WO2021061995A1

WO2021061995A1 - Arcuate energy harvesting thermionic device

Info

Publication number: WO2021061995A1
Application number: PCT/US2020/052506
Authority: WO
Inventors: Joseph Birmingham; James P. BRAINARD
Original assignee: Birmingham Technologies, Inc.
Priority date: 2019-09-25
Filing date: 2020-09-24
Publication date: 2021-04-01
Also published as: WO2021061997A1; WO2021061996A1

Abstract

Embodiments relate to an apparatus for energy converters and electric power generators, especially at the nano-scale. The apparatus includes first and second electrodes positioned proximate to each other. The first electrode has a first work function value and the second electrode has a different second work function value. A separation material is positioned between the first and second electrodes. The separation material includes a first surface in at least partial physical contact with the first electrode and a second surface positioned opposite from the first surface. The second surface is in at least partial physical contact with the second electrode. The first and second electrodes and the separation material form an at least partially arcuate energy harvesting device.

Description

Arcuate Energy Harvesting Thermionic Device

BACKGROUND

[0001] Embodiments disclosed herein relate to electric power generation, conversion, and transfer. More specifically, embodiments disclosed herein relate to a nano-scale energy harvesting thermionic device that generates electric power through thermionic energy conversion and/or thermoelectric energy conversion.

SUMMARY

[0002] The embodiments described herein are direct to an apparatus and a method to generate electric power, including in exemplary embodiments on a nanometer scale or nano- scale.

[0003] In one aspect, the apparatus is provided with first and second electrodes. The first electrode has a first work function value. The second electrode is positioned proximal to the first electrode. The second electrode has a second work function value that is different from the first work function value. A separation material is positioned between the first and second electrodes. The separation material includes a first surface in at least partial physical contact with the first electrode. The separation material also includes a second surface positioned opposite to the first surface. The second surface is in at least partial physical contact with the second electrode. The first and second electrodes and the separation material collectively define an at least partially arcuate energy harvesting thermionic device.

[0004] In another aspect, the apparatus is provided that includes a first component. The first component includes a first electrode having a first work function value. The first electrode includes a first surface and an oppositely disposed second surface. The first component also includes a separation material including a first separation material surface and an oppositely disposed second separation material surface. The separation material is positioned in at least partial communication with the first surface of the first electrode. The apparatus also includes a second electrode including a third surface and an oppositely disposed fourth surface. The third surface is positioned proximal to the second separation material surface. The second electrode has a second work function value that is different from the first work function value. The first component and second electrode collectively form an at least partially arcuate energy harvesting thermionic device. [0005] In yet another aspect, a method is provided for generating electric power. The method includes providing an apparatus comprising a first electrode having a first work function value, a second electrode having a second work function value that is different from the first work function value, and a separation material having a first surface and an opposite second surface. The second electrode is positioned proximal to the first electrode. The first surface of the separation material is positioned in at least partial physical contact with the first electrode and the second surface of the separation material is positioned in at least partial physical contact with the second electrode. At least one aperture is defined within the separation material extending from the first surface to the second surface. A nano-fluid including a medium and a plurality of particles is positioned within the aperture. The first and second electrodes and the separation material collectively define an at least partially arcuate configuration. A plurality of electrons is transmitted between the first and second electrodes via the nanoparticles.

[0006] These and other features and advantages will become apparent from the following detailed description of the exemplary embodiment(s), taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0007] The drawings referenced herein form a part of the specification. Features shown in the drawings are meant as illustrative of only some embodiments, and not of all embodiments, unless otherwise explicitly indicated.

[0008] FIG. 1 depicts a sectional view of an embodiment of a nano-scale energy harvesting thermionic device.

[0009] FIG. 2A depicts a top view of an embodiment of a spacer and adjacent electrodes for use in a nano-scale energy harvesting thermionic device.

[0010] FIG. 2B depicts a top view of an embodiment of a spacer and adjacent electrodes for use in a nano-scale energy harvesting thermionic device.

[0011] FIG. 3 depicts a schematic view of an embodiment of a nano-fluid including a plurality of nanoparticle clusters suspended in a dielectric medium. [0012] FIG. 4 depicts a schematic perspective view of an at least partially cylindrical energy harvesting thermionic device.

[0013] FIG. 5 depicts a perspective view of an at least partially cylindrical energy harvesting thermionic device.

[0014] FIG. 6 depicts a perspective view of a first repository of layered materials that may be used to manufacture the arcuate energy harvesting thermionic device.

[0015] FIG. 7 depicts a perspective view of a second repository of layered materials that may be used to manufacture the arcuate energy harvesting thermionic device.

[0016] FIG. 8 depicts an enlarged perspective view of a first portion of the arcuate energy harvesting thermionic device.

[0017] FIG. 9 depicts an enlarged perspective view of a second portion of the arcuate energy harvesting thermionic device.

[0018] FIG. 10 depicts a flow chart illustrating a process for generating electric power with the arcuate energy harvesting thermionic device.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0019] It will be readily understood that the components of the exemplary embodiments, as generally described herein and illustrated in the Figures, may be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the apparatus, system, and method of the present embodiments, as presented in the Figures, is not intended to limit the scope of the embodiments, as claimed, but is merely representative of selected embodiments.

[0020] Reference throughout this specification to “a select embodiment,” “one embodiment,” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases “a select embodiment,” “in one embodiment,” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. The embodiments described herein, including individual or multiple features, components, attributes, steps, etc. of the embodiments, may be combined with one another in various combinations and modifications.

[0021] The illustrated embodiments will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of devices, systems, and processes that are consistent with the embodiments as claimed herein.

[0022] Thermoelectric power conversion presents an avenue to harvest and convert thermal energy into electricity. Thermoelectric power generation may involve forming a junction between a first electrode and a different second electrode where the two electrodes experience a temperature gradient. Based on the Seebeck effect, the temperature gradient induces a voltage. Higher temperature differentials tend to produce higher voltages and electric currents. However, due to the transfer of heat between the two materials at the junction, a thermal backflow is introduced which reduces the efficiency of thermoelectric power conversion systems.

[0023] Thermionic power conversion also presents an avenue to convert thermal energy into electrical energy. Thermoelectric power conversion generators convert thermal energy to electrical energy by emission of electrons from a heated emitter electrode ( i.e ., a cathode). Electrons flow from an emitter electrode, across an inter-electrode gap, to a collector electrode {i.e., an anode), through an external load, and return back to the emitter electrode, thereby converting heat to electrical energy. Recent improvements in thermionic power converters pertain to material selection based on work functions and corresponding work function values for the electrodes and using a fluid to fill the inter-electrode gap. Electron transfer density is limited by the materials of the electrodes and the materials of the fluid in the inter-electrode gap {i.e., the associated work functions). Representative and exemplary materials, features, conditions, etc. associated with thermal energy harvesting thermionic devices are described in further detail below.

[0024] To provide additional details for an improved understanding of selected embodiments of the present disclosure that combine the use of thermoelectric and thermionic power conversion, reference is now made FIG. 1 illustrating a sectional view of an embodiment of an energy harvesting thermionic device (100) that is configured to convert thermal energy (or heat) into electrical power, e.g., electricity. In exemplary embodiments, the device (100) may be nano-scale and/or contain one or more nano-scale components. Each of the dimensions, including a thickness dimension defined parallel to a first-axis, also referred to herein as a vertical axis, i.e., the Y-axis in FIG. 1, a longitudinal dimension parallel to a second-axis, i.e., the X-axis in FIG. 1, also referred to herein as a horizontal axis, and a lateral dimension parallel to a third axis-axis, i.e., the Z-axis in FIG. 1, orthogonal to the first-axis and second axis, are shown for reference. The X-axis, Y-axis, and Z-axis are orthogonal to each other in physical space.

[0025] The nano-scale energy harvesting device (100) is sometimes referred to herein as a cell. In an embodiment, the energy harvesting thermionic device (100) is illustrated as a sheet or a plurality of adjacently positioned sheets or layers, e.g., that may be stacked or wound. A plurality of devices (100) may be organized as a plurality of cells, or in an embodiment a plurality of layers, with the cells or layers arranged in series or parallel, or a combination of both to generate electrical power at the desired voltage, current, and power output.

[0026] The energy harvesting thermionic device (100) includes an emitter electrode (also referred to herein as a cathode) (102) and a collector electrode (also referred to herein as an anode) (104) positioned to define an inter-electrode gap (or interstitial space) therebetween.

In an embodiment, a spacer (106) of separation material, sometimes referred to herein as a standoff or spacer, maintains separation between the electrodes (102) and (104). While the spacer (106) is referred to herein in the singular, it should be understood that the spacer (106) may comprise a plurality of components. In an embodiment, the spacer (106) is an insulator or comprises one or more materials that collectively exhibit electrically non-conductive properties. The spacer (106) is in direct contact with the electrodes (102) and (104). The electrodes (102) and (104) and the spacer (106) define a plurality of apertures (108), also referred to herein as cavities (discussed further below with respect to FIGS. 2A and 2B), in the inter-electrode gap. The apertures (108) extend in the Y direction between the electrodes (102) and (104) for a distance (110) in the range of, for example, about 1 nanometer (nm) to about 100 nm, and in an embodiment, the distance (110) is in a range of about 1 nm to about 20 nm. A fluid (112), also referred to herein as a nano-fluid (discussed further herein with reference to FIG. 3), is received and maintained within one or more, and preferably each, of the apertures (108). [0027] In another embodiment, no spacer (106) is used and only the nano-fluid (112) is positioned between the electrodes (102) and (104). Accordingly, the energy harvesting thermionic device (100) includes two opposing electrodes (102) and (104), optionally separated the spacer (106), with a plurality of apertures (108) extending between the electrodes (102) and (104) and configured to receive the nano-fluid (112).

[0028] The emitter electrode (102) and the collector electrode (104) each may be fabricated with different materials, with the different materials having separate and different work function values. As used herein, the work function of a material, or in one embodiment a combination of materials, is the minimum thermodynamic work, i.e., minimum energy, needed to remove an electron from a solid to a point in a vacuum immediately outside a solid surface of the material. The work function is a material-dependent characteristic. Work function values are typically expressed in units of electron volts (eV). Accordingly, the work function of a material determines the minimum energy required for electrons to escape the surface, with lower work functions generally facilitating electron emission.

[0029] The difference in work function values between the electrodes (102) and (104) due to the different electrode materials influences the voltage that can be achieved. Thus, to generate high power, the difference in work function values between the electrodes (102) and (104) is significant in an exemplary embodiment. In an embodiment, the emitter electrode (102) has a higher work function value than the collector electrode (104). The difference in work function values between the electrodes (102) and (104) due to the different electrode materials induces a contact potential difference between the electrodes (102) and (104) that has to be overcome, e.g., by the application of heat to the emitter electrode (102), to transmit electrons through the fluid (112) within the apertures (108) from the emitter electrode (102) to the collector electrode (104). Both electrodes (102) and (104) emit electrons; however, as explained in more detail elsewhere herein, once the contact potential difference is overcome, the emitter electrode (102) will emit significantly more electrons than the collector electrode (104). The total of the work function value of the collector electrode (104) and the contact potential difference is less than or equal to the work function of the emitter electrode (102) in an exemplary embodiment. Maximum flow occurs when the total of the work function value of the collector electrode (104) and the contact potential equals the work function of the emitter electrode (102). [0030] Both electrodes (102) and (104) emit electrons; however, once the contact potential difference is overcome, the emitter electrode (102) will emit significantly more electrons than the collector electrode (104), which is influenced by an electric field that suppresses electron production from the collector electrode (104). A net flow of electrons is transferred from the emitter electrode (102) to the collector electrode (104) through the apertures (108). This net electron current (114) causes the emitter electrode (102) to become positively charged and the collector electrode (104) to become negatively charged. Accordingly, the energy harvesting thermionic device (100) generates an electron flow (114) that is transmitted from the emitter electrode (102) to the collector electrode (104).

[0031] The emitter electrode (102) is manufactured with a first backing (116), which may comprise, for example, a polyester film, e.g., Mylar^®, and a first layer (118) extending over the first backing (116). The first layer (118) may be comprised of, for example, graphene, platinum (Pt), or other suitable materials. The emitter electrode (102) has an emitter electrode measurement (120) that in an embodiment is approximately 0.25 millimeters (mm), such value being non-limiting, and in an embodiment may have a measurement of, for example, about 2 nm to about 0.25 mm, such values being non-limiting. The first backing (116) is shown in FIG. 1 with a first backing measurement (122), and the first layer (118) is shown in FIG. 1 with a first layer measurement (124), extending in the Y direction. In an embodiment, the first backing measurement (122) and the first layer measurement (124) are in a range of, for example, about 0.01 mm to about 0.125 mm, and in an embodiment are each approximately 0.125 mm, such values being non-limiting. In an embodiment, the first backing measurement (122) and the first layer measurement (124) may have the same or different measurement values.

[0032] In an embodiment, the first layer (118) is sprayed onto the first backing (116) so as to embody the first layer (118) as a nanoparticle layer that is approximately 2 nm ( ._<?., the approximate length or diameter of a nanoparticle), where the 2 nm value should be considered non-limiting. In an embodiment, the thickness (124) of the first layer (118) may be in a range of, for example, about 1 nm to about 20 nm, or about 2 nm to about 20 nm. In another embodiment, the thickness (124) of the first layer (118) may be in a range of, for example, 0.01 mm to 0.125 mm. Generally, smaller thicknesses have higher energy densities and less wasted energy. The first backing (116) has an outer surface (128). The first backing (116) and the first layer (or the nanoparticle layer (118)) define a first interface (130) therebetween. The first layer (or the nanoparticle layer (118)) defines a first surface (132) facing the inter-electrode gap. Alternatively to spraying, the first layer (118) may be pre formed and applied to the first backing layer (116), or vice versa, i.e., the first backing layer (116) applied to the first layer (118).

[0033] A first coating (134), which in an embodiment is comprised of cesium oxide (CS2O) and discussed further herein, at least partially covers the first surface (132) to form an emitter surface (136) of the emitter electrode (102) that directly interfaces with a first spacer surface (138). Accordingly, the emitter electrode (102) of the embodiment is manufactured with a first layer (or nanoparticle layer (118)) interposed between the first backing (116) and the first coating (134).

[0034] The collector electrode (104) includes a second backing (146), which in an embodiment is comprised of, for example, a polyester film, and at least one second layer (148), which in an embodiment is comprised of, for example, graphene or aluminum (Al), extending over the second backing (146). The collector electrode has a collector electrode measurement (150) extending in the Y direction that in an embodiment is approximately 0.25 millimeters (mm), such values being non-limiting, and in an embodiment may have a measurement of, for example, about 2 nm to about 0.25 mm, such values being non-limiting. For example, in an embodiment, a second backing measurement (152) of the second backing (146) and a second layer measurement (154) of the second layer (148) are each approximately 0.125 mm, such values being non-limiting. In an embodiment, the second backing measurement (152) and the second layer measurement (154) may be in a range of, for example, about 0.01 mm to about 0.125 mm, and in an embodiment are each approximately 0.125 mm, such values being non-limiting. The second backing measurement (152) and the second layer measurement (154) may have the same or different measurement values.

[0035] In an embodiment, the second layer (148) is sprayed onto the second backing (146) to embody the second layer (148) as a nanoparticle layer that is approximately 2 nm, where the 2 nm value should be considered non-limiting. In an embodiment, the second layer measurement (154) of the second layer (148) may be in a range of, for example, about 1 nm to about 20 nm, or about 2 nm to about 20 nm. In another embodiment, the second layer measurement (154) of the second layer (148) may be in a range of, for example, 0.01 mm to 0.125 mm. As discussed above in connection with the first layer (118), generally, smaller thicknesses have higher energy densities and less wasted energy. The second backing (146) has an outer surface (158). The second backing (146) and the second layer/nanoparticle layer (148) define a second interface (160). The second layer (or the second nanoparticle layer) (148) defines a second surface (162) facing the inter-electrode gap. Alternatively to spraying, the second layer (148) may be pre-formed and applied to the second backing (146) or vice versa, i.e., the second backing (146) applied to the second layer (148).

[0036] A second coating (164), which in an embodiment is comprised of cesium oxide (CS2O) and discussed further herein, at least partially covers the second surface (162) to form a collector surface (166) that directly interfaces with a second surface (168) of the spacer (106). Accordingly, in an embodiment the collector electrode (104) is manufactured with the second layer/nanoparticle layer (148) on the second backing (146) and a CS2O coating (164) on the second surface (162).

[0037] In an exemplary embodiment, the first and second coatings, (134) and (164), respectively, are formed on the first and second surfaces (132) and (162), respectively. In an embodiment, an electrospray or nano-fabrication techniques, with one or more predetermined patterns, is employed to form or apply the first and second coatings, (134) and (164), respectively. The first and second coatings (134) and (164) can be applied in one or more predetermined patterns that may be the same as or different from one another.

[0038] In exemplary embodiments, the first surface (132) is a platinum surface and the second surface (162) is an aluminum surface. In an embodiment, a percentage of coverage of each of the first surface (132) and second surface (162) with the respective CS2O coating layers (134) and (164) is within a range of at least 50%, and up to 70%, and in at least one embodiment is about 60%. The CS2O coatings (134) and (164) reduce the work function values of the electrodes (102) and (104) from the work function values of platinum (Pt), which in an embodiment is 5.65 electron volts (eV), and aluminum, which in an embodiment is 4.28 eV. In an embodiment, the emitter electrode (102) with the CS2O coating layer (134) has a work function value ranging from about 0.5 to about 2.0 eV, and in an embodiment is approximately 1.5 eV, and the collector electrode (104) with the CS2O coating layer (164) has a work function value of about 0.5 to about 2.0 eV, and in an embodiment is approximately 1.5 eV. [0039] In an embodiment, the electrodes (102) and (104) are comprised of graphene, and are referred to herein as graphene electrodes (102) and (104). The graphene electrodes (102) and (104) can exhibit work function values below 1.0 eV when coated with cesium oxide, gold, tungsten, and other elements and compounds. Sulfur may be incorporated into the coatings (134) and (164) to improve the bonding of the coatings (134) and (164) to the graphene electrodes (102) and (104), respectively, particularly where the first and second layers (118) and (148) of the electrodes (102) and (104) comprise graphene and the sulfur creates covalent bonding between the electrodes (102) and (104) and their respective coatings (134) and (164). The respective work function values of the electrodes (102) and (104) can be made to differ, even when both are comprised of graphene, for example by incorporating different coatings (134) and (164) onto the electrodes (102) and (104). Suitable graphene electrodes are available through ACS (Advanced Chemical Suppliers) Materials, and include Trivial Transfer Graphene™ (TTG 10055).

[0040] In an embodiment, the surface area coverage on the emitter electrode (102) or the collector electrode (104) of CS2O is spatially resolved, e.g. applied in a pattern or non- uniform across the length of the corresponding surface, and provides a reduction in a corresponding work function to a minimum value. In an embodiment, the work function value, from a maximum of about 2.0 eV is reduced approximately 60-80% corresponding to the surface coverage of the CS2O, e.g. cesium oxide. Accordingly, the lower work function values of the electrodes (102) and (104) are essential to the operation of the energy harvesting thermionic device (100) as described herein.

[0041] Platinum (Pt)-coated on copper foil and aluminum (Al) materials optionally are selected for the electrodes (102) and (104), respectively, due to at least some of their metallic properties, e.g., strength and resistance to corrosion, and the measured change in work function when the thermionic emissive material of CS2O or other materials disclosed herein is layered thereon. Alternative materials may be used, such as graphene, noble metals including, and without limitation, rhenium (Re), osmium (Os), ruthenium (Ru), iridium (Ir), rhodium (Rh), and palladium (Pd), or any combination of these metals. In addition, and without limitation, non-noble metals such as gold (Au), tungsten (W), tantalum (Ta), and molybdenum (Mo), and any combination thereof, may also be used. For example, and without limitation, tungsten (W) nanoparticles may be used rather than Pt nanoparticles to form surface (132), and Au nanoparticles may be used rather than Al nanoparticles to form surface (162). Accordingly, the selection of the materials to use to form the nanoparticle surfaces (132) and (162) is principally based on the work functions of the electrodes (102) and (104), and more specifically, the difference in the work functions once the electrodes (102) and (104) are fully fabricated.

[0042] The selection of the first and second coatings (134) and (164), respectively, e.g., thermionic electron emissive material, to deposit on the first surface (132) and the second surface (162), respectively, is partially based on the desired work function value of the electrodes (102) and (104), respectively, and chemical compatibility between the deposited materials, and the deposited thermionic electron emissive materials (134) and (164). Deposition materials include, but are not limited to, thorium, aluminum, cerium, and scandium, as well as oxides of alkali or alkaline earth metals, such as cesium, barium, calcium, and strontium, as well as combinations thereof and combinations with other materials. In at least one embodiment, the thickness of the layer of patterned thermionic electron emissive material of the first and second coatings (134) and (164), respectively, is approximately 2 nm, where the 2 nm value should be considered non-limiting. Accordingly, the electrodes (102) and (104) have the desired work functions.

[0043] FIG. 2A depicts a top view of an embodiment of a spacer (200) and the adjacent electrodes (262) and (264) for use in the nano-scale energy device, such as the device (100) having electrodes (102) and (104) as shown and described in FIG. 1. The spacer (200) and electrodes (262) and (264) are not shown to scale.

[0044]

[0045] Electrodes (262) and (264) are shown in broken lines. The spacer (200), as shown and described herein, includes a plurality of interconnected edges (202). The edges (202) have a thickness or edge measurement (204) in the range of about 2.0 nm to about 0.25 mm. As shown herein, the edges (202) are interconnected. In an embodiment, the interconnected edges (202) collectively define a plurality of hexagonal apertures, also referred to herein as cavities (206), in a honeycomb array (208). The cavities (206) extend in the Y direction. In an embodiment, the spacer (200) may be configured as a uniform or relatively uniform layer, e.g., contiguous and with or without limited apertures. The apertures or cavities, either uniformly or non-uniformly provided across the width and/or length of the spacer material, may be in a range of, for example, greater than 0 mm ( e.g ., 2 nm) to about 0.25 mm in the Y- axis direction, similar to an embodiment of the spacer (106) of FIG. 1.

[0046] Referring to FIG. 2A, in an embodiment, the apertures (206) have a first dimension (210) and a second dimension (212) each having a value in a range between, for example, 2.0 nm and 100 microns. In an embodiment, the edges (202), the apertures (206), and the array (208) may form various shapes, configurations, and sizes, including the dimensions and sizing of the apertures (206), that enable operation of spacer (200) as described herein, including, without limitation, circular, rectangular, and elliptical apertures (206).

[0047] The spacers (200) and (270), shown in FIGS. 2A and 2B, respectively, also include first and second edges (214) and (216), respectively, that define the dimensions, e.g. outer edges, of the spacer (200). The spacer (200), (270) has a distance measure (218) in the lateral dimension (Z) between the lateral side edges (214) and (216). In an embodiment, the distance measure (218) has a range between about 1 nm to approximately 10 microns.

[0048] As shown in FIGS. 2A and 2B, the electrodes (262) and (264) are offset in the lateral dimension Z with respect to one another and with respect to the spacer (200), (270). Specifically, the emitter electrode (262) includes opposite first and second lateral side edges (230) and (232) separated by a first distance (234). The collector electrode (264) includes opposite third and fourth lateral side edges (240) and (242) separated by a second distance (244). The values of the first and second distances (234) and (244) may be the same or different from one another, and may be within a range of, for example, approximately 10 mm to approximately 2.0 m.

[0049] With respect to the first electrode (262), the first lateral side edge (230) extends in the lateral direction Z beyond the first lateral support side edge (214) of the spacer (200), (270) by a third distance (236), and the second lateral support side edge (216) of the spacer (200), (270) extends in the lateral direction Z beyond the second lateral side edge (232) by a fourth distance (228).

[0050] With respect to the second electrode (264), the first lateral support side edge (214) of the spacer (200), (270) extends in the lateral direction Z beyond the third lateral side edge (240) by a fifth distance (226), and the fourth lateral side edge (242) extends in the lateral direction Z beyond the second lateral support side edge (216) of the spacer (200), (270) by a sixth distance (248).

[0051] In embodiments, the third distance (236), the fourth distance (228), the fifth distance (226), and the sixth distance (248) may be the same or different from one another and within a range of, for example, approximately 1.1 nm to approximately 10 microns. The spacer (200), (270) may have a lateral measurement (218) with respect to the Z-axis greater than lateral measurements (234) and (244) of the electrodes (262) and (264), respectively.

The spacer design and measurements shown and described herein reduce a potential for electrodes, such as the electrodes (102) and (104), to contact one another when the spacer (200), (270) is incorporated into the device (100) of FIG. 1. The direct contacting of the electrodes (102) and (104) would create a short circuit.

[0052] Each of the lateral support side edges (214) and (216) may receive at least one layer of an electrically insulating sealant that electrically isolates the portions (250) and (252) of the electrodes (262) and (264), respectively, that extend beyond the lateral support side edges (214) and (216), respectively. Further, as described above, each of the electrodes (262) and (264) may be offset from the spacer (200), (270) to, along with the sealant, reduce the potential for the electrodes (262) and (264) contacting each other and creating a short circuit.

[0053] In exemplary embodiments, the at least one spacer (200) and/or (270), which in exemplary embodiments are dielectric spacers, as shown and described in FIGS. 2A and 2B, respectively, is fabricated with a dielectric material, such as, and without limitation, silica (silicon dioxide), alumina (aluminum oxide), titania (titanium dioxide), and boron-nitride.

The apertures (206) extend between the electrodes (262) and 264) for the distance (110), e.g., in the Y-dimension, in a range from about 1 nanometer (nm) to about 10 microns. A fluid, e.g., the nano-fluid (112), as shown and described in detail in FIG. 3, is received and maintained within each of the apertures (206). The dielectric spacer (200), (270) is positioned between, and in direct contact with, the electrodes (262) and (264).

[0054] Referring to FIG. 3, a diagram (300) is provided to illustrate a schematic view of an embodiment of a fluid or medium (302), also referred to herein as a nano-fluid. As shown, the nano-fluid (302) includes a plurality of gold (Au) nanoparticle clusters (304) and a plurality of silver (Ag) nanoparticle clusters (306) suspended in a dielectric medium (308). In some embodiments, and without limitation, the dielectric medium (308) is selected from one of the groups including alcohols, ketones ( e.g ., acetone), ethers, glycols, olefins, and alkanes ( i.e ., those alkanes with greater than three carbon atoms, e.g., tetradecane). In FIG. 3, each cluster (306) and (308) is embodied as a single nanoparticle, in particular a single Au nanoparticle or a single Ag nanoparticle, with a dielectric coating (discussed below). In some embodiments, and without limitation, the dielectric medium (308) is an alcohol, a ketone (e.g., acetone), an ether, a glycol, an olefin, and/or an alkane (e.g., those alkanes with greater than three carbon atoms, e.g., tetradecane). In an embodiment, the dielectric medium (308) is water or silicone oil. Alternatively, in at least one embodiment, the dielectric medium (308) is a sol-gel with aerogel-like properties and low thermal conductivity values that reduce heat transfer therethrough, e.g., thermal conductivity values as low as 0.013 watts per meter- degrees Kelvin (W/m-K) as compared to the thermal conductivity of water at 20 degrees Celsius (°C) of 0.6 W/m-K. Appropriate materials are selected to fabricate the nanoparticle clusters (304) and (306). The materials selected for the nanoparticle clusters (304) and (306) may have work function values that are greater than the work function values for associated electrodes, such as the electrodes (102) and (104) in FIG. 1. For example, the work function values of the Au nanoparticle clusters (304) and the Ag nanoparticle clusters (306) are about 4.1 eV and 3.8 eV, respectively.

[0055] At least one layer of a dielectric coating (310), such as a monolayer of alkanethiol material, is deposited on the Au and Ag nanoparticle clusters (304) and (306), respectively, to form a dielectric barrier thereon. In an embodiment, the deposit of the dielectric coating (310) is performed via electrospray. The alkanethiol material includes, but is not limited to dodecanethiol and/or decanethiol. Additionally or alternatively, the dielectric coating (310) may be a halogenoalkane or alkyl halide, in which one or more of the hydrogen atoms of the alkane are replaced by halogen atom(s), i.e., fluorine, chlorine, bromine, or iodine. The deposit of the dielectric coating (310), such as alkanethiol, reduces coalescence of the nanoparticle clusters (304) and (306). In at least one embodiment, the nanoparticle clusters (304) and (306) have a diameter in the range of about 1 nm to about 3 nm. In an embodiment, the nanoparticle clusters (304) and (306) have a diameter of about 2 nm. The nanoparticle clusters of Au (304) and Ag (306) are tailored to be electrically conductive with charge storage features (i.e., capacitive features), minimize heat transfer through the spacer apertures (206) of FIG. 2A, with low thermal conductivity values, minimize ohmic heating, eliminate space charges in the spacer apertures (206), and prevent arcing. The plurality of Au and Ag nanoparticle clusters (304) and (306), respectively, are suspended in the dielectric medium (308). The nano-fluid (302), including the suspended nanoparticle clusters (304) and (306), provides a conductive pathway for electrons to travel across the spacer apertures (206) from, for example with reference to FIG. 1, the emitter electrode (102) to the collector electrode (104) through charge transfer. Accordingly, in at least one embodiment, a plurality of Au and Ag nanoparticle clusters (304) and (306) are mixed together in a dielectric medium (308) to form a nano-fluid (302), the nano-fluid (302) residing in the apertures (108) of FIG. 1 and/or apertures (206) of FIG. 2A and/or the permeable or semi-permeable material of FIG. 2B.

[0056] The Au nanoparticle clusters (304) according to exemplary embodiments are dodecanethiol functionalized gold nanoparticles. In exemplary embodiments, the Au nanoparticle clusters (304) have an average particle size of about 1 nm to about 3 nm, at about 2% (weight/volume (grams/ml)). According to exemplary embodiments, the Ag nanoparticle clusters (306) are dodecanethiol functionalized silver nanoparticles. In some embodiments, the Ag nanoparticle clusters (306) have an average particle size of about 1 nm to about 3 nm, at about 0.25% (weight/volume percent). In an embodiment, the average particle size of both the Au and Ag nanoparticle clusters (304) and (306) is at or about 2 nm. The Au and Ag cores of the nanoparticle clusters (304) and (306) are selected for their abilities to store and transfer electrons. In an embodiment, a 50%-50% mixture of Au and Ag nanoparticle clusters (304) and (306) are used. However, a mixture in the range of 1-99% Au- to-Ag could be used as well. Electron transfers are more likely to occur between nanoparticle clusters (304) and (306) with different work functions. In an exemplary embodiment, a mixture of nearly equal (molar) numbers of two different nanoparticle clusters (304) and (306), e.g., Au and Ag, provides good electron transfer. Accordingly, nanoparticle clusters are selected based on particle size, particle material (with the associated work function values), mixture ratio, and/or electron affinity.

[0057] Conductivity of the nano-fluid (302) can be increased by increasing concentration of the nanoparticle clusters (304) and (306). The nanoparticle clusters (304) and (306) may have a concentration within the nano-fluid (302) of, for example, about 0.1 mole/liter to about 2 moles/liter. In at least one embodiment, the Au and Ag nanoparticle clusters (304) and (306) each have a concentration of at least 1 mole/liter. In at least one embodiment, a plurality of Au and Ag nanoparticle clusters (304) and (306) are mixed together in a dielectric medium (308) to form a nano-fluid (302), the nano-fluid (302) residing in, for example, the apertures (108) of FIG. 1, the apertures (206) of FIG. 2A, and/or the permeable or semi- permeable material of FIG. 2B.

[0058] The stability and reactivity of colloidal particles, such as Au and Ag nanoparticle clusters (304) and (306), are determined largely by a ligand shell formed by the alkanethiol coating (310) adsorbed or covalently bound to the surface of the nanoparticle clusters (304) and (306). The nanoparticle clusters (304) and (306) tend to aggregate and precipitate, which can be prevented by the presence of a ligand shell of the non-aggregating polymer alkanethiol coating (310) enabling these nanoparticle clusters (304) and (306) to remain suspended. Adsorbed or covalently attached ligands can act as stabilizers against agglomeration and can be used to impart chemical functionality to the nanoparticle clusters (304) and (306). Over time, the surfactant nature of the ligand coatings is overcome and the lower energy state of agglomerated nanoparticle clusters is formed. Over time, agglomeration may occur due to the lower energy condition of nanoparticle cluster accumulation and occasional addition of a surfactant may be used. Examples of surfactants include, without limitation, Tween^® 20 and Tween^® 21.

[0059] In the case of the nano-fluid (302) of FIG. 3 substituted for the nano-fluid (112) of FIG. 1, electron transfer through collisions of the plurality of nanoparticle clusters (304) and (306) is illustrated. The work function values of the nanoparticle clusters (304) and (306) are much greater than the work function values of the emitter electrode (102) (e.g., about 0.5 eV to about 2.0 eV) and the collector electrode (104) (e.g., about 0.5 eV to about 2.0 eV). The nanoparticle clusters (304) and (306) are tailored to be electrically conductive with capacitive ( i.e ., charge storage) features while minimizing heat transfer therethrough. Accordingly, the suspended nanoparticle clusters (304) and (306) provide a conductive pathway for electrons to travel across the apertures (108) from the emitter electrode (102) to the collector electrode (104) through charge transfer.

[0060] Thermally-induced Brownian motion causes the nanoparticle clusters (304) and (306) to move within the dielectric medium (308), and during this movement the nanoparticle clusters (304) and (306) occasionally collide with each other and with the electrodes (102) and (104). As the nanoparticle clusters (304) and (306) move and collide within the dielectric medium (308), the nanoparticle clusters (304) and (306) chemically and physically transfer charge. The nanoparticle clusters (304) and (306) transfer charge chemically when electrons (312) hop from the electrodes (102) and (104) of FIG. 1 to the nanoparticle clusters (304) and (306) and from one nanoparticle cluster (304) and (306) to another nanoparticle cluster. The hops primarily occur during collisions. Due to the electric field affecting the collector electrode (104), electrons (312) are more likely to move from the emitter electrode (102) to the collector electrode (104) via the nanoparticle clusters (304) and (306) rather than in the reverse direction. Accordingly, a net electron current from the emitter electrode (102) to the collector electrode (104) via the nanoparticle clusters (304) and (306) is the primary and dominant current of the nano-scale energy harvesting device (100).

[0061] The nanoparticle clusters (304) and (306) transfer charge physically ( i.e ., undergo transient charging) due to the ionization of the nanoparticle clusters (304) and (306) upon receipt of an electron, and the electric field generated by the differently charged electrodes (102) and (104). The nanoparticle clusters (304) and (306) become ionized in collisions when the clusters gain or lose an electron (312). Positive and negative charged nanoparticle clusters (304) and (306) in the nano-fluid (302) migrate to the negatively charged collector electrode (104) and the positively charged emitter electrode (102), respectively, providing an electrical current flow. This ion current flow is in the opposite direction from the electron current flow, but less in magnitude than the electron flow.

[0062] Some ion recombination in the nano-fluid (302) may occur, which diminishes both the electron and ion current flow. Electrode separation may be selected at an optimum width (or thickness in the Y direction in FIG. 1) to maximize ion formation and minimize ion recombination. In an exemplary embodiment, the electrode separation (110) is less than about 10 nm to support maximization of ion formation and minimization of ion recombination. In an embodiment, the nanoparticle clusters (304) and (306) have a maximum dimension of, for example, about 2 nm. The electrode separation distance (110) as defined by the spacer (106) (or the spacer (200) or the permeable/semi-permeable material (270) of FIGS. 2A and 2B, respectively) has an upper limit of, for example, about 1000 nm, preferably about 100 nm, and more preferably about 20 nm, and the electrode separation distance (110) of 20 nm is equivalent to approximately 10 nanoparticle clusters (304) and (306). Therefore, the electrode separation distance (110) of about 20 nm provides sufficient space within the apertures (108) for nanoparticle clusters (304) and (306) to move around and collide, while minimizing ion recombination. For example, in an embodiment, an electron can hop from the emitter electrode (102) to a first set of nanoparticle clusters (304) and (306) and then to a second, third, fourth, or fifth set of nanoparticle clusters (304) and (306) before hopping to the collector electrode (104). A reduced quantity of hops mitigates ion recombination opportunities. Accordingly, ion recombination in the nano-fluid (302) is minimized through an electrode separation distance (110) selected at an optimum width to maximize ion formation and minimize ion recombination.

[0063] In an exemplary embodiment, when the emitter electrode (102) and the collector electrode (104) are initially brought into close proximity, the electrons of the collector electrode (104) have a higher Fermi level than the electrons of the emitter electrode (102) due to the lower work function of the collector electrode (104). The difference in Fermi levels drives a net electron current that transfers electrons from the collector electrode (104) to the emitter electrode (102) until the Fermi levels are equal, i.e., the electrochemical potentials are balanced and thermodynamic equilibrium is achieved. The transfer of electrons between the emitter electrode (102) and the collector electrode (104) results in a difference in charge between the emitter electrode (102) and the collector electrode (104). This charge difference sets up the voltage of the contact potential difference and an electric field between the emitter electrode (102) and the collector electrode (104), where the polarity of the contact potential difference is determined by the material having the greatest work function. With the Fermi levels equalized, no net current will flow between the emitter electrode (102) and the collector electrode (104). Accordingly, electrically coupling the emitter electrode (102) and the collector electrode (104) with no external load results in attaining the contact potential difference between the electrodes (102) and (104) and no net current flow between the electrodes (102) and (104) due to attainment of thermodynamic equilibrium between the two electrodes (102) and (104).

[0064] The energy harvesting thermionic device (100) can generate electric power with or without additional heat input (e.g., at room temperature). Heat added to the emitter electrode (102) will raise the temperature of the emitter electrode (102) and the Fermi level of the emitter electrode (102) electrons. With the Fermi level of the emitter electrode (102) higher than the Fermi level of the collector electrode (104), a net electron current will flow from the emitter electrode (102) to the collector electrode (104) through the nano-fluid (112), (302). If the device (100) is placed into an external circuit, such that the external circuit is connected to the electrodes (102) and (104), the same amount of electron current will flow through the external circuit from the collector electrode (104) to the emitter electrode (102). Heat energy added to the emitter electrode (102) is carried by the electrons (312) to the collector electrode (102). The bulk of the added energy is transferred to the external circuit for conversion to useful work, some of the added energy is transferred through collisions of the nanoparticle clusters (304) and (306) with the collector electrode (104), and some of the added energy is lost to ambient as waste energy. As the energy input to the emitter electrode (102) increases, the temperature of the emitter electrode (102) increases, and the electron transmission from the emitter electrode (102) increases, thereby generating more electron current. As the emitter electrode (102) releases electrons onto the nanoparticle clusters (304) and (306), energy is stored in the energy harvesting thermionic device (100). Accordingly, the energy harvesting thermionic device (100) generates, stores, and transfers charge and moves heat energy associated with a temperature difference, where added energy causes the production of electrons to increase from the emitter electrode (102) into the nano-fluid (112), (302).

[0065] The nano-fluid (302) can be substituted into the device (100) of FIG. 1 and used to transfer charges from the emitter electrode (102) to one of the mobile nanoparticle clusters (304) and (306) via intermediate contact potential differences from the collisions of the nanoparticle clusters (304) and (306) with the emitter electrode (102) induced by Brownian motion of the nanoparticle clusters (304) and (306). Selection of dissimilar nanoparticle clusters (304) and (306) that include Au nanoparticle clusters (304) and Ag nanoparticle clusters (306), which have greater work function values of about 4.1 eV and about 3.8 eV, respectively, than the work functions of the electrodes (102) and (104), improves transfer of electrons to the nanoparticle clusters (304) and (306) from the emitter electrode (102) to the collector electrode (104). This relationship of the work function values of the Au and Ag nanoparticle clusters (304) and (306) improves the transfer of electrons to the nanoparticle clusters (304) and (306) through Brownian motion and electron hopping. Accordingly, the selection of materials within the energy harvesting thermionic device (100) improves electric current generation and transfer therein through enhancing the release of electrons from the emitter electrode (102) and the conduction of the released electrons across the nano-fluid (112), (302) to the collector electrode (104).

[0066] As the electrons (312) hop from nanoparticle cluster (304) and (306) to nanoparticle cluster (306) and (304), single electron charging effects that include the additional work required to hop an electron (312) onto a nanoparticle cluster (304) and (306) if an electron (312) is already present on the nanoparticle cluster (304) and (306), determine if hopping additional electrons (312) onto that particular nanoparticle cluster (304) and (306) is possible. Specifically, the nanoparticle clusters (304) and (306) include a voltage feedback mechanism that prevents the hopping of more than a predetermined number of electrons to the nanoparticle cluster (304) and (306). This prevents more than the allowed number of electrons (312) from residing on the nanoparticle cluster (304) and (306) simultaneously. In an embodiment, only one electron (312) is permitted on any nanoparticle cluster (304) and (306) at any one time. Therefore, during conduction of current through the nano-fluid (302), a single electron (312) hops onto the nanoparticle cluster (304) and (306). The electron (312) does not remain on the nanoparticle cluster (304) and (306) indefinitely, but hops off to either the next nanoparticle cluster (306) and (304) or the collector electrode (104) through collisions resulting from the Brownian motion of the nanoparticle clusters (304) and (306). However, the electron (312) does remain on the nanoparticle cluster (304) and (306) long enough to provide the voltage feedback required to prevent additional electrons (312) from hopping simultaneously onto the nanoparticle clusters (304) and (306). The hopping of electrons (312) across the nanoparticle clusters (304) and (306) avoids resistive heating associated with current flow in a media. Notably, the energy harvesting thermionic device (100) containing the nano-fluid (302) does not require pre-charging by an external power source in order to introduce electrostatic forces. This is due to the device (100) being self- charged with triboelectric charges generated upon contact between the nanoparticle clusters (304) and (306) due to Brownian motion. Accordingly, in exemplary embodiments the electron hopping across the nano-fluid (302) is limited to one electron (312) at a time residing on a nanoparticle cluster (304) and (306).

[0067] As the electron current starts to flow through the nano-fluid (302), a substantial energy flux away from the emitter electrode (102) is made possible by the net energy exchange between emitted and replacement electrons (312). The replacement electrons from an electrical conductor connected to the emitter electrode (102) do not arrive with a value of energy equivalent to an average value of the Fermi energy associated with the material of emitter electrode (102), but with an energy that is lower than the average value of the Fermi energy. Therefore, rather than the replacement energy of the replacement electrons being equal to the chemical potential of the emitter electrode (102), the electron replacement process takes place in the available energy states below the Fermi energy in the emitter electrode (102). The process through which electrons are emitted above the Fermi level and are replaced with electrons below the Fermi energy is sometimes referred to as an inverse Nottingham effect. Accordingly, a low work function value of, for example, about 0.5 eV for the emitter electrode (102) allows for the replacement of the emitted electrons with electrons with a lower energy level to induce a cooling effect on the emitter electrode (102).

[0068] As described this far, the principal electron transfer mechanism for operation of the device (100) is thermionic energy conversion or harvesting. In some embodiments, thermoelectric energy conversion is conducted in parallel with the thermionic energy conversion. For example and referring to FIG. 3, an electron (312) colliding with a nanoparticle cluster (304) and (306) with a first energy may induce the emission of two electrons at second and third energy levels, respectively, where the first energy level is greater than the sum of the second and third energy levels. In such circumstances, the energy levels of the emitted electrons are not as important as the number of electrons.

[0069] In at least one embodiment, either the emitter electrode (102) or the collector electrode (104), or both, include a material in the form of lead selenide telluride (PbSeTe) or lead telluride (PbTe). PbSeTe and PbTe are thermoelectric conversion materials that, when introduced into the emitter electrode (102) during fabrication, allows for emission of electrons from the emitter electrode (102) through thermoelectric electron emission. In some embodiments, the PbSeTe or PbTe is also introduced into the collector electrode (104) during fabrication to multiply the number of electrons being introduced into the external circuit current, as shown and described in FIG. 4. Similarly, in some embodiments, the PbSeTe or PbTe is introduced during fabrication into at least a portion of the suspended nanoparticle clusters (304) and (306) to multiply the number of electrons being introduced into the external circuit current, as shown and described in FIG. 4. For example, an electron (312) colliding with a nanoparticle cluster (304) and (306) with a first energy may induce the emission of two electrons at second and third energy levels, respectively, where the first energy level is greater than the sum of the second and third energy levels. In such circumstances, the energy levels of the emitted electrons are not as important as the number of electrons. Accordingly, the use of the PbSeTe or PbTe as described herein increases conversion of thermal energy to electrical energy through further increasing the rate of transfer of electrons through the nano-scale energy harvesting thermionic device (100).

[0070] Furthermore, the PbSeTe or PbTe used in some embodiments as described herein may be an n-type compound doped with a transition metal in the form of bismuth (Bi) or antimony (Sb). The doping of the n-type compound of PbSeTe or PbTe with the transition metal further increases conversion of thermal energy to electrical energy through further increasing the rate of transfer of electrons through the nano-scale energy harvesting thermionic device (100). Accordingly, introducing PbSeTe or PbTe, doped with the transition metal into the emitter electrode (102), the collector electrode (104), and the nanoparticle clusters (304) and (306), increases conversion of thermal energy to electrical energy through increasing the rate of transfer of electrons through the nano-scale energy harvesting thermionic device (100).

[0071] A plurality of energy harvesting thermionic devices (100) is distinguished by at least one embodiment having the thermoelectric energy conversion features described herein. The nano-fluid (112), (302) is selected for operation of the energy harvesting thermionic devices (100) within one or more temperature ranges. In an embodiment, the temperature range of the associated energy harvesting thermionic device (100) is controlled to modulate a power output of the device (100). In general, as the temperature of the emitter electrode (102) increases, the rate of thermionic emission therefrom increases. The operational temperature ranges for the nano-fluid (302) are based on the desired output of the nano-scale energy harvesting device (100), the temperature ranges that optimize thermionic conversion, the temperature ranges that optimize thermoelectric conversion, and fluid characteristics. Therefore, different embodiments of the nano-fluid (302) are designed for different energy outputs of the device (100).

[0072] For example, in an embodiment, the temperature of the nano-fluid (112), (302) is maintained at less than 250°C to avoid deleterious changes in energy conversion due to the viscosity changes of the dielectric medium (308) above 250°C. In an embodiment, the temperature range of the nano-fluid (302) for substantially thermionic emission only is approximately room temperature ( i.e ., about 20°C to about 25°C) up to about 70-80°C, and the temperature range of the nano-fluid (302) for thermionic and thermo-electric conversion is above 70-80°C, with the principle limitations being the temperature limitations of the materials. In an exemplary embodiment, the nano-fluid (302) for operation including thermoelectric conversion includes a temperature range that optimizes the thermoelectric conversion through optimizing the power density within the energy harvesting thermionic device (100), thereby optimizing the power output of the device (100). In at least one embodiment, a mechanism for regulating the temperature of the first nano-fluid (302) includes diverting some of the energy output, e.g., heat, of the device (100) into the nano- fluid (302). Accordingly, the apertures (108) of specific embodiments of the energy harvesting thermionic device (100) may be filled with the nano-fluid (302) to employ thermoelectric energy conversion with thermionic energy conversion above a particular temperature range, or thermionic energy conversion by itself below that temperature range.

[0073] As described herein, in at least one embodiment, the dielectric medium (308) has thermal conductivity values less than about 1.0 watt per meter- Kelvin (W/m K). In at least one embodiment, the thermal conductivity of the dielectric medium (308) is about 0.013 watt per meter- Kelvin (W/m K), as compared to the thermal conductivity of water at about 20 degrees Celsius (°C) of about 0.6 W/m K. Accordingly, the nano-fluid (302) minimizes heat transfer, such as through the apertures (108) of FIG. 1, with low thermal conductivity values. Since the heat transport in a low thermal conductivity nano-fluid (302) can be small, a high temperature difference between the two electrodes, e.g., the electrodes (102) and (104), can be maintained during operation. These embodiments are designed for thermal energy harvesting thermionic devices that employ thermionic emission where minimal heat transfer through the nano-fluid (112), (302) is desired.

[0074] As shown in FIG. 1, the energy harvesting thermionic device (100) has an aperture (108) with a distance (110) between electrodes (102) and (104) that is within a range of, for example, about 1 nm to about 20 nm. For electrode separation distances (110) of about 1 nm to about less than 10 nm, thermal conductivity values and electrical conductivity values of the nano-fluid (302) are enhanced beyond those conductivity values attained when the predetermined distance of the cavity (108) is greater than about 100 nm. This enhancement of thermal and electrical conductivity values of the nano-fluid (302) associated with the distance (110) of about 1 nm to 10 nm as compared to the distance (110) of greater than 100 nm is due to a plurality of factors. For example, a first factor involves, but is not limited to, enhanced phonon and electron transfer between the plurality of nanoparticle clusters (304) and (306) within the nano-fluid (302), enhanced phonon and electron transfer between the plurality of nanoparticle clusters (304) and (306) and the first electrode (102), and enhanced phonon and electron transfer between the plurality of nanoparticle clusters (304) and (306) and the second electrode (104).

[0075] A second factor is an enhanced influence of Brownian motion of the nanoparticle clusters (304) and (306) in a confining environment, e.g., a distance (110) less than about 10 nm. As the distance (110) between the electrodes (102) and (104) decreases below about 10 nm, fluid continuum characteristics of the nano-fluid (302) with the suspended nanoparticle clusters (304) and (306) is altered. For example, as the ratio of particle size to volume of the apertures (108) increases, random and convection-like effects of Brownian motion in a dilute solution dominate. Therefore, collisions of the nanoparticle clusters (304) and (306) with the surfaces of other nanoparticle clusters (304) and (306) and the electrodes (102) and (104) increase thermal and electrical conductivity values due to the enhanced phonon and electron transfer.

[0076] A third factor is the at least partial formation of matrices of nanoparticle clusters (304) and (306) within the nano-fluid (302). Under certain conditions, the nanoparticle clusters (304) and (306) will form matrices within the nano-fluid (302) as a function of close proximity to each other, with some of the nanoparticle clusters (308) remaining independent from the matrices. In an embodiment, the formation of the matrices is based on the factors of time and/or concentration of the nanoparticle clusters (304) and (306) in the nano-fluid (302). A fourth factor is the predetermined nanoparticle cluster (304) and (306) density, which in an embodiment is about one mole per liter. Accordingly, apertures (108) with a distance (110) of about 1 nm to less than about 10 nm causes an increase in the thermal and electrical conductivity values of the nano-fluid (302) therein.

[0077] In addition, the nanoparticle clusters (304) and (306) have a small characteristic length or diameter, e.g., about 2 nm, and the clusters (304) and (306) are often considered to have only one dimension. This characteristic length restricts electrons in a process called quantum confinement, which increases electrical conductivity. The collision of particles with different quantum confinement facilitates transfer of charge to the electrodes (102) and (104). The energy harvesting thermionic device (100) has an enhanced electrical conductivity value greater than about 1 Siemens per meter (S/m) as compared to the electrical conductivity of drinking water of about 0.005 S/m to about 0.05 S/m. Also, embodiments of device (100) with the enhanced thermal conductivity have a thermal conductivity value greater than about 1 W/m-K as compared to the thermal conductivity of water at 20 degrees Celsius (°C) of about 0.6 W/m-K.

[0078] Thermionic emission of electrons (312) from the emitter electrode (102) and the transfer of the electrons (312) across the nano-fluid (302) from one nanoparticle cluster (304) and (306) to another nanoparticle cluster (304) and (306) through hopping are both quantum mechanical effects. [0079] Release of electrons from the emitter electrode (102) through thermionic emission as described in embodiments disclosed herein is an energy selective mechanism. A Coulombic barrier in the apertures (108) between the emitter electrode (102) and the collector electrode (104) is induced through the interaction of the nanoparticles (304) and (306) with the electrodes (102) and (104) inside the apertures (108). The Coulombic barrier is at least partially induced through the number and material composition of the plurality of nanoparticle clusters (304) and (306). The Coulombic barrier induced through the nano-fluid (302) provides an energy selective barrier on the order of magnitude of about 1 eV. Accordingly, the nano-fluid (302) provides an energy selective barrier to electron emission and transmission.

[0080] To overcome the Coulombic barrier and allow electrons (312) to be emitted from the emitter electrode (102) above an energy level that overcomes the barrier, materials for the emitter electrode (102) and the collector electrode (104) are selected for their work function values and Fermi level values. The Fermi levels of the two electrodes (102) and (104) and the nanoparticle cluster (304) and (306) will try to align by tunneling electrons (312) from the electrodes (102) and (104) to the nanoparticle cluster (304) and (306). The difference in potential between the two electrodes (102) and (104) (described elsewhere herein) overcomes the Coulombic barrier, and the thermionic emission of electrons (312) from the emitter electrode (102) occurs with sufficient energy to overcome the Coulombic block. Notably, and in general, for cooling purposes, removing higher energy electrons from the emitter electrode (102) causes the emission of electrons (312) to carry away more heat energy from the emitter electrode (102) than is realized with lower energy electrons. Accordingly, in exemplary embodiments the energy selective barrier is overcome through the thermionic emission of electrons at a higher energy level than would otherwise occur without the Coulombic barrier.

[0081] Once the electrons (312) have been emitted from the emitter electrode (102) through thermionic emission, the Coulombic barrier continues to present an obstacle to further transmission of the electrons (312) through the nano-fluid (302). In exemplary embodiments, smaller gaps on the order of about 1 nm to about 10 nm as compared to gaps in excess of 100 nm facilitate electron hopping, i.e., field emission, of short distances across the apertures (108). Energy requirements for electron hopping are much lower than the energy requirements for thermionic emission. Therefore, the electron hopping has a significant effect on the energy generation characteristics of the device (100). The design of the nano-fluid (302) enables energy selective tunneling, e.g., electron hopping, that is a result of the barrier (which has wider gap for low energy electrons) which results in electrons above the Fermi level being a principal hopping component. The direction of the electron hopping is determined through the selection of the different materials for the electrodes (102) and (104) and their associated work function and Fermi level values. The electron hopping across the nano-fluid (302) transfers heat energy with electrons (312) across the apertures (108) while maintaining a predetermined temperature gradient such that the temperature of the nano-fluid (302) is relatively unchanged during the electron transfer. Accordingly, the emitted electrons transport heat energy from the emitter electrode (102) across the apertures (108) to the collector electrode (104) without increasing the temperature of the nano-fluid (302).

[0082] Referring to FIG. 4, a diagram (400) is provided illustrating a schematic perspective view of an embodiment of a nano-scale energy harvesting thermionic device (490) having an arcuate profile. The device (490) is not shown to scale. The device (490), also referred to herein as a power generation device, is manufactured with a plurality of layers of materials, shown in FIG. 4 as four separate layers. A first layer (402) is referred to as a casing or sheathing that protects one or more of the inner layers and facilitates heat transfer in and out of the device (490). In an embodiment, first layer (402) is referred to as a casing or sheathing that protects one or more of the inner layers and facilitates heat transfer into and out of the device (490). In an embodiment, the casing (402) is manufactured from a thermally conductive and electrically insulating material. A second layer (404) includes the emitter electrode, a third layer (406) includes the separation material (also referred to herein as a standoff and spacer), and a fourth layer (408) includes the collector electrode. In one embodiment, the third layer (406) is referred to herein as a spacer. The emitter electrode (404), the spacer (406), and the collector electrode (408) are fabricated and configured as shown and described in FIGS. 1-3. The nano-fluid (112), (302) is positioned in apertures (108) and (206) of the separation material (406), i.e., the third layer. The outer casing (402), i.e., the first layer, is in direct contact with the emitter electrode (404), i.e., the second layer. The emitter electrode (404), i.e., the second layer, and the collector electrode (408), i.e., the fourth layer, are in direct contact with the spacer (406). Layers (402), (404), (406), and (408) are shown peeled away for clarity. In an embodiment, the layers (402), (404), (406), and (408) define a composite layer (410). Accordingly, the outer casing (402) is in contact with the emitter electrode (404) to provide heat transfer, protective, and sealing features to device (490) (or (100) in relation to FIG. 1). [0083] The device (490) is shown herein with an arcuate or cylindrical configuration with defined a radius (412) extending from an axial centerline (414) to an outermost surface (416) of the device (490). The axial centerline (414) extends parallel to the Z-axis and the radius (412) is defined in a plane defined by the X-axis and Y-axis such that the radius (412) and axial centerline (414) are orthogonal. The device (490) includes an axial aperture (418), shown in broken lines or phantom, coincident with the axial centerline (414) extending from a first base area (420) to an opposing second base area (422). In an embodiment, a structural member (424) is positioned in and received by the axial aperture. The structural member (424) extends from the first base area (420) to the second base area (422), and in an embodiment, the structural member (424) protrudes from one or both of the base areas (420) and (422). In an embodiment, the structural member (424) is fabricated from one or more materials that are both thermally and electrically conductive, as well as chemically compatible with the materials of the layers (402), (404), (406), and (408). In other embodiments, the structural member (424) is fabricated from materials that are either thermally or electrically conductive. Accordingly, the structural member (424) is configured with mechanical and electrical properties, including properties to transfer heat energy and electrical energy generated within the device (490) away from the device (490).

[0084] The fourth layer (408), i.e., the collector electrode, is electrically coupled to the structural member (424) to provide at least a partial electrical flow path. The composite layer (410) extends from the structural member (424) in a spiral configuration, where the spiral configuration has a common center defined by the axial centerline (414) to further define a concentric configuration. The axial aperture (418) further defines a cylindrical configuration with respect to the spiral wound configuration of the composite layer (410). Accordingly, the arcuate electric power harvesting device (490) has features that are spiral, concentric, cylindrical, and toroidal.

[0085] In an embodiment, the arcuate energy harvesting thermionic device (490) has a length (426) measured along the Z-axis of approximately 10 mm to about 2.0 m. The radius (412) of device (490) is approximately 0.635 cm (about 0.25 inch) to about 5.1 cm (about 2.0 inches). A thickness (428) of the composite layer (410) is approximately 0.005 mm to about 2 mm. A thickness (430) of the collector electrode (408) is approximately 0.005 mm to about 2.0 mm. A thickness (432) of the spacer (406) is approximately 1.0 nm to about 10 microns.

A thickness (434) of the emitter electrode (404) is approximately 0.005 mm to about 2.0 mm. A thickness (of the outer casing (402) is approximately 0.005 mm to about 2.0 mm. A length of the composite layer (410) of the embodiment, if laid out flat from the spiral configuration, is within a range of approximately 5.1 cm (about 2.0 inches) to approximately 122 cm. (about 48.0 inches). Other embodiments include any dimensional characteristics that enable operation of the arcuate electric power harvesting thermionic device (490) as described herein.

[0086] As further shown, an electrical circuit (450) is connected to the arcuate energy harvesting thermionic device (490). The circuit (450) includes a first electrical conductor (452) that is electrically connected to the structural member (424) that is electrically connected to the collector electrode (408). The circuit (450) also includes a second electrical conductor (454) electrically connected to the emitter electrode (404). The circuit (450) further includes at least one load (456) electrically connected to the conductors (452) and (454). When the arcuate energy harvesting thermionic device (490) is in service generating electricity, current (458) is transmitted through the circuit (450), and the same amount of electron current as flowing through the circuit (450) will flow from the emitter electrode (404) to the collector electrode (408). For example, a single device (490) can generate a voltage within a range extending between about 0.5 volt and 1.0 volt, depending on the contact potential difference (discussed further herein) induced between the emitter electrode (404) and the collector electrode (408) as a function of the materials used for each. In an embodiment, the device (490) generates about 0.90 volt. The device (490) can generate an electrical current within a range of approximately 5 amperes (amps) to approximately 10 amps. In an embodiment, the device (490) generates about 7.35 amps. Further, in an embodiment, the device (490) generates approximately 2.5 watts to approximately 10 watts.

In an embodiment, the device (490) generates about 6.6 watts. A plurality of the devices (490) may be electrically connected in series for a specific voltage or in parallel for a specific current, or in series and parallel to satisfy both the voltage and current requirements. Accordingly, as described further herein, the arrangements of the devices (490) are scalable to provide sufficient electrical power from the watt range to the megawatt range for a variety of applications.

[0087] The structural member (424) performs both heat transfer and electrical conduction actions when the arcuate energy harvesting thermionic device (490) is in service generating electricity. In addition, the structural member (424) provides structural integrity, and an anchor for an end cap (not shown). The structural member (424) is electrically coupled to the circuit (450) to transmit the electrical power generated within the device (490) to loads (456). The structural member (424) is operably also coupled to a heat sink (460) through a heat transfer member (462). In an embodiment, the heat sink (460) and the heat transfer member (462) are energized to approximately the voltage of the energized structural member (424). In an embodiment, the heat transfer member (462) is fabricated from, for example, an electrically non-conductive material that has sufficient heat transfer characteristics to maintain the device (490) within a predetermined temperature range. In an embodiment, heat transfer member (462) is fabricated from, but not limited to, graphene, carbon composites, and similar materials.

[0088] The energy harvesting thermionic device (490) generates electric power through harvesting heat energy (464). As described in further detail herein, the emitter electrode (404) receives heat energy (464) from sources that include, without limitation, heat generating sources and ambient environments, and generates the electric current (458) that traverses the apertures (206) via the nanoparticle clusters (304) and (306) in the form of the electrons (312). The electric current (458) reaches the collector electrode (408) and the current (458) is transmitted through the circuit (450) to power loads (456). In an embodiment, the device (490) generates electrical power through placement in ambient, room temperature environments. Accordingly, the device (490) harvests heat energy (464), including waste heat, to generate useful electrical power.

[0089] Referring to FIG. 5, a diagram (500) is provided illustrating a perspective view of an arcuate energy harvesting thermionic device (590). In an embodiment, the device (590) is similar to the device (490). In an embodiment, an outer casing (502) includes multiple layers (402) of outer casing material to fabricate the outer casing (502) with an enhanced robustness. The outer casing (502) of the device (590) includes an external surface (540) that includes a seam (542) defined by one or more layers (402) of the outer casing (502). In an embodiment, the seam (542) is defined by the composite layer (410). The seam (542) receives a sealant (544) to prevent ingress of contaminants and egress of device materials through the seam (542). In an embodiment, the sealant (542) is non-conductive to prevent short circuiting of the electrodes (404) and (408). In an embodiment, the sealant (544) is antimony-based. In another other embodiment, the sealant (544) is manufactured from a material that enables operation of the arcuate energy harvesting thermionic devices (490) as described herein.

[0090] A first base area (520) receives a sealant (546) that extends between a rim (548) defined by the outer casing (502) and a structural member (524) that is similar to the structural member (424). In an embodiment, the sealant (546) is substantially similar to the sealant (544). In an embodiment, the sealant (546) is different from the sealant (544). The sealant (546) is also applied to a second base area (522), where the second end area (522) has a similar configuration to the first base area (520). The sealant (546) functions to provide protection of the electrodes (404) and (408), the spacer (406), and the nano-fluid (302) from environmental factors, such as debris, that may induce a short circuit between the electrodes (404) and (408) or contaminate the nano-fluid (302). In addition, as described herein with respect to FIGS. 2A and 2B, and described further in FIGS. 8 and 9, the electrodes (404) and (408) (equivalent to electrodes (262) and (264), respectively) are offset distances (236) and (248), respectively, from the spacer (406). Therefore, the non-conducting sealant (546) resides around the lateral side edges (230) of the electrode (102) and the lateral side edges (242) of the electrode (104) that extend distances (236) and (248) beyond the spacer (406), respectively. Accordingly, the device (590) is shown herein with the sealants (544) and (546) that provide environmental protections for the device (590) and electrical insulation for the electrodes (404) and (408).

[0091] In an embodiment, the devices (490) and (590) are manufactured from separate repositories of materials. Referring to FIG. 6, a diagram (600) is provided illustrating a perspective view of a first repository (690) of layered materials (602) and (604), i.e., the first and second layers, that may be used to manufacture the devices (490) and (590). Referring to FIG. 7, a diagram (700) is provided illustrating a perspective view of a second repository (790) of layered materials (706) and (708), i.e. the third and fourth layers, that may be used to manufacture the devices (490) and (590).

[0092] Referring to FIG. 6, in an embodiment the first layer (602) is equivalent to the outer casing (402) and is hereinafter referred to as outer casing (602). Similarly, a second layer (604) is equivalent to the emitter electrode (404) and is hereinafter referred to as emitter electrode (604). The outer casing (602) includes a first surface (670) that defines an external surface (540) of the arcuate devices (490) and (590). The outer casing (602) also includes a second surface (672) that is opposite to the first surface (670) and contacts the emitter electrode (604). The emitter electrode (604) includes a first surface (674) contacting the second surface (672) of the outer casing (602). The emitter electrode (604) also includes a second surface (676) that is opposite to the first surface (674). In an embodiment, the second surface (676) is at least partially coated with CS O (678), which in an embodiment is pre applied to the second surface (676). In an embodiment, the CS O (678) is applied to the second surface (676) during manufacturing of the devices (490) and (590).

[0093] Referring to FIG. 7, a third layer (706) is equivalent to the separation material (406), and is hereinafter referred to as separation material (706). Similarly, a fourth layer (708) is equivalent to the collector electrode (408) and is hereinafter referred to as collector electrode (708). The separation material (706) includes a first surface (780) that contacts the second surface (676) of the emitter electrode (604). The separation material (706) also includes a second surface (772) that is opposite to the first surface (780). The collector electrode (708) includes a first surface (774) contacting the second surface (772) of the separation material (706). The collector electrode (708) also includes a second surface (776) that is opposite to the first surface (774). In an embodiment, the second surface (776) is at least partially coated with CS O (778), which in an embodiment is pre-applied to the second surface (776). In an embodiment, the CS O (778) is applied to the second surface (776) during manufacturing of the devices (490) and (590).

[0094] In one embodiment, rather than manufacturing the energy harvesting thermionic devices (490) and (590) using the two repositories (600) and (700), each of the layers (602), (604), (706), and (708) are dispensed from an individual repository for each layer. In an embodiment, rather than using a separation material (706) in the form of a solid material, the separation material (706) is applied to either the second surface (676) of the emitter electrode (604) or the first surface (774) of the collector electrode (708). In an embodiment, the solid material is one of a sheet and a web. In an embodiment, the separation material (706) is applied to both of the surfaces (676) and (774). In an embodiment, the separation material (706) is pre-applied to the electrodes (604) and (708). In an embodiment, the separation material (706) is applied to the electrodes (604) and (708) at the time of manufacture of the devices (490) and (590). In an embodiment, the separation material (706) is applied through one or more electrospray devices (not shown). In an embodiment, the separation material (706) is applied through any method that enables operation of devices (490) and (590) as described herein. [0095] Referring to FIG. 8, a diagram (800) is provided illustrating an enlarged perspective view of a first portion (880) of the energy harvesting thermionic device (890).

The device (890) is similar to devices (490) and (590). An outer casing (802), an emitter electrode (804), a spacer (806), and a collector electrode (808) are shown with an offset (882) of the collector electrode (808) with respect to the spacer (806). The collector electrode (808) is depressed in the Z-dimension with respect to the first base area (820) at least partially defined by the outer casing (802), the emitter electrode (804), and the spacer (806). The depression of the collector electrode (808) defines a cavity (884) between each adjacent layer of the spacer (806) and each adjacent layer of the outer casing (802). In addition to the depression of the collector electrode (808), the edge of the emitter electrode (804) may extend beyond the adjacent spacer (806) in the Z-dimension. In an embodiment, rather than the collector electrode (808), the emitter electrode (804) is depressed in the Z-dimension with respect to the first base area (820). In an embodiment, the edge of the collector electrode (808) is approximately flush, e.g., co-planar, with the edge of the spacer (806) to partially define the first base area (820) with no offset. As described herein with respect to FIG. 5, a sealant (546) is applied to the first base area (820) to cover the emitter and collector electrodes (804) and (808), respectively, proximate the first base area (820) and fill in the cavity (884) with a non-conductive material to further electrical isolation between the electrodes (804) and (808).

[0096] Referring to FIG. 9, a diagram (900) is provided illustrating an enlarged perspective view of a second portion (980) of an arcuate energy harvesting device (990). The device (990) is similar to devices (490) and (590). An outer casing (902), an emitter electrode (904), a spacer (906), and a collector electrode (908) are shown with a first offset (982) of the collector electrode (908) and a second offset (984) of the emitter electrode (904) with respect to the adjacent spacer (906). The collector electrode (908) extends in the Z-dimension beyond the adjacent spacer (906) proximate to the second end area (922) at least partially defined by the outer casing (902) and spacer (906). In addition, the emitter electrode (904) is depressed in the Z-dimension with respect to the adjacent spacer (906) proximate to the second base area (922). The extension offset (982) of successive layers of the collector electrode (908), the depression offset (984) of the emitter electrode (904), and the second base area (922), define a cavity (986) between each successive layer of the collector electrode (908). In an embodiment, the emitter electrode (904) extends beyond the adjacent spacer (906) in the Z- dimension. In an embodiment, the edge of the emitter electrode (904) is approximately flush with the edge of the spacer (906) to partially define the second base area (922) with no offset. In an embodiment, the collector electrode (908) is depressed with respect to the adjacent spacer (906). In an embodiment, the edge of the collector electrode (908) is approximately flush with the edge of the spacer (906) to partially define the second base area (922) with no offset. As described herein with respect to FIG. 5, a sealant (546) is applied to the second base area (922) to cover the collector electrode (908) proximal to the second base area (922) and fill in the cavity (986) with a non-conductive material to further electrical isolation between the electrodes (904) and (908). Accordingly, in reference to FIGS. 2, 8, and 9, either of or both of the emitter electrode (804) and (904) and the collector electrode (808) and (908) is offset with respect to the adjacent spacer (806) and (906).

[0097] Referring to FIG. 10, a flow chart (1000) is provided illustrating a process for generating electric power with the energy harvesting device (490). As described herein, a first electrode having a first work function value is provided (1002) and a second electrode having a second work function value is provided (1004). The work function value of the second electrode is different than, e.g., less than, the work function value of the first electrode. The first electrode and the second electrode are proximally positioned a predetermined distance from each other, e.g., about 1 nm to less than about 20 nm, to define an opening there between (1006). A separation material is positioned within the opening (1008). A first surface of the separation material is positioned in at least partial physical contact with the first electrode (1010), and a second surface of the separation material is positioned in at least partial physical contact with the second electrode (1012). At least one aperture is defined within the separation material, with the aperture extending from the first surface to the second surface (1014). A nano-fluid including a fluid medium or media and plurality of nanoparticles is positioned within the aperture(s) (1016). The first and second electrodes, the separation material, and the nanoparticles are arranged in an arcuate configuration (1018), and electrons are transmitted between the first and second electrodes via the nanoparticles (1020).

[0098] As described herein, the present disclosure is directed generally to an energy source, similar in some respects to a battery, and more particularly is directed to a thermal energy harvesting thermionic device, including in exemplary embodiments a nano-scale energy harvesting thermionic device. Ionization is provided therein by the combination of electron tunneling and thermionic emission of the nano-scale energy harvesting thermionic device. Charge transfer therein is affected through conductive nanoparticles suspended in a fluid, e.g., a nano-fluid, undergoing collisions driven by thermally-induced Brownian motion. The design of the device enables ambient energy extraction at low and elevated temperatures (including room temperature). To this end, the electrodes are proximally positioned to allow electrons to travel the distance between them. These electrons emitted at a wide range of temperatures proceed across the gap due to the nano-fluid providing a conductive pathway for the electron emission, minimizing heat transfer to maintain a nano-scale heat engine, and preventing arcing.

[0099] With respect to thermionic converters, in exemplary embodiments the electrical efficiency of exemplary embodiments of the devices depends on low work function materials deposited on the emitter electrode (cathode) and the collector electrode (anode). The efficiency of two low work function electrodes can be increased by using cathodes with sufficient thermionic emission of electrons operating even at room temperature. These low work function cathodes and anodes provide copious amounts of electrons. Similarly, a tunneling device includes two low work function electrodes separated by a designed nano fluid. Cooling by electrode emission refers to the transport of hot electrons across the nano fluid gap, from the object to be cooled (cathode) to the heat rejection electrode (anode). Thus, certain exemplary embodiments involve the coupling of several technologies, including: the electrospray-deposited two low work function electrodes include cesium-oxide on both tungsten and gold; an energy selective electron-transfer thermionic emission and quantum hopping of electrons; a nano-fluid that is tailored as a thermoelectric element to conduct electricity while minimizing heat transfer within the device; and thermal communication from the anode electrical connection that is in thermal contact with the device and the outside heat reservoir, produces a viable thermionic power generator.

[00100] The nano-scale energy harvesting thermionic devices of exemplary embodiments described herein facilitate generating electrical energy via a long-lived, constantly recharging, battery-like device for any size-scale electrical application. The devices of exemplary embodiments provide battery-like properties while having a conversion efficiency superior to presently available single and double conversion batteries. In addition, the devices described herein may be fabricated as an integral part of, and provide electrical energy for, an integrated circuit. The devices described herein are a light-weight and compact multiple- conversion device having a relatively long operating life with an electrical power output at a useful value. Furthermore, in addition to the tailored work functions, the nanoparticle clusters described herein are multiphase nano-composites that include thermoelectric materials. In exemplary embodiments, the combination of thermoelectric and thermionic functions within a single device further enhances the power generation capabilities of the nano- scale thermal energy harvesting thermionic device.

[00101] The conversion of ambient heat energy into usable electricity enables energy harvesting capable of offsetting, or even replacing, the reliance of electronics on conventional power supplies, such as electrochemical batteries, especially when long-term operation of a large number of electronic devices in dispersed locations is required. Energy harvesting distinguishes itself from batteries and hardwire power owing to inherent advantages, such as outstanding longevity measured in years, little maintenance, and minimal disposal and contamination issues. The nano-scale energy harvesting devices of exemplary embodiments described herein demonstrate a novel electric generator with low cost for efficiently harvesting thermal energy. The devices of exemplary embodiments described herein initiate electron flow due to the differences in the Fermi levels of the electrodes without the need for an initial temperature differential or thermal gradient.

[00102] The nano-scale energy harvesting thermionic devices of exemplary embodiments described herein are scalable across a large number of power generation requirements. The devices may be designed for applications requiring electric power in the milliwatts (mW), watts (W), kilowatts (kW), and megawatts (MW) ranges. Examples of devices for the mW range include, but are not limited to, those devices associated with the Internet of Things (IoT) (home appliances, vehicles (communication only), handheld portable electronic devices ( e.g ., mobile phones, medical devices, tablets), and embedded systems (RFIDs and wearables). Examples of devices for the watts range include, but are not limited to, handheld sensors, networks, robotic devices, cordless tools, drones, appliances, toys, vehicles, utility lighting, and edge computing. Examples of devices in the kW range include, but are not limited to, residential off-grid devices (rather than backup fossil fuel generators), resilient/sustainable homes, portable generators, electric and silent transportation (including water-faring), and spacecraft. Examples of devices in the MW range include, but are not limited to, industrial/data center/institutional off-grid devices (e.g., uninterruptible power supplies), resilient complexes, urban centers, commercial and military aircraft, flying cars, and railway/locomotive/trucking/shipboard transportation. Accordingly, substantially any power demand in any situation can be met with embodiments of the devices disclosed herein. [00103] Aspects of the present embodiments are described herein with reference to one or more of flowchart illustrations and/or block diagrams of methods and apparatus (systems) according to the embodiments.

[00104] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[00105] The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present embodiments has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the embodiments. The embodiments were chosen and described in order to best explain the principles of the embodiments and the practical application, and to enable others of ordinary skill in the art to understand the embodiments for various embodiments with various modifications as are suited to the particular use contemplated. The implementation of the nano-scale energy harvesting thermionic devices as heat harvesting thermionic devices that efficiently convert waste heat energy to usable electric energy facilitates flexible uses of the minute power generators. Accordingly, the nano- scale energy harvesting thermionic devices and the associated embodiments as shown and described in FIGs. 1-10, provide electrical power through conversion of heat in most known environments, including ambient, ambient temperature environments.

[00106] It will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications may be made without departing from the spirit and scope of the embodiments. In particular, the nano-scale energy harvesting devices are shown as configured to harvest waste heat from stationary or relatively stationary conditions. Alternatively, the nano-scale energy harvesting devices may be configured to harvest waste heat while in motion. Accordingly, the scope of protection of the embodiment(s) is limited only by the following claims and their equivalents.

Claims

CLAIMS What is claimed is:

1. An apparatus comprising: a first electrode having a first work function value; a second electrode positioned proximal to the first electrode, the second electrode having a second work function value, the second work function value being different from the first work function value; a separation material positioned between the first electrode and the second electrode, the separation material comprising a first surface in at least partial physical contact with the first electrode and a second surface positioned opposite to the first surface, the second surface in at least partial physical contact with the second electrode; and the first electrode, the second electrode, and the separation material collectively defining an at least partially arcuate energy harvesting thermionic device.

2. The apparatus of claim 1, wherein at least the first electrode, the second electrode, and the separation material collectively define a spiral wound composite layer.

3. The apparatus of claim 2, wherein the spiral wound composite layer is wound about itself multiple times.

4. The apparatus of claim 2, further comprising an electrically conductive member about which the composite layer is wound.

5. The apparatus of claim 4, wherein the electrically conductive member is electrically connected to the second electrode.

6. The apparatus of claim 1, further comprising a fluid in the separation material, wherein the separation material further comprises at least one aperture extending from the first surface to the second surface, wherein the fluid is positioned in the at least one aperture to place the first electrode in fluid communication with the second electrode through the aperture.

7. The apparatus of claim 6, wherein the separation material is a dielectric material, the separation material further comprising a solid structure, a permeable material, or a semi-permeable material.

8. The apparatus of claim 7, wherein the separation material comprises silica, alumina, boron-nitride, or titania.

9. The apparatus of claim 6, wherein the at least one aperture comprises an array of apertures.

10. The apparatus of claim 6, wherein the fluid comprises a nano-fluid received in the aperture.

11. The apparatus of claim 10, wherein the nano-fluid comprises a dielectric medium.

12. The apparatus of claim 12, wherein the dielectric medium comprises an alcohol, a ketone, an ether, a glycol, an olefin, or an alkane.

13. The apparatus of claim 12, wherein the nano-fluid further comprises a plurality of nanoparticles suspended in the dielectric medium, wherein the plurality of nanoparticles have a third work function value greater than the first and second work function values.

14. The apparatus of claim 13, wherein the suspended nanoparticles comprise a conductive material with an alkanethiol coating.

15. The apparatus of claim 1, wherein the first electrode comprises at least two components, the at least two components comprising: a first material having a fourth work function value, the fourth work function value being greater than the first work function value; and a second material positioned proximal to the first material.

16. The apparatus of claim 15, wherein the first material is a noble metal, aluminum, molybdenum, tungsten, or a combination thereof.

17. The apparatus of claim 15, wherein the second material is cesium oxide.

18. The apparatus of claim 15, wherein the second electrode comprises at least two components, the at least two components comprising: a third material having a fifth work function value, the fifth work function value being greater than the second work function value; and a fourth material positioned proximal to the third material.

19. The apparatus of claim 18, wherein the third material comprises a noble metal, aluminum, molybdenum, tungsten, or a combination thereof.

20. The apparatus of claim 18, wherein the fourth material is cesium oxide.

21. The apparatus of claim 2, wherein the spiral wound layer has opposite ends, and wherein the apparatus further comprises a sealant applied to extend over at least a portion of at least one of the opposite ends.

22. The apparatus of claim 21, wherein the sealant comprises antimony.

23. The apparatus of claim 1, further comprising an external insulative casing extending about and in contact with the first electrode.

24. The apparatus of claim 1, wherein the first electrode and/or the second electrode has an edge that is offset from an edge of the separation material.

25. The apparatus of claim 1, wherein the first electrode, the second electrode, and/or the separation material has a nano-scale thickness.

26. The apparatus of claim 25, wherein the nano-scale thickness is in a range of 2 nm to 20 nm.

27. A method comprising: providing an apparatus comprising: a first electrode having a first work function value; a second electrode positioned proximal to the first electrode, the second electrode having a second work function value, the second work function value being different from the first work function value; a separation material positioned between the first electrode and the second electrode, the separation material comprising a first surface in at least partial physical contact with the first electrode and a second surface positioned opposite to the first surface, the second surface in at least partial physical contact with the second electrode, the separation having at least one opening containing a nano-fluid comprising a media and nanoparticles; and the first electrode, the second electrode, and the separation material collectively defining an at least partially arcuate energy harvesting thermionic device; and transmitting a plurality of electrons between the first and second electrodes via the nanoparticles.

28. The method of claim 27, further comprising collectively winding at least the first electrode, the second electrode, and the separation material to form a spiral wound composite layer.

29. The method of claim 28, further comprising winding the composite layer about itself multiple times.

30. The method of claim 28, wherein the winding comprises winding the composite layer about an electrically conductive member.

31. The method of claim 30, wherein the electrically conductive member is electrically connected to the second electrode.

32. The method of claim 27, wherein the apparatus further comprises comprising a fluid in the separation material, wherein the separation material further comprises at least one aperture extending from the first surface to the second surface, wherein the fluid is positioned in the at least one aperture to place the first electrode in fluid communication with the second electrode through the aperture.

33. The method of claim 27, wherein the separation material is a dielectric material, the separation material further comprising a solid structure, a permeable material, or a semi-permeable material.

34. The method of claim 33, wherein the separation material comprises at least one of silica, alumina, boron-nitride, or titania.

35. The method of claim 32, wherein the at least one aperture comprises an array of apertures.

36. The method of claim 32, wherein the fluid comprises a nano-fluid received in the aperture.

37. The method of claim 27, wherein the separation material is a nano-fluid.

38. The method of claim 36, wherein the nano-fluid comprises a dielectric medium.

39. The method of claim 38, wherein the dielectric medium comprises an alcohol, a ketone, an ether, a glycol, an olefin, or an alkanes.

40. The method of claim 38, wherein the nano-fluid further comprises a plurality of nanoparticles suspended in the dielectric medium, wherein the plurality of nanoparticles have a third work function value greater than the first and second work function values.

41. The method of claim 40, wherein the suspended nanoparticles comprise a conductive material with an alkanethiol coating.

42. The method of claim 27, wherein the first electrode comprises at least two components, the at least two components comprising: a first material having a fourth work function value, the fourth work function value being greater than the first work function value; and a second material positioned proximal to the first material.

43. The method of claim 42, wherein the first material comprises a noble metal, aluminum, molybdenum, tungsten, or a combination thereof.

44. The method of claim 42, wherein the second material is cesium oxide.

45. The method of claim 42, wherein the second electrode comprises at least two components, the at least two components comprising: a third material having a fifth work function value, the fifth work function value being greater than the second work function value; and a fourth material positioned proximal to the third material.

46. The method of claim 45, wherein the third material comprises a noble metal, aluminum, molybdenum, tungsten, or a combination thereof.

47. The method of claim 45, wherein the fourth material is cesium oxide.

48. The method of claim 28, wherein the spiral wound composite layer has opposite ends, and wherein the apparatus further comprises a sealant applied to extend over at least a portion of at least one of the opposite ends.

49. The method of claim 48, wherein the sealant comprises antimony.

50. The method of claim 27, further comprising an external insulative casing extending about and in contact with the first electrode.

51. The method of claim 27, wherein the first electrode and/or the second electrode has an edge that is offset from an edge of the separation material.

52. The method of claim 27, wherein the first electrode, the second electrode, and/or the separation material has a nano-scale thickness.

53. The method of claim 52, wherein the nano-scale thickness is in a range of 2 nm to 20 nm.