THERMOELECTRIC MATERIALS

Abstract

Disclosed is an article having: a porous thermally insulating material, an electrically conductive coating on the thermally insulating material, and a thermoelectric coating on the electrically conductive coating. Also disclosed is a method of forming an article by: providing a porous thermally insulating material, coating an electrically conductive coating on the thermally insulating material, and coating a thermoelectric coating on the electrically conductive coating. The articles may be useful in thermoelectric devices.

Claims

1. A thermoelectric article comprising: a thermally insulating material have a porous foam structure; a conformal electrically conductive coating directly on the surface of the thermally insulating material; and a conformal thermoelectric coating directly on the electrically conductive coating.

2. The article of claim 1, wherein the article contains at least 80% by mass of the thermally insulating material.

3. The article of claim 1, wherein the thermally insulating material has a thermal conductivity of at most 0.1 W m.sup.?1 K.sup.?1.

4. The article of claim 1, wherein the thermally insulating material comprises polyurethane or polystyrene.

5. The article of claim 1, wherein the electrically conductive coating comprises a material having a bulk electrical conductivity of at least 50 S cm.sup.?1.

6. The article of claim 1, wherein the electrically conductive coating comprises ruthenia.

7. The article of claim 1, wherein the electrically conductive coating is at most 100 nm thick.

8. The article of claim 1, wherein the thermoelectric coating comprises a material having a bulk Seebeck coefficient absolute value of at least 50 ?V K.sup.?1.

9. The article of claim 1, wherein the thermoelectric coating comprises one or more of tellurium, tellurium oxide, and a bismuth telluride.

10. The article of claim 1, wherein the article has a figure of merit of at least 1 at room temperature.

11. A thermoelectric element comprising: the article of claim 1; and two electrodes in contact with separated portions of the article.

12. A method comprising: placing a portion of the thermoelectric element of claim 11 in an area or on a surface to be cooled; and applying a voltage between the electrodes.

13. A method of forming a thermoelectric article comprising: providing a thermally insulating material have a porous foam structure; coating a conformal electrically conductive coating directly on the surface of the thermally insulating material; and coating a conformal thermoelectric coating directly on the electrically conductive coating.

14. The method of claim 13, wherein the thermally insulating material comprises polyurethane or polystyrene.

15. The method of claim 13, wherein the electrically conductive coating comprises ruthenia.

16. The method of claim 13, wherein the thermoelectric coating comprises one or more of tellurium, tellurium oxide, and a bismuth telluride.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] A more complete appreciation of the invention will be readily obtained by reference to the following Description of the Example Embodiments and the accompanying drawings.

[0012] FIG. 1 shows a schematic depiction of a thermoelectric article.

[0013] FIG. 2 shows fused silica (SiO.sub.2) fiber paper (a.k.a. membrane) before (left) and after deposition of RuO.sub.2 (right).

[0014] FIG. 3 shows thermopower measured for RuO.sub.2(SiO.sub.2) before (lower points) and after (higher points) electrodeposition of a Bi.sub.xTe.sub.y compound.

[0015] FIG. 4 shows thermal diffusivity of SiO.sub.2 paper, RuO.sub.2(SiO.sub.2) paper, and known thermal conductors and insulators. Thermal diffusivity differences between the standards were significant and the series trend matches the trend of thermal conductivity of the measured materials. From high-to-low thermal conductivity: Cu?Al>Pb>stainless steel>>Kapton?Teflon>SiO.sub.2 paper=RuO.sub.2(SiO.sub.2) paper.

[0016] FIG. 5 shows thermopower measured for RuO.sub.2(SiO.sub.2) before (upper curve) and after (middle curve) electrodeposition of the semi-metal Te (after prolonged exposure to air) and after (lower curve) partial reduction of the TeO.sub.x by exposure to a reductant (vapor-phase methanol).

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0017] In the following description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of the present disclosure. However, it will be apparent to one skilled in the art that the present subject matter may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and devices are omitted so as to not obscure the present disclosure with unnecessary detail.

[0018] Objects with enhanced thermoelectric properties are described in which two sequential modification steps serve to decouple the three key, normally interdependent, parameters that dictate performance metrics for thermoelectric devices, namely: electronic conductivity (?.sub.e), thermal conductivity (?), and Seebeck coefficient (S). These three parameters are decoupled by first conformally depositing nanometric films (a.k.a. shells or skins or sheets) of an electron conductor such as RuO.sub.2 onto a massive thermal insulator (mass ratio of insulator to RuO.sub.2 may be ??10.sup.2); this step creates an object in which the electronic conductivity of the object, which can only arise in the nanometric skin or shell or sheet, is inherently decoupled from the thermal conductivity, which is dominated by the thermal mass of the object, which arises from the insulator (e.g., SiO.sub.2), not the RuO.sub.2 coating. In the second step, the poor Seebeck coefficient of the coated object (for RuO.sub.2(SiO.sub.2) on the order of 10? ?V K.sup.?1) is modified by depositing a thin thermoelectric material over the electron conductor. This second material imposes its Seebeck character onto the Seebeck properties of the object, which retains the thermal mass of the thermally insulating substrate (e.g., SiO.sub.2) and the electronic wiring of the inner core of the nanoscale conductive coating.

[0019] FIG. 1 shows a schematic depiction of the scheme to decouple the dependency of electronic conductivity, thermal conductivity, and Seebeck coefficient for a thermoelectric material by creating a macroscale object in which a nanoscale electron wire (?2-nm-thick film or shell or skin of RuO.sub.2) moves electrons but not heat, while the substrate onto which it is electrolessly deposited in a conformal, self-limiting fashion is a thermal insulator thereby inherently decoupling the ratio of electronic/thermal conductivity of the object. The poor Seebeck coefficient of RuO.sub.2 (?3-4 ?V K.sup.?1 for bulk rutile RuO.sub.2; 0.5 ?V K.sup.?1 for the particular specimen of RuO.sub.2(SiO.sub.2) shown in FIG. 3) is overwritten for the object by a conformal, thin coating of a good thermoelectric material (e.g., a semi-metal such as Bi or Te or a compound such as Bi.sub.xTe.sub.y). The micrographs (FIG. 1) show the morphology of the RuO.sub.2 nanoskin on the silica fibers before (left micrograph) and after (right micrograph) electrodepositing a thin film of Bi.sub.xTe.sub.y on the RuO.sub.2-coated SiO.sub.2 fibers.

[0020] Ultrathin (2-10 nm), conformal RuO.sub.2 films exhibit high electronic conductivity (?1000 S cm.sup.?1 for planar nanosheets) and produce conductive SiO.sub.2 papers with practical object conductivity of ?0.5 S cm.sup.?1. The base object (an electronically conductive and thermally insulating paper) is produced with a simple and versatile chemical method capable of yielding uniform thickness coatings on simple and complex dielectric and insulating substrates. One aspect of the material relates to its inherent electrical conductivity as a result of the metallic properties of RuO.sub.2 as well as the conduction properties of this particular type of RuO.sub.2 produced by subambient thermal decomposition of ruthenium tetroxide following by heating. This coating has an optical transparency that spans the entire infrared region. In addition, the material can be functionalized using standard methods for oxide surfaces, thus permitting deposition of a thin, conformal coating of a material with a high Seebeck coefficient. The TE-modified base object retains the character of the high Seebeck coefficient material rather than the poor Seebeck coefficient measured for RuO.sub.2(SiO.sub.2) that is defined by the RuO.sub.2.

[0021] Thermoelectric materials with a figure of merit>2 would allow development of shipboard cooling systems as replacements for current CFC-based systems. Not only are thermoelectric cooling systems environmentally friendly, they are more suitable for a decentralized shipboard cooling plant, particularly on submarines. Thermoelectric coolers have no moving parts and hence have longer duty cycles.

[0022] A solution-based deposition method used to prepare the RuO.sub.2 nanoscale films self-limits the coating at ?2 nm on nanowires, nanoscale networks, and fibers and is a simple, economical, atom-efficient, benchtop protocol that does not rely on synthesizing elaborate nanoscale inclusions in bulk material or using chemical vapor deposition, atomic-layer deposition, or vacuum-deposition methods. The approach provides conducting nanometric films that conformally coat a wide range of substrates ranging from planar to complex 3-D morphologies. A practical advantage is that the RuO.sub.2 coating can be chemically or electrochemically modified using established techniques for metal oxide functionalization, including atomic-layer deposition, to deposit a material with a high Seebeck coefficient. The method and material provide new opportunities for thermoelectric devices without some of the limitations of homogeneous materials or bulk composites.

[0023] A key emphasis is to decouple the phononic component of thermal conductivity from the electronic component by preparing an established metallic conducting oxide as an ultrathin layer. Arranging functional matter in this fashion provides a means to decouple two transport properties critical to realizing high-performance thermoelectric devices, namely electronic conductivity and thermal conductivity. The thermally/electronically decoupled object with its too-low Seebeck coefficient is then modified by depositing a conformal thin film of a known thermoelectric material. The new compositeTE shell//electron shell//thermally insulating core (see FIG. 1)adopts a Seebeck coefficient in keeping with the outmost shell of material, the thermoelectric material, thus further decoupling the interdependency of the variables in ZT, the thermoelectric figure of merit.

[0024] A chemically synthesized RuO.sub.2 film as the first nanoscale shell has numerous potential advantages in terms of its properties, synthesis, and post-synthetic modification. Ruthenium dioxide is a well-established and technologically important electronic and electrocatalytic material with structure-dependant properties that can be tuned for applications such as electrolysis, electrocatalysis, electrochemical energy storage, and thick and thin film resistors (Adams et al., J. Phys. Chem. B 107 (2003) 6668-6697). High electronic conductivity occurs for single crystal and polycrystalline RuO.sub.2, whereas electrocatalytic behavior is most common in defective forms of ruthenia, which are generally hydrous and contain varying amounts of structural disorder at both the surface and within the bulk (RuO.sub.2.xH.sub.2O or RuO.sub.xH.sub.y). Surface-disordered RuO.sub.2 is used extensively in the chloralkali industry to electrocatalyze production of chlorine gas from brine (Kuhn et al., J. Electrochem. Soc. 120 (1973) 231-234), and has received considerable attention as an electrode material that exhibits high specific capacitance (Trasatti et al., J. Electroanal. Chem. 29 (1971) Al), particularly in its bulk disordered form (Zheng et al., J. Electrochem. Soc. 142 (1995) 2699-2703). When disordered, this versatile oxide is also a high-performance Li-ion insertion material with high specific capacity and columbic efficiency as compared to conventional electrodes (Balaya et al., Adv. Funct. Mater. 13 (2003) 621-625; Lytle et al., J. Mater. Chem. 17 (2007) 1292-1299) and when combined with nanoscopic Pt is 250-times more effective as a fuel-cell catalyst/electrode for direct oxidation of methanol than Pt.sub.?Ru.sub.? alloy (Long et al., J. Phys. Chem. B 104 (2000) 9772-9776). The use of RuO.sub.2 for less commodity-intensive processes than chloralkali is limited by the high-cost of ruthenium precursors. A current strategy for alleviating this cost-limitation is to disperse high-surface-area, nanoscale forms of RuO.sub.2 on inexpensive substrates in order to optimize the number of catalytically active sites while minimizing the weight loading (Ryan et al., Nature 406 (2000) 169-172; Chervin et al., Nano Lett. 9 (2009) 2316-2321; Chervin et al., J. Electroanal. Chem. 644 (2010) 155-163; Hu et al., Nano Lett. 6 (2006) 2690-2695; Hu et al., J. Electrochem. Soc. 151 (2004) A281-A290; Kim et al., Electrochem. Solid-State Lett. 8 (2005) A369-A372).

[0025] One of the advantages of electrochemistry is the ability to deposit films on non-planar, three-dimensional morphologies without line-of-sight control. Other than electrophoretic deposition, these types of depositions require a substrate with sufficient electronic conductivity to act as a low-resistance electrode. The conductivity of the RuO.sub.2(SiO.sub.2) paper is sufficient for it to serve as the working electrode on which to electrodeposit a conformal, thin film on top of the RuO.sub.2 nanoshell. The concept of imposing a better Seebeck coefficient onto the poor S of the RuO.sub.2(SiO.sub.2) object was demonstrated by using literature protocols to electrodeposit the semi-metal tellurium and compounds from the Bi.sub.xTe.sub.y family of chalcogenides. Recent work (Takahashi et al., Thin Solid Films 240 (1994) 70-72; Magri et al., J. Mater. Chem. 6 (1996) 773-779) described electrodeposition of films of bismuth tellurium alloys from aqueous acidic electrolyte containing TeO.sub.2 and Bi(NO.sub.3).sub.3. The X-ray diffraction (XRD) characterization of the coatings indicated that based on the electrochemical conditions either polycrystalline Bi.sub.2?xTe.sub.3+x or a mixture of Bi.sub.2Te.sub.3, a solid solution of Bi.sub.2+xTe.sub.3?x, and Te metal were formed (Magri et al., J. Mater. Chem. 6 (1996) 773-779). The Magri group showed that their Te-rich Bi.sub.2Te.sub.3 coatings had both a lower film resistivity and higher carrier concentration than single crystal Bi.sub.2Te.sub.3, which they attributed to high grain-to-grain connectivity in the polycrystalline material.

[0026] Magri et al. also reported galvanostatic (constant current) deposition rates of polycrystalline Bi.sub.2?xTe.sub.3+x films on planar stainless-steel electrodes at rates of 18.5 ?m h.sup.?1 (5 nm s.sup.?1) at 25? C. To form thin films that are conformal, three approaches can be used to control the rate of deposition: (i) electrodeposition at temperatures less than 25? C.; (ii) pulsed deposition (at constant current) using sub-second pulse trains for times that total 1-2 seconds; and (iii) potentiostatic control of the electrodeposition at potentials or constant applied current where reduction of the precursor ions (Bi.sup.3+ and HTeO.sub.2.sup.+) to metallic Bi and Te is kinetically slow. These variations on the literature electrodeposition procedures were tested and optimized by first using stainless-steel flag electrodes before attempting electrodeposition on RuO.sub.2(SiO.sub.2) paper. Structural analysis by XRD verified that solid solutions of Bi.sub.2+xTe.sub.3?x were electrodeposited. Energy-dispersive spectroscopy verified composition of Te metal films.

[0027] The article described above comprises three components: a porous thermally insulating material, an electrically conductive coating on the thermally insulating material, and a thermoelectric coating on the electrically conductive coating. As much as 80% or more of the mass of the article may be the thermally insulating material, and may be 90%, 95%, 99% or more. A higher percentage of thermally insulating material allows the thermal conductivity of the thermally insulating material to dominate the thermal conductivity of the article as a whole.

[0028] The thermally insulating material, or the bulk material of the same composition, has a thermal conductivity of at most 0.1 W m.sup.?1 K.sup.?1. A lower thermal conductivity may be more desirable to improve the thermal insulating properties of the article. For example, the thermal conductivity of a SiO.sub.2 fiber paper is about 0.025 W m.sup.?1 K.sup.?1.

[0029] Any porous thermally insulating material may be used. It may be macroscopic and large enough to manually handle, as opposed to microscopic. For example, the material may be at least 1 cm long in two of its dimensions. The thermally insulating material may be in the form of a structural scaffold supporting the electrically conductive coating and the thermoelectric coating. Scaffold forms include, but are not limited to, a foam, a particulate network, a fibrous structure, and a bonded polymer. A foam may have an open pore structure with connected pores in order to have continuous electrical conductivity in the conductive coating. Suitable materials include, but are not limited to, silica, polyurethane foam, cotton, polystyrene foam, and wool. The thermally insulating material may be amorphous or crystalline. When a polymer is used, it may be a block copolymer that may form two phases.

[0030] Silica (SiO.sub.2) is one suitable material, including silica fiber and paper or membranes made therefrom as disclosed in US Patent Application Publication No. 2009/0092834. The membrane includes a plurality of SiO.sub.2 fibers entangled to form a sheet. SiO.sub.2 fiber membranes may be macroporous (pores sized>50 nm) with a relatively low surface area (on the order of <1 m.sup.2 g.sup.?1) and may be composed of submicron-to-micron-sized SiO.sub.2 fibers. SiO.sub.2 fiber membranes are flexible and can be easily molded to form quality electronic contacts with uneven surfaces. The SiO.sub.2 fiber paper also has a degree of compressibility (softness) that helps in forming electronic contacts with other materials through pressure. Materials with higher surface areas may be used, though this can result in a larger amount of the electronically conductive coating in the article. As many electron conductors are also thermal conductors, a high surface area may reduce the thermal insulating properties of the article. It can be determined whether a particular material has an appropriate surface area for a particular application by forming the electrically conductive coating and measuring the resulting electrical conductivity of the article.

[0031] The electrically conductive coating may comprise any material considered to have metal-like conductivity. The conductive material may have a bulk electrical conductivity of at least 50 S cm.sup.?1, including at least 500 S cm.sup.?1 and at least 1000 S cm.sup.?1. The conductivity of the article, either before or after the addition of the thermoelectric coating, may be less than the conductivity of the bulk electrically conductive material. The electrically conductive coating may be created by any method that produces a thin coating. Ruthenia (RuO.sub.2) is a suitable conductor, which may be deposited on the insulator as disclosed herein and in US Patent Application Publication Nos. 2009/0092834 and 2011/0091723 and in U.S. Pat. Nos. 6,290,880 and 6,649,091. Ruthenia in particular maintains a high conductivity when deposited in a thin layer. Other suitable conductive materials include, but are not limited to, metals, graphite, and graphene. The electrically conductive coating may be at most 100 nm, 50 nm, 20 nm, 10 nm, 5 nm, 3 nm, or 2 nm thick and may be up to 1%, 5%, or 10% of the mass of the article.

[0032] The thermoelectric coating may comprise any thermoelectric material. The thermoelectric material may have a bulk Seebeck coefficient absolute value of at least 50 ?V K.sup.?1, including at least 100 ?V K.sup.?1. The Seebeck coefficient of the article may differ from that of the bulk thermoelectric material. Suitable thermoelectric materials include, but are not limited to, tellurium metal, tellurium oxide (TeO.sub.x), a bismuth telluride (Bi.sub.2Te.sub.3, Bi.sub.2+xTe.sub.3?x), uranium dioxide, a perovskite, constantan, ytterbium trialuminide, and a dirty metal. Dirty metals are known thermoelectric materials that become electronically insulating at temperatures below the cryogenic regime (<77 K) and metallic at warmer temperatures. The thermoelectric coating may be created by any method that produces a thin coating including, but not limited to, electrodeposition. The thermoelectric coating may be at most 100 nm, 50 nm, 20 nm, 10 nm, 5 nm, 3 nm, or 2 nm thick and may be up to 1%, 5%, or 10% of the mass of the article.

[0033] In some embodiments, the electrically conductive material may be the same composition as the thermoelectric material, but treated to produce different properties. For example, the conductive coating may be heated to increase its conductivity, followed by coating the same material as the thermoelectric coating, but not heated to preserve its Seebeck coefficient.

[0034] The article may have a dimensionless figure of merit, defined above as the product of electric conductivity of the article, the square of the Seebeck coefficient of the article, and the operating temperature (such as ambient), divided by the thermal conductivity of the article, of at least 1 or at least 2 at room temperature.

[0035] A thermoelectric element may be made by placing two electrodes in contact with separated portions of the article, as schematically illustrated in FIG. 3. Such a thermoelectric element may generate a thermopower having an absolute value of at least 10 ?V K.sup.?1, including 20 ?V K.sup.?1 and 50 ?V K.sup.?1. The electrodes may be placed on two extreme ends of the article. A surface may be cooled by contacting a portion of the article on the surface and applying a voltage of appropriate polarity to the electrodes. The portion of the article contacting the surface may be immediately opposed to an electrode. Alternatively, a portion of the article near one electrode may be heated or cooled relative to the other electrode to generate a voltage between the electrodes.

[0036] The following examples are given to illustrate specific applications. These specific examples are not intended to limit the scope of the disclosure in this application.

EXAMPLE

[0037] Synthesis and Characterization of RuO.sub.2 Films

[0038] Ruthenium oxide coatings were prepared via the decomposition of RuO.sub.4 from organic solutions. To prepare the precursor solution, multiple aliquots of petroleum ether, pre-chilled for 1 min in a dry ice/acetone bath, were used to extract RuO.sub.4 from an aqueous solution pre-chilled to T<5? C. (0.5 wt % solution, Strem Chemicals). After phase transfer, each aliquot of the nonaqueous precursor solution was rapidly mixed with a few milligrams of MgSO.sub.4 (or other desiccant in order to remove water), passed through a coarse filter, and collected in a flask held in a dry ice/acetone bath. The nonaqueous solution (from the combined aliquots) was then thermally equilibrated in an aqueous ice bath and decanted into a pre-chilled (aqueous ice bath) glass vial containing the substrate of interest immersed in petroleum ether. The capped vial was then removed from the bath and held at room temperature overnight (?15 h), after which a faint brown/black coating was observed. The RuO.sub.2-modified substrates were then rinsed with several aliquots of petroleum ether while sonicating followed by drying for several hours in air.

[0039] The resulting as-deposited RuO.sub.2 coating had modest conductivity. The conductivity could be increased by several orders of magnitude by heating in air or 02 to temperatures between ?150 and 250? C. The thickness of the RuO.sub.2 coatings is estimated to be similar to those previously produced on silica paper (at ?2-3 nm) or planar substrates (at ?10 nm). Additional layers of RuO.sub.2 can be deposited by subsequent deposition steps, with each layer adding another ?2-3 to 10 nm of RuO.sub.2, increasing the conductivity of the coating.

[0040] Electrodeposition of Thermoelectric Materials on RuO.sub.2(SiO.sub.2) Paper

[0041] Thin Bi.sub.xTe.sub.y coatings were electrodeposited at the conductive surfaces of RuO.sub.2(SiO.sub.2) papers, using adaptations of previously published protocols (Martin-Gonzalez et al., J. Electrochem. Soc. 149 (2002) C546-0554). Although Bi.sub.xTe.sub.y electrodeposition is seemingly simple, the resulting film composition, elemental stoichiometry, and orientation depends strongly on the solution composition and potential at which deposition occurs. The typical electrodeposition bath contained 1-5 mM of both Bi(NO.sub.3).sub.3 and TeO.sub.2 in a solution of 1 M HNO.sub.3. Both constant-potential (0 V to ?0.2 V vs. saturated calomel reference electrode) and constant-current (1-10 mA cm.sup.?2) methods were used in conjunction with a three-electrode cell configuration, with typical deposition times of 30 min to 2 h. The resulting Bi.sub.xTe.sub.y-coated RuO.sub.2(SiO.sub.2) papers were characterized by scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction. The electrodeposited films could be poised from Bi.sub.xTe.sub.y to Te as a function of potential.

[0042] Thermal Properties of RuO.sub.2 Films

[0043] The thermal diffusivity of ultrathin RuO.sub.2 films on SiO.sub.2 papers was measured on strips of the paper long enough for a separation of at least 1 cm between hot and cold ends. When characterizing a single material with relatively high thermopower, compared to metals such as Cu or Au, it is possible to determine the Seebeck coefficient of that material by correcting for the minor contribution due to the metallic contacts. Thermopower was measured with Cu contacts to the TE-coated RuO.sub.2(SiO.sub.2) paper using the set-up shown schematically in FIG. 3 (or minor variations thereon).

[0044] Obviously, many modifications and variations are possible in light of the above teachings. It is therefore to be understood that the claimed subject matter may be practiced otherwise than as specifically described. Any reference to claim elements in the singular, e.g., using the articles a, an, the, or said is not construed as limiting the element to the singular.