• Journal of Magnetics
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• 제10권1호
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• pp.23-27
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• 2005

The crystallographic and magnetic properties of a series of substitutions in nickel ferrite where the Fe³⁺ is replaced with In³⁺ have been investigated using X-ray diffraction (XRD) and Mössbauer spectroscopy. Information on the exact crystalline structure, lattice parameters, bond lengths and bond angles were obtained by refining their XRD profiles by a Rietveld method. All the crystal structures were found to be cubic with the space group Fd/3m. The lattice constants increased with In³⁺ concentration. The expansion of the tetrahedron was outstanding, indicative of the tetrahedral (A) site preference of larger indium ion. The Mossbauer spectra showed two sets of sextuplet originating from ferric ions occupying the tetrahedral sites and the octahedral (B) sites under the Neel temperature T_N. Regardless of the composition x, the electric quadrupole splitting was zero within the experimental error. At x = 0.2, the magnetic hyperfine fields increased slightly, which meant that the nonmagnetic indium ions occupied preferentially the A-site. At the same time, the intensity of the B-site sub-spectra decreased markedly at the elevated temperature, indicating that the occupation of the A site by indium induced a considerable perturbation on the B site.

• Bulletin of the Korean Chemical Society
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• 제21권7호
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• pp.679-684
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• 2000

The Sr2Co0.5Nb(Ta)0.5O4 and Sr3CoNb(Ta)O7 compounds, both with Ruddlesden-Popper structures, have been synthesized by the ceramic method at $1150^{\circ}C$ under atmospheric pressure. The crystallographic structure of the compounds was assigned to the tetr agonal system with space group 14/mmm by X-ray diffraction(XRD) Rietveld refinement. The reduced lattice volume and lattice parameters increased as the Ta with 5d substitutes for the Nb with 4d in the compounds. The Co/Nb(Ta)O bond length has been determined by X-ray absorption spectroscopic(EXAFS/XANES) analysis and the XRD refinement. The CoO6,octahedra were tetragonally distorted by elongation of Co-O bond along the c-axis. The magnetic measurement shows the compounds Sr2Co0.5Nb(Ta)0.5O4 and Sr3CoNb(Ta)O7 have paramagnetic properties and the Co ions with intermediate spin sates between high and low spins in D4h symmetry. All the compounds showed semiconducting behavior whose electrical conductivity increased with temperature up to 1000 K. The electrical conductiviy increased and the activation energy for the conduction decreased as the number of perovskite layers increased in the compounds with chemical formula An+1BnO3n+1.

• 호남수학학술지
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• 제27권4호
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• pp.649-654
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• 2005

Given operators X and Y acting on a separable complex Hilbert space ${\mathcal{H}}$, an interpolating operator is a bounded operator A such that AX = Y. We show the following: Let $Alg{\mathcal{L}}$ be a subspace lattice acting on a separable complex Hilbert space ${\mathcal{H}}$ and let $X=(x_{ij})$ and $Y=(y_{ij})$ be operators acting on ${\mathcal{H}}$. Then the following are equivalent: (1) There exists a unitary operator $A=(a_{ij})$ in $Alg{\mathcal{L}}$ such that AX = Y. (2) There is a bounded sequence {${\alpha}_n$} in ${\mathbb{C}}$ such that ${\mid}{\alpha}_j{\mid}=1$ and $y_{ij}={\alpha}_jx_{ij}$ for $j{\in}{\mathbb{N}}$.

• 호남수학학술지
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• 제36권1호
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• pp.29-32
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• 2014

Given operators X and Y acting on a separable Hilbert space $\mathcal{H}$, an interpolating operator is a bounded operator A such that AX = Y. In this article, we investigate self-adjoint interpolation problems for operators in a tridiagonal algebra : Let $\mathcal{L}$ be a subspace lattice acting on a separable complex Hilbert space $\mathcal{H}$ and let X = ($x_{ij}$) and Y = ($y_{ij}$) be operators acting on $\mathcal{H}$. Then the following are equivalent: (1) There exists a self-adjoint operator A = ($a_{ij}$) in $Alg{\mathcal{L}}$ such that AX = Y. (2) There is a bounded real sequence {${\alpha}_n$} such that $y_{ij}={\alpha}_ix_{ij}$ for $i,j{\in}\mathbb{N}$.

• 한국전산응용수학회:학술대회논문집
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• 한국전산응용수학회 2003년도 KSCAM 학술발표회 프로그램 및 초록집
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• pp.4.1-4
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• 2003

Given operators X and Y acting on a Hilbert space H, an interpolating operator is a bounded operator A such that AX= Y. An interpolating operator for n-operators satisfies the equation AXi= Yi, for i = 1,2,...,n, In this article, we showed the following : Let H be a Hilbert space and let L be a subspace lattice on H. Let X and Y be operators acting on H. Assume that rangeX is dense in H. Then the following statements are equivalent : (1) There exists an operator A in AlgL such that AX = Y, A$\^$*/=A and every E in L reduces A. (2) sup｛(equation omitted) : n $\in$ N f$\sub$I/ $\in$ H and E$\sub$I/ $\in$ L｝<$\infty$ and = for all E in L and all f, g in H.

• International Journal of Aeronautical and Space Sciences
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• 제12권1호
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• pp.63-68
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• 2011

Aerodynamic characteristics of a wing during fold motion were investigated in order to understand how variations or changes in such characteristics increase aircraft performance. Numerical simulations were conducted, and the results were obtained using the unsteady vortex lattice method to estimate the lift, drag and the moment coefficient in subsonic flow during fold motion. Parameters such as the fold angle and the fold angular velocity were summarized in detail. Generally, the lift and pitching moment coefficients decreased as the angle increased. In contrast, the coefficients increased as the angular velocity increased.

• 한국에너지공학회:학술대회논문집
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• 한국태양에너지학회, 한국에너지공학회 1993년도 춘계 공동학술발표회 초록집
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• pp.53-59
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• 1993

The interests on GaAs solar cell grown on Ge substrates as an alternative of GaAs substrate arises from its very close lattice parameters, very small difference in thermal expansion coefficients, and much higher fracture toughness between GaAs and Ge. In addition, for many space power application, it would be a most attractive solar cell with high radiation resistance of GaAs and high reliability for the reverse current damage of Ge, and expecting the theoretical efficiency limit of the tandem GaAs/Ge solar cell is 34% under 1 Sun, AM 0, and 28$^{\circ}C$ condition. In this report, we have reviewed the performance and the manufacturing technics of GaAs/Ge solar cell, and current status of research in GaAs/Ge solar cell.

• Journal of applied mathematics & informatics
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• 제19권1_2호
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• pp.447-452
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• 2005

Given operators X and Y on a Hilbert space H, an interpolating operator is a bounded operator A such that AX = Y. In this article, we investigate compact interpolation problems for vectors in a tridiagonal algebra. Let L be a subspace lattice acting on a separable complex Hilbert space H and Alg L be a tridiagonal algebra. Let X = $(x_{ij})\;and\;Y\;=\;(y_{ij})$ be operators acting on H. Then the following are equivalent: (1) There exists a compact operator A = $(x_{ij})$ in AlgL such that AX = Y. (2) There is a sequence {$\alpha_n$} in $\mathbb{C}$ such that {$\alpha_n$} converges to zero and $$y_1\;_j=\alpha_1x_1\;_j+\alpha_2x_2\;_j\;y_{2k}\;_j=\alpha_{4k-1}x_{2k\;j}\;y_{2k+1\;j}=\alpha_{4k}x_{2k\;j}+\alpha_{4k+1}x_{2k+1\;j}+\alpha_{4k+2}x_{2k+2\;j\;for\;all\;k\;\epsilon\;\mathbb{N}$$.

• 한국시뮬레이션학회논문지
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• 제22권3호
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• pp.1-6
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• 2013

생태계는 다양한 환경 내에 다양한 생물종이 서로 상호작용하고 있는 복잡계이다. 이들 상호작용은 계층적 먹이그물 구조를 이루고 있는데, 많은 경우, 포식자-피식자-식물의 관계를 보여준다. 포식자-피식자 경쟁관계는 시공간적으로 일어나는 현상이기 때문에, 초기시점에서의 개체들 분포와 밀도가 어떠한가는 매우 중요한 정보를 담고 있다. 본 연구에서는, 이들 세 단계 계층구조의 생태계를 간단한 격자 모델로 구성하고 이 모델을 사용하여 각 종의 초기 개체군 밀도가 변함에 따라 생태계 안정성이 어떻게 변하는지를 연구하였다. 격자공간은 $L{\times}L$ 크기의 L(=100) 사각격자로 구성되었다. 식물의 초기 밀도는 0.2로 고정하였다. 시뮬레이션 결과는, 포식자의 밀도가 0.4이하, 피식자의 밀도가 0.5이하일 때 두 종이 공존하는 것을 보여 주었으며, 포식자 밀도가 0.5이상, 피식자 밀도가 0.6 이상의 조건에서는 두 종이 멸종하는 것을 보여 주었다. 공존과 멸종의 두 상태가 접하는 영역의 조건에서는 확률적으로 공존하기도하고 멸종하기도 하는 비선형성이 강한 행동을 보여 주었다. 본 연구를 통해 초기종의 밀도가 생태계 안정성에 매우 중요한 역할을 한다는 것을 알 수 있었다.

• 한국항공우주학회지
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• 제50권9호
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• pp.599-608
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• 2022

로터 시스템을 사용하는 도심 항공 모빌리티(Urban Air Mobility, UAM)는 이착륙 시 버티포트(Vertiport)에서 지면 효과를 경험하게 된다. UAM의 안전한 운용을 위해서는 지면 효과가 비행체의 공력성능에 미치는 영향성이 선행적으로 분석되어야 한다. 본 연구에서는 Lattice Boltzmann Method (LBM)를 적용하여지면 효과가 동축 반전 프로펠러를 장착한 쿼드콥터 형상 전기동력 수직이착륙기 전기체의 공력성능 및 후류 구조에 미치는 영향을 분석하였다. 동축 반전 프로펠러 시스템의 상하부 프로펠러에서 지면 효과 영향성은 상이하게 관찰되었다. 지면과의 이격 거리가 변화하더라도 상부 프로펠러의 성능에는 큰 변화가 없었지만, 지면과 가까워질수록 하부 프로펠러에서는 평균 추력과 토크 값이 크게 증가하였다. 또한 이격 거리가 감소함에 따라 추력 변동의 진폭이 증가하는 경향성이 나타났다. 지면 효과에 의해 프로펠러 후류는 하류 방향으로 충분히 전파되지 못하고 지면을 따라 발달한 Outwash 흐름에 의해 확산되었다. 프로펠러 시스템 사이에서 지면 확산 유동이 충돌하는 분수 와류(Fountain Vortex)의 형성을 확인하였다.