• Journal of the Korean Society of Propulsion Engineers
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• v.12 no.2
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• pp.74-80
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• 2008

The feasibility study into the development of turbine spinners, which start up the turbo-pump, has been carried out and the design requirements and parameters ranges have been presented. Turbine spinners use the solid propellant as such composite propellant based AN compound with high energy plasticizers, coolants, and phase stabilizer which relieves a sensible volume change due to the phase transformation of AN near room temperature. Propellants which have a homing rate of $0.2{\sim}0.3\;mm/s$ and pressure exponent ranged from 0.3 to 0.6, showed stable burn-out in the standard motor tests. Both the magnitude of ignition energy and its thermal transfer mechanism have been proved to have a tangible effect on the ignition of the pyre starter, and the results of this study showed that a flame temperature of 1400K would be quite adequate to get a stable ignition for the AN composite propellant.

• Korean Journal of Materials Research
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• v.9 no.10
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• pp.956-961
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• 1999

When the 0-2 mol% of $\textrm{La}_{3+}$ were added to the nanocrystalline $BaTiO_3$ powders and sintered at $1200~1400^{\circ}C$, the nanocrystailine $BaTiO_3$ ceramics having the average grain size of 110-240 nm were obtained except for when no $\textrm{La}_{3+}$ was added and sintered at > $1350^{\circ}C$. As the average grain size decreased from 240 nm to 110 nm, the tetragonality (c/a) and volume fraction of the $90^{\circ}$ domain decreased, and all the properties such as relative dielectric constant at room temperature, maximum relative dielectric constant at the temperature of phase transformation $\textrm{T}_{c}$ and diffuseness of phase transformation at $\textrm{T}_{c}$ also decreased. The results were attributed to the internal stress induced by inhibition of the $90^{\circ}$ domain formation in the case of finer grain size.

• Transactions on Electrical and Electronic Materials
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• v.13 no.4
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• pp.171-176
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• 2012

As langasite $A_3BC_3D_2O_{14}$ compounds with piezoelectric properties exhibit no phase transition up to the melting point of 1,400-$1,500^{\circ}C$, many high temperature applications are expected for the SAW filter, temperature sensor, pressure sensor, and so on, based on the digital transformation of wider bandwidth and higher-bit rates. It has a larger electromechanical coupling factor compared to quartz and also nearly the same temperature stability as quartz. The $La_3Ga_5SiO_{14}$ (LGS) crystal with the $Ca_3Ga_2Ge_4O_{14}$-type crystal structure was synthesized and the crystal structure was analyzed by Mill et al. It is also an important feature that the growth of the single crystal is easy. In the case of three-element compounds such as $[R_3]_A[Ga]_B[Ga_3]_C[GaSi]_DO_{14}$ (R=La, Pr and Nd), the piezoelectric constant increases with the ionic radius of R. In this study, crystal structures of four-element compounds such as $[A_3]_A[B]_B[Ga_3]_C[Si_2]_DO_{14}$ (A = Ca or Sr, B = Ta or Nb) are analyzed by a single crystal X-ray diffraction, and the mechanism and properties of the piezoelectricity depending on the species of cation was clarified based on the crystal structure.

• Korean Journal of Materials Research
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• v.19 no.6
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• pp.293-299
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• 2009

The microstructures and mechanical properties of Dual Phase (DP), Transformation-Induced Plasticity (TRIP), and Quenching & Partitioning (Q&P) steels were investigated in order to define the strengthening mechanism of 0.2 C steel. An intercritical annealing between Ac1 and Ac3 was conducted to produce DP and TRIP steel, followed by quenching the DP and TRIP steel being quenched at to room temperature and by the TRIP steel being austemperingaustempered-air cooling cooled the steel toat room temperature, respectively. The Q&P steel was produced from full austenization, followed by quenching to the temperature between $M_s$ and $M_f$, and then enriching the carbon to stabilize the austenite throughout the heat treatment. For the DP and TRIP steels, as the intercritical annealing temperature increased, the tensile strength increased and the elongation decreased. The strength variation was due to the amount of hard phases, i.e., martensite and bainite, respectively in the DP and TRIP steels. It was also found that the elongation also decreased with the amount of soft ferrite in the DP and TRIP steels and with the amount of the that was retained in the austenite phasein the TRIP steel, respectively for the DP and TRIP steels. For the Q&P steel, as the partitioning time increased, the elongation and the tensile strength increased slightly. This was due to the stabilized austenite that was enriched with carbon, even when the amount of retained austenite decreased as the partitioning time increased from 30 seconds to 100 seconds.

• Corrosion Science and Technology
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• v.13 no.2
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• pp.70-80
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• 2014

Duplex stainless steels with nearly equal fraction of the ferrite(${\alpha}$) phase and austenite(${\gamma}$) phase have been increasingly used for various applications such as power plants, desalination facilities due to their high resistance to corrosion, good weldability, and excellent mechanical properties. Hyper duplex stainless steel (HDSS) is defined as the future duplex stainless steel with a pitting resistance equivalent (PRE=wt.%Cr+3.3(wt.%Mo+0.5wt.%W)+30wt.%N) of above 50. However, when HDSS is welded with gas tungsten arc (GTA), incorporation of nitrogen in the Ar shielding gas are very important because the volume fraction of ${\alpha}$-phase and ${\gamma}$-phase is changed and harmful secondary phases can be formed in the welded zone. In other words, the balance of corrosion resistance between two phases and reduction of $Cr_2N$ are the key points of this study. The primary results of this study are as follows. The addition of $N_2$ to the Ar shielding gas provides phase balance under weld-cooling conditions and increases the transformation temperature of the ${\alpha}$-phase to ${\gamma}$-phase, increasing the fraction of ${\gamma}$-phase as well as decreasing the precipitation of $Cr_2N$. In the anodic polarization test, the addition of nitrogen gas in the Ar shielding gas improved values of the electrochemical parameters, compared to the Pure Ar. Also, in the erosion-corrosion test, the HDSS welded with shielding gas containing $N_2$ decreased the weight loss, compared to HDSS welded with the Ar pure gas. This result showed the resistance of erosion-corrosion was increased due to increasing the fraction of ${\gamma}$-phase and the stability of passive film according to the addition $N_2$ gas to the Ar shielding gas. As a result, the addition of nitrogen gas to the shielding gas improved the resistance of erosion-corrosion.

• Journal of the Korean Ceramic Society
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• v.36 no.10
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• pp.1094-1101
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• 1999

Liquid phase sintered silicon carbides were obtained by sintering of $\alpha$-SiC and $\beta$-SiC powders as starting materials at 2173K and 2273K respectively. The SiCplatelet seeds of different sizes were obtained by a repeated ball milling and sedimentation. Their mean size (d50) were 2.217 ${\mu}{\textrm}{m}$ 13.67 ${\mu}{\textrm}{m}$, 22.17${\mu}{\textrm}{m}$ respectively 6wt%Al2O3-4 wt% Y2O3 was used as the sintering additives for the liquid phase sintering. The two silicon carbides had a bimodal microstructure consisting of small matrix grains and large platelike grains when the SiCplatelet seeds were added. In the case of the $\beta$-SiC the appreciable phase transformation occurred as sintering temperature increased from 2173K to 2273K and resulted in matrix shape change from equiaxed into platelike grains. In contrast there was no shape change for the $\alpha$-SiC. The size of large grains in the $\alpha$-SiC of large grains in the $\alpha$-SiC was larger than that of the large grains in the $\beta$-SiC These results suggested that the growth of the $\alpha$-SiCplatelet in the $\alpha$-SiC matrix was more favored than that of the $\alpha$-SiCplatelet in the $\beta$-SiC matix. The three point flexural strength decreased as the added seed size increased. Fracture toughness values of samples sintered at 2273K were higher than those of samples sintered at 2173K.

• Proceedings of the KIEE Conference
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• 2008.07a
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• pp.1230-1231
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• 2008

The effect of content of $Al_2O_3+Y_2O_3$ sintering additives on the densification behavior, mechanical and electrical properties of the pressureless-sintered $SiC-ZrB_2$ electroconductive ceramic composites was investigated. The $SiC-ZrB_2$ electroconductive ceramic composites were pressureless-sintered for 2 hours at 1,700[$^{\circ}C$] temperatures with an addition of $Al_2O_3+Y_2O_3$(6:4 mixture of $Al_2O_3$ and $Y_2O_3$) as a sintering aid in the range of 8${\sim}$20[wt%]. Phase analysis of $SiC-ZrB_2$ composites by XRD revealed mostly of ${\alpha}$-SiC(6H), $ZrB_2$ and In Situ YAG($Al_5Y_3O_{12}$). The relative density, flexural strength, Young's modulus and vicker's hardness showed the highest value of 89.01[%], 81.58[Mpa], 31.437[GPa] and 1.34[GPa] for $SiC-ZrB_2$ composites added with 16[wt%] $Al_2O_3+Y_2O_3$ additives at room temperature respectively. Abnormal grain growth takes place during phase transformation from ${\beta}$-SiC into ${\alpha}$-SiC was correlated with In Situ YAG phase by reaction between $Al_2O_3$ and $Y_2O_3$ additive during sintering. Compositional design and optimization of processing parameters are key factors for controlling and improving the properties of SiC-based electroconductive ceramic composites. In this paper, it is convinced that ${\beta}$-SiC based electroconductive ceramic composites for heaters or ignitors can be manufactured by pressureless sintering.

• Korean Journal of Materials Research
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• v.21 no.11
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• pp.604-610
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• 2011

The effect of $BaF_2$ flux in $Y_3Al_5O_{12}:Ce^{3+}$(YAG:Ce) formation was investigated. Phase transformation of $Y_3Al_5O_{12}$(YAG) was characterized by using XRD, SEM, and TEM-EDS, and it was revealed that the sequential formation of the $Y_4Al_2O_9$(YAM), $YAlO_3$(YAP) and $Y_3Al_5O_{12}$(YAG) in the temperature range of 1000-1500$^{\circ}C$. Single phase of YAG was revealed from 1300$^{\circ}C$. In order to find out the effect of $BaF_2$ flux, three modeling experiments between starting materials (1.5$Al_2O_3$-2.5$Y_2O_3$, $Y_2O_3$-$BaF_2$, and $Al_2O_3$-$BaF_2$) were done. These modeling experiments showed that the nucleation process occurs via the dissolution-precipitation mechanism, whereas the grain growth process is controlled via the liquid-phase diffusion route. YAG:Ce phosphor particles prepared using a proposed technique exhibit a spherical shape, high crystallinity, and an emission intensity. According to the experimental results conducted in this investigation, 5% of $BaF_2$ was the best concentration for physical, chemical and optical properties of $Y_3Al_5O_{12}:Ce^{3+}$(YAG:Ce) that is approximately 10-15% greater than that of commercial phosphor powder.

• Journal of the Korean Society for Heat Treatment
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• v.29 no.5
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• pp.220-226
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• 2016

The mechanical properties of Ti-6Al-4V can be improved by microstructural control through the heat treatment in ${\alpha}+{\beta}$ region. The heat treatment was carried out with a variety of heat treatment temperatures and holding times to find the optimized heat treatment conditions and it was analyzed by linking the microstructural characteristics and mechanical properties. The part of ${\beta}$ phase with $10{\pm}2wt%$ vanadium was transformed into ${\alpha}^{{\prime}{\prime}}$ martensite phase after quenched, so the hardness and tensile properties were decreased below $900^{\circ}C$. The higher the heat treatment temperature is, the smaller is the vanadium-rich region, which leads to transformation into hcp ${\alpha}^{\prime}$ martensite above $900^{\circ}C$. The hardness and tensile properties were improved due to the hard ${\alpha}^{\prime}$ martensite. As the holding times were longer, the hardness and tensile properties decreased below $900^{\circ}C$ because of the softening effect by the grain growth. When varying the holding times above $900^{\circ}C$, the change of mechanical properties was slight because the softening effect of grain growth and the strengthening effect of ${\alpha}^{\prime}$ phase were counteractive. Therefore, the best conditions of heat treatment, which is in the range of $920{\sim}960^{\circ}C$, 40 min, WQ, can effectively improve the mechanical properties of Ti-6Al-4V.

• Korean Journal of Metals and Materials
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• v.46 no.3
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• pp.159-168
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• 2008

Thermally-evaporated 10 nm-Ni/1 nm-Ru/(30 nm or 70 nm-poly)Si structures were fabricated in order to investigate the thermal stability of Ru-inserted nickel monosilicide. The silicide samples underwent rapid thermal anne aling at $300{\sim}1,100^{\circ}C$ for 40 seconds. Silicides suitable for the salicide process were formed on the top of the single crystal and polycrystalline silicon substrates mimicking actives and gates. The sheet resistance was measured using a four-point probe. High resolution X-ray diffraction and Auger depth profiling were used for phase and chemical composition analysis, respectively. Transmission electron microscope and scanning probe microscope(SPM) were used to determine the cross-sectional structure and surface roughness. The silicide, which formed on single crystal silicon and 30 nm polysilicon substrate, could defer the transformation of $Ni_2Si $i and $NiSi_2 $, and was stable at temperatures up to $1,100^{\circ}C$ and $1,100^{\circ}C$, respectively. Regarding microstructure, the nano-size NiSi preferred phase was observed on single crystalline Si substrate, and agglomerate phase was shown on 30 nm-thick polycrystalline Si substrate, respectively. The silicide, formed on 70 nm polysilicon substrate, showed high resistance at temperatures >$700^{\circ}C$ caused by mixed microstructure. Through SPM analysis, we confirmed that the surface roughness increased abruptly on single crystal Si substrate while not changed on polycrystalline substrate. The Ru-inserted nickel monosilicide could maintain a low resistance in wide temperature range and is considered suitable for the nano-thick silicide process.