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Sputtered Ti--rich TiNiCu alloys are known to show excellent cyclic stability. Reversibility is mostly influenced by grain size, crystallographic compatibility, and precipitates. Isolating their impact on cyclic stability is difficult. [...]

Here we report for the first time on a complete simulation assisted “material to module” development of a high performance thermoelectric generator (TEG) based on the combination of a phase change material and established thermoelectrics yielding the compositions (1 −x)(GeTe) x(Bi2Se0.2Te2.8). For the generator design our approach for benchmarking thermoelectric materials is demonstrated which is not restricted to the determination of the intrinsically imprecise ZT value but includes the implementation of the material into a TEG. This approach is enabling a much more reliable benchmarking of thermoelectric materials for TEG application. Furthermore we analyzed the microstructure and performance close to in-operandi conditions for two different compositions in order to demonstrate the sensitivity of the material against processing and thermal cycling. For x = 0.038 the microstructure of the as-prepared material remains unchanged, consequently, excellent and stable thermoelectric performance as prerequisites for TEG production was obtained. For x = 0.063 we observed strain phenomena for the pristine state which are released by the formation of planar defects after thermal cycling. Consequently the thermoelectric performance degrades significantly. These findings highlight a complication for deriving the correlation of microstructure and properties of thermoelectric materials in general. [ABSTRACT FROM AUTHOR]

Abstract: A comprehensive study on the complete process module for the fabrication of AlN-based MEMS sensors and actuators is presented. The investigations include the influence of the electrode material, AlN thickness and stress tuning during sputtering on the material parameters, especially the piezoelectric coefficients. It is shown that AlN layers deposited at only 200°C have good piezoelectric properties. The tuning of residual layer stress has little influence on the piezoelectric properties of AlN. Further two methods of wet chemical etching of AlN are compared. The complete CMOS compatible processing of AlN-based MEMS structures is demonstrated on 200mm wafers. [Copyright &y& Elsevier]

Energy harvesting and storage is highly demanded to enhance the lifetime of autonomous systems, such as IoT sensor nodes, avoiding costly and time-consuming battery replacement. However, cost efficient and small-scale energy harvesting systems with reasonable power output are still subjects of current development. In this work, we present a mechanically and magnetically excitable MEMS vibrational piezoelectric energy harvester featuring wafer-level integrated rare-earth micromagnets. The latter enable harvesting of energy efficiently both in resonance and from low-g, low-frequency mechanical energy sources. Under rotational magnetic excitation at frequencies below 50 Hz, RMS power output up to 74.11 µW is demonstrated in frequency up-conversion. Magnetic excitation in resonance results in open-circuit voltages > 9 V and RMS power output up to 139.39 µW. For purely mechanical excitation, the powder-based integration process allows the realization of high-density and thus compact proof masses in the cantilever design. Accordingly, the device achieves 24.75 µW power output under mechanical excitation of 0.75 g at resonance. The ability to load a capacitance of 2.8 µF at 2.5 V within 30 s is demonstrated, facilitating a custom design low-power ASIC.

A zero-power wakeup scheme for energy-efficient sensor applications is presented in this study based on a piezoelectric MEMS energy harvester featuring wafer-level-integrated micromagnets. The proposed setup overcomes a hybrid assembly of magnets on a chip-level, a major drawback of similar existing solutions. The wakeup device can be excited at low frequencies by frequency up-conversion, both in mechanical contact and contactless methods due to magnetic force coupling, allowing various application scenarios. In a discrete circuit, a wakeup within 30−50 ms is realized in frequency up-conversion at excitation frequencies < 50 Hz. A power loss in the off state of 0.1 nW renders the scheme virtually lossless. The potential extension of battery lifetime compared to cyclical wakeup schemes is discussed for a typical wireless sensor node configuration.

The electrochemical generation of hydrogen is a key enabling technology for the production of sustainable fuels. Transition metal chalcogenides show considerable promise as catalysts for this reaction, but to date there are very few reports of tellurides in this context, and none of these transition metal telluride catalysts are especially active. Here, we show that the catalytic performance of metallic 1T'-MoTe 2 is improved dramatically when the electrode is held at cathodic bias. As a result, the overpotential required to maintain a current density of 10 mA cm -2 decreases from 320 mV to just 178 mV. We show that this rapid and reversible activation process has its origins in adsorption of H onto Te sites on the surface of 1T'-MoTe 2 . This activation process highlights the importance of subtle changes in the electronic structure of an electrode material and how these can influence the subsequent electrocatalytic activity that is displayed.

Nanomaterials for highly selective and sensitive sensors toward specific gas molecules of volatile organic compounds (VOCs) are most important in developing new-generation of detector devices, for example, for biomarkers of diseases as well as for continuous air quality monitoring. Here, we present an innovative preparation approach for engineering sensors, which allow for full control of the dopant concentrations and the nanoparticles functionalization of columnar material surfaces. The main outcome of this powerful design concept lies in fine-tuning the reactivity of the sensor surfaces toward the VOCs of interest. First, nanocolumnar and well-distributed Ag-doped zinc oxide (ZnO:Ag) thin films are synthesized from chemical solution, and, at a second stage, noble nanoparticles of the required size are deposited using a gas aggregation source, ensuring that no percolating paths are formed between them. Typical samples that were investigated are Ag-doped and Ag nanoparticle-functionalized ZnO:Ag nanocolumnar films. The highest responses to VOCs, in particular to (CH 3 ) 2 CHOH, were obtained at a low operating temperature (250 °C) for the samples synergistically enhanced with dopants and nanoparticles simultaneously. In addition, the response times, particularly the recovery times, are greatly reduced for the fully modified nanocolumnar thin films for a wide range of operating temperatures. The adsorption of propanol, acetone, methane, and hydrogen at various surface sites of the Ag-doped Ag 8 /ZnO(0001) surface has been examined with the density functional theory (DFT) calculations to understand the preference for organic compounds and to confirm experimental results. The response of the synergistically enhanced sensors to gas molecules containing certain functional groups is in excellent agreement with density functional theory calculations performed in this work too. This new fabrication strategy can underpin the next generation of advanced materials for gas sensing applications and prevent VOC levels that are hazardous to human health and can cause environmental damages.

Chalcogenide-based thin films are employed in data storage and memory technology whereas van der Waals-bonded layered chalcogenide heterostructures are considered to be a main contender for memory devices with low power consumption. The reduction of switching energy is due to the lowering of entropic losses governed by the restricted motion of atoms in one dimension within the crystalline states. The investigations of switching mechanisms in such superlattices have recently attracted much attention and the proposed models are still under debate. This is partially due to the lack of direct observation of atomic scale processes, which might occur in these chalcogenide systems. This work reports direct, nanoscale observations of the order-disorder processes in van der Waals bonded Ge-Sb-Te thin films and GeTe-Sb2Te3-based superlattices using in situ experiments inside an aberration-corrected transmission electron microscope. The findings reveal a reversible self-assembled reconfiguration of the structural order in these materials. This process is associated with the ordering of randomly distributed vacancies within the studied materials into ordered vacancy layers and with readjustment of the lattice plane distances within the newly formed layered structures, indicating the high flexibility of these layered chalcogenide-based systems. Thus, the ordering process results in the formation of vacancy-bonded building blocks intercalated within van der Waals-bonded units. Moreover, vacancy-bonded building blocks can be reconfigured to the initial structure under the influence of an electron beam, while in situ exposure of the recovered layers to a targeted electron beam leads to the reverse process. Overall, the outcomes provide new insights into local structure and switching mechanism in chalcogenide superlattices.

The dynamic underwater chemistry seen in nature is inspiring for the next generation of eco-friendly nanochemistry. In this context, green synthesis of size-tailored nanoparticles in a facile and scalable manner via a dynamic process is an interesting challenge. Simulating the volcano-induced dynamic chemistry of the deep ocean, here we demonstrate the Leidenfrost dynamic chemistry occurring in an underwater overheated confined zone as a new tool for customized creation of nanoclusters of zinc peroxide. The hydrodynamic nature of the phenomenon ensures eruption of the nanoclusters towards a much colder region, giving rise to growth of monodisperse, size-tailored nanoclusters. Such nanoparticles are investigated in terms of their cytotoxicity on suspension and adherent cells to prove their applicability as cancer nanotherapeutics. Our research can pave the way for employment of the dynamic green nanochemistry in facile, scalable fabrication of size-tailored nanoparticles for biomedical applications.

The positive influence of crystallographic compatibility on the thermal transformation stability has been already investigated extensively in the literature. However, its influence on the stability of the shape memory effect or superelasticity used in actual applications is still unresolved. In this investigation sputtered films of a highly compatible TiNiCuCo composition with a transformation matrix middle eigenvalue of 1±0.01 are exposed to thermal as well as to superelastic cycling. In agreement with previous results the thermal transformation of this alloy is with a temperature shift of less than 0.1 K for 40 cycles very stable; on the other hand, superelastic degradation behaviour was found to depend strongly on heat treatment parameters. To reveal the transformation dissimilarities between the differently heat-treated samples, the microstructure has been analysed by transmission electron microscopy, in situ stress polarization microscopy and synchrotron analysis. It is found that good crystallographic stability is not a sufficient criterion to avoid defect generation which guarantees high superelastic stability. For the investigated alloy, a small grain size was identified as the determining factor which increases the yield strength of the composition and decreases the functional degradation during superelastic cycling.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'., (© 2016 The Author(s).)

Here we report for the first time on a complete simulation assisted "material to module" development of a high performance thermoelectric generator (TEG) based on the combination of a phase change material and established thermoelectrics yielding the compositions (1 - x)(GeTe) x(Bi(2)Se(0.2)Te(2.8)). For the generator design our approach for benchmarking thermoelectric materials is demonstrated which is not restricted to the determination of the intrinsically imprecise ZT value but includes the implementation of the material into a TEG. This approach is enabling a much more reliable benchmarking of thermoelectric materials for TEG application. Furthermore we analyzed the microstructure and performance close to in-operandi conditions for two different compositions in order to demonstrate the sensitivity of the material against processing and thermal cycling. For x = 0.038 the microstructure of the as-prepared material remains unchanged, consequently, excellent and stable thermoelectric performance as prerequisites for TEG production was obtained. For x = 0.063 we observed strain phenomena for the pristine state which are released by the formation of planar defects after thermal cycling. Consequently the thermoelectric performance degrades significantly. These findings highlight a complication for deriving the correlation of microstructure and properties of thermoelectric materials in general.