Preparation And Properties Research Of SrO-V₂O₅ System Microwave Ceramics

A detailed study on SrO-V₂O₅ system microwave dielectric ceramics has been carried out in this thesis.In order to reduce the sintering temperature and ameliorate the microwave dielectric properties of ceramics,methods such as calcium ion doping,adjusting the content of V element,and adding sodium borate were used to synthesize three kinds of ceramic materials with favorable microwave dielectric properties in the temperature range of 625℃～1000℃:Sr_0.5Ca_0.5V₂O₆、Sr₂V_1.90O₇、Sr₃（VO₄）₂+1wt.%Na₂B₄O₇·10H₂O.The analysis and detection methods used include:X-ray diffraction analysis,DSC-TG,SEM,TEM,Raman analysis,etc.Supplemented by theoretical calculations,the relationships between the extrinsic factors（secondary phase,relative density,grain morphology,pores,etc.）,intrinsic factors（crystal structure,bond length,cations order,bond valence,polarizabilities of ions）and the microwave dielectric properties of ceramics in this system were analyzed.In the study on modification of Sr V₂O₆ with Ca replacement,it was found that the second phase Ca V₂O₆ with better performance appeared after adding calcium ions.With the increase of Ca V₂O₆ phase,the sample sintered more densely.The relative density determined the Q×f value of Sr_1-xCa_xV₂O₆（0≤x≤0.5）ceramic.Compared with pure phase Sr V₂O₆ microwave dielectric ceramic,the ultra-low temperature（625℃）sintering condition was maintained after adding calcium,and the Q×f andτ_f value were significantly improved.Vanadium content is very important to the microwave dielectric properties of Sr₂V_xO₇（1.80≤x≤2.05）ceramics.Only Sr₂V₂O₇ single phase existed when V was slightly excessive,but there were too large grains and pores in Sr₂V_2.05O₇ ceramics.When V was insufficient,an appropriate amount of second phase Sr₃（VO₄）₂ made the unit cell volume of the main crystalline phase Sr₂V₂O₇ reach a relatively large value,which increased the density,changed the full width at half maximum（FWHM）of the Raman peak,and improved the cation order degree in the system,thus optimized the Q×f of the entire system.The reason why|τ_f|of single-phase Sr₂V₂O₇ approached zero is that the average bond strength decreased,which made it easier to restore the tilted state of[VO₄]tetrahedron.When there were more Sr₃（VO₄）₂ phase,the effects of external defects was greater than that of internal defects,and the positiveτ_f value of the second phase compensated the negativeτ_f value of the major phase.Polarizabilities of ions,molar volume,and the density of samples affected the permittivity.Sr₂V_1.90O₇ ceramic were sintered well at 950℃:ε_r=11.1,Q×f=36,500GHz andτ_f=-35.6ppm/℃.The sintering temperature of Sr₃（VO₄）₂ ceramics was successfully reduced from about 1000℃to 675℃by using borax as an additive.The incorporation of a small amount of borax did not damage the crystal structure of Sr₃（VO₄）₂.The appearance of low-melting-point glass phase Na₂O-SrO-V₂O₅-B₂O₃ could assisting sintering and the grains grew uniformly.Due to the changes of the vibration of V-O bond and the lattice vibration space,Q×f value was keeping at a high level.The Na₂O-SrO-V₂O₅-B₂O₃ glass phase probably hold a negativeτ_f value,and the positiveτ_f value of Sr₃（VO₄）₂ was compensated.With the sintering condition of 725℃,theτ_f value of Sr₃（VO₄）₂+1wt.%Na₂B₄O₇·10H₂O ceramic reached+2.5ppm/℃.