Investigation On Magnetic-induced Intergranular Interface Strengthening Mechanism In Microwave Sintering By SR-CT Experiment

Microwave sintering is a newly developed material preparation method,which shows advantages in optimizing the microstructure and enhancing the mechanical performance of materials.But the extreme complexity and uncontrollability of microwave sintering process has always been the bottleneck in the large-scale industrial application of microwave sintering technique.Therefore it has great significance to carry out researches on microwave-material interaction mechanisms.This paper explained the important way to study comprehensively the microwave-matter interactions,which is separating the microwave field into electric field(E-field)and magnetic field(H-field)and studying the interactions between matter and single E-field or H-field.In fact,the mechanisms of H-field has been seldom researched while the previous studies focused on the E-field.Therefore,this paper proposed a solution to solve the difficulties and challenges in studying the mechanisms of H-field.A special microwave sintering experimental setup using controllable standing wave and combining the synchrotron radiation tomography(SR-CT)technique was developed.Online microwave sintering SR-CT experiments of magnetic and non-magnetic materials were carried out.The evolution process of intergranular interface inside the materials was quantitatively characterized and analyzed.On this basis,the reinforcement mechanisms of microwave H-field on different types of intergranular interface were discussed.Finally,the research content was summarized and the future direction of this research was prospected.The main research content is as follows:Firstly,a microwave sintering experimental setup which combining the controllable standing wave and SR-CT technique was developed,which provide a way to study the microwave H-field mechanisms with matter.A special microwave resonator cavity was designed to cooperate with the SR-CT platform.The cavity could produce separate microwave H-field and maintain the field pattern in the open SR-CT environment.A sample rotating and displacement system was designed to ensure high accuracy motion of sample in the extreme microwave high-temperature environment.A linkage control scheme between the microwave field pattern and sample motion was developed,which could ensure the microwave parameters around sample to be stable in the experiment.Secondly,microwave sintering SR-CT experiments of different materials using the new experimental setup was carried out and special evolution phenomena of intergranular interface were discovered.In the experiments of magnetic materials,titanium,nickel and their mixture were chosen to be the sample,considering their different magnetic properties.It was found that the sintering temperature in H-field was lower than that in conventional sintering,the spatial orientation of intergranular interface after H-field sintering had relevance with the H-field orientation,and the heterogeneous intergranular interface inside sample showed abnormal directional movement in H-field.In the experiments of non-magnetic materials,alumina was chosen.It was found that the abnormal grain growth was suppressed and the volume of material particle became uniform during sintering in H-field.Thirdly,the interaction mechanisms between H-field and matter were analyzed based on the results of sintering SR-CT experiment and assistance of numerical simulation.For the magnetic materials,the H-field distribution in the intergranular interface region was calculated in numerical simulation.It showed that the density of H-field and magnetic energy loss was concentrated at the interface region,which might promote the sintering process at relatively low temperature.The magnetic loss at two sides of the interface was unequal so that special matter driving force was produced and the abnormal directional movement of the heterogeneous intergranular interface would occur.The orientation of H-field could alter the magnetic loss distribution,therefore the growth of intergranular interface could be related to the H-field.For the non-magnetic materials,it also showed that the density of H-field was concentrated at the interface region.Besides,a polarization force induced by H-field may lead to mass transfer from big material particle to small particle.This mechanical model could also explain the suppression of grain growth and uniformity of grain volume during H-field sintering.