• The Journal of Korean Academy of Prosthodontics
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• v.59 no.2
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• pp.248-260
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• 2021

Cobalt-chromium alloys are used to fabricate various dental prostheses, and have advantages of low cost and excellent mechanical properties compared to other alloys. Recently, selective laser melting, which is an additive manufacturing method, has been used to overcome the disadvantages of the conventional fabrication method. A local rapid heating and cooling process of selective laser melting induces fine microstructures, grain refinement, and reduction of porosities of the alloys. Therefore, it can improve mechanical properties compared to the alloys fabricated by the conventional method. On the other hand, layering process and rapid heating and cooling cause accumulation of a large amount of residual stresses that can adversely affect the mechanical properties. A heat treatment for removing residual stresses through recovery and recrystallization process caused complicated changes in mechanical properties induced by phase transformation, precipitate and homogenization of the microstructures. The purpose of this review was to compare the manufacturing methods of Co-Cr alloys and to investigate the characteristics of Co-Cr alloys fabricated by selective laser melting.

• Corrosion Science and Technology
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• v.18 no.1
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• pp.1-7
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• 2019

To find proper lathe machining parameters for SA182 Grade 304 stainless steel (SS), six kinds of samples with different machining surface states were prepared using a lathe. Surface morphologies and microstructures of near surface deformed layers on different samples were analysed. Surface morphologies and chemical composition of oxide films formed on different samples in simulated primary water with $100{\mu}g/L\;O_2$ at $310^{\circ}C$ were characterized. Results showed that surface roughness was mainly affected by lathe feed. Surface machining caused grain refinement at the top layer. A severely deformed layer with different thicknesses formed on all samples. In addition to high defect density caused by surface deformation, phase transformation, residual stress, and strain also affected the oxidation behaviour of SA182 Grade 304 SS in the test solution. Machining parameters used for # 4 (feed, 0.15 mm/r; back engagement, 2 mm; cutting speed, 114.86 m/min) and # 6 (feed,0.20 mm/r; back engagement, 1 mm; cutting speed, 73.01 m/min) samples were found to be proper for lathe machining of SA182 Grade 304 SS.

• Korean Journal of Metals and Materials
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• v.49 no.2
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• pp.161-166
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• 2011

The aim of this study was to determine the effect of high-energy mechanical milling (HEMM) time and sintering temperature on microstructure and mechanical properties of the TiAl composite fabricated by pulse current activated sintering. TiAl intermetallic powders were milled by HEMM for 1h, 4h, and 8h respectively. Thermal analysis was used to observe the phase transformation of the milled TiAl powders. The sintering time decreased with increase of milling time. The hardness and fracture toughness of the sintered specimens also was improved with increasing milling time. The grain size of the sintered specimens which was milled for 4h was in the range of 50~100 nm.

• Journal of Sensor Science and Technology
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• v.31 no.6
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• pp.423-427
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• 2022

A pressure sensor that mimics the sensing ability of human skin has emerged as high-profile technology because it shows remarkable applications in numerous fields such as robotics, human health monitoring, and artificial prosthetics. Whereas recent pressure sensors have achieved high sensitivity similar to that of human skin, they still show limited detection bandwidth. Moreover, once these e-skin are fabricated, their sensitivity and stiffness are fixed; therefore, they can be used for only limited applications. Our study proposes a new adaptive pressure sensor built with uniform gallium microdroplet-elastomer composite. Based on the phase transition of gallium microdroplets, the proposed sensor undergoes mode transformation, enabling it to have a higher sensitivity and wider detection bandwidth compared with those of human skin. In addition, we succeeded in extending a single adaptive pressure sensor to sensor arrays based on its high uniformity, reproducibility, and large-scale manufacturability. Finally, we designed an adaptive e-skin with the sensor array and demonstrated its applications on health monitoring tasks including blood pulse and body weight measurements.

• Korean Journal of Materials Research
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• v.32 no.5
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• pp.264-269
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• 2022

Ti₃C₂T_x MXene, which is a representative of the two-dimensional MXene family, is attracting considerable attention due to its remarkable physicochemical and mechanical properties. Despite its strengths, however, it is known to be vulnerable to oxidation. Many researchers have investigated the oxidation behaviors of the material, but most researches were conducted at high temperatures above 500 ℃ in an oxidation-retarding environment. In this research, we studied changes in the structural and electrical properties of Ti₃C₂T_x MXene induced by low-temperature heat treatments in ambient conditions. It was found that a number of TiO₂ particles were formed on the MXene surface when it was mildly heated to 200 ℃. Heating the material to higher temperatures, up to 400 ℃, the phase transformation of Ti₃C₂T_x MXene to TiO₂ was accelerated, resulting in a TiO₂/Ti₃C₂T_x hybrid. Consequently, the metallic nature of pure Ti₃C₂T_x MXene was transformed to semiconductive behavior upon heat-treating at ≥ 200 ℃. The results of this research clearly demonstrate that Ti₃C₂T_x MXene may be easily oxidized even at low temperatures once it is exposed to air.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.33 no.1
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• pp.15-21
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• 2023

To prepare zinc oxide powder, three types of sodium-based alkali precipitants such as NaOH, Na₂CO₃, NaOH/NaHCO₃ were compared to the differences in the manufacturing process of zinc oxide powder from zinc precipitate products like intermediates with the consideration of thermodynamic reaction. The prepared zinc precipitate products by the reaction with the sodium-based alkali precipitant were confirmed to respectively hydroxy zinc chloride (Zn₅(OH)₈Cl₂·H₂O) and zinc carbonate hydroxide (Zn₅(OH)₆(CO₃)₂·H₂O) from XRD analysis. Zinc oxide particles were compared in heat treatment at 800℃ according to sodium-based alkali precipitants. The mixed NaOH and NaHCO₃ of alkali precipitant reaction was contributed to synthesize the more uniform zinc oxide particles.

• Journal of the Korean Society for Precision Engineering
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• v.13 no.5
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• pp.84-94
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• 1996

In order to prevent shell sticking by providing sufficient lubrication between the strand and the mold, the mold oscillation has been used. Now it is well known that the shape of the oscillation curve has a decisive effect on the surface quality of the cast product. Besides, oscillation parameters such as stroke and frequency are also very important. In order to guarantee that parameters which have been found to be optimal for a certain grade of steel do not change with time, periodical checks of the physical condition of the whole equipment are necessary. The portable mold oscillation analyzer with integrated computer, developed by POSCO, records the movement of the mold in every spatial direction. The system uses the gap sensors to measure the mold movement (displacement ) in the two horizontal directions according to the mold narrow and broad faces and the vertical strokes in the four corners of mold. The gap sensor is a non-contacting minute displacement measuring device using the principle of high frequency eddy current loss. The mold oscillation diagnosis system integrates the gap sensors, their converters and the industrial portable computer with plug-in data acquisition boards. The all programs, such as the fast Fourier transformation module (amplitude and phase spectrums) and harmonic analysis module, was coded by LabVIEW$^{TM}$ software as the graphical language. In an own 'expert module' which is included in the diagnosis program, one can obtain much information about the mold oscillation equipment.

• Nuclear Engineering and Technology
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• v.55 no.2
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• pp.460-474
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• 2023

A simple but accurate mathematical model is crucial for dynamic simulations and controller design of helical coil once-through steam generator (OTSG). This paper presents a three-region movable boundary dynamic model of the helical coil OTSG. Based on the secondary side fluid conditions, the OTSG is divided into subcooled region (two control volumes), two-phase region (two control volumes) and superheated region (three control volumes) with movable boiling boundaries between each region. The nonlinear dynamic model is derived based on mass, energy and momentum conservation equations. And the linear model is obtained by using the transfer function and state space transformation, which is a 37-order model of five input and three output. Validations are made under full-power steady-state condition and four transient conditions. Results show good agreements among the nonlinear model, linear model and the RELAP5 model, with acceptable errors. This model can be applied to dynamic simulations and controller design of helical coil OTSG with constant primary-side flow rate.

• Composites Research
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• v.22 no.2
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• pp.37-42
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• 2009

This paper presents an experimental approach to design a bending-type actuator by using a shape memory alloy wire (SMA), composite strip, and spring. The SMA wire is attached to two edges of the bent strip to apply pre-stress to the SMA wire. The spring is used to provide recovery force right after actuation of the SMA wire. To investigate thermo-mechanical characteristics of the SMA wire, a series of DSC tests have been conducted and tensile tests under various levels of pre-stress and input power have been performed. Based on the measured properties of the SMA wire, bending-type actuators are designed and tested for different combination of strip, number of springs, and input power. It has been found that a bending-type actuator with a proper combination shows fast actuation performance and low power consumption.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.3C
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• pp.181-190
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• 2006

The behavior of artificially cemented sands were investigated by undrained triaxial test of isotropically consolidated sample. The cementation were induced by gypsum that is generally used for the aitificial cementation of sands. The gypsum of 5~20%(sand weight) were included in the sand and cured in the mold under the overburden pressure 55kPa. The yielding strength and stiffness of cemented sand were increased as the degree of cementation. And the dilation of sand was restricted by the cementation bonds, but after breakage of the bonds, it was increased more abrupt than the uncemented sands. The effective stress path showed that the aspects of effective pore water pressure were changed as the degree of cementation and the relative density. The effective stress ratio of cemented sand in the phase transformation line and the failure line were changed by the cementation. Generally the behavior of cemented sand more influenced by the degree of cementation than the relative density.