Dielectric Micro‐ and Sub‐Micrometric Spacers for High‐Temperature Energy Converters

Abstract

Dielectric microspacers (DMS) are important components in thermal energy converters. Engineered DMS are fabricated and characterized on different substrates by depositing patterned ceramic thin films of alumina (Al 2 O 3) and zirconia (ZrO 2) with a thickness ranging from 0.3 to 3 μm. Both Al 2 O 3 and ZrO 2 films are electrically and thermally optimized, finding zirconia more suitable as a thermal and electrical insulating material at high temperature, whereas the developed DMS are morphologically analyzed by scanning electron microscopy. The analysis of thermal simulations carried out with COMSOL Multiphysics allows identifying the best geometrical constraints for each single structure, whereas simulations carried out by the Fluent software allow identifying the best arrangement for DMS, leading to a solution with optimized pattern in terms of amount and spatial distribution so to achieve the required electrical and thermal insulation for practical applications. DMS are integrated within thermionicphotovoltaic devices to be validated experimentally, and enhanced electron emission measurements are successfully performed at a cathode temperature up to 1350 C to verify the operational feasibility and potential of this technology.

Progress Toward High Power Output in Thermionic Energy Converters

“…[ 40 ] ), ii) columns (similar to Belbachir, An, and Ono [ 57 ] and Bellucci et al. [ 60 ] ), and iii) freestanding corrugated films (similar to Nicaise et al. [ 190 ] ).…”

“…[ 83 ] Perhaps the most elegant solution, however, is to use fixed‐form inserts within the gap to define the electrode spacing. This approach has been implemented in several forms, including commercially available micron‐scale grains such as polystyrene particles, [ 202 ] alumina beads, [ 40 ] and silica spheres [ 203 ] ; through lithographically defined insulators such as silicon dioxide or zirconia columns, [ 57,60,204,205 ] quartz standoffs, [ 206 ] and photoresist pillars [ 207,208 ] ; and through freestanding structures such as corrugated ceramic films. [ 48,190 ]…”

“…[40,46,[48][49][50][51][52][53][54][55][56][57][58][59] The numbers in brackets refer to the references at the end of this article. In panel (a), the TEC constructed by Bellucci et al [59] was a proof-of-concept device whose high work function electrodes limited its current density (note that another paper by Bellucci et al [60] describes a different early prototype that falls below the range of this plot). Also in panel (a), the point behind Littau et al [40] is associated with Dick et al [52] In panel (b), several points are associated with Fitzpatrick et al [53] functions, TECs must be operated at high temperatures in order to achieve reasonable energy conversion efficiencies (𝜂 > 10%).…”

“…[57] The scientists operated their device at low temperatures (T e = 1100 K, T c = 640 K) and achieved an output power density of P = 11.5 mWcm −2 with a core efficiency of 𝜂 = 3.9 × 10 −3 %, noting that the low thermal resistance of the columns (an area average of roughly 𝜚 s ≈ 3.5 cm 2 K W −1 ) resulted in large conductive heat transfer losses. Similarly, Bellucci et al [60] fabricated zirconia columns on the surface of a GaAs collector to separate it from a tungsten emitter by a distance of d = 3 µm. This prototype produced a low open-circuit current density of J ≈ 30 nAcm −2 (peak power density P ≈ 16 nWcm −2 ) despite its high temperature difference (T e − T c = 1468 − 523 K) due to its large electrode work functions (𝜙 e ≈ 4.5 eV and 𝜙 c ≈ 3.46 eV); however, it represented progress toward achieving ultra-thin gaps in practical devices.…”

Properties of Hafnium and Aluminium Silicates Coatings Obtained by PLD

“…Hafnia and alumina are high-κ dielectrics with remarkable properties, such as high abrasion resistance and high thermal and chemical stability, which make them useful in a wide area of applications: dielectric gate in various types of transistors [1,2], mirrors or antireflective coatings. They can be also used for the development of important components in novel high-temperature energy devices, such as hybrid thermionic converters, as a promising alternative to the presently used ceramics [3]. Besides the aforementioned applications, Al 2 O 3 has also been extensively investigated as a top layer for solar cells [4,5] because it passivates the silicon surface, and it also acts as an electron blocking layer, which increases the built-in potential and reduces recombination [6].…”