ISTM is one of the seven European partners involved in a joint effort aimed at developing nano-engineered high performance thermoelectric materials and devices (Fig. 1). The European Community within the Vth Framework Program funds this team project, named NanoThermel.
Thermoelectric (TE) materials have a dual purpose: electrical generation on one side and cooling/heating on the other side. Power generation is achieved by applying a temperature difference between two ends of the TE material, while cooling/heating is obtained by applying electrical current. TE devices have very attractive features, such a small size, simplicity, reliability, and have important terrestrial and space applications. They will play an important role in the development of clean and efficient cooling and energy conversion systems. Cars loose energy directly as heat in the exhaust, which could partly be recovered with an efficient TE device and converted to useable electricity resulting in overall vehicle operation economy and sizeable impact on CO2 emission. TE coolers would lower ozone depletion since they operate without fluorocarbons.
Yet, to compete with conventional technologies, efficiency of current TE devices must be increased, mostly by improving ZT,
ZT=TS2s/k (T is the absolute temperature),
the thermoelectric figure of merit of the TE material, on which the device performance is based upon.
Discovering a material with high ZT is akin to finding the proverbial needle in a haystack, because of seemingly contradictory requirements - high thermopower S, like a semiconductor; high electrical conductivity s, like a metal; and low thermal conductivity k, like a glass.
Our project has developed a viable strategy for improving ZT, based on the use of nanotechnology as well as on the introduction of structural modifications of new classes of host-guest TE materials, like the skutterudites and the inorganic clathrates.
Nanostructuring led to a dramatic k decrease in skutterudite, due to much higher density of grain boundaries. Such an important result was partly offset by the S and s concomitant decreases. The ongoing search for optimum doping and synthetic pathways preventing oxidation of grain surfaces is aimed at opposing such a decrease.
Filling the voids of skutterudites (Fig. 2) or clathrates (Fig. 3) with atoms that are small enough to "rattle" and create dynamical disorder also reduces k, without much adverse the electronic transport. By combining voids' filling with suitable doping into the host framework, a good balance between decrease of k and possible S and s decreases may be achieved.
Modeling of such structural modifications through ab initio computations is the main task performed by ISTM. It is providing precious guidance for the design of optimum TE materials and valuable help for understanding their transport properties.
Doubling ZT of the current TE materials is a foreseen deliverable of the project.
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