Charge carrier effective mass and concentration derived from combination of Seebeck coefficient andTe125NMR measurements in complex tellurides

Abstract

Thermoelectric materials utilize the Seebeck effect to convert thermal to electrical energy. The Seebeck coefficient (thermopower), S, depends on the free (mobile) carrier concentration, n, and effective mass, m*, as In some materials the influence of m* on the Seebeck coefficient can be significant and its determination is important for better understanding of thermoelectric materials. 125 Te nuclear magnetic resonance (NMR) [4][5][6][7][8][9] can be used to study complex tellurides and derive the carrier concentration from spin-lattice relaxation time, T 1 . In a theoretical model by Selbach et al. [10], the coefficient of proportionality between T 1 and n, "Bloembergen constant", C B , depends on m*. Hence, if the effective mass in a series of materials is not constant, it can affect C B , and, therefore, the carrier concentration deriving from 125 Te NMR T 1 measurements should be different compared to that obtained if m* is constant.Here it is shown that the combination of the (Table 1) were synthesized by a direct reaction of the constituent elements in 10 mm diameter fused silica ampoules in argon backfilled up to ~17 kPa pressure atmosphere. For synthesis we used Ge (Materion G-1038, 99.999% purity), and Te (Alfa Aesar, 99.9999% purity, product #12607, lot 004). Ampoules with the constituent elements were heated up to 1323 K to melt the constituents, and after ~4 hours the melt was cooled down with the furnace at the rate of 100 K/hour. Each ingot had a mass of 20 g, diameter of 10 mm, length of ~40 mm. Analysis of each material was performed by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy, and showed that all materials are nearly single phase; only ~1 at.% Ge is present in the samples matrix due to naturally occurring process of Ge rejection from the melt [8,14]. 125 Te NMR experiments were performed at 126 MHz using a Bruker 400WB spectrometer with TOPSPIN software in a magnetic field of 9.4 T without sample spinning 4 (static regime). Powder samples were prepared from ingots and placed in 4-mm diameter insert. 125 Te NMR spin-lattice relaxation measurements were used to obtain the spin-lattice relaxation time, T 1 , using GeTe as a reference material [5,8]; the uncertainty of T 1 measurements is less than ±0.2 ms [5].More experimental details including pulse sequence for 125 Te NMR can be found in Refs. 5,8,9,14. Samples for the Seebeck coefficient measurements were prepared by cutting the ingot with a diamond saw [9]. The Seebeck coefficient was measured by a LSR-3 measuring system (Linseis Inc.) in a helium atmosphere relative to the Pt legs of a Pt-(Pt+Rh) thermocouple and then the absolute Seebeck coefficient was calculated; the uncertainty of measurements is about ±3% [9]. Table 1). 125 Te NMR spectra for these two materials are shown on inset in Fig. 1(a), demonstrating that signals can be easily detected; spectra for the rest of materials also are located between those for GeTe and TAGS-85.Relaxation curves shown on Fig. 1(a) as well as for ...

Large Power Factors in Wide Band Gap Semiconducting RFeO3 Materials for High-Temperature Thermoelectric Applications

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High Thermoelectric Performance in Two-Dimensional Janus Monolayer Material WS-X (X = Se and Te)

“…At many places, the d-orbitals of the W atom are overlapped with p-orbitals of S, Se, and Te atoms in the valence band far from the Fermi level. The flat band lines in these suggest a large effective mass for them, and the Seebeck coefficient directly depends on the effective mass ( m *) . According to that, the monolayers WS 2 , Janus WSSe, and WSTe have higher values of the Seebeck coefficient.…”

“…The flat band lines in these suggest a large effective mass for them, and the Seebeck coefficient directly depends on the effective mass ( m *). 53 According to that, the monolayers WS 2 , Janus WSSe, and WSTe have higher values of the Seebeck coefficient.…”

Impacts of Cu-Doping and Mg-Deficiency on Mg2Sn Thin Films Thermoelectric Properties

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