Patent Application
20050016575
No Government License Rights.
This invention lies in the field of thermoelectric and field emission devices and is particularly concerned with the device design of field emission enhanced thermoelectric effect chips useful for cooling and power generation applications.
The thermoelectric (TE) phenomena involve the three effects known as the Seebeck effect, the Peltier effect and the Thomson effect. These effects explain the conversion of heat energy into electrical energy and vice versa. The TE phenomena has been known for a long time and has been described in detail in many books and review articles over the last 100 years.[1-3] In 1823, Thomas Seebeck discovered that a voltage drop appears across a metal with a temperature gradient. The thermocouples and thermoelectric power generators are based on this effect. In 1883, Heinrich Lenz showed the thermoelectric cooling effect by passing current through a junction made from wires of bismuth and antimony. Passing the current in one direction caused the junction to cool. On the other hand, the junction was heated when the current direction was reversed. This was a very important discovery, however, the TE effects remained a scientific curiosity until the 1950s when Abram Ioffe found that the doped semiconductors had much large cooling effect than other materials.[4]
Similar to a thermocouple, the TE power generator is based on the Seebeck effect. When a steady temperature gradient is maintained along a finite conductor, the free carriers at the hot end will have greater kinetic energy and will diffuse to the cold end. The accumulation of these charge carriers results in a back electromotive force which opposes a further flow of charge carriers. The Seebeck voltage is the open circuit voltage when no current flows. To form a thermocouple, the junctions of two dissimilar conductors (or semiconductors due to their larger Seebeck coefficients, >100 microvolts per degree K) are maintained at two different temperatures, and an open circuit potential difference is developed. This potential difference depends on the temperature difference between the two junctions and the difference between the absolute Seebeck coefficients of the two materials.
A thermoelectric power generator consists of many thermocouples. Since a thermocouple produces low voltage and high current, a TE power generator uses a large number of thermocouples that are connected electrically in series and thermally in parallel. The module is heated at one end and is maintained at a higher temperature than the other end such that a voltage appears between the terminals of the generator. Without going through the details of the theory, in the case of TE device consisting of two arms made from n- and p-type semiconductors, the power conversion efficiency is determined by a figure of merit Z, given as
where α is the Seebeck constant of the two materials, σ is the electrical conductivity, and λ is the thermal conductivity. Since, the open circuit voltage increases with temperature difference, the factor ZT must be maximized. However, since Z also changes with temperature, it has been found that the factor ZT is a more useful figure of merit that Z in actual practice.
After a long and intensive period of development of many technologies, two main cooler technologies have emerged, namely: mechanical coolers incorporating moving parts, and thermoelectric coolers based on Peltier cooling effect.
As mentioned earlier, the electronic coolers are generally associated with Peltier or thermoelectric coolers (TEC) commonly used for electronic chip cooling and even small portable commercial coolers. After the discovery of semiconductor thermoelectrics, almost every known semiconductor was investigated and it was found that bismuth telluride alloys (Bi2Te3/Sb2Te3) were the best at room temperature. However, even at their best, they produce only moderate amount of cooling and have very poor efficiency because they are dictated by the same equations as the thermoelectric generator. Again, intuitively, this is due to the fact that the hot and cold junctions are thermally connected with the p and n type semiconductors. Higher the thermal conductivity, lower is the TEC efficiency due to heat leakage from the hot junction to the cold junction. Additionally, the efficiency drops significantly as the temperature difference between the hot and cold junctions increases. This effect also limits the maximum temperature difference that can be maintained between the hot and cold junctions. Due to these limitations, TECs have only found limited use in niche power/thermal management applications such as IR detector cooling, diode laser cooling and spacecraft cooling/electric generation.
The materials used to make a TE device determine its efficiency, and usefulness of a material is described by its figure of merit ZT, a dimensionless constant. Most materials have a ZT values between 0.4 and 1.3.[5] As a frame of reference with traditional efficiency figures, a ZT value of 3 would make TEC based home refrigerators economically competitive with compressor based refrigerators. Theoretically, there is no upper limit to the ZT.[6] However, the maximum value for ZT has been stuck around 1 in spite of serious efforts since the early 1960s.
To understand the key reason behind this lack of improvement in the coefficient of performance, let us look at the equation 1. As per equation 1, Z depends on the Seebeck coefficient (α), the electrical conductivity (σ) and the thermal conductivity (λ). Thus, good TE materials should have the following properties:
However, the ideal TE material does not exist because, unfortunately, no one has found a good thermoelectric material that has good electrical conductivity but has poor thermal conductivity at the same time. Most metals have high electrical conductivity, but also have very high thermal conductivity resulting in very low Z. Semiconductors possess larger seebeck coefficients and poorer electrical conductivity resulting in λ/σ greater than metals. This is why most TE devices currently use semiconductor materials. However, poor electrical conductivity of semiconductors results in high ohmic losses affecting overall efficiency.
After decades of research in almost all possible materials, it has been determined that lowering the thermal conductivity of the high Seebeck coefficient is the best way to reduce their thermal conductivity without affecting other parameters. The reason for this is the fact that high thermal conductivity of the materials forms a direct thermal path between the hot and the cold sides resulting in serious wastage of energy.
The oldest field emission cooler concepts are based on the Nottingham effect that revolves around the fact that as electrons are emitted from any cathode, they leave with significant energy, thereby cooling the cathode. This is true for almost all cathodes. The Nottingham effect has been known for almost 60 years. It has also been demonstrated experimentally for cathodes at elevated temperature (around 1000 degree Celsius). However, this cooling is noticeable only at very high temperatures due to the fact that high temperatures are necessary to emit any electrons from a cathode. Furthermore, as the electrons are emitted from the surface, these electrons must be replaced from the external circuit. The difference in the energies of the emitted electrons and the replacement electrons determines the net cooling/heating per emitted electron. If the average energy of emitted electrons is less than that of replacement electrons, the net result is heating of the cathode. This is the case at T=0K (absolute zero). On the other hand, if the emitted electrons have higher energy (e.g. due to electron excitation at room temperature) than the replacement electrons, the cathode is cooled due to the emission process. However, for all practical field emission materials, Nottingham cooling effect does not start until elevated temperatures. It is due to these reasons that 60 years after Nottingham described this cooling effect, no practical room temperature cooler has been successfully fabricated.
Recently, Mahan et al. have theoretically proposed the use of thermionic emitters [7] for use in coolers and electric generators. Cooling is obtained by thermionic emission of electrons over periodic barriers in a multilayer geometry. However, this type of device is difficult to fabricate and its operation at room temperature is very limited.
To bring the working temperature of these electronic coolers, several researchers have proposed use of very low work function materials. Edelson (U.S. Pat. Nos. 5,675,972, 5,722,242 and 5,994,638) describes vacuum diode based devices including vacuum diode heat pumps and vacuum thermionic generators, where the electrodes are coated with a low work function material called an electride. The fabrication of such a low work function thermionic cathode is very difficult with current techniques as has been pointed out by Edelson (U.S. Pat. No. 6,103,298), Cox (U.S. Pat. No. 6,214,651 B1) and Cox et al. (U.S. Pat. No. 6,117,344). More recently, Tavikhelidze, Edelson et al. have described methods for making a diode device with very small gap (preferably 5 nm) in between the device electrodes (U.S. Pat. Nos. 6,417,060 B2, 6,720,704 B1).
Recently, Ghoshal (U.S. Pat. Nos. 6,608,250 B2 and 6,740,600 B2) has taught a thermoelectric device with improved efficiency where tips made from thermoelectric tips provide a low resistive connection while minimizing thermal conduction between the electrical conductor and the device. In this configuration, the thermoelectric tips are directly coupled such that electrical current may pass, however, the tips increase the thermal resistivity. This approach potentially improves the device efficiency, but the device is difficult and expensive to fabricate due to difficulty in making tips from thermoelectric materials.
It is, therefore, an object of the present invention is to provide improved thermoelectric devices for use in cooling and electricity generation applications.
It is another object of the present invention to provide devices that have high electrical conductivity and low thermal conductivity.
It is yet another object of the present invention to provide high efficiency thermoelectric devices that function with electrodes made from easily available high work function materials such as silicon.
It is yet another object of the present invention to provide high efficiency thermoelectric devices that operate at substantially large gap between the electrodes.
It is yet another object of the present invention to provide thermoelectric devices that can operate at cryogenic temperatures and maintain large temperature difference across their hot and cold sides.
The present invention teaches improvements to the present thermoelectric cooling and power generation technology resulting in increased efficiency. In a preferred embodiment, a thermoelectric cooler is constructed with a high thermal resistivity device inserted in each leg of the thermoelectric cooler. In a preferred embodiment, the high thermal resistivity device is made from field emission tips and has high electrical conductivity. In a preferred embodiment, the high thermal resistivity device is made from silicon tips arranged in a triode configuration.
A typical thermoelectric cooler (TEC) taught by prior art is depicted in
Δε=<εn>−<εp>, (2)
where <εn>and εp>are the average energies of the field emitted electrons from the n- and p-type semiconductors, respectively. The two thermal breaks in the path do not allow phonon conduction and there is no other thermal flow other than that associated with the electric or field emission current. Thus the net energy flow from the cold source to the hot source is just Δε. For the typical p-n junction, the energy levels of the conduction band of the n-type semiconductor are generally higher than that of the p-type semiconductor. This implies that Δε is positive. Thus, the mechanism for cooling is a field emission process. In this discussion, we can, as a first approximation, ignore traditional thermoelectric effects in the cooling process. The reason is that in good thermoelectric coolers, the cooling term, which is related to the entropy transport parameter, is on the order of about 50-60 meV per electron at room temperature. By contrast, the cooling device in accordance with the present invention has an energy transport (i.e., heat) per electron of 500-1,000 meV or so depending on concentration and field. For example, the energy carried by each electron going around the device is the difference of Fermi energies of the n-type and p-type semiconductors. In the case of silicon, this difference is almost 1,000 meV, almost equal to the bandgap of silicon. Thus, a cooling device in accordance with the present invention will carry 10-20 times more heat with the same amount of current flowing through the device.
The field emission based cooler will be electrically biased as shown in
This shows that the device efficiency is no longer dependent on the ZT factor, because the thermal conductivity of the TE materials is no longer part of the equation. This equation also shows that the efficiency can be improved by decreasing the applied voltage V between the anode and the cathode. In addition, the over all device performance can be further improved by using a wide bandgap semiconductor to increase the Δε. For example use of n-type and p-type diamonmd will give a Δε on the order of 5 eV. Since it is difficult to fabricate n-type and p-type doped semiconductors from one wide bandgap material, it is possible to even use dissimilar materials as long as their Fermi energies are vastly different.
Another embodiment of the present invention is shown in
A similar modification to the triode device shown schematically in
When silicon tips are used, it is possible to obtain large emitted electron current density from these tips at an electric field of 0.5 MV/m (megavolts per meter). Using a device gap of 100 nm and a modest emitted current density of 1 ampere per square cm, we obtain a cooling capacity of almost 1 watt per square cm. Since the applied voltage is only 0.05 volts, the efficiency is almost 95% of the Carnot efficiency. This is much higher than 5-10% for prior art thermoelectric coolers and 40-50% for the mechanical coolers.
This application claims the benefit under 35 U.S.C. 119(e) from U.S. Provisional Application 60/478,899, filed Jun. 13, 2003.
| Number | Date | Country | |
|---|---|---|---|
| 60478899 | Jun 2003 | US |
