Thermoelectric materials can be utilized to generate electricity from a temperature gradient, thereby recycling the nowadays abundant waste heat, as well as for cooling applications by creating a temperature gradient from electricity. The former is based on the Seebeck effect, and the latter on the Peltier effect. Noticing the continuously declining fossil fuel resources and mankind's increasing need for energy, the importance for clean thermoelectric energy generation continues to climb.Traditional thermoelectric materials were based on the binary tellurides Bi 2 Te 3 and PbTe, which have been utilized for decades. The focus on tellurium as the heaviest non-radioactive chalcogen stems from the observation that heavier elements are advantageous for a reduced thermal conductivity, which is essential for the thermoelectric energy conversion. Moreover, tellurides are less ionic than sulfides or selenides, which leads to an enhanced carrier mobility that is advantageous for the desired high electrical conductivity. This review presents these traditional routes to low thermal conductivity, as well as alternatives based on the lighter analogues of tellurium, namely sulfur and selenium.
A new state-of-the-art thermoelectric material, TlAgTe, which possesses an extremely low thermal conductivity of about 0.25 W m K and a high figure-of-merit of up to 1.1 at 525 K, was obtained using a conventional solid-state reaction approach. Its subcell is a variant of the ZrFeP type, but ultimately its structure was refined as a composite structure of a TlAgTe framework and a linear Te atom chain running along the c axis. The super-space group of the framework was determined to be P6(00γ) s with a = b = 11.438(1) Å, c = 4.6256(5) Å, and that of the Te chain substructure has the same a and b axes, but c = 3.212(1) Å, space group P6(00γ) s. The modulation leads to the formation of Te and Te fragments in this chain and a refined formula of TlAgTe. The structure consists of a complex network of three-dimensionally connected AgTe tetrahedra forming channels filled with the Tl atoms. The electronic structures of four different models comprising different Te chains, TlAgTe, TlAgTe, and 2× TlAgTe, were computed using the WIEN2k package. Depending on the Te content within the chain, the models are either semiconducting or metallic. Physical property measurements revealed semiconducting properties, with an ultralow thermal conductivity, and excellent thermoelectric properties at elevated temperatures.
In this work, polycrystalline n-type Mg2Si0.30Sn0.67Bi0.03 dispersed with x wt % β-SiC nanoparticles (x = 0,
0.5, 1.0,
1.5, and 3.0) thermoelectric materials were fabricated by a solid-state
reaction in a low-cost container, consolidated by hot-pressing. We
obtained figure of merit values zT above 1.4 at 773 K along with enhanced mechanical properties by
adding β-SiC into an Mg2Si0.30Sn0.67Bi0.03 matrix. Incorporation of SiC nanoparticles has
thusly simultaneously increased toughness and, depending on the SiC
content, thermoelectric performance. The peak figure of merit was
improved from zT = 1.33 for Mg2Si0.30Sn0.67Bi0.03 to 1.45 for Mg2Si0.30Sn0.67Bi0.03 with 3 wt % at
773 K.
While continuing our investigations of thallium chalcogenides because of their outstanding thermoelectric properties, we discovered a new selenide with an interesting pnp switching behavior around 400 K. Tl 2 Ag 12 Se 7 was prepared via high temperature reaction from the elements in the stoichiometric ratio. This selenide crystallizes in a new structure type, namely a √3 × √3 × 1 super cell of the Zr 2 Fe 12 P 7 type, adopting space group P3̅ , a = b = 18.9153(18) Å, c = 4.3783(4) Å, and V = 1356.6(2) Å 3 (Z = 3). The structure consists of a complex network of three-dimensionally connected AgSe 4 tetrahedra that include linear channels filled with thallium atoms. This material is a semiconductor with an experimentally derived activation gap of 0.8 eV and extraordinarily low thermal conductivity of <0.45 W m −1 K −1 . A reversible phase transition causes a pnp switch from p-type to n-type and back to p-type conduction beginning around 390 K, with the thermopower changing fast from +230 μV K −1 at 390 K to −230 μV K −1 at 410 K, and then back up to +75 μV K −1 at 420 K.
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