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An ultrasonic nanocrystal surface modification (UNSM) technique was applied to a 1-mm thick AZ31 magnesium sheet. UNSM is a relatively new surface modification technique in which a hard, hemispherical tip (2.38 mm in diameter) strikes the surface at an ultrasonic frequency to induce plastically deformed gradient microstructures and deep compressive residual stresses through the thickness. After the UNSM treatment, the through-thickness microstructures were thoroughly investigated using electron microscopy and electron backscatter diffraction analysis. The through-thickness microstructures revealed zones that were severely deformed (down to 200 μm from the surface) and twin-dominated (200~300 μm deep from the surface). The severely deformed zone consisted of shear banding, grain subdivision and reorientation, due to the strong plastic deformation, accompanied by the formation of {1012} tensile twins (despite compressive strikes by the hemispherical tip), {1011}-{1012} double twins and {1011} compression twins. The cause for tensile twinning was examined through a literature survey. In the twin-dominated zone, the twining activity prevailed as the slip activity gradually decayed through the thickness. The UNSM-induced hardness and microstructure enhancement was found to be effective down to about 300~400 μm deep from the surface. Finally, the source of the increase in yield strength after the UNSM treatment of the AZ31 sheet was analyzed, and focused on individual cases of microstructural enhancement in the severely deformed zone and the twin zone, and the compressive residual stress. (Received 21 July 2021; Accepted 15 December 2021)

keyword : AZ31 Mg, ultrasonic nanocrystal surface modification (UNSM), through-thickness microstructures, effective depth, strength

In this study, Ti-10Mo alloy powder and sintered bodies were successfully fabricated using a threestep process of acid/organic solvent cleaning, hydrogenation/dehydrogenation (HDH) of bulk Ti-Mo scrap, and pressureless vacuum sintering of the alloy powder. This process can be used to recycle valuable Ti alloy. And unlike remelting-based recycling processes, in which elements vaporize, this process maintains the original ratio of Ti and Mo and results in no loss of elements during HDH and sintering. We note, however, that the phases in the resulting Ti-Mo alloy were changed by HDH and the sintering processes, and as a result the phases of the alloy evolved differently than those of alloys prepared by the blending of Ti and Mo powders. XRD results of the processed sintered bodies revealed different pathways for the transformation of the crystal structure. As sintering temperature increased, the dominant phase changes in the crystal structure of the sintered bodies that used commercial powders were α' → α' + α'', while the phase changes for the recycled Ti-10Mo sintered bodies were α' + α'' → α'' + β. Although the density of the sintered body using commercial materials was generally higher at a lower sintering temperature than the density of the sintered body that used recycled powder, at 1673 K their respective densities were nearly identical. Moreover, the sintered body that contained scrap showed superior Vickers hardness after the dual phase (α + β) of Ti was reached, despite its relatively low density. (Received 13 September 2021; Accepted 16 November 2021)

keyword : Ti-10Mo alloy, sintering, pre-alloyed, XRD, recycling, hydrogenation-dehydrogenation

The demand for palladium (Pd) has increased and this has made it necessary to recover pure Pd from diverse secondary resources. To recover the small amount of Pd(II) present in spent electroplating solutions, Pd(II) is concentrated by cementation with zinc metal. In this work, a hydrometallurgical process consisting of leaching and solvent extraction was developed to recover Pd from the cemented Pd. The extraction behavior of Pd(II) and Zn(II) from synthetic HCl solution by commercial extractants such as LIX 63, Cyanex301, DOS, TBP, Alamin 308, Alamine 336 and Aliquat 336 was investigated by varying HCl concentration. The results showed that LIX 63, Cyanex 301 and DOS selectively extracted Pd(II) over Zn(II). These three extractants were employed to separate Pd(II) and Zn(II) from actual leaching solutions with cemented Pd using a mixture of 7.0 M HCl and 0.5% M H2O2 with 2 g/L pulp density at 60oC for 120 min. Cyanex 301 and DOS selectively extracted Pd(II) over Zn(II) from the leaching solution, while a small amount of Zn(II) was co-extracted with Pd(II) by LIX 63. A two stage counter-current extraction of actual leaching solution with LIX 63 resulted in the co-extraction of Zn(II) with Pd(II). The complete stripping of Pd(II) from the loaded Cyanex301 was achieved using either diluted aqua regia or a mixed solution of 5 M HCl and 0.5% NaClO at an A/O ratio of ten. The hydrometallurgical process is proposed to recover a pure Pd(II) solution from cemented Pd. (Received 17 August 2021; Accepted 16 December 2021)

keyword : hydrochloric acid, palladium, zinc, solvent extraction, spent electroplating solution

In this study, we demonstrate high-performance optical wavelength-selective organic thin-film transistors (TFTs) that incorporate heterogeneous organic semiconductor materials, poly(3-hexylthiophene) (P3HT) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene). The electrical characteristics of the fabricated transistors were analyzed in the dark to determine how the P3HT:TIPS-pentacene ratio of the semiconductor affected the performance of the transistor. Specifically, the P3HT:TIPS-pentacene weight ratio was varied (1:0, 1:0.25, 1:0.5, 1:0.75, 1:1, and 0:1) by blending 1 wt% P3HT dissolved in chloroform, and 1 wt% TIPS-pentacene dissolved in anisole. The UV-visible light absorbance characteristics of the films containing the P3HT, TIPS-pentacene, and P3HT:TIPS-pentacene blends were analyzed. Monochromatic light at wavelengths of 515 and 450 nm was used to clarify the influence of irradiation on the electrical characteristics of the TFTs. The results confirmed that the TFT containing P3HT:TIPS-pentacene at a blending ratio of 1:0.5 had the largest light-to-dark current ratio, i.e., approximately 33.8 and 23.5 when exposed to monochromatic light at wavelengths of 515 nm and 450 nm, respectively. The TFT with the P3HT:TIPS-pentacene blending ratio of 1:0.5 exhibited the highest photosensitivity values of 261.9 and 49.6 upon irradiation with light at wavelengths of 515 nm and 450 nm, respectively. The observed improvement in the performance of the heterogeneously blended organic transistors is discussed in relation to the morphological structure and charge transport path of the P3HT:TIPS-pentacene blended semiconductor films. (Received 30 September 2021; Accepted 6 November 2021)

keyword : organic semiconductor, P3HT, TIPS-Pentacene, thin-film transistor, phototransistor

Graphene has been used in various fields because of its excellent mechanical, optical, electrical, and thermal properties. However, its intrinsic low sensitivity to pressure limits its sensor applications. To overcome this drawback, many researchers have tried to improve the sensitivity by controlling the defects on the graphene, but have yet to report a significant increase in sensitivity compared to the pristine graphene. Herein, we fabricated a graphene-based highly sensitive pressure sensor by flowing ammonia gas during chemical vapor deposition. The ammonia gas assisted the generation of nano-sized defects due to nitrogen doping in the graphene lattice, as well as macro-sized cracks in the graphene layer, due to the corrosion of the Cu surface. We regulated the concentration ratio of ammonia gas and methane gas during the graphene synthesis, which controlled the crack generation. These cracks weakened the in-plane force of the networks in the geometric structure of the graphene, thereby allowing them to deform more easily under external force, and changing the resistance of the sensor dramatically. As a result, the sensitivity of the pressure sensor increased about 10,000 times higher than that of the pristine graphene. These results suggest that controlling defects to improve graphene’s mechanical sensitivity provides a promising route to pressure sensor applications. (Received 1 November 2021; Accepted 7 December 2021)

keyword : graphene, chemical vapor deposition, nitrogen doping, Cu corrosion, pressure sensor, crack

Half-Heusler (HH) thermoelectric materials are promising for mid- to high-temperature applications, and MNiSn (M = Ti, Zr, Hf) is a representative n-type HH alloy. In general, the M sites are mixed with isoelectronic elements Ti, Zr, and Hf, to lower the lattice thermal conductivity, and the Sn sites are doped with Sb to adjust the electron concentration. However, Hf is a rare element in earth’s crust, and the volatility of Sb makes it difficult to maintain the initial Sb amount during material synthesis. In this study, as an alternative, Ta was added in the M sites along with the host elements Ti and Zr to produce Hf-free Zr_0.6-xTi_0.4Ta_xNiSn alloys (0 ≤ x ≤ 0.04), and the effects of Ta doping on the thermoelectric properties were analyzed. The electrical conductivity of Zr_0.6-xTi_0.4Ta_xNiSn increased with Ta content, and the electron concentration increased almost linearly, reaching 5.6 × 10²⁰ cm ^-3 at x = 0.04. By adding Ta, the maximum power factor also increased by approximately 17% to 3.94 × 10^-3 W m^-1K^-2 at x = 0.02. The lowest lattice thermal conductivity (κ_lat) was observed at x = 0.02, reaching approximately 1.9 W m^-1K^-1 at 723 K, but overall, a dramatic decrease in κ_lat was not observed with Ta doping in Zr_0.6-xTi_0.4Ta_xNiSn. This is probably due to the existing effect of Zr/Ti mixing at the M sites, which enhances phonon scattering. A maximum figure of merit (zT) of 0.91 was obtained in Zr_0.58Ti_0.4Ta_0.02NiSn at 873 K, which is a high value for ZrNiSnbased Hf-free HH materials. In conclusion, Ta doping is a viable method to replace Sb doping in ZrNiSnbased HH alloys. (Received 27 September 2021; Accepted 15 November 2021)

keyword : thermoelectric, half-Heusler, Zr_{0.6-xTi0.4TaxNiSn, Ta doping, zT}

Advanced electronic materials have attracted great interest for their potential use in flexible, largearea, and printable electronic applications. However, fabricating high-performance low-voltage thin-film transistors (TFTs) for those applications with these advanced semiconductors is still challenging because of a lack of dielectric materials which satisfy both the required electrical and physical performance. In this work, we report self-assembled hybrid multilayer gate dielectrics prepared using a facile solution procedure to achieve organic semiconductor and amorphous oxide semiconductor-based thin-film transistors with ultralow operating voltage. These self-assembled hybrid multilayer gate dielectrics were constructed by iterative selfassembly of synthesized bifunctional phosphonic acid-based organic molecules and ultrathin high-k hafnium oxide layers. The novel self-assembled hybrid multilayer gate dielectrics exhibit excellent dielectric properties with exceptionally large capacitances (up to 815 nF/ cm²) and low-level leakage current densities of < 1.56 × 10^-6 A/cm², featureless morphology (RMS roughness < 0.24 nm), and thermal stability (up to 300 ℃). Consequently, these hybrid gate dielectrics can be incorporated into thin-film transistors with pentacene as p-type organic semiconductors, and with indium oxide as n-type inorganic semiconductors. The resulting TFTs functioned at ultralow voltages (< ± 2 V) and achieved high transistor performances (hole mobility: 0.88 cm²/ V·s, electron mobility: 7.8 cm²/ V·s and on/off current ratio >10⁴, and threshold voltage: ± 0.5 V). (Received 4 November 2021; Accepted 18 November 2021)

keyword : self-assembled multilayer, hybrid gate dielectric, TFTs, ultralow-voltage operation

WC-Co cemented carbide has excellent mechanical properties and is widely used in many industrial applications including cold forging die and cutting tools. WC-Co cemented carbide is manufactured by liquid phase sintering (LPS), and a new sintering process for densification has been developed. In this study, cylindrical rod shaped tensile and fatigue specimens of WC-20wt%Co cemented carbide were fabricated using commercial sintering methods such as vacuum sintering (VS), low pressure sintering (LP), sinter HIP (S-HIP), and HIP after vacuum sintering(VS+HIP). The effect of the sintering methods on microstructure, tensile properties and fatigue life was investigated. The relative density and average size of the WC particles increased after additional HIP treatment. Also large abnormal WC particles were observed. In the tensile test, the deviation of tensile properties (maximum tensile strength, elongation) was minimized in the VS+HIP specimen due to densification. Based on the uniaxial tensile-compression fatigue test, the SN curve of WC- 20wt%Co cemented carbide was concluded to be logσ_a = -0.11244logN_f +3.66817. Furthermore, the increase in WC particle size in the HIP process caused the VS+HIP specimen to have a lower fatigue life than the VS specimen. However, when high pressure was applied to the WC-Co cemented carbide during the sintering process, internal defects and the standard deviation of fatigue life were reduced. (Received 1 October 2021; Accepted 6 December 2021)

keyword : WC-Co cemented carbide, microstructure, sintering method, tensile strength, fatigue

Recent studies have extensively studied the use of composites of carbon nanotubes (CNTs) dispersed in epoxy resin to mechanically reinforce hydrogen storage and lightweight vehicle applications. The mechanical properties of the composite are strengthened due to load transfer from the epoxy matrix to the CNTs when an external force is generated. However, there is a limit to the level of reinforcement that can be achieved by simply dispersing CNTs in the epoxy polymer. In this study, a composite was prepared by dispersing functionalized CNTs with carboxyl groups, which formed covalent bonds with the epoxy resin (EPON 862). The covalent bonding improved the load transfer from the epoxy resin to the CNTs, which increased the Young’s modulus and ultimate tensile strength of the composite. In addition, the interfacial interaction and adhesion between the matrix and the filler were improved by the covalent bonding, thereby improving the degree of dispersion. The changes in elastic modulus and ultimate tensile strength according to the nanofiller content and the presence of the functional groups in the MWCNT were observed, and exhibited consistent values. (Received December 15, 2021; Accepted January 19, 2022)

keyword : Carbon nanotube, composite, reinforcement, covalent bonding

Magnesium silicate is a porous material with a large specific surface area and is easily adsorbed. In particular, it is widely used commercially because it is very effective for adsorbing Na+, K+ ions and other catalysts. However, if the powder is used as an adsorbent as is, there are disadvantages, in that the adsorbed material is easily lost during the adsorption action and becomes difficult to recover. In this study, magnesium silicate was used as an adsorbent to remove pollutants (CO₂) from the atmosphere. In addition, in order to overcome the disadvantages of using a powder adsorbent material, an inorganic binder (clay) and a reinforcing agent (glass fiber) were added to prepare a molded article which imparted strength to the adsorbent material. Changes in the properties of the adsorbent were confirmed. Changes in the physicochemical properties of the adsorbed material according to the calcination temperature, from 400 to 800℃, were confirmed. In addition, it was confirmed that a molded body with 15 wt% inorganic binder and 4 wt% reinforcing agent had a specific surface area of about 87 m²/g and a strength of 4.63 N. The prepared molded article could adsorb about 0.41 mmol/g of CO₂ at atmospheric pressure, confirming its potential use as a CO₂ adsorbent. (Received 27 September 2021; Accepted 1 December 2021)

keyword : magnesium silicate, adsorbent, clay, glass fiber