Sang-Yul Lee

Synthesis and characteristics of Ag/Pt bimetallic nanocomposites by arc-discharge solution plasma processing

Arc discharge in solution, generated by applying a high voltage of unipolar pulsed dc to electrodes of Ag and Pt, was used as a method to form Ag/Pt bimetallic nanocomposites via electrode erosion by the effects of the electric arc at the cathode (Ag rod) and the sputtering at the anode (Pt rod). Ag/Pt bimetallic nanocomposites were formed as colloidal particles dispersed in solution via the reduction of hydrogen radicals generated during discharge without the addition of chemical precursor or reducing agent. At a discharge time of 30 s, the fine bimetallic nanoparticles with a mean particle size of approximately 5 nm were observed by transmission electron microscopy (TEM). With increasing discharge time, the bimetallic nanoparticle size tended to increase by forming an agglomeration. The presence of the relatively small amount of Pt dispersed in the Ag matrix could be observed by the analytical mapping mode of energy-dispersive x-ray spectroscopy and high-resolution TEM. This demonstrated that the synthesized particle was in the form of a nanocomposite. No contamination of other chemical substances was detected by x-ray photoelectron spectroscopy. Hence, solution plasma could be a clean and simple process to effectively synthesize Ag/Pt bimetallic nanocomposites and it is expected to be widely applicable in the preparation of several types of nanoparticle.

One-step synthesis of cellulose/silver nanobiocomposites using a solution plasma process and characterization of their broad spectrum antimicrobial efficacy

Solution plasma process (SPP) is a one-step synthesis technique which expeditiously produces ultra-pure, stable, and uniform nanoparticles in polymer solutions with plasma discharge. Silver nanoparticles (AgNPs) were synthesized in a cellulose matrix as biocomposites by discharging plasma for 180 s at 800 V with a frequency of 30 kHz using a pulsed unipolar power supply into solutions containing cellulose (1–3%) and AgNO3 (1–5 mM). 3D scaffolds of the resulting cellulose/AgNP biocomposites were prepared by lyophilization and cross-linked with UV irradiation. UV-Vis spectroscopy showed a characteristic absorbance maximum in the range of 350–440 nm for the AgNP biocomposites with increase in the intensity of the peaks as the concentration of AgNO3 increased. The peaks exhibited a red shift transition due to the AgNP formation. The nanobiocomposites were pure when examined by FTIR spectroscopy. The 3D scaffolds had a micro-porous structure with pores of (68–74) ± 2 μm in diameter when observed using a FE-SEM instrument equipped with an EDS function. TEM analysis showed that spherical AgNPs in the size range of 5–30 nm were well distributed in the biocomposites of C3Ag3 and C3Ag5. The nanobiocomposites had a broad spectrum of antimicrobial activity against various pathogens with a minimal inhibition concentration of 5.1–20.4 μg ml−1 for bacteria and 81.6–255.0 μg ml−1 for fungi. They killed gram negative bacteria most effectively, but did not affect fungal growth very well, implying their potential as topical antimicrobial agents for the topical treatment of wounds. SPP seems to be the most effective and safest method to synthesize various biocompatible polymer–metal nanoparticle biocomposites.

Influence of target power density and substrate bias voltage on the electrochemical properties of type 304 SS films prepared by unbalanced magnetron sputtering

Abstract Type 304 SS coatings were deposited at 200 °C onto AISI 1045 carbon steel substrate using unbalanced magnetron sputtering (UBMS) with an austenitic AISI 304 stainless steel (SS) target of 100 mm diameter. The deposition was performed at a total pressure in the active Ar gas of 2.7×10 −3 mbar and at various target power densities and bias voltages. The structure and the morphology were investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Corrosion properties of the coated specimens were examined using electrochemical polarization measurements and electrochemical impedance spectroscopy in a deaerated 3.5% NaCl solution. The porosity was obtained from a comparison of the d.c. polarization resistance of the uncoated and coated substrates. Scratch adhesion testing was used to compare the critical loads for different coatings. The sputtered films exhibit a ferritic b.c.c. α-phase. The corrosion performance of coatings could be controlled by coating porosity, penetration of the electrolyte, and coating adhesion as a function of depositional parameters. The corrosion current density decreased with the increase of the power density. The protective action of the coating should be related to the values of the capacitance and charge transfer resistance of the coating. The results of the adhesion test correlated well with the electrochemical data. Finally, an optimized deposition condition for corrosion protection was found at 40 W/cm 2 and −50 V.

Studies on the application of laser surface texturing to improve the tribological performance of AlCrSiN-Coated surfaces

Recently, laser texturing has received lots of attention for improving the tribological performance of various surfaces, and this laser texturing of surfaces could be applied to improve the tribological performance of PVD coated surfaces. In this work two different patterns of laser surface texturing were performed on H13 substrate, and PVD AlCrSiN coatings with various N2 contents were made by unbalanced magnetron sputtering on the textured H13 specimens. The coating characteristics such as crystalline structure, surface morphology, hardness, and friction coefficient of the coatings as a function of the N2 partial pressure were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), microhardness tester, and wear test. Synthesis of the AlCrSiN thin films containing various N2 contents was successful using the unbalanced magnetron sputtering process. A dense and compact microstructure, as well as a very smooth surface, were observed from the synthesized thin films, which could be attributed to the addition of Si into the AlCrN films, producing an amorphous phase in the films. Also the hardness of the AlCrSiN films increased with increasing N2 partial pressures and the maximum hardness of approximately 33 GPa was measured from the AlCrSiN film synthesized with N2 partial pressures of 0.16 Pa. From the wear tests against an Al2O3 counterpart ball at room temperature without lubrication, the average friction coefficient of the non-textured AlCrSiN films of 0.42 was decreased to 0.38 and 0.36 for dimple-textured and honeycomb-textured AlCrSiN films, respectively. Laser surface texturing on the AlCrSiN films reduced the friction coefficients by approximately more than 15%, and further improvement on the tribological performance of the textured surfaces could be possible by optimizing the honeycomb-textured surfaces.

Size-tunable palladium nanoparticles synthesized using the solution plasma process and their electrocatalytic activities

In this work, for fuel cell applications, size-tunable Pd nanoparticles were synthesized by adjusting the applied voltage in a range from 2.4 to 3.2 kV in solution plasma. The crystalline structure and morphology of the size-tuned Pd nanoparticles were examined using X-ray diffractometer (XRD) and high-resolution transmission electron microscopy (HR-TEM). The results revealed that the size of the Pd nanoparticles could be varied by tuning the Pd concentration in the plasma, which could be achieved by changing the applied voltages. The size of the Pd nanoparticles gradually decreased with increasing voltage, and the smallest Pd nanoparticles of 4.7 nm in diameter were observed at the highest voltage of 3.2 kV. According to the cyclic voltammetry and chronoamperometry tests, the Pd nanoparticles of 5.7 nm in diameter exhibited the enhanced catalytic activity and durability for methanol oxidation since the Pd surface was less covered by OH species.

Seed step-coverage enhancement process for a high-aspect-ratio through-silicon via using a pyrophosphate solution

A seed step-coverage enhancement process (SSEP) was investigated in order to improve the poor stepcoverage of the Cu seed layer when it is deposited by physical vapor deposition with a high-aspect-ratio through-silicon via (TSV) to inhibit void generation during a subsequent electroplating TSV gap-fill process. The SSEP was performed by means of electroplating after the deposition of the Cu seed layer in TSV using an alkaline pyrophosphate solution, which is effective when used to prevent the dissolution of the Cu seed layer. The molar ratio of pyrophosphate ions to Cu ions in the plating solution and the current density were optimized for the SSEP by considering the surface uniformity of the electroplated Cu film through electroplating on the defective seed layer. In order to improve the wettability of the pyrophosphate solution in the TSV pattern, polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether was added to the solution. Finally, we significantly strengthened the seed step-coverage at the bottom-corner of the TSV, the weakest point in terms of seed step-coverage, and achieved a void-free filling of the TSV in the subsequent TSV gap-fill process.

Effects of interlayer thickness and the substrate material on the adhesion properties of CrZrN coatings

To confirm the influence of the interlayer thickness and substrate material on adhesion properties, CrZrN coatings with various Cr interlayer thickness were deposited on AISI H13, high speed steel, and tungsten carbide using unbalanced magnetron sputtering. The adhesion strength showed maximum value at 300 nm of the interlayer, but as the interlayer increased further to 450 nm, the adhesion strength decreased. The adhesion properties of the coatings were dependent upon not only interlayer thickness but also the substrate materials. The adhesion strength of the coating were measured 12, 32, 53 N on the tungsten carbide, AISI H13 steel, high speed steel, respectively and three different failure modes such as buckling spallation, wedging spallation, and chipping were observed on each substrate. The difference in adhesion properties could be attributed to the difference in value of elastic strain to failure (H/E) among the CrZrN coating, the interlayer, and the substrates material.

An evidence of C16 fatty acid methyl esters extracted from microalga for effective antimicrobial and antioxidant property

Fatty acid methyl esters (FAME) derived from lipids of microalgae is known to have wide bio-functional materials including antimicrobials. FAME is an ideal super-curator and superior anti-pathogenic. The present study evaluated the efficiency of FAME extracted from microalgae Scenedesmus intermedius as an antimicrobial agent against Gram positive (Staphylococcus aureus, Streptococcus mutans, and Bacillus cereus) Gram negative (Escherichia coli and Pseudomonas aeruginosa) bacteria and Fungi (Aspergillus parasiticus and Candida albicans). The minimal inhibitory concentration (MIC) for the gram negative bacteria was determined as 12-24 μg mL-1, whereas MIC for gram positive bacteria was 24-48 μg mL-1. MIC for the fungi was as high as 60-192 μg mL-1. The FAME profiles determined by gas chromatography showed 18 methyl esters. Among them, pharmacologically active FAME such as palmitic acid methyl ester (C16:0) was detected at high percentage (23.08%), which accounted for the bioactivity. FAME obtained in this study exhibited a strong antimicrobial activity at the lowest MIC than those of recent reports. This result clearly indicated that FAME of S. intermedius has a strong antimicrobial and antioxidant property and that could be used as an effective resource against microbial diseases.

Effects of Annealing Heat Treatment on the Corrosion Resistance of Zn/Mg/Zn Multilayer Coatings

Zn coatings alloyed with magnesium offer superior corrosion resistance compared to pure Zn or other Zn-based alloy coatings. In this study, Zn/Mg/Zn multilayer coatings with various Mg layer thicknesses were synthesized using an unbalanced magnetron sputtering process and were annealed to form Zn-Mg intermetallic phases. The effects of the annealing heat treatment on the corrosion resistance of the Zn/Mg/Zn multilayer coatings were evaluated using electrochemical measurements. The extensive diffusion of magnesium species into the upper and lower zinc layer from the magnesium layer in the middle of the coating was observed after the heat treatment. This phenomenon caused (a) the porous microstructure to transition into a dense structure and (b) the formation of a MgZn2 intermetallic phase. The results of the electrochemical measurements demonstrated that the heat treated Zn/Mg/Zn multilayer coatings possessed higher levels of corrosion resistance than the non-heat treated coatings. A Zn/Mg/Zn multilayer coating with MgZn2 and (Zn) phases showed the best corrosion resistance among the heat treated coatings, which could be attributed to the reduced galvanic corrosion effects due to a small potential gradient between the MgZn2 and zinc.

The Study on the Corrosion Property of the Zn-Mg Alloy Coatings with Various Mg Contents using EIS Measurement

In this study, the Zn-Mg alloy coatings with various Mg contents were deposited using an unbalanced magnetron sputtering process. Their surface microstructure, chemical composition, phase, and corrosion property were investigated. The microstructure of the Zn-Mg coatings changed from porous microstructure to dense one with increasing Mg contents in the coatings. As Mg contents in coatings increased, intermetallic phases such as Mg2Zn11 and MgZn2 were detected from X-ray diffraction (XRD) results. The corrosion resistance of the Zn-Mg alloy coatings was investigated quantitatively using electrochemical impedance spectroscopy (EIS) measurement with 3.5% NaCl solution. The results of EIS measurement showed that the charge transfer resistance and the phase angle of the Zn-Mg alloy coatings were increased from 162.1 Ω·cm 2 to 558.8 Ω·cm2 and from about 40o to 60o with increasing Mg contents from 5.1 wt.% to 15.5 wt.% in the coatings. These results demonstrate that the Zn-Mg coatings with increasing Mg contents showed an enhanced corrosion resistance.