Relationship among customer equity, customer lifetime value and attitude toward luxury brand

쉬빙 Changwon National Univ. 2009 국내석사

Railway security detection system by image processing technology

바이즈궈 Changwon National Univ. 2012 국내석사

(A) study on the antimicrobial metabolites isolated from Streptomyces strains

최혜정 Changwon National Univ. 2012 국내박사

Algorithmic approach to the retrial queueing systems

추택식 Changwon National Univ. 2007 국내박사

재시도 대기체계는 다음과 같이 묘사된다. 외부에서 고객이 도착했을 때 서비스시설(service facility)에 idle server가 있으면 server로부터 즉시 서비스를 받지만, 만약 모든 server가 서비스중이면 고객은 다음과 같은 행동 양식을 보인다. 즉, 서비스시설에 있는 대기실에 여유 공간이 있다면 기다렸다가 서비스를 받든지, 대기실의 여유 공간이 없으면 시스템을 영원히 떠나든지 혹은 서비스를 받을 목적으로 재시도를 하는 경우이다. 이 때 재시도를 하는 경우의 고객은 orbit라고 불리는 가상의 공간에 들어간다고 가정한다. orbit에 있는 고객이 랜덤시간 후 재시도를 할 때는 외부에서 시스템에 도착한 처음 고객과 같은 형태의 행동양식을 보인다. 재시도 대기체계 이론은 특히 전화교환기시스템, 컴퓨터통신, 통신망분석 등에서 많이 응용되고 있다. 본 논문에서는 콜 센터(call center)와 관련되어 나타나는 현상을 모형화하고 보킹(balking)과 포기(abandonment)를 갖는 재시도 대기체계를 이용하여 고객의 수의 분포를 구하는 효과적인 알고리즘을 제시하였다. 또한 마코프 도착과정(MAP)을 따라서 도착하고 서비스시간이 지수분포이며 유한의 대기실과 다수의 서버를 갖는 대기체계모형에 대한 분석을 하였다. 본 논문의 1장에서는 대기체계 및 재시도 대기체계에 관한 일반적인 소개를 하였고, 2장에서는 본 논문 예비결과들을 기술하였다. 3장에서는 콜 센터(call center)와 관련된 모형으로서 보킹(balking)과 포기(abandonment)를 갖는 재시도 대기체계를 고려한다. 이 모형을 분석하기 위하여 2개의 node를 갖는 네트워크와 보킹(balking)과 포기(abandonment)를 갖는 재시도 대기체계의 간단한 모형으로 분리한다. 여기서 사용된 알고리즘은 Neuts와 Rao(1990)에 의하여 제안된 Generalized Truncation Method (GTM)를 기반으로 Bright와 Taylor(1995)의 boundary 벡터 계산법을 이용하여 고객의 수의 분포를 구한다. 한편 가우스 소거법을 이용하여 boundary 벡터들을 구할 경우 계산의 복잡성이 인데 비하여 여기서 제시한 알고리즘을 사용하면 로 효율적인 계산을 할 수 있다. 4장에서는 마코프 도착과정(MAP)을 따라서 고객이 도착하고 서비스 시간이 지수분포이며 유한 서버 및 유한 서비스 시설 용량 를 가지는 재시도 대기체계를 고려한다. 본 장에서 다루는 알고리즘의 주요한 내용은 다음과 같다. 즉, 먼저 transient generator에 있는 한 component의 특수한 구조를 바탕으로 rate matrix들을 간단한 형태로 표현한다. 다음으로 행렬을 partition하고 역행렬을 구한다. 이 때 나타나는 블록삼대각형태의 행렬에 대한 역행렬 계산을 위하여 Shin(2006)에 의하여 제안된 알고리즘을 적용한다.

Thermal and mechanical properties of thermal barrier coating with thermal fatigue system

송광수 Changwon National Univ. 2012 국내석사

Thermal barrier coatings (TBCs) have been applied to superalloys to increase their resistance for the high temperature oxidation or corrosion in industrial applications. The thermal durability and facture behavior of TBCs are closely related with its microstructure. Therefore, in this study, the relationship between microstructure evolution and mechanical properties in the bond and top coats has been investigated through different thermal fatigue systems, electric thermal fatigue (ETF) and flame thermal fatigue (FTF), including the thermal stability at the interface of the bond and top coats. The TBC system with the thicknesses of 300 and 300 ??m in the top and bond coats, respectively, were prepared with METECO 204 NS and AMDRY 962, respectively, with an air plasma spray (APS) system using 9MB gun. To observe the oxidation resistance and thermal stability of TBC, the thermal exposure tests were performed with both thermal fatigue tests at a surface temperature of 850 ?XC with a temperature difference of 200 ?XC between the surface and bottom of sample, with a dwell time of 1 h for 1144 cycles (24000 EOH; Equivalent Operating Hour) in a specially designed apparatus?Xone side of the sample is exposed and the other side air cooled. With increasing the thermal exposure time, the hardness values are slightly increased due to the densification, and then decreased by the formation and coalescence of cracks in the both thermal fatigue tests. However, the toughness values show a reverse tendency. The mechanical properties represent a certain relationship with the thermal fatigue species, indicating that the TBC in the ETF show a more severe degradation of mechanical properties at same EOH than that in the FTF. The microstructure of TBC in the ETF is more dramatically changed than that in the FTF, showing a relatively thicker TGO layer in the ETF. The influences of thermal fatigue condition on the microstructural evolution and interfacial stability of TBC is discussed. Keywords: Thermal barrier coating; Cyclic thermal fatigue; Thermal fatigue test; Thermal fatigue system; Hardness

Microstructures and deformation behavior of Mg-Sn based alloys

청웨이리 Changwon National Univ. 2011 국내박사

It is generally regarded that the main obstacle that must be overcome in Mg extrusions is their low extrusion speed, which leads to high production costs. Normally, high-strength Mg alloys such as AZ80 and ZK60 alloys are extrudable only at speeds of 0.5~2.5 m/min, well below those attainable with Al alloys. This is mainly due to an increase in the susceptibility to hot shortness during extrusion with increases in alloying elements such as Al and Zn caused by the incipient melting of second-phase particles. From this respect, it is believed that Mg-Sn based alloys have strong potential for use in high-speed extrusion processes, as they usually have higher thermal stability than conventional AZ and ZK series Mg alloys. In addition, their susceptibility to hot shortness can be decreased by applying an indirect extrusion process, since it can alleviate frictional heat during extrusion better than a conventional direct extrusion process. The hot deformation behaviors of homogenized Mg-8Sn-1Al-1Zn (wt.%) and AZ31 alloys were investigated in the temperature range of 523~723 K and at the strain rate range of 0.1~10 s-1 with a view to evaluate the optimum processing parameters for extrusion through processing map. The activation energy Q and flow stress of Mg-8Sn-1Al-1Zn alloy are lower compared to that of AZ31 in the studied conditions. The processing-map results indicated that homogenized TAZ811 can be deformed at lower temperature and similar strain rate ranges. Indirect extrusion of the binary Mg-Sn alloys with 6-10% Sn has been performed at ram speeds of 1.3 and at a temperature of 523K. Tensile and compressive strengths increased and yield asymmetry reduced with Sn addition. These changes are attributed to grain refinement and fine Mg2Sn precipitates. However, Mg-10%Sn alloy showed relatively poor tensile ductility due to the presence of coarse particles. A novel high strength wrought Mg-8Sn-1Al-1Zn developed for high-speed extrusion was successfully extruded at speeds in a range of 2-10 m/min and temperature of 523 K. The effects of extrusion speed on the microstructures and tensile properties of the extruded alloys were investigated. Grain size, recrystallization fraction and texture were found to be greatly affected by the extrusion speed, resulting in tensile properties showing lower strength and ductility as the extrusion speed increased. The strength and ductility of the extruded alloys are also discussed in terms of the formation of double twins during the tensile test. The extruded Mg-8Sn and Mg-8Sn-1Al-1Zn alloys present superior tensile properties compared to the commercial AZ31 alloy extruded at same condition due to the fine-grain strengthening, particle strengthening and solid solution strengthening as well as texture strengthening. The contribution when the alloy was strengthened fully with four strengthening mechanism was calculated. Compression tests of the extruded Mg-8Sn-1Al-1Zn alloys were conducted in the temperature range of 298~523 K and strain rate range of 0.001~10 s−1 with a view to evaluate the optimum processing parameters for post deformation by processing map. The different efficiency domains and flow instability region corresponding to various microstructural characteristics have been identified as follows: 423K/0.001-0.01s-1 and 473~523K/0.001-3.3s-1. The compression behaviors of the extruded Mg-8Sn-1Al-1Zn alloy were also analyzed by sin hyperbolic constitutive equations. The activation energy and stress exponent are 130.3kJ/mol and 6.73, respectively. Microstructure analyses indicate that LTDRX, TDRX and CDRX is the main deformation mechanisms of extrude TAZ811 alloy at 298-523 K. The newly formed DRX grains are perpendicular with the compression direction, namely original extrusion direction.

Preparation and characterization of super hard ceramic layers on Al alloys by electrolytic plasma processing

왕카이 Changwon National Univ. 2011 국내박사

(A)characterization of regular graphs embedded in a surface

김상범 Changwon National Univ. 2000 국내박사

Transition metal (TM)-doped ZnO nanostructures

Ahmed, Faheem Changwon National Univ. 2013 국내박사

The interest in ZnO nanostructures has increased drastically in recent years due to its unique properties and versatile applications in transparent electronics, ultraviolet (UV) light emitters, piezoelectric devices, chemical sensors, biomedical and spintronics. Based on these remarkable properties, it is very important to realize the controllable growth of ZnO nanostructures and investigate their properties. This thesis presented controllable growth of pure and transition metal (TM)-doped ZnO nanostructures by microwave irradiation method and investigated their structural, morphological, optical, and magnetic properties. The optimization of pure ZnO growth was performed by changing the solution molar ratio (zinc acetate: KOH) and reaction time. It was found that the shape of nanostructures controlled from sheetlike to rodlike by adjusting the molar ratio of zinc acetate and KOH in the solution and a molar ratio of 1:20 was found to be appropriate. However, the aspect ratio and shape was controlled by changing the reaction time, in which the nanorods prepared for 20 min have higher aspect ratio and good crystallinity. Furthermore, TM-doped ZnO nanostructures, Zn1-xTMxO (TM=Cr, Mn, Fe, Co, Ni and Cu) were prepared and studied in order to analyze the effect of concentration and atomic number of TM dopant on the magnetic and material properties of ZnO. X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and selected area diffraction pattern (SAED) results showed that the nanostructures have homogeneous distribution, well crystalline, and single phase for a limited concentration of the dilution of TM ions in ZnO lattice. In particular, we have obtained single phases of Zn1-xTMxO nanostructures for up to 5at.% of Cr, Fe, Ni, Cu, and the secondary phases of ZnCr2O4, ZnFe2O4 , NiO and CuO, respectively were detected in higher concentration of Cr, Fe, Ni and Cu-doped ZnO, respectively. While, Mn and Co-doped ZnO samples showed single phase nature upto 10% TM doping, hence, higher solubility limit was obtained. Additionally the change in lattice volume was in good agreement with the difference in ionic radii of Zn and TM ions. Raman studies revealed that the confinement of optical phonons in the disorder lattice was explained using a spatial correlation (SC) model. The detailed analysis of the optical phonon behavior in disorder lattice confirmed the substitution of the TM ions in Zn2+ site of the ZnO host lattice. The optical properties showed the reduction of bandgap for Cr and Mn-doped ZnO with the increase of dopant concentration, while for Fe and Cu:ZnO, the bandgap was first decreased upto 5% and then increased upon further doping. However, the opposite behavior was observed for Ni:ZnO. For Co:ZnO, bandgap was continuously increased with the concentration of TM dopants. Hysteresis curves were obtained at RT for all the Zn1-xTMxO nanostructures which revealed the room temperature ferromagnetism (RTFM) and indicated that there is a high tendency for TMs to cluster in ZnO at higher doping concentration (>5% for Cr, Fe, Ni and Cu) which ruined the RTFM. Furthermore, a fix concentration (3%) of TM elements was incorporated into ZnO nanostructures to obtain a better understanding of the impact of atomic number on the properties of the ZnO. It was determined that by increasing the atomic number from Cr to Fe, lattice volume, bandgap, aspect ratio and saturation magnetization initially increased and then decreased upon further increasing the atomic number. Thus, a correlation between these properties exists. Microwave-hydrothermal growth studies have shown that TM-doped ZnO resulted in an enhancement of all the functional properties. XRD, SAED and HRTEM analyses revealed that all the TM-doped ZnO nanostructures have wurtzite structure and no secondary phase was detected. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) results confirmed a higher aspect ratio of nanostructures than that of simple microwave. Raman spectra reveled that no defect related mode was observed which indicated that the nanostructures have high quality and negligible defects as compared with the simple microwave. The value of bandgap was found to be close to the standard value of ZnO, which indicated that the TM ions were substituted in ZnO more uniformly as compared with the simple microwave. RTFM was observed in all the TM-doped ZnO nanostructures and the value of Ms and Mr were higher while Hc was lower than that of the TM-doped ZnO prepared by simple microwave. This work provides the effective way to fabricate various pure and TM-doped ZnO nanostructures, some beneficial results in aspects of their structural, morphological, optical and magnetic properties, which builds theoretical and experimental foundation for much better understanding fundamental physics and broader applications of low-dimensional ZnO and related structures. Hence, these nanostructures pave the way for development of multifunctional spintronics and optoelectronic devices that integrate structural, morphological, optical, and magnetic properties.

(A) study on novel simulation methods for grid-connected fixed speed wind power generation system by using EMTDC and RTDS

박대진 Changwon National Univ. 2008 국내석사

This thesis focused on the grid connected fixed Wind Power Generation System(WPGS) modeling and analysis at transient state using PSCAD/EMTDC(Power System Computer Aided Design/ Electro Magnetic Transients DC analysis program) and RTDS(Real Time Digital Simulator). In this work, a 3[MW] WPGS connected to power grid is modeled. For the purpose of accurate simulation, real wind speed data gathered by anemometer are used for both RTDS and EMTDC simulations. To apply the real wind velocity, Mod-2 wind turbine is modeled in RTDS and EMTDC. The pitch control systems which are divided into power and speed constant mode are applied for minimization of generator output power fluctuation and stabilization of WPGS system. To verify pitch system operation, several simulation cases are implemented under the various wind speed. EMTDC and RTDS simulation results are also compared in detail. When fault occurs in the grid, it affects to wind generator. As the results, generator speed increases and its terminal voltage and output power are collapsed. Power constant mode in pitch control does not operate due to generator output power collapse. Therefore, the specific stabilization methods are required for wind power generator stability. The author is sure that the speed constant mode in pitch control can stabilize WPGS under transient state. The speed constant mode in pitch control is the most cost effective method among several kinds of stabilization methods. Finally, simulation is carried out to compare its results with several cases for verifying the accurate operation of power and speed constant mode in pitch control. The results demonstrate the effectiveness of the proposed RTDS and EMTDC based simulation methods of WPGS. 화석연료의 고갈과 환경오염 문제, 에너지의 자급자족의 가능성이라는 측면에서 대체에너지에 대한 연구가 활발히 진행되어지고 있다. 이러한 대체에너지원 가운데 풍력발전 시스템은 환경 친화적이고 무한한 에너지원을 보유하고 있으며, 타 대체에너지원에 비해 발전단가가 저렴하고 실용성이 높아 가장 주목받고 있다. 2006년 풍력 발전 협회 보고서에 따르면, 우리나라의 풍력발전 보유 현황은 173MW로 세계 최대 보유국인 독일과 스페인의 약 1%밖에 되지 않는다는 것을 알 수 있지만 최근 풍력발전 시스템의 민간 및 국가적인 보급 사업으로 인하여 점차 보유 용량이 증가하고 있다. 풍력 발전 시스템은 실제 풍력 발전시스템의 고가의 가격 때문에 시스템을 구축하기 전 다양한 기상조건에서 일어날 수 있는 과도현상들을 시뮬레이션을 통한 분석이 필요하게 되었다. 따라서 실제 풍력발전 시스템의 구축 전 과도현상의 시뮬레이션을 할 수 있는 풍력발전 시뮬레이터의 필요성이 요구되고 있다. RTDS는 실제 기상조건을 이용하여 실시간으로 시스템의 과도 상태를 분석할 수 있다는 점에서 가장 정확하고 실제 시스템과 유사한 시뮬레이터라고 할 수 있다. 그리고 과도현상 시뮬레이터인 EMTDC를 이용하여 RTDS의 결과와 비교하였다. 시뮬레이션은 창원대학교 55호관 옥상에 설치되어진 풍속계를 통해 측정되어진 풍속 Data가 실시간으로 RTDS의 아날로그 입력부로 전송되게 된다. 그리고 RTDS는 실시간으로 측정되어진 풍속 Data를 연동 프로그램인 RSCAD의 풍력 터빈의 입력부로 입력하게 되어, 실제 기상 조건을 이용한 실시간 시뮬레이션이 가능하게 된다. 즉 실제 풍속 Data를 이용하여 실시간으로 시뮬레이션이 가능하다는점에서 가장 정확하고 유용한 시뮬레이터로 고려되어진다. 풍력 발전 시스템의 Circuit은 RSCAD와 EMTDC에서 각각 구현되어졌다. 풍력 터빈은 IEEE규정 모델인 Mod-2 wind turbine을 모델링하였고 풍력 터빈에 발전기 출력의 Fluctuation을 최소화하기 위하여 Power constant mode를 적용하였다. 그 이유는 농형 유도발전기(Squirrel cage induction generator)를 사용한 풍력발전시스템의 경우는 계통과 직접 연결되기 때문에 발전기의 출력이 항상 일정해야 한다. Pitch control part에서 실제 Mod-2 Turbine과 유사한 특성을 만들기 위하여 초당 pitch angle의 최대 변화각을 -10˚~10˚로 설정하고 최소 pitch각을 0˚, 최대 pitch각을 90˚로 설정하였다. RSCAD와 EMTDC에 구현되어진 농형 유도 발전기는 항상 고정속으로 회전을 하고, Converter가 필요 없이 계통에 직접 연결되어지기 때문에 구조가 가장 간단한 풍력 발전시스템이지만 발전기가 회전을 할 때 계통의 무효전력을 소비하기 때문에 무효전력 보상을 위한 캐패시터 뱅크가 필요하게 된다. 이와 같이 Mod-2 wind turbine과 농형 유도발전기를 사용하여 계통에 연결되어진 풍력 발전 시스템을 모델링하였고, 풍속계를 통해 측정된 실시간 풍속 Data가 적용된 계통에 연결되어진 풍력발전 시뮬레이션이 가능하게 되었다. 그리고 구현되어진 풍력 발전 시스템에서 22.9kV 배전 선로에 고장이 발생하게 될 경우 Pitch control system으로 안정화가 가능하다는 것을 RTDS와 EMTDC로 시뮬레이션 하였다. Pitch control mode는 크게 발전기의 출력의 Fluctuation을 최소화 하기위한 Power constant mode와 고장 발생시, 풍력 발전시스템의 안정화를 위한 Speed constant mode로 나눌 수 있다. 기존의 피치 컨트롤 부분에서는 발전기의 출력을 안정화를 위하여 터빈의 토크를 Pitch control의 입력으로 사용하는 Power constant mode를 사용한다. 하지만 Power constant mode 만을 사용 하였을 경우, 선로에 고장이 발생하게 되면 Pitch control system은 동작하지 않게 되고 발전기의 속도는 증가할 뿐만 아니라 발전기의 출력은 감소하게 된다. 따라서 고장 발생시, 제안된 Pitch system에서 발전기의 속도를 Pitch control의 입력부로 사용하는 Speed constant mode를 사용하면 풍력발전 시스템의 안정화가 가능하게 된다. 따라서 고장이 발생하지 않은 경우는 Power constant mode를 사용하여 발전기의 출력의 Fluctuation을 최소화시키고, 고장이 발생하였을 경우는 Speed constant mode를 사용하여 안정화도 가능하게 된다. 본 연구를 통하여 실제 기상상태를 이용한 실시간 시뮬레이션이 가능함에 따라서 다양한 기상조건에서 정확한 시뮬레이션이 가능하게 되었고, 정격풍속 이상일 경우 Power constant mode를 사용하면 발전기의 출력의 변화를 최소화 시킨다. 그리고 계통에 고장이 발생하였을 경우 Pitch control system인 Power constant mode와 Speed constant mode가 결합된 Pitch control system을 사용함으로써 정상상태시에는 Power constant mode 가 동작하고 과도상태시에는 Speed constant mode가 동작하여 풍력발전 시스템의 안정화가 가능하다.