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Characterization of Eutectic Lead-Bismuth Alloys Used in Liquid Metal Batteries

Rakan F. Ashour

Materials Science Program, University of Rochester

Introduction:

Liquid metal batteries were developed in 2012 by Donald Sadoway and his group from MIT and is now being commercialized for worldwide grid scale energy storage by Ambri. Our goal in the mixing-lab is to be able to enhance the performace of liquid metal batteries by monitoring and controlling the flow using ultrasound velocimetry. The setup used in ultrasound measurements is shown bellow

Ultrasound probeBattery Velocity Profile
Experimental setup of ultrasound measurement in liquid metal electrodes (left). ultrasound velocity profile (left).  images obtained from [1]
 the setup is shown for a single electrode. However, the same setup can be used for a full operating battery. The Cu plate provides the vessel with the necessary temperature to maintain battery components in liquid state. The entire vessel is placed inside an insulating container and an ultrasound probe is inserted for the side. The entire setup is placed in a glove box to minimize metal oxide formation, contamination and chemical reactivity.


Although successful measurements were obtained using this setup, there are still many obstacles with using ultrasound in liquid metal batteries.One of the major obstacles in this technique is the formation of a solid layer of metal oxide on the surface of the ultrasound probe which can interfere with the signal. This can make the process of signal aquisition and processing quite problematic.
To understand the driving force of metal oxide formation on the surface of the probe in argon filled evironment, we analyzed the interface between solid ePbBi  and the epoxy probe using scanning electron microscopy in both the secondary electrons and backscattered electrons imaging modes. To obtain compositional analysis, EDAX data is obtained at the interface and on the bulk of the ePbBi sample. Analysis of the depth of signal generation in the sample is obtained using electron flight simulation




Materials & Methods
Epoxy ultrasound probes are cleaned with acetone and placed under vacuum for 24 hourse before entering the glove box. ePbBi is mealted at a temperature of T=150 C. The melted metal is poured on to the different surfaces and left to solidify. The samples are then cut and mechanically polished using sand paper to reveal the interface between ePbBi and epoxy.
Zeiss Auriga CrossBeam SEM-FIB equipped with EDXA spectrometer is used for imaging and x-ray analysis.

Imaging and Results:


Low-mag-BSD
Low Magnification backscattered electron image of polished ePbBi interface. The dark regions represent the surface of the epoxy

SE2-tilted interface
Secondary electrons (SE2) image
at large working distance showing the debree of the polished surface.

small WD SE2

SE2 image of the polished surface of the alloy at small working distance

Low-mag-BSD-interface

Low magnification and large working distance Backscattered electrons (BSD) image of the ePbBi and epoxy interface

BSD-interface

large working distance  Backscattered electrons (BSD) image of the ePbBi and epoxy interface

SE2-image of the interface

SE2 image of the interface at large working distance (Charging is observed since that the surface is not coated)


Energy Dispersive X-ray Analysis (EDAX):



interface 1st day Spectrum

EDAX spectrum at 10KV beam energy. The spectrum is obtained using a reduced box on the polished interface

Away from the interface


EDAX spectrum at 10 KV taken on the bulk of ePbBi away from the interface


EDAX analysis indicate that the relative concentration of oxygen is greater at the interface than the bulk of the alloy. This might the result of adsorbed moisture and contaminants on the surface causing metal oxidation at the probe interface.

Electron flight simulation:

In this section Monte Carlo flight simulation is performed on the PbBi and epoxy samples . Understanding the interaction of beam electrons with the sample provides useful information regarding the size of the interaction volume which in turn allows for the determination of the depth of x-ray and the resolution of the signal.


simulation

Electron flight simulation of the interaction of beam electrons with PbBi at 10Kv

simulation-epoxy

Electron flight simulation of the interaction of beam electrons with Epoxy at 10Kv

Optical Microscope Images:

interface
Optical Microscope Image of ePbBi interface with epoxy

real color
Optical Microscope Image of ePbBi interface with epoxy


Silicon Adhesive interface

In many ultrasound velocity measurements, the difference in impedence between different mediums can cause the ultrasound signal to be reflected. To minimize this effect a coupling fluid is applied to ultrasound probe. In this case, a silicon based adhesive is applied as an attempt to minimize difference in impedence between the two mediums.

Si-interface

SE2 image of the Si- adhesive interface with ePbBi

si-interface-1

SE2 image of the Si- adhesive interface with ePbBi


Clearly, the Si adhesive provides a very rough surface which can degrade the wettability of PbBi on the ultrasound probe.


Artistic view of PbBi and its interface:


"All great scientists have, in a certain sense, been great artists; the man with no imagination may collect facts, but he cannot make great discoveries" Karl Pearson

beach

Inlens image of PbBi interface with polished epoxy. The image was colored using ImageJ. The sandy beach represents the surface of PbBi and the water in this case is the epoxy.

Hell

SE2 image of the Si-Adhesive interface with the PbBi. The image was colorized using ImageJ. The blazing fire of hell is the effect of electron charging on the surface of the Si.
 
night  mid-day  sun-rise

SE2 image resembling a day in the desert sunrise (right) mid-day (center) and night time (left)



Conclusions:

The goal of this project was to study the interface between eutectic lead bismuth (ePbBi) and epoxy in order to get a closer look at the oxide layer forming near  the interface. By taking EDAX spectrum, it seems that the relative concentration of oxygen is greater at the interface than that on the bulk of the sample. The increased oxygen concentration near the interface provides a hint that moisture and adsorbed contaminant might be driving the oxidation near the interface.

Acknowledgements:

I would like to thank Brian McIntyre for his unique method of teaching and good humor. I would also like to thank Douglas Kelley for the valuable discussions and for providing the necessary samples.

References

1.  D. H. Kelley and D. R. Sadoway, “Mixing in a liquid metal electrode,” Phys. Fluids, vol. 26(5), 057102, 2014.

2.  K. Wang, K. Jiang, B. Chung, T. Ouchi, P. J. Burke, D. a. Boysen, D. J. Bradwell, H. Kim, U. Muecke, and D. R. Sadoway, “Lithium–antimony–lead liquid metal battery for grid-level energy storage,” Nature, vol. 514, no. 7522, pp. 348–350, 2014.


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