Seung Zeon Han

Bonding structure and optical bandgap of rf sputtered hydrogenated amorphous silicon carbide alloy films

Abstract Hydrogenated amorphous silicon carbide (a-Si1−xCx:H) alloy filmswere deposited by radio frequency (rf) magnetron sputtering at room temperature with variable rf powers. Effects of deposition parameters on the bonding structure and optical properties of the films were measured. Infrared (IR) absorption and optical transmittance measurements of films deposited under methane gas flow rates of 2 and 3 sccm showed that they differed. In the case of 2 sccm flow rate, the optical bandgaps gradually decreased with increasing rf power and slopes, B, of the Tauc plot increased up to a power of 5 W/cm2. In the case of 3 sccm flow rate, optical bandgaps increased and the slopes were invariant with power. This difference is attributed to the differences in the bond structure of the films. Under the condition of flow methane gas flow rate as in the former case, the Siue5f8C bonding concentration is saturated and those of Siue5f8Hn bonds still increase while those of Cue5f8Hn bonds decrease with power. However in the latter, both concentrations of Siue5f8C and Siue5f8Hn bonds increase and those of Cue5f8Hn bonds decrease. As a result, optical bandgaps of rf sputtered films which have a high fraction of carbon, greater than 0.65, are more influenced by the Siue5f8C bonds than by the hydrogen-related bonds.

Aging characteristics of thermomechanically processed Cu-9Ni-6Sn alloy

Cu-Ni-Sn alloys have gathered a considerable amount of interests over the past 20 years, since the strength level of Cu-Ni-Sn alloys with a proper thermomechanical processing equals to those obtained by high-strength, precipitation-hardened Cu-Be alloys. In the present study, aging characteristics of heavily swaged Cu-9Ni-6Sn alloy, the composition of which is known to have the best combination of mechanical and physical properties, were examined. The hardening behaviors of cu-9Ni-6Sn alloys, either solutionized and aged or directly aged, are discussed with SEM fractographic studies and TEM micrographic observations.

Coarsening behavior of L12 precipitates in melt-spun AlTiVZr alloys

Aging studies of two melt-spun Al-2at.% (Ti,V,Zr) alloys showed that the would-be metastable L12 Al3 (Ti,V,Zr) precipitates did not transform to stable D023 ones, and the average radius is 5–7 nm and the interparticle spacing is 20–40 nm at 698 K up to 400 h. The coarsening rate of spherical Al3(ti0.2V0.4Zr0.4) precipitates was observed to be five times as fast as that of Al3(Ti0.1V0.4Zr0.5) precipitates. The coarsening behavior in both alloys obeyed the Lifshitz-Slyozov-Wagner (LSW) prediction well. Owing to the low coarsening rate and the high thermal stability of the precipitated phase, Alue5f8Tiue5f8Vue5f8Zr systems show promise as bases for high-temperature high-strength Al alloys.

Design of exceptionally strong and conductive Cu alloys beyond the conventional speculation via the interfacial energy-controlled dispersion of γ-Al2O3 nanoparticles

The development of Cu-based alloys with high-mechanical properties (strength, ductility) and electrical conductivity plays a key role over a wide range of industrial applications. Successful design of the materials, however, has been rare due to the improvement of mutually exclusive properties as conventionally speculated. In this paper, we demonstrate that these contradictory material properties can be improved simultaneously if the interfacial energies of heterogeneous interfaces are carefully controlled. We uniformly disperse γ-Al2O3 nanoparticles over Cu matrix, and then we controlled atomic level morphology of the interface γ-Al2O3//Cu by adding Ti solutes. It is shown that the Ti dramatically drives the interfacial phase transformation from very irregular to homogeneous spherical morphologies resulting in substantial enhancement of the mechanical property of Cu matrix. Furthermore, the Ti removes impurities (O and Al) in the Cu matrix by forming oxides leading to recovery of the electrical conductivity of pure Cu. We validate experimental results using TEM and EDX combined with first-principles density functional theory (DFT) calculations, which all consistently poise that our materials are suitable for industrial applications.

Phase equilibria of Al3(Ti,V,Zr) intermetallic system

Abstract In Al,(Ti,V,Zr) alloy systems investigated in this work, both tetragonal DO,, and DO,, phases were found to form. The site fractions (solubilities) of transition elements Ti, V and Zr in Al,(Ti,V,Zr) phase were measured. These data were used to optimize and assess thermodynamic parameters which were needed to describe equilibria conditions of Al,(Ti,V,Zr) compounds. Isothermal phase diagrams of the Al,Ti-A&V-Al,Zr system were calculated at 1173,1273 and 1373°K and they were compared with experimental measurements. There was a good agreement.

Alloy design and coarsening phenomenon of L12 precipitates in high-temperature Al-2 At. Pct (Ti,V,Zr) systems

Aging works of two melt-spun Al-2 at pct (Ti,V,Zr) alloys showed that metastable L12 Al3(Ti,V,Zr) precipitates were dominant and did not transform to stable D023 ones: the average radius was 3 to 4 nm and the interparticle spacing was 10 to 30 nm at 698 K up to 400 hours. Coarsening kinetics was found to be very sluggish and was coincident with the low lattice mismatch. Due to the low coarsening rate and the high thermal stability of the precipitated phase, rapidly so-lidified Al-Ti-V-Zr systems show promise as base of high-strength Al alloys for high-temperature applications.

Effects of Ti addition and heat treatments on mechanical and electrical properties of Cu-Ni-Si alloys

The mechanical and electrical properties of Cu-5.98Ni-1.43Si and Cu-5.98Ni-1.29Si-0.24Ti alloys under heat treatment at 400 and 500 °C after hot- and cold-rolling were investigated, and a microstructural analysis using transmission electron microscopy was performed. Cu-5.98Ni-1.29Si-0.24Ti alloy displayed the combined Vickers hardness/electrical conductivity value of 315.9 Hv/57.1%IACS. This was attributed to a decrease of the solution solubility of Ni and Si in the Cu matrix by the formation of smaller and denser δ-Ni2Si precipitates. Meanwhile, the alloyed Ti was detected in the coarse Ni-Si-Ti phase particles, along with other large Ni-Si phase particles, in Cu-5.98Ni-1.29Si-0.24Ti.

Effect of alloyed Ti:Zr ratio on phase stability of Al66Mn9(Ti, Zr)25 intermetallic compounds

The tetragonal Al3(Ti1 − xZrx) phase is transformed by the addition of 9% Mn to the cubic L12-type Al66Mn9(Ti1 − xZrx)25 with a small amount of second phases such as Al8Mn5 and Al2Zr still existing in the as-cast condition. Second phases disappear when intermetallic compounds are homogenized at 1000 °C for 24 h, and alloys are of the monolithic L12 phase with compositional variations still existent. The combined addition of Ti and Zr is found to reduce the hardness of intermetallic alloys compared to the addition of Ti or Zr alone. The Al66Mn9(Ti, Zr)25 phase is found to be structurally stable but compositionally unstable.

Coarsening phenomenon of Li2 precipitates in rapidly solidified Al-3at%(Ti,V,Zr) system

It is of interest how to design the Al-Ti-V-Zr system and optimize the compositions of added transition elements to minimize the lattice misfit, and thus an interfacial energy, and maximize the coarsening resistance and, at the same time, optimize the volume fraction of the precipitated stable D0{sub 23} Al{sub 3}M phase. These all can be beneficial to the high temperature strength. The coarsening kinetics of the L1{sub 2} precipitate in the present study were very slow. Furthermore, the transformation of the L1{sub 2} phase to the D0{sub 23} phase was not observed. The free energy change for the L1{sub 2}-D0{sub 23} transformation was probably reduced due to the reduction in lattice misfit and strain. In the previous works, the authors designed alloys assuming the stable DO{sub 23} phase would play an active role in the aging condition, but this prediction was not observed. Instead, L1{sub 2} precipitates were dominant. Owing to the low coarsening rate and these high thermal stability of the precipitated phases, Al-Ti-V-Zr systems show promise as bases for high-temperature high-strength Al alloys.

Reliable and cost effective design of intermetallic Ni2Si nanowires and direct characterization of its mechanical properties.

We report that a single crystal Ni2Si nanowire (NW) of intermetallic compound can be reliably designed using simple three-step processes: casting a ternary Cu-Ni-Si alloy, nucleate and growth of Ni2Si NWs as embedded in the alloy matrix via designing discontinuous precipitation (DP) of Ni2Si nanoparticles and thermal aging, and finally chemical etching to decouple the Ni2Si NWs from the alloy matrix. By direct application of uniaxial tensile tests to the Ni2Si NW we characterize its mechanical properties, which were rarely reported in previous literatures. Using integrated studies of first principles density functional theory (DFT) calculations, high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDX) we accurately validate the experimental measurements. Our results indicate that our simple three-step method enables to design brittle Ni2Si NW with high tensile strength of 3.0u2009GPa and elastic modulus of 60.6u2009GPa. We propose that the systematic methodology pursued in this paper significantly contributes to opening innovative processes to design various kinds of low dimensional nanomaterials leading to advancement of frontiers in nanotechnology and related industry sectors.