• Journal of Korea Foundry Society
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• v.21 no.4
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• pp.239-245
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• 2001

Microstructure and mechanical properties of Sn-3.1 wt.%Ag-6.9 wt.%Bi system solders on Cu-substrate were studied. The Sn3.1 wt.%Ag-6.9 wt.%Bi alloy was designed by phase diagram and chemical properties and was prepared by melting in argon atmosphere. The mechanical properties of solder/Cu joints were examined by shear strength test, and also creep test. The microstructure of Sn-3.1 wt.%Ag-6.9 wt.%Bi alloy consists of Bi-rich phase and $Ag_3Sn$ precipitate in {\beta}-Sn$ matrix phase. The shear strength of the joint was decreased with aging treatment. Crack path under shear test was through the solder. Similar crack path change mode was observed at the creep test of solder/Cu joint. The creep behavior of Sn-3.1 wt.%Ag-6.9 wt.%Bi alloy represented the inverse primary creep behavior at all test condition. It is suggested that the inverse primary creep behavior is induced from Bi solute atoms in Sn-matrix. The creep resistance of Sn-3.1Ag-6.9Bi alloy is better than that of Sn-3.5 wt.%Ag alloy at all test conditions.

• Journal of the Korea Institute of Information Security & Cryptology
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• v.30 no.5
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• pp.859-870
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• 2020

The MQTT-SN (Message Queuing Telemetry Transport-Sensor Network) protocol is a message transmission protocol used in a sensor-based Internet of Things (IoT) environment. This MQTT-SN protocol is a publish-subscribe model with a broker in the middle of message transmission, and each IoT device sends and receives messages through an intermediary when delivering messages. However, the MQTT-SN protocol does not provide security-related functions such as message security, mutual authentication, access control, and broker security. Accordingly, various security problems have recently occurred, and a situation in which security is required has emerged. In this paper, we review the security requirements of MQTT-SN once again, and propose a modified protocol that improves security while satisfying the constraints in the environment where the resource of IoT to which this protocol is applied is limited. Unlike the existing protocol, the security field and authentication server have been added to satisfy the security requirements. In addition, the proposed protocol is actually implemented and tested, and the proposed protocol is evaluated for practical use in terms of energy consumption.

• Journal of the Korean institute of surface engineering
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• v.40 no.5
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• pp.225-233
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• 2007

Effects of alloying elements on the corrosion layer formation of Pb-grid/active materials interface has been researched for improvement of corrosion resistance of Pb-Ca alloy. For this research, various amounts of alloying elements such as Sn, Ag and Ba were added to the Pb-Ca alloys and investigated their corrosion behaviors. Batteries fabricated by using these alloys as cathode grids were subjected to life cycle test. Overcharge life cycle test was carried out at $75^{\circ}C$, 4.5 A, for 110 hrs. with KS standard (KSC 8504). And then, after keeping the battery with open circuit state for 48 hr, discharge was carried out at 300A for 30 sec. Corrosion morphology and interface between Pb-grid and active materials were investigated by using ICP, SEM, WDX, and LPM. Corrosion layer of Pb-Ca alloy got thicken with increasing Ca content. For Pb-Ca-Sn alloy, thickness of corrosion layer decreased as Sn and Ag content increased gradually. In case of Pb-Ca-Sn-Ba alloy, thickness of corrosion layer decreased up to 0.02 wt% Ba addition, whereas, it was not changed in case of above 0.02 wt% Ba addition.

• Journal of Korea Foundry Society
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• v.37 no.4
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• pp.101-107
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• 2017

This study investigated the effect of Pd on the microstructure, tensile and creep properties of Mg-4Al-2Sn (AT42) alloy at a high temperature for transportation-related industrial applications. AT42-xPd (x = 0, 1 and 2 wt. %) alloys were prepared using a permanent mould casting method. The microstructures of the as-cast alloys were characterized by the presence of the intermetallic phases $Mg_{17}Al_{12}$, $Mg_2Sn$ and $Al_4Pd$. The addition of Pd was found to improve the tensile properties of AT42 at room and at elevated temperatures, and to increase the creep resistance at elevated temperatures. A small amount of Pd could markedly improve the tensile properties of AT42 by means of grain-refinement and the dispersion of secondary phase strengthening. Moreover, the thermally stable phase $Al_4Pd$ effectively improves the creep resistance of AT42 due to the strengthened grain boundaries and the suppressed formation of $Mg_{17}Al_{12}$.

• Journal of Korea Foundry Society
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• v.31 no.5
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• pp.293-301
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• 2011

Representative magnesium alloys applied to the die-casting are AZ91, AM60, etc., and the application of these alloys is restricted to components operating at moderate temperatures, due to grain boundary siding of ${\beta}$-phase($Mg_{17}Al_{12}$) at temperatures above $120^{\circ}C$. Heat-resistant magnesium alloys such as AE42, AE44 have been developed, but that have been too burdensome to produce because of the expensive rare earth materials. Research work for the development of low-priced heat-resistant magnesium alloy is actively in progress and positive results are being reported. This study aims to investigate the effect of Ca additions on mechanical properties of Mg-4Al-2Sn heat resistant magnesium alloys. Mg-4Al-2Sn alloys with Ca (0wt.%, 0.3wt.%, 0.7wt.%, 1wt.%) have been produced through the die-casting process for the development of low-priced heat-resistant magnesium alloy, and high temperature tensile tests are performed using the specimens. The results showed that mechanical properties of Mg-4Al-2Sn-xCa increased with the addition of Ca up to 0.7wt.% Ca and further addition of Ca deteriorated the mechanical properties of the alloys. A significant amount of porosity was observed at the sample with 1wt%. Ca and the longer freezing range of the alloy was believed to cause the formation of porosity.

• Journal of the Korean institute of surface engineering
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• v.52 no.2
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• pp.90-95
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• 2019

Methylammonium lead bromide ($MAPbBr_3$) has attracted a lot of attention due to their excellent optoelectronic properties such as the compositional flexibility relevant to photoluminescence (PL) and UV-Vis absorbance spectrum, high diffusion length, and photoluminescence quantum yield (PLQY). Despite such advantages of organic-inorganic perovskite materials, more systematic study on manipulation of their optoelectronic properties in homo- or heterovalent metal ions doped halide perovskite nanocrystals is lacking. In this study, we systematically investigated the optical properties of colloidal $CH_3NH_3Pb_{1-x}Sn_xCl_{2x}Br_{3-2x}$ particles by addition of $SnCl_2$ into the typical methylammonium lead tribromide ($CH_3NH_3PbBr_3$) precursor solution. We found that only 1% addition of $SnCl_2$ shows a significant blue-shift from 540 nm to 420 nm in UV-Vis absorbance spectrum due to the strong quantum confinement effect. Furthermore, continuous blue-shift in photoluminescence spectra was observed as the amount of Cl increases. These experimental results provide new insights into the replacement of Pb within $MAPbBr_3$, required for the broadening of their application.

• Journal of the Korean Electrochemical Society
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• v.17 no.2
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• pp.86-93
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• 2014

Mesoporous tin oxide (meso-$SnO_2$) with 5 nm mesopore and well-aligned $SnO_2$ nanowire-bundles with 5~7 nm diameters were prepared by template synthesis method. In addition to meso-$SnO_2$, meso-$SnO_2$/$SiO_2$, which has almost the same structure as meso-$SnO_2$ including $SiO_2$ used as the template were prepared by the modification of template synthesis. X-ray diffraction, N2 adsorption-desorption isotherms, transmission electron microscopy observed structures of meso-$SnO_2$ and meso-$SnO_2$/$SiO_2$. Although the meso-$SnO_2$/$SiO_2$ showed some positive evidences to suppress the volume change of meso-$SnO_2$ through cyclic voltammogram, electrochemical impedance spectroscopy, and voltage profiles during cycling, its cycle life was not improved highly to address modified structural effects. Thus, further study might be done to control the nanostructure of meso-$SnO_2$/$SiO_2$ for enhanced cycle performance.

• Journal of Electrochemical Science and Technology
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• v.6 no.3
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• pp.95-105
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• 2015

In the present work, Carbon supported Pt₁₀₀, Pt₈₀Sn₂₀, Pt₈₀Ni₂₀ and Pt₈₀Sn₁₀Ni₁₀ electrocatalysts with different atomic ratios were prepared by ethylene glycol-reduction method to study the electro-oxidation of ethanol in membraneless fuel cell. The electrocatalysts were characterized in terms of structure, morphology and composition by using XRD, TEM and EDX techniques. Transmission electron microscopy measurements revealed a decrease in the mean particle size of the catalysts for the ternary compositions. The electrocatalytic activities of Pt₁₀₀/C, Pt₈₀Sn₂₀/C, Pt₈₀Ni₂₀/C and Pt₈₀Sn₁₀Ni₁₀/C catalysts for ethanol oxidation in an acid medium were investigated by cyclic voltammetry (CV) and chronoamperometry (CA). The electrochemical results showed that addition of Ni to Pt/C and Pt-Sn/C catalysts significantly shifted the onset of ethanol and CO oxidations toward lower potentials. The single membraneless ethanol fuel cell performances of the Pt₈₀Sn₁₀Ni₁₀/C, Pt₈₀Sn₂₀/C and Pt₈₀Ni₂₀/C anode catalysts were evaluated at room temperature. Among the catalysts investigated, the power density obtained for Pt₈₀Sn₁₀Ni₁₀/C (37.77 mW/cm² ) catalyst was higher than that of Pt₈₀Sn₂₀/C (22.89 mW/cm² ) and Pt₈₀Ni₂₀/C (16.77 mW/ cm² ), using 1.0 M ethanol + 0.5 M H₂SO₄ as anode feed and 0.1 M sodium percarbonate + 0.5 M H₂SO₄ as cathode feed.

• Journal of Welding and Joining
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• v.33 no.3
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• pp.19-24
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• 2015

Joint properties of electric control unit (ECU) module using Sn-Cu-(X)Al(Si) lead-free solder alloy were investigated for automotive electronics module. In this study, Sn-0.5Cu-0.01Al(Si) and Sn-0.5Cu-0.03Al(Si) (wt.%) lead-free alloys were fabricated as bar type by doped various weight percentages (0.01 and 0.03 wt.%) of Al(Si) alloy to Sn-0.5Cu. After fabrications of lead-free alloys, the ball-type solder alloys with a diameter of 450 um were made by rolling and punching. The melting temperatures of 0.01Al(Si) and 0.03Al(Si) were 230.2 and $230.8^{\circ}C$, respectively. To evaluation of properties of solder joint, test printed circuit board (PCB) finished with organic solderability perseveration (OSP) on Cu pad. The ball-type solders were attached to test PCB with flux and reflowed for formation of solder joint. The maximum temperature of reflow was $260^{\circ}C$ for 50s above melting temperature. And then, we measured spreadability and shear strength of two Al(Si) solder materials compared to Sn-0.7Cu solder material used in industry. And also, microstructures in solder and intermetallic compounds (IMCs) were observed. Moreover, thickness and grain size of $Cu_6Sn_5$ IMC were measured and then compared with Sn-0.7Cu. With increasing the amounts of Al(Si), the $Cu_6Sn_5$ thickness was decreased. These results show the addition of Al(Si) could suppress IMC growth and improve the reliability of solder joint.

• Journal of Sensor Science and Technology
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• v.4 no.2
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• pp.37-44
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• 1995

A coprecipitation method was used for preparing Ca and Pt doped $SnO_{2}$-based material. Crystallite size and specific surface area were investigated by TEM, XRD and BET analysis. $SnO_{2}(Ca)/Pt$ based thick film devices were prepared by a screen printing technique for hydrocarbon gas detecting. Then the electrical and sensing characteristics of devices were investigated. As Ca and Pt addition, the crystal growth of $SnO_{2}$ was suppressed during calcining and sintering, and the sensitivity of $SnO_{2}(Ca)/Pt$ thick film to gas was enhanced. Also any difference in the sensing properties has to be attributed to the Pt and Au electrode. For the 2000 ppm $CH_{4}$, the sensitivity of $SnO_{2}(Ca)/Pt$ thick film devices were about 83% at an operating temperature of $400^{\circ}C$.