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ZnO-based thermoelectrics : modelling, electrochemical thick film growth, and characterization

University of British Columbia

The thermoelectric performance of electrodeposited, aluminum doped zinc oxide was assessed. In this work, wurtzite ZnO was first modelled using Mueller-Plathe to compare the effectiveness of different nanostructured configurations on reducing thermal conductivity. A new analysis technique, Local Vibrational Density of States Equilibrium Molecular Dynamics (LVDOS-EMD), was created to study localized lattice vibrations around nanostructural features of silicon and ZnO, and was used to predict thermal properties in materials of similar composition 17× faster than conventional thermal modelling methods. A 30% void density was determined to yield the best reduction in thermal conductivity by volume of voids in bulk Al:ZnO with a computed thermal conductivity of 0.77 W m-¹ K-¹ at room temperature, 3× below the threshold achieved through established experimental means with high electrical conductivity Al:ZnO. Thick film, electrodeposited Al:ZnO was grown using a nitrate system. Experiments on solution pH using various counter electrodes demonstrated that inert electrodes caused acidification of the growth solution, limiting film thickness. Chloride contamination from commonly used Ag/AgCl reference electrodes was also determined to affect thick film opacity, morphology, crystallinity, and electrical properties. Aluminum integration and activation was explored by adding Al(NO₃)₃ to the growth solution during film synthesis, yielding aluminum integration molar ratios of up to 1.72% (Al.₀₃₄Zn.₉₆₆₀). Partially doped films in excess of 95 µm thick, 4× the thickness reported elsewhere, were electrochemically grown and characterized. Sub-micron voids were integrated into the films using sacrificial material and annealing. A new electrochemical chromium etching methodology was developed and successfully used to free 20 films from their growth substrates for thermoelectric characterization. A new, reusable thermoelectric test apparatus for thin film thermoelectric testing was designed, implemented, calibrated, and successfully deployed to characterize ZnO and Al:ZnO thin films grown 79 – 95 µm in thickness. Extremely low thermal conductivity of 11 mW m-¹ K-¹ at room temperature was demonstrated concurrently with a Seebeck coefficient of -88 µV K-¹. Polycrystallinity and poor dopant activation yielded a low electrical conductivity of 0.75 mS/cm and corresponding low room temperature ZT of 1.3×10-⁵ for the Al:ZnO films.

Text

2016-10-12

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/59418

Electrical and Computer Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/