Electrical, Thermal, and Thermoelectric Transport Properties of Co-Doped n-type Cu0.008Bi2Te2.6Se0.4 Polycrystalline Alloys

Okmin Park,TaeWan Kim,Minsu Heo,Sang Jeong Park,Se Woong Lee,Hyun Kyu Cho,Sang-il Kim 대한금속·재료학회 2023 대한금속·재료학회지 Vol.61 No.3

Bi2Te3-based alloys have been extensively studied as thermoelectric materials near roomtemperature. In this study, the electrical, thermal, and thermoelectric transport properties of a series of Codopedn-type Cu0.008Bi2Te2.6Se0.4 polycrystalline alloys (Cu0.008Bi2−xCoxTe2.6Se0.4, x = 0, 0.03, 0.06, 0.09 and 0.12)are investigated. The electrical conductivity of the Cu0.008Bi1.97Co0.03Te2.6Se0.4 (x = 0.03) sample wassignificantly enhanced, by 34%, to 1199 S/cm compared to 793 S/cm of the pristine Cu0.008Bi2Te2.6Se0.4 (x = 0)sample at 300 K, and gradually decreased to 906 S/cm for x = 0.12 upon further doping. Power factors of theCo-doped samples decreased compared to the 3.26 mW/mK2 of the pristine Cu0.008Bi2Te2.6Se0.4 sample at 300K. Meanwhile, the power factor of the Cu0.008Bi1.97Co0.03Te2.6Se0.4 (x = 0.03) sample became higher at 520 K. The lattice thermal conductivities of the Co-doped samples decreased due to additional point defect phononscattering by the Co dopant. Consequently, the zT for the Cu0.008Bi1.97Co0.03Te2.6Se0.4 alloy at 520 K was 0.83,which is approximately 15% larger than that of pristine Cu0.008Bi2Te2.6Se0.4, while the zT of the Cu dopedsamples at 300 K was smaller than that of the pristine Cu0.008Bi2Te2.6Se0.4 sample. Electrical transportproperties of the Co-doped Cu0.008Bi2−xCoxTe2.6Se0.4 samples were analyzed by experimental phenomenologicalparameters, including the density-of-state, effective mass, weighted mobility, and quality factor.

Thermomechanical Behavior and Microstructure Properties of Carbon Fiber Reinforced Polymer at Elevated Temperatures

Okmin Park,Seulgi Han,Sanghyun Park,Jamil Ur Rahman,Sang-il Kim,Sungmo Choi 대한금속·재료학회 2024 대한금속·재료학회지 Vol.62 No.1

Carbon fiber reinforced polymer (CFRP) has been extensively used in civil engineering forapplications such as reinforcing and retrofitting various architectural materials. Therefore, understanding thedegradation of CFRP under high temperatures is important. This study aims to investigate the thermomechanicaland microstructural properties of CFRP plates at elevated temperatures up to 350 oC. The platetypeCFRP composites were subjected to temperatures of 50, 100, 150, 200, 250, 300, and 350 oC, and thencompared with pristine CFRP samples. X-ray diffraction analysis was conducted to examine the crystalstructures of the carbon fibers and epoxy resin matrices in the CFRP. At temperatures higher than 150 oC,the FWHM increased due to the degradation and softening of the resin matrix. Delamination and debondingbetween the matrix and fibers were observed in samples exposed to temperatures above 200 oC. The maximumtensile strength of the CFRP plates exposed at 350 oC significantly decreased to 0.605 GPa, a reduction ofapproximately 40% compared to the pristine sample. On the other hand, Young's modulus remained relativelyunchanged across the different temperatures. This suggests that the polymer matrix degradation plays acrucial role in the mechanical properties of CFRP, as the matrix layers contribute significantly to thedistribution of forces.

Electrical, Thermal, and Thermoelectric Transport Properties of Se-doped Polycrystalline Re2Te5

Se Woong Lee,Okmin Park,Hyun-Sik Kim,Won-Seon Seo,Sang-il Kim 대한금속·재료학회 2022 대한금속·재료학회지 Vol.60 No.12

Re2Te5 is considered a potential thermoelectric material because of its intrinsically low thermalconductivity, due to its complex crystal structure. Herein, a series of Se-doped Re2Te5 (Re2Te5-xSex, x = 0, 0.2,1, and 2) samples were synthesized, and their electrical and thermal transport properties were investigated. Pure orthorhombic Re2Te5 phases were successfully synthesized without any impurities for all compositions,and the continuous decrease in the calculated lattice parameters confirmed the substitution of Se atoms atthe Te sites. A maximum power factor of 0.135 mW/mK2was achieved for the sample with x = 0.2 at 880 K,mainly due to the increase in carrier concentration and electrical conductivity. The lattice thermalconductivity significantly decreased for all doped samples, which was attributed to the point defect phononscattering caused by Se doping. The thermoelectric figure of merit, zT reached a maximum value of 0.20 at880 K for Re2Te4.8Se0.2 (x = 0.2) sample, which was approximately 22% higher than that of the pristine Re2Te5sample. The weighted mobility, quality factor, and expected zT were calculated to evaluate the optimizationof the power factor and zT.

Physicochemical characteristics of mineral trioxide aggregate depending on the ratio of di- and tricalcium silicates

Lee Se Woong,Park Okmin,Kang Seungmin,Rho Hyung Tay,Kim Sang-il 한국세라믹학회 2023 한국세라믹학회지 Vol.60 No.6

Mineral trioxide aggregate (MTA) has gained attention as a bioactive endodontic material used for root-end filling, root repair, pulp capping, and iatrogenic root perforations, owing to its biocompatibility, high strength, and good sealing ability. Herein, physicochemical properties of MTA depending on the ratio of tricalcium silicate (C3S) and dicalcium silicate (C2S) was examined. A series of calcium silicate mixtures CSM of (100–x)C3S-xC2S (x = 0, 10, 14, 25, and 100) were prepared with 18 wt.% ZrO2 addition. The pH of the hydrated CSM samples in Dulbecco’s modified Eagle’s medium (DMEM) solvent were increased to 10.7–12.1 from 7.55 of free DMEM, primarily owing to Ca(OH)2 production during setting reaction of C3S and C2S. The solubilities of the hydrated CSM samples in deionized water were examined; the samples with high C2S content exhibited relatively low solubility mainly due to the longer setting time and smaller amount of Ca(OH)2 produced by C2S setting reaction compared to C3S. The flow of the CSM samples was measured by thickness of the hydrated samples under 120 g (ISO 6876:2012), which decreased with increasing C2S content, which indicates that higher C2S contents enhance penetration ability and workability of the MTA.

Influence of Elevated Temperature on the Mechanical and Microstructural Properties of Glass Fiber-Reinforced Polymers

Se Woong Lee,Seulgi Han,Okmin Park,Sang-il Kim,Sungmo Choi 대한금속·재료학회 2023 대한금속·재료학회지 Vol.61 No.10

Glass fiber-reinforced polymers (GFRPs) have attracted significant attention as structuralmaterials because of their high fatigue resistance, corrosion resistance, strength, and stiffness. This studyexamined the effect of elevated temperatures (150, 250, 350, and 450oC) on the microstructural andmechanical properties of GFRP plates. The number of bubbles increased as the firing temperature increased,and the bubbles burst at 250oC or higher, forming pores on the surface. A tensile test was conducted, andthe maximum stress of the GFRP plates fired at 150, 250, and 350oC was reduced from 54.2 to 52.2, 40.3,and 24.0 MPa, respectively, compared to that of the unfired GFRP plate. Meanwhile, the elastic moduli ofthe GFRP plates fired at 150, 250, and 350oC reduced from 19.1 to 18.3, 16.1, and 12.1 GPa, respectively,compared to that of the unfired GFRP plate. This reduction in the mechanical properties of the GFRP platesat elevated temperatures was attributed to the degradation of the mechanical properties of the resin matrixdue to glass transition and decomposition, debonding, and an increase in surface defects. The maximum straindecreased gradually with increasing firing temperature, suggesting that the brittleness of the GFRP platesincreased at elevated temperatures.

Transition to n-type Thermoelectric Conduction in Ni-doped FeSe2 Alloys

Young Bin An,Sang Jeong Park,Okmin Park,Sang-il Kim 대한금속·재료학회 2022 대한금속·재료학회지 Vol.60 No.12

We investigated the effect of Ni doping on the electrical, thermal, and thermoelectric transportproperties of FeSe2 polycrystalline alloys. FeSe2 alloys exhibit both n-type and p-type conduction at differenttemperatures and thus their practicality as a thermoelectric material is somewhat limited. In this study,Fe1-xNixSe2 polycrystalline samples with x = 0, 0.01, 0.05, 0.075, 0.1, and 0.125 were synthesized, and it wasshown that Ni doping in FeSe2 alloys induces n-type conduction in the entire temperature range. The electricalconductivity is gradually increased with an increase in carrier concentration as Ni doping increases, from 8.52(x = 0) to 329 S/cm (x = 0.125) at 300 K. The maximum power factor of 0.61 mW/mK2was observed in theFe1-xNixSe2 sample at x = 0.01 at 600 K. Other Ni-doped samples exhibited power factors between 0.31 and0.34 mW/mK2at 600 K. The thermal conductivity gradually decreased from 6.9 (x = 0) to 4.2 W/mK (x = 0.125)with Ni doping because of the additional point-defect phonon scattering of the Ni substitutes. As a result, azT of 0.11 was observed in x = 0.01 at 600 K, while the zT of the other Ni-doped samples exhibited 0.056-0.069 at 600 K.

Thermoelectric Properties of Cu-added Polycrystalline Bi2O2Se Oxyselenide

Si-young Bae,Se Woong Lee,Hyun-Sik Kim,Okmin Park,Hyunjin Park,Won-Seon Seo,Sang-il Kim 대한금속·재료학회 2022 대한금속·재료학회지 Vol.60 No.12

Bi2O2Se oxyselenide has been actively studied as a potential n-type thermoelectric material becauseof its intrinsically low thermal conductivity and high Seebeck coefficient (S). However, Bi2O2Se has very lowelectrical conductivity (σ), resulting in relatively poor thermoelectric performance. Herein, we investigate theeffect of Cu addition on the electrical and thermal transport of n-type polycrystalline Bi2O2Se. A series ofCuxBi2O2Se (x = 0, 0.0025, 0.005, and 0.0075) polycrystalline samples were synthesized by a conventional solid state reaction. Tetragonal Bi2O2Se was successfully synthesized, and its lattice parameters graduallydecreased with the addition of Cu. Further, σ decreased and the magnitude of S increased with increasingCu content, according to the trade-off relationship between these parameters. Consequently, a maximumpower factor of 0.106 mW m-1 K-2 was achieved for the sample with x = 0.0025 at 300 K, owing to the increasein the magnitude of S. The Hall carrier concentration decreased exponentially with the addition of Cu, whichis mainly attributed to the possible enlargement of the band gap of the Cu-added samples. The lattice thermalconductivity decreased with increasing x, which was attributed to point-defect phonon scattering via Cuaddition. Therefore, a maximum zT of 0.222 was obtained at 790 K for the Cu0.0025Bi2O2Se (x = 0.0025) sample,which was approximately 8% higher than that of the pristine Bi2O2Se sample.