Intrinsic nano-diffusion-couple for studying high temperature diffusion in multi-component superalloys

We present a new approach for the quantitative study of high-temperature diffusion in compositionally complex superalloys on the nano-scale. As key element, the approach utilizes the γ/γ\u27-microstructure itself as intrinsic nano-diffusion-couple (NDC). By establishing equilibrium at one temperature followed by annealing at a different temperature, well-defined transient states are generated which are studied using STEM-EDXS. We demonstrate this approach for a multi-component superalloy of CMSX-4 type. The temporal evolution of element concentrations is consistently revealed for γ- and γ\u27-forming elements and is compared to diffusion simulations based on DICTRA. Excellent agreement is obtained for Ni, Co, and Cr whereas diffusion of Al and, in particular, Re lacks behind in experiment. Finally, it is demonstrated that transient states can also be captured by in situ TEM using chip-based heating devices. The NDC approach offers great opportunities for diffusion studies in compositionally complex superalloys and might be extended to other two-phase multi-component systems

In Situ Pyrolysis of 3D Printed Building Blocks for Functional Nanoscale Metamaterials

This study presents a novel approach for investigating the shrinkage dynamics of 3D-printed nanoarchitectures during isothermal pyrolysis, utilizing in situ electron microscopy. For the first time, the temporal evolution of 3D structures is tracked continuously until a quasi-stationary state is reached. By subjecting the 3D objects to different temperatures and atmospheric conditions, significant changes in the resulting kinetic parameters and morphological textures of the 3D objects are observed, particularly those possessing varying surface-to-volume ratios. Its results reveal that the effective activation energy required for pyrolysis-induced morphological shrinkage is approximately four times larger under vacuum conditions than in a nitrogen atmosphere (2.6 eV vs. 0.5–0.9 eV, respectively). Additionally, a subtle enrichment of oxygen on the surfaces of the structures for pyrolysis in nitrogen is found through a postmortem electron energy loss spectroscopy study, differentiating the vacuum pyrolysis. These findings are examined in the context of the underlying process parameters, and a mechanistic model is proposed. As a result, understanding and controlling pyrolysis in 3D structures of different geometrical dimensions not only enables precise modification of shrinkage and the creation of tensegrity structures, but also promotes pyrolytic carbon development with custom architectures and properties, especially in the field of carbon micro- and nano-electromechanical systems

High‐Sensitivity Flexible Thermocouple Sensor Arrays Via Printing and Photonic Curing

Despite remarkable advances, high-performance thermoelectric (TE) materials-based thermocouples (TCs) lack mechanical robustness and flexibility. Hence, conventional low Seebeck coefficient metals (Ni, Cu, Fe) wire-junction-based TCs are used for temperature sensor applications. However, not only the sensitivity of the metal-based TC sensor is low, but also it is very difficult to fabricate a pixelated sensor using the conventional approach. In this study, high-performance (SbBi)$_2$(TeSe)$_3$-based printed flexible TE materials are employed to fabricate two types of shape-conformable TC-based temperature sensor arrays with 25 pixels (TSA I and TSA II). Bi$_{0.5}$Sb$_{1.5}$Te$_3$-based p-type (p-BST) printed TE film and copper are used to fabricate TSA I, and the p-type film together with the Bi$_2$Te$_{2.7}$Se$_{0.3}$-based n-type (n-BT) film is used to fabricate TSA II. A high average sensitivity (S$_{exp}$) value of ≈124 µV °C$^{−1}$ for TSA I and ≈319 µV °C$^{−1}$ for TSA II is attained with high linearity

Realizing High Thermoelectric Performance of Bi-Sb-Te-Based Printed Films through Grain Interface Modification by an In Situ-Grown β-Cu2-δSe Phase

It has been a substantial challenge to develop a printed thermoelectric (TE) material with a figure-of-merit ZT > 1. In this work, high ZT p-type Bi0.5Sb1.5Te3-based printable TE materials have been advanced by interface modification of the TE grains with a nonstoichiometric β-Cu2-δSe-based inorganic binder (IB) through a facile printing–sintering process. As a result, a very high TE power factor of ∼17.5 μW cm–1 K–2 for a p-type printed material is attained in the optimized compounds at room temperature (RT). In addition, a high ZT of ∼1.2 at RT and of ∼1.55 at 360 K is realized using thermal conductivity (κ) of a pellet made of the prepared printable material containing 10 wt % of IB. Using the same material for p-type TE legs and silver paste for n-type TE legs, a printed TE generator (print-TEG) of four thermocouples has been fabricated for demonstration. An open-circuit voltage (VOC) of 14 mV and a maximum power output (Pmax) of 1.7 μW are achieved for ΔT = 40 K for the print-TEG

Electrospun carbon nanofibre-assisted patterning of metal oxide nanostructures

This work establishes carbon nanofibre-mediated patterning of metal oxide nanostructures, through the combination of electrospinning and vapor-phase transport growth. Electrospinning of a suitable precursor with subsequent carbonization results in the patterning of catalyst gold nanoparticles embedded within carbon nanofibres. During vapor-phase transport growth, these nanofibres allow preferential growth of one-dimensional metal oxide nanostructures, which grow radially outward from the nanofibril axis, yielding a hairy caterpillar-like morphology. The synthesis of metal oxide caterpillars is demonstrated using zinc oxide, indium oxide, and tin oxide. Source and substrate temperatures play the most crucial role in determining the morphology of the metal oxide caterpillars, whereas the distribution of the nanofibres also has a significant impact on the overall morphology. Introducing the current methodology with near-field electrospinning further facilitates user-defined custom patterning of metal oxide caterpillar-like structures

High Figure‐of‐Merit Telluride‐Based Flexible Thermoelectric Films through Interfacial Modification via Millisecond Photonic‐Curing for Fully Printed Thermoelectric Generators

The thermoelectric generator (TEG) shows great promise for energy harvesting and waste heat recovery applications. Cost barriers for this technology could be overcome by using printing technologies. However, the development of thermoelectric (TE) materials that combine printability, high-efficiency, and mechanical flexibility is a serious challenge. Here, flexible (SbBi)2(TeSe)3-based screen-printed TE films exhibiting record-high figure of merits (ZT) and power factors are reported. A high power factor of 24 µW cm−1 K−2 (ZTmax ≈ 1.45) for a p-type film and a power factor of 10.5 µW cm−1 K−2 (ZTmax ≈ 0.75) for an n-type film are achieved. The TE inks, comprised of p-Bi0.5Sb1.5Te3 (BST)/n-Bi2Te2.7Se0.3 (BT) and a Cu-Se-based inorganic binder (IB), are prepared by a one-pot synthesis process. The TE inks are printed on different substrates and sintered using photonic-curing leading to the formation of a highly conducting β-Cu2−δSe phase that connects “microsolders,” the grains resulting in high-performance. Folded TEGs (f-TEGs) are fabricated using the materials. A half-millimeter thick f-TEG exhibits an open-circuit voltage (VOC) of 203 mV with a maximum power density (pmax) of 5.1 W m−2 at ∆T = 68 K. This result signifies that a few millimeters thick f-TEG could power Internet-of-Things (IoTs) devices converting low-grade heat to electricity

Inkjet‐Printed Tungsten Oxide Memristor Displaying Non‐Volatile Memory and Neuromorphic Properties

Printed electronics including large-area sensing, wearables, and bioelectronic systems are often limited to simple circuits and hence it remains a major challenge to efficiently store data and perform computational tasks. Memristors can be considered as ideal candidates for both purposes. Herein, an inkjet-printed memristor is demonstrated, which can serve as a digital information storage device, or as an artificial synapse for neuromorphic circuits. This is achieved by suitable manipulation of the ion species in the active layer of the device. For digital-type memristor operation resistive switching is dominated by cation movement after an initial electroforming step. It allows the device to be utilized as non-volatile digital memristor, which offers high endurance over 12 672 switching cycles and high uniformity at low operating voltages. To use the device as an electroforming-free, interface-based, analog-type memristor, anion migration is exploited which leads to volatile resistive switching. An important figure of merits such as short-term plasticity with close to biological synapse timescales is demonstrated, for facilitation (10–177 ms), augmentation (10s), and potentiation (35 s). Furthermore, the device is thoroughly studied regarding its metaplasticity for memory formation. Last but not least, the inkjet-printed artificial synapse shows non-linear signal integration and low-frequency filtering capabilities, which renders it a good candidate for neuromorphic computing architectures, such as reservoir computing

Formation and thermal stability of two-phase microstructures in Al-containing refractory compositionally complex alloys

Phase separation into an A2+B2 two-phase microstructure in refractory compositionally complex alloys (RCCA) has been speculated as being spinodal in nature with continuous chemical distribution during the separation. However, these reactions might instead occur as precipitation by nucleation and growth. In order to unequivocally elucidate the distinct nature of phase separation sequence in RCCA from the system Ta-Mo-Ti-Cr-Al, atom probe tomography and electron microscopy techniques were utilized on samples that were annealed over multiple orders of magnitude in time. The composition 82(TaMoTi)-8Cr-10Al (at.%) was chosen, as it exhibits a two-phase microstructure, with a desired A2 matrix and embedded B2 phase. Quenching the samples from 1200°C resulted in a microstructure consisting of ordered clusters (2 nm) of distinct chemical composition. Subsequent annealing at 800°C to 1000°C leads to an increase in the volume fraction of the precipitating phase, which saturates after 10 h. Further annealing leads to the ripening of the microstructure; however, the absolute size of the precipitates stays <100 nm even after 1000 h. For the investigated conditions, the interface between matrix and precipitate can be considered sharp within the resolution of the applied techniques and no significant change in the transition of chemical composition across the interface is observed. Therefore, the phase separation mechanism is confirmed to be phase nucleation and growth in contrast to the possible spinodal decomposition, as hypothesized for other RCCA systems. The impact of precipitation and coarsening on the hardness of the alloy is discussed

A Review on 3D Architected Pyrolytic Carbon Produced by Additive Micro/Nanomanufacturing

Additive micro/nano-manufacturing of polymeric precursors combining with a subsequent pyrolysis step enables the design-controlled fabrication of micro/nano-architected 3D pyrolytic carbon structures with complex architectural details. Pyrolysis results in a significant geometrical shrinkage of the pyrolytic carbon structure, leading to a structural dimension significantly smaller than the resolution limit of the involved additive manufacturing technology. Combining with the material properties of carbon and 3D architectures, architected 3D pyrolytic carbon exhibits exceptional properties, which are significantly superior to that of bulk carbon materials. This article presents a comprehensive review of the manufacturing processes of micro/nano-architected pyrolytic carbon materials, their properties, and corresponding demonstrated applications. Acknowledging the “young” age of the field of micro/nano-architected carbon, this article also addresses the current challenges and paints the future research directions of this field

Microarchitected Compliant Scaffolds of Pyrolytic Carbon for 3D Muscle Cell Growth

The integration of additive manufacturing technologies with the pyrolysis of polymeric precursors enables the design-controlled fabrication of architected 3D pyrolytic carbon (PyC) structures with complex architectural details. Despite great promise, their use in cellular interaction remains unexplored. This study pioneers the utilization of microarchitected 3D PyC structures as biocompatible scaffolds for the colonization of muscle cells in a 3D environment. PyC scaffolds are fabricated using micro-stereolithography, followed by pyrolysis. Furthermore, an innovative design strategy using revolute joints is employed to obtain novel, compliant structures of architected PyC. The pyrolysis process results in a pyrolysis temperature- and design-geometry-dependent shrinkage of up to 73%, enabling the geometrical features of microarchitected compatible with skeletal muscle cells. The stiffness of architected PyC varies with the pyrolysis temperature, with the highest value of 29.57 ± 0.78 GPa for 900 °C. The PyC scaffolds exhibit excellent biocompatibility and yield 3D cell colonization while culturing skeletal muscle C2C12 cells. They further induce good actin fiber alignment along the compliant PyC construction. However, no conclusive myogenic differentiation is observed here. Nevertheless, these results are highly promising for architected PyC scaffolds as multifunctional tissue implants and encourage more investigations in employing compliant architected PyC structures for high-performance tissue engineering applications