Hanshik Chung

Dye-Sensitized Solar Cell Counter Electrodes Based on Carbon Nanotubes

Dye-sensitized solar cells (DSSCs) have received significant attention from the scientific community since their discovery in 1991. However, the high cost and scarcity of platinum has motivated researchers to seek other suitable materials for the counter electrode of DSSCs. Owing to their exceptional properties such as high conductivity, good electrochemical activity, and low cost, carbon nanotubes (CNTs) have been considered as promising alternatives to expensive platinum (Pt) in the counter electrode of DSSCs. Herein, we provide a Minireview of the CNTs use in the counter electrode of DSSCs. A brief overview of Pt-based counter electrodes is also discussed. Particular attention is given to the recent advances of counter electrodes with CNT-based composite structures.

Structural and magnetic study of a diluted magnetic semiconductor: Fe-doped CeO2 nanoparticles.

This paper reports the effect of Fe doping on the structure and room temperature ferromagnetism of CeO2 nanoparticles. X-ray diffraction and the selective area electron diffraction measurements performed on the Ce(1-x)Fe(x)O2 (0 < or = x < or = 0.07) nanoparticles showed a single-phase nature with a cubic structure, and none of the samples showed the presence of any secondary phase. The mean particle size, which was calculated using transmission electron microscopy, increased with the increase in the Fe content. The DC magnetization measurements that were performed at room temperature showed that all the samples exhibited ferromagnetism. The saturation magnetic moment increased with the increase in the Fe content.

Optimum design of vaporizer fin with Liquefied Natural Gas by numerical analysis

Generally, the temperature drop under 0°C on vaporizer surface creates frozen dews. This problem seems to increase as the time progress and humidity rises. In addition, the frozen dews create frost deposition. Consequently, heat transfer on vaporizer decreases because frost deposition causes adiabatic condition. Therefore, it is very important to solve this problem. This paper aims to study of the optimum design of used vaporizer at local LNG station. In this paper, experimental results were compared with numerical results. Geometries of numerical and experimental vaporizers were identical. Studied parameters of vaporizer are angle between two fins (Φ) and fin thickness (THF). Numerical analysis results were presented through the correlations between the ice layer thickness (THICE) on the vaporizer surface to the temperature distribution of inside vaporizer (TIN), fin thickness (THF), and angle between two fins (Φ). Numerical result shows good agreement with experimental outcome. Finally, the correlations for optimum design of vaporizer are proposed on this paper.

Ultra-fine grinding of inorganic powders by stirred ball mill: Effect of process parameters on the particle size distribution of ground products and grinding energy efficiency

A series of wet grinding experiments have been carried out using a stirred ball mill to systematically investigate the effect of processing conditions on the final particle size distribution and grinding kinetics. A sub-micron particle size down to 350 nm was achieved, while the grinding energy efficiency or enhanced grinding rate was shown to be improved with the addition of grinding aids and with the use of smaller grinding media. A simple exponential relation was then proposed to quantitatively describe the observed grinding behavior by introducing the production rate constant,K. It is suggested that by usingK as a guideline, ways of improving the grinding energy efficiency can be established, e.g. use of smaller grinding media, choosing larger particle size class materials, etc.

Experimental investigation of the mechanical grinding effect on graphene structure

Graphene has been proven to be a promising material for various applications due to its outstanding chemical, physical, optical as well as mechanical properties. To further improve these properties of graphene, here we apply a grinding method with various speeds (100–600 rpm) of a planetary ball mill under wet conditions in graphene based aqueous solution. Therefore, the improvements in dispersion and thermal characteristics of the graphene–water solution were investigated based on the morphological and structural changes. The best dispersibility and highest thermal conductivity of graphene–water solution were observed for a grinding speed of 500 rpm. As a result, the grinding speed of 500 rpm is found as the optimum condition of planetary ball milling in the case study. The reason for the grinding speed of 500 rpm revealing the best condition is attributed to the reduced ratio (ID/IG = 0.221) of the D band and the G band in Raman spectroscopy. We believe that structurally upgraded graphene in this study would greatly improve the performance of the graphene based devices.

Is metal nanofluid reliable as heat carrier

A pre- and post experimental analysis of copper-water and silver-water nanofluids are conducted to investigate minimal changes in quality of nanofluids before and after an effective heat transfer. A single loop oscillating heat pipe (OHP) having inner diameter of 2.4mm is charged with aforementioned nanofluids at 60% filling ratio for end to end heat transfer. Post experimental analysis of both nanofluids raises questions to the physical, chemical and thermal stability of such suspension for hazardless uses in the field of heat transfer. The color, deposition, dispersibility, propensity to be oxidized, disintegration, agglomeration and thermal conductivity of metal nanofluids are found to be strictly affected by heat transfer process and vice versa. Such degradation in quality of basic properties of metal nanofluids implies its challenges in practical application even for short-term heat transfer operations at oxidative environment as nano-sized metal particles are chemically more unstable than its bulk material. The use of the solid/liquid suspension containing metal nanoparticles in any heat exchanger as heat carrier might be detrimental to the whole system.

Study on the Thermal and Flow Characteristics on the Periodically Arranged Semi-Circular Ribs in a Rectangular Channel

The article investigates turbulent flow characteristics and heat transfer enhancement on periodically arranged semicircular ribs fitted in a rectangular channel. The channel was fabricated as the aspect ratio of AR 5, rib height to hydraulic diameter ratio was 0.07, and rib height to channel height ratio was set as e/H 0.117 for various rib pitch to rib height ratios between 8 and 14. In order to predict realistic phenomena of heat transfer and flow characteristics near the wall, shear stress transport k-ω turbulence and v2-f turbulence models were chosen. The results of numerical analysis show turbulent flow characteristics, heat transfer enhancement, and the effect of friction as observed experimentally. The results shows that turbulent kinetic energy (k) is closely related to diffusion of recirculation flow, and results from v2-f turbulence model simulation are in agreement with experimental values.

Convective Heat Transfer in Ventilated Space with Various Partitions

The laminar convective heat transfer in ventilated space with various horizontal partitions was studied numerically and experimentally. For the numerical study, the governing equations were solved by using a finite volume method for various numbers Re, Gr, Pr and partition numbers. The experimental study was conducted by using a holographic interferometer. The isotherms and velocity vectors have been presented for various parameters. As the number and length of partition ncreased, convective heat transfer decreased. Based on the numerical data, correlation equations were obtained for the mean Nusselt number in term of Gr/Re2. In the region ofGr/Re2<-1, the mean Nusselt number was small, but in the region ofGr/Re2>1, the mean Nusselt number was constant.

Study on natural convection in a rectangular enclosure with a heating source

The natural convective heat transfer in a rectangular enclosure with a heating source has been studied by experiment and numerical analysis. The governing equations were solved by a finite volume method, a SIMPLE algorithm was adopted to solve a pressure term. The parameters for the numerical study are positions and surface temperatures of a heating source i.e., Y/H=0.25, 0.5, 0.75 and 11°C≦ΔT≦59°C. The results of isotherms and velocity vectors have been represented, and the numerical results showed a good agreement with experimental values. Based on the numerical results, the mean Nusselt number of the rectangular enclosure wall could be expressed as a function of Grashof number.

Electronic structure and magnetic properties of the Ni0.2Cd0.3Fe(2.5-x)A1(x)O4 (0 < or = x < or = 0.4) ferrite nanoparticles.

The structural, magnetic, and electronic structural properties of Ni0.2Cd0.3Fe(2.5-x)Al(x)O4 ferrite nanoparticles were studied via X-ray diffraction (XRD), transmission electron microscopy (TEM), DC magnetization, and near-edge X-ray absorption fine-structure spectroscopy (NEXAFS) measurements. Nanoparticles of Ni0.2Cd0.3Fe(2.5x)Al(x)O4 (0 < or = x < or = 0.4) ferrite were synthesized using the sol-gel method. The XRD and TEM measurements showed that all the samples had a single-phase nature with a cubic structure, and had nanocrystalline behavior. From the XRD and TEM analysis, it was found that the particle size increases with Al doping. The DC magnetization measurements revealed that the blocking temperature increases with increased Al doping. It was observed that the magnetic moment decreases with Al doping, which may be due to the dilution of the sublattice by the doping of the Al ions. The NEXAFS measurements performed at room temperature indicated that Fe exists in a mixed-valence state.