(735p) Reactive Wetting between Gallium-Based Liquid Metals and Cu, Ni, Ti, and Au Substrates

Growing global energy demand, coupled with the urgent need to achieve net-zero carbon emissions by 2050, requires revolutionary advances in energy storage and management systems. Researchers have long utilized conventional phase change materials (PCMs), such as paraffins and salt hydrates, to increase thermal energy storage and absorb peak energy loads. However, these materials lack sufficient thermal conductivity to achieve sufficient thermal charge and discharge rates (greater than 1 W/cm²), especially for use in concentrated solar receivers, and high-power transient electromagnetic systems [1], [2], [3]. As a strategy to quantify the relative capacity of a PCM to rapidly charge or discharge thermal energy, a figure of merit (FOM) has been derived from the analytical solution to the 1D two-phase Neumann—Stefan problem which relates the rate of thermal energy storage to specific PCM thermophysical properties [4]. This performance metric suggests that low melting point liquid metals exhibit 1 to 2 orders of magnitude greater cooling capacity compared to conventional PCMs when utilized in heat sinks with similar boundary conditions [5]. Among these, gallium-based liquid metals (GBLMs) represent one candidate metallic PCM system of great interest due to their following material properties: 1) large thermal conductivities, greater than 30 W/m/K [4]; 2) large volumetric latent heat of melting, greater than 400 MJ/m³ [6]; and 3) low melting point temperatures, less than 30 ℃ [6].

Despite this interest, the interfacial characteristics of GBLMs with thermally conductive metallic heat exchanger surfaces or containment vessels are underdeveloped, and have previously demonstrated problematic characteristics including (1) formation of brittle intermetallic phases which can spall off, and dissolution of gallium and subsequent embrittlement of metallic substrates, such as Al alloys [7]; (2) depletion and composition modulation of GBLMs due to selective diffusion of GBLMs elements into the metallic substrate and the formation of intermetallic phases; and (3) the large surface tension of GBLMs, which generally exceeds 700 mN/m [8], thus inhibiting wetting in metallic surfaces; and (4) the near-instantaneous formation of oxide layers on GBLMs when handled in the air that pins the liquid’s fluidity [9].

This study is motivated by the hypothesis that there exists a quantifiable maximum intermetallic layer thickness due to a self-limiting intermetallic reaction that also promotes reactive wetting (a progressive decrease in contact angle and surface tension over time due to the formation of highly wettable intermetallic phases between the GBLM and the metallic substrate), see Fig. 1. Cu, Ni, Au, and Ti, metallic substrates were chosen to represent a range of reactivity and thermal conductivities. The GBLM’s contact angle and baseline length (the baseline length being the droplet’s diameter that contacts the interface) was measured via a dynamic contact angle meter [10]. Additionally, GBLM droplets were placed on the substrates for various times and temperatures. The passivating nature and stability of the intermetallic reactions were discovered using back-scatter electron microscopy and measurement of diffusion profile thicknesses in polished cross-sections by wavelength dispersive spectroscopy. The activation energy and time exponents of these reactions will be calculated. We used X-ray diffraction and X-ray photoelectron spectroscopy to analyze the chemical composition of the intermetallic layer. Perturbations of the system included using various bulk metallic substrate compositions, thicknesses of thin-film metallic substrates, and surface roughness of the sputter-deposited thin-film substrates. Through a systematic exploration of the interfacial dynamics governing GBLM-metallic substrate interactions, this project has the potential to increase the technology readiness level for utilizing GBLMs in metallic thermal management systems.

References:

[1] Woods, J., and et al., 2021, “Rate capability and Ragone plots for phase change thermal energy storage,” Nature Energy, 6(1), pp. 295-302.

[2] Ge, H., and et al., 2013, “Low melting point liquid metal as a new class of phase change material: An emerging frontier in energy area,” Renewable and Sustainable Energy Reviews, 21(1), pp. 331-346.

[3] Tripathi, B. M., and et al., 2023, “A comprehensive review on solar to thermal energy conversion and storage using phase change materials,” J. of Energy Storage, 72 pp. 108280.

[4] Shamberger, P. J., 2016, “Cooling Capacity Figure of Merit for Phase Change Materials,” J. Heat Transfer, 138(2), pp. 024502-024509.

[5] Hoe, A., and et al., 2020, “Conductive heat transfer in lamellar phase change material composites,” Applied Thermal Engineering 178 pp. 115553.

[6] Hale, D. V., and et al., 1971, “Phase Change Materials Handbook,” Lockheed Missiles and Space Co., Huntsville, AL, Report No. NASA CR-61363.

[7] Senel, E., and et al., 2014, “Liquid metal embrittlement of aluminum by segregation of trace element gallium,” Corrosion Science, 85(1), pp. 167-173.

[8] Song, M., and et al., 2021, “Interfacial Tension Modulation of Liquid Metal via Electrochemical Oxidation,” Adv. Intelligent Systems, 3(8), Article 2100024.

[9] Liu, T., and et al., 2012, “Characterization of Nontoxic Liquid-Metal Alloy Galinstan for Applications in Microdevices,” IEEE, 21(2), pp. 443-450.

[10] Seo, K., and et al., 2015, “Re-derivation of Young’s Equation, Wenzel Equation, and Cassie-Baxter Equation Based on Energy Minimization,” IntechOpen.