Swedish Neutron Education for Science & Society

Yifei Zhang, Chinese

PhD student in Anders Palmqvist's research group, Chalmers University of Technology, Sweden. I finished my master degree also in Chalmers University of Technology, with the thesis “simulation model for the synthesis of thermoelectric materials by spark plasma sintering”. My interests lie in the nanotechnology and materials for the sustainable energy, especially thermoelectric materials and photoelectric materials.

Thermoelectric (TE) materials enable conversion between thermal energy and electrical energy, and are promising for sustainable energy applications, such as waste heat recovery, solar heat utilization and solid-state cooling.

The performance of TE materials is characterized by the dimensionless thermoelectric figure of merit, , where  is the Seebeck coefficient,  is the electrical conductivity,  is the thermal conductivity. For better TE performance, higher  and lower  are favorable, but these properties are interrelated, which makes it challenging to optimize the TE performance of a material.

Since thermal conductivity comes from two sources: (1) charge carrying electrons and holes ( ) and (2) phonons travelling through the crystalline lattice ( ), it is possible to reduce the thermal conductivity by lowering ( ) without necessarily affecting  and . Thus, it’s important to understand the factors that influence lattice thermal conductivity, and neutron scattering provides tools for this. Composites of embedded nanoparticles or low-dimension materials in the bulk matrix, nanostructured bulk materials, achieve low thermal conductivity by enhancing phonon scattering. These can be studied with neutron scattering by analyzing the phonon density of state. Thus, the aim is to better understand the ‘phonon-glass electrical-crystal’ concept, and design TE materials with improved performance.

Another important tool is spark plasma sintering (SPS), which is a powder consolidation technique that is often used to consolidate and synthesize TE materials. Using SPS, the final product can reach full theoretical density at lower sintering temperatures compared to conventional sintering techniques. Due to the fast heating rate, powders with nanosized features can be densified without much grain growth, which offers the possibility to prepare and study nanostructured bulk materials in a systematic fashion. In addition, the product synthesized by SPS is favorable for the subsequent properties measurement, especially the measurement of thermal conductivity using the Hot Disk method.