Sunjong Lee

Comparative study of the tribological behavior of thermal sprayed quasicrystalline coating layers

To investigate the role of tribological reactions on the friction and wear of quasicrystalline materials, coatings with two alloy compositions have been prepared by plasma and HVOF spraying techniques. The tribolayers were characterized by the formation of a transfer film on the counterface and densification of the coating subsurfaces. It was observed that the thickness of the transfer film and pore-free region were dependent on the composition and process used for the deposition of the coatings as well as the sliding velocity. As the sliding velocity increased, the growth rate of the transfer film decreased, resulting in a decrease of the coefficient of friction. On the other hand, the wear rate appeared to be controlled by the thickness of the pore-free region formed within the coating surface zone.

Tribological properties of Al–Ni–Co–Si quasicrystalline coatings against Cr-coated cast iron disc

Abstract The tribological behavior of Al–Ni–Co–Si quasicrystalline (QC) coatings prepared by air plasma spray (APS) and high velocity oxy-fuel (HVOF) techniques was investigated in dry sliding condition against a Cr-coated cast iron disc. Tests were performed for different parameters of the load, sliding velocity, and temperature under a reciprocal motion. Our results demonstrated the influence of the coating microstructure on the frictional and wear behavior. Values of the coefficient of friction measured for HVOF coatings against Cr-coated cast iron disc were found to be about 14% lower than those of APS coatings. Under the standard condition, HVOF coatings exhibited a better wear resistance than APS coatings, however, this difference was reduced for tests performed under the highest values of the load and temperature investigated in this study. The sliding conditions imposed in this study led to significant modifications of the contacting surfaces characterized by the formation of a transfer film on the counterpart material, densification of the coating subsurface, and generation of wear particles. The variations of the coefficient of friction with the testing parameters were explained by the third body concept. It was also shown that the wear performance was dependent on the local surface properties of the QC coatings.

Quasicrystal-forming ability of the icosahedral phase in Al–Cu–Fe–Be alloys

Abstract The effect of Be (beryllium) addition and the influence of cooling rate on the solidification path of the icosahedral (i) phase in conventional casting Al 62− x Be x Cu 25.5 Fe 12.5 ( x =0,1,3,5,7 at.%) alloys were investigated by means of X-ray diffraction, differential thermal analysis, SEM and EDX analyses. The addition of Be has been found to modify the i-phase formation mechanism from peritectic reaction to primary solidification. The volume fraction of the i-phase has been observed to increase from 45% for x =0 to 90% for x =7. These results suggested that the substitution of Al by Be is favorable for the increase of the quasicrystal-forming ability (QFA) of the i-phase. Two parameters have been proposed for defining and assessing the QFA for i-phase formed by peritectic reaction. These parameters can be expressed by the reduced quasicrystal transition temperature T rq = T p / T l and the reduced undercooling Δ T r =Δ T lp / T l , respectively, ( T l : liquidus temperature, T p : peritectic peak temperature, and Δ T lp = T l − T p : freezing range of primary phase).

Formability of ZK60A Magnesium Alloy Determined by Compression and Backward Extrusion

Magnesium alloys have been in the spotlight due to their high specific strength and stiffness. In order to manufacture forged products with magnesium alloys, it is crucial to determine the dependence of the formability on processing conditions. In this study, the formability of ZK60A manufactured by a continuous casting was experimentally investigated by uniaxial compression and backward extrusion. Processing parameters such as temperature and compressing speed varied in experiments. The surface quality and internal defects of the deformed specimens were carefully inspected for comparison purposes. A damage criterion is employed to quantify the formability under these two different testing routes. Furthermore, our results were compared to the damage data of the similar material reported in the literature.

A polydiacetylene-based colorimetric chemosensor for malondialdehyde detection: a food spoilage indicator

A colorimetric chemosensor adopting polydiacetylene derivatives was rationally designed to detect malondialdehyde (MDA) as a potential food spoilage indicator. A newly developed diacetylene monomer containing an active methylene group effectively reacted with the aldehyde units of the MDA molecule via Knoevenagel-type condensation and resulted in a CC bond, judging from the results of FT-IR spectroscopy. When the monomer was incorporated into polydiacetylene liposomes, they exhibited a strong change in color from blue to red in the presence of MDA molecules, even though their sensitivity was partly affected by the feed ratio of the applied diacetylene monomers during liposome preparation. In addition, it was verified that the devised MDA chemosensor selectively responded to the divalent MDA, rather than the monovalent butanal. The detection limit of the optimized MDA chemosensor was about 250 μM in solution, but when a film-type MDA chemosensor was fabricated through the accumulation of the liposomes on top of a nylon membrane and then applied, the detection limit improved dramatically, reaching down to 10 μM at room temperature, even by judging the change in color with the naked eye. To confirm that the change of the resulting Hue angle can reflect the detection sensitivity of the MDA chemosensor under different experimental conditions, CIELAB analysis was conducted with the film-type MDA sensor. The results indicated that the Hue angle changed according to the MDA concentration, and the resulting slope can be quantitatively correlated with the sensitivity of the colorimetric detection system.

On the phase transitions of the quasicrystalline phases in the Al-Cu-Fe-Co alloy

Abstract Microstructural change with Co content together with heat treatment was studied in rapidly solidified Al–Cu–Fe–Co alloys using X-ray diffractometry and transmission electron microscopy. With an increase in Co content in the as-cast Al 65 Cu 20 Fe 15− x Co x alloys, the relative amount of the icosahedral phase drastically decreased and that of the decagonal phase increased. In the as-melt spun alloys, effects of Co content are critically changed at around 5 at% Co. For the less Co containing alloys (below 5 at%), the icosahedral phase proved to be the major phase, while the 5 at% Co containing alloy showed the coexistence of the icosahedral and decagonal phases. The 8 at% containing alloy showed the monolithic decagonal phase in the as-melt spun state. Unlike as-cast alloys, all the as-melt spun alloys showed no trace of the λ-phase. However, after heat treatment of ribbons, λ-phase newly appeared in the Al 65 Cu 20 Fe 10 Co 3 and Al 65 Cu 20 Fe 10 Co 5 alloys. The λ-phase showed orientation relationships with surrounding icosahedral or decagonal phases.

Huge Enhancement of Luminescence from a Coaxial-Like Heterostructure of Poly(3-methylthiophene) and Au

Recently, the light-matter interaction at nanoscale has attracted great interest from physicists, chemists and material scientists, as it gives peculiar optical properties that couldn’t be observed at the bulk scale. The synthesis and characterization of organic-inorganic heterostructures forming quantum dots, nanowires or nanotubes provide opportunities to understand their photophysical mechanism and to apply optoelecronic devices. Herein, we report a huge enhanced luminescence in a coaxial-like heterostructured poly (3-methylthiophene) (P3MT) with Au. We electrochemically synthesized P3MT nanowires (NWs) on a nanoporous template, and sequentially deposited Au on the surface of P3MT NWs. The diameter of heterostructured P3MT/Au NWs was about 200 nm, where the cladding-shape Au were about 10 nm. The visible range absorbance, with two new absorption peaks of P3MT/Au NWs, was significantly increased compared with that of P3MT NWs. Accordingly, the photoluminescence (PL) of a P3MT/Au NW was enormously increased; up to 170 times compared to that of P3MT NWs. More interestingly, an unexpected enhancement of PL was observed from cross-junction point of P3MT/Au NWs. The abnormal PL properties of P3MT/Au NWs were attributed to the charge transfer and the surface plasmon resonance between the cladding-shape Au and the core-shape P3MT, which resulted in the enhanced quantum yield. This incites us to reconsider the light-matter interaction in polymer-metal hybrid structures applicable for high-performance optoelectronic devices.

Optical transition and amplification of organic phosphor coupling with graphene plasmon (Conference Presentation)

Light-matter interactions in two dimensional (2D) materials have given new momentum to nano optoelectronics since the observation of localized surface plasmons interacting with the excitons. Graphene, a typical metallic 2D crystal with high optical absorbance, can provide surface plasmon effects to proximate molecules as nanostructured metals do. The spontaneous emission rate can be enhanced by the coupling of plasmonic modes with the emission frequencies of organic molecules. However, most experimental and theoretical studies report graphene plasmonics in the terahertz to mid-infrared range. Here, we demonstrate the optical transition and significant amplification of singlet emission from phosphoric molecule on a graphene substrate, with simultaneous enhancement of triplet emission in the visible regime. The spectroscopic investigations ascribe these phenomena to the coupling of graphene plasmonic modes with molecular transient dipole. The modulation of emission channel and quantum efficiency is achieved by specifically controlling the organic molecular surface density on graphene. The single layer graphene is the most efficient substrate for plasmon coupling, however, remarkable strong PL intensity is achieved by forming multi-stacks of the organic molecule-graphene hybrid layer. This work suggests a novel route for the manipulation of organic molecular emissions using graphene plasmonics, and can be applied in developing photonic devices with high quantum efficiency.

Inhomogeneous Chemical Composition of Semi-Continuously Casted ZK60A Billet

Applying continuous casting to magnesium alloys is one of the recent issues due to relatively less defects inside the as-cast billet. In this study, homogeneity in the chemical composition of a commercial continuously-casted ZK60A billet was investigated. Density distribution depending on the distance from the center of the billet was measured, and the difference in density was analyzed based on the measured chemical composition.