Research On Process And Mechanism Of Joining Lithium Ferrite And Al₂O₃ Ceramic By Magnetic Glass-ceramic

Lithium ferrite is a kind of magnetic composite oxide ceramics,which is often used as the gyromagnetic medium for microwave devices.The preparation of large-size or complex-structure devices needs to be connected to itself;Al₂O₃ ceramics are often used to make substrates for carrier components in microwave devices,and it also needs to be reliably connected with ferrite.In this paper,by designing the composition of the glass solder,the composite joint structure of magnetic crystallites precipitated in situ in the glass is formed in the ferrite/ferrite and ferrite/Al₂O₃ ceramic connection joints to realize the structure-function integration of the joint.The crystallization behavior of the solder and the wettability of the solder to the ferrite were studied using SEM,XRD and high-temperature wetting angle measuring instrument,and the influence of the process parameters on the micro-morphology of the ferrite connection joint was analyzed.The first-principle simulation was used to study the magnetic source and mechanical properties of the ferrite self-connected joints,and the formation mechanism of the magnetic joints was summarized;the thermal stress distribution of the ferrite/Al₂O₃ ceramic joints under different process parameters was investigated using finite element simulation,study and analyze the ways of joint strengthening.Three-component glass solders were designed,named LFBi5 B,LFBi15B,and LFBi25 B according to the different molar content of Bi elements.The three solders prepared by the melt-water quenching method were all amorphous.The LFBi5 B solder was heat-treated at 700°C and 800°C.At both temperatures,the solder precipitated crystals.As the temperature increased,the precipitated crystal phase gradually refined,in which Li Fe O2 was magnetic.The three solders have good wetting properties on the surface of the ferrite.As n?Bi:B?increases,the viscosity of the glass becomes larger and larger.Three kinds of solders are used to connect the ferrite itself.With the increase of n?Bi:B?,the number of precipitated crystals decreases first and then increases.The crystals of Li Fe O2 and Mg2Fe7O47 are magnetic;when the temperature increases,the precipitated crystals basically dissolve and the reaction generates a small amount of bismuth titanate whiskers;the common component in the precipitated crystals under different conditions is the Li Fe O2 phase,which is magnetically simulated using Materials Studio's first-principles module.The upper and lower spin energy bands are not completely symmetrical,the spin state density exhibits spin-split characteristics,which together indicate that the crystal is magnetic,and the main source of magnetism is the p-d state electron exchange of Fe and O atoms,explain that the precipitated crystal can introduce magnetic properties;Li Fe O2 crystal meets the mechanical stability criterion,and the cubic shear constant is far greater than 0,indicating that the precipitation of crystals can improve the mechanical properties of the joint.The formation mechanism of the magnetic joint is the joint interface reaction and the in situ precipitation of magnetic crystallites.According to the ANSYS finite element simulation,the thermal stress distribution of the ferrite/Al₂O₃ ceramic joint is approximately the same at different temperatures.The temperature increases and the thermal stress gradually increases;the maximum thermal stress is located at the interface between the glass-ceramic solder and the Al₂O₃ ceramic;as the thickness of the brazing material increases,the low-stress area inside the brazing material gradually narrows.The optimal thickness of the brazing material should be 500?m;butt joint and miter joint can disperse the thermal stress throughout the joint,and the overlap will increase the stress concentration.The joint can significantly relieve the stress concentration,and the stress concentration appears at the connection interface between the base material and the brazing material.Butt joint and socket joint are more suitable for practical engineering applications.According to the simulation results,the joint strengthening method is that as the grains in the material increase and the size decreases,the total grain boundary area becomes larger and larger,and the resistance to the formation of microcracks increases.