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- ItemUltrahigh Power Factor in Thermoelectric System Nb0.95M0.05FeSb (M = Hf, Zr, and Ti)(Chichester : John Wiley and Sons Ltd, 2018) Ren, W.; Zhu, H.; Zhu, Q.; Saparamadu, U.; He, R.; Liu, Z.; Mao, J.; Wang, C.; Nielsch, K.; Wang, Z.; Ren, Z.Conversion efficiency and output power are crucial parameters for thermoelectric power generation that highly rely on figure of merit ZT and power factor (PF), respectively. Therefore, the synergistic optimization of electrical and thermal properties is imperative instead of optimizing just ZT by thermal conductivity reduction or just PF by electron transport enhancement. Here, it is demonstrated that Nb0.95Hf0.05FeSb has not only ultrahigh PF over ≈100 µW cm−1 K−2 at room temperature but also the highest ZT in a material system Nb0.95M0.05FeSb (M = Hf, Zr, Ti). It is found that Hf dopant is capable to simultaneously supply carriers for mobility optimization and introduce atomic disorder for reducing lattice thermal conductivity. As a result, Nb0.95Hf0.05FeSb distinguishes itself from other outstanding NbFeSb-based materials in both the PF and ZT. Additionally, a large output power density of ≈21.6 W cm−2 is achieved based on a single-leg device under a temperature difference of ≈560 K, showing the realistic prospect of the ultrahigh PF for power generation.
- ItemPolarization manipulation of surface acoustic waves by metallization patterns on a piezoelectric substrate(Melville, NY : AIP Publishing, 2020) Weser, R.; Darinskii, A.N.; Schmidt, H.Surface acoustic waves (SAWs) with large normal (vertical) surface displacement at the surface are commonly utilized in microfluidic actuators in order to provide the desired momentum transfer to the fluid. We present an alternative concept using a SAW with comparatively small vertical displacement. Such a SAW passes underneath the microfluidic vessel walls with minimum losses but it needs to be converted inside the vessel into surface vibrations with large vertical displacements. The principal operability of the above idea is illustrated by experimental and numerical studies of the polarization conversion of a leaky SAW on 64° rotated Y-cut of lithium niobate owing to the partial metallization of the substrate surface. In particular, it is found that vertical displacements on the metallized surface can be up to 3.5 times higher as compared to their values on the free surface. Results of computations agree reasonably well with measurements carried out with a laser Doppler vibrometer and allow the clarification of some specific features of this polarization conversion by means of spatial frequency analysis. © 2020 Author(s).
- ItemMagnetically induced anisotropy of flux penetration into strong-pinning superconductor/ferromagnet bilayers(Bristol : Institute of Physics Publishing, 2019) Simmendinger, J.; Hanisch, J.; Bihler, M.; Ionescu, A.M.; Weigand, M.; Sieger, M.; Hühne, R.; Rijckaert, H.; Van Driessche, I.; Schütz, G.; Albrecht, J.We studied the impact of soft ferromagnetic permalloy (Py) on the shielding currents in a strong-pinning superconductor - YBa2Cu3O7-δ with Ba2Y(Nb/Ta)O6 nano-precipitates - by means of scanning transmission x-ray microscopy. Typically and in particular when in the thin film limit, superconductor/ferromagnet (SC/FM) bilayers exhibit isotropic properties of the flux line ensemble at all temperatures. However, in elements with small aspect ratio a significant anisotropy in flux penetration is observed. We explain this effect by local in-plane fields arising from anisotropic magnetic stray fields originated by the ferromagnet. This leads to direction-dependent motion of magnetic vortices inside the SC/FM bilayer. Our results demonstrate that small variations of the magnetic properties can have huge impact on the superconductor.
- ItemSlowness curve surface acoustic wave transducers for optimized acoustic streaming(Cambridge : Royal Society of Chemistry, 2020) O'Rorke, R.; Winkler, A.; Collins, D.; Ai, Y.Surface acoustic waves can induce force gradients on the length scales of micro- and nanoparticles, allowing precise manipulation for particle capture, alignment and sorting activities. These waves typically occupy a spatial region much larger than a single particle, resulting in batch manipulation. Circular arc transducers can focus a SAW into a narrow beam on the order of the particle diameter for highly localised, single-particle manipulation by exciting wavelets which propagate to a common focal point. The anisotropic nature of SAW substrates, however, elongates and shifts the focal region. Acousto-microfluidic applications are highly dependent on the morphology of the underlying substrate displacement and, thus, become dependent on the microchannel position relative to the circular arc transducer. This requires either direct measurement or computational modelling of the SAW displacement field. We show that the directly measured elongation and shift in the focal region are recapitulated by an analytical model of beam steering, derived from a simulated slowness curve for 128° Y-cut lithium niobate. We show how the negative effects of beam steering can be negated by adjusting the curvature of arced transducers according to the slowness curve of the substrate, for which we present a simple function for convenient implementation in computational design software. Slowness-curve adjusted transducers do not require direct measurement of the SAW displacement field for microchannel placement and can capture smaller particles within the streaming vortices than can circular arc IDTs.