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.
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机译：在热电子能量转换器中，如果通过在电极之间使用小于10 µm的间隙来消除空间电荷损失，则绝对效率可以提高至40％。在较大的器件面积上实现如此小的间隙的一种实用方法是在两个电极之间使用坚硬且隔热的垫片。我们报道了基于薄膜的氧化铝基垫片的设计，制造和表征，该垫片在平面基板之间提供了稳固的3-8μm间隙，并且具有比气凝胶更低的有效导热系数。垫片是在硅模具上制造的，释放后可以手动转移到任何基板上。在大规模压缩测试中，它们承受的压缩应力为0.4–4 MPa，没有破裂。在实验中，热导为10–30 mWcm −2 K −1 ，并且令人惊讶的是，它与膜厚度（100–800 nm）和垫片高度无关。为了解释这种独立性，我们开发了一个模型，该模型包括压力分布的电导率，局部分布的凹凸不平和整个隔离物结构的稀疏接触点，表明只有0.1-0.5％的隔离物-电极界面在传导热量。我们的垫片在多种长度范围内均显示出非凡的功能性，使用纳米级薄膜可在厘米级区域提供绝缘的微米间隙。这些创新技术可应用于在狭小空间中要求高热阻的其他技术，例如热光电转换器，航天器绝缘和低温设备。
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