In their quest for new materials, nanotechnologists have to learn how submicroscopic particles get together. The way particles measuring only billionths of a meter self-assemble can be as important as the elements of which they are made. For example, new insight into the self-assembly of metal particles has produced a metallic material that can be molded like putty and then hardened like pottery. In addition, a study of how cement particles stack suggests how to significantly cut down carbon-dioxide emissions produced in its manufacture.
We make cement the way the Romans did: Heat crushed limestone and clay to 1,500 degrees C and mix the resulting powder with water, then chemistry transforms it into calcium-silicate-hydrate.
Call it C-S-H for short. It binds sand and gravel together to make concrete.
Franz-Josef Ulm and associates at the Massachusetts Institute of Technology think there's probably a better way to make cement. To find it, they examined what C-S-H particles are doing on a microscopic scale.
What they are finding gives a new twist to the old adage about not trying to compare apples and oranges. When grocers stack apples or oranges in a pyramid, they are using one of the highest packing densities for spheres that nature allows. The other high-density scheme for packing spheres involves their random distribution inside a box. Dr. Ulm's group reported in January that its study of a variety of cement samples shows that C-S-H nanoparticles pack in only these two ways.
Ulm said it is this packing density – not the chemical nature of the C-S-H nanoparticles – that gives cement its properties. He suggests that substituting another material with the same packing density as C-S-H particles could produce a cement that would not need high temperatures in its manufacture. That would eliminate the hot kiln firing that currently produces large amounts of CO2. Ulm estimates it could cut the global CO2 emissions from making cement by 10 percent. His group is looking at magnesium as a substitute for calcium. He notes that, currently, "it is a waste material that people must pay to dispose of."
Bartosz Grzybowski at Northwestern University in Evanston, Ill., and colleagues have solved a different nanoscale problem. Metal and metal-alloy nanoparticles have chemical and optical properties that the bulk metals don't necessarily exhibit. Assembling these particles into larger useful products is tricky. They tend to form rigid structures that are hard to fabricate into useful shapes. Dr. Grzybowski's group is studying how the nanoparticles join up with each other and with other materials in a solution.
The team's report in Science this month described a chemical scheme they have developed in which the nanoparticles self-assemble into a puttylike material that can easily be molded into any shape – say, a tiny gear. Heating the part to 50 degrees C (122 degrees F.) hardens it like pottery. No CO2-producing kiln is needed. This opens up the prospect of new metallic electrically conducting materials that can be used for many purposes.
MIT's Ulm compares the utility of understanding nanomaterials at the basic self-assembly level to the insights biologists gain by studying how biological materials work together under genetic control. He calls his research "identification of the geogenomic code of materials."