Jaewoo Kim

Effect of particle size, dispersion, and particle–matrix adhesion on W reinforced polymer composites

W dispersed mixed polymers of ethylene propylene monomer and high density polyethylene were prepared by means of a twin-screw extruder by the conventional technique using a co-rotated two-roll mill. The W nanoparticles used as filler were prepared by pulse wire evaporation then coated with low-density polyethylene (LDPE) as polymeric surfactant. Surface treatment of the nanoparticles with LDPE was conducted to enhance the wettability and lubrication of the fillers in the polymer matrix. According to SEM images and mechanical properties, dispersion of W/LDPE nanoparticles in the polymer matrix was homogeneous, and adhesion of the nanoparticles to the matrix was strong. The polymer nanocomposites had better mechanical properties than those containing dispersed micro-W powder. The γ-ray attenuation factor of nanofiller-reinforced composites was substantially enhanced compared with that containing micro filler.

The Fabrication of PVA Polymer Coated on the Surface of B 4 C Nanocomposite by High Energy Ball Mill

Mechanical coating process was applied to form 89 %-hydrolyzed poly vinyl alcohol (PVA) onto boron carbide () nanopowder using one step high energy ball mill method. The polymer layer coated on the surface of B4C was changed to glass-like phase. The average particle size of core/shell structured /PVA was about 50 nm. The core/shell structured /PVA was formed by dry milling. However, the hydrolyzed PVA of with high glass transition temperature () was rarely coated on the powder. The of polymer materials was one of keys for guest polymer coating on to the host powder by solvent free milling.

Isotope separation using condensation repression of the laser excited gaseous CHCl3 molecules colliding with a cold wall

Abstract Cold surface condensation characteristics of vibrationally excited gaseous chloroform (CHCl3) molecules have been investigated. Continuous-wave CO2 laser emission lines between 934.9 and 929.0 cm-1 were used for excitation of the carbon atom dependent binary vibration of the gaseous chloroform molecules mixed with He or N2 carrier gases. Gas mixtures were subsonically flowed through a coaxial cylindrical irradiation chamber (IC). The cold IC surface escaped fractions (cuts) of the vibrationally excited chloroform were increased more than 15% when natural chloroform molecules, whose 12C isotopic abundance is 98.9%, were used with a N2 carrier gas. With a He carrier gas, however, changes in the cut were not observed. Separations of isotopic chloroform by selective vibrational excitation were also observed with the enrichment factors between 1.01 and 1.15 under certain IC temperature conditions.

FABRICATIONS OF PVA COATED NANO-B4C REINFORCED HDPE COMPOSITES

Nano-B4C powder whose surface was coated with polyvinylalcohol (PVA) was prepared by using a ball milling mechanical activation process. As-prepared powders exhibit the structure of PVA layer on the surface of nano-sized B4C and the size of the produced core/shell structured nano-B4C/PVA particles was in the range of 50 to 200 nm. The sheets of the HDPE composite reinforced by the nano- and micro-B4C fillers were fabricated by hot pressing following the melt mixing process respectively. The structures of prepared nano-B4C/PVA powders and the degree of particles dispersion in HDPE were observed by means of X-ray diffraction as well as the SEM/TEM images. It was found that the dispersion of the core/shell structured nano-B4C/PVA fillers in HDPE was more homogeneous than the surface-untreated micro-B4C fillers in HDPE.

PARTICLE SIZE-DEPENDENT PULVERIZATION OF B4C AND GENERATION OF B4C/STS NANOPARTICLES USED FOR NEUTRON ABSORBING COMPOSITES

Pulverization of two different sized micro-B4C particles (∼10 μm and ∼150 μm) was investigated using a STS based high energy ball milling system. Shapes, generation of the impurities, and reduction of the particle size dependent on milling time and initial particle size were investigated using various analytic tools including SEM-EDX, XRD, and ICP-MS. Most of impurity was produced during the early stage of milling, and impurity content became independent on the milling time after the saturation. The degree of particle size reduction was also dependent on the initial B4C size. It was found that the STS nanoparticles produced from milling is strongly bounded with the B4C particles forming the B4C/STS composite particles that can be used as a neutron absorbing nanocomposite. Based on the morphological evolution of the milled particles, a schematic pulverization model for the B4C particles was constructed.

Effect of Fe Magnetic Nanoparticles in Rubber Matrix

A new kind of magnetic rubber, Fe dispersed ethylene propylene monomer (EPM), was prepared by a conventional technique using a two roll mill. The magnetic fillers of Fe-nanoparicles were coated by low density polyethylene (LDPE). The purpose of surface treatment of nanoparticles by LDPE is to enhance wettability and lubricancy of the fillers in a polymer matrix. The mechanical strength and microstructure of the magnetic rubber were characterized by tensile strength test and scanning electron microscopy (SEM). Results revealed that the Fe nanoparticles were relatively well dispersed in an EPM matrix. It was found that the nano- Fe dispersed magnetic rubber showed higher coercivity and tensile strength than those of micron- Fe dispersed one.

High Purity and Yield of Boron Nitride Nanotubes Using Amorphous Boron and a Nozzle-Type Reactor

Enhancement of the production yield of boron nitride nanotubes (BNNTs) with high purity was achieved using an amorphous boron-based precursor and a nozzle-type reactor. Use of a mixture of amorphous boron and Fe decreases the milling time for the preparation of the precursor for BNNTs synthesis, as well as the Fe impurity contained in the B/Fe interdiffused precursor nanoparticles by using a simple purification process. We also explored a nozzle-type reactor that increased the production yield of BNNTs compared to a conventional flow-through reactor. By using a nozzle-type reactor with amorphous boron-based precursor, the weight of the BNNTs sample after annealing was increased as much as 2.5-times with much less impurities compared to the case for the flow-through reactor with the crystalline boron-based precursor. Under the same experimental conditions, the yield and quantity of BNNTs were estimated as much as ~70% and ~1.15 g/batch for the former, while they are ~54% and 0.78 g/batch for the latter.

A Report on the ESR and Magnetic Structure of Bi x Ca 1-x MnO₃ (x = 0.15, 0.22)

The electron spin resonance signals of Bi x Ca 1-x MnO₃ have been acquired for two samples of x = 0.15 and 0.22. ESR signal of the sample of x = 0.15 clearly shows signal shape change into Dysonian with g-value shift at around 165 K so that the charge ordering temperature can be identified. The general features of ESR signal of the two samples well correlate with magnetic susceptibility measurement and also confirm the validity of former investigations.