Patent Application
20110062420
The present invention relates to thermoelectric modules and in particular to low-cost, high-temperature high-efficiency quantum well modules.
The Seebeck coefficient of a bulk thermoelectric material is defined as the open circuit voltage produced between two points on a conductor, where a uniform temperature difference of 1 K exists between those points. The figure-of-merit Z of a thermoelectric material is defined as:
Z=α
2/ρκ
where α is the Seebeck coefficient of the material, ρ is the electrical resistivity of the material and κ is the thermal conductivity of the material. A dimensionless figure of merit is found by multiplying Z by an average temperature. Greater values of ZT indicate greater efficiency of the thermoelectric material.
A large number of semiconductor materials were being investigated by the late 1950's and early 1960's, several of which emerged with Z values significantly higher than similar values for metals or metal alloys. No single compound semiconductor evolved that exhibited a uniform high figure-of-merit over a wide temperature range, so research focused on developing materials with high figure-of-merit values over relatively narrow temperature ranges. Of the great number of materials investigated, those based on bismuth telluride, lead telluride and silicon-germanium alloys emerged as the best for operating in various temperature ranges. Much research has been done to improve the thermoelectric properties of the above three thermoelectric materials. For example n-type bismuth telluride, Bi2Te3 typically contains 5 to 15 mol percent Bi2Se3 and p-type Bi2Te3 typically contains 70-90 mol percent Sb2Te3. Lead telluride is typically doped with sodium for P type and iodine (PbI2) for N type.
Electric power generating thermoelectric modules are well known. These modules typically are comprised of a number of thermoelectric elements called n-legs and p-legs connected electrically in series. The effect is that a voltage differential of a few millivolts is created in the presence of a temperature difference at the two junctions of p-type thermoelectric semiconductor elements and n-type thermoelectric semiconductor elements. Since the voltage differential is normally small, many of these elements (such as about 100 elements) are typically positioned in parallel between a hot surface and a cold surface and are connected electrically in series to produce potentials of a few volts. Electrons flow from the hot side to the cold side through the n-legs and from the cold side to the hot side through the p-legs. Many references refer to the current in the p-legs as holes flowing from the hot side to the cold side. These references also refer to current flowing from the cold side to the hot side through the n-legs and from the hot side to the cold side through the p-legs.
For example Hi-Z Technology, Inc. offers a Model HZ-14 bismuth telluride thermoelectric module designed to produce about 14 watts at a load potential of 1.66 volts with a 200° C. temperature differential. Its open circuit potential is 3.5 volts. The module contains 49 n-legs and 49 p-legs connected electrically in series. It is a 0.5 cm thick square module with 6.27 cm sides. The legs are p-type and n-type bismuth telluride semiconductor legs and are positioned in an egg-crate type structure that insulates the legs from each other except where they are intentionally connected in series at the top and bottom surfaces of the module. That egg-crate structure which has spaces for 100 legs is described in U.S. Pat. No. 5,875,098 which is hereby incorporated herein by reference. The egg-crate is injection molded in a process described in detail in the patent. This egg-crate has greatly reduced the fabrication cost of these modules and improved performance for reasons explained in the patent. Insulating walls keep the electrons flowing in the desired series circuit. Other Bi2Te3 thermoelectric modules that are available at Hi-Z are designed to produce 2.5 watts, 9 watts, 14 watts and 20 watts at the 200° C. temperature differential.
The egg-crates for the above described Bi2Te3 modules are injection molded using a thermoplastic supplied by Dupont under the trade name “Zenite”. Zenite melts at a temperature of about 350° C. The ZT thermoelectric properties of Bi2Te3 peak at about 100° C. and are greatly reduced at about 250° C. For both of these reasons, use of these modules are limited to applications where the hot side temperatures are lower than about 250° C. to 300° C.
Despite the fact that there exists a great need for non-polluting electric power and the facts that there exists a very wide variety of un-tapped heat sources, thermoelectric electric power generation in the United States and other countries is minimal as compared to other sources of electric power. The reason primarily is that thermoelectric efficiencies are typically low compared to other technologies for electric power generation and the cost of thermoelectric systems per watt generated is high relative to other power generating sources. Generally the efficiencies of bismuth telluride thermoelectric power generating systems are in the range of about 5 percent. Proposals to increase these efficiencies by segmenting different types of materials have been made but these stacked designs become complicated and expensive to produce and the resulting efficiencies are not much better than about 10 percent.
Workers in the thermoelectric industry have been attempting to improve performance of thermoelectric devices for the past 20-30 years with some success, but much more is needed. Most of the effort has been directed to reducing the thermal conductivity (κ) without adversely affecting the electrical conductivity. Experiments with super-lattice quantum well materials have been underway for several years.
A super-lattice is defined as a periodic structure of layers of two (or more) materials, typically with thickness per layer of several nanometers. The layers may be completely amorphous or partially amorphous and partially crystalline or completely crystalline. A quantum well is a potential well that confines particles, (which were originally free to move in three dimensions) to two dimensions, forcing them to occupy a planar region. The effects of quantum confinement take place when the quantum well thickness becomes comparable at the de Broglie wavelength of the carriers (generally electrons and holes). The de Broglie wavelength, λ, is:
λ=h/p
where h is Planks constant and p is the momentum of the particle. For an electron with a velocity of 2.18×106 m/s, the de Broglie wavelength is about 3 Angstroms (0.3 nanometers). For an electron with a velocity of 1×105 m/s, the de Broglie wavelength is about 60 Angstroms (6 nanometers).
These super-lattice quantum well materials were discussed in a paper by Gottfried H. Dohler which was published in the November 1983 issue of Scientific American. This article presents an excellent discussion of the theory of enhanced electric conduction in super-lattices. These super-lattices contain alternating conducting and barrier layers and create quantum wells that improve electrical conductivity. These super-lattice quantum well materials are crystals grown by depositing semiconductors in layers with thicknesses generally less than about 10 nm (100 Angstroms). Thus, each layer is generally less than 100 atoms thick. (These quantum well materials are also discussed in articles by Hicks, et al and Harman published in Proceedings of 1992 1st National Thermoelectric Cooler Conference Center for Night Vision & Electro Optics, U.S. Army, Fort Belvoir, Va. The articles project theoretically very high ZT values as the layers are made progressively thinner.) The idea being that these materials laid down as super-lattices might provide very great increases in electric conductivity without adversely affecting Seebeck coefficient or the thermal conductivity.
The present inventors have actually demonstrated that high ZT values can definitely be achieved with Si/Si0.8Ge0.2 super-lattice quantum well n-legs and p-legs (see U.S. Pat. Nos. 5,436,467, 5,550,387, 6,096,964, 6,096,965, 6,914,343, 7,038,234, 7,342,170 and 7,400,050). They have also demonstrated that these very high ZT values can be achieved with super-lattice quantum well modules having Si and SiC n-legs and B4C and B9C p-legs (see, for example U.S. Pat. Nos. 6,828,579, 6,914,343 and 7,342,170). Most of the efforts to date with super-lattice quantum well material have involved alloys that are known to be good thermoelectric materials, many of which are difficult to manufacture as extremely thin layers. The present inventors have had issued to them United States patents which disclose such materials and explain how to make them. These patents (which are hereby incorporated by reference herein) include U.S. Pat. Nos. 5,436,467; 5,550,387; 6,096,964; 6,096,965; 7,038,234 and 7,342,170. The '234 patent describes n-legs utilizing Si and SiGe layers and p-legs utilizing B4C and B9C layers. The '170 patent discloses similar legs in which the n-legs utilize Si and SiC layers with the p-legs also utilizing B4C and B9C layers. A large number of very thin layers (in the '234 patent, more than 3 million layers per leg) together produce a thermoelectric leg about 0.4 cm thick. In the embodiment shown in the figures all the legs are connected electrically in series and otherwise are insulated from each other in an egg-crate type thermoelectric element as indicated in
For thermoelectric modules of the type described above in order to be generally competitive with other power generating methods must be made at costs in the range of about $1.00 per watt with efficiencies in the range of 20 to 30 percent or greater. Prior art thermoelectric devices with legs made of bulk materials cost at least several dollars per watt. In addition, the efficiencies of the thermoelectric devices with bulk material legs are in the range of about 5 percent. In general the costs of products that are mass produced tend to gravitate toward the cost of the materials from which they are made. Germanium differs from silicon in that the supply of silicon is limited only by production facilities whereas germanium availability is limited by the resource. As a result silicon could be purchased in 1998 for less than $10/kg, but the price of germanium was then $800/kg. In 2008 the price of germanium had increased to $1,400/kg. Recently the prices of both silicon and germanium have fallen due to the current economic recession.
What is needed is a method of making quantum well thermoelectric modules at costs of less than about $1.00 per watt. With costs in this range and efficiencies in the range of 20 to 30 percent thermoelectric modules could become extremely popular even for large scale electric power production.
The present invention provides quantum well thermoelectric modules with quantum well n-legs and p-legs and a low cost method of mass producing the modules. The n-legs and p-legs are each comprised of layers of quantum well material in the form of very thin alternating layers of high-temperature thermoelectric semiconductor materials having different band-gaps. Applicants have recently discovered after years of work on super-lattice materials, that the super-lattices combined with a very large number of potential dopants, provide Applicants the opportunity to create a huge variety of new thermoelectric materials which can be (in effect) made-to-order. In preferred embodiments the super-lattice materials are silicon and silicon carbide and in other preferred embodiments the materials are poly-types of boron carbide. Applicants believe also that silicon/silicon nitride n-legs and p-legs can be fabricated as very high-temperature thermoelectric materials. In preferred embodiments to improve the efficiency and lower the module cost the legs also include layers of low cost electrical and thermal insulating material. In some of these preferred embodiments these insulating layers may be substrates on which the quantum well super-lattice layers are deposited.
Applicants have built, tested and patented quantum well super-lattice prototype samples with silicon carbide and silicon n-legs. (See U.S. Pat. No. 6,828,579, Thermoelectric Devices with Si/SiC Superlattice N-Legs.) In the parent application (Ser. No. 12/460,424) Applicants disclose techniques for making low cost modules with silicon and silicon carbide n-legs. The module specifically described in that patent utilized boron carbide for the p-legs. These p-legs utilized alternating layers of B4C and B9C. The prior art to the best of Applicants' knowledge does not disclose any techniques for making silicon and silicon carbide super-lattice p-legs. However Applicants have now discovered techniques for making thermoelectric quantum well modules with both n-legs and p-legs made using silicon and silicon carbide super-lattice layers. Applicants have determined that having both n-legs and p-legs made from the same materials is advantageous in that many problems associated with differential thermal expansion and contraction are avoided.
In a special preferred embodiment utilizing super-lattice n-legs, the super-lattice layers of the p-legs are comprised of alternating layers of n-doped silicon carbide and p-doped silicon. In one of these embodiments, actually fabricated and tested by Applicants, the p-legs of the silicon carbide layers were produced with sputtering targets of silicon carbide lightly doped n-type with nitrogen at about 1016 to 1018 atoms per cc and the silicon layers were produced with silicon more heavily doped with boron at about 1021 atoms per cc. The n-type SiC layers were 10 nm thick and the p-type Si layers were 4 nm thick. Seebeck coefficient for the resulting super-lattice material was measured at about 985 microvolts per degree centigrade. This compares to 180 microvolts per degree centigrade for prior art p-doped bulk bismuth telluride thermoelectric material. Electrical resistivity for the new super-lattice material was also a substantial improvement over the prior art material; i.e. 0.31 milliohm-centimeter for the p-type super-lattice material as compared to 0.8 milliohm-centimeter for the bismuth telluride. Other techniques for producing p-type silicon-silicon carbide quantum well thermoelectric legs are proposed. In an alternate preferred embodiment, silicon carbide powders are mixed with about one percent aluminum and are hot pressed then heated to 2300° C. to evenly distribute and activate the aluminum dopant to form p-type silicon carbide sputtering targets which are then used with silicon sputtering targets to produce the super-lattice quantum well layers. Boron can be substituted for the aluminum.
Another proposed technique utilizes reactive sputtering. In this case the sputtering target is silicon. To produce the silicon carbide layers the gas mixture is argon and methane (CH4). To produce the silicon layers, the methane flow is stopped so the gas flow is only argon. The sputtering chamber is maintained at a pressure of about 20 mm of mercury. RF power is controlled within 100 to 200 watts. The composition of the films can be controlled by varying the relative pressures of the argon and the methane. The n and p features of the nano-structured Si/SiC materials can be controlled by proper doping of the Si targets
Silicon carbide films can also be produced by co-sputtering the carbon and the silicon from separate targets at the same time. The n and p dopants are preferably added to the silicon targets. Silicon layers can be applied using the same target used for the co-sputtering of the silicon carbide or if a different dopant concentration is required a separate silicon target must be used. Co-sputtering equipment is available from 4 Wave, Inc.
There exists a wide variety of semiconductor materials that are potentially available for fabrication of quantum well thermoelectric modules. However many of these materials are rare and as a result become expensive when demand is high. Applicants preferred quantum well material for their preferred quantum modules is a one-to-one ratio of silicon and carbon, two of the most abundant materials on earth. With the utilization of these materials costs, over a period of years, should continue to go down as demand goes up.
In U.S. Pat. No. 6,828,579 referred to above the p-legs were comprised of alternating layers of B4C and B9C and the n-legs were comprised of silicon carbide and silicon. As explained above Applicants have determined that there are important advantages in having both types of legs comprised of the same basic materials. Therefore, Applicants have disclosed in this specification techniques for making modules with polytypes of boron carbide in both the n-legs and the p-legs.
In preferred embodiments the quantum well material is produced with a sputter process in a web coater on an insulating substrate to produce quantum well film which is stacked with insulating spacers to produce a quantum well stack which is then sliced and diced to produce the quantum well legs. In preferred embodiments layers are about 4 nm to 10 nm thick. In order to reduce costs of the modules, insulating material may be added to the legs. In many applications the insulating material will also improve the efficiencies of the modules. In preferred embodiments, the volume of insulating material in each leg is at least 12 times the volume of quantum well super-lattice layers. In other preferred embodiments the ratio is about 50 which results in a module cost of about $0.85 per watt.
Quantum well material can be deposited with sputtering machines at a rate of about 10 nanometers per minute. A typical thermoelectric module designed in accordance with the present invention may contain only about 0.14 cm3 of the quantum well material. Using a prior art sputtering machines quantum well material could be produced at the rate of about 0.25 cm3 per day. Applicants have performed demonstration runs on a two-target web coating sputtering machine showing that with this prior art machine 1.4 cm3 of quantum well material could be produced per day, enough material per day for about 10 modules of a preferred module design. Applicants have developed a preliminary design of a multiple target web coating machine to produce about 29 cm3 of super-lattice film per day, enough material to produce per day more than 200 modules of the preferred design. Details of a prior art web coating sputtering machine is described in detail in the parent patent application identified above along with designs for modifications that would greatly increase production. These details have been incorporated by reference.
In a first preferred embodiment, 400 quantum well silicon and silicon carbide super-lattice layers are grown on a 200 micron thick substrate in a web coating sputtering machine. The preferred substrate is Kapton coated with a 100 nm buffer layer of silicon. On the silicon buffer layer are deposited alternating 10 nm layers of silicon carbide and silicon. The thickness of the quantum well material on the 200 micron thick substrate is about 8 microns, so the quantum well film and substrate is about 208 microns. The 8 micron film on the 200 micron substrate and a 200 micron spacer are assembled in sets with thicknesses of about 408 microns and the sets are stacked 12 high (12×408 microns) to produce sheets of thermoelectric material about 0.49 cm thick. The stack of quantum well film and spacers is cut into legs with dimensions of about 0.3 cm×0.5 cm×0.49 cm. The legs are treated at both hot and cold ends with an ion implantation procedure and sputter coated at both hot and cold ends with molybdenum and silver to improve electrical connections between the legs. The legs are then assembled into a thermoelectric egg-crate similar to prior art thermoelectric egg-crates as shown in
The ratio of insulating material to quantum well material in the legs is about 50. The estimated maximum efficiency of the module is about 21.4 percent. This preferred embodiment is a thermoelectric 10×10 egg crate type module about 5.55 cm×5.55 cm×0.7 cm. The module has 98 active thermoelectric legs, with each leg having more than 4,800 super-lattice quantum well layers. Applicants expect to be able to produce more than two hundred of these modules per day per web coating machine. Applicants expect to manufacture the modules for about $40 per module at a cost per watt of about $0.85/watt or less.
Applicants' experiments and research have shown that there is a large number of potential semiconductor super-lattice combinations in addition to the ones referred to above that could result in excellent quantum well thermoelectric properties with proper doping. In this specification Applicants have included lists of potential candidate combinations of these semiconductors.
Other preferred substrate and spacers include Kapton, glass, silicon-coated glass and porous silicon. Substrates that can be dissolved (i.e. NaCl), evaporated or etched away (metals) can also be used.
As explained in the Background section, Applicants have disclosed techniques for placing the thin alternating layers on film substrates. In these patents the alternating layers specifically described include layers comprised of silicon and silicon-germanium to produce both n-legs and p-legs. The Si layers were referred to as insulating or barrier layers and the SiGe layers are appropriately doped to produce n legs and p legs and are referred to as conducting layers. Also as described in the Background section Applicants have disclosed how to fabricate silicon and silicon carbide n-legs with p-legs made from different semiconductor materials. Applicants have recently discovered how to fabricate p-legs using heavily p-doped silicon and low cost lightly n-type silicon carbide with excellent results and have proposed other combinations of silicon and silicon carbide which could prove equally effective.
As explained in the Dohler article, in these very thin layers, electrons made available for electrical conduction in the n-doped conduction layer can migrate to the boundary layer to make conduction possible there. Applicants are not certain as to the reason for the excellent electrical conducting properties of these quantum well materials. However their research confirms that these super lattice quantum well layers do produce excellent thermoelectric results including low electrical resistance and increased Seebeck coefficients. These extremely thin films can be varying degrees of amorphous to crystalline and still produce quantum well effects.
In 2002 Applicants produced a small test quantum well thermoelectric couple with 11 microns of Si/SiGe thermoelectric layers on a 5-micron silicon film that has operated at 14 percent conversion efficiency. This efficiency was calculated by dividing the power out of the couple by the power in to an electric heater with no correction for extraneous heat losses. The accuracy of the experimental set-up used was validated by measurement of the 5 percent efficiency of a couple fabricated of bulk Bi2Te3 alloys.
Measurements at University of California at San Diego on behalf of Applicants indicate that the thermal conductivity of the Si—SiGe multi-layer films are significantly reduced in comparison with the bulk value. The use of the UCSD low value for the in-plane thermal conductivity leads to a factor of three enhancement in the performance (i.e., figure of merit) of the material.
Applicants have now actually demonstrated that with an existing large web coater type sputtering machine similar to the one shown in
The equipment used was a web coater sputtering machine of the type described in U.S. Pat. No. 4,204,942.
As illustrated in
Apparatus 10 further includes a refrigeration unit 32 which is communicated to the chamber 12 via a conduit 34 to provide the support cylinder 20 and deposition stations 22-26 with a coolant. For clarity, conduits communicating the coolant to the individual deposition stations are not fully illustrated in the Figures.
A vacuum pump 38 is communicated to chamber 12 via exhaust conduit 40. With chamber door 11 securely attached to chamber 12 so that it hermetically seals the chamber, the pump 38 can partially evacuate the chamber to pressures of 1 millitorr. Sensor 42 is provided to monitor the chamber pressure.
A gas supply unit 44 communicates an admixture of reactive gas to stations 22 and 26 to provide a reactive sputtering process. Similarly, gas supply unit 44 supplies an inert or nonreactive gas to deposition station 24 for generating the gas-discharge plasma that will provide the sputtering environment for that station. In addition, each deposition station 22, 24, 26 is provided with a separate source of electrical power to control the sputtering taking place at each station. Accordingly, there is provided power supplies 48 to supply the corresponding deposition stations with the appropriate high voltage required for the sputtering process. Again for clarity, the conduits and electrical lines which communicate the gases and electrical power to each deposition station are not fully illustrated in the Figures.
Referring now to
Inlet and outlet coolant lines 54 and 56, respectively, carry a coolant to and from the support cylinder. The lines 54, 56 pass through a rotating coaxial seal 58 of known construction to communicate water (cooled to about 22.degree. C.) to and from the interstitial area between jackets 50 and 52 of support member 20, thereby cooling the support cylinder.
In an actual demonstration run, high volume quantum well type multi-layer films were fabricated on a large area web coater sputtering machine. Alternating layers of Si0.8Ge0.2 and Si were grown on Kapton® and Mylar® substrates from two targets in a magnetron sputtering system with the web coater. (Applicants are confident that the success they had with the silicon and silicon germanium will apply equally well to silicon and silicon carbide layers as described below.) The plasma, web speed, pulse power frequency, pulse duration and power were set to yield a deposition rate of 10 nm/min and 50 alternate Si0.8Ge0.2 layers of 10 nm and individual alternating layers of Si each 10 nm to give a total thickness of 1 micron. Prior to deposition, Kapton® and Mylar® substrates were cleaned and a 50 nm thick Si buffer layer was applied to the Kapton® and Mylar® substrates by magnetron sputtering in a web coater. A thin 300 μm thick Si substrate also was used to demonstrate use of a crystalline substrate for web coating. The pulse power supply operated at a frequency of 15 kHz and a pulse width of 2.2 pee for both the Si0.8Ge0.2 and Si source targets. The Si0.8Ge0.2 source targets power was 3,000 Watts at a belt speed of 3.6 ft/min and the Si source target's power was 3,000 Watts at a belt speed of 2.5 ft/min.
The actual deposition configuration is illustrated schematically in
In preferred embodiments the deposition target source 24 is pure Si and deposition target source 26 is Si80Ge20 doped to ˜1019 phosphorous carriers per cc for n-type film. For P type film, boron was used as the dopant also at ˜1019 atoms per cc. Antimony could also be used as the N type dopant. The sputtering should be operated using an argon pressure between 0.001 and 0.1 torr. During deposition of films, the substrate should be about five centimeters from the sputtering targets. Preferred processes utilize two 5 kW pulse power magnetrons, one having a source target of Si0.8Ge0.2 that is 5 cm×100 cm with a 0.375 cm thickness, and the other having a source target of Si with the same sizes. Substrate 30 on supply roll 14 could be about 1 meter wide×300 meters long. Many other supply roll substrate materials are possible.
Substrates and films can be heated and cooled prior to deposition, during deposition and subsequent to deposition as a means to control structure of individual layers of crystalline films.
As explained above the prior art web coater can produce enough quantum well material per day for about 10 modules of a preferred design. Applicants have developed preliminary designs for high volume sputter machines for greatly increasing production rates of the quantum well film. An important limiting factor in quantum well film production is that the quality of the film decreases substantially if the deposition rate exceeds about 10 nm per minute. The area of the substrate covered by deposition is approximately equal to the effective area of the targets. Therefore, to increase the production rate the target area should be increased. This can be accomplished by increasing the number of targets or increasing the size of the targets. The targets are just like the ones described above for the two-target machine, each one meter long and 5 cm wide other specifications are also the same or similar. With this machine the production rate of quantum well film could be increased by a factor of eight from the two-target machine described above from 1.44 cm3 per day to about 11.5 cm3 per day (enough for 48 modules per machine per day). In another design the substrate film (having a length approximately equal to the circumference of the drum is mounted on a large 4-meter diameter cylindrical drum and targets are spaced entirely around the drum. In the drawing shown, 24 targets are provided which, assuming the same size targets as in the two-target embodiment described above, would provide for a factor or 12 improvement in the film production rate resulting in a daily production rate of about 72 modules per day per sputter machine. Applicants have also proposed a larger machine with forty 5 cm×100 cm targets to provide a factor of 20 improvement in the production rate which would result in a production per machine of 28.8 cm3 per day. This is enough quantum well material for 120 modules of the type 3 preferred design. If we assume a cost of about $2,000 per 24-hour operation of the machine, the estimated cost of the quantum well material would be roughly $70/cm3.
A preferred thermoelectric module is an egg-crate type module approximately 5.55 cm×5.55 cm square and 0.7 cm thick. The module consists of a 10×10 matrix of thermoelectric elements with each element being an approximate cube about 5 mm×4.9 mm square and about 3 mm thick. Forty nine of the thermoelectric elements are P type conductors and forty nine are N type conductors and they are connected in such a way that they are electrically in series but thermally in parallel. Two of the corners are used to fasten power leads and so do not contain thermoelectric elements. The electrical connectors are formed by thermally spraying molybdenum and aluminum (or copper) metal as described in Applicants' employer's prior art U.S. Pat. No. 5,875,098 (see FIGS. 19A and 19B in that patent and the related text). The fabrication of the egg-crate itself is also described in the U.S. Pat. No. 5,875,098.
Each thermoelectric element consists of twelve layers of quantum well films with a spacer layer of Kapton film in between each quantum well film stack. The Kapton film serves two purposes; it bonds the layers together and also acts as a thermal insulator to reduce the heat flux. Reducing the heat flux permits the use of fewer elements that are shorter. This means that less quantum well material is required resulting in a significantly lower cost with only a small sacrifice in efficiency due to some bypass heat loss through the Kapton.
The quantum well film for this module and the process for making it is described above. A preferred quantum well film is comprised of a Kapton substrate that is 200 μm thick and 400 alternating layers of silicon and silicon carbide deposited on one surface of the Kapton substrate. The thickness of each layer of silicon is 10 nm and each layer of SiC is also 10 nm. The total thickness of the 800 layers is about 8,000 nm or 8 microns so the quantum well film including its substrate is about 208 microns thick. A step-by-step procedure for making the egg-crate module is described in the section below.
When thermoelectric modules are fabricated in high volumes, following the twelve steps previous described, the fabrication costs of these modules should be about $30 per module. When operated at a hot side temperature of 350° C. and a cold side temperature of 50° C., this module will produce more than 46.8 watts of electrical power for a cost per watt of less than $0.65. The efficiency of the module is expected to be about 21.4 percent.
N-type SiC is readily available for industrial integrated circuit applications and was utilized in the fabrication of the modules described in the Applicants' prior art patent covering the modules with n-type Si/SiC legs. For those prior art demonstrations the n-type SiC and silicon are alternately bombarded with argon ions to alternately deposit n-type SiC and silicon. However, p-type SiC material for sputtering targets are not to the best of Applicants' knowledge in early 2010 sold commercially. For those prior art demonstrations the p-legs were prepared using B4C and B9C for the super-lattice legs. Applicants could have used p-type SiGe for the p-legs.
Since high purity p-type SiC targets have not been commercially available, Applicants decided to try to use n-type SiC and p-doped silicon to produce p-type SiC quantum well legs. To have a chance at success, Applicants used heavily p-doped (to 1021 atoms /cc) silicon targets. The net effect is that the large amount of p-type dopant (boron in this case) in the silicon layer resulted in a p-type silicon and silicon carbide quantum well leg with excellent thermoelectric properties. Sputtering rates of 4 minutes, 27 seconds for each 10 nanometer layer were used for n-type SiC and 1 minute for each 10 nanometer for p-type silicon. Thicknesses of 4 nm for the silicon and 10 nm for the SiC were deposited. The deposition temperature was about 625° C. A total of 50 layers of Si and 50 layers of SiC are typically deposited on a silicon single crystal substrate.
The thermoelectric qualities (Seebeck coefficient and electrical resistivity) of the silicon carbide—p-doped silicon samples were measured and the results of the measurements are compared to the Seebeck coefficient and electrical resistivity of commercially available bulk p-type bismuth telluride material which is currently considered the best commercially available p-type thermoelectric material. The Seebeck coefficient for the new p-type silicon carbide super-lattice layer was 985 microvolts per ° C. as compared to 180 microvolts per ° C. for the bismuth telluride sample. This Seebeck coefficient of 985 microvolts per ° C. is about the same as the Seebeck coefficients that Applicants have measured for p-type silicon germanium quantum well super-lattice samples which are in the range of about 1,000 microvolts per ° C.
As indicated above n-type silicon carbide sputter targets are available from many sources. However, p-type sputter targets are not generally available. Applicants propose to fabricate their own p-type SiC sputter targets. Silicon carbide powders are mixed with about one percent aluminum and are hot pressed at temperatures of about 1500 to 1700° C. then oven heated to 2300° C. to evenly distribute and activate the aluminum to form p-type silicon carbide sputtering targets which are then used with silicon sputtering targets doped to about 1016 to produce the super-lattice quantum well layers. Boron can be substituted for the aluminum.
Another proposed technique utilizes radio frequency reactive sputtering. In this case the sputtering target is silicon. To produce the silicon carbide layers a gas mixture of argon and methane (CH4) is used to produce the SiC layers. The composition of the films can be controlled by varying the relative pressures of the argon and the methane. To produce the silicon layers, the methane flow is stopped so the gas flow is only argon. The sputtering chamber is maintained at a pressure of 3×10−7 ton. RF power is controlled within 100 to 200 watts. The n and p features of the nano-structured Si/SiC materials can be controlled by the choice of dopants for the silicon targets.
Silicon carbide films can also be produced by co-sputtering the carbon and the silicon from separate targets at the same time. The n and p dopants are preferably added to the silicon targets. Silicon layers can be applied using the same target used for the co-sputtering of the silicon carbide or if a different dopant concentration is required a separate silicon target must be used. Co-sputtering equipment is available from 4 Wave, Inc.
There exist a wide variety of semiconductor materials that are potentially available to fabrication of quantum well thermoelectric modules. However many of these materials are rare and as a result become expensive when demand is high. Applicants preferred quantum well material for their preferred quantum modules is silicon and carbon, two of the most abundant materials on earth. The price of high purity silicon and silicon carbide can fluctuate with economic conditions but there will never be a shortage of the raw materials. Factories to refine the materials can be built to meet any demand. Therefore, with the utilization of these materials, costs, at least over a period of years, should continue to go down as demand goes up.
In U.S. Pat. No. 6,828,579 referred to above the p-legs were comprised of alternating layers of B4C and B9C and the n-legs were comprised of silicon carbide and silicon. As explained above Applicants have determined that there are important advantages in having both types of legs comprised of the same basic materials. Therefore Applicants have disclosed in this specification techniques for making modules with poly-types of boron carbide in both the n-legs and the p-legs.
As indicated in the Background section for many years, poly-types of boron carbide have been known as a good p-type semiconductor material. Attempts to create n-type boron carbide included attempts to dope the boron carbide with nickel, but at about 50° C. the material reverted to p-type. More recently however researchers in various semi-conductor fields have developed methods of making n-type boron carbide materials. For example a few references are listed below:
Applicants expect to utilize these n-type materials as sputtering targets to prepare super-lattice quantum well boron carbide n-legs.
For example a proposed boron carbide thermoelectric module would include p-legs comprised of alternating super-lattice layers of B4C and B9C and n-legs comprised of alternating super-lattice layers, at least one of the alternating layers being comprised of an n-doped polytype of boron carbide and the other layer being a different semiconductor. Alternatively, super-lattice layers of p-type boron carbide could be combined with super-lattice layers of an n-doped polytype of boron carbide or the other super-lattice layers could be n-doped silicon. The use of n-doped silicon layers to produce an n-type leg is an extrapolation of Applicants' success in producing a p-type leg with p-doped silicon super-lattice layers in combination with super-lattice layers of n-type silicon carbide.
The major cost of the thermoelectric module described above is expected to be the cost of the quantum well material. The volume of quantum well material in the preferred type 3 egg-crate module is 0.144 cm3/module. (The quantum well thickness is 0.0008 cm. The film area per film in each leg is 0.5 cm×0.3 cm or 0.15 cm2. So the volume of quantum well material per film in each leg is 0.0001 cm3. The number of films per leg in the preferred embodiment is 12 so the volume of quantum well material per leg is about 0.0014 cm3. There are approximately 100 legs per module so the volume of quantum well material per module is about 0.144 cm3.)
With an estimate of $70/cm3 for the cost of the quantum well material, the quantum well material in the preferred embodiment would be about $10.00. This $10.00 is similar to the cost of the material used in the 14 watt bismuth telluride module currently being marketed by the Applicants' employer but the quantum well module will generate more than 40 watts of power. If we use $10/cm3 for the cost of the quantum well material, the quantum well material in the preferred embodiment would be about $1.50. So the advantages of the present invention compared to current commercial thermoelectric modules is better by factors of somewhere between about 3 and 18 depending primarily on the production costs.
In a first preferred embodiment, 400 quantum well silicon and silicon carbide super-lattice layers are grown on a 200 micron thick substrate in a web coating sputtering machine. The preferred substrate is Kapton coated with a 100 nm buffer layer of silicon. On the silicon buffer layer are deposited alternating 10 nm layers of silicon carbide and silicon. The thickness of the quantum well material on the 200 micron thick substrate is about 8 microns, so the quantum well film is about 208 microns. The 8 micron film on the 200 micron substrate and a 200 micron spacer are assembled in sets with thicknesses of about 408 microns and the sets are stacked 12 high (12×408 microns) to produce sheets of thermoelectric material about 0.49 cm thick. The stack of quantum well film and spacers is cut into legs with dimensions of about 0.3 cm×0.5 cm×0.49 cm. The legs are treated at both hot and cold ends with an ion implantation procedure and sputter coated at both hot and cold ends with molybdenum and silver to improve electrical connections between the legs. The legs are then assembled into a thermoelectric egg-crate, similar to prior art thermoelectric egg-crates as shown in
The ratio of insulating material to quantum well material in the legs is about 50. The estimated maximum efficiency of the module is about 21.4 percent. This preferred embodiment is a thermoelectric 10×10 egg-crate type module about 5.55 cm×5.55 cm×0.7 cm. The module has 98 active thermoelectric legs, each leg having more than 4,800 super-lattice quantum well layers. Applicants expect to be able to produce more than two hundred of these modules per day per web coating machine.
With an estimate of $70/cm3 for the cost of the quantum well material, the quantum well material in the preferred embodiment would be about $10.00. This $10.00 is similar to the cost of the material used in the 14-watt bismuth telluride module currently being marketed by Applicant's employer but the quantum well module will generate more than 40 watts of power. If we use $10/cm3 for the cost of the quantum well material the quantum well material in the preferred embodiment would be about $1.50. So the advantage of the present invention compared to current commercial thermoelectric modules is better by factors of somewhere between about 3 and 18 depending primarily on the production costs. Applicants expect to manufacture the modules for about $40 per module at a cost per watt of about $0.85/watt.
The reader will note that according to the above description, the amount of quantum well material in each module is very small compared to the substrate and spacer material. There are many advantages associated with the relatively small volume of quantum well material compared to the spacer and substrate material. The main advantage is cost. The quantum well material cost is many orders of magnitude greater than the substrate and spacer material when figured on a volume basis. A second advantage is the thermal conductivity of the substrate and spacer material is orders of magnitude lower on a volume basis than that of the quantum well material. This has the effect of reducing greatly the thermal flux through the thermoelectric module. The down-side of reducing the relative amount of quantum well material in the thermoelectric legs is that ideally most of the thermal energy must pass through the quantum well layers. When the area of the quantum well layer is too small compared to the area of the leg, then the heat flux becomes too high and the desired temperature difference can not be achieved. This means that there is a practical lower limit on the amount of quantum well material needed in a module that will depend in large part on the specific application. Use of the spacers and the inclusion of the insulating substrates in the design of the modules as indicated in
Applicants have performed calculations to estimate the effects of varying the quantity of quantum well material in the egg-crate described above. The results are shown in
The calculations showing how the performance of the preferred embodiment was calculated and the assumptions that went into the calculations are shown below.
In summary the Seebeck coefficient of these films has been measured repeatedly by Applicants and is in the range of about 1000 microvolts/° C. The open circuit voltage per leg is the product of the Seebeck coefficient and the temperature difference (assumed to be 300 C). The film resistance is estimated to be 0.750 ohms. The film resistance per leg is 1/12 of that at 0.0625 ohms and the module resistance is 100 times that or 6.25 ohms. The heat flow through the module is estimated based on known thermal conductivity of the module materials and the result for the preferred egg-crate module is 258 watts. Estimated current at maximum power is estimated by assuming that the operating voltage will be ½ the open circuit voltage and that all of the current will flow through the quantum well film. The operating current is then obtained by dividing the operating voltage by the module “film” resistance and the electric power produced by the module is estimated to be the product of the operating current and the operating voltage or 46.8 watts. The total power flowing through the module in watts is the sum of the electric power plus the heat flow in watts through all of the components of the module which is estimated to be 258 watts. The efficiency of the module is the electric power divided by the total power flowing through the module which is estimated to be 21.4 percent.
As described in U.S. patent '467, '387, '964 and '965, quantum well thermoelectric material is preferably deposited in layers on substrates. For a typical substrate as described in those patents, heat loss through the substrate can greatly reduce the efficiency of a thermoelectric device made from the material. If the substrate is removed some of the thermoelectric layers could be damaged and even if not damaged the process of removal of the substrate could significantly increase the cost of fabrication of the devices. The present invention provides a substrate that can be retained. The substrate preferably should have a low thermal and electrical conductivity with good thermal stability and strong and flexible.
Kapton is a product of DuPont Corporation. According to DuPont bulletins:
Kapton® polyimide film possesses a unique combination of properties that make it ideal for a variety of applications in many different industries. The ability of Kapton® to maintained its excellent physical, electrical, and mechanical properties over a wide temperature range has opened new design and application areas to plastic films.
Kapton® is synthesized by polymerizing an aromatic dianhydride and an aromatic diamine. It has excellent chemical resistance; there are no known organic solvents for the film. Kapton® does not melt or burn as it has the highest UL-94 flammability rating: V-0. The outstanding properties of Kapton® permit it to be used at both high and low temperature extremes where other organic polymeric materials would not be functional.
Adhesives are available for bonding Kapton® to itself and to metals, various paper types, and other films.
Kapton® polyimide film can be used in a variety of electrical and electronic insulation applications: wire and cable tapes, formed coil insulation, substrates for flexible printed circuits, motor slot liners, magnet wired insulation, transformer and capacitor insulation, magnetic and pressure-sensitive tapes, and tubing. Many of these applications are based on the excellent balance of electrical, thermal, mechanical, physical, and chemical properties of Kapton® over a wide range of temperatures. It is this combination of useful properties at temperature extremes that makes Kapton® a unique industrial material.
Applicants have demonstrated that Kapton can be useful as a substrate film for super-lattice thermoelectric layers when high temperature (i.e. greater than 350 C) use is not planned. Applicants have shown that an amorphous silicon layer laid down with short crystalline range orders between the Kapton® substrate and the series of very thin conducting and barrier layers greatly improve thermoelectric performance especially for n-type layers. The preferred technique is to lay it on about 100 nm thick in an amorphous form then to at least partially crystallize it by one or more of a variety of techniques such as thermal or laser or metal-induced or explosion or microwave induced re-crystallization techniques or combinations of these techniques. When Kapton® is used as a substrate it can be mounted on a crystalline base that can be sand blasted off of the Kapton® after the thermoelectric film is deposited.
Silicon is a potential substrate material, but its thermal conductivity is much greater than Kapton. Si has also been used by Applicants as a substrate for depositing Si/SiGe alloys. Si was available commercially in films as thin as 5 microns from suppliers such as Virginia Semiconductor with offices in Fredricksburg, Va. The silicon film is stable at much higher temperatures than Kapton. Silicon film may be attractive in some applications especially very high temperature applications especially if it can be obtained in extremely thin sheets. Also Applicants have experimented with porous silicon which has very low thermal conductivity properties as compared to silicon. If the pores beginning on one side of the film can be controlled to within a micron or less from the other surface, the porous silicon film could make a very good substrate material. Alternatively the entire substrate could be removed by etching the Silicon to the point where the quantum well layers begin. In this case it may be necessary to bond the quantum well films to Kapton or glass with a low thermal conductivity to provide structural support to the films.
Many other organic materials such as Mylar, polyethylene, NaCl and polyamide, polyamide-imides and polyimide compounds could be used as substrates. Other potential substrate materials are a variety of silicon on glass or silicon on insulator materials as well as oxide films such as SiO2, Al2O3 and TiO2. Mica could also be used for a substrate. As stated above, the substrate preferably should be very thin and a very good thermal and electrical insulator with good thermal stability, strong and flexible. At very high temperatures substrates glass or ceramics with low electric and thermal conductivity could be used. A Si3N4 layer can be employed between the substrate and the SiC layer to improve the quality of the SiC layers.
It is possible to deposit the n and p materials at the same times on opposite sides of the substrate. One technique is to coat one side of the Kapton as explained above then remove the film and coat the other side. Another technique is to arrange the film on a web coater as a continuous Mobius strip so that both sides can be coated at the same time without removing the film.
The advantage of this process is to balance out the stresses that are developed as the films are deposited and also the stresses the form by the differences between the thermal expansion of the SiGe alloys and the high thermal expansion of Kapton or the low thermal expansion of Si. Also, the cost of the sputtering operation is reduced. Samples can also be prepared with the coatings separately deposited. Such samples were able to endure excellent adhesion when rolled up in the reverse direction so the second deposition could be performed.
Applicants' tests and theoretical studies have shown that there is a strong correlation between the crystallinity of the quantum well legs and the thermoelectric properties. Their studies show that if the semiconductor material in the legs is amorphous there is no significant improvement in the thermoelectric properties. If the legs are near perfect crystals the thermoelectric properties are greatly enhanced. There tests and studies however further suggests that the substantial improvement in performance is between amorphous and about 25 percent crystalline. (
Persons skilled in the thermoelectric art will recognize that many other egg-crate designs are possible that will provide the advantages of the thermoelectric egg-crate which include the electrical isolation of the legs except where they need to be connected and to permit the electrical connections to be simply sprayed onto the hot and cold surfaces of the module. Many sizes are possible. The number of legs could be tailored as desired. Series and parallel connections can be easily designed into the modules.
The egg-crate describe in the U.S. Pat. No. 5,875,098 is molded utilizing a thermoplastic material that is satisfactory for operation at temperatures up to about 35° C. For modules designed for higher temperature operation will require a higher temperature egg-crate. Such a higher temperature egg-crate is described in detail in Applicants' U.S. patent application Ser. No. 12/590,653 filed Nov. 12, 2009, which is incorporated herein by reference.
A preferred embodiment easily adapted for use with these quantum well film is a one-dimensional egg-crate as compared to the two-dimensional 10×10 egg-crate described above and shown in
Applicants' experiments have shown that the ability to produce super-lattices with combinations of semiconductor materials along with appropriate doping provides a means for creation of a large variety of new materials with quantum well properties. Applicants' techniques basically include choosing materials or combinations of materials that constitute semiconductor materials with significant band gaps. Materials with differing band gaps are appropriately doped to produce n-properties and formed into thin super-lattices to produce n-legs. And materials with differing band gaps are appropriately doped to produce n-properties and formed into thin super-lattices to produce n-legs. Preferably the materials chosen for the p-legs are similar to the materials chosen for the n-legs. This helps to minimize problems associated with differing thermal expansion during operation.
While the above description contains many specificities, the reader should not construe these as limitations on the scope of the invention, but merely as exemplifications of preferred embodiments thereof. Those skilled in the art will envision many other possible variations within its scope. The preferred layer thickness is about 10 nm; however, layer thickness could be somewhat larger or smaller such as within the range of 20 nm down to about 5 nm. It is not necessary that the layers be grown on film. For example, they could be grown on thicker substrates that are later removed. There are many other ways to make the connections between the legs other than the methods discussed. Efficiency values referred to in this specification could were generally based on a delta T of about 200° C. Substantially higher efficiencies could be realized at higher delta T's. Accordingly, the reader is requested to determine the scope of the invention by the appended claims and their legal equivalents, and not by the examples which have been given.
The present invention is a continuation-in-part of Ser. No. 12/460,424 filed Jul. 17, 2009 which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12460424 | Jul 2009 | US |
| Child | 12806359 | US |
