LONDON Ideas of how to exploit miniaturized vacuum tubes have emerged regularly and without much success, since the glass-envelope valve was replaced for most applications by the transistor. However, researchers at the University of Illinois at Urbana-Champaign, reckon the vacuum tube could be basis of a high energy density battery, or a non-volatile memory. In theory arrays of nanometer-scale vacuum tubes should be able to achieve three times the energy density of lithium-ion batteries.
The researchers Alfred Hubler and Onyeama Osuagwu have prepared a paper entitled Digital quantum batteries: energy and information storage in nanovacuum tube arrays . However, the work is theoretical and it remains to be seen whether the technology can live up to its theoretical potential.
The paper builds on work on field-emission devices which contain pointed structures expressly to allow current flow. However, the authors claim that with appropriate design at the nanometer-scale, electric breakdown can be suppressed with quantization phenomena, while the capacitance is very large due to the small gap sizes, of the order of 10-nm. The tubes, structure in a cathode, anode, cathode arrangement are shown as being fabricated vertically with a pitch slightly larger than the critical gap of about 10-nm, and therefore of the same order as today's leading-edge lithography.
The energy density of the tubes is limited by vacuum breakdown. At gas pressures of less than 10?6 torr the breakdown field does not depend on the residual gas, but on the properties of the electrode surfaces, the authors state.
They go on to report that the energy density and power densities in nanovacuum tubes are large compared to lithium batteries and electrochemical capacitors. They give a gravimetric energy density of about 1-Mjoule per kilogram and volumetric energy density of about 3-Gjoules per cubic meter.
Such digital quantum batteries, even when requiring arrays of billions of tubes, have a number of practical advantages. They could be built using conventional lithographic techniques using wafer fab friendly metal materials within a conventional silicon substrate and using silicon dioxide for the insulating side walls of the tubes. Carbon nanotubes are proposed for the anode material and tungsten for the cathodes. In addition such batteries could easily be included within integrated circuit substrates as an on-chip rechargeable battery.
The arrangement has other advantages. The charge, discharge rates of such nanotubes should exceed all other devices, the authors calculate, but at the same time vacuum nanotubes should be able to retain electrical energy without losses for many years, giving rise to the possibility of configuring the nanotube as a nonvolatile memory device.
The electric field in a nanovacuum tube can be sensed with a MOSFETs built in to the silicon dioxide insulating walls. The arrangement would not be dissimilar to a capacitor-based dynamic RAM but would be able to dispense with refresh cycles. Thus random access arrays of nanovacuum 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, the authors assert.
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The complete paper could be found here when this story was first posted.
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