Applications and advanced sintering techniques of functionally graded ZnO-based thermoelectric material
Date
2017
Authors
Cramer, Corson Lester, author
Ma, Kaka, advisor
James, Susan P., advisor
Williams, John D., committee member
Sampath, Walajabad, committee member
Neilson, Jamie R., committee member
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Abstract
Thermoelectric generator (TEG) materials provide a unique solid-state energy conversion from heat to electricity. Nanostructured TEGs experiencing transient thermal loads at medium to high-temperatures are susceptible to degradation due to thermal stress cracking, which subsequently causes decreased lifetime. Previous efforts to prevent the thermal degradation have led to the following approaches: geometric pinning, compositional gradients, and segmentation of different materials. In the present research, functionally graded zinc oxide (ZnO) materials with graded grain size distribution were fabricated using a water sintering strategy via spark plasma sintering (SPS) with a thermal gradient in combination with modified tooling and strategic mechanical load schedules. Samples with homogeneous grain size distribution were also fabricated as a baseline for comparison. The primary objective of the work is to investigate the correlation between the processing conditions, formation of graded microstructure, and the resultant thermoelectric (TE) output performance and lifetime of the ZnO materials. The fundamental understanding of this correlation will contribute to future design of TEG materials using the approach of graded microstructure. The hypothesis is as follows: in a TEG material with graded grain size distribution, one side that consists of coarse (micron-sized) grains is exposed to the heat source. This coarse-grained side of the material can mitigate thermal stress cracking by spreading the heat more quickly during transient heating and thus provide improved thermal stability. The other side of the TEG material consists of fine grains (submicron-sized) and still exhibits high efficiency. In the current study, both continuously graded ZnO materials and a five-layer discretely graded ZnO material were fabricated. Microstructural characterization shows that the grain size gradient of the continuously graded materials across a 10-mm thickness goes from submicron scale (average size ~ 180 nm) to micron scale (~1.2 μm). The thermoelectric properties of the baseline ZnO materials with uniform grain sizes were measured. Using the data obtained from those samples with uniform grain sizes, the peak efficiencies of the continuously graded materials and the five-layer graded materials were simulated and compared to the experimentally measured values. The lifetime of the ZnO samples was evaluated from the electrical resistance at the cycling temperature. The results of the final efficiencies suggest that the thermoelectrical performance of the ZnO materials benefit from the grain size gradation. In addition, the sintering behavior of the continuously graded ZnO system is investigated and compared to that of the isothermally sintered samples to establish a predictive model of the microstructure (density-grain size-time relation). A discrepancy is observed between the prediction of the continuously graded materials and the experimental results. This discrepancy is attributed to a stress shielding that develops during sintering due to differential sintering from the temperature gradient. The stress shielding occurs when denser, and thus stiffer material develops adjacent to less dense and less stiff material causing the stress to vary because the stress is not evenly distributed. The stress shielding effect during sintering is further investigated through theoretical sintering equations. Using the viscoelastic analogy in sintering, the stress to be added to the sample during sintering in a thermal gradient is quantified to compensate the discrepancy from the samples sintered isothermally based on an average strain rate difference.
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Subject
functionally graded material
spark plasma sintering
thermoelectric material
pressure-shielded sintering
advanced sintering
thermal fatigue