• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2006년도 춘계학술대회
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• pp.96-98
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• 2006

Proper water management is vital to achieve high performance and durability of PEFC (Polymer Electrolyte Fuel Cell). The effects of the residual water from PEFC after purge in shut-down processes on GDL/MEAs were investigated with freeze/thaw cycles Freeze/thaw cycle tests were conducted with single cells which were designed from transparent acryl plates. Single cells which contain several amount of residual water were cycles from $80^{\circ}C$ to $-28^{\circ}C$. The resistance changes of the single cells which have various amount of residual water were evaluated by ac-impedance analysis with 24 times of freeze/thaw cycles. Also, after the freeze/thaw cycles, the property changes were characterized by visual methods such as SEM, EPMA. Though it was difficult to observe noticeable property changes in the visual characterizations, the resistance of cells dramatically increased with the amount of remained water.

• 한국축산식품학회지
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• 제41권6호
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• pp.923-935
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• 2021

Fresh grass carp was used to produce surimi samples that were supplemented with 50 g/kg, 100 g/kg, or 150 g/kg pork back fat. The lipid composition, lipase activity, lipid oxidation index, and lipoxygenase activity of samples subjected to repeated freezethaw process were determined to assess the effects of the added fat on lipolysis and lipid oxidation of grass carp surimi. Freeze-thaw treatment increased free fatty acid content, mainly due to the decomposition of phospholipids and some neutral lipids by lipase. With repeated freeze-thaw treatment, the levels of free fatty acids and phospholipids were correlated with the lipid oxidation indexes and lipoxygenase activity, indicating that lipid degradation can promote lipid oxidation. In the same freeze-thaw cycle, surimi products with high fat content are more vulnerable to oxidative damage, neutral lipids are the main source of free fatty acids in the early stage of freeze-thaw, and phospholipids are the main source of free fatty acids in the late stage.

• 대한수의학회지
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• 제61권4호
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• pp.29.1-29.7
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• 2021

Resveratrol (RSV, 3,5,4'-trihydroxytrans-stilbene) protects sperm from cryo-induced damage in various animal and human species. In this study, we aimed to assess the effect of dog sperm cryoprotective extender containing RSV on the quality of post-thaw dog sperm. Sperm were collected from 4 Beagles and supplemented with different concentrations of RSV (0, 100, 200, and 400 µM). After thawing, apoptosis index, and reactive oxygen species (ROS) levels were assessed to determine post-thaw sperm quality. Dog sperm cryopreserved with 400 µM RSV showed significant improvement in post-thaw sperm quality with lower apoptosis index and ROS levels (p < 0.05). Our results showed that the supplementation of dog sperm cryoprotective extender with RSV at a concentration of 400 µM improved the post-thaw dog sperm quality in the term of sperm ROS production and apoptosis. In addition, we emphasize the necessity of testing the ROS levels and apoptosis index using flow cytometry to determine the quality of post-thaw semen.

• Steel and Composite Structures
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• 제48권3호
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• pp.251-262
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• 2023

There are many examples of steel structures subjected to severe environmental conditions with bolted connections directly exposed to extreme climatic agents such as freeze-thaw cycles or low temperatures. Some examples are: steel bridges, mining transfer towers, wind towers... These service conditions neither are included in Eurocode 3 or EN1090-2, nor there are references in other international standards. In this experimental research, 46 specimens of non-slip joints with HV M20 bolts and four different types of contact surfaces have been studied. Half of the specimens were subjected to fourteen twelve-hours freeze-thaw cycles, with periodic immersion in water and temperature oscillation. Subsequently, half of the connections were subjected to a slip test under monotonic load at temperature of -20 ± 0.5 ℃ and the other half at room temperature. The results were compared with others equal joints not subjected to freeze-thaw cycles and kept at room temperature for the same time. This finally resulted in 4 sets of joints by combining the freeze-thaw degradation or not with the low-temperature conditions or not in the slip testing. Therefore, a total of 16 different conditions were studied by also considering 4 different contact surfaces between the joined plates in each set. The results obtained show influence of environmental conditions on the slip resistant capacity of these joints.

• 한국도로학회논문집
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• 제18권6호
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• pp.123-130
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• 2016

OBJECTIVES : The objective of this research is to determine the moisture resistance of the freeze-thaw process occurring in low-noise porous pavement using either hydrated-lime or anti-freezing agent. Various additives were applied to low-noise porous asphalt, which is actively paved in South Korea, to overcome its disadvantages. Moreover, the optimum contents of hydrated-lime and anti-freezing agent and behavior properties of low-noise porous asphalt layer are determined using dynamic moduli via the freeze-thaw test. METHODS : The low-noise porous asphalt mixtures were made using gyratory compacters to investigate its properties with either hydrated-lime or anti-freezing agent. To determine the dynamic moduli of each mixture, impact resonance test was conducted. The applied standard for the freeze-thaw test of asphalt mixture is ASTM D 6857. The freeze-thaw and impact resonance tests were performed twice at each stage. The behavior properties were defined using finite element method, which was performed using the dynamic modulus data obtained from the freeze-thaw test and resonance frequencies obtained from non-destructive impact test. RESULTS : The results show that the coherence and strength of the low-noise porous asphalt mixture decreased continuously with the increase in the temperature of the mixture. The dynamic modulus of the normal low-noise porous asphalt mixture dramatically decreased after one cycle of freezing and thawing stages, which is more than that of other mixtures containing additives. The damage rate was higher when the freeze-thaw test was repeated. CONCLUSIONS : From the root mean squared error (RMSE) and mean percentage error (MPE) analyses, the addition rates of 1.5% hydrated-lime and 0.5% anti-freezing agent resulted in the strongest mixture having the highest moisture resistance compared to other specimens with each additive in 1 cycle freeze-thaw test. Moreover, the freeze-thaw resistance significantly improved when a hydrated-lime content of 0.5% was applied for the two cycles of the freeze-thaw test. Hence, the optimum contents of both hydrated-lime and anti-freezing agent are 0.5%.

• Clinical and Experimental Reproductive Medicine
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• 제20권2호
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• pp.165-176
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• 1993

These studies were carried out to investigate the optimal freezing protocol for 2 cell mouse embryos and to find the probability of quality control with 2-cell embryos frozen. The embryos showed the best survival by the protocol composed of a freezing solution with the cryoprotectants(1.5M propanediol + 0.1M sucrose), and a 2-steop thawing method(room temperature, 20 sec-37$^{\circ}C$, 20 sec). The developmental ability of frozen-thaw 2-cell embryos did not differ from that of fresh 2-cell embryos in m-KRB medium with 0.4% bovine serum albumin. But development of frozen-thaw embryos was depended on the supplements of the medium. In the albumin-free medium, the developmental rate(rate of blastocysts) was significantly reduced, compared with that in the medium with 0.4% BSA. Also, when frozen-thaw embryos were cultured in the meduim with human fetal cord serum(HCS), the developmental rate of frozen-thaw embryos was sligtly reduced, compared with that of fresh 2-cell embryos. Finally, frozen-thaw 2-cell mouse embryos were more sensitive to the toxic agent of disposable-plastic syringe. Therefore, toxicity of medium could be effectively detected by frozen-thaw 2-cell mouse embryos.

• International Journal of Concrete Structures and Materials
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• 제11권1호
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• pp.59-68
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• 2017

Mercury intrusion and nitrogen sorption porosimetry were employed to investigate the pore structure of calcium sulfoaluminate ($C{\bar{S}}A$) and portland cement pastes with cement-to-water ratio (w/c) of 0.40, 0.50, and 0.60. A unimodal distribution of pore size was drawn for $C{\bar{S}}A$ cement pastes, whereas a bimodal distribution was established for the portland cement pastes through analysis of mercury intrusion porosimetry. For the experimental results generated by nitrogen sorption porosimetry, the $C{\bar{S}}A$ cement pastes have a smaller and coarser pore volume than cement paste samples under the same w/c condition. The relative dynamic modulus and percentage weight loss were used for investigation of the concrete durability in freeze-thaw condition. When coarse aggregate with good freeze-thaw durability was mixed, air entrained portland cement concrete has the same durability in terms of relative dynamic modulus as $C{\bar{S}}A$ cement concrete in a freeze-thaw environment. The $C{\bar{S}}A$ cement concrete with poor performance of durability in a freeze-thaw environment demonstrates the improved durability by 300 % over portland cement concrete. The $C{\bar{S}}A$ concrete with good performance aggregate also exhibits less surface scaling in a freeze-thaw environment, losing 11 % less mass after 297 cycles.

• 콘크리트학회논문집
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• 제22권3호
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• pp.399-407
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• 2010

이 연구에서는 동결융해 사이클을 경험한 2가지 종류의 휨부재의 거동특성을 평가하였다. 이 연구의 목적은 동결융해에 따른 철근콘크리트 보의 거동특성을 검토하는 것이다. 이를 위해 일부 실험체는 동결융해를 경험하기 전, 인장철근이 항복되기까지 손상을 입도록 계획되었다. 또한 반복하중 재하시 강성저하 특성을 평가하기 위하여 단조 및 반복재하 실험을 실시하였다. 재료 실험 결과, 동결융해 300사이클을 경험한 콘크리트의 상대동탄성계수는 86.8%까지 감소되었으나 내동해저항성은 충분히 가지고 있는 것으로 평가되었다. 단조재하 실험 결과, 동결융해 사이클에 따른 휨 강도, 연성 및 강성은 상대적으로 감소하는 것으로 나타났다. 특히, 인위적 균열손상을 경험한 BDF13 시리즈는 현행 콘크리트설계기준에서 요구하는 공칭모멘트를 만족하지 못하는 것으로 나타났다. 반복재하시 BF75 시리즈에서 동결융해를 경험함에 따라 10% 이상의 반복강성 저하를 나타내었다. 따라서 내진부재와 같이 반복하중을 받게 되는 부재를 설계할 경우, 동결융해로 인한 압축측 콘크리트의 변형 특성도 고려되어야 할 것으로 판단된다.

• 비파괴검사학회지
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• 제20권4호
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• pp.269-275
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• 2000

동결-융해 피로로부터 발생되는 손상이 콘크리트의 열화 및 붕괴를 초래하는 중대한 문제가 된다는 것은 이미 널리 알려져 있다. 일반적으로 동결-융해 피로가 지속되면 콘크리트의 미시조직에 내부응력과 크랙을 유발시킨다. 본 연구에서는 콘크리트에서의 동결-응해 손상을 평가하기 위해 종파와 표면파속도를 동시에 측정하는 one-sided 응력파 속도측정기법이라는 새로운 기법을 사용하였다. 상업용 시험장치를 이용하여 ASTM C666에 따라 제작된 $400{\times}350{\times}100mm$ 크기의 콘크리트 시편에 동결-융해 손상을 발생시켰다. 1사이클은 온도변화를 -14에서 $4^{\circ}C$로 하였고, $4{\sim}5$시간이 소요되었다. 매 5사이클마다 one-sided 응력파 속도측정기법에 기초하여 종파 및 표면파속도를 측정하였다. 동결-융해 손상이 증가함에 따라 종파 및 표면파속도의 변화가 있었고 이것은 동결-등채 피로 손상과정을 보다 유효하게 나타낸다는 것을 입증하였다. 또한 one-sided 기법에 의해 측정된 종파속도의 변화를 투과법을 이용하여 측정된 결과와 비교하였다.

• 한국지리정보학회지
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• 제26권4호
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• pp.40-55
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• 2023

식목일은 식재를 권장하고 식재 시기가 도래했음을 상징하는 날로서 우리나라의 경우 4월 5일로 정해져 있지만, 지구온난화로 인해 식목일이 실제 식재 시기와 큰 차이를 보이고 있다. 본 연구에서는 1991~2020년 기간을 대상으로 인공위성 기반의 토양 융해 관측 자료(FT-ESDR)를 이용하여 식재 가능 시기의 시작과 현행 식목일 간의 불일치 현상을 확인하고 식재를 위한 대안 시기를 검토하였다. 연구 결과, 한반도의 토양 융해일은 평균적으로 1991~2000년 기간 중 3월 24일이었던 것이 2011~2020년 기간에는 3월 17일로 변화하였다. 토양 융해일을 고려하여 식목일을 변경할 경우, 시·군 수와 면적을 모두 고려할 때 3월 중순을 채택하는 것이 식재 시기의 적합성 측면에서 가장 포괄성이 큰 것으로 분석되었다. 따라서 현행 식목일의 대안으로 적합도가 높은 날은 식수절(3월 14일)과 세계 산림의 날(3월 21일)인데, 이는 시기적으로 한반도 평균 토양 융해일인 3월 19일에 가깝고, 토양 융해일과의 편차가 10일 이내 범위에 놓이는 면적이 한반도 중부를 중심으로 전체의 52.5~58.8%에 달한다는 장점을 갖는다. 기후 변화의 영향으로 토양 융해 시기는 변화할 것으로 예측되기 때문에 식목일 일자를 변경하고자 할 경우에는 미래의 식재 적합 시기가 지속적으로 앞당겨지는 경향을 반영할 필요가 있다.