Catalytic growth of semiconductor micro- and nano-crystals using transition metal catalysts

The catalytic reaction concept was introduced in the growth of semiconductor micro- and nano-crystals. It was found that gallium nitride (GaN) micro- and nano-crystal structures, carbon nanaotubes, and silicon carbide (SiC) nanostructures could be efficiently grown using transition metal catalysts. The use of Ni catalyst enhanced the growth rate and crystallinity of GaN micro-crystals. At 1,100 ‡C, the growth rate of GaN micro-crystals grown in the presence of Ni catalyst was over nine times higher than that in the absence of the catalyst. The crystal quality of the GaN microcrystals was almost comparable to that of bulk GaN. Good quality GaN nanowires was also grown over Ni catalyst loaded on Si wafer. The nanowires had 6H hexagonal structure and their diameter was in the range of 30–50 nm. Multiwall nanotubes (MWNTs) were grown over 20Fe : 20Ni : 60Al₂O₃ catalyst. However, single wall nanotubes (SWNTs) were grown over 15Co : 15Mo : 70MgO catalyst. This result showed that the structure of CNTs could be controlled by the selection of catalysts. The average diameters of MWNTs and SWNTs were 20 and 10 nm, respectively. SiC nanorod crystals were prepared by the reaction of catalytically grown CNTs with tetrametysilane. Structural and optical properties of the catalytically grown semiconductor micro- and nano-crystals were characterized using various analytic techniques. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Microstructure, electrical properties, and degradation behavior of praseodymium oxides-based zinc oxide varistors doped with Y2O3

The microstructure, electrical properties, and degradation behavior of Pr-based zinc oxide varistors, which are composed of Zn-Pr-Co-Cr-Y oxides were investigated according to Y₂O₃ additive content in the range 0.5–4.0 mol%. The majority of the Sadded Y₂O₃ were segregated at the multiple ZnO grain junctions and grain boundaries. The average grain size was markedly decreased in the range 27.3–8.6 m with increasing Y₂O₃ additive content. Y₂O₃ acted as an inhibitor of grain growth. Additions of Y₂O₃ increased the varistor voltage in the range 36.90–686.58 V/mm, increased the nonlinear exponent in the range 3.75–87.42, decreased the leakage current in the range 115.48–0.047A, increased the barrier height in the range 1.06–2.16 eV, and decreased the donor concentration in the rang 1.87 × 10¹⁸–0.19 × 10¹⁸ cm^–3. Y₂O₃ acted as an acceptor, as a result of the decrease of donor concentration. All Pr-based ZnO varistors doped with Y₂O₃ exhibited very predominant degradation characteristics, which show a nearly symmetric I-V after the stress. In particular, since 4.0 mol% Y₂O₃-added ZnO varistor has not only very excellent non-ohmicity, but also very stable degradation behavior, it is estimated to be sufficiently used to various application fields.     

Anomalous Defect Dependence of Thermal Conductivity in Epitaxial WO3 Thin Films

Lattice defects typically reduce lattice thermal conductivity, which has been widely exploited in applications such as thermoelectric energy conversion. Here, an anomalous dependence of the lattice thermal conductivity on point defects is demonstrated in epitaxial WO₃ thin films. Depending on the substrate, the lattice of epitaxial WO₃ expands or contracts as protons are intercalated by electrolyte gating or oxygen vacancies are introduced by adjusting growth conditions. Surprisingly, the observed lattice volume, instead of the defect concentration, plays the dominant role in determining the thermal conductivity. In particular, the thermal conductivity increases significantly with proton intercalation, which is contrary to the expectation that point defects typically lower the lattice thermal conductivity. The thermal conductivity can be dynamically varied by a factor of ≈ 1.7 via electrolyte gating, and tuned over a larger range, from 7.8 to 1.1 W m^?1 K^?1, by adjusting the oxygen pressure during film growth. The electrolyte‐gating‐induced changes in thermal conductivity and lattice dimensions are reversible through multiple cycles. These findings not only expand the basic understanding of thermal transport in complex oxides, but also provide a path to dynamically control the thermal conductivity.     

The characteristics of pulverized coal combustion in the two stage cyclone combustor

Numerical investigations on air staging and fuel staging were carried out with a newly designed coaxial cyclone combustor, which uses the method of two stage coal combustion composed of pre-combustor and main combustor. The pre-combustor with a high air/fuel ratio is designed to supply gas at high temperature to the main combustor. To avoid local high temperature region in this process, secondary air is injected in the downstream. Together with the burned gas supplied from the pre-combustor and the preheated air directly injected into main combustor, coals supplied through the main burner react rapidly at a low air/fuel ratio. Strong swirling motion of cyclone combustor keeps the wall temperature high, which makes slagging combustion possible. Alaska, US coal is used for calculations. Predictions were made for various coal flow rates in the main combustor for fuel staging and for the various flow rate of secondary air in the pre-combustor for air staging. In-scattering angles are also chosen as a variable to increase residence times of coal particles. Temperature fields and particle trajectories for various conditions are described. Predicted temperature variations at the wall of the combustor are compared with corresponding experimental data and show a similar trend. The in-scattering angle of 20° is recommended to increase the combustion efficiency in the main chamber.     

Synthesis of carbon-nitrogen nanostructures by hot isostatic pressure apparatus and their field emission properties

Carbon-nitrogen (CN) nanofibers were synthesized in argon-nitrogen gas mixture at 75 MPa by high isostatic pressure (HIP) apparatus using a graphite resistive heater. The CN nanofibers were grown in random with the diameter of about 200 nm and the length over 5 microm. The structures obtained can be divided bamboo-like, spring-like, and bead necklace-like CN nanofibers. The nitrogen content of up to 8.4% was found in CN nanofibers by EELS analysis. Field emission results showed that the density of field emitters and the field enhancement factors changed by surface treatments and that CN nanofibers contained glass frit. The screen-printed CN nanofiber had a turn-on field of 2 V/microm.     

Optical study of Mn-doped Bi4Ti3O,12 thin films by spectroscopic ellipsometry

Mn-doped Bi4Ti3O12(B4T3) thin films grown at 400 degrees C on a Pt/Ti/SiO2/Si substrate through pulsed laser deposition (PLD) were analyzed via spectroscopic ellipsometry (SE). The PLD targets were produced through the conventional solid-state sintering method, and the film samples were annealed at 600 degrees C. The SE spectra of B4T3 films were measured using a rotating analyzer type ellipsometer within the 1.12 to 6.52 eV energy range, with the various incidence angles. The optical properties of the B4T3 films with increasing Mn-mol concentration were extracted using a multilayer model for the whole structure and the Tauc-Lorentz (TL) dispersion relation for the B4T3 film layer. The analysis results clearly showed that the significant changes in optical properties of B4T3 films are caused by thermal annealing procedure and the Mn-mol concentrations. X-ray diffraction (XRD) measurement was also performed to confirm the results of SE analysis.     

An experimental study for qualifying hydrogen compatibility of austenitic stainless steel under low temperature

Journal of Mechanical Science and Technology - Hydrogen compatibility of materials refers to the ability to exhibit reliable mechanical integrity and a probability of failure in a given...