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Inorganic islands on a highly stretchable polyimide substrate
Sun, Jeong-Yun,Lu, Nanshu,Yoon, Juil,Oh, Kyu-Hwan,Suo, Zhigang,Vlassak, Joost J. Cambridge University Press (Materials Research Soc 2009 Journal of materials research Vol.24 No.11
<P>For a flexible electronic device integrating inorganic materials on a polymer substrate, the polymer can deform substantially, but the inorganic materials usually fracture at small strains. This paper describes an approach to make such a device highly stretchable. A polyimide substrate is first coated with a thin layer of an elastomer, on top of which SiN<I>x</I> islands are fabricated. When the substrate is stretched to a large strain, the SiN<I>x</I> islands remain intact. Calculations confirm that the elastomer reduces the strain in the SiN<I>x</I> islands by orders of magnitude.</P>
High-throughput analysis of thin-film stresses using arrays of micromachined cantilever beams.
Kim, Hyun-Jong,Han, Jun-Hyun,Kaiser, Roy,Oh, Kyu Hwan,Vlassak, Joost J American Institute of Physics 2008 Review of scientific instruments Vol.79 No.4
<P>We report on a technique for making high-throughput residual stress measurements on thin films by means of micromachined cantilever beams and an array of parallel laser beams. In this technique, the film of interest is deposited onto a silicon substrate with micromachined cantilever beams. The residual stress in the film causes the beams to bend. The curvature of the beams, which is proportional to the residual stress in the film, is measured by scanning an array of parallel laser beams generated with a diffraction grating along the length of the beams. The reflections of the laser beams are captured using a digital camera. A heating stage enables measurement of the residual stress as a function of temperature. As the curvature of each beam is determined by the local stress in the film, the film stress can be mapped across the substrate. This feature makes the technique a useful tool for the combinatorial analysis of phase transformations in thin films, especially when combined with the use of films with lateral composition gradients. As an illustration, we apply the technique to evaluate the thermomechanical behavior of Fe-Pd binary alloys as a function of composition.</P>
Pharr, Matt,Choi, Yong Seok,Lee, Dongwoo,Oh, Kyu Hwan,Vlassak, Joost J. Elsevier 2016 Journal of Power Sources Vol.304 No.-
<P><B>Abstract</B></P> <P>We measure stresses that develop in sputter-deposited amorphous Ge thin films during electrochemical lithiation and delithiation. Amorphous Li<SUB>x</SUB>Ge electrodes are found to flow plastically at stresses that are significantly smaller than those of their amorphous Li<SUB>x</SUB>Si counterparts. The stress measurements allow for quantification of the elastic modulus of amorphous Li<SUB>x</SUB>Ge as a function of lithium concentration, indicating a much-reduced stiffness compared to pure Ge. Additionally, we observe that thinner films of Ge survive a cycle of lithiation and delithiation, whereas thicker films fracture. By monitoring the critical conditions for crack formation, the fracture energy is calculated using an analysis from fracture mechanics. The fracture energies are determined to be <I>Γ</I> = 8.0 J m<SUP>−2</SUP> for a-Li<SUB>0.3</SUB>Ge and <I>Γ</I> = 5.6 J m<SUP>−2</SUP> for a-Li<SUB>1.6</SUB>Ge. These values are similar to the fracture energy of pure Ge and are typical for brittle fracture. Despite being brittle, the ability of amorphous Li<SUB>x</SUB>Ge to flow at relatively small stresses during lithiation results in an enhanced ability of Ge electrodes to endure electrochemical cycling without fracture.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Stresses were measured in-situ in a-Li<SUB>x</SUB>Ge during electrochemical cycling. </LI> <LI> a-Li<SUB>x</SUB>Ge was found to flow plastically at significantly lower stresses than a-Li<SUB>x</SUB>Si. </LI> <LI> The elastic modulus was measured in a-Li<SUB>x</SUB>Ge as a function of lithium concentration. </LI> <LI> The fracture energy of a-Li<SUB>x</SUB>Ge was measured, indicating a brittle material. </LI> <LI> a-Li<SUB>x</SUB>Ge exhibits an unusual combination of plastic flow and brittle fracture. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>