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In this study the correlation between bainitic microstructure and the low-temperature toughness of high-strength API pipeline steels was discussed in terms of crack initiation and propagation in the microstructure. Three types of API pipeline steels with different bainitic microstructures were fabricated using varying alloying elements and thermo-mechanical processing conditions, and then their microstructure was characterized by optical and scanning electron microscopy, and electron backscatter diffraction (EBSD). In particular, the effective grain size and microstructure fraction of the steels were quantitatively measured by EBSD analysis. Although all the steels were composed of polygonal ferrite (PF), and complex bainitic microstructures such as acicular ferrite (AF), granular bainite (GB), and bainitic ferrite (BF), they had different effective grain sizes and microstructure fraction, depending on the alloying elements and thermomechanical processing conditions. Charpy impact test results showed that when the martensite-austenite constituent fraction was lowest, it resulted in higher upper-shelf energy, and absorbed energy at room temperature due to the decrease in crack initiation. In contrast, excellent low-temperature toughness, such as lower ductile-brittle transition temperature and higher absorbed energy at low temperatures, could be achieved with a bainitic microstructure with fine effective grain size and high fraction of high-angle grain boundaries, which act as obstacles to prevent cleavage crack propagation.

keyword : pipeline steel, bainitic microstructure, effective grain size, ductile-brittle transition, low-temperature toughness

This study investigated the removal of the organic dye like methylene blue using porous particles in a batch adsorption process. Porous silica microparticles were synthesized with macropores or mesomacropores and used as adsorbent particles. The hierarchically porous structures with additional mesopores were found to be more effective adsorbent than simple macroporous particles. The size of the macropores affected the adsorption capacity of the porous particles, and smaller pore size resulted in a larger amount of organic dye being adsorbed on the particle surface, due to increased surface area. Silica microparticles with wrinkled surfaces were also tested, and their adsorption capacities were studied in relation to their morphologies. Unlike previous studies using mesoporous silica as adsorbents, in this study the methylene blue adsorption kinetics could be explained by the second to fourth order reaction, implying that faster removal of dye molecules is possible compared to photocatalytic reactions, with first order kinetics. The higher order kinetics can be advantageous, providing a faster removal rate, compared to the conventional photocatalytic removal of dyes and other adsorbents.

keyword : porous particles, self-assembly, nanostructured materials, adsorption

In recent years, the metal casting industry has sought to meet ever more stringent environmental standards. Inorganic binders based on silicate chemistry have many advantages with respect to environmental issues, but often suffer from reduced strength, burn-on and poor water-resistance. In particular, when sand is mixed with a water glass based inorganic binder, it adversely affects fluidity. In this study, a Powder flow test (PFT) analysis was conducted to confirm the effect of the surface tension of water glass on mixed sand. Certain additives were selected as surfactants to lower the surface tension of the water glass. The characteristics of the samples were measured using the PFT. A correlation between the surface tension of the water glass and the fluidity of the mixed sand was established. We then evaluated the effect of the inorganic binder on core strength. Using the surfactants, the fluidity of the mixed sand increased by 66%. However, strength and water resistance were reduced by approximately 45%. As a result, it was found that when 1.5% of surfactant was added, the resulting fluidity and humidity strength characteristics produced a mixed sand with good properties. Water glass with improved fluidity can produce a high quality core and mold. Finally, we used a practical application prove that an inorganic binder can replace organic binders in foundry cores.

keyword : Inorganic binder, mixed-sand, fluidity, core, sand core

A reliable and efficient numerical method is presented for the design of broadband absorbers, fabricated by layering two square patch-frequency selective surfaces (SP-FSS) with different geometries on a grounded dielectric substrate. The circuit parameters of the inductance and capacitance of the SP-FSS were retrieved using the strip wire conductor model. Due to the high capacitance and low inductance of the SPFSS, a nearly constant resonance frequency (f₀ = 37 GHz) is observed, irrespective of patch size at a given unit cell periodicity of 7.5 mm. For the SP-FSS, the circuit is capacitive below f0 and inductive above f0. For a grounded substrate with a quarter wavelength thickness, however, the input impedance is inductive below f₀, resulting in impedance matching over a wide frequency range, with the controlled FSS resistance matched to the free-space impedance. The double-layer absorber was designed by optimizing the surface resistance and layer thickness of two SP-FSSs with different geometries, and demonstrated a 10 dB absorption bandwidth of 6.1-41.4 GHz with a total thickness of 5 mm, which is equal to the theoretical limit. A test sample was prepared by screen printing method, and the free space measurement demonstrated a wide-bandwidth absorption result (4.7-40.0 GHz for -10 dB reflection loss) with a small total thickness (5.4 mm). The simulation and experimental results

keyword : Electromagnetic wave absorbers, Wide bandwidth, Frequency selective surfaces

We report the thermoelectric transport properties of n-type Bi₂Te_2.7Se_0.3 compounds prepared by oxide-reduction process. The oxide-reduction process is a recently developed method that can synthesize Bi₂Te₃-based compounds using only oxide starting materials, which is advantageous in terms of the diversity of the starting materials and cost efficiency. In this study, starting materials of Bi₂O₃,TeO₂ and SeO₂ were ball-milled for homogenous mixing, and then oxidized to prepare mixed oxide powders. The mixed oxide powders were reduced at different reduction temperatures (573, 603, 633 and 663 K) for 7 h under hydrogen atmosphere. A single phase of Bi₂Te_2.7Se_0.3 could be achieved when the reduction temperature was higher than 603 K, and the shape of the particles changed from granules to flakes with increasing reduction temperature. Furthermore, the formation of antisite defects of Bi_Se was promoted by increasing reduction temperature, which strongly affected the electrical and thermal transport properties of the Bi₂Te_2.7Se_0.3 compounds, which were prepared by spark plasma sintering of the reduced powders. The anisotropic microstructures in the sintered bodies were significantly influenced by the shape of the reduced powders, which was also controlled by the reduction temperature. The thermoelectric transport properties were characterized at room temperature, and the detailed effects of the reduction temperature on these properties are discussed in terms of the anisotropic microstructure and antisite defects.

keyword : thermoelectric, Bi₂Te₃, oxide reduction process, spark plasma sintering

Tetrahedrite (C₁₂Sb₄S₁₃) has attracted attention as a p-type thermoeletric material with very low thermal conductivity induced by the anharmonic oscillation of Cu due to the lone-pair electrons of Sb. Many studies have been conducted to improve its thermoelectric performance by partially substituting the transition elements for the Cu sites. In this study, Fe-doped tetrahedrites Cu_12-xFe_xSb₄S₁₃ (x = 0.1-0.4) were prepared by mechanical alloying and hot pressing. The tetrahedrite phase was successfully synthesized by mechanical alloying without post-annealing and exhibited stability even without phase transition after hot pressing. Moreover, the Fe content was observed to be directly proportional to the lattice constant, which confirmed the Fe substitutions on the Cu sites. The electrical conductivity was observed to decrease with the increase in the Seebeck coefficient due to the charge compensation caused by Fe doping (electron donation). The highest power factor was 0.84 mWm^-1K^-2 at 723 K for the specimen with x = 0.1; however, it decreased with an increase in Fe content. In addition, as the Fe content increased, the electronic thermal conductivity decreased. Thus, the lowest thermal conductivity value was obtained for the specimen with x = 0.4 (0.45-0.64 Wm^-1K^-1) in the temperature range of 323-723 K. As a result, the maximum value of the dimensionless figure of merit (ZT = 0.80) was achieved at 723 K for the specimen with x = 0.2.

keyword : thermoelectric, tetrahedrite, mechanical alloying, hot pressing

Layered post transition metal chalcogenides such as SnSe, SnSe₂, In₂Se₃, and In₄Se₃ have attracted attention as promising thermoelectric materials due to their intrinsically low lattice thermal conductivities. Recently, n-type indium selenide (InSe) based materials have also been suggested as good candidates for thermoelectric materials by optimizing their electrical properties, i.e., increasing carrier concentration. Here, we report enhancement of the thermoelectric properties of n-type InSe by Sn substitutional doping at the In site. A series of In_1-xSn_xSe for x = 0, 0.03, 0.05, 0.15 and 0.2 was examined. The carrier concentration and electrical conductivity increased due to the Sn substitution, since Sn behaves as a shallow electron donor in InSe, while the Seebeck coefficient decreased moderately. In addition, it was found that effective mass was increased by more than 10 times by Sn doping. As a result, the power factor was enhanced from 0.07 mW/ mK² to 0.13 mW/mK² at 800 K. The total thermal conductivity was unchanged despite Sn doping because the electrical contribution to the total thermal conductivity was very small. Consequently, Sn doping in InSe enhanced the dimensionless thermoelectric figure of merit zT from 0.04 to 0.14 at 800 K, mainly due to enhanced electrical properties.

keyword : InSe, thermoelectric, chalcogenide, doping, density-of-states effective mass

TiO₂ nanostructures were synthesized using a thermal evaporation method without a catalyst. TiO powders mixed with graphite powders were used as the source materials. The synthesis process was performed in air atmosphere at 1000 °C. When the mass ratio of TiO/graphite in the source material was 2:1, TiO₂ nanowires and nanobelts started to form. As the mass ratio of graphite to TiO increased to 1:1, TiO₂ nanowires and nanobelts were formed in large quantity. The nanowires had an average diameter of 80 nm and lengths in the range of 3 ~ 11 μm. The average width and length of the nanobelts were 500 nm and 3.4 μm, respectively. However, with further increase in the mass ratio of TiO/graphite to 1:2, no nanostructures were observed. The mass ratio of graphite to TiO in the source material had an important effect on the formation of the TiO₂ nanowires and nanobelts. The X-ray diffraction data confirmed that the TiO₂ nanostructures had a rutile crystal structure. Two emission bands centered at 410 nm and 510 nm were observed in the room temperature cathodoluminescence spectrum of the TiO₂ nanostructures. The emission at 410 nm is attributed to the electron transition from the conduction band to the valence band in rutile TiO₂ crystal, which is indicative of the high crystallinity of the TiO₂ nanostructures.

keyword : titanium dioxide, nanostructures, thermal evaporation, atmospheric air, violet emission

Polyurethane foam (PUF) is one of the most well-known cellular materials and is widely employed in various industrial and biomedical fields thanks to its many advantages. These include mechanical and material characteristics such as low density and thermal conductivity, and high specific elastic modulus and strength. Despite of these advantages, the PUF has extremely complex material nonlinearity, with changes in density and strain rate, which is a major obstacle to material design and the application of PUF-based structures. PUF has elasto-viscoplastic behavior including three stages of material features, linear elasticity, softening/plateau with stress drop and densification. These phenomena depend strongly on strain rate and density. Therefore, in this study, a phenomenological constitutive model, namely, an elasto-viscoplastic model, was proposed to describe the density- and strain rate-dependent material nonlinear behavior of PUF. The yield surface-independent plastic multiplier, and the hardening- and softening-associated internal state variables proposed by Frank and Brockman, and Zairi et al. were adopted in the constitutive model, respectively. The proposed constitutive model was discretized using the implicit time integration algorithm and was implemented into a user-defined subroutine of the commercial finite element analysis program, ABAQUS. At the same time, a deterministic identification method for material parameters of the constitutive model was introduced to predict the precise material response of PUF under arbitrary densities and strain rates. To do this, the three-dimensional constitutive model was contracted to a one-dimensional equation, and the explicit equation for each material parameter was derived. Then, the strain hardening- and softeningdependent material parameters were calculated using experimental results, such as the work hardening ratestress curve and the yield stress-strain rate curve. After analyzing the obtained material parameters, it was found that the material parameters were strongly dependent on the density and the strain rate. Consequently, the macroscopic material response of PUF, such as a uniaxial compressive stress-strain curve, can be predicted based on the proposed method in this study.

keyword : Polyurethane foam, Infinitesimal strain-based elasto-viscoplastic constitutive model, Deterministic identification method of material parameters, ABAQUS user-defined subroutine UMAT