Patent Application
20070006583
1. Field of the Invention
The invention relates generally to thermoelectric heat transfer. More particularly, the invention relates to nanotube-based heat transfer devices.
2. Background Art
Increasingly, electronic components along with their associated circuits and systems are getting smaller, while their thermal power density and heat production are increasing. To avoid thermal damage, it is necessary to remove the heat generated by these devices by some appropriate thermal management or heat transfer means. Common “passive” heat transfer techniques are based on the physical principles of heat conduction, convection, or radiation. Unfortunately, cooling electronics downhole by these passive heat transfer techniques is difficult to accomplish because borehole environments typically experience high temperatures, as high as 150° C.
“Active” heat transfer techniques include compression-evaporation techniques and thermoelectric techniques. Compression-evaporation techniques are based on the fact that compression of a gas releases heat, while gas expansion absorbs heat. Compression-evaporation refrigerators tend to be bulky. Recently, a miniature compressor was developed at the University of Illinois. See Shannon, et al., “Integrated Mesoscopic Cooler Circuits (IMCCs),” 1999 ASME International Mechanical Engineering Congress and Exhibition, Nashville, Tenn., Nov. 15-20, 1999, Proceedings of the ASME, Advanced Energy System Division, AES-Vol. 39, p. 75-82. However, refrigerator approaches have all the drawbacks and problems inherent with moving parts and moving fluids.
Thermoelectric techniques are based on Peltier effects (see
U.S. Pat. No. 6,089,311 issued to Edelson discloses the use of thermionic vacuum diodes having very low work function electrodes to construct heat pumps. According to methods disclosed in this patent, a negative potential bias is applied to the cathode relative to the anode, and electrons are emitted. In the process of emission, the electrons carry off kinetic energy, carrying heat away from the cathode and dissipating it at an opposing anode.
Recently, researchers at Stanford University reported a nanometer-scale approach for refrigeration based on combined tunneling and thermionic emission in vacuum. See Hishinuma et al., “Refrigeration by Combined Tunneling and Thermionic Emission in Vacuum: Use of Nanometer Scale Design,” Applied Physics Letters, 78(17): 2572-74, 2001. A product based on “Electron Tunneling Through Large Area Vacuum Gap” is being commercialized under the trade name of Cool Chip™ by Cool Chips Plc. (Gibraltar). See Tavkhelidze et al., “Electron Tunneling Through Large Area Vacuum Gap-Preliminary Results,” ICT 2002 Conference Proceedings. In order for electron tunneling or thermionic emission to occur, the vacuum gap must be small (on the order of several nanometers). The fabrication and reliability of such devices presents a big challenge because it requires maintaining electrical insulation across extremely close-spaced (about 10 nanometers) metal plates over a relatively large surface area.
While the above described prior art devices are capable of providing cooling or heating in certain applications, there is still a need for heat transfer apparatus and methods that can provide more efficient heat transfer and are reliable at high temperatures.
One aspect of the invention relates to nanotube-based heat transfer devices. A nanotube-based heat transfer device in accordance with one embodiment of the invention includes a substrate layer having a first conductive layer, wherein the substrate layer is adapted to conduct heat from an object to be cooled; a plurality of carbon nanotubes disposed on the first conductive layer; and an anode plate having a second conductive layer, wherein the anode plate is arranged substantially parallel to the substrate layer with the second conductive layer facing the first conductive layer, wherein the second conductive layer is disposed at a selected distance from the first conductive layer such that a gap exists between the second conductive layer and the plurality of carbon nanotubes, wherein the first conductive layer and the second conductive layer are adapted to connect to a power source to provide a bias electrical potential for inducing electron emission from the plurality of carbon nanotubes.
One aspect of the invention relates to thermal energy management systems. A thermal energy management system in accordance with one embodiment of the invention includes a heat transfer assembly comprising a plurality of nanotube-based heat transfer devices; and a first heat exchanger coupled to the heat transfer assembly; wherein at least one of the plurality of nanotube-based heat transfer devices comprises: a substrate layer having a first conductive layer, wherein the substrate layer is adapted to conduct heat from an object to be cooled; a plurality of carbon nanotubes disposed on the first conductive layer; and an anode plate having a second conductive layer, wherein the anode plate is arranged substantially parallel to the substrate layer with the second conductive layer facing the first conductive layer, wherein the second conductive layer is disposed at a selected distance from the first conductive layer such that a gap exists between the second conductive layer and the plurality of carbon nanotubes, wherein the first conductive layer and the second conductive layer are adapted to connect to a power source to provide a bias electrical potential for inducing electron emission from the plurality of carbon nanotubes.
One aspect of the invention relates to nanotube-based heat-driven engines. A nanotube-based heat-driven engine in accordance with one embodiment of the invention includes an engine comprising a fluid, wherein controlled cooling and heating of the fluid provides energy to run the engine; a cooling mechanism; and a heating mechanism, wherein at least one selected from the cooling mechanism and the heating mechanism comprises a carbon nanotube-based heat transfer device, wherein the carbon nanotube-based heat transfer device comprising: a substrate layer having a first conductive layer, wherein the substrate layer is adapted to conduct heat from an object to be cooled; a plurality of carbon nanotubes disposed on the first conductive layer; and an anode plate having a second conductive layer, wherein the anode plate is arranged substantially parallel to the substrate layer with the second conductive layer facing the first conductive layer, wherein the second conductive layer is disposed at a selected distance from the first conductive layer such that a gap exists between the second conductive layer and the plurality of carbon nanotubes, wherein the first conductive layer and the second conductive layer are adapted to connect to a power source to provide a bias electrical potential for inducing electron emission from the plurality of carbon nanotubes.
One aspect of the invention relates to methods for heat transfer. A method for heat transfer in accordance with one embodiment of the invention includes placing a carbon nanotube-based heat transfer device in contact with an object, wherein the carbon nanotube-based heat transfer device comprises: a substrate layer having a first conductive layer, wherein the substrate layer is adapted to conduct heat from the object; a plurality of carbon nanotubes disposed on the first conductive layer; and an anode plate having a second conductive layer, wherein the anode plate is arranged substantially parallel to the substrate layer with the second conductive layer facing the first conductive layer, wherein the second conductive layer is disposed at a selected distance from the first conductive layer such that a gap exists between the second conductive layer and the plurality of carbon nanotubes, wherein the first conductive layer and the second conductive layer are adapted to connect to a power source to provide a bias electrical potential for inducing electron emission from the plurality of carbon nanotubes; and applying a bias electrical potential across the first conductive layer and the second conductive layer of the carbon nanotube-based heat transfer device to induce electron emission from the plurality of the carbon nanotubes.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
Embodiments of the invention relate to apparatus and methods for thermal energy transfer based on nanotube electron emission, electrical, and thermal conductivity properties. Apparatus in accordance with embodiments of the invention may function as nanotube-based coolers (or heater) or nanotube-based power generators.
Carbon nanotubes (CNT) are seamless tubes of graphite sheets with fullerene caps. CNT were first discovered as multi-layer concentric tubes (i.e., multi-walled carbon nanotubes, MWCNT). Subsequently, single-walled carbon nanotubes (SWCNT) were prepared in the presence of transition metal catalysts. Embodiments of the invention may use SWCNT, MWCNT, or a mixture of the two. CNT have shown promising potentials in applications including, for example, nano-scale electronic devices, high strength materials, electron field emission, tips for scanning probe microscopy, and gas storage.
Main approaches to the synthesis of CNT include: laser ablation of carbon, electric arc discharge of graphite rod, and chemical vapor deposition (CVD) of hydrocarbons. See U.S. Pat. No. 6,333,016 B1 issued to Resasco et al. and references cited therein. Among these approaches, CVD coupled with photolithography has been found to be the most versatile in the preparation of various CNT devices. In a CVD method, a transition metal catalyst is deposited on a silicon wafer in the desired pattern, which may be fashioned using photolithography followed by etching. The silicon wafer having the catalyst deposits is then placed in a furnace in the presence of a vapor-phase mixture of, for example, xylene and ferrocene. Carbon nanotubes typically grow on the catalyst deposits in a direction normal to the substrate surface. Various carbon nanotube materials and devices are now available from commercial sources, including Molecular Nanosystems (Palo Alto, Calif.) and Bucky, USA (Houston, Tex.).
An improved CVD method was recently reported by Wei et al. in “Organized Assembly of Carbon Nanotubes,” Nature, 416, pp. 495-496, Apr. 4, 2002, which discloses methods for preparing carbon nanotubes on silica (SiO2) and silicon surfaces without using a transition metal catalyst. According to this method, areas of silica (SiO2) are patterned on a silicon wafer, by photolithography and etching. Carbon nanotubes are then grown on the silica (SiO2) areas in a CVD or a plasma-enhanced CVD (PECVD) process. These methods permit the production of carbon nanotube bundles in various shapes. Carbon nanotubes suitable for embodiments of the invention may be prepared according to this method.
As noted above, carbon nanotubes have unique physical and electrical properties. As electron field emitters, carbon nanotubes have the characteristics of low work function, durability, and thermal stability. See Dean et al., “Carbon Nanotube Field Emission Electron Source,” New Diamond and Frontier Carbon Technology, vol. 12, No. 4, (2002). Accordingly, an electron field emitter based on CNT can be driven at relatively low voltages. In addition, the chemical resistance of such devices to reactions with gases, which may be generated during the operation of the device, is improved, thereby increasing the life span of the emitters. Examples of the use of CNT as electron field emitters and the methods of preparing CNT-based field emission arrays, for example, may be found in U.S. Pat. No. 6,440,761 issued to Choi.
CNT-based field emitters in the prior art are often used as cold cathode displays, which may replace the conventional cathode ray tubes (CRT). See e.g., U.S. Pat. No. 6,664,727 B2 issued to Nakamoto and Kim et al., “Full Color, Large Area Field Emission Displays Using Carbon Nanotube Emitters,” EUROFE 2000, Sep. 25-19, 2000, Segovia, Spain. In contrast, embodiments of the invention relate to the use of carbon nanotube electron emitters in thermal management. In accordance with embodiments of the invention, hot electrons are emitted by the carbon nanotubes and travel to an anode. In the process, heat is removed from the “cathode” to the “anode,” much like a Peltier cooler or the Cool Chip™.
Because the substrate layer 21 is adapted to conduct heat from the object to be cooled, the base layer 21a is preferably a good heat conductor. In some embodiments, the base layer 21a is made of a good heat conductor that does not conduct electricity, such as ceramic. In some embodiments, the base layer 21a is made of a good conductor for both heat and electricity, such as metal. Similarly, the base layer 24a is preferably a good heat conductor. In addition, the substrate layer 21 may adapt a configuration for optimal contact with the object to be cooled.
The layer of carbon nanotubes 22 preferably comprises an ordered array of parallel carbon nanotubes (CNT). These carbon nanotubes can be either single-walled (SWCNT) or multi-walled (MWCNT), or a mixture of the two. Both MWCNT and SWCNT can be manufactured to have narrow size distributions, large-scale periodicities, and high array densities. These attributes make it possible to manufacture a stable, predictable, and uniformly dense electron emitter.
In accordance with embodiments of the invention, gaps between the substrate layers 21 and the anode plates 24 are on the order of a few micrometers (μm) to a few millimeters (mm), preferably on the order of 10-100 micrometer (μm). A nano-cooler in accordance with embodiments of the invention is preferably sealed in vacuum. However, the vacuum requirement for the nano-cooler is less stringent than a conventional vacuum tube (or cathode ray tube; CRT) because carbon nanotubes are less influenced by the atmosphere or residual gas. The voltage required for the nanotubes to emit electrons to anode is low, typically a few volts per μm gap (e.g., 1-5 V/μm). See e.g., U.S. Pat. No. 6,605,894 B2 issued to Choi et al. Thus, a nano-cooler in accordance with the invention may require a voltage on the order of about 100 volts to operate. This voltage requirement can be further reduced if the nano-cooler includes a control or screen grid 23.
In accordance with embodiments of the invention, a screen grid 23 may be maintained at an appropriate voltage to assist electron emission from the carbon nanotubes 22 and to assist the emitted electrons to reach the anode plate 24. In this particular embodiment that includes the screen grid 23, the structure of the nano-cooler is similar to a cathode ray tube or vacuum tube. One of ordinary skill in the art would appreciate that the potential of the screen grid 23 may be regulated to lower the bias potential needed for electron emission from the carbon nanotubes. In addition, the screen grid potential may be used to turn the electron emission on and off (i.e., as in the control grid of a vacuum tube to regulate electron emission or to pulse electron emission). One of ordinary skill in the art would appreciate that an additional electrode, e.g., the suppressor grid in a vacuum tube, can be introduced and appropriate voltage control can be applied to it in order to reduce, or suppress, the return of electrons from the anode plate back to the cathode, a phenomenon known as “secondary emission” (see Spangenburg, K. R., Fundamentals of Electron Devices, McGraw-Hill, 1957 or see Valley, G. E. & Wallman, H., Vacuum Tube Amplifiers, MIT Press, 1946, reprinted by Boston Technical Publishers Inc., 1964). Furthermore, in the nano-cooler, the screen grid 23 may also be used to control the electron flow to optimize the device's performance or to operate more than one device in a series or parallel combination of circuits.
The nano-cooler 20 shown in
A single nano-cooler cell 20 shown in
In addition to arranging the nano-cooler cells side-by-side (i.e., in parallel) to increase heat transfer, the nano-cooler cells may also be arranged in series to increase the efficiency per area.
With the nano-coolers stacked in series, thermal electrons emitted by the first nano-cooler are passed along the various nano-coolers in the series. In each step along the series, the electrons will carry some heat forward. Therefore, the serial nano-cooler stack can produce a larger temperature drop. Note that embodiments of the invention may use a plurality of nano-coolers in parallel, in series, or a combination of the two.
Some embodiments of the invention relate to the use of the nano-coolers in a heat exchanger or thermal management system.
Some embodiments of the invention relate to heat-driven engines powered by pulsing the nano-coolers. Heat-driven engines, such as Sterling engines, are known in the art. Various configurations of Sterling engines are available.
Some embodiments of the invention relate to methods for heat transfer using nano-coolers.
Advantages of embodiments of the invention may include one or more of the following. A nano-cooler of the invention has no moving parts, is fabricated with robust and stable materials, and is therefore more reliable, and can withstand high temperature. A nano-cooler or an assembly of the nano-coolers can be used to cool electronic devices or circuits in tight spaces, for example, on a logging tool or on the downhole electronics in an oil well monitoring and control system. Nano-coolers of the invention may be coupled with conventional heat exchangers or heat pipes to form a thermal management system. In addition, the nano-coolers of the invention may be used to power heat-driven engines, such as a Sterling engine.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
