Crystal structure of one of the selenides prepared solvothermally. Pink represents the indium and yellow represents the selenium. Disordered protonated amine molecules (not shown) are located within these channelsA UK-based scientist is exploring low temperature solvothermal synthesis in an attempt to create materials with improved thermoelectric efficiency for power generation and cooling applications.

Dr Paz Vaqueiro-Rodriguez, at Heriot-Watt University in Edinburgh, is preparing tellurides and selenides containing cages or tunnels in which inorganic (alkali and alkali earth cations) and organic (amines) species are located.

A low temperature (~100-250ºC), rather than the traditional high temperature (~700ºC) route of combining elements, enables the development of materials with a complex framework structure, says the researcher. This is because the work is not restricted to thermodynamically stable phases that often have simple lattices.

Having a reaction

‘The solvothermal method itself is not new. It is widely used for synthesising zeolites and other open framework materials. The novelty lies in exploiting this for thermoelectric materials,’ says Vaqueiro-Rodriguez.

This involves superheating solvents in a sealed vessel to facilitate the crystallisation of new materials with microporous structures. While water is often used as a solvent, in this case, the reaction contains metal sources (indium, germanium, antimony and bismuth), chalcogens (selenium and tellurium) and an organic amine.

‘As reactions take place, polymeric anions remain intact in solution to [produce] solid state phases,’ describes Vaqueiro-Rodriguez. ‘Organic amines, or inorganic species such as NH4+ [ammonia ion] can then be employed as templates to direct the assembly of these polymeric anions to form low density frameworks, with the template retained in the pores of the structures.’

Controlled architecture

The work is part of broader research in the field to explore complex materials containing nanoinclusions, which manipulate the electrical and thermal transport properties by interface scattering at the grain boundaries. Materials with cage-like open structures are another line of investigation. ‘The cages can be filled with atoms that “rattle”.

The motion is believed to reduce the thermal conductivity of the materials,’ explains Vaqueiro-Rodriguez.

High performance in thermoelectric materials is achieved by simultaneously combining high electrical conductivity and thermoelectric power with low thermal conductivity. The efficiency of converting thermal energy into electric power, and vice versa, is measured by a material’s figure of merit.

Vaqueiro-Rodriguez says, ‘The best commercial materials, based on bismuth telluride, have a figure of merit of around one. This is only enough for niche applications, in which size, reliability or convenience are more important than economy’.

However, she notes, ‘The vast majority of research is focused on improving existing materials, not in searching for new ones. Work on solvothermal synthesis has resulted in new phases’.

The thermal stability range of these tellurides and selenides is currently limited to around 300ºC, which may be more suitable for thermoelectric cooling rather than power generation. The next phase is to affect improvements in thermoelectric efficiency via doping of the materials to optimise electrical transport. ‘The selenides tend to have band gaps that are too high. However, it may be possible to tune them by preparing mixed chalcogen phases,’ says Vaqueiro-Rodriguez.

The team will also explore how to incorporate these materials into thermoelectric devices, by finding compatible p- and n-type materials. She adds, ‘At this stage, establishing the conditions for good electrical contacts, as well as preparing mechanically robust ingots, becomes important. Toxicity or world-wide availability of certain elements may also be factors to consider prior to commercial application’.

Vaqueiro-Rodriguez has also explored high-temperature routes for preparing a novel family of unfilled ternary skutterudites, metal-rich tellurides and quaternary phases for high-temperature applications.