Young-Jo Park

A Survey of the HCN J = 1–0 Hyperfine Lines toward Class 0 and I Sources

The HCN 1-0 hyperfine lines have been observed toward 24 young stellar objects (YSOs) of class 0 and I. The hyperfine lines are well separated in most cases and show such rich structures as asymmetric double peaks and strong wings. We examined how their line shapes and velocity shifts vary along with their relative optical depths, and compared them with those of the CS 2-1, H2CO 212-111, and HCO+ 4-3 and 3-2 transitions previously observed by Mardones et al. and Gregersen et al. We find that all these molecular species do not always exhibit the same sense of line asymmetry, and the correlation of velocity shift is better between HCN and CS than between HCN and H2CO. The most opaque transition of HCN F = 2-1 has about the same velocity shift as that of CS despite the larger beam size of this study, which suggests that the HCN F = 2-1 line could be more sensitive to the internal motion of YSOs than CS line. Systematic changes of the velocity shift are noted for many sources as one goes from F = 0-1 to 2-1. The monotonic decrease of velocity (blueshift) is apparently more frequent. A detailed model of radiative transfer allowing line overlap of HCN is employed to L483, which shows convincing signatures of infall on a scale of ~0.1 pc. It appears that the observed line is not compatible with the standard Shu model, but is fitted with augmentations of density and infall velocity, by factors of 6 and 0.5, respectively, and with the inclusion of a diffuse, static, turbulent, and geometrically thick envelope. The distribution of hyperfine line intensity ratios for these YSOs does not accord with the LTE condition and is essentially the same as ones previously noted in cold dark clouds or small translucent cores. Although this anomaly may be explained in terms of radiative transfer effects in the cores, which are either static or under systematic motion, some of them seem to invoke the existence of a scattering envelope. It is confirmed that HCN is detected more selectively in class 0 and I sources than in starless cores or class II objects, which implies that the YSO(s) form a dense (~106 cm-3) envelope with a significant HCN abundance in a narrow time span of their evolution.

The Synthesis and Pore Property of Hydrogen Membranes Derived from Polysilazane as Inorganic Polymer

We investigated the pore properties of inorganic membranes applied for hydrogen separation industry. Inorganic membranes were derived from polysilazanes. The thermal reactions involved were studied using thermogravimetry(TG) and IR spectroscopy(FTIR) of the solids. To determine the thermal effect of pore properties, polysilazanes were pyrolysed in inert atmosphere. Pore volume and BET surface area showed the maximum value at a pyrolysis temperature of 500℃. For amorphous SiCN membrane derived from polysilazanes, selectivity of H₂/N₂ was 4.81 at 600℃.

Characterization of Pore Structures for Porous Sintered Reaction-Bonded Silicon Nitrides with Varied Pore-Former Content

The effect of pore former content on both porosity and pore structure was investigated for porous sintered reaction-bonded silicon nitrides (SRBSNs). A spherical PMMA with d 50 =8 μm was employed as a pore-former. Its amount ranged from 0 to 30 part. Porous SRBSNs were fabricated by post-sintering at various temperatures where the porosity was controlled at 12~52%. The strong tendency of increasing porosity with PMMA content and decreasing porosity with sintering temperature was observed. Measured pore-channel diameter increased (0.3→1.1 μm) with both PMMA content and sintering temperature.

Porous silica aerogel/honeycomb ceramic composites fabricated by an ultrasound stimulation process

The synthesis behavior of nanoporous hydrophobic silica aerogel in honeycomb-type ceramics was observed using TEOS and MTES. Silica aerogel in the honeycomb ceramic structure was synthesized under ultrasound stimulation. The synthesized aerogel/honeycomb ceramic composites were dried under supercritical CO2 drying conditions. The values for the line shrinkage of the wet gels during supercritical CO2 drying declined from 19% to 4% with an increase in the H2O/TEOS molar ratio from 8 to 24. Low shrinkage was a key factor in increasing the interface compatibility with the aerogel/honeycomb ceramic composites. The optimum condition of silica aerogel in the honeycomb-type ceramic structure had a TEOS:MTES: H2O:glycerol ratio equal to 1:1.2:24:0.05 (mol%).

The Effect of Processing Variables and Composition on the Nitridation Behavior of Silicon Powder Compact

The effect of compositional and processing variables on a nitriding reaction of silicon powder compact and subsequent post sintering of RBSN (Reaction-Bonded Silicon Nitride) was investigated. The addition of a nitriding agent enhanced nitridation rate substantially at low temperatures, while the formation of a liquid phase between the nitriding agent and the sintering additives at a high temperature caused a negative catalyst effect resulting in a decreased nitridation rate. A liquid phase formed by solely an additive, however, was found to have no effect on nitridation for the additive amount used in this research. The original site of a decomposing pore former was loosely filled by a reaction product (Si₃N₄), which provided a specimen with nitriding gas passage. For SRBSN (Sintered RBSN) specimens of high porosity, only a marginal dimensional change was measured after post sintering. Its engineering implication for near-net shaping ability is discussed.

Synthesis of SiAlON Ceramics with Novel Magnetic Properties

This paper presents a study on the magnetic behaviour of selected doped SiAlONs with various compositions including Y, Yb, Sm, Gd, and Er. The resulting crystalline phases were confirmed by X-ray diffraction. The magnetic hysteresis data for the samples were collected at room temperature using a vibrating sample magnetometer. The study revealed that doped SiAlONs experience an appreciable level of magnetic hysteresis. Although the parameters corresponding to hysteresis loops in doped SiAlONs are less than those of common ferrites, their magnetic properties of SiAlONs may open up new potential areas of application as the host SiAlON ceramics have excellent structural properties.

Fabrication of β-SiAlONs by a Reaction-Bonding Process Followed by Post-Sintering

A cost-effective route to synthesize β-SiAlONs from Si mixtures by reaction bonding followed by post-sintering was investigated. Three different z values, 0.45, 0.92 and 1.87, in Si 6-z Al z O z N 8-z without excess liquid phase were selected to elucidate the mechanism of SiAlON formation and densification. For RBSN (reaction-bonded silicon nitride) specimens prior to post-sintering, nitridation rates of more than 90% were achieved by multistep heating to 1400℃ in flowing 5%H₂/95%N₂; residual Si was not detected by XRD analysis. An increase in density was acquired with increasing z values in post-sintered specimens, and this tendency was explained by the presence of higher amounts of transient liquid phase at larger z values. Measured z values from the synthesized β-SiAlONs were similar to the values calculated for the starting compositions. Slight deviations in z values between measurements and calculations were rationalized by a reasonable application of the characteristics of the nitriding and post-sintering processes.

Microstructure and Thermal Conductivity of Sintered Reaction-Bonded Silicon Nitride: The Particle Size Effects of MgO Additive

The particle size effect of MgO as a sintering additive on the thermal conductivity of sintered reaction-bonded silicon nitride (SRBSN) was investigated. It was revealed that the size of MgO is critical for thermal conductivity with regard to the microstructural evolution process. That is, the abnormal grain growth promoted by an inhomogeneous liquid-phase distribution led to higher thermal conductivity when coarser MgO was added, whereas a relatively homogeneous liquid-phase distribution induced moderate grain growth and lower thermal conductivity when finer MgO was added.

The Synthesis of Hydrophobic Silica Aerogel in the Macroporous Ceramic Structure by Ambient Drying Process

The synthesis behavior of nanoporous silica aerogel in the macroporous ceramic structure was observed using TEOS as a source material and glycerol as a dry control chemical additive (DCCA). Silica aerogel in the macroporous ceramic structure was synthesized via sono-gel process using hexamethyldiazane (HMDS) as a modification agent and n-hexane as a main solvent. The wet gel with a modified surface was dried at under ambient pressure. The addition of glycerol appears to give the wet gel a more homogeneous microstructure. However, glycerol also retarded the rate of surface modification and solvent exchange. Silica aerogel completely filled the macroporous ceramic structure without defect in the condition of surface modification (20% HMDS/nhexane at 36hr).

The Effect of the Sintering Additives on the Fabrication and Thermal Conductivity of Porous Sintered RBSN

The nitriding and post-sintering behavior of silicon powder compact containing sintering additives of 2.3 wt% and 7 wt% were investigated in this study. Regardless of the liquid phase content, elongated large grains of a typical morphology evolved in the post-sintered specimens. Phase analysis revealed a complete phase transformation into β-Si₃N₄ in both porous systems. Oxynitride second phases (mellilite) precipitated in the latter, while those were free in the former containing less amount of liquid phase. The post-sintering condition that yielded a favorable microstructure for a filter application was achieved when the specimens were soaked at 1800℃ for 2 h. It was found that the thermal conductivity of porous Si₃N₄ ceramics is dominated by the porosity more than this factor is influenced by the addition of Al₂O₃.