Optimization of the electronic, thermal and mechanical properties of Bi2Te3-based room temperature thermoelectric materials

The utilization of energy sources is a highly critical issue regarding the environmental concerns and insufficient natural resources. In addition to regulations promoting the use of renewable energy sources, waste energy management has gained attention since the mid-20th century. Sources of waste heat are ubiquitous such that unused heat reaches a total of two-thirds of all energy input. Thermoelectric materials allow direct conversion of dissipated heat into electricity and vice versa by using nontoxic, lightweight and emission free materials. Improving the efficiency of such materials requires the increment of thermoelectric figure of merit, 𝑧𝑇 = %&' ( 𝑇 where S, σ, and κ symbolize the Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. Nonetheless, decoupling of these transport coefficients is quite difficult as they are interdependent, making the optimization of zT challenging.Bi2Te3 - based thermoelectric materials are heavily used in thermoelectric cooling systems owing to their high band degeneracy along with relatively low thermal conductivity resulting in a zT > 1 at room temperature (323K) This thesis study focuses on the significant enhancement of thermal, electronic and mechanical properties of both n- and p-type Bi2Te3-based materials by optimizing the carrier concentration and reducing the thermal conductivity. For the first time, rickardite mineral (Cu2.9Te2) was doped into p-type (Bi,Sb)2Te3 to increase the electrical conductivity without significant deleterious effects on the thermal conductivity.Results indicate that as-doped Bi2Te3 attains a zT of 1.32 at 467 K, which is 106% higher than the undoped pristine sample at the same temperature. Besides this record-high increment in peak zT, room temperature zT and microhardness were elevated by 18% and 64%, respectively. Doping of carbon-coated nano boron (cB) was found to have a similar contribution to the efficiency of (Bi,Sb)2Te3. For example, 0.5 and 0.8 wt% cB additions resulted in a zT of 1.21 and 1.22, which are 19% and 14% higher than their undoped counterparts. On the other hand, mechanical hardness values were recorded to be 56% and 14% higher than pristine samples. Furthermore, cB addition was a facile method to optimize the thermoelectric and mechanical properties of n-type Bi2Te2Se by 17% and 75%, respectively. These extraordinarily promising results can contribute to the widespread use of n- and p-type Bi2Te3 in thermoelectric cooling applications that are free of greenhouse gases.