Hyun-Koo Moon

Stoichiometry dependence of electrochemical performance of thin-film SnOx microbattery anodes deposited by radio frequency magnetron sputtering

Thin-film SnOx microbattery anodes, with various oxygen deficiencies, are deposited from a SnO2 target on to an ambient temperature substrate by radio frequency (RF) magnetron sputtering. The high reversible capacity and cycle performance characteristics of SnOx are described. RF power density and process gas pressure during deposition are fixed at 2.5 W/cm2 and 10 mTorr, respectively. The SnOx films are characterized by energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Constant-current galvanostatic charge–discharge tests of half cells are also performed. The stoichiometric parameter x increases with the increase in oxygen partial pressure, but decreases when the number of Sn chips placed on the target material in an argon atmosphere are increased. It is observed that SnOx transforms to lithium oxide and metallic Sn after an initial Li intercalation reaction. The charge–discharge performance of the tin oxide films is found to be dependent on stoichiometry. In the present work, SnO1.43 is the optimum stoichiometry, exhibiting the highest reversible capacity (498.33 μA h/cm2 μm) and the lowest irreversible capacity (301.79 μA h/cm2 μm).

A study on the subsidence risk evaluation using 3-D rock mass collapse simulation for abandoned mines

Abstract This paper suggests a way to evaluate the subsidence risk applicable to abandoned mines. For the subsidence risk evaluation, a diagram modified from Sakurai’s tunnel stability evaluation diagram was developed. To check the feasibility of the modified diagram, two typical mines to represent Korean mines, one gold mine including one steep goaf and one coal mine including two gently sloped goafs were selected and 3-D numerical analyses for the sections in which actual subsidence was occurred were performed. In the numerical analyses, the variation in relative displacement strain between the observation point and reference point placed over the goafs according to the location of the points was analyzed, and the strain was applied to the modified diagram to evaluate the subsidence risk. The results show that the critical factors in the subsidence risk evaluation are the relative locations of the observation and reference points from the goafs as well as joints existing between goafs and the ground.

An Adaptive Security Approach According to the Reliability Level of Drones Using Blockchain

In drone environments using batteries, it is important to reduce unnecessary overhead. Also, for security of data, encryption is also an important factor. However, in the case of encryption, overhead is generated, but encryption and overhead are always in a trade-off relationship in this way. In this paper, in order to reduce overhead of drone, the concept of reliability was applied. We applied low level security to drone with high reliability to reduce overhead and conversely applied high level security to drone with low reliability. In order to adjust the security level, adjust the size of Key used for encryption and reduce the weight of the security algorithm itself. The reliability of the drone was determined based on the number of times of receiving data and by applying group reliability through drone grouping, it was made possible to determine hierarchical reliability. Moreover, by storing the communication records of all drones in the blockchain, the integrity of the reliability is guaranteed.

A study on the development of an artificial neural network model for the prediction of ground subsidence over abandoned mines in Korea

Abstract The subsidence over abandoned mines has led to big problems in the local areas. Hence, many attempts have been made to predict subsidence and to check the stability of an abandoned mine. This paper focuses on the development of an artificial neural network (ANN) model to predict mine subsidence, and the verification of prediction ability, based upon the investigation of 247 subsidence areas over 27 abandoned mines in Korea. The analysis is divided into two stages. In stage 1 where the data of 229 cases are used for learning as well as prediction of ANN, it is found that the correct answer percentage of final output values to the actual values shows 99, 95, 100 and 100% for four output parameters of subsidence type, final decision, filling method, and local reinforcement respectively. In stage 2 where the information of 18 other cases is used for prediction after learning with the 229 cases, the correct answer percentage of final output values is 89, 78, 100, and 94% for the four parameters, respectively. Finally, it is thought that this ANN model can be used for the prediction of unknown situations regarding subsidence over an abandoned mine with high reliability.

Stress analysis for shotcrete lining on uneven tunnel perimeter

ABSTRACT: When a tunnel is constructed by drilling and blasting operation, the excavated perimeter becomes uneven due to overbreak at the drill holes so that the thickness of shotcrete tends to be irregular. In this case, the conventional stress analyses for tunnel lining of a uniform thickness cannot be readily applicable. In this study, the profile of tunnel perimeter assumed to be sinusoidal in order to simulate the uneven tunnel perimeter and to control the thickness of shotcrete by using the amplitude and wavelength. By adopting the sinusoidal function to a theoretical solution of stress analysis for uniform lining, the range of axial stress of irregular shotcrete can be estimated. The applicability of the approximate solution has be en verified by performing a series of numerical analyses for various conditions. It is shown that the axial stress of shorcrete is highly dependent upon the irregularity of shotcrete, together with the ground property and initial stress conditions. It is also shown the shear stress is dependent upon the wavelength, and the stress condition becomes unfavorable where the thickness of shotcrete is relatively small. The approach developed in this study shows that the stress state where the thicknes s is relatively small is unfavourable, and it is necessary to take complementary measures when installing shotcrete after blasting.Keywords: Shotcrete, Stress analysis, Uneven perimeter, Irregular lining, Sinusoidal function

A Comparative Study on Five Assessment Systems of TBM Tunnelling

ABSTRACT Five assessment systems for planning and evaluation of TBM tunnelling are introduced with detailed calculation steps. Their characteristic features and necessary input data sets are discussed and compared to cope with different levels of rock and rock mass information tunnel engineers have at different stages of project execution. Two systems are adequate for pre-planning and two are for post-evaluation of TBM performance. Another system provides probabilistic planning and evaluation. This study gives tunnel engineers the freedom to choose a particular system depending on their objectives and available input data.