• 한국자기학회:학술대회 개요집
• /
• 한국자기학회 2012년도 자성 및 자성재료 국제학술대회
• /
• pp.104-105
• /
• 2012

The temperature dependence of the effective magnetic anisotropy constant K(T) of ferrite nanoparticles is obtained based on the measurements of SQUID magnetometry. For this end, a very simple but intuitive and direct method for determining the temperature dependence of anisotropy constant K(T) in nanoparticles is introduced in this study. The anisotropy constant at a given temperature is determined by associating the particle size distribution f(r) with the anisotropy energy barrier distribution $f_A(T)$. In order to estimate the particle size distribution f(r), the first quadrant part of the hysteresis loop is fitted to the classical Langevin function weight-averaged with the log?normal distribution, slightly modified from the original Chantrell's distribution function. In order to get an anisotropy energy barrier distribution $f_A(T)$, the temperature dependence of magnetization decay $M_{TD}$ of the sample is measured. For this measurement, the sample is cooled from room temperature to 5 K in a magnetic field of 100 G. Then the applied field is turned off and the remanent magnetization is measured on stepwise increasing the temperature. And the energy barrier distribution $f_A(T)$ is obtained by differentiating the magnetization decay curve at any temperature. It decreases with increasing temperature and finally vanishes when all the particles in the sample are unblocked. As a next step, a relation between r and $T_B$ is determined from the particle size distribution f(r) and the anisotropy energy barrier distribution $f_A(T)$. Under the simple assumption that the superparamagnetic fraction of cumulative area in particle size distribution at a temperature is equal to the fraction of anisotropy energy barrier overcome at that temperature in the anisotropy energy barrier distribution, we can get a relation between r and $T_B$, from which the temperature dependence of the magnetic anisotropy constant was determined, as is represented in the inset of Fig. 1. Substituting the values of r and $T_B$ into the $N{\acute{e}}el$-Arrhenius equation with the attempt time fixed to $10^{-9}s$ and measuring time being 100 s which is suitable for conventional magnetic measurement, the anisotropy constant K(T) is estimated as a function of temperature (Fig. 1). As an example, the resultant effective magnetic anisotropy constant K(T) of manganese ferrite decreases with increasing temperature from $8.5{\times}10^4J/m^3$ at 5 K to $0.35{\times}10^4J/m^3$ at 125 K. The reported value for K in the literatures is $0.25{\times}10^4J/m^3$. The anisotropy constant at low temperature region is far more than one order of magnitude larger than that at 125 K, indicative of the effects of inter?particle interaction, which is more pronounced for smaller particles.

• 한국수산과학회지
• /
• 제51권5호
• /
• pp.590-594
• /
• 2018

We developed a time-temperature indicator (TTI) that rapidly determines whether frozen mackerel Scomber japonicus has thawed during its distribution. Our TTI was made from filter paper capable of absorbing mackerel exudate that was soaked in a 20% citric acid solution at pH 4.0 and then dried. The dried absorbent paper was cut and attached to blue litmus paper with a 2 mm overhang. The fixed litmus paper was covered with a polypropylene film and sealed. The indicator was placed inside a polyvinyl vacuum package containing the mackerel sample. The vacuum-sealed packaged was frozen at $-20^{\circ}C$ for 24 h. After freezing, the color change and time dependence of the indicator were observed at room temperature ($25^{\circ}C$) and demonstrated the utility of this TTI for rapidly determining whether frozen mackerel underwent thawing during distribution.

• Journal of Electrical Engineering and Technology
• /
• 제6권5호
• /
• pp.671-676
• /
• 2011

A multilayer square-type piezoelectric transformer with a hole at the center was investigated in this paper. Temperature distribution at the center was improved by using this construction, therefore increasing input voltage and output power. This model was simulated and investigated successfully by applying a finite element method (FEM) in ATILA software. An optimized structure was then fabricated, examined, and compared to the simulation results. Electrical characteristics, including output voltage and output power, were measured at different load resistances. The temperature distribution was also monitored using an infrared camera. The piezoelectric transformer operated at first radial vibration mode and a frequency area of 70 kHz. The 16 W output power was achieved in a three-layer transformer with 96% efficiency and $20^{\circ}C$ temperature rise from room temperature under 115 V driving voltage, 100 ${\Omega}$ matching load, $28{\times}28{\times}1.8mm$ size, and 2 mm hole diameter. With these square-type multilayer piezoelectric transformers, the temperature was concentrated around the hole and lower than in piezoelectric transformers without a hole.

• 조명전기설비학회논문지
• /
• 제27권1호
• /
• pp.10-16
• /
• 2013

This study performed a subjective evaluation of the Semantic Differential (SD) method for living room activities to evaluate the living room lighting environment and investigated the relationship between these through luminance distribution in the space. As a result, three factors-"lightness", "emotion" and "calmness"-have been grouped together. According to the analysis of experimental variables, difference has been observed depending on color temperature, the dimming of the ceiling & cove lights and the use of down lights and a bracket. 'Conversation with Family', 'Having Fun with Family', 'Entertaining Guests' and 'Reading a Book or Newspaper' requires "lightness". In terms of "emotion"-centered activities, 'Watching TV' is the highest, but all three factors are related. In terms of "calmness"-centered activities, 'Relaxing' is the highest.

• 한국전산유체공학회지
• /
• 제14권2호
• /
• pp.39-45
• /
• 2009

In engine room, proper enclosure system is preferable for reducing noise level but the enclosure system in the engine room causes bad influence on cooling performance due to poor ventilation. Cooling efficiency of the enclosure system can be improved by varying fan speed and proper flow path for ventilation. In this study, numerical analysis is performed to assess cooling effect of the enclosure system using finite volume method. The RNG k-$\varepsilon$ model is adopted for turbulence model along with heat exchanger model and porous media model for heat exchanger analysis, and moving reference frame model for rotational fan. Verification result shows reasonable agreement with experimental data. Analysis results show direct effect of velocity and temperature distribution on cooling ability in the enclosure system. Enclosure system of case B shows high heat transfer coefficient and has the smallest area ratio of opened flow passages which is good for noise level reduction.

• 태양에너지
• /
• 제17권2호
• /
• pp.13-22
• /
• 1997

본 연구에서는 실내로 유입되는 온기와 실내공기사이의 온도차 및 유입속도에 따라 발생되는 부력의 영향 및 실내로 유입된 온기의 혼합특성에 관한 열유동특성을 파악하였다. 혼합도의 정의로부터 임계속도($1.2{\sim}1.6m/s$)전 후로 기하학적영향에 대하여서 혼합과의 관계를 파악할 수 있었다. 그리고 축열표율과의 관계는 속도가 크고 기준온도차가 작을수록 부력의 영향을 적게 받아서 아네모스형, 베인형 모두 높은 축열효율을 나타냈다. 그리고 속도가 클 때는 베인형이 전반적으로 우수한 축열효율을 나타냈다. 따라서 유입속도에 의한 영향이 기준온도차에 의한 영향보다 지배적임을 알 수 있었다.

• 대한설비공학회:학술대회논문집
• /
• 대한설비공학회 2004년도 하계학술발표 논문집
• /
• pp.146-146
• /
• 2004

• 설비공학논문집
• /
• 제20권11호
• /
• pp.711-717
• /
• 2008

Despite the fact that UFAD(Under Floor Air Distribution) systems have many benefits and are being applied in the field in increasing numbers, there is a strong need for an improved fundamental understanding of several key performance features of these systems. This study numerically investigates the effect of supplied air temperature and supplied flow rate on the performance of UFAD, especially focused on thermal comfort. Also this study has compared UFAD with conventional overhead air distribution system. In contrast to the well-mixed room air conditions of the conventional overheat system, UFAD system produces an overall floor-to-ceiling airflow pattern that takes advantage of the natural buoyancy produced by heat sources in the occupied zone and more efficiently removes heat loads and contaminants from the space. Thermal comfort parameters were evaluated by CFD approach and then PMV was computed to detect the occupants' thermal sensation. Results show that radiative mean temperature plays crucial role on the evaluating PMV. Until now, the radiative temperature has been the missing link between CFD and thermal comfort, but the present study paves the way for overcoming this weakness.

• 한국환경복원기술학회지
• /
• 제17권5호
• /
• pp.1-17
• /
• 2014

Research was initiated to investigate early establishment characteristics and germination pattern of Kentucky bluegrass (KB, Poa pratensis L.) under natural room temperature and to analyze differences between alternative and natural conditions for a practical application. Fifteen cultivars of KB were evaluated in the study. Daily and cumulative germination pattern were measured and analyzed on a daily basis. Differences were observed in germination percentage, germination pattern, days to the first germination and days to 75% germination. Germination percentage was variable with cultivars at the end of study. It was 75.75 to 90.25% under natural condition. There were considerable variations in early germination characteristics and germination pattern among KB cultivars according to germination conditions. The first germination in all entries tested was initiated between 6 and 9 DAS (days after seeding) under ISTA (International Seed Testing Association) condition. But it was observed between 9 and 12 DAS under natural condition, being 3 to 5 days later as compared with ISTA condition. On the first date of a germinating process, the germination rate was 0.25% to 25.00% under ISTA condition, while 0.50% to 13.25% under natural condition. Days to the 75% germination were 15.08 to 28.80 DAS under ISTA condition and 17.78 to 28.75 DAS under natural condition. Midnight II, Excursion and Midnight were the fastest cultivars under ISTA condition, while Odyssey the fastest one under natural condition. Regardless of germination condition, the slowest cultivar was Voyager II, being over 4 weeks to 75% germination percentage. For the most cultivars, days to 75% germination under natural room temperature condition were 1 to 8 days slower than under ISTA condition. From this study information on differences in germination characteristics and pattern among cultivars would be usefully applied for KB establishment such as golf course construction. Prior to on-site seeding, a comprehensive germination test is practically recommended due to a possible decline in germination capability during domestic distribution and storage after import.

• Journal of Magnetics
• /
• 제19권4호
• /
• pp.323-326
• /
• 2014

Superparamagnetic maghemite nanoparticles were prepared by chemical co-precipitation, followed by a temperate oxidation stage, and investigated using FE-SEM, XRD, TGA, VSM, and M$\ddot{o}$ssbauer spectroscopy. Through SEM image and XRD analysis, its average particle size was found to be 13.9 nm. While VSM magnetic measurement showed typical superparamagnetic behavior at room temperature, M$\ddot{o}$ssbauer spectroscopic investigation revealed that non-vanishing magnetic hyperfine structure were retained. Cation distribution estimated from M$\ddot{o}$ssbauer spectroscopy confirmed the formation of maghemite nanophase in the sample.