Youngkwon Kim

Heterogeneous copper catalyst for the cycloaddition of azides and alkynes without additives under ambient conditions.

A new copper catalyst was developed by immobilizing copper nanoparticles in aluminum oxyhydoxide fiber. The catalyst showed high catalytic activity for the (3+2) Huisgen cycloaddition of nonactivated alkynes as well as activated ones with various azides at room temperature. The catalyst was recycled five times without significant loss of activity.

Architectural Engineering of Rod–Coil Compatibilizers for Producing Mechanically and Thermally Stable Polymer Solar Cells

While most high-efficiency polymer solar cells (PSCs) are made of bulk heterojunction (BHJ) blends of conjugated polymers and fullerene derivatives, they have a significant morphological instability issue against mechanical and thermal stress. Herein, we developed an architecturally engineered compatibilizer, poly(3-hexylthiophene)-graft-poly(2-vinylpyridine) (P3HT-g-P2VP), that effectively modifies the sharp interface of a BHJ layer composed of a P3HT donor and various fullerene acceptors, resulting in a dramatic enhancement of mechanical and thermal stabilities. We directly measured the mechanical properties of active layer thin films without a supporting substrate by floating a thin film on water, and the enhancement of mechanical stability without loss of the electronic functions of PSCs was successfully demonstrated. Supramolecular interactions between the P2VP of the P3HT-g-P2VP polymers and the fullerenes generated their universal use as compatibilizers regardless of the type of fullerene acceptors, including mono- and bis-adduct fullerenes, while maintaining their high device efficiency. Most importantly, the P3HT-g-P2VP copolymer had better compatibilizing efficiency than linear type P3HT-b-P2VP with much enhanced mechanical and thermal stabilities. The graft architecture promotes preferential segregation at the interface, resulting in broader interfacial width and lower interfacial tension as supported by molecular dynamics simulations.

High Temperature Superconducting Motor Cooled by On-Board Cryocooler

HTS (high temperature superconducting) motor that utilizes superconductor as the field winding has advantage of compactness, light weight and high efficiency, but it has an inherent disadvantage that the rotor must be cooled down at cryogenic temperature. Conventionally, HTS rotor was cooled by circulation of cryogenic fluid such as helium or neon, and the cooling system was installed outside of the motor to prevent it from being rotated. In this paper, however, the prototype of the HTS motor with an on-board cryocooler, is fabricated and tested. The experimental system is composed of the stator with conventional copper winding, the rotor with superconductor, and the rotating cryocooler as a novel concept of cooling system. The rotor is fabricated as the race-track coil with 2G, YBCO tape and situated in the 110 mm diameter cryostat. It is designed to minimize heat invasion from room temperature environment and be compatible with the coaxial structure of the cryocooler. The cryocooler is in physically and thermally contact with the HTS rotor while being rotated together. Two kinds of refrigerator, a Stirling refrigerator and an inline-type PTR (Pulse Tube Refrigerator) are used individually as the on-board cryocooler. The whole system is carefully integrated and fabricated for rotating stability. Several temperature sensors and voltage taps are installed to monitor the conditions of the HTS rotor and the cryocooler while their signals are acquired by the bluetooth data acquisition system during the operation. The HTS motor is successfully operated with 120 rpm of rotating speed. In the rotating test, 70 A current is supplied to the superconducting rotor. Further developmental design issues for on-board cryocooling system are addressed and discussed in this paper.

Cycle analysis of an air-cooled LiBr/H2O absorption heat pump of parallel-flow type

Abstract A gas-fired absorption heat pump with cooling capacity of 2 RT was analysed as an air-conditioner for domestic use during the summer. The absorption heat pump considered was an air-cooled, double-effect, LiBr/H2O system of parallel-flow type. The performance of the absorption heat pump in the cooling mode of operation was investigated through cycle simulation to obtain the system characteristics depending on the inlet temperature of air to the absorber, the working solution concentrations, the solution distribution ratio of the mass of solution into the first generator to the total mass of solution from the absorber, and the LTDs (leaving temperature differences) of the heat-exchanging components. When the predicted results were compared with the measured data for similar design conditions, reasonable agreement was observed. The optimum design and operating conditions of an air-cooled absorption system are suggested based on this cycle simulation analysis.

Architectural Effects on Solution Self-Assembly of Poly(3-hexylthiophene)-Based Graft Copolymers

While solution assembly of conjugated block copolymers has been widely used to produce long 1-D nanowires (NWs), it remains a great challenge to provide a higher level of control over structure and function of the NWs. Herein, for the first time, we report the solution assembly of graft copolymers containing a conjugated polymer backbone in a selective solvent and demonstrate that their self-assembly behaviors can be manipulated by the molecular structures of the graft copolymers. A series of poly(3-hexylthiophene)-graft-poly(2-vinylpyridine) (P3HT-g-P2VP) copolymers was designed with two different architectural parameters: grafting fraction (fg) and molecular weight of P2VP chains (Mn,P2VP) on the P3HT backbone. Interestingly, crystallization of the P3HT-g-P2VP copolymers was systematically modulated by changes in fg and Mn,P2VP, thus allowing for control of the growth kinetics and curvatures of solution-assembled NWs. When Mn,P2VP (4.4 to 15.1 kg/mol) or fg (2.8 to 9.2%) of the P3HT-g-P2VP polymers was increased, the crystallinity of the copolymers was reduced significantly. Steric hindrance from the grafted P2VP chains apparently modified the growth of NWs, leading to shorter NWs with a greater degree of curvature for graft copolymers with more hindrance. Therefore, we envision that such conjugated chain-based graft copolymers can be versatile building blocks for producing NWs with controlled length and shape, which can be important for tailoring the optical and electrical properties of NW-based devices.

Nitrogen-doped titanium oxide microrods decorated with titanium oxide nanosheets for visible light photocatalysis

Nitrogen-doped titania with a unique two-level hierarchical structure and visible light photocatalytic activity is reported. Thus, nitrogen-doped titanium oxide microrods decorated with N-doped titanium oxide nanosheets were synthesized by a hydrothermal reaction in NH 4 OH and postcalcination. During the calcination, the in situ incorporation of nitrogen atoms of ammonium ion into titania lattice was accompanied by the structural evolution from titanate to anatase titania. The morphological and structural evolution was monitored by scanning electron microscopy (SEM), x-ray diffraction (XRD), thermogravimetric analysis/differential thermal analysis (TGA/DTA), Raman, Fourier transform infrared (FTIR), x-ray absorption near edge structure (XANES), x-ray photoelectron spectroscopy (XPS), and adsorption isotherms. The N-doping brought visible light absorption, and the material exhibited high photocatalytic activity in the decomposition of Orange II under visible light irradiation (λ ≥ 400 nm), especially when it was loaded with 1 wt% Pt as a cocatalyst.

Superfluid transport and its applications in space

Abstract Various transport modes in superfluid helium are discussed in this paper. They include zero net mass flow (ZNMF) and finite mass flow (FMF) for pure superfluid and normal fluid flow. An attempt is made to characterize these transport modes in a common frame of reference. Two dimensionless numbers are used, namely the dimensionless heat flux number and the dimensionless driving force number. The equations are generalized by the use of a characteristic length so that they can be applied to the transport of He II in any geometry. The theories are then extended to applications in space. In particular, fountain effect pumps (FEPs) and superfluid management at zero g by vapour—liquid phase separators (VLPSs) will be discussed in detail. While transport in a phase separator is close to that of ZNMF, the flow in a FEP belongs to the FMF mode. The transport modes of the above systems using porous media are found to be strongly size dependent. For VLPSs, the heat rejection rate is proportional to the square root of plug permeability, K P . As in the case of FEPs, the volumetric flow rate is inversely proportional to K P . These findings are supported by ZNMF data in capillaries and FMF (gravitational flow) data in millipores, respectively.

Investigation on dynamic behavior of linear compressor in stirling-type pulse tube refrigerator

This paper describes the experimental study of the dynamic behavior of a linear compressor in a Stirling-type pulse tube refrigerator (PTR). The dynamic behavior of the piston is closely coupled with the hydraulic force of gas and, therefore, directly influenced by the specific load condition of the pulse tube refrigerator. In the experiment, the frequency response of the pressure at each component, the cooling performance and the piston displacement are measured while an alternate current with fixed magnitude is supplied to the linear motor. The linear compressor in this study was originally designed for a Stirling cryocooler and its maximum input power is approximately 200 W. The pulse tube refrigerator is configured as an in-line type and an inertance tube is incorporated as the phase control device in the pulse tube refrigerator. The pressure difference between both ends of the piston imposes additional stiffness and the PV power in the compression space can be considered a damping effect in the vibra...

Investigation on the two-stage active magnetic regenerative refrigerator for liquefaction of hydrogen

An active magnetic regenerative refrigerator (AMRR) is expected to be useful for hydrogen liquefaction due to its inherent high thermodynamic efficiency. Because the temperature of the cold end of the refrigerator has to be approximately liquid temperature, a large temperature span of the active magnetic regenerator (AMR) is indispensable when the heat sink temperature is liquid nitrogen temperature or higher. Since magnetic refrigerants are only effective in the vicinity of their own transition temperatures, which limit the temperature span of the AMR, an innovative structure is needed to increase the temperature span. The AMR must be a layered structure and the thermophysical matching of magnetic field and flow convection effects is very important. In order to design an AMR for liquefaction of hydrogen, the implementation of multi-layered AMR with different magnetic refrigerants is explored with multi-staging. In this paper, the performance of the multi-layered AMR using four rare-earth compounds (GdNi2...