Prospects for the development of microwave power applications in the new century[Copy link]
Contents 0 Introduction 1 Microwave heating principle and microwave power application equipment 2 Current status of microwave power application in China 3 Microwave power application is moving towards high-tech field 4 Strengthening basic research on microwave application is the basic condition for further development 5 Multi-disciplinary joint efforts to turn research work to industrialization as soon as possible 6 Combination of microwave and traditional heating and drying technology
0 Introduction Microwave technology matured during World War II for the development of radar. When the war was about to end, American engineers who adjusted radar found that the chocolate in their pockets often melted ! They immediately understood that this was caused by the effect of electromagnetic waves on matter, which was the same as the heat loss of insulating dielectric in high-power cables. Curiosity drove them to use microwave devices to make successfully. This is the prototype of microwave power application equipment. As early as the 1930s, when debugging high-power radio transmitters , flies or insects were often found to be dead and shrunken in hollow solenoids. These accidental discoveries clearly revealed to people that microwaves and radio waves can cause heating and drying. In fact, microwaves and radio waves are both electromagnetic waves, but the frequency of microwaves is above , while the frequency of radio waves is below 300 MHz. Of course, the technical focus of the early work of developing radio technology was to use electromagnetic waves of various frequencies to carry information or obtain information to construct a modern colorful life. In the initial stage, it was impossible to use expensive radio and radar equipment for heating and drying. The use of radio wave heating as an industrial application was earlier than microwave heating and was called radio frequency heating. With the development of microwave technology, the microwave power generated has been continuously improved and the cost has been reduced. It is possible to convert the energy of microwave electromagnetic fields into the energy of material molecules as a means of scientific research, production and medical treatment. This method of penetrating into the interior of the material and instantly converting it into molecular thermal energy has changed the traditional concept of heating from the outside to the inside and created a new technology for rapid heating. 1 Microwave Heating Principle and Microwave Power Application Equipment Microwave electromagnetic waves have two transmission states. The first is that the antenna is directed to propagate into space, and like light, it is a straight line propagation. The second is the guided transmission state set by humans, that is, restricting the transmission of electromagnetic waves in a hollow pipe. This hollow pipe is called a waveguide, which is generally rectangular or circular and made of good conductors such as copper or aluminum. The cross-sectional size of the waveguide is related to the frequency of the microwave used . Microwave electromagnetic waves propagating in a hollow waveguide are energy enclosed for transmission. It can be transmitted over long distances with minimal energy loss. If non-metallic materials are filled in the waveguide, it will cause the loss of transmission power and the transmission distance will be limited. It is due to the interaction between the electromagnetic field and the material that part of the energy of the electromagnetic wave has been converted into the energy of the material molecules. The conversion ratio is related to the frequency of the electromagnetic wave and the loss factor of the material. In principle, the energy of the electromagnetic wave introduced into the waveguide for closed transmission can be completely converted into the energy of the material molecules. The increase in temperature is the main sign of the increase in energy of the material molecules. Electromagnetic waves propagate at the speed of light, and the speed at which electromagnetic waves penetrate into the material is also close ; and the time to convert the energy of the electromagnetic wave into the energy of the material molecules is approximately instantaneous, and the conversion time in the microwave frequency band is faster than one ten-millionth of a second. This is the principle of microwaves that can form rapid . Traditional heating of solid materials must be in a heated environment, and then gradually transferred from the surface to the inside of the solid to obtain the conditions of thermal equilibrium, which requires a long time. The heating environment is generally not strictly insulated and closed, and a lot of heat may be emitted to the environment during a long heating time. Microwave power is completely sealed and penetrates into the interior of the object , and is immediately converted into heat, thus saving heat loss during the long heating process. This is the energy-saving principle of microwave heating. The characteristics of microwave heating compared with radio frequency heating are: a. The ratio of field energy converted into heat energy is high; b. It is easy to shield electromagnetic waves and prevent them from escaping . The actual microwave power equipment generally consists of four parts: (1) microwave power source (2) application device (3) waveguide element and application device feeding structure (4) sensing and control. The microwave power source that generates microwave power generally uses a magnetron as a vibrator. In this tube, the hot cathode emits electrons. Under the action of a strong constant magnetic field, the electrons move in a circular motion; the resonant decelerates the electrons, so that the kinetic energy of the electrons is converted into electromagnetic wave energy, which is accumulated in the resonant cavity and sent to the waveguide and then to the application device for use. The magnetron requires a DC high-voltage power supply , a filament heating power supply and a constant magnetic field coil power supply, and requires corresponding protection and control circuits to form the entire microwave power source . The excitation current of DC high voltage or constant magnetic field can control the output of microwave power. Microwave applicator is an enlarged waveguide tube, which is used as a place for electromagnetic waves and materials to interact. The design consideration is to adapt to the shape and processing requirements of the processed materials, which can be divided into traveling wave type and resonance type. Waveguide element is the connecting component between microwave power source and applicator, which is to solve the problem of both allowing magnetron to obtain the best load working conditions and enabling the applicator to obtain effective feeding effect. From the perspective of microwave technology, it is completed through a variety of waveguide elements and feeding structures. At the same time, the waveguide element provides data on the amount of incident power and the amount of overflow reflected power. The configuration of sensors is to detect the degree of interaction between the field and the material, whether it meets the processing requirements, such as temperature sensing and humidity sensing. The equipment can effectively control the output and transmission speed of the power source according to the real-time sensor data and the real-time working status of the microwave power source. It is very necessary to divide the microwave power application equipment into four parts. Generally speaking, application equipment is produced in single pieces or small batches, and must be designed . It is a process similar to "tailoring to the body". Microwave power application equipment is divided into four parts, of which microwave power source and waveguide components are microwave engineering design, sensing and control engineering design. These three parts are highly versatile and do not change with different application objects. Over the years, we have strengthened these three parts. The standardization and serialization of parts have improved the reliability and stability of these components, provided a strong foundation for the reliability of the whole machine, and shortened the development cycle of the whole machine. The design of microwave application devices is highly targeted. It varies according to the processing requirements, different states, shapes . It is a multidisciplinary engineering design. In recent years, we have adopted the method of microwave application device series design. Different systems are assembled with specific common components, reducing the number of newly designed components to a minimum. In this way, the development cycle of the equipment is further shortened and the reliable quality of the equipment is guaranteed. 2 The current status of microwave power application in China In the early 1970s, China paid attention to the development of foreign microwave power application technology. As early as the end of 1972, the Ministry of Electronics Industry held a microwave electron tube technology seminar at Nanjing Factory 772 (i.e. Sanle Electric Corporation), focusing on the possible development of microwave electron tubes in new fields. The new fields are industrial applications of microwave heating and drying, microwave therapy, microwave diagnosis, and microwave plasma. The pioneer of research work in new fields must be the development of high-power continuous wave magnetrons. That year, Factory 772 started to develop 915MHz and 2450MHz continuous wave magnetrons. On the basis of the successful development of two frequency band continuous wave magnetrons, it also developed China's first 2450MH z microwave physiotherapy instrument and 915MHz microwave heating equipment. In the spring of 1974 , the first microwave heating equipment was exhibited in Beijing. The microwave rapid heating phenomenon displayed attracted widespread attention from people in the industry. In November 1974, the Ministry of Electronics Industry held a symposium on microwave energy application technology at Factory 772 in Nanjing. The conference introduced the application of foreign microwave power in industrial production, agricultural production and medical care, and discussed the prospects for development in China. After more than 20 years of efforts, China has initially established a foundation for the research field of microwave power application. The microwave power equipment developed by Factory 772 has gained a firm foothold in industrial production such as food, wood and bamboo product processing, pharmaceuticals, paper products, brewing, rubber, and chemicals, improving production conditions and product quality. The various microwave plasma equipment, microwave high temperature equipment and microwave vacuum drying equipment developed by the factory have become important scientific research means in many disciplines. As far as the national situation is concerned, the promotion of microwave power application technology in China has been a very difficult journey for more than 20 years. At present, initial achievements have been made, laying a foundation for continued development. The main signs of this foundation are: (1) Microwave heating and drying, microwave food processing, microwave sterilization and insecticide have been widely used in various industries (2) Household microwave ovens have formed the capacity for large-scale production; (3) The clinical application of microwave medical instruments has achieved general success; (4) At present, many cutting-edge topics in many fields have made many gratifying progress by using microwave power as a powerful tool, expanding new research fields and keeping up with the pace of the world. 3 Microwave power application is moving towards high-tech fields. From the research trends of various countries in the world, microwave power application is in a period of development towards new fields, that is, the focus has shifted from traditional heating and drying and food processing to multiple high-tech fields as a new tool for research work. The main fields include: microwave catalytic chemical reactions, microwave processing , and research on various applications of microwave gas discharge. Experimental research on microwave chemistry covers almost all fields of chemistry and chemical engineering. A large number of selected reports show that microwave electromagnetic fields can accelerate chemical reactions and shorten the reaction time to one tenth to one thousandth of the original time , which has brought attractive prospects to the chemical industry. Microwave high-temperature technology can sinter fine ceramics, weld ceramics, and process and treat materials, such as thermal shaping of polymer materials and heat treatment of non-metallic materials. Microwave methods are superior to conventional methods. Microwave gas discharge, that is, the formation of low-temperature plasma with microwave electromagnetic fields, is a major . Microwave plasma chemical vapor deposition (MPCVD) and plasma etching are the main process methods for microelectronics processing. The preparation of diamond films and optical fibers also adopts the MPCVD method. The preparation of ultrafine powders and microwave plasma methods have many advantages. In microwave plasma, the activity of many particles is stronger than that of radio frequency plasma, which has more favorable conditions for chemical reactions and material processing. In addition, strong lighting sources (such as sulfur lamps) and microwave odor generators composed may gradually move towards industrialization. According to preliminary literature research, the hot spots of research work in the new field of microwave power in China are as follows : (1) Research on microwave ore dressing (microwave-assisted pyrolysis), the conversion of non-ferrous metal sulfides into oxides, the conversion of nickel carbonates into nickel oxides, the desulfurization of gold ore sands and the conversion of radioactive isotope nitrates into oxides have achieved laboratory results. (2) Microwave-assisted organic and inorganic chemical reactions have been used to improve the chemical reaction rate, and a lot of laboratory work has been done. (3) Research on microwave-assisted extraction technology. Microwave electric fields can accelerate the dissolution rate and improve solubility, and have achieved remarkable results in many laboratories . It will be an inevitable trend to use microwave-assisted extraction methods and scientifically standardize production. (4) Using natural gas instead of petroleum to produce chemical raw materials such as ethylene, microwave-assisted catalytic chemical reactions and microwave plasma technology have achieved good yields in the laboratory. (5) Microwave regeneration methods for activated carbon and diesel filters have achieved good experimental results. (6) As early as the early 1980s, China began to develop a variety of microwave plasma equipment, such as MPCVD equipment, plasma etching equipment, and laser microwave pump sources. In recent years , microwave radio electrode discharge sulfur lamp strong light source has been developed, and microwave gas discharge constitutes ozone generator (in an ozone environment, it is an effective method to extend the shelf life of food). (7) Microwave high temperature technology is used for sintering ceramics and welding ceramics. China has achieved many experimental results. The trend abroad is to combine conventional heating technology with microwave high temperature methods to improve the production of the ceramic industry, and small-scale production equipment is already available. China also has the foundation and experience in this regard, and should start this work as soon as possible. (8) Since the early 1980s, a project has been launched to conduct research on microwave desulfurization of coal and oil, with the aim of forming it as soon as possible.
Clean emissions during combustion. Preliminary experimental research has also been conducted on solid, liquid and gas waste treatment in combination with microwave methods. From the content of these topics, we can see that the gap between China and foreign countries in research work in new fields is not too large. At present, many practical problems should be solved in order to strengthen the transformation to industrialization , so as to achieve practical results in China's economic construction and improvement of the ecological environment. 4 Strengthening basic research on microwave applications is the basic condition for further development As mentioned above, China's pioneering experimental research work on microwave power applications in new fields covers a wide range, has accumulated a lot of experience, and has achieved many pioneering research results, which is encouraging. However, it is not optimistic to transform the results of these laboratories into productivity and make the process of industrialization catch up with the pace of world progress. From the perspective of microwave engineering, the equipment used is still in a relatively primitive state, and the processing capacity is generally small. There are still many specific difficulties to be solved when expanding to the scale of industrialization, and there is still a long way to go to industrialization . Experimental research in new fields abroad shows that there is a need for higher requirements for microwave power application equipment. Equipment that remains at the original level is difficult to adapt to the needs of research in new fields. First of all, there are high requirements for microwave power sources, requiring high reliability, high stability and readjustment accuracy , low ripple factor, and modulation function to adapt to changing conditions , achieve better experimental results, and have reliable repeatability. Second, the equipment needs real-time sensing, monitoring and high speed . Sensing settings are the weak links of domestic equipment at present, and these functions need to be improved. In order to ensure the accuracy of monitoring and the reliability of adjustment, it is necessary to improve the performance of current high-power waveguide components and develop a variety of adaptable feeding structures for application devices. It can be seen that in order to meet the needs of application research in new fields, there is still a lot of improvement work to be done on the three basic components with strong universality, namely microwave power sources, waveguide components, and sensing and control , so that the technical indicators of these basic components can reach the advanced level of foreign countries. The microwave power production equipment that has been successfully used at present still needs to be improved and improved. The improvement of the quality of these universal basic components will provide a solid foundation for improving current application equipment. It is possible to make the current microwave power application equipment more standardized, make the work more stable and reliable, and heat more evenly. With reliable sensing functions, the working status of the equipment can be monitored in real time, and the state of interaction between the electromagnetic field and the material can be perceived. It has the basic conditions for configuring closed-loop automatic control. The equipment is becoming more and more perfect, which can narrow the gap with similar advanced foreign products, and make the rubber microwave vulcanization equipment and printing drying equipment, which are currently mainly imported equipment, all adopt domestic equipment. Once the quality of our application equipment leaps to a higher level, it is possible to continuously expand the application of microwave power in the heating field. For example, the drying of chemical materials and glass fiber, the drying and shaping of ceramic blanks , the application of textile, printing and dyeing, and printing industries, and the application of large-scale frozen meat thawing can completely use microwave power equipment to improve production conditions and shorten production time. The improvement and improvement of existing microwave power application equipment and its reliable use will undoubtedly provide useful experience and basic components for application equipment in new fields. 5. Multidisciplinary convergence to tackle key problems so that research work can be turned to industrialization as soon as possible. Microwave power application is a multidisciplinary subject. As far as the research in new fields is concerned, the full penetration of multiple disciplines is even more important. More than 20 years of experience have shown that if many research works in applied disciplines are not clear about the basic framework, main principles and current status of microwave principles, the research topics they set up will often be repeated many times , lingering in the laboratory for a long time, and the research work will not progress quickly. If the basic requirements of specific topics in applied disciplines, the main links of processing, and the engineering design of microwave application devices are not clear, improper use will caused, resulting in a lot of waste. Although these are all past history, these experiences that should be learned must be given high attention, and they are of greater significance for opening up new application fields . In the research topics of microwave applications, there must be a process in which multiple disciplines can fully communicate and penetrate, and then find the meeting point of multidisciplinary laws. The necessity of in-depth dialogue between multiple disciplines from a theoretical perspective cannot be ignored, because the theoretical laws of multiple disciplines can predict many directional principles, and can more reliably give the specific requirements of application device design and the actual operation procedures of the experiment, avoiding detours and saving time. The convergence of multiple disciplines should include research work strategies, plans and designs, as well as discussions and summaries of failed and successful experiences , so as to cover the complete laws of research work and find the direction for progress . It is expected that the scientific research results will be transformed into productivity, and more attention should be paid to the consideration from the perspective of multidisciplinary convergence. The results of a scientific research project should also pay full attention to the successful experience of similar industries or existing similar-scale production industries, and make plans for industrialization. Microwave technology cannot replace all the rules of the original production process. Microwave technology can find the weak links of the original production process, supplement, strengthen or replace this link , and make a large-scale improvement in the original process. The development of microwave rubber vulcanization equipment is an obvious example. The vulcanization temperature and vulcanization time have a certain optimal range for a certain product . The conventional method is the contradiction between the heating time and the vulcanization time. For large-volume products, conventional heating from the surface to the inside requires a long time to make the inside reach the predetermined vulcanization temperature. The surface rubber has exceeded the vulcanization time and reached the over-vulcanization state, while the inside is still under-vulcanized. Traditional methods make it difficult to achieve uniform and consistent . The design of microwave rubber vulcanization equipment and microwave heating are only used to quickly heat up to a predetermined vulcanization temperature range. After reaching the vulcanization temperature, it is immediately kept warm by traditional heating methods . With this arrangement, microwave rubber vulcanization equipment not only speeds up production time and saves energy, but also achieves uniform vulcanization treatment and improves product quality. Another example is the research work on microwave high-temperature sintering of ceramics, which shows that the microwave high-temperature method heats up quickly, can shorten production time, and improve the quality of ceramic products. However, the experimental study of the sintering process has a low sintering rate .
, it is difficult to master the various links of rapid heating. Thinking calmly, improving the process of ceramic sintering should also be a combination of conventional heating and microwave heating. The initial heating does not need to use the microwave method. When it reaches above 500°C (the coefficient of microwave energy absorption by the ceramic body increases with the increase in temperature), the microwave method can be used to make the inside and outside of the body reach the sintering temperature uniformly, while the insulation and cooling still use conventional methods. This idea coincides with the report I have seen. Foreign countries have developed a small-scale transmission kiln combining microwave sintering and traditional sintering under the premise of this idea , and have achieved good sintering effects. The many scientific studies conducted using microwave power are actually working hard . The technical work in microwave power engineering is accumulating experience, gradually exploring and coordinating with related application disciplines, and jointly finding ways to overcome the difficulties in research work in different fields, so that various high-tech research can be transformed into industrialization as soon as possible. 6 Combination of microwave and traditional heating and drying technology At present, large-scale microwave power application equipment is mainly used in the production of heating, drying and food processing. However, from the current demand situation, microwave power application equipment has not yet met the needs of multiple fields . Due to the popularity of household microwave ovens, many companies have conducted feasibility , or have seen signs of improvement, which needs to be further promoted. However, the existing microwave power equipment cannot fully meet these requirements. In other words, as far as microwave heating and drying are concerned, microwave power engineering still has a lot of pioneering work to do . These fields are roughly high-temperature treatment of non-metallic materials, polymer thermal setting, absolute drying of chemical materials, de -crystallization water, drying of glass fibers, various biochemical materials, low-temperature drying of food, vacuum dehydration drying, etc. In some fields of heating and drying, traditional methods have been used for a lot of research work . For example, the drying method focuses on the most effective drainage Spray drying, fluidized bed drying, vibrating fluidized bed drying, boiling drying, and vacuum drying are all based on the different states of the material and the contact with the hot air to discharge moisture. If microwave energy is properly introduced , it is entirely possible to speed up the drying process and improve the drying quality. In these fields, microwave methods should be combined with traditional methods to supplement the weakness of insufficient heat supply to the material, and microwave heating can be used. The method of draining water should also take the advantages of traditional methods, which requires the reform of the original equipment to be compatible with the feeding microwave power and the measures to prevent microwave leakage. The application of absolute dry treatment of many materials and heat treatment of non-metallic materials is not high enough at present. The design of high field density equipment is daunting. Microwave power equipment can improve the heat treatment method of non-metallic materials. Theoretically, it is estimated that the absolute dry treatment of chemical materials will achieve good results. Unify the electromagnetic field power engineering method to improve production conditions and make efforts for the progress of cutting-edge research. According to many reported documents, the design method of foreign RF heating equipment is intended to gradually approach microwave power, that is, to develop the so-called 50Ω RF industrial heating technology. The standard RF equipment should consist of the following four parts: (1) RF oscillator with 50Ω output impedance; (2) 50Ω coaxial line connecting the RF oscillator and the matching box; (3) Matching box with control and discrimmatic; (4) Applicator. In other words, the development direction of RF power equipment is no longer a unified device. It can also be assembled with general parts , and the oscillator source and the applicator can be separated as needed (the current f industrial equipment cannot meet this requirement at all). This working method is actually consistent with the method of developing microwave power equipment; that is, the method of assembling equipment according to standard parts. At the same time, the RF power output is planned to use a crystal oscillator to feed the amplifier to stabilize the frequency and control the power; this scheme is consistent with the idea of further improving the microwave power source; using an orthogonal field amplifier, and the oscillation circuit composed of an active microwave network is called a stabililotron. The output power of the stabililotron is 10-50KW and 10-100KW at 2450MHz. The typical Rf use frequency is 13.56MHz, 27.12MHz, and 40.68MHz. There is a trend to increase the use frequency. The trial-produced Klystron uses 267MHz for high-power industrial applications, while the frequency of microwave power applications is currently 2450MHz, 915MHz, and is currently developing a trend of 434MHz. As shown above, from the perspective of equipment design methods, RF equipment and microwave equipment are gradually approaching, while the frequency of microwave use is expanding downward and the frequency of RF use is expanding upward. That is, the two extend up and down , and further connect into a unified electromagnetic field power device. In fact, microwave power equipment and RF power equipment are the two leaves of electromagnetic field power equipment. The solution should be handled from a unified perspective of electromagnetic field power application . RF and microwave have their own strengths and weaknesses. They should use their strengths and avoid their weaknesses to make the electromagnetic field more reasonable and practical. The main fields of microwave and RF electromagnetic field power engineering are heating and drying, material processing and gas discharge. The application range is very wide and very close to production practice. It is not only an improvement on the traditional heating and drying method, but also an important tool for many important research methods at present. At present, some overall planning should be made on the basis of investigation and research . Which industries have weak links in heating and drying, and which frequency bands should electromagnetic field power equipment use which technical means to deal with these links? These links are more reasonable. At present, the frontier research work using RF and microwave methods, what problems exist in the weak links of the equipment foundation, and the infrastructure construction should be gradually strengthened to effectively promote the frontier research work.
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