Bismuth-boron Glass Braze Design And Brazing Mechanism Of Lithium Ferrite And Magnesium Titanate Ceramic

Due to high electrical resistivity,low dielectric constant,hith breakdown voltage,high reliability,good temperature stability,low cost and so on,lithium ferrites have been widly used in microwave devices.Magnesium titanate ceramic is an excellent traditional microwave dielectric material.The highly reliable and functional bonding of ferrites themselves and between ferrites and magnesium titanate ceramic substrates is of great significance to promote the continuous progress of aerospace technology and electronics industry.In this work,based on the properties of base ceramics and the designability of glass braze,the bismuth-boron glass braze system with low melting point is suggested to join ferrite and magnesium titanate ceramic.The bismuth-boron glass braze system is designed and prepared.The relationship between the compositions of glass brazes and their performance is studied.The effect of bonding process and compositions of glass brazes on?i?the element diffusion and interfacial reaction and?ii?the shear strength and dielectric properties of joints is researched emphatically.Firstly,the amorphous bismuth-boron glass braze is prepared by the methods of melting-water quenching.Based on the content of Bi2O3 and the kind of glass modifier,the present glasses are named Bi40,Bi35,Bi25,Bi20,Bi30-BF?addition of Ba O and Fe2O3?and Bi25-BC?addition of Ba O and Ca O?.The researches show that the number of [Bi O3] and [Bi O6] units increases with rising the Bi2O3 content.Moreover,either the increase of Bi2O3 content,the decrease of Si O2 content or the addition of an appropriate network modifier induce a progressive conversion of [BO4] into the [BO3] unit.The coefficients of thermal expansion,characteristic temperature and mechanical properties of glass brazes directly correlate to the glass network structure and the number of non-bridging oxygen.Both the content of Bi2O3 and the density of glass network structure combine to determine the dielectric properties of glass brazes.A high content of Bi2O3 and Zn O promotes the formation of Bi-rich and Zn-rich crystals during heat-treated process,respectively.An addition of network modifier helps stabilize the glass structure against a crystallization.The formation of bulky crystals in the glass matrix or at the interface is unfavorable for the spreading of glass brazes.However,the dissolution of lithium ferrite and interfacial reaction between ferrite and glass braze provide an additional driving force for spreading process.Based on the above research of the properties and wetting behavior of glass brazes,a series of bismuth-boron glass brazes are used to join lithium ferrite.Upon brazing,a large number of Bi-rich phases?Bi46Fe2O72,Bi12 Si O2 and Bi24B2O39?with big size and a few Zn-rich phase?Zn O?are observed in the ferrite/Bi40 and ferrite/Bi35 joints.The metastable Bi-rich phases?Bi4Zn B2O10 and Bi2O2 Si O3?are observed in the ferrite/Bi25 joint bonded at 650 ?.With rising the bonding temperature,the metastable Bi-rich phases disappear and a few Zn2 Si O4 phases are founded in the joint.There is no interfacial reaction in the ferrite/Bi20 joint bonded at 650 ?.However,a lot of granular Zn Fe2O4 phases are observed in the seam.No crystalline phase is detected in the ferrite/Bi30-BF joint.Only when the bonding temperature is equal to or higher than 725 ?,Bi25-BC reacts with ferrite and then the Bi₅Ti₃FeO₁₅ whisker forms.Except Bi₅Ti₃FeO₁₅ whisker,other crystal phases and interfacial reaction products are proved to be bad for or have no effect on the joint strength.The joint strength has the maximum value of 86 MPa for a lithium ferrite/Bi25-BC couples.The main impact is attributed to the strengthening effect of Bi₅Ti₃FeO₁₅ whiskers.In order to further analyze the formation mechanism of in-situ Bi₅Ti₃FeO₁₅ whisker,the composite braze?Bi25-BC and ferrite particles?is prepared and the Ti content of lithium ferrite is changed.This is attributed to the fact that the Ti element in the seam diffusing from the ferrite ceramic to the seam determines whether the interfacial reaction between Bi25-BC and ferrite occurs or not.Meanwhile,both the crystal structure of Bi₅Ti₃FeO₁₅ phase and the Ti concentration gradient in the seam combine to determine the morphology and orientational distribution of Bi₅Ti₃FeO₁₅ phase.If there is an apparent Ti concentration gradient in the seam,the Bi₅Ti₃FeO₁₅ nucleuses grow into whiskers along the c-axis orientation promoted by this Ti concentration gradient.The orientational distribution of Bi₅Ti₃FeO₁₅ whiskers is consistent with the Ti concentration gradient.On the contrary,the Bi₅Ti₃FeO₁₅ nucleuses grow into plates along the ab crystalline plane orientation.The distribution of Bi₅Ti₃FeO₁₅ plates is random.The layered strip-like or rob-like BTF phase is a transition from whisker to plate.Either the content of ferrite particles in the composite braze or the Ti substitution in the ferrite influence the morphology,quantity,size and distribution of Bi₅Ti₃FeO₁₅ phase by determining the Ti content and Ti concentration gradient in the seam.The analysis of mechanical properties of joints shows neither the Bi₅Ti₃FeO₁₅ plates nor the agminated layered Bi₅Ti₃FeO₁₅ phases have an ability to resist a crack expansion or induce a crack deflection.This is bad for the joint strength.However,the Bi₅Ti₃FeO₁₅ whisker array and the dispersive layered Bi₅Ti₃FeO₁₅ phases could induce microcrack shift in a large angle offset or do a interlaminar crack.Then,the crack propagation path and the fracture work of microcrack could increase.Therefore,the glass matrix and the interface are toughened effectively.The toughening effect of Bi₅Ti₃FeO₁₅ whisker is more obvious than that by Bi₅Ti₃FeO₁₅ strip or rob.Research on the electrical properties of lithium ferrite with or without heat-treatment implies that a high bonding temperature appears to decrease the dielectric constant and increase the dielectric loss tangent?electrical resistivity?of lithium ferrite.The electrical properties of joints are determined by the bonding temperature,glass matrix and interfacial reaction.The dielectric constant of ferrite/Bi25-BC joint near that of the original ferrite at a high frequency.The dielectric loss tangent?electrical resistivity?of ferrite/each glass braze joint bonded at different temperature near that of the original ferrite at a high frequency,but is higher than that of ferrite heat-treated at corresponding temperature.The formation of Bi₅Ti₃FeO₁₅ whiskers decreases the dielectric loss tangent?electrical resistivity?of joints to some extent.Compared with other glass brazes,the bonding effect of lithium ferrite and magnesium titanate ceramic by Bi30-BF is the best.Due to the crystallization of Bi30-BF and the interfacial reaction between Bi30-BF and ferrite,the bulk phases?Bi12Si O2 and Bi46Fe2O72?are observed in the seam at 625 ?,which decreases the shear strength of joints.There is no interfacial reaction between Bi30-BF and ferrite with increasing the bonding temperature.The white rod Bi4Ti3O12 phase and rich magnesium ceramic particles form due to the interface reaction of glass braze and magnesium titanate ceramic.The degree of this interface reaction increases with rising the bonding temperature.The shear strength of joint increases first and then decreases with an increase in the bonding temperature.A maximum in the shear strength of 35 MPa is achieved at 700 ?.