Microstructure investigations of Yb- and Bi-doped Mg2Si prepared from metal hydrides for thermoelectric applications

Abstract

Within the field of thermoelectric materials for energy conversion magnesium silicide, Mg 2 Si, is an outstanding candidate due to its low density, abundant constituents and low toxicity. However electronic and thermal tuning of the material is a required necessity to improve its Figure of Merit, zT. Doping of Yb via reactive YbH 2 into the structure is performed with the goal of reducing the thermal conductivity. Hydrogen is released as a byproduct at high temperature allowing for facile incorporation of Yb into the structure. We report on the properties of Yb-and Bi-doped Mg 2 Si prepared with MgH 2 and YbH 2 with the focus on the synthetic conditions, and samples' microstructure, investigated by various electron microscopy techniques. Yb is found in the form of both Yb 3 Si 5 inclusions and Yb dopant segregated at the grain boundary substituting for Mg. The addition of 1 at.% Yb concentration reduced the thermal conductivity, providing a value of 30 mW/cm•K at 800 K. In order to adjust carrier concentration, the sample is additionally doped with Bi. The impact of the microstructure on the transport properties of the obtained material is studied. Idealy, the reduction of the thermal conductivity is achieved by doping with Yb and the electronic transport is adjusted by doping with Bi. Large grain microstructure facilitates the electronic transport. However, the synthetic conditions that provide the optimized microstructure for electrical transport do not facilitate the additional Yb dopant incorporation. Therefore, the Yb and Bi containing sample with the optimized microstructure provides a zT = 0.46 at 800 K.

From NaZn4Sb3 to HT-Na1–xZn4–ySb3: Panoramic Hydride Synthesis, Structural Diversity, and Thermoelectric Properties

“…In this model the sum of site occupancy factors (s.o.f.) for an atomic site and its split counterparts was constrained to 100%, while the atomic displacement parameters (APDs) were constrained to be the same: Sb(17)−Sb (25), Sb(21)− Sb (26), Sb(24)−Sb (27), Zn(16)−Zn (32), Zn(20)−Zn (33), Zn(27)−Zn (34), Zn(29)−Zn (35), Zn(30)−Zn (36), and Zn(31)− Zn(37) (Table S2). The refined s.o.f.…”

“…The hydride synthetic route, which uses mixable salt-like hydride precursors (e.g., alkali metal hydrides A H, A = Li, Na, K), instead of ductile alkali metals, was successfully utilized for the synthesis of binary and ternary borides, ,, antimonides, , arsenides, , silicides, − and germanides. , This method is particularly applicable for the compositional screening in the discovery of new ternary alkali zinc antimonides, providing composition control and high purity samples . Traditional solid state synthesis using alkali metal precursors is hampered by their ductility, high reactivity, and vapor pressure at elevated temperatures, as well as side reactions with crucible materials.…”

Odyssey of thermoelectric materials: foundation of the complex structure

“…The low lattice thermal conductivity is due to low velocity of optical modes caused by the large mass density [192]. zT reached the value of 0.46 in Mg2Si microstructure via Yb and Bi doping because Yb doping lowers the thermal conductivity and Bi doping adjusts the electronic transport properties [193]. zT of 1.4 was obtained in Mg 2 Sn 0.73 Bi 0.02 Ge 0.25 at 673 K which shows that the charge donors are much more effective at Sn-site than Mg site [34].…”

Inverse Perovskite Oxysilicides and Oxygermanides as Candidates for Nontoxic Infrared Semiconductor and Their Chemical Bonding Nature

“…In this regard, silicides, germanides, and stannites, using group IV elements as a tetravalent anion in a formal charge state, have recently attracted interest in the search for nontoxic semiconductors. One such example is magnesium silicide (Mg 2 Si), − which is formed by using silicon as a tetravalent anion. However, the Mg 2 Si band structure is of indirect-transition-type and, hence, it cannot be useful as a light emitter. , In contrast, there are some silicide semiconductors with a direct-transition-type band structure, such as Sr 2 Si and Ba 2 Si, but, to the best of our knowledge, their luminescence properties have not yet been reported.…”

“…In this regard, silicides, germanides, and stannites, using group IV-elements as a tetravalent anion in a formal charge state, have recently attracted interest in search for non-toxic semiconductors. One such example is magnesium silicide (Mg2Si), [16][17][18][19][20][21] which is formed by using silicon as a tetravalent anion. However, the Mg2Si band structure is of indirect-transition-type and, hence, it cannot be useful as a light emitter.…”