• Bulletin of the Korean Chemical Society
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• v.32 no.3
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• pp.836-840
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• 2011

This study examines the effects of a carbon coating on the electrochemical performances of $LiFePO_4$. The results show that the capacity of bare $LiFePO_4$ decreased sharply, whereas the $LiFePO_4$/C shows a well maintained initial capacity. The Li ion diffusivity of the bare and carbon coated $LiFePO_4$ is calculated using cyclic voltammetry (CV) to determine the correlation between the electrochemical performance of $LiFePO_4$ and Li diffusion. The diffusion constants for $LiFePO_4$ and $LiFePO_4$/C measured from CV are $6.56{\times}10^{-16}$ and $2.48{\times}10^{-15}\;cm^2\;s^{-1}$, respectively, indicating considerable increases in diffusivity after modifications. The Li ion diffusivity (DLi) values as a function of the lithium content in the cathode are estimated by electrochemical impedance spectroscopy (EIS). The effects of the carbon coating as well as the mechanisms for the improved electrochemical performances after modification are discussed based on the diffusivity data.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.2
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• pp.302-309
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• 2002

A new method of measuring the thermal diffusivity of solid material at room temperature using photothermal displacement is proposed. The influence of the parameters, such as radius and modulation frequency of the pump beam and the sample thickness, was studied. In previous works, thermal diffusivity was determined by the deformation angle and phase angle as the relative position between the heating and probe beams. In this study, however, we proposed the new analysis method based on the real part of deformation angle as the relative position between two beams. From the zero-crossing position of real part of deformation angle with respect to the pump beam, the thermal diffusivity of the materials can be obtained. The experimental values for different samples obtained by applying the new method are in good agreement with the literature values.

• Nuclear Engineering and Technology
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• v.49 no.3
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• pp.459-465
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• 2017

The goal of this study is to evaluate the effective thermal conductivity and diffusivity of containment walls as heat sinks or passive cooling systems during nuclear power plant (NPP) accidents. Containment walls consist of steel reinforced concrete, steel liners, and tendons, and provide the main thermal resistance of the heat sinks, which varies with the volume fraction and geometric alignment of the rebar and tendons, as well as the temperature and chemical composition. The target geometry for the containment walls of this work is the standard Korean NPP OPR1000. Sample tests and numerical simulations are conducted to verify the correlations for models with different densities of concrete, volume fractions, and alignments of steel. Estimation of the effective thermal conductivity and diffusivity of the containment wall models is proposed. The Maxwell model and modified Rayleigh volume fraction model employed in the present work predict the experiment and finite volume method (FVM) results well. The effective thermal conductivity and diffusivity of the containment walls are summarized as functions of density, temperature, and the volume fraction of steel for the analysis of the NPP accidents.

• Journal of the Korean Statistical Society
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• v.19 no.2
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• pp.176-181
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• 1990

Sung [1990] presented an analogue of the classical Cramer-Rao inequality for median-unbiased estimators with continuous multivariate densities depending upon a vector parameter. In the process, diffusivity, a new dispersion measure relevant to median-unbiased estimators, was defined to be a function of median-unbiased estimator's density height. In this paper we shall elaborate these ideas by defining a second kind of diffusivity and discuss the role of model-unbiasedness in median-unbiased estimation in connection with this seconde kind of diffusivity. In addition, median-unbiased estimation will be compared to mean-unbiased estimation.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.10a
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• pp.531-536
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• 2002

Silica fume influences concrete diffusivity and permeability as well as strength by densifying the microstructure of the interfacial transition zone (ITZ) of high strength concrete, by reducing the capillary porosity of cement paste and by producing less diffusible and permeable pozzolanic CSH gel than CSH gel of conventional cement hydration. This paper presents a procedure to predict the chloride ion diffusivity and water permeability of the high strength concrete containing silica fume. Water binder ratio, silica fume addition, degree of hydration and volume fraction of aggregates are considered as the major factors influencing concrete diffusivity and permeability in the procedure. Analytical results using the procedure are shown and verified with other data.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.10 no.6
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• pp.795-802
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• 1998

For measuring the thermal diffusivity by laser flash method, raw data have to be calibrated using temperature data. We have developed in-process calibration method and polynomial calibration in which thermal diffusivity can be calibrated during measuring, This method is different from existing temperature pre-process calibration method and exponential calibration having various source of error. Using this new calibration method, measurement accuracy was improved about 1∼2% compare to the value by the existing method. We also studied more accurate fitting curve as in Figure 4 was shown the result of measuring output characteristics of IR radiometer with temperature. As illustrated in data, in-process calibration method and polynomial calibration equation is proper than pre-process calibration method and exponential calibration.

• Proceedings of the Korea Concrete Institute Conference
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• 2000.10a
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• pp.425-428
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• 2000

When concrete structures are exposed to sulfate or marin environments, sulfate ions penetrated into concrete make it deteriorate. An accelerated test under potential difference method was performed to evaluate not only the sulfate ion diffusivity in ordinary portland cement and ground granulated blast-furnace slag cement concretes but the effect of slag replacement and water-cement ratio on the sulfate ions diffusivity. As the result of this study, we assumed the sulfate ion diffusivity was significantly related with total passed charge and initial current in concrete. Moreover sulfate ions penetration resistance of ordinary portland cement concrete was superior to that of ground granulated blast-furnace slag cement concrete.

• Proceedings of the Korea Concrete Institute Conference
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• 2003.11a
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• pp.541-544
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• 2003

Chloride diffusivity is one of the important properties of concrete affecting the durability of a structure. The diffusivity for porous materials is determined conventionally by immersion in a solution. However, this method is complicate and time-consuming, often requiring months or years to obtain results. Thus, the application of colorimetric method to estimate the apparent diffusivity of chloride ion was verified in this study. The result reveals that the apparent diffusivity of chloride ion can be predicted to use colorimetric method. Additionally the colorimetric method is capable to predict the profile of chloride ion.

• Transactions of the Korean Society of Mechanical Engineers
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• v.8 no.5
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• pp.462-468
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• 1984

The variation of thermal diffusivity with respect to rolling reduction is experimentally determined by flash technique. Copper alloy is used as an experimental and specimens are cut from the slab produced by rolling process with different percent reduction in thickness along the rolling direction and with 30.deg. 60.deg. and 90.deg. angle to the rolling direction. It is found that thermal diffusivity is slightly decreased as rolling reduction increased, and it has tendency that thermal diffusivity is increased slightly as its angle approached to 90.deg. at same rolling reduction.

• Transactions of the Korean Society of Mechanical Engineers
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• v.7 no.3
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• pp.319-327
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• 1983

The thermal diffusivity one of the series of standard pieces for spark test are determined by the flash method. The standard pieces are composed with carbon steels, structural carbon steels, alloy steels and high speed tool steels. In order to compute the thermal conductivity of the standard pieces, their specific heats are measured by a differential scanning calorimeter. The thermal conductivities are calculated from the data of specific heat, density and thermal diffusivity. To increase the accuracy of data for the thermal diffusivity by data reduction excursion method in the flash method, the governing heat diffusion equation, which is closely described experimental conditions with the finite pulse and the heat loss from the sample surfaces, is solved. In this analysis an integral transform is used.