Hydrogen storage materials
Magnetic suspension balance for gravimetric study of adsorption and desorption behaviour of hydrogen storage materials
When using hydrogen as an energy carrier, effective and safe storage plays a central role. Storing hydrogen in its liquid state requires low temperatures and in turn, expends a great deal of energy. In a gaseous state, very high pressure is required. This means that much more sophisticated designs are needed in order to keep the weight of the containers low enough to ensure acceptable storage densities. In addition, such pressure vessels pose a hazard and exhibit significant leakage losses. Therefore, storing hydrogen in solids offers an interesting alternative. For example, light metal hydrides and alanates have a high theoretical storage density for hydrogen. However, with these materials, other complications arise. At present, the main problems are the slow hydrogen sorption kinetics and the lack of reversibility.
As part of IEK-1's ongoing research projects, our expertise on the synthesis of powder-shaped materials is brought together with the experimental possibilities for volumetric and gravimetric characterization of the sorption behaviour (Sievert instrument, magnetic suspension balance). As a result, an innovative high-energy wet grinding process was developed that allowed the specific surface of magnesium hydride (MgH2) to be significantly enlarged. Thus, the surface-dependent mechanisms that play a crucial role in restricting sorption kinetics are considerably quicker and occur at lower temperatures. Further studies are concerned with improving the kinetics by means of doping and catalysts. Current work involves the use of complex hydrides, such as lithium boron hydride (LiBH4), as well as alanates, such as sodium alanate (NaAlH4).
High-performance ceramic structural materials
All-ceramic turbine blades
This project aims to identify the prospects for targeted development of an all-ceramic turbine blade for use at temperatures above 1400 °C. This would allow a higher turbine inlet temperature to be achieved. Along with a significant reduction in the volume of cooling air required, this would permit a further significant increase in the efficiency of a gas turbine.
The composite concept under study consists of a three-dimensional textile reinforcement structure made of ceramic fibres and a ceramic matrix. The expertise in the area of three-dimensional fibre structures is provided by the Chair for Textile Machinery (ITA) at RWTH Aachen University headed by Univ.-Prof. Dipl.-Wirt. Ing. Thomas Gries. Years of experience in processing ceramics and developing high-temperature materials are supplied by RWTH Aachen University's Department of Mineral Engineering (GHI, Univ.-Prof. Dr. rer. nat. Rainer Telle) and Forschungzentrum Jülich's Institute of Energy and Climate Research IEK-1: Materials Synthesis and Processing (Univ.-Prof. Dr. rer. nat. Detlev Stöver, Univ.-Prof. Dr. Robert Vaßen).
The work focuses on the oxide fibre ZrO2-based ceramics system. Central questions especially concern damage tolerance and adequate solidity of complex-shaped components. For our research, the interdisciplinary cooperation of the participating institutes with their diverse areas of expertise is a fundamental prerequisite for successful work on the project.
The aim of the work currently under way is to determine whether customized three-dimensional textile reinforcement structures and ceramic matrices reinforced to withstand high temperatures constitute a successful method for a new attempt at developing an all-ceramic turbine blade. In addition, it explores whether at RWTH Aachen University and Forschungszentrum Jülich, in cooperation with industrial companies, the competence can be demonstrated to implement a large-scale collaborative project for refining the concept up to and including the development of a component as a prototype.
Barrier coatings for ceramic matrix composites
Environmental barrier coatings (EBCs) are employed when high thermal stressing and corrosive atmospheres make the use of components difficult or impossible. They are used in thermal barrier systems of space re-entry vehicles, as seeker head covers for highly agile missiles, or as coatings for combustion chamber shingles in aircraft gas turbines. When used as barrier coatings in gas turbines, they not only provide protection for the particular components, but frequently also boost efficiency and reduce emissions.
The nickel base superalloys used as substrates today have only limited potential for higher temperatures. For this reason, we are investigating materials with higher temperature stability. One option in this area is ceramic matrix composites (CMCs), which consist of fibres (in this case, oxide ceramic fibres, for instance, frequently NextelTM 610Al2O3, as well as Mullit), and ceramic, porous matrix (e.g. Al2O3, SiC). A well-known problem associated with these substrate materials is that they are highly sensitive to water vapour corrosion, which makes them dependent on a protective coating.
Stable barrier coatings can also have the function of an abradable coating, which is used to reduce the air gap between the rotating and the static components of a gas turbine. In this case, EBCs must thus demonstrate good abradability or be friable.
Due to the extremely high thermal stressing present in all of the above-mentioned areas of application, good thermoshock behaviour must be guaranteed that allows a sufficiently long lifetime at high temperatures.
Materials used for EBCs include various silicates (e.g. yttrium monosilicate), yttrium-stabilized zirconia (YSZ), yttrium monosilicates, mullites, or Mg spinel. Dense YSZ coatings demonstrate characteristics such as good resistance to water vapour corrosion. Spinel coatings are characterized by their good abradability.
The adhesion of the coating to the porous substrate material is a critical aspect. Applying the coatings using atmospheric plasma spraying (APS) is possible.
In order to achieve the best possible adhesion to the substrate material, the coatings are frequently designed as double layers (e.g. mullite/yttrium monosilicate or also as YSZ/yttrium monosilicate).
At IEK-1, research is conducted on oxide ceramic protective coatings. In so doing, one priority is the production of these coatings. Depending on the particular material, the process parameters are optimized during atmospheric plasma spraying. Microstructural analyses carried out subsequently provide information on the suitability of the coatings developed. Another priority involves lifetime tests. New EBCs or those which have been optimized are tested under alternating thermal stress using the institute's test stands.
Porous titanium-base implants
For ceramic moulding, placeholder materials have become established, for instance, in order to create defined pore channels for gas transport in fuel cell substrates. This approach has been successfully transferred to powder metallurgy.
If suitable placeholder materials such as ammonium hydrogen carbonate are mixed and pressed with a metal powder, the compacted pellets then have adequate stability in an unsintered state (green state) to be worked into a near-net shape by means of conventional mechanical processing (drilling, turning, milling). If metal powders that can be pressed well are used, this eliminates the need for organic binders or pressing aids. The placeholder is removed through degradation in air at temperatures < 150 °C, leaving only minimal oxygen and carbon contamination behind. After the degradation of the placeholder, the porous shaped body is sintered, which gives it the stability required for later use. Fractionation of the placeholder allows defined pore sizes in the range of 100 µm to 1000 µm and porosities of up to a maximum of 80 % to be created.
A popular area of application for the method is the production of porous implant materials. For this purpose, the preferred class of materials is titanium and its alloys. The method's potential was demonstrated with the prototype of a cup for a hip implant (see figure). The method was transferred to a dental implant with porous exterior surface (see figure). A licence for the method was granted to Synthes, which brought a vertebral implant manufactured with this method onto the market in 2007 (see figure). One technological feature is the stepwise graduation of the implant. The highly porous part ensures reliable anchoring of the implant because it grows into the adjacent vertebrae, while the dense part provides adequate stability during the implantation. In 2009, the interdisciplinary work was awarded the Erwin Schrödinger Prize of the Stifterverband–a joint German industry initiative for promoting science and higher education.
An alternative method for forming near-net shape, porous components is metal injection moulding combined with suitable placeholder materials. Since the placeholder must withstand temperatures of up to 150 °C and pressures of several hundred MPa during the injection moulding process, NaCl was introduced as an alternative placeholder. The fundamental suitability of the method was demonstrated for simple geometries (see figure). Proof of functionality was provided with titanium powders, since this process engineering technique also has great potential for biomedical applications. The placeholder is added to the injection-moulded materials and after the moulding process is dissolved in a water bath. Desalination is carried out between the removal of the first binder components (solvent extraction) and the second binder component (thermal debinding). After the debinding, the component is sintered under inert gas.
At present, work is under way to fabricate vertebral implants by means of metal injection moulding (MIM). In order to achieve graduation of the porosity, two-component injection moulding is used. This is a new, innovative moulding technique used in the field of powder metallurgy. Suitable plant engineering and tool design permit two different injection-moulded materials to be injected into an injection mould with the appropriate cavities immediately after each other, thus allowing graduation of the porosity to be achieved directly.
Nickel-titanium-base shape memory alloys
Powder metallurgy of NiTi
Within the framework of the special collaborative research project SFB459 "Shape Memory Technology" at Ruhr-Universität Bochum funded by the German Research Foundation (DFG), the institute concerns itself with powder-metallurgic fabrication of shape memory alloys. Powder metallurgy offers the advantage of near-net-shape forming of this alloy, which can only be processed mechanically with a great deal of effort.
The studies focus on:
- Optimizing the sintering processes from prealloyed powders
- Near-net-shape forming by means of metal injection moulding (MIM)
- Forming by means of hot isostatic pressing (HIP)
- Functional coating by means of plasma spraying
- Porous NiTi shape memory alloys
Powder metallurgy of ternary NiTi-X alloys
Optimizing the sintering processes from prealloyed powders
Production of NiTi shape memory alloys using powder metallurgy based on Ni and Ti elemental powders is complicated by the fact that during the formation of the alloy, developing thermodynamically stable secondary phases without shape memory properties cannot be fully prevented. This is due to an exothermic reaction between Ni and Ti.
For this reason, the focus of the work at the institute is on the use of prealloyed NiTi powders, which are manufactured by a external industrial partners through gas atomization of NiTi bars or from the melt. At the institute, the sintering parameters for prealloyed NiTi powders were optimized. Furthermore, it was determined in what form the uptake of oxygen and carbon, which cannot be prevented during the process flow, influences the microstructure that has formed. Since the NiTi matrix has only a limited solubility for oxygen and carbon, Ti4Ni2Ox is formed and TiC is deposited (see figure). These deposits remove Ti from the matrix and can cause the conversion temperatures to drop. In addition, the oxidic phase causes cracks (see figure).
Near-net-shape forming by means of metal injection moulding (MIM)
Metal injection moulding (MIM) has become an established method for the large-scale fabrication of small components with complex shapes. Especially for NiTi shape memory alloys, it has a great advantage, because with conventional mechanical machining, these alloys can cause high tool wear.
The basic feasibility of the MIM method was demonstrated using prototypes of a biomedical staple and a clamping sleeve. In both cases, the application is based on the one-way effect. For the characterization of the mechanical properties, a tensile test specimen was also produced.
Forming by means of hot isostatic pressing (HIP)
One alternative to compressing prealloyed NiTi powders is hot isostatic pressing (HIP). For this procedure, the powder is filled into a metallic capsule (for example, made of steel 1.4571). The capsule is evacuated and sealed. Afterwards, compression in the hot isostatic pressure is carried out by simultaneously applying pressure and temperature. In contrast to the NiTi samples sintered without pressure, as a result of the procedure, the residual porosity is nearly completely eliminated.
The advantage of the method is that the theoretical density is nearly achieved and the uptake of additional oxygen and carbon contamination is avoided in comparison to the starting powder.
The disadvantage of the method is that near-net-shape forming is only possible to a limited extent. However, manufacturing simple geometries such as tubes is possible in principle.
In addition, by means of spark erosion, tensile test specimens can be fabricated to determine the mechanical properties of hot isostatic pressed NiTi alloys.
Functional NiTi coating by means of plasma spraying
The process of vacuum plasma spraying is suitable for producing wear protection coatings from prealloyed NiTi powders, with thicknesses in the range of several 100 µm. With this production method, the coarse particles resulting from the powder manufacture can be used.
During the coating process, the powder particles are dissolved by heat in the plasma and encounter the substrate material to be coated in a molten state. If the coatings have pseudoelastic properties after coating, an increased resistance to wearing processes such as cavitation is expected.
Porous NiTi shape memory alloys
The classic placeholder method (pressing a powder placeholder mixture, near-net-shape forming in an unsintered state) can be applied to NiTi powders to only a limited extent, since these powders have a spherical shape during the preferred method of production by means of gas atomization, which makes them difficult to press. In addition, pseudoelastic properties of the starting powder impair the compression process.
For this reason, the placeholder method was applied to metal injection moulding, in order to produce porous NiTi components in a near-net shape. NaCl is the placeholder material of choice, because it has sufficient thermal and mechanical stability to withstand the temperatures and pressures. Figure 10 shows a porous NiTi shape body used to identify mechanical properties, as well as the associated microstructure.
The following figure shows the compression behaviour of a porous NiTi shape body. In the case of compression up to approx. 6 %, a nearly complete return to shape resulting from pseudoelastic behaviour is observed.
The nearly complete return to shape with low compression makes porous NiTi interesting for implant applications, since the material copies the elastic behaviour of the human bone better than other metallic implant materials.
Powder metallurgy of ternary NiTi-X shape memory alloys
In recent years, the development of ternary and quaternary NiTi-X shape memory alloys has increased, since alloy elements such as Mo, Al, Hf, Zr, Co, Cr, Cu, and Nb can be used to selectively influence the conversion behaviour and the conversion temperatures. This allows application areas to be covered that are not accessible with binary NiTi alloys. At present, experiments on powder metallurgical processing of ternary shape memory alloys are under way. The alloy systems under investigation are NiTi-Nb, NiTi-Ag, and NiTi-Cu.