• Transactions of the Korean Society of Mechanical Engineers A
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• v.32 no.11
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• pp.1003-1009
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• 2008

In this study, liquid phase sintered SiC (LPS-SiC) materials were made by hot pressing method. The particle size of nano-SiC powder was 30nm. Alumina ($Al_2O_3$), yttria ($Y_2O_3$) and silica ($SiO_2$) were used for sintering additives. To investigate effects of $SiO_2$, ratios of $SiO_2$ contents were changed by five kinds. Materials have been sintered for 1 hour at $1760^{\circ}C$, $1780^{\circ}C$ and $1800^{\circ}C$ under the pressure of 20MPa. The system of sintering additives which affects a property of sintering as well as the influence depending on compositions of sintering additives were investigated by measurement of density, mechanical properties such as flexural strength, vickers hardness and sliding wear resistance were investigated to make sure of the optimum condition which is about matrix of $SiC_f$/SiC composites. The abrasion test condition apply to load of 20N at 100RPM for 20min. Sintered density, flexural strength of fabricated LPS-SiC increased with increasing the sintering temperature. And in case of LPS-SiC with low $SiO_2$, sliding wear resistance has very excellent. Monolithic SiC $1800^{\circ}C$ sintering temperatures and 3wt% have excellent wear resistance.

• Journal of the Korean Ceramic Society
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• v.46 no.2
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• pp.189-199
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• 2009

The Powder characteristics and sintering behavior of $SiO_2$ coated $BaTiO_3$ were studied. $BaTiO_3$ powders were synthesized by the liquid mix method developed by Pechini, and silica coating was prepared by alkoxide hydrolysis method with TEOS and ethanol. The particle size of the $BaTiO_3$ powders was 35 nm and the thickness of the $SiO_2$ coating layer was 5 nm. As the $SiO_2$ content increased, the $SiO_2$ layers improved the powder dispersion by increasing electrostatic repulsion between the $BaTiO_3$ particles. Effects of MgO coating on microstructure and dielectric properties of $BaTiO_3$ have been studied compared with mechanically MgO mixed $BaTiO_3$. MgO coated $BaTiO_3$ particles were prepared by a homogeneous precipitation method using $MgCl_2\cdot 6H_2O$ and urea. MgO coated $BaTiO_3$ exhibited homogeneous microstructure compared with mixed samples. XRD analysis revealed that Mg substitution for the Ti site in the MgO mixed sample was much greater than in the coated one. Electrical properties of MgO mixed and coated $BaTiO_3$ were affected by the diffusion behavior of Mg in $BaTiO_3$ lattice.

• Elastomers and Composites
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• v.45 no.2
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• pp.122-128
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• 2010

We measured shear viscosity of copoly(styrene(St)/butyl methacrylate(BMA)) (co-PSB) particles, with a capillary rheometer at $170^{\circ}C$, prepared by suspension polymerization with hydrophobic silica as a stabilizer. co-PSB particles with the weight average molecular weights of lower than 74,800 g/mol displayed a Newtonian behavior at low shear rates. With the weight average molecular weight exceeding 136,800 g/mol, co-PSB particles showed shear thinning against shear rates and the absolute value of the slopes between shear viscosity vs. shear rate increased. When the ratio between St and BMA changed from 7/3 to 5/5 to 3/7, shear viscosity and glass transition decreased despite similar molecular weights. When the ratio was 1/9, it showed a large increase in initial shear viscosity despite reduced glass transition. Shear viscosity exhibited an increase in proportion to carbon black concentration. The effect of carbon black concentration on the shear viscosity of co-PSB composites was less pronounced compared to varying molecular weights and/or compositional ratio.

• Proceedings of the Korean Geotechical Society Conference
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• 2008.10a
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• pp.1475-1486
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• 2008

Bentonite-based grouting has been popularly used to seal a borehole installed for a closed-loop vertical ground heat exchanger in a geothermal heat pump system (GHP) because its high swelling potential. However, if the bentonite-based grouting is conducted in coastal areas, the salinity of groundwater changes in the mineral fabric of bentontie. In order words, an increase of cation concentration in groundwater leads to a reduction in the diffuse double-layer thickness in the bentonite mineral structure, and thus the volume of bentointe-based grouts will decrease proportional to the salinity of groundwater. In this paper, the effect of salinity (i.e., NaCl 0.5M, 0.25M, and 0.1M) on the change of swelling potential for bentonite-based grouts has been quantitatively evaluated for seven bentonite grouts from different product sources. In addition, in case of using addictives such as a silica sand to increase the thermal conductivity of bentonite-based grouts, the possibility of particle segregation has been studied considering the viscosity of grouts and salinity of groundwater.

• Korean Chemical Engineering Research
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• v.45 no.6
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• pp.611-618
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• 2007

We demonstrate the sol-gel coating technique of colloidal clusters for producing hollow micro-particles with complex morphologies. Cross-linked amidine polystyrene (PS) microspheres were synthesized by emulsifier-free emulsion copolymerization of styrene and divinylbenzene. The amidine PS particles were self-organized inside toluene-in-water emulsion droplets to produce large quantities of colloidally stable clusters. These clusters were coated with thin silica shell by sol-gel reaction of tetraethylorthosilicate (TEOS) and ammonia, and the organic polystyrene cores were removed by calcination at high temperature to generate nonspherical hollow micro-particles with complex morphologies. This process can be used to prepare hollow particles with shapes such as doublets, tetrahedra, icosahedra, and others.

• Journal of the Korea Concrete Institute
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• v.13 no.2
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• pp.139-145
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• 2001

One of ways to make high-strength concrete is for the mix contain particles graded down to the finest size : this is achieved by the use of fly ash, silica fume which fills the spaces between the cement particle and between the aggregate and the cement particles. And, the mix needs a sufficient workability. This is achieved by the use of a superplasticizer. This study is to investigate frost resistance of high-strength concrete at early age, with ratio of tensile strength and recovery of compressive strength, when high-strength concrete is placed in cold climates. According to this study, it is necessary to ensure 4 % of air content, 5 kgf/$\textrm{cm}^2$ of tensile strength, at least, for frost resistance of high-strength concrete at early age.

• Korean Journal of Materials Research
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• v.12 no.7
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• pp.573-579
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• 2002

Stoichiometric mullite ($3Al_2$$O_3$. $2SiO_2$) precursor sol has been prepared by sol-gel method. The effects of the precursor pH and sintering temperature on the synthesizing behavior and morphology of mullite have been studied. Mullite precursor sol was prepared by dissolution of aluminum nitrate enneahydrate (Al($NO_3$)$_3$.9H$_2O) into the mixture of silica sol. Precursor pH of the sols was controlled to acidic condition ($PH\leq$ 1~1.5) and to basic condition ($pH\geq$8.5~9). The synthesized aluminosilicate sols were formed under 20 MPa pressure after drying at $150^{\circ}C$ for 24 hours, and then sintered for 3hours in the temperature range of $1100~1600^{\circ}C$. From TGA/DTA analysis, total weight loss in the aluminosilicate gel of the acidic sample was (equation omitted) 56% and that of the basic sample was (equation omitted) 85%, indicating that the synthesizing temperature of mullite phase for acidic and basic samples was above $1200^{\circ}C$ and $1300^{\circ}C$, respectively. The morphologies of the synthesized mullite were fine and needle-like (or rod-like) for acidic sample, and granular for basic sample that has been sintered above $1300^{\circ}C$. It was found that the morphology of mullite particle was predominantly governed by precursor pH and sintering temperature.

• Journal of the Korean Recycled Construction Resources Institute
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• v.10 no.3
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• pp.211-218
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• 2022

Coal ash is being considered as a source of silica and alumina for cement clinker. The purpose of this study was to investigate the effect on cement clinker sintering by confirming the high-temperature microstructural change according to the firing temperature in the cement clinker sintering process of coal ash. In the coal ash used as a raw material for cement clinker, the shape change of the particle surface was confirmed from the sintering tem perature of 950 ℃. The shape of the coal ash disappeared from the sintering temperature higher than 1250 ℃. It was confirmed that the Al and Fe components of the coal ash were converted to the cement interstitial phase at a temperature higher than 1350 ℃. In addition, the clinker using a large amount of coal ash as a raw material showed a low content of Lime and a high content of Belite in the sintering tem perature range of 1150~1200 ℃. From this, it was confirmed that the formation of calcium silicate mineral proceeds more easily at the initial sintering temperature by the application of coal ash.

• Computers and Concrete
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• v.29 no.6
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• pp.375-391
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• 2022

This paper examined the impact of the cross-sectional structure on the structural results under different loading conditions of reinforced concrete (RC) members' management limited in Carbon Fiber Reinforced Polymers (CFRP). The mechanical properties of CFRC was investigated, then, totally 32 samples were examined. Test parameters included the cross-sectional shape as square, rectangular and circular with two various aspect rates and loading statues. The loading involved concentrated loading, eccentric loading with a ratio of 0.46 to 0.6 and pure bending. The results of the test revealed that the CFRP increased ductility and load during concentrated processing. A cross sectional shape from 23 to 44 percent was increased in load capacity and from 250 to 350 percent increase in axial deformation in rectangular and circular sections respectively, affecting greatly the accomplishment of load capacity and ductility of the concentrated members. Two Artificial Intelligence Models as Extreme Learning Machine (ELM) and Particle Swarm Optimization (PSO) were used to estimating the tensile and flexural strength of specimen. On the basis of the performance from RMSE and RSQR, C-Shape CFRC was greater tensile and flexural strength than any other FRP composite design. Because of the mechanical anchorage into the matrix, C-shaped CFRCC was noted to have greater fiber-matrix interfacial adhesive strength. However, with the increase of the aspect ratio and fiber volume fraction, the compressive strength of CFRCC was reduced. This possibly was due to the fact that during the blending of each fiber, the volume of air input was increased. In addition, by adding silica fumed to composites, the tensile and flexural strength of CFRCC is greatly improved.

• The korean journal of orthodontics
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• v.53 no.3
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• pp.139-149
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• 2023

Objective: To assess the color stability and translucency of full cubic stabilized zirconia (FSZ) following orthodontic bonding with different surface treatments and coffee thermocycling (CTC). Methods: This in vitro study was conducted on 120 disc-shaped specimens of FSZ. Thirty specimens were selected as the control group and remained intact. The remaining specimens were randomly divided into three groups based on the type of surface treatment (n = 30): airborne particle abrasion (APA), silica-coating (CoJet), and carbon dioxide (CO₂) laser. After metal bracket bonding in the test groups, debonding and polishing were performed. Subsequently, all specimens underwent CTC (10,000 cycles). Color parameters, color difference (ΔE₀₀), and translucency parameter (TP) were measured three times at baseline (t0), after debonding and polishing (t1), and after CTC (t2). Data were statistically analyzed (α = 0.05). Results: Significant difference existed among the groups regarding ΔE₀₀t0t2 (p < 0.001). The APA group showed minimum (ΔE₀₀ = 1.15 ± 0.53) and the control group showed maximum (ΔE₀₀ = 0.19 ± 0.02) color stability, with no significant difference between the laser and CoJet groups (p = 0.511). The four groups were significantly different regarding ΔTPt0t2 (p < 0.001). Maximal increases in TP were noted in the CoJet (1.00 ± 0.18) and APA (1.04 ± 0.38) groups while minimal increase was recorded in the control group (0.1 ± 0.02). Conclusions: Orthodontic treatment makes zirconia restorations susceptible to discoloration and increased translucency. Nonetheless, the recorded ΔE₀₀ and ΔTP did not exceed the acceptability threshold.