Patent Application
20100024437
The present invention relates to thermoelectric modules and especially to high temperature thermoelectric modules.
The Seebeck coefficient of a 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 of a thermoelectric material is defined as:
where α is the Seebeck coefficient of the material (measured in microvolts/K), σ is the electrical conductivity of the material and λ is the total thermal conductivity of the 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 in 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 percent Bi2Se3 and p-type Bi2Te3 typically contains 80 Mol percent Sb2Te3. Lead telluride is typically doped with sodium for P type and Pbl2 iodine for N type.
Electric power generating thermoelectric modules are well known. These modules produce electricity directly from a temperature differential utilizing the thermoelectric effect. 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. These thermoelectric elements are called n-legs and p-legs. Since the voltage differential is 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.
For example Hi-Z Technology, Inc. offers a Model HZ-14 thermoelectric 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 98 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. FIG. 1 is a drawing of the egg-crate and
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 thermoelectric properties of Bi2Te3 peak at about 100° C. and are greatly reduced at about 250° C. For both of these reasons, uses of these modules are limited to applications where the hot side temperatures are lower than about 250° C.
Lead telluride thermoelectric modules are also known in the prior art. A prior art example is the PbTe thermoelectric module described in U.S. Pat. No. 4,611,089 issued many years ago to two of the present inventors. This patent is hereby incorporated herein by reference. That module utilized lead telluride thermoelectric alloys with an excess of lead for the n-legs and lead telluride with an excess of tellurium for the p-legs. The thermoelectric properties of lead telluride thermoelectric alloys peak in the range of about 425° C. The egg-crate for the module described in the above patent was fabricated using a technique similar to the technique used many years ago for making chicken egg crates from cardboard spacers. For the thermoelectric egg-crate the spacers were mica which was selected for its electrical insulating properties at high temperatures. Mica, however, is marginal in strength and cracks easily. In addition the walls of the egg-crate made from the mica spacers all had gaps at the intersections of the walls that could lead to short circuits. A more rugged high temperature egg-crate with gapless walls is needed.
That lead telluride module was suited for operation in temperature ranges in excess of 500° C. But the cost of fabrication of this prior art module is greatly in excess of the BiTe module described above. Also, after a period of operation of about 1000 hours some evaporation of the p-legs and the n-legs at the hot side would produce cross contamination of all of the legs which would result in degraded performance.
What is needed is a low cost, compact, high-temperature thermoelectric module with gapless walls designed for operation at hot side temperatures in excess of 500° C. preferably with thermoelectric properties substantially in excess of prior art high-temperature thermoelectric modules.
The present invention provides a high-temperature thermoelectric module. The module includes a two-part molded egg-crate for holding in place and providing insulation and electrical connections for a number of thermoelectric n-legs and p-legs. A first part of the egg-crate is comprised of a ceramic material capable of operation at temperatures in excess of 500° C. and a second part comprised of a thermoplastic material having very low thermal conductivity. In preferred embodiments the high temperature ceramic is zirconium oxide and the thermoplastic material is a DuPont Zenite available from DuPont in the form of a liquid crystal polymer resin. The thermoelectric legs of preferred embodiments are also comprised of high-temperature and low-temperature material. In preferred embodiments the high-temperature thermoelectric material is lead telluride and the low-temperature material is bismuth telluride thermoelectric alloys described in the background section. In preferred embodiments metal felt spacers are provided in each leg to maintain proper electrical contacts notwithstanding substantial temperature variations. Preferably the module is sealed in an insulating capsule.
A first preferred embodiment of the present invention can be described by reference to
Egg-crate 70 is injection molded using a technique similar to that described in U.S. Pat. No. 5,875,098. However, the molding process in substantially more complicated. The egg-crate comes in two molded together sections. It includes an upper section (which will lie adjacent to a hot side) molded from stabilized zirconium oxide (ZrO2). ZrO2 has a very high melting point of 2715° C. and a very low thermal conductivity for an oxide. The egg-crate also includes a lower side (which will lie adjacent to a cold side) molded from Zenite Model 7130 available from Dow Chemical that has a melting point of 350° C. but very low thermal conductivity.
The ZrO2 portion of the egg-crate is fabricated by injection molding of the ZrO2 powders with two different binder materials. Some of the binder material is removed by leaching prior to sintering. The ZrO2 portion is then sintered to remove the second binder and produce a part with good density and high temperature strength. The ZrO2 portion will typically shrink about 20 percent during sintering. This sintered section is then placed in a second mold and a subsequent injection molding of the Zenite portion of the egg-crate is then performed, thereby bonding the Zenite to the ZrO2. While the thermal conductivity of the ZrO2 is among the lowest of any known oxide, its thermal conductivity of 2 W/mK is much higher than the thermal conductivity of Zenite which is 0.27 W/mK. An objective of the present invention is to minimize any loss of heat through the egg-crate material. Also the Zenite is flexible and will allow the two-section egg-crate to endure significant rough handling. The mica of the prior art patent is a relatively weak material that cracks easily.
The PbTe/Bi2Te3 thermoelectric legs of this preferred embodiment are segmented as shown at 72 and 74 in
Egg-crate 70 contains spaces for 80 legs, 40 n-legs and 40 p-legs. The components of these legs are shown blown-up in
As indicated in the background section life testing by Applicants of PbTe modules has shown that some degradation of the module occurs after approximately 1,000 hours of operation. Applicants have discovered that the degradation can be attributed to “cross-talk” between the n-legs and the p-legs near the hot junction caused by evaporation of tellurium from the p-leg contaminating the n-leg. (As explained in the background section an excess of lead in the n-leg is what provides the n-leg with its thermoelectric doping properties.) The problem is prevented in this embodiment with two techniques: First, as shown in
Compliant Metal Parts
The thermoelectric module of this preferred embodiment will typically be placed between a hot surface of about 600° C. and a cold surface of about 50° C. In many applications these temperatures may vary widely with temperature differentials swinging from 0° C. to 550° C. Therefore the module and its components should be able to withstand these temperatures and these changes in temperature which will produce huge stresses on the module and its components. This embodiment is designed to meet these challenges.
This preferred embodiment includes at the bottom of each leg at the cold side a fiber metal felt pad 86 comprised of copper or bronze wool. These materials provide good thermal conductivity and are able to deform when the module is placed in compression between the heat source and heat sink. These materials are resilient to respond to thermal cycling and to the inevitable warping that the module will undergo because of the thermal gradient imposed across it. These compliant materials are also able to cushion the brittle lead telluride yet maintain intimate contact between it and the heat sink. The fiber metal felt pads can be impregnated with an elastomer such as silicone rubber to reduce the risk of creep and to add deflection and compliance. Silicone rubber can operate at temperatures up to 300° C.
The n and p type PbTe legs are creeping or pushing up towards the Fe hot shoe. A load of 1,000 psi is initially used and this load can be reduced to 500 psi after the module is operated for approximately 100 hours and proper seating of the module is obtained. After this time a lower load of 50 to 100 psi can be used to maintain the low contact resistance joints.
The module is specifically designed to endure considerable thermal cycling or steady state behavior. The hot and cold side joints are free to slide and relieve thermal stresses. The module needs to be held in compression at approximately 50 to 100 psi after it reaches its design operating temperatures.
Lead telluride based alloys have been used since the 1960s and the alloys and recommended doping levels are well documented in the prior art literature. Their thermoelectric properties versus temperature are given in many publications such as Chapter 10, “Lead Telluride Alloys and Junctions” of Thermoelectric Materials and Devices, Cadoff and Miller, published by Reinhold Publishing Corporation of New York.
In the past four years newer PbTe based alloys have evolved that have better properties than the conventional PbTe based alloys noted above. For example a Jul. 25, 2008 article in Science Daily reported on a lead telluride material developed at Ohio State University having substantial improvements in efficiency over prior art lead telluride materials. This new material is doped with thallium instead of sodium. The article suggests that efficiency of the new material may be twice the efficiency of prior art lead telluride. Applicants are seeking to prepare bulk lead telluride thermoelectric material with a finer grain size than has previously been achieved. Fine grain size is expected to lower the thermal conductivity of the material without significant impact on resistivity or Seebeck coefficient, thus raising its ZT and efficiency. In previous attempts others have made to produce a fine-grained PbTe, the grain size was observed to coarsen rapidly, even near room temperature, so the benefit of small grain size could not be retained. In this study, Applicants seek to preserve a fine grain structure by additions of very fine alumina powder, which is expected to produce a grain boundary pinning effect, thus stabilizing the fine grain size. Applicants' recent results indicate that the PbTe grain size can indeed be held below 2 μm, even with processing at 800° C.
A second preferred embodiment of the present invention is just like the first preferred embodiment except the entire legs are comprised of lead telluride thermoelectric alloys. Preferably, the lead telluride alloy is one of the newer very high efficient alloys.
The high-temperature module of the preferred embodiment requires encapsulation to prevent oxidation of the n and p alloys with an accompanying decrease in thermoelectric properties. An example of encapsulating PbTe modules would be the 1 kW generator for diesel trucks shown at 16 in
The casting of the support structure has two flanges 24 and 26, one large and one small which are perpendicular to the main part of the support structure. The large flange 24 is about 10 inches in diameter while the small flange 26 is about 8 inches in diameter.
The large flange contains feed-throughs for both the two electrical connections and four water connections. The two electric feed-throughs 28 are electrically isolated with alumina insulators from the support structure. Both the large and small flange will contain a weld preparation so a metal dust cover 30 can be welded in position.
The four water feed-throughs 32 consists of one inch diameter tubes that are welded into the flange. The inside portion of the water tubes are welded to a wire reinforced metal bellows hose with the other end connected to the heat sink by a compression fitting or a stainless to aluminum bimetallic joint. Two of the tubes are inlets and the other two tubes are outlets for the cooling water.
Once the generator is assembled and the flanges between the support structure and the dust cover are welded as shown at 34, the interior volume will be evacuated and back filled with an inert gas such as Argon through a small ⅜ inch diameter tube 36 in the dust cover to about 75% of one atmosphere when at normal room temperature (˜20√ c.). Once filled, the fill tube will be pinched off and welded.
In a preferred embodiment nine thermoelectric modules 20 of the first preferred embodiment are mounted on each of the eight sides of the hexagonal structure for a total of 96 modules. Applicants estimate a total electrical output of about 1.1 kilowatts. This estimate is based on prior performance with the structure described in the U.S. Pat. No. 5,625,245 patent, utilizing modules of the first preferred embodiment and assuming an exhaust hot side temperature of about 550° C. and cold side cooling water temperature of about 100° C.
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. For example:
Some of the other thermoelectric alloys that are attractive over high-temperature ranges are:
All of these bulk alloys and others under development can be used in the new ZrO2/Zenite egg-crate design shown in
The egg-crate of the present invention could be utilized with thin film quantum well thermoelectric p and n legs of the type described in detail in U.S. Pat. No. 5,550,387 which is incorporated herein by reference. That patent describes n and p thermoelectric legs that are fabricated using alternating layers 10 nanometers thick of Si/Si0.8Ge0.2 layers grown on silicon substrates. In applications with temperatures above 500° C. these legs would be used on the cold side. That patent and U.S. Pat. No. 6,828,579 also disclose high temperature lattices comprised of thin layers of B4C/B9C and Si/SiC can also be operated at very high temperatures up to about 1100° C. Details for fabricating B4C/B9C thermoelectric legs are provided in U.S. Pat. No. 6,828,579 (assigned to Applicants' employer) which is also incorporated herein by reference. See especially Col. 3 where high temperature performance is discussed. These B4C/B9C and Si/SiC materials could also be used alone to make thermoelectric legs which could be used in the egg-crate of the present invention or on the hot side of the legs along with bismuth telluride, lead telluride or quantum well Si/SiGe for the cold side.
A large number of 10 nm quantum well layers are built up on a compatible substrate that has a low thermal conductivity to produce quantum well thermoelectric film. Kapton is a good substitute candidate if the temperature is not too high. For higher temperature operation silicon is a preferred choice of substrate material as described in Col. 11 of U.S. Pat. No. 6,828,579. Other substrate materials are discussed in Col. 7. A good substrate material not disclosed in the patent is porous silicon. Porous silicon can survive very high temperatures and has extremely low thermal conductivity. The pores can be produced in silicon film such as 5 micron thick film from one side to extend to within a fraction of a micron of the other side. The pores can be produced either before the thermoelectric layers are laid down or after they are laid down.
The quantum well thermoelectric film is cut and combined to make n and p type legs of the appropriate size and each leg is loaded into one opening of the
Cyano Glue
A technique that can simplify cold side lay-up involves the use of cyano acrylate glue (called super glue) which can be used to temporarily hold the parts in position during lay-up but is burned off at a seating step after the thermoelectric legs are assembled in the egg-crate.
Other Compliant Members
Other compliant members could be substituted for the copper felt discussed above. These include woven steel felt with a boron nitride slurry to enhance thermal conductivity. A thin Beliville spring could be used. Boron nitride powder could be stirred into the spring material before the spring is cast.
Other Interfaces
Instead the tab-socket technique, other techniques for joining the hot and cold portions of the egg-crate could be used, such as a roughen surface of the ceramic portion.
Segmented Legs
The PbTe portions of the p-legs could be cold pressed and sintered separately from the BiTe portions. When they are subject to hot operating conditions they will diffusion bond. The same applies to the n-legs.
Hot Pressing of the Legs
Another option is to hot press the thermoelectric materials in bulk then slice and dice them into legs.
Aerogel Crate
In one variation, an aerogel material is used to fill all unoccupied spaces in the egg-crate. A module assembly will be sent to Aspen Aerogels. They will immerse it in silica sol immediately after dropping the pH of the sol. The sol converts to silica gel over the next few days. It is then subjected to a supercritical drying process of tightly controlled temperature and pressure condition in a bath of supercritical liquid CO2. This process removes all the water in the gel and replaces it with gaseous CO2. The Aerogel serves multiple functions: (1) helping to hold the module together, (2) reducing the sublimation rate of the PbTe and (3) providing thermal and electrical insulation around the legs.
In another variation, a portion of the egg-crate could be made of non-woven refractory oxide fiber material, possibly with a fugitive polymer binder, and having the consistency of stiff paper or card stock is used. After assembly, the binder is burned away, leaving a porous fiber structure that is them infiltrated with Aerogel. The fiber reinforcement of the aerogel gives added strength and toughness.
Those skilled in the art will envision many other possible variations within its scope. 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 claims the benefit of Provisional Patent Application, Ser. No. 61/137,206, filed Jul. 17, 2008.
| Number | Date | Country | |
|---|---|---|---|
| 61137206 | Jul 2008 | US |
