Scientific Events

Defects and Grain boundaries have a remarkable effect on the thermal and electrical transport properties of polycrystalline materials but are often ignored by prevailing physical theories. Grain boundaries and interfaces can adversely alter the properties of Power Electronics, Solar Cells, Batteries and Thermoelectrics such as interfacial electrical and thermal resistance (Kapitza resistance) and even an interfacial Seebeck effect. Interfacial thermal resistance limits the performance of power electronics because of overheating. New scanning thermal reflectance techniques can image the thermal resistance of interfaces and boundaries directly. The Thermal conductivity suppression at grain boundaries can even be imaged showing that different grain boundaries can have very different thermal resistances with high energy grain boundaries having more resistance and low energy boundaries having lower thermal resistance. Electrical grain boundary resistance can be so high in some thermoelectric materials it is the dominant property that limits zT. While small grains are usually considered beneficial for thermoelectric performance due to reduced thermal conductivity, Mg₃Sb₂ based thermoelectric materials, so far at least, contradict that trend. Indeed, atomic segregation has been recently observed at the nanometer scale in grain boundaries in many materials suggesting interfacial or complexion phases should be specifically considered when understanding nearly all thermoelectric materials. The concentration of point defects, such as vacancies, interstitial and substitutional atoms can now be predicted with DFT allowing defects to be included in phase diagram analysis for prediction of materials processing for particular properties. [more]

In recent decades, extensive efforts have been done to develop new and more efficient alternative energy sources, which can substitute conventional sources like gas, petrol, and carbon, have been made. Due to the increase in energy consumed by society, we not only need an alternative to conventional energy sources, but also a reduction in energy consumption. Therefore, it is necessary to investigate different methods for energy-recovery and energy-saving as such as thermoelectric materials and radiative coolers. The thermoelectric materials can convert heat into electricity and vice versa. The efficiency of these materials is related to the figure of merit (zT) and it is defined as zT=(σ·S²/k)·T, where σ is the electrical conductivity, S is the Seebeck coefficient, k is the thermal conductivity, and T is the absolute temperature. Nowadays, the application of inexpensive and scalable materials in the industry for thermoelectric applications has received great interest. In this sense electrodeposition is one of the most interesting techniques. It is performed at room temperature, so it is compatible with polymeric substrates, it does not require vacuum conditions, and it allows perfect control over the composition, morphology, and crystallographic structure. In this presentation, I will provide an overview of different thermoelectric materials such as Bi₂Te₃¹, CuNi², and Ag₂Se³ grown by electrodeposition and their thermoelectric properties. In the case of silver selenide, a thermoelectric power generator was produced and characterized. Radiative cooling is the process by which temperature decreases due to an excess of emitted radiation above absorber radiation. To achieve cooling, it is necessary to reduce and keep the temperature below the ambient air temperature. The requirements of radiative coolers to have maximum cooling power, to be able to reduce the temperature sufficiently, and to function 24 hours a day anywhere, are high solar reflectance and high infrared emittance, close to the atmosphere’s window (between 8 and 13μm wavelengths). Different approaches have been explored and porous nanostructures have shown the best results to this respect. In this sense, porous anodic aluminium oxide (AAO) nanostructures on Al was demonstrated to be a great candidate⁴. AAO is an amorphous material with an isotropic permittivity, a strong acoustic resonance absorption at the far IR (15 – 25 µm), and high transparency in the UV‑Vis‑NIR range. In this presentation, I will highlight the possibilities to use AAO nanostructure as radiative cooling. In addition, strcutural cellulose will be also analysed for the same purpose. [more]

Due to its high diffusivity hydrogen atoms alloy with metals even at room temperature. At this temperature, the materials microstructure remains rather stable. When the system size is reduced to the nano-scale, microstructural defects as well as mechanical stress significantly affect the thermodynamics and kinetics properties of the system.[1-6] Effects will be demonstrated on Niobium-H and Palladium-H thin films.Hydrogen absorption in metal systems commonly leads to lattice expansion. The lateral expansion is hindered when the metal adheres to a rigid substrate, as for thin films. Consequently, high mechanical stresses arise upon hydrogen uptake. In theory, these stresses can reach about -10 GPa for 1 H/M. Usually, metals cannot yield such high stresses and deform plastically. Thereby, maximum compressive mechanical stress of -2 to -3 GPa is commonly measured for 100 nm Nb thin films adhered to Sapphire substrates. It will be shown that phase transformations change in the coherency state upon film thickness reduction. The coherency state affects the nucleation and growth behaviour of the hydride phase as well as the kinetics of the phase transformation.[1] It will be further demonstrated that plastic deformation can be hindered and even suppressed upon film thickness reduction. In this case the system behaves purely elastic and ultra-high stress of about -10 GPa can be experimentally reached.[2] These high mechanical stresses result in changes of the materials thermodynamics. In the case of Nb-H thin films of less than 8 nm thickness, the common phase transformation from the α-phase solid solution to the hydride phase is completely suppressed, at 300 K.[3,4,5] The experimental results go in line with the σDOS model that includes microstructural and mechanical stress effects on the chemical potential [6]. [1] V. Burlaka, K. Nörthemann, A. Pundt, „Nb-H Thin Films: On Phase Transformation Kinetics“, Def. Diff. Forum 371 (2017) 160. [2] M. Hamm, V. Burlaka, S. Wagner, A. Pundt, “Achieving reversibility of ultra-high mechanical stress by hydrogen loading of thin films”, Appl. Phys. Letters 106 (2015) 243108. [3] S. Wagner, A. Pundt, “Quasi-thermodynamic model on hydride formation in palladium-hydrogen thin films: Impact of elastic and microstructural constraints “, Int. J. Hydrog. Energy 41 (2016) 2727. [4] V. Burlaka, S. Wagner, M. Hamm, A. Pundt, “Suppression of phase transformation in Nb-H thin films below switchover-thickness”, Nano Letters 16 (2016) 6207. [5] S. Wagner, P. Klose, V. Burlaka, K: Nörthemann, M. Hamm, A. Pundt, Structural Phase Transitions in Niobium Hydrogen Thin Films: Mechanical Stress, Phase Equilibria and Critical Temperatures, Chem. Phys. Chem. 20 (2019) 1890–1904. [6] S. Wagner, A. Pundt, Hydrogen as a probe for defects in materials: Isotherms and related microstructures of palladium-hydrogen thin films, AIMS Materials Science 7 (2020), 399–419. [more]

Materials play a crucial role in driving the twin transition, a key strategy of the European Union to address current and future environmental challenges. Currently, improving the efficiency of solar cells and the capacity of battery storage is essential for achieving a Net Zero Carbon society, underlining the growing demand for innovative materials. Nanotechnology played a major role in development of the necessary hardware, such as sensors, data storage systems, and actuators, needed for advanced digital solutions. Still, the push to enhance performance and replace outdated technologies is accelerating the research into always-new materials and solutions. Performance-oriented development also expanded the range of metals utilized by humanity, leading to a scenario where a single smartphone necessitates a broader spectrum of elements than the entire biosphere [1], [2]. Due to the fast development, the raw materials for such technologies are changing by day - a pace that the supply chain cannot follow satisfactory. In addition, the environmental impact of raw materials production, especially metals, varies significantly. For example, steel production—accounting for 1.9 billion tons annually—contributes to 8% of global CO₂ emissions, but its per mass environmental footprint is one of the lowest (2 kg of CO₂ per kg of primary steel, and 0.7 kg of CO₂ when recycled). In contrast, the production of platinum, crucial element for hydrogen economy with a limited production of 200 t/y, is one of the most carbon-intensive (60 tons of CO₂ per kg of Pt). Limiting the challenge of the Green and digital revolutions to the simple cost-performance paradigm would be somehow repeating the mistake of the Oil Age during which the resource was considered as infinite and the impact on the environment had been long time neglected. Making the twin transition successful requires changing the mindset of innovators (from lower TRL) to a binary trade-off (price-performance) towards multi-criteria decision-making [3]. In the presentation, we will introduce a straightforward multi-criteria assessment methodology for evaluating the sustainability of metallic alloys. [more]

The use of nanoindentation-based techniques to study high strain rate deformation behavior of materials is of immense scientific interest because it enables investigating the strain rate dependence of individual grains and small-scale structures. While nanoindentation impact tests, capable of reaching high strain rates, have been used for over two decades, they suffer from lack of indentation profile control and rapidly varying strain rate during impact. This makes extraction of reliable mechanical properties, for e.g. hardness, and determination of the representative strain rate rather difficult. It is only recently that advances in nanoindentation instrumentation have enabled reaching constant strain rates > 100 s^-1 in both micropillar compression [1] and indentation [2]. In this talk, I will present our progress in performing controlled, constant strain rate nanoindentation tests up to 10⁴ s^-1 for reliable extraction of mechanical properties and deformation activation parameters, particularly at high strain rates. Typically, high speeds and fast unloading rates excite the resonance of the nanoindenter, which affects the extracted hardness and modulus values. Novel experimental protocols and calibration procedures were developed to circumvent this issue, which will be discussed. Case studies of high strain rate nanoindentation testing on multiple material systems – single and ultrafine grained metals, amorphous glasses and polymers – will be presented. Deformation activation parameters, for e.g. activation volumes and strain rate sensitivity exponents, were successfully extracted at high strain rates to probe possible changes in the underlying deformation mechanism(s). It is hoped that this study will pave the way for routine high strain rate nanoindentation testing. [more]

Refractory metals and their alloys are considered a versatile group of high temperature resistant materials. Promising design approaches to extend their application range include the modification of both existing commercial materials, but also novel alloys that are still at earlier stages of development. Additionally, rapid solidification processes offer interesting possibilities in this respect. In this presentation, selected research activities in this combined area will be discussed and possible approaches for further research will be presented, while the focus is on nano-oxide composites and titanium-based alloys. [more]

Nanoscale metal-graphene nanolayered composites are known to have ultra-high strength due to the ability of graphene to effectively block dislocations from penetrating through the metal-graphene interface. The same graphene interface can deflect generated cracks, thereby serving as a toughening mechanism. In this talk, the role of graphene interfaces in strengthening and toughening the Cu-graphene nanolayered composite will be discussed. In-situ TEM tensile testing of Cu-graphene showed that the dislocation plasticity was strongly confined by the graphene interfaces and the grain boundaries. The weak interfacial bonding between Cu-graphene induced an interesting stress decoupling effect, which resulted in independent deformation of each Cu layer. MD simulations confirmed such independent deformation of each Cu layer and also showed that the graphene interfaces effectively block crack propagation as delamination occurs at the Cu- graphene interfaces to allow for elastic strain energy dissipation. Bending fatigue testing was also conducted on Cu-graphene nanolayered composites that indicated ~5 times enhancement in robustness against fatigue-induced damage in comparison to the conventional Cu only thin film. Such an enhancement in reliability under cyclic bending was found to be due to the ability of the graphene interface to stop fatigue-induced crack propagations through thickness of the thin film, which is contrary to how a metal only thin film fails under cyclic loadings. [more]