• 한국전기전자재료학회논문지
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• 제11권12호
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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.

• 열처리공학회지
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• 제13권4호
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• pp.239-245
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• 2000

Effect of tin and iron content on the recrystallization behavior of temary Zr-0.8Sn-x%Nb alloys were studied. The specimens with 0.1, 0.2 and 0.4 wt.% of iron were prepared under various annealing temperatures from $400^{\circ}C$ to $800^{\circ}C$ and times from 30 to 5000 minutes after vacuum arc remelting. The recrystallization behavior was observed using a polarized optical microscope, TEM and micro-vickers hardness tester. The recrystallization temperature of the alloys slightly increased with iron content due to increase of activation energy. The grain growth of the alloys with 0.1 and 0.2 wt.% of iron occured rapidly, however, that of the alloys with 0.4 wt.% iron was gradually retarded due to precipitation. The hardness of the alloy with a high iron slightly increased by the precipitation of beta phase after annealing at $800^{\circ}C$.

• Nuclear Engineering and Technology
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• 제53권8호
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• pp.2576-2581
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• 2021

The thermal stability and crystallization behaviors of La₂O₃ containing B₂O₃-CaO-Al₂O₃ glass waste forms were investigated to evaluate the stability of waste form during emergencies in deep geological disposal. For glasses containing 15% La₂O₃, LaBO₃ phases were observed as major crystals from 780 ℃ and exhibited needlelike structures. Al, Ca, and O were homogeneously distributed throughout the entire specimen, while some portions of B and La were concentrated in some parts. By differential thermal analysis at various heating rates, the activation energy for grain growth and the crystallization rate of LaBO₃ were calculated to be 12.6 kJ/mol and 199.5 kJ/mol, respectively. These values are comparable to other waste forms being developed for the same purpose.

• Journal of Korean Vacuum Science & Technology
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• 제3권2호
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• pp.116-120
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• 1999

Possible role of hydrogen atoms on the formation of microcrystalline silicon films was schematically investigated using a plasma enhanced chemical vapor deposition system. A layer-by-layer technique that can alternate deposition of ${\alpha}$-Si thin film and then exposure of H2 plasma was used for this end. The experimental process was extensively carried out under different hydrogen plasma times (t2) at a fixed number of 20 cycles in the deposition. structural properties, such as crystalline volume fractions and grain shapes were analyzed by using a Raman spectroscopy and a scanning electron microscopy. Electrical transports were characterized by the temperature dependence of the dark conductivity that gives rise to the calculation of activation energy (Ea). Optical absorption was measured using an ultra violet spectrophotometer, resulting in the optical energy gap (Eopt). Our experimental results indicate that both of the hydrogen etching and the structural relaxation effects on the film surface seem to be responsible for the growth mechanism of the crystallites in the ${\mu}$c-si films.

• 한국재료학회지
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• 제21권5호
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• pp.277-282
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• 2011

Negative temperature coefficient (NTC) materials have been widely studied for industrial applications, such as sensors and temperature compensation devices. NTC thermistor thick films of $Ni_{1+x}Mn_{2-x}O_{4+{\delta}}$ (x = 0.05, 0, -0.05) were fabricated on a glass substrate using the aerosol deposition method at room temperature. Resistance verse temperature (R-T) characteristics of the as-deposited films showed that the B constant ranged from 3900 to 4200 K between $25^{\circ}C$ and $85^{\circ}C$ without heat treatment. When the film was annealed at $600^{\circ}C$ 1h, the resistivity of the film gradually decreased due to crystallization and grain growth. The resistivity and the activation energy of films annealed at $600^{\circ}C$ for 1 h were 5.203, 5.95, and 4.772 $K{\Omega}{\cdot}cm$ and 351, 326, and 299 meV for $Ni_{0.95}Mn_{2.05}O_{4+{\delta}}$, $NiMn_2O_4$, and $Ni_{1.05}Mn_{1.95}O_{4+{\delta}}$, respectively. The annealing process induced insulating $Mn_2O_3$ in the Ni deficient $Ni_{0.95}Mn_{2.05}O_{4+{\delta}}$ composition resulting in large resistivity and activation energy. Meanwhile, excess Ni in $Ni_{1.05}Mn_{1.95}O_{4+{\delta}}$ suppressed the abnormal grain growth and changed $Mn^{3+}$ to $Mn^{4+}$, giving lower resistivity and activation energy.

• 대한금속재료학회지
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• 제50권9호
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• pp.659-667
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• 2012

Alloy 617 is a Ni-base superalloy and a candidate material for the intermediate heat exchanger (IHX) of a very high temperature gas reactor (VHTR) which is one of the next generation nuclear reactors under development. The high operating temperature of VHTR enables various applications such as mass production of hydrogen with high energy efficiency. Alloy 617 has good creep resistance and phase stability at high temperatures in an air environment. However, it was reported that the mechanical properties decreased at a high temperature in an impure helium environment. In this study, high-temperature corrosion tests were carried out at $850^{\circ}C-950^{\circ}C$ in a helium environment containing the impurity gases $H_2$, CO, and $CH_4$, in order to examine the corrosion behavior of Alloy 617. Until 250 h, Alloy 617 specimens showed a parabolic oxidation behavior at all temperatures. The activation energy for oxidation in helium environment was 154 kJ/mol. The SEM and EDS results elucidated a Cr-rich surface oxide layer, Al-rich internal oxides and depletion of grain boundary carbides. The thickness and depths of degraded layers also showed a parabolic relationship with time. A normal grain growth was observed in the Cr-rich surface oxide layer. When corrosion tests were conducted in a pure helium environment, the oxidation was suppressed drastically. It was elucidated that minor impurity gases in the helium would have detrimental effects on the high-temperature corrosion behavior of Alloy 617 for the VHTR application.

• 방사성폐기물학회지
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• 제16권4호
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• pp.411-420
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• 2018

공기 분위기하 $UO_2$의 독특한 산화거동을 모사하기 위해 기존 Crackling Core Model (CCM)을 개선하였다. $UO_2$가 $U_3O_8$으로 전환될 때 시간-전환율 곡선에서 나타나는 실험적 sigmoid 거동을 근사하게 재현할 수 있도록 모델 개선에 파편화 효과로 인한 반응 표면적 증대 및 결정립 가변 전환시간 개념을 고려하였다. $UO_2$는 $U_3O_7$을 거쳐 $U_3O_8$으로 전환되며 최종 결정립 산화소요 시간은 초기 결정립 산화 소요 시간의 10배에 해당한다는 가정을 도입했을 때, 개선된 모델은 599 - 674 K에서의 $UO_2$ 구형 입자의 실험적 산화거동과 근사한 계산결과를 나타내었으며 핵종성장모델(Nucleation and Growth Model) 및 자촉매반응모델(AutoCatalytic Reaction Model)과 비교할 때 가장 작은 오차를 보여주었다. 개선된 모델을 통해 $U_3O_8$으로의 100% 전환시 계산된 활성화에너지값은 $57.6kJ{\cdot}mol^{-1}$로 자촉매반응모델로 계산된 값인 $48.6kJ{\cdot}mol^{-1}$보다 크며, 외삽에 의해 결정된 실험값에 더 근사함이 밝혀졌다.

• Nuclear Engineering and Technology
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• 제15권3호
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• pp.197-206
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• 1983

핵연료로서 사용 가능한 O/U비 범위인 2.005～2.01의 이산화 우라늄의 소결체를 환원 공정을 거치지 않고 직접 $CO_2$/CO 혼합깨스 분위기에서 소결하였다. O/U비 변화가 소결속도에 미치는 영향을 소결후기에서 조사하였으며, 일정 O/U비에 있어서의 소결 온도-시간-밀도-입도 간의 관계를 나타내는 소결 다이아그램을 결정하였다. 그결과 소결분위기중의 산소분압만을 조절하여, 이론밀도의 95%이상, 평균입도 7$\mu\textrm{m}$ 이상의 소결체를 1050$^{\circ}$～120$0^{\circ}C$의 저온에서 쉽게 얻을 수 있었다. 소결후기의 결정입도의 성장속도는 D=(Kt)$^{1}$4/의 실험식에 따르고, 결정립성장에 대한 활성화 에너지는 O/U비가 2,005, 2.01, 2.10일 때 각각 75, 64, 62Kcal/mol이었다. O/U비가 변화해도 활성화 에너지는 크게 변하지 않았지만, 소결은 산소 분압의 증가에 따라 크게 증가하였다.

• 한국전기전자재료학회논문지
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• 제11권1호
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• pp.48-56
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• 1998

Pr\ulcornerO\ulcorner-based ZnO varistors were fabricated in the range of $Y_2$O$_3$additive content from 0.5 to 4.0mol%, and its microstructure and electrical properties were investigated. Yttrium was distributed nearly in the grain boundaries and the cluster phase formed at nodal point but more in cluster phase. The average grain size was decreased markedly from 34.9 to 8.6${\mu}{\textrm}{m}$ with increasing $Y_2$O$_3$additive content. It is believed that the decrease of grain size is attributed to the formation of cluster phase and the weakening of driving force for liquid sintering. As a result, $Y_2$O$_3$was acted as the inhibitor of the grain growth. With increasing $Y_2$O$_3$additive content, the varistor voltage, the activation energy, and the nonlinear exponent increased whereas the leakage current decreased, especially 4.0mol% $Y_2$O$_3$-added varistor exhibited very good I-V characteristics; nonlinear exponent 87.42 and leakage current 46.77nA. On the other hand, as $Y_2$O$_3$additive content increases, the varistor showed tendency of the salient decrease for donor concentration and the increase for barrier height. Conclusively, it is estimated that ZnO:Pr varistor compositions added more than 2.0mol% $Y_2$O$_3$are to be used to fabricate useful varistors.

• 한국세라믹학회지
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• 제22권2호
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• pp.63-67
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• 1985

$BaTiO_3$ and $SrTiO_2$ were mixed with the mole ratio of 36:65:50, 50: 50 and 65:35 and then heated at 110$0^{\circ}C$~130$0^{\circ}C$ for 1~64 hrs. The solid state reactions and dielectric properties were investigated as a function of amount of solid solution. Activation energy of solid solution decreased with increasing amount of $BaTiO_3$ due to fast diffusion of $Ba^{2+}$ ions. Dielectric constants increased with increasing the soaking time at 125$0^{\circ}C$and 130$0^{\circ}C$ and Curie Temperature shifted to higher temperature with increasing the soaking time at 125$0^{\circ}C$ and 130$0^{\circ}C$. It attributes to the am-ount of solid solution and grain growth, Dielectric constants decreased and Curie Temperature shifted to lower temperature due to decreasing polari-zability.