Fig. 4.

Metamaterial-based approaches. a Design of a metamaterial-inspired cold electron emitter incorporating the dense array of emitting elements (e.g., carbon nanotubes or metal-tipped nanocones) grown directly on the electrode, and the gate electrode with µ-sized orifices. Due to strong electric field enhancement on the long nanosized structures, significant current densities could be obtained without external heating. b Design of the nanoscaled metamaterial capable of reversal heat transfer by electron emission. This structure may include a nanoporous membrane (a porous insert) capable of containing some amount of a highly emissive material usually working in a liquid state (e.g., cesium). The nanoporous membrane with the emissive material in pores is placed directly onto the solid cathode, and the spacer separates this membrane and the anode with the low-emissive material. The low-emissive material (e.g., tungsten) covers the anode surface. The cathode (the cold electrode in green) has a temperature lower than that of the anode (the hot electrode in red). Electron current emitted from the cathode flows through the inter-electrode gap to the anode. It is evident that such metamaterials could create strong heat barriers in the systems where heating over some limit (e.g. Curie temperature) should be avoided. Reproduced from ref. ¹³⁸. (Copyright 2016, Wiley)