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- 저자 : Pharr, G. M. (검색결과 3 건)
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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>
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Effective Poisson’s ratio from combined normal and lateral contacts of single crystals
Lee, J.H.,Gao, Y.F.,Pharr, G.M. Cambridge University Press (Materials Research Soc 2012 Journal of materials research Vol.27 No.1
<▼1><B>Abstract</B><P/></▼1><▼2><P>When an elastic half-space is subjected to both normal and tangential contact, the ratio of normal and tangential contact stiffnesses can be measured by various scanning force microscopy techniques. For elastically isotropic solids, this stiffness ratio depends on Poisson’s ratio as given by the Mindlin solution. An anisotropic elastic contact analysis here shows the difference between the effective Poisson’s ratio as defined from the stiffness ratio and its uniaxial counterpart with respect to various crystal structures and various normal/tangential contact directions. Closed-form analytical solutions of effective indentation moduli are derived for materials with at least one plane of transverse isotropy. Since the Sneddon (normal contact) and Mindlin (lateral contact) solutions are derived under different frictional conditions, finite element simulations were performed which show that the effects of elastic dissimilarity and contact shape are generally small but not negligible. The predicted dependence on crystallographic orientation and elastic anisotropy has been compared favorably with previously reported multiaxial contact experiments for a number of cubic single crystals. Implications for atomic force microscopy based experiments are also discussed.</P></▼2>
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