265 research outputs found
Self-healing of deformation-induced defects in Fe-based alloys using positron annihilation techniques
No full textThis thesis investigates the self-healing mechanisms of deformation-induced defects in ferritic Fe-Au(-B-N) and Fe-Mo alloys, by segregation and precipitation of Au elements and Fe2Mo compounds. For this purpose, cyclic tensile tests on the Fe-Au(-B-N) alloys are conducted interrupted with isothermal ageing steps at 550 °C. Coincidence Doppler Broadening (CDB) measurements are performed to study the defect evolution and precipitation behaviour occurring at these defect sites, to clarify the correlation between precipitation and strain-induced defects. Using this technique, the Fe-Mo samples are studied for various ageing times up to 64 h. These measurements are conducted with 8% deformed and undeformed samples, to study the influence of dislocations on the Fe2Mo precipitation. Furthermore, Vickers hardness tests are conducted on undeformed and 8 % deformed Fe-Mo alloy samples for various ageing times, as well as creep tests for the Fe-Mo and Fe-Au(-B-N) alloys in combination with SEM observations. Next, diffusion-based finite element model calculations are performed on filling of a creep cavity for the Fe-Mo alloy. It is found from the model that the filling of a creep cavity on a grain boundary can be well compared with the growth model of a precipitate on a grain boundary. From the CDB measurements, it is found for the Fe-Au(-B-N) alloys that in a major part of the deformation-ageing cycles healing takes place after introducing defects. Being consistent with previous research [1], it is found that the addition of B and N to the Fe-Au alloy retards the Au precipitation. For the Fe-Mo alloy, it is found from CDB measurements that the segregation and precipitation of Fe2Mo is enhanced by induced dislocations and other defects. The creep lifetime of this alloy is improved compared with that of the Fe-Au alloy, together with a higher ductility. Based on the CDB measurements, supported by the creep measurements and SEM studies, it is shown that Mo solutes have a tendency to segregate selectively to dislocations and open-volume defects. The site-selective precipitation makes Mo a promising alternative for Cu and Au to act as an efficient self-healing agent for deformation-induced defects in Fe-based structural alloys for use at elevated temperatures.Novel Aerospace MaterialsAerospace Structures and MaterialsAerospace Engineerin
Uitdam, Praktijkproef Dijk op Veen. Gemeente Waterland (N.H.): Een Inventariserend Archeologisch Veldonderzoek
No full textVoor een plangebied ten noorden van Uitdam, gemeente Waterland, is een archeologisch bureauonderzoek met uitbreiding tot een booronderzoek uitgevoerd ten behoeve van de geplande aanleg van een proefopstelling voor een dijk op veen. Bij de graaf-werkzaamheden die gepaard gaan met deze aanleg wordt mogelijk het bodemarchief geschaad. Doel van het bureauonderzoek is het opstellen van een verwachting en advies op basis van de bekende gegevens met betrekking tot archeologie en fysische geografie.
Doel van een dergelijk onderzoek is het vaststellen van de mate waarin de bodem intact is, om zo te kunnen localiseren waar en en in welke mate intacte archeologische resten moeten worden verwacht.
Het plangebied ligt niet in een terrein van bijzondere archeologische waarde zoals aangeduid op de Archeologische Monumenten Kaart (AMK). Ook komen in het Centraal Archeologisch Archief (CAA) geen vondstmeldingen voor uit het plangebied. In het Centraal Monumenten Archief (CMA) is wel een aantal (vermoede) middeleeuwse woonplaatsen bekend, die bij een eerder archeologisch onderzoek uit 1986 zijn aangewezen. Uit dit onderzoek zijn vondsten van onder meer middeleeuws aardewerk en bewerkt hout bekend.
Het terrein heeft op de Indicatieve Kaart Archeologische Waarden (IKAW) een lage trefkans op archeologische sporen. Op de gemeentelijke beleidsadvieskaart geldt dat bij werkzaamheden dieper dan 40 centimeter in een plangebied groter dan 500 m2 rekening moet worden gehouden met de aanwezigheid van archeologische waarden.
Het is niet uit te sluiten dat ook binnen het plangebied middeleeuwse woonplaatsen en/of vondstmateriaal aanwezig zijn. Om de eventuele aanwezigheid en conservering hiervan vast te stellen, werd geadviseerd om een karterend booronderzoek met een dichtheid van tien boringen per hectare uit te laten voeren.
Op 27 juni 2011 is het veldonderzoek middels grondboringen uitgevoerd. Intussen was het plangebied verkleind tot alleen het oostelijke deel. Er zijn 15 boringen uitgevoerd. De bodem bleek te bestaan uit een dunne bouwvoor van veraard veen, gevolgd door een veenpakket met dunne kleilaagjes. Er zijn geen archeologische indicatoren of cultuurlagen waargenomen. Wij adviseren daarom geen archeologisch vervolgonderzoek
Magnetic structure and phase formation of magnetocaloric Mn-Fe-P-X compounds
No full textThis thesis presents a study of the crystal and magnetic structure, the magnetocaloric effect and related physical properties in Mn-Fe-P-X compounds. The influences of boron addition in (Mn,Fe)2(P,As) compounds have been studied. It is found that boron atoms occupy interstitial sites within the basal plane. First order magnetoelastic phase transitions with small thermal and magnetic hysteresis are observed in all these compounds. The ferromagnetic ordering temperatures increase by boron addition. The optimal working temperatures can be finely adjusted by varying the boron content without losing the good magnetocaloric properties. Both the maximal magnetic entropy changes and the Relative Cooling Power (RCP) are slightly enhanced. All these features make boron addition a good tool to tune and improve magnetic and magnetocaloric properties in (Mn,Fe)2(P,As) compounds. (Chapter 4) The effect of transition metal substitution on TC and ?Thys has been studied in (Mn,Fe,T)1.95P0.50Si0.50 (T = Co, Ni and Cu) compounds. X-ray diffraction patterns imply that all the compounds crystallize in the hexagonal Fe2P type of structure. It is found that all these transition metal substitutions for Mn(3g)/Fe(3f) weaken the ferromagnetic ordering. Ni substitutions reduce the thermal hysteresis, while the Cu substitutions enhance the thermal hysteresis. Moreover, the Co substitutions for Mn(3g) reduce thermal hysteresis, while the Co substitutions for Fe(3f) result in hardly any change in thermal hysteresis. (Chapter 5) Single phase compounds Mn0.66Fe1.29P1-xSix (0 ? x ? 0.42) have been synthesized using the melt-spinning (rapid solidification) technique. All the compounds form in the Fe2P type hexagonal structure, except a Co2P type orthorhombic structure of the Si free Mn0.66Fe1.29P compound. The compounds with 0.24 ? x ? 0.42 present a FM-PM phase transition, while the compounds with lower Si content show an AFM-PM phase transition. By increasing the Si content from x = 0.24 to 0.42, TC increases from 195 to 451 K and ?Thys is strongly reduced from ~61 to ~1 K. TC increases and ?Thys decreases with increasing magnetic field. It is also found that TC and ?Thys are not only Si content dependent, but also magnetic field dependent. Mn0.66Fe1.29P1-xSix compounds show large spontaneous magnetic moments with values up to 4.57 ?B/f.u.. A large MCE with a small thermal hysteresis is obtained simultaneously in Fe-rich Mn0.66Fe1.29P1-xSix melt-spun ribbons. The compounds with a high working temperature may also be useful for other applications, e.g. thermomagnetic generators and heat pumps. (Chapter 6) Single phase Mn1.95 xFexP2/3Si1/3 compounds with 1.0 ? x ? 1.95 have been synthesized using ball-milling technology and solid state reactions. All the compounds show a FM PM phase transition. The compounds with x ? 1.6 crystallize in Fe2P based hexagonal structure, for the higher Fe content compounds, a bco hex structural transition is observed. In contrast to the Mn rich Mn Fe P-Si system, no coupling between the magnetic and structural transition is found. The TC and ?Thys in the Mn1.95 xFexP2/3Si1/3 compounds can be easily tuned by adjusting the Fe/Mn ratio. By increasing the Fe content from x = 1.0 to 1.95, TC increases from ~269 to ~647 K and the ?Thys strongly reduces from ~65 to ~1 K. The reduction in the magnetic moment of the Fe-rich Mn1.95 xFexP2/3Si1/3 compounds, suggests that the dominant effect on the size of the moment is the change in local electron configuration rather than the interlayer exchange coupling, thus confirming a more localized magnetism on the 3g sites. (Chapter 7) High resolution neutron diffraction has been employed to determine the crystal and magnetic structure, the magnetic moment and the interatomic distances in the melt-spun ribbons Mn0.66Fe1.29P1-xSix. Introducing site disorder at the 3g site by replacing 1/3 of Fe with Mn appears to enhance the magnetic interaction, while the strong magnetoelastic coupling is maintained. The Mn substitution also shows a stabilizing effect on the hexagonal crystal structure, which is maintained to a high Si content. The moment alignment within the crystallographic unit cell is affected when the Si content increases from x = 0.34 to 0.42 in Mn0.66Fe1.29P1 xSix compounds, as the canting angle with respect to the c direction increases. The canted magnetic moment alignment confirms a low magnetic anisotropy, which ensures soft magnetic properties of Fe rich Mn Fe P Si compounds. (Chapter 8) The effect of varying the Mn/Fe ratio in the Mn1.95-xFexP2/3Si1/3 compounds has been studied using high resolution neutron diffraction. The alignment of the magnetic moment is canted from nearly the a b plane towards c axis with increasing Fe content in the Fe rich Mn1.95 xFexP2/3Si1/3 compounds with 1.0 ? x ? 1.4. The canted magnetic moment alignment confirms a low magnetic anisotropy of the Mn Fe P Si compounds, which display properties close to that of soft magnets. The magnetic moments are enhanced on both the 3f and 3g sites by introducing Mn atoms into the 3g sites. Decrease of the Mn/Fe ratio on the 3g sites does not affect the Mn/Fe(3g) magnetic moment, but results in a gradual decrease in the magnitude of the Fe(3f) magnetic moment, supporting the description of the 3f sites as weakly magnetic. Microstrains develop with decreasing Mn content, and are proposed to be caused by the inconsistent change of the intralayer and interlayer interatomic distances with respect to the changes in a and c lattice constants, respectively. Hence, the Mn favors not only the high magnetic moment, but also the hexagonal crystal structure of the Fe-rich Mn Fe P Si compounds. (Chapter 9)Radiation, Radionuclides & ReactorsApplied Science
Thermomagnetic studies on transition metal pnictides
No full textThe magnetocaloric effect (MCE) consists, in simple terms, in the heating of a magnetic material by the application of an external magnetic field. This can be easily understood if one but imagines a magnetic material with randomly applied spins; as a magnetic field is applied the spins will tend to alight with this field and as a result the overall Entropy of the system decreases, which consequently results in an exchange in Heat. This idea then becomes analogous to the vapor compression cycle used in our current household refrigerators, making the possibility of assembling a magnetic refrigerator quite tangible. The current thesis is in part an ample exploratory research of several material families with the aim of determining their magnetocaloric potential. In this scope the (Mn,Fe)3(Si,P), (Mn,Co)3(Si,P), (Fe,Co)3(Si,P) and (Mn,Fe)2(P,Ge) systems are studied and characterized. The study in (Mn,Fe)3(Si,P) resulted in the full magnetostructural mapping of this system, revealing many possible applications in a wide variety of physical areas. The (Mn,Co)3(Si,P) and (Fe,Co)3(Si,P), although not wielding positive results for MCE applications, still revealed the existence of other related material systems with extremely relevant physical properties, such as the inverse MCE. Finally the (Mn,Fe)2(P,Ge) revealed a very promising MCE potential, with the added discovery of a definite potential for permanent magnet applications. Finally, the current thesis also covers the assembly of microcalorimetry setup using commercial Xensor chips designed for the measurement of key physical properties for the understanding of the transition phenomenon in materials of MCE interest.Reactor Institute DelftApplied Science
Apomixis in Taraxacum : an embryological and genetic study = Apomixie in Taraxacum : een embryologische en genetische studie
No full textApomixis is asexual plant reproduction through seeds that are produced with no prior genome reduction or fertilisation. The use of apomixis holds enormous potential for future plant breeding and production. The outcome of the research described in this thesis suggests that the genetic basis for apomixis in Taraxacum (dandelions) comprises three major dominant genes whose timely expression may be influenced by modifier genes. Its also suggests that genome interactions effect successful establishment of apomixis in polyploid hybrids of sexual x apomictic dandelions. The work consisted of cytogenetic and embryological studies performed at the Laboratory of Genetics, Wageningen University, and the analysis of inheritance of apomixis in sexual x apomict crossings performed at the Netherlands Institute of Ecology, Centre for Terrestrial Ecology, (NIOO-CTO), Heteren, the Netherlands. A theoretical model for the Taraxacum type of apomixis is described. As a conclusion, we discuss several aspects of the evolution of apomixis in Taraxacum which serve to explain why Taraxacum is a very successful genus.</p
Modelling Magnetic Phase Transitions: with Density Functional Theory and the Finite Element Method
No full textThe present thesis discusses two modelling endeavours that serve to provide insights into both the magnetic and structural dynamics associated with a first order magnetic phase transition. Firstly, density functional theory has been used to model the lithiation of a supercell of 2x2x2 the conventional unit cell of the ferromagnet Fe2P. Subsequently, the magnetic moments in the resulting structure were set to proxy a paramagnetic state and compute the energy difference between the ferromagnetic and paramagnetic state. It has been found that non-magnetic Li-atoms substitute for magnetic Fe-atoms at 3g-positions in the a,b-plane. Moreover, since these are the positions of Fe-atoms that carry a high magnetic moment and induce a magnetic moment on the Fe-atoms in the 3f-layer, the lithiation of the respective positions leads to a reduction of the magnetic moment per formula unit. The energy difference between the ferromagnetic state and the selected proxy of a paramagnetic state showed to decrease linearly in the fraction of lithiated 3g-positions. This deviates from experimental findings suggesting an increase of Tc of the structure upon lithiation. This difference can stem from the fact that the lithiation fractions considered here were higher than those achieved experimentally or that the selected proxy of a paramagnetic state was incorrect. Secondly, the finite element method has been applied to approximate the displacement field of a 2D structure consisting of grains distributed over two structural phases with different associated lattice parameters. The development of shear stress discontinuities across grain boundaries has been studied in relation to the degree of porosity in a domain. The replacement of grain boundaries by voids appears, after correction for the orientation of edges at these grain boundaries, to lead to a reduction in the values for these discontinuities. More delicate development of the grid to correspond to physically realistic orientations of grain boundaries is needed to gain more robust quantitative measures for this relation. Moreover, the relation between the orientation of grain boundaries and shearing stresses at these boundaries forms an interesting venue for further research to which the present FEM-model can be readily applied.Applied Mathematics | Applied Physic
Landau model evaluation of the magnetic entropy change in magnetocaloric materials
No full textThe field exponent for the magnetic entropy change n=dlnΔS/dlnH in a magnetic field was evaluated using the Landau model to provide a classification for the nature of the ferromagnetic-to-paramagnetic transitions at the Curie temperature TC in magnetocaloric materials. The magnetic phase transition can be classified as a first-order magnetic transition (FOMT), a second-order magnetic transition (SOMT) or a critical point (CP) at the border between the first and second order transitions. The value of n is mapped as a function of the reduced temperature and the reduced magnetic field for the FOMT, SOMT and CP. The influence of the magnetic field on the transition temperature was found to be relatively weak for the SOMT and corresponds to a power law for the CP and FOMT.RST/Fundamental Aspects of Materials and Energ
Tuning Giant Magnetocaloric Materials: A Study of (Mn,Fe)2(P,Si) and NiCoMnTi Heusler Compounds
No full textSolid‐state caloric effects as intrinsic responses from different physical external stimuli (magnetic‐, uniaxial stress‐, pressure‐ and electronic‐ fields) have been evaluated near magnetic phase transformations. In the last decades the magnetically driven caloric changes in various magnetocaloric materials (MCMs) have been exploited extensively for magnetic refrigeration and magnetic heat pumping scenarios near room temperature. This thesis systematically investigates the magnetocaloric effect (MCE) for the representative magnetoelastic (Mn,Fe)2(P,Si) system. Special emphasis has been directed towards the giant MCE in nanoscale particles and the influence of doping with elements that show a strong electronegativity on the magnetic properties of this metal‐metalloid system. Meanwhile, two optimization strategies (decoupling and light element B doping) are successfully introduced to regulate the thermal hysteresis ΔThys, the ferromagnetic phase transition TC and improve the reversibility of the MCE for magnetostructural transition in the all‐d‐metal NiCoMnTi Heusler alloys.RST/Fundamental Aspects of Materials and Energ
Influence of compositions and size on the giant magnetocaloric effect in (Mn,Fe)2(P,Si)-based compounds
No full textRST/Fundamental Aspects of Materials and Energ
Modelling self healing creep steel
No full textAdvances in self-healing creep steels are driven by the understanding of its mechanism. Previous work on self-healing creep metals has shown selective precipitation inside the creep cavities, which has a beneficial effect on the creep lifetime. This effect occurswhen a solute supersaturation exists at the creep temperature. In the case of self-healing creep steels, the precipitation of gold was found to have an exceptional effect. Gold precipitation was found to be efficient because of the atomic size mismatch of gold and iron, which induces a high selectivity for precipitates to form inside creep cavities and not in the bulk. However, gold is not the most widely used alloying element in steel, to put it mildly. Ideally the self-healing effect could be achieved by another solute element dissolved in steel. In order to identify some other possible solute element for self healing creep steels, the mechanism must be understood. In this thesis the mechanism of self healing creep steels, andmetals in general, is investigated.RST/Fundamental Aspects of Materials and EnergyMaterials Science and Engineerin
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