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Expeditious low-temperature sintering of copper nanoparticles with thin defective carbon shells.
Kim, Changkyu,Lee, Gyoungja,Rhee, Changkyu,Lee, Minku RSC Pub 2015 Nanoscale Vol.7 No.15
<P>The realization of air-stable nanoparticles, well-formulated nanoinks, and conductive patterns based on copper is a great challenge in low-cost and large-area flexible printed electronics. This work reports the synthesis of a conductively interconnected copper structure via thermal sintering of copper inks at a low temperature for a short period of time, with the help of thin defective carbon shells coated onto the copper nanoparticles. Air-stable copper/carbon core/shell nanoparticles (typical size 23 nm, shell thickness 1.0 nm) are prepared by means of an electric explosion of wires. Gaseous oxidation of the carbon shells with a defective structure occurs at 180 C, impacting the choice of organic solvents as well as the sintering conditions to create a crucial neck formation. Isothermal oxidation and reduction treatment at 200 C for only about 10 min yields an oxide-free copper network structure with an electrical resistivity of 25.1 μω cm (14.0 μω cm at 250 C). Finally, conductive copper line patterns are achieved down to a 50 μm width with an excellent printing resolution (standard deviation 4.0%) onto a polyimide substrate using screen printing of the optimized inks.</P>
Design of Model-based VCU Software for Driving Performance Optimization of Electric Vehicle
Changkyu Lee,Youngho Koo,Kwangnam Park,Gwanhyung Kim 한국정보통신학회JICCE 2023 Journal of information and communication convergen Vol.21 No.4
This study designed a model-based Vehicle Control Unit (VCU) software for electric vehicles. Electric vehicles have transitioned from conventional powertrains (e.g., engines and transmissions) to electric powertrains. The primary role of the VCU is to determine the optimal torque for driving control. This decision is based on the driver’s power request and current road conditions. The determined torque is then transmitted to the electric drive system, which includes motors and controllers. The VCU employs an Artificial Neural Network (ANN) and calibrated reference torque to enhance the electric vehicle’s performance. The designed VCU software further refines the final reference torque by comparing the control logic with the torque calculation functions and ANN-generated reference torque. Vehicle tests confirmed the effective optimization of vehicle performance using the model-based VCU software, which includes an ANN.
The effect of the Internet on commercial bank profitability: Global bank-level panel data evidence
( Changkyu Choi ),( Myung Hoon Yi ) 한양대학교 경제연구소 2017 JOURNAL OF ECONOMIC RESEARCH Vol.22 No.3
The Internet has changed every aspect of economic life. Much re-search has studied whether the Internet affects cross-border trade, service trade, foreign direct investment, economic growth and in-flation. In addition to these studies, our paper analyzed the effect of the Internet on bank profitability. A bank profitability equation is estimated by using global bank-level panel data of 58 countries from 1996 to 2008. The use of the Internet turned out to improve bank profitability after controlling for total assets, GDP growth rate, net interest margin, number of employees, equity ratio and loan-loss reserves.
Analysis on a Drop-Shipping Method for Supplier-Retailer Coordination
Changkyu Park 대한산업공학회 2019 Industrial Engineeering & Management Systems Vol.18 No.4
In a two-stage supply chain, a retailer forecasts a future demand, determines an order quantity, and sends the order tothe supplier. Then the supplier fulfills the order accordingly. The supplier may provide a drop-shipping method fororder fulfillment to increase profits for the both parties, while the retailer takes inventory-related risks. In order to investigate the effect of a drop-shipping method for supplier-retailer coordination, we build the profit models for thesupplier, the retailer, and the total system with/without a drop-shipping method. Then we analyze the independent andvertically integrated models using Stackelberg game and demonstrate equilibrium solutions. Through numerical experiments, we also illustrate the effectiveness of newly developed models and derive some observed implications. Studyfindings imply that (i) a drop-shipping method for shortages improve the supplier-retailer coordination performance,(ii) in general, decision power give an advantage to the supplier, (iii) the Nash equilibrium is not appealing to anybody, and (iv) the vertically integrated models outperform the independent models.
Purchase dependent partial backordering inventory model
Changkyu Park(박창규) 대한산업공학회 2015 대한산업공학회 춘계학술대회논문집 Vol.2015 No.4
Purchase dependence is a frequent phenomenon in retail shops and is characterized by the purchase of certain items together due to their unknown interior associations. Although this concept has been significantly examined in the marketing field (e.g, market basket analysis),it has largely remained unaddressed in operations management Since purchase dependence is an important factor in designing inventory replenishment policies, this paper demonstrates the means of applying it to the partial backordering inventory model.
An EOQ Approach to Purchase-Dependent Demands When Unmet Orders Are Canceled
Changkyu Park(박창규),Minji Sung(성민지) 한국경영과학회 2021 韓國經營科學會誌 Vol.46 No.1
We investigate the inventory models for purchase-dependent demands in the case in which customer orders are canceled if all items in the order are not delivered all at once. The exact objective function of an inventory model is quite difficult to derive when purchase dependence exists. Determining the optimal order quantity and reorder point is also a challenging task. Thus, we first analyze the properties of optimal solutions using an EOQ model to derive the condition of optimal solution. Then, we extend the condition to continuous review models. Through computer simulations, we demonstrate the usefulness of this study’s approach. In general, the inventory model, which considers the purchase dependence, reduces the loss of sales by holding more inventories through more frequent orders, thus eventually minimizing the total cost.
Partial Backordering Inventory Model under Purchase Dependence
Changkyu Park 대한산업공학회 2015 Industrial Engineeering & Management Systems Vol.14 No.3
Purchase dependence is a frequent phenomenon in retail shops and is characterized by the purchase of certain items together due to their unknown interior associations. Although this concept has been significantly examined in the marketing field (e.g. market basket analysis), it has largely remained unaddressed in operations management. Since purchase dependence is an important factor in designing inventory replenishment policies, this paper demonstrates the means of applying it to the partial backordering inventory model. Through computational analyses, this paper compares the performance of inventory models that either consider or ignore purchase dependence; the results demonstrate that inventory models that ignore purchase dependence incur more average cost per unit time than the model that considers purchase dependence, and the impact of purchase dependence can increase in significance as the item set becomes more closely correlated with regard to order demand.