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March 28th, 2013

There’s more to multifunctional ultra-flyweight aerogel produced at a Zhejiang U. lab than just ‘world’s lightest material’ record

Published on March 28th, 2013 | By: Eileen De Guire

(a) Photograph of new UFAs with diverse shapes; (b) a 100 cm3 cylinder standing on a flower like “dog’s tail”; (c) a ~1000 cm3 pie pan-shaped cylinder (21 cm in diameter and 3 cm in thickness). Credit: Gao et al.; Advanced Materials.

I suspect this story may seem a little like old news to some readers, but a lot of the pop-sci reporting in the last few days about a new ultralight aerogel (actually, a UFA or ultra-flyweight aerogel, i.e., less than 1 mg cm−3) has missed the point by a wide margin. The big takeaway from this story, as documented in a new Advanced Materials paper, is that the graphene–carbon nanotube (CNT) aerogel is relatively easy to make, appears to be easily scalable, and is moldable into any shape. Moreover, the new UFA, besides having ultralow density, also is extremely flexible and elastic—with the elasticity independent of temperature—and is thermally stable, a good electrical conductor, hydrophobic, and capable of absorbing extremely high capacities for organic liquids and phase change materials.

Now, certainly, there is a significant gee-whiz factor for gaining the “world’s lightest material” title, an achievement based on a density of 0.16 mg cm−3, accomplished by the research team at Zhejiang University headed by Gao Chao.

The group’s material is not the first UFA. Within the past 18 months, other researchers have constructed nickel foams with density of 0.9 mg cm−3 (via electroless plating and subsequently etching away a polymer template), and another group constructed an aerographite with a density of 0.18 mg cm−3 (via a ZnO template-based chemical vapor deposition approach). But, both groups’ dependency on a template also creates enormous limitations to scalability. Sol-gel-derived, low-density aerogels can be made on fairly large scales, but with sol-gel processes it is difficult to control the dimensions of the structures.

Gao’s group, instead, uses a process that involves freeze-drying aqueous solutions of CNTs and giant graphene oxide (GGO) sheets, followed by chemical reduction of graphene oxide into graphene using hydrazine vapor. The researchers named their method a “sol-cryo” approach and note in their paper that it is easy to make large samples:

Because of the simplicity of assembly process in our template-free “sol-cryo” methodology and the large-scale availability of GGO and CNTs, the integrated all-carbon aerogels with desired densities and shapes such as rods, cylinders, papers and cubes were readily accessible. More significantly, UFAs can be easily manufactured in a large-scale. For example, a UFA cylinder up to 1000 cm3 was made with a mold of 1-liter plate.

A story on the university’s website reports quotes Gao saying, “With no need for templates, its size only depends on that of the container. [A] bigger container can help produce the aerogel in bigger size, even to thousands of cubic centimeters or larger.” The story also reports that Gao believes “the value of this achievement lies not in the record but in its simple way in developing the material and the superior performance exhibited.”

Briefly speaking, the microstructure of the material is a 3D porous framework constructed with cell walls of randomly oriented, crinkly graphene sheets and CNT “ribs.” The macropores ranged from hundreds of nanometers to tens of micrometers. The authors of the paper say the properties of the UFA derive from the graphene-CNT synergy: “Giant graphene flakes build a framework with macro-pores, making the aerogel ultralight; the coating of CNTs reinforces the relatively flexible graphene substrate and endows their intrinsic elasticity to the coorganized aerogel.”

One of the UFA’s interesting properties is its elasticity that allows it to be repeatedly compressed and returned to its nearly original size. This sponginess comes in handy in combination with another property: It can rapidly absorb up to 900 times its own weight in oil or other organic liquids. For example, one gram can absorb 68.8 grams of organics per second. The elasticity remained the same in tests that ranged from −190 to 300°C. The elasticity also remained after researchers annealed the UFA at 900°C for five hours.

There are probably many ways the elasticity and absorbability can be useful, not the least of which is that, because of its hydrophobicity, it could be a reusable medium to soak up oil spills on lakes and oceans. Gao says in the university story, “Maybe one day when oil spill occurs, we can scatter them on the sea and absorb the oil quickly. Due to its elasticity, both the oil absorbed and the aerogel can be recycled.” (Another Chinese research group working at Tsinghua and Peking Universities published a paper in 2010 about the use of CNTs for oil spills, and I suspect that there is some overlap between the work of the two groups.)

Another property of the UFA is that it has elasticity-dependent electrical conductivity. For example, they report connecting an LED lamp top the UFA bulk, and “its brightness fluctuates upon compressing and releasing the aerogel. This phenomenon promises the application of UFAs as pressure-responsive sensors.”

They also say that by loading the UFA with tiny amounts of certain liquids (say, CCl4 or 1-hexadecanol), they can make conductive composites with very high electrical conductivity compared to just CNT- or graphene-based composites.

[flash http://ceramictechweekly.org/wp-content/video/ufa_aerogel_led.flv mode=1 f={image=http://ceramictechweekly.org/wp-content/video/ufa_aerogel_led.jpg}]

Video of variable conductivity of UFA demonstrating current change to LED when aerogel is compressed and released. Credit: Gao et al.; Advanced Materials.

Gao and the other authors also suggest the UFA could find use as supercapacitors and catalyst beds, but another intriguing application they only hint at is its use as a medium that enhances phase-change energy storage materials. For example, unlike other composites, a UFA-paraffin combination delivers higher phase-change enthalpy (ΔH) than ordinary paraffin.

These applications may only be the beginning. When it comes to learning how to leverage the properties of their fluffy stuff, the researchers say their new UFA is “just like a new-born baby.” A baby that was, perhaps, born with a silver spoon in its mouth.


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