Thermoelectric transport in ionic conducting copper and silver chalcogenides
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Abstract
In order to avoid a serious energy crisis in the future, alternative energy sources are required and new, powerful approaches have to be explored to use current energy technologies more efficiently. Based on Seebeck and Peltier effects, thermoelectric ma-terials either convert thermal to electrical energy or vice versa, providing a perspective for both power generation and refrigeration applications, respectively.
The thermoelectric efficiency is governed by the thermoelectric figure of merit zT = S2(ρκ)-1T, and the last decade has seen the discovery of many suitable materials for thermoelectric power generation. In this context the “phonon-glass electron-crystal” (PGEC) concept has proven to be very useful for the exploration of new thermoelectric materials. According to this concept, a good thermoelectric material requires an opti-mized charge carrier concentration and a low lattice thermal conductivity which can be fulfilled by merging two fractional or structural units responsible for the electronic and thermal properties. In recent years, the PGEC concept has been applied to superionic compounds like Cu2Se, Cu2S, Ag2Se which achieve very high thermoelectric efficiencies. Here, the concept of “phonon-liquid electron-crystal” (PLEC) thermoelectrics has been introduced as an extension of the “phonon-glass electron-crystal” concept because these materials are usually built up of a very simple anion network in which the cations are highly disordered with liquid-like mobility leading to very low lattice thermal conductiv-ities.
It is the goal of this work to discuss structural aspects of several ionic conducting materials and the relations to the thermoelectric transport properties. In order to obtain a deeper understanding of structure-property relationships in these materials, we have in-vestigated different series of solid solutions and discussed the influence of dopants in ionic conducting materials.
The superionic conductor Cu2-δSe has proven to be a promising thermoelectric at higher temperatures. Being the best-studied ionic conducting thermoelectric material, it is also the subject of critical discussions, since certain issues concerning chemical stabil-ity need to be addressed before a possible application. After a general introduction we present the potential of copper selenide to achieve a high figure of merit at room temper-ature, if the intrinsically high hole carrier concentration can be reduced. Using bromine as a dopant we show that reducing the charge carrier concentration in Cu2-δSe is in fact
possible. Furthermore we provide profound insight in the complex defect chemistry of bromine doped Cu2-δSe via various analytical methods and investigate the consequential influences on the thermoelectric transport properties. Here we show, for the first time, the effect of copper vacancy formation as compensating defects when moving the Fermi level closer to the valence band edge. These compensating defects provide an explanation for frequently observed doping inefficiencies in thermoelectrics via defect chemistry and guiding further progress in the development of new thermoelectric materials.
Building on the good thermoelectric performances of the binary superionic com-pounds, a better and more detailed understanding of PLEC thermoelectric materials is desirable. Therefore, we investigate the thermoelectric transport properties of the com-pound Cu7PSe6 as the first representative of the class of more complex Argyrodite-type copper ion conducting thermoelectrics. The Argyrodite family is well known for a huge variety of compositions, which can even be increased by varying the oxidation states leading to both ternary and quaternary compounds. Therefore, the Argyrodites provide a huge playground for the investigation of structure-property relationships, making this class of materials a well-suited model system for PLEC thermoelectric materials.
The further research on Argyrodite-type compounds addresses the effects of struc-tural aspects on the thermoelectric transport. For this purpose the series of solid solutions (Cu,Ag)7PSe6 is considered. In this context both the influence of the substitution of cop-per and silver and the influence of the order-disorder-transitions above room temperature on the thermoelectric properties can be investigated. Here, the Argyrodites are well suit-able, since cation substitutions are possible which is much more difficult in binary sys-tems. Within the scope of the investigation of Argyrodite type samples, the influence of the amount of mobile cations per formula unit on the thermoelectric properties can also be investigated. Here, Ag8SiSe6 is regarded, which shows outstanding thermoelectric transport properties at temperatures close to room temperature, in combination with very high charge carrier mobility and a complex structure.
The remainder of this thesis focusses on compounds similar to chalcopyrite, which crystallizes in a tetrahedrally bonded network structure. Chalcopyrite-type compounds achieve extremely high thermoelectric efficiencies. AgGaTe2, CuGaTe2 and CuInTe2 are prominent examples for ternary chalcopyrite-type thermoelectric materials with high ef-ficiencies. CuFeTe2, however, crystallizes in a layered structure. In addition to the tetra-hedral bonded ions within the layers, it provides additional pyramidal cation sites between
the layers leading to compositions with variable amounts of cations (Cu1+xFe1+yTe2). As the phase pure bulk synthesis of this compound is difficult, there is hardly any report on the physical properties of these compounds. In this context a method for the phase pure bulk synthesis of this compound is developed and the thermoelectric properties of this series of solid solutions Cu1+xFe1+yTe2 are investigated with regard to changes in x and y.