• Published May 1st, 2017

S. Cai¹, J. E. Schaffer^{1
Research and Development Engineer, Fort Wayne Metals Research Products Corp}

Shape memory alloy (SMA) wires, such as nitinol, with diameters less than 8µm (~.0003”) can be produced by the accumulative drawing/rolling and bonding technique, which has been used to make other microscale metallics [1-2]. Further, Nitinol wire or fiber forms have already been explored for use in complex textile production for advanced function [3]. In this working example, as shown in Figure 1 (a,b), over 900 NiTi wires with a diameter of 2.5 µm (approx. 0.0001 in) were produced by drawing inside a specially tuned and sacrificial, deformable matrix. The NiTi wire diameter can be further reduced to nanoscale diameters with length to diameter ratios exceeding 10⁶. Processing based subtraction of the sacrificial matrix reveals a robust and ultrafine NiTi microwire bundle (Fig. 1c). These NiTi microwires can be spun, separated or shape set by heat treatment in aggregate to achieve shape memory or super-elastic properties as illustrated in Figure 1d.

While the full physical and mechanical properties of these material forms are yet to be investigated, it is believed that the ultrafine NiTi wire bundle/yarn will provide improved high cycle, strain-control fatigue performance and better thermal mechanical reaction. This is due to the finer wire diameter and higher-surface-to-volume ratio driving rapid heat transfer to and from the environment. The new SMA form has great potential to be used in spun, woven and other fiber and textile technology for medical devices and actuators.

References:

1. Mirvakili, Seyed M., et al. "Niobium nanowire yarns and their application as artificial muscles." Advanced Functional Materials 23.35 (2013): 4311-4316.
2. Kanithi, Hem C. "Process for producing fine and ultrafine filament superconductor wire." U.S. Patent No. 5,088,183. 18 Feb. 1992.
3. Abel, J., Luntz, J., & Brei, D. (2012). A two-dimensional analytical model and experimental validation of garter stitch knitted shape memory alloy actuator architecture. Smart Materials and Structures, 21(8), 085011.

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