• Journal of the Korean Ceramic Society
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• v.23 no.1
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• pp.33-43
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• 1986

The synthesis of high-purity AlN by a vapor-phase reaction was investigated using the $NH_3-AlCl_3-H_2$ reacting system. The theoretical yields of AlN were determined from th thermodynamic equilibrium composi-tions. It was shown that the yields above 90% could by obtained even in the range of relatively low temper-ature of 600-1200K. The reaction temperature and the initial amounts/ratios of the reacting gases showed significant effects on the yields but the total pressure did not. The experimental results showed that a high-purity AlN having a needle shape was the only product as a solid phase and its amount produced increased with the reaction temperature. While the degree of agglmera-tion of the synthesized AlN increased with the reaction temperature the size of each particle consisting of the agglomerates was independent of the temperature but grew from 0.09 to 0.115${\mu}{\textrm}{m}$ with the flow rate of NH3. These experimental results were compared with the theoretical aspects for the synthesis of a high-purity AlN.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.5
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• pp.501-509
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• 2004

Unipolar diffusion charging of non-spherical particles was investigated for various particle shapes. We researched with TiO$_2$agglomerates produced by the thermal decomposition of titanium tetraisopropoxide (TTIP) vapor. TTIP was converted into TiO$_2$, in the furnace reactor and was subsequently introduced into the sintering furnace. Increasing the temperature in the sintering furnace, aggregates were restructured into higher fractal dimensions. The aggregates were classified according to their mobility using a differential mobility analyzer. The projection area and the mass fractal dimension of particles were measured with an image processing technique performed by using transmission electron microscope (TEM) photograph. The selected aggregates were charged by the indirect photoelectric-charger and the average number of charges per particle was measured by an aerosol electrometer and a condensation particle counter. For the particles of same mobility diameter, our results showed that the particle charge quantity decreases as the sintering temperature increases. This result is understandable because particles with lower fractal dimension have larger capacitance and geometric surface area.

• Journal of Korean Society for Atmospheric Environment
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• v.27 no.6
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• pp.681-691
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• 2011

The size distributions of smoke particles from fire are prerequisite for the studies on fire detection and adverse health effects. Above the flame of the fire, coagulation dominates and the smoke particles grow from 1 to 50 nm up to 100 to 3,000 nm, sizes ranging from the free-molecular regime to the continuum regime. The characteristics of the agglomeration of the smoke particles are well known, independently for each of the free-molecular and continuum regimes. However, there are not many systematic studies in the entire regime by the complexity of the mechanisms. The purpose of this work is to find the characteristics of the development of the size distribution of smoke particles by agglomeration in the entire size range covering the free-molecular regime, via transition regime, to the near-continuum and continuum regime for each variation of parameters such as fractal dimension, primary particle size and dimensionless coagulation time. In this work, the dynamic equation for the discrete-size spectrum of the particles was solved using a nodal method based on the modification of a sectional method. In the calculation, the collision frequency function for the entire regime, which is derived by using the concept of collision volume and general enhancement function, was applied. The self-preserving size distribution for the entire regime is compared with the ones for the free-molecular or continuum regimes for each variation of the parameters.

• Journal of Powder Materials
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• v.14 no.2 s.61
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• pp.145-149
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• 2007

Nanocrystalline transient aluminas (${\gamma}$-alumina) were coated on core particles (${\gamma}$-alumina) by a carbonate precipitation and thermal-assisted combustion, which is environmentally friend. The ammonium aluminum carbonate hydroxide (AACH) as a precursor for coating of transient aluminas was produced from precipitation reaction of ammonium aluminum sulfate and ammonium hydrogen carbonate. The crystalline size and morphology of the synthetic, AACH, were greatly dependent on pH and temperature. AACH with a size of 5 nm was coated on the core alumina particle at pH 9. whereas rod shape and large agglomerates were coated at pH 8 and 11, respectively. The AACH was tightly bonded coated on the core particle due to formation of surface complexes by the adsorption of carbonates, hydroxyl and ammonia groups on the surface of the core alumina powder. The synthetic precursor successfully converted to amorphous- and ${\gamma}$-alumina phase at low temperature through decomposition of surface complexes and thermal-assisted phase transformation.

• Journal of Powder Materials
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• v.14 no.5
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• pp.287-291
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• 2007

Oxidation behavior and microstructural characteristics of nano-sized Sn powder were studied. DTA-TG analysis showed that the Sn powder exhibited an endothermic peak at $227^{\circ}C$ and exothermic peak at $560^{\circ}C$ with an increase in weight. Based on the phase diagram consideration of Sn-O system and XRD analysis, it was interpreted that the first peak was for the melting of Sn powder and the second peak resulted from the formation of $SnO_2$ phase. Microstructural observation revealed that the $SnO_2$ powder, heated to $1000^{\circ}C$ under air atmosphere, consisted of agglomerates with large particle size due to the melting of Sn powder during heat treatment. Finally, fine $SnO_2$ powders with an average size of 50nm can be fabricated by controlled heat treatment and ultrasonic milling process.

• Macromolecular Research
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• v.10 no.4
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• pp.187-193
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• 2002

Several methods have been used to synthesize polymer-clay nanocomposites. In-situ polymerization with clay belongs to a classical way to develop nano-structured materials, while melt intercalation is being recognized as another useful approach due to its versatility and environmentally benign character. In this research, we prepared polymer-clay nanocomposites based on the poly (methyl methacrylate) and organically modified montmorillonite via two-stage sonication process. According to the unique mode of power ultrasonic wave, the sonication during processing led to enhanced breakup of the clay agglomerates and reduction in size of the dispersed phase. Optimum conditions to form stable exfoliated nanocomposites were studied for various compositions and conditions. It was found that a novel attempt carried out in this study yielded further improvement in the mechanical performance of the nanocomposites compared to those produced by the conventional melt mixing process, as revealed by DMA, XRD and TEM. And rheological properties of nanocomposites were measured by ARES. As a result, sonicated PMMA-clay nanocomposites exhibits enhanced properties such as storage modulus and thermal stability than that of neat PMMA.

• Journal of the Korean Ceramic Society
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• v.51 no.2
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• pp.57-63
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• 2014

Bottom ash can be used as pelletizing seeds in unsintered artificial lightweight aggregates, so it can be called as 'cold-bonded aggregates'. In the present study, the mechanical and chemical characteristics of bottom ash aggregates cold-bonded with fly ash were investigated. The crushing strength and the water transfer characteristic of the aggregates, which may affect the strength gain of the concrete, were evaluated. Moreover, the degree of hydration and the hydration products of the aggregates were analyzed to verify the chemical stability of the aggregates. Compared to commercialized artificial lightweight aggregates manufactured by sintering process, cold-bonded fly/bottom ash aggregates had similar levels of water transfer characteristics, while having lower strengths. The calcium hydroxide in the aggregates was almost completely consumed after 28 days moist curing.

• Bulletin of the Korean Chemical Society
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• v.30 no.2
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• pp.373-377
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• 2009

A pathway for the formation of normal $CoAl_2O_4$ particles is suggested. The optimal amount of malonic acid was determined, and the characteristics of the obtained powders were investigated. Normal $CoAl_2O_4$ powders were prepared using solutions of malonic acid and metal nitrates. X-ray diffraction, Brunauer-Emmett-Teller (BET) and scanning electron microscope (SEM) measurements, as well as Fourier transform infrared (FTIR) and ultraviolet/visible (UV-Vis) spectroscopy were carried out. Normal $CoAl_2O_4$ crystallites were formed by a solid state reaction between $CoAl_2O_4$ and amorphous aluminum oxide. The optimal molar ratio of malonic acid to the nitrate anions present in the initial solution was found to be 0.30~0.35. The particles were composed of agglomerates of primary particles. The primary particles were 40 nm in size. This size was relatively constant regardless of the preparation temperature. However, the size of the agglomerated particles increased to 220 nm with increasing temperature.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.10
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• pp.1326-1338
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• 1997

Electrical characteristics of soot particles in a LPG diffusion flame were studied for the control of soot particle coagulation. When a DC voltage was applied between two electrodes installed parallel to gas flow, ionic wind effect caused soot deposition on the cathode, implying that most of the soot particles were positively charged. Soot deposit on the cathode linearly increased and was saturated with respect to the strength of the applied voltage. The possibility of applying an AC voltage to enhance the particle coagulation was then investigated and the efficiency of the size control was checked with transmission electron microscope photographs. For the amplitude of 2 kV AC field, primary (spherical) soot particle size decreased from 30 ~ 40 nm to around 20 nm when the frequency of the applied AC voltage was 60 Hz and higher. Collisions between the soot particles in such a selected AC condition could lead to the formation of much bigger agglomerates of roughly 1-5 .mu.m in size.

• Journal of Powder Materials
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• v.8 no.4
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• pp.245-252
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• 2001

Recent remarkable progress in the semiconductor industry has promoted smaller size of semiconductor chips and increased amounts of heat generation. So, the demand for a substrate material to meet both the characteristics of thermal expansion coefficient and heat radiation has been on the increase. Under such conditions, tungsten(W)-copper(Cu) has been proposed as materials to meet both of the above characteristics. In the present study, the W-10wt.%Cu powders were synthesised by the mixing and hydrogen reduction of the starting mixture materials such as W-Cu, $W-CuCl_2$and $WO_3-CuCl_2$ in order to obtain the full densification. The W-10wt.%Cu produced by hydrogen reduction showed the higher interparticle friction than the simple mixed W-10wt%Cu because of the W agglomerates. In the dilatometric analysis the W-10wt.%Cu prepared from the $W-CuCl_2$was largely shrank by heating up $1400^{\circ}C$ at the constant heating rate of $5^{\circ}C$/min. The possibility of application of metal injection molding (MIM) was also investigated for mass production of the complex shaped W-Cu parts in semiconductor devices. The relationship between the temperature of molding die and the pressure of injection molding was analyzed and the heating up stage of 120-$290^{\circ}C$ in the debinding process was controlled for the most suitable MIM condition.