Progress and Prospect of Microwave Sintering Technology

- May 08, 2019 -

Microwave sintering of < P > materials began in the mid-1960s. W.R. Tinga first proposed microwave sintering technology for ceramics. By the mid-1970s, J.C. Badot and A.J. Berteand in France began to systematically study microwave sintering technology. Since the 1980s, various high-performance ceramics and metal materials have been widely used, and the corresponding preparation technology has become the focus of attention. Microwave sintering, with its unique advantages of energy-saving and time-saving, has received extensive attention from the governments, industry and academia of developed countries such as the United States, Japan, Canada, Britain and Germany. China also included it in 1988. "863" plan. During this period, microwave theory, optimization design of microwave sintering device system, 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 has entered the stage of industrialization, and developed countries such as the United States, Canada and Germany began to produce ceramic products in small batches. Among them, the United States has the ability to produce microwave continuous sintering equipment. The principle of microwave sintering

1

Microwave sintering is to use microwave heating to sinter materials. It is different from the traditional heating method. Traditional heating depends on the convection, conduction or radiation of heat energy from the heater to the heated material to reach a certain temperature, heat transfer from outside to inside, sintering time is long, and fine grains can also be obtained. Microwave sintering is a method to achieve densification by coupling the special band of microwave with the basic micro-structure of the material and heating the material to the sintering temperature due to the dielectric loss of the material. Electromagnetic energy dissipation in

1.1 materials

P>microwave absorption is achieved by coupling microwave electric field or magnetic field to convert microwave energy into heat energy. Huang Xiangdong et al. analyzed the interaction mechanism between microwave and matter by Maxwell's electromagnetic theory. He pointed out that the absorption of microwave by medium originated from the conductivity loss and polarization loss of medium to microwave, and that the conductivity loss would occupy the main position at high temperature. In conductive materials, the main loss of electromagnetic energy is conductive loss. In dielectric materials (such as ceramics), the polarization response of electric dipoles formed by a large number of space charge energies is significantly lagged behind that of rapidly changing external electric fields, resulting in polarization relaxation. In this process, the energy exchange between micro-particles is shown as energy loss in macro-level. The results show that microwave radiation can promote densification, grain growth and chemical reaction. Because in sintering, microwave is not only a kind of heating energy, but also an activated sintering process. M.A.Janny et al. first analyzed the phenomenon of microwave-assisted structure and measured the apparent activation energy Ea in the sintering process of high purity Al2O3. It was found that Ea in microwave-assisted sintering was only 170kj/mol, while in conventional resistance-heated sintering, Ea = 575kj/mol. It can be inferred that microwave-assisted atom diffusion. M. A. Janny et al. further measured the diffusion process of Al2O3 single crystal by 18O tracer method. It was also proved that the diffusion coefficient under microwave heating was higher than that under conventional heating. The experimental results of S.A. Freeman et al. show that the microwave field can enhance the ionic conductivity. It is believed that the high frequency electric field can promote the migration of charged vacancies in the grain surface, resulting in plastic deformation similar to diffusion creep, thus promoting the sintering process. The distribution of microwave field between two contacting dielectric sphere particles was analyzed by

Birnboin et al. It was found that in the sintering neck formation region, the electric field was focused, and the electric field intensity in the neck region was about 10 times that of the external field, while the electric field intensity in the neck gap was about 30 times that of the external field. In addition, within the angle of 0 80 between the field and the two particle centers, it is found that the electric field polarizes parallel to the line, which promotes the mass transfer process to proceed at a very fast speed. In addition, the highly focused electric field in the sintering neck may ionize the local region and further accelerate the mass transfer process. This ionization is particularly important for accelerated mass transfer in covalent compounds. The results show that the acceleration mass transfer process caused by local ionization is the fundamental reason for microwave-assisted sintering. The technical characteristics of microwave sintering

2

2.1 The direct coupling of microwave and material leads to the whole heating

because of the volume heating of microwave, the zero gradient uniform heating of large area of material can be realized, the thermal stress in material can be reduced, and the tendency of cracking and deformation can be reduced. At the same time, because microwave can be directly absorbed by materials and converted into heat energy, the energy utilization rate is very high, saving about 80% energy than conventional sintering. Microwave sintering

2.2 has fast heating rate and short sintering time

When the temperature of some materials is higher than the critical temperature, its loss factor increases rapidly, which leads to very fast heating. In addition, the existence of microwave decreases the activation energy, speeds up the sintering process and shortens the sintering time. The short-time sintered grain is not easy to grow, and it is easy to obtain uniform fine grain structure with fewer internal voids and better ductility and toughness than the traditional sintered round voids. At the same time, the sintering temperature also decreased in varying degrees. Microwave

< P > 2.3 can selectively heat the phase, and

< P > Because different materials and different substances have different absorption to microwave, new materials and structures can be obtained by selective heating or selective chemical reaction. The heating area can also be controlled by adding absorbing phase, and the microwave transparent material can be preheated by using strong absorbing material, and the low loss material can be sintered by mixing heating. In addition, microwave sintering is easy to control, safe and pollution-free. Technological progress of microwave sintering

3

3.1 Research progress of microwave sintering mechanism

P>3.1 Microwave can promote sintering of ceramics, but its micro-mechanism is still unclear. Huang Xiangdong et al. analyzed the effect of microwave on diffusion from the point of view of directional movement of charged defects such as vacancies and interstitial ions caused by microwave electric field. It was pointed out that in microwave sintering of ceramic products, microwave only promoted densification parallel to electric field direction, and macroscopically polarized electromagnetic waves which did not change with time were parallel to electric field direction. The shrinkage is greater than that in the direction of vertical electric field. S.A. Freeman et al. have studied the charge transport of NaCl in microwave field. The results show that the existence of microwave field does not improve the motion ability of the original vacancy, but improves the driving force of charge transport. In addition, S.A. Freeman also simulated the transport of ions in solids in microwave field. The development of equipment and technology for microwave sintering

3.2

P>microwave sintering equipment plays an important role in the development of microwave sintering technology. H.D. Kimmery designed a microwave continuous sintering system with a frequency of 28 Hz in 1988, and the field intensity distribution was less than 4%. In addition, they designed a mode agitator for the microwave continuous sintering system with a frequency of 2.45 GHz to improve the field distribution uniformity. The multi-mode resonant scheme of dielectric excited by convergent antenna designed by Shenyang Institute of Metals, Chinese Academy of Sciences and No. 772 Factory has achieved remarkable results by uniform beam of microwave energy in sintering zone. In recent years, with the support of the National New Technology 863 Program of Shenyang Metal Institute, CAS has developed several MFM-863 series microwave sintering equipments, whose main performance indexes are: power supply, 380V, 50Hz; power, 0.5-10kW continuously adjustable; working frequency, 2.45GHz; working temperature: more than 1800 C; size of sintering zone, 120mm*120mm; average time consumption, 0.5-2h/oven. In terms of technology, H.D. Kimmery et al. proposed a hybrid heating method combining conventional radiation or conduction heating with microwave direct heating. When H.D. Kimmery sintered ZrO 2 (Y2O3 with 8% molar fraction), SiC rod was used as a heat sensor to mix the heat and eliminate the thermal runaway of ZrO 2. The application of microwave sintering

<3.3

has been extended for a long time. The main research and application of microwave sintering are limited to ceramic products. In recent years, the application of microwave sintering technology has emerged many new starvation growth points. Nanomaterials

are the hotspot of materials research nowadays, and the microwave sintering of nanomaterials has also made gratifying progress. Li Yunkai et al. used nano-Al2O3 and ZrO 2 (3Y) nano-powder as raw materials to study the microwave sintering of Al2O3-ZrO 2 (3Y) composite ceramics with different proportions, and obtained high density and improved fracture toughness. J.A. Eastman et al. sintered titanium dioxide with an average particle size of 14 mm using 6 kW and 2.45 GHz microwave, and obtained good sintering properties. CuTi-diamond composites were prepared by microwave sintering by Cheng Yuehang et al. The results show that there is no graphitization transformation of diamond particles in the sintering process. The diamond particles in CuTi-diamond composites can form a good bond with CuTi matrix. Self-propagating high temperature formation by microwave heating is another important aspect of microwave applications. In 1990, R.C. Dalton of Virginia State University first proposed the application of microwave heating in self-propagating high-temperature synthesis, and synthesized nine materials such as TiC by this technology. Then, British, German and American scientists used this method to synthesize YBCuO, Si3C4, Al2O3-TiC and other materials. In 1996, J. K. Bechtholt et al. of the United States carried out numerical simulation and analysis of ignition process in microwave self-propagating high temperature synthesis. The ignition time was accurately calculated by simulation. In 1999, S. Gedevabshvili and D. Agrawal of the United States synthesized Ti-Al, Cu-Zn-Al and other intermetallic compounds and alloys by this technology. Rustum Roy, Dinesh Agrawal of Pennsylvania State University, USA, produced powder metallurgical stainless steel, copper-iron alloy, tungsten-copper alloy and nickel-based superalloy by microwave sintering. Among them, the fracture modulus of Fe-Ni is 60% higher than that of conventional sintering. In addition, microwave sintering under high magnetic field can prepare long bone amorphous magnetic materials, which can turn materials with significant hard magnetic properties (such as NdFeB permanent magnet) into soft magnetic materials. The development of microwave sintering technology has gone through several decades. Although there are still many immature and imperfect aspects, it has incomparable advantages compared with conventional technology, which indicates its broad development prospects. Firstly, as a time-saving, energy-saving, labor-saving and pollution-free technology, microwave sintering can meet the requirements of energy saving and environmental protection nowadays; secondly, its activated sintering characteristics are conducive to obtaining excellent microstructures, thereby improving material properties; thirdly, the characteristics of microwave-material coupling determine that microwave can be used for selective heating, thus enabling the system. Structural materials with special structures, such as functionally gradient materials, are obtained. These advantages make microwave sintering have broad prospects in the preparation of high-tech ceramics and cermet-metal composites. Dielectric loss characteristics of

materials vary with frequency, temperature and impurity content. Due to the need of automatic control, relevant databases need to be established. The principle of microwave sintering also needs further study. Because of the strong selectivity of microwave sintering furnace to products, the parameters of microwave oven for different products are quite different.

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