Research On Microstructure And Properties Control Of YIG Ferrite And Magnesium Titanate Ceramic Joint Brazed By Bismuth-Based Glass

As an indispensable device in electronic communication systems and equipment,microwave ferrite devices play an important role in national defense technology,satellite communications,and other fields.Yttrium iron garnet（YIG）ferrite owns high-quality factor,low magnetic loss and dielectric loss.As a microwave dielectric material,magnesium titanate ceramic（MTC）owns a low dielectric constant and dielectric loss,which guarantees the signal transmission speed.Thus,the reliable connection between YIG ferrite and magnesium titanate ceramic has become an important prerequisite for the long-term stable service of microwave ferrite devices.Bismuth-based glass,bismuth composite glass,and bismuth glass-ceramic were designed to join YIG ferrite and magnesium titanate ceramics,and a new method of pre-cladding amorphous glass and glass-ceramic brazing was invented.The evolution process and reaction mechanism of the joint microstructure were studied,the mechanical and electromagnetic properties of the joint were investigated.The bismuth-based amorphous glass（Bi34）with gradient distribution of thermal expansion coefficient compared with base materials was designed to join YIG ferrite and magnesium titanate ceramics.The YIG/MTC joint domain was composed of YBO₃ lamellar phase,YBO₃ whisker,Fe₂O₃ block phase,glass matrix,Mg Fe₂O₄whisker,Bi₄Ti₃O₁₂ whisker,and（Zn,Mg）₂Ti O₄ reaction layer.[BO₃]/[BO₄]in the glass matrix reacted with Y³⁺to form the YBO₃ phase.Fe³⁺diffused from YIG ferrite reacted with Mg₂Ti O₄,forming Mg Fe₂O₄.Additionally,[Bi O₆]in the glass matrix reacted with Ti⁴⁺,forming Bi₄Ti₃O₁₂.And Zn in[Zn O₄]substitutes a part of Mg in Mg₂Ti O₄ to preferentially form（Zn,Mg）₂Ti O₄ layer.Increasing the bonding temperature and prolonging the holding time could promote the growth of YBO₃,Mg Fe₂O₄,and Bi₄Ti₃O₁₂ whiskers.This is attributed to that the atomic concentration gradient could be increased via increasing the bonding pressure,to promote the growth of the whiskers and improve the density of the whiskers.The shear strength of the YIG/MTC joint processed at 725 ^oC,30 min,and 7.5 k Pa could reach 57 MPa.Mg Fe₂O₄ whisker,YBO₃ lamellar phase,and thin YBO₃ whisker performed favorable mechanical properties,which played an important role in improving the joint strength.For the aim of strengthening the mechanical properties of the glass seam,nano CoO-doped bismuth-based composite glass was designed to realize the formation of magnetic in-situ whiskers in YIG/MTC joint.The shear strength of the YIG/MTC joint was increased to 67 MPa.The effect of magnetic whiskers on the mechanical properties of the joint was revealed that whiskers with appropriate size were conducive to the crack deflection,which could improve the joint strength,while the crack could directly pass through the coarsened whiskers,weakening the joint mechanical properties.The formation mechanism of whiskers was revealed that Co₂Fe O₄ nuclei were formed via two-step heterogeneous nucleation and continued to grow along the axis[001].The MFM analysis indicates that Co₂Fe O₄ whiskers were magnetic.The joint dielectric loss tangent was under 0.001 at high frequency.The excellent mechanical properties of glass-ceramics make it a great choice for glass bonding.To further increase the joint mechanical properties and electromagnetic matching property,Bi₂O₃-CoO-Fe₂O₃-B₂O₃ glass-ceramic was designed for the joining of YIG ferrite and MTC.The crystallization of the glass-ceramic was clarified through thermal analysis,Bi₂B₄O₉,Bi_xFe_yO_z(Bi₁₀Co₁₆O₃₈ and Bi₃₄CoO₄₀),and Bi_uCo_vO_w(Bi34Fe O₄₀,Bi₂₄Fe₂O₃₉,and Bi₄₆Fe₂O₇₂)were precipitated.As the temperature increased,the crystals gradually decomposed,meanwhile,Co Fe₂O₄ nuclei formed and grew up.After the crystallization process was completed,the glass-ceramic was magnetic and the saturation magnetization was 4.2emu/g.At the interface of MTC base material,the Co Fe₂O₄ reaction layer was formed,which formed a coherent interface with the base material,effectively reducing the interfacial stress.Moreover,Mg Fe₂O₄ and Bi₄Ti₃O₁₂ phases were generated at the local interface.Nano-scale and micro-scale Co Fe₂O₄ crystals and Bi₉Ti₃Fe₅O₂₇ strip phases.The shear strength of the YIG/MTC joint brazed at 800 ^oC reached a maximum of 72 MPa.The glass-ceramic improved the mechanical properties of the joint,the fracture located at the interface of YIG ferrite.The dielectric constant was between that of the base materials,and the dielectric loss tangent was about 0.001,close to that of the base materials,which guarantees good electromagnetic matching between the glass seam and base materials.To reduce the interfacial stress,a new method of pre-cladding amorphous glass and glass-ceramic brazing was invented.The bonding temperature could be reduced from 800 ^oC to 725 ^oC.The YBO₃ reaction layer was formed at the interface of YIG ferrite,and short Mg Fe₂O₄ whiskers were generated at the interface of MTC.The glass seam consisted of glass matrix,micro,and nanoscale Co Fe₂O₄ phases.The joint shear strength reached 117 MPa,which was 205%of the joint brazed by the amorphous glass.Combine with finite element simulation,the stress distribution state of the joint was revealed,and the interfacial residual stress decreased,which was the important reason for the improvement of YIG/MTC joint strength.