March 27, 2007 – A new technique to make metals with varying-sized grains could result in thin films for use in more energy-efficient oscillators and resonators in electronic circuits, according to researchers at the U. of Illinois.
In studying manipulation of metals films, Prof. Taher Saif and graduate students found that metals typically change shape permanently when bent, but metals that contain a precise microstructure will return to their original shape, and applying heat accelerates the process.
“It’s as though the metal has a memory of where it came from,” said Saif, in a paper to be published in the March 30 issue of the journal Science, noting that the process is independent of whatever the metal is.
The key is creating a precise size and distribution of grains in the metal’s crystalline microstructure. Grain sizes in metal microstructures typically are 1/3-1/2 the thickness of the metal film. The researchers determined that raising temperature by about 50°C caused the grains to grow larger. If grains are uniformly too small the metal will become brittle and break while being bent; grains that are too large will make the metal stay in the bent position.
Working with aluminum (300-360µm length x 50-60µm width x 200nm thickness, with 65nm grains) and gold films (185µm x 12-20µm x 200nm, 50nm grains), researchers varied the microstructures to create plastic deformation in the larger grains and elastic accommodations in the smaller grains — i.e., the bigger grains bend, pushing and pulling on the smaller grains which become elastically deformed like a spring. If left alone, the metal grains will release this energy, forcing the larger grains back into their original shapes over time, a process than can be accelerated by applying heat, the researchers said.
Aside from curious consumer uses listed by the research — crumpled kitchen foil that lays flat for reuse, dinged-up car bumpers that straighten overnight — the technique may also have application in electronic devices. Controlling the crystalline microstructure of thin films could reduce energy loss in oscillators and resonators, in devices such as air bag sensors, camcorders, and GPS devices. “If the grains that constitute the metal films in these devices are between 50-100nm they can be very lossy,” Saif said, but “if we decrease the grain size, we can reduce much of the energy loss.”