Bum Sung Kim

Size-dependent piezoelectric and mechanical properties of electrospun P(VDF-TrFE) nanofibers for enhanced energy harvesting

Piezoelectricity-based energy harvesting from wasted mechanical energies has garnered an increasing attention as a clean energy source. Especially, flexible organic piezoelectric materials provide an opportunity to exploit their uses in mechanically challenging areas where brittle inorganic counterparts have mechanical limitations. In this regard, electrospinning has shown its advantages of producing poly(vinylidene fluoride) (PVDF)-based nanofibrous structures without the necessity of a secondary processing to induce/increase piezoelectric properties. However, the effects of electrospun fiber dimension, one of the main morphological parameters in electrospun fibers, on piezoelectricity have not been fully understood. In this study, two dependent design of experiments (DOEs) were utilized to systematically control the dimensions of electrospun poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) to produce nanofibers having their diameter ranging from 1000 to sub-100 nm. Such a dimensional reduction resulted in the increase of piezoelectric responsible electroactive phase content and the degree of crystallinity. These changes in crystal structure led to approximately 2-fold increase in piezoelectric constant as compared to typical P(VDF-TrFE) thin films. More substantially, the dimensional reduction also increased the Youngs modulus of the nanofibers up to approximately 80-fold. The increases in piezoelectric constant and Youngs modulus collectively enhanced piezoelectric performance, resulting in the exponential increase in electric output of nanofiber mats when the fiber diameters were reduced from 860 nm down to 90 nm. Taken together, the results suggest a new strategy to improve the piezoelectric performance of electrospun P(VDF-TrFE) via optimization of their electromechanical and mechanical properties.

Combinatorial influence of bimodal size of B2 TiCu compounds on plasticity of Ti-Cu-Ni-Zr-Sn-Si bulk metallic glass composites

We report on the formation of Ti-Cu-Ni-Zr-Sn-Si bulk metallic glass composites containing bimodal size of B2 TiCu compounds. The small B2 TiCu compound with a size of 1 to 10 μm has a strong influence on the oscillation of the shear stress, thus causing wavy propagation of the shear bands. In contrast, the large B2 TiCu compound with a size of 70 to 150 μm dissipates the shear stress by branching and multiplication of the shear bands. By forming the bimodal size of B2 TiCu compound, it is possible to determine the harmonic influence to further enhance the plasticity of the Ti-Cu-Ni-Zr-Sn-Si bulk metallic glass composites.

Fabrication and sensing property for conducting polymer nanowire-based biosensor for detection of immunoglobulin G

Conducting polymers are excellent sensing materials in the design of bioanalytical sensors because of their electronic conductivity, low energy optical transitions, biocompatibility, and room temperature operation. Among them, Polypyrrole (Ppy) is one of the most extensively used conducting polymers because of a number of properties such as redox activity, rapid electron transfer, and ability to link a variety of biomolecules to pyrrole groups by chemical treatment. In this study, Ppy nanowires were synthesized by an electrospinning method. The nanowires were prepared from a solution mixture of Ppy and poly(ethylene oxide). The method of detection in such a device is based on the selective binding of antigen onto an antibody that is covalently attached to the nanowires. Thus, anti-IgG was immobilized on Ppy nanowires using an EDC {[N-(3-dimethyl aminopropyl)-N2-ethylcarbodiimide hydrochloride]}-NHS(N-hydrosuccinimide) modified technique. Fluorescence images of BSA–FITC (fluorescein isothiocyanate labeling of bovine serum albumin) conjugation demonstrated that antibody was functionalized on the Ppy nanowires without non-specific binding and facilitated selective detection of antigen. Current–voltage (I–V) characterization was used to monitor the change in the conductivity of nanowires while the specific binding interaction occurred. These results of electrical properties enable Ppy nanowire-based biosensors to detect biomolecules in real-time.

Transformative piezoelectric enhancement of P(VDF-TrFE) synergistically driven by nanoscale dimensional reduction and thermal treatment

Despite the significant potential of organic piezoelectric materials in the electro-mechanical or mechano-electrical applications that require light and flexible material properties, the intrinsically low piezoelectric performance as compared to traditional inorganic materials has limited their full utilization. In this study, we demonstrate that dimensional reduction of poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) at the nanoscale by electrospinning, combined with an appropriate thermal treatment, induces a transformative enhancement in piezoelectric performance. Specifically, the piezoelectric coefficient (d33) reached up to -108 pm V-1, approaching that of inorganic counterparts. Electrospun mats composed of thermo-treated 30 nm nanofibers with a thickness of 15 μm produced a consistent peak-to-peak voltage of 38.5 V and a power output of 74.1 μW at a strain of 0.26% while sustaining energy production over 10k repeated actuations. The exceptional piezoelectric performance was realized by the enhancement of piezoelectric dipole alignment and the materialization of flexoelectricity, both from the synergistic effects of dimensional reduction and thermal treatment. Our findings suggest that dimensionally controlled and thermally treated electrospun P(VDF-TrFE) nanofibers provide an opportunity to exploit their flexibility and durability for mechanically challenging applications while matching the piezoelectric performance of brittle, inorganic piezoelectric materials.

Tribological Behaviors of Dense Gelcasting Nanocomposites

Gelcasting/pressure less sintered Al2O3/SiC nanocomposites has a low sinterability. Also, mechanical and wear properties of these nanocomposites was degraded. Wear mechanism of low sinterability gelcasting nanocomposites was dominated by fracture mode of surface during wear tests. In this study, gelcasting processing and followed plressureless (PL) and hot isostatic pressing (HIP) sintering was attempted to fabricate dense Al2O3/SiC nanocomposites. Wear behaviors of high densities gelcasting nanocomposites were investigated under the identical wear test condition. The comparative specimen was used to hot pressed nanocomposites. Wear rates of dense gelcasting nanocomposites were related to closely initial friction coefficient.

Controlled Linear Assemblies of Graphite Flakes Anchoring Polysiloxane-Based Nanocomposite Films and Enhancement of Thermal Properties

Polysiloxane-based nanocomposites were fabricated by controlled linear assemblies of graphite flakes (GFs) while applying electric and magnetic fields. The orientation of the longitudinal axis of GFs can be controlled and constructed linear assemblies of GFs (LAGFs) by anchoring composite film surfaces with increased thickness of LAGF structure by the field-induced assembly. The different effects of applying magnetic and electric fields during the assembly of GFs in a polymer matrix are investigated. Analysis revealed that the formation of densely packed LAGFs induced by modulating the electric field is directly related to a significant enhancement in the thermal diffusivity and optical transmission of the composites.

Mechanical Properties of Metallic Glass Matrix Composites Synthesized by Powder Consolidation as Precursor of Porous Material

Mechanical properties of Cu-based metallic glass matrix composites reinforced by ductile fugitive phases, synthesized by warm extrusion of gas atomized powders, has been investigated under the uniaxial compression condition at room temperature. The ductile fugitive phases are well distributed in the metallic glass matrix and enhanced macroscopic plasticity is observed due to the formation of multiple shear bands, initiated from the interface between ductile fugitive phase and metallic glass matrix, as well as their confinement between the reinforcements, stemming from the constrained plastic deformation of the reinforcing ductile phase.