US20150316298A1

US20150316298A1 - Thermoelectric Device And Method For Fabrication Thereof

Info

Publication number: US20150316298A1
Application number: US14/268,653
Authority: US
Inventors: Mahmoud Al Ahmad
Original assignee: United Arab Emirates University
Current assignee: United Arab Emirates University
Priority date: 2014-05-02
Filing date: 2014-05-02
Publication date: 2015-11-05
Also published as: WO2015166474A1

Abstract

A thermoelectric device and a method for fabrication thereof are disclosed. The thermoelectric device includes one or more thermocouples electrically connected in series. Each thermocouple includes at least one first thermoelectric element and at least one second thermoelectric element, wherein the thermoelectric elements are maintained in a spaced apart relationship forming an internal cavity there between.

Description

TECHNICAL FIELD

The present invention generally relates to thermoelectric devices. More specifically, the present invention relates to a thermoelectric device with relatively high voltage output, compact form factor, and enhanced portability; and a fabrication method therefor.

DESCRIPTION OF THE RELATED ART

Thermoelectric devices used for thermoelectric power generation, thermoelectric heating/cooling, and sensing and measurement applications are well known in the state of the art.
The thermoelectric generators, in accordance with the Seebeck effect, convert the temperature gradient across two ends of a thermoelectric element to generate an electrical potential, which may be used to drive an electrical current through an electrical load.
In a reverse principle, the thermoelectric heating/cooling devices, in accordance with the Peltier effect, draw an electrical current from an external power supply, and provide controlled heating/cooling of a desired surface.
The thermoelectric devices also find several applications in the fields of sensing and measurement by way of facilitating temperature sensing and/or constituting a power supply for driving sensing devices based on other sensing modalities.
Referring now to FIG. 1, a conventional semiconductor based thermoelectric generator 100 is shown.
The thermoelectric generator 100 typically includes multiple thermocouple modules 102 arranged between a heat source 104 and a heat sink 106. Each thermocouple module 102 includes a first type (for example, n-type) of semiconductor element 108 a and a second type (for example, p-type) of semiconductor element 108 b electrically interconnected through a metallic bridge 110. The thermocouple modules 102 are electrically interconnected in series using additional metallic bridges 112.
As will be apparent from the adjoining figure, the semiconductor elements 108 are thermally connected in parallel and electrically connected in series.
Owing to intrinsic material properties of the semiconductor elements 108, when a temperature gradient is maintained across the heat source 104 and the heat sink 106, free charge carriers therein, namely, electrons in the n-type semiconductor element 108 a and holes in the p-type semiconductor element 108 b diffuse towards the heat sink 106 resulting in an electrical potential across the length of the semiconductor elements 108 between the heat source 104 and the heat sink 106. As the n-type semiconductor element 108 a and the p-type semiconductor element 108 b have mutually opposite electrical conductivity types, the electrical potentials therein build in opposite directions as free charge carriers have opposite polarity, the resulting electrical potential is aggregated across the series combination of the n-type semiconductor element 108 a and the p-type semiconductor element 108 b.
The electrical potential developed across the ends of a thermoelectric element is proportional to the thermal gradient with Seebeck coefficient as the constant of proportionality. The Seebeck coefficients of well known thermoelectric materials typically range from about 100 μV/degree C. to about 300 μV/degree C. Owing to low values of Seebeck coefficients, multiple thermocouple modules 102 are electrically connected in series to obtain a sufficiently high output voltage for use in practical applications. With increased output voltage requirement, the thermoelectric devices are forced to be expanded laterally and thereby, tend to become bulky and attain a cumbersome form factor.
The thermal efficiency of a thermoelectric material is defined in terms of its figure of merit, which depends not only on its ‘thermal power’ (absolute value of Seebeck coefficient) but also a complex interplay of its electrical and thermal properties. The thermoelectric material should ideally have high electrical conductivity and low thermal conductivity; and this requirement proves to be contradictory with regard to most of the commonly available materials. The best known thermoelectric materials currently available provide a figure of merit of about 1.
In recent years, significant efforts have been made in developing new thermoelectric materials with improved figure of merit such that various theoretical applications of thermoelectric devices may indeed be realized.
While research and development of new thermoelectric materials is important, efforts to improve design of thermoelectric devices to enhance output voltage without adversely affecting their form factor is also desirable.
Moreover, the state of the art thermoelectric devices typically rely on a heat source and/or a heat sink and need to be thermally coupled thereto in order to generate electrical power, and hence, imposing constraints on spatial positioning and thereby, diminishing portability and ease-of-use of such thermoelectric devices.
In light of the foregoing, there is a need for a thermoelectric device with increased output voltage, compact form factor, and enhanced portability.

SUMMARY OF THE PRESENT INVENTION

It is an object of the present invention to provide a thermoelectric device with an improved form factor for a specified output voltage.
It is another object of the present invention to provide a thermoelectric device with improved portability.
It is yet another object of the present invention to provide a method for fabrication of such thermoelectric device.
The object is achieved by providing a thermoelectric device according to claim 1 and a method for fabricating the same according to claim 11. Further embodiments of the present invention are addressed in respective dependent claims.
The underlying concept of the present invention is to fabricate a thermoelectric device in a manner that one or more layers of thermoelectric elements may be stacked one atop another in such manner that an internal cavity is created therein. The external boundary includes two thermally conducting substrates arranged substantially parallel to each other with the thermoelectric device being configured such that a direction of thermal flux within the thermoelectric device is substantially orthogonal to each substrate.
The internal cavity is accessible from an external boundary of the thermoelectric device, and is configured to receive thermally active materials therein. In one embodiment, the thermoelectric device is configured as a flow-through device such that the thermally active material enters the internal cavity through an inlet interface and exits from an outlet interface. In another embodiment, the thermally active materials are placed inside a cartridge, which has such form factor that is suitable for inserting the cartridge in the thermoelectric device. As used herein, the term ‘thermally active’ is intended to characterize a subject as endothermic or exothermic.
When such a thermally active cartridge is present within the internal cavity of the thermoelectric device, a thermal gradient is created across the thermoelectric elements, which extend between the internal cavity and at least one of the substrates. Accordingly, an electrical potential is created across individual thermoelectric elements. The individual thermoelectric elements are electrically connected in series and an aggregate electrical potential across all thermoelectric elements included in the thermoelectric device is tapped through a suitable electrical interface provided therein.
In a first aspect of the present invention, a thermoelectric device is provided. The thermoelectric device includes at least a first substrate and at least a second substrate arranged substantially parallel to a reference plane. The reference plane defines three mutually orthogonal axes, namely, a first axis, a second axis, and a third axis. The first axis is orthogonal to the reference plane and the second and the third axes are aligned to the reference plane. The first and the second substrates are retained in a spaced apart relationship along the first axis. The thermoelectric device further includes at least one thermocouple. The thermocouple includes at least one first thermoelectric element and at least one second thermoelectric element.
The first thermoelectric element is disposed on the first substrate and extends towards the second substrate along the first axis. The first thermoelectric element includes a first basal surface facing towards the first substrate, a first apical surface substantially opposite to the first basal surface, and a first lateral surface there between.
The second thermoelectric element is disposed on the second substrate and extends towards the first substrate along the first axis. The second thermoelectric element includes a second basal surface facing towards the second substrate, a second apical surface substantially opposite to the second basal surface, and a second lateral surface there between.
The first thermoelectric element and the second thermoelectric element are disposed such that the first and the second apical surfaces are spaced apart along the first axis, whereby an internal cavity is defined there between.
In a second aspect of the present invention, a method for fabricating a thermoelectric device, as described in accordance with the first aspect of the present invention, is provided.
At a first step, a first substrate arranged substantially along a reference plane is provided. The reference plane defines three mutually orthogonal axes, namely, a first axis, a second axis, and a third axis. The first axis is orthogonal to the reference plane and the second and the third axes are aligned to the reference plane.
At least a first thermoelectric element is formed on the first substrate. The first thermoelectric element extends along the first axis and includes a first basal surface facing towards the first substrate, a first apical surface substantially opposite to the first basal surface, and a first lateral surface there between. A sacrificial layer is disposed on the first thermoelectric element. After disposing the sacrificial layer, at least a second thermoelectric element is formed on the sacrificial layer. The second thermoelectric element extends along the first axis and includes a second basal surface facing away from the first substrate, a second apical surface substantially opposite to the second basal surface and facing towards the sacrificial layer, and a second lateral surface there between. After forming the second thermoelectric element, a second substrate is disposed on the sacrificial layer. The second substrate is retained in a spaced apart relationship from the first substrate along the first axis and extends in a spatial plane substantially parallel to the reference plane. Finally, the sacrificial layer is removed such that the first and the second apical surfaces of the first thermoelectric element and the second thermoelectric element respectively are spaced apart along the first axis, whereby an internal cavity is defined there between.
Thus, at least one thermocouple is formed between the first and the second substrates provided in the thermoelectric device.
Accordingly, the present invention provides a thermoelectric device and a method for fabrication thereof such that a relatively compact form factor is achieved for a specified output voltage. The techniques of the present invention provide a thermoelectric device that is not constrained to be spatially positioned such as to be coupled to a heat source and/or a heat sink, thereby greatly enhances ease-of-use and portability thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which:

FIG. 1 illustrates a schematic view of a thermoelectric device in accordance with the state of the art,

FIG. 2 illustrates a perspective view of a partially cut-away thermoelectric device in accordance with an exemplary embodiment of the present invention,

FIGS. 3A-3C illustrate cross-sectional views of a thermoelectric device in accordance with various exemplary embodiments of the present invention, and

FIGS. 4-18 illustrate cross-sectional views of a thermoelectric device depicting a method for fabrication thereof in accordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Various embodiments are described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident that such embodiments may be practised without these specific details.
The present invention will hereinafter be explained primarily in the context of solid-state thermoelectric generation. However, it should be noted that various techniques of the present invention are not limited in any manner to semiconductor based thermoelectric generation per se, and are equally applicable to thermoelectric devices based on other materials and further, to any desired application of thermoelectric effect, as presently known or as may be developed in future.
Referring now to FIGS. 2 and 3A through 3C, a perspective view of a partially cut-away thermoelectric device 200 and cross-sectional views thereof are respectively shown in accordance with exemplary embodiments of the present invention.
The thermoelectric device 200 includes one or more thermocouples 202 arranged between a first substrate 204 and a second substrate 206. Each thermocouple 202 includes a first thermoelectric element 208 and a second thermoelectric element 210. The thermoelectric device 200 further includes an internal cavity 212.
The structure of thermoelectric device 200 will now be explained with respect to a reference plane (P), X-Y plane in the adjoining figure. The reference plane (P) defines three mutually orthogonal axes, namely, a first axis (z-axis), a second axis (x-axis), and a third axis (y-axis). The first axis (z-axis) is orthogonal to the reference plane (P) whereas the second axis (x-axis) and the third axis (y-axis) are aligned to the reference plane (P).
Each substrate 204, 206 is substantially planar and is arranged substantially parallel to the reference plane (P). The first substrate 204 and the second substrate 206 are retained in a spaced apart relationship along the first axis (z-axis).
The first thermoelectric element 208 is disposed on the first substrate 204 and extends towards the second substrate 206 along the first axis (z-axis). The first thermoelectric element 208 includes a first basal surface 208(b) facing towards the first substrate 204, a first apical surface 208(a) substantially opposite to the first basal surface 208(b), and a first lateral surface 208(l) there between.
The second thermoelectric element 210 is disposed on the second substrate 206 and extends towards the first substrate 204 along the first axis (z-axis). The second thermoelectric element 210 includes a second basal surface 210(b) facing towards the second substrate 206, a second apical surface 210(a) substantially opposite to the second basal surface 210(b), and a second lateral surface 210(l) there between.
As can be seen from the adjoining figure, the relative spacing between the first substrate 204 and the second substrate 206 and the respective dimensions of the first thermoelectric element 208 and the second thermoelectric element 210, in particular, the respective dimensions along the first axis (z-axis), are regulated in such manner that the first apical surface 208(a) and second apical surface 210(a) are spaced apart along the first axis (z-axis), whereby an internal cavity 212 is defined there between.
Having broadly described the structure of the thermoelectric device 200, further structural details thereof will now be explained. The flow of thermal and electrical currents within the thermoelectric device 200 will also be explained.
The first thermoelectric element 208 and the second thermoelectric element 210 are formed using thermoelectric material of mutually opposite types of electrical conductivity. In one example, the first thermoelectric element 208 is an n-type semiconducting material and the second thermoelectric element 210 is a p-type semiconducting material.
In various exemplary embodiments of the present invention, any suitable thermoelectric material may be used. In order to achieve a better figure of merit, generally, heavily doped semiconducting materials are used. In the current state of the art, thermoelectric materials formed using alloys of Bismuth telluride and Antimony telluride are shown to display good thermoelectric properties. However, the present invention is not limited to any specific examples of thermoelectric materials, and any suitable thermoelectric material currently available or developed in future may be used to practice the techniques of the present invention.
In an exemplary embodiment of the present invention, each of first lateral surface 208(l) and the second lateral surface 210(l) of the first thermoelectric element 208 and the second thermoelectric element 210 respectively is electrically and thermally insulated such that a direction of electrical and thermal currents through the first thermoelectric element 208 and the second thermoelectric element 210 is aligned with the first axis (z-axis). In one example, the desired insulation may be achieved using Silicon nitride.
The first thermoelectric element 208 and the second thermoelectric element 210 are electrically interconnected through disposing an intra-couple bridge 214 between the first apical surface 208(a) and the second apical surface 210(a). The intra-couple bridge 214 includes a first electrically connecting layer 216, a second electrically connecting layer 218, and an intra-couple metallization layer 220.
The first electrically connecting layer 216 is deposited on the first apical surface 208(a). Similarly, the second electrically connecting layer 218 is deposited on the second apical surface 210(a). The intra-couple metallization layer 220 extends along the first axis (z-axis) and interconnects the electrically connecting layers 216, 218. Any suitable metal such as aluminum, gold, and so on may be used to form the intra-couple bridge 214.
The intra-couple bridge 214 is at least electrically insulated at a non-contact surface thereof. The term non-contact surface refers to an external surface of the intra-couple bridge 214 excluding such portion of the electrically connecting layers 216, 218 that are in electrical contact with the first apical surface 208(a) and second apical surface 210(a). Such electrical insulation ensures that the electrical current flowing from the first thermoelectric element 208 to the second thermoelectric element 210 is prevented from any leakage.
In an exemplary embodiment of the present invention, the electrically connecting layers 216, 218 entirely cover the first apical surface 208(a) and the second apical surface 210(a) respectively. In an alternative exemplary embodiment of the present invention, the electrically connecting layers 216, 218 partially cover the respective apical surfaces 208(a) and 210(a), the remaining portions of the first apical surface 208(a) and the second apical surface 210(a) are electrically insulated. It should be noted that the material used for electrical insulation of the remaining portions of the first apical surface 208(a) and the second apical surface 210(a) should be thermally conducting. In this embodiment, the intra-couple bridge 214 is provided both electrical and thermal insulation such that not only the electrical but the thermal currents are also aligned with the first axis (z-axis).
The thermoelectric device 200 further includes an inter-couple bridge 222. The inter-couple bridge 222 includes a first electrically coupling layer 224, a second electrically coupling layer 226, and an inter-couple metallization layer 228.
The first electrically coupling layer 224 is disposed between the first basal surface 208(b) and the first substrate 204. Similarly, the second electrically coupling layer 226 is disposed between the second basal surface 210(b) and the second substrate 206. As will now be apparent, an aggregate of individual electrical potentials across the first thermoelectric element 208 and the second thermoelectric element 210 is accessible across the first electrically coupling layer 224 and the second electrically coupling layer 226.
Each of the first substrate 204 and the second substrate 206 are formed of a thermally conducting and electrically insulating material. In one example, each substrate 204, 206 is formed using alumina, which is a thermal conductor but an electrical insulator. In general, any ceramic material or any other suitable material may be used for forming the substrates 204, 206.
In an exemplary embodiment of the present invention, each of the first substrate 204 and the second substrate 206 are provided with one or more structural features 232 and/or one or more material coatings 234 to permit heat dissipation, as shown in FIGS. 3B and 3C. Such structural features 232 may, for example, include fins to increase the heat dissipation surface area.
In an alternative exemplary embodiment of the present invention, each of the first substrate 204 and the second substrate 206 are provided with one or more structural features 232 and/or one or more material coatings 234 to permit heat absorption, as shown in FIGS. 3B and 3C. In particular, any heat absorbing material generally known in the art may be used to provide the material coating on the first substrate 204 and the second substrate 206.
These embodiments of the present invention will be further understood in relation to operation of the thermoelectric device 200, as will be explained later.
As described earlier, the Seebeck coefficient of currently available thermoelectric materials is quite low. Accordingly, multiple thermocouples 202 are usually formed and are electrically connected in series to provide desired output voltage. This aspect of the present invention will now be explained.
In an embodiment of the present invention, at least a first thermocouple 202(a) and at least a second thermocouple 202(b) are formed. Each of the first thermocouple 202(a) and the second thermocouple 202(b) has the same configuration as that of the thermocouple 202, as described in the preceding description.
Thus, the individual aggregate electrical potentials across the first thermocouple 202(a) and second thermocouple 202(b) are formed across corresponding first electrically coupling layer 224 and the second electrically coupling layer 226.
The first thermocouple 202(a) and the second thermocouple 202(b) are adjacently disposed between the first substrate 204 and the second substrate 206.
The first electrically coupling layer 224 of the first thermocouple 202(a) is electrically interconnected to the second electrically coupling layer 226 of the second thermocouple 202(b) through an inter-couple bridge 222. Thus, the first thermocouple 202(a) and the second thermocouple 202(b) are electrically connected in series.
The inter-couple bridge 222 includes an inter-couple metallization layer 228 which extends along the first axis (z-axis) from substantially adjacent to the first substrate 204 to substantially adjacent to the second substrate 206, and electrically interconnects the first and the second electrically conductive layers 224, 226.
The inter-couple bridge 222 is at least electrically insulated on a non-contact surface thereof in a manner similar to that of the intra-couple bridge 214.
Similar to the manner explained in the context of intra-couple bridge 214, the term non-contact surface refers to an external surface of inter-couple bridge 222 excluding such portion of the electrically coupling layers 224, 226 that are in electrical contact with the first basal surface 208(b) and the second basal surface 210(b). Such electrical insulation ensures that the electrical current flowing from the first thermocouple 202(a) to the second thermocouple 202(b) is prevented from any leakage. In one embodiment, the inter-couple bridge 222 is at least partially provided with thermal insulation also such as to avoid formation of an undesirable thermal flow paths between the first substrate 204 and the second substrate 206 and various intermediate components.
The aggregate electrical potential of the first thermocouple 202(a) and the second thermocouple 202(b) are available across the first electrically coupling layer 224 of the first thermocouple 202(a) and the second electrically coupling layer 226 of the second thermocouple 202(b).
As will be readily understood, the thermoelectric device 200 is suitable for scaling up to include multiple thermocouples 202 a, 202 b, 202 c through 202 n.
Accordingly, in an exemplary embodiment of the present invention, multiple thermocouples 202 a through 202 n (collectively, thermocouples 202) are disposed between the first substrate 204 and the second substrate 206 in such manner as to form a two dimensional matrix relative to the reference plane (P). While arranging the thermocouples 202 in a two-dimensional matrix, a set of thermocouples 202 are arranged in individual rows extending along the second axis (x-axis) and multiple such rows are formed along the third axis (y-axis). The thermocouples 202 along an individual row are electrically interconnected in series and individual rows of thermocouples 202, are in turn, electrically connected in series, as can be seen in FIG. 2.
When the thermocouples 202 are arranged in the manner described above, the internal cavities of corresponding thermocouples 202 in adjacent rows are in continuum.
In accordance with the second aspect of the present invention, the method for fabrication of the thermoelectric device 200 will now be explained in conjunction with FIGS. 4 through 18.
Referring to FIG. 4, the first substrate 204 is arranged substantially along the reference plane (P) is provided. As mentioned earlier, the reference plane (P) defines three mutually orthogonal axes, namely, the first axis (z-axis), the second axis (x-axis), and the third axis (y-axis). The first axis (z-axis) is orthogonal to the reference plane (P) and the second axis (x-axis) and the third axis (y-axis) are aligned to the reference plane (P).
The first substrate 204 is formed of a thermally conducting and electrically insulating material. As mentioned previously, the first substrate 204 may be formed using ceramic materials, for example, alumina and so on; or any material exhibiting the desired properties.
The first substrate 204 is provided with one or more structural features and/or one or more material coatings either to permit heat dissipation, for example, by providing fins to increase the heat dissipation surface area, or to permit heat absorption, for example, by providing a material coating using any heat absorbing material generally known in the art.
Still referring to FIG. 4, the first electrically coupling layer 224 is disposed on the first substrate 204. The first electrically coupling layer 224 may be formed using any standard metallization technique used in semiconductor fabrication.
Referring now to FIG. 5, the first thermoelectric element 208 is then formed. As can be seen in the adjoining figure, the first thermoelectric element 208 extends along the first axis (z-axis) and includes the first basal surface 208(b) facing towards the first substrate 204, the first apical surface 208(a) substantially opposite to the first basal surface 208(b), and a first lateral surface 208(l) there between.
As will now be evident, the first electrically coupling layer 224 is disposed between the first basal surface 208(b) and the first substrate 204.
The first thermoelectric element 208 is formed using a suitable thermoelectric material. In one example, the first thermoelectric element 208 is formed using a heavily doped n-type semiconducting material.
Any suitable technique may be used to form the first thermoelectric element 208. In one example, the n-type first thermoelectric element 208 is grown on the first substrate 204 on top of the first electrically coupling layer 224 using a deposition technique.
Referring now to FIGS. 6 and 7, after forming the first thermoelectric element 208, an electrically insulating material is deposited on the first thermoelectric element 208 to partially form an insulating layer 230. In one example, silicon nitride is used as the insulating material. The silicon nitride is first deposited and subsequently, etched and polished to form the structure shown in FIG. 7.
As will be easily understood, such insulation is required to ensure that direction of electrical current through the first thermoelectric element 208 is aligned with the first axis (z-axis) and there is no leakage of electrical current.
Referring now to FIG. 8, a metallization step is performed such that a first electrically connecting layer 216 is created on the first apical surface 208(a) of the first thermoelectric element 208. In addition, the inter-couple metallization layer 228 is also partially formed.
Referring now to FIGS. 9 and 10, another iteration of deposition of insulating material and metallization layer is performed such that the intra-couple metallization layer 220 is also partially formed while the inter-couple metallization layer 228 is further grown. In addition, an external surface of the first electrically connecting layer 216 which is not in contact with the first thermoelectric element 208, namely, the so-called non-contact surface, is provided with the insulation layer 230.
Referring now to FIGS. 11 and 12, a sacrificial layer (SL) is disposed on the first thermoelectric element 208. The insulation layer 230, and the intra-couple metallization layer 220 and the inter-couple metallization layer 228 are further grown through the sacrificial layer (SL) such that part of the intra-couple metallization layer 220 and the inter-couple metallization layer 228 are covered with the insulating layer 230.
Referring now to FIGS. 13 and 14, the insulating layer 230, and the metallization layers 220 and 228 are further grown. In addition, the second electrically connecting layer 218 is formed.
Referring now to FIG. 15, the second thermoelectric element 210 is formed.
As evident from the adjoining figures, the second thermoelectric element 210 extends along the first axis (z-axis). The second thermoelectric element 210 includes the second basal surface 210(b) facing away from the first substrate 204, the second apical surface 210(a) substantially opposite to the second basal surface 210(b) and facing towards the sacrificial layer (SL), and the second lateral surface 210(l) there between.
As with the first thermoelectric element 208, the second thermoelectric element 210 is formed using a suitable thermoelectric material. However, the second thermoelectric element 210 has an opposite type of electrical conductivity with respect to that of the first thermoelectric element 208. Thus, in one example, while the first thermoelectric element 208 is formed using a heavily doped n-type semiconducting material, the second thermoelectric element 210 is formed using a heavily doped p-type semiconducting material.
Any suitable fabrication technique may be used to form the second thermoelectric element 210. In one example, the p-type second thermoelectric element 210 is grown using a deposition technique.
Referring now to FIGS. 16 and 17, the insulating layer 230 is grown further such that the second lateral surface 210(l) is electrically insulated. In addition, the inter-couple metallization layer 228 is also further grown and the second electrically coupling layer 226 is also disposed on the second thermoelectric element 210.
Referring now to FIG. 18, after forming the second thermoelectric element 210, the second substrate 206 is disposed in the manner shown in the adjoining figure.
As evident from the adjoining figure, the second substrate 206 extends in a spatial plane substantially parallel to the reference plane (P).
Subsequently, the sacrificial layer (SL) is removed using a suitable technique. Thus, at this stage, the first apical surface 208(a) of the first thermoelectric element 208 and the second apical surface 210(a) of the second thermoelectric element 210 respectively are spaced apart along the first axis (z-axis) and the internal cavity 212 is formed there between.
In various embodiments of the present invention, the second substrate 206 is retained in a spaced apart relationship from the first substrate 204 along the first axis (z-axis). This is achieved through disposing additional columnar structures, extending along the first axis (z-axis), along the peripheral boundary of the first substrate 204 and the second substrate 206. Such columnar structures not only provide necessary spaced relationship between the first substrate 204 and the second substrate 206 but also provide mechanical stability and robustness to the resulting thermoelectric device 200.
The entire thermoelectric device 200 is then packaged using generally known packaging techniques to provide further mechanical stability thereto.
As evident from the preceding description, the intra-couple bridge 214 extends along the first axis (z-axis) and interconnects the first electrically connecting layer 216 and the second electrically connecting layer 218. The first and the second electrically connecting layers 216, 218 are in electrical contact with the first and the second thermoelectric elements 208, 210 through the respective apical surfaces 208(a), 210(a). Thus, the intra-couple bridge 214 electrically interconnects the first thermoelectric element 208 and the second thermoelectric element 210. The intra-couple bridge 214 is electrically insulated at the non-contact surface thereof.
It should be noted that for ease of understanding, a single insulating layer 230 has been referred to while describing the fabrication method. In various embodiments of the present invention, different insulating layers with desired electrical and thermal properties may be used in different regions of the thermoelectric device 200, for example, to achieve the insulating layer properties as explained in conjunction with FIGS. 2 and 3.
As evident from FIGS. 4 through 18, multiple thermocouples 202 are formed between the first substrate 204 and the second substrate 206.
As explained in conjunction with FIGS. 2 and 3, the thermocouples 202 are arranged in a two dimensional matrix relative to the reference plane (P). While arranging the thermocouples 202 in a two-dimensional matrix, a set of thermocouples 202 are arranged in individual rows extending along the second axis (x-axis) and multiple such rows are formed along the third axis (y-axis). The thermocouples 202 along an individual row are electrically interconnected in series and individual rows of thermocouples 202, are in turn, electrically connected in series. When the thermocouples 202 are arranged in the manner described above, the internal cavities of corresponding thermocouples 202 in adjacent rows are in continuum.
A suitable electrical interface, for example two electrodes, is provided such that the aggregate electrical potential of the thermoelectric device 200 is available for driving external electrical loads.
In a further aspect of the present invention, the thermoelectric device 200 is provided with an external housing. The external housing of the thermoelectric device 200 includes one or more removable covers in an external packaging substantially along X-Z plane such that the removable covers may be removed to access the internal cavity 212.
In one example of the present invention, the thermoelectric device 200 of the present invention is operated in flow-through mode. In this mode of operation, the external housing is provided with removable covers on two opposite sides thereof such that the internal cavity 212 is accessible from each of the two sides, one side acts as an inlet interface and the other side acts as an outlet interface. Referring to FIGS. 4 through 18, these sides are substantially parallel to the plane of the drawings, that is, X-Z plane. In this mode of operation, a heat source, such as heated water, steam, oil, and so on enters the internal cavity through the inlet interface and exits from the outlet interface. As will now be readily understood, the thermoelectric device 200 in flow-through configuration may be used for electrical power generation in myriads of waste heat recovery applications such as industrial, power plants, automotive, oil and water supply lines, and so on.
In another example, the thermoelectric device 200 is operated in a stand-alone mode. In this mode of operation, the external housing is provided with removable covers on at least one side, which is substantially parallel to the plane of the drawing, that is, X-Z plane. An external cartridge may be inserted inside the thermoelectric device 200 through opening one of the covers.
The cartridge is made using thermally conducting material and has a structure which is essentially a collection of tubular elements arranged is a grid-like manner on a support such that tubular elements may be filled with a desirable thermally active material and the cartridge inserted inside the thermoelectric device 200. In one exemplary embodiment, the cartridge is further configured to serve the purpose of removable cover by way of locking onto the external housing of the thermoelectric device 200.
As per techniques of the present invention, the first thermoelectric element 208 and the second thermoelectric element 210 are formed using thermoelectric material of opposite conductivity types. Thus, while the first thermoelectric element 208 is formed using an n-type material, the second thermoelectric element 210 is formed using a p-type material. The charge carriers within the first thermoelectric element 208 and the second thermoelectric element 210 diffuse towards the respective adjoining substrates. Thus, the electrical potentials across are aligned and an aggregate electrical potential is formed across the first basal surface 208(b) and the second basal surface 210(b). As already explained, the electrical potentials across individual thermocouples 202 are aggregated through a series combination of these thermocouples 202 using inter-couple bridges 222.
It should be noted that the first and the second electrically connecting layers and also, the first and the second electrically coupling layers respectively interface with the first thermoelectric element 208 and the second thermoelectric element 210 through a diffusion barrier preventing diffusion of charge carrier in metals into the thermoelectric material and hence preventing a functional failure of the thermoelectric element.
Several such standard aspects associated with fabrication of thermoelectric devices are generally known and have not been explained herein for sake of brevity.
The operation of the thermoelectric device 200 will now be explained.
In accordance with various exemplary embodiments of the present invention, a thermally active material, either directly or through use of a cartridge, is inserted within the internal cavity 212 of the thermoelectric device 200.
The thermally active material may be any such material that is endothermic or exothermic in nature. The present invention contemplates use of any such materials and/or mixtures that may lead to sustained heat generation over prolonged time periods. Various examples include, but are not limited to, phase change materials, biochemical mixtures, biological materials, and so on. If so desired, any suitable material that may act as a heat source for even for short time periods may be employed. Such examples may include hot water, soil, sand, and so on.
The operation of thermoelectric device 200 will now be explained with the internal cavity 212 filled with such material that acts as a heat source.
Owing to presence of a heat source within the internal cavity 212, the temperature therein increases above the ambient temperature. Accordingly, a thermal gradient is established across each of the first thermoelectric element 208 and the second thermoelectric element 210 wherein the respective apical surfaces 208(a) and 210(a) are maintained at higher temperature while the respective basal surfaces 208(b) and 210(b) are maintained at an ambient temperature.
As previously explained, owing to intrinsic material properties of the thermoelectric materials, when a thermal gradient is applied across a thermoelectric element, free charge carriers diffuse along the direction of thermal flux and accordingly, an electrical potential is developed.
The first substrate 204 and the second substrate 206 act as heat sinks and are provided with structural features such as fins to ensure rapid dissipation of heat.
In case an endothermic material is inserted inside the internal cavity 212, the direction of thermal and electrical currents is reversed. However, the basic principle of operation remains the same.
In this case, the first substrate 204 and the second substrate 206 are provided with features to absorb heat from ambient environment, for example, the thermoelectric device 200 may be operated through exposure to sunlight whereby the first substrate 204 and the second substrate 206 are configured to absorb infra-red component of the solar radiation. Alternatively, the thermoelectric device 200 may be thermally coupled to a heat source at the first substrate 204 and the second substrate 206.
Thus, the present invention provides a thermoelectric device and a method for fabrication thereof such that a relatively compact form factor is achieved for a specified output voltage. The techniques of the present invention provide a thermoelectric device that is easily portable and does not necessarily need to be coupled to a heat source or a heat sink, thereby greatly enhancing ease-of-use.
The present invention offers several advantages over prior art techniques. In particular, owing to stacking of individual thermoelectric elements considerable substrate real estate is preserved. The form factor of the thermoelectric device becomes more compact and convenient. The ease-of-use and portability of the thermoelectric device significantly improves not only because of improved form factor but also due to an autonomous mode of operation, which is to say that the potential uses of the thermoelectric device is not constrained by the need to couple the thermoelectric device to a heat source or a heat sink. The required heat source (or sink) is integrated with the thermoelectric device, while the ambient environment acts as the heat sink (or source).
While the present invention has been described in detail with reference to certain embodiments, it should be appreciated that the present invention is not limited to those embodiments. In view of the present disclosure, many modifications and variations would present themselves, to those of skill in the art without departing from the scope of various embodiments of the present invention, as described herein. The scope of the present invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope.

Claims

What is claimed is:

1. A thermoelectric device comprising at least a first substrate and at least a second substrate arranged substantially parallel to a reference plane, said reference plane defining a first axis, a second axis, and a third axis, said axes being mutually orthogonal, wherein said first axis is orthogonal to said reference plane and said second and said third axes are aligned to said reference plane, wherein said first and said second substrates are retained in a spaced apart relationship along said first axis, and at least one thermocouple, said thermocouple comprising:

at least one first thermoelectric element disposed on said first substrate extending towards said second substrate along said first axis, wherein said first thermoelectric element comprises a first basal surface facing towards said first substrate, a first apical surface substantially opposite to said first basal surface, and a first lateral surface there between,

at least one second thermoelectric element disposed on said second substrate extending towards said first substrate along said first axis, wherein said second thermoelectric element comprises a second basal surface facing towards said second substrate, a second apical surface substantially opposite to said second basal surface, and a second lateral surface there between,

wherein said first thermoelectric element and said second thermoelectric element are disposed such that said first and said second apical surfaces are spaced apart along said first axis, whereby an internal cavity is defined there between.

2. The thermoelectric device according to claim 1, wherein each of said first and said second lateral surfaces of said first and said second thermoelectric elements respectively is electrically and thermally insulated such that a direction of electrical and thermal flux through each of said first and second thermoelectric elements is aligned with said first axis.

3. The thermoelectric device according to claim 1, wherein said first and said second substrates are formed of a thermally conducting and electrically insulating material.

4. The thermoelectric device according to claim 3, wherein said first and said second substrates are provided with one or more structural features and/or one or more material coatings to permit heat dissipation.

5. The thermoelectric device according to claim 3, wherein said first and said second substrates are provided with one or more structural features and/or one or more material coatings to permit heat absorption.

6. The thermoelectric device according to claim 1, wherein said first and said second thermoelectric elements correspond to mutually opposite types of electrical conductivity.

7. The thermoelectric device according to claim 1, wherein said first and said second apical surfaces are electrically interconnected through an intra-couple bridge, wherein said intra-couple bridge comprises individual electrically connecting layers deposited on said first and said second apical surfaces and an intra-couple metallization layer extending along said first axis and interconnecting said individual electrically connecting layers, wherein said intra-couple bridge is at least electrically insulated at a non-contact surface thereof.

8. The thermoelectric device according to claim 1, wherein a first electrically coupling layer is disposed between said first basal surface and said first substrate; and a second electrically coupling layer is disposed between said second basal surface and said second substrate, such that an aggregate of individual electrical potentials across said first and said second thermoelectric elements is accessible across said first and said second electrically coupling layers.

9. The thermoelectric device according to claim 8 further comprising at least a first thermocouple and at least a second thermocouple adjacently disposed thereto between said first and said second substrates, wherein individual aggregate electrical potentials across said first and said second thermocouples are formed across corresponding said first and said second electrically coupling layers, and further wherein, said first electrically coupling layer of said first thermocouple is electrically interconnected to said second electrically coupling layer of said second thermocouple through an inter-couple bridge, whereby said first and said second thermocouples are electrically connected in series; wherein said inter-couple bridge comprises an inter-couple metallization layer electrically extending along said first axis from substantially adjacent to said first substrate to substantially adjacent to said second substrate, and electrically interconnecting said first and said second electrically coupling layers, and wherein said inter-couple bridge is at least electrically insulated at a non-contact surface thereof.

10. The thermoelectric device according to claim 1, wherein a plurality of thermocouples extending between said first substrate and said second substrate are arranged in a two dimensional matrix relative to said reference plane, wherein a set of thermocouples arranged in individual rows are electrically interconnected in series and further, individual rows of thermocouples are electrically connected in series, wherein said internal cavity of corresponding thermocouples in adjacent rows is in continuum.

11. A method for fabricating a thermoelectric device, said method comprising:

providing a first substrate arranged substantially parallel to a reference plane, said reference plane defining a first axis, a second axis, and a third axis, said axes being mutually orthogonal, wherein said first axis is orthogonal to said reference plane and said second and said third axes are aligned to said reference plane,

forming at least a first thermoelectric element on said first substrate, wherein said first thermoelectric element extends along said first axis and comprises a first basal surface facing towards said first substrate, a first apical surface substantially opposite to said first basal surface, and a first lateral surface there between,

disposing a sacrificial layer on said first thermoelectric element,

forming at least a second thermoelectric element on said sacrificial layer, wherein said second thermoelectric element extends along said first axis and comprises a second basal surface facing away from said first substrate, a second apical surface substantially opposite to said second basal surface and facing towards said sacrificial layer, and a second lateral surface there between,

disposing a second substrate on said second thermoelectric element such that said second substrate is retained in a spaced apart relationship from said first substrate along said first axis and extends in a spatial plane substantially parallel to said reference plane, and

removing said sacrificial layer such that said first and said second apical surfaces of said first thermoelectric element and said second thermoelectric element respectively are spaced apart along said first axis, whereby an internal cavity is defined there between,

whereby at least one thermocouple is formed between said first and said second substrates.

12. The method according to claim 11 further comprising electrically and thermally insulating each of said first and said second lateral surfaces of said first and said second thermoelectric elements respectively such that a direction of electrical and thermal flux through each of said first and second thermoelectric elements is aligned with said first axis.

13. The method according to claim 11, wherein said first and said second substrates are formed of a thermally conducting and electrically insulating material.

14. The method according to claim 13 further comprising providing one or more structural features and/or one or more material coatings on said first and said second substrates to permit heat dissipation therefrom.

15. The method according to claim 13 further comprising providing one or more structural features and/or one or more material coatings on said first and said second substrates to permit heat absorption therefrom.

16. The method according to claim 11, wherein said first and said second thermoelectric elements correspond to mutually opposite types of electrical conductivity.

17. The method according to claim 11 further comprising forming an intra-couple bridge electrically interconnecting said first and said second apical surfaces, wherein forming said intra-couple bridge comprises:

disposing a first electrically connecting layer on said first apical surface of said first thermoelectric element prior to disposing said sacrificial layer,

forming an intra-couple metallization layer through said sacrificial layer, and

disposing a second electrically connecting layer on said sacrificial layer prior to disposing said second thermoelectric element,

wherein said intra-couple bridge extends along said first axis and interconnects said first and said second electrically connecting layers and further wherein, said intra-couple bridge is at least electrically insulated at a non-contact surface thereof.

18. The method according to claim 11 further comprising

disposing a first electrically coupling layer on said first substrate prior to disposing said first thermoelectric element such that said first electrically coupling layer is disposed between said first basal surface and said first substrate, and

disposing a second electrically coupling layer on said second thermoelectric element prior to disposing said second substrate such that said second electrically coupling layer is disposed between said second basal surface and said second substrate, further such that an aggregate of individual electrical potentials across said first and said second thermoelectric elements is accessible across said first and said second electrically coupling layers.

19. The method according to claim 18 further comprising

forming at least a first thermocouple and at least a second thermocouple adjacently disposed thereto between said first and said second substrates, wherein individual aggregate electrical potentials across said first and said second thermocouples are formed across corresponding said first and said second electrically coupling layers,

electrically interconnecting said first electrically coupling layer of said first thermocouple to said second electrically coupling layer of said second thermocouple through an inter-couple bridge, whereby said first and said second thermocouples are electrically connected in series; wherein said inter-couple bridge comprises an inter-couple metallization layer extending along said first axis from substantially adjacent to said first substrate to substantially adjacent to said second substrate, and electrically interconnecting said first and said second electrically coupling layers, and wherein said inter-couple bridge is at least electrically insulated at a non-contact surface thereof.

20. The method according to claim 11 further comprising forming a plurality of thermocouples extending between said first and said second substrates, said plurality of thermocouples being arranged in a two dimensional matrix relative to said reference plane, wherein a set of thermocouples arranged in individual rows are electrically interconnected in series and further, individual rows of thermocouples are electrically connected in series, wherein said internal cavity of corresponding thermocouples in adjacent rows is in continuum.