HyukSang Kwon

Self-Powered Cardiac Pacemaker Enabled by Flexible Single Crystalline PMN-PT Piezoelectric Energy Harvester

A flexible single-crystalline PMN-PT piezoelectric energy harvester is demonstrated to achieve a self-powered artificial cardiac pacemaker. The energy-harvesting device generates a short-circuit current of 0.223 mA and an open-circuit voltage of 8.2 V, which are enough not only to meet the standard for charging commercial batteries but also for stimulating the heart without an external power source.

Photoelectrochemical analysis on the passive film formed on Fe–20Cr in pH 8.5 buffer solution

Abstract The structure and composition of a passive film formed on Cr in pH 8.5 buffer solution were explored through the photo-electrochemical and impedance analysis of the film. The passive film on Cr was confirmed to be a single layer or duplex layers depending on the film formation potential. At low film formation potentials such as −300 mV versus saturated calomel electrode (SCE), a single Cr(OH) 3 layer was formed on Cr. In contrast, the passive film formed on Cr at potentials noble to 0 V SCE was composed of duplex layers; inner CrOOH and outer Cr(OH) 3 layers. Specifically, photocurrent spectra for the passive film could be resolved into two spectral components responsible, respectively, for the inner and the outer layers. The outer Cr(OH) 3 layer, exhibiting an n-type semiconductor with a band gap energy ( E g ) of 2.75 eV at 300 mV SCE , changed to a p-type semiconductor by decreasing the applied potential to 0 V SCE or lower. In contrast, the inner CrOOH layer revealed an n-type semiconductor with E g of 3.0 eV, irrespective of the applied potential. However, the Mott–Schottky behavior for the passive film on Cr revealed a p-type semiconductor above −100 mV SCE . An electronic band structure model for the passive film on Cr was proposed to explain the photocurrent spectral responses and the Mott–Schottky behavior of the film.

Effects of aging at 475 °C on corrosion properties of tungsten-containing duplex stainless steels

Abstract Effects of aging at 475 °C on the corrosion and mechanical properties of Fe–25Cr–7Ni–0.25N–xMo–yW (x=0–3, y=0–6) duplex stainless steels were investigated by an anodic polarization test in HCl solution, a modified double-loop electrochemical potentiodynamic reactivation (DL-EPR) test, and an impact test. Corrosion resistance of the alloys was degraded with aging at 475 °C due to the depletion of Cr around α′ precipitates where numerous micropits were formed during the anodic polarization test. Especially for over-aged alloys, a second anodic current loop appeared in the passive region during the anodic polarization in 1 M HCl solution. The peak value of the second anodic current loop as well as the ratio of the maximum current in reactivation loop to that in anodic loop (ir/ia) determined from the modified DL-EPR test were found to be an effective measure of the precipitation of α′-phase during the aging. However, the degradation in corrosion resistance and impact toughness of the alloys during the aging was retarded with an increase in the W content of the designed DSS, suggesting that W in duplex stainless steels delays the precipitation rate of α′-phase due to a slower diffusion rate of W compared with that of Mo in ferrite. Influences of aging on the galvanic corrosion behaviors between austenite and ferrite phases were discussed by atomic force microscopy observation.

Effects of deposition condition on the ionic conductivity and structure of amorphous lithium phosphorus oxynitrate thin film

Thin film rechargeable lithium batteries, in forms of Li{vert_bar}Li{sup +} solid electrolyte{vert_bar}Li intercalation cathode, have been studied for their applications in electronic devices such as built-in power sources for RAM, CMOS chips and smart cards. Recently, it was reported that a new amorphous lithium electrolyte, lithium phosphorus oxynitride (LIPON), showed an ionic conductivity as high as 2.0 x 10{sup {minus}6} {Omega}{sup {minus}1} cm{sup {minus}1} at room temperature, and also excellent long term stability when in contact with lithium. In spite of the advantages of LIPON as a solid electrolyte for microbatteries, the influence of deposition parameters on the structure and ionic conductivity on the LIPON is not yet known. the objective of the present study is to examine the effect of deposition conditions on the ionic conductivity and structural change of the LIPON solid electrolyte films, and hence to elucidate the mechanism by which the incorporation of nitrogen into phosphate glass improves the Li{sup +} ionic conductivity of LIPON.

Electrochemical Insertion of Lithium into Multiwalled Carbon Nanotube/Silicon Composites Produced by Ballmilling

Multiwalled carbon nanotube (MWNT)/silicon composite with different weight ratios were produced from purified MWNTs and Si powder with a maximum particle size of 45 μm by high-energy ballmilling and then electrochemically inserted with lithium using Li/(MWNT/Si) cells. The charge/discharge properties for the ballmilled MWNT/Si composite anode were found to be very sensitive to the weight ratio of MWNT to Si. The highest C rev and lowest C irr of the composite anode were measured to be 1770 mAh/g and 469 mAh g -1 , respectively, at the ratio of MWNT 0.5 /Si 0.5 . During charging/discharging processes, most of the Li ions were inserted into the ballmilled MWNT/Si composites by alloying with Si particles at potentials positive to 0.25 V vs Li and extracted from the ballmilled MWNT/Si composites by dealloying with Si particles at potentials positive to 0.5 V vs Li. The good capacity and cycle performance of the MWNT 0.5 /Si 0.5 composite anode is due primarily to the fact that MWNTs encapsulating fine Si particles by strong contact prevent Si particles from electrical insulation caused by the crumbling of the Si particles as well as buffer the volume expansion caused by the formation Li-Si compound during Li insertion.

Role of Chloride Ion in Passivity Breakdown on Iron and Nickel

�on the electronic properties of the n-type passive film on Fe and the p-type passive film on Ni in pH 8.5 buffer solution were investigated using photocurrent spectroscopy and Mott–Schottky analysis in order to elucidate the mechanism of passivity breakdown on metals. Metastable pitting events were observed in the potentiostatic current transients chronoamperograms for Fe measured at 0.7 VSCE when 0.1 M of NaCl was added to the solution, demonstrating that Cl � induces passivity breakdown. However, the Mott–Schottky analysis revealed chloride ion, present at a concentration of 0.10 M, did not change the concentration of oxygen vacancies in the passive film on Fe, indicating that the oxygen vacancy is not responsible for the passivity breakdown. It was observed by Mott–Schottky analysis that as the concentration of Cl � in the solution increased, the concentration of cation vacancy in the passive film on Ni also increased, which is in concert with the view that metal vacancies are responsible for the breakdown of passivity on nickel. Finally, by using electrochemical impedance spectroscopy to interrogate the point defect generation and annihilation reactions that occur at the metal/film and film/solution interfaces, we show that the observed increase in cation vacancy concentration in the passive film on Ni is due to chloride-catalyzed ejection of cations from the film/solution interface. These findings are inconsistent with chloride-catalyzed film dissolution and chloride penetration mechanisms for passivity breakdown, but they are entirely consistent with cation vacancy generation at the barrier layer/solution interface and subsequent cation vacancy condensation at the metal/barrier layer interface, as postulated in the point defect model.

Template-Free Electrochemical Synthesis of Sn Nanofibers as High-Performance Anode Materials for Na-Ion Batteries

Sn nanofibers with a high aspect ratio are successfully synthesized using a simple electrodeposition process from an aqueous solution without the use of templates. The synthetic approach involves the rapid electrochemical deposition of Sn accompanied by the strong adsorption of Triton X-100, which can function as a growth modifier for the Sn crystallites. Triton X-100 is adsorbed on the {200} crystallographic planes of Sn in an elongated configuration and suppressed the preferential growth of Sn along the [100] direction. Consequently, the Sn electrodeposits are forced to grow anisotropically in a direction normal to the (112) or (1̅12) plane, forming one-dimensional nanofibers. As electrode materials for the Na-ion batteries, the Sn nanofibers exhibit a high reversible capacity and an excellent cycle performance; the charge capacity is maintained at 776.26 mAh g(-1) after 100 cycles, which corresponds to a retention of 95.09% of the initial charge capacity. The superior electrochemical performance of the Sn nanofibers is mainly attributed to the high mechanical stability of the nanofibers, which originate from highly anisotropic expansion during sodiation and the pore volumes existing between the nanofibers.

The influences of microstructure and nitrogen alloying on pitting corrosion of type 316L and 20 wt.% Mn-substituted type 316L stainless steels

Abstract The effects of nitrogen alloying on pitting corrosion of type 316LN and 20 wt.% Mn-substituted type 316LN stainless steels were investigated by potentiodynamic polarization tests in Cl − ion-bearing neutral and acidic solutions. Pitting resistance was markedly improved through the nitrogen alloying in both types of alloys, compared with the nitrogen-free alloys. It was confirmed that the added nitrogen was solid-solutioned in the austenitic phase without forming any nitrides under 20 min heat treatment at 1150°C. From the in situ observation on the initiation and growth of pits, pitting was found to occur consistently at the sites of inclusions. The pitting corrosion behaviors in both types of alloys were discussed with respect to the role of sulfides as initiators of pitting corrosion, and the changes of repassivation properties due to the nitrogen alloying in the alloys.

Self-powered deep brain stimulation via a flexible PIMNT energy harvester

Deep brain stimulation (DBS) is widely used for neural prosthetics and brain–computer interfacing. Thus far in vivo implantation of a battery has been a prerequisite to supply the necessary power. Although flexible energy harvesters have recently emerged as alternatives to batteries, they generate insufficient energy for operating brain stimulation. Herein, we report a high performance flexible piezoelectric energy harvester by enabling self-powered DBS in mice. This device adopts an indium modified crystalline Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIMNT) thin film on a plastic substrate to transform tiny mechanical motions to electricity. With slight bending, it generates an extremely high current reaching 0.57 mA, which satisfies the high threshold current for real-time DBS of the motor cortex and thereby could efficiently induce forearm movements in mice. The PIMNT based flexible energy harvester could open a new avenue for future in vivo healthcare technology using self-powered biomedical devices.

Structural enhancement of Na3V2(PO4)3/C composite cathode materials by pillar ion doping for high power and long cycle life sodium-ion batteries

Structurally stabilized Na3V2(PO4)3/C composite cathode materials with excellent electrochemical performance can be obtained by incorporating functional pillar ions into the structure. As pillar ions, K-ions have a larger ionic radius compared to Na-ions, and play an important role in enlarging the Na-ion diffusion pathway and in increasing the lattice volume by elongating the c-axis, thereby improving the rate performance. Furthermore, since the incorporated K-ions rarely participate in the electrochemical extraction/insertion reactions, they can stabilize the Na3V2(PO4)3 structure by suppressing significant lattice volume changes or structural distortion, even in a wide range of voltage windows accompanying multiple transitions of V ions and phase distortions. We investigated how the K-ion doping level affected the crystal structure and electrochemical properties of Na3V2(PO4)3 cathode materials for Na-ion batteries.