High-Entropy Oxides for Thermoelectric Application

High-entropy oxides (HEOs) are a new class of single-phase inorganic materials with a high specific capacity, high structural stability, and super-electronic conductivity and exhibit a wide range of useful properties. HEOs are better semiconductor materials compared to traditional ones due to their lattice distortion. Because parameters, such as crystal symmetry, different lattice parameters, etc., have a significant influence on the thermal conductivity of the material, lowering it via phonon-phonon or phonon-electron scattering. The entropy stabilization produces the high stability of the phase but also can result in interesting properties of the materials due to the contribution of different elements through four main effects: high-entropy effect, severe lattice distortion, sluggish diffusion effect, and cocktail effect. This thesis identified potential HEOs with the chemical composition Co-Cr-Fe-Mn-Ni-O by doing a thorough literature review. During the research, we have focused on the synthesis process and electrical properties of the HEOs (Co0.33Cr0.22Fe0.22Mn0.11Ni0.11)3O4, (Co0.33Cr0.22Fe0.22Mn0.11Cu0.11)3O4, and (Co0.2Cr0.2Fe0.2Mn0.2Cu0.2)3O4. Oxides were synthesized via Spark Plasma Sintering and Solid-State Reaction resulting in obtaining two or more phases with different crystal structures for the materials (Co0.33Cr0.22Fe0.22Mn0.11Ni0.11)3O4, and single-phased for the (Co0.33Cr0.22Fe0.22Mn0.11Cu0.11)3O4 and (Co0.2Cr0.2Fe0.2Mn0.2Cu0.2)3O4 at specific synthesis conditions. As expected, obtained single-phased materials exhibit higher values of electrical conductivity, which is probably due to the less electron-phonon scattering. Two types of semiconductors are needed for thermoelectric applications: p- and n-type. Due to the different synthesis temperatures, materials with Ni were obtained in both types. This can lead to the production of the Peltier module with the same chemical composition inside. With the Ni-Cu substitution, it became easier to produce single-phased materials, probably due to the melting point of the reagents. These materials also presented higher electrical properties, which the changes in carrier concentration can explain due to the differences in the electronic structures. All obtained samples exhibit low values of the electronic part of thermal conductivity, which can lead to low values of total thermal conductivity. It shows that the main contributor to the thermal conductivity will be from the phonons (lattice thermal conductivity). Overall, the expected thermal conductivity for these materials should be lower compared to the traditional semiconductor materials due to the crystal distortion, which can lead to higher phonon-phonon and phonon-electron scattering. Furthermore, this research shows that HEOs with unequal content of metals can be produced as single-phase materials and have even better or similar electrical properties compared to known compositions. Also, these oxides with impurities still exhibit promising electrical properties.