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In this study, the liquation cracking temperature range (LCTR) of over-aged 247LC superalloy repair welds was quantitatively evaluated using spot-Varestraint testing, especially for application in the repair weld heat-affected zone (HAZ). The metallurgical mechanism of the evaluated liquation cracking susceptibility and variation of the LCTRs of the over-aged 247LC superalloys was clarified by analyzing the microstructural characteristics at the liquation cracking surface, and using thermodynamic calculations. From the spot-Varestraint tests of the over-aged materials, the LCTR was quantitatively evaluated to be 410 and 620 K for the 500 and 1000 h over-aged specimens, respectively. These results demonstrate that the over-aged 247LC superalloys face significant and serious liquation cracking susceptibility in repair weld HAZ. Based on local element distributions at the liquation cracking surface, analyzed by transmission electron microscopy and the Thermo-Calc calculations, the underlying metallurgical mechanism of high LCTRs of over-aged materials could be attributed to the precipitation of fine MC and M₆C carbides during the over-aging treatment. The carbides promote local liquation at the interface between the gamma phase at temperatures lower than the equilibrium solidus (1530 K) of the 247LC. The estimated liquation initiation temperatures at the MC/γ and γ/M₆C interfaces were 1125 and 1420 K, respectively. (Received 3 November, 2022; Accepted 28 November, 2022)

keyword : 247LC superalloy, repair welding, liquation cracking, Varestraint test, carbides

This article shows how fractures in the Si₃N₄ layer, which comprises the top layer of semiconductor devices encapsulated utilizing a lead-on-chip (LOC) packaging technique, are influenced by changes in the lead-frame materials and thermal-cycling test conditions. Using thermal-cycling tests, it was found that fractures in the Si₃N₄ layer are the most sensitive to changes in the lead-frame materials at the early stage of thermal-cycling, between -65 ℃ and 150 ℃. Through SEM examinations and stress simulations, this work shows that adopting a copper lead-frame with a CTE-value similar to that of a package body effectively prevents filler-driven Si₃N₄ damage, providing semiconductor devices with better reliability margins during thermal-cycling. (Received 14 July, 2022; Accepted 14 November, 2022)

keyword : semiconductor, chip, stress, fracture, reliability

We fabricated an additive-free MAX(Ti₃AlC₂) phase Li-ion battery (LIB) electrode using the electrophoretic deposition (EPD) method. In this study, MAX, a precursor of MXene, which has recently been receiving great attention as a negative electrode material for LIBs, was manufactured as a coin cell through EPD rather than the conventional slurry system. We excluded the effect of additives on the electrochemical performance, enabling evaluation of the intrinsic electrochemical properties related to battery charging and discharging. As a result, the battery using MAX as an anode material showed a large specific capacity of 148.2 mAh/g in the first discharge and superior cycle stability. Enhanced cycle stability and reversible electrochemical reactions were attributed to activation of faradaic and non-faradaic behavior, i.e., pseudo-capacitive behavior, caused by delamination of the MAX(Ti₃AlC₂) into MXene (Ti₃C₂). This was confirmed by the decrease in the charge transfer resistance and the increase in total capacitance at the interface, using electrochemical impedance spectroscopy and cyclic voltammetry measurements. In addition, the activation of pseudocapacitive behavior was confirmed by the change in kinetic mechanism, as evidenced from a significant increase in the Li ion diffusivity with cycles. These results demonstrate that MAX(Ti₃AlC₂) is promising as an anode material for LIBs and at the same time shows potential for tuning electrochemical properties through the electrochemical delamination process. (Received 1 July, 2022; Accepted 8 November, 2022)

keyword : EPD, MAX, pseudocapacitive behavior, Mxene

With increasing interest in energy harvesting using heat as a next-generation eco-friendly energy source, organic-inorganic perovskite materials have emerged as promising materials for thermoelectric devices. In particular, the photo-Seebeck effect of halide perovskite materials has attracted attention due to their wide optical absorption spectrum and large diffusion length, depending on their composition. MAPbI₃, a representative organic perovskite component, has been reported to have a Seebeck coefficient of only hundreds of μV/K. In this manuscript, we report a photo-Seebeck effect for bulk MAPbI₃ perovskite in which the magnitude of the Seebeck coefficient significantly increased by 700 μV/K under illumination with a green laser diode. An air-stable perovskite pellet was synthesized using the alcohol substitution synthesis method, and both the Seebeck coefficient and the photocurrent increased in air, proving that enhanced Seebeck coefficient is associated with the formation of excitons in MAPbI₃. X-ray diffraction analysis found that the remnant PbI₂ led to n-type electronic transport characterized by a negative Seebeck coefficient. Photo-induced electron transfer from MAPbI₃ to the PbI₂-rich phase under illumination led to dedoping of electrons, to form an MAPbI₃ pellet. The significant enhancement in the Seebeck coefficient was found to depend on the composition of the remnant PbI₂, which alters the majority carrier type in the bulk MAPbI₃. (Received 2 July, 2022; Accepted 8 November, 2022)

keyword : organic-inorganic hybrid perovskite, photo-seebeck effect, thermoelectric, seebeck coefficient

Environmentally sustainable thermoelectric technologies can be more broadly applied in industries once the performance of thermoelectric materials is improved. Several approaches have been proposed to improve the electronic transport properties of thermoelectric materials. The effects of each approach on the electronic properties can be evaluated by changes in the band parameters. The Single Parabolic Band (SPB) model has been widely used to determine the effect of different materials engineering strategies on band parameters, such as the density-of-states effective mass and deformation potential. However, when the material has a narrow band gap, the Two-Band (TB) model better describes the changes in band parameters, as it includes the bipolar conduction from the minority carrier band. Here, the band parameters of previously reported Fe(Se_1-xTe_x)₂ (x = 0, 0.2, 0.6, 0.8, 1) alloys, whose band gap significantly decreases with x, have been estimated using the SPB and TB models. While the x-dependent band parameters obtained via the SPB model varied abruptly with x, all the band parameters estimated by the TB model changed linearly with x. The abruptness observed in the band parameters of the SPB model can be attributed to artifacts reflected in the single band, which occurs when the minority carrier band and band gap change are not included. The bipolar thermal conductivity of Fe(Se_1-xTe_x)₂ alloys was also calculated using the TB model, and is understood in terms of changes in the weighted mobility ratio, Hall carrier concentration, and band gap in terms of alloy composition, x. (Received 24 October, 2022; Accepted 28 November, 2022)

keyword : thermoelectric, single parabolic band model, two-band model, FeSe₂-FeTe₂, bipolar thermal conductivity

In this study, a mechanical exfoliation method was developed for the synthesis of high purity Bi₂Te₃, Sb₂Te₃, and Bi_0.5Sb_1.5Te₃ flakes. The synthesized Bi₂Te₃, Sb₂Te₃, and Bi_0.5Sb_1.5Te₃ flakes were characterized by atomic force microscope, Raman spectroscopy, and photoluminescence (PL). The effect of the thickness of the Bi₂Te₃, Sb₂Te₃, and Bi_0.5Sb_1.5Te₃ flakes on PL and Raman spectra was investigated. As the thickness increased in the Bi₂Te₃ flakes, the out of plane vibration mode (A²_1g) shows a blue shift. For the thicker Sb₂Te₃ flakes, the A¹_1g and E²_g modes indicated a blue shift and a red shift, respectively. When the thickness of Bi_0.5Sb_1.5Te₃ flakes decreased, the in-plane vibration mode (E²_g) shifted to lower frequencies. A new Raman peak has been observed in Bi_0.5Sb_1.5Te₃ flakes, which is not active in the thin films. PL measurements of Bi_0.5Sb_1.5Te₃ flakes with various thickness revealed PL peaks in the range of 2.15 - 2.54 eV at room temperature (300 K). A Bi_0.5Sb_1.5Te₃ flakes-based photodetector exhibited photoresponsivity as high as 661.5 A/W at a 0.02 mW power density with an 1800 nm laser at room temperature (300 K). Compared to the optoelectronic properties of Bi₂Te₃ flake, a twice higher responsivity at a wavelength of 1800 nm was observed with the Bi_0.5Sb_1.5Te₃ flake-based photodetector. (Received 30 October, 2022; Accepted 28 November, 2022)

keyword : Bi₂Te₃, Bi_0.5Sb_1.5Te₃, raman spectroscopy, optoelectric properties, photocurrent

As interest in hydrogen energy grows, eco-friendly methods of producing hydrogen are being explored. CuBi₂O₄ is one of the p-type semiconductor cathode materials that can be used for photoelectrochemical hydrogen production via environment-friendly water electrolysis. CuBi₂O₄ has a bandgap of 1.5 - 1.8 eV which allows it to photogenerate electrons and holes from the absorption of visible light. This study investigated the effect of sulfur doping on the bandgap and photoelectrochemical water reduction properties of CuBi₂O₄. Sulfur-doped CuBi₂O₄ thin films were electrochemically synthesized using a nitrate-based precursor solution with thiourea. This was followed by two-step annealing in an Ar atmosphere, which effectively prevented the oxidation of sulfur. Sulfur doping up to 0.1 at% led to the expansion of the lattice volume of the CuBi₂O₄. The bandgap was reduced from 1.9 eV to 1.5 eV with increasing doping concentration, which resulted in the enhancement of photoelectrochemical current density by ~240%. X-ray photoelectron spectroscopy showed that sulfur-doping reduced oxygen vacancies with increasing doping concentration, confirming that the enhanced photoelectrochemical properties resulted from the reduction in bandgap, not from any extrinsic factor such as oxygen vacancies. Further studies of sulfur-doped CuBi₂O₄ to improve surface coverage are expected to lead to a more promising photoelectrochemical cathode material. (Received 28 December, 2022; Accepted 4 January, 2023)

keyword : CuBi₂O₄, photoelectrochemical water splitting, hydrogen production, sulfur-doping, bandgap reduction

In the present study, we analyzed the thermal and mechanical properties of Cu-graphite composites (CGCs) with spatially anisotropic graphite layers. These composites were fabricated using a combination of electroless plating and spark plasma sintering (SPS) processes. Thermal conductivities and thermal expansion coefficients of the composites were measured using differential thermal analysis (DTA) and a laser flash method. In particular, the thermal expansion coefficients of the composites were investigated by comparative analysis, which was conducted by processing prototypes in a vertical direction with a parallel sector in the direction of the upper and lower axial pressure. The Cu reinforced with the graphite flake showed better thermal properties compared to the graphite fiber, while the graphite fiber led to better mechanical properties. This investigation was conducted to better understand the dependence of thermal properties on the morphologies of the graphite layers (i.e., flake and fiber types) in the Cu matrix. The bending strength and friction coefficient of the composites were also investigated. Taken together, the results from this work offer fruitful, yet practical information to utilize the CGCs as a thermal management material, essential for electric and electronic devices. (Received 2 June, 2022; Accepted 11 November, 2022)

keyword : Cu-graphite composite, spatial anisotropy, thermal and mechanical properties, spark plasma sintering

The efficiency of gas turbines depends on the gas turbine working temperature. Single crystal blades are being applied more often than equiaxed blades in gas turbine engines, to increase the turbine inlet temperature, resulting in enhanced turbine engine efficiency. Single crystal blades endure creep conditions at high temperature better than polycrystal blades because the single crystals do not include grain boundaries. The single crystal process is a breakthrough technology, however, production yield is relatively low compared with polycrystal, and their mechanical properties depend on the crystallographic orientation of the single crystals. In this study, a thermal simulation model, the 3D cellular automation-finite element (CA-FE), was used on the single crystal process with the Bridgman method. The simulation model was well expected, by analysis of the microstructure and EBSD, on the grain selection in the single crystal process. The evolution of single crystal grains was analyzed on process of grain selection in start block and spiral selector. Single crystal orientation was also investigated to determine the effect of nucleation density, forming in the initial stage of solidification. (Received 20 September, 2022; Accepted 7 November, 2022)

keyword : superalloy, directional solidification, CMSX-4, single crystal, procast

There is substantial concern about the phase change to artificial ferrite or martensite in commercial austenitic stainless steels during intense focused-ion-beam milling. In this work, severe Ga ion milling to prepare microscale samples was found to produce thin transformed layers on the surface of austenitic stainless steel. The surface BCC phase transformation layer was clearly distinguished on the specimen processed with the focused ion beam using cross-sectional analysis. It was determined that Ga ions had accumulated in the surface BCC layer. However, it was also confirmed that the Ga ion accumulation was localized at the top of the surface. This is expected to be caused by the decrease in the formation energy of the body-centered crystalline phase, which may result from the considerable changes in chemical composition and stress state by the accumulation of implanted Ga atoms. We propose an effective method to overcome the phase change problem caused by FIB, by using low-energy Ar ion milling. It was determined that low-energy FIB milling was insufficient by itself to clear the phase transformation layer. High quality micro-scale experimental samples of austenitic stainless steels can be achieved by additional low-energy Ar ion milling at an energy of 1 keV or less after high-energy FIB milling. (Received 29 August, 2022; Accepted 8 November, 2022)

keyword : FIB-induced phase change, low-energy Ar ion milling, focused ion beam, transmission electron microscopy, austenitic stainless steel