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From the Labs: Materials

New publications, experiments and breakthroughs in materials science–and what they mean.
February 24, 2009

Nanotube Super-capacitors
A method for making electrodes doubles electrical storage capacity

Carbon power: Twenty layers of carbon nanotubes are assembled on a polymer backing.

Source: “Layer-by-Layer Assembly of All Carbon Nanotube Ultrathin Films for Electrochemical Applications”
Paula Hammond et al.
Journal of the American Chemical Society
131: 671-679

Results: MIT researchers have developed a new technique for making thin films of multi­walled carbon nanotubes. The materials have low electrical resistance and can store about 160 farads of electrical charge per gram–a capacitance more than twice that of other carbon nanotube films and an order of magnitude higher than that of conventional carbon materials.

Why it matters: Since the films can store large amounts of electrical charge and discharge it rapidly, they are promising materials for supercapacitors, long-lasting batterylike devices that charge up quickly. The way they’re made gives the researchers a great deal of control over their thickness and porosity, and thus over their electrical properties. That means the materials could be useful in diverse applications, including microbatteries for medical implants and flexible electrodes for electronics.

Methods: The researchers treated carbon nanotubes with either positively or negatively charged surface molecules, then put them into separate water suspensions. They dipped a substrate, such as a silicon wafer, alternately in the positive and negative nanotube solutions; the difference in charge created electrostatic attraction, causing the nanotubes to cling to one another without the need for chemical binders. (Previous nanotube films, which required such binders, did not have electrical properties as impressive as those displayed by a pure mat of nanotubes.) The researchers have now made nanotube films of varying thicknesses, released them from their substrates, and tested their electrical properties.

Next steps: The researchers will modify the nanotubes so that the materials can store even more charge. They are also developing faster assembly methods based on spraying rather than dipping.

Tough Ceramics
Materials with a seashell-like microstructure resist fracturing

Source: “Tough, Bio-Inspired Hybrid Materials”
Robert Ritchie et al.
322: 1516-1520

Results: A composite ceramic whose microscale structure mimics that of nacre, or ­abalone shell, is the toughest (that is, the most resistant to fracturing) ever made. Composed of microscale bricks of an aluminum oxide ceramic cushioned by a polymer filling, it has properties comparable to those of aluminum alloys and is twice as tough as the best structural ceramics.

Why it matters: Ceramics are lightweight and strong, but when pushed past their limits, they fail catastrophically–fracturing rather than bending, as materials such as steel would. That has limited their use as structural materials. The new materials, which exceed the toughness of nacre, could replace heavier structural materials in ­vehicles, improving fuel efficiency. They could also do double duty as insulation and structural support for buildings.

Methods: Researchers at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory created the material with the help of directional freezing of ice, a technique that one of them, Antoni ­Tomsia, refined. The researchers mixed aluminum oxide with water and froze the mixture by drawing the heat out from one side, which caused the ice to form distinct shapes. The ice served as a template, producing multiple layers of long, thin crystals of aluminum oxide, with microscopic bridges of the ceramic between the layers. After removing the water, the researchers crushed the aluminum oxide into tiny bricklike structures. Then they added a polymer “mortar” (polymethyl methacrylate) that created a cushion between the brittle bricks. The composite material is 300 times tougher than either constituent alone.

Next steps: The structure of the ceramic very closely mimics that of nacre, but nacre’s structural elements are on the order of nanometers, not micrometers. By making the bricks smaller and closer together, the Berkeley researchers hope to achieve a tougher material. They are also exploring ways to replace the polymer with other materials, in order to increase the ceramic’s tolerance of high temperatures.

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