Papers by Author: Kap Jin Kim

Abstract: Thin films of polyvinylidene (PVDF) and its copolymers containing 50 to 30 mole % of trifluoroethylene (TrFE) demonstrate rapid reversal of polarization under applied electric field, and can be used as ferroelectric materials in non-volatile memory devices. In this study, Fourier-transform infrared transmission (FTIR-TS) and grazing incident reflection absorption (FTIR-GIRAS) spectroscopy techniques were used to examine the changes in Curie transition temperature (T_c), chain and dipole orientation in selected VDF copolymer spun-cast films as a subjected to a reversible thermal-cycling treatment. A reversible solid-to-solid β-crystalline phase transition with thermal hysteresis and a favorable C-F dipole orientation along the electric field direction were observed for the sample subjected to thermal-cycling below its melting point (T_m). An irreversible β-crystalline phase transition with unfavorable C-F dipole orientation was observed for the sample treated to heating-cooling cycle above T_m. The results are reported in detail here.

Abstract: Titanium(IV) complexes of the type [(acac)₂Ti(O-G-O] and [(acac)₂Ti(OCH₂CH₂SH)₂] {where G = (CH₂)₂, CH₂CH(CH₃), CH₂CH(C₂H₅), CH(CH₃)CH(CH₃) have been synthesized in high yield by the interaction of the precursor [(acac)₂Ti( OR)₂] {where R = Pri , Et} with a variety of glycols and thioglycols in 1:1 and 1:2 molar ratios in refluxing benzene under anhydrous condition yield heteroleptic derivatives. On the basis of physico-chemical analyses, a cis-octahedral environment around Ti (IV) is proposed.

Abstract: Polyvinylidene fluoride (PVDF) based electrospun nanoweb fibers with outstanding piezo-, pyro- and ferroelectric behavior are being intensely studied by many researchers, especially for touch-sensor applications. In order to further improve the advantageous characteristics of PVDF nanoweb fibers, we focused our attention on studying the effect of filling PVDF solution with calculated amount of calcium chloride (CaCl2) or multi-walled carbon nanotube (MWCNT), and their electrospun nanoweb fibers were analyzed for the changes in β-crystalline phase, and its associated piezoelectric characteristics using a custom-made sensor set-up developed in our lab. FT-IR spectroscopy was used to confirm the changes in the β-crystalline content with varying content of CaCl2 and MWCNT. SEM data revealed the reducing fiber diameter with increasing CaCl2 content. PVDF nanoweb subjected to pressure showed changes in touch sensing property as analyzed using an oscilloscope integrated with Labview program. Overall, the PVDF nanoweb containing the additives used in our study exhibited greater sensitivity-in-touch for use in smart apparel applications compared to unmodified PVDF nanoweb, and the results are reported in detail here.

Abstract: In this study, the dipole switching and non-volatile memory functionality of poly(vinylidene fluoride-trifluoroethylene) (PVDF/TrFE)(72/28 mol%) random copolymer ultrathin films were analyzed. PVDF/TrFE(72/28) used as ferroelectric insulator in varying memory device architectures such as metal-ferroelectric polymer-metal (MFM), MF-insulator-semiconductor (MFIS), MIS and ferroelectric field-effect transistors (FeFET) were examined using different electrical measurements. A maximum data writing speed of 1.69 MHz was calculated from the switching time measured using MFM architecture. Compared to MFM, MFIS device architecture was found to be more suitable for distinguishing the ‘0’ and ‘1’ state using the capacitance-voltage measurement. With FeFET, the measured drain current (Id) as well as its memory window increased with decreasing channel length, thereby enabling the easier identification of ‘0’ and ‘1’ state comparable to the MFIS case. The data obtained from this study will be useful in the fabrication of non-volatile random access memory (NVRAM) devices operating at lower voltage with faster data R/W/E speed and memory retention capability.

Abstract: In this paper, the surface crystalline morphology and piezoelectricity of P(VDF/TrFE)(72/28) copolymer ultrathin films were studied using different scanning probe microscopy (SPM) techniques. Atomic force microscopy (AFM) was used to study the changes in their crystalline morphology with varying temperature conditions. From electric force microscopy (EFM) studies, the change in dipole moment vectors along the applied field direction and the resultant change in amplitude image for '1 or 0' state were monitored and used to 'write/erase and read' the data on the memory bit. These results indicate the possibility of using SPM-based high density data storage with these copolymer ultrathin films.

Abstract: Flexible piezoelectric polymer materials for smart apparel and wearable computer applications are of great interest. Among known ferroelectric and piezoelectric polymers, polyvinylidene fluoride (PVDF) exhibit β-phase under poling and is known to give highest piezo-, pyro-, and ferroelectric properties. Previous reports suggests that, during corona poling of the PVDF film, a high surface electric potential is generated resulting in a high internal electric field within the polymer film causing the polarization of the dipoles along the direction of the applied electric field. The resultant phase change from α- to β-phase and the dipole switching generates displacement of charges or piezoelectricity. And also mechanical variation would change dipole density of PVDF film. In this report, we measured human heartbeat signal from an DAQ interfaced with a custommade voltage-amplifier with specific frequency filtering function using the corona-poled PVDF film of various sizes and thickness as a piezoelectric sensor and analyzed it. We employed elastic textile band to sensor system for comfortable fit on wrist or ankle. And then, we found the feasibility of applying flexible PVDF film sensor to smart apparel application which can sense heartbeat rate, blood pressure, respiration rate, accidental external impact on human body, etc.