Development and Characterisation of Novel Coatings and Thin Films for High-Efficiency Smart Thermoelectric Materials and Modules

Gurtaran, Mikdat ORCID: 0000-0002-8953-7458 (2024). Development and Characterisation of Novel Coatings and Thin Films for High-Efficiency Smart Thermoelectric Materials and Modules. University of Birmingham. Ph.D.

Gurtaran2024PhD_Redacted.pdf
Text - Accepted Version
Available under License All rights reserved.
Download (12MB)

Abstract

This PhD study was focused on improving the heat-to-energy conversion efficiency at high and low temperatures in thermoelectric materials and modules by developing advanced coatings and thin films. Firstly, the oxidation behaviour and mechanism of P-type (Zr,Ti)Co(Sn,Sb) and N-type (Zr,Ti)Ni(Sn,Sb) half-Heusler thermoelectric (TE) materials (used in this PhD project) at medium-to-high-temperatures were examined using scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), and X-ray diffraction (XRD). It was observed that the surface morphology and composition of these TE materials completely changed after cyclic oxidation at 500°C. Multilayer structures of SnO2+Ni3Sn4+(Zr,Ti)O2 and CoSb+SnO2+Sb2O4+(Zr,Ti)O2 formed on the surface of the N-type and P-type TE materials, respectively. Additionally, thick double-layers formed on the surface of N-type and P-type TE materials (SnO2+Ni3Sn4 and SnO2+CoSb, respectively) when statically oxidised at 600°C for 50h. Based on the findings obtained, advanced oxidation-resistant CrSi coatings were developed and deposited on these TE materials using the magnetron sputtering physical vapour deposition (PVD) technique. The CrSi coatings can effectively reduce oxidation kinetics, protecting both N-type ((Zr,Ti)Ni(Sn,Sb)) and P-type ((Zr,Ti)Co(Sn,Sb)) TE materials against oxidation at 500°C and 600°C, regardless of the oxidation methods (static or cyclic).
To increase the efficiency of thermoelectric materials and modules, the interface of P-type half-Heusler TE materials ((Zr,Ti)Co(Sn,Sb)) was metallised by Ni/NiGO coatings with high electrical and thermal conductivities. These coatings were deposited using the electro-brush plating method, which is more flexible and cost-effective than other coatings methods. The findings showed that metallising Ni/NiGO coatings significantly reduced the interfacial and sheet resistances of P-type TE material although no significant effect of graphene oxide (GO) addition on increasing the electrical conductivity of the coating layer. Additionally, Ni coatings protected the material from oxidation at high temperatures.
To improve the TE performance of Bismuth Telluride (BiTe-based) TE materials, which are practically used in low-temperature applications, BiTe-based thin films were produced on different substrates using the magnetron sputtering PVD system. The outcome of the study has indicated that the power output of thin films deposited on different substrates (glass, Kapton, and silicon) is closely related to how the active layers are connected. However, the proportional increase in thermal conductivity along with electrical conductivity with increasing temperature limited the further improvement of the figure of merit (zT) value.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):