Steve Lien-Chung Hsu

Enhanced thermal conductivity of polyimide films via a hybrid of micro- and nano-sized boron nitride.

A new thermally conductive polyimide composite film has been developed. It is based on a dispersion of different particle sizes of boron nitride (BN) in a polyimide (PI) precursor, polyamic acid (PAA). Subsequently, thermal imidization of PAA at 350 degrees C produced the corresponding polyimide composites. 3-Mercaptopropionic acid was used as the surfactant to modify the BN surface for the dispersion of BN in the polymer. The PI/BN composites showed different thermal conductivities at different proportion of BN particle sizes and contents. The thermal conductivity of the PI/BN composite was up to 1.2 W/m-k, for a mixture containing 30 wt % of micro and nanosized BN fillers in the polyimide matrix. The PI/BN composites had excellent thermal properties. Their glass transition temperatures were above 360 degrees C, and thermal decomposition temperatures were over 536 degrees C.

A novel asymmetric polybenzimidazole membrane for high temperature proton exchange membrane fuel cells

A novel asymmetric polybenzimidazole (PBI) membrane used for high temperature proton exchange membrane fuel cells has been successfully fabricated by a soft-template method using the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) as the porogen. Typically, the asymmetric PBI membrane exhibits a double-layer structure comprising a dense layer and a porous layer with a distinguishable boundary. The morphology and asymmetry of the porous structure have been characterized by SEM micrographs. The density difference between the polymer matrix and the porogen can be considered as the driving force for developing the asymmetrical structure. Phosphoric acid-doped asymmetric PBI with a high porosity exhibited considerably enhanced doping levels and proton conductivity. For example, a doping level of up to 23.6 and a proton conductivity as high as 6.26 × 10−2 S cm−1 were achieved. Moreover, the crosslinking modification of asymmetric PBIs had beneficial effects on the mechanical strength and oxidative stability, which were investigated. We have also demonstrated the fuel cell performance of a membrane electrode assembly (MEA) based on the asymmetric PBI at elevated temperatures under anhydrous conditions in the present work.

Nanoimprinting-induced efficiency enhancement in organic solar cells

The performance of bilayer heterojunction, pentacene and perylene tetracarboxylic diimide organic solar cells (OSC) is dramatically enhanced by performing nanoimprinting on the hole transport layer in which poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) nanogratings are built. A standard OSC embeded with PEDOT:PSS gratings enables photocarriers to move toward electrodes. In particular, the visualization of morphologies for these heterojunction layers reveals pillar-like grains that are induced by the geometric effect of PEDOT:PSS gratings. Toghther with analyzing absorption and time-resolved photoluminescence spectra, a consistent correlation between OSC performance, photophysical data, and pillar-like morphology has been obtained for the device with imprinted PEDOT:PSS nanogratings.

Preparation and properties of conductive polyimide nanocomposites with assistance of a metallo-organic compound

Highly reflective and electrical conductive polyimide films have been developed by incorporation of surface modified silver flakes and a metallo-organic compound, silver 2-ethylhexanoate, into a polyimide matrix. The surface conductivity can be obtained during the imidization process of polyimide, because the metallo-organic compound can decompose into silver atoms at a low temperature of 150 °C. The optimum weight ratio of silver 2-ethylhexanoate to silver flakes was 2/8, and the surface resistivity of the conductive polyimide film, which contained 30 wt% of silver fillers, was 10−2 ohm sq−1. The metallized films had excellent thermal properties. Their glass transition temperatures were above 276 °C, and the thermal decomposition temperatures were over 512 °C. The optical reflectivity of the films was up to 90%.

Effects of O2 and N2 Plasma Treatment on 6FDA-BisAAF Fluorine-Contained Polyimide

Polymer materials, with their stable chemistry, excellent electrical properties, and easy processability, have drawn a lot of attention in the microelectronic industry. Among them, polyimides, owing to their superior properties, have been used in many applications, such as passivation layer, substrate materials, optical devices, interlayer dielectric, etc. Among them, fluorine-contained polyimides with their low dielectric constants, good thermal properties, superior resistances to chemicals, high transparencies, and excellent mechanical properties were considered as one of the best materials for use the microelectronic devices. 1-4 Although fluorine-contained polyimides have such advantages and are promising material for microelectronic applications, the integration of fluorinated-polyimides in devices still faces many challenges. Plasma modification is widely used to enhance the surface property for polymers. In general, different gases used and the variety of treated polymers will lead to different effects. O2 contained plasma were commonly used for etching and also to improve the adhesion property. Plasmas of N2, ammonia, and their mixtures are always used to enhance the wettability. Noble gases such as He and Ar can improve the wettability of various polymers. 5-8 Ar plasma treatment was also demonstrated to improve the bondability of polymer surface. 9

Enhanced spontaneous light emission by multiple surface plasmon coupling

Photoluminescence of polyfluoren copolymers, a white-light material, was demonstrated to be enhanced selectively by coupling with either localized or propagating modes of surface plasmon resonance (SPR). The silver sub-micron cylinders with 75nm height fabricated by e-beam lithography followed by e-beam evaporation and lift-off process. The enhanced light emissions at 500nm and 533nm are attributed to the low frequency branch of localized SPR. Furthermore, a 50nm silver thin film between these cylinders and the substrate provides propagating surface plasmons under excitation and enhances the blue emission band of the polyfluoren copolymer at 438nm. This delocalized SPR is sufficient for effective plasmon to light conversion. Moreover, by effectively coupling the localized and propagating SPR, we can experimentally demonstrate that the photoluminescence of polyfluoren copolymers is enhanced by 4 to 5.4 times at different wavelengths compared to enhancement by either single mode.

Thermo-curable epoxy systems for nanoimprint lithography

In this work, we have used solvent-free thermo-curable epoxy systems for low-pressure and moderate-temperature nanoimprint lithography (NIL). The curing kinetic parameters and conversion of diglycidyl ether of bisphenol A (DGEBA) resin with different ambient-cure 930 and 954 hardeners were studied by the isothermal DSC technique. They are useful for the study of epoxy resins in the imprinting application. The DGEBA/930 and DGEBA/954 epoxy resists can be imprinted to obtain high-density nano- and micro-scale patterns on a flexible indium tin oxide/poly(ethylene terephthalate) (ITO/PET) substrate. The DGEBA/930 epoxy resin is not only suitable for resist material, but also for plastic mold material. Highly dense nanometer patterns can be successfully imprinted using a UV-curable resist from the DGEBA/930 epoxy mold. Using the replicated DGEBA/930 epoxy mold instead of the expensive master can prevent brittle failure of the silicon molds in the NIL.

Synthesis of new silicon-containing copolybenzimidazoles

19 Polybenzimidazoles (PBIs) are rather promising materials for preparing high temperature (above 100°C) membranes due to their high thermal and mechanical characteristics. Moreover, PBIs doped with strong acids show stable proton conduction at temperatures above 100°C [1–5]. It was noted, however [6, 7], that the doping of PBI with orthophosphoric acid leads to a sharp deteriora tion of the mechanical properties of membranes. One of approaches to stabilize the mechanical strength of polybenzimidazole membranes consists in the preparation of hybrid, in particular polybenzimi dazole–silica, composite systems prior to doping [8–10]. Therefore, we suggested in this work to obtain using sol–gel process new copolybenzimidazole–silica copolymers from organosoluble copolymeric PBIs containing diphenyl ether fragments in the main chain and hydroxy groups in side groups.

Preparation of highly concentrated and stable suspensions of silver nanoparticles by an organic base catalyzed reduction reaction

This study successfully prepared highly concentrated and stable suspensions of silver nanoparticles using an organic base, triethylamine, as the promoter and by chemical reduction of silver nitrate in formaldehyde with TSA as the stabilizer. The average size of the silver particles is less than 10 nm. The suspensions are stabilized by excess triethylamine. The suspensions of silver nanoparticles prepared by this method are free from any metal ions contamination, and are suitable for use in semiconductor industry.

Synthesis and properties of ABPPQ for high-temperature proton exchange membrane fuel cells

In this study, the one-pot synthesis of self-polymerizable quinoxaline monomer was developed. 3-(4-hydroxyphenyl)-2-phenyl-6-fluoroquinoxaline and 2-(4-hydroxyphenyl)-3-phenyl-6-fluoroquinoxaline mixture (M1a,b) was synthesized from benzyl 4-hydroxyphenyl ketone and 1,2-diamino-4-fluorobenzene, catalyzed by 1,4-diazabicyclo[2.2.2]octane. Then, an ether-containing AB type polyphenylquinoxaline (ABPPQ) was synthesized successfully from the monomer M1a,b. The ether-containing ABPPQ is organosoluble, and has good proton conductivity at high temperatures after doping with phosphoric acid. It is suitable for use in high-temperature proton exchange membrane fuel cells. Compared to polybenzimidazole (PBI), ABPPQ has higher acid doping level at the same doping time, because it has more sites that can be doped with phosphoric acid in the PPQ’s molecular structure than PBI.