Yale researcher studies metal memory

by Abram Katz

A car made out of shape memory alloys could slide into a fender bender and then smooth itself back to showroom condition.

Auto manufacturers do not use these "memory metals" because the materials are much too expensive, difficult to machine and altogether unsuitable as body panels or bumpers.

However, tiny amounts of these remarkable materials may soon be incorporated into mechanical and biological sensors, space probe instruments, chemical plants, doctor’s offices and aerospace gear.

In these "microelectromechanical systems," or MEMS, memory metals could supply the force to push or pull microscopic parts.

To make shape-memory metals useful to industry, engineers need to know how to calculate the properties of different sizes, shapes and thicknesses of the compounds and custom-make alloys for specific uses.

Ainissa G. Ramirez, assistant professor of mechanical engineering at Yale University, studies memory metals and currently is working with extremely thin layers of a particular memory alloy composed of nickel and titanium.

"I’m interested in using memory metal, nickel titanium, to move or activate a part in a nano-machine. Shape memory provides the biggest force to move something," she said.

As part of that research, Ramirez and colleagues determined a new mathematical method for calculating an important quality of nickel titanium alloy using two independently measurable metallurgical factors. Results were published in Applied Physics Letters.

Ramirez keeps a small bundle of perfectly straight nickel titanium wires in her desk. The short wires are easy to twist into a complicated shape.

She clicks a butane lighter and pulls the distorted wire over the flame. It immediately unbends into a straight wire. How?

About 20 different alloys have memory, including copper-zinc-aluminum, copper-aluminum-nickel and iron-manganese-silicon. They all work in similar ways.

The nickel titanium alloy crystallizes in a certain pattern. Imagine theater seats. At low temperatures, the atoms in the crystal want to be offset so that each seat is behind the junction of two other seats, Ramirez said.

At high temperatures, the atoms want to line up in straight columns so each seat is in front of another seat.

The difference is a phase transformation between monoclinic (cool) and cubic (hot) crystal structures, she said.

A piece of nickel-titanium is not one large crystal but is made out of many smaller crystals, or grains. Strength and other properties depend on grain size.

Ramirez wants to find out how very thin films of nickel-titanium behave. By "thin" she means about 0.2 microns. A human hair is about 100 microns in diameter. Memory shape actuators in nano-machines will be about this thick.

When a small amount of heat is supplied, the film will curl up, performing mechanical work. When it cools, the film will straighten out. Memory shape metal requires far less energy than electronically actuated devices.

The only way to make a layer of metal that thin is by vaporizing it and allowing it to condense on a plate.

Once in a film, the atoms are in disarray and the material has no shape memory. The film is heated to 500 degrees centigrade for 30 minutes to permit the atoms to crystallize.

Ramirez said she is exploring how atoms grow into crystals in the very thin film. Watching the atoms would be ideal, but there aren’t very many ways to "see" atoms.

Ramirez used the $1 million transmission electron microscope at the Yale medical school. Ramirez and colleagues used a small sample of alloy and heated it in a $40,000 holder to prevent damage to the instrument.

Electrons, which share the features of both particles and waves, can resolve a cluster of a few atoms. This is sufficient detail to observe crystal structure. Electrons were "shined" on the sample and reflected electrons were collected by a detector and converted into a computer image.

Ramirez could see crystals expanding in disks. She measured how rapidly the disks grew. She also counted how fast new disks appeared. These numbers, in technical talk, are the crystallization rate and nucleation rate.

Crystallization is usually described using a 70-year-old equation now known as the Johnson-Mehl-Avrami-Kolmogorov, or JMAK, model. JMAK is useful but has the shortcoming of combining the growth and nucleation rates, Ramirez said.

By photographing the samples every two seconds while they were heated, Ramirez was able to observe growth rate and nucleation rate.

By plugging these numbers into a modified JMAK equation Yale researchers were able to predict grain size. They also calculated a standard crystallization curve using the straight JMAK that engineers have depended on for decades. The new results agreed with the old.

The new crystallization equation allows engineers to predict the grain size of nickel-titanium, depending on growth and nucleation rate, or to create a certain grain size by controlling the two rates, Ramirez said.

"It’s helpful to relate structure to properties. We can predict the structure and hope to be able to predict the properties," Ramirez said. "The next step is to relate grain size to actuation force."

Nickel-titanium actuators will be useful in dozens of miniaturized applications, Ramirez predicted, from laboratories on a chip to gyroscopes, to accelerometers, other instruments, pressure sensors and chemical and biological sensors.

Actually, if your vehicle has air bags, there’s a good chance it already contains a number of miniaturized accelerometers to trigger air bags.

Abram Katz can be reached at akatz@nhregister.com or 789-5719.