Vacuum tunnel diodes
on the temperature difference between the electrodes in the mode of electricity generation shows promise for their use in the electrical generators, working on a variety of heat sources in the least used temperature range of 300-600°C.
In 2000 the firm Cool Chips declared the beginning of development of VTDs as basic elements for solid-state coolers and electric power generators. But after 2003 no new investigation results were reported.
A typical VTD consists of two metal or semi-conductor electrodes separated with several nanometer vacuum gap. At heating one of electrode there appears a tunnel electron flow directed from hot electrode to cold electrode creating electromotive force that is proportional to the temperature difference between electrodes.
According to Cool Chips information the cooling VTDs have the following advantages:
high efficiency – about 55 % connected with reduction of back heat flow through a vacuum gap (this figure was calculated in theoretical department of Stanford university), for comparison, efficiency of cooling systems on the basis of compressors is about 45%, and on the basis of thermoelectric devices – 5–8%;
Up till now practical applications of VTDs are restrained by absence of VTD creation technology with electrodes of large area and vacuum gap width of 5–10 nanometers.
Several years ago the firm Cool Chips pls offered the engineering solution to manufacture large area VTD by sputtering metal film on semiconductor substrate with their subsequent separation so that the relief of a metal film completely repeated a relief of a substrate. The distance between electrodes was adjusted by piezoceramic ring elements. Some experimental structure of VTDs with 20–50 nm vacuum gap and 1–2 cm2 working area of electrodes was manufactured. However, till now there is no information of methods of gap width control in process of VTD operation, methods of compensation of vibrations and thermal deformations of electrodes. There is no information of VTD application for the cooling of real objects.
There is a lack of information now concerning VTD application for electric energy production. That is why the authors have made preliminary quantum-mechanical calculations of VTD characteristics to evaluate their prospects as electric current generators.
Dependences of electric current, electromotive force, electric power, heat flow and VTD efficiency on the vacuum gap width and temperature difference on electrodes have been calculated.
Fig.1–3 present calculated dependences of generated electric power Qe(d) at the matched load (electrical load resistance is equal to the internal VTD resistance) and the heat flow Qh(d) on the distance between electrodes d for different temperatures on the cold and hot electrode Tc and Th respectively.
Fig.4 shows the dependence of the VTD efficiency (COP(d) = Qe(d)/Qh(d)) on the distance between electrodes d.
Calculations have been made with the following assumptions:
the potential barrier has trapezoidal form;
the width of a barrier is equal to vacuum gap width (reduction of width and heights of a barrier due to the induced charge on surfaces of electrodes was not considered);
electrodes are made of the same metal;
the work function of electrons is equal to 1.0 eV, the temperature of a cold electrode is 50°C;
the temperature of a hot electrode is 350°C, 450°C and 550°C.
Results of calculations show that VTD electric power and efficiency in practically interesting range of vacuum gap width of 1–5 nm exceed similar characteristics of thermoelectric devices and that VTDs can be used in electric power generators in the temperature range of 300–600°C.
The induced charge on the electrode surface reduces the gap width and the height of potential barrier and increases the tunnel current – thus the main VTD characteristics are improved as well.
Preliminary calculations show that VTDs can be used as high efficiency coolers and power generators.
In order to create effective large area VTD for power generators it is necessary to carry out a number of theoretical and research works, including:
selection of materials for electrodes (metals or semiconductors) with optimum work functions;
optimization of surface shape and microrelief of electrodes;
development of technology of creation and maintenance of gap width in the range of 1–5 nanometers;
development of materials deposition technology on electrodes that reduces work function;
development of materials deposition technology on electrodes with discrete levels in band-gap or conduction band located a few kBTh above the Fermi level in hot electrode for resonant tunneling of electrons only in the interesting range of electron energy;
development of technology of electrodes manufacturing made of thermoelectric materials, combining the effect of electron tunneling and the Seebeck effect;
development of dynamic systems.
It is necessary to note that creation of large area VTDs can be realized only on the basis of new ideas and nanotechnology application.
At the first stage we are planning to conduct the following work: measurements of small area VTD basic characteristics with use of tunnel microscopes; development of technical requirements to materials of electrodes, their shape and quality of surface processing; conducting of calculations of various variants of VTDs and experimental test to realize original ideas of large area VTDs manufacturing.
At the second stage we are planning to develop processes of VTDs manufacture including development of the special process, control and measuring equipment.
At the third stage we are planning to develop cooling systems and power generators on the basis of large area VTDs. ■