(Hafnium zirconium) nitride/scandium nitride metal/semiconductor superlattices for thermionic energy conversion
Abstract
Nitride metal/semiconductor superlattices are a promising materials system for high temperature (>800K) thermionic energy conversion devices. This dissertation specifically investigates various properties of the (HfxZr 1-x)N/ScN metal/semiconductor materials system and introduces a novel approach for fabricating bulk-like thermoelectric devices. (HfxZr 1-x)N/ScN superlattices were deposited on magnesium oxide, sapphire, and silicon substrates by reactive DC magnetron sputtering and characterized by field emission scanning electron microscopy, high resolution x-ray diffraction, and transmission electron microscopy. Magnesium oxide and sapphire substrates produce epitaxial superlattice, whereas films deposited on silicon are characterized as textured-polycrystalline with superlattices within each grain. In addition to thin film characterization, a novel laminate approach was developed that allows for bulk-like devices to be fabricated from nanostructured superlattices, bridging the nano/micro divide. The laminate approach provides a means to simultaneously characterize all of the thermoelectric parameters, (Seebeck coefficient, electrical conductivity, thermal conductivity) of superlattices via a Z-meter characterization system and also provides a scalable process for industrial applications. Parasitic analysis of laminates revealed that low electrical contact resistivity contacts (<2·10-8 Ω-cm 2) are a critical factor for successful implementation of laminate metal/semiconductor superlattice devices. Electrical contact resistivity values for various contact schemes were characterized by the transfer length method, with values as low as 4·10-8 Ω-cm2 achieved. The high uncertainty in the characterization of contacts with low electrical contact resistivity is a challenging roadblock that can be partially overcome through careful design of the transfer length method pattern. Temperature dependent thermal conductivity analysis of HfN/ScN, (Hf 0.5Zr0.5)N/ScN, and ZrN/ScN superlattices was performed. Increasing thermal conductivity with increasing temperature revealed a high electronic component to thermal conduction, an indicator of thermionic emission. However, thermal conductivity values (6 to 10 W/m-K) were significantly higher than required (1 W/m-K) for an efficient thermoelectric device, so additional work must be directed at reducing the thermal conductivity. HfN/ScN and ZrN/ScN also exhibited unusually high thermal interface conductance (600-1200 MW/m 2-K), which contributes additional challenges towards creating high ZT materials. The results detailed in this dissertation, while not exhaustive, provide a stepping stone along the long path towards creation of a new class of high ZT materials.
Degree
Ph.D.
Advisors
Sands, Purdue University.
Subject Area
Materials science
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