EL COLAPSO PACÍFICO

Ni octet meso-lattices were fabricated using the methods described in Chapter 5.2, with the exception of the use of an electroplating bath containing 240 g/l NiSO4"H2O, 45 g/l

NiCl2"6H2O, and 40 g/l H3BO3"2H2O. Ni meso-lattices were created with either 6 µm or 8 µm-

wide unit cells, with side lengths of 24 to 24 µm and heights of 17 to 20 µm (Fig. 5.10). The minor axis of the elliptical lattice beam, d, ranged from 860 nm to 1.5 µm.

Figure 5.10: SEM images of Ni meso-lattices with (A) 6 µm unit cell and (B) 8 µm unit cell.

Scale bars are 10 µm in size.

Ni meso-lattice TEM samples were prepared according to the methods in Chapter 5.2. A protective layer of Pt was deposited on the meso-lattice using a scanning electron beam (FEI, Nova 600) prior to thinning lattice beams using focused ion beam. TEM microstructural analysis of 22 grains in two lattice beams revealed the average grain size in the Ni meso-lattices to be 119 ± 74 nm, but sample thickness and interference between the magnetic field of the Ni sample and the transmission electron beam obscured some microstructural details (Fig. 5.11). Further TEM investigation is warranted to obtain a more accurate measurement of grain size.

Figure 5.11: TEM images of Ni meso-lattice beam in (A) bright field mode, (B) dark field mode and (C) the corresponding diffraction pattern.

Uniaxial compression experiments were carried out on the Ni meso-lattices using the methods described in Chapter 5.2. One obstacle to obtaining meaningful Ni meso-lattice mechanical data was that the load limit of the nanoindenter used for micro-compressions was reached before the yield point of the meso-lattices for higher relative density lattices, as is shown in Figure 5.12a. Figure 5.12b shows the stress-strain response for meso-lattices with 8 µm unit cell size and 𝜌 of 0.41 and 0.42, which had yield stresses of 412 and 436 MPa respectively.

Figure 5.12: Stress-strain response of Ni meso-lattices with (A) 6 µm unit cell and 𝜌 = 0.53

The yield strength of bulk electroplated Ni with similarly sized (~100 nm diameter) grains was found to be ~600 MPa by Ebrahimi et al.135 This bulk yield strength is higher than the measured Ni meso-lattice yield strengths shown in Figure 5.12b, but more meso-lattice mechanical data must be obtained for a larger range of 𝜌 and unit cell size before conclusions are drawn about dependence of Ni meso-lattice strength on structural geometry or microstructure.

5.6 Summary

We developed a fabrication process for monolithic 3D Cu lattices with structural (unit cell size) and microstructural (grain size) features on the micron-scale. Compression experiments on meso-lattices with 𝜌 > 0.5 and 6 µm unit cell size revealed strengths that were 1.8 times higher than that of monolithic bulk Cu. We deduce that the single crystalline regions in the lattice beams exhibit the “smaller is stronger” size effect which elevates the overall structural strength of the meso-lattice. These findings may have significant implications for the processing of engineering metals and alloys for structural purposes. For example, Cu meso-lattices have similar densities (~4.5 kg/m3) and strengths (~350 MPa) as lightweight Ti-based alloys but maintain the intrinsic material properties of Cu, such as electrical and thermal conductivity.115 Porous metallic nanostructures are also of great interest in catalysis, as battery electrodes, and in photonic devices.136-138 _{Some of these applications require mechanical stability for prolonged}

operation during chemical, electrochemical or mechanical cycling, which could be aided by using principles from cellular solids in combination with size-dependent strengthening. This work demonstrates that removing material from a monolithic metal block and architecting it into

a 3D meta-material with microscale features, while maintaining a well-defined microstructure, provides a pathway to attaining high strength, low-density structural materials.

The meso-lattice fabrication process is versatile enough to accommodate many metals, which we show by fabricating Ni meso-lattices with similar geometry to the Cu meso-lattices. The Ni meso-lattices have an average grain size of 119 nm, which means that lattice beams are likely to be spanned by many grains, unlike in the case of the Cu meso-lattices. Preliminary mechanical results showed that Ni mesolattices with 8 µm unit cell size and 𝜌 of 0.41 and 0.42 had yield strengths that are 30% lower than the bulk yield strength of electroplated Ni with similar microstructure. It is of interest to compare the behavior of the Ni and Cu meso-lattices in order to gauge the effect of materials chemistry and microstructure. For example, the Ni meso- lattice is not expected to exhibit the “smaller is stronger” size effect because its microstructure does not consist of grains large enough to span the lattice beams. Scaling of Ni meso-lattice strength with 𝜌 is expected to follow the size-independent octet scaling law more closely (assuming Ni meso-lattice microstructure is the same for different 𝜌), and can be contrasted with Cu meso-lattice strength scaling in order to separate the effects of structural topology and materials size effects. More mechanical tests must be performed on Ni meso-lattices before meaningful conclusions can be made.

Chapter 6. In-situ SEM Lithiation of 3D Architected