Tae-Hyuk Lee

Mechanical and asymmetrical thermal properties of Al/Cu composite fabricated by repeated hydrostatic extrusion process

In this work, an Al/Cu composite was fabricated using repeated hydrostatic extrusions with a semi-die angle of 45° at a temperature of 200°C. During the repeated hydrostatic extrusion process, an intermetallic layer of 1 μm was generated at the Al/Cu interface. The intermetallic reinforcement layer formation was studied via field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy. The reinforcement layers consisted of Al2Cu and Al4Cu9 phases. The thermal conductivity and coefficient of thermal expansion of the Al/Cu composites were determined via laser flash analyzer and thermal mechanical analyzer. The thermal conductivities in the longitudinal direction and cross direction were 267.8 W/mK and 209.9 W/mK at room temperature, respectively. The coefficient of thermal expansion at temperatures between room temperature and 450°C was 18.3 μm/m°C. The ultimate tensile strength of the Al/Cu composite is 102.8 MPa. The anisotropic physical properties of the prepared composite were mainly a result of the unique directional microstructure formed during the repeated extrusion process.

Few-atomic-layer boron nitride nanosheets synthesized in solid thermal waves

In this study, we demonstrate the synthesis of few-atomic-layer hexagonal boron nitride (h-BN) sheets in a solid thermal wave implemented in a B2O3 + (3 + 0.5k)Mg + kNH4Cl exothermic mixture (here, k is the mole number of NH4Cl). The maximum synthesis temperature, developed using a thermal wave, was between 1030 and 1250 °C as k was changed from 5 to 7 moles. The phase content, morphology, and optical properties of the products were characterized. It is shown that BN sheets synthesized at the given k were 1.5–3 nm thick and had a hexagonal structure. The number of atomic layers in one sheet ranged from 5 to 10; the lateral dimension of individual sheets ranged from 50 to 1000 nm. The developed method allowed the synthesis of a large amount of uniform and high quality BN nanosheets (tens of grams in laboratory-scale experiments); this method will reduce the overall production cost.

Iron-assisted electroless deposition reaction for synthesizing copper and silver dendritic structures

Electroless deposition reaction (EDR) is a promising route for the synthesis of copper (Cu) and silver (Ag) dendritic structures with a controllable morphology. Here, we demonstrate the use of an electroless deposition reaction on the interface between an iron foil and aqueous solutions of either Cu or Ag salts to produce well-defined Cu and Ag dendritic nanostructures at room temperature. The dendritic structures and morphology were characterized based on the reaction time, Cu and Ag salt concentration, and diethylene glycol (DEG) content in the aqueous solution. We also demonstrate control over the dendrite sizes, morphologies and growth mechanisms. The electrical resistivity and improved sheet resistance were also evaluated in an anisotropic conductive film prepared from dendritic silver powder.

Aluminothermic Reduction of K2TiF6 to Prepare TiC, TiB2, and TiN Nanoparticles

Aluminothermic reduction of K2TiF6 in the presence of C, B, or N2 was investigated as a low exothermic combustion process for synthesizing nanoparticles of titanium ceramics such as TiC, TiB2, and TiN. Sample pellets were prepared by mixing powders of K2TiF6, Al, and non-metal (C, B) powders in stoichiometric ratios. Experiments were conducted at room temperature under either an Ar or N2 atmosphere. The combustion parameters were estimated from the temperature-time profiles, measured in the combustion wave using thermocouples. The reaction drastically proceeds within seconds, raising the temperature to 890–1170°C. The size of TiC, TiB2, and TiN nanoparticles prepared by the developed approach were, respectively, 5–30 nm, 10–60 nm, and 70–100 nm.

Effect of applied current on the formation of defect in PWR nuclear fuel rods in resistance pressure welding process

The welding of zirconium alloy components is one of the most critical processes in the fabrication of nuclear fuel rods used in pressurized water reactors. For this, various welding processes, such as gas tungsten arc welding, electron beam welding, laser beam welding, and resistance pressure welding (RPW), are used around the world. In Korea, the RPW process is being used to fabricate nuclear fuel assembly fuel rods. This study investigated changes in the weldment shape owing to welding conditions such as welding current, welding force, and overlapping. The welding soundness of the weldment was evaluated by hydraulic burst test. The welding temperature of the weld zone was measured using a thermal infrared method. Discontinuous black spots in the weld line, regarded as a non-bonding defect, were confirmed as spots caused by the carbide precipitation of zirconium during welding.

Effect of intermetallic compound thickness on anisotropy of Al/Cu honeycomb rods fabricated by hydrostatic extrusion process

Abstract The effect of intermetallic compound (IMC) thickness on the thermal and mechanical properties of Al/Cu honeycomb rods was investigated. The Al/Cu honeycomb rods were fabricated using repeated hydrostatic extrusions at 200 °C. During the process, an IMC layer with 1 μm in thickness was generated at the Al/Cu interface. Different IMC thicknesses were obtained by post-heat treatment at 420 °C for 0.5 to 2 h. The IMC thickness increased to 10.1 μm. The IMC layers were identified as Al2Cu (θ), AlCu (η2), and Al4Cu9 (γ1) phases. The thermal conductivities in the longitudinal direction and cross direction decreased by 11.9% ((268±4.8) to (236±4.4) W/(m·K)) and 10.4% ((210±3.2) to (188±2.8) W/(m·K)), respectively, with increasing IMC thickness. The ultimate tensile strength and elongation of the Al/Cu honeycomb rod are (103±8.4) MPa and (73±6.2)%, respectively. The ultimate tensile strength increased to (131±6.5) MPa until the IMC thickness reached 7.7 μm. It subsequently decreased to (124±3.9) MPa until the IMC thickness reached 10.1 μm. The elongation of the Al/Cu honeycomb rod then sharply decreased to (29±2.5)% with increasing IMC thickness.

Tailoring the morphology of AlN: from 6-fold patterned crystals to multilayer hierarchical structures

Combustion of inorganic powder mixtures is not only one of the chemical routes of fabrication of advanced inorganic materials but is also drawing attention as a high-temperature process to grow inorganic nanocrystals of various shapes and morphologies. Here, we demonstrate the formation of AlN 3-D structures with diverse morphologies: from 6-fold patterned crystals to multilayer hierarchical structures. We examined the possible chemical mechanisms governing the formation of AlN crystals and the growth behaviour based on FESEM and TEM observations.

Effects of fabrication process on microstructure and texture of Inconel 690 tubes for steam generator

The main goal of this research was to investigate the relationship between the grain boundary misorientation and the precipitation of intergranular M23C6 carbides during the pilgering process and the heat treatment of Inconel 690 tubes for steam generators. The M23C6 carbides behavior is obviously influenced by the grain boundary character and interfacial energy. The grain boundary misorientation of the Inconel 690 tubes was investigated by electron backscattered diffraction of carbide precipitates at these grain boundaries. Numerous M23C6 carbide precipitate at the large angle grain boundaries with high interfacial energy.

Analysis of the Aluminum Extrusion Process Equipped with the Continuous Heat Treatment System

In this study, the heat flow of the plant scale aluminum extrusion process was investigated to establish optimum continuous heat treatment conditions. During the extrusion of 6061 aluminum alloy, processing parameters such as the extrusion pressure, speed and temperature histories of billets were logged as a function of time. The surface temperature of the billets increased at constant ram speed, while it decreased with decreases of the ram speed. In order to maintain the billet temperature within a solutionizing temperature range prior to the succeeding water quenching step, the ram speed or the temperature of the blower should be controlled. The temperature histories of the billets during the extrusion and hot air blowing processes were successfully simulated by using the velocity boundary model in ANSYS CFX. The methodology to design an optimum process by using a commercial simulation program is described in this study on the basis of the metallurgical validation results of the microstructural observation of the extrudates. The developed model allowed the advantages of taking into account the motion of the extrudate coupled with the temperature change based on empirical data. Calculations were made for the extrudate passing through the isothermal chamber maintained at appropriate temperature. It was confirmed that the continuous heat treatment system is beneficial to the productivity enhancement of the commercial aluminum extrusion industry.