This application claims priority to U.S. Provisional Application No. 61/674,816, filed Jul. 23, 2012, commonly assigned and incorporated by reference herein for all purposes.
Additionally, this application is related to U.S. patent application Ser. No. 12/943,134 and Ser. No. 13/299,179, and U.S. Provisional Application No. 61/597,254 and No. 61/607,739, which are incorporated by reference herein for all purposes.
The present invention is directed to thermoelectric module assembly. More particularly, the invention provides a method for forming an assembly in series of thermoelectric (TE) unicouples. Merely by way of example, the invention has been applied for sizing and assembling silicon-based thermoelectric unicouples bonded thermally in parallel and electrically in series with silicon-based contact wafers and heat sinks to form a 3D package of thermoelectric modules capable of generating electrical energy from high temperature waste heat. It would be recognized that the invention has a much broader range of applicability without limiting to specific material based TE unicouples, in various fields including but not limited to automobile combustion, industrial hot exhaust, nuclear power plants, and aircraft turbines.
An actual thermoelectric device must transport significant amounts of current from one electrode to another in the case of power generation, where a temperature gradient is applied to the thermoelectric material and the Seebeck effect is employed to drive a gradient in voltage and in turn the flow of electrical current. Conversely, an actual thermoelectric device used for refrigeration must carry an appreciable amount of heat with an applied electric current by way of the Peltier effect. In both of these thermoelectric device configurations, the thermoelectric figure of merit ZT of the thermoelectric material is one indicator of the material's efficiency in either converting heat to electricity (Seebeck effect, or thermopower) or pumping heat with electricity (Peltier effect).
In a thermoelectric device, electrodes must be placed on either ends of a thermoelectric material in order to collect current from it or transmit current through it. These electrodes must be made such that they form low resistance electrical and thermal contact to the thermoelectric material with high ZT value, and furthermore allow each TE “leg,” or single element of either p-type or n-type semiconductor material, to be wired together among other TE legs and external circuitry. A TE unicouple (or simply referred as “unicouple”) is a building block used for assembling an actual thermoelectric device with corresponding electrodes. In particular, the unicouple is a three-dimensional structure comprising a first conductive shunt material coupled on a hot side of a p-type thermoelectric leg and an n-type thermoelectric leg and a second conductive shunt material coupled to a cold side of either one or the p-type thermoelectric leg or the n-type thermoelectric leg.
Many efforts for improving thermoelectrics have been made to search for new advanced thermoelectric materials with high ZT value, to determine optimum unicouple structure associated with the thermoelectric material, and to develop feasible processes for forming the unicouples and assembling them to thermoelectric devices. Conventional high ZT thermoelectric materials such as bismuth telluride (Bi2Te3), either in bulk or nanostructured form or alloy form combined with other materials (Ce, Fe, Sb, etc.), have been used in some thermoelectric applications. However, other than the high cost and complexity of manufacturing these materials, which also have toxic characteristics, the poor high-temperature adaptability of such thermoelectric materials also substantially limits these devices to applications in relatively low temperature environments. This drives efforts in research and development on advanced, low-cost, silicon-based TE unicouples for assembling a thermoelectric module that can be used for wide range of temperatures, especially for waste-heat power generation application at high temperature greater than 600° C. as well as refrigeration application for electronic system.
The present invention is directed to thermoelectric module assembly. More particularly, the invention provides a method for forming an assembly in series of thermoelectric (TE) unicouples. Merely by way of example, the invention has been applied for sizing and assembling silicon-based thermoelectric unicouples bonded thermally in parallel and electrically in series with silicon-based contact wafers and heat sinks to form a 3D package of thermoelectric modules capable of generating electrical energy from high temperature waste heat. It would be recognized that the invention has a much broader range of applicability without limiting to specific material based TE unicouples, in various fields including but not limited to automobile combustion, industrial hot exhaust, nuclear power plants, and aircraft turbines.
According to one or more embodiments of the present invention, methods are provided for assembling the thermoelectric unicouples in a series configuration based on silicon wafer processing technology although the method should be applicable to assemble suitably metalized legs of bismuth telluride or other TE materials. The method includes processes of determining optimum sizes of individual n-type or p-type thermoelectric leg, forming p-n singulated unicouples, picking up and re-disposing the singulated unicouples thermally in parallel and electrically in series with pre-determined spacing and arrangement, and bonding the unicouples with metalized shunts and further coupling the shunts with thermally matched heat sinks.
In a specific embodiment, the present invention provides a method for assembling a plurality of thermoelectric unicouples. The method includes providing a plurality of blocks with a rectangular shape having a width and a length made by either n-type or p-type thermoelectric functional semiconductor material. The method further includes disposing the plurality of blocks on a first shunt wafer in a 2D array wherein each n-type block is alternately disposed next to a p-type block. Additionally, the method includes performing a first cutting operation along the length of the block to reduce the width of each block and increase a gap spacing between two neighboring blocks while substantially free from removing any material of the first shunt wafer. The method further includes performing a second cutting operation along the width of each block through the first shunt wafer below to divide each block along the length into a column of multiple units; furthermore; the method includes performing a third cutting operation along middle line of each column of multiple units through the first shunt wafer below to further cut each unit into two thermoelectric functional legs respectively attached on two separate remaining pieces of the first shunt wafer. The combination of the first cutting operation, the second cutting operation, and the third cutting operation causes a formation of a plurality of singulated unicouples. Each singulated unicouple is made of a n-type thermoelectric functional leg attached at one end and a p-type thermoelectric functional leg attached at another end of a same remaining piece of the first shunt wafer. The method also includes re-arranging the plurality of singulated unicouples in one or more serial chains by bonding the remaining piece of the first shunt wafer of each singulated unicouple onto a first base plate such that every singulated unicouple in the serial chain comprises a same spatial orientation of a n-type thermoelectric functional leg on one end and a p-type thermoelectric functional leg on other end of a same piece of the first shunt wafer. The method continues to include bonding a second shunt wafer onto the n-type thermoelectric functional leg and the p-type thermoelectric functional leg of each singulated unicouple in the one or more serial chains. Moreover, the method includes performing a fourth cutting operation to remove material of the second shunt wafer partially from regions beyond two longitudinal edges of each serial chain and regions between the n-type thermoelectric functional leg and the p-type thermoelectric functional leg of each singulated unicouple while substantially free from removing any material of the first shunt wafer and the first base plate. The method further includes attaching a second base plate from above to bond each and every remaining piece of the second shunt wafer.
In another specific embodiment, the present invention provides a method for assembling thermoelectric unicouples to form a thermoelectric module. The method includes disposing a plurality of thermoelectric blocks with either n-type or p-type semiconductor characteristic onto a first shunt wafer. The method further includes resizing the plurality of thermoelectric blocks and a gap spacing between a n-type block and a p-type block without removing any material from the first shunt wafer. Additionally, the method includes removing partially materials of the thermoelectric block and the first shunt wafer along a middle region of each thermoelectric block to form a plurality of singulated unicouples comprising a separate partial piece of the first shunt wafer with a n-type thermoelectric leg attached on one end and a p-type thermoelectric leg attached on another end. The method further includes rearranging the plurality of singulated unicouples to form one or more daisy chains on a first heat sink plate. Each singulated unicouple within each of the one or more daisy chains has a same spatial orientation of the n-type thermoelectric leg and the p-type thermoelectric leg and is disposed at a predetermined space from a neighboring singulated unicouple. Furthermore, the method includes bonding a second shunt wafer onto the plurality of singulated unicouples. The method further includes resizing the second shunt wafer to retain a partial piece of the second shunt wafer connecting the n-type thermoelectric leg of one singulated unicouple with the p-type thermoelectric leg of the neighboring singulated unicouple. Moreover, the method includes attaching a second heat sink plate onto each retained partial piece of the second shunt wafer.
In yet another specific embodiment, a method for forming a plurality of singulated unicouples for assembling a thermoelectric module is provided. The method includes providing a plurality of blocks with a rectangular shape having a width and a length made by either n-type or p-type thermoelectric functional semiconductor material. Additionally, the method includes disposing the plurality of blocks on a conductive shunt wafer in a 2D array wherein each n-type block is alternately disposed next to a p-type block. The method further includes performing a first cutting operation along the length of the block to reduce the width of each block and increase a gap spacing between two neighboring blocks while substantially free from removing any material of the conductive shunt wafer. Furthermore, the method includes performing a second cutting operation along the width of each block through the conductive shunt wafer below to divide each block along the length into a column of multiple units. Moreover, the method includes performing a third cutting operation along middle line of each column of multiple units through the conductive shunt wafer below to further cut each unit into two thermoelectric functional legs respectively attached on two separate remaining pieces of the conductive shunt wafer, thereby forming a plurality of singulated unicouples each comprising a n-type thermoelectric functional leg attached at one end and a p-type thermoelectric functional leg attached at another end of a same remaining piece of the conductive shunt wafer.
In still another specific embodiment, a method for assembling a plurality of singulated unicouples to form a thermoelectric module is provided. The method includes providing a plurality of singulated unicouples. Each singulated unicouple is formed with a n-type thermoelectric functional leg and a p-type thermoelectric functional leg respectively attached to two ends of a stripe-shaped piece of a first shunt material. The method further includes arranging the plurality of singulated unicouples in one or more serial chains by bonding each piece of the first shunt material onto a first base plate, each serial chain comprising a same spatial orientation such that the n-type thermoelectric functional leg of a singulated unicouple is spatially opposed to a p-type thermoelectric functional leg of a next singulated unicouple with a predetermined spacing. Additionally, the method includes bonding a wafer piece of a second shunt material from above with each of the plurality of singulated unicouples in the one or more serial chains. Furthermore, the method includes performing a cutting operation to remove the second shunt material partially from regions beyond two longitudinal edges of each serial chain and regions between the n-type thermoelectric functional leg and the p-type thermoelectric functional leg of each singulated unicouple while substantially free from removing any first shunt material and the first base plate. Moreover, the method includes attaching a second base plate from above to bond with each and every remaining piece of the second shunt material.
The Si-based TE material can be fabricated directly out of a silicon wafer material to bear high-ZT characteristics. It is desirable to transform as much as possible of a starting wafer material into functionalized thermoelectric units. The ultimate commercial performance, and therefore usefulness, of a power generation thermoelectric is governed by its cost-per-Watt. It is beneficial to process a single piece of material, for example, a silicon wafer, in such a fashion as its use as a thermoelectric is maximized, since processing steps for most two-dimensional semiconductor material or the like cost the same amount regardless of the material thickness. In a specific embodiment, the Si-based TE units with high-ZT characteristics can be realized by transforming bulk crystal silicon material into nanostructured material. More details on methods for forming the bulk-sized nano-structured TE material units, including array of nanowires, nanoholes, or nano-porous blocks and transforming these nanostructures to form thermoelectric elements from a finite silicon wafer can be found in U.S. patent application Ser. No. 12/943,134 and Ser. No. 13/299,179, and U.S. Provisional Application No. 61/597,254 and No. 61/607,739, commonly assigned to Alphabet Energy, Inc. of Hayward, Calif., incorporated as references for all purposes.
FIG. 1 shows a schematic diagram of a thermoelectric unicouple for assembling a thermoelectric module according to an embodiment of the present invention.
FIG. 1A shows a schematic diagram of one of a series of thermoelectric unicouples being bonded between two (a hot and a cold) heat sinks as a part of an assembled thermoelectric module according to embodiments of the present invention.
FIG. 2 schematically shows one or more steps of a method for forming a plurality of thermoelectric unicouples by cutting a wafer with basic thermoelectric material to proper sized blocks according to an embodiment of the present invention.
FIG. 2A schematically shows two cut blocks respectively bearing a n-type and a p-type semiconductor characteristics from corresponding wafers according to a specific embodiment of the present invention.
FIG. 3 schematically shows one or more steps of the method for assembling thermoelectric module according to an embodiment of the present invention.
FIG. 3A is a cross-sectional view of FIG. 3 including a closer view of local structures of the thermoelectric module assembly according to an embodiment of the present invention.
FIG. 4 is a schematic diagram showing another process of the method for assembling thermoelectric module according to an embodiment of the present invention.
FIG. 4A is a cross-sectional view of FIG. 4 including a closer view of local structures of the thermoelectric module assembly according to an embodiment of the present invention.
FIG. 5 is a schematic diagram showing additional processes with a series of wafer cutting operations for forming the thermoelectric unicouples according to an embodiment of the present invention.
FIG. 5A is a cross-sectional view of FIG. 5 including a closer view of local structures of the thermoelectric module assembly according to an embodiment of the present invention.
FIG. 6 is a schematic diagram showing a process of re-distributing multiple singulated unicouples according to an embodiment of the present invention.
FIG. 6A is a cross-sectional view of FIG. 6 including a closer view of local structures of the thermoelectric module assembly according to an embodiment of the present invention.
FIG. 7 is a schematic diagram showing additional processes of the method for forming the thermoelectric unicouples according to an embodiment of the present invention.
FIG. 7A is a cross-sectional view of FIG. 7 including a closer view of local structures of the thermoelectric module assembly according to an embodiment of the present invention.
FIG. 8 is a schematic diagram showing a subsequent process of the method for assembling thermoelectric unicouples to form a thermoelectric module according to an embodiment of the present invention.
FIG. 8A is a cross-sectional view of FIG. 8 including a closer view of local structures of the thermoelectric module assembly according to an embodiment of the present invention.
FIG. 9 is a simplified chart showing a method of forming and assembling a plurality of thermoelectric unicouples according to an embodiment of the present invention.
The present invention is directed to thermoelectric module assembly. More particularly, the invention provides a method for forming an assembly in series of thermoelectric (TE) unicouples. Merely by way of example, the invention has been applied for sizing and assembling silicon-based thermoelectric unicouples bonded thermally in parallel and electrically in series with silicon-based contact wafers and heat sinks to form a 3D package of thermoelectric modules capable of generating electrical energy from high temperature waste heat. It would be recognized that the invention has a much broader range of applicability without limiting to specific material based TE unicouples, in various fields including but not limited to automobile combustion, industrial hot exhaust, nuclear power plants, and aircraft turbines.
FIG. 1 shows a schematic diagram of a thermoelectric unicouple as a material/structure unit for assembling a thermoelectric module in a daisy chain configuration according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown, a couple of pre-fabricated thermoelectric legs are provided. One leg 121 is functionalized as a p-type semiconductor characteristic and another leg 122 is functionalized as an n-type semiconductor characteristic. In a specific embodiment, the thermoelectric legs 121 and 122, are made from silicon based nanostructures having very low thermal conductivity and high electrical conductivity. The method of fabricating the thermoelectric legs includes forming metallization layers on two contact faces of the TE legs. As shown, a first metallization layer 132 is formed on a first ending face of a TE leg 121 or 122 and a second metallization layer 133 is formed on a second ending face of the TE leg 121 or 122. The two TE legs 121 and 122 are bonded commonly with a shunt material 112. In an embodiment, the shunt material 112 is a silicon-based sheet material (flexible or rigid) processed to be a good thermal conductor and a good electrical conductor. The silicon-based shunt material 112 has its contact face coated with another metallization layer 131 that is bonded with the first metallization layer 132 of the two TE legs 121 and 122. In a specific embodiment, this shunt material 112 is configured to attach to a heat sink located at hot side (or high temperature side), namely it is called hot-side shunt. The second ending faces of the two TE legs 121 and 122 are respectively bonded to a cold-side shunt material 111. In another specific embodiment, the cold-side shunt material 111 is also a silicon-based wafer having a metallization layer 134 that is bonded with the metallization layer 133 at the second ending face of either TE leg 121 or TE leg 122. This forms a unit structure for creating a thermoelectric device and is called a thermoelectric unicouple 100.
One advantage of the present invention lies in the substantial usages of silicon as a main TE material for assembling the thermoelectric module that can be adaptable to high temperature environment with improved thermal contact and easy-to-match coefficient of thermal expansion between TE legs and shunt materials. It has also been shown (see references in US patent applications cited above) that the silicon-based thermoelectric leg, made by creating one or more novel nanostructures out of a bulk silicon material, can be very cost-effective and high in efficiency. Silicon-based material applied as a shunt material and a heat sink provides naturally excellent matching of its coefficient of thermal expansion with that of the silicon-based TE legs, minimizing associated contact stress especially in an environment with temperature greater than 600° C. Additionally, as a technologically well established electronic material, silicon can be processed easily with low cost for many aspects associated with the thermoelectric unicouple manufacture and module assembly. For example, the polished surface of silicon wafer/sheet/film provides an improved thermal contact interface. With proper doping treatment, it can be used either as a good insulator or a material that conducts electrical current. Other advantages of use of silicon for thermoelectric modules lie in the ability of silicon-based material to withstand both high and low temperature without inducing electronic property deficiency or mechanical failure of the thermoelectrics and to form improved contact conformity with the flexible silicon shunts. Many advantages can be found throughout the specification and in particularly below.
FIG. 1A shows a schematic diagram of one of a series of thermoelectric unicouples being bonded between two (a hot and a cold) heat sinks as a part of the assembled thermoelectric module according to embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown, each thermoelectric unicouple 100 acts as a unit element of the assembled module. The top surface of the hot-side shunt 112 is bonded via a metallization layer 135 with a hot-side heat sink material 109. The hot-side heat sink 109, in fact, is an extended piece of wafer or sheet material configured to bond a series of unicouples at their hot-side shunts. Also, the bottom surface of the cold-side shunt 111 is bonded via another metallization layer 136 with a cold-side heat sink material 101. Again, the cold-side heat sink is an extended piece of wafer or sheet material configured to bond a series of unicouples at their cold-side shunts. In an example, either the hot-side or cold-side heat sink material is a silicon-based wafer or a piece of sheet material that is configured to provide substantially matching of thermal expansion coefficients of the shunt material versus sink material and enhance thermal conductance through their junctions. The heat sink material, however, is processed to maintain substantially low electrical conductivity to restrain the thermoelectric current passing along the shunt-leg-shunt pathway only through the series of TE unicouples. The structure as shown provides a single unit of an assembled thermoelectric module. The hot-side heat sink 109 and the cold-side heat sink 101 will respectively be in contact with or by itself be part of a hot-side and a cold-side heat exchanger when the assembled thermoelectric module is applied for power generation or for cooling purpose. Of course, the sink material can be changed if the shunt material changes or modified. For example, the shunt material may use good conductor like Cu or metalized ceramic and the sink material may use anodized Aluminum or stain-less steel or ceramic. Alternatively, an electrically insulated thermal interface material may be inserted between the shunt material and sink material.
In an embodiment, the metallization layers, associated with either the thermoelectric leg or the shunt material, include multilayers of metals or metal-alloy films, for example, Ti/TiN/Ni/Au/Au—Sn-alloy for the cold-side shunt, or Ti/TiN/W/W—Pt—B-alloy for the hot-side shunt. The Au—Sn alloy and the W—Pt-alloy containing 2% of B are provided as a form of nanofoil overlying the rest of the metallization layers. Nanofoil provides a unique brazing media for coupling the two metallization layers and forming excellent electrical and thermal contact at a preset stress level. Of course, there are many variations, alternatives, and modifications in layer thickness, stacking order, and specific material selection. For example, Ni layer may be replaced by a Ti layer. The metallization layers associated with the thermoelectric leg is bonded with another metallization layer of the shunt material by a bonding process. The bonding process employs localized heating of one or more layers of metal/alloy material inserted at the interface. The heating temperature locally is reached slightly above its melting point of the inserted layer to cause bonding between two metallization layers. Depending on the bonding location at the hot side or cold side of the thermoelectric leg, the inserted layer is chosen to have a melting point closely matching with the estimated working temperature correspondingly at the hot side or the cold side of the final assembled thermoelectric module. This bonding process substantially reduces the mechanical stress induced by the thermal process and enhances the application reliability of the assembled thermoelectric module.
FIGS. 2-8 schematically show a method for forming one or more thermoelectric unicouples and assembling a plurality of unicouples thermally in parallel and electrically in series for a thermoelectric module according to an embodiment of the present invention. These diagrams are merely examples, which should not limit the scope of the claims herein. One ordinary skilled in the art should recognize many alternatives, variations, and modifications. The following descriptions about each figure among the above figures are merely for schematic illustrations of one or more processes of the method using a snap shot of the physical appearance of the structures, arrangements, results of steps partially or completely executed according to the method. The claims herein should not be limited by the scope described in the figures or by any features such as dimensions, shapes, orientations, placement orders, markings for alignment or other purposes, etc. shown in the figures. They should also not be limited to terminologies used to describe the features illustrated, steps performed, tools used along with particular processes based on those exemplary figures. Each figure should not be considered as an exact matching of one step/process of the method and should not be limited in terms of the order or exclusion of additions or subtractions of one or more steps/processes.
FIG. 2 is a schematic diagram showing one or more steps for assembling a thermoelectric module according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. In an embodiment, the method includes forming a basic thermoelectric material out of a finite semiconductor material, for example, a standard silicon wafer. The basic thermoelectric material is characterized by high figure-of-merit ZT in a form of an either n-type or p-type functionalized block having bulk-like dimension. As shown, a silicon wafer is provided for fabricating a silicon-based thermoelectric material. In an embodiment, the silicon wafer is configured to be functionalized as either n-type or p-type material. The as-formed silicon-based thermoelectric material can be in one or more forms of nanostructures created out of the whole (functionalized or standard) silicon wafer. The nanostructures may include arrays of nanowires, or arrays of nanotubes/nanoholes, or a bulk nanohole structure, or a bulk nanoporous structure, or any combination of the as-mentioned nanostructures. In another embodiment, the basic thermoelectric material is characterized as a nanocomposite material, which can be synthesized from a plurality of nano- or micro-sized particles, nanowires, nanotubes, nanoporous structured single-element or alloy materials made by various separate processes including mixing, melting, re-crystallizing, milling, doping, annealing, sintering, and more. The as-formed silicon-based thermoelectric material also bears either n-type or p-type functionalized characteristics, which is either inherited from the functionalized silicon wafer or separately treated after the formation of the one or more forms of nanostructures using doping, ion-implantation, or direct mixing of various electronic impurity elements at a desired density and polarity. Then, a filler material with low thermal/electric conductivity may be added to substantially fill the void spaces within the nanostructures, leading to the formation of the basic thermoelectric material.
FIG. 2 also shows that, after the formation of the basic thermoelectric material, the wafer can be cut into smaller sized blocks as illustrated by the rectangular shaped grid pattern. The size is predetermined in a range of a few millimeters or smaller for the purpose of assembling the thermoelectric module. FIG. 2A just shows two individual blocks after the cut. One block is indicated as n-type which is cut from an n-type functionalized wafer. Another block is p-type bearing property from a p-type functionalized wafer. Each block (either n-type or p-type) is shown in a rectangular shape w×l as a simplest choice although it is not limited to that shape. In a specific embodiment, the cutting process can be performed using known wafer-dicing or sawing technique with proper alignment. In an example, a single block has a width w=w1 of about 11.5 mm and a length l ranging from 30 mm to 100 mm. The particular size here should not limit the scope of claims herein. Other dimensions of the blocks should be applicable depending on the material handled and dicing/sawing tool used. The embodiments as shown provides flexibility to determine the block width w in particular to obtain an optimum size w0 of a thermoelectric leg. An optimum size is partially depended on intrinsic structure and material that forms the leg, which may be varied during development but should not be affected by applying the current method as the block disposition are very adaptive to adjust without changing the tool of the method. Although not shown in the figure, one embodiment of the present invention provides that either of these blocks contains silicon-based nanostructures and corresponding filler material to make it a unique basic thermoelectric material suitable for forming the thermoelectric unicouples and module assembly thereof. Of course, the method for assembling thermoelectric module according to the present invention should not be limited for the silicon-based thermoelectric material. Instead, the method should also be applicable to non-silicon-based basic thermoelectric material.
FIG. 3 is a schematic diagram showing a process of the method for assembling thermoelectric module according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The process is to pick the as-formed functionalized thermoelectric blocks and bond them on to a conductor shunt material. As shown as a top view over the shunt material 300, the n-type thermoelectric blocks are disposed one by one in a column and the p-type thermoelectric blocks are disposed one by one correspondingly in a next column. Further, more columns of alternative n-type and p-type blocks are further disposed over a major surface area of the conductor shunt material. The column to column gap is arranged with a predetermined spacing w2 for properly sizing the final thermoelectric leg through a material-cutting procedure. In certain examples, final optimized size w0 of a thermoelectric leg is about 0.5 mm×0.5 mm=0.25 mm2 to hundreds of millimeter square. The final size w0 will be much smaller than the block size w1 of the as-formed thermoelectric material. But handling larger block size w1 of the thermoelectric material is easier and causes much less chance to damage the nanostructures associated with the basic thermoelectric material. In another embodiment, while sizing the final thermoelectric leg around 0.25 mm2 to hundreds of millimeter square is designed for making suitable dimensions of the thermoelectric unicouples used for assembling a module with nearly the highest thermoelectric efficiency for particular types of thermoelectric material used for each leg. For example, a nanostructured Si-based leg, or SiGe alloy based leg, or conventional bismuth telluride alloy based leg, can result in different optimum size in leg dimension.
In a specific embodiment, the conductor shunt material is a silicon-based thin wafer or sheet material that is chosen for matching the silicon-based thermoelectric material. The shunt material can be rigid or flexible in mechanical characteristics. Alternatively, the shunt material can be a metal and metal alloy sheet material or a conductive ceramic material. The shape of the shunt material used should not be a limiting element and it can be round shape as shown or a rectangular shape in an actual operation. In one example, the conductor shunt material may be called cold-side shunt as it is configured to be associated with a cold-side heat sink of final assembled thermoelectric module. FIG. 3A is a cross-sectional view along a BB′ line of the FIG. 3 showing a plurality of thermoelectric material blocks having a first width w1 disposed on a wafer of shunt material 300. A closer view of the circled portion in FIG. 3A illustrates alternative disposition of n-type block and p-type block having the first width w1 and a first gap w2 spacing between each other. In an example, the first width w1 of each block is about 11.5 mm, and the first gap w2 spacing between them is about 4.5 mm.
In another specific embodiment, bonding the thermoelectric material on the shunt material is performed using a bonding process. The silicon-based shunt wafer is processed to have its surface metalized with a bonding material. The bonding material for bonding with the metallization layer of the thermoelectric blocks is selected to have its working temperature substantially close to a cold-side working temperature of the thermoelectric module having the same shunt material in use. As the bonding material is partially melted at the working temperature by locally applied heat, the metallization layer of thermoelectric material blocks are bonded to the surface of the shunt material, reducing substantially potential thermal stress around the bonding region during the application of the thermoelectric module in the designated temperature environment.
FIG. 4 is a schematic diagram showing another process of the method for assembling thermoelectric module according to an embodiment of the present invention. It is a top view of the FIG. 3 having multiple columns of n- and p-type blocks subjected to one or more first cutting operations along a CC′ direction. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown, the first cutting operation is executed with multiple cuts in a zone defined by a shaded rectangular region covering an extended range across the gap between two columns of thermoelectric material blocks. Additionally, each cut is substantially confined to a depth of the thickness of the thermoelectric material bonded on the shunt material 300. Therefore, the first cutting operation in the defined zone results in removals of part of the n-type thermoelectric block on one side of the first gap w2 and part of the p-type thermoelectric block on the other side of the gap w2, resulting a reduced block width (a.k.a. a second width w3) and an enlarged block-to-block gap (a.k.a. a second gap w4).
FIG. 4A shows a cross-sectional view and closer view of multiple columns of thermoelectric blocks having the reduced second width w3 and the second gap w4. The first cutting operation can be conveniently programmed to adjust the process parameters such as the second width and second gap based on one or more embodiments of the present method. In a specific example, the first cutting operation reduces 4 mm width of the n-type block and also reduces 4 mm width of the next p-type block. The block-to-block gap spacing may be enlarged from about 4.5 mm to about 12.5 mm. In an embodiment, the first cutting operation is one of a series of cutting operations for forming the thermoelectric unicouples with optimum and suitable dimensions in a 3D configuration for assembling thermoelectric modules.
FIG. 5 is a schematic diagram showing additional processes with a series of wafer cutting operations for forming the thermoelectric unicouples according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The top view diagram shows that a second cutting operation is performed to execute a plurality of parallel cuts along DD′ direction. In an example, the DD′ cut direction is perpendicular to the CC′ cut direction (in FIG. 4). The cutting/sawing tool for the second cutting operations can be the same as for the first cutting operation. The second cutting operation is configured to cut to a depth at least through both the thermoelectric material block and the shunt material 300 underneath along the DD′ direction, which leads to a formation of multiple parallel stripes of a serial chain of alternate n-type and p-type blocks. Each stripe has a predetermined width d0 ranging from a few millimeters or greater. The width d0 becomes one dimension (a.k.a. the depth) of a final thermoelectric leg. Although it should not be a limiting factor and the width d0 can be adjusted with leg size optimization process. Cutting width of the second cutting operation is configured to be minimum (limited by the tool) so as to minimize material loss along the cutting line. In an embodiment, the shunt material can be pre-disposed over a tape material before the second cutting operation. The tape material still holds the cut stripes after the second cutting operation.
FIG. 5 also shows that at least a third cutting operation is performed along EE′ direction with a cutting line set to a central line through each n-type and p-type block with the second width w3. The EE′ cut direction is substantially parallel to the CC′ cut direction. In an embodiment, the third cutting operation is configured to cut through the thermoelectric material as well as the shunt material 300 underneath with a finite cutting width w5. Therefore, the third cutting operation further reduces the second width of each n-type or p-type block to a third width and separates stripe of shunt material 300 into multiple sections 300A. It is intended to obtain a final width w0 of a pair of thermoelectric legs defined by w0=w3−w5. In a specific example, the cutting width w5 of the third cutting operation is set to be about 2.5 mm. As the result on this cutting width w5, each n-type block or p-type block is reduced from a single block of 3.5 mm or greater to two n-type legs or two p-type legs of about w0=0.5 mm or greater, as seen in a detailed view in FIG. 5A. One n-type leg and one p-type leg are respectively attached to two ends of each section of shunt material 300a, forming a singulated unicouple 500.
FIG. 5A shows a cross-sectional view along the second cutting line and a detailed view of the circled region with several n-type legs and p-type legs on isolated sections of shunt material in a serial chain configuration. After the third cutting operation, each of a plurality of parallel serial chains of alternate n-type and p-type legs along the DD′ direction is separated into multiple individual singulated unicouples 500 and 500A with an alternate n-p and p-n sequential order of the leg spatial orientation. The singulated unicouple 500A is substantially the same as singulated unicouple 500 except that the former one is oppositely oriented. Because the shunt material 300 has been pre-attached on a tape material (from bottom, not shown), each singulated unicouple 500 remains being attached with the tape material in its position.
FIG. 6 is a schematic diagram showing a process of re-distributing the singulated unicouples according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown, the individual singulated unicouples 500 resulted from the third cutting operation are respectively picked up from the tape material and re-disposed on a heat sink material 600 one after another in multiple thermally in parallel electrically in series chain configurations along FF′ direction. Process equipment used for this operation is a die bonder consisting of a dispenser which provides uniform epoxy or silver glass dispensing with consistent material thickness and precise die placement in a given location. Then it is used to pick, align and dispose a singulated unicouple 500 in the same location where a preformed epoxy or dispensed material has been placed along FF′ lines. The process for redistributing the singulated unicouples includes aligning multiple singulated unicouples 500 to form a serial chain in an n-p n-p sequential order along the FF′ lines (switching orientation of singulated unicouple 500A, see FIG. 5A). The process further bonds each singulated unicouple 500, including an n-type leg and a p-type leg respectively on two ends of each section of shunt material 300A, to a metallization layer overlying the heat sink material 600 to complete the chain configuration. The heat sink material can also be selected from a silicon-based wafer or sheet material for matching coefficient of thermal expansion of the silicon-based shunt material and silicon nanostructured thermoelectric legs. The heat sink material, in an example, is a cold-side heat sink configured to be in contact with a cold-side heat exchanger as the to-be-formed thermoelectric module is disposed. In the above serial chain configuration, each singulated unicouple 500 is separated from its nearest neighbor by a pitch distance w6 (see FIG. 6A). In a specific example, each singulated unicouple 500 has two legs located on two ends of a section of shunt material 300A with a gap spacing of about 12.5 mm. Each leg has a width w0=0.5 mm and a depth d0 ranging from 0.5 mm to a few millimeters depending on settings in the second cutting operation. The serial chain configuration of these singulated unicouples has a pitch distance of w6=10 mm. Of course, these parameters associated with the chain configuration and leg dimensions can be varied and easily adjusted under the same tooling for the current method depending on thermoelectric leg material and structural optimization as well as the thermal environment for specific applications. Additionally, as shown in FIG. 6, the spacing between each parallel serial chains of singulated unicouples is operably set to constant value of about 20 mm, increased from row 1 to row N (N>1), decreased from row 1 to row N, or in other orders depending on embodiments.
FIG. 6 also shows a subsequent process of bonding a second shunt material 400 on top ending faces of the n-type or p-type legs among all the individual singulated unicouples 500 already disposed on the heat sink material 600 in the redistribution process mentioned above. In an embodiment, this second shunt material 400 can be selected from a silicon based wafer or sheet material, which is configured for further forming thermal contact with a hot-side heat sink (to be seen in FIG. 8 below) associated the physical configuration of the to-be-formed thermoelectric module. In an embodiment, the hot-side shunt material 400 includes Silicon (shown as darker colored wafer) with a proper metallization layer on its surface that is bonded with the pre-coated metallization layers on the top ending faces of the n-type legs and p-type legs of all singulated unicouples 500 mentioned above (also see FIG. 1 for metallization layer for bonding the thermoelectric legs). FIG. 6A shows a cross-sectional view along a chain of singulated unicouples 500 in FF′ direction of FIG. 6 after the hot-side shunt material 400 is disposed thereon. As shown, a three-dimensional configuration of the thermoelectric module comprising multiple unicouples has been primarily formed.
FIG. 7 is a schematic diagram showing additional processes of the method for forming the thermoelectric unicouples according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown, more cutting operations are performed for assembling the chains of thermoelectric unicouples. In particular, a fourth cutting operation is performed to cut through the hot-side shunt material 400 along a GG′ line direction in designated positions associated with the singulated unicouples 500. For each singulated unicouple 500, two GG′ line cuttings are performed. One cutting line G1 is aligned to near an inner side edge of an n-type leg of a singulated unicouple 500 and another cutting line G2 is aligned to near an inner side edge of p-type leg of the same singulated unicouple 500. The two cuttings are both stopped as their depths just surpass the thickness of the hot-side shunt material so that a section of the hot-side shunt material 400 is removed while leaving the two legs and the section of cold-side shunt material 300A below in tack. All GG′ cutting operations are applied to other similar regions to leave one or more bridge section of hot-side shunt material 400A bonded over two legs, one p-type leg of one singulated unicouple and one n-type leg of a next singulated unicouple, as shown as seen in a cross-sectional view in FIG. 7A. As the result, a thermoelectric unicouple structure (e.g., 100 shown in FIG. 1A) is formed around the bridge section of the hot-side shunt material bonded commonly with a p-type leg and an n-type leg which are respectively bonded to two sections of the cold-side shunt material, each of them being configured to couple a next unicouple with the same structure.
FIG. 7 also shows a sequential process of the method, wherein the fourth cutting operation is further performed along HH′ direction according to an embodiment of the present invention. Multiple cuts are performed in this cutting operation along the HH′ direction. For each serial chain numbered from 1 through N, two cuts are executed along HH′ lines near the two longitudinal edges of the unicouples. In an embodiment, each of these two cuts is stopped as its depth just surpasses the thickness of the hot-side shunt material 400 to make sure its removal from two sides of each serial chain. As the results of this HH′ cutting operation across the whole hot-side shunt material, several parallel serial chains of thermoelectric unicouples numbered from 1 through N are formed. Each serial chain of thermoelectric unicouples is also known as a daisy chain structure. In a specific example, FIG. 7A is a cross-sectional view along FF′ line in FIG. 7 showing a plurality of thermoelectric unicouples formed in a serial chain configuration according to an embodiment of the present invention. Multiple serial chains of thermoelectric unicouples can be laid out in a 2D arrangement (as seen in FIG. 7) for assembling a macroscopic-scaled thermoelectric module. Total number of unicouples in each serial chain can be varied depending on embodiments.
FIG. 8 is a schematic diagram showing a subsequent process of the method for assembling thermoelectric unicouples to form a thermoelectric module according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The formation of one or more daisy chain structures of thermoelectric unicouples leaves multiple sections of hot-side shunt material 400A on top of each unicouple along the chain. A hot-side heat sink (wafer or sheet) material 700 is applied to bond with the top-most sections of the hot-side shunt material 400A of all the daisy chain structures to form the assembly of all the thermoelectric unicouples according the embodiment of the invention.
FIG. 8A is a cross-sectional view of the assembly of the thermoelectric unicouples of FIG. 8 along one daisy chain structured unicouples. As shown, a three-dimensional arrangement of each unicouple includes an n-type thermoelectric leg and a p-type thermoelectric leg having their one ending faces respectively bonded to two sections of a first shunt and their another ending faces separately bonded to two end regions of a same section of a second shunt (See FIG. 1). In an embodiment, the first shunt is configured to couple with a cold-side heat sink which is designed for attaching to a low-temperature heat exchanger and the second shunt is configured to couple with a hot-side heat sink which is designed for attaching to a high-temperature heat exchanger (see FIG. 1A). In a specific embodiment, a daisy chain configuration is formed with a series of such thermoelectric unicouples being connected linearly such that all the thermoelectric unicouples are coupled electrically in series and thermally in parallel (see FIG. 7A). Multiple such daisy chain structures can be laid out thermally in parallel and electrically in series and coupled one by one electrically, leading to a formation a thermoelectric module having their cold-side shunt materials commonly coupled to a cold-side heat sink material and their hot-side shunt materials commonly coupled to a hot-side heat sink material (see FIG. 8).
In a specific embodiment, a method for assembling a plurality of thermoelectric unicouples is provided. FIG. 9 is a simplified chart showing a method of forming and assembling a plurality of thermoelectric unicouples according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown, the method 900 includes a step 905 of providing a plurality of blocks made by n-type and p-type thermoelectric material. In an implementation of this step, a plurality of material blocks is provided with a rectangular shape having a width and a length cut from a wafer (see FIG. 2 and FIG. 2A) made by either n-type or p-type thermoelectric functional semiconductor material. For example, the wafer can be a silicon-based wafer having a plurality of nanowires or nanotubes being etched directly from a crystalline silicon wafer and further doped with proper electronic impurities to give desired n-type and p-type semiconductor characteristics. Each material block is cut from the wafer. In another example, each material block can be a sintered nano-composite material independently fabricated from a plurality of nano-particles pre-manufactured based various processes including melting, mixing, solidification, milling, and doping, etc.
The method 900 further includes a step 910 of disposing the n-type blocks and p-type blocks alternatively on a first shunt wafer. As shown in FIG. 3, the plurality of material blocks is disposed on a first shunt wafer in a 2D array wherein each n-type block is alternately disposed next to a p-type block. The first shunt wafer is an electrically conductive sheet material. For example, it can be a Cu lead frame coated with Ni material. The disposing the plurality of material blocks may include process of bonding each material block onto the first shunt wafer via a metalized material made by a multilayered film of Ti/TiN/Ni/Au/Au—Sn alloy or Ni/TiN/Ni/Au/Au—Sn Alloy using a brazing process. In specific embodiment, the first shunt wafer is designated for being used for attaching with a cold side heat sink, thereby the material selection and process treatment all are designed to make the internal stress across the thermoelectric material block and the shunt to be adaptive to relative low temperature environment for the thermoelectric application.
Additionally, the method 900 includes a step 915 of performing a first cutting operation to change sizes of and spacing between the n-type and p-type blocks. As shown in FIG. 4, in a specific implementation of the step 915, the first cutting operation is performed along the length of the blocks (e.g., along CC′ direction) to reduce the width of each block and increase a gap spacing between two neighboring blocks while substantially free from removing any material of the first shunt wafer (see FIG. 4A also). A process of taping the first shunt material including the plurality of blocks thereon onto a removable tape is introduced after the first cutting operation.
The method 900 further includes a step 920 of performing a second cutting operation to form multiple units per block. As shown in FIG. 5, in a specific implementation of the step 920, the second cutting operation is performed along the width of each block (e.g., along DD′ direction) to cut the thermoelectric material and through the first shunt wafer material below to divide each block along the length into a column of multiple units. This is to redefine the thermoelectric blocks in sizes and gap spacing for facilitating volume production utilizing common semiconductor wafer processing technology.
Furthermore, the method 900 includes a step 925 of performing a third cutting operation to divide each unit to two legs, causing a formation of a plurality of singulated unicouples having a n-type leg and a p-type leg on two ends of a piece of first shunt. As shown in FIG. 5, in a specific implementation of the step 925, the third cutting operation is performed along middle line of each column of multiple units through the first shunt wafer below (e.g., along EE′ direction) to further cut each unit into two thermoelectric functional legs respectively attached on two separate remaining pieces of the first shunt wafer (see also FIG. 5A). Accordingly a plurality of singulated unicouples is formed and each singulated unicouple is made of a n-type thermoelectric functional leg attached at one end and a p-type thermoelectric functional leg attached at another end of a same remaining piece of the first shunt wafer.
The method 900 also includes a step 930 of rearranging the plurality of singulated unicouples on a first base plate in one or more serial chains having same spatial orientation of the n- and p-type legs. As shown in FIG. 6, the plurality of singulated unicouples 500 is rearranged in one or more serial chains numbered from 1 through N by bonding the remaining piece of the first shunt wafer 300 of each singulated unicouple 500 onto a first base plate 600 such that every singulated unicouple 500 in the serial chain (e.g., chain #1) comprises a same spatial orientation of a n-type thermoelectric functional leg on one end and a p-type thermoelectric functional leg on other end of a same piece of the first shunt wafer (see FIG. 6A). The first base plate 600 is an electrical insulating material used as a heat sink for the thermoelectric legs. For example, a hard anodized aluminum is used for making the first base plate.
The method 900 continues to include a step 935 of bonding a second shunt wafer on each n-type and p-type legs of the plurality of singulated unicouples. As shown in FIG. 6 and FIG. 6A, the second shunt wafer 400 is attached onto the n-type thermoelectric functional leg and the p-type thermoelectric functional leg of each singulated unicouple in the one or more serial chains numbered from 1 through N. The process of bonding the second shunt wafer on the thermoelectric leg includes brazing a multilayer of metalized film made by Ni/TiN/W/W—Pt Alloy with 2% of B, or Ti/TiN/W/W—Pt Alloy including a Pd—Al nanofoil for facilitating the brazing process.
Moreover, the method 900 includes another step 940 of performing a fourth cutting operation to remove partially the second shunt wafer for forming chains of thermoelectric unicouples. As shown in FIG. 7, in a specific implementation of the step 940, the fourth cutting operation is performed to remove material of the second shunt wafer partially from regions beyond two longitudinal edges (along HH′ direction) of each serial chain and regions between the n-type thermoelectric functional leg and the p-type thermoelectric functional leg (along GG′ direction) of each singulated unicouple while substantially free from removing any material of the first shunt wafer and the first base plate (also see FIG. 7A). Thus, a plurality of thermoelectric unicouples (e.g., 100 shown in FIG. 1A) is formed along each of the one or more serial chains numbered from 1 through N. The method 900 further includes a step 945 of attaching a second base plate from above to bond each and every remaining piece of the second shunt wafer. As shown in FIG. 8 and FIG. 8A, the second base plate 700 is attached onto each remaining piece of the second shunt wafer (400A) of the plurality of thermoelectric unicouples.
The method further may include a fifth cutting operation for forming one or more daisy chains of thermoelectric unicouples assembled between the first base plate and the second base plate. Each daisy chain of thermoelectric unicouples is characterized at least by a serial electrical conduction path from at least one piece of the first shunt wafer, through one or more n-type thermoelectric functional legs, through at least one piece of the second shunt wafer, to one or more p-type thermoelectric functional legs, and a parallel thermal conduction path from the first base plate through all pieces of the first shunt wafer, through all the n-type thermoelectric functional legs and all the p-type thermoelectric functional legs, through all pieces of the second shunt wafer, to the second base plate.
It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the claims.
Scullin, Matthew L., Aguirre, Mario
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