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Magnetism and magnetocaloric effect in LaFe11.9-xCoxSi1.1

Hansen, Britt Rosendahl; Kuhn, Luise Theil; Bahl, Christian Robert Haffenden; Ancona-Torres, CarlosEugenio; Katter, Matthias

Publication date:2009

Link back to DTU Orbit

Citation (APA):Hansen, B. R., Kuhn, L. T., Bahl, C. R. H., Ancona-Torres, C. E., & Katter, M. (2009). Magnetism andmagnetocaloric effect in LaFe11.9-xCoxSi1.1. Poster session presented at International Conference onMagnetism 2009, Karlsruhe, Germany.

Magnetism and magnetocaloric effect in LaFe Co Si11.9-x x 1.1

MagCool project

(a) (a) (a) (a)Britt Rosendahl Hansen , L. Theil Kuhn , C.R.H. Bahl , C. Ancona Torres .(b), M. Katter

(a) Fuel Cells and Solid State Chemistry Division,

Risø DTU, National Laboratory for Sustainable Energy(B)

Vacuumschmelze GmbH & Co. KG

Crystal structure: cubic

The 13 atoms of Fe, Co and Si form icosahedra arounda central atom, while La sits interstitially (blue).

(purple)

Cooling fluid

Piston

Magnetocaloric material(plates)

Good

Magnetic entropy change, DSM

No significant lattice expansion

Lack of hysteresis

Ease of production

Can be shaped into plates

Bad

Brittleness

Corrosivity

Adiabatic temperature change, Dtad

When compared to themagnetocaloric material, Gd, La(Fe,Co,Si) is seen 13

to have a larger DS . The M

peak in DS is slimmerM

making the range of opera-tion narrower, but this is remedied by the possibility for a composite material.

X-ray diffractometry confirms that the samples, which were made using powders that are pressed and then sintered (Katter et al.) for four to eight hours, have La(Fe,Co,Si) as 13

the main phase with small amounts of LaFeSi and a-Fe.

After 20 hours in demineralisedwater, the sample has oxidized.However, if the fluid is a mix-ture of anti-freeze and water, corrosion is greatly reduced.

The material is very brittle andcan be broken by hand, when made into plates.

References:Katter et al., IEEE Trans. Magn., 22 (2008) 3044

For an applied field of 1.1Ta plate of LaFe Co Si11.06 0.86 1.08

(not one of the seven samples)

is seen to have a DT that isad

less than that of the magneto-caloric material Gd.

H

A magnetic field is applied adiabatically heating the plates and thus the water.

What’s happening with the magneto-caloric material:

H

The cooling fluid ismoved between the plates to move heat to the ‘hot end’ and expel it.

The magnetic field is removed adiabaticallycooling the platesand thus the water.

The cooling fluid is moved over the plates and a ‘cold end’ is established.

Heat

T=T0 The spins align withthe field and the mag-netic entropy decrease. The total entropy is preserved and therefore the lattice entropy increases.

T = T + Dt0 ad

Heat is removed, while the spins are still aligned with the field.

The spins becomedisordered and the magnetic entropy increases, causing the lattice entropyto decrease.

T = T - DT0 ad

Seven samples of LaFe Co Si were characterized. The materials 11.9-x x 1.1

are interesting for magnetic refrigeration at room temperature.

Samples:

No. Composition T (K)c

1 LaFe Co Si 255.9 11.25 0.65 1.1

2 LaFe Co S 267.211.14 0.76 1.1

3 LaFe Co Si 282.111.05 0.85 1.1

4 LaFe Co Si 293.510.92 0.98 1.1

5 LaFe Co Si 311.610.77 1.13 1.1

6 LaFe Co Si 327.810.61 1.29 1.1

7 LaFe Co Si 347.310.45 1.45 1.1

Magnetism: itinerant electron ferromagnetismand localized moments on Fe and Co atoms

La(Fe,Co,Si) 13

- as a magnetic refrigerant

How to cool using magnets

Measurement of the temperature at the hot and cold end when using plates of the materialLaFe Co Si , which has a transition 10.96 0.98 1.06

temperature around 15°C. Without a heat load

a temperature difference of 8°C is achieved.

..and repeat

X-ray diffraction at a range of tempera-tures and dilatometry show a lattice ex-

pansion on the order of 0.4 %.

What’s next?

Conclusion

ndThe materials LaFe Co Si for 0.65<x<1.45 show a 2 11.9-x x 1.1

order ferromagnetic-to-paramagnetic transition at tem-peratures between ~254 K and ~336 K.

The materials were studied as possible magnetic refrige-rants for magnetic cooling at room temperature. Forthis purpose they are promising. The corrosiveness and brittleness should, however, be resolved.

Further studies on the La(Fe,Co,Si) series: Thermal13

conductivity measurements and a study of the micro-structure.

The magnetic entropy change is given byH2 ó

DS (T, H) = ô (¶M/¶T) dHM H,P

õH1 and calculated from a series of initialcurves.

The adiabatic temperature change was mea-sured directly by a thermocouple attached tothe magnetocaloric material.

Brittleness

Corrosiveness

Lattice expansion

Hysteresis

The plates were cut from blocks using wire EDM (Electrical Discharge Machining).

Heat

X-ray diffraction at room temperature Plates

Magnetic entropy change, DSM

No magnetic or thermal hysteresis is observed in mag-netization or calorimetry data. Presented are magnetiza-tion as a function of temperature for sample 1, hysteresis curves for sample 1 and heat capacity with and withoutan applied magnetic field for sample 2 and 4.

Adiabatic temperature change, DTad