• Proceedings of the Korea Concrete Institute Conference
• /
• 2001.11a
• /
• pp.445-450
• /
• 2001

The paper investigates the relationships between dynamic elastic modulus and static elastic modulus or compressive strength according to curing temperature, aging, and cement type. Based on this investigation, the new model equations are proposed. Impact echo method estimates the resonant frequency of specimens and uniaxial compression test measures the static elastic modulus and compressive strength. Type I and V cement concretes, which have the water-cement ratios of 0.40 and 0.50, are cured under the isothermal curing temperature of 10, 23, and 50 $^{\circ}C$. Cement type and aging have no large influence on the relationship between dynamic and static elastic modulus, but the ratio of dynamic and static elastic modulus comes close to 1 as temperature increases. Initial chord elastic modulus, which is calculated at lower strain level of stress-strain curve, has the similar value to dynamic elastic modulus. The relationship between dynamic elastic modulus and compressive strength has the same tendency as the relationship between dynamic and static elastic modulus. The proposed relationship equations between dynamic elastic modulus and static elastic modulus or compressive strength properly estimates the variation of relationships according to cement type, temperature, and aging.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2001.11b
• /
• pp.467-471
• /
• 2001

DC accelerated aging characteristics of $Pr_{6}O_{11}$-based ZnO varistors, which are composed of $ZnO+Pr_{6}O_{11}+CoO+Cr_{2}O_{3}+Dy_{2}O_{3}$ ceramics were investigated with CoO content in the range of 0.5 - 5.0 mol%. The varistors doped with 1.0 mol% revealing maximum value(66.61) in the nonlinear exponent exhibited excellent stability, in which the variation rates of the varistor voltage, the nonlinear exponent and leakage current are -1.93%, -10.48%, and +288.79%, respectively, under DC accelerated aging stress, such as $(0.85V_{lmA}/115^{\circ}C/24h)+(0.90V_{lmA}120^{\circ}C/24h)+(0.95V_{lmA}/125^{\circ}C/24h)+(0.95V_{lmA}/150^{\circ}C/24h)$. Next the varistors doped with 2.0 mol% exhibiting the nonlinear exponent of 47.39 showed high stability.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2002.11a
• /
• pp.330-333
• /
• 2002

The stability against DC accelerated aging stress of $Dy_{2}O_{3}-doped$ $ZnO-Pr_{6}O_{11}-based$ varistor ceramics was investigated. The calculated nonlinear exponent$(\alpha)$ in varistor ceramics without $Dy_{2}O_{3}$ was only 4.9, whereas the $\alpha$ value of the varistors with $Dy_{2}O_{3}$ was abruptly increased in the range of 48.8 to 58.6. The varistor ceramics with $Dy_{2}O_{3}$ content of 1.0 mol% exhibited maximum ${\alpha}$, reaching 58.6, whereas they exhibited very poor stability. While, The varistor ceramics doped with 0.5 mol% $Dy_{2}O_{3}$ exhibited not only the high nonlinearity, which the ${\alpha}$ is 55.3 and the leakage current is $0.1{\mu}A$， but also the highest stability, which the variation rates of varistor voltage and nonlinear exponent are -0.8% and -14.3%, respectively, under DC accelerated aging stress, $0.95 V_{1mA}/150^{\circ}C/24h$.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
• /
• v.15 no.9
• /
• pp.776-782
• /
• 2002

The microstructure, V-Ι characteristics, and stability of ZnO-P $r_{6}$ $O_{11}$ CoO-C $r_2$ $O_3$- $Y_2$ $O_3$-based varistor ceramics were investigated with cooling rate in the range of 2~8$^{\circ}C$/min. The cooling rate relatively weakly affected the microstructure, the varistor voltage, and the leakage current in the V-Ι characteristics. But the nonlinear exponent relatively strongly affected by cooling rate. The cooling rate also greatly affected the stability of V-Ιand dielectric characteristics for DC accelerated aging stress. On the whole, the varistors cooled with 4$^{\circ}C$/fin exhibited the highest performance in the densification, nonlinearity, and stability. Especially, they exhibited a high stability, in which the variation rate of the varistor voltage( $V_{1㎃}$), the nonlinear exponent($\alpha$), and the dissipation factor(tan $\delta$) is -1.4%, -4.9%, and ＋60.0%, respectively, under DC accelerated aging stress such as 0.95 $V_{1㎃}$15$0^{\circ}C$/12 h)

• Journal of the Korean Solar Energy Society
• /
• v.40 no.2
• /
• pp.11-23
• /
• 2020

The application of the high-performance insulation materials for buildings seems to be an essential measure for reducing energy use in buildings. Phenolic foam is a readily available insulation material with thermal conductivity of about 0.018 to 0.020 W/(mK). It has the advantage of higher thermal resistance and better fire resistance compared to other conventional building insulation materials. Insulation material used for building envelope is regarded as one of the decisive factors for building's energy load. Furthermore, the degradation of its thermal performance over time increasingly affects the building's energy use demand. Generally, the life span of conventionally built buildings is expected to be more than 50 years, so the long-term performance of insulation materials is critical. This paper aims to evaluate the long-term performance of phenolic form boards through an accelerated aging test. The tests were conducted according to BS EN 13166 and KS M ISO 11561. Based on the results of the accelerated aging test, the thermal performance variation of the material was analyzed, and then its aged value after 25 years was computed. Also, the characteristics of the phenolic foam board's long-term performance were also examined based on the standard testing methods adopted.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 1999.11a
• /
• pp.479-482
• /
• 1999

Cured epoxy resins are extensively used for the electrical insulation in high-voltage equipments. The bisphenol A-based epoxy resins lured with azine show, especially, good thermal properties and mechanical resistances. For the technical and economic reasons, varing amount of inorganic fillers are added to endow the required special properties. In the large generators and motors of power plants, epoxy insulation is disclosed to the harsh conditions like the superheated steam and abrupt temperature variation. Hygrothermal aging at elevated temperatures tends to induce degradation in epoxy resins. To predict the effect of this degradation in DGEBA/MDA/SN/zeolite system, we proceeded the forced moisture absorption experiment using the autoclave. The thermal properties of the untreated and treated specimens were analyzed by DSC and TGA under the nitrogen flowing condition. The moisture absorption results showed a weight increase during hygrothermal aging at 1207. At the initial aging period, the system leaded to more or less postcuring but more prolonged environmental aging leaded the discoloration of specimen and lowering the T$_{g}$./.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2001.11a
• /
• pp.467-471
• /
• 2001

DC accelerated aging characteristics of Pr$_{6}$O$_{11}$-based ZnO varistors, which are composed of ZnO+Pr$_{6}$O$_{11}$+CoO+Cr$_2$O$_3$+Dy$_2$O$_3$ ceramics were investigated with CoO content in the range of 0.5~5.0 mol%. The varistors doped with 1.0 mol% revealing maximum value(66.61) in the nonlinear exponent exhibited excellent stability, in which the variation rates of the varistor voltage, the nonlinear exponent and leakage current are -1.93%, -10.48%, and 288.79%, respectively, under DC accelerated aging stress, such as (0.85 V$_{1mA}$/115$^{\circ}C$/24h)+(0.90 V$_{1mA}$/12$0^{\circ}C$/24h)+(0.95 V$_{1mA}$/1$25^{\circ}C$/24h)+(0.95 V$_{1mA}$/15$0^{\circ}C$/24h). Next the varistors doped with 2.0 mol% exhibiting the nonlinear exponent of 47.39 showed high stability,ity,ability,ity,

• The Transactions of The Korean Institute of Electrical Engineers
• /
• v.56 no.9
• /
• pp.1561-1565
• /
• 2007

According to thermal degradation on power transformers, it is known that electrical, mechanical and chemical characteristics for power transformer's oil-paper are changed. In the chemical property, especially, when the kraft paper is aged, the cellulose polymer chains break down into shorter lengths. It causes decrease in both tensile strength and degree of polymerization of paper insulation. Also the paper breakdown is accompanied by an increase in the content of various furanic compounds within the dielectric liquid. It is known that furanic components in transformer oil come only from the decomposition of insulating paper rather than from the oil itself. Therefore the analysis of furanic degradation products provides a complementary technique to dissolved gas analysis for monitoring transformers when we evaluate the aging of insulating paper by the total concentration of carbon monoxide and carbon dioxide dissolved in oil only. Recently, the analysis of furanic compounds by high performance liquid chromatography(HPLC) using IEC 61198 method for estimating degradation of paper insulation in power transformers has been used more conveniently for assessment of oil-paper. It is know that the main products which is produced by aging are 2-furfuryl alcohol, 2-furaldehyde(furfural), 2-furoic acid, 2-acetylfuran, 5-methyl-2-furaldehyde, and 5-hydroxymethyl-2-furaldehyde. For investigating the accelerated aging process of oil-paper samples we manufactured accelerating aging equipment and we estimated variation of insulations at $140^{\circ}C$ temp. during 500 hours. Typical transformer proportions of copper, silicon steel and iron have been added to oil-paper insulation during the aging process. The oil-paper insulation samples have been measured at intervals of 100 hours. Finally we have analyzed that 2-furoic acid and 2-acetylfuran products of furanic compounds were detected by HPLC, and their concentrations were increased with accelerated aging time.

• The Korean Journal of Food And Nutrition
• /
• v.31 no.4
• /
• pp.559-564
• /
• 2018

Rehmannia glutinosa root (R. root) has been used as an traditional medicine, and is important resource for natural medicines and functional foods. However, R. root contains catalpol which is bitter, and undigested sugars, including stachyose and raffinose, which can cause diarrhea. Therefore, this study was performed to identify the changes in reducing sugar from undigested sugars and in catalpol contents in R. root slurry induced by aging treatments. R. root slurry was treated at $10{\sim}70^{\circ}C$ for up to 72 hr; and extracted with a 50% ethanol solution. The catalpol content was analyzed using HPLC-UVD. Reducing sugar content generated from undigested sugars was measured by the Nelson-Somogyi methods, and the reaction rates were calculated from their variation according to aging time and temperature. During the aging treatment, reducing sugar increased and catalpol decreased. Their formation and degradation rates were highest at $50^{\circ}C$ and $30{\sim}40^{\circ}C$, and their rates were $2.05mg/g{\cdot}hr$ and 23.09 to 23.33%/hr, respectively. These results indicated that aging treatment can positively affect the sweetness and digestibility of R. root slurry. Therefore, an aging treatment could be considered for improving the taste and digestibility of R. root.

• Journal of the Korean Magnetics Society
• /
• v.19 no.3
• /
• pp.100-105
• /
• 2009

The high-temperature isothermal aging is studied in ultra-supercritical steel, which is attractive to the next generation of power plants. The effects of microstructure on reversible permeability are discussed. Isothermal aging was observed to coarsen the tempered carbide ($Cr_{23}C_6$), generate the intermetallic ($Fe_2W$) phase and grow rapidly during aging. The dislocation density also decreases steeply within lath interior. The dynamic coercivity, measured from the peak position of the reversible permeability profile decreased drastically during the initial 500 h aging period, and was thereafter observed to decrease only slightly. The variation in dynamic coercivity is closely related to the decrease in the number of pinning sites, such as dislocations, fine precipitates and the martensite lath.