Research

Currently, the research program involves three directions:

1. Magnetocaloric phases (materials for magentic refrigeration)

Magnetocaloric materials utilize magnetocaloric effect, a change in material's temperature upon a change in an applied magnetic field. Such materials heat up when the magnetic field is increased and cool down when magnetic field is reduced. They can be used for magnetic refrigeration (figure on the right) and can offer a greater efficiency than the current vapor-cycle refrigeration.
A conventional magnetocaloric effect is based on ordering of magnetic moments, i.e. on magnetic transition only. The effect can be significantly increased (by a factor of ~2) when a magnetic ordering is coupled to a structural transition. A combined effect is known as a giant magnetocaloric effect.

Our research in this area focuses on discovery of new metal-rich magnetocaloric materials and on manipulating the physical and structural properties of known materials.
We have successfully used valence electron concentration to tune structure and magnetic properties of Gd₅X₄ phases, with X being a p-element. As an example, through the substitution of P for Si in Gd₅Si₄, we have broken interslab dimers in Gd₅(Si,P)₄, and through the substitution of Sb, we have increased the Curie temperature of the Gd₅(Si,Sb)₄ phases with broken dimers up to room temperature.

2. Thermoelectric phases

Thermoelectric materials can convert heat into electricity (Seebeck effect, figure on the right) or perform cooling/heating when electrical current is passed through them (Peltier effect). Thermoelectric materials are used to generate electricity when other source of electricity are not available (i.e. deep space missions) or to convert waste heat into electricity thus reducing fuel consumption (i.e. in cars). They are also used to perform cooling when other cooling techniques cannot be easily applied (i.e. car seats, spot cooling in electronics).
One of the challenges in the thermoelectric research is to reduce thermal conductivity of materials in order to optimize their performance.

Our group tackles this challenge by utilizing a natural superlattice approach for the material design. We combine two structures with different properties to obaine a material known as "phonon-glass electron-crystal". We also prepare unique suboxide phases, in which a band gap is opened in the original semimetallic material through the incorporation of oxide fragments.

3. Phosphor materials

Phosphor materials are used for lighting applications (LEDs), where they downconvert the high-energy blue radiation into lower energy one (yellow, green and red) to yield the white light. We explore Cr³⁺ and Mn⁴⁺ phosphors that emit the red light and can be employed for LEDs.

We also explore Cr³⁺ and Mn⁴⁺ phosphors for temperature sensing. The life time of the Cr³⁺ and Mn⁴⁺ emission is temperature dependent and can be correlated to the temparature.

Resources