• Journal of Sensor Science and Technology
• /
• v.28 no.4
• /
• pp.256-261
• /
• 2019

In this study, mid-infrared transparent zinc sulfide (ZnS) ceramics were fabricated through hydrothermal synthesis with different molar ratios of S/Zn (S/Zn = 0.8, 1.0, 1.2, 1.4, and 1.6). The ZnS ceramics were sintered at a relatively low temperature of $850^{\circ}C$ to prevent the occurrence of the hexagonal phase featuring optical anisotropy. The phase composition, microstructure, and optical properties of the ZnS ceramics were subsequently investigated by employing X-ray diffraction, scanning electron microscopy, and Fouriertransform infrared spectroscopy. The results obtained indicate that the ZnS nanoparticles feature the cubic phase, without the hexagonal phase. Moreover, with increasing S, the crystallinity and particle size of the ZnS nanoparticles increased. The crystallinity and density of the ZnS ceramics improved when the molar ratio of S was higher than the molar ratio of Zn, thereby enhancing the transmittance. Furthermore, the ZnS ceramic with an S/Zn value of 1.2 was found to exhibit the highest transmittance of approximately 69% owing to the reduced occurrence of the hexagonal phase and a high density of 99.8%.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2001.07a
• /
• pp.143-148
• /
• 2001

PbWO$_4$ can be densified at 85$0^{\circ}C$ and it shows fairy good microwave dielectric properties; dielectric constant($\varepsilon$$_{r}$) of 21.5, quality factor(Q $\times$f$_{0}$) of 37,224 GHz, and temperature coefficient of resonant frequency($\tau$/suf f/) of -31ppm/$^{\circ}C$. Due to its low sintering temperature, PbWO$_4$ can be used as a multilayered chip component at microwave frequency with high electrical performance by using high conductive electrode metals such as Ag and Cu. However, in order to use this material for microwave communication devices, the $\tau$$_{f}$ of PbWO$_4$ must be stabilized to near zero with high Q$\times$f$_{0}$. In present study, PbWO$_4$ was modified by adding TiO$_2$, B$_2$O$_3$, and CuO in order to improve the microwave dielectric properties without increasing the sintering temperature. The addition of TiO$_2$ increased the $\tau$$_{f}$ and $\varepsilon$$_{r}$, due to its high rr(200ppm/$^{\circ}C$) and $\varepsilon$$_{r}$(100). However, the addition of TiO$_2$ reduced the Q$\times$f$_{0}$ value. When the mot ratio of PbWO$_4$ and TiO$_2$ was 0.913:7.087, near zero $\tau$$_{f}$(0.2ppm/$^{\circ}C$) was obtaibed with $\varepsilon$$_{r}$=22.3, and Q$\times$f/$_{0}$=21,443GHz. With this composition, various amount of B$_2$O$_3$ and CuO were added in order to improve the quality factor. The addition, of B$_2$O$_3$ decreased the $\varepsilon$$_{r}$. However, increased Q$\times$f$_{0}$ and $\tau$$_{f}$. When 2.5 wt% of B$_2$O$_3$ was added to the 0.913PbWO$_4$-0.087TiO$_2$ ceramic, $\tau$$_{f}$ =8.2, $\varepsilon$$_{r}$=20.3, Q$\times$f$_{0}$=54784 GHz. When CuO added to the 0.913PbWO$_4$-0.087TiO$_2$ ceramic, $\tau$$_{f}$ was continuously decreased. And $\varepsilon$$_{r}$ . and Q$\times$f$_{0}$ were increased up to 1.0 wt% then decreased. At 0.1 wt% of CuO addition, the 0.913PbWO$_4$-7.087Ti0$_2$ Ceramic Showed $\varepsilon$$_{r}$=23.5, $\tau$$_{f}$=4.4ppm/$^{\circ}C$, and Q$\times$f$_{0}$=32,932 GHz.> 0/=32,932 GHz.X>=32,932 GHz.> 0/=32,932 GHz.

• Journal of Sensor Science and Technology
• /
• v.6 no.4
• /
• pp.316-327
• /
• 1997

Polycrystalline $Sr_{2}Nb_{2}O_{7}$ and $La_{2}Ti_{2}O_{7}$ ceramics with very high Curie temperatures were synthesized by the chemical coprecipitation method (CCP). The powders synthesized were identified by XRD and their sintering behavior and physical properties were studied. The grain-orientation and electrical properties of sintered ceramics were investigated as a function of firing temperature. Single phase could be obtained by CCP method at temperature lower than that of the conventional method by 100 - $150^{\circ}C$. Strontium niobate, $Sr_{2}Nb_{2}O_{7}$, powder was Prepared by CCP method at temperatures as low as $800^{\circ}C$ via intermediate phase of $Sr_{5}Nb_{4}O_{15}$ formed at $700^{\circ}C$. The resulting CCP-derived powder was observed to have finer and more uniform particle size distribution than those obtained through the conventional or the molten salt synthesis method. Sintering of CCP-derived $Sr_{2}Nb_{2}O_{7}$ powder at $1500^{\circ}C$ yielded a highly dense ceramics with 97% theoretical density. Very high grain-orientation developed along the (0k0) direction was observed by SEM, which resulted in anisotropic dielectric properties of the sintered samples, with the dielectric constant values approaching those for single crystal.

• Journal of the Korean Ceramic Society
• /
• v.39 no.2
• /
• pp.159-163
• /
• 2002

A single phase perovskite relaxor ferroelectric PMN-PT-BT was prepared by a single calcination and the modified mixed oxide process. It was accomplished by ball-milling PbO, $Nb_2O_5,\;Ti(OC_3H_7)_4,\;BaCO_3,\;and\;Mg(NO_3)_2$ instead of MgO, removing the solvent, and then followed by calcination at 900$^{\circ}C$ for 2h. The specimen sintered at 1100$^{\circ}C$/2h showed the sintered density of 7.83 g/$cm^3$, room temperature dielectric constant of 22000, and dielectric loss of 2.5%. Addition of 1.0 mole% (0.3 wt%) of Ag as $AgNO_3$ and followed by calcination at 550$^{\circ}C$/2h lowered the sintering temperature to 900$^{\circ}C$. It still showed the sintered density of 7.88 g/$cm^3$, room temperature dielectric constant of 20000 and dielectric loss of 2.4%.

• Korean Journal of Soil Science and Fertilizer
• /
• v.41 no.6
• /
• pp.379-386
• /
• 2008

Calcined clay granules (pellet) have been used as a soil conditioner. The space among the pellets can secure drainage of water in soil and, simultaneously, can keep water for plants in the inner pore of that. However, the usage of the pellet has been restrained because fabrication of that requires a high energy and cost for heating over the temperate of $1000^{\circ}C$. Recently, SCS(Self-propagating Combustion and Sintering) method was developed and this method use the combustion energy of the preliminary mixed combustible. The SCS method is suitable to fabrication of small porous aggregate and requires a very low cost. This research applied the SCS method to coal refuses for fabrication of soil conditioner pellets. The coal refuses were pulverized under the size of $100{\mu}m$ and the pulverized powders were pelletized to the size of 4~6mm. The pellets were heated at the temperature of $1200^{\circ}C$ in the SCS furnace that was specially prepared for this research. Characteristics of the pellets were investigated and were compared with that of ordinary calcined clay pellet of kaolin; porosity, pore size distribution, bulk density, pH and etc.. Characteristics of the moisture retention in the pellets were measured by the centrifugal method: ASTM D425-88. The pellets of the coal refuses showed the higher values of the field capacity and the plant-available water than that of kaolin pellet. These results suggest the very low cost process that can utilize the coal refuses and can fabricate the lightweight porous soil conditioner of the very high plant-available water.

• Journal of the Korean Applied Science and Technology
• /
• v.35 no.3
• /
• pp.757-767
• /
• 2018

In the propane dehydrogenation reaction proceeding at high temperature, the main cause of deactivation of the catalyst is coke deposition and sintering. In order to investigate the catalysts for reducing such inactivation, we have investigated the applicability of $MgAl_2O_4$ as a carrier for the catalytic dehydrogenation reaction. $MgAl_2O_4$ was prepared by Alcohthermal method at calcination temperature of 800, 900, $1000^{\circ}C$, and $Pt-Sn/MgAl_2O_4$ catalyst was prepared by supporting Pt and Sn by co-impregnation method. The reaction temperature was conducted at a high temperature of 650, $600^{\circ}C$ to confirm the thermal stability. As a result of the reaction experiment, it was confirmed that the conversion rate and yield of propane dehydrogenation reaction test were higher than that of the carrier-applied catalyst having a carrier calcination temperature of 900 and $1000^{\circ}C$, when the carrier-applied catalyst having a calcination temperature of $800^{\circ}C$ was used, It was found that the yield was higher than that of $Pt-Sn/{\theta}-Al_2O_3$ at $650^{\circ}C$. TGA, BET, XRD, CO-chemisorption, and SEM-EDS analyzes were performed for characterization. $MgAl_2O_4-800^{\circ}C$ was correlated with the relationship between good yield, Pt dispersion and low deactivation rate.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2008.06a
• /
• pp.52-52
• /
• 2008

The positive temperature coefficient of resistivity (PTCR) effect in (1-x)$BaTiO_3$ - $x(Bi_{0.5}K_{0.5})TiO_3$ doped with $Nb_2O_5$ was investigated. $(Bi_{1/2}K_{1/2})TiO_3$ (BKT) is more environment-friendly than $PbTiO_3$ in order to use in PTC thermistors. The incorporation of 1 mol% BKT to $BaTiO_3$ increased the Curie temperature (Tc) to $148^{\circ}C$. Doping of $Nb_2O_5$ to $Ba_{0.99}(Bi_{0.5}K_{0.5})_{0.01}TiO_3$ (BaBKT) ceramic has enhanced its PTCR effects. For the sample containing 0.025 mol% $Nb_2O_5$, it showed good PTCR properties; low resistivity at room temperature (${\rho}_r$) of 30 $\Omega{\cdot}cm$, a high PTCR intensity of approximately $3.3\times10^3$, implying the ratio of maximum resistivity to minimum resistivity (${\rho}_{max}/{\rho}_{min}$) in the measured temperature range, and a large resistivity temperature factor (a) of 13.7%/$^{\circ}C$ along with a high Curie temperature (Tc) of $167^{\circ}C$. In addition, the cooling rate of the samples during the sintering process had an influence on their PTCR behavior. All the samples showed the best ${\rho}_{max}/{\rho}_{min}$ ratio when they have cooled down at a rate of $600^{\circ}C$/min.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
• /
• v.31 no.5
• /
• pp.324-329
• /
• 2018

Red phosphor in glasses (PiGs) for automotive light-emitting diode (LED) applications were fabricated with 620-nm $CaAlSiN_3:Eu^{2+}$ phosphor and Pb-free silicate glass. PiGs were synthesized and mounted on high-power blue LED to make a monochromatic red LED. PiGs were simple mixtures of red phosphor and transparent glass powder. After being fabricated with uniaxial press and CIP at 300 MPa for 20 min, the green bodies were thermally treated at $550^{\circ}C$ for 30 min to produce high dense PiGs. As the phosphor content increased, the density of the sintered body decreased and PiGs containing 30% phosphor had a full sintered density. Changes in photoluminescence spectra and color coordination were studied by varying the thickness of plates that were mounted after optical polishing. As a result of the optical spectrum and color coordinates, PiG plate with $210{\mu}m$ thickness showed a color purity of 99.7%. In order to evaluate the thermal stability, the thermal quenching characteristics were measured at temperatures of $30{\sim}150^{\circ}C$. The results showed that the red PIG plates were 30% more thermally stable compared to the AlGaInP red chip.

• Journal of the Korean Ceramic Society
• /
• v.29 no.12
• /
• pp.935-941
• /
• 1992

The LAS system with good thermal properties has a narrow range of firing and sintering temperature near the melting point. So it is difficult to sinter LAS to dense sintered body. In this study, the petalite (Li2O.Al2O3.8SiO2) with good thermal properties, was taken as a base composition, and zironia was added in this composition to broaden the firing range, increase the mechanical strength, and control the thermal expansion. The thermal and mechanical properties were investigated. The results are as follows; 1. Zirconia phase was formed in LAS matrix and apparent porosity was decreased from 0.9% to 0.5%, and the mechanical strength was kincreased from 112 MPa to 190 MPa, by the densification of body. 2. The composition Li2O.Al2O3.8SiO2 has a negative thermal expansion, but the thermal expansion was changed from negative to positive with the densification and the increase of amount of synthesized zircon phase which had positive thermal expansion. The coefficient of thermal expansion, with the increase of the amount of additives, was low as -0.74~9.06$\times$10-7/$^{\circ}C$ in 20~$600^{\circ}C$, and 7.95~20.13$\times$10-7/$^{\circ}C$ in 20~80$0^{\circ}C$. 3. The mechanical strength of LZ15 (added with ZrO2.SiO2 15 wt%) composition thermal-shocked was stable in the temperature range of 0~$600^{\circ}C$, but rapidly decreased due to the increase of thermal expansion above $600^{\circ}C$.

• Journal of the Korean Ceramic Society
• /
• v.34 no.3
• /
• pp.281-288
• /
• 1997

The effects of 0 to 6 mol% excess 91PbO-9WO3 addition on the microstructure and the dielectric pro-perties in 40Pb(Mg1/3Nb2/3)O3-30PbTiO3-30Pb(Mg1/2W1/2)O3 ternary system were investigated. Excess 91PbO-9WO3 addition enhanced densification at relatively lower temperature due to the formation of liquid phase. The dielectric constant of the specimen with standard composition was 16,400 and that of specimen with 1 mol% excess additive was the maximum of 18,500. And more than 2 mol% excess addition decreased dielec-tric constant. Specimens with 2~4 mol% 91PbO-9WO3 addition showed dual peak maxima in the tem-perature dependence of dielectric constant. In the specimens which have more than 5 mol% excess addition a new phase with W-rich composition was formed at grain boundary.