Researchers Develop Material that Expands Under Pressure

Researchers at the Argonne National Laboratory have developed a new kind of material that expands on squeezing instead of contracting.

"It's like squeezing a stone and forming a giant sponge. Materials are supposed to become denser and more compact under pressure. We are seeing the exact opposite. The pressure-treated material has half the density of the original state. This is counterintuitive to the laws of physics," said Karena Chapman, a chemist at the U.S. Department of Energy laboratory.

The discovery is not only expected to rewrite textbooks on behavior of materials under pressure, but also change the way materials are manufactured.

Chapman and team spent many years trying to understand how this material works. "The bonds in the material completely rearrange," Chapman said. "This just blows my mind."

The new material has holes in it - just like a sponge does - that traps and stores material. These gaps in the material can be used to store selected molecules. This material can be used as water filters and compressible storage for hydrogen fuel cells.

For the study, researchers put zinc cyanide in a diamond-anvil cell and applied high pressure of 0.9 to 1.8 gigapascals. The pressure used is about 9,000 to 18,000 times the atmospheric pressure at sea-level.

As the material contracted, researchers used different fluids around it and were able to create five new phases of the material; two of which had the pores at normal pressure. This is the first time that hydraulic pressure has been used to make dense material into new kinds of porous materials.

Diamond anvil cells are used to create very high pressure. This pressure changes the chemical bond structure in materials, which in turn change the way a material behaves. In this study, the material's porosity was changed under pressure. 

"By applying pressure, we were able to transform a normally dense, nonporous material into a range of new porous materials that can hold twice as much stuff," Chapman said in a news release. "This counterintuitive discovery will likely double the amount of available porous framework materials, which will greatly expand their use in pharmaceutical delivery, sequestration, material separation and catalysis."

The study article, "Exploiting High Pressures to Generate Porosity, Polymorphism, And Lattice Expansion in the Nonporous Molecular Framework Zn (CN)2", is published in Journal of the American Chemical Society.