Poisson's Ratio Of Thin Films Research Articles

On the poisson ratio of thin films of Ce0.8Gd0.2O1.9 II: Strain-dependence

We have adapted the X-ray diffraction technique of measuring crystal lattice parameters as a function of sample orientation in order to investigate the Poisson ratio (ν) of thin films of Ce 0.8 Gd 0.2 O 1.9 with different degrees of in-plane compressive strain. Since Gd-doped ceria is mechanically inelastic, the unstrained lattice constant is not well-defined. Therefore in order to determine the Poisson ratio, more than one strain state of the same sample must be characterized. We have accomplished this with a home-built, diffractometer-compatible sample-bender. Using this device, we find that the Poisson ratio of as-deposited thin films of Gd-doped ceria is a decreasing function of the in-plane compressive strain. At total strain ≈ −0.9 %, ν is > 0.45, indicating that the material tends to preserve volume. Increase of the in-plane strain to ≈ −2 % produces a monotonic decrease in the Poisson ratio to ~0.2. Such low values are indicative of reduced volume. We explain this behavior by noting earlier results from our group suggesting that, at temperatures below 200 °C and at low strain values, Ce 0.8 Gd 0.2 O 1.9 responds to non-isotropic mechanical stress by reorienting the locally distorted CeCe-VO –O7 units. At higher strain values this mechanism is apparently no longer effective. Measurement of the Poisson ratio of inelastic thin films by this technique is straightforward and not diffractometer-dependent. It therefore represents a practical method for ceramic thin film characterization.

Simultaneous measurements of the magnetostrictive coefficient, Young’s modulus, and Poisson ratio of thin films

A method, which determines simultaneously the magnetostrictive coefficient, Young’s modulus, and Poisson ratio of a thin film utilizing the minimization of the total elastic energy of a cantilever film–substrate system, is suggested. An inaccuracy for the magnetostrictive coefficient, caused by assuming the elastic properties of the film as those of the corresponding bulk material, could be avoided and only a single elastic isotropic substrate is employed in the present method. The experimental data of an Fe-based amorphous thin film were analyzed by using the model. The calculated dependence of the magnetostrictive coefficient on the external magnetic field was compared with the experiment, and the discrepancy between both results were explained. The elastic properties of the film were also obtained.