| Summary: | Thermoelectrics can convert heat energy into electrical energy (Seebeck effect) and vice-versa (Peltier effect) without any sort of pollution. That is because there are no moving parts that can cause noise pollution and no liberation of gas or chemical residue that could degrade the environment. On the contrary, application of the thermoelectric concept will help to deal with two main global issues, the increasing demand for energy with all the developments (increasing number of vehicles on the road, construction of more building, urbanisation of rural areas in developing countries etc.) and the drastic climate changes which are a result of those developments. The application of thermoelectrics in Peltier coolers has already helped to decrease the ozone depletion problem by replacing the CFC’s in the refrigerators.
The Seebeck effect could help fulfil the increasing global demand for fuel and decrease significantly greenhouse gases if applied to exhaust systems of vehicles to convert the lost heat energy into useful electricity. The efficiency of thermoelectrics depends on the dimensionless figure of merit, ZT (ZT=T (S^2 σ)⁄κ, T = temperature, S = Seebeck coefficient, σ = electrical conductivity, κ = thermal conductivity); the higher the ZT value, the higher will be the efficiency. The best suited materials for thermoelectric are semiconductors as they have a compromised high S and high , however, depends on the materials themselves, some have low and some have high. Lots of research has been done on reduction of of semiconductors with good electronic properties.
This research is about investigating the structure and the thermoelectric properties of thallium lanthanoid tellurides. Thallium lanthanoid tellurides might be promising thermoelectrics exhibiting small , as selected thallium-based tellurides have outstanding properties. The ZT values of both Tl9AgTe5 and Tl9BiTe6 are 1.2 at 700 K and 500 K respectively; the state-of-the-art thermoelectric materials, SiGe, Bi2Te3 and LAST have a ZT value of ~ 0.5 at temperatures greater than 900 K, 0.6 at RT and ~ 1.7 at 700 K respectively. The low of ~ 0.4 W/(mK) is responsible for the good ZT value of Tl9BiTe6 and Tl9AgTe5. However, thallium based semiconductors might never be commercialized due to the toxicity of Tl element. Nevertheless, from a scientific point of view, the study of the thallium lanthoid tellurides, Tl10-xLnxTe6, will still contribute in understanding the relation between the structure, stoichiometry and the properties of these thallium based semiconductors.
In the present study, thallium lanthanide tellurides, Tl10-xLnxTe6, Ln = La, Nd, Gd, Tb, Dy, Ho, Er; 0.25 ≤ x ≤ 1.32 are investigated. All the thallium lanthanoid tellurides are isostructural to Tl9BiTe6, adopting the space group I4/mcm with mixed occupancy of Tl+/Ln3+ at the 4c site. Substitution of Tl by Ln at the 4c site in the Tl10Te6 structure, changes the lattice parameters and unit cell volume of the compounds as expected. The unit cell volume increases as the lanthanide content increases within a particular phase (more of the f-block Ln elements incorporated in the structure) and decreases across the lanthanides series for a specific stoichiometry (across the Ln series, atomic size decreases due to lanthanide contraction). Thermoelectric property measurements on sintered pellet showed that the Seebeck coefficient, S, increase as the lanthanide content increases for a particular phase (carrier concentration in the compound decreases as more Ln is incorporated). On the other hand, electrical conductivity, , (due to decrease in carrier concentration) and thermal conductivity, , (due to decrease in carrier concentration, increase in mass fluctuation and lattice vibration) decrease as the lanthanide content increase. The opposite trend is true for Tl10-xLnxTe6, x 1, across the lanthanide series, S decreases whereas and increase. Consequently, the dimensionless figures of merit increase within a particular phase but decreases across the lanthanide series, the highest ZT value of ~ 0.20 was obtained for Tl9LaTe6 and Tl8.98Nd1.02(6)Te6 at 553 K.
Moreover, thermoelectric properties were studied on a hot-pressed pellet of Tl9LaTe6. The Seebeck coefficient for the cold-pressed and the hot-pressed pellets were almost same for the whole temperature range studied. A slight increase in the values for hot-pressed compared to the data of the sintered pellet was observed for the same temperature range. Similarly, values for the hot-pressed pellet were higher that of the cold-pressed pellet. The higher electrical and thermal conductivity of the hot pressed pellet with respect to the sintered pellet is due to improved compactness. The highest ZT for the hot-pressed pellet was ~ 0.32 around 555 K.
The second part of the project was to study the thermoelectric and magnetic properties on the Ce, Sm and Pr compounds of the Tl10-xLnxTe6 family. The thermoelectric properties of those compounds with temperature were in agreement to the observations made for other compounds studied in this thesis. As temperature increases, S increases while decreases, was basically temperature independent as for the rest of this study. However, the study of the thermoelectric properties of compounds of Ce, Pr and Sm phases did not correlate with the rest of the thallium lanthanoid tellurides both within the phases and across the lanthanide series.
The unit cell volume of Tl9CeTe6 was found to be unexpectedly smaller compared to the general decreasing tend ain the unit cell volume of Tl9LnTe6, across the Ln series. On the contrary, the unit cell volume of Tl9PrTe6 was found to be unexpectedly bigger compared to the general decreasing tend in the unit cell volume of Tl9LnTe6, across the Ln series. The physical properties of Tl9CeTe6 and Tl9PrTe6 compounds were anomalous as well, with respect to the rest of the Tl9LnTe6 series. There is a general increase in S from Tl9CeTe6 to Tl9SmTe6 compounds instead of a decreasing trend. Instead of an increasing tendency in the and values across the Tl9LnTe6 series, both parameters decrease from Tl9CeTe6 to Tl9SmTe6. Curie-Weiss Law and Modified Curie-Weiss Law were applied to their magnetic property measurement data. The magnetic property measurements revealed a magnetic moment of 2.02 eff/B for Tl9CeTe6 which is lower than the expected value of 2.54 eff/B. This points out towards the possibility of some Ce4+ in the structure unlike the Tl9PrTe6 compounds which had only Pr3+ ion in the structure. The magnetic data of Tl9SmTe6 was not conclusive as there was a temperature dependence of the magnetic field.
This study therefore reveals that the thallium lanthanoid tellurides, Tl10-xLnxTe6, 0.25 x 1.32, Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho and Er, do have low ( 3 W/(mK)), especially when x ~ 1 ( 0.5 W/(mK)). The best ZT of this series is ~ 0.20 around 550 K exhibited by Tl9LnTe6, Ln = La, Nd, Sm compounds. The hot-pressed sample of Tl9LaTe6 exhibited a ZT value of ~0.32 in the around 550 K.
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