• Journal of the Korean Ceramic Society
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• v.40 no.12
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• pp.1208-1212
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• 2003

CaCO$_3$ powders were synthesized by aqueous solution reaction of CaC1$_2$ㆍ2$H_2O$-(NH$_4$)$_2$CO$_3$ system with NH$_4$OH at 45$^{\circ}C$ and pHs 8, 9, 10, and 11 and in the concentration range of 0.1∼5 M and its polymorphism, morphology and size were investigated. In order to investigate the influence of pH on nucleation, pH was adjusted before and after reaction respectively. When pH was adjusted after reaction a formation ratio of vaterite was increased with increasing pH and concentration but vaterite was formed with calcite. But, when pH was adjusted before reaction, the formation rate of vaterite was increased with increasing pH and concentration. resulting in a phase-pure vaterite with a spherical shape and 2∼5 $\mu\textrm{m}$ in size. It was found that solubility of alkaline vaterite was decreased with increasing OH- ions in the high pH solution. When pH was adjusted before nucleation in the high concentration range, in particular, decreasing of solubility disturbed transformation of initially formed numerous vaterite to calcite.

• 한국신재생에너지학회:학술대회논문집
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• 2006.06a
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• pp.425-428
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• 2006

Natural gas hydrate has been a major problem for its plugging nature in the pipeline. With the demand of deep-water production, the importance of flow assurance technology, preventing hydrate, asphaltene and wax in the pipeline becomes bigger Kinetic models combined with the flow simulator are being developed to explain the nature of hydrate plug formation in the pipeline. To simulate the hydrate plug formation, each stage including the nucleation, growth and agglomeration should be considered. The hydrate nucleation is known to be stochastic and is believed hard to be predicted. Recent publications showed hydrate growth and agglomeration can be observed rigorously using a particle size analyzer. However properties of the hydrate should be investigated to model the growth and agglomeration. The attractive force between hydrate particles, supposed to be the capillary force, was revealed to be stochastic. Alternative way to model the hydrate agglomeration is to simulate by the discrete element method. Those parameters, particle size distribution, attractive force, and growth rate are embedded into the kinetic model which is combined Into the flow simulator. When compared with the flowloop experimental data, hydrate kinetic model combined into a flow simulator showed good results. With the early results, the hydrate kinetic model is promising but needs more efforts to improve it.

• Journal of the Korean Ceramic Society
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• v.28 no.2
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• pp.167-175
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• 1991

In synthesizing hydrated zirconia powder by hydrolysis of Zr-alkoxides using ethanol as mutual solvent, three experimental parameters, namely, concentration of alkoxides and hydrolysis water and addition rate of hydrolysis water were varied systematically. Spherical, monodispersed, nonagglomerated and submicrometer sized powders were prepared at 0.3 M of Zr(n-OPr)4 and 0.05M of Zr(n-OBu)4 with wide ranges of hydrolysis water conditions i.e. 0.5-2.0M concentration and 1-20ml/min addition rate. During the hydrolsis, careful attention have to be paid to maintain homogeneous reaction by controlling the agitation of the reactant and the addition of the hydrolysis water. For more improved condition of monodispersity it was found that the key point is to shorten the self-nucleation time within several seconds as rapid as possible. In both alkoxides system, with higher concentration of alkoxide and hydrolysis water and with slow addition rate of hydrolysis water, hydrated zirconia powders synthesized showed tendency to fall in worse powder conditions.

• Journal of Korea Foundry Society
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• v.5 no.3
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• pp.19-26
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• 1985

This study has been carried out to examine into wettability of Cu-5%Sn alloy in $Al_2O_3$, MgO, $SiO_2$ and graphite, respectively and investigated into the change in macrostructure of Cu-5%Sn alloy according to kind and mixing rate of mold-coating. The results obtained from the experiment are summerized as follows; 1. Cu Cu-5%Sn alloy, wettabilities of $Al_2O_3$ and MgO were good, on the other hand, wettabilities of $SiO_2$ and graphite were bad. 2. The fine equiaxed zone was created because of the role of $Al_2O_3$ and MgO as preferential nucleation sites. 3. Notwithstanding change of mixing rate of $SiO_2$ in mold coating the equixed zone was not created. 4. The area of equiaxed zone was varied according to mixing rate in the case of using $Al_2O_3$ and MgO in mold-coating.

• Applied Microscopy
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• v.44 no.3
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• pp.105-109
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• 2014

The microstructure of as-quenched $Al_{70}Cu_{18}Mg_{12}$ alloy has been investigated in detail using transmission electron microscopy. Al nano-crystals about 5 nm with a high density are distributed in the amorphous matrix, indicating amorphous matrix nano-composite can be synthesized in Al-Cu-Mg alloy. The high density of Al nano-crystals indicates very high nucleation rate and sluggish growth rate during crystallization possibly due to limited diffusion rate of solute atoms of Cu and Mg during solute partitioning. The result of hardness measurement shows that the mechanical properties can be improved by designing a nano-composite structure where nanometer scale crystals are embedded in the amorphous matrix.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 1998.10b
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• pp.10-10
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• 1998

The growth process of solid grains in a liquid matrix is usually explained in tem1S of Ostwald ripening. The variation of growth (dissolution) rate as a function of grain size during Ostwald ripening predicted that the dissolution rate becomes very large as grain size decreases but the growth rate of a large grain is rather limited. Therefore. a rather uniform size distribution of grain size is maintained once after the quasi-equilibrium state is reached. Quite frequently, however, the exaggerated grain growth (EGG) is observed to occur: only a limited number of grains grow exceptionally. From the observation that the EGG occurs only for the faceted grains with apparently straight solid-liquid interfaces, the EGG is suggested to be the consequence of growth process controlled by 2-dimensional nucleation. In this study, the result by computer calculation on the grain growth process controlled by various mechanisms will be given.

• 전기의세계
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• v.28 no.5
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• pp.39-43
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• 1979

The aging of permittivity of a barium titanate dielectrics doped with La$_{2}$O$_{3}$ under zero and low DC field has been studied. The aging rate was decreased as the amount of La$_{2}$O$_{3}$ addition is increased to 3 mole%. The zero field aging rate of barium titahate doped with La$_{2}$O$_{3}$. 3TiO$_{2}$ was similar to that doped with La$_{2}$O$_{3}$.5V/mil DC field aging rate of La$_{2}$O$_{3}$.3TiO$_{2}$ doped sample, howeve, was lowered to that of La$_{2}$O$_{3}$ doped sample. When the phase transformation is occured from the paraelectric state to the ferroelectric state, 90.deg. domains are mucleated in order that the system becomes thermodynamically more stable. It is concluded that the aging phenomenon is occured as the dielectric constant is decreasing by the nucleation and growth of 90.deg. domains.

• Journal of Korea Foundry Society
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• v.5 no.3
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• pp.13-18
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• 1985

Decarburization phenomena were investigated at $800^{\circ}C$ by the $PH_2O/PH_2$ + Ar gas mixture in the case iron range which contains Si, Mn and Cr as an alloying elements. Dissociation of cementite in a matrix which contains graphitizer as Si begins at the carbon rich cementite dendrite arms. Several primary austenite $({\gamma})$ skeletons are surrounded by those nucleated graphite nodules, and that forms a limited area of nucleation region. Decarburization reactions at $800^{\circ}C$ in Fe-C, Fe-Mn-C and Fe-Cr-C alloy are followed by parabolic rate law under the gas mixture of $PH_2O/PH_2=0.01$ and the modified rate const. ${\kappa}$ were in the range of $1{\sim}6{\times}10^{-10}cm^2/s$.

• Steel and Composite Structures
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• v.47 no.3
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• pp.355-363
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• 2023

The main purpose of this study is to characterize the size-dependent strain rate sensitivity in structural steel using the continue stiffness measurement (CSM) indentation. A series of experiments, such as CSM indentation and optical microscope examination, has been performed at the room temperature at different rate conditions. The results indicated that indentation hardness, strain rate, and flow stress showed size-dependent behavior. The dependency of indentation hardness, strain rate, and flow stress on the indentation size was attributed to the transition of the dislocation nucleation rate and the dislocation behaviors during the indentation process. Since both hardness and strain rate showed the size-dependent behavior, SRS tended to depend on the indentation depth. The results indicated that the SRS was quite high over 2.0 at the indentation depth of 240 nm and quickly dropping to 0.08, finally around 0.046 at large indents. The SRS values at large indentations strongly agree with the general range reported for several types of low-carbon steel in the literature (Chatfield and Rote 1974, Nguyen et al. 2018b, Luecke et al. 2005). The results from the present study can be used in both static and dynamic analyses of structures as well as to assess and understand the deformation mechanism and the stress-state of material underneath the indenter tip during the process of the indentation testing.

• Applied Chemistry for Engineering
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• v.9 no.1
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• pp.38-43
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• 1998

It was realized that the nucleation rate in the synthesis of M-ZSM-5 using various alkali cations such as $Li^+$, $Na^+$, $K^+$ and $Cs^+$ at low temperature and atmospheric pressure was decreased in the order of $Na^+>K^+>Li^+>Cs^+$. Unlike conventional synthesis method at high temperature and pressure, the results showed that at low temperature and atmospheric pressure, the higher the nucleation rate is, the larger the crystal size of M-ZSM-5 obtained ; that is, the crystal size in the order of $K^+>Na^+>Cs^+>Li^+$. In addition, it also suggests that regardless of alkali cations to be used, the current synthesis method can produce M-ZSM-5 with BET surface area greater than $300m^2/g$ within 52hrs. of reaction time, in particular greater than $400m^2/g$ within 32hrs, for $Na^+$ cation.