Theoretical and Experimental Studies on Early Transition Metal Nitrides for Thermoelectrics

Abstract: Thermoelectricity transforms temperature gradients across thermoelectric material into an external voltage through a phenomenon known as the Seebeck effect. This property has resulted in niche applications such as solid-state cooling for electronic and optoelectronic devices which exclude the need for a coolant or any moving parts and long-lasting, maintenance-free radioisotope thermoelectric generators used for deep-space exploration. However, the high price and low efficiency of thermoelectric generators have prompted scientists to search for new materials and/or methods to improve the efficiency of the already existing ones. Thermoelectric efficiency is governed by the dimensionless figure of merit ??, which depends on the electrical conductivity, thermal conductivity and Seebeck coefficient value of the material and has rarely surpassed unity.In order to address these issues, research conducted on early transition metal nitrides spearheaded by cubic scandium nitride (ScN) thin films showed promising results with high power factors close to 3000 μWm−1K−2 at 500 °C. These results are the main motivation behind my thesis where the conducted research is separated into two different routes:• the synthesis and characterization of chromium nitride thin films and its alloys• the study of hypothetical ternary nitrides equivalent to scandium nitrideRock-salt cubic chromium nitride (CrN) deposited in the form of thin films by reactive magnetron sputtering was chosen for its large Seebeck coefficient of approximately -200 μV/K and low thermal conductivity between 2 and 4 Wm−1K−1. The results show that CrN in single crystal form has a low electrical resistivity below 1 mΩcm, a Seebeck coefficient value of -230 μV/K and a power factor close to 5000 μWm−1K−2 at room temperature. These promising results could lead to CrN based thermoelectric modules which are cheaper and more stable compared to traditional thermoelectric material such as bismuth telluride (Bi2Te3) and lead telluride (PbTe).Although cubic CrN has been shown to be a promising material for research with a large power factor, the electrical resistivity limits applications in pure form as the ?? is estimated to be slightly below 0.5. To overcome this issue, I enhanced the thermoelectric power-factor of CrN by alloying it with a conductor, Rock-salt cubic vanadium nitride (VN). VN is a suitable choice as both materials share the same crystal structure and have almost equal lattice constants. Through deposition at 720 °C, where a small amount of VN (less than 5%) and Cr2N is introduced into the film, a reduced electrical resistivity averaged around 0.8 × 10-3 Ωcm, Seebeck coefficient value of 270 μV/K and a power-factor of 9.1 × 10-3 W/mK2 is measured at room temperature, which surpasses the thermoelectric properties of Bi2Te3. Hexagonal dichromium nitride (Cr2N) nano-inclusions increase the charge carrier concentration and act as phonon scattering sites. Single crystal Cr2N was also studied separately, as it shows interesting elastic-plastic mechanical properties and high resistance to oxidation at high temperatures for long periods of time.In the second part of this thesis, hypothetical ternary nitrides equivalent to ScN are investigated for their prospective thermoelectric properties. Scandium nitride has a relatively high thermal conductivity value (close to 10 Wm−1K−1), resulting in a low ??. A hypothetical ternary equivalent to ScN may have a similar electronic band structure and large power factor, but with a lower thermal conductivity value leading to better thermoelectric properties. Thus, the elements magnesium, titanium, zirconium, and hafnium were chosen for this purpose. DFT calculations were used to simulate TiMgN2, ZrMgN2 and HfMgN2. The results show the MeMgN2 stoichiometry to be stable, with two rivaling crystal structures: trigonal NaCrS2 and monoclinic LiUN2. The calculated electronic band structure of these compounds shows a direct band-gap for the monoclinic and an indirect band-gap for the trigonal crystal structures. These findings, coupled with predicted Seebeck coefficient values, encourages actual synthesis of such materials. DFT calculations were also used to study (Zr, Mg)N and (Hf, Mg)N alloys based on the SQS model. The transition temperature between the ordered monoclinic structure of ZrMgN2 and HfMgN2 and the disordered (Zr, Mg)N and (Hf, Mg)N alloys is calculated to be approximately 800 K and 1050 K respectively. Density of State (DoS) calculations show that similar to (Ti, Mg)N, (Zr, Mg)N and (Hf, Mg)N are also semiconducting. The thermoelectric properties of both compounds are also predicted, and that in the range of a moderate change in the Fermi level, high Seebeck coefficient values at room temperature can be achieved.Finally, in order to complete the mentioned study on hypothetical ternaries, I deposited (Ti, Mg)N thin film alloys by reactive magnetron sputtering. These films, which were deposited at 400 °C, are porous and are crystallized in the rocksalt cubic structure. As-deposited films show an electrical resistivity of 150 mΩcm and a Seebeck coefficient of -25 μV/K, which shows semiconducting properties. In order to initiate a phase transformation, these films when annealed at approximately 800 °C, where nano-inclusions of a titanium/magnesium oxynitride are formed in a LiTiO2-type superstructure are identified by XRD and TEM analysis.