Young-Cheon Kim

Stretchable, Transparent Electrodes as Wearable Heaters Using Nanotrough Networks of Metallic Glasses with Superior Mechanical Properties and Thermal Stability

Mechanical robustness, electrical and chemical reliabilities of devices against large deformations such as bending and stretching have become the key metrics for rapidly emerging wearable electronics. Metallic glasses (MGs) have high elastic limit, electrical conductivity, and corrosion resistance, which can be promising for applications in wearable electronics. However, their applications in wearable electronics or transparent electrodes have not been extensively explored so far. Here, we demonstrate stretchable and transparent electrodes using CuZr MGs in the form of nanotrough networks. MG nanotroughs are prepared by electrospinning and cosputtering process, and they can be transferred to various desired substrates, including stretchable elastomeric substrates. The resulting MG nanotrough network is first utilized as a stretchable transparent electrode, presenting outstanding optoelectronic (sheet resistance of 3.8 Ω/sq at transmittance of 90%) and mechanical robustness (resistance change less than 30% up to a tensile strain of 70%) as well as excellent chemical stability against hot and humid environments (negligible degradation in performance for 240 h in 85% relative humidity and 85 °C). A stretchable and transparent heater based on the MG nanotrough network is also demonstrated with a wide operating temperature range (up to 180 °C) and excellent stretchability (up to 70% in the strain). The excellent mechanical robustness of these stretchable transparent electrode and heater is ascribed to the structural configuration (i.e., a nanotrough network) and inherent high elastic limit of MGs, as supported by experimental results and numerical analysis. We demonstrate their real-time operations on human skin as a wearable, transparent thermotherapy patch controlled wirelessly using a smartphone as well as a transparent defroster for an automobile side-view mirror, suggesting a promising strategy toward next-generation wearable electronics or automobile applications.

Estimation of principal directions of Bi-axial residual stress using instrumented Knoop indentation testing

We propose a novel method for estimating stress directionality p and principal direction θp using four Knoop indentations. Indentation load-depth curves with a Knoop indenter are shifted with respect to indenter orientation to stress direction. Based on this phenomenon, we derived p and θp theoretically in terms of the indentation load differences between a stressed and an unstressed sample. Plastic zone beneath the Knoop indenter was theoretically estimated based on expanding cavity model for confirming separate plastic zones of four Knoop indentations. The proposed models and algorithms were verified by indentation experiments under various applied stress states that are generated using a stress-generating jig with two independent orthogonal loading axes. Estimated principal directions showed good agreement with applied principal directions. For small target area when considering in-field application, we proposed an estimation method of one load difference with other three load differences, and it was verified as feasible method with experimental results.

Assessment of surface-local strains from remnant microindents on a Zr-based metallic glass

In this study, a morphological modeling was done for remnant microindents on a Zr-based metallic glass for avoiding an overestimation in the indented surface area by applying the Riemann integral and for calculating quantitative values of the stretch strain which is defined by the indented surface area divided by its projected area according to Milman et al. A discrete pixel image for an indent was fitted into a continuous ellipsoidal cap and its indented surface area by a spherical indenter was deterministically calculated by integrating the ellipsoidal cap surface. The calculated stretch strains were lower than their upper limits and quantitatively close to the conventional indentation strains. The stretch strains were generally higher than the conventional indentation strains but decreased to their lower values at a shallow indentation of less than 50 μm. This phenomenon was attributed to severe elastic recovery in shallow indentations. From an overall viewpoint, the strain overestimation by the Riemann integral was clearly resolved, and this result confirmed the validity of the new approach for estimating the indented surface area and the stretch strain.

Contact morphology and constitutive equation in evaluating tensile properties of austenitic stainless steels through instrumented spherical indentation

We evaluate representative stress and strain of austenitic stainless steels using instrumented indentation tests with a spherical indenter by taking into account the real contact depth and effective radius. We investigate the relation between material pileup underneath the spherical indenter and the strain-hardening exponent in uniaxial tensile tests for these steels. We evaluate the suitability of three constitutive equations, the Hollomon, Ludwigson, and Swift equations, for describing linear-type strain-hardening of austenitic stainless steels. Using the real contact depth and effective radii developed for the austenitic stainless steels, we find good agreement between representative stress and strain in instrumented indentation and uniaxial tensile tests.

Wall-thickness-dependent strength of nanotubular ZnO

We fabricate nanotubular ZnO with wall thickness of 45, 92, 123 nm using nanoporous gold (np-Au) with ligament diameter at necks of 1.43 μm as sacrificial template. Through micro-tensile and micro-compressive testing of nanotubular ZnO structures, we find that the exponent m in

Directionality of residual stress evaluated by instrumented indentation testing using wedge indenter

\bar{\sigma }\propto {\bar{\rho }}^{m}

Nondestructive Measurement of Non-Equibiaxial Welding Residual Stresses Using Instrumented Indentation Technique With Knoop Indenter

σ¯∝ρ¯m, where