Ho Jin Ryu

Scalable exfoliation process for highly soluble boron nitride nanoplatelets by hydroxide-assisted ball milling.

The scalable preparation of two-dimensional hexagonal boron nitride (h-BN) is essential for practical applications. Despite intense research in this area, high-yield production of two-dimensional h-BN with large-size and high solubility remains a key challenge. In the present work, we propose a scalable exfoliation process for hydroxyl-functionalized BN nanoplatelets (OH-BNNPs) by a simple ball milling of BN powders in the presence of sodium hydroxide via the synergetic effect of chemical peeling and mechanical shear forces. The hydroxide-assisted ball milling process results in relatively large flakes with an average size of 1.5 μm with little damage to the in-plane structure of the OH-BNNP and high yields of 18%. The resultant OH-BNNP samples can be redispersed in various solvents and form stable dispersions that can be used for multiple purposes. The incorporation of the BNNPs into the polyethylene matrix effectively enhanced the barrier properties of the polyethylene due to increased tortuosity of the diffusion path of the gas molecules. Hydroxide-assisted ball milling process can thus provide simple and efficient approaches to scalable preparation of large-size and highly soluble BNNPs. Moreover, this exfoliation process is not only easily scalable but also applicable to other layered materials.

Generalized shear-lag model for load transfer in SiC/Al metal-matrix composites

The load-transfer efficiency of reinforcement, in cylindrical forms in metal-matrix composite (MMC), was analyzed based on the shear-lag model. Both the geometric shape and alignment of reinforcement were considered. The stress transferred to a misaligned whisker was calculated from differential equations based on the force equilibrium in longitudinal and transverse directions. A new parameter, defined as effective aspect ratio, was used to indicate the load-transfer efficiency of misaligned reinforcement. The effective aspect ratio was formulated as a function of aspect ratio and misorientation angle of reinforcement in MMC. A probability density function of misorientation distribution was used to estimate the strengthening effect of all misaligned whiskers distributed in the matrix. Considering the contributions of both effective aspect ratio and misorientation distribution on load-transfer efficiency, a generalized shear-lag model was proposed to explain the mechanical anisotropy of discontinuous reinforced MMC.

Effect of thermomechanical treatments on microstructure and properties of Cu-base leadframe alloy

The effect of thermomechanical treatments (TMT) on the microstructuresand properties of Cu-1.5Ni-0.3Si-0.03P-0.05Mg leadframe alloy wasinvestigated. The Cu-base leadframe alloy was received as hot rolledplates with 8 mm thickness. The hot rolled plates were solutiontreated at 700°C or 800°C for 1 hour, and coldrolled with 40–85% reduction, then followed by aging treatment at450°C. The leadframe alloy solution treated at 800°Cshowed larger grain size of 15 μm comparing with the grain size of10 μm in leadframe alloy solution treated at 700°C. Theleadframe alloy with smaller grain size of 10 μm showed highertensile strength and lower electrical resistivity than that withlarger grain size of 15 μm. The dislocation density increased withincreasing reduction ratio of cold rolling from 40% to 85% andresulted in finer Ni2Si precipitates. Tensile strength increasedand electrical resistivity decreased with increasing reduction ratioof cold rolling due to the formation of finer Ni2Si precipitates.Two types of thermomechanical treatments were performed to enhance theproperties of leadframe alloy. One type of thermomechanical treatmentis to refine the grain size through the overaging, cold rollingfollowed by recrystallization. The recrystallization process improvedthe tensile strength to 540 MPa and elongation to 15% by reducing thegrain size to 5 μm. The other type of thermomechanical treatmentis to refine the precipitate size by two-step aging process. Thetwo-step aging process increased the tensile strength to 640 MPa andreduced the electrical resistivity to1.475 × 10−8 Ωm by reducing the size of Ni2Si precipitates to 4 nm.

Reaction layer growth and reaction heat of U–Mo/Al dispersion fuels using centrifugally atomized powders

Abstract The growth behavior of reaction layers and heat generation during the reaction between U–Mo powders and the Al matrix in U–Mo/Al dispersion fuels were investigated. Annealing of 10 vol.% U–10Mo/Al dispersion fuels at temperatures from 500 to 550 °C was carried out for 10 min to 36 h to measure the growth rate and the activation energy for the growth of reaction layers. The concentration profiles of reaction layers between the U–10Mo vs. Al diffusion couples were measured and the integrated interdiffusion coefficients were calculated for the U and Al in the reaction layers. Heat generation of U–Mo/Al dispersion fuels with 10–50 vol.% of U–Mo fuel during the thermal cycle from room temperature to 700 °C was measured employing the differential scanning calorimetry. Exothermic heat from the reaction between U–Mo and the Al matrix is the largest when the volume fraction of U–Mo fuel is about 30 vol.%. The unreacted fraction in the U–Mo powders increases as the volume fraction of U–Mo fuel increases from 30 to 50 vol.%.

Microstructure and mechanical properties of mechanically alloyed and solid-state sintered tungsten heavy alloys

The mechanical properties of solid-state sintered 93W‐5.6Ni‐1.4Fe tungsten heavy alloys fabricated by mechanical alloying were investigated. Blended W, Ni and Fe powders were mechanically alloyed in a tumbler ball mill at a milling speed of 75 rpm employing a ball-to-powder ratio of 20:1 and a ball filling ratio of 15%. A nanocrystalline size of 16 nm and fine lamellar spacings of 0.2 mm were obtained in mechanically alloyed powders at a steady state milling stage. Mechanically alloyed powders were consolidated into green compacts and solid-state sintered at 1300°C fo r1hi n ahydrogen atmosphere. The alloys sintered from mechanically alloyed powders showed fine tungsten particles (about 3 mm in diameter) and a relative density above 99%. The volume fraction of the matrix phase was 11% and the tungsten:tungsten contiguity was determined to be 0.74. The alloys exhibited high yield strengths (about 1100 MPa) due to their fine microstructures, but exhibited reduced elongation and impact energy due to a large area fraction of tungsten:tungsten boundaries and the low volume fraction of matrix phase.

Mechanical alloying process of 93W-5.6Ni-1.4Fe tungsten heavy alloy

Abstract The mechanical alloying process of 93W-5.6Ni-1.4Fe tungsten heavy alloy from the elemental powders of W, Ni and Fe by a high energy ball mill in argon atmosphere was investigated. The mechanical alloying process parameters such as milling speed, milling time, ball-to-powder ratio and ball filling ratio were varied in order to investigate their influence on the microstructural evolution of mechanically alloyed powders. The mechanical alloying process proceeded following five distinct stages such as flattening stage, welding dominant stage, equiaxed particle fprming stage, random lamellar forming stage and steady state stage with increasing the milling time. The steady state stage of mechanical alloying was reached after milling for 48 hours with milling speed of 75 rpm, ball-to-powder ratio of 20:1 and ball filling ratio of 15%. Nanocrystalline grain size of 16 nm was obtained at the steady state stage of mechanical alloying. Mechanically alloyed powders were consolidated by cold isostatic pressing and followed by sintering at temperature ranged 1300–1485°C for 1 hour in hydrogen atmosphere. When liquid phase sintered at 1485°C, tungsten heavy alloy from mechanically alloyed powders showed finer tungsten particles about 27μm than that from conventionally blended powders. The density of liquid phase sintered tungsten heavy alloy decreased with increasing the milling time due to the swelling during sintering. When solid state sintered below 1430°C, tungsten heavy alloy from mechanically alloyed powders showed ultra-fine tungsten particles about 3μm and showed high relative density above 97% insensitive to the milling time.

USE OF A CENTRIFUGAL ATOMIZATION PROCESS IN THE DEVELOPMENT OF RESEARCH REACTOR FUEL

A centrifugal atomization process for uranium fuel was developed in order to fabricate high uranium density dispersion fuel for advanced research reactors. Spherical powders of and U-Mo were successfully fabricated and dispersed in aluminum matrices. Thermal and mechanical properties of dispersion fuel meat were characterized. Irradiation tests at the research reactor HANARO confirm the excellent performance of high uranium density dispersion fuel.

PERFORMANCE EVALUATION OF U-Mo/Al DISPERSION FUEL BY CONSIDERING A FUEL-MATRIX INTERACTION

Because the interaction layers that form between U-Mo particles and the Al matrix degrade the thermal properties of U-Mo/Al dispersion fuel, an investigation was undertaken of the undesirable feedback effect between an interaction layer growth and a centerline temperature increase for dispersion fuel. The radial temperature distribution due to interaction layer growth during irradiation was calculated iteratively in relation to changes in the volume fractions, the thermal conductivities of the constituents, and the oxide thickness with the burnup. The interaction layer growth, which is estimated on the basis of the temperature calculations, showed a reasonable agreement with the post-irradiation examination results of the U-Mo/Al dispersion fuel rods irradiated at the HANARO reactor. The U-Mo particle size was found to be a dominant factor that determined the fuel temperature during irradiation. Dispersion fuel with larger U-Mo particles revealed lower levels of both the interaction layer formation and the fuel temperature increase. The results confirm that the use of large U-Mo particles appears to be an effective way of mitigating the thermal degradation of U-Mo/Al dispersion fuel.

Irradiation-enhanced interdiffusion in the diffusion zone of U-Mo dispersion fuel in Al

Uranium-molybdenum (U-Mo) alloy fuel particles dispersed in an aluminum (Al) matrix, designated as U-Mo/Al dispersion fuel, is in the development stage in the worldwide RERTR (Reduced Enrichment for Research and Test Reactors) program. The main issue in developing U-Mo/Al dispersion fuel is the diffusion reaction occurring at the interface between the fuel particles and matrix. To accurately analyze fuel performance, a model to predict the diffusion kinetics is necessary. For this purpose, the authors developed a diffusion layer growth rate correlation for out-of-pile annealing tests and a similar correlation for in-reactor tests. The correlation for in-reactor tests is considerably different from that of out-of-pile tests because it contains factors that amplify diffusion kinetics by fission damage in the diffusion reaction zone. This irradiation enhancement was formulated by a combination of the fission rate in the fuel and fission fragment damage distribution in the diffusion reaction zone. Using a computer code, fission damage factors were obtained as a function of diffusion reaction layer thickness and composition. The model correlation was established and fitted to the in-reactor data. As a result of this data fitting, the interaction layer growth rate is found to be proportional to the square root of the fission fragment damage rate and to have a temperature dependence characterized by the effective activation energy of 46 to 76 kJ/mole, which is smaller by a factor of 4 to 7 than that of out-of-pile tests.

Enhanced Electrical Networks of Stretchable Conductors with Small Fraction of Carbon Nanotube/Graphene Hybrid Fillers

Carbon nanotubes (CNTs) and graphene are known to be good conductive fillers due to their favorable electrical properties and high aspect ratios and have been investigated for application as stretchable composite conductors. A stretchable conducting nanocomposite should have a small fraction of conductive filler material to maintain stretchability. Here we demonstrate enhanced electrical networks of nanocomposites via the use of a CNT-graphene hybrid system using a small mass fraction of conductive filler. The CNT-graphene hybrid system exhibits synergistic effects that prevent agglomeration of CNTs and graphene restacking and reduce contact resistance by formation of 1D(CNT)-2D(graphene) interconnection. These effects resulted in nanocomposite materials formed of multiwalled carbon nanotubes (MWCNTs), thermally reduced graphene (TRG), and polydimethylsiloxane (PDMS), which had a higher electrical conductivity compared with MWCNT/PDMS or TRG/PDMS nanocomposites until specific fraction that is sufficient to form electrical network among conductive fillers. These nanocomposite materials maintained their electrical conductivity when 60% strained.