Understanding and Improving Mechanical Stability in Electrodeposited Cu and Bi for Dynamic Windows Based on Reversible Metal Electrodeposition |
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Authors: | Gabriel R McAndrews Andrew L Yeang Yuchun Cai Christopher J Barile Michael D McGehee |
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Institution: | 1. Materials Science and Engineering Program, University of Colorado Boulder, Boulder, CO, 80303 USA;2. Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, CO, 80303 USA;3. Department of Chemistry, University of Colorado Boulder, Boulder, CO, 80303 USA;4. Department of Chemistry, University of Nevada Reno, Reno, NV, 89503 USA |
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Abstract: | Dynamic windows based on reversible metal electrodeposition (RME) can electronically adjust light transmission from ≈70% to <0.1% to improve building aesthetics and energy efficiency by controlling light and heat flow. For RME devices using Cu and Bi, the windows reach “privacy state” (<0.1% transmission) when ≈180 nm of metal is electrodeposited on the transparent conducting electrode. When films with a plated atomic Cu–Bi ratio of ≈2:1 rest in the privacy state, sinusoidal cracks form across the entire film, and the metal delaminates in <1 day. This mechanical failure renders the window unusable as specks of metal are visually unattractive and reduce the dynamic range of the window. The Cu–Bi film is stress free upon deposition, but after 4 h of resting, 38 MPa of tensile stress develops. The tension in Cu–Bi and Cu films combined with the Cu(ClO4)2 in the electrolyte results in severe, widespread fractures and delamination due to stress corrosion cracking. In contrast, electrodeposited Bi films have compressive stress, likely due to high self-diffusion and insertion of atoms into grain boundaries while plating, which results in a Bi-based dynamic window with crack-free resting stability that exceeds 9 weeks. |
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Keywords: | dynamic windows mechanical stability reversible metal electrodeposition stress corrosion cracking |
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