Patent Application
20070277866
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
In some embodiments, the present invention is directed to thermoelectric devices comprising thermoelectric elements comprising nanotubes of thermoelectric material. The present invention is also directed to methods of making such thermoelectric elements and devices, particularly wherein the nanotubes are formed electrochemically in templates. The present invention is also directed to systems and applications incorporating and using such devices, respectfully.
With respect to such above-mentioned thermoelectric elements and devices comprising nanotubes, the most important nanostructure dimension is the tube wall thickness, so that the outer tube diameter is not as critical and the arrays are simpler to fabricate than very narrow diameter nanowires. Methods in accordance with some embodiments of the present invention allow for excellent control over the tube wall thickness and composition. This approach is also suitable for manufacturing dense arrays of nanotubes over large areas, which is critical for the fabrication of practical devices. In addition, a wide range of thermoelectric nanotube materials can be fabricated, allowing one to tailor the material choice to a particular temperature range of interest.
In the following description, specific details are set forth such as specific quantities, sizes, etc. so as to provide a thorough understanding of embodiments of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In many cases, details concerning such considerations and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art.
Referring to the drawings in general, it will be understood that the illustrations are for the purpose of describing a particular embodiment of the invention and are not intended to limit the invention thereto.
In the above-described embodiment, the thermoelectric module 12 comprises n-type semiconductor legs 18 and p-type semiconductor legs 20 that function as thermoelements, whereby heat generated by charge transport is transferred away from the object 14 towards the object 16. In this embodiment, the n-type and p-type semiconductor legs (thermoelements) 18 and 20 are disposed on patterned electrodes 22 and 24 that are coupled to the first and second objects 14 and 16, respectively. In certain embodiments, the patterned electrodes 22 and 24 may be disposed on thermally conductive substrates (not shown) that may be coupled to the first and second objects 14 and 16. Further, interface layers 26 and 28 are employed to electrically connect pairs of the n-type and p-type semiconductor legs 18 and 20 on the patterned electrodes 22 and 24.
In the embodiment described above and as depicted in
Moreover, a plurality of thermoelements (i.e., thermoelectric elements) 74 and 76 are established between the first and second patterned electrodes 64 and 68. Further, each of the plurality of thermoelements 74 and 76 comprises an array (i.e., a plurality) of nanotubes 70 comprised of a thermoelectric material, wherein the material is a doped semiconductor material, and where thermoelements 74 comprise nanotubes of p-doped material and thermoelements 76 comprise nanotubes of n-doped material (or vice versa). Examples of suitable thermoelectric materials include, but are not limited to, InP, InAs, InSb, silicon germanium based alloys, bismuth antimonide based alloys, lead telluride based alloys (e.g., PbTe), bismuth telluride based alloys (e.g., Bi2Te3), or other III-V, IV, IV-VI, and II-VI semiconductors, or any combinations thereof having substantially high thermoelectric figure-of-merit, and their combinations thereof. Typically, the thermoelements 74 and 76 further comprise a porous template 75 in which the nanotubes 70 have been electrodeposited. Such porous templates may optionally comprise a substrate 72.
Regarding the template 75, the template material is not particularly limited save for the requirement that it accommodate pores. Suitable materials include, but are not limited to, anodized aluminum oxide (AAO), nanochannel glass, self-organized di-block copolymers, and the like. Typically, the template is a substantially two-dimensional planar template. The pores are substantially aligned (with respect to each other) and generally perpendicular to the plane of the template. In some embodiments the pores are roughly cylindrical in shape and generally possess a diameter between about 5 nm and about 500 nm. The template thickness is generally between about 10 μm and about 500 μm. Pore density within the template is generally between about 109/cm2 and about 1012/cm2.
Regarding the nanotubes 70, the nanotubes are generally electrochemically-deposited in the pores of the template 75 (vide infra). Consequently, their dimensions and density within the template array largely parallel that of the pores. They generally possess an outer diameter between about 5 nm and about 500 nm, and a tube wall thickness between about 1 nm and about 20 nm. Their height is generally between about 10 μm and about 500 μm, and their density within the template is generally between about 109/cm2 and about 1012/cm2. As mentioned above, compositionally, the nanotubes 70 comprise a doped semiconducting material, the bulk of which can include, but is not limited to, InP, InAs, InSb, silicon germanium based alloys, bismuth antimonide based alloys, lead telluride based alloys (e.g., PbTe), bismuth telluride based alloys (e.g., Bi2Te3), or other III-V, IV, IV-VI, and II-VI semiconductors, or any combinations thereof having substantially high thermoelectric figure-of-merit (including, e.g., ternary and quaternary semiconductors), and their combinations thereof. Within a particular thermoelement (i.e., a nanotube array), the nanotubes will comprise either a n-doped or a p-doped semiconducting composition. The nanotubes can be deposited by electrochemical codeposition, where a compound material is deposited from one solution. Alternatively, the nanotubes can be deposited by electrochemical atomic layer epitaxy (ECALE), where a monolayer or sub-monolayer of each element is deposited sequentially from separate baths. In order to obtain smooth films with excellent control over the film thickness, ECALE offers significant advantages over codeposition. See Stickney et al for examples of ECALE of thin films (Stickney et al., “Electrochemical atomic layer epitaxy,” Electroanalytical Chemistry, vol. 21, pp. 75-209, 1999).
The thermal transfer device 60 also includes a joining material 78 disposed between the plurality of thermoelements 74 and 76 and the first and second patterned electrodes 64 and 68 for reducing the electrical and thermal resistance of the interface. In certain embodiments, the joining material 78 between the thermoelements 74 and 76 and the first patterned electrode 64 may be different than the joining material 78 between the thermoelements 74 and 76 and the second patterned electrode 68. In one embodiment, the joining material 78 includes silver epoxy. It should be noted that other conductive adhesives may be employed as the joining material 78. In particular, the joining material 78 is disposed between the substrate 72 and the patterned electrode 64.
In some other embodiments, the thermoelements 74 and 76 may be bonded to the patterned electrodes 64 and 68 by diffusion bonding through atomic diffusion of materials at the joining interface or other techniques such as wafer fusion bonding for semiconductor interfaces. As will be appreciated by one skilled in the art, diffusion bonding causes micro-deformation of surface features leading to sufficient contact on an atomic scale to cause the two materials to bond. In certain embodiments, gold may be employed as an interlayer for the bonding and the diffusion bonds may be achieved at relatively low temperatures of about 300° C. In certain other embodiments indium or indium alloys may be employed as an interlayer for the bonding at temperatures of about 100° C. to about 150° C. Further, a typical solvent cleaning step may be applied on the surfaces to achieve flat and clean surfaces for applying diffusion bonding. Examples of solvents for the cleaning step include acetone, isopropanol, methanol and so forth. Further, metal coatings may be disposed on the top and bottom surfaces of the thermoelements 74 and 76 and the substrate 72 to facilitate the bonding between the thermoelements and the first and second substrates 62 and 66. In one embodiment, the thermoelements 74 and 76 may be bonded to the patterned electrodes 64 and 68 through direct diffusion bonding. Alternatively, the thermoelements 74 and 76 may be bonded to the patterned electrodes 64 and 68 via an interlayer, such as gold, metal, or solder metal alloy foil. In certain embodiments, the bonding between the thermoelements 74 and 76 and the first and second substrates 62 and 66 may be achieved through an interface layer such as silver epoxy. However, other joining methods may be employed to achieve the bonding between the thermoelements 74 and 76 and the first and second substrates 62 and 66.
While not intending to be bound by theory, in a presently contemplated configuration, the thermoelements 74 and 76 comprise nanotubes having wall thicknesses where quantum effects (e.g., quantum or surface confinement) are dominant. Typically, this involves wall thicknesses between about 1 nm and about 20 nm. Further, the electronic density of states of the charge carriers and phonon transmission characteristics can be controlled by altering the dimensions and composition of the nanotubes within thermoelements 74 and 76, thereby enhancing the efficiency of the thermoelectric devices that is characterized by the figure-of-merit (ZT) of the thermoelectric device.
In some embodiments, the thermal transfer device of
The metal layer can be any metal or combination of metals that can be conformally deposited over the template surface so as to serve as an electrode for the electrodeposition of thermoelectric nanotubes within the pores. Suitable materials include, but are not limited to, gold (Au), copper (Cu), nickel (Ni), and combinations thereof. Typically, this metal layer is deposited via electroless means, and the layer generally has a thickness between about 10 nm and about 100 nm. Removal of the metal layer after nanotube deposition can be accomplished by selective etching techniques such as, but not limited to, wet chemical etching of gold by a potassium iodide/iodine solution, wet chemical etching of copper or nickel by an iron chloride solution, or dry etching processes, and the like. For a general (non-specific) discussion of electrochemical deposition of metal in a porous (polymer) membrane, see Ku et al. “Fabrication of Nanocables by Electrochemical Deposition Inside Metal Nanotubes,” J. Am. Chem. Soc. vol. 126, pp. 15022-15023, 2004. See above for details on the template and nanotube materials. Alternatively, the metal can be deposited by a vapor phase process, such as atomic layer deposition (ALD). ALD could be used to deposit a metal layer on the nanoporous template, such as copper, iron, nickel, gold, etc., or another type of conducting material that could act as an electrode, such as indium tin oxide. These vapor deposited electrodes could be removed after depositing the thermoelectric material by a wet or dry selective chemical etch. For an example of nanotubes deposited by ALD onto anodic alumina templates see Elam et al., “Conformal Coating on Ultrahigh-Aspect-Ratio Nanopores of Anodic Alumina by Atomic Layer Deposition,” Chem. Mater. vol. 15, pp. 3507-3517, 2003).
In some embodiments, it is envisioned that an entirely metal template is utilized instead of a ceramic template covered by a metal layer. In such an embodiment, the entire metal template would have to be removed after nanotube deposition and replaced by an insulating material, such as a ceramic or polymer, in order to provide mechanical stability.
In some or other embodiments, the nanotubes 70 are formed using a variation on one or more of the above-described embodiments or using a method other than those described above. For example, in some embodiments the nanotubes are deposited by electrodeposition in templates coated not with a metal layer, but rather having pore walls coated with a metal nanoparticle seed layer or functional molecular layer. See, e.g., Brumlik et al., “Template Synthesis of Metal Microtubules,” J. Am. Chem. Soc., vol. 113, pp. 3174-3175, 1991. In other embodiments, very fast electrodeposition can result in the deposition of nanotubes in porous templates rather than nanowires. See, e.g., Yuan et al. “Highly Ordered Platinum-Nanotubule Arrays for Amperometric Glucose Sensing,” Adv. Funct. Mater., vol. 15(5), pp. 803-809, 2005. In some or other embodiments, the electrode layer only partially coats one side of the template pores, thereby permitting electrochemical deposition of nanotubes within the pores. See, e.g., Li et al., “A Facile Route to Fabricate Single-crystalline Antimony Nanotube Arrays,” Chem. Lett., vol. 34(7), pp. 930-931, 2005; Lee et al., “A Template-Based Electrochemical Method for the Synthesis of Multisegmented Metallic Nanotubes,” Angew. Chem. Int. Ed., vol. 44, pp. 6050-6054, 2005. In still other embodiments, templates are coated with a sacrificial layer (e.g., carbon nanotubes or polymer) and filled with metal nanowires. The sacrificial layer is then removed and nanotubes are electrodeposited in the resulting open spaces of the template. See, e.g., Mu et al., “Uniform Metal Nanotube Arrays by Multistep Template Replication and Electrodeposition,” Adv. Mater., vol. 16, pp. 1550-1553, 2004.
In fabricating such above-mentioned thermoelements, in some embodiments a particular doping density within the nanotubes is chosen for particular thermoelectric performance (typically, such doping densities are ca. 1017-1018 cm−3). The doping can be accomplished by intrinsic doping to produce an excess of one of the elements of the compound. For example, an excess of Te in Bi2Te3 deposition results in an n-type material (see, e.g., Yoo et al., “Electrochemically deposited thermoelectric n-type Bi2Te3 thin films,” Electrochimica Acta vol. 50(22), pp. 4371-4377, 2005). An excess of one of the elements can be obtained, for example, by altering the electrodeposition conditions, including deposition potential. Alternatively, an extrinsic dopant can be introduced into the nanotubes by adding a small amount of a dopant precursor to the electrochemical deposition solution or by integrating a cycle into the deposition process for the dopant.
As mentioned above, the critical dimension with respect to thermoelectric properties in the above-described nanotubes is the tube wall thickness. By depositing the nanotube walls using a controlled deposition process, the nanotube wall thickness can be controlled with sub-nanometer resolution. Because the nanotube wall thickness is the critical dimension, any distribution in the pore diameters in the template will be fairly unimportant (this is in contrast to conformal deposition of nanowires in porous templates, where larger wires will tend to dominate the device behavior). It is also not necessary to fabricate templates with very small pore diameters (e.g., <10 nm). Since the critical dimension is the wall thickness, it is possible to have outer tube diameters (corresponding to template pore diameters) with larger, and more easily fabricated dimensions (e.g., >10 nm). Again, this is an advantage compared to nanowires, where conformal deposition would require fabrication of templates with pore diameters corresponding to the critical thermoelectric property dimensions, which are typically less than 10-20 nm. Because the thermoelectric material deposits as a thin film over the entire surface simultaneously, the composition of the deposit can be carefully controlled. This avoids the potential problems of variation in composition along the length of a nanowire, which are anticipated for very high aspect ratio nanowire deposition, e.g., <10 nm diameter by >100 um tall. By depositing the nanotubes conformally over the surface of the template, it is possible to obtain nanotubes in nearly 100% of the pores. This avoids any difficulties that may be encountered for the deposition of nanowires, where obtaining high pore filling ratios is potentially difficult for high aspect ratio structures. Additionally, such electrochemical deposition techniques are easily scalable.
Various aspects of the techniques described above find utility in a variety of heating/cooling systems, such as refrigeration, air conditioning, electronics cooling, industrial temperature control, and so forth. The thermal transfer devices as described above may be employed in air conditioners, water coolers, climate controlled seats, and refrigeration systems including both household and industrial refrigeration. For example, such thermal transfer devices may be employed for cryogenic refrigeration, such as for liquefied natural gas (LNG) or superconducting devices. Further, the thermal transfer devices as described above may be employed for cooling of components in various systems, such as, but not limited to vehicles, turbines and aircraft engines. For example, a thermal transfer device may be coupled to a component of an aircraft engine such as, a fan, or a compressor, or a combustor or a turbine case. An electric current may be passed through the thermal transfer device to create a temperature differential to provide cooling of such components.
Alternatively, the thermal transfer device described herein may utilize a naturally occurring or manufactured heat source to generate power. For example, the thermal transfer devices described herein may be used in conjunction with geothermal based heat sources where the temperature differential between the heat source and the ambient (whether it be water, air, etc.) facilitates power generation. Similarly, in an aircraft engine the temperature difference between the engine core air flow stream and the outside air flow stream results in a temperature differential through the engine casing that may be used to generate power. Such power may be used to operate or supplement operation of sensors, actuators, or any other power applications for an aircraft engine or aircraft. Additional examples of applications within which thermoelectric devices described herein may be used include gas turbines, steam turbines, vehicles, and so forth. Such thermoelectric devices may be coupled to photovoltaic or solid oxide fuel cells that generate heat thereby boosting overall system efficiencies.
The thermal transfer devices described above may also be employed for thermal energy conversion and for thermal management. It should be noted that the materials and the manufacturing techniques for the thermal transfer device may be selected based upon a desired thermal management need of an object. Such devices may be used for cooling of microelectronic systems such as microprocessor and integrated circuits. Further, the thermal transfer devices may be employed for thermal management of semiconductor devices, photonic devices, and infrared sensors.
The following examples are included to demonstrate particular embodiments of the present invention. It should be appreciated by those of skill in the art that the methods disclosed in the examples that follow merely represent exemplary embodiments of the present invention. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments described and still obtain a like or similar result without departing from the spirit and scope of the present invention.
This Example serves to illustrate the formation of thermoelectric elements comprising nanotubes for use in thermoelectric devices, in accordance with some embodiments of the present invention.
A nanoporous alumina template is fabricated by anodization of aluminum foil. The pores created during the anodization are nearly parallel to one another and run through the length of the template. The average pore diameter and spacing are determined by the anodization conditions, including potential, acid, etc. (this is a well-established procedure). The pores of the anodized alumina membrane are coated by gold metal using an electroless plating process (Kohli et al., “Template Synthesis of Gold Nanotubes in an Anodic Alumina Membrane,” J. Nanosci. Nanotech. vol. 4, pp. 605-610, 2003). Next, one side of the membrane is coated with a thick gold electrode layer by fast electroless plating. The membrane is then placed into an electrochemical flow cell, and thermoelectric nanotubes are deposited concentrically onto the gold nanotubes of the membrane. The thermoelectric material is deposited by an electrochemical atomic layer epitaxy process. For example, Bi2Te3 can be deposited by using a modification of the procedure described by Zhu et al., “Optimization of the formation of bismuth telluride thin film using ECALE,” J. Electroanalytical Chemistry, 585, 83-88, 2005. In that case, they deposited thin films. In order to deposit a film over the surface of the high aspect ratio gold nanotubes, it may be necessary to increase the deposition cycle times, etc. After thermoelectric nanotube deposition, metal films are deposited onto one or both sides of the membrane. Then the gold nanotubes are removed by a selective chemical etch. The remaining structure comprises thermoelectric nanotubes embedded in the pores of the nanoporous alumina template and connected at the top and bottom sides by deposited metal layers.
This Example serves to illustrate how a plurality of thermoelectric elements, comprising electrochemically-deposited nanotubes, can be integrated into the manufacture of a thermoelectric device, in accordance with some embodiments of the present invention.
Metal electrodes (Cu or Al) are patterned on two thermally conductive substrates (AlN or SiC) using standard photolithography. The metal electrodes are patterned on each substrate so that when the two substrates are facing each other with thermoelectric elements in between, the electrodes and thermoelectric elements are electrically in series from one corner of the first substrate to the opposite corner of the second substrate. To connect the thermoelements to the metal electrodes, indium foil is used as a joining layer. Pieces of indium foil are sandwiched between the metal electrodes and the thermoelements, and then the entire substrate/thermoelement assembly is subjected to pressure and heat to cause the indium foil to diffusion bond between the metal electrodes on the substrates and the metal layers on the ends of each of the thermoelements. In this final thermoelectric module, the patterned electrodes on each substrate are electrically connected in series with the joining layers and alternating n-type and p-type thermoelements sandwiched between the two substrates. The thermoelements are thermally connected in parallel between the two substrates.
It will be understood that certain of the above-described structures, functions, and operations of the above-described embodiments are not necessary to practice the present invention and are included in the description simply for completeness of an exemplary embodiment or embodiments. In addition, it will be understood that specific structures, functions, and operations set forth in the above-described referenced patents and publications can be practiced in conjunction with the present invention, but they are not essential to its practice. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without actually departing from the spirit and scope of the present invention as defined by the appended claims.
