Junkyung Kim

Preparation and characterization of epoxy composites filled with functionalized nanosilica particles obtained via sol–gel process

Abstract To investigate the interfacial effect on properties of epoxy composites, uniform sized silica particles (S) were synthesized by sol–gel reaction and then modified either by substituting surface silanol groups into epoxide ring (S–epoxide), amine (S–NH 2 ) or isocyanate (S–NCO) groups or by calcinating them to remove surface silanol groups (CS). The modified particles are identified by infrared and raman spectroscopy, differential scanning calorimetry (DSC), and particle size analyzer. It has been found that surface modified particles can be chemically reacted with epoxy matrix, which is confirmed by exothermic peaks in DSC thermograms. In scanning electron micrographs of fractured composites, it is observed that the particle dispersion and interface are considerably affected by functional groups of fillers. Weak interfaces and aggregation of particles are observed for composites filled with CS or S–NCO. However, the aggregation of fillers is highly suppressed in composites filled with S–epoxide and S–NH 2 particles. Generally, the coefficients of thermal expansion (CTE) of composites are reduced with an increase of filler contents. Moreover, composites with strong interface exhibit an additional reduction of CTEs. Composites with weak interface show essentially no change in glass transition temperature ( T g ) and damping with filler contents, while composites with strong interface show an increase of T g and a decrease of damping with filler content.

Quantitative analysis of microstructure and its related electrical property of SOFC anode, Ni–YSZ cermet

Abstract The microstructural and electrical properties of Ni–YSZ composite anode of solid oxide fuel cells (SOFC) were investigated. We measured the electrical conductivity via 4-probe DC technique as a function of Ni content (10–70 vol.%) in order to examine the correlation with the microstructure of Ni–YSZ cermet. Image analysis based on quantitative microscopic theory was performed to quantify the microstructure of Ni–YSZ composite. The size and distribution, contiguity and interfacial area of each phase or between the phases could be obtained from the image analysis. According to image analysis, contiguity between the same phases was simply dependent on the amount of that phase itself while the contiguity between different phases was greatly influenced by the amount of Ni phase because the overall microstructural changes were mainly controlled by the coarsening of Ni phase. These similarly quantified microstructural properties were used for the characterization of the electrical properties of Ni–YSZ cermet.

The changes of membrane performance with polyamide molecular structure in the reverse osmosis process

Abstract In order to develop high performance membranes for the reverse osmosis process, thin film composite membranes were prepared by the interfacial polymerization with various acyl chloride solutions and amine solutions containing poly(m-aminostyrene), (PmAS). PmAS was prepared by reducing poly(m-nitrostyrene) that was obtained by the free radical polymerization of 3-nitrostyrene. The results related to changes in the membrane performance with interfacial reaction time and thickness measured with ESCA indicate that the thickness of the skin layer obtained from various reactants is nearly constant. Membranes obtained by the reaction of trimesoyl chloride (TMC) with various amines, i.e. aromatic diamines and polyaminostyrenes showed performance advantage over those obtained from the difunctional acyl chlorides and amines. Membranes prepared by the reaction of TMC with aromatic diamines, PmAS, or poly(p-aminostyrene) exhibited a trade-off trend between salt rejection and water flux. Membranes prepared by the reaction of TMC with a mixture solution of PmAS and aromatic diamines showed performance advantage over the trade-off trend. Membranes prepared from PmAS and a mixed solution of TMC and benzoyl chloride (BC) also exhibited a performance advantage. The active skin layer of thin film composite membrane, which showed the best performance in the reverse osmosis process could be obtained by reacting mixed amine solution containing PmAS and m-phenylenediamin (MPDA) with mixed acyl chloride solution containing TMC and BC. Membranes having a continuous range of properties as well as a performance advantage over usual membranes could be developed by blending the reactants or by controlling the polyamide chain structure.

Enhanced performance of inverted polymer solar cells with cathode interfacial tuning via water-soluble polyfluorenes

Enhanced performance of inverted polymer solar cells (PSCs) is demonstrated by indium tin oxide (ITO) interfacial tuning via a water-soluble polyfluorene (WPF-6-oxy-F). Kelvin probe studies and dark current-voltage curves demonstrated that the WPF-6-oxy-F layer reduces the ITO work-function because of the favorable interfacial dipole formed by the WPF-6-oxy-F interlayer, thereby enhancing the built-in potential and reducing the interface resistance. As a result, introduction of the WPF-6-oxy-F by simple solution processing into the inverted PSCs dramatically enhanced cell-performances. This approach could be very beneficial and an important step for the future development of all-solution-processed or roll-to-roll processed PSCs.

Effects of the polyamide molecular structure on the performance of reverse osmosis membranes

Thin-film composite reverse osmosis membranes of polyamides were prepared by interfacial polymerization. Various benzenediamines and poly(aminostyrene) were interfacially reacted with various acyl chlorides to prepare a skin layer of composite membranes. Among the membranes prepared from the structural isomeric monomers of benzenediamines and acyl chlorides, i.e., the same chemical composition but different in the position of functional groups on the aromatic ring, the membrane with the best salt rejection was obtained when the reacting groups forming amide are located at the same position on the aromatic ring. Membranes prepared by interfacially reacting various diamines with trimesoyl chloride revealed that the salt rejection depends on the linear chain structure of polyamides and network formed by crosslinking. Membranes obtained by interfacial polymerization of poly(aminostyrene) with trimesoyl chloride showed higher water flux but lower salt rejection than those obtained by interfacial polymerization of various benzenediamines with trimesoyl chloride. Membranes obtained here showed the typical trade-off behavior between salt rejection and water flux. However, membranes prepared by interfacially reacting trimesoyl chloride with a mixture of poly(aminostyrene) and m-phenylenediamine or a mixture of poly(aminostyrene), m-phenylenediamine, and diaminobenzoic acid showed a performance advantage over usual membranes, i.e., a large positive deviation from the usual trade-off trend.

The effects of diluent molecular weight on the structure of thermally-induced phase separation membrane

Abstract The effects of molecular weight of a homologous series of diluent on the droplet size of the thermally-induced phase separation membranes were investigated for the mixtures of poly(ethylene glycol) used as a diluent and nylon 12 as a matrix. All the phase diagrams obtained for different molecular weights of the diluent ranging from 200 to 600 showed the upper critical solution temperature type phase boundaries. When molecular weight of PEG was higher than 1000, nylon 12 did not form miscible blends with PEG below the thermal degradation temperature. Interaction energy denssty for each pair was evaluated from the phase separation temperature by the Flory-Huggins theory. The chain end group effects on the interaction energy were also considered using a binary interaction model and a Massa plot. The equilibrium domain size increased as the molecular weight of the diluent increased and then approached an asymptotic value. The increase of the average domain size with the increase of the diluent molecular weight was suppressed to a great extent for a higher content of nylon 12 in the mixture suggesting that there exist specific interactions between PEG and nylon 12.

Removal of aromatic compounds in the aqueous solution via micellar enhanced ultrafiltration: Part 1. Behavior of nonionic surfactants

The effects of nonionic surfactants having different hydrophilicity and membranes having different hydrophobicity and molecular weight cut-off on the performance of micellar-enhanced ultrafiltration (MEUF) process were examined. A homologous series of polyethyleneglycol (PEG) alkylether having different numbers of methylene groups and ethylene oxide groups was used for nonionic surfactants. Polysulfone membranes and cellulose acetate membranes having different molecular cut-off were used for hydrophobic membranes and hydrophilic membranes, respectively. The concentration of surfactant added to pure water was fixed at the value of 100 times of critical micelle concentration (CMC). The flux through polysulfone membranes decreased remarkably due to adsorption mainly caused by hydrophobic interactions between surfactant and membrane material. The decline of solution flux for cellulose acetate membranes was not as serious as that for polysulfone membranes because of hydrophilic properties of cellulose acetate membranes. The surfactant rejections for the cellulose acetate membranes increased with decreasing membrane pore size and with increasing the hydrophobicity of surfactant. On the other hand the surfactant rejections for polysulfone membranes showed totally different rejection trends with those for cellulose acetate membranes. The surfactant rejections for the polysulfone membranes depend on the strength of hydrophobic interactions between surfactant and membrane material and molecular weight of surfactants.

Effect of molecular weight between crosslinks on the fracture behavior of rubber-toughened epoxy adhesives

The effect of molecular weight between crosslinks, Mc, on the fracture behavior of rubber-toughened epoxy adhesives was investigated and compared with the behavior of the bulk resins. In the liquid rubber-toughened bulk system, fracture energy increased with increasing Mc. However, in the liquid rubber-toughened adhesive system, with increasing Mc, the locus of joint fracture had a transition from cohesive failure, break in the bond layer, to interfacial failure, rupture of the bond layer from the surface of the substrate. Specimens fractured by cohesive failure exhibited larger fracture energies than those by interfacial failure. The occurrence of transition from cohesive to interfacial failure seemed to be caused by the increase in the ductility of matrix, the mismatch of elastic constant, and the agglomeration of rubber particles at the metal/epoxy interface. When core-shell rubber, which did not agglomerate at the interface, was used as a toughening agent, fracture energy increased with Mc.

Variations of cell performance in ITO-free organic solar cells with increasing cell areas

This study examined the effects of a cell area on the cell performances in ITO-free organic solar cells (OSCs) based on poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM). Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films with two different sheet resistances (Rsh) were used as polymeric transparent anodes for cost-effective ITO-free OSCs. Changes in the power conversion efficiency (PCE), the fill factor (FF), the short-circuit current (Jsc), and the open-circuit voltage (Voc) that resulted from changing the cell area or sheet resistance of transparent electrodes were systematically investigated. With increasing cell area from 4.5 to 49.5 mm2, the device performance of ITO-free OSCs was continuously decreased mainly due to the decrease in the FF and the series resistance (Rs). In addition, the performance of OSCs was critically dependent on Rsh of the PEDOT:PSS electrode. Upon reducing Rsh of the polymer anode from ~200 to ~90 Ω/, the FF and PCE showed better values at an identical large cell area and exhibited a relieved cell performance degradation with increasing cell area, suggesting that the sheet resistance of transparent electrodes is a dominant factor to limit cell efficiencies in practical large-area solar cells.

Electrical conductivity and defect structure of yttria-doped ceria-stabilized zirconia

AbstractThe electrical property and defect structure of 12 mro CeO –ZrO , which has been known for good mechanical and 22 thermal properties, were studied in order to investigate the possibility of its use as a solid electrolyte for electrochemicaldevice, like electrolyser, sensor and fuel cell. The electrical conductivity of 12 mro CeO –ZrO was greatly enhanced and 22 electrolytic domain boundary EDB was also extended to lowŽ. P with yttria doping, which were readily comparable with O 2 those of commercial electrolyte, YSZ, CSZ. Unlike normal stabilized zirconia, saturation point of electrical conductivityappeared at higher doping concentration 12–15 mŽ.ro yttria and the activation energy was not also changed seriously withdoping till 15 mro yttria doping. q2001 Elsevier Science B.V. All rights reserved. Keywords: Zirconia; Ceria; Yttria; Electrical conductivity; Defect structure 1. IntroductionIn the last few decades, zirconia-based ceramichas been studied with great interests for many appli-cation fields due to its good mechanical, chemicaland electrolytic properties. Especially, doped zirco-nia with a lower valence cation like Ca