Mechanical behavior of 2G REBCO HTS at 77 and 300 K

El gran esfuerzo realizado durante la ��ltima d��cada con el fin de integrar los diferentes materiales superconductores en el campo de los sistemas el��ctricos y en otras aplicaciones tecnol��gicas ha dado lugar a un campo de investigaci��n amplio y prometedor. El comportamiento el��ctrico de los Superconductores de Alta Temperatura (SAT) cr��tica (masivo y cintas) depende de diferentes par��metros desde su fabricaci��n hasta la aplicaci��n final con imanes o cables. Sin embargo, las aplicaciones pr��cticas de estos materiales est��n fuertemente vinculadas con su comportamiento mec��nico tanto a temperatura ambiente (manipulaci��n durante fabricaci��n o instalaci��n) como a temperaturas criog��nicas (condiciones de servicio). En esta tesis se ha estudiado el comportamiento mec��nico de materiales masivos y cintas de alta temperatura cr��tica a 300 y 77 K (utilizando nitr��geno l��quido). Se han obtenido la resistencia en flexi��n, la tenacidad de fractura y la resistencia a tracci��n a la temperatura de servicio y a 300 K. Adicionalmente, se ha medido la dureza mediante el ensayo Vickers y nanoindentaci��n. El m��dulo Young se midi�� mediante tres m��todos diferentes: 1) nanoindentaci��n, 2) ensayos de flexi��n en tres puntos y 3) resonancia vibracional mediante grindosonic. Para cada condici��n de ensayo, se han analizado detalladamente las superficies de fractura y los micromecanismos de fallo. Las propiedades mec��nicas de los materiales se han comparado con el fin de entender la influencia de las t��cnicas de procesado y de las caracter��sticas microestructurales de los monocristales en su comportamiento mec��nico. Se ha estudiado el comportamiento electromec��nico de cintas comerciales superconductoras de YBCO mediante ensayos de tracci��n y fatiga a 77 y 300 K. El campo completo de deformaciones en la superficie del material se ha obtenido utilizando Correlaci��n Digital de Im��genes (DIC, por sus siglas en ingl��s) a 300 K. Adem��s, se realizaron ensayos de fragmentaci��n in situ dentro de un microscopio electr��nico con el fin de estudiar la fractura de la capa superconductora y determinar la resistencia a cortante de la intercara entre el substrato y la capa cer��mica. Se ha conseguido ver el proceso de la fragmentaci��n aplicando tensi��n axial y finalmente, se han implementado simulaciones mediante elementos finitos para reproducir la delaminaci��n y el fen��meno de la fragmentaci��n. Por ��ltimo, se han preparado uniones soldadas entre las capas de cobre de dos cintas superconductoras. Se ha medido la resistencia el��ctrica de las uniones con el fin de evaluar el metal de soldadura y el proceso. Asimismo, se ha llevado a cabo la caracterizaci��n mec��nica de las uniones mediante ensayos "single lap shear" a 300 y 77 K. El efecto del campo magn��tico se ha estudiado aplicando campo externo hasta 1 T perpendicular o paralelo a la cinta-uni��n a la temperatura de servicio (77 K). Finalmente, la distribuci��n de tensiones en cada una de las capas de la cinta se estudi�� mediante simulaciones de elementos finitos, teniendo en cuenta las capas de la cinta mec��nicamente m��s representativas (Cu-Hastelloy-Cu) que influyen en su comportamiento mec��nico. The strong effort that has been made in the last years to integrate the different superconducting materials in the field of electrical power systems and other technological applications led to a wide and promising research field. The electrical behavior of High Temperature Superconducting (HTS) materials (bulk and coated conductors) depends on different parameters since their processing until their final application as magnets or cables. However, practical applications of such materials are strongly related with their mechanical performance at room temperature (handling) as well as at cryogenic temperatures (service conditions). In this thesis, the mechanical behavior of HTS bulk and coated conductors was investigated at 300 and 77 K (by immersion in liquid nitrogen). The flexural strength, the fracture toughness and the tensile strength were obtained at service temperature as well as at 300 K. Furthermore, their hardness was determined by Vickers measurements and nanoindentation and the Young's modulus was measured by three different techniques: 1) nanoindentation, 2) three-point bending tests and 3) vibrational resonance with a grindosonic device. The fracture and deformation micromechanics have been also carefully analyzed for each testing condition. The comparison between the studied materials has been performed in order to understand the influence of the main sintering methods and the microstructural characteristics of the single grains on the macroscopic mechanical behavior. The electromechanical behavior of commercial YBCO coated conductors was studied. The mechanical behavior of the tapes was studied under tensile and fatigue tests at 77 and 300 K. The complete strain field on the surface of the sample was obtained by applying Digital Image Correlation (DIC) at 300 K. Addionally, in situ fragmentation tests inside a Scanning Electron Microscope (SEM) were carried out in order to study the fragmentation of the superconducting layer and determine the interfacial shear strength between substrate and ceramic layer. The fragmentation process upon loading of the YBCO layer has been observed and finally, Finite Element Simulations were employed to reproduce delamination and fragmentation phenomena. Finally, joints between the stabilizing Cu sides of two coated conductors have been prepared. The electrical resistivity of the joints was measured for the purpose of qualifying the soldering material and evaluating the soldering process. Additionally, mechanical characterization under single lap shear tests at 300 and 77 K has been carried out. The effect of the applied magnetic field has been studied by applying external magnetic field up to 1 T perpendicular and parallel to the tape-joint at service temperature (77 K). Finally, finite element simulations were employed to study the distribution of the stresses in earch layer, taking into account the three mechanically relevant layers of the coated conductor (Cu-Hastelloy-Cu) that affect its mechanical behavior

Theoretical and numerical investigation of plasmon nanofocusing in metallic tapered rods and grooves

Effective focusing of electromagnetic (EM) energy to nanoscale regions is one of the major challenges in nano-photonics and plasmonics. The strong localization of the optical energy into regions much smaller than allowed by the diffraction limit, also called nanofocusing, offers promising applications in nano-sensor technology, nanofabrication, near-field optics or spectroscopy. One of the most promising solutions to the problem of efficient nanofocusing is related to surface plasmon propagation in metallic structures. Metallic tapered rods, commonly used as probes in near field microscopy and spectroscopy, are of a particular interest. They can provide very strong EM field enhancement at the tip due to surface plasmons (SP���s) propagating towards the tip of the tapered metal rod. A large number of studies have been devoted to the manufacturing process of tapered rods or tapered fibers coated by a metal film. On the other hand, structures such as metallic V-grooves or metal wedges can also provide strong electric field enhancements but manufacturing of these structures is still a challenge. It has been shown, however, that the attainable electric field enhancement at the apex in the V-groove is higher than at the tip of a metal tapered rod when the dissipation level in the metal is strong. Metallic V-grooves also have very promising characteristics as plasmonic waveguides. This thesis will present a thorough theoretical and numerical investigation of nanofocusing during plasmon propagation along a metal tapered rod and into a metallic V-groove. Optimal structural parameters including optimal taper angle, taper length and shape of the taper are determined in order to achieve maximum field enhancement factors at the tip of the nanofocusing structure. An analytical investigation of plasmon nanofocusing by metal tapered rods is carried out by means of the geometric optics approximation (GOA), which is also called adiabatic nanofocusing. However, GOA is applicable only for analysing tapered structures with small taper angles and without considering a terminating tip structure in order to neglect reflections. Rigorous numerical methods are employed for analysing non-adiabatic nanofocusing, by tapered rod and V-grooves with larger taper angles and with a rounded tip. These structures cannot be studied by analytical methods due to the presence of reflected waves from the taper section, the tip and also from (artificial) computational boundaries. A new method is introduced to combine the advantages of GOA and rigorous numerical methods in order to reduce significantly the use of computational resources and yet achieve accurate results for the analysis of large tapered structures, within reasonable calculation time. Detailed comparison between GOA and rigorous numerical methods will be carried out in order to find the critical taper angle of the tapered structures at which GOA is still applicable. It will be demonstrated that optimal taper angles, at which maximum field enhancements occur, coincide with the critical angles, at which GOA is still applicable. It will be shown that the applicability of GOA can be substantially expanded to include structures which could be analysed previously by numerical methods only. The influence of the rounded tip, the taper angle and the role of dissipation onto the plasmon field distribution along the tapered rod and near the tip will be analysed analytically and numerically in detail. It will be demonstrated that electric field enhancement factors of up to ~ 2500 within nanoscale regions are predicted. These are sufficient, for instance, to detect single molecules using surface enhanced Raman spectroscopy (SERS) with the tip of a tapered rod, an approach also known as tip enhanced Raman spectroscopy or TERS. The results obtained in this project will be important for applications for which strong local field enhancement factors are crucial for the performance of devices such as near field microscopes or spectroscopy. The optimal design of nanofocusing structures, at which the delivery of electromagnetic energy to the nanometer region is most efficient, will lead to new applications in near field sensors, near field measuring technology, or generation of nanometer sized energy sources. This includes: applications in tip enhanced Raman spectroscopy (TERS); manipulation of nanoparticles and molecules; efficient coupling of optical energy into and out of plasmonic circuits; second harmonic generation in non-linear optics; or delivery of energy to quantum dots, for instance, for quantum computations.

Early stages of growth of YBa2Cu3O7-�� high Tc superconducting films on (001) Y-ZrO2 substrates

The early stages of growth of high quality YBa2Cu 3O7-�� (YBCO) films grown on (001) Y-ZrO2 (YSZ) substrates by pulsed laser deposition have been studied using a combination of atomic force microscopy and transmission electron microscopy. A one unit cell thick YBCO layer and relatively large CuO particles formed in the initial stages. Additional YBCO grew on top of the first layer in the form of one or a few unit cell high c-axis oriented islands about 30 nm in diameter. The rounded islands subsequently coalesced into faceted domains. Elongated Y 2BaCuO5 particles nucleated after the first layer of YBCO. A highly textured BaZrO3 layer formed between the YSZ and the YBCO with a cube-on-cube dominant orientation relationship with respect to the YBCO film.

Effect of substrate surface on electromigration-induced sliding at hetero-interfaces

Electromigration (EM)-induced interfacial sliding between a metal film and Si substrate occurs when (i) only few grains exist across the width of the film and (ii) diffusivity through the interfacial region is significantly greater than diffusivity through the film. Here, the effect of the substrate surface layer on the kinetics of EM-induced interfacial sliding is assessed using Si substrates coated with various thin film interlayers. The kinetics of interfacial sliding, and therefore the EM-driven mass flow rate, strongly depends on the type of the interlayer (and hence the substrate surface composition), such that strongly bonded interfaces with slower interfacial diffusivity produce slower sliding.

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Mechanism on Effect of Surface Plasmons Coupling with InGaN/GaN Quantum Wells: Enhancement and Suppression of Photoluminescence Intensity

The photoluminescence (PL) intensity enhancement and suppression mechanism on surface plasmons (SPs) coupling with InGaN/GaN quantum wells (QWs) have been systematically studied. The SP-QW coupling behaviors in the areas of GaN cap layer coated with silver thin film were compared at different temperatures and excitation powers. It is found that the internal quantum efficiency (IQE) of the light emitting diodes (LEDs) varies with temperature and excitation power, which in turn results in anomalous emission enhancement and suppression tendency related to SP-QW coupling. The observation is explained by the balance between the extraction efficiency of SPs and the IQE of LEDs

Green-light-emitting ZnSe nanowires fabricated via vapor phase growth

Stoichiometric ZnSe nanowires have been synthesized through a vapor phase reaction of zinc and selenium powder on the (100) silicon substrate coated with a gold film of 2 nm in thickness. The microstructures and the chemical compositions of the as-grown nanowires have been investigated by means of electron microscopy, the energy dispersive spectroscopy, and Raman spectroscopy. The results reveal that the as-grown materials consist of ZnSe nanowires with diameters ranging from 5 to 50 nm. Photoluminescence of the sample demonstrates a strong green emission from room temperature down to 10 K. This is attributed to the recombination of electrons from conduction band to the medium deep Au acceptors. (C) 2003 American Institute of Physics.

Molecular Dynamics Simulation of Methane Hydrate Dissociation Process in the Presence of Thermodynamic Inhibitor

The dissociation of methane hydrate in the presence of ethylene glycol (11.45 mol.L-1) at 277.0 K was studied using canonical ensemble (NVT) molecular dynamics simulations. Results show that hydrate dissociation starts from the surface layer of the solid hydrate and then gradually expands to the internal layer. Thus, the solid structure gradually shrinks until it disappears. A distortion of the hydrate lattice structure occurs first and then the hydrate evolves from a fractured frame to a fractional fragment. Finally, water molecules in the hydrate construction exist in the liquid state. The inner dissociating layer is, additionally, coated by a liquid film formed from outer dissociated water molecules outside. This film inhibits the mass transfer performance of the inner molecules during the hydrate dissociation process.

Contrastive study of two SESAMs for passive mode-locking in Nd:YVC>4 laser with low pump power

Two semiconductor saturable absorber mirrors (SESAMs), of which one is coated with 50% reflection film on the top and the other is not, were contrastively studied in passively mode-locked solid-state lasers which were pumped by low output power laser diode (LD). Experiments have shown that reducing the modulation depth of SESAM by coating partial reflection film, whose reflectivity is higher than that between SESAM and air interface, is an effective method to get continuous wave (CW) mode-locking instead of Q-switched mode-locking (QML) in low power pumped solid-state lasers. A simple Nd:YVO4 laser pumped by low power LD, in which no water-cooling system was used, could obtain CW mode-locking by the 50% reflector coated SESAM with average output power of ~ 20 mW

Electrochemical study of ion channel behavior in incorporated poly L-glutamate bilayer lipid membranes

The lipid layer membranes were fabricated on the glassy carbon electrode (GC) and demonstrated to be bilayer lipid membranes by impedance spectroscopy. The formation of incorporated poly L-glutamate bilayer lipid membrane was achieved. The ion channel behavior of the incorporated poly L-glutamate membrane was determined. When the stimulus calcium cations were added into the electrolyte, the ion channel was opened immediately and exhibited distinct channel current. Otherwise, the ion channel was closed. The cyclic voltammogram at the GC electrode coated with incorporated poly L-glutamate DMPC film response to calcium ion is very fast compared with that at the GC electrode coated only with DMPC film. Ion channel current is not dependent on the time but on the concentration of calcium. The mechanism of the ion channel formation was investigated.

A study of the electrochemical characteristics of the Co(salen)-Nafion modified electrode

A Co(salen)-Nafion modified electrode was prepared by immersing a glassy carbon electrode coated with the Nafion film into the aqueous solution with saturated Co(salen), The modified electrode showed a stable electrochemical reaction of Co(salen) at about 0 V(vs, SCE), The result of XPS indicated the valence of cobalt in Co(salen) changes from +2 before to +3 after Co(salen) enters the Nafion film, It is due to forming axis coordination of cobalt with sulfonic group in Nafion film, It was found that the mode of electron transfer in Co(salen)-Nafion modified electrode was controlled by physical diffusion and electron hopping, It was also found that the modified electrode could catalyze the reduction of O-2 to H2O2. The real catalyst may be the adduct of Co-I(salen) and O-2.

FLOW-INJECTION ANALYSIS OF GLUCOSE AT AN AMPEROMETRIC GLUCOSE SENSOR-BASED ON ELECTROCHEMICAL CODEPOSITION OF PALLADIUM AND GLUCOSE-OXIDASE ON A GLASSY-CARBON ELECTRODE

A glassy carbon electrode (GCE) modified with palladium provides excellent electrocatalytic oxidation of hydrogen peroxide. When the electrolyte contains palladium chloride and glucose oxidase, the GCE can be modified by electrochemical codeposition at a given potential. The resulting modified surface was coated with a thin film of Nation to form a glucose sensor. Such a glucose sensor was successfully used in the flow-injection analysis of glucose with high stability and anti-poisoning ability. It gave a detection limit of 1 X 10(-7) M injected glucose, with a linear concentration range of 0.001-8 mM. There is no obvious interference from substances such as ascorbate and saccharides.

An intelligence ink for photocatalytic films

An ink is described which, when printed or coated onto a photocatalyst film, changes colour irreversibly and rapidly upon UV activation of the photocatalyst film and at a rate commensurate with its activity.

Lorentz TEM investigation of the switching behavior in artificial spin ice building blocks

Frustration ��� the inability to simultaneously satisfy all interactions ��� occurs in a wide range of systems including neural networks, water ice and magnetic systems. An example of the latter is the so called spin-ice in pyrochlore materials [1] which have attracted a lot of interest not least due to the emergence of magnetic monopole defects when the ���ice rules��� governing the local ordering breaks down [2]. However it is not possible to directly measure the frustrated property ��� the direction of the magnetic moments ��� in such spin ice systems with current experimental techniques. This problem can be solved by instead studying artificial spin-ice systems where the molecular magnetic moments are replaced by nanoscale ferromagnetic islands [3-8]. Two different arrangements of the ferromagnetic islands have been shown to exhibit spin ice behaviour: a square lattice maintaining four moments at each vertex [3,8] and the Kagome lattice which has only three moments per vertex but equivalent interactions between them [4-7]. Magnetic monopole defects have been observed in both types of lattices [7-8]. One of the challenges when studying these artificial spin-ice systems is that it is difficult to arrive at the fully demagnetised ground-state [6-8]. <br/>Here we present a study of the switching behaviour of building blocks of the Kagome lattice influenced by the termination of the lattice. Ferromagnetic islands of nominal size 1000��nm by 100��nm were fabricated in five island blocks using electron-beam lithography and lift-off techniques of evaporated 18��nm Permalloy (Ni80Fe20) films. Each block consists of a central island with four arms terminated by a different number and placement of ���injection pads���, see Figure 1. The islands are single domain and magnetised along their long axis. The structures were grown on a 50��nm thick electron transparent silicon nitride membrane to allow TEM observation, which was back-coated with a 5��nm film of Au to prevent charge build-up during the TEM experiments.<br/>To study the switching behaviour the sample was subjected to a magnetic field strong enough to magnetise all the blocks in one direction, see Figure 1. Each block obeys the Kagome lattice ���ice-rules��� of ���2-in, 1-out��� or ���1-in, 2-out��� in this fully magnetised state. Fresnel mode Lorentz TEM images of the sample were then recorded as a magnetic field of increasing magnitude was applied in the opposite direction. While the Fresnel mode is normally used to image magnetic domain structures [9] for these types of samples it is possible to deduce the direction of the magnetisation from the Lorentz contrast [5]. All images were recorded at the same over-focus judged to give good Lorentz contrast.<br/>The magnetisation was found to switch at different magnitudes of the applied field for nominally identical blocks. However, trends could still be identified: all the blocks with any injection pads, regardless of placement and number, switched the direction of the magnetisation of their central island at significantly smaller magnitudes of the applied magnetic field than the blocks without injection pads. It can therefore be concluded that the addition of an injection pad lowers the energy barrier to switching the connected island, acting as a nucleation site for monopole defects. In these five island blocks the defects immediately propagate through to the other side, but in a larger lattice the monopoles could potentially become trapped at a vertex and observed [10].<br/>References<br/><br/>[1] M J Harris et al, Phys Rev Lett 79 (1997) p.2554.<br/>[2] C Castelnovo, R Moessner and S L Sondhi, Nature 451 (2008) p. 42.<br/>[3] R F Wang et al, Nature 439 (2006) 303.<br/>[4] M Tanaka et al, Phys Rev B 73 (2006) 052411.<br/>[5] Y Qi, T Brintlinger and J Cumings, Phys Rev B 77 (2008) 094418.<br/>[6] E Mengotti et al, Phys Rev B 78 (2008) 144402.<br/>[7] S Ladak et al, Nature Phys 6 (2010) 359.<br/>[8] C Phatak et al, Phys Rev B 83 (2011) 174431.<br/>[9] J N Chapman, J Phys D 17 (1984) 623.<br/>[10] The authors gratefully acknowledge funding from the EPSRC under grant number EP/D063329/1.