A diamond is forever. Sure, a diamond may be the strongest material on the earth, but a group of scientists at UW-Madison have taken away its crown as the stiffest material around.
Don Stone, UW-Madison professor of materials science and engineering, describes the structure of the new composite.
Photo Credit: Eyleen ChouStiffness is a property of a material that characterizes its resistance to deformation. Roderic Lakes, UW-Madison professor of engineering physics and mechanical engineering, published a paper in 2001 outlining the theory behind creating a material with extreme stiffness. The idea was to create an ultra-stiff composite by combining two materials—one with positive stiffness and one with negative stiffness. A material with positive stiffness pushes back in a direction opposing an applied force (think of a spring). Conversely, a material with negative stiffness doesn’t push back at all; instead, it increases the deformation in the direction of the applied force.
Although it would intuitively seem that combining these two materials would create a composite with minimal stiffness, it actually breaks the limits and creates a very stiff material.
“It’s like putting anti-sugar into espresso and getting something sweeter than sugar,” Professor Lakes says.
Previous attempts to create stiff composites used materials with only positive stiffness. For several years, a diverse team of engineers has been working to create a composite of tin and barium titanate that would exhibit extreme stiffness. There have been many technical challenges in physically creating the composite since the constituents must be combined very uniformly to make use of the phase transformation that the barium titanate undergoes.
Tim Jaglinski, a former UW-Madison Ph.D. student in materials science, has been credited with finally creating the composite in the lab. Currently, researchers are testing the only sample of this material ever created.
The composite’s extraordinary stiffness is derived from the technique used to combine the tin and barium titanate. Small amounts of barium titanate, a molecule which undergoes a solid phase transformation when it is heated or cooled, are suspended in a tin matrix. As the barium titanate attempts to exhibit its phase changing qualities within the tin, potential energy is stored within the composite. This potential energy counters the effects of pressure on the composite and is responsible for the composite’s exceptional resistance to pressure.
One of the challenges for the team of engineers in preparing the composite for commercial use is the narrow temperature range at which the material displays this extreme stiffness. The composite has been shown to be stiffer than diamond within a 3 degree Fahrenheit range and stronger than steel within a 10 degree Fahrenheit range. However, recent theoretical experiments conducted by Walter Drugan, UW-Madison professor of engineering physics, have shown that it may be possible to extend the temperature range far beyond this.
Although the material is stiff, there is no reason to suspect it will be as strong as a diamond, and therefore it is unable to replace diamond cutting tools. However, there may be useful applications for such a material in structures, airplanes, hard drive platters and robotic arms where increased stiffness could lead to more efficient designs. For instance, bridges could be built using less material if it was stiffer than diamond or even steel.
“You wouldn’t build a bridge with diamonds,” Drugan says.
In addition to being stiff, the new composite has a high phase angle resulting in high damping effects.
Professor Lakes demonstrated a metallic ringing sound by tapping on the platters of a dissected hard drive to demonstrate characteristics of a material with a low phase angle. This ringing is due to vibrations within the material, which reduce the accuracy of the drive’s reading head, thereby reducing the maximum possible data density. The new composite could be used to create a hard drive with less vibration, capable of a higher data density
This new composite has a promising future in industry, but for now it is relegated to a lab on campus while the synthesizing process and temperature characteristics are improved.
Even though your next piece of jewelry may not be tin and barium titanate composite, the material may find its way into your next computer hard drive.