Microwave sintering oxides
Microwave sintering the ceramic material to absorb the microwave energy into kinetic energy and heat energy within the molecule , so that the whole material is heated to a uniform temperature sintering densification is achieved . And conventional sintering , microwave sintering with rapid heating , rapid sintering , staining material organization , improving performance and energy-efficient materials , etc., which is called the 1990s a new generation of sintering method .
     1976 , Berteaud and Badot had a single rectangular cavity 2.45GHz sintered alumina and zirconia , etc., they noted that the microwave sintering has many advantages , such as high thermal efficiency and rapid sintering process , etc., but also found a many problems such as difficult to control the temperature , easily forming a thermal runaway phenomenon . 1978 , Colomban other single-mode cavity sintering of R a A12O3, they observed a rapid sintering phenomenon, but did not observe the phenomenon of grain growth inhibition .
     In 1983 , Schubring spark plug insulator made ​​of microwave sintering of alumina , experiments showed that microwave sintering reduces the sintering time , the equipment is much smaller than the conventional sintering furnace , sintering time is reduced from 24h to 3h-6h, energy is only gas-fired conventional sintering furnace half. Although the 186 boxes have some products sintered sintered density is different, but still get an acceptable product. Krage made ​​barium ferrite microwave sintering work . Similarly , he believes that barium ferrite microwave sintering sintering rate to accelerate , microwave sintered barium ferrite particles of smaller size than the traditional methods .
 1988 , Sutton the sintered alumina forming the microwave sintering , the use of microwave equipment for the power 13.5kW, multi-cavity frequency of 2.45GHz , the weight of the sintered samples 62kg - 1360kg. The results showed that injection molded alumina firing release immediately after treatment with microwave , reducing the conventional air-drying process required 16h. Compared with traditional methods , the microwave treatment in time to save 50% on energy consumption was reduced by 20 % to 30%.
1988 to 1991 , Y. L. Tian and Johnson single-mode cavity such as sintered alumina 4mm diameter round bar , in order to avoid thermal runaway effect , they are in the sintering process is controlled by adjusting the sintering temperature of the microwave power , succeeded to obtain a dense sintered alumina round bar the relative density of 99.8 % and a particle size of 25m. They also had a diameter of 4mm TiC Al203 a round bar composed of a mixture of sintering, that if the content of more than 20% of TiC , it can prevent the occurrence of thermal runaway effect .
     In 1990 , M. A. Janney and H. D. Kimrey with frequency 28GHz and 2.45GHz microwave sintering of oxide.
     Microwave sintering accelerates the densification of the sample rate, the microwave and conventional sintering temperature has 300'C - 400 ℃ gap. Microwave and conventional sintering of the two curves are not significantly different , as the temperature rises , microwave sintering densification of the sample rate ratio faster under conventional sintering method , resulting in the microwave sintering of the sample particle size than the conventional sintering method under much smaller.
The microwave ( frequency of 28GHz) the sintering temperature of 950 ° 1000 ° , 1100 ° ; conventional sintering temperature of 1 250 ° , 1300 ° , 1350 ° ; sintering time 5min-120min, the relative density of the sintered samples under the conditions of 80% , calculated the activation energy of the sintering process , the obtained high purity alumina conventional sintering activation energy of 575kJ/mol, the value range is controlled by diffusion ; activation energy of the microwave sintering 160kJ/mol, conventional sintering method is only 1 / 3 .
   Microwave sintering densification rate accelerated , reducing the activation energy of the mechanism is not very clear, the electric field of microwave sintering process there must be a factor , frequency, perhaps also a factor, such as Y. V. Bykov , etc. with 82GHz microwave sintering activation energy is about the time of high purity alumina 100KJ/mol, microwave frequencies from 28GHz to 82GHz, the activation energy never 160kJ/mol down 100kJ/mol.