1 About electronic cooling technology Cooling technology involves many fields. In the field of electronics, cooling technology has also penetrated into every aspect. From the emergence of electronic technology, there have been cooling problems. The development of electronic technology is advancing by leaps and bounds, and cooling technology is also accompanied by it, which always plays a basic and supporting role.
1.1 The necessity of electronic cooling There are mainly the following reasons for cooling in the electronic field: Take away the heat of the device: the processing process of the electronic device (in this case, "information" can be any form of current, electromagnetic oscillation, sound, light, etc.) is essentially It is the process of energy conversion. This will always be accompanied by heat generation. The source of heat generation is that any energy conversion process cannot be 100% efficiency. Less than 100% of the energy is all or most turned into heat, which cannot be accumulated in electronic devices. , Must emanate. The development of electronic devices towards smaller, higher speed, and greater power density all means greater heat flux density, and cooling is becoming more and more important.
Typical examples are: transistors, power devices, vacuum electronics, ICs, lasers, etc. The illustrated electronic device A processes energy or energy-bearing signals X1, Y1 into energy or energy-bearing signals X2Y2, Q is the heat emitted; from an energy perspective, the following relationship should be satisfied. Greater than 0, that is, there is always heat to be dissipated.
Lower the temperature of the device to improve the device performance related to temperature. The working temperature of electronic devices often has a great influence on its performance. Thermal noise or dark current are the most obvious performances affected by temperature, such as in sensor devices, infrared detectors and various photon detectors, amplifier devices, INA Wait, this is the case in high-speed digital devices. Cooling down has a thermal noise suppression or blocking effect on electronic devices.
Cooling helps to increase the working life of the device.
The characteristics of some electronic devices only appear when working below a certain temperature. If the superconducting sub-device is based on superconducting phenomena, it must work below the superconducting transition temperature.
1.2 The electronic cooling object can be divided into the following two types according to the level of the object to be cooled: cooling the electronic machine or system so that the working environment temperature of the machine or system is within the required range. This method requires not only cooling, but sometimes heating. The temperature requirements must take into account both the machine and the person operating the machine. So it is more appropriate to call it air conditioning or temperature regulation or constant temperature. It regards the entire system as an object, and sometimes requires temperature adjustment measures for specific heating points.
Cool electronic devices or modules. This method is highly targeted, wherever it heats, it cools wherever it needs to be cooled. This article focuses on this situation and is referred to as electronic cooling for short.
1.3 According to the temperature and method of sub-cooling, the method of electronic cooling can be divided into the following categories: cooling method name heat dissipation refrigeration low temperature refrigeration meaning to transfer the heat inside the cooled object to the surface, and then to the heat sink, Disseminated to the outside world.
Reduce the temperature of the object to be cooled to a temperature lower than the heat sink or ambient temperature.
Reduce the temperature of the object to be cooled to a temperature that is much lower than the heat sink or ambient temperature. Generally refers to temperatures below 120K.
Features do not need to add energy, the power of cooling comes from the temperature difference between the object to be cooled and the heat sink.
Energy must be added to proceed. Larger energy must be added to cool the target electronic machine or system electronic devices, components, or modules.
Examples of cooling objects of electronic whole machines or system electronic devices, components or modules. Electronic devices are radar, various electronic chassis of communication systems, various power devices or power module radars, communication systems, various electronic chassis, various power devices and modules, micro Electronic device such as IC or DSP, optoelectronic device, vacuum electronic device infrared device, LNA, superconducting device cooling method Natural convection-radiator, chassis cooling forced convection fan; cooling liquid circulation conduction a thermally conductive solid, thermally conductive liquid, heat pipe
The surface of the object to be cooled uses high emissivity materials to radiate heat outward.
In practice, these methods are often used in combination.
Thermoelectric cooler (semiconductor cooler) vapor compression cooler gas cooler absorption type and absorption cooler thermoelectric emission cooler gas low temperature cooler such as Stirling cooler. GM refrigerator.
Pulse tube refrigerator. Throttling refrigerators use low-temperature liquids or solids (such as IN2LH, etc.) to evaporate to cool radiant refrigerators (such as space-use radiant refrigerators, vapor compression low-temperature refrigerators, solid refrigerators (adiabatic demagnetizing refrigerators, optical refrigerators) 2 There are only three methods of heat transfer in cooling technology: conduction, convection and radiation. In most heat dissipation structures, not only one heat transfer method is used, but three or two methods are used in combination. Existing heat dissipation methods are relatively mature, There are many products designed and manufactured for heat dissipation, typical of which are: various heat sink radiators) various disturbance sources that disturb the heat transfer medium to produce convection such as fans and pumps; various thermally conductive solids or liquids, etc. The rational selection and matching technology of these products and the thermal design methods for the objects to be cooled have been perfected. However, the development of electronic technology continues to provide new requirements for heat dissipation. These requirements mainly include: the cooling product has a smaller volume and weight in order to adapt to the electronic The volume and weight of the whole machine or device are getting smaller and smaller; cooling products are required to be more efficient, heat transfer is faster, and the temperature difference is smaller, and small heat dissipation products can take away more heat; cooling products are required to be more convenient to use, including installation Convenient, easy to use, low energy consumption, good adaptability to the object to be cooled (no interference, etc.), etc.
To this end, a number of cooling technologies that meet the new cooling requirements are now discussed as follows: 2.1 Micro-channel cooling technology Micro-channel heat exchanger refers to the use of photolithography or other etching methods to produce cross-sectional dimensions on the substrate of only a few dozen to With hundreds of micrometers of channels, the heat exchange medium flows through these small channels with the base of the heat exchanger and exchanges heat with other heat exchange media through the base. The base material of the heat exchanger can be metal, glass, silicon or any other suitable material. The outstanding advantages of this heat exchanger are: â‘ Large heat exchange coefficient and good heat exchange effect. Due to the extremely small geometric dimensions, the flow state when the fluid flows through the channel is very different from conventional heat exchangers. The Reynolds number is generally one order of magnitude larger, so the heat transfer coefficient is significantly larger. The temperature difference between the heat exchange medium and the substrate is very small.
The manufacturing process uses electronic device manufacturing process, which is conducive to cost reduction and mass production.
Due to the small temperature difference between the heat exchange medium and the substrate, and the short distance between the channels, the thermal conductivity of the substrate itself has little effect on the total heat transfer derivative. Heat Exchanger.
It is a miniature cryogenic refrigerator, its heat exchanger is made on the glass substrate by photolithography, the size is only about 7X 1.5cm. The refrigerator can be cooled at 77K, used to cool optoelectronic devices or low temperature electronic devices .
It is a schematic diagram of micro-channel heat exchanger cooling for semiconductor laser array. Microchannels have caused great changes in the thermal field of the laser display substrate.
Low-temperature refrigerator using micro-channel heat exchanger Micro-channel heat exchanger is used to cool the laser array. The heat transfer medium used for micro-channel is generally purified air, nitrogen, â‘´2, water, etc. The microchannel can make the heat flux density up to 100 ~ 150W / cm2, while the general traditional heat exchange form can only reach 10 ~ 20W / cm2, and the gap between them is as high as 50 times. While dissipating the large heat flow, the surface temperature only increases by 1 / 50. Microchannel heat exchange technology is used for cooling of multi-chip components, laser diode display, radar solid-state devices, high-speed digital devices, etc. The application in optoelectronic devices is relatively mature. The current high-power laser display needs to keep the temperature in the 0.001 liter volume below 100C and dissipate hundreds of watts of heat, so a huge circulating water cooler must be used.
The heat dissipation capacity of the micro-channel heat exchanger shown can reach 100W / cm2. The size of the micro-channel is 501X500Mm = width parent height. Some researchers use a single mask method to make width X height = (5 ~ 10rtn) X (8 ~ 10rtn ) Of the microchannel. Because of its smaller size, its performance is better.
Tests show that the thermal resistance of the air-cooled silicon microchannel heat sink is less than 1cm2.K / W, and the thermal resistance of the water-cooled silicon heat sink is less than 0.1cm2.K / W, which means that the temperature difference between the water and the chip is 150W / cm2 when dissipating heat on a 1cm2 chip It can be maintained below 15 ° C. The thermal resistance of the liquid nitrogen-cooled silicon microchannel heat sink is less than that in the case of single-layer microchannel heat exchangers becoming mature. Double-layer microchannels have also been studied. The latter is helpful to reduce the pressure drop and improve the uniformity of the chip temperature. To reduce thermal stress.
~ 2006, the heat of high-speed IC may reach 200W / cm2, the development of micro-channel heat sink that can adapt to this power has made progress.
Micro-jet heat transfer refers to the injection of heat transfer medium from many micropores to the surface to be cooled. The heat transfer coefficient between the medium and the surface is kept at a very high level due to strong disturbances. Under certain conditions, this cooling method The thermal conductivity is 1000 times higher than copper.
It is an enlarged view of a micro-nozzle, which shows the intensity of the disturbance.
The integrated heat path of the micro-nozzle heat exchanger 2.3 The integrated heat path is a closed-loop cooling system composed of a micro-channel condenser, a micro-pump drive, and a micro-jet evaporator. This is a modular micro-mechanical silicon heat dissipation system. It is called "integrated hot circuit" and corresponds to an integrated circuit. This name reflects the thought of solving the heat dissipation and thermal management problems of ICs and other electronic devices from the system, and at the same time, solving the thermal problems from the microscopic point of view, from the source of heat.
For the high-power integrated heat circuit of power electronic devices (such as IGBT) research, the goal is to achieve a heat flux density of 600W / cm2. Some researchers theoretically calculate the heat dissipation capacity of up to 1kW / cm2. 2.4 New heat pipe heat transfer technology Used in the field of electronic cooling. Because of its small heat transfer temperature difference and large heat transfer, it does not need to pump heat transfer medium, and it has been maturely used in power electronic devices and avionics.
In view of the small size of microelectronic devices and multi-chip components, an embedded miniature ceramic heat pipe was developed. Several micro heat pipes are buried in the chip substrate, and the heat pipe is filled with water. The capillary core in the heat pipe is made of ceramic material and axially grooved. The manufacturing process is fully compatible with the current chip substrate manufacturing process. The thermal conductivity of this kind of heat pipe is better than that of diamond with high thermal conductivity and much better than the thermal conductivity of existing substrate materials.
A considerable part of the CPU cooling in notebook computers uses micro heat pipes, generally about 3 mm in diameter. It has obvious advantages compared with the existing fan heating sink structure (see). For the specific requirements of electronic cooling, gravity has been developed With auxiliary heat pipe flexible loop heat pipe, flat type electronic cooling heat pipe, miniature air-to-air heat exchange tube, etc.
Miniature heat pipes embedded directly in the silicon substrate of the chip have appeared. Researchers call it a "heat spreader" to replace the diamond film that plays a role of heat conduction in the integrated circuit.
The volume of this miniature heat pipe is so small that the size of the steam and liquid interface in the heat pipe is comparable to the hydraulic radius of the heat pipe. The steady-state computer model of this miniature heat pipe has been developed to calculate the heat transfer of the heat pipe.
3 Refrigeration technology under development 0 is a refrigerator using the principle of absorption refrigeration, it also has all the components of the ordinary absorption refrigerator.
1 is the mesoscopic refrigerator using the Stirling cycle, which contains the compressor, expander, cooler and corresponding channels. The working fluid always works in gas mode without phase change.
The compressors, expanders, valves and other moving parts of the above-mentioned refrigerators are all manufactured using micro-technology processes, and in most cases are made of silicon materials. They have a wide range of applications in MEMS, optoelectronic devices, and micro-electric 31 mesoscopic cooler sub-device cooling.
32Thermal electron emission refrigeration technology When the kinetic energy of a solid is heated to a certain degree by heating internal electrons, some electrons will overcome the work function and escape. The refrigerator made by the principle of thermal electron emission is an all-solid-state refrigerator. A refrigerator in the form of a film. Much research has been done on the thin film thermoelectron emission refrigerator because it can be directly plated on the surface of optoelectronic microelectronic devices, which is extremely convenient to manufacture and use. 1 The mesoscopic refrigerator (Stirling cycle) is made by the principle of heat absorption and heat release at the joints when direct current is applied.
The efficiency of the thermoelectron emission cooler can reach 60% ~ 70% of the Changnuo efficiency, while the traditional cooling method is only 30 ~ 50%, and the thermoelectric cooler is only about 8%.
In a thermionic emission refrigerator, the "refrigerant" is mass electrons. The cathode and anode are separated by a vacuum. When a voltage is applied, electrons are emitted from the cathode and pass through the vacuum to the anode. Heat absorption at the cathode and heat release at the anode (see 2) The thermal electron emission refrigeration management is the same as the cathode ray management, except that the latter must heat the cathode to a very high temperature to use electrons to overcome the beam emission. The materials used are usually Work function materials. When used for cooling, low work function materials must be used so that the cathode can emit electrons at low temperatures (see 3). 4 Conclusion Electronic cooling technology has been greatly developed, laying a foundation for the development of electronic technology and industry; Adapting to the rapid development of electronic technology, many new concepts of cooling technology have emerged. Among them, micro-channel heat exchange, micro heat pipes, micro jets, mesoscopic refrigerators, and micro electron emission refrigerators have been or are maturing. Development plays an extremely important role in promoting. We believe that these technologies will be more perfect and improved, towards practical and large-scale use, and at the same time, more advanced cooling technologies will appear to adapt to more advanced electronic technologies.
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