Research highlights from Fraunhofer IFAM

Aluminum fiber structures for highly efficient heat pumps

As part of the project “Development of a Gas Adsorption Heat Pump with a Crystallized Zeolite Heat Exchanger and a New Evaporator- Condenser Apparatus (A DOSO)”, funded by BMWi, the Fraunhofer IFAM in Dresden is working with Stiebel Eltron GmbH & Co. KG, SorTech AG and the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg to develop a highly efficient adsorption heat pump for the provision of hot water and heat in residential buildings.

Efficient technology for the provision of hot water and heat in residential buildings is an essential cornerstone of a resource-efficient energy supply. Conventional heat pumps run on electricity, whereas these heat pumps use environmental heat sources such as geothermal sources, as well as solar or ambient air heat. In seeking to offer the next generation of natural gas technology, the successor to the gas condensing boilers currently available on the market, we have combined a gas-condensing boiler with a thermally driven heat pump module for use in gas heat pumps. The goal of the ADOSO project is to develop a zeolite-based heater using water. This will be significantly more compact and cost-efficient than current heaters on the market while producing comparable results, thanks to a novel adsorption heat exchanger, which will enable it to optimize the operation of the evaporator and condenser, whilst attaining an annual performance factor of higher than 1.3.

For the development of the adsorption heat exchanger, we have focused on porous but still thermally conductive materials that have a highly coatable surface. To this end, the Fraunhofer IFAM is working on structures made of sintered aluminum fibers. The short fibers made of AlSi1 aluminum-silicon alloy are firmly bonded together using liquid phase sintering. This creates open porous structures with a highly specialized surface on which the zeolite is crystallized. At 75 percent, the high proportion of surface pores allows steam to access the zeolite surface as well as the interior structure. The steam is captured on the zeolite and releases a large amount of heat, which is then transferred to the water pipes by the aluminum fiber structure. Desorption of the steam reverses this process, so that as the zeolite is heated, it releases the water molecules.

The large surface area of the metal fiber material in combination with the zeolite layer accelerates the dynamic of the adsorption and desorption in comparison to current technology. The first prototype, which has not been further optimized, has already doubled the performance density at 300 W/l. This brings the reality of compact, cost-efficient adsorption heat pumps within reach. This same principle can also be applied to refrigeration using waste heat, opening up another very interesting market. SorTech AG will soon begin field tests of the new generation of adsorber in their own line of refrigeration machines.

Electron Beam Melting - versatile and productive

Selective Electron Beam Melting  (SEBM) is a powder bed-based additive manufacturing process for the production of complex metallic components. It has so far been industrially introduced primarily for the processing of titanium-based materials in the aerospace and medical technology sectors.

The advantages of  SEBM for metallic components lie in particular in the powders and a high degree of freedom with regard to component design and material selection. Powder costs, for example, are significantly lower than the prices for laser beam-based processes. Deviations in particle shape can also be better tolerated. The targeted preheating of the powder bed to temperatures of over 1000 °C reduces the tendency to residual stresses and, thus, to component distortion. Therefore, the comparatively short support structures do not have to be tied up to the starting plate. The resulting possibility of making components more complex and stacking them makes SEBM a productive additive process.

A further advantage of the increased powder bed temperatures is in particular the processability of materials that are difficult to weld and, thus, an extension of the range of materials used in common beam-based processes. Therefore, a special potential lies in the processing of steels for applications in mechanical and plant engineering as well as in tool making. In the newly opened "Innovation Center Additive Manufacturing ICAM" in Dresden, Fraunhofer IFAM is working together with industrial partners in various projects on this attractive complex of topics.

The additive processing of corrosion-resistant steels by SEBM into components with internal structures has already been successfully demonstrated several times. In the AGENT-3D_FunGeoS* project, the housing of a servo-hydraulic valve was redesigned for production and realized in austenitic stainless steel (Figure 1). SEBM creates a specific structure with advantageous mechanical properties. The additive processing of duplex steel and the construction of pump impellers are a focal point of the RessFAST* project.

From the important class of tool steels, one cold work steel and one high-speed steel were qualified in the AddMaTs* project. The challenge with these materials is to avoid cracking of these high carbon materials. The processed material is characterized by a fine and homogeneous microstructure. Heat treatment can be carried out in a similar way to conventionally processed steel and, therefore, provides a good basis for hybrid production. Another development object is alloys with a high hard material content for applications requiring special wear resistance. Finally, the investigations also consider the limits of realizable geometries, such as wall thicknesses, channel diameters and overhang angles. *

* Projects AGENT-3D_FunGeoS, RessFAST and AddMaTs are funded by BMBF

High-efficiency electrode materials for gas-generating reactions

For political, security, and energy strategy reasons it is vital to guarantee the availability of raw materials. Hydrogen is one of the irreplaceable base chemicals of the chemical industry and, being an energy source, is one of the central pillars of the shift to renewable energy. The CO₂-neutral production of hydrogen can only be achieved industrially via the electrochemical splitting of water in an electrolyzer that is powered by renewable energy sources.

Improving electrolysis technology for “green” hydrogen

The further development of electrolysis technology has high strategic importance for Germany in order to generate “green” hydrogen for commercial use. At present, the conventional way to generate hydrogen is via gas reforming. This is economically favorable due to the availability of natural gas but is accompanied by very high CO₂ emissions. Fraunhofer IFAM in Dresden is undertaking development work to improve electrolysis technology for commercial hydrogen manufacture. The main costs (investment costs and operational costs) of electrodes and stacks are the focus. The aim of the development work is to considerably reduce the plant and hydrogen production costs using electrodes that have long-term efficiency.

Fraunhofer IFAM in Dresden is evaluating novel gas-generating electrode materials using electrochemical methods – combined with structural analysis. Degradation mechanisms of the electrodes are being elucidated, allowing conclusions to be drawn for improved electrode composition and manufacture. Favorable-cost nanocrystalline electrodes (Fe and Ni alloys) have been developed, whose high catalytic activity can be maintained by activation in the electrolyzer. In addition, the electrode materials are being tested under real conditions in single-cell test stands to gain information about gas bubble transport in the cells. This then allows adaptation of the electrode and cell geometry. The end result is the development of customized electrodes with advantageous gas bubble transport. Our in-depth knowledge of electrochemistry, metallic materials, and production technologies make Fraunhofer IFAM an expert partner for manufacturers of electrolyzers.

Metallic materials for implants – highly porous and degradable

An ongoing medical problem is how best to treat major bone damage. Such damage does not heal spontaneously and requires implants. The best bone replacement is, and always has been, the patient’s own bone. However, only limited amounts of this are available and its removal also involves risks. The use of a synthetic bone replacement is also accompanied by various risks. The ideal solution is a degradable material, namely implants which disappear after successful bone healing.

Magnesium comes very close to being the ideal material. It degrades in biological surroundings, has excellent biocompatibility, and bonds very readily to bone. Fraunhofer IFAM in Dresden has developed a magnesium implant material whose structure also gives it other favorable properties. The metallic fiber structures form a highly porous lattice which assists bone growth and also the growth of blood vessels. Such structures are, however, of particular interest due to their reduced stiffness, very closely resembling the biomechanical properties of bone. This stimulates bone growth. The starting point for the development work was the production of short magnesium fibers via extraction from the melt. These fibers are then homogeneously deposited and sintered. The particular challenge for manufacturing magnesium fiber structures is the sintering, which the material with extremely high oxygen affinity resists due to the stable surface oxides. The heat treatment is hence undertaken in a partial melt phase. Precise knowledge of the melt phase composition is decisive for the sintering result. The best sintering regime was determined using computer simulation methods. The resulting implants possess favorable mechanical properties and also excellent corrosion properties. Multiple depositions of Y₂O₃ at the grain boundaries allowed for a degradation behavior with reduced corrosion rate to meet the physiological requirements. In the animal model, slow corrosion was measured after 12 weeks but after 24 weeks the majority of the metal implant had disappeared.

Realisation

As a winner of the Medical Technology Innovation Competition, the research was funded by the BMBF (Federal Ministry of Education and Research). The favorable properties of the implant material have in the meantime also convinced industry, with Botiss Dental GmbH now licensing the patent. The company plans to use this material in oral surgery and is currently evaluating the setting up of a suitable production chain.

PowerPaste – Hydrogen without storage pressure

The most important advantages of hydrogen as a secondary energy carrier have long been known. However, hydrogen has not yet been widely established on the market, for example for fuel-cell applications in the mid-performance range, because until now the available hydrogen storage solutions could not be established in the market due to costs, problems with their technical implementation, or lack of hydrogen infrastructure. PowerPaste, a new development from Fraunhofer IFAM in Dresden, has the potential to fundamentally change this situation.

A well-known alternative to conventional hydrogen storage methods, such as high-pressure or cryostorage, is hydrogen production using so-called hydrolysis reactions. One challenge facing this technology is comparably low energy densities of the materials that have been previously used for hydrolysis. Above all, it is problematic that, when producing hydrogen by hydrolysis, the required amount of hydrogen needs to be assessed in advance, because the material continuously reacts with the hydrogen as soon as the components have been mixed. Also, in principle, only a relatively small predetermined performance range can be covered.

Fraunhofer IFAM in Dresden has developed a new technology that solves all three of these problems for the first time. The most important component of the technology is the PowerPaste, a high-energy pasty material mixture based on magnesium hydride (MgH₂), which releases hydrogen on contact with water according to the following chemical reaction:

MgH₂ + 2 H₂O à Mg(OH)₂ + 2 H₂

Releasing hydrogen in this way was not possible in the past, because magnesium hydride forms passivation layers when in contact with water that significantly reduce the reaction speed. However, after developing the technology further, Fraunhofer IFAM has succeeded in making magnesium hydride usable for technically relevant applications. Powdered magnesium hydride is first modified by adding inexpensive, non-toxic substances that effectively prevent the formation of the troublesome passivation layers. In the next step, esters are added to create a pasty material – the so-called PowerPaste. The PowerPaste reacts dynamically with water, causing a fully controlled hydrogen-producing reaction, which can power the fuel cell as needed, directly generating electrical power.

The feasibility of functional power generators based on this technology has already been demonstrated with a stationary 50W demonstrator at a technological maturity level of 4 as well as with a mobile 300W power generator at a technological maturity level of 5, which were both developed at Fraunhofer IFAM in less than a year. As a whole, the technology has huge economic potential, especially for power-hungry mobile and portable fuel-cell applications such as light electromobility (e.g. electric bikes for consumers and fleets), drones, and emergency power systems.

Recycling of rare earth magnets and production waste

The imposition of export duties by China in 2010 and 2011 for certain raw materials made lowering the need for rare earth imports a key economic goal of the German government. The three main ways of achieving this goal are exploiting own primary deposits, reducing the use of or substituting certain rare earth elements, and recovering rare earths via recycling.

Although neodymium, praseodymium, and dysprosium are the most industrially important rare earths due to their use for the manufacture of high-performance NdFeB magnets, there has been insufficient emphasis up until now to recycle these elements. Indeed, there are very few approaches for processing old magnets and magnetic waste that allow effective separation of the rare earth elements and removal of contaminants (e.g. oxides, organic compounds).

Fraunhofer IFAM in Dresden has developed a hydrometallurgical recycling process for the effective recovery of neodymium, praseodymium, and dysprosium from material mixtures. By exploiting physical relationships and having optimum process control, the principle is to have such a high overall selectivity that there is no need for complex and costly fine separation of rare earths using ion exchangers or liquid-liquid extraction plants. In order to produce new high-quality NdFeB magnets it is, however, essential that samarium, which is present in mixtures of magnetic materials, is removed and also oxides and organic compounds.

Project work has demonstrated that the required selectivity can be achieved for recycling magnetic materials on a laboratory scale by ensuring suitable physical pretreatment and preseparation, plus controlled hydrometallurgical digestion. This recycling method is in the process of being patented.

Efficient use of energy using thermoelectric materials

Increased energy efficiency, resource conservation, and the reduction of CO₂ emissions are some of the most important social and economic challenges of our time. When energy is produced, up to 50 % of the primary energy is lost as waste heat, however. Thermoelectric generators (TEG) can contribute to more efficient and low-emission energy usage by recovering energy from this waste heat.

Current technology uses waste heat in heat exchangers, heat storage devices, burners for air heating, heat pumps, and cooling machines. To produce electricity from waste heat, the ORC (organic rankine cycle) process, steam turbines, and Stirling engines can be used. Electricity is generated indirectly via intermediate media in liquid or gaseous form.

TEGs on the other hand convert waste heat directly into electrical energy, working without moving parts. They are silent, maintenance-free, and scalable, adapting easily to the space available. In comparison to the technologies mentioned above, they have significant advantages and cover a broader spectrum of applications.

Since 2007, Fraunhofer IFAM has been developing n-Mg₂ Si_0.4Sn_0.6 and p-MnSi_1.8 as thermoelectric (TE) materials; these are made from inexpensive, non-toxic elements easily found in nature and can be used in applications at up to 550 °C. For TE modules made from these silicides, efficiency levels of up to 7 % have been reported in the literature. This dramatically exceeds the operating temperature and the efficiency of commercially available modules made from Bi₂Te₃-based connections (250 °C and ~ 3 %). Fraunhofer IFAM has focused its research activities on scaling up material production (currently up to 1 kg starting powder per batch and 0.25 kg per sintered body) in the manufacture of TE components (TE legs) in various dimensions with regard to industrial-scale production. To cover the entire value chain, we are currently developing configuration and connection technology for module production from the components.

The TE silicide chip modules developed by Fraunhofer IFAM (chip thickness of only 0.5 mm) will be installed in diesel locomotives in a BMWi-funded project on energy recovery from waste heat.

To win over users with this technology and to establish TEGs on the market, the positive properties of the materials as well as high-volume, cost-effective industrial production of TE modules must be demonstrated. For this reason, a partially automated production chain for silicide-based modules is being built at Fraunhofer IFAM in Dresden, in order to significantly reduce the hitherto high unit costs

Customers: DFG, BMBF, BMWi, SAB

Project partners: Max-Planck-Institut für Chemische Physik fester Stoffe, FZJ, Universität Tübingen, Universität Hamburg, TU Chemnitz, Fraunhofer IWM, Fraunhofer IPM, Fraunhofer IKTS, TU Dresden, companies: Mahle, Tenneco, Curamik, O-Flexx, Bombardier

New concept for environmentally friendly production of aluminium

Aluminium is an indispensable part of our daily lives. Whether as household foil, in cooking appliances or for the lightweight construction of cars and airplanes - the list is endless. However, large quantities of carbon dioxide are generated in the production of this important material.

Within the AGRAL project, funded by the European Commission, researchers from Fraunhofer IFAM in Dresden, together with partners from industry and research under the direction of the mining company Rio Tinto, have developed a composite material and its manufacturing process for anodes within the framework of the AGRAL project, which is funded by the European Commission.

The electrolytic Hall-Héroult process from the 19th century, which has remained the most economical process to this day, is used as standard for the production of the light metal aluminium.

The reduction of aluminium oxide to aluminium takes place in electrolysis furnaces by applying a low DC voltage between anode and cathode. The current flow keeps the process temperature constant on the one hand and causes the chemical process by electrolysis on the other hand. The electrolyte used is molten cryolite with aluminium oxide, which allows the reduction to take place at a temperature level of 900 - 1000°C. The electrolysis process is carried out at a temperature level of 900 - 1000°C. The electrolysis process is carried out by means of an electrolysis process. The liquid aluminium collects at the bottom of the furnace and acts as a cathode.

The anodes consist of pre-baked blocks of petroleum coke and tar pitch, i.e. essentially carbon, and are consumed during the process by the reaction with the released oxygen of the aluminium oxide to form carbon monoxide (CO) and carbon dioxide (CO₂). For each ton of aluminium produced, about half a ton of anode coal is consumed, resulting in the emission of about 1.8 tonnes of CO₂, accompanied by the release of fluorinated carbons.

In order to avoid these greenhouse gases and pollutants, the current joint project aims to develop a stable, oxygen-developing anode (inert anode) for aluminium electrolysis on a prototypical scale.

The newly developed composite material can meet extreme requirements such as good electrical conductivity, high mechanical strength and resistance to thermal shock. The composite material, consisting of a mixture of iron oxide and a metal alloy, applied as a millimetre-thick layer to a metal substrate made of a nickel alloy, shows extremely high stability in aluminium electrolysis. This anode has already been successfully tested for several hundred hours.

In addition, suitable powder metallurgical production routes based on a pressure-assisted sintering process (HIP - hot isostatic pressing) or a high-speed flame spraying process were developed and tested in the joint project.

In order to further develop and commercialise this process, the Australian-British raw materials group Rio Tinto and its American competitor Alcoa have now founded the Elysis joint venture with financial support from Apple.