Creating ecological and efficient thermoelectric materials

In a new study, environmentally benign inverse perovskites with high energy conversion efficiency have been reported by Tokyo Tech scientists, with potential for practical application as thermoelectric materials (TEMs). Addressing the limitations typically encountered with TEMs, such as insufficient energy conversion efficiency and environmental toxicity due to heavy elements, novel TEMs provide a suitable alternative to toxic element-based TEMs with better thermoelectric properties than traditional TEMs. Conventional environmentally friendly TEMs.

Thermoelectric materials (TEM) capable of converting thermal energy into electrical energy and vice versa have become an essential part of our world, which needs better waste energy harvesting systems and cooling systems for electronic gadgets.

The energy conversion efficiency of TEMs depends on a dimensionless figure of merit (ZT), which is the product of two different factors: the reciprocal of thermal conductivity (k) and power factor (PF).

A high-performance TEM has a high ZT if it has a low k and a high PF. Over the years, scientists have developed several high-performance heavy metal chalcogenide TEMs, such as Bi₂You₃ and PbTe, which meet these criteria. Although these materials were ideal for energy conversion, they were toxic to the environment and the health of living organisms: they contained toxic heavy elements, such as lead (Pb) and tellurium (Te), which limited their practical applications.

On the other hand, although oxide-based TEMs, such as SrTiO₃have several advantages of non-toxicity and abundant natural resources, their ZT has been limited due to their high k.

To address this problem, a research team led by Associate Professor Takayoshi Katase of the Tokyo Institute of Technology explored efficient yet environmentally friendly TEMs without toxic elements. In their study published in Advanced scienceresearchers present high ZT TEMs based on “reverse” perovskite with chemical formula Ba₃BO, where B refers to silicon (Si) and germanium (Ge).

“Unlike normal perovskites, such as SrTiO₃the positions of the cationic and anionic sites are reversed in inverse Ba perovskites₃BO. Thus, they contain a large amount of the heavy element, Ba, and their crystal structure is formed by a soft flame composed of weak O-Ba bonds. These characteristics enable the low k of inverse perovskites to be realized,” explains Dr. Katase, detailing the remarkable properties of the materials.

The research team clarified the synthesized bulk polycrystals of Ba₃BO has an extremely low k of 1.0 to 0.4 W/mK at a T of 300 to 600 K, which is lower than that of Bi₂You₃ and bulk PbTe. Consequently, the Ba₃The BO bulks present a rather high ZT of 0.16 to 0.84 at T = 300 to 623 K.

Additionally, the team performed theoretical calculations that predicted a maximum ZT potential of 2.14 for Ba.₃SiO and 1.21 for Ba₃GeO at T = 600 K by optimizing the concentration of holes. The maximum ZT of these non-toxic TEMs is much higher than that of other environmentally friendly TEMs and comparable to those of toxic heavy elements in the same temperature range.

Additionally, the team clarified that the high ZT of Ba₃BO is due not only to its low k but also to its high PF: the B ion, which generally behaves as a positively charged cation but as a negatively charged anion in Ba.₃BO. B anions are responsible for carrier transport, which allows high PF to be achieved.

In summary, this study validates the potential of the newly designed Ba.₃BO as an efficient and ecological alternative to conventional toxic TEM based on heavy elements.

The results establish inverse perovskites as a promising option for the development of advanced and environmentally benign TEMs. In this regard, Dr. Katase concludes: “We believe that our unique vision of designing high ZT materials without using toxic elements would have a great impact on the materials science and chemistry communities as well as the innovators seeking to expand the horizons of thermoelectric materials. applications beyond laboratories in our daily lives.