The energy density and the power density of nano vacuum tubes in comparison to other energy storage devices (a). The volumetric energy density and the gravimetric energy density of nano vacuum tubes in comparison with combustion engines and several types rechargeable batteries, including those based on lead, nickel-cadmium, nickel-metal hydride, and lithium (b). Nano vacuum tube arrays are lighter and smaller energy storage devices than other batteries.
Dielectric material between capacitor electrodes increases the capacitance. However, when the electric field exceeds a threshold, electric breakdown in the dielectric discharges the capacitor suddenly and the stored energy is lost. We show that nano vacuum tubes do not have this problem because (i) electric breakdown can be suppressed with quantization phenomena, and (ii) the capacitance is large at small gap sizes. We find that the energy density and power density in nano vacuum tubes are large compared to lithium batteries and electrochemical capacitors. The electric field in a nano vacuum tube can be sensed with MOSFETs in the insulating walls. Random access arrays of nano vacuum tubes with an energy gate, to charge the tube, and an information gate attached to the MOSFET, to sense the electric field in the tube, can be used to store both energy and information.
Capacitors built as nanoscale arraysâwith electrodes spaced at about 10 nanometers (or 100 atoms) apartâquantum effects ought to suppress arcing between electrodes. Hubler claims the resulting power density (the speed at which energy can be stored or released) could be orders of magnitude greater, and the energy density (the amount of energy that can be stored) two to 10 times greater than possible with todayâs best lithium-ion and other battery technologies.
Digital quantum batteries could be fabricated using existing lithographic chip-manufacturing technologies using cheap, nontoxic materials, such as iron and tungsten, atop a silicon substrate, he says. The resulting devices would, in principal, waste little or no energy as they absorbed and released electrons. Hubler says it may be possible to build a benchtop prototype in one year.
The concept represents a variation on existing micro- and nanoelectronic devices. âIf you look at it from a digital electronics perspectiveâitâs just a flash drive,â says Hubler. âIf you look at it from an electrical engineering perspective, you would say these are miniaturized vacuum tubes like in plasma TVs. If you talk to a physicist, this is a network of capacitors.â
The maximum energy density at the anode versus the apex radius r for three gap sizes d. Tips with a small apex radius have a larger maximum energy density at the anode. The dashed lines indicate the tensile strength of typical anode materials. The tensile strength of the anode material limits the energy density (a). The shaded region indicates where a nano vacuum tube with a carbon nanotube anode is mechanically and electrically stable (b).