Effect of nanocrystallinity on lattice dynamics in Bi₂Te₃based thermoelectrics

Abstract

The lattice dynamics in as‐cast and nanocrystalline thermoelectric Bi2Te3 based p‐type and n‐type material were investigated using inelastic neutron scattering. Generalized densities of phonon states show substantial agreement between the lattice dynamics in as‐cast samples and previous studies. The lattice dynamics in the nanocrystalline materials differ significantly from its as‐cast counterparts in the acoustic phonon regime. In nanocrystalline p‐type and n‐type compounds, the average acoustic phonon group velocity was found to be reduced to 80(5)% and 95(2)% of the value in as‐cast material. It is argued that point‐defect and strain contrast scattering may play an important role for the understanding of lattice thermal conductivity in (nanocrystalline) Bi2Te3 based thermoelectrics beside the observed decrease of sound velocity. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Lattice dynamics and thermoelectric properties of nanocrystalline silicon–germanium alloys

“…Concentrating on coarse-grained competitors [38,39] first, nanocrystalline samples prepared by gas-phase synthesis exhibit significantly reduced thermal conductivities, lower also than, e.g., 3.1 W m −1 K −1 reported for small, <5 m, grained hot-pressed Si 63.5 Ge 36.5 [40]. This can be ascribed to an increase both of the density of phonon scattering centers upon nanostructuring and to an increase of the DPS in the low-energy region yielding a reduced speed of sound but also to point defect scattering similar to recent findings for Bi 2 Te 3 [13] and Si-Ge nanocomposites [41]. In contrast, charge carrier concentration and Seebeck coefficient are mainly unaffected by the nanocyrstalline state produced by gas-phase synthesis and dc hot pressing.…”

“…These modes are attributed to vibrations of atoms located in the grain boundaries where the atomic structure is less defined than in the crystalline core of the material. This results in a change of the force field and softening of the force constants [5][6][7][8][9][10][11][12][13].…”

From thermoelectric bulk to nanomaterials: Current progress for Bi2Te3 and CoSb3

“…Vibrational properties of bulk and nanomaterials were analyzed by nuclear inelastic scattering (NIS), inelastic neutron scattering (INS), and Mössbauer spectroscopy as explained in Refs. .…”

“…The generally low thermal conductivity in Pn 2 Te 3 type compounds, where Pn is a pnictide atom, was attributed to the low acoustic cutoff energy and the rather low average phonon group velocity as derived from Te and Sb partial densities of phonon states . Extending such investigations to the nano‐regime, binary Bi 2 Te 3 nanowires and nanocrystalline p‐ and n‐type bismuth telluride based alloys were investigated using NIS by 125 Te and using inelastic neutron scattering , respectively. Further results were obtained by 121 Sb NIS on thin films of nano‐alloyed Sb 2 Te 3 and are reported elsewhere .…”

“…). This entails a reduction of the average phonon group velocity in nanowires and nanocrystalline alloys , which contributes to the reduction of lattice thermal conductivity. With respect to the investigation of the nanowire ensemble, the derived reduction of about 6.5 % might be comparable to recent calculations .…”