Browsing by Author "Panagiotopoulos, Nikolaos T."

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    Advanced ultra-light multifunctional metallic-glass wave springs
    (Elsevier, 2020-05-01) Panagiotopoulos, Nikolaos T.; Georgarakis, Konstantinos; Jorge Jnr, Alberto Moreira; Aljerf, M.; Botta, Walter José; Greer, Lindsay; Yavari, Ahmad
    We show that, using thermo-elastic processing, metallic-glass foils can be shaped, without being embrittled, into linear and annular wave springs. These springs exhibit an undulatory behaviour, unique to metallic-glass foils, in which under compression the number of arcs in the spring increases, increasing the load-bearing capacity and the spring constant. We evaluate the performance limits of the metallic-glass wave springs, and consider how the undulatory behaviour can be exploited. The metallic-glass springs can operate over the same load-ranges as commercially available crystalline wave springs, but have material volumes (and therefore weights) that are one to two orders of magnitude less. Their energy storage per unit material volume is as high as 2600 kJ m−3. We suggest that the undulatory behaviour is important in rendering the springs fail-safe in case of overload. We discuss the range of applicability of thermo-elastic processing, the likely working limit of metallic-glass wave springs, and the potential for application of metallic-glass springs in MEMS devices.
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    The effect of Ni or Co additions on the structure of Zr60Cu30Al10 bulk metallic glass revealed by high-energy synchrotron radiation
    (Elsevier, 2022-04-13) Stiehler, Martin E.; Panagiotopoulos, Nikolaos T.; Keeble, Dean S.; Ivanov, Yurii P.; Menelaou, Melita; Jolly, Mark R.; Greer, A.Lindsay; Georgarakis, Konstantinos
    The effect of substituting Cu by elemental additions of Ni or Co on the atomic structure of the Zr60Cu30Al10 ternary bulk metallic glass (BMG) is studied using high-energy synchrotron radiation X-ray diffraction. Analyses of the structural features in reciprocal and real space using the structure factors S(Q) and pair-distribution functions (PDF) point to an increase in the structural disorder for the Ni- or Co-bearing quaternary alloys. This is consistent with the “confusion principle” since upon alloying the initially nearly identical atomic sizes of Cu, Ni and Co diversify due to local electronic interactions. In real space, the disordering is manifested by a reduced deviation from the average particle density visible in the nearest-neighbour (NN) atomic shell structure over the complete short- and medium-range order region. Despite their similar atomic size, enthalpies of mixing with the main alloy elements and apparent disordering of the structure, the additions of Ni or Co have different effects on thermal stability of the ternary “mother” alloy.
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    In-situ TEM study of the crystallization sequence in a gold-based metallic glass
    (Elsevier, 2020-06-18) Ivanov, Yu P.; Meylan, Carolina M.; Panagiotopoulos, Nikolaos T.; Georgarakis, Konstantinos; Greer, Lindsay A.
    The composition Au49Ag5.5Pd2.3Cu26.9Si16.3 (at.%) is of interest as the basis for the development of gold-based bulk metallic glasses for application in jewellery. In-situ heating in transmission electron microscopy (TEM) and differential scanning calorimetry (DSC, both conventional and fast) are used to obtain a comprehensive characterization of the decomposition on heating a melt-spun glass of this composition. Linking TEM with DSC over a range of heating rates 0.083‒2000 K s‒1, allows the sample temperature in the TEM heating stage to be calibrated. On heating up to melting, the glass decomposes in up to four stages: (1) complete transformation to single-phase nanocrystalline (Au,Cu)7Si; (2) grain growth of this phase; (3) precipitation of (Pd,Ag)Si, reducing the supersaturation of silicon in the (Au,Cu)7Si matrix; (4) with the precipitate phase remaining stable, decomposition of the matrix to a mixture of (Au,Ag)8Cu2, AuCu and Cu3Au phases. At all stages, grain diameters remain sub-micrometre; some of the stable nanocrystalline microstructures may themselves be of interest for applications. The characterization of the decomposition can assist in the optimization of the glass composition to improve tarnish-resistance, while retaining adequate glass-forming ability, formability in thermoplastic processing, and resistance to crystallization. For materials in general, the close correlation of in-situ TEM and DSC results should find wide use in characterizing complex transformation sequences.
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    Nanoporous silver for electrocatalysis application in alkaline fuel cells
    (Elsevier, 2016-09-10) Barsuk, Daria; Zadick, Anicet; Chatenet, Marian; Georgarakis, Konstantinos; Panagiotopoulos, Nikolaos T.; Champion, Yannick; Moreira Jorge, Alberto
    Self-supporting porous silver foils with an average pore size < 100 nm were produced from a crystalline silver-based ternary alloy as a precursor by removing second phases present in the silver matrix. The final Ag-based porous foil shows good mechanical properties when comparing to its previous amorphous analogues. Its activity for direct electrochemical oxidation of ammonia-borane (AB), a fuel of interest for direct liquid fuel cells, has been investigated in alkaline media. The material exhibits promising electrochemical properties in long-term operation; indeed, material composition and nanostructure remain similar after 15,000 cyclic-voltammetries between − 0.3 and 0.5 V vs. RHE in a 0.1 M NaOH + 5 mM AB solution thermostated at 25 °C. Nanoporous materials, and in particular nanoporous silver, can therefore represent a relevant choice as anode in direct ammonia-borane fuel cell.
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    On a new electromechanical switch using the reversible wavy elastic response of metallic glass ribbons
    (Elsevier, 2017-08-16) Yousfi, Mohamed Abdelbasset; Ammari, Cheima; Hajlaoui, Khalil; Panagiotopoulos, Nikolaos T.; Georgarakis, Konstantinos; Tourki, Zoubeir
    For the first time, practical applications as an alarm device and automatic filling of an aquarium using an electromechanical switch manufactured from metallic glass (MG) ribbon is proposed. The elastic response of an initial arc-shaped MG ribbon-based Fe90.65B3.9Cr2.75Si2.7 is studied and exploited. Under the applied load F, the amorphous material exhibits a reversible elastic wavy response. During the elastic deformation and multiplication of harmonic undulations, a perfect linear contact between the waves and support is established. This contact position is the same for the pair waves, and can be employed to ensure the passage of an electric current, since the ribbon is Fe-based. The reversible elastic wavy response of MG ribbon can be used as an electromechanical switch. The lifespan of the ribbon used as a switch is also considered.
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    On the undulatory behaviour of metallic glass foils: a novel spring-type behaviour
    (Cranfield University, 2022-11-08) Shahin Elzoubi, O.; Panagiotopoulos, Nikolaos T.; Stiehler, Martin E.; Salonitis, Konstantinos; Georgarakis, Konstantinos
    The undulatory behaviour is a unique type of mechanical response that was recently observed for metallic glass foils in geometric confinement. It is manifested when normal load is applied on the top of an arc-shaped thin foil of metallic glass; the foil then deforms elastically and its shape changes by progressively increasing the number of formed sinusoidal arcs. This behaviour results from a combination of successive elastic bending and buckling events and can be utilized for developing novel types of non-linear springs. In this work, the undulatory behaviour of a Ni-Fe-Si-B-Mo metallic glass foil has been systematically studied and compared with that of the previously reported Fe-Cr-Si-B foil. The results indicate that the alloy composition and the foil thickness can significantly affect the load required for the formation of the harmonic undulations. The initial geometry of the formed sinusoidal arc including its amplitude and boundary length, can also be used to tune the load and displacement response of the foils. Upon unloading, the foil returns to its initial shape, as long as the loading remains in the elastic deformation range of the metallic glass. The findings suggest that the undulatory behaviour of thin metallic glass foils can be potentially exploited for a wide range of engineering applications including micro-springs, sensors, actuators, and shock absorbers.