• The Journal of Advanced Prosthodontics
• /
• v.14 no.2
• /
• pp.96-107
• /
• 2022

PURPOSE. Microstructural and physico-mechanical characterization of highly translucent zirconia, prepared by milling technology (CAD-CAM) and repeated firing cycles, was the main aim of this in vitro study. MATERIALS AND METHODS. Two groups of samples of two commercial highly-translucent yttria-stabilized dental zirconia, VITA YZ-HT^White (Group A) and Zolid HT + White (Group B), with dimensions according to the ISO 6872 "Dentistry - Ceramic materials", were prepared. The specimens of each group were divided into two subgroups. The specimens of the first subgroups (Group A₁ and Group B₁) were merely the sintered specimens. The specimens of the second subgroups (Group A₂ and Group B₂) were subjected to 4 heat treatment cycles. The microstructural features (microstructure, density, grain size, crystalline phases, and crystallite size) and four mechanical properties (flexural strength, modulus of elasticity, Vickers hardness, and fracture toughness) of the subgroups (i.e. before and after heat treatment) were compared. The statistical significance between the subgroups (A₁/A₂, and B₁/B₂) was evaluated by the t-test. In all tests, P values smaller than 5% were considered statistically significant. RESULTS. A homogenous microstructure, with no residual porosity and grains sized between 500 and 450 nm for group A and B, respectively, was observed. Crystalline yttria-stabilized tetragonal zirconia was exclusively registered in the X-ray diffractograms. The mechanical properties decreased after the heat treatment procedure, but the differences were not statistically significant. CONCLUSION. The produced zirconia ceramic materials can be safely (i.e., according to the ISO 6872) used in extensive fixed prosthetic restorations, such as substructure ceramics for three-unit prostheses involving the molar restoration and substructure ceramics for prostheses involving four or more units. Consequently, milling technology is an effective manufacturing technology for producing zirconia substructures for dental fixed all-ceramic prosthetic restorations.

• Journal of Platform Technology
• /
• v.9 no.4
• /
• pp.60-70
• /
• 2021

This paper proposes a method capable of stably tracking the coordinates of a patterned laser image in a curved screen shooting system using two or more channels of multiple cameras. This method can track and acquire target points very effectively when applied to a multi-screen shooting method that can replace the HMD shooting method. Images of curved screens with severe deformation obtained from individual cameras are corrected through image normalization, image binarization, and noise removal. This corrected image is created and applied as an Euclidean space map that is easy to track the firing point based on the matching point. As a result of the experiment, the image coordinates of the pattern laser were stably extracted in the curved screen shooting system, and the error of the target point position of the real-world coordinate position and the broadband Euclidean map was minimized. The reliability of the proposed method was confirmed through the experiment.

• Journal of Technologic Dentistry
• /
• v.43 no.4
• /
• pp.168-174
• /
• 2021

Purpose: This study was conducted to investigate alloys spectrophotometrically including yttrium of nickel-chromium (Ni-Cr) alloys, which are used as substitutes for the regulation of beryllium and provide helpful improvements in Ni-Cr alloys. Methods: Four groups of specimens (ZN, ZY, SN and EM) were prepared for analysis. Color parameters were measured with a spectrophotometer, and color difference (∆E^*) was calculated. The t-test and one-way analysis of variance test were used to determine significant difference, and the Tukey test was used to identify where the differences were. To measure the spectroscopic reflectivity, the spectroscopic reflectance was measured and converted into CIE L^*, a^*, b^* color system. Results: The ∆E^* value of each metal ceramic group after opaque firing of Ni-Cr alloy with and without yttrium was <2, and the total group color difference (∆E^*) was below 1 in the dentin ceramic all experimental group. However, the a^* and b^* values of the metal ceramic groups were higher than that in the lithium disilicate all ceramic group, and the chroma was higher than the natural tooth. The brightness of all experimental groups was similar to that of the shade guide sample. Conclusion: Yttrium added to Ni-Cr alloys showed similar CIE L^*, a^*, b^* values to Ni-Cr alloys that did not contain yttrium, indicating that yttrium had no effect on color in metallic ceramic systems.

• The Journal of the Korea Contents Association
• /
• v.22 no.10
• /
• pp.546-553
• /
• 2022

This research has focused on identifying a significant cause of the pavement cracks and irregularities of roads in Changwon city which have been constructed using steel slag, an auxiliary-based material. It is important to note that the cracks and irregularities yield logistics inconvenience, the risk of traffic accidents, and increased road maintenance costs. X-ray diffraction analysis tests have been conducted in this study on the sample collected by pavement cutting and excavating the three target roads. It is well known that the primary cause of the expansion of steelmaking slag is the hydration reaction between CaO and MaO. While the reaction of CaO is completed within a few months, that of MgO is pretty slow depending on the firing temperature. The test results reveal that the MgO content of the testing samples is approximately 47% of the total average, and that of CaO is around 14% of the total average. Hence, these results make it possible to be understood that the expansion induced by the slow hydration reaction of MgO results in road uplift in the long term, resulting in the cracks and irregularities of roads.

• Journal of Sensor Science and Technology
• /
• v.31 no.1
• /
• pp.51-56
• /
• 2022

In this study an efficient method for detecting and monitoring engine misfiring, focusing on minute speed changes in the crankshaft is proposed., Its validity is verified using various misfiring cases. Typically, the crankshaft speed fluctuates around the normal value depending on the engine misfiring status. Even a minute speed change in the crankshaft can be estimated by measuring the rotation time of each tooth of the 118-tooth flywheel attached to the crankshaft with a 2-MHz timer. Therefore, a speed pattern for an in-line six-cylinder engine consists of 236 tooth rotation speeds corresponding to the two rotations of the crankshaft, in which all the cylinders complete four-stroke cycle. FFT analysis can reduce the number of components of a speed pattern from 236 to just four major components: - fundamental frequency_(f), 2f, 3f, 6f., - This makes the comparison of the misfiring cases simpler and faster. In the experiment, five engine status cases (one normal firing and, four misfiring cases) were simulated. While the 6f component was the largest for the normal case, the f component increased as misfiring occurred one, two apart, and two consecutive times. The 3D FFT pattern comprising the ratio of f, 2f, and 3f, 6f showed that the distance between the misfiring and normal states was larger

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

• Journal of the Korean Society of Propulsion Engineers
• /
• v.24 no.4
• /
• pp.1-11
• /
• 2020

In this study, study on the internal ballistic analysis method for single-chamber dual-thrust rocket motors meeting a dual-thrust profile requirement by tailoring the grain burning area is presented. The analysis method, which can acquire variables required for the performance prediction, considering gradual change of burning rate correction factor and specific impulse in the transition phase, is proposed. Improvements compared to the analysis method in the previous study, which do not consider change in the transition phase, are verified through comparison between the newly proposed method and the method in the previous study. Internal ballistic variables are obtained for four different ground firing test conditions using the proposed method, and the performance prediction for each condition is conducted using these variables. These prediction results and the ground test data are in good agreement, so it is confirmed that the performance prediction of dual-thrust motors with same design geometries based on the proposed analysis method is available.

• Journal of the Korean Recycled Construction Resources Institute
• /
• v.10 no.1
• /
• pp.15-22
• /
• 2022

In this study, the hydration reaction and strength characteristic of cement mortar with spent coffee ground(SCG) was investigated. As a result of the study, it was found that as the firing temperature of the SCG increased, the mass loss due to the combustion of organic matter increased, but the density increased. In addition, when the SCG were mixed, SCG interfered with the hydration reaction and the compressive strength was significantly lowered. On the other hand, the coffee grounds ash(SCG_Ash) calcined at 800 ℃ showed a hydration reaction and a compressive strength equivalent to or higher than that of OPC mortar.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.26 no.6
• /
• pp.34-42
• /
• 2022

A correlation between propellant supply condition and chamber pressure in GCH₄-LOx small rocket engine was explored and hot-firing tests were conducted to analyze the engine performance characteristics according to the equivalence ratio variation at a constant chamber pressure. Correlation studies have shown that chamber pressure is linearly proportional to oxidizer supply pressure. As a result of the test, the thrust, specific impulse and characteristic velocity that are the main performance parameters of a rocket engine, were found to be enhanced as the equivalence ratio starting from a fuel-lean condition approached the stoichiometric ratio, but the efficiencies of characteristic velocity and specific impulse were on the contrary, in their dependency on the equivalence ratio.

• Journal of Korean Port Research
• /
• v.3 no.1
• /
• pp.35-55
• /
• 1989

Today, about 99% of total import and export cargo in Korea is being transported through the port. The general trends of cargo handling show increases in capacity and speed, In order to cope with these trends, it is not only required to raise the efficiencies of port operation and function but also necessary to decide the optimal amount of the skilled labor force for cargo handling in the port. Cargo handling in the port is basically relied on the cargo handling facilities. Therefore, it is very important to reserve the amount of labor force for cargo handling system has been developed up to a certain level but the personnel management system which is the superior structure has not been followed well. In this study, therefore, we show a method to determine the required amount of labor force for cargo handling considering the amount of cargo and type of cargo handling work per each cargo, and the optimal amount labor force in cope with the fluctuation of the basic cargo handling labor force with respect to the time of in and out cargo flow in the viewpoint of minimizing the expences due to reservation of extra labor force than needed and firing employment of labor force using the Dynamic Programming. The derived algorithm is introduced into the computer simulation for Pusan port with the analyzed real data such as amount of cargo handling in the port with respect to working hour, cargo capacity, working step, the ratio of cargo handling facility and actual number of workers and we estimated the required labor force. As a result of analysis the labor force of Pusan port showed the over-employment such as maximum 21.4%, minimum 8.2% when we assumed that the averages of actual working hours and days were 8 hours in a day and 20 day in a month.