Universiẗat, Diss., 2003--Wuppertal.

No description available.

The purpose of this thesis is to analyse which factors drive migration from Turkey towards Europe and whether the welfare benefits play a major role in the decision making process. The analysis is based on a gravitation model of migration in log-log form. The FE and RE methods were employed as estimation techniques and the Hausman test enabled to distinguish them. The present problem of heteroscedasticity was solved by adjusting the model with robust standard errors. The most important determinants appear to be individual income which immigrants can earn in the states of the EU-15 and welfare benefits provided by the EU-15. The number of acquisition of citizenship, as a proxy for migration policy of countries the EU-15, plays also important role. The limitation of the model is that the rest of the variables are not statistically significant and therefore we do not consider them as important determinants.

The development of new techniques for control of magnetic objects by external magnetic fields has been in constant improvement. These advancements range from the design and fabrication of magnetic nanoparticles to design and control of actuators that enable their manipulation. The ability to guide such magnetic objects at a distance without any direct mechanical contact is an attractive feature with great potential in medical applications. Magnetic fields are not distorted by their interaction with nonmagnetic materials, like those in the human body; and pose no harm to living tissues, which make them convenient tools for minimally invasive techniques and treatments. Moreover, several actuator configurations have been proposed to achieve the remote motion of a magnetic particle or magnetic fluids. Arrays of electromagnets have been widely utilized due to their lack of mechanical parts and flexibility to accurately and rapidly change their magnetic field by controlling the current through their coils. However, they are relatively weak for their size and electrical power, making them inefficient for medical applications which need large magnetic forces at relatively long distances. On the other hand, permanent magnets have a much higher strength-to-size ratio than electromagnets and allow for control from larger distances. The disadvantage is that their magnetic fields cannot be turned off and a mechanical actuator is needed to modify their position and orientation to change their field. In this work, a magnetic manipulator used as a testbed to manipulate a magnetic bead is designed. It consists of an array of six diametrically magnetized cylindrical permanent magnets evenly spaced around a petri dish, following the work in [11]. Servomotors are utilized to precisely adjust the direction of the magnets according to a control law developed by other researchers in the past. A monochromatic camera located above the petri dish provides the feedback on the position of the bead and a set of hall-effect sensors provides the location of the poles of the magnets. The dynamics of the system is modeled by a linearized set of state-space equations where the magnetic field is estimated with an analytical expression for the geometry of the magnets. The testbed has been designed with the CAD software SolidWorks and its structure has been completely 3D printed with polylactic acid (PLA) filament. The design is tested under different speeds of the servomotors and initial orientations of the magnets. Some recommendations are presented at the end for improvement and considerations for future designs.

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Magnet schools were developed in the early 1970s when a large number of urban school districts began seeking alternatives to court-ordered desegregation mandates (Levine and Steel, 1994). Since that time, numerous studies have been conducted on the effectiveness of magnet schools in providing a racially balanced learning environment as well as increasing academic achievement. The purpose of the causal-comparative study was to determine if the math and science magnet program at a middle school affected achievement, attendance, and parent perceptions. This study conducted three different analyses. A chi square analysis of the student population was conducted to determine racial balanced on attendance data from the school years 1993-94 through 1996-97, and if the racial balance of the magnet program mirrored that of the district. Three-way ANCOVA analyses, with a 2x2x2 factorial design were performed on attendance and the five components of the 1997 Stanford Achievement Test Form 9-TA results for the eighth grade population at the targeted middle school enrolled during the 1996-97 school year. Complete data for 177 eighth grade students was utilized. Attendance and achievement served as the dependent variables. The independent variables tested were group membership (magnet, non-magnet), gender (male, female), and race/ethnicity (black, white). Socio-economic status (SES) and Literacy Passport Test (LPT) scores served as the covariates in the study. A survey of school effectiveness was sent to a random sample of parents. A t-test was performed to determine if there was a statistically significant difference between the perceptions of parents of magnet students and parents of student not enrolled in the program on school effectiveness. The racial balance of the magnet program did not mirror that of the district. There was more of an equally distributed number of blacks and whites in the magnet program. Within the district, approximately 68% of the student enrollment was black, the white enrollment was approximately 31%. Magnet students achieved statistically significantly higher scores on each of the five components of the Stanford Achievement Test Form 9-TA than non-magnet students. Gender and race/ethnicity differences were statistically significant in science achievement in that male and white students achieved higher scores than female and black students. There was a statistically significant difference in attendance between magnet students and non-magnet students. Magnet students attended school more than non-magnet students. There was no significant difference in perceptions of parents of magnet and non-magnet students. Both groups felt that the school was very good. Implications for future avenues of research were also suggested. / Ed. D.

Single molecule magnets (SMMs) are molecules of mesoscopic scale which exhibit quantum properties such as quantum tunneling of magnetization, quantum interference, spin filtering effects, strong spin-phonon coupling and strong hyperfine Stark effects. These effects allow applications of SMMs to high-density information storage, molecular spintronics, and quantum information science. Therefore, SMMs are of interest to physicists, chemists, and engineers. Recently, experimental fabrication of individual SMMs within transistor set-ups have been achieved, offering a new method to examine magnetic properties of individual SMMs. In this thesis, two types of SMMs, specifically Eu2(C8H8)3 and Ni9Te6(PEt3)8, are theoretically investigated by simulating their electron transport properties within three-terminal transistor set-ups. An extended metal atom chain (EMAC) consists of a string of metallic atoms with organic ligands surrounding the string. EMACs are an important research field for nanoelectronics. Homometallic iron-based EMACs are especially attractive due to the high spin and large magnetic anisotropy of iron(II). We explore the exchange coupling of iron atoms in two EMACs: [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. Chapter 1 provides an introduction to SMMs, electron transport experiments via SMMs and an introduction to density functional theory (DFT). Chapter 2 presents a theoretical study of electron transport via Eu2(C8H8)3. This type of molecule is interesting since its magnetic anisotropy type changes with oxidation state. The unique magnetic properties lead to spin blockade effects at zero and low bias. In other words, the current through this molecule is completely suppressed until the bias voltage exceeds a certain value. Chapter 3 discusses a theoretical study of electron transport via Ni9Te6(PEt3)8. The magnetic anisotropy of this magnetic cluster has cubic symmetry, which is higher than most SMMs. With appropriate magnetic anisotropy parameters, in the presence of an external magnetic field, uncommon phenomena such as low-bias blockade effects, negative conductance and discontinuous conductance lines, are observed. In Chapter 2 and 3 DFT-calculated magnetic anisotropy parameters are used and electron transport properties are calculated by solving master equations at low temperature. Chapter 4 examines the exchange coupling between iron ions in EMACs [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. The exchange coupling constants are calculated by using the least-squares fitting method, based on the DFT-calculated energies from different spin configurations. / Ph. D. / Single molecule magnets (SMMs) are molecules of mesoscopic scale which exhibit quantum properties. Its quantum effects are used to describe the behavior of SMMs at the smallest scales. These quantum properties could also be used to reveal possible applications of SMMs to high-density information storage, molecular spintronics, and quantum information science. Thus SMMs are of interest to physicists, chemists, and engineers. Recently, electron transport via individual SMMs was achieved in experiments. Electron transport is obviously affected by the magnetic properties of the SMM, thus one can examine magnetic properties of an SMM indirectly by measuring electron transport via the SMM. In this thesis, two types of SMMs, Eu2(C8H8)3 and Ni9Te6(PEt3)8, are investigated theoretically by simulating their electron transport properties. An extended metal atom chain (EMAC) consists of a string of metallic atoms with organic ligands surrounding the string. EMACs are an important research field for nanoelectronics. Homometallic iron-based EMACs are especially attractive due to the high spin and large magnetic anisotropy of iron(II). If a molecule has magnetic anisotropy, its magnetic properties change with the direction of its magnetic moment. We explore how iron atoms interact with each other in the EMACs [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. Chapter 1 provides an introduction to SMMs, electron transport experiments via SMMs and an approximation method, density functional theory (DFT). DFT is a method to approximate electronic structure and magnetic properties of various many-body systems. Chapter 2 investigates theoretical electron transport via Eu2(C8H8)3. Eu2(C8H8)3 changes its type of magnetic anisotropy when it obtains an extra electron, which is different from most SMMs. If the Eu2(C8H8)3 is short of an extra electron, its magnetization direction is in-plane, that is, its magnetic energy is lowest when its magnetic moment is along any direction in a specific plane. If an extra electron is captured by Eu2(C8H8)3, its magnetization direction becomes out-of-plane, and its lowest energy is obtained when its magnetic moment is along the direction normal to the specific plane. The unique magnetic properties lead to blockade effects at low bias: the current through this molecule is completely suppressed until the bias voltage exceeds a certain value. The bias voltage on a molecule equals the electrical potential difference between two ends of the molecule. Chapter 3 investigates theoretical electron transport via Ni9Te6(PEt3)8. Magnetic anisotropy of Ni9Te6(PEt3)8 is cubic symmetric, and its symmetry is higher than most SMMs. With appropriate magnetic anisotropy parameters, in the presence of an external magnetic field, uncommon phenomena are observed. These phenomena include (1) current is completely suppressed when bias is low; (2) current via SMM decreases while bias on SMM increases; (3) there are discontinuous lines in the figures that describe electrical conductance of current. Chapter 4 examines the iron atoms’ interaction strength in both [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. Reasonable spin Hamiltonians are used to describe the energy of EMACs. Considering all possible directions of the spins of iron atoms in two EMACs, we calculate the energy of every possible spin configuration using DFT. The energy of each spin configuration can be expressed as an equation containing one or more coupling constants. We apply the least-squares fitting method to obtain the values of the coupling constants in the spin Hamiltonians.

Thesis (MScEng) -- Stellenbosch University, 2003. / ENGLISH ABSTRACT: Axial Flux Permanent Magnet (AFPM) machines have become attractive because of significant improvements in permanent magnets over the past decade, improvements in power electronic devices, and the ever increasing need for more efficient machines in electric vehicle systems. In comparison with the cylindrical radial flux motor, the AFPM machine is better in a number of aspects: short frame; compact construction; high efficiency; brush less construction; good starting torque and high-power density. The common modes of failure and typical operating conditions of AFPM machines are discussed further. The focus of this research project is a prototype AFPM machine developed by the Electrical Engineering Department of The University of Stellenbosch. The machine considered has a power rating of 300 kW and an operating efficiency of 95 % at a speed of 2300 rpm. This specific machine is used as an example to illustrate the thermal characteristics of geometrically similar AFPM machines. The thermal characterization was achieved with the use of two numerical computer models. Firstly a fluid model was specially developed and experimentally verified. The objective of the fluid model was to calculate the mass flow rate of air through any geometrically similar AFPM machine. The fluid model was further used to investigate the effects of different magnet thickness and axial gaps between the stator and the rotor plates on the mass flow rate of air through the machine. The fluid model was verified with experimental testing that was done on a half-scale Perspex model. During the experimental testing the magnet thickness was varied between 2.5 mm, 5.0 mm, and 7.5 mm along with axial gaps of 6.5 mm, 7.5 mm, 8.5 mm, and 9.5 mm. The fluid model showed a correlation to within 10 % of the experimental mass flow rates. The results of these tests showed that the magnet thickness and axial gap between the stator and the rotor plates had no significant effect on the mass flow rate of air. The fluid model was based on one-dimensional, steady-state, and incompressible flow. The second numerical computer model was a thermal model. This model was used to calculate the transient temperature response of the AFPM machine. The model was based on a twodimensional transient finite difference solution technique. Experimental temperatures taken from the prototype AFPM machine were used to verify the thermal model. Correlations between the experimental and theoretical temperatures were within 5.8 % of each other. The thermal model was used to investigate the effect of geometrical changes on the temperatures in the AFPM machine. It was found that these geometrical changes had no significant effect on the temperatures in the AFPM machine. It was also established that increasing the air mass flow rate over about I kg/s had no further effect on lowering the temperatures. The stator was also identified as being the most critical component as it reached its maximum temperature limit before any other component. Heat pipes were considered as an alternative thermal management technique. The location of the heat pipe was limited to the stator. Further simulations were done to investigate the effect of the heat pipe properties on the amount of heat removed from the stator. Recommendations were made concerning the thermal management of the current and possible future prototype AFPM machines. It was recommended that a further more detailed investigation into the use of heat pipes be considered. This recommendation is substantiated by the fact that in this research project only one type of heat pipe was considered and its location was limited to within the stator. / AFRIKAANSE OPSOMMING: AFPM masjiene het meer aantreklik geword weens betekenisvolle verbeteringe in permanente magnete gedurende die laaste dekade, verbeteringe in elektroniese toestelle en die vraag na meer effektiewe masjiene in elekriese voertuigstelsels. Die AFPM masjien is beter as die Silindriese Radiale Fluksie Motor wat die volgende aspekte betref: die kort raamwerk; kompakte konstruksie; hoe effektiwiteit; borsellose konstruksie; goeie aanvangsdraaimoment; en hoe-krag digtheid. Die algemene vorms van faling en ook die tipiese werkstoestande van die AFPM word verder bespreek. Hierdie navorsingsprojek fokus op die prototipe AFPM masjien wat ontwikkel is deur die Elektriese Ingenieurs Departement van die Universiteit van Stellenbosch. Die masjien onder bespreking wek 300 kW per uur op en is 95% effektief teen 'n spoed van 2300 rpm. Hierdie masjien word gebruik om die termiese kenmerke van geometries-gelyksoortige masjiene te illustreer. Die termiese eienskappe is bepaal deur die gebruik van twee numeriese rekenaarmodelle. Eerstens is 'n vloeistofmodel spesiaal ontwerp en eksperimenteel geverifieer. Die doel van die vloeistofmodel was om die massa vloeitempo van lug deur enige geometries-gelyksoortige AFPM masjien te bereken. Die vloeistofmodel is verder gebruik om die uitwerking van verskillende magneetdiktes en aksiale gapings tussen die stator en die rotorplate op die massa vloeitempo van lug deur die masjien te ondersoek. Die vloeistofmodel is geverifieer deur eksperimentele toetsing wat gedoen is op 'n halfskaal Perspex model. Tydens die toetsing het magneetdiktes gewissel tussen 2.5 mm, 5.0 mm en 7.5 mm en die aksiale gapings tussen 6.5 mm, 7.5 mm en 9.5 mm. Die vloeistof model het 'n korrelasie van binne 10 % van die eksperimentele massa vloeistempo getoon. Die resultate van hierdie toetse het getoon dat die magneetdiktes en die aksiale gapings tussen die stator en die rotorplate geen noemenswaardige uiterking op die massa vloeitempo van lug gehad het nie. Die vloeistofmodel is gebaseer op een-dimensionele, gestadigde, onsamedrukbare vloei. Die tweede numeriese model was 'n termiese model. Hierdie model is gebruik om die transiente temperatuur respons van die AFPM masjien te bereken. Die model is gebaseer op 'n tweedimensionele, transiente eindige-verskil oplossingstegniek. Eksperimentele temperature gemeet op die prototipe AFPM masjien is gebruik om die termiese model te verifeer. Die eksperimentele en teoretiese temperature het binne 5.8% met mekaar gekorrelleer. Die termiese model is gebruik om die uitwerking van geometriese veranderinge op die temperatuur in die AFPM masjien te ondersoek. Daar is gevind dat hierdie geometriese veranderinge geen noemenswaardige uitwerking op die temperature van die AFPM masjien gehad het nie. Daar is ook vasgestel dat 'n vermeerdering in die lug massa vloeitempo yerby I kg/s geen verdere uitwerking het op die verlaging van die temperatuur gehaad het nie. Die stator is ge-identifiseer as die mees kritiese komponent aangesien dit sy maksimum temperatuur limiet bereik het voor enige ander komponent, Hittepype is oorweeg as 'n alternatiewe termiese bestuurstegniek. Die plasing van die pype is tot die stator beperk. Verdere simulasies is uitgevoer om die uitwerking van die hittepyp eienskappe op die hoeveelheid hitte wat verwyder word van die stator te ondersoek. Aanbevelings is gemaak m.b.t die termiese bestuur van die huidige en moontlike toekomstige prototipes van AFPM masjiene. Daar is aanbeveel dat daar in meer besonderhede ondersoek ingestel word na die gebruik van hittepype. Die rede hiervoor is dat daar in hierdie studie net gebruik gemaak is van een tipe hittepyp en dat die plasing daarvan beperk is tot binne die stator.

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

No description available.