Low thermal conductivity of Al-doped ZnO with layered and correlated grains

An alternative, faster and simpler method for the formation of hierarchically porous ZnO particles and their thermoelectric performance

“…4e-h and d-g), considerably larger than the mean free path of $30 nm for phonons in ZnO. 2,22,48 The (k) thermal conductivity value (especially T-NZnOP-750C and TNZnOP-950C) is considerably lower compared with the previous reported data of normal bulk ZnO-based materials ($100 W m À1 K À1 at RT). 6,7,[17][18][19] We also note that for our present T-ZnOP samples, the sintering temperatures of T-ZnOP were lower compared to typical bulk ZnO-based material wherein their sintering temperature required to reach as high as 1200-1400 C. 6,[12][13][14][15] Generally, the low (k) thermal conductivity achieved by the T-ZnOP samples were mainly attributed to the relative suppression of grain growth due to our low-temperature sintering and the formation of macro/nanopore structuring owing to our (synthesized) starting nanoporous ZnO powders.…”

Improvement in the thermoelectric properties of porous networked Al-doped ZnO nanostructured materials synthesized via an alternative interfacial reaction and low-pressure SPS processing

“…The electrical conductivity of these sintered samples (T-NZnOP and T-NZn 1−x Al x OP) that were synthesized from fine, porous-networked powders is found to be superior to what was reported in the literature for the same compositions that were prepared by other different synthesis routes. 44,54,55,57,58,82,83,90 This difference is more pro-nounced when these values of electrical resistivity of our sintered (under an Ar atmosphere) samples are compared with the other literature reports of ZnO-based ceramics that were sintered in air. 29,40,56,91 It is known that the sintering of ZnO in air can result in intrinsic acceptor states, i.e., zinc vacancies (V Zn ) and oxygen interstitials (O i ), at the grain boundaries and hence the formation of electrostatic Schottky barriers at the grain boundaries with a breakdown voltage of ∼0.1 V and higher, which can markedly increase the resistivity of the ceramics.…”

“…A low resistance n-type Al-doped ZnO bulk material is usually obtained by doping with ∼0.5-5 mol% Al 2 O 3 followed by sintering at ∼1673 K. 28,38 Several attempts and strategies have been adopted in the past with varying degrees of success to reduce the thermal transport in ZnO materials such as doping with homologous compounds like InO 1.5 49,50 and GaO 1.5 , 39,51 for nanoscale engineering by creating nanoinclusions of planar defects, superstructuring, pores/ voids, nano-graining, grain boundaries, point defects, etc. [52][53][54][55][56][57][58][59][60] However, in most cases, the preparation of such ZnO-based nanocomposites involves the laborious process of mechanical alloying (high energy ball milling) or solid-state reactions followed by hot pressing.…”

Effect of powder size on the microstructure and dielectric properties of ZnO ceramics

“…Decreased powder size enhances the vapor transport, which simultaneously reduces the driving force for densification due to its coarsening effect. Therefore, coarsening by the vapor transport mechanism increases the grain size accompanied by the pore growth as well, leading to the correlated grain structure with relatively high porosity [16]. Curvature-driven boundary migration also promotes the motion of pores trapped between the agglomerates.…”