Hyewon Lee

Effects of Al–21Ti–23Cr coatings on oxidation and mechanical properties of TiAl alloy

Abstract Ti–48Al specimens were coated with Al–21Ti–23Cr film of approximately 10 μm thickness at 200 W, 0.8 Pa and 300 °C using RF magnetron sputtering. The oxidation behavior of the coated specimens was investigated through isothermal and cyclic oxidation tests, and the effect of coating on the mechanical properties of TiAl alloy was, in particular, assessed by means of focusing on the tensile properties, which also provided the key to investigate the adhesion between TiAl substrate and Al–21Ti–23Cr coating and between Al–21Ti–23Cr coating and Al 2 O 3 scale. The isothermal and cyclic oxidation curves showed that the Al–21Ti–23Cr coating was very effective in improving the oxidation resistance of Ti–48Al at 1000 °C. This excellent oxidation resistance is attributable to the formation of a protective Al 2 O 3 scale on the surface of the Al–21Ti–23Cr film. Although extensive cracking in a transverse direction was observed on the surface of the film, the delamination of the coating from the substrate and the spallation of the Al 2 O 3 scale were not found. From the tensile test, it was confirmed that the Al–21Ti–23Cr coating enabled Ti–48Al to maintain its tensile properties regardless of exposure to oxidizing atmosphere. In addition, the result of the microhardness test indicated that the Al–21Ti–23Cr coating was very effective in suppressing the embrittlement of the Ti–48Al when exposed at 1000 °C. Thus, it could be suggested that the Al–21Ti–23Cr coating provides the key, not only to improve oxidation resistance, but also to maintain the mechanical properties of TiAl alloy.

Principles and Comparative Studies of Various Power Measurement Methods for Lithium Secondary Batteries

리튬이차전지의 수요가 소형 휴대용 전자기기에서 중대형 전기자동차와 에너지저장장치로 변화함에 따라, 요구되는 전지 특성도 크게 달라지고 있다. 10년 이상의 장기 신뢰성, 팩 수준의 안전성, 가격 경쟁력 확보 등 기존 소형 전지보다 크게 강화된 요구 조건뿐만 아니라, 고출력 특성도 함께 필요로 한다. 그런데, 출력...

Effects of Lithium Bis(Oxalate) Borate as an Electrolyte Additive on High-Temperature Performance of Li(Ni 1/3 Co 1/3 Mn 1/3 )O 2 /Graphite Cells

The effects of electrolyte additives, lithium bis(oxalate)borate (LiBOB), fluoroethylene carbonate (FEC), vinylene carbonate (VC), 2-(triphenylphosphoranylidene) succinic anhydride (TPSA), on high-temperature storage properties of /graphite are investigated with coin-type full cells. The 1 wt.% LiBOB-containing electrolyte showed the highest capacity retention after high temperature () storage for 20 days, 86.7%, which is about 5% higher than the reference electrolyte, 1.15M lithium hexafluorophosphate () in ethylene carbonate/ethyl methyl carbonate (EC/EMC, 3/7 by volume). This enhancement is closely related to the formation of semi-carbonate compounds originated from anions, thereby resulting in lower SEI thickness and interfacial resistance after storage. In addition, the 1 wt.% LiBOB-containing electrolyte also exhibited better cycle performance at 25 and than the reference electrolyte, which indicates that LiBOB is an effective additive for high-temperature performance of /graphite chemistry.

Cross-linkable Polymer Matrix for Enhanced Thermal Stability of Succinonitrile-based Polymer Electrolyte in Lithium Rechargeable Batteries

A polymer electrolyte was prepared by using polyvinylidenefluoride-co-hexafluoropropylene (PVdF-HFP) or poly(ethylene glycol) dimethacrylate (PEGDMA) as polymer matrices, succinonitrile as an additive, and lithium perchlorate as a lithium salt. Compared to the polymer electrolyte employing PVdF-HFP, the PEGDMA-based polymer electrolyte exhibits substantially superior thermal stability when exposed to high temperatures. Nonetheless, the ionic conductivity of the PEGDMA-based polymer electrolyte was preserved in a wide temperature range between and .