‘Metallic wood’ as strong as steel

Editorial

Rebecca Pool

Tuesday, January 29, 2019 - 12:45
SEM image of 'metallic wood'; its porous structure is responsible for its high strength-to-weight ratio, just like wood.
 
An international team of researchers has developed a high strength, extremely light 'metallic' wood based on a sheet of nickel with nanoscale pores.
 
Professor James Pikul from the University of Pennsylvania, and colleagues from University of Illinois at Urbana-Champaign, US, Middle East Technical University, Turkey, and UK-based Cambridge University, claim the new material is as strong as titanium, but four to five times lighter.
 
Likening the porous metal to a natural material, the researchers reckon its pores could be infused with other materials, such as electrode materials, to give a dual-purpose composite that could serve as a plane wing or prosthetic leg, as well as a battery.
 
“The reason we call it metallic wood is not just its density, which is about that of wood, but its cellular nature,” highlights Pikul. “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.”
 
As Pikul and colleagues write in Nature Scientific Reports, the load-bearing nickel struts have a diameter as small as 17 nm yet a yield strength that exceeds 8 GPa, some four times greater than bulk nickel. 
 
"The mechanical properties of this material can be controlled by varying the nanometre-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," says Pikul.
 
To fabricate the material, the researchers suspend plastic spheres, a few hundred nanometres in diameter, in water.
 
The water is then slowly evaporated, so the spheres stack in a orderly, crystalline framework.
 
Using electroplating, the researchers infiltrate the plastic spheres with nickel and then dissolve the plastic spheres with a solvent, to leave an open network of metallic struts.
 
(a) Fabrication of a unit cell of the 'metallic wood'; stacked plastic spheres, white, provide a framework for nickel, blue, and are ultimately dissolved away. Once there is an open lattice of nickel, other functional coatings, yellow, can be added. (b–g) Cross section SEM images. (h) A 2 cm2 piece of material with 500 nm pores and 15 µm thickness grown on a gold/chromium coated glass slide. (i) Metallic wood foil on a plastic backing. [Pikul et al, Scientific Reports, 9, 719 (2019)]
 
The researchers have fabricated metallic wood foils up to a square centimetre in size, which according to Pikul, will contain around 1 billion struts.
 
What's more, the material is around 70% porous, so its density is extremely low, relative to its strength, and brick-sized structures would float.
 
The researchers now hope to fabricate the material at larger, commercially-relevant sizes and also intend to carry out macro-scale tests on larger samples.
 
“We don’t know, for example, whether our metallic wood would dent like metal or shatter like glass,” says Pikul. “Just like the random defects in titanium limit its overall strength, we need to get a better understanding of how the defects in the struts of metallic wood influence its overall properties.”
 
“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,” he adds. “And you could one day fill that space with other things, like living organisms or materials that store energy.”
 
Research is published in Nature Scientific Reports.
 
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