Research On Microwave Thermal Runaway Mechanisms Based On SR-CT Experiments

Microwave thermal runaway is a bottleneck problem that hinders the application of microwave sintering technique,which is efficient,energy-save and green.Research on the thermal runaway behaviors and its thermodynamic principles is important for the prevention,suppression and regulation of microwave thermal runaway.Firstly,the characteristics of microwave technology and the challenges of studying the principle of thermal runaway were explained.It states that carrying out time-resolved synchrotron radiation tomography(SR-CT)microwave sintering experiments and numerical simulation based on the real experimental data is the key to studying microwave thermal runaway.Then,the existing microwave sintering SR-CT equipment and methods were improved.A time-resolved SR-CT equipment and corresponding methods for the characterization of rapid microstructure evolution during microwave thermal runaway were developed.The time-resolved SR-CT experiment of metal and alloy materials were carried out.Based on the quantitative characterization of thermal runaway behavior of metals,the dominant mechanism leading to thermal runaway was analyzed.The evolution rule of the dominant driving force in metal's microwave sintering process was discussed.Next,the regulation mechanism of ceramic to the thermal runaway behavior of metal was analyzed by introducing ceramic phase into metal samples.Finally,the research content was summarized,the direction of further research was discussed.The main research content is as follows:Firstly,a time-resolved SR-CT microwave sintering equipment and supporting experimental methods were developed to provide a way for studying the thermal runaway mechanisms.The existing microwave heating device was improved,and a concentrated assisting heating system was designed to achieve ultra-rapid temperature rise under the condition of maintaining the microwave field.The problem of sample instability in the time-resolved SR-CT process was solved by improving the rotating drive system.Using the AFBP-TVM reconstruction algorithm and a special designed three-dimensional matching algorithm,high-quality microstructure evolution data during thermal runaway were obtained.Secondly,based on the on-line tracking of microwave thermal runaway process,the local plasma sintering mechanism was proposed.It was found that there were different sintering driving forces in the thermal runaway process through the quantitative analysis of the microstructure evolution process.By extracting microstructure features and kinetic parameters and combining the finite element simulation method,the eddy current loss mechanism of metal particles was verified.Moreover,it was proposed that the local plasma mechanism is the dominant driving mechanism of thermal runaway.The evolution rule of the dominant mechanism in metal's microwave sintering was also discussed.Thirdly,the regulation mechanism of ceramics on the micro structure evolution of metal was studied by introducing ceramic phase into metals.It was found that the ceramics could greatly influence the sintering process of metal by introducing ceramics with different dielectric properties.The low dielectric loss ceramic could improve the sintering efficiency by changing the microwave field distribution and generating microwave passage or microwave lens.The high dielectric loss ceramic could generate hot spots in the sample through selective heating,thereby suppressing the microwave thermal runaway and stabilizing the sintering process.Moreover,with the aid of finite element simulation of adding ceramics with different electromagnetic properties,the regulation mechanism of ceramics were systematically analyzed.