• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2002.07a
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• pp.82-85
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• 2002

High-quality Sr-ferrite sintered magnets have been studied by using mill scale added SrCO$_3$ and oxidents before oxidation process. The pre-added SrCO$_3$ powders were improved the degree of oxidation and crush of mill scale and the magnetic properties of Sr-ferrite sintered magnets. The small added NaNO$_3$ oxidant was also highly improved the degree of oxidation and crush of mill scale and the magnetic properties of Sr-ferrite sintered magnets; 3805 G of remanent flux density, 3240 Oe of intrinsic coercivity, and 3.45 MGOe of maximum energy product.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.828-833
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• 2003

Rigid plate elastically supported at the edges is modeled and the performance of the optimal vibration control under sinusoidal excitation is tested. The controller based on the linear quadratic regulator with output feedback is designed to control the multi-degree of freedom vibration. Relative weighting parameters are considered as design constraints to determine the limitation of maximum control force and state parameters. Control force calculated by proportional output feedback of the displacement and velocity is used to suppress the vibration induced by the sinusoidal external force. The active vibration control of vibrating plate by the LQR controller is examined through the numerical simulations that show the effectiveness of optimal control scheme on the three degrees of freedom structure.

• Journal of The Korean Astronomical Society
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• v.40 no.4
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• pp.113-118
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• 2007

Recently far infra-red (FIR) polarization of the $850{\mu}m$ continuum emission from T Tauri disks has been detected. The observed degree of polarization is around 3 %. Since thermal emission from dust grains dominates the spectral energy distribution at the FIR regime, dust grains might be the cause of the polarization. We explore alignment of dust grains by radiative torque in T Tauri disks and provide predictions for polarized emission for disks viewed at different wavelengths and viewing angles. In the presence of magnetic field, these aligned grains produce polarized emission in infrared wavelengths. When we take a Mathis-Rumpl-Nordsieck-type distribution with maximum grain size of $500-1000{\mu}m$, the degree of polarization is around 2-3 % level at wavelengths larger than ${\sim}100{\mu}m$. Our study indicates that multifrequency infrared polarimetric studies of protostellar disks can provide good insights into the details of their magnetic structure.

• Journal of applied mathematics & informatics
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• v.22 no.1_2
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• pp.435-440
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• 2006

Let G be a simple graph with vertex set V(G) and edge set E(G). A subset S of E(G) is called an edge cover of G if the subgraph induced by S is a spanning subgraph of G. The maximum number of edge covers which form a partition of E(G) is called edge covering chromatic number of G, denoted by X'c(G). It is known that for any graph G with minimum degree ${\delta},\;{\delta}-1{\le}X'c(G){\le}{\delta}$. If $X'c(G) ={\delta}$, then G is called a graph of CI class, otherwise G is called a graph of CII class. It is easy to prove that the problem of deciding whether a given graph is of CI class or CII class is NP-complete. In this paper, we consider the classification of nearly bipartite graph and give some sufficient conditions for a nearly bipartite graph to be of CI class.

• Journal of applied mathematics & informatics
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• v.22 no.1_2
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• pp.307-316
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• 2006

Incomplete LU factorization preconditioning techniques often have difficulty on indefinite sparse matrices. We present hybrid reordering strategies to deal with such matrices, which include new diagonal reorderings that are in conjunction with a symmetric nondecreasing degree algorithm. We first use the diagonal reorderings to efficiently search for entries of single element rows and columns and/or the maximum absolute value to be placed on the diagonal for computing a nonsymmetric permutation. To augment the effectiveness of the diagonal reorderings, a nondecreasing degree algorithm is applied to reduce the amount of fill-in during the ILU factorization. With the reordered matrices, we achieve a noticeable improvement in enhancing the stability of incomplete LU factorizations. Consequently, we reduce the convergence cost of the preconditioned Krylov subspace methods on solving the reordered indefinite matrices.

• Journal of applied mathematics & informatics
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• v.32 no.3_4
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• pp.297-302
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• 2014

A well known result due to Ankeny and Rivlin [1] states that if $p(z)={\sum}^n_{j=0}a_jz^j$ is a polynomial of degree n satisfying $p(z){\neq}0$ for |z| < 1, then for $R{\geq}1$ $$\max_{{\mid}z{\mid}=R}{\mid}p(z){\mid}{\leq}{\frac{R^n+1}{2}}\max_{{\mid}z{\mid}=1}{\mid}p(z){\mid}$$. It was proposed by Professor R.P. Boas Jr. to obtain an inequality analogous to this inequality for polynomials having no zeros in |z| < k, k > 0. In this paper, we obtain some results in this direction, by considering polynomials of degree $n{\geq}2$, having all its zeros on the disk |z| = k, $k{\leq}1$.

• Journal of Korea Foundry Society
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• v.4 no.4
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• pp.14-19
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• 1984

In order to clarify the solidification mechanism of Al-Si alloy, Mushy Degree of Eutectic Solidification (MDE) and Centerline Feeding Resistance (CFR) were systematically studied by casting with various compositions of $Al-(6{\sim}18%)$ Si alloys into several kinds of molds having different cooling rates. The results are as follows: 1. CFR% increases slightly as solute concentration increases, but decreases remarkably as the cooling rate of the mold increases, that is, the composition dependence of the alloys has more effect on the change of CFR% than that of the mold cooling rate. 2. The composition dependence of MDE value has the same tendency as that of Degree of Eutectic Solidification (DES). MDE value within the range of hypereutectic composition is larger than that of hypoeutectic and it represents the maximum value at eutectic composition. The higher the cooling rate is, the less the MDE value is.

• Journal of Korea Technical Association of The Pulp and Paper Industry
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• v.32 no.1
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• pp.26-32
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• 2000

The impact of major surface chemical properties of silica sols on the retention and drainage performance of the silica based micrparticle system, Compozil was investigated using four different silica sols. And the effect of silica properties on the interactions with cationic starch and cationic plyacrylamide has also been identified. The surface charge density and the stability over pH of silica sols were increased by introducing aluminosilicate anions at surface. It was found that the charge density of silica sols determined the addition level necessary to attain the maximum retention and drainage. When silica sols were combined with cationic starch, the change density of the product was the critical properties and the degree of microagregation was of minor importance. In the cationic polyacrylamide system the degree of colloid structure appeared to be a more critcial property than the charge density of silica sols.

• Journal of Korea Technical Association of The Pulp and Paper Industry
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• v.33 no.4
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• pp.1-6
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• 2001

Today's paper industry tries to use greater amount of high yield pulp and recycled fiber and to close mill water system, which results in higher fines content and buildup of organic and inorganic contaminants in white water system. Researches are being focused to develop chemical additives that provide good retention and drainage in a closed papermaking system. A microparticle retention system consisted of cationic guar gum and anionic colloidal silica so has been developed to meet the requirements for improving machine speed and product quality. The objective of this investigation was to determine the effects of the degree of cationic guar gums, charge density and structure of anionic colloidal silica sols, and the degree of system closure on the performances of this microparticle retention system. Cationic guar gums and anionic colloidal silica sols with higher charge densities showed better retention performance. Particularly, wider maximum in retention was obtained when structure colloidal silica was used suggesting as mechanism of microparticle bridging is functioning in this system.

• Journal of the Korean Society for Railway
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• v.8 no.1
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• pp.57-62
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• 2005

The frost heaving pressure can be a problem for weakening of the railway roadbed material. This study was initiated to investigate the soils frost heaving pressure and physical characteristics(Liquid limit, permeability, SEM analysis) resulting from freezing and freezing-thawing cycle process. Therefore, upon freezing a saturated soil in a closed-system from the top, a considerable pressure was developed. Weathered granite soils, sandy soil were used in the laboratory freezing test which sometimes subjected to thermal gradients under closed-systems. The frost heaving pressure arising within the soil samples and the temperature of the samples inside were monitored with elapsed time. The degree of saturation versus heaving pressure curve is also presented for weathered granite soil and the maximum pressure is closely related to this curve. Based on the laboratory test results, fine-grained soils with strong attractive forces between soil grains md water molecules, and additional water is attracted into the pores leading to further volume changes and ice segregation.