Patent Application
20060068611
The invention relates generally to heat transfer devices, and particularly, to solid state heat transfer devices.
Heat transfer devices may be used for a variety of heating/cooling and power generation/heat recovery systems, such as refrigeration, air conditioning, electronics cooling, industrial temperature control, waste heat recovery, and power generation. These heat transfer devices are also scalable to meet the thermal management needs of a particular system and environment. Unfortunately, existing heat transfer devices, such as those relying on refrigeration cycles, are relatively inefficient and environmentally unfriendly due to mechanical components such as compressors and the use of refrigerants.
In contrast, solid-state heat transfer devices offer certain advantages, such as the potential for higher efficiencies, reduced size and weight, reduced noise, and being more environmentally friendly. For example, thermotunneling devices transfer heat by tunneling hot electrons from one electrode to another electrode across a nanometer-scale barrier. The heat transfer efficiency of these thermotunneling devices depends upon various factors, such as material characteristics (for electrodes and barrier), electrode alignment, electrode spacing, and thermal losses. For efficient operation of these thermotunneling devices, the electrodes should have a low work function, the barrier should ideally be in vacuum, and the spacing between the electrodes should be on the order of 4-20 nanometers. Unfortunately, electrode spacing is particularly difficult to achieve and maintain in these thermotunneling devices. Thus, achieving efficient thermotunneling devices can be problematic.
Accordingly, a need exists for creating a heat transfer device with low work function electrodes and a controlled spacing between the electrodes.
In accordance with certain embodiments, a method of manufacturing a heat transfer device includes providing first and second thermally conductive substrates that are substantially atomically flat, providing a patterned electrical barrier on the first or second thermally conductive substrates and disposing a low work function material on the first or second thermally conductive substrates in an area oriented between the patterned electrical barrier in a configuration in which the first and second thermally conductive substrates are positioned opposite from one another. The method also includes bonding the first and second thermally conductive substrates in the configuration and extracting a plurality of units having opposite sections of the first and second thermally conductive substrates, each unit having a portion of the patterned electrical barrier disposed about the low work function material.
In accordance with certain embodiments, the present technique has a heat transfer device including first and second thermally conductive substrates that are positioned opposite from one another, wherein the first and second thermally conductive substrates are each substantially atomically flat and a patterned electrical barrier is disposed between the first and second thermally conductive substrates on the first or second thermally conductive substrates. The heat transfer device also includes a low work function material disposed between the first and second thermally conductive substrates on the first or second thermally conductive substrates in an area oriented between the patterned electrical barrier, wherein introduction of a current flow between the first and second thermally conductive substrates enables heat transfer between the first and second thermally conductive substrates via a flow of electrons between the first and second thermally conductive substrates.
In accordance with certain embodiments, the present technique has a method of operation of a heat transfer device including passing hot electrons across a thermotunneling gap between first and second thermally conductive substrates, wherein the thermotunneling gap is formed by a patterned electrical barrier disposed about a low work function material on one of the first or second thermally conductive substrates.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Referring now to the drawings,
In this embodiment, the thermotunneling gap 26 is formed by vacuum that provides a minimum thermal back path to enhance the efficiency of the thermotunneling device 12. In certain embodiments, the thermotunneling gap 26 has a spacing ranging between approximately 4 nanometers to about 20 nanometers. The nanometer gap between the first and second electrodes 20 and 22 facilitates a substantial reduction in the tunneling of cold electrons across the thermotunneling gap and facilitates a substantial increase the tunneling of hot electrons across the thermotunneling gap 26. Further, the nanometer gap between the first and second electrodes 20 and 22 advantageously reduces a high voltage requirement across the first and second electrodes 20 and 22 for facilitating the tunneling of electrons. Thus, a nanometer gap between the first and second electrodes 20 and 22 enables the tunneling of electrons at a relatively lower voltage, thereby enhancing the efficiency of the thermotunneling device 12.
The nanometer spacing and a bias voltage across the thermotunneling gap 26 ensure that the heat flow is substantially unidirectional. In the illustrated embodiment, the heat flow is unidirectional from the object 14 towards the heat sink 16, thus making the object 14 cooler by transferring the heat to the heat sink 14. In certain embodiments, the thermotunneling device 12 may facilitate the heating or cooling of a closed environment. It should be noted that the thermotunneling device 12 may be operable at or near room temperature. In certain embodiments, the first and second electrodes 20 and 22 comprise dissimilar materials that enhance the tunneling of electrons because of a peltier effect, thereby, enhancing the efficiency of the thermotunneling device 12. However, the direction of current flow may be selected based upon a desired direction of the thermotunneling of electrons between the first and second electrodes 20 and 22.
Referring first to
At block 44, a patterned electrical barrier layer (e.g., a plurality of frames or borders) is provided on at least one of the first and second thermally conductive substrates. The patterned electrical barrier layer provides perimeter support to open areas on the first and second thermally conductive substrates and facilitates control of alignment of the first and second thermally conductive substrates during subsequent bonding of the first and second thermally conductive substrates. The bonding of the first and second thermally conductive substrates will be described in detail below. The thickness of the patterned electrical barrier layer is adjusted to a pre-determined value based upon the work function of the first and second thermally conductive substrates and a desired thermotunneling gap. It should be noted that, as used herein, the term “work function” is defined by the least amount of energy required to remove an electron from the surface of a solid material, to a point outside the solid material. In certain embodiments, the thickness of the patterned electrical barrier layer is in a range from about 4 nanometers to about 20 nanometers. Further, the thickness of the patterned electrical barrier layer provides the desired thermotunneling gap for facilitating the tunneling of electrons between the first and second thermally conductive substrates. In certain embodiments, where the work function of the first and second thermally conductive substrates is substantially low, the thickness of the patterned electrical barrier layer may be greater than 10 nanometers.
Moreover, embodiments of the patterned electrical barrier layer are grown on at least one of the first and second thermally conductive substrates. The patterned electrical barrier may be grown or deposited on at least one of the first and second thermally conductive substrates by techniques such as thermal oxidation, chemical vapor deposition, enhanced plasma assisted chemical vapor deposition, sputtering, evaporation and spin coating. In certain embodiments, the patterned electrical barrier layer comprises a material having a low thermal conductivity. Examples of such materials include oxides, polymers, nitrides, and silica-based aerogels.
Next, a low work function material is disposed on at least one of the first and second thermally conductive substrates to form first and second electrodes (block 46). In certain embodiments, the low work function material is disposed on the first and second thermally conductive substrates in an area disposed between the patterned electrical barrier layer. In other words, the patterned electrical barrier layer is disposed about the low work function material on the same one of the first or second thermally conductive substrates. Alternatively, the low work function material may be disposed on either first or second thermally conductive substrates in an area aligned with the area between the patterned electrical barrier layer on either first or second thermally conductive substrates. For example, one embodiment has the patterned electrical barrier layer disposed on the first thermally conductive substrate, while the low work function material is disposed on the second thermally conductive substrate in an area opposite from and within the borders of the patterned electrical barrier layer.
The low work function material may be deposited on the first and second thermally conductive substrates by metal deposition techniques, such as ion implantation, evaporation, sputtering, vapor deposition, plasma enhanced chemical vapor deposition, chemical vapor deposition, pulsed laser deposition and so forth. In various embodiments, the low work function material may comprise an alkalide, an electride, a rare-earth sulfide, an oxide of barium, strontium, calcium and their combinations thereof. In certain embodiments, the low work function material comprises a multilayer thin film structure. In this embodiment, the multilayer thin film structure comprises at least one metal layer disposed adjacent to at least one wide band-gap semiconductor layer. In addition, the low work function material may be deposited on the entire substrate either below or above the patterned electrical barrier. In certain embodiments, a low emmissivity material may be disposed on the low work function material to minimize the radiation parasitic thereby enhancing the efficiency of the thermotunneling device.
Moreover, the process 40 includes bonding the first and second thermally conductive substrates in a wafer bonder (block 48). In certain embodiments, the first and second thermally conductive substrates are placed inside a vacuum chamber and are bonded at a desired temperature, thus forming a vacuum within the tunneling gap to enhance the efficiency of the heat transfer device. Alternatively, the bonding of the first and second thermally conductive substrates may be performed in an inert gas environment, thus filling the tunneling gap with an inert gas such as xenon. In some embodiments, a thermal anneal of the first and second thermally conductive substrates may be performed at a desired temperature to improve the bond between the first and second thermally conductive substrates. Embodiments of this bonding step 48 comprise positioning the first and second thermally conductive substrates, such that at least one portion (e.g., frame or border) of the patterned electrical barrier and at least one portion (e.g., square or block) of the low work function material on the first and second thermally conductive substrates are substantially opposite each other.
Further, at block 50, a plurality of units having opposite sections of the bonded first and second thermally conductive substrates are extracted to form a plurality of heat transfer devices. Each of these extracted units has a portion of the patterned electrical barrier disposed about the low work function material between the bonded first and second thermally conductive substrates. The extracted units also may be coupled electrically and assembled as a heat transfer module to provide a desired heating or cooling capacity based on certain thermal management needs.
Turning now to
Next, at block, 56 the first and second thermally conductive substrates are bonded together. In certain embodiments, the bonding step 54 may be performed in vacuum at a desired temperature, thus forming a vacuum within a tunneling gap between the first and second thermally conductive substrates to enhance the efficiency of the heat transfer device. Moreover, the first and second thermally conductive substrates may be positioned to align the low work function material surrounded by the patterned electrical barrier (e.g., a frame or border) with the low work function material disposed on the opposite one of the first and second thermally conductive substrates. Subsequently, at block 58, the process 52 continues by extracting a plurality of units from the bonded first and second thermally conductive substrates, wherein each of the units has a portion of the patterned electrical barrier (e.g., frame or border) surrounding a portion of the low work function material (e.g., a square or block). As mentioned above, these extracted units may be used individually or collectively for use as a heat transfer device for a system, such as a refrigeration system, an electronics cooling system, and so forth.
Referring now to
The insulative boundaries 72 comprise a material having a low thermal conductivity typically in the range of about 0.01 W/cm K to about 0.15 W/cm K. Examples of such materials include oxides, nitrides and silica-based aerogels. In one embodiment, the insulative boundaries 72 may be deposited on the substrate 70. In another embodiment, the insulative boundaries 72 may be grown on the substrate 70. Further, the patterning of the insulative boundaries 72 may be achieved by techniques such as etching, photoresist, shadow masking, lithography and so forth. It should be noted that embodiments of the heat transfer device manufactured by the exemplary techniques described above are passive devices that enable a heat transfer between two electrodes by maintaining a desired thermotunneling gap. Thus, certain embodiments of the heat transfer device do not use any active components or a feed back control circuitry for controlling the thermotunneling gap between the electrodes.
In certain embodiments, the low work function material 76 is disposed selectively on the substrate 70 by using techniques such as etching, photo resist, lithography, and so forth. Further, the low work function material 76 may be deposited on the substrate via techniques such as ion implantation, evaporation, sputtering, vapor deposition, plasma enhanced chemical vapor deposition (PECVD), chemical vapor deposition (CVD), pulsed laser ablation (PLA), or combinations thereof. In this embodiment, the low work function material 76 comprises an alkalide. In various embodiments, the low work function material 76 may comprise an electride, an oxide of barium, a rare-earth sulfide, strontium, calcium and their combinations thereof. In certain embodiments, the low work function material comprises a multilayer thin film structure. In this embodiment, the multilayer thin film structure comprises at least one metal layer disposed adjacent to at least one wide band-gap semiconductor layer. In addition, the low work function material may be deposited on the entire substrate either below or above the patterned electrical barrier.
As discussed above, another thermally conductive substrate (not shown) is bonded to the thermally conductive substrate 70 over the insulatively bounded low work function pattern 74, as illustrated in
It should be noted that, a plurality of units 78 may be extracted from the foregoing bonded structure to form the thermotunneling devices of
The various aspects of the technique described hereinabove find utility in a variety of heating/cooling systems, such as refrigeration, air conditioning, electronics cooling, industrial temperature control, power generation, and so forth. These include air conditioners, water coolers, refrigerators, heat sinks, climate control seats and so forth. The heat transfer device as described above may be employed in refrigeration systems such as for household refrigeration and industrial refrigeration. In addition, such heat transfer devices may be employed for cryogenic refrigeration, such as for liquefied natural gas (LNG) or superconducting devices. Further, the heat transfer device may be employed in systems for ventilation and air conditioning. Examples of such systems include air conditioners and dehumidifiers. In addition, the heat transfer device may be employed for power generation/waste heat recovery in different applications by maintaining a thermal gradient between two electrodes. Examples of such applications include gas turbine exhausts, furnace exhausts, exhausts of vehicles and so forth.
The passive heat transfer device described herein above may also be employed for thermal energy conversion and for thermal management. It should be noted that the materials and the manufacturing techniques for the heat transfer device may be selected based upon a desired thermal management need of an object. Such devices may be used for cooling of microelectronic systems such as microprocessor and integrated circuits. Further the heat transfer devices may also be employed for thermal management of semiconductors, photonic devices and infrared sensors. As noted above, the method described here may be advantageous in relatively precise control of the spacing and alignment between adjacent electrodes of a heat transfer device to meet the desired thermal management needs in the environments mentioned above.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
