11,791 research outputs found
Critical current oscillations in strong ferromagnetic pi junctions
No full textWe report magnetic and electrical measurements of Nb Josephson junctions with strongly ferromagnetic barriers of Co, Ni, and Ni80Fe20 (Py). All these materials show multiple oscillations of critical current with a barrier thickness implying repeated 0-pi phase transitions in the superconducting order parameter. We show, in particular, that the Co barrier devices can be accurately modeled using existing clean limit theories and that, despite the high exchange energy (309 meV), the large IcRN value in the pi state means Co barriers are ideally suited to the practical development of superconducting pi-shift devices.</p
Fabrication and characterization of sub-micron thin film intrinsic Josephson junction arrays
No full textGiant triplet proximity effect in superconducting pseudo spin valves with engineered anisotropy
No full textThe proximity coupling of a thin superconducting layer and an inhomogeneous ferromagnet can lead to a significant reduction of the critical temperature due to the generation of spin-polarized triplet Cooper pairs. We report critical temperature measurements of Co/Cu/NiFe(Py)/Cu/Nb superconducting pseudo spin valves (PSVs) in which the magnetization of the soft layer (Py) can be independently rotated in-plane with a magnetic field to create an angle (θ) between it and the magnetization of Co. Here we observe results consistent with spin-triplet theory and demonstrate large changes in ΔTC up to -120 mK as the Py layer is rotated from 0° (Co and Py are parallel) to 90° (Co and Py are orthogonal), which offers the potential for active control of the superconducting state. The key to this achievement is engineered magnetic anisotropy in Py, which enables well-defined control over the magnetization configuration of the PSV. © 2014 American Physical Society.The work was funded by the Royal Society, the Leverhulme Trust through an International Network Grant and the European Research Council (AIG “Superspin”).Peer Reviewe
Study on the Mg-Li-Zn ternary alloy system with improved mechanical properties, good degradation performance and different responses to cells
No full textNovel Mg-(3.5, 6.5wt%)Li-(0.5, 2, 4wt%)Zn ternary alloys were developed as new kinds of biodegradable metallic materials with potential for stent application. Their mechanical properties, degradation behavior, cytocompatibility and hemocompatibility were studied. These potential biomaterials showed higher ultimate tensile strength than previously reported binary Mg-Li alloys and ternary Mg-Li-X (X=Al, Y, Ce, Sc, Mn and Ag) alloys. Among the alloys studied, the Mg-3.5Li-2Zn and Mg-6.5Li-2Zn alloys exhibited comparable corrosion resistance in Hank's solution to pure magnesium and better corrosion resistance in a cell culture medium than pure magnesium. Corrosion products observed on the corroded surface were composed of Mg(OH)2, MgCO3 and Ca-free Mg/P inorganics and Ca/P inorganics. In vitro cytotoxicity assay revealed different behaviors of Human Umbilical Vein Endothelial Cells (HUVECs) and Human Aorta Vascular Smooth Muscle Cells (VSMCs) to material extracts. HUVECs showed increasing nitric oxide (NO) release and tolerable toxicity, whereas VSMCs exhibited limited decreasing viability with time. Platelet adhesion, hemolysis and coagulation tests of these Mg-Li-Zn alloys showed different degrees of activation behavior, in which the hemolysis of the Mg-3.5Li-2Zn alloy was lower than 5%. These results indicated the potential of the Mg-Li-Zn alloys as good candidate materials for cardiovascular stent applications. Statement of significance: Mg-Li alloys are promising as absorbable metallic biomaterials, which however have not received significant attention since the low strength, controversial corrosion performance and the doubts in Li toxicity. The Mg-Li-Zn alloy in the present study revealed much improved mechanical properties higher than most reported binary Mg-Li and ternary Mg-Li-X alloys, with superior corrosion resistance in cell culture media. Surprisingly, the addition of Li and Zn showed increased nitric oxide release. The present study indicates good potential of Mg-Li-Zn alloy as absorbable cardiovascular stent material.Accepted Author ManuscriptBiomaterials & Tissue Biomechanic
Universal time relaxation behavior of the exchange bias in ferromagnetic/antiferromagnetic bilayers
No full textAuthor name used in this publication: C. W. Leung2005-2006 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishedVoR allowe
“Mg” Bone: Ink extrusion-based additive manufacturing of Mg implants
No full textThe treatment of largy bone defect remains challenging in clinics. All the clinically available bone grafts have their own limitations and are not ideal for the treatment. Therefore, developing a new generation of suitable bone substitutes is urgently needed. In the recent years, porous magnesium (Mg) has been extensively studied for orthopedic applications owing to its biodegradability, favorable mechanical properties, and osteopromotive ability. The recent advances in additive manufacturing (AM) provide unprecedented opportunities to design and fabricate porous Mg scaffolds with interconnected porous structures that are favorable for the adhesion and proliferation of bone cells. However, powder bed fusion AM, which is the most commonly used AM technique for fabricating metal structures, has encountered many difficulties in manufacturing Mg due to safety concerns, excessive oxidation, and undesirable compositional variation due to the low boiling temperature of Mg. To alleviate these difficulties, alternative AM techniques that can create highly porous structures at room temperature are highly sought after. The aim of this research was to develop a room-temperature AM technique for manufacturing porous Mg and to characterize the fabricated Mg-based scaffolds in different aspects relevant to their potential applications as bone implants.In this thesis work, we, for the first time, successfully employed extrusion-based 3D printing techniques to fabricate biodegradable porous Mg and Mg-based scaffolds for application in orthopedics. We started with the optimization of the formulated binder system, the printing process, and the subsequent liquid-phase sintering process for the AM of Mg and Mg-based scaffolds. On this basis, a series of Mg and Mg-based porous scaffolds, including Mg alloy and Mg matrix composite scaffolds were successfully fabricated. Then, we conducted comprehensive studies on the microstructure, geometrical characteristics, in vitro biodegradation behavior, mechanical properties, and the in vitro biodegradation and the responses of preosteoblast MC3T3-E1 cells to the fabricated scaffolds to evaluate the ability of the fabricated scaffolds to satisfy the requirements of ideal bone-substituting biomaterials. By modifying the alloy composition and adding bioceramic components, the properties of the Mg scaffolds required were significantly improved as compared to those of the pure Mg specimens. The fabricated Mg-matrix composite scaffolds were shown to be the most promising materials to be further developed for bone substitution. Surface modification could also contribute to bringing the fabricated Mg scaffolds closer to meeting the requirements. Therefore, with proper material design and surface modification, the Mg-based scaffolds fabricated using extrusion-based 3D printing technique constitute a new category of porous Mg-based biomaterials that hold great promise for application as bone substitutes
Extrusion-based additive manufacturing of Mg-Zn alloy scaffolds
No full textPorous biodegradable Mg and its alloys are considered to have a great potential to serve as ideal bone substitutes. The recent progress in additive manufacturing (AM) has prompted its application to fabricate Mg scaffolds with geometrically ordered porous structures. Extrusion-based AM, followed by debinding and sintering, has been recently demonstrated as a powerful approach to fabricating such Mg scaffolds, which can avoid some crucial problems encountered when applying powder bed fusion AM techniques. However, such pure Mg scaffolds exhibit a too high rate of in vitro biodegradation. In the present research, alloying through a pre-alloyed Mg-Zn powder was ultilized to enhance the corrosion resistance and mechanical properties of AM geometrically ordered Mg-Zn scaffolds simultaneously. The in vitro biodegradation behavior, mechanical properties, and electrochemical response of the fabricated Mg-Zn scaffolds were evaluated. Moreover, the response of preosteoblasts to these scaffolds was systematically evaluated and compared with their response to pure Mg scaffolds. The Mg-Zn scaffolds with a porosity of 50.3% and strut density of 93.1% were composed of the Mg matrix and MgZn2 second phase particles. The in vitro biodegradation rate of the Mg-Zn scaffolds decreased by 81% at day 1, as compared to pure Mg scaffolds. Over 28 days of static immersion in modified simulated body fluid, the corrosion rate of the Mg-Zn scaffolds decreased from 2.3 ± 0.9 mm/y to 0.7 ± 0.1 mm/y. The yield strength and Young's modulus of the Mg-Zn scaffolds were about 3 times as high as those of pure Mg scaffolds and remained within the range of those of trabecular bone throughout the biodegradation tests. Indirect culture of MC3T3-E1 preosteoblasts in Mg-Zn extracts indicated favorable cytocompatibility. In direct cell culture, some cells could spread and form filopodia on the surface of the Mg-Zn scaffolds. Overall, this study demonstrates the great potential of the extrusion-based AM Mg-Zn scaffolds to be further developed as biodegradable bone-substituting biomaterials.Biomaterials & Tissue BiomechanicsTeam Peyman TaheriTeam Arjan Mo
Ca-modified Al–Mg–Sc alloy with high strength at elevated temperatures due to a hierarchical microstructure
No full textAl-Mg alloys are normally prone to lose part of their yield and tensile strength at high temperatures due to insufficient thermal stability of the microstructure. Here, we present a Ca-modified Al–Mg–Sc alloy demonstrating high strength at elevated temperatures. The microstructure contains Al4Ca phases distributed as a network along the grain boundary and Al3(Sc,Zr) nano-particles dispersed within the grains. The microstructure evolution and age-hardening analysis indicate that the combination of an Al4Ca network and Sc-rich nano-particles leads to excellent thermal stability even upon aging at 300 °C. The tensile strength of the alloy for temperatures up to 250 °C is significantly improved by an aging treatment and is comparable with the commercial heat-resistant aluminum alloys, i.e., A356 and A319. At a high temperature of 300 °C, the tensile strength is superior to the above-mentioned commercial alloys, even more so when expressed as the specific strength due to the low density of Ca-modified Al–Mg–Sc alloy. The excellent high-temperature strength results from a synergistic effect of solid solution strengthening, grain boundary strengthening and nanoparticle order strengthening.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Novel Aerospace Material
Influence of surface pretreatment on phosphate conversion coating on AZ91 Mg alloy
No full textSurface pretreatment is generally applied before application of protective coatings on Mg alloys, which influences surface microstructure and electrochemical activity of the substrate and has an effect on the coating properties. The effect of various pretreatment processes (sand-blasting, grinding and polishing) on the microstructure and corrosion protection performance of phosphate conversion coating (PCC) on AZ91D Mg alloy was investigated in the present study. Sand-blasting cleaning significantly increases the surface roughness and electrochemical activity of the substrate, leading to formation of a porous PCC with inferior corrosion protection performance. In the case of ground/polished Mg alloy, the uniformity and corrosion resistance of the resultant conversion coating are mainly related to the surface roughness. Relatively low surface roughness of the substrate facilitates formation of a corrosion protective PCC.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.(OLD) MSE-
Sub-micron thin film intrinsic Josephson junctions
No full textWe have fabricated sub-micron intrinsic Josephson junctions in thin films of Tl-Ba-Ca-Cu-O using two differing techniques suited to different applications. By using lateral focussed ion-beam milling we have created arrays of intrinsic junctions in c-axis oriented films. Such arrays, with areas as low as 0.25 mum(2), display large hysteresis comparable to that observed in single-crystal intrinsic junctions. By using normal focussed ion-beam milling we have created arrays in mis-aligned films grown on vicinal substrates. In arrays of area less than 0.4 mum(2) we observe Josephson phase diffusion and a suppressed critical current, showing that charging effects may be significant in these junctions
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