The common refrigerators work using liquid refrigerants that boil around ambient temperatures. These substances may be toxic, flammable, ozone-depleting or greenhouse active.
We investigate solid refrigeration materials, so-called caloric materials, which are a promising alternative. These materials can cool down or heat up when an external field - e.g. a magnetic field or a mechanical stress - is applied.
The magnetocaloric effect
A magnetic material contains elementary magnetic moments on the atomic scale. A magnetic field will bring all these moments into an ordered state, which causes the material to heat up. The heat is dissipated into the environment. When the field is then removed, the magnetic moments disorder and the crystal lattice - and hence the material cools down and can absorb heat from the environment. In this way, and with a suitable material, a magnetocaloric fridge can be built.
Materials that cool by several Kelvin are called "giant magnetocaloric materials". Examples are Lanthanum-Iron-Silicon, Gadolinium or Nickel-Manganese-Indium and Nickel-Manganese-Gallium-Cobalt alloys. In our group, we concentrate on rare-earth-free alloys.
The elastocaloric effect
Certain materials undergo a phase transition, e.g. a sudden change of the crystal structure, when an external mechanical stress or a hydrostatic pressure is applied. The phase transition generates heat, which can be dissipated into a heat sink. When the force (or stress) is removed, the material transforms back and absorbs heat from the environment. This cycle can be used for cooling.
Examples for elastocaloric materials with high effects are Nickel-Titanium- or Nickel-Manganese-based alloys.
We concentrate on sputter-deposited Ni-Mn-Ga-Co films on various substrates.
Often, materials with good magnetocaloric properties show also elastocaloric properties. Recently, materials were developed that can be used in both, magnetic and elastic fields. We use a combination of a Ni-Mn-Ga-Co film that shows magneto- and elastocaloric effects and a piezoelectric substrate. By applying a voltage to the substrate, we can strain the film and induce the phase transformation that is responsible for the effect.
Why thin films?
Thin films are a very interesting geometry to study functional materials.
- Thin films are good model systems for the fundamental properties of the materials.
- They have a fast heat exchange and can be used in high frequency and micro-systems.
- They can be grown on variable substrates. Piezoelectric materials for examples allow to exploit the multicaloric effects.
- Sputter deposition is a fast and easy way to grow single-crystalline (epitaxial) films.
- In contrast to bulk, the surface morphology can be studied without any further preparation and is very significant for the entire film.
|An indent into the surface of a magnetocaloric film promotes the nucleation of martensite (light stripes), which may increase reversibility. |
R. Niemann et al., APL Mater. (2016)
|In substrate-constrained and epitaxial Ni-Mn-Ga-Co-films, the martensitic transformation leads to a magnetocaloric effect shown by calculation the entropy change from magnetization measurements. |
A. Diestel et al., J. Appl. Phys. (2015)
|Structural phase transition (from austenite to martensite) in Ni-Mn-Ga-Co thin films obtained through (T-dependent) pole figure measurements.|
B. Schleicher et al., J. Appl. Phys. (2015)
|Reorientation of martensitic variants can be misinterpreted as a magnetcaloric effect in magnetic shape memory alloys. |
R. Niemann et al., Int. J. Refrig. (2014)