• Journal of the Korean Society for Heat Treatment
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• v.26 no.3
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• pp.113-119
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• 2013

Grain growth behaviors of hot-rolled AZ31 (Mg-3%Al-1%Zn) and AZ31-0.3%CaO alloys at elevated temperatures have been investigated in order to clarify the effect of CaO addition on grain stability of Mg-Al-based wrought alloy. The grain size of CaO-free alloy increased steeply from 673 K with an increase in annealing temperature from 573 to 773 K, whereas the grains of CaO-containing alloy were relatively stable up to 723 K. The activation energies for grain growth ($E_g$) were 12.2 and 18.3 kJ/mole between 573 and 673 K and 119.2 and 126.9 kJ/mole between 673 and 773 K in the AZ31 and AZ31-0.3%CaO alloys, respectively. This result indicates that grains in the CaO-added alloy possess higher thermal stability than CaO-free alloy. SEM observations on the annealed alloy samples revealed that higher grain stability resulting from CaO addition would be associated with the suppression of grain growth by Ca-related precipitate particles distributed in the microstructure.

• Advances in concrete construction
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• v.5 no.3
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• pp.241-255
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• 2017

Concrete is known to be the most used construction material worldwide. The environmental and economic aspects of Ordinary Portland Cement (OPC) containing concrete have led research studies to investigate the possibility of incorporating supplementary cementitious materials (SCMs) in concrete. Metakaolin (MK) is one SCM with high pozzolanic reactivity generated throughout the thermal activation of high purity kaolinite clay at a temperature ranging from $500^{\circ}C$ to $800^{\circ}C$. Although many studies have evaluated the effect of MK on mechanical properties of concrete and have reported positive effects, limited articles are considering the effect of MK on durability properties of concrete. Considering the lifetime assessment of concrete structures, the durability of concrete has become of particular interest recently. In the present work, the influences of MK on mechanical and durability properties of concrete mixtures are evaluated. Various experiments such as slump flow test, compressive strength, water permeability, freeze and thaw cycles, rapid chloride penetration and surface resistivity tests were carried out to determine mechanical and durability properties of concretes. Concretes made with the incorporation of MK revealed better mechanical and durability properties compared to control concretes due to combined pozzolanic reactivity and the filler effect of MK.

• Journal of Powder Materials
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• v.11 no.4
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• pp.322-327
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• 2004

Ultrafine titanium carbide particles were synthesized by the reaction of liquid-magnesium and vaporized TiCl$_{4}$+C$_{x}$Cl$_{4}$(x = 1 and 2) solution. Fine titanium carbide particles with about 50 nm were successfully produced by combining Ti and C atoms released by chloride reduction of magnesium, and vacuum was then used to remove the residual phases of MgCl$_{2}$ and excess Mg. Small amounts of impurities such as O, Fe, Mg and Cl were detected in the product, but such problem can be solved by more precise process control. The lattice parameter of the product was 0.43267 nm, near the standard value. With respect to the reaction kinetics, the activation energy for the reactions of TiCl$_{4}$+C$_{2}$Cl$_{4}$and Mg was found to 69 kJ/mole, which was about half value against the use of TiCl$_{4}$+CCl$_{4}$, and such higher reactivity of the former contributed to increase the stoichiometry until the level of TiC$_{0.96}$ and decrease the free carbon content below 0.3 wt.%.

• Journal of Microbiology and Biotechnology
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• v.25 no.12
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• pp.2016-2023
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• 2015

Xylanase plays important roles in a broad range of industrial production as a biocatalyst, and its applications commonly require immobilization on supports to enhance its stability. Aluminum hydroxide, a carrier material with high surface area, has the advantages of simple and low-cost preparation and resistance to biodegradation, and can be potentially used as a proper support for xylanase immobilization. In this work, xylanase from Thermomyces lanuginosus was immobilized on two types of aluminum hydroxide particles (gibbsite and amorphous Al(OH)₃) through adsorption, and the properties of the adsorbed enzymes were studied. Both particles had considerable adsorptive capacity and affinity for xylanase. Xylanase retained 75% and 64% of the original catalytic activities after adsorption to gibbsite and amorphous Al(OH)₃. Both the adsorptions improved pH and thermal stability, lowered activation energy, and extended lifespan of the immobilized enzyme, as compared with the free enzyme. Xylanase adsorbed on gibbsite and amorphous Al(OH)₃ retained 71% and 64% of its initial activity, respectively, after being recycled five times. These results indicated that aluminum hydroxides served as good supports for xylanase immobilization. Therefore, the adsorption of xylanase on aluminum hydroxide particles has promising potential for practical production.

• Journal of Microbiology and Biotechnology
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• v.20 no.2
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• pp.332-339
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• 2010

The lipase from Mucor racemosus NRRL 3631 was partially purified by fractional precipitation using 60% ammonium sulfate, which resulted in a 8.33-fold purification. The partially purified lipase was then immobilized using different immobilization techniques: physical adsorption, ionic binding, and entrapment. Entrapment in a 4% agar proved to be the most suitable technique (82% yield), as the immobilized lipase was more stable at acidic and alkaline pHs than the free enzyme, plus 100% of the original activity was retained owing to the thermal stability of the immobilized enzyme after heat treatment for 60 min at $45^{\circ}C$. The calculated half-lives (472.5, 433.12, and 268.5 min at 50, 55, and $60^{\circ}C$, respectively) and the activation energy (9.85 kcal/mol) for the immobilized enzyme were higher than those for the free enzyme. Under the selected conditions, the immobilized enzyme had a higher $K_m$ (11.11 mM) and lower $V_{max}$ (105.26 U/mg protein) when compared with the free enzyme (8.33 mM and 125.0 U/mg protein, respectively). The operational stability of the biocatalyst was tested for both the hydrolysis of triglycerides and esterification of fatty acids with glycerol. After 4 cycles, the immobilized lipase retained approximately 50% and 80% of its original activity in the hydrolysis and esterification reactions, respectively.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1993.11a
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• pp.75-79
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• 1993

The Thermally Stimulated Current(TSC) method has been allied to study the influence of the structural change and interface on the electrical properties of epoxy composites. Three DGBA- MeTHPA matrix model samples mixed different ratios arts silica(SiO$_2$) filled sample and silaln treating-filled sample have been studied. Above room temperature, the relaxation mode ${\alpha}$ peak associated with T$\_$g/ has been located at 110$^{\circ}C$. Below glass transition temperature(T$\_$g/), three relaxation modes are observed in all samples : a ${\beta}$ mode situated at 10$^{\circ}C$, a ${\gamma}$ mode located at -40$^{\circ}C$ and a $\delta$mode appeared in -120$^{\circ}C$, which may be due to segmental motion, side chains, substitution and terminal groups. The analysis of its fine structure indicates that constitution of elementary processes is characterized by the activation energy and relaxation time. Also the change of the molecular structure and their thermal motion are compared with the relaxation mode and conduction mechanism in TSC spectra through the dielectric properties and FTIR measurements.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2001.09a
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• pp.52-59
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• 2001

The photoluminescence and the photo-current from p-type GaN films were investigated on both room- and low-temperatures for various Mg doping concentrations. At a low Mg doping level, there exists a photoluminescence center of the donor and the acceptor pair transition of the 3.28-eV band. This center is correlated with the defects for a shallow donor of the VGa and for an acceptor of MgGa. The acceptor level shows the binding energy of 0.2-0.25 eV, which was observed by the photon energy of the photo-current signal of 3.02-3.31 eV. At a high Mg doping level, there is a photoluminescence center of a deep donor and an acceptor pair transition of the 2.76-eV blue band. This center is attributed to the defect structures of MgGa-VN for the deep donor and MgGa for the acceptor. For low. doped samples, thermal annealing provides an additional photo-current signal for an unoccupied deep acceptor levels of 0.87-1.35 eV above valence band, indicating the p-type activation.

• Journal of radiological science and technology
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• v.21 no.2
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• pp.53-56
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• 1998

Highly sensitive $CaSO_4:Tm$ TLD is fabricated and their trap parameters are determined. The glow curve of $CaSO_4:Tm$ consists of three glow peaks and these peaks are isolated by the thermal bleaching method. The isolated glow peaks are fitted by a least squares method. The activation energies are 0.68, 0.82 and 1.03 eV. The frequency factors are $8.09{\times}10^8,\;9.14{\times}10^8$ and $1.03{\times}10^9/s$, and the kinetic orders are 1.37, 1.54 and 1.68, respectively. The optimum temperature range of the main peak for radiation dosimetry is between 220 and $290^{\circ}C$.

• Korean Journal of Materials Research
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• v.12 no.6
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• pp.431-435
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• 2002

The dark current and photocurrent(PC) spectrum of Mg-doped GaN thin film were investigated with various bias voltages and temperatures. At high temperature and small bias, the dark current is dominated by holes thermally activated from an acceptor level Al located at about 0.16 eV above the valence band maximum $(E_v)$, The PC peak originates from the electron transition from deep level A2 located at about 0.34 eV above the $E_v$ to the conduction band minimum $(E_ C)$. However, at a large bias voltage, holes thermally activated from A2 to Al experience the field-in-duces tunneling to form one-dimensional defect band at Al, which determines the dark current. The PC peak associated with the transition from Al to $E_ C$ is also observed at large bias voltages owing to the extended recombination lifetime of holes by the tunneling. In the near infrared region, a strong PC peak at 1.20 eV appears due to the hole transition from deep donor/acceptor level to the valence band.

• Journal of the Korean Institute of Telematics and Electronics A
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• v.30A no.8
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• pp.42-48
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• 1993

Ar ions were implanted at 1 MeV into (100)Cz Si wafers with dose of 1 * 10$^{15}$ ions/cm$^{2}$. Damage induced by high energy implantation and its annealing behavior during rapid thermal annealing for 10sec at temperatures from 550 to 1100${\circ}C$ were investigated by crosssection transmission electron microscopy study. It can be clearly seen from the observation that the SPE(Solid Phase Epitaxy) regrowth of the buried amorphous layer induced by ion implantation proceeds from both upper and lower amorphous/crystalline (a/c) interfaces, and the activation energy for SPE from interfaces were both 1.43eV. Misfit dislocation where two interfaces met was formed and it coalesced into the hair pin dislocation in the upper regrown region. At the higher temperature after annealing out of the misfit dislocation, hair pin dislocations showed considerable drop in its bandwidth. However, they were not disappeared even at the temperature 1100${\circ}C$ with the end of range dislocation loops which were formed at the original lower a/c interface.