발간논문

Home > KJMM 논문 > 발간논문

γ-TiAl alloys have attracted interest as lightweight materials for structural applications at temperatures below 800 ℃. With recent improvements in microstrutural control of the γ-TiAl alloys, resulting in excellent creep properties and high temperature strength, their use can be extended to temperatures higher than 800 ℃. To increase their application temperature range, the oxidation resistance of γ-TiAl alloys must be improved through alloy design or coating techniques. To improve the high temperature oxidation resistance of γ-TiAl alloys, it is important to suppress the growth of TiO₂ oxide by forming a dense coating layer to block oxygen atoms. In this study, we tried to improve the high temperature oxidation resistance of a conventional γ-TiAl alloy, Ti-48Al-2Nb-2Cr (Ti 4822) using a pack cementation process, with carbon sources and activating materials, XRD and EDS analysis results for the surface products indicated the formation of coating layers comprising oxides and carbides including TiC, Al₄C₃, Ti₂AlC, and Ti₃AlC. Based upon the sequential formation of coating layers and phase diagrams, the diffusion path of the carbon during the pack cementation process is proposed. Also, detailed carburization and oxidation behaviors of the γ-TiAl alloys in the temperature range of 800-1000 ℃ are discussed in terms of microstructural observations. Enhanced high temperature oxidation resistance was obtained in Ti-48Al-2Nb-2Cr (Ti 4822) through a proper carbon pack cementation process. (Received 9 May, 2022; Accepted 16 July, 2022)

keyword : TiAl, carburizing, pack cementation, oxidation, diffusion path

Silica fibers were fabricated by sol-gel reaction and an electrospinning process. A high voltage source of electricity was applied to the prepared spinning solution to form the fibers. Macroporous silica fibers were prepared using polystyrene (PS) nanospheres as templates after calcination. The pore size could be controlled by adjusting the diameter of the PS nanospheres in the spinning solution. PS nanospheres with different diameters (250, 430, 600, 870, and 1000 nm) were synthesized for this purpose using the dispersion polymerization method. Silica fibers have a hydrophilic surface. A coating film applied to the fibers showed superhydrophilicity, which is not suitable for adsorbing oil contaminant. Thus, silane coupling agents containing methyl groups were used to modify the surface of the porous fibers to obtain hydrophobic and water-repellent properties. The amount of oil adsorbed by the porous silica fibers modified with various kinds of coupling agent or PS nanospheres with different sizes was investigated, to determine their effects on oil adsorption. When the size of the macropores in the fibers increased, the amount of oil adsorption increased, because oil infiltration through the pores became easier. Small hydrophobic groups of the silane coupling agent, like methyl groups, were able to adsorb more oil compared to bulky functional groups. The measured oil adsorption capacity of the porous fibers was found to be larger than that of mesomacroporous silica particles, since the voids between the fibers might provide additional space for oil adsorption. (Received 9 May, 2022; Accepted 10 July, 2022)

keyword : porous silica fibers, electrospinning, silane coupling agents, oil adsorption

Lithium is a representative rare metal and ranks 32nd in abundance among elements in the earth’s crust. Lithium is used in a variety of applications, including the production of organolithium compounds, as an alloying addition to aluminum and magnesium, and as the anode in rechargeable lithium ion batteries especially for electronic devices and electric vehicles. Today, lithium is an indispensable metal in our daily lives. It is important to recycle lithium from used lithium-ion batteries to prepare for lithium shortages and protect lithium resources. The active cathode material of a lithium ion battery contains other valuable metals including Ni, Co, and Mn. In this study, the effect of carbonation temperature on Li recovery from NCM (LiNi_xCo_yMn_zO₂) powder as Li₂CO₃ was investigated. First, a carbonation roasting was performed to convert the Li in the NCM powder into Li₂CO₃ at various temperature using a thermo-gravimetric analyzer. The roasted cinder leached into the water to dissolve the Li₂CO₃. The results showed that in Ar gas atmosphere the NCM phase was decomposed into Li₂O and Li_1-xM_1+xO₂ phases and the weight decreased by 4.7%, but in a CO₂ atmosphere Li₂CO₃ was formed, resulting in a 12.1% increase in weight. In the isothermal experiment, the weight and carbon concentration of cinder increased with temperature, and the Li ratio in the NCM gradually decreased. The NCM powder was able to react with CO₂ above 853 K, while some nickel, cobalt and manganese were regenerated into different Li_1-xM_1+xO₂ crystalline phases. The maximum Li recovery rate of 76% wsa achieved for 2 h carbonation roasting at 1073 K followed by water leaching, filtering and an evaporative crystallization process. (Received 29 March, 2022; Accepted 26 July, 2022)

keyword : lithium ion battery, recycling, carbonation, water leaching, lithium carbonate

The smelting reduction of spent LIBs produces metallic alloys containing Co, Cu, Fe, Mn, and Ni. These metallic alloys can be completely dissolved by 2 M HCl solution containing H₂O₂ as an oxidizing agent. Extraction of the leaching solution with D2EHPA results in a Fe(III) free raffinate. In this work, solvent extraction experiments were performed with a mixture of Aliquat 336 and LIX 63 to separate Cu(II) from the Fe(III) free raffinate. This mixture showed selectivity for Cu(II) over Co(II), Mn(II), Ni(II), and Si(IV). The mixture of 0.3 M Aliquat 336 and 0.2 M LIX 63 had the highest synergistic coefficient for the extraction of Cu(II) from the solution. The Cu(II) in the loaded mixture was easily stripped by weak sulfuric acid solution, which facilitates the recovery of copper sulfate from the stripping solution. McCabe-Thiele diagrams for the extraction of Cu(II) by the mixture and for the stripping with sulfuric acid solution were constructed. Three stages of counter-current extraction and stripping experiments verified that only Cu(II) was completely extracted, and the purity of Cu(II) in the stripping solution was higher than 99.9%. Considering the selectivity and easy stripping of Cu(II) from the loaded organic, this mixture can be employed in real operation for the recovery of valuable metals from the leaching solutions of spent LIBs. (Received 16 May, 2022; Accepted 26 July, 2022)

keyword : lithium-ion battery, solvent extraction, extractant mixture, copper ion

Nanostructured transparent conductors have been studied intensively for application to flexible and transparent electronic devices. Among diverse nanomaterials, electrospun nanofibers provide several advantages over other nanomaterials, including a high aspect ratio, uniform distribution, and a facile fabrication process. In the present study, an electrode composed of a core-shell (PVDF)-(Al) structured nanofiber mesh film was fabricated via electrospinning and sputtering processes, without any high-temperature heat treatment or wet chemical treatment. A free-standing circular collector was used to form a PVDF nanofiber mesh film, to perform the conformal coating of Al onto the PVDF surfaces. The parameters of the tip-to-collector distance and the electrospinning voltage needed to stably form a PVDF nanofiber mesh film were determined. The sheet resistance of the mesh film was dramatically reduced from 1870 Ω/sq. to 154 Ω/sq. when the sputtering time was increased from 2 min to 4 min, an outcome explained by the changed surface morphology of the Al coating layer. The sheet resistance decreased from 657 Ω/sq. to 15.4 Ω/sq. with an increase in the electrospinning time from 2 min to 8 min due to the increased numbers of nanofibers and junctions. When the transmittance at 550 nm with respect to the sheet resistance was plotted for these mesh films, the graph was divided into the bulk and percolation regimes. Mesh films with transmittance rates that exceeded 85% exhibited sheet resistance that clearly increased with an increase in transmittance, indicating a percolation network. Finally, the durability of the core-shell (PVDF)-(Al) structured nanofiber mesh film was evaluated using a repetitive bending test, the results of which clearly showed superior performance over Al thin film. With inexpensive metal Al, a competitive flexible and transparent electrode was fabricated in the form of a core-shell (PVDF)-(Al) structured nanofiber mesh film. (Received 22 July, 2022; Accepted 8 August, 2022)

keyword : aluminum, PVDF, nanofiber, electrode, transmittance, flexible

Ultrathin MgO nanosheets were successfully synthesized by thermal evaporation of a mixture of Mg and graphite powders as the source material. The synthesis was performed at 1000 ℃ in air. Scanning electron microscopy showed that the two-dimensional MgO nanosheets had widths of several micrometers and the thickness of less than 20 nm. X-ray diffraction analysis revealed that the MgO nanosheets had a cubic crystal structure and high purity. Zero-dimensional MgO nanocubes were formed at temperatures below 1000 ℃ and one-dimensional MgO nanowires were grown at a temperature higher than 1000 ℃. As the synthesis temperature increased, the morphology of the Mg nanocrystals changed from cube to sheet and then wire. The experimental results suggested that the difference in Mg vapor concentration could be responsible for the morphological change in the MgO nanocrystals. When Mg vapor concentration was low, MgO nanocrystals were grown with a cubic shape. A relatively high concentration of Mg vapor led to the growth of sheet-like MgO nanocrystals. A very high Mg vapor concentration favored the growth of MgO nanowires. The growth mechanism is discussed based on the Mg vapor concentration and the crystal structures of Mg and MgO. Visible emissions, which were attributed to lattice defects such as oxygen vacancies, were observed in the cathodoluminescence spectra of the MgO nanocrystals. (Received 13 June, 2022; Accepted 25 July, 2022)

keyword : magnesium oxide, nanosheets, thermal evaporation, air, magnesium vapor concentration

Metallurgical remains such as crucible, mould, slag and tuyere are generated in the production of bronze. The analysis of slag in particular can be used to identify the raw materials, processes and environments (temperature and condition) employed in the bronze fabrication. The Gwanbuk-ri site in Buyeo is known as the location of a royal palace during the Baekje period, in the 6-7th centuries, and crucible fragments and slags used to make bronze were excavated there. The purpose of this study is to understand the process of bronze production and the origin of the raw ores used in the Baekje period, through micro-structure and micro-chemical analyses. Using chemical analysis and observation of the microstructure of the crucible and slag samples, we identified evidence of the bronze alloying and melting conditions. The melting process would have taken place under a variable redox atmosphere at 1,100 ℃. Cu-Sn-Pb ternary bronze might have been produced by alloying Cu-Sn metal with lead ore (galena) in the Baekje period. The lead isotope composition ratios of 2 bronze slags were located in zone 2 and zone 3 of the lead isotope map in the southern Korean peninsula, which indicates the use of galena from south Chungcheong and north Gyeongsan provinces. For efficiency and cost-effectiveness, a concentrate of lead ore might have been transported from the mine to the consumer site. (Received 1 February, 2021; Accepted 3 August, 2022)

keyword : bronze, slag, microstructure, lead isotope, technology, production

The influence of trace amounts of sulfur (S) on the microstructural evolution, tensile and creep properties in directionally solidified (DS) Ni-based superalloy GTD-111 were systematically investigated. Doping of S in the range of < 1 to 154 ppm resulted in the formation of a Ti₂SC-type carbosulfide phase near the coarse eutectic γ/γ and η (eta, Ni₃Ti) phases in interdendritic regions and grain boundaries (GB). The morphology of the Ti₂SC was found to change from discrete particles to curved and elongated (film-like) shapes with increasing S content. The measurement of GB line fractions revealed that the GB fraction of Ti₂SC increases with S content and reaches about 20% in 154 ppm S alloy. Tensile test results showed that the tensile properties at room temperature were not influenced by S doping, while the tensile elongation at 650-980 ℃ significantly decreased with increasing S content. The creep life and rupture elongation were also found to decrease with S content. The normalized creep life of 154 ppm S alloy was only in the 69%-74% range, compared to that of < 1 ppm S alloy. Careful microstructural observation of the fracture surface and longitudinal section of the creep ruptured 154 ppm S alloy revealed that Ti₂SC plays a significant role in crack formation at the matrix γ/ Ti₂SC interface, and leads to brittle facet-like traces on the fracture surface. It was concluded from these results that the formation of film-like Ti₂SC in high S alloys might be responsible for the reduction in creep life and rupture elongation of DS GTD-111 alloy. (Received 6 July, 2022; Accepted 29 July 2022)

keyword : ni-based superalloys, GTD-111, sulfur, Ti_{2SC, tensile property, creep property}

For medical device and artificial skin applications, etc., large-area tactile sensors have attracted strong interest as a key technology. However, only complex and expensive manufacturing methods such as fine pattern alignment technology have been considered. To replace the existing smart sensor, which has to go through a complicated process, a new approach including a simple piezoresistive patch based on artificial intelligence has been suggested. Specifically, a 16-electrode terminal was connected to the edge of a polydimethylsiloxane pad where multi-walled carbon nanotube sheets are well dispersed, and a voltage input to the center of the specimen. The collected data was calculated using a voltage divider circuit to collect the voltage data. 54 random positions were marked on the pad. 4 positions were configured as the validation data set and 50 positions as the training data set. We examined whether it was possible to determine points in untrained positions using a deep neural network (DNN) and 12 different machine learning (ML) algorithms. The result of a deep neural network for untrained point location identification was MSE: 0.00026, R2: 0.991158, and the result of Random Forest, an ensemble model among ML algorithms, was MSE: 0.00845, R2: 0.971239. Real-time position detection is possible using smart sensors created by combining simple bulk materials and artificial intelligence models from research results. (Received 27 June, 2022; Accepted 18 July, 2022)

keyword : carbon nanotube, piezoresistive materials, tactile sensing, artificial intelligence, machine learning