발간논문

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In this study, artificially formed corrosion and paint layers on 304L stainless steel (SS304L) specimen were removed using a kilowatt level high-power Nd:YAG laser. A laser power of 1140 W and a pulse duration of 89 ns were used for the corrosion removal, and a laser power of 850 W and a pulse duration of 54 ns were adopted for the paint removal. Surface composition was analyzed via electron probe micro-analysis (EPMA) and x-ray diffraction (XRD) analysis before and after the laser cleaning process, confirming that the corrosion and paint layers were successfully removed from the SS304L surface by the laser cleaning process. The effect of the laser cleaning process on the mechanical properties of SS304L was investigated. The hardness and tensile strength of the laser-cleaned specimens were analyzed to determine their mechanical properties. After the laser cleaning process, the surface hardness was slightly increased up to depths of 250 μm for the corrosion removal and up to 50 μm for the paint removal. However, only marginal changes in tensile strength and elongation were detected after the laser cleaning process. These results confirmed that laser cleaning can effectively remove surface contaminants (i.e., corrosion and paint) on SS304L with only a minimum modification of microstructure and mechanical properties. (Received 31 August, 2023; Accepted 8 November, 2023)

keyword : Laser cleaning, SS304L, Corrosion removal, Paint removal

Inconel 718 nickel-based alloy is extensively used in the aerospace industry (e.g., gas turbine engine components) because of its excellent corrosion resistance and high mechanical properties at elevated temperatures. However, there is a certain limit to manufacturing the alloy through plastic deformation due to its high deformation resistance and complicated deformation behaviors. In this study, the hot deformation behavior of Inconel 718 alloy was investigated to establish how processing conditions of flow stress-strain, at strain rates from 0.001 to 10 s^-1, and temperatures from 850 to 1200℃, affected dynamic recrystallization. The regression-based material model was utilized to calculate the strain-rate sensitivity, and subsequently depict the efficiency of the power dissipation and instability criterion of hot deformation. The processing map and instability criterion predicted by the developed 3^rd-order polynomial regression model corresponded with the experimental results and in particular, showed a better prediction for instability regime compared to the existing discrete derivative approach. Predicting the strain-rate sensitivity values on a continuous scale with regression analysis covered the additional instability region of the high strain rate near 10 s^-1. The dynamic recrystallization deformation was also characterized by microstructural analysis along with the processing map. Consequently, ring-rolled aviation parts were manufactured with the optimum processing parameters, which conform to the AMS 5663 standard (Aerospace material specifications for Inconel 718). (Received 6 October, 2023; Accepted 11 November, 2023)

keyword : Inconel 718, hot deformation, model regression, processing map, dynamic recrystallization

Lead selenide (PbSe) diodes were fabricated using a magnetron sputtering process system with a pulsed DC power supply and a 2-inch PbSe target with a purity of 5N. For p-type PbSe thin films, the process variable was the oxygen ratio in the mixed gas of argon and oxygen. The electrical characteristics of the thin films were observed after heat treatment. For the n-type PbSe, nickel (Ni) was used as a doping material. The deposition and doping were performed simultaneously using a co-sputtering method. During co-sputtering, the input power of the Ni sputter gun was adjusted as a process variable. Hall measurement experiments were performed to measure the doping concentration and resistivity of both the p-type and n-type PbSe semiconducting films. The maximum doping concentration was 2.33×10¹⁹ cm^-3 for p-type PbSe and 7.55×10²⁰ cm^-3 for n-type PbSe thin films, respectively. The p-n junction IV curve showed that the lowest forward voltage generation point, V_f, was 1.5 V and the reverse breakdown voltage was -4.3 V. In the photocurrent measurement, the photo sensitivities of the heat-treated samples were higher than that of the non-treated sample, and the maximum value was 5.148. Photo responsivity was also higher in the heat-treated samples. Its maximum was 0.7306 mA / W. (Received 9 June, 2023; Accepted 8 November, 2023)

keyword : PbSe, Co-sputtering, Nickel doping, p-n PbSe Diode, Hall measurement, I-V Curve, Photo Current

This research elucidates the intricate interplay of lithium content, oxygen pressure, and temperature and their influence on the electrical properties of Lithium-doped zinc oxide (LZO). The electrical behavior of zinc oxide (ZnO), a prominent semiconductor material, can be modulated when doped with lithium (Li). Through systematic experimentation, we demonstrate that varying lithium content in the ZnO matrix leads to notable shifts in carrier concentration and mobility, which in turn impacts the material's conductivity and overall electrical performance. Furthermore, oxygen pressure during synthesis plays a pivotal role in defect formation, especially oxygen vacancies, which interact dynamically with lithium dopants to further modulate electrical behavior. Introducing the variable of temperature, our study reveals a synergistic effect, where temperature not only affects intrinsic carrier concentration but also the interactions between lithium dopants and inherent defects in ZnO. Under optimized conditions of oxygen pressure and temperature, the influence of Li content on crystallinity was pronounced, consequently impacting mobility. In contrast, under unoptimized conditions, as Li concentration increased, particularly beyond optimal levels (0.75 mol%), introduced Li atoms assumed the role of compensating centers by capturing or neutralizing carriers, reducing mobility. The findings presented herein provide a comprehensive understanding of how these factors collectively determine the electrical properties of LZO, paving the way for tailored applications in optoelectronic devices, sensors, and more. (Received 6 October, 2023; Accepted 9 November, 2023)

keyword : zinc oxide, Li-doping, mobility, field effect transistor, carrier concentration

Many studies have been conducted on methods to remove air pollutants. Among them, a representative commercial technology for reducing nitrogen oxides (NO_X) is selective catalytic reduction (SCR) using NH₃ as a reductant. However, the NH₃-SCR technology has problems, such as the need for additional NH₃ gas (Active-SCR), the fact that unreacted NH₃ (slipped-NH₃) in the reaction process can be re-oxidized to NO_X, and the facility system cost according to the added NH₃ gas. In order to solve these problems, CO gas that is naturally generated in industrial processes can be used, and NO and CO can be removed at the same time using passive-SCR technology that does not require a separate injection of a reductant. In this study, transition metals Ni and Fe were used as catalytic materials to replace expensive precious metals. The results confirmed that both NO_X and CO removal efficiencies were about 90% or more in the catalyst with combined Ni-Fe, compared to the single catalyst. The addition of Ni to the Fe catalyst increased the catalytic reducing power, which also affected the increase in acid sites. Therefore, it is expected that this catalyst can simultaneously remove NO and CO without the use of NH₃ gas. These results could be explained through XRD, FT-IR, TEM, BET, H₂-TPR and CO-TPD analyses, and compared to single catalysts, it was confirmed that the catalyst had higher low-temperature desorption ability for CO as a reductant and reducing capability. (Received 18 August, 2023; Accepted 9 November, 2023)

keyword : transition metal oxides, selective catalytic reduction, CO-SCR, bimetallic oxides catalysts

Titanium alloys are currently used in offshore industries. The titanium alloys used in offshore plants need to have excellent corrosion resistance and mechanical properties because offshore plants are exposed to harsh corrosive environments. At this time, the Ti-6Al-4V ELI alloy is mainly used in offshore plants because it has excellent mechanical properties and corrosion resistance. However, Ti-6Al-4V ELI alloy has the disadvantage of poor formability. To improve this, studies on metastable beta-Titanium alloys with a BCC structure are being actively conducted. In this study, a metastable beta-titanium alloy was fabricated using Mo, which is inexpensive and improves corrosion resistance, and Fe, which improves strength, among the titanium beta stabilizer elements. After the Ti-6Al-4V ELI and Ti-xMo-2Fe alloys solution treatments, electrochemical corrosion experiments were conducted to analyze corrosion characteristics, and mechanical properties were also analyzed through compression tests at room temperature and Vickers hardness measurement. Corrosion properties and the mechanical properties of Ti-xMo-2Fe alloy were considered in connect experiments such as microstructure analysis and hardness measurement. It was confirmed that the corrosion resistance of the Ti-xMo-2Fe alloy was better than that of the Ti-6Al-4V ELI alloy, and it was confirmed that the compressive strength and elongation rate of Ti-xMo-2Fe with its BCC structure get higher than Ti-6Al-4V ELI alloy. (Received 6 October, 2023; Accepted 13 November, 2023)

keyword : Ti-Mo-Fe alloy, Beta stabilizer, Corrosion properties, Mechanical properties

The effect of solution temperature and quench delay on the microstructure and mechanical properties of an extruded Al-Mg-Si-Cu-Mn alloy was investigated by employing differential scanning calorimetry, transmission electron microscopy, hardness test, and tensile test. The extruded specimens were held at 500 and 540 ℃, respectively, for 80 min for solutionizing and then immediately quenched in water or exposed to air for 30 sec before water quenching. Each specimen was further heat treated at 180 ℃ for 6 hr for artificial aging. Quantitative analysis of the precipitates in the artificially aged specimens was performed using high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and fast Fourier transforms (FFT) analysis. Solution treatment at 540 ℃ led to higher number density of precipitates and higher strengths after artificial aging, compared with solution treatment at 500 ℃. Quench delay resulted in a lower number density of precipitates and lower strengths after artificial aging. The specimen solution treated at 540 ℃ with no quench delay showed the largest peak area of precipitate formation in the differential scanning calorimetry, and concurrently exhibited the highest strengths after artificial aging. Strength reduction by quench delay was higher in the specimen solution treated at 500 ℃ than that treated at 540 ℃; this was consistent with the results of higher reduction in the number density of precipitates. (Received 7 September, 2023; Accepted 8 November, 2023)

keyword : Al-Mg-Si-Cu-Mn alloy, Extrusion, Solution temperature, Quench delay, Microstructure, Mechanical properties

In the modern era, advanced industrial and process technologies in various fields are known to produce harmful gases. These gases typically leak at low concentrations, but continuous exposure can cause serious harm to health and safety. Since human sensory organs cannot detect these risks or leaking hazardous gases, there has been significant interest in developing high-performance gas sensors capable of rapidly detecting harmful and hazardous gas leaks. Metal oxides possess several advantages for such applications, including ease of manufacturing, affordability, and high sensitivity to harmful gases. In this review, we highlight recent progress in the field of chemo-resistive gas sensors based on metal-oxide nanostructures. It begins by discussing the features of various nanostructure morphologies such as 0-dimensional nanospheres to nanocomposites of nanowires. It then describes methods for fabricating metal-oxide nanostructures. Finally, we describe recent advances in the design and fabrication of nanostructures for metal-oxide gas sensors using four different morphologies: 0-dimensional (0 D, nanosphere and nanoparticle), 1-dimensional (1 D, nanowire and nanorod), 3-dimensional (3 D, nanoflower and nanocomposite), and heterojunctions. Based on their morphology-dependent sensing properties, there are various fields of application for gas sensors, with different target gases. This review can serve as an overview of recent research trends and offer insights into the future development of next-generation high-performance gas sensors. (Received 6 October, 2023; Accepted 14 November, 2023)

keyword : metal oxide, chemoresistive gas sensors, sensing materials, nanostructure, gas sensing

In recent years, the exploration of van der Waals materials has taken center stage in the field of advanced materials research. Studies have delved deeply into the multifaceted characteristics of these materials, and in particular their potential applications in solar cell technology. Van der Waals materials, named after the unique forces binding their layers, are of high interest because of their unparalleled physical and chemical properties. Their two-dimensional (2D) nature offers a plethora of advantageous features, including a high surface-to-volume ratio, which is crucial to enhanced light absorption and electron transport. The tunable electronic properties of these materials also offer a versatile platform for customizing behavior to meet the specific requirements of solar cell applications. Their distinctive optical characteristics can also be enhanced to improve light absorption and reduce energy losses, thereby increasing overall solar cell efficiency. Recent research has not only investigated the inherent properties of van der Waals materials but also explored various techniques to combine and stack them. Such methodologies aim to exploit and optimize the synergistic effects of different materials, paving the way for next-generation solar cells with improved performance metrics. A comprehensive examination of the operating principles of these materials provides insights into their interaction with light and their subsequent electrical behavior. The study concludes with a forward-looking perspective, emphasizing how the integration of van der Waals materials could very well revolutionize the solar cell industry. Their potential to significantly boost the efficiency and stability of solar cells positions them as promising candidates for future sustainable energy solutions. (Received 21 September, 2023; Accepted 20 November, 2023)

keyword : Van der Waals, 2D Transition Metal Dichalcogenides, Interface Engineering, Photovoltaics