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

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 −2 K −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. Thermionic converters: insulated micron-scale gaps An aluminum oxide spacer effectively insulates one electrode from another in a thermionic device. Energy conversion devices such as thermionic and thermophotovoltaic devices require thermal and electrical insulation across two electrodes separated by a small space. However, finding materials that can effectively occupy this gap and minimize parasitic heat flow is challenging. Now, a team led by Igor Bargatin from University of Pennsylvania demonstrate that deposited aluminum oxide spacers can be integrated between electrodes in a thermionic device, achieving a thermal conductivity in a vacuum of just 5 mW m –1 K –1 —below that of aerogel—and can sustain compressive stresses of 0.4–4 MPa while avoiding electrical or thermal shorts. It is found that only 0.2–0.5% of the surface area of the spacer is conducting heat, and thermal conductivity is nearly independent of both spacer thickness and gap distance.