Single-phase bulk solids heat management

Materials World magazine
1 May 2007

Multicomponent materials with electronic properties exceeding those of their individual nanocrystals have become more achievable, thanks to research by a team of scientists at the IBM T J Watson Research Center and the Lawrence Berkeley National Laboratory, both in the USA. This could have potential applications in the recovery of waste heat.

Traditional single-phase bulk solids have only achieved limited thermoelectric efficiency, explains Jeffrey Urban, a post-doctoral researcher at T J Watson’s Nanoscale Materials and Devices Group. ‘Our goal was to design new composite materials which would harness the properties of multiple classes of semiconductors in one binary superlattice material.’

The thermoelectric efficiency of a material depends on several factors, including the operating temperature, the square of the thermopower that it generates, its electrical conductivity and the inverse of its thermal conductivity. Often, optimising one parameter adversely affects another.

Multicomponent materials offer a solution to this problem. Each parameter can be separately optimised within each component.

The binary nanocrystalline thin films created by Urban’s group make use of solution-phase chemistry to create a synthesis of individual quantum dots of lead telluride and silver telluride from their molecular precursors. Both solutions were mixed before undergoing a hydrazine and thermal treatment to remove ligands and decrease interparticle spacing.

Lead telluride was specifically chosen because it ‘has the highest ZT [thermoelectric figure-of-merit] of any single-phase bulk material in the temperature range of interest to us (over 500ºC for waste heat recovery). We believed that the performance could be further enhanced by incorporating it [with] silver telluride, as nanostructured materials, into a binary superlattice,’ explains Urban.

The resulting material displayed conductivities approximately 100 times greater than that of the nanocrystals alone, while hole transport conductivities can go as high as 4.8 siemens/cm.

‘We were elated to discover that this combination did indeed exhibit synergism in electrical transport,’ Urban adds. ‘The unique feature of this study is that it demonstrates the potential of using the motif of binary nanocrystal superlattices to design materials for targeted application.’

Urban believes the technique can be recreated using spin-coating, roll-to-roll processing, inkjet printing, or other methods amenable to mass production.

For now, the research team is exploring the optical and thermal properties of these materials, and have produced at least 12 different symmetries of superlattices using several different nanocrystal combinations.