Introduction

Magnetic refrigeration

Energy efficient cooling will be one of the most important challenges for our future society. Energy demands for refrigeration and air conditions are expected to grow dramatically in the coming decades. All refrigeration technologies have one thing in common: they contain a refrigerant which changes state, and in doing so, changes temperature. Magnetic refrigeration is the only alternative cooling technology which would simultaneously eliminate the need for harmful refrigerant gases and reduce the energy requirements, and hence carbon dioxide emissions. Magnetic cooling offers up to 30% improved efficiency as compared to conventional refrigeration technologies [1,2].

The magnetocaloric effect (MCE) originates in the partial alignment of the magnetic moments of the material by an external magnetic field. Therefore a decrease in the entropy of the magnetic moments is observed. When applying a magnetic field under adiabatic conditions, the reduction of the magnetic entropy is compensated by an increase of the entropy of the crystal lattice. This transfer of entropy is causing the warming of the material. By removing the magnetic field again, the opposite effect is observed and the material cools down. A cooling machine can be designed by utilizing the magnetocaloric effect in a cyclic operation as illustrated in the schematic.

 
 

The magnetocaloric cooling cycle

The magnetocaloric cooling cycle consists of four steps. First the refrigerant is exposed to a magnetic field which results in a heating of the material. This excess heat is expelled to the surroundings. The removal of the magnetic field causes a cooling of the refrigerant and heat can be absorbed from the cooling compartment.

 
 

History of Magnetic Cooling

1917: Discovered by P. Weiss
1933: Giauque and MacDougall reached temperatures below 1 K using adiabatic demagnetisation of Gd2(SO)4 8H2O
1949: Giauque received Nobel Prize in chemistry
1976: G.V. Brown: first magnetic refrigerator near room temperature
1997: Pecharsky and Gschneidner, Jr. discovered “giant” magnetocaloric effect in Gd5Si2Ge2 paving the way for new “1st order” transition refrigerants
 

Research topics

The number of publications related to magnetic refrigeration drastically increased after the discovery of the giant magnetocaloric effect in 1997. There are only few material families showing a first-order magnetostructural transition near room temperature being mostly based on rare earth elements like Gd-Si-Ge and La-Fe-Si. Hydrogenated La-Fe-Si-Mn shows very large magnetocaloric effects with a very small thermal hysteresis.

The ideal magnetic refrigerant displays the following attributes:

• large magnetic entropy jump at Tt and a broad operating temperature range

• large ΔTad / ΔH ⇒ driven by moderate magnetic field (permanent magnet)

• small thermal and magnetic hysteresis, fast dynamics

• material criticality: low cost, non-toxic, etc. (avoid e.g. Gd or As)

• high thermal and small electrical conductivity

• mechanical and chemical stability, high ductility

 
 

There are many first- and second-order magnetocaloric materials showing large adiabatic temperature changes near room temperature [2].

 
 
 
Hydrogenated La-Fe-Si-Mn is one of the most promising magnetocaloric materials showing much larger isothermal entropy changes than Gadolinium or La-Fe-Si-Co [5].
Hydrogenated La-Fe-Si-Mn is one of the most promising magnetocaloric materials showing much larger isothermal entropy changes than Gadolinium or La-Fe-Si-Co [5].

The big challenge for this material family is the production of efficient heat exchanger geometries for application. Useful techniques are powder metallurgy and machining or the net-shape preparation of a composite refrigerant with a binder in order to obtain thin and mechanically stable designs with best performance [3,4,5].

 
 
Large reversible temperature changes in limited magnetic fields can also be obtained in the rare earth free Heusler alloys Ni-Mn-In-Co. After a very large drop of the temperature of -8 K in the first magnetization process, the ΔTad accounts to 3K respectively [7].
Large reversible temperature changes in limited magnetic fields can also be obtained in the rare earth free Heusler alloys Ni-Mn-In-Co. After a very large drop of the temperature of -8 K in the first magnetization process, the ΔTad accounts to 3K respectively [7].

Beside the Fe2P-type materials [6], Heusler alloys are among the most promising rare earth free magnetic refrigerants. Unfortunately, a large thermal hysteresis is typically observed in Heusler compounds, which has negative impact on a cyclic operation. We could however demonstrate that the thermal hysteresis can be partly overcome by multiple stimuli or by moving in minor loops of the transition instead of completely transforming the material from one phase to the other [2,7]. A reversible ΔTad of 3K could be achieved in 1.9 Tesla for our Heusler compound.

 
 

As a summary we are working on:

  • Synthesis, comprehensive characterization and evaluation of all relevant materials by e.g. direct ΔTad measurement leading to material libraries
  • Understanding and control of critical properties (first-order transition, required fields, hysteresis, time dependency) by nano-architectures
  • Development and scaling of efficient processing and shaping techniques (incl. raw materials and processing and annealing steps)
  • Demonstrator building allowing material testing under real application conditions accompanied by material and system modelling utilizing the material libraries
 

Some selected publications from the group, ( for the complete list please see: Publications)

[1] O. Gutfleisch, J.P. Liu, M. Willard, E. Brück, C. Chen, S.G. Shankar, Magnetic Materials and Devices for the 21st Century: Stronger, Lighter, and More Energy Efficient (review), Advanced Materials 23 (2011) 821.

[2] J. Liu, T. Gottschall, K. P. Skokov, J. D. Moore, O. Gutfleisch, Giant magnetocaloric effect driven by structural transitions, Nat. Mater. 11 (2012) 620.

[3] J. Liu, J.D. Moore, K.P. Skokov, M. Krautz, K. Löwe, O. Gutfleisch, Exploring La(Fe,Si)13-based magnetic refrigerants towards application, View Point Paper, Scripta Mat. 67 (2012) 584.

[4] O. Gutfleisch, A. Yan and K.-H. Müller, Large magnetocaloric effect in melt-spun LaFe13-xSix, J. Appl. Phys. 97 (2005) 10M305.

[5] I.A. Radulov, K. P.Skokov, D. Yu.Karpenkov, T. Gottschall and O. Gutfleisch, On the preparation of La(Fe,Mn,Si)13Hx polymer-composites with optimized magnetocaloric properties, J. Mag. Mag. Mater. 396 (2015) 228.

[6] A. Yan, K. H. Müller, L. Schultz and O. Gutfleisch, Magnetic entropy changes melt-spun MnFePGe, J. Appl. Phys. 99 (2006) 08K903.

[7] T. Gottschall, K. P. Skokov, B. Frincu and O. Gutfleisch, Large reversible magnetocaloric effect in Ni-Mn-In-Co, Appl. Phys. Lett. 106 (2015) 021901.

[8] J. Liu, S. Aksoy, N. Scheerbaum, M. Acet, O. Gutfleisch, Large magnetostrain in polycrystalline Ni-Mn-In-Co, Appl. Phys. Lett. 95 (2009) 232515.

[9] M.D. Kuz’min, K.P. Skokov, D.Yu. Karpenkov, J.D. Moore, M. Richter, O. Gutfleisch, Magnetic field dependence of the maximum adiabatic temperature change, Appl. Phys. Lett. 99 (2011) 012501.

[10] J. Lyubina, M.D. Kuz’min, K. Nenkov, O. Gutfleisch, M. Richter, D.L. Schlagel, T.A. Lograsso, K.A. Gschneidner, Jr., Magnetic field dependence of the maximum magnetic entropy change, Phys. Rev. B 83 (2011) 012403.

[11] M. Krautz, K. Skokov, T. Gottschall, C.S. Teixeira, A. Waske, J. Liu, L. Schultz, O. Gutfleisch, Systematic investigation of Mn substituted La(Fe, Si)13 alloys and their hydrides for room-temperature magnetocaloric application, J. Alloys and Comp. 598 (2014) 27.

[12] M.E. Gruner, W. Keune, B. Roldan Cuenya, C. Weis, J. Landers, S. Makarov, D. Klar, M. Y. Hu, E.E. Alp, J. Zhao, M. Krautz, O. Gutfleisch, H. Wende, Element-resolved thermodynamics of magnetocaloric LaFe13-xSix, Phys. Rev. Lett. 114 (2015) 057202.