What is the advantage of metallic hydrogen
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BAM has extensive expertise in the field of materials, especially in the field of property characterization and the assessment of suitability for safe and sustainable use of the corresponding components, plants and systems. The focus is on the characterization and modeling of the hydrogen-dependent material properties over the entire service life. In addition, material changes are examined under extreme conditions, e.g. at high and low temperatures and pressures in different hydrogen-carrying media. The basis for this is provided by scientific approaches and modeling of material engineering mechanisms through to the life cycle engineering of components, plants and systems.
Selected examples of our work are described below. You can find detailed information on this in our brochure "Hydrogen: Our Contribution to Safety" (PDF).
Tribology - Which lubricants do we need when using hydrogen?
The generic term "tribology" summarizes the entire area of friction, wear and lubrication. Examples of tribologically stressed components are bearings, piston rings, seals and joints. The hydrogen environment places special demands on such components. BAM has special test equipment to determine friction and wear parameters in liquid and gaseous hydrogen. Currently polymer composite materials as well as friction-reducing, wear-resistant coatings are mainly investigated. For practical use, however, components such as B. Test ball bearings. A pump for liquid hydrogen is also available, in which, among other things, Materials for piston seals can be tested under real conditions.
The results so far show that some materials in hydrogen, even in cryogenic liquefied form, have more favorable properties than in air. These are e.g. B. some high-performance plastics in their pure form or as components of composite materials. Solid lubricants such as graphite or molybdenum disulfide - whether as a thin coating or as an admixture to composite materials - have also proven to be well suited. In the case of coatings, those made of amorphous diamond-like carbon are particularly interesting, as they sometimes even achieve coefficients of friction and a service life in the range of grease or oil-lubricated systems. Austenitic stainless steels, which are used as standard for containers and pipelines, since they are considered uncritical against hydrogen-related degradation, were examined from the field of metals. In friction systems, however, structural changes were detected, which then led to the formation of cracks again.
Materials for hydrogen
The production, storage and transport of hydrogen as a future energy carrier place high demands on the materials used and require careful processing.
Such negative effects should not prevent the useful use of hydrogen as an energy carrier. Phenomenologically, they are dependent on the triggering local mechanical stress, the existing microstructure and the level of the hydrogen concentration. BAM deals scientifically with both the phenomena and the mechanisms of hydrogen in metals in order to identify possibilities and application limits for the respective components, in particular in order to largely exclude unexpected and brittle hydrogen-assisted cracking and the associated safety risks.
Hydrogen absorption, diffusion and measurement
The crack-critical hydrogen concentration for a certain structure depends on how much is absorbed under the various manufacturing or operating conditions and how the hydrogen is distributed in the often very heterogeneous structure. In several projects at BAM, values for hydrogen transport in a wide variety of metallic structures are continuously determined, including the materials for hydrogen technologies. These values are used, among other things, in numerical simulations of hydrogen distribution and crack formation in heterogeneous microstructures.
Analyzers based on carrier gas hot extraction (TGHE) are available for such investigations. This technology originally comes from welding technology and was expanded to include mass spectrometry as part of standardization work (ISO 3690), which now enables the precise determination of hydrogen diffusion and concentrations in the ppb and ppm range in the temperature range from 20 ° C to 950 ° C . Together with the specially designed experimental microstructure simulation, the hydrogen transport behavior in a wide variety of metallic materials can be investigated. These investigations based on existing experiences with hydrogen-assisted crack corrosion and cold crack formation are gaining in importance for the new hydrogen technologies, especially in the area of high pressure applications and power-to-gas.
Hydrogen-dependent mechanical properties and crack formation
The degradation of the mechanical material properties by hydrogen mainly manifests itself in a reduced ductility, which is sometimes imprecisely referred to as hydrogen embrittlement. At very high concentrations, the strength of the materials can also be impaired. Depending on the type and direction of the mechanical stress, brittle material separations can occur, which are referred to as hydrogen-assisted crack formation.
In order to determine criteria under which no damage to a metallic microstructure is to be expected, and vice versa for a better understanding of how hydrogen reduces the mechanical material properties, the hydrogen-dependent material properties for metallic materials, in particular for steels, are determined in various projects. Using the experimental microstructure simulation, this enables the microstructure-specific sensitivity to be determined in the form of decreasing ductility. In this way, hydrogen-dependent crack criteria can be determined, which are used in corresponding simulations for crack initiation and crack growth. In order to investigate in-situ the mechanical properties and crack behavior of materials in various ambient media from which hydrogen can be absorbed, tensile tests are carried out with correspondingly slow elongation.
Localization of hydrogen using high-resolution imaging methods
BAM is currently the technology leader in imaging processes for the visualization of hydrogen distributions and cracking in metallic microstructures. In combination with conventional test methods, such imaging techniques make it possible to study the distribution and the effect of hydrogen in metallic microstructures in-situ and they are therefore increasingly attracting industrial interest with regard to the safe design of components for new hydrogen technologies.
Two types of imaging processes are currently being promoted at BAM in order to provide more detailed information on hydrogen distribution and crack formation in metallic materials and in additively manufactured and welded microstructures. This involves time-of-flight secondary ion mass spectrometry (ToF-SIMS) as a 2D technique using deuterium with a relatively high resolution and neutron tomography (NT) as a 3D technique with a somewhat lower resolution of the hydrogen distribution.
Self-healing of cracks in seals of high-temperature fuel cells
High-temperature fuel cells heat up to 900 ° C during operation. That is why they place particularly high demands on the seals that connect all components with one another. These must be resistant to high temperatures, mechanically stable, electrically insulating and impermeable to hydrogen molecules.
Many manufacturing companies therefore use glass to manufacture the seals. Two concepts are being pursued here. Glassy solders retain their amorphous structure during the assembly of the cell components. Vitreous-crystalline solders crystallize during joining and only have a small glassy content. In both cases, however, the thermal loads in cyclical operation can cause fine cracks to appear during cooling, as the individual components contract to different degrees. A defective seal cannot be replaced because it is part of up to 150 individual cells. In the event of damage, it is therefore inevitable to change all cells - an expensive and not very sustainable method. For this reason, it is desirable that cracks close again on their own during operation of the fuel cell. Seals made of glass that does not crystallize show particularly good crack healing. The ideal seal should contain enough crystal phase to make corrosion more difficult, but not the desired crack healing. In order to find the right ratio of glass and crystalline components, model structures were produced at BAM as part of a doctoral thesis from a sodium-calcium silicate glass with a low tendency to crystallize and zirconium dioxide (ZrO2) as an inert, crystalline filler.
Hydrogen storage project - Novel storage materials for hydrogen filling stations
Compressed gas up to 900 bar is still the state of the art for H2 storage in filling stations or in fuel cell vehicles (FCV). With liquid hydrogen - as an alternative to these massive systems in terms of handling and storage - it is possible to increase the hydrogen density, but temperatures below -240 ° C are necessary. Cryo-adsorption in porous materials is an attractive alternative between the two techniques mentioned above, as it can bring the storage densities at a higher temperature (close to -196 ° C, the temperature of boiling nitrogen) close to that of liquid hydrogen. Another advantage of this mechanism is a lower pressure than in compressed containers (80 bar compared to 900 bar), which increases the safety of these systems.
Implementing this technology in lightweight mobile FCVs is challenging. However, it could be attractive for larger-scale storage of hydrogen, e.g. B. in filling stations for H2 vehicles. In cooperation with H2 MOBILITY GmbH, BAM is involved in a project to develop the next generation of FCV filling stations based on H2 cryo-adsorption on modern porous materials. Intensive scientific research over the past decade has produced a variety of suitable materials. The best candidates for H2-cryo-adsorption are the so-called MOF materials (Metal-Organic Frameworks), whose remarkable specific surface (up to 10,000 m² / g) leads to the highest gravimetric storage capacities among the porous materials.
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