• Proceedings of the Polymer Society of Korea Conference
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• 2006.10a
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• pp.210-211
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• 2006

A series of L-LA polymerizations initiated by $Sn(Oct)_{2}\;([LA]_{0}/[Sn]_{0}=200)$ were carried out in scR22 at $130^{\circ}C$ and 300 bar, where $[LA]_{0}$ is the initial L-lactide concentration and $[Sn]_{0}$ is the initial $Sn(Oct)_{2}$ concentration. The reaction time dependences of monomer conversion and PLLA MW improved. The monomer conversion and PLLA MW increased with increasing reaction time. The effect of temperature on monomer conversion and PLLA MW was investigated in a series of polymerizations conducted at temperatures ranging from 90 to $150^{\circ}C$ and at a constant pressure of 200 bar. In all of these experiments, the ratio of monomer to R22 concentration was held constant at 12.4 wt.-%. Increasing the reaction temperature from 90 to $130^{\circ}C$ resulted in increased monomer conversion from 11.5 to 72.2 %.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.22 no.6
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• pp.484-488
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• 2009

In this study, in order to develop the lead-free piezoelectric ceramics with high piezoelectric and dielectric properties, $[(K_{0.5}Na_{0.5})_{0.95}Li_{0.05}(Nb_{0.96}Sb_{0.04})]O_3$ ceramics were fabricated using $Ag_2O$ as sintering aid and a conventional mixed oxide process and their piezoelectric and dielectric characteristics were investigated according to the $SrTiO_3$ substitution. $SrTiO_3$ substitution enhanced density, dielectric constant(${\varepsilon}_r$) and electromechanical coupling factor($k_p$). However, mechanical quality factor was deteriorated. And also, Curie temperature ($T_c$), and phase transition temperature($T_p$) were rapidly decreased. At the 0.5 mol% $SrTiO_3$ substitution, density, electromechanical coupling factor($k_p$), dielectric constant(${\varepsilon}_r$) and piezoelectric constant($d_{33}$) of specimen showed the optimum value of $4.437\;g/cm^3$, 0.457, 1294, 265 pC/N, respectively.

• Communications of the Korean Mathematical Society
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• v.35 no.4
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• pp.1221-1244
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• 2020

In this paper we consider L_K-conjecture introduced in [5, 6] for hypersurface Mⁿ in space form Rⁿ⁺¹(c) with three principal curvatures. When c = 0, -1, we show that every L₁-biharmonic hypersurface with three principal curvatures and H₁ is constant, has H₂ = 0 and at least one of the multiplicities of principal curvatures is one, where H₁ and H₂ are first and second mean curvature of M and we show that there is not L₂-biharmonic hypersurface with three disjoint principal curvatures and, H₁ and H₂ is constant. For c = 1, by considering having three principal curvatures, we classify L₁-biharmonic hypersurfaces with multiplicities greater than one, H₁ is constant and H₂ = 0, proper L₁-biharmonic hypersurfaces which H₁ is constant, and L₂-biharmonic hypersurfaces which H₁ and H₂ is constant.

• Electrical & Electronic Materials
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• v.8 no.5
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• pp.594-600
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• 1995

Dielectric properties at microwave frequencies of ($Zr_{0.65}$, $Sn_{0.35}$) $Ti_{1.04}$ $O_{4.04}$ ceramics with additives, NiO as an agent to improve dielectric properties and $B_{2}$ $O_{3}$ as a firing agent were investigated. When 0.5 - 1.5 wt% of NiO is add, the grain growth is inhibited and the shape of the grain is uniformed, Dielectric constant(Fr) and bulk density are increased with raising amount of NiO at sintering temperature of 1330 - 1360.deg. C, but the temperature coefficient of resonant frquency(.epsilon.$_{r}$) decreased gradually as the NiO content increased. The value of Qx $f_{o}$ was increased as the amount of NiO was increased in the range of 0.5 to 1.0 wt% and the Qx $f_{o}$, was decreased slightly with raising sintering temperature. With NiO of 1.0 wt% and at sintering temperature of 1360.deg. C, this ceramics was found to have excellent microwave properties of .epsilon.$_{r}$=37.8, Qx $f_{o}$ = 48.600 and .tau.$_{f}$ = 7 ppm/.deg. C.C.. C.. C.C.. C.. C.

• Bulletin of the Korean Chemical Society
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• v.17 no.4
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• pp.347-348
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• 1996

A set of substituent constants, σc+, has been widely used comparing the stability of carbocation in super acid condition. Plotting of 13C chemical shifts of cationic carbon (Δδc+) against σc+ constants yields an excellent correlation with a good correlation coefficient (r=0.999) for a cyclopentyl cation system. But these σc+ constants show poor linearity in norbornenyl cations that well known to show homoallylic stabilization in a cationic condition. We calculated the new substituent constant, σc+π, and found that the new substituent constants give an excellent correlation in π- and πσ-participating cation systems.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.10
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• pp.2122-2129
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• 2002

Creep damage using a reference stress(RS) was analyzed for type 316LN stainless steel. The generalized K-R equation was reconstructed into the RS equation using a critical stress value $\sigma$. The RS equation was derived from the critical stress in failure time $t_f$ instead of material damage parameter $\omega$, which indicates the critical condition of collapse or approach to gross instability of materials during creep. For obtaining the reference stress, a series of creep tests and tensile tests were conducted with at 55$0^{\circ}C$ and $600^{\circ}C$. The stress-time data obtained from creep tests were applied to the RS equations to characterize the creep damage of type 316LN stainless steel. The value of creep constant r with stress levels was about 18 at 55$0^{\circ}C$ and 21 at $600^{\circ}C$. This value was almost similar with r = 24 in the K-R equation, which was obtained by using damage parameter $\omega$. Relationship plots of creep failure strain and life fraction $(t_f /t_r)$ were also obtained with different λ values. The RS equation was therefore more convenient than the generalized K-R equation, because the measuring process to quantify the damage parameter $\omega$ such as voids or micro cracks in crept materials was omitted. The RS method can be easily used by designers and plant operator as a creep design tool.

• Bulletin of the Korean Mathematical Society
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• v.40 no.4
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• pp.583-591
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• 2003

For a weighted composition operator $uC_{\varphi}$ on C(X), we determine its essential norm and the constant for its Hyers-Ulam stability, in terms of the set $\varphi(\{x\;\in\;X\;:\;$\mid$u(x)$\mid$\;\geq\;r\})$ (r > 0).

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.9 no.2
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• pp.198-209
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• 1997

This paper is concerned about the performance of alternative refrigerants of R-502 which has been widely used in transport and low temperature refrigeration. A breadboard heat pump was constructed with counterflow heat exchangers and tested with R-502 and transitional alternatives of R-402A and R-402B and long-term alternatives of R-404A and R-507 to compare the performance of each refrigerant. Measurements were conducted at two different condensing temperatures of $43.3^{\circ}C$ and $52.0^{\circ}C$ and the evaporating temperature was varied over a range from $-25^{\circ}C$ to $-5^{\circ}C$. The evaporator superheat and condenser subcooling were maintained constant at about $5^{\circ}C$ for all tests. Test results showed that all alternative fluids tested in this research work can be used as 'drop-in' fluids to replace R-502 without any major problem. It is also found that in the long run HFC alternatives are to be used due to their favorable environmental characteristics and better performance.

• Journal of the Korean Ceramic Society
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• v.40 no.6
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• pp.539-546
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• 2003

The GaN/sapphire layered structure is a potential candidate for high frequency devices due to high acoustic velocity of sapphire. Generally, the GaN thin films are epitaxially grown on c, a, and r-plane sapphire substrates. In this study, wave equations of GaN/sapphire structure were calculated according to crystallographic relationship between GaN layer and sapphire substrate. On each plane, the shear velocity was changed by the kH of GaN layer and propagation direction on sapphire substrate. We found electromechanical coupling constant of r-plane was better than the others. As a result, elastic stiffness and electromechanical coupling constant of materials are affected by a cut and an orientation of substrate. GaN/r-plane sapphire structure is more advantageous for high frequency SAW devices.

• East Asian mathematical journal
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• v.34 no.5
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• pp.643-649
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• 2018

Let M be a connected n-dimensional submanifold of a Euclidean space $E^{n+k}$ equipped with the induced metric and ${\Delta}$ its Laplacian. If the position vector x of M is decomposed as a sum of three vectors $x=x_1+x_2+x_0$ where two vectors $x_1$ and $x_2$ are non-constant eigenvectors of the Laplacian, i.e., ${\Delta}x_i={\lambda}_ix_i$, i = 1, 2 (${\lambda}_i{\in}R$) and $x_0$ is a constant vector, then, M is called a 2-type submanifold. In this paper we proved that a connected 2-type hypersurface M in $E^{n+1}$ whose postion vector x satisfies ${\langle}{\Delta}x,x-x_0{\rangle}=c$ for a constant c, where ${\langle}$, ${\rangle}$ is the usual inner product in $E^{n+1}$, is of null 2-type and has constant mean curvature and scalar curvature.