Magnetic materials for energy and raw material economization
Rare Earth Metal-Free Permanent Magnetic Materials
The development of environmental friendly energy sources largely relies upon the access of high quality permanent magnets (PM). A wind turbine, e.g., uses 300 kg of magnets. Today, the most powerful PMs are based on rare-earth (RE) metals. However, export quotas posed by the main producer China, have resulted in a dramatic prize increase of RE materials. This has stimulated intense effort towards the search for new non-RE containing PM.
Figures of merit for PMs are the maximum energy product (BH)max and the coercivity, Hc. (BH)max is the highest available energy that can be stored in the PM and it is dependent on the remanent magnetic induction, Br. Hc gives the robustness of the PM against demagnetization due to external magnetic fields. Currently, the best values are (BH)max = 400 kJ/m3 and Hc = 2 MA/m, obtained for RE based PMs. Equally important is the critical temperature, Tc, above which the material ceases to be magnetic. There exist a number of families of PMs as shown in the figure below. The Nd2Fe14B magnets are by far the best while ferrites are the most widely used due to their low price. However, since ferrites have low Br, much more material is needed in applications compared to RE magnets.
One objective in this project is to find a PM with (BH)max above 100 kJ/m3, which is much higher than ferrite PMs but lower than the best RE based PM. This will require Br close to 0.7 T and Hc close to 1 MA/m. The targeted Tc is > 600 K. These objectives suggest that we should begin our search for new PM materials by looking for low symmetry, iron rich intermetallic compounds. The low symmetry materials are needed since they are known to exhibit large magnetic anisotropy (which is a prerequisite for a large Hc). Our quest is conducted in a cross-disciplinary effort, involving experimental chemistry and physics, materials theory and industry.
One material investigated is (Fe1-xCox)2B, having uniaxial magnetic anisotropy, Tc > 900 K and a potential Br = 1.3T. We will also study tetragonally distorted iron carbides. Theoretically, it has recently been shown that alloying additions have marked impact on the crystal lattice, which possibly would increase the magnetic anisotropy. But for the moment it is MnAl compounds that attract our main interest. Mn has a large magnetic moment per atom, but the moments are antiferromagnetically coupled. Forming an alloy with Al, the distance between the Mn atoms increases and the interaction between the Mn moments turns ferromagnetic. Adding e.g. carbon to the alloy stabilizes the structure. The research project has already produced Mnal-based materials with highly promising magnetic properties.
The outcome of this project will be a novel RE-free PM material with the capability of being formed using metallurgical production routes.
Contact: Senior lecturer Klas Gunnarsson
Cooling and heating systems using vapor-compression techniques totally dominate the market for refrigeration, heating, ventilation and air-conditioning (RHVAC). Alternative techniques that are more energy efficient and do not use greenhouse gases (hydrofluorocarbon (HFC) refrigerants) are requested. Magnetocaloric thermodynamic processes are more efficient than vapor-compression processes and do not need HFC refrigerants. Energy savings of more than 20% would be gained if magnetocaloric systems were to substitute current RHVAC systems.
In an inter-institutional research project supported by the Swedish Research Council (VR) we are investigating and searching for suitable magnetic materials for magnetocaloric applications. The system (Fe1-yMny)2P1-xSix experiences a composition dependent 1st order ferro- to paramagnetic transition near room temperature (cf. figure for y=0.5). Extensive research on this material system in Uppsala and elsewhere indicates that certain compositions of (Fe1-yMny)2P1-xSix will have applicable magnetocaloric properties.
Contact: Professor Per Nordblad