James PIKUL, et al.
Metallic Wood
January 28, 2019
'Metallic wood' has the strength of titanium and the density of water

Summary: Researchers have built a sheet of nickel with nanoscale pores that make it as strong as titanium but four to five times lighter.

High-performance golf clubs and airplane wings are made out of titanium, which is as strong as steel but about twice as light. These properties depend on the way a metal's atoms are stacked, but random defects that arise in the manufacturing process mean that these materials are only a fraction as strong as they could theoretically be. An architect, working on the scale of individual atoms, could design and build new materials that have even better strength-to-weight ratios.

In a new study published in Nature Scientific Reports, researchers at the University of Pennsylvania's School of Engineering and Applied Science, the University of Illinois at Urbana-Champaign, and the University of Cambridge have done just that. They have built a sheet of nickel with nanoscale pores that make it as strong as titanium but four to five times lighter.

The empty space of the pores, and the self-assembly process in which they're made, make the porous metal akin to a natural material, such as wood.

And just as the porosity of wood grain serves the biological function of transporting energy, the empty space in the researchers' "metallic wood" could be infused with other materials. Infusing the scaffolding with anode and cathode materials would enable this metallic wood to serve double duty: a plane wing or prosthetic leg that's also a battery.

The study was led by James Pikul, Assistant Professor in the Department of Mechanical Engineering and Applied Mechanics at Penn Engineering. Bill King and Paul Braun at the University of Illinois at Urbana-Champaign, along with Vikram Deshpande at the University of Cambridge, contributed to the study.

Even the best natural metals have defects in their atomic arrangement that limit their strength. A block of titanium where every atom was perfectly aligned with its neighbors would be ten times stronger than what can currently be produced. Materials researchers have been trying to exploit this phenomenon by taking an architectural approach, designing structures with the geometric control necessary to unlock the mechanical properties that arise at the nanoscale, where defects have reduced impact.

Pikul and his colleagues owe their success to taking a cue from the natural world.

"The reason we call it metallic wood is not just its density, which is about that of wood, but its cellular nature," Pikul says. "Cellular materials are porous; if you look at wood grain, that's what you're seeing? -- ?parts that are thick and dense and made to hold the structure, and parts that are porous and made to support biological functions, like transport to and from cells."

"Our structure is similar," he says. "We have areas that are thick and dense with strong metal struts, and areas that are porous with air gaps. We're just operating at the length scales where the strength of struts approaches the theoretical maximum."

The struts in the researchers' metallic wood are around 10 nanometers wide, or about 100 nickel atoms across. Other approaches involve using 3D-printing-like techniques to make nanoscale scaffoldings with hundred-nanometer precision, but the slow and painstaking process is hard to scale to useful sizes.

"We've known that going smaller gets you stronger for some time," Pikul says, "but people haven't been able to make these structures with strong materials that are big enough that you'd be able to do something useful. Most examples made from strong materials have been about the size of a small flea, but with our approach, we can make metallic wood samples that are 400 times larger."

Pikul's method starts with tiny plastic spheres, a few hundred nanometers in diameter, suspended in water. When the water is slowly evaporated, the spheres settle and stack like cannonballs, providing an orderly, crystalline framework. Using electroplating, the same technique that adds a thin layer of chrome to a hubcap, the researchers then infiltrate the plastic spheres with nickel. Once the nickel is in place, the plastic spheres are dissolved with a solvent, leaving an open network of metallic struts.

"We've made foils of this metallic wood that are on the order of a square centimeter, or about the size of a playing die side," Pikul says. "To give you a sense of scale, there are about 1 billion nickel struts in a piece that size."

Because roughly 70 percent of the resulting material is empty space, this nickel-based metallic wood's density is extremely low in relation to its strength. With a density on par with water's, a brick of the material would float.

Replicating this production process at commercially relevant sizes is the team's next challenge. Unlike titanium, none of the materials involved are particularly rare or expensive on their own, but the infrastructure necessary for working with them on the nanoscale is currently limited. Once that infrastructure is developed, economies of scale should make producing meaningful quantities of metallic wood faster and less expensive.

Once the researchers can produce samples of their metallic wood in larger sizes, they can begin subjecting it to more macroscale tests. A better understanding of its tensile properties, for example, is critical.

"We don't know, for example, whether our metallic wood would dent like metal or shatter like glass." Pikul says. "Just like the random defects in titanium limit its overall strength, we need to get a better understand of how the defects in the struts of metallic wood influence its overall properties."

In the meantime, Pikul and his colleagues are exploring the ways other materials can be integrated into the pores in their metallic wood's scaffolding.

"The long-term interesting thing about this work is that we enable a material that has the same strength properties of other super high-strength materials but now it's 70 percent empty space," Pikul says. "And you could one day fill that space with other things, like living organisms or materials that store energy."

Sezer Özerinç and Runyu Zhang of the University of Illinois at Urbana-Champaign, and Burigede Liu of the University of Cambridge, also contributed to the study.

The research was supported by the Department of Energy Office of Science Graduate Fellowship Program, made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under contract no. DE-AC05-06OR23100.
Scientific Reports volume 9, Article number: 719 (2019)

High strength metallic wood from nanostructured nickel inverse opal materials
James H. Pikul, Sezer Özerinç, Burigede Liu, Runyu Zhang, Paul V. Braun, Vikram S. Deshpande & William P. King


This paper describes a nickel-based cellular material, which has the strength of titanium and the density of water. 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. 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. 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.


Cellular materials with spatially organized and repeating pore geometries can have dramatic strength improvements as structural elements shrink to the nanometer scale1,2,3,4. This paper describes a nanostructured cellular material based on electroplated nickel (8,900 kg/m3 bulk density), which has the strength of titanium and the density of water (1,000 kg/m3). The high strength arises from size-dependent strengthening of load-bearing nickel struts whose diameter is as small as 17 nm and whose strength is as high as 8 GPa. 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.

Cellular materials with nanometer-scale structural elements exhibit remarkable material properties5, for example high strength3,6,7,8,9,10, high energy absorption4, ultra-low density1,11,12,13,14, and high specific strength1,15,16. To understand the origin of these enhanced properties, we can look at the performance of single nanopillars which approximate a single strut in a nanostructured cellular material. Compression tests on nanopillars made from single crystal metals showed that reducing pillar diameter increased pillar strength by up to an order of magnitude more than the bulk material’s strength17,18,19. This size-dependent strength enhancement has been widely studied in single crystals17,18,20,21,22. In nanopillars made from nanocrystalline metals, however, both an increase in strength (for Ni and Au)23,24 and decrease in strength (for Ni and Pt)25,26 have been observed as geometric dimensions were reduced to the size of a single grain diameter. When nanocrystalline gold was used in the struts of a nanostructured cellular material, the cellular solid showed dramatic strength enhancement as the strut diameter decreased3,6, but the conflicting results in nanopillars make it difficult to predict if nanostructured cellular materials made from nanocrystalline metals will exhibit a strength enhancement or reduction.

In addition to nanoscale enhancement, the macroscopic properties of nanostructured cellular materials are important when considering them for engineering applications. Most articles on nanostructured cellular materials report specific strength below 100 MPa/(Mg m3), which is comparable to polymers and common metals1,3,4,6,7,8,9,10,11,12,13,14,15,16,27,28,29. Materials with specific strength above 100 MPa/(Mg/m3) are attractive because their strength approaches that of engineering metals and ceramics1,15,16. However, the density of these materials can be low, in the range of 6–410 kg/m3, which prohibits their use in some applications. Consider, for example, a metal panel modeled as a simply supported beam with length 1 m, cross-sectional thickness 1 mm, specific strength 100 MPa/(Mg/m3), and density 100 kg/m3. For an application in which the panel supports a 100 N load at the center, the sheet would require a minimum width of 15 m. In contrast, a panel with specific strength 100 MPa/(Mg/m3) and density 1,000 kg/m3 would require a minimum width of 1.5 m, which is in general a more reasonable solution. Many structural materials have density near 1,000 kg/m3, including engineering polymers and naturally occurring materials such as wood. Only a few articles report nanostructured cellular materials with density near 1,000 kg/m3, and these articles focus on ceramics and carbon based materials rather than metals4,16,29,30. In general, there is a lack of published research on strong nanostructured cellular materials that have enough mass to support loads found in industrial applications.

Here we report a cellular material based on nanostructured nickel inverse opal materials. The mechanical properties of this material are governed by the size-dependent strengthening of nanometer-scale structural elements, allowing large specific strengths up to 230 MPa/(Mg/m3) in porous nickel. This specific strength is larger than most high strength metals including high strength stainless steel and Ti-6Al-4V31,32. The mechanical properties of this material can be varied between natural materials and high strength metal alloys by controlling the geometric parameters within the cellular architecture; we demonstrate materials with yield strengths over the range 90–880 MPa, specific moduli 7–25 GPa/(Mg/m3), and densities 880–14500 kg/m3. Using finite element simulations and well established micropillar compression and nanoindentation testing, we find that the material strength increases as the nanometer-scale strut diameter decreases. The strut yield strengths increase from 3.8 to 8.1 GPa as the strut diameter decreases from 115 to 17 nm, which is a 4X increase over the 2 GPa bulk deposited nickel yield strength.

Figure 1a shows the fabrication of the inverse opal material. First, monodisperse polystyrene (PS) particles of diameter 260–930 nm were self-assembled onto a gold/chromium coated substrate in a face centered cubic (FCC) orientation. The PS was sintered at 96 °C to improve stability and increase the interconnect diameter between PS spheres. Nickel, 99.9%, was electrodeposited into the voids of the PS structure, followed by PS etching in tetrahydrofuran. The result was an open cell inverse opal material having interconnected spherical pores in a face-centered cubic orientation. For some samples, conformal electrodeposition of either additional nickel or rhenium-nickel alloy (80 wt% rhenium) increased the solids volume fraction, strut diameter, and mass. The additional deposited nickel had the same composition as the nickel used to fabricate the inverse opal structure. Mechanically tested samples with rhenium are labeled Re. The average grain sizes of the nickel film and nickel inverse opals were 12.4 nm and 15.1 nm, calculated from the XRD data shown in the supplementary information using the Scherrer equation. The grain size was similar for coated and un-coated nickel samples. A 15 nm grain size in pure nickel corresponded to a ≈6.4 GPa hardness33,34, near the peak hardness values of pure nickel, which agreed well with our electrodeposited nickel thin film nanoindentation hardness measurements of 5.8 GPa. The self-assembly based fabrication method can produce material samples larger than 100 mm2, while allowing ~10 nm control of structure and chemistry by altering the polystyrene diameter, processing temperature, and electroplating parameters. In comparison, most methods for fabricating nanostructured cellular solids with controlled and repeated unit cell morphologies are limited to sample sizes with areas smaller than about 5 mm2 1,11,12,15,27,28,35. Figure 1b–g show SEM images of the material with 500 nm pores fractured on the (111) plane. Figure 1b,c show the inverse opal material with no coating and 0.16 solids volume fraction. Figure 1d,e show the material uniformly coated with 21 nm of nickel, which increased the solids volume fraction from 0.16 to 0.35. Figure 1f,g show the nickel inverse opal material uniformly coated with 25 nm of rhenium-nickel alloy, which increased the solids volume fraction to 0.46. Figure 1h shows a material with 500 nm pores, 2 cm2 area, and 15 µm thickness fabricated on a gold/chromium coated glass slide. Figure 1i shows a material with 300 nm pores and 20 µm thickness fabricated on gold/chromium-coated polyimide. The material is flexible and when prepared on a polyimide sheet could be bent past a 0.5 cm radius (Fig. 1i). Table 1 shows the fabricated materials and their geometric attributes such as coating thickness and volume fraction...


In conclusion, we present metallic wood fabricated from nickel inverse opals, which has the strength of titanium and the chemical properties of a metal, while having the density of water and the cellular nature of natural materials like wood. The high strength of the metallic wood results from the size-dependent strengthening of the inverse opal struts, which have up to 4X the yield strength of bulk electrodeposited nickel and enable high specific strengths of 230 MPa/(Mg/m3). The cellular structure can be controlled to tune the modulus and strength each by a factor of 10X.

The metallic wood can be easily fabricated over 100 mm2 areas, can be processed at room temperature, and can be combined with additional functional materials, as demonstrated with the rhenium coatings49. The high strength continuous metallic architecture with isotropic elasticity, high hardness, and high strain energy storage could be important for a variety of applications such as energy storage50,51,52, heat transport53, and sensors54,55. Future work could explore improvements in specific strength above 230 MPa/(Mg/m3) by incorporating lightweight metals such as titanium or aluminum and developing roll-to-toll processing of high strength porous metals from self-assembly..

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The present invention includes a plurality of α-Fe in the base material2O3Graphene inverse opal (α-Fe2O3/ GIO) is an α-Fe stacked in three dimensions2O3Graphene inverse opal (α-Fe2O3Relates to / GIO) structure, according to the present invention α-Fe2O3Yes forming a fin inverse opal structure (1) after laminating a polystyrene (PS) particles in the base material, followed by electrodeposition of nickel (Ni) 3-dimensional polystyrene (PS) opal (opal) structure; (2) forming a three-dimensional nickel inverse opal (inverse opal) structure (3D-NIO) by removing the 3D-opal structure PS; (3) forming the three-dimensional nickel inverse inverse opal structure an opal structure and carbide using a polyol solution in the presence of a Ni catalyst, to remove the three-dimensional nickel inverse opal structure, a three-dimensional graphene (3D-GIO); And (4) the three-dimensional inverse opal structure yes FeCl the pin3Then immersed in an aqueous solution, heating the α-Fe2O3Graphene inverse opal (α-Fe2O3/ GIO) can be produced according to the manufacturing method for forming a structural body, α-Fe according to the invention 2O3Graphen

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