Browsing by Author "Yeo, Chang Dong (TTU)"
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Item Analytical modeling and simulation of electrical contact resistance for elastic rough electrode surface contact including frictional temperature rise(2022) Talukder, Sujoy (TTU); Yeo, Chang Dong (TTU); Hong, Yang Ki; Choi, Minyeong; Flicek, Robert C.; Bishop, Joseph E.An improved electrical contact resistance (ECR) model for elastic rough electrode contact is proposed, incorporating the effects of asperity interactions and temperature rise by frictional and joule heating. The analytical simulation results show that the ECR decreases steeply at the beginning of the contact between Al and Cu. However, it becomes stabilized after reaching a specific contact force. It is also found that the longer elapsed sliding contact time, the higher ECR due to the increase in electrical resistivity of electrode materials by the frictional temperature rise at the interface. The effects of surface roughness parameters on ECR are studied through the 32 full-factorial design-of-experiment analysis. Based on the two representative roughness parameters, i.e., root-mean-square (rms) roughness and asperity radius, their individual and coupled effects on the saturated ECR are examined. The saturated ECR increases with the rms roughness for a rough machined surface condition, but it is hardly affected by the asperity radius. On the other hand, the saturated ECR increases with both the rms roughness and the asperity radius under a smooth thin film surface condition.Item Change of Electrical and Transport Properties of Nickel Oxide by Carrier Concentration and Temperature through First-Principle Calculations(2023) Shah, Nayem Md Reza (TTU); Yeo, Chang Dong (TTU); Choi, Minyeong; Hong, Yang Ki; You, Jeong H.Nickel is typically used as one of the main components in electrical contact devices or connectors. Nickel oxide (NiO) is usually formed on the surfaces of electrodes and can negatively impact system performance by introducing electrical contact resistance. The thermal, electrical, and transport properties of NiO, as a Mott insulator or a p-type semiconductor, can be altered by operating and environmental conditions such as temperature and stress/strain by contact. In this study, we investigate the fundamental material properties of NiO through the first-principle calculations. First, we obtain and compare the lattice parameter, magnetic moment, and electronic structure for NiO via the WIEN2K simulations with four different potentials (i.e., GGA, GGA + U, LSDA, and LSDA + U). Then, using the WIEN2K simulation results with LSDA + U potential that produces a highly accurate bandgap for NiO, we calculate the electrical conductivity and electrical part of the thermal conductivity of nickel and NiO as a function of temperature and carrier concentration through the BoltzTraP simulations. Systematic simulation results revealed that the electrical conductivity relative to the relaxation time for NiO increases with the carrier concentration, while it shows a slightly decreasing trend with temperature under a fixed carrier concentration. By contrast, the electrical part of the thermal conductivity shows an increasing trend considering carrier concentration and temperature.Item Electronic structures of MnB soft magnet(2016) Park, Jihoon; Hong, Yang Ki; Kim, Hyun Kyu; Lee, Woncheol; Yeo, Chang Dong (TTU); Kim, Seong Gon; Jung, Myung Hwa; Choi, Chul Jin; Mryasov, Oleg N.We have calculated the electronic structure of MnB using first-principles calculations based on the density functional theory within the local-spin-density approximation. The temperature dependence of saturation magnetization [Ms(T)] was calculated by mean field approximation. The calculated density of states (DOS) shows that the energy region near the Fermi energy (EF) is mostly attributed to the d bands of Mn. The saturation magnetizations (Ms) of MnB were calculated to be 964.5 emu/cm3 (1.21 T) at 0 K and 859.3 emu/cm3 (1.08 T) at 300 K. The calculated Ms at 300 K is in good agreement with experimental Ms of 851.5 emu/cm3.Item Friction and wear of pd-rich amorphous alloy (Pd43cu27ni10p20) with ionic liquid (IL) as lubricant at high temperatures(2019) Lee, Jaeho (TTU); Yeo, Chang Dong (TTU); Hu, Zhonglue (TTU); Thalangama-Arachchige, Vidura D. (TTU); Kaur, Jagdeep (TTU); Quitevis, Edward L. (TTU); Kumar, Golden; Koh, Yung P. (TTU); Simon, Sindee (TTU)The friction and wear behavior of palladium (Pd)-rich amorphous alloy (Pd43Cu27Ni10P20) against 440C stainless steel under ionic liquids as lubricants, i.e., 1-nonyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([C9C1im][NTf2]), were investigated using a ball-on-disc reciprocating tribometer at ambient, 100 and 200◦C with different sliding speeds of 3 and 7 mm/s, whose results were compared to those from crystalline Pd samples. The measured coefficient of friction (COF) and wear were affected by both temperature and sliding speed. The COF of crystalline Pd samples dramatically increased when the temperature increased, whereas the COF of the amorphous Pd alloy samples remained low. As the sliding speed increased, the COF of both Pd samples showed decreasing trends. From the analysis of a 3D surface profilometer and scanning electron microscopy (SEM) with electron dispersive spectroscopy (EDS) data, three types of wear (i.e., delamination, adhesive, and abrasive wear) were observed on the crystalline Pd surfaces, whereas the amorphous Pd alloy surfaces produced abrasive wear only. In addition, X-ray photoelectron spectroscopy (XPS) measurements were performed to study the formation of tribofilm. It was found that the chemical reactivity at the contacting interface increased with temperature and sliding contact speed. The ionic liquids (ILs) were effective as lubricants when the applied temperature and sliding speed were 200◦C and 7 mm/s, respectively.Item Magnetocrystalline anisotropy of interstitially and substitutionally Sn-doped MnBi for high temperature permanent magnet applications(2023) Choi, Minyeong; Hong, Yang Ki; Won, Hoyun; Yeo, Chang Dong (TTU); Choi, Byung Chul; Park, Jihoon; Lee, WoncheolFirst-principles calculations were performed to calculate the electronic structures of low temperature phase (LTP) MnBi (Mn50Bi50) and substitutionally and interstitially Sn-doped MnBi [Mn50Bi25Sn25, (Mn0.5Bi0.5)66.7Sn33.3]. Brillouin function predicts the temperature dependence of saturation magnetization M(T). Sn substitution for Bi in MnBi (Mn50Bi25Sn25) changes the magnetocrystalline anisotropy constant (Ku) from −0.202 MJ/m3 (the in-plane magnetization) for LTP MnBi to 1.711 MJ/m3 (the out-of-plane magnetization). In comparison, the Ku remains negative but slightly decreases to −0.043 MJ/m3 when Sn is interstitially doped in MnBi [(Mn0.5Bi0.5)66.7Sn33.3]. The Curie temperature (TC) decreases from 716 K for LTP Mn50Bi50 to 445 K for Mn50Bi25Sn25 and 285 K for (Mn0.5Bi0.5)66.7Sn33.3. Mn50Bi25Sn25 has a lower magnetic moment of 5.034 μB/f.u. but a higher saturation magnetization of 64.2 emu/g than (Mn0.5Bi0.5)66.7Sn33.3 with a magnetic moment of 6.609 μB/f.u. and a saturation magnetization of 48.2 emu/g because the weight and volume of the substitutionally Sn-doped MnBi are smaller than the interstitially Sn-doped MnBi. The low Curie temperature and magnetization for Sn-doped MnBi are attributed to the high concentration of Sn. Thus, future study needs to focus on low Sn-concentrated MnBi.