Kisun Kim

Rapid, High‐Resolution 3D Interference Printing of Multilevel Ultralong Nanochannel Arrays for High‐Throughput Nanofluidic Transport

3D interference printing enables the single-step production of multilayered ultralong nanochannel arrays with nanoscale regularity. The superior depth-of-focus of this technique realizes a state-of-the-art nanostructure which has intensively stacked 32 layers of inch-long, horizonontal nanochannels with sub-100 nm holes in a monolithic matrix (≈15 μm). This exceptional structure can be integrated into microfluidic devices, facilitating high-flux rheological platforms using nanocapillarity.

Monolithic Bi1.5Sb0.5Te3 ternary alloys with a periodic 3D nanostructure for enhancing thermoelectric performance

The selective reduction of thermal conductivity while preserving the Seebeck coefficient and electrical conductivity is regarded as a key strategy for achieving the high dimensionless figure-of-merit (ZT) of thermoelectric materials. Here, we newly propose a periodic three-dimensional (3D) nanostructure that has an ability to significantly reduce thermal conductivity, resulting in an improved ZT value of thermoelectric materials near room temperature. A 3D nanostructured thermoelectric monolith is developed by electrochemical deposition of a Bi–Sb–Te ternary alloy into a highly ordered, interstitial porous network in an epoxy template predefined by advanced lithography. The resultant inch-scale, bicontinuous nanocomposite monolith released from a substrate can be easily transferred to a customized reliable platform for evaluating thermoelectric properties. The measured thermal conductivity is only ∼0.89 W mK−1 at 350 K due to greatly increased phonon boundary scattering without any degradation in the Seebeck coefficient and electrical conductivity, leading to an enhanced ZT value (∼0.56) which is ∼50% higher than that of an ordinary film with the same elemental composition. The 3D nanostructure developed here will provide new design opportunities for nanostructured thermoelectric materials, potentially usable in flexible thermoelectric coolers and wearable energy harvesting systems.

Metal-induced fluorescence properties of three-dimensionally ordered macroporous silver inverse opal platforms

This study examined the metal-induced fluorescence properties of three-dimensionally ordered macroporous silver inverse opal (IO) films. Electrochemically synthesized silver IO films with a micrometer cavity exhibited notable fluorescence enhancement at the silver frame, and a decrease in fluorescence lifetime. Numerical calculations supported the observations of a higher fluorescence efficiency at the frame than in the cavity.

A Study on Determination of Complex Stiffness of Frame Bush for Ride-comfort Improvement of Body-on-frame Vehicle

Body-on-frame type vehicle has a set of frame bushes between body and frame for vibration isolation. Such frame bushes are important vibration transmission paths to passenger space for excitations during driving. In order to reduce the vibration level of passenger space, therefore, change of complex stiffness of the frame bushes is more efficient than modification of other parts of the vehicle such as body, frame and suspension. The purpose of this study is to reduce the vibration level for ride comfort by optimization of complex stiffness of frame bushes. In order to do this, a simple finite element vehicle model was constructed and complex stiffness of the frame bushes was set to be design variables. The objective function was defined to reflect frequency dependence of passenger ride comfort. Genetic algorithm and sub-structure synthesis were applied for minimization of the objective function. After optimization level at a position of interest on the car body was reduced by about 43.7 % in RMS value. Causes for optimization results are discussed.

Low-Cost Black Phosphorus Nanofillers for Improved Thermoelectric Performance in PEDOT:PSS Composite Films

In recent years, two-dimensional black phosphorus (BP) has seen a surge of research because of its unique optical, electronic, and chemical properties. BP has also received interest as a potential thermoelectric material because of its high Seebeck coefficient and excellent charge mobility, but further development is limited by the high cost and poor scalability of traditional BP synthesis techniques. In this work, high-quality BP is synthesized using a low-cost method and utilized in a PEDOT:PSS film to create the first ever BP composite thermoelectric material. The thermoelectric properties are found to be greatly enhanced after the BP addition, with the power factor of the film, with 2 wt % BP (36.2 μW m-1 K-2) representing a 109% improvement over the pure PEDOT:PSS film (17.3 μW m-1 K-2). A simultaneous increase of mobility and decrease of the carrier concentration is found to occur with the increasing BP wt %, which allows for both Seebeck coefficient and electrical conductivity to be increased. These results show the potential of this low-cost BP for use in energy devices.

Emergence of New Density-Strength Scaling Law in 3D Hollow Ceramic Nano-Architectures

Density-strength tradeoff appears to be an inherent limitation for most materials and therefore design of cell topology that mitigates strength decrease with density reduction has been a long-lasting engineering pursue for porous materials. Continuum-mechanics-based analyses of mechanical responses of conventional porous materials with bending-dominated structures often give the density-strength scaling law following the power-law relationship with an exponent of 1.5 or higher, which consequentially determines the upper bound of the specific strength for a material to reach. In this work, a new design criterion capable of significantly abating strength degradation in lightweight materials is presented, by successfully combining the size-induced strengthening effect in nanomaterials with the architectural design of cellular porous materials. Hollow-tube-based 3D ceramic nanoarchitectures satisfying such criterion are fabricated in large area using proximity field nano-patterning and atomic layer deposition. Experimental data from micropillar compression confirm that the strengths of these nanoarchitectural materials scale with relative densities with a power-law exponent of 0.93, a hardly observable value in conventional bending-dominated porous materials. This discovery of a new density-strength scaling law in nanoarchitectured materials will contribute to creating new lightweight structural materials attaining unprecedented specific strengths overcoming the conventional limit.

Suggestion of a polynomial model with a convolution integral term for a vibration isolation element with hysteresis and non-linearity for force estimation

Modelling of the force transmitted through vibration isolation elements in terms of the displacement and the velocity is very useful in transfer path or vibration power analysis of dynamic systems. Modelling using the complex stiffness in the frequency domain can be a good solution for linear elements, and force-state mapping can be a good solution for non-linear elements. In this study, it is noted that conventional force-state mapping with a linear part simply consisting of a linear spring and a viscous damper does not work sufficiently well for vibration isolation elements typically made of rubber materials; therefore, modelling of the linear part using the complex stiffness is proposed. The non-linear force-state model with the complex stiffness model for the linear part is applied to a set of simulation data for validation of the proposed approach and to another set of data measured from a cabin mount of an excavator for illustration. A form of the fractional polynomial function of the frequency is utilized for curve fitting of the linear complex stiffness in the frequency domain. The constraints for stability and causality on the coefficients and the order of this form are discussed. Procedures for the construction of the whole model are presented. Based on the modelling of the simulated and measured data, it is claimed that the suggested modelling is promising as a representation of the frequency- and amplitude-dependent complex stiffness of vibration isolation elements.