Development of Nanostructures in Thermoelectric Pb-Te-Sb Alloys

ICT: 2005 24th International Conference on Thermoelectrics, 2005

A solidification processing approach to the refinement of lead-tellurium-antimony alloy microstructure is described. Liquid alloys with eutectic, hyper-eutectic and hypo-eutectic compositions (relative to lead) were cooled to the solid state in three distinct ways, i.e. by water quenching, air cooling and furnace cooling. The structures of the alloys resulting from the three different solidification paths were examined using electron microscopy and the micrographs were quantified. Classical solidification methods were used to interpret the structures in relation to the cooling histories.

Acta Materialia, 2011

Crystals, 2017

We investigate the microstructure evolution of Ag-alloyed PbTe compounds for thermoelectric (TE) applications with or without additions of 0.04 at. % Bi. We control the nucleation and temporal evolution of Ag 2 Te-precipitates in the PbTe-matrix applying designated aging heat treatments, aiming to achieve homogeneous dispersion of precipitates with high number density values, hypothesizing that they act as phonon scattering centers, thereby reducing lattice thermal conductivity. We measure the temperature dependence of the Seebeck coefficient and electrical and thermal conductivities, and correlate them with the microstructure. It is found that lattice thermal conductivity of PbTe-based compounds is reduced by controlled nucleation of Ag 2 Te-precipitates, exhibiting a number density value as high as 2.7 × 10 20 m −3 upon 6 h aging at 380 • C. This yields a TE figure of merit value of ca. 1.4 at 450 • C, which is one on the largest values reported for n-type PbTe compounds. Subsequent aging leads to precipitate coarsening and deterioration of TE performance. Interestingly, we find that Bi-alloying improves the alloys' thermal stability by suppressing microstructure evolution, besides the role of Bi-atoms as electron donors, thereby maintaining high TE performance that is stable at elevated service temperatures. The latter has prime technological significance for TE energy conversion.

Journal of the American Chemical Society, 2007

The solid-state transformation phenomena of spinodal decomposition and nucleation and growth are presented as tools to create nanostructured thermoelectric materials with very low thermal conductivity and greatly enhanced figure of merit. The systems (PbTe)1-x(PbS)x and (Pb0.95Sn0.05Te)1-x(PbS)x are not solid solutions but phase separate into PbTe-rich and PbS-rich regions to produce coherent nanoscale heterogeneities that severely depress the lattice thermal conductivity. For x > ∼0.03 the materials are ordered on three submicrometer length scales. Transmission electron microscopy reveals both spinodal decomposition and nucleation and growth phenomena the relative magnitude of which varies with x. We show that the (Pb0.95Sn0.05Te)1-x(PbS)x system, despite its nanostructured nature, maintains a high electron mobility (>100 cm 2 /V‚s at 700 K). At x ∼ 0.08 the material achieves a very low room-temperature lattice thermal conductivity of ∼0.4 W/m‚K. This value is only 28% of the PbTe lattice thermal conductivity at room temperature. The inhibition of heat flow in this system is caused by nanostructure-induced acoustic impedance mismatch between the PbTe-rich and PbS-rich regions. As a result the thermoelectric properties of (Pb0.95Sn0.05Te)1-x(PbS)x at x ) 0.04, 0.08, and 0.16 were found to be superior to those of PbTe by almost a factor of 2. The relative importance of the two observed modes of nanostructuring, spinodal decomposition and nucleation and growth, in suppressing the thermal conductivity was assessed in this work, and we can conclude that the latter mode seems more effective in doing so. The promise of such a system for high efficiency is highlighted by a ZT ∼ 1.50 at 642 K for x ∼ 0.08.

Science of Advanced Materials, 2014

PbTe is a premiere mid-range temperature thermoelectric material and recent studies have proven nanostructing as an effective approach to reduce thermal conductivity of alloys. Whereas, little attention has been given to long-term thermal stability of secondary phases and microstructural evolution. Interestingly, replacing Te with S in PbTe provides an opportunity to form nanostructures in the bulk material through spinodal decomposition, which appears in binary phase diagrams of the PbTe-PbS system. Herein, the critical composition of PbTe 0.38 S 0.62 alloy is fabricated to n-type by chlorine doping. Thermoelectric transport properties of the alloy are investigated in the 300-850 K temperature range and the maximum zT achieved at 800 K is 0.75 with a predicted zT ∼ 0.85 at 750 K from single parabolic band model. The microstructure of the sintered samples was studied by FEG-SEM for both the as sintered and post transport properties measurement. The experimental results are compared with estimates from the parallel and series models for heterogeneous composites of single phase PbTe and PbS. The Seebeck coefficient is in agreement with the models predictions, but the resistivity is higher and the thermal conductivity is much lower than predicted values. We propose that this is attributed to the phonon and electron scattering on solute atoms in solid solutions and at interfaces.

MRS Proceedings, 2009

Dimensional nanocomposites of PbTe with varying carrier concentrations were prepared from undoped and Ag doped PbTe nanocrystals synthesized utilizing an alkaline aqueous solution-phase reaction. The nanocrystals were densified by Spark Plasma Sintering (SPS) for room temperature resistivity, Hall, Seebeck coefficient, and temperature dependent thermal conductivity measurements. The nanocomposites show an enhancement in the thermoelectric properties compared to bulk PbTe with similar carrier concentrations, thus demonstrating a promising approach for enhanced thermoelectric performance.

Acta Materialia, 2009

Unidirectional solidification experiments have been performed with a thermoelectric alloy with a starting composition of Pb 10.5 Sb 31.6 Te 57.9 in the pseudobinary PbTe-Sb 2 Te 3 system. The bottom of the resulting rod consists of PbTe phase with Widmanstätten precipitates of Sb 2 Te 3 . The precipitation is due to a decrease in the solubility of Sb 2 Te 3 with temperature: the solubility at 450°C was determined to be 2.1 ± 0.2 at.% Sb. The average thickness of plates was estimated to be $100 nm. The spacings between neighboring plates has a distribution in the 200-3000 nm range, peaking around 900 nm. The habit planes of precipitation are of the {1 1 1}PbTe family. An orientation relationship of (0 0 0 1)Sb 2 Te 3 //{1 1 1}PbTe and <1 1 2 0>Sb 2 Te 3 //<1 1 0>PbTe was found with a maximum misorientation of 15°. The Seebeck coefficient after annealing at 450°C was À50 ± 10 lV°C À1 at room temperature. To improve the thermoelectric properties, tuning of the carrier concentration would be necessary.

Journal of Materials Chemistry, 2012

The microstructures and Seebeck coefficients of thermoelectric alloys in the pseudo-ternary PbTe-Ag 2 Te-Sb 2 Te 3 system were examined using samples that were compositionally graded by unidirectional solidification by the Bridgman method and diffusion couples. At compositions near the middle of the pseudo-binary PbTe-AgSbTe 2 line, a compositionally modulated microstructure has been found. From diffusion couple experiments, it is found that the PbTe-AgSbTe 2 system exhibits a miscibility gap at low temperatures while it forms a complete solid solution at high temperatures; the critical temperature is between 400 C and 450 C. The modulated microstructure originates from the decomposition of the high-temperature solid solution during cooling. Scanning Seebeck coefficient measurement on these samples covers a wide compositional space of the pseudo-ternary system. The Seebeck coefficient transitions from positive values at AgSbTe 2 -rich compositions to negative values at PbTe-rich compositions on the pseudo-binary PbTe-AgSbTe 2 line. Composition-graded samples prepared by the Bridgman method are thus useful to investigate thermoelectric materials in multicomponent systems.

Chemistry of Materials, 2015

In this paper, we propose a heterogeneous material for bulk thermoelectrics. By varying the quenching time of Na doped PbTe, followed by hot pressing, we synthesized heterogeneous nanocomposites, a mixture of nanodot nanocomposites and nanograined nanocomposites. It is well-known that by putting excess amounts of Na (i.e., exceeding the solubility limit) into PbTe, nanodots with sizes as small as a few nanometers can be formed. Nanograined regions with an average grain size of ca. 10 nm are observed only in materials synthesized with an extremely low quenching rate, which was achieved by using a quenching media of iced salt water and cold water. Dimensionless thermoelectric figures of merit, zT, of those heterogeneous nanocomposites exhibited a zT around 2.0 at 773 K, which is a 25% increase compared to zT of a homogeneous nanodot nanocomposite with the largest quenching time in our experiment, i.e. furnace cooled. The power factor increase is 5%, and the thermal conductivity reduction is 15%; thus, zT increase mainly comes from the thermal conductivity reduction.

Journal of Applied Physics, 2009

A suspended nanogap formed by field-induced atomically sharp tips Appl. Phys. Lett. 101, 183106 (2012) Generalized interface models for transport phenomena: Unusual scale effects in composite nanomaterials J. Appl. Phys. 112, 084306 (2012) Thermoelectric properties of highly doped n-type polysilicon inverse opals J. Appl. Phys. 112, 073719 Enhanced thermoelectric performance through energy-filtering effects in nanocomposites dispersed with metallic particles Appl.

Applied Physics Letters, 2009

This work proved the incorporation of Cu12Sb4S13 nanoparticles was an effective way to improve the thermoelectric properties of Pb0.97Sb0.03Te, which was of great importance for the study of the regulation of the thermoelectric properties of n-type PbTe. [20]

ACS Applied Energy Materials

SnTe exhibits poor thermoelectric figure of merit owing to Sn vacancies that give rise to a low p-type Seebeck coefficient and high electrical−thermal conductivities. Here, we reported thermoelectric properties of a composite material synthesized by vacuum hot pressing a mixture of preformed p-type SnTe and n-type PbTe. Detailed characterization revealed that the composite material has SnTe alloyed with PbTe along with n-PbTe nanoinclusions. The cumulative effect of alloyinginduced valence band convergence along with energy filtering of charge carriers at the SnTe−PbTe (p−n) interface resulted in an enhanced Seebeck coefficient in the composite material. A significant lowering of thermal conductivity was achieved by phonon scattering at coherent nano p−n junctions and substitution point defects due to alloying. The high Seebeck coefficient along with depressed thermal conductivity in the composite (SnTe) 0.5 (PbTe) 0.5 resulted in the highest figure of merit (ZT) of ∼1.2 at 750 K (i.e., 724% higher compared to pure SnTe) and average ZT of ∼0.5 in a temperature range of 300−750 K. A single-leg thermoelectric power generator fabricated using optimized (SnTe) 0.5 (PbTe) 0.5 showed a conversion efficiency of ∼4.9% for a temperature difference of 400 K.

Acta Materialia, 2007

The effects of composition and cooling rate on the microstructures of alloys in the pseudo-binary PbTe-Sb 2 Te 3 system were investigated as a first step towards the design of nanostructured materials with enhanced thermoelectric properties. Liquid alloys of three different compositions were cooled in three distinct ways: water quenching, air cooling and furnace cooling. The resultant structures and phases were examined by electron microscopy, electron microprobe chemical analysis and electron backscatter diffraction. The compound Pb 2 Sb 6 Te 11 precipitated as a metastable phase (in conjunction with PbTe and/or Sb 2 Te 3 ) under all conditions. Furthermore, whereas PbTe exhibited dendritic morphology, Sb 2 Te 3 and Pb 2 Sb 6 Te 11 crystallized as lamellar platelets with preferred (0 0 1) orientation. The range of cooling rates was from $1 to 26 K/s, while the characteristic microstructural feature size ranged from 10 to 35 lm for dendrites, and from 15 to 50 lm for lamella. The prospects for achieving nanoscale structure are discussed.

Advanced Functional Materials, 2009

Recently, nanoscale inclusions were discovered in alloys of the form AgPb m SbTe 2þm , or (AgSbTe 2 )(PbTe) m , (LAST-m ¼ lead-antimony-silver-tellurium) that were proposed to correlate