Patent Application
20140360549
Devices for cooling and power generation based on thermoelectric effects are known in the art. Solid-state devices that employ the Seebeck effect or Peltier effect for power generation and heat pumping are known. For power generation, for example, a thermoelectric converter relies on the Seebeck effect to convert temperature differences into electricity. A thermoelectric generator (TEG) module includes a first (hot) side, a second (cold) side, and a plurality of thermoelectric converters disposed there between (e.g., pairs of p-type and n-type legs of thermoelectric material). The TEG module may include suitable electrical interconnections between each of the thermoelectric converters, as well as electrical leads for extracting electrical energy generated by the module.
Embodiments include a thermoelectric module that includes a plurality of row couples each comprising interconnected pairs of n-type and p-type thermoelectric material legs between a first bonding area and a second bonding area, a first connector bonded to each of the first bonding areas of the plurality of row couples, and a second connector bonded to each of the second bonding areas of the plurality of row couples, wherein the first and second connectors provide mechanical support for and electrical connection between the plurality of row couples. In embodiments, the first and/or second connector may be a connector member having a patterned conductive surface that defines a circuit configuration for the module.
Further embodiments include methods of fabricating a thermoelectric module that include providing a plurality of row couples each comprising interconnected pairs of n-type and p-type thermoelectric material legs between a first bonding area and a second bonding area, bonding a first connector to each of the first bonding areas of the plurality of row couples, and bonding a second connector to each of the second bonding areas of the plurality of row couples, wherein the first and second connectors provide mechanical support for and electrical connection between the plurality of row couples.
The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate exemplary embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the features of the invention.
The various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes, and are not intended to limit the scope of the invention or the claims.
Multiple methods exist for generating electricity from heat energy. Various embodiments may include thermoelectric conversion elements. Thermoelectric conversion relies on the Seebeck effect to convert temperature differences into electricity. Thermoelectric converters operate more efficiently under greater temperature differences.
For example, solar thermoelectric generators include solar radiation absorbers that transfer solar energy to the high-temperature sides of thermoelectric converters such that a temperature differential is achieved across the thermoelectric converters that may be converted to electricity. Examples of this type of device are disclosed in U.S. Published Patent Application No. 2012/0160290, published on Jun. 28, 2012, the entire contents of which are incorporated herein by reference for all purposes. In addition to solar energy, various other heat sources may be used to provide a temperature difference across thermoelectric conversion elements, such as, for example, a hot fluid flow stream, boiler heat, automobile exhaust, industrial waste heat, etc. A heat exchanger may be used to transfer heat from the flow stream to a first side (i.e., the “hot” side) of the thermoelectric conversion elements.
In many of these systems, the temperature at the “hot” side of the thermoelectric element may be relatively high, such as 400° C. or more (e.g., ≧600° C., such as 600-800° C.). Furthermore, the temperature differential between the “hot” and “cold” sides of the elements may also be quite large, such as up to about 500° C. or more. At these temperatures and temperature differentials, thermal stress is a key challenge for thermoelectric generator reliability.
Various embodiments include thermoelectric modules and methods of fabricating thermoelectric modules. The module design of the various embodiments may provide a cost-effective, universal approach that can be used in any thermoelectric material-based system, and may be readily scaled up or down to any suitable module size or configuration.
In embodiments, the metal headers 107 may include an electrically conductive material (e.g., metal) on a first (e.g., bottom) surface of the header 107 that faces the thermoelectric legs 105A, 105B, and an electrically insulating material on a second surface of the header 107 (e.g., the top surface of the header in
In embodiments, the first and second connector members 201, 203 may comprise rigid or semi-rigid members made of one or more structural materials that provide mechanical support and electrical interconnection for the respective row couples 101-1, 101-2, . . . 101-6. The first and second connector members 201, 203 may be bonded to the bonding regions 103, 104 using any suitable technique, such as via brazing, soldering, welding, solid state diffusion, or by a suitable adhesive material. The connector members 201, 203 may provide a mechanical frame that provides structural integrity and maintains a desired spacing between the row couples 101-1, 101-2, . . . 101-6 of the thermoelectric module 100.
In an alternative embodiment, the first and/or second connector members 201, 203 can comprise multiple element members.
Each of the connector members 201, 203 may comprise an electrically conductive material 205, such as a metal, over at least a portion of the surfaces of the connector members 201, 203 that interface with the respective bonding regions 103, 104 of the row couples 101-1, 101-2, . . . 101-6. The electrically conductive material 205 may be continuous over the entire interfacing surface of the connector member 201, 203, or in some embodiments, may be patterned to include non-continuous regions of electrically conductive material 205 separated by non-conductive regions or gaps 209, as shown in
In embodiments, the connector members 201, 203 may include a first layer 207 of non-conductive material and a second layer 205 of conductive material under the first layer 207. A third layer 208 is shown over the second layer 205, and may be, for example, an electrically insulating or conductive layer. In other embodiments, layer 208 may be omitted. Portions of the second layer 205 may be removed to provide a desired pattern of conductive regions separated by non-conductive gaps 209. The connector members 201, 203 may then be bonded to the respective first and second bonding regions 103, 104 of the row couples, with the conductive second layer 205 electrically connected to the bonding regions 103, 104. The non-conductive gaps 209 may be located between select row couples 101-1, 101-2, . . . 101-6 to define a circuit pattern through the row couples 101-1, 101-2, . . . 101-6. Alternatively, a continuous conductive second layer 205 may be bonded to the bonding regions 103, 104, and portions of the second layer 205 may be removed between select row couples 101-1, 101-2, . . . 101-6 to define the circuit pattern. Any conductor (e.g., metal or metal alloy, such as copper, nickel, titanium, etc., or combinations or alloys of the same, may be used for the electrically conductive material 205 of connector members 201, 203.
In one embodiment, the connector members 201, 203 may be formed using a direct bonded copper (DBC) technique. Direct bonded copper (DBC) substrates include a ceramic tile (e.g., alumina, aluminum nitride, beryllium oxide, etc.) with a sheet of copper bonded to one or both sides by a high-temperature oxidation process (e.g., heating the copper and substrate in controlled atmosphere of nitrogen and about 30 ppm oxygen to form a copper-oxygen eutectic which bonds to both the copper layer and the oxide(s) of the substrate layer). DBC substrates are often used in power modules due to their high thermal conductivity. The copper surface layer may be patterned prior to firing and/or portions of the copper layer may be removed after firing (e.g., etched using printed circuit board technology) to form the patterned conductive layer 205 and non-conductive gaps 209. The copper surface layer may be formed into any desired pattern for electrically connecting a plurality of row couples 101-1, 101-2, . . . 101-6 in a series and/or parallel circuit configuration. The headers 107 and connectors 109 that connect the thermoelectric legs may also be formed using a DBC structure (e.g., to provide a metal/ceramic/metal sandwich structure).
In some embodiments, a DBC substrate may be formed and optionally patterned to define a circuit pattern in the copper surface layer, and the DBC substrate may then be diced into strips to form the individual connector members 201, 203. The length and width of the strips may be variable depending on the size of the module 100 and bonding regions 103, 104. As shown in
Mechanically and electrically connecting a plurality of discrete row couples 101 using connector members 201, 203 may provide a thermoelectric module 100 having an improved thermal mechanical arrangement relative to conventional designs. In a typical design for a thermoelectric module, one or both of the hot and cold sides of the module is anchored to a supporting substrate, such as a ceramic board. In such a design, thermal stress due to large temperature gradients and coefficient of thermal expansion (CTE) mismatches between the materials can damage or even break the module especially at high temperature. In the device of
The thermoelectric modules 100 of the various embodiments may also provide a relatively high filling factor, which is defined as the ratio of the total cross-sectional area of all thermoelectric material legs 105A, 105B in the module 100 to the total area of the module 100 through a cross-section of the module 100 intersecting each of the legs 105A, 105B (e.g., cross-section parallel to the plane of headers 107). In various embodiments, the filling factor of the module 100 may be 80% or more, such as 85-100% (e.g., ≧90%, such as 95-100%).
As discussed above, the interfacing conductive surface layer 205 of the connector members 201, 203 may be formed into any desired pattern for electrically connecting the plurality of row couples 101-1, 101-2, . . . 101-6 in a series and/or parallel circuit configuration. Various examples of circuit configurations for a thermoelectric module 100 are schematically illustrated in
Although the embodiments have been described in terms of a thermoelectric generator module 100 for converting temperature differences into electricity using the Seebeck effect, the various embodiments may also be used for a heat pumping device that uses the Peltier effect to convert electrical power to a temperature differential across a hot side and a cold side of the module 100.
In various embodiments, the thermoelectric material legs 105A, 105B may be made from a variety of bulk materials and/or nanostructures. The thermoelectric materials can comprise, but are not limited to, one of: half-Heuslers, Bi2Te3, Bi2Te3-xSex (n-type)/BixSe2-xTe3 (p-type), SiGe (e.g., Si80Ge20), PbTe, skutterudites, Zn3Sb4, AgPbmSbTe2+m, Bi2Te3/Sb2Te3 quantum dot superlattices (QDSLs), PbTe/PbSeTe QDSLs, PbAgTe, and combinations thereof. The materials may comprise compacted nanoparticles or nanoparticles embedded in a bulk matrix material. For example, see U.S. patent application Ser. No. 11/949,353 filed Dec. 3, 2007, which is incorporated herein by reference for all purposes, for a description of exemplary materials.
In preferred embodiments, the thermoelectric elements comprise half-Heusler materials. Suitable half-Heusler materials and methods of fabricating half-Heusler thermoelectric elements are described in U.S. patent application Ser. No. 13/330,216 filed Dec. 19, 2011 and Ser. No. 13/719,96 filed Dec. 19, 2012, the entire contents of both of which are incorporated herein by reference for all purposes. Half-Heuslers (HHs) are intermetallic compounds which have great potential as high temperature thermoelectric materials for power generation. HHs are complex compounds: MCoSb (p-type) and MNiSn (n-type), where M can be Ti or Zr or Hf or combination of two or three of the elements. Sn and Sb can be substituted by Sn/Sb; Co and Ni by Ir and Pd. They form in cubic crystal structure with a F4/3m (No. 216) space group. These phases are semiconductors with 18 valence electron count (VEC) per unit cell and a narrow energy gap. The Fermi level is slightly above the top of the valence band. The HH phases have a fairly decent Seebeck coefficient with moderate electrical conductivity. The performance of thermoelectric materials depends on ZT, defined by ZT=(S2σ/κ)T, where σ is the electrical conductivity, S the Seebeck coefficient, κ the thermal conductivity, and T the absolute temperature. Half-Heusler compounds may be good thermoelectric materials due to their high power factor (S2σ).
The dimensionless thermoelectric figure-of-merit (ZT) of conventional HHs is lower than that of many other state-of-the-art thermoelectric materials. Recently, enhancements in the dimensionless thermoelectric figure-of-merit (ZT) of n-type half-Heusler materials using a nanocomposite approach has been achieved. A peak ZT of 1.0 was achieved at 600-700° C., which is about 25% higher than the previously reported highest value. The materials may be made by ball milling ingots of composition Hf0.75Zr0.25NiSn0.99Sb0.1 into nanopowders and hot pressing (e.g., DC hot pressing or without the application of current) the powders into dense bulk samples. The ingots may be formed by arc melting the constituent elements. The ZT enhancement mainly comes from reduction of thermal conductivity due to increased phonon scattering at grain boundaries and crystal defects, and optimization of antimony doping.
By using a nanocomposite half-Heusler material, a greater than 35% ZT improvement from 0.5 to 0.8 in p-type half-Heusler compounds at temperatures above 400° C. has been achieved. Additionally, a 25% improvement in peak ZT, from 0.8 to 1.0 at temperatures above 400° C., in n-type half-Heusler compounds by the same nanocomposite approach has been achieved. The ZT enhancement is not only due to the reduction in the thermal conductivity but also an increase in the power factor. These nanostructured samples may be prepared, for example, by hot pressing a ball milled nanopowder from ingots which are initially made by an arc melting process. The hot pressed, dense bulk samples may be nanostructured with grains having a mean grain size less than 300 nm in which at least 90% of the grains are less than 500 nm in size. In some cases, the grains have a mean size in a range of 10-300 nm, such as a mean size of around 200 nm. Typically, the grains have random orientations. Further, many grains may include 10-50 nm size (e.g., diameter or width) nanodot inclusions within the grains.
Embodiments of the half-Heusler materials may include varying amounts of Hf, Zr, Ti, Co, Ni, Sb, Sn depending on whether the material is n-type or p-type. Other alloying elements such as Pb may also be added. Example p-type materials include, but are not limited to, Co containing and Sb rich/Sn poor Hf0.5Zr0.5CoSb0.8Sn0.2, Hf0.3Zr0.7CoSb0.7Sn0.3, Hf0.5Zr0.5CoSb0.8Sn0.2+1% Pb, Hf0.5Ti0.5CoSb0.8Sn0.2, and Hf0.5Ti0.5CoSb0.6Sn0.4. Example n-type materials include, but are not limited to, Ni containing and Sn rich/Sb poor Hf0.75Zr0.25NiSn0.975Sb0.025, Hf0.25Zr0.25Ti0.5NiSn0.994Sb0.006, Hf0.25Zr0.25NiSn0.99Sb0.01(Ti0.30Hf0.35Zr0.35)Ni(Sn0.994Sb0.006), Hf0.25Zr0.25Ti0.5NiSn0.99Sb0.01, Hf0.5Zr0.25Ti0.25NiSn0.99Sb0.01 and (Hf,Zr)0.5Ti0.5NiSn0.998Sb0.002.
The ingot may be made by arc melting individual elements of the thermoelectric material in the appropriate ratio to form the desired thermoelectric material. Preferably, the individual elements are 99.9% pure. More preferably, the individual elements are 99.99% pure. In some cases, two or more of the individual elements may first be combined into an alloy or compound and the alloy or compound used as one of the starting materials in the arc melting process. Ball milling may result in a nanopowder with nanometer size particles that have a mean size less than 100 nm in which at least 90% of the particles are less than 250 nm in size. In one example, the nanometer size particles have a mean particle size in a range of 5-100 nm.
It has been discovered that the figure of merit of thermoelectric materials improves as the grain size in the thermoelectric material decreases. In one example of a method for fabricating thermoelectric materials, thermoelectric materials with nanometer scale (less than 1 micron) grains are produced, i.e., 95%, such as 100% of the grains have a grain size less than 1 micron. Preferably, the nanometer scale mean grain size is in a range of 10-300 nm. This method may be used to fabricate any thermoelectric material and includes making half-Heusler materials with nanometer scale grains. The method may be used to make both p-type and n-type half-Heusler materials. In one example, the half-Heusler material is n-type and has the formula Hf1+δ−x−yZrxTiyNiSn1+δ−zSbz, where 0≦x≦1.0, 0≦y≦1.0, 0≦z≦1.0, and −0.1≦δ≦0.1 (to allow for slightly non-stoichiometric material), such as Hf1-x-yZrxTiyNiSn1-zSbz, where 0≦x≦1.0, 0≦y≦1.0, and 0≦z≦1.0 when δ=0 (i.e., for the stoichiometric material). In another example, the half-Heusler is a p-type material and has the formula Hf1+δ−x−yZrxTiyCoSb1+ε−zSnz, where 0≦x≦1.0, 0≦y≦1.0, 0≦z≦1.0, and −0.1≦δ≦0 (to allow for slightly non-stoichiometric material), such as Hf1-x-yZrxTiyCoSb1-zSnz, where 0≦x≦1.0, 0≦y≦1.0, and 0≦z≦1.0 when δ=0 (i.e., for the stoichiometric material).
The preceding description of the disclosed aspects is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects without departing from the scope of the invention. Thus, the present invention is not intended to be limited to the aspects shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
This application claims the benefit of priority to U.S. Provisional Application No. 61/833,169, filed on Jun. 10, 2013, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61833169 | Jun 2013 | US |
