Strain on the Fly
Uday Singh and Shireen Adenwalla
Strain refers to the expansion or contraction of a material and can have dramatic effects on the properties of materials, even at very small strain levels of well below 1%. This sensitivity has been exploited to alter material behavior. Thin-film materials can be strained by growing them on a substrate with a slight mismatch in the atomic lattice. The slightly altered atomic spacings in the strained thin films can result in dramatically different behavior from bulk crystals, changing the magnetic or superconducting ordering temperatures, for example. Usually, strain sensitivity is explored by growing a series of samples on different substrates, with slight variations in the atomic spacing, but this method results in strains that are fixed by the choice of the substrate and do not allow for any variation on the fly.
Nebraska MRSEC researchers are exploring methods to strain thin films using intense, high frequency sound waves, which alternately squeeze and stretch a thin film. This allows variation of the strain on the fly, as well as investigating what happens to a material when the strain is varied very quickly. The researchers produce these high frequency strains by patterning a series of curved electrodes on a piezoelectric material – a material that can convert electricity to strain. The curved electrodes focus all the sound wave energy into a central spot. The figure shows light reflecting off a surface excited by a high frequency standing strain wave produced by such an arrangement, with the strain varying from compressive to tensile at close to one hundred millions times a second. The strain distorts the surface, and the reflected light beam wavers in synchrony with the distorted surface, producing a map of high strain and low strain regions, with the highest strain occurring between the two darkest red areas.
The pattern is reminiscent of a stone was dropped into still water. In this case, however, the wave moves inward to the center rather than outward from the center. Moreover, because the piezoelectric material is not isotropic, the inward moving wave forms elliptical rather than circular rings.
These programs are supported by the National Science Foundation, Division of Materials Research, Materials Research Science and Engineering Program, Grant 1420645.
The figure shows optical reflectivity measurements of a standing strain waves produced on a piezoelectric surface at high frequencies (88MHz). The dark red regions are regions of high strain gradient and the blue regions represent low strain gradients. The highest strain is produced in the regions between the two darkest red areas and may be as high as 1%.
Highlight InfoDate: Aug. 2015
IRG1: Magnetoelectric Materials and Functional Interfaces