Thermoelectric property studies on thallium-doped lead telluride prepared by ball milling and hot pressing

Abstract

Thallium doping into lead telluride has been demonstrated to increase the dimensionless thermoelectric figure-of-merit ͑ZT͒ by enhancing Seebeck coefficient due to the creation of resonant states close to Fermi level without affecting the thermal conductivity. However, the process is tedious, energy consuming, and small in quantities since it involves melting, slow cooling for crystal growth, long time annealing, post-crushing and hot pressing. Here we show that a similar ZT value about 1.3 at 400°C is achieved on bulk samples with grain sizes of 3 -7 m by ball milling a mixture of elemental thallium, lead, and tellurium and then hot pressing the ball milled nanopowders. © 2010 American Institute of Physics. ͓doi:10.1063/1.3452323͔Thermoelectrics ͑TEs͒, as one of the most promising approaches for solid-state energy conversion between heat and electricity, is becoming increasingly important within the last decade as the availability and negative environmental impact of fossil fuels draw increasing attention. Various TE materials with a wide working temperature range ͑from 25 to 1000°C͒ for different applications in cooling and power generation have been extensively studied. The efficiency of TE devices is determined by a dimensionless figure-of-merit ZT = ͑S 2 / ͒T, where S is the Seebeck coefficient, the electrical conductivity, the thermal conductivity ͑sum of the electronic part e and the lattice part L ͒, and T the absolute temperature of the material. 1 Lead telluride ͑PbTe͒ is one of the most studied and used intermediate temperature TE materials with a ZT of about 1 at 380°C, 2 which is suitable for power generation applications such as waste heat recovery 3 and potentially in solar energy conversion. 4 However, the toxicity of Pb is a big concern, which slowed down the applications of such materials. Nevertheless, the interest in PbTe has significantly increased due to the demonstration of a ZT higher than 1.5 by creating nanosized inclusions in bulk crystalline materials ͑lead antimony silver telluride͒ using the traditional crystal growth technique, in which the improvement comes from thermal conductivity reduction due to the increased phonon scattering by the nanosized inclusions in the materials. 5 The formation of such inclusions is believed to be caused by substituting some Pb with Ag and Sb.Recently, a ZT of about 1.5 at around 500°C was also reported through improving the Seebeck coefficient 6 via creating resonant states close to Fermi level by using a 2 at. % thallium ͑Tl͒ to replace Pb ͑Tl 0.02 Pb 0.98 Te͒. However, the process is very tedious since it involves melting, slow cooling for crystal growth, long time annealing, post-crushing, and hot pressing for compacting the powders into dense bulk samples. Such a slow process is not practical to large scale production of multiple tons per year required by industrial applications.In the last few years, we have successfully demonstrated that large quantities of bulk nanostructured TE materials can be easily produced by ball milling and hot press...

Effect of Ce-Doping on Thermoelectric Properties in PbTe Alloys Prepared by Spark Plasma Sintering

“…Tl-doped PbTe (Tl 0.02 Pb 0.98 Te) can achieve a maximum ZT of 1.3 at 673 K by enhancing the Seebeck coefficient due to the creation of resonant states close to the Fermi level without affecting the thermal conductivity. 2 The addition of excess Pb and Sb to PbTe leads to a large enhancement of the power factor, achieving a ZT of 1.4 at 673 K. 3 The substitution or addition of other elements may lead to the formation of nanodots or nanoregions, which is beneficial in terms of reducing the lattice thermal conductivity or total thermal conductivity. Substitutions of Ag and Sb for Pb in PbTe result in the compound AgPb m SbTe 2+m , exhibiting an outstanding ZT value of 2.2 at 800 K due to the distribution of Ag-Sb nanodots in the PbTe matrix.…”

Complex role for thallium in PbTe:Tl from local probe studies

“…The pure compound is already a good thermoelectric, but when doped with a small amount of Tl (Pb 1−x Tl x Te), ZT improves significantly to ∼ 1.5 near 800K for 2% Tl; much of this increase is attributed to an enhancement in S at high hole concentrations for the Tl doped samples, as a result of a change of the density of states near the top of the valence band. 4,5 At much lower Tl concentrations (∼ 0.2% Tl) S is larger 6 but the electrical resistivity is much higher. Unfortunately for x > 2%, ZT again decreases.…”

Enhancement of thermoelectric figure-of-merit by resonant states of aluminium doping in lead selenide

“…It is reported that the modification of band edge also can be realized by nano inclusion 28,29 , such as the AgSbTe 2 nanoinclusion in the PbTe matrix 30 . Since such nano inclusions are widely observed in nanocomposite made by ball milling and hot pressing 3,21,22 , the Al related nano inclusion may be another possible reason for the unchanged Seebeck coefficient increasing carrier concentration. Besides the possible band structure changing by resonant state or nano inclusion, there is another possibility, i.e.…”

“…The alloyed nanopowders were then loaded into a graphite die with an inner diameter of 12.7 mm, and finally consolidated by direct current (DC) induced hot pressing 3,21,22 . The thermal diffusivities of the as-pressed disk samples were measured by a laser flash system (NETZSCHLFA 457), and the specific heat was determined by a DSC system (NETZSCH DSC 200-F3).…”