Universal Scaling Law for the Size Effect on Superelasticity at the Nanoscale Promotes the Use of Shape‐Memory Alloys in Stretchable Devices

Abstract

stretchable materials for wearable electronics, [1,2] including a diversity of materials such as flexible semiconductors, [3] or flexible meta-materials, [4] for instance, giving place to a plethora of unforeseen applications. [5] New advances in small-scale soft robots, [6] and particularly in stretchable electronic skin, [7] are driving new fields like functional neural interfaces, [8] and implantable bioelectronics. [9] With a growing worldwide forecast market, which will exceed 200 billion dollars per year by 2027, [10] flexible and stretchable electronic and wearable technologies constitute a new paradigm in the materials science era. Most of these technologies rely on organic materials, [2,11] including shape memory polymers. [12] Nevertheless, in a recent road map in this field, [8] the key points for electrical connectors or electrode materials were well identified: exceptional electrical conductivity, high stretchability, long-term functionality, the ability to withstand high stresses, and the capability of working with low cross-sectional area. While polymers and metals currently used in flexible technologies exhibit several drawbacks (for instance low electrical conductivity in polymers and low stretching capabilities in metals), shape memory alloys (SMAs), [13] could fulfill all the above requirements. Indeed, Cu-based SMAs have an extremely high electrical conductivity, 10 7 S m −1 , [14] close to pure copper and silver (6 × 10 7 S m −1 ). [15] Thanks to the superelastic behavior, [13] network-shaped devices designed with SMAs will be dramatically stretchable and their superelastic functionality could exceed 10 7 cycles, [16] withstanding stresses above 100 MPa even at small scale. [17,18] Then, SMAs could be excellent candidates to be used as electrical connectors in flexible electronic devices, provided that the last requirement could be fulfilled, this means that the above properties would be exhibited in a very low cross-sectional area, down to 1 µm 2 section or even less.Among different families of SMAs, Ti-Ni is the most widespread SMA but shows a loss of reversibility during superelastic effect at small scale, [19] while, on the contrary, Cu-Al-Ni SMAs exhibit a good reproducible superelastic behavior at the nanoscale, [17,18] even after long-term cycling. [20,21] However, until now, Cu-Al-Ni has been the only Cu-based SMA family tested for superelasticity at nanoscale, [17,18,[20][21][22] and in order to explore new SMA families, Cu-Al-Be becomes a very attractive family because at macroscopic scale, it exhibits a longer functional (superelastic) fatigue life than the Cu-Al-Ni family. [23] Shape-memory alloys (SMAs) are the most stretchable metallic materials thanks to their superelastic behavior associated with the stress-induced martensitic transformation. This property makes SMAs of potential interest for flexible and wearable electronic technologies, provided that their properties will be retained at small scale. Nanocompression experiments on Cu-Al-Be SMA single crystals demonstrate...