14 results
Search Results
Now showing 1 - 10 of 14
- ItemThe Role of Al2O3 ALD Coating on Sn-Based Intermetallic Anodes for Rate Capability and Long-Term Cycling in Lithium-Ion Batteries(Weinheim : Wiley-VCH, 2022) Soltani, Niloofar; Abbas, Syed Muhammad; Hantusch, Martin; Lehmann, Sebastian; Nielsch, Kornelius; Bahrami, Amin; Mikhailova, DariaThe electrochemical performances of CoSn2 and Ni3Sn4 as potential anode materials in lithium-ion batteries (LIBs) are investigated using varying thicknesses of an alumina layer deposited by the atomic layer deposition (ALD) technique. Rate capability results showed that at high current densities, Al2O3-coated CoSn2 and Ni3Sn4 electrodes after 10-ALD cycles outperformed uncoated materials. The charge capacities of coated CoSn2 and Ni3Sn4 electrodes are 571 and 134 mAh g−1, respectively, at a high current density of 5 A g−1, while the capacities of uncoated electrodes are 363 and 11 mAh g−1. When the current density is reduced to 1 A g−1, however, the cycling performances of Al2O3-coated CoSn2 and Ni3Sn4 electrodes fade faster after almost 40 cycles than uncoated electrodes. The explanation is found in the composition of the solid-electrolyte interface (SEI), which strongly depends on the current rate. Thus, X-ray photoelectron spectroscopy analysis of SEI layers on coated samples cycles at a low current density of 0.1 Ag−1, revealed organic carbonates as major products, which probably have a low ionic conductivity. In contrast, the SEI of coated materials cycled at 5 Ag−1 consists mostly of mixed inorganic/organic fluorine-rich Al-F and C-F species facilitating a higher ionic transport, which improves electrochemical performance.
- ItemTransparent Power-Generating Windows Based on Solar-Thermal-Electric Conversion(Weinheim : Wiley-VCH, 2021) Zhang, Qihao; Huang, Aibin; Ai, Xin; Liao, Jincheng; Song, Qingfeng; Reith, Heiko; Cao, Xun; Fang, Yueping; Schierning, Gabi; Nielsch, Kornelius; Bai, Shengqiang; Chen, LidongIntegrating transparent solar-harvesting systems into windows can provide renewable on-site energy supply without altering building aesthetics or imposing further design constraints. Transparent photovoltaics have shown great potential, but the increased transparency comes at the expense of reduced power-conversion efficiency. Here, a new technology that overcomes this limitation by combining solar-thermal-electric conversion with a material's wavelength-selective absorption is presented. A wavelength-selective film consisting of Cs0.33WO3 and resin facilitates high visible-light transmittance (up to 88%) and outstanding ultraviolet and infrared absorbance, thereby converting absorbed light into heat without sacrificing transparency. A prototype that couples the film with thermoelectric power generation produces an extraordinary output voltage of ≈4 V within an area of 0.01 m2 exposed to sunshine. Further optimization design and experimental verification demonstrate high conversion efficiency comparable to state-of-the-art transparent photovoltaics, enriching the library of on-site energy-saving and transparent power generation.
- ItemCurrent State-of-the-Art in the Interface/Surface Modification of Thermoelectric Materials(Weinheim : Wiley-VCH, 2021) He, Shiyang; Lehmann, Sebastian; Bahrami, Amin; Nielsch, KorneliusThermoelectric (TE) materials are prominent candidates for energy converting applications due to their excellent performance and reliability. Extensive efforts for improving their efficiency in single-/multi-phase composites comprising nano/micro-scale second phases are being made. The artificial decoration of second phases into the thermoelectric matrix in multi-phase composites, which is distinguished from the second-phase precipitation occurring during the thermally equilibrated synthesis of TE materials, can effectively enhance their performance. Theoretically, the interfacial manipulation of phase boundaries can be extended to a wide range of materials. High interface densities decrease thermal conductivity when nano/micro-scale grain boundaries are obtained and certain electronic structure modifications may increase the power factor of TE materials. Based on the distribution of second phases on the interface boundaries, the strategies can be divided into discontinuous and continuous interfacial modifications. The discontinuous interfacial modifications section in this review discusses five parts chosen according to their dispersion forms, including metals, oxides, semiconductors, carbonic compounds, and MXenes. Alternatively, gas- and solution-phase process techniques are adopted for realizing continuous surface changes, like the core–shell structure. This review offers a detailed analysis of the current state-of-the-art in the field, while identifying possibilities and obstacles for improving the performance of TE materials.
- ItemMagnetic Properties of GaAs/NiFe Coaxial Core-Shell Structures(Basel : MDPI, 2022) Monaico, Eduard V.; Morari, Vadim; Kutuzau, Maksim; Ursaki, Veaceslav V.; Nielsch, Kornelius; Tiginyanu, Ion M.Uniform nanogranular NiFe layers with Ni contents of 65%, 80%, and 100% have been electroplated in the potentiostatic deposition mode on both planar substrates and arrays of nanowires prepared by the anodization of GaAs substrates. The fabricated planar and coaxial core-shell ferromagnetic structures have been investigated by means of scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). To determine the perspectives for applications, a comparative analysis of magnetic properties, in terms of the saturation and remanence moment, the squareness ratio, and the coercivity, was performed for structures with different Ni contents.
- ItemHow to grow single-crystalline and epitaxial NiTi films in (100)- and (111)-orientation(Bristol : IOP Publishing, 2023) Lünser, Klara; Undisz, Andreas; Nielsch, Kornelius; Fähler, SebastianUnderstanding the martensitic microstructure in nickel-titanium (NiTi) thin films helps to optimize their properties for applications in microsystems. Epitaxial and single-crystalline films can serve as model systems to understand the microstructure, as well as to exploit the anisotropic mechanical properties of NiTi. Here, we analyze the growth of NiTi on single-crystalline MgO(100) and Al2O3(0001) substrates and optimize film and buffer deposition conditions to achieve epitaxial films in (100)- and (111)-orientation. On MgO(100), we compare the transformation behavior and crystal quality of (100)-oriented NiTi films on different buffer layers. We demonstrate that a vanadium buffer layer helps to decrease the low-angle grain boundary density in the NiTi film, which inhibits undesired growth twins and leads to higher transformation temperatures. On Al2O3(0001), we analyze the orientation of a chromium buffer layer and find that it grows (111)-oriented only in a narrow temperature range around 500 ∘C. By depositing the Cr buffer below the NiTi film, we can prepare (111)-oriented, epitaxial films with transformation temperatures above room temperature. Transmission electron microscopy confirms a martensitic microstructure with Guinier Preston-zone precipitates at room temperature. We identify the deposition conditions to approach the ideal single crystalline state, which is beneficial for the analysis of the martensitic microstructure and anisotropic mechanical properties in different film orientations.
- ItemExchange Bias Effect of Ni@(NiO,Ni(OH)2) Core/Shell Nanowires Synthesized by Electrochemical Deposition in Nanoporous Alumina Membranes(Basel : MDPI, 2023) García, Javier; Gutiérrez, Ruth; González, Ana S.; Jiménez-Ramirez, Ana I.; Álvarez, Yolanda; Vega, Víctor; Reith, Heiko; Leistner, Karin; Luna, Carlos; Nielsch, Kornelius; Prida, Víctor M.Tuning and controlling the magnetic properties of nanomaterials is crucial to implement new and reliable technologies based on magnetic hyperthermia, spintronics, or sensors, among others. Despite variations in the alloy composition as well as the realization of several post material fabrication treatments, magnetic heterostructures as ferromagnetic/antiferromagnetic coupled layers have been widely used to modify or generate unidirectional magnetic anisotropies. In this work, a pure electrochemical approach has been used to fabricate core (FM)/shell (AFM) Ni@(NiO,Ni(OH)2) nanowire arrays, avoiding thermal oxidation procedures incompatible with integrative semiconductor technologies. Besides the morphology and compositional characterization of these core/shell nanowires, their peculiar magnetic properties have been studied by temperature dependent (isothermal) hysteresis loops, thermomagnetic curves and FORC analysis, revealing the existence of two different effects derived from Ni nanowires’ surface oxidation over the magnetic performance of the array. First of all, a magnetic hardening of the nanowires along the parallel direction of the applied magnetic field with respect their long axis (easy magnetization axis) has been found. The increase in coercivity, as an effect of surface oxidation, has been observed to be around 17% (43%) at 300 K (50 K). On the other hand, an increasing exchange bias effect on decreasing temperature has been encountered when field cooling (3T) the oxidized Ni@(NiO,Ni(OH)2) nanowires below 100 K along their parallel lengths.
- ItemEncapsulation of locally welded silver nanowire with water-free ALD-SbOx for flexible thin-film transistors(Melville, NY : American Inst. of Physics, 2022) Yang, Jun; Bahrami, Amin; Ding, Xingwei; Lehmann, Sebastian; Nielsch, KorneliusTransparent conductive electrodes are essential in the application of flexible electronics. In this work, we successfully demonstrated a novel strategy for improving mechanical/electrical properties of indium tin oxide (ITO)-free flexible silver nanowire (Ag NW) thin films. To reduce the contact resistance of Ag NWs, an ethanol-mist was used to weld the cross junction of wires at room temperature. The nano-welded Ag NWs (W-Ag NWs) were then coated with an aluminum-doped ZnO (AZO) solution, which significantly reduce the roughness of the Ag NW thin film. Finally, an ultrathin SbOx thin film of 2 nm was deposited on the film surface using a water-free low-temperature atomic layer deposition technique to protect the W-Ag NW/AZO layer from water or oxygen degradation. The treated Ag NWs have a high transmittance of 87% and a low sheet resistance of about 15 ω/sq, which is comparable with the ITO electrode's property. After 1000 cycles of bending testing, the W-Ag NW/AZO/SbOx film practically retains its initial conductivity. Furthermore, the samples were immersed in a solution with pH values ranging from 3 to 13 for 5 min. When compared to untreated Ag NWs or those coated with AlOx thin films, W-Ag NW/AZO/SbOx had superior electrical stability. The W-Ag NW/AZO/SbOxlayer was integrated as a gate electrode on low-power operating flexible Ti-ZnO thin film transistors (TFTs). The 5% Ti-ZnO TFT has a field-effect mobility of 19.7 cm2 V s-1, an Ion/Ioff ratio of 107, and a subthreshold swing of 147 mV decade-1.
- ItemMagneto-thermal transport indicating enhanced Nernst response in FeCo/IrMn exchange coupled stacks(Melville, NY : American Inst. of Physics, 2022) Martini, Mickey; Reichlova, Helena; Lee, Yejin; Dusíková, Dominika; Zemen, Jan; Nielsch, Kornelius; Thomas, AndyWe present an analysis of magneto-thermal transport data in IrMn/FeCo bilayers based on the Mott relation and report an enhancement of the Nernst response in the vicinity of the blocking temperature. We measure all four transport coefficients of the longitudinal resistivity, anomalous Hall resistivity, Seebeck effect, and anomalous Nernst effect, and we show a deviation arising around the blocking temperature between the measured Nernst coefficient and the one calculated using the Mott rule. We attribute this discrepancy to spin fluctuations at the antiferromagnet/ferromagnet interface near the blocking temperature. The latter is estimated by magnetometry and magneto-transport measurements.
- ItemWater-Free SbOx ALD Process for Coating Bi2Te3 Particle(Basel : MDPI, 2023) Lehmann, Sebastian; Mitzscherling, Fanny; He, Shiyang; Yang, Jun; Hantusch, Martin; Nielsch, Kornelius; Bahrami, AminWe developed a water-free atomic layer deposition (ALD) process to homogeneously deposit SbOx using SbCl5 and Sb-Ethoxide as precursors, and report it here for the first time. The coating is applied on Bi2Te3 particles synthesized via the solvothermal route to enhance the thermoelectric properties (i.e., Seebeck coefficient, thermal and electrical conductivity) via interface engineering. The amorphous character of the coating was shown by the missing reflexes on the X-ray diffractograms (XRD). A shift from the oxidation state +III to +V of the Sb species was observed using X-ray photoelectron spectroscopy (XPS), indicating increased thickness of the SbOx coating layer. Additionally, a peak shift of the Sb 3d5/2 + O 1s peak indicated increased n-type doping of the material. Electrical measurements of spark plasma-sintered bulk samples confirmed the doping effect on the basis of decreased specific resistivity with increasing SbOx layer thickness. The Seebeck coefficient was improved for the coated sample with 500 cycles of SbOx, while the total thermal conductivity was reduced, resulting in enhancement of the zT. The results distinctly show that surface engineering via powder ALD is an effective tool for improving key properties of thermoelectric materials like electrical conductivity and the Seebeck coefficient.
- ItemGrain Boundary Phases in NbFeSb Half-Heusler Alloys: A New Avenue to Tune Transport Properties of Thermoelectric Materials(Weinheim : Wiley-VCH, 2023) Bueno Villoro, Ruben; Zavanelli, Duncan; Jung, Chanwon; Mattlat, Dominique Alexander; Hatami Naderloo, Raana; Pérez, Nicolás; Nielsch, Kornelius; Snyder, Gerald Jeffrey; Scheu, Christina; He, Ran; Zhang, SiyuanMany thermoelectric materials benefit from complex microstructures. Grain boundaries (GBs) in nanocrystalline thermoelectrics cause desirable reduction in the thermal conductivity by scattering phonons, but often lead to unwanted loss in the electrical conductivity by scattering charge carriers. Therefore, modifying GBs to suppress their electrical resistivity plays a pivotal role in the enhancement of thermoelectric performance, zT. In this work, different characteristics of GB phases in Ti-doped NbFeSb half-Heusler compounds are revealed using a combination of scanning transmission electron microscopy and atom probe tomography. The GB phases adopt a hexagonal close-packed lattice, which is structurally distinct from the half-Heusler grains. Enrichment of Fe is found at GBs in Nb0.95Ti0.05FeSb, but accumulation of Ti dopants at GBs in Nb0.80Ti0.20FeSb, correlating to the bad and good electrical conductivity of the respective GBs. Such resistive to conductive GB phase transition opens up new design space to decouple the intertwined electronic and phononic transport in thermoelectric materials.