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- ItemThe impact of chemical short-range order on the thermophysical properties of medium- and high-entropy alloys(Amsterdam [u.a.] : Elsevier Science, 2024) Andreoli, Angelo F.; Fantin, Andrea; Kasatikov, Sergey; Bacurau, Vinícius P.; Widom, Michael; Gargarella, Piter; Mazzer, Eric M.; Woodcock, Thomas G.; Nielsch, Kornelius; Coury, Francisco G.The unusual behavior observed in the coefficient of thermal expansion and specific heat capacity of CrFeNi, CoCrNi, and CoCrFeNi medium/high-entropy alloys is commonly referred to as the K-state effect. It is shown to be independent of the Curie temperature, as demonstrated by temperature-dependent magnetic moment measurements. CoCrFeNi alloy is chosen for detailed characterization; potential reasons for the K-state effect such as texture, recrystallization, and second-phase precipitation are ruled out. An examination of the electronic structure indicates the formation of a pseudo-gap in the Density of States, which suggests a specific chemical interaction between Ni and Cr atoms upon alloying. Hybrid Monte Carlo/Molecular Dynamic (MC/MD) simulations indicate the presence of non-negligible chemical short-range order (CSRO). Local lattice distortions are shown to be negligible, although deviations around Cr and Ni elements from those expected in a fully disordered structure are experimentally observed by X-ray absorption spectroscopy. The determined bonding distances are in good agreement with MC/MD calculations. A mechanism is proposed to explain the anomalies and calorimetric experiments and their results are used to validate the mechanism.
- ItemInfluence of Lattice Mismatch on Structural and Functional Properties of Epitaxial Ba0.7Sr0.3TiO3 Thin Films(Basel : MDPI, 2023) Wawra, Jonas; Nielsch, Kornelius; Hühne, RubenSubstrate-induced strains can significantly influence the structural properties of epitaxial thin films. In ferroelectrics, this might lead to significant changes in the functional properties due to the strong electromechanical coupling in those materials. To study this in more detail, epitaxial Ba0.7Sr0.3TiO3 films, which have a perovskite structure and a structural phase transition close to room temperature, were grown with different thicknesses on REScO3 (RE–rare earth element) substrates having a smaller lattice mismatch compared to SrTiO3. A fully strained SrRuO3 bottom electrode and Pt top contacts were used to achieve a capacitor-like architecture. Different X-ray diffraction techniques were applied to study the microstructure of the films. Epitaxial films with a higher crystalline quality were obtained on scandates in comparison to SrTiO3, whereas the strain state of the functional layer was strongly dependent on the chosen substrate and the thickness. Differences in permittivity and a non-linear polarization behavior were observed at higher temperatures, suggesting that ferroelectricity is supressed under tensile strain conditions in contrast to compressive strain for our measurement configuration, while a similar reentrant relaxor-like behavior was found in all studied layers below 0°C.
- 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.
- ItemOn-Chip Micro Temperature Controllers Based on Freestanding Thermoelectric Nano Films for Low-Power Electronics(Berlin ; Heidelberg [u.a.] : Springer, 2024) Jin, Qun; Guo, Tianxiao; Pérez, Nicolás; Yang, Nianjun; Jiang, Xin; Nielsch, Kornelius; Reith, HeikoDense and flat freestanding Bi2Te3-based thermoelectric nano films were successfully fabricated by sputtering technology using a newly developed nano graphene oxide membrane as a substrate. On-chip micro temperature controllers were integrated using conventional micro-electromechanical system technology, to achieve energy-efficient temperature control for low-power electronics. The tunable equivalent thermal resistance enables an ultrahigh temperature control capability of 100 K mW−1 and an ultra-fast cooling rate exceeding 2000 K s−1, as well as excellent reliability of up to 1 million cycles.
- ItemSolving the puzzle of hierarchical martensitic microstructures in NiTi by (111)-oriented epitaxial films(Amsterdam : Elsevier, 2023) Lünser, Klara; Undisz, Andreas; Wagner, Martin F.-X.; Nielsch, Kornelius; Fähler, SebastianThe martensitic microstructure decides on the functional properties of shape memory alloys. However, for the most commonly used alloy, NiTi, it is still unclear how its microstructure is built up because the analysis is hampered by grain boundaries of polycrystalline samples. Here, we eliminate grain boundaries by using epitaxially grown films in (111)B2 orientation. By combining scale-bridging microscopy with integral inverse pole figures, we solve the puzzle of the hierarchical martensitic microstructure. We identify two martensite clusters as building blocks and three kinds of twin boundaries. Nesting them at different length scales explains why habit plane variants with ⟨011⟩B19' twin boundaries and {942} habit planes are dominant; but also some incompatible interfaces occur. Though the observed hierarchical microstructure agrees with the phenomenological theory of martensite, the transformation path decides which microstructure forms. The combination of local and global measurements with theory allows solving the scale bridging 3D puzzle of the martensitic microstructure in NiTi exemplarily for epitaxial films.
- ItemA multifunctional highway system incorporating superconductor levitated vehicles and liquefied hydrogen(Melville, NY : AIP Publishing, 2023) Vakaliuk, O.; Song, Shaowei; Floegel-Delor, U.; Werfel, F.; Nielsch, Kornelius; Ren, ZhifengMagnetic levitation for the transport of people and goods using bulk superconductors and electrical power transmission using superconductors have both been demonstrated, but neither has been developed for daily use due to technological deficiencies and high costs. We envision combining the transport of people and goods and energy transmission and storage in a single system. Such a system, built on existing highway infrastructure, incorporates a superconductor guideway, allowing for simultaneous levitation of vehicles with magnetized undercarriages for rapid transport without schedule limitations and lossless transmission and storage of electricity. Incorporating liquefied hydrogen additionally allows for simultaneous cooling of the superconductor guideway and sustainable energy transport and storage. Here, we report the successful demonstration of the primary technical prerequisite, levitating a magnet above a superconductor guideway.
- 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.
- ItemDensity-Dependence of Surface Transport in Tellurium-Enriched Nanograined Bulk Bi2Te3(Weinheim : Wiley-VCH, 2023) Izadi, Sepideh; Bhattacharya, Ahana; Salloum, Sarah; Han, Jeong Woo; Schnatmann, Lauritz; Wolff, Ulrike; Perez, Nicolas; Bendt, Georg; Ennen, Inga; Hütten, Andreas; Nielsch, Kornelius; Schulz, Stephan; Mittendorff, Martin; Schierning, GabiThree-dimensional topological insulators (3D TI) exhibit conventional parabolic bulk bands and protected Dirac surface states. A thorough investigation of the different transport channels provided by the bulk and surface carriers using macroscopic samples may provide a path toward accessing superior surface transport properties. Bi2Te3 materials make promising 3D TI models; however, due to their complicated defect chemistry, these materials have a high number of charge carriers in the bulk that dominate the transport, even as nanograined structures. To partially control the bulk charge carrier density, herein the synthesis of Te-enriched Bi2Te3 nanoparticles is reported. The resulting nanoparticles are compacted into nanograined pellets of varying porosity to tailor the surface-to-volume ratio, thereby emphasizing the surface transport channels. The nanograined pellets are characterized by a combination of resistivity, Hall- and magneto-conductance measurements together with (THz) time-domain reflectivity measurements. Using the Hikami-Larkin-Nagaoka (HLN) model, a characteristic coherence length of ≈200 nm is reported that is considerably larger than the diameter of the nanograins. The different contributions from the bulk and surface carriers are disentangled by THz spectroscopy, thus emphasizing the dominant role of the surface carriers. The results strongly suggest that the surface transport carriers have overcome the hindrance imposed by nanoparticle boundaries.