• 한국재료학회지
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• 제16권3호
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• pp.188-192
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• 2006

The microstructures and mechanical properties of continuously porous $SiC-Si_3N_4$composites fabricated by multi-pass extrusion were investigated at different Si levels added. Si-powder with different weight percentages (0%, 5%, 10%, 15%, 20%) was added to the SiC powder to make the raw mixture powders, with $6wt%Y_2O_3-2wt%Al_2O_3$ as sintering additives, carbon ($10-15{\mu}m$) as a pore-forming agent, ethylene vinyl acetate as a binder and stearic acid ($CH_3(CH_2)_{16}COOH$) as a lubricant. In the continuously porous $SiC-Si_3N_4$ composites, $Si_3N_4$ whiskers like the hairs of nostrils were frequently observed on the wall of the pores. In this study, the morphology of the $Si_3N_4$ whiskers was investigated with the silicon addition content. In the composites containing of 10 wt% Si, a large number of $Si_3N_4$ whiskers was found at the continuous pore regions. In the sample to which 15 wt% Si powder was added, maximum values of about 101 MPa bending strength and 57.5% relative density were obtained.

• 센서학회지
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• 제23권5호
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• pp.337-341
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• 2014

For various additives doped-$VO_2$ critical temperature sensors using the nature of semiconductor to metal transition, the crystallinity, microstructure, and temperature vs. resistance characteristics were systematically investigated. As a starting material of $VO_2$ sensor, vanadium pentoxide ($V_2O_5$) powders were used, and CaO, SrO, $Bi_2O_3$, $TiO_2$, and PbO dopants were used, respectively. The $V_2O_5$ powders with dopants were mixed with a vehicle to form paste. This paste was silk screen-printed on $Al_2O_3$ substrates and then $V_2O_5$-based thick films were heat-treated at $500^{\circ}C$ for 2 hours in $N_2$ gas atmosphere for the reduction to $VO_2$. From X-ray diffraction analysis, $VO_2$ phases for pure $VO_2$, and CaO and SrO-doped $VO_2$ thick films were confirmed and their grain sizes were 0.57 to $0.59{\mu}m$. The on/off resistance ratio of the $VO_2$ sensor in phase transition temperature range was $5.3{\times}10^3$ and that of the 0.5 wt.% CaO-doped $VO_2$ sensor was $5.46{\times}10^3$. The presented critical temperature sensors could be commercialized for fire-protection and control systems.

• 한국분말재료학회지
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• 제27권2호
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• pp.126-131
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• 2020

The grain growth behavior of M-type Sr hexaferrite (SrM) grains is investigated with the addition of MnCO₃. First, the SrM powder is synthesized by a conventional solid-state reaction. The powder compacts of SrM are sintered at 1250℃ for 2 h with various amounts of MnCO₃ (0, 0.5, 1.0, and 4.0 mol%). There is no secondary solid phase in any of the sintered samples. Relative density increases when MnCO₃ is added to the SrM. Obvious abnormal grain growth does not appear in any of the SrM samples with MnCO₃. The average grain size increases when 0.5 mol% MnCO₃ is added to the SrM. However, as the amount of MnCO₃ increase to over 0.5 mol%, the average grain size decreases. These observations allow us to conclude that the growth of SrM grains is governed by the two-dimensional nucleation grain growth mechanism, and the critical driving force for the growth of a grain decreases as the amount of MnCO₃ increases.

• Transactions on Electrical and Electronic Materials
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• 제10권5호
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• pp.152-155
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• 2009

The microstructure and the electrical properties of a ZnO varistor, which was composed of a ZnO-$Bi_2O_3$-$Sb_2O_3$-CoO- $MnO_2$ -NiO-$Nd_2O_3$ system, were investigated at various $Y_2O_3$ addition concentrations. $Y_2O_3$ played a role in the inhibition of the grain growth. As the $Y_2O_3$ content increased, the average grain size decreased from $6.8{\mu}m$ to $4{\mu}m$, and the varistor voltage($V_{1mA}$) greatly increased from 275 to 400 V/mm. The nonlinearity coefficient ($\alpha$) decreased from 72 to 65 with increasing $Y_2O_3$ amount. On the other hand, the leakage current ($I_L$) increased from 0.2 to 0.9 ${\mu}A$. These results confirmed that doping the varistors with $Y_2O_3$ is a promising production route for production of a higher fine-grained varistor voltage ($V_{1mA}$) which can dramatically reduce the size of the varistors.

• Corrosion Science and Technology
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• 제12권6호
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• pp.280-287
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• 2013

Flow accelerated corrosion(FAC) of the carbon steel piping in pressurized water reactors(PWRs) has been major issue in nuclear industry. Severe accident at Surry Unit 2 in 1986 initiated the worldwide interest in this area. Major parameters influencing FAC are material composition, microstructure, water chemistry, and hydrodynamics. Qualitative behaviors of FAC have been well understood but quantitative data about FAC have not been published for proprietary reason. In order to minimize the FAC in PWRs, the optimal method is to control water chemistry factors. Chemistry factors influencing FAC such as pH, corrosion potential, and hydrazine contents were reviewed in this paper. FAC rate decreased with pH up to 10 because magnetite solubility decreased with pH. Corrosion potential is generally controlled dissolved oxygen (DO) and hydrazine in secondary water. DO increased corrosion potential. FAC rate decreased with DO by stabilizing magnetite at low DO concentration or by formation of hematite at high DO concentration. Even though hydrazine is generally used to remove DO, hydrazine itself thermally decomposed to ammonia, nitrogen, and hydrogen raising pH. Hydrazine could react with iron and increased FAC rate. Effect of hydrazine on FAC is rather complex and should be careful in FAC analysis. FAC could be managed by adequate combination of pH, corrosion potential, and hydrazine.

• 콘크리트학회논문집
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• 제25권3호
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• pp.305-312
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• 2013

이 연구는 물-결합재(W/B) 비에 따른 실리카 퓸이 알칼리 활성화 슬래그 시멘트(AASC)에 미치는 유동성과 강도 특성에 대한 연구이다. W/B비는 0.50에서 0.60까지 0.05 단위로 일정하게 변화시켰다. 실리카 퓸은 고로슬래그 중량의 0%에서 50%까지 치환시켰다. 활성화제는 수산화나트륨(NaOH)를 사용하였고, 농도는 3M로 하였다. W/B 비에 따른 강도는 1, 3, 7 그리고 28일을 측정하였다. 유동성 측정 결과는 실리카 퓸의 치환율이 증가할수록 감소하였다. 압축강도는 실리카 퓸의 치환율이 증가할수록 증가하였다. 또한 W/B 비가 증가할수록 모든 재령에서 강도는 감소하였다. 실리카 퓸은 W/B 비와 실리카 퓸의 치환율에 따라서 활성화 반응을 증대시켜 강도를 증가시키는 것을 알 수 있었다.

• Nuclear Engineering and Technology
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• 제52권2호
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• pp.344-351
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• 2020

AgInCd alloy is widely used as neutron absorber in nuclear reactors. However, the AgInCd control rods may fail during service due to the irradiation swelling. In the present study, a calculational method is proposed to calculate the composition change of the AgInCd absorber. Calculated results show that neutron fluence has significant impact on the chemical compositions. Ag and In contents gradually decrease while Cd and Sn conversely increases from the center to the rim of AgInCd absorber due to the depression of neutron flux. The composition change at the surface is higher almost two times than that at the center. Based on the calculated compositions, six simulated AgInCdSn alloys were prepared and examined. With the increase of Cd and Sn, the simulated AgInCdSn alloys transform from a single fcc phase into the mixed fcc and hcp phases, and finally into the single hcp phase. The atomic volume of the hcp phase is obviously larger than the fcc phase. The fcc-hcp transformation results in considerable volume swelling of the AgInCd absorber. Moreover, the lattice parameters of the fcc and hcp phases gradually increase with Cd and Sn contents, which also can induce small volume swelling.

• 한국세라믹학회지
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• 제34권1호
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• pp.70-78
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• 1997

3Y TZP/SUS316계 경사기능재료를 슬립캐스팅법을 이용하여 제조하였다. 슬립캐스팅 공정에서 석고몰드를 대체하기 위해 알루미나몰드를 제조하였고, 3Y-TZP/SUS316 2상 슬러리의 최적 분산조건을 ESA, 점도계, 침전거동의 관찰 등을 통해 결정하였으며, 석고몰드와 알루미나몰드로 캐스팅한 시험편의 제반 특성을 소결수축율변화, 건조 및 소결거동, 미세구조 관찰 등으로 조사하였다. 그 결과 알루미나몰드를 사용하여 제조된 시험편에서는 석고몰드 사용시 나타나는 표면에서의 오염이 관찰되지 않았으며, 각 층의 두께조절이 쉬웠고 높은 재현성을 나타냄을 알 수 있었다. 특히 알루미나몰드를 사용하여 제조한 SUS316에서는 어떤 열화현상도 관찰되지 않았다. 결국 슬립캐스팅 공정으로 3Y-TZP/SUS316계 경사기능재료를 제조함에 있어서 기존의 석고몰드보다 다공질 알루미나몰드의 사용이 바람직하다는 것을 알 수 있었다.

• 한국세라믹학회지
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• 제40권7호
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• pp.702-708
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• 2003

포틀랜드 시멘트 양(중량)을 플라이 애쉬로 50 wt%까지 치환한 페이스트, 모르터 내지 콘크리트 시편을 만들어 플라이 애쉬가 모르터/콘크리트의 온도, 자유석회량 및 강도 등에 미치는 영향을 조사하였다. 페이스트의 수화열을 측정하여 환산식에 따라 일반 포틀랜드 시멘트 콘크리트의 경우와 비교할 때, 플라이 애쉬를 50 wt% 함유한 콘크리트의 양생 7일 온도(상승)는 약 45% 저하되었다. 전자의 경우 온도 상승은 33.4$^{\circ}C$이며 후자의 경우는 18.7$^{\circ}C$였다. 한편 철근 콘크리트의 제조에 사용되는 시멘트의 플라이 애쉬 함유량은 철근부식의 방지를 위하여 30 wt%를 초과하지 않는 게 바람직하다. 플라이 애쉬를 혼합한 콘크리트의 강도가 순수 포틀랜드 시멘트 콘크리트의 강도보다 양호하기 위해서는 양생기간이 28일 이상 되어야 한다. 그리고 플라이 애쉬 50 wt%를 함유한 시멘트를 가지고도 360 kg/$\textrm{cm}^2$ 이상의 28일 강도를 얻을 수 있었다.

• 한국주조공학회지
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• 제25권1호
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• pp.16-22
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• 2005

This study aims to investigate the effects of the microstructures and nodule type on the fatigue characteristics of cast iron. Fatigue tests were carried out in tension-tension mode using a servo-hydraulic testing machine with load control mode operating at a frequency of 15 Hz. The tests were conducted at stress ratio R=Kmin/Kmax, of 0.1. Initial crack ${\Dalta}K$ values were highly performed with increase in tensile strength of DCI fatigue specimens. ${\Dalta}K_{th}$ region, fatigue crack propagation was primarily advanced through cell boundary and in periphery of near nodule. Fatigue crack propagation rate of D2 consisted with 2Phase(Ferrite+Pearlite) was slow due to crack closure enhanced by crack deflection and occurred crack branching. The generation of crack branch was occurred due to interaction of crack-nodule. At Threshold and Paris zone, the fractographs of the fatigue fracture surface for DCI show typical striations of a ductile fracture and isolated cleavage planes near graphite. The effect of microstructure on fatigue crack propagation of GC strongly depends on the type of flake. The generation of crack branch occurred due to interaction of crack-nodule. The fractographs of the fatigue fracture surface for GC show cleavage plane along the flake graphite.