• The Journal of Engineering Geology
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• v.8 no.3
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• pp.207-213
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• 1998

A study was conducted to determine the characteristics of acoustic emission(AE) events generated by the expansive cement induced rock fracturing. The dominant frequency and the maximum amplitude of the AE events are changed in relation to the rate of expansive pressure development in the hole. The dominant frequencies are in the range of 150∼230kHz for the small hole tests and 400∼500kHz for the large hole test. The maximum amplitudes are in the range of 0.015∼0.050cm/sec and 0.025∼0.064cm/sec, respectively. The fact that AE events of higher amplitude with higher frequency on the large hole test and lower amplitude with lower frequency on the small hole tests were detected, may strongly imply that the amount of energy consumed for a macro-crack in both tests may be similar. The expansive cement induced crack propagates stably without any distinguished event having higher amplitude and this implies that a macro-crack is a result of stable growth of micro cracks.

• Journal of Periodontal and Implant Science
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• v.32 no.4
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• pp.697-709
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• 2002

The present study was performed to evaluate the effect of citric acid on the change of implant surface microstructure according to application time. Implants with pure titanium machined surface, titanium plasma-sprayed surface, and sand-blasted, large grit, acid etched surface were utilized. Implant surface was rubbed with pH 1 citric acid for $\frac{1}{2}$ min., 1 min., 1 $\frac{1}{2}$ min., 2 min., and 3min, respeaively in the test group and implant surface was not treated in the control group. Then, the specimens were processed for scanning electron microscopic observation. The following results were obtained. 1. Both test and control group showed a few shallow grooves and ridges in pure titanium machined surface implants. There were not significant differences between two groups. 2. In titanium plasma-sprayed surfaces, round or amorphous particles were deposited irregularly. The irregularity of titanium plasma-sprayed surfaces conditioned with pH 1 citric acid was lessened and the cracks were increased relative to the application time of pH 1 citric acid. 3. Sand-blasted, large grit, acid etched surfaces showed the macro/micro double roughness. The application of pH 1 citric acid didn't change the characteristics of the sand-blasted, large grit, acid etched surfaces. In conclusion, the application of pH 1 citric acid to titanium plasma-sprayed surface is improper. And pure titanium machined surface implants and sand-blasted, large grit, acid etched surface implants can he treated with pH 1 citric acid for peri-implantitis treatment if the detoxification of these surfaces could be evaluated.

• Journal of Periodontal and Implant Science
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• v.36 no.3
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• pp.731-744
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• 2006

The present study was performed to evaluate the effect of tetracycline-HCl on the change of implant surface microstructure according to application time. Implant with pure titanium machined surface, HA-coated surface and dual acid etched surface were utilized. Implant surface was rubbed with $50mg/m{\ell}$ tetracycline-HCL solution for ${\frac{1}{2}}$min., 1min., $1{\frac{1}{2}}$min., 2min., and $2{\frac{1}{2}}$min. respectively in the test group. Then, specimens were processed for scanning electron microscopic observation. The results of this study were as follows. 1. Both test and control group showed a few shallow grooves and ridges in pure titanium machined surface implants. There were not significant differences between two groups. 2. In HA-coated surfaces, round particles were deposited irregularly. The roughness of surfaces conditioned with tetracycline-HCL was lessened and the cracks were increased relative to the application time. 3. The etched surfaces showed the honey comb structures. The surface conditioning with tetracycline-HCI didn't influence on its micro-morphology. In conclusion, the detoxification with $50mg/m{\ell}$ tetracycline-HCI must be applied respectively with different time according to various implant surfaces.

• Journal of the Korean Ceramic Society
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• v.39 no.12
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• pp.1149-1152
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• 2002

Indentation was used to analyze high pressure phases of silicon. Phase transformations on both loading time and loading rate were studied. Micro-raman spectroscopy was used to observe the indentation-induced transformations. As the loading time increased, Si-III and Si-XII disappeared and only a-Si was observed in (111) samples. As the loading time increased, the residual stress was removed by creation of cracks or dislocations. At 0.1 mm/min loading rate, pop-in . At 5 mm/min loading rate pop-in was observed in force/displacement curve of (111) sample, but pop-in was not observed in force/displacement curve of (100) sample. This result indicates that the loading rate affects the volume of phase transformation in silicon.

• Journal of the Korea Concrete Institute
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• v.14 no.3
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• pp.331-340
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• 2002

In this paper, finite element analysis is applied for simulation of cracks due to restraining autogenous and drying shrinkage at early-age concrete. A micro-level heat hydration model and a shrinkage prediction model along with a moisture diffusion model are adopted for the finite element analysis. Then, an axial restraint test is carried out for concrete specimens containing different amounts of chloride ions to evaluate stress development and cracking due to the restraining shrinkages at early ages. Test results show that the increase of contents of chloride ions increases restrained stress, but does not increase strength. By this increase of shrinkage strain at early-age, time to occur the crack is accelerated. Finally, stress development and cracking of concrete specimens containing different amount of chloride ions we simulated using the finite element analysis. Results of the analysis using the Proposed model are verified by comparison with test results.

• Journal of the Korea Concrete Institute
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• v.24 no.2
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• pp.121-127
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• 2012

Cement has been traditionally used as a main binding material of high ductile fiber reinforced cementitious composites. The purpose of this paper is to investigate the feasibility of using alkali-activated slag and polyvinyl alcohol (PVA) fibers for manufacturing high ductile fiber reinforced cementless composites. Two mixture proportions with proper flowability and mortar viscosity for easy fiber mixing and uniform fiber dispersion were selected based on alkali activators. Then, the slump flow, compression, uniaxial tension and bending tests were performed on the mixes to evaluate the basic properties of the composites. The cementless composites showed an average slump flow of 465 mm and tensile strain capacity of approximately 2% of due to formation of multiple micro-cracks. Test results demonstrated a feasibility of manufacturing high ductile fiber reinforced composites without using cement.

• Journal of Periodontal and Implant Science
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• v.35 no.4
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• pp.891-905
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• 2005

The present study was performed to evaluate the effect of tetracycline-HCL on the change of implant surface microstructure according to application time. Implant with pure titanium machined surface. HA-coated surface and TiUniteTM surface were utilized. Implant surface was rubbed with 50mg/ml tetracycline-HCL solution for $\frac{1}{2}min.$, 1min., $1\frac{1}{2}min.$, 2min., and $2\frac{1}{2}min.$ respectively in the test group. Then, specimens were processed for scanning electron microscopic observation. The results of this study were as follows. 1. Both test and control group showed a few shallow grooves and ridges in pure titanium machined surface implants. There were not significant differences between two groups. 2. In HA-coated surfaces, round particles were deposited irregularly. The roughness of surfaces conditioned with tetracycline-HCL was lessened and the cracks were increased relative to the application time. 3. The anodic oxidized surfaces showed the craterous structures. The surface conditioning with tetracycline-HCl didn't influence on its micro-morphology. In conclusion, the detoxification with 50mg/ml tetracycline-HCL must be applied respectively with different time according to various implant surfaces.

• Journal of the Korean Ceramic Society
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• v.36 no.4
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• pp.391-402
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• 1999

Characteristics of composite electrolytes which were prepared by coating a thin film of YSZ (yttria sta-bilized zirconia : (ZrO2)0.92 (Y2O3)0.08) on YDC (yttria doped ceria : Ce0.8Y0.2O1.9) with mixed conductivity have been investigated in order to develop the low-temperature solid oxide fuel cell. The thickness (t) of spin-coated YSZ thin films after the heat-treatment at 600$^{\circ}C$ was increased proportionally to the sol con-centrations (C) while the decrease in its thickness with the spin rate ($\omega$) could be expressed in the e-quation of ln t=9.49-0.53 ln $\omega$(0.99mol//s sol conc.) When the sol concentration and the spin rate being less than 0.99 mol/l and higher than 1000 rpm respectively reliable YSZ/YDC composite electrolytes could be obtained by multi-coating although several micro-cracks were observed in singly coated YSZ film surfaces. The dense YSZ film with a 1$\mu\textrm{m}$ thickness was prepared by coating of 0.99 mol/l YSZ sol five-times at 2000 rpm followed by heat-treatment at 1400$^{\circ}C$ for 2h, The adhesion between YSZ film and YDC substrate was found to be very good. The open circuit voltages of H2/O2 single cell with YSZ/YDC composite electrolytes were 0.79∼0.82 V at 800$^{\circ}C$ and 0.75∼0.77V at 900$^{\circ}C$ The open circuit voltage was inversely proportioned to the thickness ratio of YSZ thin film (1$\mu\textrm{m}$) to YDC substrate(0.28-2.22 mm)

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.10 no.6
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• pp.389-393
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• 2000

High-quality $Er^{3+}$ doped Mg : $LiNbO_3$single crystal fibers were grown by a micro-pulling down ($\mu$-PD) method. Single crystal fibers were pulled down through the nozzle, at a pulling down rate of 0.5 mm/min and using a Pt crucible with a nozzle 1 mm in diameter in air atmosphere. Defects such as bubbles, cracks and inclusions were not detected in any of the grown crystals. The optical transmission of Er : Mg : $LiNbO_3$crystal was measured and the energy levels of $Er_2O_3$ ion could be calculated. The photoluminescence spectrum of crystal fibers showed an energy band emission with the strongest line corresponding to the $^4I_{3/2}{\to}^4I_{15/2}$transition. The concentration dependence of the entire wavelength region emission intensity upon excitation intensity measured emission intensity for the 3 mol% MgO doped fibers was larger than that for the 1, 5 mol% MgO doped fibers.

• Journal of the Korean Ceramic Society
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• v.51 no.6
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• pp.549-553
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• 2014

$CoSb_3$-based skutterudite compounds are candidate materials for thermoelectric power generation in the mid-temperature range (600 - 900 K) because their thermoelectric properties can be enhanced by doping and filling. The joining property of thermoelectric module electrodes containing thermoelectric materials is of great importance because it can dominate the efficiency of the thermoelectric module. This study examined the properties of $CoSb_3$/Al/Ti/CuMo joined by the spark plasma sintering technique. Titanium thin foil was used to prevent the diffusion of copper into $CoSb_3$ and Aluminum thin foil was used to improve the adhesion between $CoSb_3$ and Ti. The insertion of an Aluminum interlayer between the Ti and $CoSb_3$ was effective for joining $CoSb_3$ to Ti by forming an intermediate layer at the Al-$CoSb_3$ boundary without any micro cracks. Specifically, the adhesion strength of the Ti/Al/$CoSb_3$ joining interface showed a remarkable improvement compared with our previous results, without deterioration of electrical property in the interface.