a follow-up machine in the Center for Plasma-Material Interactions Laboratory at Illinois, and further examine the phenomena with the support of his advisor, NPRE Prof. David N. Ruzic. In 2006 Jaworski wrote a white paper proposing the apparatus that was built and then culminated in the machine used for the liquid metal stirring experiments.
Said Jaworski, “It took a while since the electron beam is peculiar to the experiment and had to be designed, fabricated and tested. Additionally, the liquid metal container technology and apparatus needed to be designed and fabricated. It was definitely worth it in the end with these results.”
Now, as a research associate at Princeton’s Plasma Physics Laboratory, Jaworski continues to examine the behavior of liquid metals in their use as plasma facing components in fusion experiments.
Working with Jaworski on the Illinois experiments were NPRE Adjunct Assistant Prof. Martin J. Neumann, October 2009 PhD Travis K. Gray, graduate student Wenyu Xu, 2008 NPRE bachelor’s degree graduates Michael K. Antonelli and Cheuk Y. Lau, and Molecular and Cellular Biology undergraduates Jason J. Kim and Matthew B. Lee.