Yujin Hwang

Color variation of ZnGa2O4 phosphor by reduction-oxidation processes

The color of the emission of zinc gallate (ZnGa2O4) oscillates between ultraviolet and blue by hydrogen ambient reduction and air ambient oxidation heat treatments. The photoluminescence spectra and electron paramagnetic resonance signals show that ultraviolet emission of reduced ZnGa2O4 always accompanies 680 nm emission originating from single oxygen vacancies (VO*). The increasing difference in binding energy between Ga3+ and O2− in reduced ZnGa2O4 indicates that the configuration of octahedral sites is distorted due to VO* generation and it becomes more ionic which shifts the emission band from 430 to 360 nm. The x-ray diffraction patterns and Raman scattering spectra show that β-Ga2O3 from ZnGa2O4 is formed in both reduction and oxidation processes which suggests the vaporization of Zn ions. We propose a model in which the origin of 360 nm emission is the Ga–O transition at distorted octahedral sites with VO* in ZnGa2O4, whereas 430 nm emission originates from the Ga–O transition of regular octahedra...

Resistance to High-Fat Diet–Induced Obesity but Exacerbated Insulin Resistance in Mice Overexpressing Preadipocyte Factor-1 (Pref-1): A New Model of Partial Lipodystrophy

OBJECTIVE—White adipose tissue is a critical regulator of whole-body glucose metabolism. Preadipocyte factor-1 (Pref-1) is a secreted protein that inhibits adipocyte differentiation, both in vitro and in vivo. In this study, we have investigated the effects of Pref-1 overexpression on whole-body glucose homeostasis and its contribution to the development of insulin resistance. RESEARCH DESIGN AND METHODS—To gain insight into the role of Pref-1 on the onset of insulin resistance and type 2 diabetes, we measured body composition and whole-body insulin-stimulated glucose metabolism during a hyperinsulinemic-euglycemic clamp in Pref-1 transgenic and wild-type control mice fed a high-fat diet. RESULTS—Mice overexpressing Pref-1 were resistant to high-fat diet–induced obesity, as reflected by a marked reduction in adipose tissue mass. However, Pref-1–overexpressing mice were severely insulin resistant, mainly because of a reduction in insulin-stimulated glucose uptake in skeletal muscle and adipose tissue. The aggravated insulin resistance was associated with impaired insulin signaling and increased diacylglycerol content in skeletal muscle. CONCLUSIONS—Mice overexpressing Pref-1 are insulin resistant despite being protected from diet-induced obesity and may provide a new rodent model for the study of lipodystrophic disorders.

Resistance to high fat diet-induced obesity but exacerbated insulin resistance in mice overexpressing Pref-1: a new model of partial lipodystrophy

OBJECTIVE—White adipose tissue is a critical regulator of whole-body glucose metabolism. Preadipocyte factor-1 (Pref-1) is a secreted protein that inhibits adipocyte differentiation, both in vitro and in vivo. In this study, we have investigated the effects of Pref-1 overexpression on whole-body glucose homeostasis and its contribution to the development of insulin resistance. RESEARCH DESIGN AND METHODS—To gain insight into the role of Pref-1 on the onset of insulin resistance and type 2 diabetes, we measured body composition and whole-body insulin-stimulated glucose metabolism during a hyperinsulinemic-euglycemic clamp in Pref-1 transgenic and wild-type control mice fed a high-fat diet. RESULTS—Mice overexpressing Pref-1 were resistant to high-fat diet–induced obesity, as reflected by a marked reduction in adipose tissue mass. However, Pref-1–overexpressing mice were severely insulin resistant, mainly because of a reduction in insulin-stimulated glucose uptake in skeletal muscle and adipose tissue. The aggravated insulin resistance was associated with impaired insulin signaling and increased diacylglycerol content in skeletal muscle. CONCLUSIONS—Mice overexpressing Pref-1 are insulin resistant despite being protected from diet-induced obesity and may provide a new rodent model for the study of lipodystrophic disorders.

Investigation of pore formation for polystyrene electrospun fiber: Effect of relative humidity

Electrospinning method uses electrical force to produce a polymer nanofiber from a polymer solution. The surface morphology and the pore formation of e-spun fiber have been studied by many variables that are involved in different polymer concentrations and solvent mixing ratios. Another major factor affecting fiber morphology and size distribution is the relative humidity. The interaction between the relative humidity and the solvent evaporation affects the distribution of electric charge on the surface of the e-spun fiber. The higher the electric density, the thinner the fiber that can be produced in low humidity conditions. The relative humidity and solvent evaporation can create pores on the fiber surface. The pores can be formed under the condition of 30% relative humidity using 100% of THF solvent. The boundary of the pores has expanded and becomes formless due to the agglomeration of each pore, which can decrease the evaporating capacity.

The morphology of electrospun polystyrene fibers

Electrospinning is a process of electrostatic fiber formation which uses electrical forces to produce polymer nanofibers from polymer solution. The electrospinning system consists of a syringe feeder system, a collector system, and a high power supplier. The important parameters in the morphology of electrospun polystyrene fibers are concentration, applied voltage, and solvent properties. Higher concentrations of the polymer solution form thicker fibers and fewer beads. When the concentration is 7 wt%, electrospun fibers have an average diameter of 340 nm, but as the concentration of PS increases to 17 wt%, the fiber diameter gradually thickens to 3,610 nm. The fiber morphology under different solvent mixture ratios and solvent mixtures has also been studied.

An Experimental Study of the Air-side Particulate Fouling in Fin-and-Tube Heat Exchangers of Air Conditioners

An experimental study was conducted to investigate the chronological performance variation such as pressure drop across a heat exchanger and cooling capacity due to the air-side particulate fouling of fin-and-tube heat exchangers for air conditioner evaporators used. Thirty samples of air conditioners used in the field such as inns, restaurants, and offices are collected in chronological order of use. This study was intended to provide factual long-term fouling data under actual operating conditions. It was found that the important parameters to influence the fouling of heat exchangers are the concentration and size of indoor pollutants, the filter efficiency, the hydrophilicity of fin surfaces, fin spacing, and the structure of fins. The pressure drop of heat exchangers increases from year to year due to the deposition of indoor pollutants larger than 1 Μm in size and increases up to 44% in the samples used for 7 years. Also, the air-side particulate fouling degrades the cooling capacity by 10-15% in the samples used for 7 years.

Mn2+ site behaviors in CdxZn1−xGa2O4 and SrxBa1−xAl12O19 green phosphors

Abstract Manganese ion (Mn 2+ ) sites in phosphors, ZnGa 2 O 4 :Mn 2+ and SrAl 12 O 19 :Mn 2+ , were studied by utilizing the host lattices of solid solutions which are the spinel structure of the Cd x Zn 1− x Ga 2 O 4 :Mn 2+ and the magnetoplumbite of the Sr x Ba 1− x Al 12 O 19 :Mn 2+ , along with utilizing a crystal field as a probing tool. The green emitting centers of Mn 2+ in ZnGa 2 O 4 and SrAl 12 O 19 are assigned unequivocally to be tetrahedrally coordinated ones.

Design and Performance Evaluation of Plasma Air Cleaning Systems for Removing Yellow Sand Dust

Yellow sand dust (Asian dust storms) causes harmful damage indoors and outdoors during the springtime, and the removal of Yellow sand dust has become an issue for suitable indoor conditions. An air cleaner is required to remove Yellow sand dust efficiently to improve indoor air quality, and the removal characteristics of Yellow sand dust should be studied. The size distribution and mass concentration of Yellow sand dust observed in China and Korea are analyzed, and the removal efficiency of a plasma air cleaning system based on the principle of electrostatic precipitation is evaluated by using Yellow sand dust. Mass median diameter of Yellow sand dust sampled in Beijing and Seoul ranges from 7.0 to 8.0 μm with a mass concentration of 300-1,462 μg/m3. For a single-pass test, the efficiency of dust removal increases with increasing particle size and decreasing flow rate. The removal efficiency of Yellow sand dust in a plasma air cleaning system at a face velocity of 1.0 m/s is higher than 80%. For a multi-pass test in occupied spaces, the operation time required to reduce Yellow sand dust concentration from an initial concentration of 300 μg/m3 to 150 μg/m3 is 10 minutes for a test room of 27 m3.

Enhancement of the phase stability in ZnGa2O3.994Se0.006: Li+, Mn2+ phosphors

Abstract The degradation behavior of ZnGa 2 O 3.994 Se 0.006 : Li, Mn caused by thermal annealing and acceleration voltage was investigated. Thermal degradation of ZnGa 2 O 3.994 Se 0.006 : Li, Mn initiated from the volatilization of Se, whereas, commercial green phosphor, ZnS: Cu, Al, transformed from ZnS to ZnO. The cathodoluminescence measurements showed that the saturation characteristics of the luminance of ZnGa 2 O 3.994 Se 0.006 : Li, Mn at higher voltage were a little better than that of commercial green phosphor (ZnS: Cu, Al). These results indicate that the phase stability of ZnGa 2 O 3.994 Se 0.006 : Li, Mn in both thermal and acceleration voltage dependent degradations is superior to that of ZnS: Cu, Al.

Characteristic Analyses of Plasma Air Cleaning Systems for the Removal of Indoor Air Pollutants

Tobacco smoke is one of the most common manmade aerosols. Yellow sand dust and pollen are the particular and regional pollutants generated by natural phenomena. These pollutants have different removal characteristics, respectively, when the air cleaning system is operated. It is well known that tobacco smoke particles are removed effectively with electrostatic precipitators. But it is necessary to evaluate whether the plasma air cleaning system has good performance of removing Yellow sand dusts and pollens simultaneously and also to establish the operation modes for efficient removal of those particular air pollutants by controlling the air flow rates passing the electrostatic precipitator and operating times of air cleaning system. In this study, the performance evaluation of plasma air cleaning systems is investigated with tobacco smoke particles, Yellow sand dusts, and pollens. For the multi-pass test in occupied spaces of 150 m3, the operation time required to reduce dust concentration from the initial concentration of 300 µg/m3 to 150 µg/m3, the criteria of indoor air quality in Korea, are 40 min for tobacco smoke, 28 min for Yellow sand dust, and 5 min for pollen when the flow rate is 17 m3/min. Also, the optimal operation modes for each pollutant are suggested for the efficient removal of indoor air pollutants. At first, most particles are removed by maximum flow operation. Second, the rest of the particles are removed by medium flow operation. Next, the plasma air cleaning systems are maintained by minimum flow for tobacco smoke mode and by repeating minimum flow and medium flow for Yellow sand and pollen modes. Edit to “Because the Yellow sand dust and the pollen flow into the room continuously and settle down … noise reduction.” Because the Yellow sand dust and the pollen flow into the room continuously and settle down. The plasma air cleaning system is suitable for the removal of the tobacco smoke, the Yellow sand dust, and the pollen for maintaining suitable indoor air quality, and, if it is operated through the suitable operation mode, energy efficiency will improve noise reduction.