• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.26 no.1
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• pp.22-26
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• 2013

PTCR ceramics of $(Ba_{0.998}Sm_{0.002})TiO_3+0.001MnCO_3+xSiO_2$ (x=1, 2, 3, 4, 5, 6 mol%) were fabricated by solid state method. Disk samples of diameter 5 mm and thickness about 1mm were sintered at $1,290^{\circ}C$ for 2 h in reduced atmosphere of $5%H_2-95%N_2$ followed by re-oxidation at $600^{\circ}C$ for 30 min. in $20%O_2-80%N_2$.and their microstructures and electrical properties were investigated with SEM and Multimeter. The color of sintered samples was strongly dependent on $SiO_2$ content showing that the color of samples with $SiO_2$ of 1~2 mol% was gray but that of samples with $SiO_2$ of 4~6 mol% was changed from gray to blue, which seems to be related with the reduction of samples due to the oxygen vacancies created during the sintering in reduced atmosphere. $SiO_2$ content had a great influence on the microstructure and the electrical properties. With increasing $SiO_2$ content, the grain size of samples increased and the resistivity as well as the resistivity jump ($R_{285}/R_{min}$) decreased, which is considered to be attributed to the resistivity change at grain interior and grain boundary due to the fast mass transfer through $SiO_2$ liquide phase during the sintering. Samples with 2 mol% $SiO_2$ has the resistivity of $202{\Omega}cm$ and the resistivity jump of 3.28. It is expected that $SiO_2$ doped $BaTiO_3$ based PTC ceramics can be used for multilayered PTC thermistor due to the resistance to the sintering in reduced atmosphere.

• Korean Journal of Materials Research
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• v.1 no.2
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• pp.65-70
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• 1991

The effect of inclusion particles on the microstructure development and electrical characteristics in the systems $ZnO-Bi_2O_3-CoO-Sb_2O_3\;and\;ZnO-Bi_2O_3-CoO-Sb_2O_3-Cr_2O_3 were investigated. The growth of ZnO grains, which was controlled by the spinel particles during sintering, decreased with increasing amount of spinel particles. Addition of $Cr_2O_3(0.5mol\%) increased the breakdown voltage without affecting the non-linear characteristics. The calculated barrier voltage of the $ZnO-Bi_2O_3-CoO-Sb_2O_3\;-and\;ZnO-Bi_2O_3-CoO-Sb_2O_3-Cr_2O_3$ systems were about 3.1V and 2.9V, respectively.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.11 no.12
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• pp.1099-1107
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• 1998

In this study, the microstructure and electric conductivity of 5mol% $Gd_2O_3$-5mol% $Y_2O_3-ZrO_2$ system(5G5YZ) with a variation of sintering time at $1600^{\circ}C$ were investigated. By the result of TEM analysis of 5G5YZ sintered for 12h, a microcrack was observed near grain boundary. The change of the sintering time did not affect the lattice conductivity, but the grain boundary contribution was varied with the sintering time. The grain boundary conductivity of the sample sintered for 1h showed the highest value. Furthermore, the activation energy of the total conductivity was independent upon the sintering time and showed approximately 1.01eV. The highest conductivity measured at $1000^{\circ}C$ was 0.0197S/cm with the sample sintered for 1h. Comparing to 0h’s, the thickness ration of grain boundary as a function of sintering time were 0.88, 1.11 and 1.29 for 1h, 5h and 12h, respectively. In case of the sample sintered for 1h, the thickness of the grain boundary showed the lowest value. The increase of the sintering time over 1h made the decrease of the electric conductivity as well as the increase of the grain growth and the thickness of the grain boundary. As a result, it seemed that the proper sintering time for 5G5YZ composition was 1h.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.12 no.12
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• pp.5405-5411
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• 2011

The effects of heat input (1.5~3.6 kJ/mm) and post weld heat treatment (PWHT, $600^{\circ}C$, 40hr.) on the TMCP HSB600 steel weldments made by GMAW process were investigated. The tensile strengths and hardness of as-welded specimens were decreased as heat input increased, but CVN (Charpy V-Notch) impact energy did not show any differences. The fine-grained acicular ferrite was mainly formed in the low heat input while polygonal and side plate ferrites were dominated in the high heat inputs. Meanwhile, tensile strength and hardness of PWHT weldments were decreased due to the coarsening and globular of microstructure as well as reduction of residual stresses.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2007.11a
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• pp.281-281
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• 2007

Low Temperature Co-fired Ceramic (LTCC) technology has been used in electronic device for various functions. LTCC technology is to fire dielectric ceramic and a conductive electrode such as Ag or Cu thick film below the temperature of $900^{\circ}C$ simultaneously. The glass-ceramic has been widely used for LTCC materials due to its low sintering temperature, high mechanical properties and low dielectric constants. To obtain the high strength, addition of filler, the microstructure should have various crystals and low pores in a composite. In this study, two glass frits were mixed with different alumina size(0.5, 2, 3.7um) and sintered at the range of $850{\sim}950^{\circ}C$. The microstructure, crystal phases, thermal and mechanical properties of the composites were investigated using FE-SEM, XRD, TG-DTA, Dilatomer. When the particle size of $Al_2O_3$ filler increased, the starting temperatures for the densification of the sintered bodies, onset point of crystallization, peak crystallization temperature in the glass-ceramic composites decreased gradually. After sintered at $900^{\circ}C$, the glass frits were crystallized as $CaAl_2Si_2O_8\;and\;CaMgSi_2O_6$. The purpose of our study is to understand the relationship between the $Al_2O_3$ particle size and thermal properties in composites.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2007.11a
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• pp.181-181
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• 2007

PTCR 세라믹스를 적층형 부품으로 제조할 경우 소형화, 저 저항화 및 과전류 유입 시 빠른 응답특성을 갖는다는 장점을 가지고 있으며, 이러한 적층형 부품제조시에는 내부전극재가 부품소자의 물성에 중요한 영향을 미친다. 특히 우수한 옴성 접촉(Ohmic Contact)을 갖는 Zn, Fe, Sn, Ni 등의 적층 PTC용 전극재는 높은 산화특성으로 인해 재산화 과정에서의 비옴성 접촉(Non-ohmic contact)을 갖게 되어 PTC 특성을 저하시킬 우려가 있다. 따라서 본 연구에서는 적층형 PTCR 세라믹스의 내부전극재와 반도체 세라믹층의 동시소성거동 및 적층 PTCR 세라믹스의 전기적 특성을 평가하였다. 본 연구에 적용된 내부전극재로는 Ni 전극을 사용하였고, Ni 전극용 paste로는 무공제 paste, 반도체 세라믹공제 paste, $BaTiO_3$ 공제 paste의 3종 전극재가 이용되었다. 적층형 PTCR 세라믹스의 제조공정은 테이프 캐스팅(Tape casting), 내부전극인쇄, 적층 및 동시소성을 포함하는 적층화공정을 적용하였다. 각각의 전극 paste를 적용하여 제조된 chip은 미세구조관찰, I-V특성, R-T특성 등을 평가하여 내부전극내 세라믹공제의 영향을 고찰하였다.

• Nuclear Engineering and Technology
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• v.48 no.2
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• pp.518-524
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• 2016

The thermal aging effects on mechanical properties and microstructures in China low-activation martensitic steel have been tested by aging at $550^{\circ}C$ for 2,000 hours, 4,000 hours, and 10,000 hours. The microstructure was analyzed by scanning and transmission electron microscopy. The results showed that the grain size and martensitic lath increased by about $4{\mu}m$ and $0.3{\mu}m$, respectively, after thermal exposure at $550^{\circ}C$ for 10,000 hours. MX type particles such as TaC precipitated on the matrix and Laves-phase was found on the martensitic lath boundary and grain boundary on aged specimens. The mechanical properties were investigated with tensile and Charpy impact tests. Tensile properties were not seriously affected by aging. Neither yield strength nor ultimate tensile strength changed significantly. However, the ductile-brittle transition temperature of China low-activation martensitic steel increased by $46^{\circ}C$ after aging for 10,000 hours due to precipitation and grain coarsening.

• Korean Journal of Materials Research
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• v.22 no.10
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• pp.513-518
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• 2012

Recently, automobile parts have been required to have high strength and toughness to allow for weight lightening or improved stability. But, traditional micro-alloyed steel cannot be applied in automobile parts. In this study, we considered the influence of quenching temperature and cooling rate for specimens fabricated by vacuum induction furnace. Directly quenched micro-alloyed steel for hot forging can be controlled according to its micro structure and the heat-treatment process. Low carbon steel, as well as alloying elements for improvement of strength and toughness, was used to obtain optimized conditions. After hot forging at $1,200^{\circ}C$, the ideal mechanical properties (tensile strength ${\geq}$ 1,000 MPa, Charpy impact value ${\geq}\;100\;J/cm^2$) can be achieved by using optimized conditions (quenching temperature : 925 to $1,050^{\circ}C$, cooling rate : ${\geq}\;5^{\circ}C/sec$). The difference of impact value according to cooling rate can be influenced by the microstructure. A fine lath martensite micro structure is formed at a cooling rate of over $5^{\circ}C/sec$. On the other hand, the second phase of the M-A constituent microstructure is the cause of crack initiation under the cooling rate of $5^{\circ}C/sec$.

• Journal of the Korean Society for Heat Treatment
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• v.17 no.3
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• pp.173-179
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• 2004

In this study, the influence of heat treatment and Ca contents on the electrochemical behavior was investigated. Mg-Ca alloys, i.e., Mg-0.22wt%Ca, Mg-0.56wt%Ca, Mg-1.31wt%Ca are prepared by ingot metallurgy. As-cast Mg-Ca alloys exhibited better electrochemical properties than pure Mg. Especially, Mg-0.22wt%Ca alloy improves its anode efficiency up to 62% and lowers the OCP up to -1.72VSCE. Microstructure and XRD patterns of Mg-Ca alloys show that additive Ca element is mainly solid-solutioned. While, the others show the microstructure and XRD pattern with large $Mg_2Ca$ at grain boundary. To assess the effect of heat treatment on the as-cast Mg-alloy, the specimens were heat treated at $200^{\circ}C$ for 2 hours under $CO_2$ gas atmosphere. Although corrosion properties of Mg-Ca alloys are somewhat deteriorated by heat treatment at $200^{\circ}C$ Mg-0.22wt%Ca alloy with uniformly distributed nano-sized $Mg_2Ca$ phase in ${\alpha}$-Mg matrix show still better corrosion properties than pure Mg specimen.

• Journal of the Korean Society for Heat Treatment
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• v.31 no.5
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• pp.220-230
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• 2018

The objective of this study is to investigate the effect of solution treatment on the microstructure and corrosion behavior of cast AZ91-4%RE magnesium alloy. In the as-cast state, microstructure of the AZ91-4%RE alloy was characterized by intermetallic ${\beta}(Mg_{17}Al_{12})$, $Al_{11}RE_3$ and $Al_2RE$ phase particles distributed in ${\alpha}-(Mg)$ matrix. After solution treatment, the ${\beta}$ particles with low melting point dissolved into the matrix, but Al-RE phases still remained due to their high thermal stabilities. It was found from the immersion and potentiodynamic polarization tests that corrosion rate of the AZ91-4%RE alloy increased after the solution treatment. On the contrary, EIS tests and EDS compositional analyses on the surface corrosion products indicated that the stability of the corrosion product was improved after the solution treatment. Examinations on the corroded microstructures for the ascast and solution-treated samples revealed that dissolution of the ${\beta}$ particles which play a beneficial role in suppressing corrosion propagation, would be responsible for the deterioration of corrosion resistance after the solution treatment. This result implies that the microstructural features such as amount, size and distribution of secondary phases that determine corrosion mechanism, are more influential on the corrosion rate in comparison with the stability of surface corrosion product.