Seung Tae Choi

Chung-Ang University · School of Mechanical Engineering

This study unravels nickel-iron chloride/oxychloride as a unique precatalyst for oxygen evolution reaction. Chloride/oxychloride layers are electrodeposited on a permalloy surface upon sodium chloride etching to prepare the precatalyst. After chloride etching for 20 cycles (NiFe-NaCl-20) in NaCl, the oxygen evolution reaction (OER) activity of the...

Full comprehension of the pyrolysis of polymer materials is crucial for the design and application of thermal protection systems; however, it involves complex phenomena at different spatial and temporal scales. To bridge the gap between the abundant atomistic simulations and continuum modeling in the literature, we perform a novel mesoscale study o...

We studied the effect of femtosecond (FS) laser-texturing of 316 L stainless steel (SS) plates on their oxygen evolution reaction (OER). Here, the FS laser texturing is adapted to create oxyhydroxide layers with dimple structures on the pristine SS (denoted as B-SS) electrode. The single-side textured SS electrode is represented as FSSS-SS, and the...

Heteroatom-doped porous carbon material (H-PCM) was synthesized using Anacardium occidentale (cashew) nut’s skin by a simple pyrolysis route. The resulting H-PCM was thoroughly characterized by various analytical techniques such as field emission scanning electron microscopy (FE-SEM) with energy-dispersive X-ray (EDX) spectroscopy, high-resolution...

Stainless steel, a cost-effective Fe, Ni, and Cr alloy, is a promising electrode material for green electrochemical energy conversion systems. Accordingly, this study investigated the effect of potassium hydroxide concentration (KOH; 0.1, 1.0, and 6.0 M) on the oxygen evolution reaction (OER) activity of pristine stainless-steel mesh (pre-SSM) used...

In this study, resistive-type stretch sensors based on embroidered conductive threads were investigated to determine the best combinations of various conductive threads and fabrics, as well as the ideal embroidery patterns and their optimal geometric parameters. Among various conductive threads, a silver-plated nylon fiber of 280 denier was selecte...

In this study, an amorphous poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) copolymer with a high yield strain (approximately 18 %) is proposed as a cladding material for highly flexible and reliable piezoelectric ribbon fibers. Macro preforms are fabricated for thermal drawing (TD) processes, in which a poly(vinylidene fluoride-trifluor...

Rationale: Functional abdominal pain is an intractable medical condition that often reduces quality of life. Celiac plexus block is a representative intervention for managing intractable abdominal pain. However, celiac plexus block can be technically difficult to perform and carries the risk of potential complications. During erector spinae plane...

Background: Holmium laser enucleation of the prostate (HoLEP) has become an important treatment modality for benign prostate hypertrophy. The aim of the present study was to compare regional anesthesia methods for HoLEP operation and to determine the optimal technique. Methods: Sixty patients with American Society of Anesthesiologists scores of...

Objective To investigate the ability of kobusone to reduce high glucose levels and promote β-cell proliferation. Methods Four-week-old female db/db mice were assigned to the kobusone (25 mg/kg body weight, intraperitoneally twice a day) or control group (same volume of PBS). Glucose levels and body weight were measured twice a week. After 6 weeks,...

Crystalline metals are typically coated with materials such as natural oxides for engineering applications, wherein the coating surface has nanoscale roughness. Although the nanoscale asperities of material surfaces typically play a key role in the evolution of surface plasticity under contact loading, the dislocation activities interplaying with a...

Atomic-scale mixed-mode intergranular fracture, featured by non-local non-linear discrete atomic debonding processes near a crack-tip along a grain boundary (GB), is modeled with a cohesive zone in a continuum scale controlled by cohesive-zone dual constitutive relations, a balanced traction-separation relationship, i.e., a conventional cohesive-zo...

Electrospray (ES) cooling is a promising technique for dissipating high heat fluxes due to its excellent heat transfer characteristics and high energy efficiency. Conventional sprays exhibit a low efficiency due to droplet rebound and require a high pumping power. These issues are resolved with electrospraying, in which fine droplets are dispersed...

In this study, the creep deformation of ferritic/martensitic 9Cr-1Mo (Grade T91) steel boiler tubes that occurs during the operation of thermal power plants is studied using crystal elasto-viscoplasticity finite element analysis (CEV-FEA). The material properties and creep behaviour of Grade T91 were obtained through experimentation and computation...

The creep deformation behavior of virgin and long-term service-aged (77000 h) T91 steel of superheater tubes in a power plant was assessed via creep tests along with microstructure examination. A series of miniature tensile creep tests were conducted at 590 °C under applied stresses ranging from 120.0 to 224.1 MPa for both material types. The creep...

Thin films, coatings, and layered structures are ubiquitous in contemporary micro- and nanoelectronics, optoelectronics, and electromechanical systems, for which adhesion between heterogeneous materials is one of the important mechanical properties to be considered in fabrication and operation. In this study, the adhesive contact between elastic la...

Creep deformation of Super304H austenitic stainless steel boiler tubes in thermal power plants is evaluated using crystal elasto-viscoplastic finite element analysis (CEV-FEA). The material properties of Super304H at elevated temperatures are obtained from experiments and computations at different scales as follows: anisotropic elastic constants of...

Rationale: Midazolam is known as a safe drug and is widely used as a sedative and an anesthetic adjuvant. Therefore, there is a lack of awareness that midazolam can cause anaphylaxis. Midazolam anaphylaxis is rare, and only a few cases have been reported, but such a risk is always present. In this study, we report a case of midazolam anaphylaxis b...

This study proposes a novel skinny button with multimodal audio and haptic feedback to enhance the touch user interface of electronic devices. The active material in the film-type actuator is relaxor ferroelectric polymer (RFP) poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] blended with poly(vinylidene fluoride-t...

Tactile perception in large-area displays is currently attracting substantial research attention, since in conjunction with visible and auditory sensations, it provides more immersive and realistic interactions with displayed contents. Here, a new vibrotactile display based on the fretting phenomenon is first developed to provide localized tactile...

Two contacting surfaces grafted with polymer brushes have potential applications due to their extraordinary lubricating behavior. However, the polymer brushes may have poor mechanical stability under high normal and shear stresses, which is a challenge for practical usage of polymer brushes systems. In this study, we propose the use of grafted nano...

In this study, thermomechanical and creep analyses of boiler tubes are performed and a procedure for cost-based optimization of boiler tube geometries is presented. First, analytical thermal convection and conduction heat transfer under ultra-supercritical (USC) and advanced ultra-supercritical (A-USC) conditions as well as numerical evolution of c...

In article number 1801054, Dongseob Kim, Sangmin Lee, and co‐workers present a gyroscopic generator that can generate high rotation speeds above 8000 rpm with only hand input. The triboelectric nanogenerator (TENG) and poled poly(vinylidene fluoride‐co‐trifluoroethylene) (P(VDF‐TrFE))‐based hybrid generator can produce electricity from every mechan...

Uniform pattern transformation can be observed in some structures with periodic arrays of pores at a critical compressive load because of buckling of the constituents of the structures. This pattern transformation can be exploited to design structures for various potential applications. Previous studies have focused on the instability of periodic p...

In this study, the fabrication process of a piezoelectric poly[(vinylidenefluoride-trifluoroethylene) [P(VDF-TrFE)] film was optimized to maximize its crystallinity and piezoelectric properties and to improve its thermal stability. The effect of purification of P(VDF-TrFE) solution dissolved in methyl ethyl ketone [MEK] and uniaxial stretching on t...

In this study, we developed a computational scheme based on the atomic-level J-based mutual integral (or two-state conservation integral) to analyze the mixed-mode fracture along grain boundaries (GB) in polycrystalline solids. Discrete atomic information, obtained from molecular dynamics simulation of crack propagation along GBs in polycrystalline...

A novel method to drill stepwise vias on a multilayered metal/polymer structure by using a CO2 laser percussion drilling process is developed, in which the Gaussian intensity distribution of the CO2 laser and the differences in CO2 laser absorption and melting characteristics between the metal and polymer layers are understood to be the key factors...

Roll-to-roll (R2R) transfer of a chemical vapor deposition (CVD) graphene is an inevitable step for large scale and high throughput manufacturing of graphene transparent electrodes on flexible substrates. The damages on graphene induced by high contact pressure of nip rolls during the roll transfer degrade the electrical properties of the transferr...

Atomically thin molybdenum disulfide (MoS2) shows great potential for use in nano-devices due to its remarkable electronic, optoelectronic, and mechanical properties. These material properties are often dependent on the thickness or the number of layers, and hence Raman spectroscopy is widely used to characterize the thickness of atomically thin Mo...

The effect of copper and/or nickel nanoclusters, generally formed by neutron irradiation, on fracture mechanisms of ferrite iron was investigated by using molecular statics simulation. The equilibrium configuration of nanoclusters was obtained by using a combination of an on-lattice annealing based on Metropolis Monte Carlo method and an off-lattic...

This study extended the conventional JKR theory on adhesive contact between elastic cylinders to the frictionless adhesive contact between elastic films coated on rigid cylindrical rollers. The plane-strain elasticity problem of indentation on an elastic film perfectly bonded to a rigid half-plane was revisited, with which the force-depth relations...

In this paper, we will present electromechanical behavior of the poly(vinylidene fluoridetrifluoroethylene- 1,1-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer under hydrostatic pressure. The dielectric data obtained under controlled hydrostatic pressure suggest a reversible pressure-induced crossover from the relaxor state to a ferroelectric st...

A vari-focal fluidic lens is provided. The fluidic lens includes a frame, an first membrane, a second membrane, and an optical fluid. The frame defines an inner space of the fluidic lens including a driving portion and a lens portion that are connected to each other. The elastic membrane is attached to one side of the frame to cover at least the le...

A touch panel is provided. The touch panel includes an electroactive polymer (EAP) actuator which is deformable when protruded or depressed locally at a part to which a driving voltage is applied; and a flexible touch sensor which is disposed on the EAP actuator. The flexible touch sensor is deformable locally in correspondence with a deformation o...

A varifocal lens structure, a method of manufacturing the varifocal lens structure, an optical lens module, and a method of manufacturing the optical lens module. The varifocal lens structure includes a liquid lens unit including a silicone membrane that includes a first silicone elastomer, a polymer actuator disposed on an upper surface of the sil...

To provide tactile feedback on flexible touch screens, transparent relaxor ferroelectric polymer film vibrators were designed and fabricated in this study. The film vibrator can be integrated underneath a transparent cover film or glass, and can also produce acoustic waves that cause a tactile sensation on human fingertips. Poly(vinylidene fluoride...

The work of adhesion between a polydimethylsiloxane (PDMS) elastomer film and a flat diamond tip was measured by instrumented indentation. The results showed that the apparent work of adhesion between the tip and the PDMS film increases with increasing dwell time and retreating velocity; on the other hand, the indentation depth has no significant e...

Nonlinear large deformation of a transparent elastomer membrane under hydraulic pressure was analyzed to investigate its optical performance for a variable-focus liquid-filled membrane microlens. In most membrane microlenses, actuators control the hydraulic pressure of optical fluid so that the elastomer membrane together with the internal optical...

We designed and fabricated multilayered relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-clorotrifluoroethylene) [P(VDF-TrFE-CTFE)] actuators in this study. To minimize the driving voltage of the P(VDF-TrFE-CTFE) actuator, which produces bending deformation, we designed a multilayered structure and analyzed the structure using compo...

An LED package includes a substrate having an electrically conductive portion and an electrically non-conductive portion composed of an oxide of the conductive portion; an LED mounted on the conductive portion and electrically connected to the conductive portion; a first electrode disposed on the non-conductive portion and electrically connected to...

A multilayered electroactive polymer (EAP) device and a method of manufacturing the same is provided. The multilayered EAP device includes a plurality of unit layers. Each unit layer includes an EAP layer formed of an electroactive polymer (EAP), a protecting layer configured to prevent a material from penetrating into the EAP layer, and an active...

Provided are a polymer and a polymer actuator including the polymer. The polymer is cross-linked by a cross-linking agent. When the polymer is used in the polymer actuator, the polymer actuator shows a high strain and may be stably operated at high temperatures.

A fluidic lens and method for manufacturing the same are provided. The fluidic lens includes a transparent optical fluid and a double elastomer membrane. An outer membrane of the double elastomer membrane that is externally exposed includes a Poly DiMethyl Siloxane (PDMS) elastomer, and an inner membrane of the double elastomer membrane that makes...

An optical probe and an optical system therefor are provided. The optical probe is includes a housing configured to house the optical system and the housing has a transparent window therein. the optical system includes a light emitting unit, a collimation lens, and a focusing lens. A numerical aperture of the optical system is adjustable by adjusti...

A three-dimensional subsurface penny-shaped crack in an elastic half-space subjected to a compressive moving load is analyzed using the finite element method. The compressive load is applied through a spherical asperity, which moves from left to right on the top surface of the half-space. Normal contact between the crack faces of the penny-shaped c...

The conventional JKR theory was extended to the adhesive contact of a rigid sphere onto an elastic film perfectly bonded to a rigid substrate. An elasticity problem of axisymmetric indentation on an elastic film was revisited, in which the force–depth relations for both flat and spherical indentations were obtained in a simple form. With the obtain...

We designed and fabricated the multilayered P(VDF-TrFE-CTFE) actuators, which can be operated by the applied voltage of only 40 V. To fabricate P(VDF-TrFE-CTFE) films having a thickness of about 1~2 μm, we also developed a new film transfer method, with which the P(VDF-TrFE-CTFE) films of about 1.5 μm thickness were fabricated and stacked. The defl...

We designed, fabricated, and characterized varifocal microlenses, whose focal length varies along with the deformation of a transparent elastomer membrane under hydraulic pressure tailored by electroactive polymer actuators. The microfluidic channel of the microlens was designed to be embedded between silicon and glass so that transient fluctuation...

Microfluidic design and fabrication was developed for wafer-scale varifocal liquid lens which is slim less than 0.9mm. The liquid-filled varifocal lens has advanced functions such as auto macro and focusing to obtain a high quality of image. This varifocal lens is similar to human eye and it consists of main Si frame which has penetrated inner hole...

In this study we developed a liquid-filled varifocal lens operated by electroactive polymer actuators. A silicon wafer was structured with micromachining processes to have four microfluidic chambers and a circular hole working as an aperture. The structured silicon wafer (opaque frame) was bonded to a glass wafer (transparent frame), and thus micro...

A systematic method of flat indentation was developed to measure the elastic and viscoelastic properties of polymer films. A flat indentation problem on an elastic film perfectly bonded to a rigid substrate was revisited, from which the relationship between the applied force and the penetration depth was obtained in a simple form. Application of th...

Singularities interacting with a coated circular inhomogeneity are analyzed with the method of analytic continuation and the Schwarz-Neumann's alternating technique. It is shown that the solution for singularities in a homogeneous medium can be used as a building block of the solution for the same singularities interacting with a coated circular in...

We revisited an elasticity problem of flat indentation on an elastic film bonded to a rigid substrate and obtained the force–depth relation in a simple form. With the obtained force–depth relation, Kendall's elastic equilibrium theory of adhesion was extended to the adhesion between a flat tip and a compressible elastic film. Thus, the thermodynami...

We have developed a modified-creep experiment with nanoindenter to measure the viscoelastic properties of elastomer films. Load-controlled experiments on PDMS (polydimethylsiloxane) films were performed using the Nano Indenter® XP with a flat-ended tip. By adapting the force–depth relation obtained by Choi et al. [S.T. Choi, S.R. Lee, Y.Y. Earmme,...

Cracking phenomena in tensile-strained InxGa1−xAs epitaxial film on an InP substrate are analyzed via the formulation given in Part I [Lee, S., Choi, S.T., Earmme, Y.Y., 2006. Analysis of vertical cracking phenomena in tensile-strained epitaxial film on a substrate: Part I. Mathematical formulation. International Journal of Solids and Structures 43...

The method of analytic continuation and Schwarz-Neumann 's alternating technique were applied to the thermoelastic interaction problems of singularities and interfaces in an anisotropic "trimaterial," which denotes an infinite body composed of three dissimilar materials bonded along two parallel interfaces. It was assumed that the linear thermoelas...

This paper presents an analysis of a single vertical crack and periodically distributed vertical cracks in an epitaxial film on a semi-infinite substrate where the cracks penetrate into the substrate. The film and substrate materials have different anisotropic elastic constants, necessitating Stroh formalism in the analysis. The misfit strain due t...

The field projection method of Hong and Kim (2003) to identify the crack-tip cohesive zone constitutive relations in an isotropic elastic solid is extended to a nanoscale planar field projection of a cohesive crack tip on an interface between two anisotropic solids. This formulation is applicable to the elastic field of a cohesive crack tip on an i...

In order to measure the mechanical properties of gold films on silicon substrate, two types of specimens, i.e., bridged films and circular membranes, are manufactured. Using a wedge tip, the bridged gold films are indented so that the films are pushed off, which is called as V-peel test. The load-deflection curves obtained by the V-peel test are an...

The elastic strain and stress fields associated with nanoscale compositional modulation in an anisotropic epitaxial film on an anisotropic substrate are obtained by using Stroh formalism and the Eshelby-type inclusion method. The composition of the epitaxial film is considered to periodically fluctuate in a surface soft mode, with the amplitude of...

The authors regret that several sign and notation errors were made in the above article. The errors were mainly introduced when changing notation systems for anisotropic elasticity, which are not unique in the literature.

This paper presents an analysis of a single vertical crack and periodically distributed vertical cracks in an epitaxial film on a semi-infinite substrate where the cracks penetrate into the substrate. The film and substrate materials have different anisotropic elastic constants, necessitating Stroh formalism in the analysis. The misfit strain due t...

The GaAs wafer bonding process is investigated to reduce the mechanical failures of GaAs wafer based on strength design concept. Three-point bending experiment is performed to measure the fracture strength of GaAs wafer, of which cleavage takes place on (110) plane. We propose a simple method for minimizing the thermal residual stress in a three-la...

Quality issue in cold forging industry means the important feature in terms of both manufacturing technology and economy. In the present study, authors try to discuss on how to achieve the cold forging technology to produce better parts with economy. In the first part of paper, we show a new approach to design the processing step and die system in...

The co-cured joint is more efficient than the adhesively bonded joint for composite structures because of its several advantages. However, failure analysis of the co-cured joint has just a little been reported since the co-cured joining method was introduced. Observing based on the experimental results, failure starts at the interface corner of the...

The equivalence between anisotropic and isotropic elasticity is investigated in this study for two-dimensional deformation under certain conditions. That is, the isotropic elasticity can be reconstructed in the same framework of the anisotropic elasticity, when the interface between dissimilar media lies along a straight line. Therefore, many known...

Using LSM (laser scanning method), the radius of curvature due to thermal deformation in polyimide film coated on Si substrate is measured. Since the polyimide film shows viscoelastic behavior, i.e., the modulus and deformation of the film vary with time and temperature, we estimate the relaxation modulus and the residual stresses of the polyimide...

Using LSM (laser scanning method), the radius of curvature due to thermal deformation in polyimide film coated on Si substrate is measured. Since the polyimide film shows viscoelastic behavior, i.e., the modulus and deformation of the film vary with time and temperature, we estimate the relaxation modulus and the residual stresses of the polyimide...

A problem of a circular elastic inhomogeneity interacting with a crack under uniform loadings (mechanical tension and heat flux at infinity) is solved. The singular integral equations for edge and temperature dislocation distribution functions are constructed and solved numerically, to obtain the stress intensity factors. The effects of the materia...

The elastic solution for a singularity in an anisotropic trimaterial with perfectly bonded interfaces was obtained in the previous work by Choi and Earmme. The term "trimaterial" denotes an infinite body composed of three dissimilar materials bonded along two parallel interfaces. It is shown in this paper that when the interfaces of an anisotropic...

Proposed in this paper is the equivalence between anisotropic and isotropic elasticity for two-dimensional deformation under certain conditions. That is, the isotropic elasticity can be reconstructed in the same framework of the anisotropic elasticity, when the interface between dissimilar media lies along a straight line. Therefore, many known sol...

Singularity problems in an isotropic trimaterial are analyzed by the same procedure as in an anisotropic trimaterial of Part I [Int. J. Solids Struct. 39, 943–957]. `Trimaterial' denotes an infinite body composed of three dissimilar materials bonded along two parallel interfaces. Linear elastic isotropic materials under plane deformations are assum...

Schwarz–Neumann's alternating technique is applied to singularity problems in an anisotropic `trimaterial', which denotes an infinite body composed of three dissimilar materials bonded along two parallel interfaces. Linear elastic materials under general plane deformations are assumed, in which the plane of deformation is perpendicular to the two p...

Using LSM (laser scanning method), the radius of curvature due to thermal deformation in polyimide film coated on Si substrate is measured. Since the polyimide film shows viscoelastic behavior, i.e., the modulus and deformation of the film vary with time and temperature, we estimate the relaxation modulus and the residual stresses of the polyimide...