Kon-Bae Lee

A cost-effective route to produce Al/AlN composites with low coefficient of thermal expansion

In this study, a novel process was developed to produce Al/AlN composites by infiltrating molten Al ingot into a mixture of Al and lamp carbon powders. The findings revealed that, during the process, Al2O3 on the surface of Al powder reacted with nitrogen gas and was transformed to AlN. The degree of nitridation was greatly enhanced by adding only less than 3 wt.% lamp carbon, because lamp carbon could act as a dispersion agent as well as a reduction agent. The Al-based composites containing in situ AlN phases showed coefficient of thermal expansion values of ∼11 × 10−6/℃, which were comparable to those of composites containing 60% ex situ AlN particles.

Effects of a Ni alloying element on Al?Ni metallization

The effect of the Ni content (2?18 at.% Ni) in Al thin films on their resistivity, hillock formation and Al3Ni compound formation was investigated. The as-deposited Al?Ni-alloy films showed high elastic strains which increased with increasing Ni content. In addition, the annealing of the supersaturated Al?Ni-alloy thin films yielded two phases: Al3Ni and Al with strong (2?2?0) and (1?1?1) textures, respectively, suggesting that the nucleation of (2?2?0) Al3Ni is closely associated with (1?1?1) Al. The resistivity of the as-annealed Al?Ni-alloy films varied as functions of the volume fraction and grain size of the two phases, which were determined by the Ni content and annealing temperature, respectively. The hillock formation was effectively suppressed when a small amount of Ni was added to the Al alloy. The results showed that a Ni content of less than approximately 4.5 at.% produced hillock-free Al-alloy thin films with a low resistivity of less than 6.0 ?? cm upon annealing at 350 ?C.

Nitridation-assisted Al infiltration for fabricating Al composites

In this study, we developed an Mg-, vacuum-, and pressure-free process for manufacturing Al-based composites via nitridation-assisted spontaneous infiltration of molten Al. In this process, the wettability between molten Al and ceramic reinforcement was significantly enhanced by nitridation via heating and surface modification of the Al particles; this provided a sufficient capillary force (i.e., negative curvature) for spontaneous infiltration of molten Al. The proposed process can generate composites based on Mg-free Al matrices and is relatively versatile in terms of the type, volume fraction, and morphology of the reinforcement material. Furthermore, the properties of the final composites can be modified by controlling their degree of nitridation, thereby expanding the material scope and their corresponding application fields.

Effect of magnesium on nitridation and infiltration of aluminum powder

We investigated the effect of Mg on the nitridation behavior of Al particles during thermal treatment for a range of Mg contents and temperatures. The addition of Mg stimulates the initiation of nitridation because it diffuses and forms an Al-Mg alloy with a lower melting point than that of pure Al. In the initial stages of the nitridation, nitrides without a perfect crystallographic structure are formed on the surface of Al particles and then AlN with a superlattice structure is formed within the reaction product. Since the Al-Mg powder bed is rapidly densified during heat treatment compared to its pure Al counterpart, the pathways in it for nitrogen diffusion are limited in comparison and further nitridation is much suppressed, resulting in much lower total nitridation compared to a pure Al powder bed. This behavior is observed regardless of the Mg content or the nitridation temperature (up to 900 °C), demonstrated in experiment for Al powder beds containing <5 wt% Mg.

Effects of Magnesium Catalyst on the Nitridation of Aluminum Melt in the Synthesis of Aluminum Nitride Powder

Aluminum nitride (AlN) powder was easily synthesized by the direct nitridation of Al melt containing ~20 wt.% Mg catalyst and the nitriding behavior was investigated by thermodynamic calculation and through observations of electron microscopy and X-ray diffraction. The addition of Mg catalyst decreased the nitriding temperature below , which is comparable to the high nitriding temperature of required in carbothermal method. It was caused by a significant increase of the solubility of nitrogen gas due to the increase of Mg catalyst in Al melt. The dissolved nitrogen gas met Mg catalyst and was transformed into metastable . Finally the metastable phase reacted with Al to AlN.

IN-SITU TEM INVESTIGATION ON NUCLEATION AND GROWTH BEHAVIOR OF P-DOPED Si THIN FILMS

The nucleation and growth behaviors of undoped and phosphorus doped polycrystalline Si thin films were investigated by in-situ TEM observations. Polycrystalline Si thin films were partially changed to amorphous by ion implantations. A normal grain growth was observed in the undoped Si thin films during heating. On the other hand, the P-doped sample showed the recovery and growth at grain boundary as well as the nucleation of Si nanocrystals at amorphous regions. Although the amorphous hindered the grain growth and acted as the nucleation source of Si nanocrystals at lower temperature, the final grain size of polycrystalline Si at 650°C was larger in the P-doped sample. The carrier mobility of the P-doped Si thin films not only increased with heat treatments, but also was corresponding to the microstructural evolution.

Applications of Nanomanipulator in Nanowires

The combination of focused ion beam (FIB) and 4 point probe nanomanipulator could make various nano manufacturing and electrical measurements possible. In this study, we manufactured individual ZnO nanowire devices and measured those electrical properties. In addition, tensile experiments of metallic Au and Pd nanowires was performed by the same directional alignment of two nanomanipulators and a nanowire. It was confirmed from I-V curves that Ohmic contact is formed between electrodes and nanomanipulators, which is able to directly measure the elelctrical properties of a nanowire itself. In the mechanical tensile test, Au and Pd nanowires showed a totally different fracture behavior except the realignment from to . The deformation until the fracture was governed by twin for Au and by slip for Pd nanowires, respectively. The crystallographic relationship and fracture mechanism was discussed by TEM observations.