High-strength and lightweight materials

Materials with controlled architectures at the nanometer scale can have dramatically improved optical, mechanical, and transport properties compared to their bulk counterparts. However, it has been difficult to maintain nanoscale control of material architecture for samples larger than cm². We seek to use a combination of self-assembly and scalable manufacturing technologies to enable manufacturing of m² of material while controlling nanoscale geometry and chemistry.

High-strength nanostructured cellular solids

We have developed a nickel-based cellular material which has the strength of titanium and the density of water (Fig. 1). The material’s strength arises from size-dependent strengthening of load-bearing nickel struts whose diameter is as small as 17 nm and whose 8 GPa yield strength exceeds that of bulk nickel by up to 4X (Fig. 2). The mechanical properties of this material can be controlled by varying the nanometer-scale geometry, with strength varying over the range 90–880 MPa, modulus varying over the range 14–116 GPa, and density varying over the range 880–14500 kg/m3 (Fig. 3). We refer to this material as a “metallic wood,” because it has the high mechanical strength and chemical stability of metal, as well as a density close to that of natural materials such as wood. In addition, its cellular nature allows for future multifunctional material integration.

Figure 1
Figure 1: Overview of metallic wood. (a) The fabrication process for a unit cell of the nickel inverse opal material. (bg) Cross section SEM images of nickel inverse opal material. (b,c) A nickel inverse opal with no coating. (d,e) A nickel inverse opal material with a 21 nm coating of additional electrodeposited nickel. (f) A nickel inverse opal material with a 25 nm coating of additional electrodeposited rhenium-nickel. (g) A closer image of one of the struts in (f). (h) A 2 cm2 nickel inverse opal material with 500 nm pores and 15 µm thickness grown on a gold/chromium coated glass slide. (i) A nickel inverse opal material with 300 nm pores grown on gold/chromium coated 20 µm thick polyimide. From ref. [18]

Figure 3
Figure 2: Mechanical measurements. (a) SEM images of inverse opal micropillars with 500 nm pores, before and after compression testing. The uncoated sample is 84% porous and the 19 nm nickel coated sample is 58% porous. Failure occurs in the [111] direction, parallel to the compression axis. Scale bars are 3 µm. (b) Engineering stress, σ*, versus engineering strain for micropillars under compression. The nickel sample is bulk electroplated nickel. (c) Strut yield strength, σy, as a function of strut diameter, d, for nickel inverse opal materials with 260, 500, and 930 nm pores. Blue data are nickel inverse opals measured with micropillar compression tests. Green data are nickel inverse opals measured with nanoindentation. Orange data are 500 nm pore nickel inverse opals coated with 19 and 33 nm of additional nickel. Shaded regions represent standard deviations. The fit line was only applied to nickel samples with no coatings. From ref. [18]

Figure 3: Metallic wood compared to other engineering materials. Ashby plot of material strength, calculated at a 0.2% strain offset, versus density for the fabricated materials and several reference materials. For comparison, common Ti, Al, Ni, and Fe high strength alloys are labeled as follows 1 – CP Ti, 2 – 2024-T4, 3 – Inconel 718, 4 – 7075-T6, 5 – HSSS steel, 6 – Ti-6Al-4V, 7 – Ti-10V-2Fe-3Al. Error bars show standard deviations. From ref. [18]

Understanding how cracks form in self-assembled particles

Cracking in the drying opal templates used to fabricate inverse opals, however, is a major hindrance to the use of self-assembled nanolattices for practical and fundamental studies. In work published in ref. [25], we conduct desiccation experiments on polystyrene particle opals self-assembled on indium-tin-oxide coated substrates to study their fracture mechanisms, which we describe using an energy-conservation fracture model. The model incorporates film yielding, particle order, and interfacial friction to explain several experimental observations, including thickness-dependent crack spacings, cracking stresses, and order-dependent crack behavior. Guided by this model, we are the first to fabricate 120 μm thick free-standing metallic inverse opals. This work improves our understanding of fracture mechanics in drying particle films, provides guidelines to reduce crack formation in opal templates, and enables the fabrication of free-standing large-area single-crystal inverse opals.

Figure 4: (A) Phase diagram of the crack types that occur in desiccated opal films, defined by Eq. 6 and 7. nD/nC is a ratio of the water bridge density on D-crack surface to C-crack surface. (B) Optical image of a thick opal assembled on an ITO-SO3̅ substrate with 552 ± 13 nm diameter particles (top), and its 45° view SEM image (bottom). (C) Optical image of a 552 ± 13 nm particle opal assembled on a PET film without D-cracks (top), and its SEM image shows a high long-range order (bottom). From ref. [25]

Enabling large area crack-free nanolattices with record strength and scalability

In work published in Nature Materials (Ref. [30]), we demonstrate a crack-free self-assembly approach to fabricate cm-scale multifunctional metallic nanolattices with 100 nm periodic features and 30 nm grain sizes, which corresponds to a 20,000X increase in crack-free area and 1,000X the number of unit cells in the loaded direction than prior nanolattices. These nanolattices have 257 MPa tensile strengths at 1.12% strain and a density of 2.67 g/cm3, which is 2.6 times the strength of the strongest porous metals with the same relative density at any scale. We eliminated cracks during self-assembly by maintaining a wet template and utilizing electrostatic forces to assist metal electrodeposition through the template. The resulting nickel nanolattices have excellent photonic coloration and approach their macroscopic theoretical tensile strength. The high absolute strength and the low density would allow nickel nanolattices to replace sandwich panel cores with 50% smaller volume than porous titanium, 50% lower mass than porous iron, and, importantly, 10X less volume than other nanolattices.

Figure 5: The Wet-Eelectrostatic (WE) method for fabricating metallic nanolattices without inverted cracks using positively charged particles and a glycerol additive. The inset is a top-view optical image of a wet opal template without cracks. SEM images of nickel nanolattices without inverted cracks fabricated by WE method. From Ref. [30]
Figure 6: Physical and optical characterization of inverted-crack-free nickel nanolattices. (a, bottom left) A cross-section SEM image of an inverted-crack-free and free-standing nickel nanolattice. The blue mark represents the prior thickness limit of inverted-crack-free silica nanolattices, equivalent to 16 particle layers. The inset is an image of a large-area nickel nanolattice without inverted cracks. (a, top left) Dogbone samples of inverted-crack-free nickel nanolattices with different internal pore sizes. The colored areas are nickel nanolattices. No false coloring was used. From Ref. [30]
Figure 7: The nickel nanolattice properties compared to other porous metals and nanolattices. (a) Ultimate tensile strength versus relative density of porous Ag, Al, Au, Cu, Fe, Ni, and Ti. Dashed lines follow equation (1) with  from 0 to 1.8 GPa. The black dashed line denotes the empirical limit of prior fabrication methods, including particle sintering, dealloying, Gasar, slurry foaming, and syntactic foaming. (b) Ultimate tensile strength versus relative density of nanolattices, nanoporous graphene, and aerogels. (c) Characteristic volume (-1/2) versus characteristic mass (ρ*-1/2) of a rectangular beam to bear a bending moment for different porous materials. The dashed lines are defined by specific tensile strengths between 0.005 and 0.08 MPa/(kg·m-3). (D) Specific tensile strength versus the ratio of sample loaded length to unit cell length of nanolattices. From Ref. [30]


[30] Zhimin Jiang and James H. Pikul. “Centimetre-scale crack-free self-assembly for ultra-high tensile strength metallic nanolattices” Nature Materials, 1-7, 2021.

[25] Zhimin Jiang, Zakaria Hsain, and James H. Pikul. “Thick free-standing metallic inverse opals enabled by new insights into the fracture of drying particle films” Langmuir, 2020.

[18] James H. Pikul, Sezer Özerinç, Burigede Liu, Runyu Zhang, Paul V. Braun, Vikram S. Deshpande, William P. King, “High strength metallic wood from nanostructured nickel inverse opal materials“, Scientific Reports, vol. 9, January 2019. Top 100 downloaded materials science papers for Scientific Reports in 2019.

[8] James H. Pikul, Paul V. Braun, and William P. King, “Micromechanical devices with controllable stiffness fabricated from regular 3D porous materials”, Journal of Micromechanics and Microengineering, vol. 24, 105006, 2014