Seung Won Lee

Effects of nanofluids containing graphene/graphene-oxide nanosheets on critical heat flux

The superb thermal conduction property of graphene establishes graphene as an excellent material for thermal management. In this paper, we selected graphene/graphene oxide nanosheets as the additives in nanofluids. The authors interestingly found that the highly enhanced critical heat flux (CHF) in the nanofluids containing graphene/graphene-oxide nanosheets (GON) cannot be explained by both the improved surface wettability and the capillarity of the nanoparticles deposition layer. Here we highlights that the GON nanofluid can be exploited to maximize the CHF the most efficiently by building up a characteristically ordered porous surface structure due to its own self-assembly characteristic resulting in a geometrically changed critical instability wavelength.

Optimal synthesis and characterization of Ag nanofluids by electrical explosion of wires in liquids

Silver nanoparticles were produced by electrical explosion of wires in liquids with no additive. In this study, we optimized the fabrication method and examined the effects of manufacturing process parameters. Morphology and size of the Ag nanoparticles were determined using transmission electron microscopy and field-emission scanning electron microscopy. Size and zeta potential were analyzed using dynamic light scattering. A response optimization technique showed that optimal conditions were achieved when capacitance was 30 μF, wire length was 38 mm, liquid volume was 500 mL, and the liquid type was deionized water. The average Ag nanoparticle size in water was 118.9 nm and the zeta potential was -42.5 mV. The critical heat flux of the 0.001-vol.% Ag nanofluid was higher than pure water.

CRITICAL HEAT FLUX ENHANCEMENT IN FLOW BOILING OF Al₂O₃ AND SiC NANOFLUIDS UNDER LOW PRESSURE AND LOW FLOW CONDITIONS

Critical heat flux (CHF) is the thermal limit of a phenomenon in which a phase change occurs during heating (such as bubbles forming on a metal surface used to heat water), which suddenly decreases the heat transfer efficiency, thus causing localized overheating of the heating surface. The enhancement of CHF can increase the safety margins and allow operation at higher heat fluxes; thus, it can increase the economy. A very interesting characteristic of nanofluids is their ability to significantly enhance the CHF. Nanofluids are nanotechnology-based colloidal dispersions engineered through the stable suspension of nanoparticles. All experiments were performed in round tubes with an inner diameter of 0.01041 m and a length of 0.5 m under low pressure and low flow (LPLF) conditions at a fixed inlet temperature using water, 0.01 vol.% Al2O3/water nanofluid, and SiC/water nanofluid. It was found that the CHF of the nanofluids was enhanced and the CHF of the SiC/water nanofluid was more enhanced than that of the Al2O3/water nanofluid.

High-Performance Strain-Compensated Multiple Quantum Well Planar Buried Heterostructure Laser Diodes with Low Leakage Current

The dependence of leakage current in a planar buried heterostructure laser diode (PBH-LD) on the operating temperature was analyzed by taking the effects of the connection width between a p-InP clad layer and a p-InP blocking layer into account. A two-step etching process comprising nonselective mesa etching followed by InP selective etching is proposed for obtaining a narrow connection width and high controllability of an active layer width. The performance was compared for LDs fabricated using the two-step etching process and those fabricated using conventional nonselective etching process. The average threshold current and the slope efficiency of the 1.3 µ m strain-compensated multiple quamtum well (MQW) PBH-LD fabricated using the two-step etching process were 5.6 mA and 0.27 mW/mA, respectively, for a cavity length of 400 µ m. However, using the nonselective etching process, the average threshold current was 14.5 mA and the slope efficiency was 0.22 mW/mA, given the same cavity length. A higher differential gain and characteristic temperature were also obtained due to the lower leakage currents and strain-compensated multiple quantum well active layers.

InAsP phase formations during the growth of a GalnAsP/lnP distributed feedback laser diode structure on corrugated lnP using metalorganic vapor phase epitaxy

An InAsP phase formed during the heatup time to the growth temperature of MOVPE was investigated by transmission electron microscopy and energy dispersive spectroscopy. The thickness of the InAsP phase on the concave regions of corrugation is increased with increased AsH3 partial pressure and heat‐up time. The arsenic composition in InAsP was also increased with the increase of AsH3 partial pressure during the heat‐up time. Dislocations and defects were not generated below an AsH3 partial pressure of 2.4×10−3 Torr, although strain was induced according to the thickness and composition of InAsP formed on the concave regions of corrugation.

Uniform and high coupling efficiency between InGaAsP-InP buried heterostructure optical amplifier and monolithically butt-coupled waveguide using reactive ion etching

We obtained uniform and high coupling efficiency for InGaAsP-InP buried heterostructure (BH) optical amplifiers integrated with butt-coupled waveguides using reactive ion etching (RIE) for mesa definition and low-pressure metalorganic vapor phase epitaxy (LPMOVPE) for waveguide layer regrowth. Measured average coupling efficiency was over 91% across a quarter of 2-in InP wafer. RIE etched vertical facet and a subsequent chemical etching using HBr-based solution for relief of RIE damage enabled us to reduce the coupling loss due to anomalous regrowth shape at the interface. RIE and selective regrowth processes are promising techniques for the fabrication of the photonic integrated circuit (PIC).

Effects of SiC and Graphene Oxide Nanoparticle–Coated Surfaces on Quenching Performance

Abstract Quenching experiments were conducted to investigate the effect of deposition of SiC and graphene oxide (GO) nanoparticles on heat transfer during rapid cooling in vertical tubes. Temperature histories during quenching were measured for each test section to confirm the effect of the nanoparticle-coated layer on quenching performance. Boiling curves for each test were obtained by using the inverse heat transfer method. Quenching performance was enhanced ∼20% to 31% for nanoparticle-coated tubes compared to the bare tube. Scanning electron microscope images of the inner surfaces of the tubes following the experiments were acquired, and the contact angles were measured to observe the effect of surface structures and wettability on quenching performance. In the case of tubes coated with GO nanoparticles for 900 s, quenching performance and critical heat flux (CHF) were enhanced although the contact angle increased. To confirm the surface effect on the enhanced quenching performance and CHF of GO nanoparticle–coated tubes, FC-72 refrigerant was used as the working fluid of the quenching experiment to reduce the wettability effect on the heat transfer.

Reflood Heat Transfer in SiC and Graphene Oxide Coated Tubes

Quenching experiments were conducted to investigate the effect of nanoparticle deposition on boiling heat transfer during rapid quenching in long vertical tubes. SiC and graphene oxide (GO) nanoparticles were deposited by boiling 0.01 vol% SiC/water and GO/water nanofluids in the vertical tube for 600 and 900 s to observe the repeatability of the nanoparticle deposition. Reflood tests were performed by passing water through bare tube and nanoparticle-coated tube at a constant flow rate (3 cm/s). Quenching curves (temperature vs. time) and saturated boiling curves were obtained at atmospheric pressure. We observed a more enhanced cooling performance in nanoparticle-coated tubes. The quenching time of tubes coated with SiC nanoparticles for 600 and 900 s were reduced by more than 20 and 25 s, respectively, compared to that of the bare tube. For the tubes boiled with GO nanoparticles for 600 and 900 s, the quenching times decreased by 10 and 12 s, respectively, compared to that of the bare tube. Scanning Electron Microscopy (SEM) images were acquired, and the contact angles were measured to observe the effects of surface structures and wettability on the cooling performance.Copyright

Performance Evaluation of a High Thermal Conductivity Fuel With Graphene Additives During the LBLOCA

The fuel rod performance of enhanced thermal conductivity UO2/graphene composites is investigated through a LBLOCA analysis. The benefits increased monotonically with increasing thermal conductivity in terms of reduced fuel center temperature and PCT. The performance of the UO2/graphene composite fuel is assessed in OPR-1000 (Optimized Power Reactor-1000) during a LBLOCA. Graphene can be a promising material for developing advanced nuclear fuel because of its property about the high thermal conductivity and low absorption cross section. The results confirm a LBLOCA performance related to PCT of the UO2/graphene composite fuel and its potential while maintaining large safety margins.Copyright

Growth of defect-free butt-coupled InGaAsP/InGaAsP strain compensated multiple quantum well by metal-organic vapor phase epitaxy

The electro-absorption modulator integrated distributed feedback (DFB) laser is an attractive device for a high bit rate and long haul optical communication system due to its narrow modulated spectral linewidth compared to a direct drive laser diode. In this report we present the successful growth of InGaAsP/InGaAsP strain compensated MQW butt-coupled to MQW laser by using a thick InP buffer layer growth.