Development and Current Situation of Microwave Sintering Technology

- May 08, 2019 -

The civilization based on modern industry depends on the application of special materials. Traditional powder metallurgical materials and new ceramic materials are indispensable foundations for modern industrial production. Sintering technology is often an important link to determine the properties of powder metallurgical products and ceramic products. Microwave

is widely used as an input energy because of its special band coupling with the basic micro-structure of most materials, which can produce peculiar electromagnetic heating effect on materials. Overview of Microwave Sintering Technology

Microwave Intering is a kind of material sintering by microwave heating. Microwave sintering technology is a method to achieve densification by coupling the special band of microwave with the basic micro-structure of materials. The dielectric loss of materials makes the whole material heated to sintering temperature. It is an important technical means to rapidly prepare high-quality new materials and to prepare traditional materials with new properties. It has the advantages of low sintering temperature, short sintering time, high energy utilization and heating efficiency, safety, sanitation and pollution-free. Compared with the workpiece produced by traditional sintering process, the workpiece made by microwave sintering has higher density, hardness and toughness. Short-time sintering produces uniform fine grain structure with few internal voids, and the shape of voids is more round than that of traditional sintering, so it has better ductility and toughness. Microwave heating can make the workpiece heating uniform, heating speed can be as high as 15,000 c/min, high efficiency and energy saving, for some materials can even achieve high temperature above 2000 C with very little input energy. Because microwave has great penetration to most powdered ceramic materials, it can heat workpieces evenly and reduce the temperature gradient in high temperature sintering process, so as to reduce the material deformation caused by non-uniform expansion and make rapid temperature rise possible. Moreover, the residence time at high temperature can be greatly shortened, the growth of grains can be inhibited, and the physical and mechanical properties of materials can be improved. The sintering technology of ceramics, engineering ceramics, magnetic materials and cemented carbides by microwave sintering furnace has a rapid heating and sintering process; the process time is shortened by more than 50%; because microwave energy is directly used to heat workpieces, the energy consumption can be reduced to 3.6 kilowatt-hours per kilogram, which is only 10% of the traditional sintering process under the same productivity conditions; there is no radiation at high temperature in microwave sintering. Shadow effect of conduction reduces thermal deformation; materials sintered by microwave have very fine microstructures; it makes sintered nanomaterials possible; microwave sintering reduces the possibility of changes in surface composition of workpieces; microwave sintering reduces sintering temperature; improves sintering density; and improves product quality. Microwave sintering technology has proved to be the best way to heat and sinter functional ceramics, engineering ceramics, magnetic materials and cemented carbides. The birth and development of microwave sintering technology

has caused a breakthrough in the traditional concept of sintering, which has been hailed as a "revolution in sintering technology" by the material industry. Microwave sintering technology has fundamentally changed the status of material sintering technology, which will become an effective means to create materials with special properties, and has great development potential and application prospects. The concept of microwave sintering was put forward by Tinga.W.R. in the late 1960s. The development of microwave sintering technology can be roughly divided into three stages: in the mid-1970s and early 1980s, it entered the preliminary experimental research stage. In 1976, Berteaud and Badot first reported the success of microwave sintering materials in the laboratory. During this period, the research and experimental work were mainly confined to some ceramic materials which were easy to absorb microwave and had lower sintering temperature, such as BaTi03 and U02. From the mid-1980s to the mid-1990s, the United States, Canada, Germany and other countries have invested a lot of financial and human resources in the research and development of microwave sintering technology. Since 1985, the American Society for Materials Research (MRS) and the Ceramic Society have sponsored an International Conference on microwave sintering. Since 1988, the MRS conference in the United States has included microwave sintering technology as a topic for discussion. After that, it has been held every two years. So far, it has been conducted five times and has published many collections of papers. The Journal of Material Syn-thesisand Processing, which was first published in the United States in 1991, also takes microwave sintering technology as one of the important contents. During this period, microwave theory, optimization design of microwave sintering device system and sintering process, dielectric parameter measurement, interaction mechanism between materials and microwave, computer numerical simulation of electromagnetic field and temperature field were mainly explored and studied, and many different types of materials were sintered.

In the late 1990s, microwave sintering industrialization began, and developed countries such as the United States, Canada and Germany began to put into small batch production. The United States has the capacity to produce microwave continuous sintering equipment, mainly for cemented carbides. Now they are developing microwave continuous sintering equipment for varistor ceramic devices. Since the United States is not a producer of cemented carbides and magnetic materials, the main research direction of microwave sintering is high-performance electronic ceramics. The Microwave Technology Research Center of Pennsylvania State University has proved that microwave can also be used to sinter powder metallurgical products such as stainless steel, copper-iron alloy, copper-zinc alloy, tungsten-copper alloy and nickel-based superalloy. INDEXABLETOOLSLTD Canada has its own microwave sintering equipment for the production of silicon nitride cutting tools. It can produce 20,000 high-quality half-inch silicon nitride blades a day. The products mainly meet the needs of North America. The company has also made a fruitful attempt to sinter cemented carbide by microwave. The author has helped INDEXABLETOOLSLTD to establish a microwave sintering model to improve equipment efficiency. At present, the U.S. military has invested in the research of microwave sintering of ceramic ballistic-proof armor materials. Under high magnetic field, microwave can produce completely amorphous magnetic materials, which can change materials with significant hard magnetic properties into soft magnetic materials. This discovery means that the application of microwave energy has begun to develop from simple sintering and synthesis to microwave modification. This project has also received huge financial support from the US military. The main research direction of Germany is focused on the industrialization of microwave sintering cemented carbide. Willert Porada of Deutsche University has proposed the shrinkage mechanism model of microwave sintering, which has been approved by most peers.

P>In 1988, Wuhan University of Technology took the lead in the research of microwave sintering technology in China and was listed as the "863 Program" of the state. The Shenyang Institute of Metals, Shanghai Institute of Silicate and Tsinghua University of the Chinese Academy of Sciences have successively carried out the research of this technology, which has promoted the development of this technology in China. According to incomplete statistics, at present, nearly thirty universities, research institutes and production institutes in China have been engaged in the research of microwave sintering and microwave synthetic materials. The representative units are Institute of Metals, Chinese Academy of Sciences, Wuhan University of Technology, Tsinghua University and Shanghai Institute of Silicate, Chinese Academy of Sciences. Wuhan Polytechnic University undertook the project of "863 Program", "Research and Development of Small Continuous Microwave Sintering System". Thermocouple bushing, high alumina ceramic tube, ceramic roller used in roller kiln and other long strip-shaped non-metallic materials passed through the sintering chamber of the system vertically and evenly to realize continuous sintering. It has the advantages of high speed, high yield, high quality, energy saving and consumption reduction. The advantages of series conventional sintering can not be replaced. The exterior diameter of the sintered material is not more than 60 mm, the length is not more than 3 meters, and the yield is up to 90%.

Shenyang Institute of Metals, Chinese Academy of Sciences, has developed several MFM-863 series microwave sintering equipment under the support of the National High-tech 863 Plan for two consecutive five-year projects. Its main technical indicators are: power supply: 380V, 50Hz; power: 0.5-10KW continuously adjustable; working frequency: 2450MHz; working temperature: & gt; 1800-C; sintering zone: 120 *120ram; average time consumption: 0.5-2h/oven. The equipment is mainly used in ceramic sintering, reactive sintering, ceramic/metal welding, raw material synthesis, ultra-fine powder preparation, rapid aeration, heat treatment and so on. In addition, the basic principle of microwave sintering is studied. It is believed that the polarization relaxation mechanism of the interaction between dielectric materials and microwave can transform microwave energy into heat energy instantaneously in the material, and make the whole material heated efficiently and rapidly at the same time. In addition, Tsinghua University and Shanghai Silicate Research Institute undertake projects of the National Natural Science Foundation of China in microwave sintering. The research results of the above research institutes and the whole research work are basically in the laboratory stage. The main factors restricting the development of microwave sintering technology in China are the mechanism of microwave sintering, the data of material properties, the manufacture of microwave sintering equipment and automatic control. So far, China's real industrial application-oriented equipment has not been made. In 2001, Changsha Longtai Technology Co., Ltd. was established in China. As a professional microwave sintering enterprise, the company is committed to the research, development and mass production of industrial microwave sintering equipment, microwave sintering technology and preparation of new materials. The company has independent intellectual property rights of microwave sintering technology, and now has a number of Chinese patents and foreign patents. In the field of microwave sintering equipment, the use of special equipment with fixed furnace body and process parameters for batch production of a sintering material can reduce costs and improve production efficiency, and has been industrialized in the field of special metallurgy and ceramics. Therefore, different microwave design equipment should be designed and manufactured for different material requirements. Modular design is used for main components to facilitate maintenance and replacement. The whole microwave sintering equipment consists of power supply and energy input system, furnace body and computer control system. In order to improve production efficiency, microwave continuous sintering equipment or several furnace bodies are used to share a set of microwave power supply. In principle, a set of equipment is controlled by a computer. When the production scale is large, a computer can be used to control many sets of sintering equipment in the whole sintering workshop. Based on the idea of modular sintering, only the furnace body and computer control software need to be replaced when the sintering materials need to be changed. The microwave sintering technology of silicon nitride, piezoelectric ceramics, varistor resistors, multi-layer ceramic capacitors, inductors, soft magnets of manganese-zinc ferrite, Nd-Fe-B permanent magnets and cemented carbides has been successfully developed. The industrialization prospect of microwave sintering technology

has broken through the main technical difficulties of microwave sintering technology. Through the scientific design of furnace chamber and process, it has been able to manufacture large-scale equipment which can be compared with the output of traditional design furnace, and the equipment can completely realize computer process control, which makes the industrial application prospect become reality. Microwave sintering technology is a major breakthrough in the field of materials, especially the sintering of nano-materials. Its performance and cost-effective ratio are much better than the traditional thermistor sintering technology. Pushing microwave sintering technology into the market will not only greatly reduce the sintering cost of materials, but also promote the industrialization of new materials.

According to market research, microwave sintering technology involves sintering of cemented carbides, technical ceramics, engineering ceramics, magnetic materials and nanomaterials, and is a recognized large emerging industry in the 21st century with a value of billions or even tens of billions of dollars. The industrialization of microwave sintering technology will bring a revolution to the material industry. With the further understanding of the principle and equipment characteristics of microwave in material field, the characteristics and advantages of microwave sintered materials are further understood.

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