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In order to improve the degree of matrix densification of SiCf/SiC composites based on liquid silicon infiltration (LSI) process, the microstructure and mechanical properties of composites according to various pyrolysis temperatures and melt infiltration temperatures were investigated. Comparing the microstructures of SiCf/C carbon preform by a one-step pyrolysis process at 600 °C and two-step pyrolysis process at 600 and 1600 °C, the width of the crack and microcrack formation between the fibers and matrix in the fiber bundle increased during the two-step pyrolysis process. For each pyrolysis process, the density, porosity, and flexural strength of the SiCf/SiC composites manufactured by the LSI process at 1450–1550 °C were measured to evaluate the degree of matrix densification and mechanical properties. As a result, the SiCf/SiC composite that was fabricated by the two-step pyrolysis process and LSI process showed an 18% increase in density, 16%p decrease in porosity, and 150% increase in flexural strength on average compared to the composite fabricated by the one-step pyrolysis process. In addition, among the SiCf/SiC specimens fabricated by the LSI process after the same two-step pyrolysis process, the specimen that underwent the LSI process at 1500 °C showed 30% higher flexural strength on average than those at 1450 or 1550 °C. Furthermore, under the same pyrolysis temperature, the mechanical strength of SiCf/SiC specimens in which the LSI process was performed at 1500 °C was higher than that of the 1550 °C although both porosity and density were almost similar. This is because the mechanical properties of the Tyranno-S grade SiC fibers degraded rapidly with increasing LSI process temperature.

Seaweed fibre is usually discarded as biomass waste after extraction of useful ingredients from seaweed. However this seaweed fibre, a natural abundant cellulose material with uniform dimensions 10 times smaller than other plant-based fibre can be utilized as electrode material for energy storage. In this work, we converted seaweed fibre into conductive carbon electrodes by a thermal carbonisation method. The morphology, chemical composition and conductivity are highly influenced by the carbonisation temperature. In comparison to other biomass sources such as cotton pulp, seaweed fibre is finer, smoother and more conductive at low carbonisation temperature. These carbonized seaweeds were then used as a supercapacitor, giving a high supercapacitance (226.3 Fg−1) at the carbonisation temperature of 900°C, and good stability within 2400 cycles. This specific capacitance is significantly higher than values obtained from filter paper or cotton pulp.

The present work involved the application of a two-step sintering (TSS) technique for typical Ce0.9Gd0.1O2−δ–La0.6Sr0.4Co0.2Fe0.8O3–δ (GDC–LSCF) composites and its effect on the microstructure and resultantly mechanical and O2-permeating properties. The samples obtained by conventional sintering (CS) performed at 1100, 1250, and 1400 °C for 3 h exhibited maximum flexural strength and hardness values of 142 MPa and 8.71 GPa, respectively. However, the application of a TSS procedure produced fine-grained microstructures with improved mechanical properties. In particular, with the use of a typical 1250/1200 TSS protocol, increases of approximately 31 % and 7% in the flexural strength and hardness values, respectively, were observed compared to those obtained using the CS method. Moreover, thick-film (∼ 60 μm) GDC–LSCF membrane prepared by tape casting and the TSS technique showed remarkably 1.5–2 times higher oxygen permeation flux than membrane sintered by CS method. The results are discussed and explained in detail.

In this paper, we report on the selection of an optimal sintering temperature for Ce0.9Gd0.1O2−δ–La0.6Sr0.4Co0.2Fe0.8O3–δ composites through characterization of their microstructural, mechanical, and elastic properties. For this selection, sintering was performed mainly at three different temperatures—1100, 1250, and 1400 °C—for 3 h. The samples were systematically investigated in terms of their relative density, microstructure, flexural strength, hardness, elastic modulus, and Poisson's ratio. An increase in density with an increase in sintering temperature was observed to cause a significant improvement in the flexural strength, hardness, and elastic modulus. However, an excessively high temperature (>1250 °C) was detrimental to the flexural strength and hardness because of the occurrence of grain growth. The optimal sintering temperature was identified to be 1250 °C from an analysis of samples with superior mechanical properties and adequate elastic properties. The optimal flexural strength and hardness were found to be 211 ± 21 MPa and 9.34 ± 0.5 GPa, respectively, at room temperature.

The phase purity and crystal structure of dual-phase Ce .9 Gd .1 O 2–δ –La .6 Sr .4 Co .2 Fe .8 O 3–δ (GDC–LSCF) composites were refined using data obtained from X-ray diffraction (XRD) by employing the Rietveld method. Rietveld analysis indicated that the structures of GDC and LSCF phases are well crystallized as cubic Fm 3 m and rhombohedral R 3 c space groups, respectively. Scanning electron microscopy images showed smooth and dense structures, depicting a homogeneous crystalline structure of the samples. When the composites were cooled from their sintering temperature (1250 °C), compressive stresses were generated in the GDC and corresponding tensile stresses were generated in the LSCF due to differences in thermal expansion coefficients. The compressive residual stresses of the composites were investigated by high-angle XRD measurements using the well-known sin 2 ψ method. The average compressive residual stresses in GDC phase are estimated to be − 312 and − 290 MPa for 80 GDC–20 LSCF and 50 GDC–50 LSCF, respectively. The aim of this study is to provide a better understanding of the crystal structures and residual stresses in GDC–LSCF composites through XRD and the suitability of these composites for oxygen transport membranes.

Nanocarbon materials have an important role in future energy storage devices, but their production has a significant energy demand and contributes to pollution. This work introduces a new method to carbonise natural biomass into nanocarbon materials at low temperatures via a simple nickel treatment. The materials obtained are covered homogeneously with nickel nanoparticles, and show excellent electrochemical properties. A material which has a unique structure with micro- and macro-pores is obtained after removal of the nickel nanoparticles, leading to outstanding supercapacitor performance. In addition, we show that activation of the nickel particles converts the material into an effective catalyst for the hydrogen evolution reaction.

Porous carbon materials are promising candidates for anode materials in rechargeable potassium-ion batteries. However, their high surface area and low crystallinity usually cause side reactions with electrolytes and slanted charge/discharge profiles. Herein, we report the synthesis of porous carbon microspheres with highly graphitized structure and enhanced potassium-ion storage properties. The prepared carbon microspheres exhibit a low working potential of ~0.2 V, high Coulombic efficiency, and a stable reversible capacity of 292.0 mAh/g after 100 cycles, which is significantly higher than that of commercial graphite (137.5 mAh/g after 100 cycles). These desirable performances are attributed to the high crystallinity of carbon and its porous structure, which provide active sites for potassium-ion storage and alleviate the stress caused by the large volume change during the insertion and extraction of potassium ions.

In this study, for application in oxygen transport membranes, an LSCF (La0.6Sr0.4Co0.2Fe0.8O3−δ)–GDC (Ce0.9Gd0.1O2−δ) composite was manufactured, and its mechanical properties were studied. Generally, it is known that LSCF has a nonlinear modulus due to changes in its microstructure when a critical stress is loaded. To improve the mechanical properties of this material, which has a perovskite structure, a study was conducted to evaluate whether its properties change according to the rule of mixtures when it is formed into a composite with GDC. The results showed that the nonlinearity of the modulus of the composite for each specimen composition was largely reduced and stable under fatigue loading. The fracture toughness was superior to that of the LSCF (1.05 MPa m1/2) or GDC (1.28 MPa m1/2) monolithic materials when the composites (1.63 MPa m1/2) was manufactured. The mechanisms for these were explained by finite element analysis and the observation of crack propagation. It was also confirmed that when these composite materials are used for oxygen transport membranes, they show stable properties.

Although carbon fiber-reinforced silicon carbide matrix composites fabricated using the liquid silicon infiltration method exhibit high thermal and oxidation resistances, their physical characteristics are limited because of the presence of unreacted, free Si within the materials. To resolve this problem, ingots prepared by alloying Cr with Si in ratios of 0, 5, 10, 25, and 50 at% were melted and made to infiltrate the composite, resulting in the formation of CrSi2 in the unreacted, free Si region without degrading the composite’s properties. The CrSi2 in the composite material reduced the amount of free Si and caused minimal variation in the flexural strength while significantly improving the fracture toughness of the composite. The results of scanning electron microscopy and transmission electron microscopy analyses indicated that the improvement in the fracture toughness was due to the presence of an amorphous interlayer between the Si and CrSi2 phases, as well as because of a stress field surrounding the CrSi2 phase.

The oxidation behavior of zirconium boride composites with various SiC contents (0–40 vol%) at 1500 °C in air (pO 2 = 10 4 Pa) and under low pO 2 (10 –8 Pa) was investigated. Due to different oxidation kinetics calculated from the oxidation depths, the oxidized composites exhibited different layered structures. In addition, the composites of ZrB 2 -30 vol% SiC (one of the typical compositions) oxidized at 1500 °C for 10 h in air and under low pO 2 conditions were analyzed using TEM (transmission electron microscope). In air, the oxidation depth as a function of time indicated a parabolic kinetic behavior, and the ZrB 2 -40 vol% SiC composite exhibited the lowest parabolic rate constant ( k P ) of 232 μm 2 /h. Under low pO 2 , the oxidation depth as a function of time indicated a parabolic to linear transition kinetic behavior, except for monolithic ZrB 2 . The monolithic ZrB 2 exhibited the lowest parabolic rate constant ( k P ) of 811 μm 2 /h.

Natural fibers have been actively investigated for the reinforcement of biocomposite, absorbent, and functional fibers, because of their environmentally friendly properties. We manufactured carbon materials based on two different natural fibers by heat treatment at 700, 900, and 1100 °C, respectively. The thermal behavior, surface morphology, and electrical properties of these carbon materials were investigated by thermogravimetric analysis (TGA) and atomic force microscopy (AFM); specifically, the electrical properties were studied through three-dimensional current images, and the current-voltage curve (I–V) characteristics using current-atomic force microscopy (c-AFM) on a nano-ampere scale. The thermal stability, roughness, and local current of the carbon materials were improved by increasing the carbonization temperature, and carbonized filter paper (FP) showed higher values in all characterization analyses than carbonized henequen (HQ) due to the higher content of cellulose in FP. Moreover, the currentvoltage characteristics of carbonized FP at 1100 °C and commercial carbon fiber were similar at high voltages (−5 or 5 V). Therefore, we expected that carbon materials from FP would be more suitable than HQ for use as environmentally friendly electrode materials.

MVNO(Mobile Virtual Network Operator) promotes service competition in mobile telecommunication market and provides various service and opportunities for consumers. This study attempts to identify MVNO Service users' motivational and dissatisfying factors in relation to their willingness for continued use of the service 새 activate MVNO service. Use motivation factors are found as customer service, user interface, easiness, economical efficiency, and dissatisfying factors are handset quality, customer service, social influence. Economic efficiency factor had an affirmative influence on the sustained use motivation of the service users. Among dissatisfying factors, users had a negative influence when they recognize that their neighboring people does not use MVNO service. Policy authorities concerned must excavate a new plan to improve distribution structure of handset and reduce wholesaling cost of the MVNO service. It is necessary for MVNO service provider to develop a new business model so that users can recognize MVNO service not as inferior service but as rational and attractive service.

The oxidation behaviors of ZrB2- 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 104 and 10−8 Pa, respectively. The oxidation of ZrB2 and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica-rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB2 was oxidized to ZrO2 accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO2 with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB2–SiC composites tested under air and low oxygen partial pressure, the ZrB2 begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.

HfB₂-HfC composites were prepared by reactive hot pressing using Hf and B₄C at temperatures of 1800 and 1900℃ for 60 min under 32 ㎫ in an Ar atmosphere. The reaction sequences of the HfB₂-HfC composite were studied through series of pressureless heat treatments ranging from 800 to 1600℃. The effect of size reduction of the starting powders on densification was investigated by vibration milling. Fully dense HfB₂-HfC composites were obtained by size reduction of the starting powders via vibration milling. The oxidation behaviour of the HfB₂-HfC composites at 1500℃ in air showed formation of a non-protective HfO₂ scale with linear mass gain. Examination of the mechanical properties showed that particle size reduction via vibration milling also led to improved flexural strength, hardness and fracture toughness.

The dielectric properties of -SiAlON and various cations doped -SiAlON bulk ceramics prepared by a hot-press method were investigated. The dielectric properties (dielectric constant and tangent loss) were characterized by a post-resonator method (Hakki-Coleman method) at room temperature in the microwave frequency range. The effect of z-values about -SiAlON was examined, and also the effects of various interstitial cations on dielectric properties of -SiAlON were studied. Dielectric properties of -SiAlON were compared with those of Si3N4 and -SiAlON and their relationship between the dielectric properties and the cationic species of SiAlON were discussed.

The family of (Sr,Mg)-doped LaGaO₃ compounds, which exhibit high ionic conductivity at 600-800℃ over a wide range of oxygen partial pressure, appears to be promising as the electrolyte for intermediate temperature solid oxide fuel cells. Conventional synthesis routes of (Sr,Mg)-doped LaGaO₃ compounds based on solid state reaction have some problems such as the formation of impurity phases, long sintering time and Ga loss during high temperature sintering. Phase stability problem especially, the formation of additional phases at the grain boundary is detrimental to the electrical properties of the electrolyte. From this point of view, we focused to synthesize single phase (Sr,Mg)-doped LaGaO₃ electrolyte at the stage of powder synthesis and to apply relatively low heattreatment temperature using novel synthesis route based on combustion method. The synthesized powder and sintered bulk electrolytes were characterized by XRD, TG-DTA, FT-IR and SEM. AC impedance spectroscopy was used to characterize the electrical transport properties of the electrolyte with the consideration of the contribution of the bulk lattice and grain boundary to the total conductivity. Finally, relationship between synthesis condition and electrical properties of the (Sr, Mg)-doped LaGaO₃ electrolytes was discussed with the consideration of phase analysis results.

-SiAlON with various z-values (z = 0.5~4.0) were produced by hot pressing. The dielectric properties (dielectric constant and tangent loss) of -SiAlON were characterized by the post-resonator method at room temperature and by the perturbation method from room temperature to 1200 oC at 2.45 GHz, respectively. Effect of z-values and temperatures with -SiAlON were investigated.