Micron-gap spacers with ultrahigh thermal resistance and mechanical robustness for direct energy conversion

Abstract

In thermionic energy converters, the absolute efficiency can be increased up to 40% if space-charge losses are eliminated by using a sub-10-µm gap between the electrodes. One practical way to achieve such small gaps over large device areas is to use a stiff and thermally insulating spacer between the two electrodes. We report on the design, fabrication and characterization of thin-film alumina-based spacers that provided robust 3–8 μm gaps between planar substrates and had effective thermal conductivities less than those of aerogels. The spacers were fabricated on silicon molds and, after release, could be manually transferred onto any substrate. In large-scale compression testing, they sustained compressive stresses of 0.4–4 MPa without fracture. Experimentally, the thermal conductance was 10–30 mWcm−2K−1 and, surprisingly, independent of film thickness (100–800 nm) and spacer height. To explain this independence, we developed a model that includes the pressure-dependent conductance of locally distributed asperities and sparse contact points throughout the spacer structure, indicating that only 0.1–0.5% of the spacer-electrode interface was conducting heat. Our spacers show remarkable functionality over multiple length scales, providing insulating micrometer gaps over centimeter areas using nanoscale films. These innovations can be applied to other technologies requiring high thermal resistance in small spaces, such as thermophotovoltaic converters, insulation for spacecraft and cryogenic devices.

Progress Toward High Power Output in Thermionic Energy Converters

“…[59] With regard to new methods to overcome space charge effects, voltage-biased gates can be used to modify the intragap electric field [56] and microfabricated corrugated thin ceramic films can be used to maintain micronscale electrode gap distances. [190] Future publications should include descriptions of the efficiency metrics used for TEC evaluation; we recommend estimating both the core efficiency, = P Q in × 100%, which compares the output power density to the input heat flux (Equation ( 6)), and the system efficiency, s =P d E c × 100%, which compares the output power to the total energy consumed by the TEC and its supporting infrastructure (Equation (18)). Additional research efforts are needed to develop electrode materials that have low work functions and high Richardson constants, have low electronic resistance, and are tolerant of high temperatures.…”

“…The spacers were manufacturable using standard microfabrication processes and were shown to be mechanically robust, such that they could be produced individually and then subsequently compressed between electrodes in TECs to establish temperature differences of several hundred kelvins while only permitting a few watts of heat flow. [ 48,190 ] In one demonstration, a 2.3 µm tall alumina‐hafnia spacer was placed between two 1.27‐cm diameter molybdenum electrodes with an emitter temperature of K, producing a peak power density of Wcm −2 (including ideal resistive lead losses from m, see Equation () and Figure ). [ 48 ] The ability of dielectric spacer films to sustain high temperatures and support high temperature gradients offers the promise of mass‐producible, high‐efficiency, high power output TECs.…”

“…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 ]…”

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

“…[9][10][11] Among the different attempts reported in the literature, some aspects result relevant: 1) the spacers should be distributed over all the electrode area to prevent shorting and 2) the total area has to be optimized, because small-scale area single structures ((1 mm 2 ) could not sustain the significant unavoidable compressive force, but, conversely, centimeter-scale structures negatively affect the thermal and electrical insulation. Very recently, Nicaise et al [12] proposed a different solution, consisting in modular and efficient thermal insulating spacers, fabricated on silicon molds, that, after the release of the sacrificial silicon mechanical templates, could be manually transferred onto any flat substrate. However, the technological complexity of this solution and the relatively low versatility in its spatial arrangement on the desired surface, where in some zones the spacers must be densified and in other rarified depending on the heat fluxes, make this approach very complex to be practically applied.…”

Extending the Thermal Near Field Through Compensation in Hyperbolic Waveguides

“…The increased power density [2,3] and frequency tunable [4] nature of near-field transport may benefit technologies such as thermo-photovoltaic/photonic/radiative devices [5][6][7][8] and infrared sensing [9]. Proof-of-concept approaches that leverage near-field enhancements have been realized using precise experimental instrumentation [10][11][12][13], but scaling up these approaches remains challenging [14].…”