WO2017136793A1

WO2017136793A1 - Electrode structure for magnesium silicide-based bulk materials to prevent elemental migration for long term reliability

Info

Publication number: WO2017136793A1
Application number: PCT/US2017/016604
Authority: WO
Inventors: Lindsay MILLER; John REIFENBERG
Original assignee: Alphabet Energy, Inc.
Priority date: 2016-02-05
Filing date: 2017-02-03
Publication date: 2017-08-10

Abstract

Thermoelectric elements comprising a thermoelectric substrate and an electrode structure are disclosed.

Description

TITLE

ELECTRODE STRUCTURE FOR MAGNESIUM SILICIDE-BASED BULK MATERIALS TO PREVENT ELEMENTAL MIGRATION FOR LONG TERM

RELIABILITY

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit under 35 U.S.C. §1 19(e) of the earlier filing date of United States Provisional Patent Application No. 62/292,1 13, entitled METALLIZATION STRUCTURE TO PREVENT DIFFUSION FOR MAGNESIUM SILICIDE-BASED BULK MATERIALS FOR LONG TERM RELIABILITY, filed on February 5, 2016, the entire disclosure of which is hereby incorporated by reference. BACKGROUND

[0002] Thermoelectric devices can convert heat energy into electrical energy. A thermoelectric device can comprise a hot junction, or hot side, a cold junction, or cold side, and one or more thermoelectric elements positioned between the hot junction and the cold junction. Oftentimes, the hot junction and the cold junction each comprise a plate, for example, positioned against and/or bonded to the opposite sides of the thermoelectric elements. The thermoelectric elements are comprised of thermoelectric materials, such as semiconductors, for example. When such thermoelectric devices are subjected to a temperature differential between their hot junction and cold junction, they can generate a voltage potential which is utilizable for any suitable purpose. Such thermoelectric devices are often referred to as Seebeck devices. Some thermoelectric devices can convert electrical energy to heat energy. When such thermoelectric devices are subjected to a voltage potential, they can generate a temperature differential between a first junction and a second junction. Such thermoelectric devices are often referred to as Peltier devices. In either event, the ability of a thermoelectric material to convert heat into electricity and vice versa can be measured by its "thermoelectric figure of merit" ZT, where ZT is equal to TS²O/K and where T is the temperature, S the Seebeck coefficient, a the

300₄₄2816 6 electrical conductivity, and / the thermal conductivity of the thermoelectric material utilized by the thermoelectric device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] Various features of the embodiments described herein, together with advantages thereof, may be understood in accordance with the following description taken in conjunction with the accompanying drawings:

[0004] FIG. 1 is an exploded view of a thermoelectric system in accordance with at least one embodiment;

[0005] FIG. 2 is a plan view of a thermoelectric sub-assembly of the thermoelectric system of FIG. 1 illustrated with some components removed for the purpose of illustration;

[0006] FIG. 3 is a schematic of a thermoelectric element in accordance with at least one embodiment;

[0007] FIG. 4 comprises an array of panels including pictures, or micrographs, of a thermoelectric element;

[0008] FIG. 5 comprises an array of panels including pictures, or micrographs, of a thermoelectric element after it has been heated;

[0009] FIG. 6 is a picture of several thermoelectric elements including the thermoelectric element of FIG. 5;

[0010] FIG. 7 comprises an array of panels including pictures, or micrographs, of a thermoelectric element in accordance with at least one embodiment;

[0011] FIG. 8 is a picture of several thermoelectric elements including the thermoelectric element of FIG. 7;

[0012] FIG. 9 comprises an array of panels including pictures, or micrographs, of a thermoelectric element in accordance with at least one embodiment;

[0013] FIG. 10 comprises an array of panels including pictures, or micrographs, of a thermoelectric element in accordance with at least one embodiment;

[0014] FIG. 1 1 is a process flow diagram in accordance with at least one

embodiment; and

[0015] FIG. 12 is a process flow diagram in accordance with at least one

embodiment.

[0016] Corresponding reference characters indicate corresponding parts

throughout the several views. The exemplifications set out herein illustrate various embodiments of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION

[0017] Thermoelectric systems generally comprise a hot side, a cold side, and a thermoelectric assembly positioned therebetween. The hot side of the

thermoelectric system often comprises a plate facing a heat source, i.e., a hot-side plate, and, similarly, the cold side often comprises a plate facing a heat sink, i.e., a cold-side plate. In use, heat flows through the thermoelectric assembly from the hot- side plate toward the cold-side plate which, in turn, generates electrical power within the thermoelectric assembly. In various instances, a thermoelectric system can be configured to harvest thermal energy from more than one heat source and/or discharge thermal energy to more than one heat sink. Moreover, a thermoelectric system can comprise more than one thermoelectric assembly configured to convert thermal energy to electrical energy.

[0018] A thermoelectric system, or thermoelectric generating unit (TGU), 100 is illustrated in FIG. 1 . The TGU 100 comprises a first cold-side plate 1 10, a hot-side heat exchanger 120, and a second cold-side plate 130. The TGU 100 further comprises a first thermoelectric assembly 160 and a second thermoelectric assembly 170. The first thermoelectric assembly 160 is positioned intermediate the first cold-side plate 1 10 and a first side 126 of the hot-side heat exchanger 120. The second thermoelectric assembly 170 is positioned intermediate the second cold-side plate 130 and a second side 127 of the hot-side heat exchanger 120. The TGU 100 also comprises lateral sides 125 positioned intermediate the first cold-side plate 1 10 and the second cold-side plate 130. The entire disclosure of International

Publication Number WO 2016/054333, entitled THERMOELECTRIC GENERATING UNIT AND METHODS OF MAKING AND USING SAME, which published on April 7, 2016, is incorporated by reference herein.

[0019] The TGU 100 further comprises a first insulation layer 150, a second insulation layer 180, and a plurality of fasteners 1 15. The first insulation layer 150 is positioned intermediate the first thermoelectric assembly 160 and the first cold-side plate 1 10. The second insulation layer 180 is positioned intermediate the second thermoelectric assembly 170 and the second cold-side plate 130. Fasteners 1 15 are positioned within apertures which extend through the first cold-side plate 1 10, the first insulation layer 150, the first thermoelectric assembly 160, the hot-side heat exchanger 120, the second thermoelectric assembly 170, the second insulation layer 180, and the second cold-side plate 130 and can clamp these components together such that satisfactory thermal contact between these components is maintained under a variety of operating conditions.

[0020] The hot-side heat exchanger 120 comprises a plurality of discrete channels 121. Each channel 121 is configured to receive a fluid carrying waste heat such as, for example, exhaust from an engine. Each channel 121 comprises a fluidic inlet, a fluidic outlet 128, and a lumen that fluidically couples the fluidic inlet and the fluidic outlet 128. Each channel 121 is sealed from the other channels 121 and,

concurrently, sealed from the other internal portions of the TGU 100. The lumen is configured to efficiently extract heat from a fluid passing there through in the direction indicated by arrow 1 12, for example. In at least one instance, the channels 121 comprise fins disposed within and extending into the lumens defined therein. The fins can be arranged in a fin pack in the lumen and can comprise any suitable configuration, as described below.

[0021] Further to the above, any suitable arrangement, number, and density of fins within the channels 121 can be used. For instance, the density of the fins within the channels 121 can be at least 12 fins per inch, for example. In various instances, the channels 121 and/or the fins disposed therein are comprised of stainless steel, nickel plated copper, and/or stainless steel clad copper, for example. Such designs are configured to increase the contact area between the hot fluid and the sidewalls of the channels 121 which, as a result, increases the heat transfer between the hot fluid and the hot-side heat exchanger 120. Moreover, such designs are configured to disrupt the boundary layer of the fluid flowing through the channels 121 which also increases the heat transfer between the hot fluid and the hot-side heat exchanger 120.

[0022] In at least one instance, the hot-side heat exchanger 120 comprises a high efficiency hot-side heat exchanger. As used herein, a high efficiency hot-side heat exchanger is intended to mean a hot-side heat exchanger characterized by a thermal resistance of less than about 0.0015m²K/W, for example. In at least one such instance, the thermal resistance of a hot-side heat exchanger is 0.00025 m²K/W, for example. In various instances, the cold-side plates 1 10 and 130 comprise high efficiency cold-side heat exchangers. As used herein, a high efficiency cold-side heat exchanger is intended to mean a cold-side heat exchanger characterized by a thermal resistance of less than about 0.0001 m²K/W, for example.

[0023] The first cold-side plate 1 10 and the second cold-side plate 130 are flat, or at least substantially flat. As used herein, a substantially flat plate is intended to mean that the first and second major surfaces are substantially planar and parallel to one another. In at least one instance, a substantially flat plate is characterized by a flatness and planarity specification of about 0.010" or less across the major surfaces, for example. The cold-side plates 1 10 and 130 comprise a substantially flat slab of a thermally conductive material, such as a metal and/or a ceramic, for example.

Metals that are suitable for use in the first cold-side plate 1 10 and/or the second cold-side plate 130 can be selected from the group consisting of aluminum, copper, molybdenum, tungsten, copper-molybdenum alloy, stainless steel, nickel, and/or alloys of one or more of these materials, for example. Ceramics that are suitable for use in the first cold-side plate 1 10 and/or the second cold-side plate 130 can be selected from the group consisting of silicon carbide, aluminum nitride, alumina, silicon nitride and/or combinations thereof, for example. In at least one embodiment, one of the cold-side plates 1 10 and 130 is comprised of a metal and the other of the cold-side plates 1 10 and 130 is comprised of a ceramic, for example.

[0024] In various instances, further to the above, the first thermoelectric assembly 160 and the second thermoelectric assembly 170 are part of an electrical circuit of the TGU 100. The thermoelectric assemblies 160 and 170 are electrically connected in series with one another. Alternatively, the thermoelectric assemblies 160 and 170 are electrically connected in parallel with one another. In either event, the electrical circuit of the TGU 100 further comprises an electrical connector comprising at least a first electrical terminal and a second electrical terminal. In use, the thermoelectric assemblies 160 and 170 create a voltage differential between the first electrical terminal and the second electrical terminal.

[0025] The thermoelectric assembly 160, further to the above, is comprised of a plurality of sub-assemblies, or cards, wherein each sub-assembly comprises a plurality of thermoelectric elements 190 mounted thereto. Similarly, referring to FIG. 2, the thermoelectric assembly 170 is comprised of a plurality of sub-assemblies, or cards, 170' wherein each sub-assembly 170' also comprises a plurality of

thermoelectric elements 190 mounted thereto. The sub-assemblies 170' are mounted to and supported by a printed circuit board (PCB) of the thermoelectric assembly 170. The thermoelectric assembly 170 comprises 80 sub-assemblies 170', for example; however, a thermoelectric assembly can comprise any suitable number of sub-assemblies 170'. The sub-assemblies 170' of the thermoelectric assembly 170 are electrically connected in series as part of an electrical circuit extending through the thermoelectric assembly 170. That said, the sub-assemblies 170' can be electrically connected in parallel and/or in series with one other in any suitable arrangement. It should also be appreciated that a sub-assembly 170' can be used by itself, i.e., without other sub-assemblies 170'.

[0026] Further to the above, each thermoelectric sub-assembly 170' comprises a substrate and a plurality of thermoelectric elements 190 mounted to the substrate. The substrate of each sub-assembly 170' can comprise a PCB and/or any suitable dielectric material. As described in greater detail below, the substrate comprises a trace circuit and the thermoelectric elements 190 are bonded to the trace circuit. Each sub-assembly 170' comprises 48 thermoelectric elements 190 mounted thereto; however, a thermoelectric sub-assembly can comprise any suitable number of thermoelectric elements 190. The thermoelectric elements 190 mounted to a subassembly 170' are electrically connected to each other in series. That said, the thermoelectric elements 190 mounted to a thermoelectric sub-assembly can be electrically connected in parallel and/or in series with one other in any suitable arrangement.

[0027] Further to the above, the thermoelectric elements 190 of each

thermoelectric sub-assembly 170' are arranged in a rectangular array of columns and rows between the second cold-side plate 130 and the second side 127 of the hot-side heat exchanger 120. That said, any suitable arrangement can be used.

[0028] Thermoelectric elements can comprise any suitable configuration. Each thermoelectric element 190 comprises two thermoelectric legs; however, a

thermoelectric element can comprise one or more thermoelectric legs. Each thermoelectric leg comprises a thermoelectric material disposed between first and second conductive materials. A thermoelectric material can be selected from the group consisting of tetrahedrite, magnesium silicide (Mg₂Si), magnesium silicide stannide (Mg₂(SiSn)), silicon, silicon nanowire, bismuth telluride (Bi₂Te₃), a skutterudite material, lead telluride (PbTe), TAGS (tellurium-antimony-germanium- silver alloys), a zinc antimonide, silicon-germanium (SiGe), a half-Heusler alloy, and combinations thereof, for example. [0029] A thermoelectric leg can comprise a p-type thermoelectric material or a n- type thermoelectric material. A p-type thermoelectric material is comprised of at least one p-doped semiconductor material, for example. A n-type thermoelectric material is comprised of at least one n-doped semiconductor material, for example. Turning now to FIG. 2, each thermoelectric element 190 of the thermoelectric assemblies 160 and 170 comprises a n-type thermoelectric leg 194 and a p-type thermoelectric leg 196. In at least one such embodiment, the p-type thermoelectric legs 196 are larger than the n-type thermoelectric legs 194. In alternative

embodiments, the legs 196 are n-type thermoelectric legs and the legs 194 are p- type thermoelectric legs. In at least one instance, one or more of the n-type thermoelectric legs 194 are connected electrically in series and thermally in parallel with one or more of the p-type thermoelectric legs 196 so as to generate an electrical current responsive to a temperature differential across the thermoelectric assemblies 160 and 170.

[0030] Further to the above, the quantity of thermoelectric elements 190 in the first thermoelectric assembly 160 and the quantity of thermoelectric elements 190 in the second thermoelectric assembly 170 are the same. In at least one such instance, the first thermoelectric assembly 160 and the second thermoelectric assembly 170 have the same number of sub-assemblies, or cards (such as sub-assemblies 170'), wherein the sub-assemblies each have the same number of thermoelectric elements 190 mounted thereto. Moreover, the thermoelectric assemblies 160 and 170 each have an equal number of n-type legs 194 and p-type legs 196; however, other embodiments are envisioned in which the quantities of n-type legs 194 and p-type legs 196 in a thermoelectric assembly are different. The above being said, embodiments are envisioned in which the quantity of thermoelectric elements 190 in the first thermoelectric assembly 160 and the quantity of thermoelectric elements 190 in the second thermoelectric assembly 170 are different.

[0031] Further to the above, the fasteners 1 15 can extend through gaps defined between the thermoelectric elements 190 and/or gaps defined between the sub- assemblies, or cards, of the thermoelectric assemblies 160 and 170, for example. Also, further to the above, the fasteners 1 15 can be tightened to clamp the first cold- side plate 1 10, the first insulation layer 150, the first thermoelectric assembly 160, the hot-side heat exchanger 120, the second thermoelectric assembly 170, the second insulation layer 180, and the second cold-side plate 130 together such that the thermoelectric elements 190 are compressed against the hot-side heat exchanger 120 without interrupting the electrical connection between the

thermoelectric elements 190 and/or between the sub-assemblies, or cards, of the thermoelectric assemblies 160 and 170.

[0032] A thermoelectric sub-assembly 170' of the thermoelectric assembly 170 is illustrated in FIG. 2. The thermoelectric sub-assembly 170' comprises a substrate 172 and a plurality of metal pads 192 mounted to the substrate 172. The metal pads 192 comprise direct bond copper (DBC) pads, for example, which are part of the electrical circuit of the thermoelectric sub-assembly 170'. In addition to or in lieu of the DBC pads, the metal pads 192 can comprise active metal brazing (AMB) pads, for example. In addition to or in lieu of the above, glass frit pads and/or glass frit joining materials, for example, can be used to bond the thermoelectric elements to one or more of the substrates. The substrate 172 is comprised of a dielectric material and does not conduct current between the thermoelectric elements 190 and the metal pads 192. The thermoelectric legs 194 and 196 of the thermoelectric elements 190 are electrically and mechanically connected to the metal pads 192 through a bonding material.

[0033] In at least one instance, further to the above, the substrate 172 is comprised of alumina and the thermoelectric legs 194 and 196 are comprised of magnesium silicide (Mg₂Si) blocks which are soldered to the metal pads 192, for example. In at least one instance, the substrate 172 is comprised of alumina and the thermoelectric legs 194 and 196 are comprised of tetrahedrite blocks which are soldered to the metal pads 192, for example. In either event, the solder may be any suitable solder, such as lead/tin eutectic solder, lead-free solders, and/or silver solders, for example. A reflow soldering process, for example, is utilized to bond the thermoelectric legs 194 and 196 to the metal pads 192. In at least one such instance, the thermoelectric sub-assembly 170' is positioned in a reflow oven which exposes the thermoelectric sub-assembly 170' to a temperature equal to or in excess of the reflow temperature of the solder. In addition to or in lieu of a reflow oven, an infrared lamp could be used, for example. In any event, the thermoelectric sub-assembly 170' is permitted to cool and/or is actively cooled after it has been removed from the reflow oven.

[0034] Further to the above, substrates comprising metal pads, such as metal pads 192, for example, are referred to as DBC and AMB substrates owing to the joining techniques used to attach the metal pads to the substrate. DBC and AMB substrates provide a desirable mechanical and electrical attachment for the thermoelectric devices 190 but can be expensive to manufacture given current manufacturing processes. Such manufacturing processes utilize high temperature vacuum processing, chemical etching, and additional metal plating, for example. Such manufacturing processes can also limit the thickness of the metal pads 192 that can be applied to the substrate 172, the size of the metal pads 192, and/or the spacing between the metal pads 192. In various instances, the high temperature processing may lead to high levels of built in mechanical stress within the metal pads 192 and/or the substrate 172 which, ultimately, can be detrimental to the reliability and efficiency of the thermoelectric sub-assembly 170'. In such instances, the thermal resistance of the thermoelectric sub-assembly 170' can increase when cracks develop during the operation of the TGU 100, thereby causing unwanted parasitic thermal resistances therein.

[0035] Other technologies can be used to attach the thermoelectric legs 194 and 196 to the metal pads 192. In at least one instance, high temperature braze materials are used to attach the thermoelectric legs 194 and 196 to the metal pads 192. In this context, a high temperature braze material is exposed to temperatures in excess of 500°C to bond the thermoelectric materials of the legs 194, 196 to the metal pads 192. Such processes can expose the thermoelectric materials comprising the legs 194 and 196 to significant stresses owing to the higher processing temperature necessary to achieve the liquid phase of the high

temperature braze materials. Such stresses can create cracks within the

thermoelectric materials which can create electrical and/or thermal opens within the legs 194 and 196, for example. Moreover, such processes can often cause the thermoelectric materials to oxidize and/or sublimate. In addition, some materials are not stable at such high assembly temperatures.

[0036] Other methods for mechanically and electrically coupling the thermoelectric elements 190 to the substrate 172 are contemplated. For instance, individual metal foils can be brazed to the thermoelectric legs 194 and 196 in lieu of using the metal pads 192. Such individual metal foils, however, add cost to the assembly process. Moreover, such individual metal foils would require a mechanically-compliant, thermally-conductive dielectric material that is stable at temperatures up to 400°C in order to obtain efficient thermal coupling and electrical isolation between the thermoelectric elements 190 and their surroundings. It is believed that such a material is not currently available on the commercial market. In addition to or in lieu of the above, spring-loaded contacts can be mounted to the substrate 172 which are configured to engage the hot side of the thermoelectric legs 194 and 196 and electrically couple the legs 194 and 196 to the electrical circuit of the thermoelectric sub-assembly 170'. Spring-loaded contacts allow for low mechanical stress within the thermoelectric materials but currently suffer from high cost and complexity.

[0037] A thermoelectric element 300 is depicted in FIG. 3. The thermoelectric element 300 comprises a body, or substrate, 310 comprised of at least one thermoelectric material. The thermoelectric substrate 310 is comprised of a magnesium silicide-based material (Mg₂Si), although any suitable material can be used. In at least one instance, the magnesium silicide-based material is magnesium silicide stannide (Mg₂(SiSn)), for example. The magnesium silicide stannide may contain anywhere from 1 atomic percent (at%) to 99 atomic percent (at%) of magnesium stannide with the remaining bulk of the material being made up of magnesium silicide. In at least one instance, the amount of magnesium stannide is greater than 5 atomic percent (at%) with magnesium silicide making up the rest of the material. In at least one instance, the amount of magnesium stannide is greater than 50 atomic percent (at%) with magnesium silicide making up the rest of the material. The thermoelectric substrate 310 is formed by sintering a powder comprising magnesium silicide-based material, although any suitable manufacturing process can be used. In some instances, the sintering process produces a suitable shape for the thermoelectric substrate 310. In other instances, an additional shaping process, such as a slicing and/or dicing process, for example, is utilized to produce a suitable shape for the thermoelectric substrate 310. In various instances, a block of sintered material is formed which is then cut to form a plurality of substrates 310. Each thermoelectric substrate 310 comprises a rectangular, or an at least substantially rectangular, shape, although any suitable shape can be used such as square, cylindrical, and/or trapezoidal shapes, for example.

[0038] Referring to FIG. 1 1 , a method 1 100 is illustrated which comprises a step 1 1 10 of manufacturing a thermoelectric powder and a step 1 120 of sintering the thermoelectric powder into a usable shape. In at least one instance, the usable shape comprises a block of sintered thermoelectric material. The method 1 100 further comprises a step 1 130 of slicing the usable shape into a wafer shape, for example, and a step 1 140 of preparing the surfaces of the shape to be coated with an electrode in step 1 150. Steps 1 130, 1 140, and 1 150 can be performed in any suitable order. The electrode deposition step 1 150 can actually comprise one or more steps of applying one or more electrode layers to the thermoelectric material, as described in greater detail below. The method 1 100 further comprises a step 1 160 of dicing the coated thermoelectric material into one or more thermoelectric elements. In some instances, the step 1 160 can occur before and/or during the step 1 150. Thereafter, the thermoelectric elements are bonded to one or more

substrates, such as a hot-side plate and a cold-side plate, for example, during steps 1 170 and 1 180, which can occur in any suitable order, to form a thermoelectric assembly. The thermoelectric assembly can be integrated into any suitable system during a step 1 190.

[0039] Referring to FIG. 12, a method 1200 is illustrated which comprises the step 1 1 10 and, in addition, a step 1220 of sintering and metallizing the thermoelectric material. In various instances, the metallizing process is entirely completed during the sintering process while, in other instances, only portions of the metallizing process are completed during the sintering process while other portions of the metallizing process are completed after the sintering process. In at least one instance, one or more metal layers are applied to the thermoelectric material during the sintering process and one or more metal layers are applied to the thermoelectric material after the sintering process. In any event, the method 1200 further comprises a step 1230 of cutting the thermoelectric material into its final size. The step 1230 can occur during and/or after the step 1220. The method 1200 further comprises a step 1240 of bonding the thermoelectric material to a hot-side substrate and a cold- side substrate in any suitable order to create a thermoelectric assembly. The thermoelectric assembly can be integrated into any suitable system during a step 1250.

[0040] The thermoelectric element 300 further comprises an electrode structure. Although an electrode structure with three layers is used in this description to illustrate the three functions that are provided by an electrode structure, the electrode structure could just as easily be comprised of one, two, three, or more layers. In one embodiment, the diffusion barrier also makes good contact to the thermoelectric element and also creates a good bond to the substrate, in which case only the diffusion barrier layer is needed. In another embodiment, the diffusion barrier layer can make good contact to the thermoelectric element but still requires a bonding layer. In another embodiment, the diffusion barrier layer makes a good bond to the substrate but not to the thermoelectric element, so a contact layer is still required. The electrode structure of thermoelectric element 300 includes a contact layer 320, a diffusion barrier layer 330, and a bonding layer 340. As described in greater detail below, the contact layer 320 is positioned intermediate the

thermoelectric substrate 310 and the diffusion barrier layer 330. As also described in greater detail below, the diffusion barrier layer 330 is positioned intermediate the contact layer 320 and the bonding layer 340. In various embodiments, the electrode structure can comprise additional layers or fewer layers while, in other embodiments, the electrode structure does not comprise any layers in addition to the contact layer 320, the diffusion barrier layer 330, and the bonding layer 340.

[0041] The contact layer 320 is comprised of nickel, although any suitable material can be used. In various instances, the contact layer 320 comprises a transition metal such as titanium, iron, cobalt, and/or chromium, for example. In certain embodiments, the contact layer 320 only comprises nickel. In some embodiments, the contact layer 320 does not comprise nickel. In any event, the contact layer 320 is applied to the thermoelectric substrate 310 utilizing a sputtering process, although any suitable process can be used. For instance, the contact layer 320 can be applied to the thermoelectric substrate 310 utilizing any suitable thin film deposition process, for example. Further to the above, the contact layer 320 is applied to the block of sintered material before the thermoelectric substrate 310 is cut from the block. That said, the contact layer 320 could be applied to the thermoelectric substrate 310 after the thermoelectric substrate 310 has been cut into shape. In either event, the surface of the sintered material may be cleaned and/or otherwise prepared before the contact layer 320 is applied thereon.

[0042] As discussed above, the contact layer 320 can be formed before and/or after the thermoelectric substrate 310 has been cut into its final shape. Notably, however, the contact layer 320, and/or any other layer of the electrode structure, is present on less than all of the sides of the thermoelectric substrate 310 when the layer is applied to the sintered thermoelectric material before the sintered

thermoelectric material is cut into shape. In various instances, as a result, the contact layer 320 is present on only the top and bottom surfaces of the

thermoelectric substrate 310. In at least one instance, the thermoelectric element 300 comprises a first electrode structure on a first side of the thermoelectric element 300 and a second electrode structure on a second side of the thermoelectric element 300 which is separate and distinct from the first electrode structure. The first electrode structure is not electrically connected to the second electrode structure.

[0043] The diffusion barrier layer 330 is a compound comprising chromium, aluminum, and yttrium and may also comprise cobalt, nickel, and/or iron. In one sense, the diffusion barrier layer 330 is a MCrAIY-based compound, where M is cobalt, nickel, and/or iron. In certain embodiments, M is only cobalt. In at least one embodiment, the composition of CoCrAIY is 50-70 weight percent (wt%) Co, 15-30 wt% Cr, 5-25 wt% Al, and 0.1 -5 wt% Y, for example. In at least one embodiment, the composition of CoCrAIY is 45-75 weight percent (wt%) Co, 10-35 wt% Cr, 1 -30 wt% Al, and 0.01 -10 wt% Y, for example. In various embodiments, as described in greater detail below, the diffusion barrier layer 330 comprises a compound of titanium and boron, such as titanium diboride (TiB₂), for example. In certain embodiments, as also described in greater detail below, the diffusion barrier layer 330 comprises indium tin oxide (ITO). In various embodiments, the diffusion barrier layer 330 can also include gold and/or silver. In certain embodiments, the diffusion barrier layer 330 comprises an electrically-conductive compound that is highly resistant to reaction with at least one of magnesium, oxygen, nickel, transition metals, and/or noble metals. In at least one embodiment, the diffusion barrier layer 330 can comprise a conductive oxide and/or a conductive amorphous material. In any event, the diffusion barrier layer 330 is applied to the contact layer 320 utilizing a sputtering process, although any suitable process can be used. For instance, the diffusion barrier layer 330 can be applied to the contact layer 320 utilizing any suitable thin film deposition process, for example. Further to the above, the diffusion barrier layer 330 can be formed before and/or after the thermoelectric substrate 310 has been cut into its final shape.

[0044] The bonding layer 340 mechanically couples the thermoelectric element 300 to a substrate, such as a hot plate 380 or a cold plate 390, for example. The bonding layer 340 also electrically couples the thermoelectric element 300 to a circuit defined in and/or on the substrate. In at least one instance, further to the above, the circuit comprises metal pads and the bonding layer 340 is soldered to a metal pad utilizing at least one solder material 370, for example. In various instances, the solder material 370 which bonds the thermoelectric element 300 to the hot plate 380 is the same as the solder material 370 which bonds the thermoelectric element 300 to the cold plate 390. In various instances, the solder material 370 which bonds the thermoelectric element 300 to the hot plate 380 is different than the solder material 370 which bonds the thermoelectric element 300 to the cold plate 390. In addition or in lieu of soldering, a bonding layer 340 can be brazed to a metal pad utilizing at least one brazing material. In at least one instance, the brazing material comprises an alloy including silver, copper, zinc, and/or tin, for example. The bonding layer 340 is comprised of gold and/or silver, although any suitable material can be used. In any event, the bonding layer 340 is applied to the diffusion barrier layer 330 utilizing a sputtering process, although any suitable process can be used. For instance, the bonding layer 340 can be applied to the diffusion barrier layer 330 utilizing any suitable thin film deposition process, for example. Further to the above, the bonding layer 340 can be formed before and/or after the thermoelectric substrate 310 has been cut into its final shape.

[0045] The contact layer 320, the diffusion barrier layer 330, and the bonding layer 340 can have any suitable thickness. The contact layer 320 comprises a thickness which is greater than 10 nm and less than 10 microns. The diffusion barrier layer 330 comprises a thickness which is greater than 10 nm and less than 10 microns. The bonding layer 340 comprises a thickness which is greater than 10 nm and less than 10 microns. In at least one embodiment, the contact layer 320 comprises a thickness which is greater than 1 nm and less than 100 microns, the diffusion barrier layer 330 comprises a thickness which is greater than 1 nm and less than 100 microns, and the bonding layer 340 comprises a thickness which is greater than 1 nm and less than 100 microns, for example.

[0046] The layers of an electrode structure, such as the layers 320, 330, and/or 340 of the thermoelectric element 300, for example, may be deposited by physical vapor deposition techniques such as sputtering, for example, and/or by a thick film deposition process such as electroplating, thermal spraying, flame synthesis, laser melting, diffusion bonding, and/or monoblock co-sintering, for example. It is also contemplated that some electrode layers of a thermoelectric element could be deposited using a thick film process while other metal layers of the thermoelectric element could be deposited using a thin film process.

[0047] In at least one embodiment, the elements comprising the contact layer 320, the elements comprising the diffusion barrier layer 330, and the elements comprising the bonding layer 340 are mutually exclusive. Also, the elements of the thermoelectric substrate 310 can be mutually exclusive with respect to the elements of the contact layer 320, the diffusion barrier layer 330, and the bonding layer 340. As described herein, one or more layers can be used to prevent, or at least control, the flow of elements between layers in the electrode structure and/or between the electrode structure and the thermoelectric substrate 310 when the thermoelectric element 300 is heated.

[0048] In certain embodiments, at least one element in the thermoelectric substrate 310 is also present in the contact layer 320, the diffusion barrier layer 330, and/or the bonding layer 340 before the thermoelectric element 300 is heated. In certain embodiments, at least one element in the contact layer 320 is also present in the thermoelectric substrate 310, the diffusion barrier layer 330, and/or the bonding layer 340 before the thermoelectric element 300 is heated. In certain embodiments, at least one element in the diffusion barrier layer 330 is also present in the thermoelectric substrate 310, the contact layer 320, and/or the bonding layer 340 before the thermoelectric element 300 is heated. In certain embodiments, at least one element in the bonding layer 340 is also present in the thermoelectric substrate 310, the contact layer 320, and/or the diffusion barrier layer 330 before the thermoelectric element 300 is heated.

[0049] The electrode structure of the thermoelectric element 300 allows

magnesium silicide-based thermoelectric materials, such as magnesium silicide stannide, to be reliably used at high temperatures, such as above 250°C, for instance, by preventing diffusion between certain layers of the electrode structure and the bulk thermoelectric material and, also, by preventing diffusion between the bulk thermoelectric material into certain layers of the electrode structure. In at least one instance, the diffusion barrier layer 330 prevents elements in the bonding layer 340 and/or elements in the solder material 370 from migrating, or at least

substantially migrating, into the contact layer 320 and/or the thermoelectric substrate 310. Moreover, the diffusion barrier layer 330 prevents elements in the

thermoelectric substrate 310, especially magnesium and tin, and/or elements in the contact layer 320, especially nickel, from migrating, or at least substantially migrating, into the bonding layer 340 and/or the solder material 370. Previously, there was no electrode scheme for magnesium silicide stannide that could prevent the diffusion of magnesium and/or tin, for example, within the thermoelectric system and therefore there was no reliable way to produce power over a long timeframe with thermoelectric systems using magnesium silicide.

[0050] The degree to which the migration of elements readily occurs increases as the ratio of magnesium stannide, Mg₂Sn, to magnesium silicide, Mg₂Si, increases in the magnesium silicide-based material. Because the addition of the tin compound improves the ZT of the magnesium silicide significantly, there is a high demand for long term reliable operation of the magnesium silicide stannide class of materials by incorporating a diffusion barrier into the electrode structure. In one embodiment, the magnesium silicide stannide material has a composition with at least 5% Mg₂Sn, for example. In another embodiment, the magnesium silicide stannide material has a composition with at least 50% Mg₂Sn, for example.

[0051] Diffusion is a temperature-dependent phenomenon that involves migration of elements away from their starting locations and occurs more quickly and severely the higher the temperature. Diffusion is a problem for long-term reliability of thermoelectric systems that operate at elevated temperature, such as thermoelectric generators, because of several issues that result from diffusion. First of all, diffusion can interfere with the carefully-engineered doping levels of thermoelectric materials, "poisoning" the thermoelectric materials by changing the doping of the thermoelectric materials, and/or causing phase segregation. Diffusion can also result in

undesirable and uncontrollable intermetallic formation, where the intermetallic may be mechanically weak, have a non-compatible thermal expansion coefficient, or have different electrical and/or thermal transport properties than the originally engineered materials. Diffusion has also been shown to cause a porous layer where enough atoms moved from their original location to leave behind a layer of pores, which can then become crack initiation sites and/or cause a loss in overall strength. Moreover, even if one element diffuses through another without reacting into an intermetallic, it can cause a change in the surface bonding physics. For example, gold is often used in the bonding layer 340, for example, partly due to its resistance to forming oxide layers in air, but if an element such as nickel and/or magnesium, for example, diffuses to the surface of the bonding layer 340, the element may oxidize and prevent a good bond between the thermoelectric element 300 and the adjacent substrate.

[0052] Using a MCrAIY (M = Co, Ni, and/or Fe) compound and/or a titanium-boron compound, such as TiB₂, for example, as a diffusion barrier in the electrical pathway of a thermoelectric system represents a significant departure from previously-known uses of these compounds. More specifically, previous uses of MCrAIY or TiB₂ compounds primarily comprised of corrosion protection coatings in high-temperature (>650°C) gaseous oxygen-containing and/or sulfur containing environments, such as on turbine blades and/or jet nozzles, for example, and were applied to such components using plasma spray and/or thermal spray processes instead of sputtering and/or evaporation deposition processes. Additionally, indium tin oxide is an oxide - and oxides are not typically considered as a diffusion barrier or as an electrode layer. Moreover, previous known uses of magnesium silicide in

thermoelectric systems did not comprise electrode structures including cobalt, iron, chromium, aluminum, yttrium, boron, indium, tin, gold, and/or silver. Rather, such known uses comprised a nickel electrode which is now known by the inventors to suffer from the element migration discussed above. Additionally, it is very

challenging to identify a material or compound that is electrically conductive enough to serve in an electrode contact structure but is also a diffusion barrier and is non- reactive enough to prevent reaction with magnesium, tin, and/or other elements in the thermoelectric system.

[0053] FIG. 4, which comprises seven panels labeled SEM, 0, Mg, Si, Ni, Au, and Sn, illustrates an example of a thermoelectric element 400 in accordance with at least one embodiment. The thermoelectric element 400 consists of a substrate 410 comprised of magnesium silicide stannide Mg₂Sio.₄Sn₀.6, a contact layer 420 comprised of nickel applied to the substrate 410, and a bonding layer 440 comprised of gold applied to the contact layer 420. The thermoelectric element 400 does not comprise a MCrAIY compound, and/or any other diffusion barrier layer, positioned intermediate the nickel of the contact layer 420 and the gold of the bonding layer 440. Also, notably, the thermoelectric element 400 has not undergone a heating process above room temperature and represents a control sample to be compared with the thermoelectric element 500 of FIG. 5, the thermoelectric element 700 of FIG. 7, the thermoelectric element 900 of FIG. 9, and the thermoelectric element 1000 of FIG. 10, which are discussed in greater detail further below.

[0054] The panel of FIG. 4 labeled SEM is a picture, or micrograph, of the thermoelectric element 400 using a scanning electron microscope (SEM) after it has been cross-sectioned. In fact, all of the seven panels of FIG. 4 comprise pictures of the thermoelectric element 400 taken with a scanning electron microscope or an energy dispersive x-ray spectroscopy system (EDS). The panel of FIG. 4 labeled 0 illustrates the oxygen (0) atoms in the thermoelectric element 400 as light dots and the non-oxygen atoms as black dots. The amount of oxygen atoms in the

thermoelectric element 400 represents the oxygenation of the thermoelectric element 400, which is minimal, after being exposed to air. The panel of FIG. 4 labeled Mg illustrates the magnesium (Mg) atoms in the thermoelectric element 400 as light dots and the non-magnesium atoms as black dots. Upon considering the O and Mg panels of FIG. 4 in tandem, it is readily apparent that a magnesium oxide layer has not formed in or on the gold bonding layer 440. The panel of FIG. 4 labeled Si illustrates the silicon (Si) atoms in the thermoelectric element 400 as light dots and the non-silicon atoms as black dots. The panel of FIG. 4 labeled Sn illustrates the tin (Sn) atoms in the thermoelectric element 400 as light dots and the non-tin atoms as black dots. As can be seen in FIG. 4, the magnesium, silicon, and tin atoms have not migrated, or at least substantially migrated, from the substrate 410 into the contact layer 420 and/or the bonding layer 440.

[0055] Further to the above, the panel of FIG. 4 labeled Ni illustrates the nickel (Ni) atoms in the thermoelectric element 400 as light dots and the non-nickel atoms as black dots. As can be seen in FIG. 4, nickel atoms have not migrated, or at least substantially migrated, from the contact layer 420 into the substrate 410 and/or the bonding layer 440. Similarly, the panel of FIG. 4 labeled Au illustrates the gold (Au) atoms in the thermoelectric element 400 as light dots and the non-gold atoms as black dots. As can be seen in FIG. 4, gold atoms have not migrated, or at least substantially migrated, from the bonding layer 440 into the substrate 410 and/or the contact layer 420. Stated another way, the elements of the thermoelectric element 400 are precisely, or at least essentially, where they are expected to be.

[0056] FIG. 5, which comprises seven panels labeled SEM, O, Mg, Si, Ni, Au, and Sn,

illustrates an example of a thermoelectric element 500 in accordance with at least one embodiment. The thermoelectric element 500 consists of a substrate 510 comprised of magnesium silicide stannide Mg₂Sio.₄Sn₀.6, a contact layer 520 comprised of nickel applied to the substrate 510, and a bonding layer 540 comprised of gold applied to the contact layer 520. The thermoelectric element 500 does not comprise a MCrAIY compound, and/or any other diffusion barrier layer, positioned intermediate the nickel of the contact layer 520 and the gold of the bonding layer 540. In contrast with the thermoelectric element 400 of FIG. 4, the thermoelectric element 500 of FIG. 5 has undergone a heating process. More specifically, the thermoelectric element 500 was heated in a vacuum at about 400°C for about 50 hours before the pictures of FIG. 5 were taken. FIG. 6 is a photograph of several thermoelectric element 500 samples after the heating process was performed and it was observed that the bonding surfaces 540 of the thermoelectric elements 500 were grey in color rather than gold as they were prior to heating. The post-heating grey color of the bonding surfaces 540 is the result of the magnesium and/or nickel migration into the bonding surfaces 540 and subsequent oxidation of those metals, which is discussed in greater detail below.

[0057] The panel of FIG. 5 labeled SEM is a picture, or micrograph, of a

thermoelectric element 500 using a scanning electron microscope (SEM) after it has been cross-sectioned. In fact, all seven panels of FIG. 5 comprise pictures of the thermoelectric element 500 taken with a scanning electron microscope or an energy dispersive x-ray spectroscopy system (EDS). The panel of FIG. 5 labeled O illustrates the oxygen (O) atoms in the thermoelectric element 500 as light dots and the non-oxygen atoms as black dots. The amount of oxygen atoms in the

thermoelectric element 500 represents the oxygenation of the thermoelectric element 500 even though the thermoelectric element 500 was heated in a vacuum. Notably, magnesium and oxygen react so readily that even < 200 ppm 0₂ could result in an oxide layer. The panel of FIG. 5 labeled Mg illustrates the magnesium (Mg) atoms in the thermoelectric element 500 as light dots and the non-magnesium atoms as black dots. The panel of FIG. 5 labeled Si illustrates the silicon (Si) atoms in the

thermoelectric element 500 as light dots and the non-silicon atoms as black dots. The panel of FIG. 5 labeled Sn illustrates the tin (Sn) atoms in the thermoelectric element 500 as light dots and the non-tin atoms as black dots. As can be seen in FIG. 5, the silicon and tin atoms have not migrated, or at least substantially migrated, from the substrate 510 into the contact layer 520 and/or the bonding layer 540;

however, the magnesium atoms have substantially migrated into the contact layer 520 and the bonding layer 540 and have reacted with oxygen at the surface of the bonding layer 540, which causes a problem due to having a non-conductive layer in the electrode structure. The O, Mg, and Au panels of FIG. 5 show that a magnesium oxide phase has formed throughout the gold layer. In fact, the magnesium migration within the thermoelectric element 500 is so substantial that the proper phases of the magnesium silicide stannide Mg₂Sio.₄Sn₀.₆ in the substrate 510 have been substantially depleted. This can be a significant problem for the thermoelectric material and the performance of the thermoelectric system after a long enough time at high temperatures.

[0058] Further to the above, the panel of FIG. 5 labeled Ni illustrates the nickel (Ni) atoms in the thermoelectric element 500 as light dots and the non-nickel atoms as black dots. As may be seen in FIG. 5, nickel atoms have migrated from the contact layer 520 into the substrate 510 and the bonding layer 540. That said, quantitative analysis using EDS was performed to verify the presence of nickel in the gold bonding layer 540. The panel of FIG. 5 labeled Au illustrates the gold (Au) atoms in the thermoelectric element 500 as light dots and the non-gold atoms as black dots. As can be seen in FIG. 5, gold atoms have not migrated, or at least substantially migrated, from the bonding layer 540 into the substrate 510 and/or the contact layer 520.

[0059] FIG. 7, which comprises ten panels labeled SEM, O, Mg, Si, Ni, Au, Sn, Co, Cr, and Al, illustrates an example of a thermoelectric element 700 in accordance with at least one embodiment. The thermoelectric element 700 consists of a substrate 710 comprised of magnesium silicide stannide Mg₂Sio.₄Sno.6, a contact layer 720 comprised of nickel applied to the substrate 710, a diffusion barrier layer 730 comprised of a CoCrAIY compound applied to the contact layer 720, and a bonding layer 740 comprised of gold applied to the diffusion barrier layer 730. Similar to the thermoelectric element 500 of FIG. 5, the thermoelectric element 700 of FIG. 7 has undergone a heating process. More specifically, the thermoelectric element 700 was heated in a vacuum at about 400°C for about 50 hours before the pictures of FIG. 7 were taken. FIG. 8 is a photograph of several thermoelectric element 700 samples after the heating process was performed and it was observed that the bonding surfaces 740 of the thermoelectric elements 700 were gold in color, as they were before the heating process. Notably, FIG. 8 is presented in grey-scale, but the reader should understand that the bonding surfaces 740 are actually gold in color. The gold bonding surfaces 740 were not discolored by diffusion, or at least substantial diffusion, of the magnesium and/or nickel atoms, which is discussed in greater detail below.

[0060] The panel of FIG. 7 labeled SEM is a picture, or micrograph, of a

thermoelectric element 700 using a scanning electron microscope (SEM) after it has been cross-sectioned. In fact, all ten panels of FIG. 7 comprise pictures of the thermoelectric element 700 taken with a scanning electron microscope or an energy dispersive x-ray spectroscopy system (EDS). The panel of FIG. 7 labeled 0 illustrates the oxygen (0) atoms in the thermoelectric element 700 as light dots and the non-oxygen atoms as black dots. The amount of oxygen atoms in the

thermoelectric element 700 represents the oxygenation of the thermoelectric element 700. Notably, an oxide layer has not formed in or on the gold bonding surface 740. The panel of FIG. 7 labeled Mg illustrates the magnesium (Mg) atoms in the thermoelectric element 700 as light dots and the non-magnesium atoms as black dots. The panel of FIG. 7 labeled Si illustrates the silicon (Si) atoms in the

thermoelectric element 700 as light dots and the non-silicon atoms as black dots. The panel of FIG. 7 labeled Sn illustrates the tin (Sn) atoms in the thermoelectric element 700 as light dots and the non-tin atoms as black dots. As can be seen in FIG. 7, the magnesium, silicon, and tin atoms have not migrated, or at least substantially migrated, from the substrate 710 into the bonding layer 740 as a result of the diffusion barrier layer 730.

[0061] Further to the above, the panel of FIG. 7 labeled Ni illustrates the nickel (Ni) atoms in the thermoelectric element 700 as light dots and the non-nickel atoms as black dots. As can be seen in FIG. 7, nickel atoms have not migrated, or at substantially migrated, from the contact layer 720 into the bonding layer 740 as a result of the diffusion barrier layer 730. The panel of FIG. 7 labeled Au illustrates the gold (Au) atoms in the thermoelectric element 700 as light dots and the non-gold atoms as black dots. As can be seen in FIG. 7, gold atoms have not migrated, or at least substantially migrated, from the bonding layer 740 into the substrate 710 and/or the contact layer 720. The gold layer is quite distinct from the magnesium layer and does not contain any oxygen. The panel of FIG. 7 labeled Co illustrates the cobalt (Co) atoms in the thermoelectric element 700 as light dots and the non-cobalt atoms as black dots. The panel of FIG. 7 labeled Cr illustrates the chromium (Cr) atoms in the thermoelectric element 700 as light dots and the non-chromium atoms as black dots. The panel of FIG. 7 labeled Al illustrates the aluminum (Al) atoms in the thermoelectric element 700 as light dots and the non-aluminum atoms as black dots. As can be seen in FIG. 7, the cobalt, chromium, and aluminum atoms have not migrated, or at least substantially migrated, into the bonding layer 740. [0062] In stark contrast to the thermoelectric element 500 sample, the thermoelectric 700 sample showed no diffusion of magnesium or nickel in either direction, no gradient in the density of the bulk magnesium silicide stannide material, and no concentrated oxide layer in the electrode structure even though the

thermoelectric element 700 sample was heated in the same vacuum furnace run, i.e., at the same time, as the thermoelectric element 500 sample. Moreover, all layers appear distinct and in their original, or at least substantially original, positions.

[0063] Further to the above, a reliable electrode structure can comprise a nickel layer, followed by a compound of MCrAIY (M = Co, Ni, and/or Fe), such as cobalt chromium aluminum yttrium (CoCrAIY), for example, that serves as an effective diffusion barrier, followed by a gold layer. The composition of the MCrAIY is 50-70 weight percent (wt%) M, 15-30 wt% Cr, 5-25 wt% Al, and 0.1 -5 wt% Y, although any suitable composition can be used. This electrode structure prevents magnesium diffusion through the nickel and/or gold layers, prevents nickel diffusion through any subsequent layer such as the gold layer, and prevents formation of nickel

intermetallics in the bulk Mg₂SiSn, and prevents formation of oxide layers in the electrode structure. The two-way diffusion of braze or solder materials into the bulk Mg₂SiSn or vice versa is also prevented. All of those things are very important for the long-term reliability of a thermoelectric system. While several reports have been made of using nickel as a contact metallization for magnesium silicide-based materials, as discussed above, no reports have been made on how to prevent the diffusion of elements that causes degradation of the thermoelectric system over time when exposed and/or operated at high temperatures.

[0064] FIG. 9, which comprises nine panels labeled SEM, O, Mg, Si, Ni, Sn, Au, B, and Ti, illustrates an example of a thermoelectric element 900 in accordance with at least one embodiment. The thermoelectric element 900 consists of a substrate 910 comprised of magnesium silicide stannide Mg₂Sio.₄Sn₀.6, a contact layer 920 comprised of nickel applied to the substrate 910, a diffusion barrier layer 930 comprised of titanium diboride (TiB₂) applied to the contact layer 920, and a bonding layer 940 comprised of gold applied to the diffusion barrier layer 930. The

thermoelectric element 900 has undergone a heating process. More specifically, the thermoelectric element 900 was heated in air at about 400°C for about 100 hours before the pictures of FIG. 9 were taken. After the heating process was performed, it was observed that the bonding surfaces 940 of the thermoelectric elements 900 were gold in color. The gold bonding surfaces 940 were not discolored by migration, or at least substantial migration of the magnesium and/or nickel atoms, which is discussed in greater detail below.

[0065] The panel of FIG. 9 labeled SEM is a picture, or micrograph, of the thermoelectric element 900 using a scanning electron microscope (SEM) after it has been cross-sectioned. In fact, all nine panels of FIG. 9 comprise pictures of the thermoelectric element 900 taken with a scanning electron microscope or an energy dispersive x-ray spectroscopy system (EDS). The panel of FIG. 9 labeled O illustrates the oxygen (O) atoms in the thermoelectric element 900 as light dots and the non-oxygen atoms as black dots. The amount of oxygen atoms in the

thermoelectric element 900 represents the oxygenation of the thermoelectric element 900. Notably, an oxide layer has not formed in or on the gold bonding surface 940 despite the thermoelectric element 900 being heated in air. The panel of FIG. 9 labeled Mg illustrates the magnesium (Mg) atoms in the thermoelectric element 900 as light dots and the non-magnesium atoms as black dots. The panel of FIG. 9 labeled Si illustrates the silicon (Si) atoms in the thermoelectric element 900 as light dots and the non-silicon atoms as black dots. The panel of FIG. 9 labeled Sn illustrates the tin (Sn) atoms in the thermoelectric element 900 as light dots and the non-tin atoms as black dots. As can be seen in FIG. 9, the magnesium, silicon, and tin atoms have not migrated, or at least substantially migrated, from the substrate 910 into the gold bonding layer 940 as a result of the diffusion barrier layer 930.

[0066] Further to the above, the panel of FIG. 9 labeled Ni illustrates the nickel (Ni) atoms in the thermoelectric element 900 as light dots and the non-nickel atoms as black dots. As can be seen in FIG. 9, nickel atoms have not migrated, or at substantially migrated, from the contact layer 920 into the bonding layer 940 as a result of the diffusion barrier layer 930. The panel of FIG. 9 labeled Au illustrates the gold (Au) atoms in the thermoelectric element 900 as light dots and the non-gold atoms as black dots. As can be seen in FIG. 9, gold atoms have not migrated, or at least substantially migrated, from the bonding layer 940 into the substrate 910 and/or the contact layer 920. The panel of FIG. 9 labeled B illustrates the boron (B) atoms in the thermoelectric element 900 as light dots and the non-boron atoms as black dots. The panel of FIG. 9 labeled Ti illustrates the titanium (Ti) atoms in the thermoelectric element 900 as light dots and the non-titanium atoms as black dots. As can be seen in FIG. 9, the boron and titanium atoms have not migrated, or at least substantially migrated, into the substrate 910, the contact layer 920, and the bonding layer 940.

[0067] In stark contrast to the thermoelectric element 500 sample, the

thermoelectric 900 sample showed no diffusion of magnesium or nickel in either direction, no oxidation of the bulk magnesium silicide stannide material, and no concentrated oxide layer on the gold surface even though the thermoelectric element 900 was heated in air. Moreover, all layers appear distinct and in their original, or at least substantially original, positions.

[0068] FIG. 10, which comprises seven panels labeled SEM, O, Mg, Si, Ni, In, and Sn, illustrates an example of a thermoelectric element 1000 in accordance with at least one embodiment. The thermoelectric element 1000 consists of a substrate 1010 comprised of magnesium silicide stannide Mg₂Sio.₄Sno.6, a contact layer 1020 comprised of nickel applied to the substrate 1010, a diffusion barrier layer 1030 comprised of indium tin oxide (ITO) applied to the contact layer 1020, and a bonding layer 1040 comprised of nickel applied to the diffusion barrier layer 1030. The thermoelectric element 1000 has undergone a heating process. More specifically, the thermoelectric element 1000 was heated in air at about 400°C for about 100 hours before the pictures of FIG. 10 were taken.

[0069] The panel of FIG. 10 labeled SEM is a picture, or micrograph, of the thermoelectric element 1000 using a scanning electron microscope (SEM) after it has been cross-sectioned. In fact, all seven panels of FIG. 10 comprise pictures of the thermoelectric element 1000 taken with a scanning electron microscope or an energy dispersive x-ray spectroscopy system (EDS). The panel of FIG. 10 labeled O illustrates the oxygen (O) atoms in the thermoelectric element 1000 as light dots and the non-oxygen atoms as black dots. The amount of oxygen atoms in the

thermoelectric element 1000 represents the oxygenation of the thermoelectric element 1000. Notably, an oxide layer has not formed in or on the gold bonding surface 1040 despite the thermoelectric element 1000 being heated in air. The panel of FIG. 10 labeled Mg illustrates the magnesium (Mg) atoms in the

thermoelectric element 1000 as light dots and the non-magnesium atoms as black dots. The panel of FIG. 10 labeled Si illustrates the silicon (Si) atoms in the thermoelectric element 1000 as light dots and the non-silicon atoms as black dots. The panel of FIG. 10 labeled Sn illustrates the tin (Sn) atoms in the thermoelectric element 1000 as light dots and the non-tin atoms as black dots. As can be seen in FIG. 10, the magnesium, silicon, and tin atoms have not become oxidized and have not migrated, or at least substantially migrated, from the substrate 1010 into the bonding layer 1040 as a result of the diffusion barrier layer 1030.

[0070] Further to the above, the panel of FIG. 10 labeled Ni illustrates the nickel (Ni) atoms in the thermoelectric element 1000 as light dots and the non-nickel atoms as black dots. The panel of FIG. 10 labeled In illustrates the indium (In) atoms in the thermoelectric element 1000 as light dots and the non-indium atoms as black dots. As can be seen in FIG. 10, the indium atoms have not migrated, or at least substantially migrated, into the substrate 1010, the contact layer 1020, or the bonding layer 1040.

[0071] In stark contrast to the thermoelectric element 500 sample, the

thermoelectric 1000 sample showed no diffusion of magnesium or nickel in either direction, no oxidation of the bulk magnesium silicide stannide material, and no concentrated oxide layer even though the thermoelectric element 1000 was heated in air. Moreover, all layers appear distinct and in their original, or at least

substantially original, positions.

[0072] In addition to the above, the resistance of the thermoelectric elements 500, 700, 900, and 1000 were evaluated and compared to the resistance of the

thermoelectric element 400. The resistance of the thermoelectric element 500, which comprises an electrode structure of Ni/Au and was heated at approximately 400°C for approximately 50 hours in a vacuum, is approximately 100% greater than the resistance of the thermoelectric element 400, which comprises an electrode structure of Ni/Au and was not heated. The resistance of the thermoelectric element 700, which comprises an electrode structure of Ni/CoCrAIY/Au and was heated at approximately 400°C for approximately 50 hours in a vacuum, is the same, or at least substantially the same, as the resistance of the thermoelectric element 400. Stated another way, the thermoelectric element 700 exhibited no increase in resistance as compared to the thermoelectric element 400 even though it was heated. The resistance of the thermoelectric element 900, which comprises an electrode structure of Ni/TiB₂/Au and was heated at approximately 400°C for approximately 100 hours in air, is less than 10% greater than the resistance of the thermoelectric element 400. The resistance of the thermoelectric element 1000, which comprises an electrode structure of Ni/ITO/Ni and was heated at

approximately 400°C for approximately 100 hours in air, is less than 10% greater than the resistance of the thermoelectric element 400. An additional thermoelectric sample comprising an electrode structure of Ni/Au was heated at approximately 400°C for approximately 100 hours in air and exhibited an approximately 1000% increase in resistance as compared to the thermoelectric element 400. Notably, this thermoelectric sample had a gold bonding surface that turned black as a result of the heating process. These results demonstrate the effectiveness of the stated diffusion barrier layers at blocking diffusion and preventing oxidation of the samples. Namely, MCoCrAIY, TiB₂, and ITO are effective barriers to diffusion between magnesium silicide-based materials and the bonding layers that connect the magnesium silicide- based materials to the system package.

[0073] The thermoelectric systems disclosed herein can be adapted for use with automotive systems. Certain automotive systems comprise a propulsion system including an internal combustion engine which generates exhaust heat. One or more of the thermoelectric systems disclosed herein can be adapted to reclaim that exhaust heat. In at least one instance, a thermoelectric system is mounted to and/or downstream of a catalytic converter which treats the exhaust stream from the internal combustion engine. The thermoelectric system can be mounted within the catalytic converter and/or to an exterior housing of the catalytic converter, for example. In certain instances, the thermoelectric system can be embedded within the exterior housing of the catalytic converter. In various instances, a voltage potential generated by a catalytic converter thermoelectric system can be used to power one or more sensor systems configured to evaluate the exhaust passing through the catalytic converter, for example.

[0074] Further to the above, heat generated by an internal combustion engine is often discharged to the surrounding environment through an air-cooled heat exchanger via a fluidic thermodynamic circuit. One or more of the thermoelectric systems disclosed herein can be adapted to reclaim that discharged heat. In at least one instance, a thermoelectric system is mounted to a heat exchanger, or radiator, of the fluidic thermodynamic circuit which cools the fluid flowing through the circuit. In various instances, a voltage potential generated by a radiator thermoelectric system can be used to power one or more sensor systems configured to evaluate the fluid passing through the radiator, for example. In various instances, a thermoelectric system can be mounted to any suitable portion of the fluidic thermodynamic circuit and/or mounted directly to the block of the internal combustion engine, for example. In at least one instance, further to the above, a thermoelectric system can be mounted to an exhaust manifold which connects the exhaust system to the engine block, for example.

[0075] The entire disclosures of the following patents are incorporated by reference herein:

- U.S. Patent No. 8,603,940, entitled AUTOMOBILE EXHAUST GAS CATALYTIC CONVERTER, which issued on December 10, 2013;

- U.S. Patent No. 8,650,864, entitled COMBINATION LIQUID-COOLED EXHAUST MANIFOLD ASSEMBLY AND CATALYTIC CONVERTER ASSEMBLY FOR A MARINE ENGINE, which issued on February 18, 2014;

- U.S. Patent No. 8,544,257, entitled ELECTRICALLY STIMULATED CATALYTIC CONVERTER APPARATUS, AND METHOD OF USING SAME, which issued on October 1 , 2013;

- U.S. Patent No. 7,858,052, entitled CATALYTIC CONVERTER OPTIMIZATION, which issued on December 28, 2010;

- U.S. Patent No. 7,767,622, entitled CATALYTIC CONVERTER WITH IMPROVED START-UP BEHAVIOR, which issued on August 3, 2010;

- U.S. Patent No. 7,051 ,522, entitled THERMOELECTRIC CATALYTIC

CONVERTER TEMPERATURE CONTROL, which issued on May 30, 2006;

- U.S. Patent No. 9,276, 188, entitled THERMOELECTRIC-BASED POWER

GENERATION SYSTEMS AND METHODS, which issued on March 1 , 2016;

- U.S. Patent No. 9,006,556, entitled THERMOELECTRIC POWER GENERATOR FOR VARIABLE THERMAL POWER SOURCE, which issued on April 14, 2015;

- U.S. Patent No. 8,646,261 , entitled THERMOELECTRIC GENERATORS

INCORPORATING PHASE-CHANGE MATERIALS FOR WASTE HEAT

RECOVERY FROM ENGINE EXHAUST, which issued on February 1 1 , 2014;

- U.S. Patent No. 6,986,247, entitled THERMOELECTRIC CATALYTIC POWER GENERATOR WITH PREHEAT, which issued on January 17, 2006; and

- U.S. Patent No. 4,029,472, entitled THERMOELECTRIC EXHAUST GAS

SENSOR, which issued on June 14, 1977.

[0076] Certain automotive systems, further to the above, comprise a propulsion system including an electric motor powered by one or more batteries. In use, the batteries can generate a significant amount of thermal energy owing to high power demands from the electric motor. Similarly, the electric motor can generate a significant amount of thermal energy during use. Such thermal energy can be harvested and reclaimed by one or more of the thermoelectric systems disclosed herein. In various instances, a battery comprises one or more battery cells positioned within an outer housing. The battery cells comprise lithium-ion battery cells, for example. In use, the heat generated by the battery cells radiates through the outer housing of the battery. In certain instances, the thermoelectric elements of a thermoelectric system are mounted to the outer housing of the battery. In various instances, the thermoelectric elements of a thermoelectric system are positioned intermediate two battery cells.

[0077] In addition to or in lieu of the above, a thermoelectric system disclosed herein can be used to cool a battery, for example. In such instances, the

thermoelectric system is operated as a Peltier device. In at least one such instance, the thermoelectric elements of the thermoelectric system are positioned on, at, and/or near the hottest portions of the battery, for example, to prevent, or at least reduce the possibility of the battery entering into a thermal runaway condition.

[0078] The entire disclosures of the following patents are incorporated by reference herein:

- U.S. Patent No. 7,781 ,097, entitled CELL THERMAL RUNAWAY PROPAGATION RESISTANCE USING AN INTERNAL LAYER OF INTUMESCENT MATERIAL, which issued on August 24, 2010;

- U.S. Patent No. 7,763,381 , entitled CELL THERMAL RUNAWAY PROPAGATION RESISTANCE USING DUAL INTUMESCENT MATERIAL LAYERS, which issued on July 27, 2010;

- U.S. Patent No. 7,736,799, entitled METHOD AND APPARATUS FOR

MAINTAINING CELL WALL INTEGRITY DURING THERMAL RUNAWAY USING AN OUTER LAYER OF INTUMESCENT MATERIAL, which issued on June 15, 2010;

- U.S. Patent No. 8,168,315, entitled METHOD FOR DETECTING BATTERY THERMAL EVENTS VIA BATTERY PACK ISOLATION MONITORING, which issued on May 1 , 2012;

- U.S. Patent No. 8,154,256, entitled BATTERY THERMAL EVENT DETECTION SYSTEM USING AN ELECTRICAL CONDUCTOR WITH A THERMALLY INTERRUPTIBLE INSULATOR, which issued on April 10, 2012; - U.S. Patent No. 8,153,290, entitled HEAT DISSIPATION FOR LARGE BATTERY PACKS, which issued on April 10, 2012;

- U.S. Patent No. 8,1 17,857, entitled INTELLIGENT TEMPERATURE CONTROL SYSTEM FOR EXTENDING BATTERY PACK LIFE, which issued on February 21 , 2012;

- U.S. Patent No. 8,082,743 , entitled BATTERY PACK TEMPERATURE

OPTIMIZATION CONTROL SYSTEM, which issued on December 27, 201 1 ;

- U.S. Patent No. 8,092,081 , entitled BATTERY THERMAL EVENT DETECTION SYSTEM USING AN OPTICAL FIBER, which issued on January 10, 2012;

- U.S. Patent No. 8,059,007, entitled BATTERY THERMAL EVENT DETECTION SYSTEM USING A THERMALLY INTERRUPTIBLE ELECTRICAL CONDUCTOR, which issued on November 15, 201 1 ;

- U.S. Patent No. 7,940,028, entitled THERMAL ENERGY TRANSFER SYSTEM FOR A POWER SOURCE UTILIZING BOTH METAL-AIR AND NON-METAL-AIR BATTERY PACKS, which issued on May 10, 201 1 ;

- U.S. Patent No. 7,939, 192, entitled EARLY DETECTION OF BATTERY CELL THERMAL EVENT, which issued on May 10, 201 1 ;

- U.S. Patent No. 7,820,319, entitled CELL THERMAL RUNAWAY PROPAGATION RESISTANT BATTERY PACK, which issued on October 26, 2010;

- U.S. Patent No. 7,789, 176, entitled ELECTRIC VEHICLE THERMAL

MANAGEMENT SYSTEM, which issued on September 7, 2010;

- U.S. Patent No. 8,178,227, entitled METHOD FOR DETECTING BATTERY THERMAL EVENTS VIA BATTERY PACK ISOLATION RESISTANCE MONITORING, which issued on May 15, 2012;

- U.S. Patent No. 8, 168,315, entitled METHOD FOR DETECTING BATTERY

THERMAL EVENTS VIA BATTERY PACK ISOLATION MONITORING, which issued on May 1 , 2012;

- U.S. Patent No. 7,890,218, entitled CENTRALIZED MULTI-ZONE COOLING FOR INCREASED BATTERY EFFICIENCY, which issued on February 15, 201 1 ;

- U.S. Patent No. 8,481 ,191 , entitled RIGID CELL SEPARATOR FOR MINIMIZING THERMAL RUNAWAY PROPAGATION WITHIN A BATTERY PACK, which issued on July 9, 2013;

- U.S. Patent No. 8,402,776, entitled THERMAL MANAGEMENT SYSTEM WITH DUAL MODE COOLANT LOOPS, which issued on March 26, 2013; - U.S. Patent No. 8,367,233, entitled BATTERY PACK ENCLOSURE WITH

CONTROLLED THERMAL RUNAWAY RELEASE SYSTEM, which issued on

February 5, 2013;

- U.S. Patent No. 8,313,850, entitled METHOD FOR DETECTING BATTERY

THERMAL EVENTS VIA BATTERY PACK PRESSURE MONITORING, which issued on November 20, 2012;

- U.S. Patent No. 8,263,250, entitled LIQUID COOLING MANIFOLD WITH MULTIFUNCTION THERMAL INTERFACE, which issued on September 1 1 , 2012;

- U.S. Patent No. 8,541 , 127, entitled OVERMOLDED THERMAL INTERFACE FOR USE WITH A BATTERY COOLING SYSTEM, which issued on September 24, 2013;

- U.S. Patent No. 8,968,949, entitled METHOD OF WITHDRAWING HEAT FROM A BATTERY PACK, which issued on March 3, 2015;

- U.S. Patent No. 8,907,594, entitled COOLING SYSTEMS AND METHODS, which issued on December 9, 2014;

- U.S. Patent No. 8,906,541 , entitled BATTERY MODULE WITH INTEGRATED THERMAL MANAGEMENT SYSTEM, which issued on December 9, 2014;

- U.S. Patent No. 8,899,492, entitled METHOD OF CONTROLLING SYSTEM TEMPERATURE TO EXTEND BATTERY PACK LIFE, which issued on December 2, 2014;

- U.S. Patent No. 8,875,828, entitled VEHICLE BATTERY PACK THERMAL

BARRIER, which issued on November 4, 2014;

- U.S. Patent No. 8,758,924, entitled EXTRUDED AND RIBBED THERMAL

INTERFACE FOR USE WITH A BATTERY COOLING SYSTEM, which issued on June 24, 2014;

- U.S. Patent No. 9,093,726, entitled ACTIVE THERMAL RUNAWAY MITIGATION SYSTEM FOR USE WITHIN A BATTERY PACK, which issued on July 28, 2015; and

- U.S. Patent No. 9,030,063, entitled THERMAL MANAGEMENT SYSTEM FOR USE WITH AN INTEGRATED MOTOR ASSEMBLY, which issued on May 12, 2015.

[0079] The entire disclosures of the following patents are incorporated by reference herein:

- U.S. Patent No. 9,306, 143, entitled HIGH EFFICIENCY THERMOELECTRIC GENERATION, which issued on April 5, 2016; - U.S. Patent No. 9,293,680, entitled CARTRIDGE-BASED THERMOELECTRIC SYSTEMS, which issued on March 22, 2016; and

- U.S. Patent No. 9,276, 188, entitled THERMOELECTRIC-BASED POWER

GENERATION SYSTEMS AND METHODS, which issued on March 1 , 2016.

[0080] The entire disclosures of the following patent applications are incorporated by reference herein:

- U.S. Patent Application Publication No. 2014/0190185, entitled SYSTEM AND METHOD FOR PREVENTING OVERHEATING OR EXCESSIVE BACKPRESSURE IN THERMOELECTRIC SYSTEMS, which published on July 10, 2014;

- U.S. Patent Application Publication No. 2013/0276849, entitled TEG-POWERED COOLING CIRCUIT FOR THERMOELECTRIC GENERATOR, which published on October 24, 2013; and

- U.S. Patent Application Publication No. 2013/0255739, entitled PASSIVELY COOLED THERMOELECTRIC GENERATOR CARTRIDGE, which published on October 3, 2013.

[0081] The Applicant of the present application also owns the patents and patent applications identified below, the entire disclosures of which are incorporated by reference herein:

- U.S. Patent Application Serial No.1 1/645,236, entitled METHODS OF

FABRICATING NANOSTRUCTURES AND NANOWIRES AND DEVICES

FABRICATED THEREFROM, now U.S. Patent No. 7,834,264;

- U.S. Patent Application Serial No. 12/487,893, entitled IMPROVED MECHANICAL STRENGTH & THERMOELECTRIC PERFORMANCE IN METAL CHALCOGENIDE MQ (M=GE,SN,PB AND Q=S, SE, TE) BASED COMPOSITIONS, now U.S. Patent No. 8,277,677;

- U.S. Patent Application Serial No. 12/882,580, entitled THERMOELECTRICS COMPOSITIONS COMPRISING NANOSCALE INCLUSIONS IN A CHALCOGENIDE MATRIX, now U.S. Patent No. 8,778,214;

- U.S. Patent Application Serial No. 12/943, 134, entitled UNIWAFER

THERMOELECTRIC MODULES, now U.S. Patent Application Publication No.

201 1/01 14146;

- U.S. Patent Application Serial No. 13/299,179, entitled ARRAYS OF LONG NANOSTRUCTURES IN SEMICONDUCTOR MATERIALS AND METHODS THEREOF, now U.S. Patent No. 9,240,328; - U.S. Patent Application Serial No. 13/308,945, entitled LOW THERMAL

CONDUCTIVITY MATRICES WITH EMBEDDED NANOSTRUCTURES AND METHODS THEREOF, now U.S. Patent No. 8,736,01 1 ;

- U.S. Patent Application Serial No. 13/331 ,768, entitled ARRAYS OF FILLED NANOSTRUCTURES WITH PROTRUDING SEGMENTS AND METHODS

THEREOF, now U.S. Patent Application Publication No. 2012/0152295;

- U.S. Patent Application Serial No. 13/364, 176, entitled ELECTRODE

STRUCTURES FOR ARRAYS OF NANOSTRUCTURES AND METHODS THEREOF, now U.S. Patent Application Publication No. 2012/0247527;

- U.S. Patent Application Serial No. 13/749,470, entitled MODULAR

THERMOELECTRIC UNITS FOR HEAT RECOVERY SYSTEMS AND METHODS THEREOF, now U.S. Patent No. 9,318,682;

- U.S. Patent Application Serial No. 13/760,977, entitled BULK NANOHOLE

STRUCTURES FOR THERMOELECTRIC DEVICES AND METHODS FOR MAKING THE SAME, now U.S. Patent Application Publication No. 2013/0175654;

- U.S. Patent Application Serial No. 13/786,090, entitled BULK NANO-RIBBON AND/OR NANO-POROUS STRUCTURES FOR THERMOELECTRIC DEVICES AND METHODS FOR MAKING THE SAME, now U.S. Patent No. 9,051 , 175;

- U.S. Patent Application Serial No. 13/947,400, entitled METHOD AND

STRUCTURE FOR THERMOELECTRIC UNICOUPLE ASSEMBLY, now U.S.

Patent No. 9,257,627;

- U.S. Patent Application Serial No. 14/053,452, entitled STRUCTURES AND METHODS FOR MULTI-LEG PACKAGE THERMOELECTRIC DEVICES, now U.S. Patent Application Publication No. 2014/0182644;

- U.S. Patent Application Serial No. 14/059,362, entitled NANOSTRUCTURED THERMOELECTRIC ELEMENTS AND METHODS OF MAKING THE SAME, now U.S. Patent No. 9,082,930;

- U.S. Patent Application Serial No. 14/062,803, entitled BULK-SIZE

NANOSTRUCTURED MATERIALS AND METHODS FOR MAKING THE SAME BY SINTERING NANOWIRES, now U.S. Patent Application Publication No.

2014/01 16491 ;

- U.S. Patent Application Serial No. 14/297,444, entitled SILICON-BASED

THERMOELECTRIC MATERIALS INCLUDING ISOELECTRONIC IMPURITIES, THERMOELECTRIC DEVICES BASED ON SUCH MATERIALS, AND METHODS OF MAKING AND USING SAME, now U.S. Patent Application Publication No.

2014/0360546;

- U.S. Patent Application Serial No. 14/469,404, entitled THERMOELECTRIC DEVICES HAVING REDUCED THERMAL STRESS AND CONTACT RESISTANCE, AND METHODS OF FORMING AND USING THE SAME, now U.S. Patent No.

9,065,017;

- U.S. Patent Application Serial No. 14/679,837, entitled FLEXIBLE LEAD FRAME FOR MULTI-LEG PACKAGE ASSEMBLY, now U.S. Patent Application Publication No. 2015/0287901 ;

- U.S. Patent Application Serial No. 14/682,471 , entitled ULTRA-LONG SILICON NANOSTRUCTURES, AND METHODS OF FORMING AND TRANSFERRING THE SAME, now U.S. Patent Application Publication No. 2016/0035829;

- U.S. Patent Application Serial No. 14/686,641 , entitled MODULAR

THERMOELECTRIC UNITS FOR HEAT RECOVERY SYSTEMS AND METHODS THEREOF, now U.S. Patent Application Publication No. 2015/0287902;

- U.S. Patent Application Serial No. 14/823,738, entitled TIN SELENIDE SINGLE CRYSTALS FOR THERMOELECTRIC APPLICATIONS, now U.S. Patent

Application Publication No. 2016/0049568;

- U.S. Patent Application Serial No. 14/872,681 , entitled THERMOELECTRIC GENERATING UNIT AND METHODS OF MAKING AND USING SAME;

- U.S. Patent Application Serial No. 14/872,898, entitled THERMOELECTRIC GENERATORS FOR RECOVERING WASTE HEAT FROM ENGINE EXHAUST, AND METHODS OF MAKING AND USING SAME, now U.S. Patent Application Publication No. 2016/0099398;

- U.S. Patent Application Serial No. 14/971 ,337, entitled ELECTRICAL AND

THERMAL CONTACTS FOR BULK TETRAHEDRITE MATERIAL, AND METHODS OF MAKING THE SAME, now U.S. Patent Application Publication No.

2016/0190420;

- International Application Patent No. PCT/US2015/053434, entitled

THERMOELECTRIC GENERATING UNIT AND METHODS OF MAKING AND USING SAME, now WO Publication No. 2016/054333;

- International Patent Application No. PCT/US2016/054791 , entitled MECHANICAL ADVANTAGE IN LOW TEMPERATURE BOND TO A SUBSTRATE IN A THERMOELECTRIC PACKAGE; - International Patent Application No. PCT/US2016/056558, entitled OXIDATION AND SUBLIMATION PREVENTION FOR THERMOELECTRIC DEVICES; and

- International Patent Application No. PCT/US2016/066029, entitled MULTI-LAYER THERMOELECTRIC GENERATOR.

[0082] Although various devices have been described herein in connection with certain embodiments, modifications and variations to those embodiments may be implemented. Also, where materials are disclosed for certain components, other materials may be used. Furthermore, according to various embodiments, a single component may be replaced by multiple components, and multiple components may be replaced by a single component, to perform a given function or functions. The foregoing description and following claims are intended to cover all such

modifications and variations.

[0083] Numerous specific details are set forth to provide a thorough understanding of the overall structure, function, manufacture, and use of the embodiments as described in the specification and illustrated in the accompanying drawings. Well- known operations, components, and elements have not been described in detail so as not to obscure the embodiments described in the specification. The reader will understand that the embodiments described and illustrated herein are non-limiting examples, and thus it can be appreciated that the specific structural and functional details disclosed herein may be representative and illustrative. Variations and changes thereto may be made without departing from the scope of the claims.

[0084] The terms "comprise" (and any form of comprise, such as "comprises" and "comprising"), "have" (and any form of have, such as "has" and "having"), "include" (and any form of include, such as "includes" and "including") and "contain" (and any form of contain, such as "contains" and "containing") are open-ended linking verbs. As a result, a thermoelectric system, device, or apparatus that "comprises," "has," "includes" or "contains" one or more elements possesses those one or more elements, but is not limited to possessing only those one or more elements.

Likewise, an element of a system, device, or apparatus that "comprises," "has," "includes" or "contains" one or more features possesses those one or more features, but is not limited to possessing only those one or more features.

[0085] It is to be understood that certain descriptions of the embodiments described herein have been simplified to illustrate only those elements, features, and aspects that are relevant to a clear understanding of the disclosed embodiments, while eliminating, for purposes of clarity, other elements, features, and aspects. Persons having ordinary skill in the art, upon considering the present description of the disclosed embodiments, will recognize that other elements and/or features may be desirable in a particular implementation or application of the disclosed

embodiments. However, because such other elements and/or features may be readily ascertained and implemented by persons having ordinary skill in the art upon considering the present description of the disclosed embodiments, and are therefore not necessary for a complete understanding of the disclosed embodiments, a description of such elements and/or features is not provided herein. As such, it is to be understood that the description set forth herein is merely exemplary and illustrative of the disclosed embodiments and is not intended to limit the scope of the claims.

[0086] Also, any numerical range recited herein is intended to include all subranges subsumed therein. For example, a range of "1 to 10" is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited herein is intended to include all higher numerical limitations subsumed therein. Accordingly, Applicants reserve the right to amend the present disclosure, including the claims, to expressly recite any sub-range subsumed within the ranges expressly recited herein.

[0087] The grammatical articles "one", "a", "an", and "the", as used herein, are intended to include "at least one" or "one or more", unless otherwise indicated.

Thus, the articles are used herein to refer to one or more than one (i.e., to at least one) of the grammatical objects of the article. By way of example, "a component" means one or more components, and thus, possibly, more than one component is contemplated and may be employed or used in an implementation of the described embodiments.

[0088] Any patent, publication, or other disclosure material that is said to be incorporated, in whole or in part, by reference herein is incorporated herein only to the extent that the incorporated material does not conflict with existing definitions, statements, or other disclosure material set forth in this disclosure. As such, and to the extent necessary, the disclosure as set forth herein supersedes any conflicting material incorporated herein by reference. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material set forth herein is only incorporated to the extent that no conflict arises between that incorporated material and the existing disclosure material.

[0089] The present disclosure includes descriptions of various embodiments. It is to be understood that all embodiments described herein are exemplary, illustrative, and non-limiting.

Claims

WHAT IS CLAIMED IS:

1 . A thermoelectric assembly, comprising:

a first substrate;

a second substrate; and

a thermoelectric element positioned intermediate said first substrate and said second substrate, wherein said thermoelectric element comprises:

a thermoelectric substrate comprised of a magnesium silicide-based material; and

an electrode structure, comprising:

a first electrode layer in contact with said thermoelectric substrate;

a second electrode layer; and

a third electrode layer, wherein said second electrode layer is positioned intermediate said first electrode layer and said third electrode layer, wherein said second layer serves as a diffusion barrier between said third electrode layer and said thermoelectric substrate, and wherein said third electrode layer bonds said thermoelectric element to said first substrate.

2. The thermoelectric assembly of Claim 1 , wherein said first electrode layer is comprised of nickel.

3. The thermoelectric assembly of Claim 1 , wherein said first electrode layer is a transition metal that is not nickel.

4. The thermoelectric assembly of Claims 1 , 2, or 3, wherein said first electrode layer comprises at least one of titanium, iron, cobalt, and chromium.

5. The thermoelectric assembly of Claims 1 , 2, 3, or 4, wherein said second layer is a MCrAIY-based compound, where M is at least one of cobalt, nickel, and iron.

6. The thermoelectric assembly of Claim 5, wherein M is cobalt, and wherein the composition of the MCrAIY-based compound is 50-70 percent by weight cobalt, 15-

300₄₄2816 6 30 percent by weight chromium, 5-25 percent by weight aluminum, and 0.1 -5 percent by weight yttrium.

7. The thermoelectric assembly of Claims 1 , 2, 3, or 4, wherein said second layer is a compound of cobalt, chromium, aluminum, and yttrium.

8. The thermoelectric assembly of Claims 1 , 2, 3, or 4, wherein said second layer comprises an electrically-conductive compound that is highly resistant to reaction with at least one of magnesium, oxygen, nickel, transition metals, and noble metals.

9. The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, or 8, wherein said third layer comprises at least one of gold and silver.

10. The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, or 9, wherein said first layer is applied to said thermoelectric substrate by a sputtering process, wherein said second layer is applied to said first layer by a sputtering process, and wherein said third layer is applied to said second layer by a sputtering process.

1 1 . The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, 9, or 10, wherein said first layer is applied to said thermoelectric substrate by a thin film deposition process, wherein said second layer is applied to said first layer by a thin film deposition process, and wherein said third layer is applied to said second layer by a thin film deposition process.

12. The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, or 1 1 , wherein said first layer is applied to said thermoelectric substrate during a sintering process.

13. The thermoelectric assembly of Claim 12, wherein said second layer is applied to said first layer by a thin film deposition process, and wherein said third layer is applied to said second layer by a thin film deposition process.

14. The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, or

13, wherein said magnesium silicide-based material comprises magnesium silicide stannide.

15. The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, or 14, wherein said first layer comprises a thickness which is greater than 10 nm and less than 10 microns, wherein said second layer comprises a thickness which is greater than 10 nm and less than 10 microns, and wherein said third layer comprises a thickness which is greater than 10 nm and less than 10 microns.

16. The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, or 14, wherein said first layer comprises a thickness which is greater than 1 nm and less than 100 microns, wherein said second layer comprises a thickness which is greater than 1 nm and less than 100 microns, and wherein said third layer comprises a thickness which is greater than 1 nm and less than 100 microns.

17. The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13,

14, 15, or 16, wherein said thermoelectric element further comprises a second electrode structure, comprising:

a fourth electrode layer in contact with said thermoelectric substrate;

a fifth electrode layer; and

a sixth electrode layer, wherein said fifth electrode layer is positioned intermediate said fourth electrode layer and said sixth electrode layer, wherein said fifth electrode layer serves as a diffusion barrier between said sixth electrode layer and said thermoelectric substrate, and wherein said sixth electrode layer bonds said thermoelectric element to said second substrate.

18. The thermoelectric assembly of Claim 17, wherein said fourth electrode layer is comprised of nickel, wherein said fifth layer is a MCrAIY-based compound, where M is at least one of cobalt, nickel, and iron, and wherein said sixth layer comprises at least one of gold and silver.

19. The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, or 18, wherein said thermoelectric assembly is a Seebeck device.

20. The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, or 18, wherein said thermoelectric assembly is a Peltier device.

21 . The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, or 20, wherein said magnesium silicide-based material comprises at least five atomic percent (5 at%) of Mg₂Sn.

22. The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, or 20, wherein said magnesium silicide-based material comprises at least fifty atomic percent (50 at%) of Mg₂Sn.

23. The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , or 22, further comprising a second said thermoelectric element positioned intermediate said first substrate and said second substrate.

24. The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, or 23, wherein said second electrode layer is comprised of titanium and boride.

25. The thermoelectric assembly of Claims 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, or 24, wherein said second electrode layer is comprised of indium and tin.

26. A thermoelectric assembly, comprising:

a first substrate;

a second substrate; and

a plurality of thermoelectric elements positioned intermediate said first substrate and said second substrate, wherein each said thermoelectric element comprises:

a thermoelectric substrate comprised of magnesium silicide stannide; and an electrode structure, comprising:

a contact layer in contact with said thermoelectric substrate, wherein said contact layer is comprised of nickel;

a diffusion barrier layer, wherein said diffusion barrier layer is a CoCrAIY-based compound; and

a bonding layer comprising gold, wherein said diffusion barrier layer is positioned intermediate said contact layer and said bonding layer, wherein said diffusion barrier layer serves as a diffusion barrier between said bonding layer and said thermoelectric substrate, and wherein said bonding layer bonds said thermoelectric element to said first substrate.

27. A method of manufacturing a thermoelectric element, comprising the steps of: obtaining a thermoelectric material comprised of a magnesium silicide-based material;

coating at least one surface of the thermoelectric material with an electrode; applying a diffusion barrier layer to the electrode, wherein the diffusion barrier layer is comprised of chromium, aluminum, and yttrium; and

applying a bonding layer to the diffusion barrier layer.

28. The method of Claim 27, wherein the magnesium silicide-based material is magnesium silicide stannide.

29. The method of Claims 27 or 28, wherein one or more of said electrode coating step, said diffusion barrier layer applying step, and said bonding layer applying step are performed by at least one process including at least one of electroplating, thermal spraying, flame synthesis, diffusion bonding, co-sintering, and laser melting.

30. The method of Claims 27, 28, or 29, wherein the diffusion barrier layer is further comprised of cobalt.

31 . The method of Claims 27, 28, 29, or 30, wherein the bonding layer is comprised of at least one of silver and gold.

32. The method of Claims 27, 28, 29, 30, or 31 , wherein said step of applying the diffusion barrier layer comprises sputtering the diffusion barrier layer onto the contact layer.

33. The method of Claims 27, 28, 29, 30, 31 , or 32, wherein said step of applying the diffusion barrier layer onto the metal comprises using a thin film deposition process.

34. The method of Claims 27, 28, 29, 30, 31 , 32, or 33, wherein said step of applying the bonding layer comprises sputtering the diffusion barrier layer onto the diffusion barrier layer.

35. The method of Claims 27, 28, 29, 30, 31 , 32, 33, or 34, wherein said step of applying the bonding layer onto the diffusion barrier layer comprises using a thin film deposition process.

36. The method of Claims 27, 28, 29, 30, 31 , 32, 33, 34, or 35, wherein said step of obtaining a thermoelectric material comprises the step of sintering a powder.

37. The method of Claim 36, wherein said sintering step and said electrode coating step are performed at the same time.

38. The method of Claim 36, wherein said electrode coating step is performed after said sintering step.

39. The method of Claims 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, or 38, wherein said electrode coating step comprises sputtering the metal onto the thermoelectric material.

40. The method of Claims 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, or 39, wherein said electrode coating step comprises using a thin film deposition process.

41 A thermoelectric assembly, comprising:

a first substrate; a second substrate; and

a thermoelectric substrate comprised of magnesium silicide stannide; and

an electrode structure, comprising:

a diffusion barrier layer, wherein said diffusion barrier layer is comprised of titanium diboride; and

42. A thermoelectric assembly, comprising:

a first substrate;

a second substrate; and

a thermoelectric substrate comprised of magnesium silicide stannide; and

an electrode structure, comprising:

a diffusion barrier layer, wherein said diffusion barrier layer is comprised of indium tin oxide; and

43. A thermoelectric assembly, comprising:

a first substrate;

a second substrate; and

an electrode structure which is used to mechanically and electrically bond said thermoelectric substrate to said first substrate.

44. The thermoelectric assembly of Claim 43, wherein said electrode structure consists of only one electrode layer that serves as a contact layer, a diffusion barrier layer, and a bonding layer.

45. The thermoelectric assembly of Claim 43, wherein said electrode structure consists of two electrode layers where a first layer serves as a contact and diffusion barrier layer and a second layer serves as a bonding layer.

46. The thermoelectric assembly of Claim 43, wherein said electrode structure consists of two electrode layers where a first layer serves as a contact layer and a second layer serves as a diffusion barrier layer and a bonding layer.

47. The thermoelectric assembly of Claims 44, 45, or 46, wherein said diffusion barrier layer comprises titanium and boron.

48. The thermoelectric assembly of Claim 47, wherein said diffusion barrier layer is titanium diboride.

49. The thermoelectric assembly of Claims 44, 45, 46, 47, or 48, wherein said diffusion barrier layer comprises an oxide of indium and tin.

50. The thermoelectric assembly of Claim 49, wherein said diffusion barrier layer is indium tin oxide.

51 . The thermoelectric assembly of Claims 44, 45, 46, 47, 48, 49, or 50, wherein said diffusion barrier layer comprises an electrically-conductive oxide.

52. The thermoelectric assembly of Claims 44, 45, 46, 47, 48, 49, 50, or 51 , wherein said diffusion barrier layer comprises a material with amorphous grain structure.

53. The thermoelectric assembly of Claims 43, 44, 45, 46, 47, 48, 49, 50, 51 , or

52, wherein said electrode structure comprises one, two, or three distinct layers, and wherein at least a portion of said electrode structure is applied to said thermoelectric substrate by a thick film deposition process.

54. The thermoelectric assembly of Claim 53, wherein said thick film deposition process comprises at least one of electroplating, thermal spraying, flame synthesis, diffusion bonding, co-sintering, and laser melting.

55. The thermoelectric assembly of Claims 43, 44, 45, 46, 47, 48, 49, 50, 51 , 52,

53, or 54, wherein said electrode structure comprises a plurality of layers, wherein one or more of said layers is applied to said thermoelectric substrate by a thick film deposition process, and wherein one or more of said layers is applied to said thermoelectric substrate by a thin film deposition process.

56. A method of manufacturing a thermoelectric element, comprising the steps of: obtaining a thermoelectric material comprised of a magnesium silicide-based material;

coating at least one surface of the thermoelectric material with an electrode; applying a diffusion barrier layer to the electrode, wherein the diffusion barrier layer is comprised of titanium and boron; and

applying a bonding layer to the diffusion barrier layer.

A method of manufacturing a thermoelectric element, comprising the steps of: obtaining a thermoelectric material comprised of a magnesium silicide-based material;

coating at least one surface of the thermoelectric material with an electrode; applying a diffusion barrier layer to the electrode, wherein the diffusion barrier layer is comprised of indium and tin; and

applying a bonding layer to the diffusion barrier layer.

58. A thermoelectric element, comprising:

an electrode structure which is used to mechanically and electrically bond said thermoelectric substrate to a substrate, wherein said electrode structure comprises:

a first electrode layer applied to said thermoelectric substrate; a second electrode layer;

a third electrode layer, wherein said second electrode layer is positioned intermediate said first electrode layer and said third electrode layer; and a fourth electrode layer, wherein said third electrode layer is positioned intermediate said second electrode layer and said fourth electrode layer.

59. The thermoelectric element of Claim 58, wherein said first electrode layer is a diffusion barrier layer.

60. The thermoelectric element of Claims 58 or 59, wherein said second electrode layer is a diffusion barrier layer.

61 . The thermoelectric element of Claims 58, 59, or 60, wherein said third electrode layer is a diffusion barrier layer.

62. The thermoelectric element of Claims 58, 59, 60, or 61 , wherein said fourth electrode layer is a diffusion barrier layer.

63. The thermoelectric element of Claims 58, 59, 60, 61 , or 62, further comprising a fifth electrode layer, wherein said fourth electrode layer is positioned intermediate said third electrode layer and said fifth electrode layer.

64. The thermoelectric element of Claim 63, wherein said fifth electrode layer is a diffusion barrier layer.