Transition metal based magneto caloric materials for energy efficient heat pumps
Ekkes Brück, Hargen Yibole, Van Thang Nguyen, Xuefei Miao, Maurits Boeije, Luana Caron, Lian Zhang, Francois Guillou and Niels Van Dijk
Fundamental Aspects of Materials and Energy, Department of Radiation Science and Technology, Faculty of Applied Sciences, Delft University of Technology, Delft, The Netherlands
Email:email@example.com, web site: http://www.rst.tudelft.nl/fame
Domestic refrigeration and air-conditioning contributes to about 30 % of the electricity bill of an US household, in upcoming economies in subtropical areas this share is growing fast. The majority of cooling devices nowadays utilizes the vapor refrigeration cycle which works as follows; first the gas is compressed in a compressor, the heat produced in the compression stage is released to the environment and the gas condenses to form a liquid. In a throttling stage the pressure of the liquid is lowered and the fluid cools down forming a mixture of liquid and gas. Evaporation from the cold fluid takes up the heat from the substance that needs to be cooled and the gas is fed back to the compressor.
This refrigeration cycle can be made energy-efficient when certain gases are utilized. However, these gases are extremely strong greenhouse gases. Currently refrigerant gases are the fastest growing source of greenhouse gas emissions. If left unchanged, it is expected that in 2050 refrigerant gases represent 9-19 % of global greenhouse gas emission.
A similar but more energy-efficient refrigeration cycle as described above, can be achieved with magnetic materials that show a large magnetocaloric effect. These materials heat up when a magnetic field is applied. After this heat is transferred to the environment, they cool down on removing the magnetic field and can take up heat from the substance that needs to be cooled. The processes as described are highly reversible and therefore very energy-efficient, which can lead to a much lower utilities bill. Additionally, these magnetic materials are solids that can easily be recycled and do not contribute to the atmospheric greenhouse effect. Thus, this solid-state technology has the potential to strongly reduce the environmental impact of cooling technology.
With the advent of giant magnetocaloric effects (MCE) that occur in conjunction with magneto-elastic or magneto-structural phase transition of first order(FOT), room temperature applications became feasible. In this context the MnFe(P,X) system is of particular interest as it contains earth abundant ingredients that are not toxic. This material family derives from the Fe2P compound, a prototypical example known since a long time to exhibit a sharp but weak FOT at 210 K (-63°C).
In this hexagonal system, the Fe atoms occupy two in equivalent atomic positions referred as 3f (in a tetrahedral environment of non-metallic atoms) and 3g (pyramidal). One intriguing aspect is the disappearance of the magnetic moments of iron atoms on the 3f sites when crossing TC, whereas there is only a limited decrease on the 3g site. This observation has led to a cooperative description of the FOT linking the loss of long range magnetic order at TC with the loss of local moments on 3f. This mechanism has recently been shown to be at the origin of the G-MCE observed in MnFe(P,Si).
The disappearance of the magnetic moments has been ascribed to a conversion from non-bonding 3f d electrons into a distribution with a pronounced hybridization with the surrounding Si/P atoms. Therefore, one can expect to adjust the properties of these compounds by substitutions on the non-metallic site. This solution has been used to optimize the properties of MnFe(P,Si) materials: .
1.Nguyen H. Dung., et al. Mixed Magnetism for Refrigeration and Energy Conversion. Advanced Energy Materials 1 (6), 2011 pp. 1215-1219.
2.Guillou F., et al., Taming the First-Order Transition in Giant Magnetocaloric Materials. Advanced Materials 26 (17), 2014 pp. 2671–2675..