Filtern
Erscheinungsjahr
Dokumenttyp
- Vortrag (94) (entfernen)
Sprache
- Englisch (94)
Referierte Publikation
- nein (94)
Schlagworte
- Hydrogen (94) (entfernen)
Organisationseinheit der BAM
- 9 Komponentensicherheit (43)
- 9.0 Abteilungsleitung und andere (24)
- 3 Gefahrgutumschließungen; Energiespeicher (22)
- 3.5 Sicherheit von Gasspeichern (19)
- 9.4 Integrität von Schweißverbindungen (18)
- 2 Prozess- und Anlagensicherheit (11)
- 2.1 Sicherheit von Energieträgern (11)
- 8 Zerstörungsfreie Prüfung (9)
- 9.5 Tribologie und Verschleißschutz (9)
- 1 Analytische Chemie; Referenzmaterialien (6)
Forest fires start mostly in partial combustion. In this paper we present a hydrogen sensor to detect early stages of forest fires. First indoor experiments in a smoke density chamber with wood samples, proved that the sensor produce a strong signal change when the smouldering of wood starts, already before the CO concentration, measured in FTIR spectrometry, exceeds 10 ppm. Indoor experiments in a Smoke Density Chamber with wood samples show, that our sensor produces an earlier signal difference than a carbon monoxide detector. In outdoor experiments we were able to detect forest fire in a distance of 110 m.
The tribological behaviour of polyetheretherketone (PEEK) and polyimide (PI) composites was investigated in air, vacuum and hydrogen environments. Polymers were filled with graphite, submicron particles or carbon nano-tubes. Experiments reveal that hydrogen environment has a major influence on both polymer structure and fillers during sliding. The sliding behaviour of these polymer composites is much improved in hydrogen compared to air. In particular, graphite and nano-particles have a remarkable effect on friction and wear, respectively.
The polymer chemical structure of the polyimide has a major influence on the tribological behaviour. The addition of graphite in PI2 has a beneficial effect in hydrogen on the friction and wear. The low friction of graphite is associated with a lubricant film in hydrogen. The influence of hydrogen on graphite is more effective than humidity. CNTs have a similar effect to that of graphite in PEEK composites. TiO2 particles improve significantly the wear rate both in vacuum and hydrogen environment. In LH2 friction and wear decrease for unfilled polymers. Friction of graphite filled composites increases slightly and wear rate is stable.
Trapping in T24 steel weld joints – Effects on activation energy for hydrogen diffusion during TDA
(2017)
Failure cases in the past decade exhibited severe cracking in T24 welds and showed that generally hydrogen-assisted cracking (HAC) occurring up to 200°C cannot be excluded. A basic understanding is necessary on how hydrogen diffusion is influenced by the weld process. In this regard, both weld microstructures HAZ and weld metal have particular influence on hydrogen diffusion compared to the base material. In general, hydrogen diffusion at a certain temperature is described by diffusion coefficients representing an effective value of combined lattice diffusion and effects of reversible hydrogen traps. Those traps are typically precipitates, interstitials, grain boundaries and so on. A common approach to describe the trap character and its effect on diffusion is the determination of so-called activation energy. This can be done by respective thermal desorption analysis (TDA) with linear heating. In the present study, different T24 as-welded microstructures (BM, HAZ, WM) were investigated. For that purpose, electrochemically hydrogen charged specimens were analyzed by TDA with linear heating using a mass spectrometer for detection of ultra-low hydrogen amounts. The results showed that typically the as-welded HAZ had higher energy traps than the tempered base material. Nonetheless two important effects were ascertained: (1) it is strictly necessary to monitor the sample temperature due to its great impact on the hydrogen desorption peak temperature and (2) the real heating rate in the specimen vs. the applied heating rate has to be considered. Both influence the calculated activation energy, i.e. the assigned hydrogen trap character (moderate or strong trap), which changed up to a factor of two in terms of the calculated activation energy. This effect can be much more important compared to the microstructure effect itself. Hence, suitable experimental boundary conditions should be mandatory for the determination of hydrogen trap kinetics.
The use of duplex stainless steels (DSS) in energy related applications is well known. Nowadays, DSS steels become more favorable than austenitic steels due to the outstanding mechanical properties, the good corrosion resistance and the lower nickel content. However, the use of the duplex grade in acidic environments such as seawater often leads to severe degradation of the structural integrity of the steel by hydrogen-induced/assisted cracking (HAC) phenomena, which can eventually result in premature failure. Hydrogen assisted degradation and cracking of steels are active fields of research even though this topic is intensively studied for more than a century. A bottleneck is the analytical validation of the theoretical models proposed ion the literature at the sub-micron scale.
Industrial and the research communities see a need for an accurate analytical method by which it is possible to image the distribution of hydrogen in the microstructure of a steels or and other alloys. Among the very few available methods hydrogen imaging methods, Time-of-Flight secondary ion mass spectrometry (ToF-SIMS) has the principal capability for mapping of hydrogen in a steel’s microstructure. The combination of ToF-SIMS with multivariate data analysis (MVA), electron microscopy (SEM) and electron-backscattered diffraction (EBSD) is a powerful approach for providing chemical and structural information. The use of data fusion techniques has been shown recently to enhance the better understanding of the hydrogen induced degradation processes in in a DSS steel.
Metallic organic frameworks are the materials stirring great interest by their large variety of structure types and chemical composition as well as low density and very high surface area, which are desirable properties for gas storage such as for hydrogen, methane or carbon dioxide. Using different so called secondary building units offer an opportunity to form tunable structures with channels of various geometry and allow in this way to study the absorption of the molecular hydrogen, methane or carbon dioxid in the systematic way.
Current interest in hydrogen gas as an energy source is growing due to its attractive properties such as high chemical energy density and the fact that its combustion produces water as by-product, qualifying this gas as an appealing clean energy source. In particular, the use of molecular hydrogen in automotive applications such as the hydrogen internal combustion engine (HICE) or hydrogen fuel cells is an excellent clean alternative.
Despite these promising opportunities, hydrogen gas has a distinct disadvantage as an everyday energy medium, that is, it is highly flammable. Hydrogen processing apparatuses should therefore be constantly monitored so that leaks are immediately detected. The implementation of direct sensing systems for H2(g) is thus a decisive factor for its application and acceptance as a clean energy source. Because the automotive market alone is already a mass market, these sensors do not only have to be reliable, robust and dimensionally small, but operation has to be simple and the device itself cost-effective. In addition, very low detection limits are a must as already escape of small quantities of H2(g) is directly related to public health and safety. Besides electrochemical or semiconductor sensors, optical sensors are especially appealing because the equipment is usually simple and accessible, easily miniaturized and measurements can be performed in situ and in real time.
If one cannot rely for instance on metallic palladium as interacting matrix, a major challenge for optochemical hydrogen sensors is to find a suitable material that fulfils all the requirements mentioned above and undergoes a dedicated indication reaction with H2(g). One such alternative can be Metal-Organic Frameworks (MOFs) which constitute a predefined, organized, mesoporous structure built up from metal ions and organic bridging ligands. With a myriad of building blocks being available, MOFs can be equipped with internal H2 reception sites that shall allow for selective and sensitive indication. For instance, if Pd ions are implemented in such a way that they express open metal sites (OMS), these OMS shall possess a strong affinity for hydrogen. If light absorbing and emitting organic ligands are additionally chosen, luminescent MOFs (LMOFs) can result that are perfect candidates for optical sensors, as changes at the analyte reception site can be effectively transduced into a measurable signal.
In this contribution, we intended to discuss the development of a new luminescent sensor material for the detection of the highly flammable H2(g). This sensor material is based on an LMOF assembled from Pd(II) and aromatic bridging ligands, the advent of hydrogen at the OMS in the LMOF producing distinct variations in the material’s optical properties.
In order to satisfy the growing requirements towards lightweight design and resource efficiency in modern steel constructions, e.g. mobile cranes and bridges, high-strength steels with typical yield strength ≥ 690 MPa are coming into use to an increasing extent. However, these steels require special treatment in welding. The susceptibility for degradation of the mechanical properties in presence of hydrogen increases significantly with increasing yield strength. In case of missing knowledge about how and which amount of hydrogen is uptaken during welding, hydrogen assisted cracking (HAC) can be a negative consequence. Moreover, modern weld technology like the modified spray arc process enables welding of narrower weld seams. In this context, a reduced number of weld beads, volume and total heat input are technical and economic benefits. This work presents the influence of welding parameters on the diffusible hydrogen content in both singlepass and multi-layer welds. Different hydrogen concentrations were detected by varied contact tube distance, wire feed speed, arc length as well as varied arc type (transitional arc and modified spray arc). The results show, that all welding parameters have significant influence on the diffusible hydrogen concentration in the single-pass welds. By increasing the number of weld beads in case of multi-layer welding, the hydrogen concentration have been substantially reduced. Whereby, differences in hydrogen concentrations between both arc types are present.
Conventional approval requirements exclusively ask for minimum strength values, which have to be met. The probabilistic approach estimates how likely none of the comparatively manufactured units fails during operation.
Both questions are juxtaposed and compared here with respect to the load cycle tests. The influence of the sample sizes is discussed additionally.