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 ove...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 3–8μm 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 spacerelectrode 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.展开更多
基金The information,data,or work presented herein was funded in part by the Advanced Research Projects Agency-Energy(ARPA-E),U.S.Department of Energy,under Award No.DE-AR0000664This work was carried out in part at the Singh Center for Nanotechnology at the University of Pennsylvania,a member of the National Nanotechnology Coordinated Infrastructure(NNCI)network,which is supported by the National Science Foundation(Grant No.ECCS-1542153).
文摘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 3–8μm 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 spacerelectrode 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.
