A Novel CaAl₂B₂O₇ Microwave Ceramics With Low Dielectric And Low Losson

With the development of 5G technology and Io T,the communication frequency has entered the era of high frequency,where communication speed is required to be faster and transmission loss is as small as possible.Microwave dielectric materials with low dielectric constant and low dielectric loss are widely concerned in order to meet the requirements of high frequency and high speed.Therefore,it is of great significance to study the new microwave materials with high performance that can be applied to 5G to support the development of wireless communication technology.For the borate system,the low cost,low sintering temperature and low dielectric constant of these materials make the system more attractive.At present,borate systems such as Mg₂B₂O₃,La₂O₃-B₂O₃ have been widely studied.However,the current research on CaAl₂B₂O₇ is limited to its structure and luminescence properties.Due to the stability,phase composition during sintering and microwave dielectric properties of this material are still unclear.This paper mainly focuses on a new CaAl₂B₂O₇ microwave dielectric ceramics.The selection of this ceramic system material is due to its low dielectric constant and high quality factor of microwave properties.Not only through the experimental process control（such as adjusting the pre-firing and sintering temperature of Al element non-stoichiometric design）to find out the best process conditions,the preparation of low dielectric ceramics with excellent microwave dielectric properties.At the same time,in order to make CaAl₂B₂O₇ based ceramic system suitable for LTCC technology field,the sintering temperature was successfully reduced to 850^oC by mixing sintering additives to form glass-bonded ceramics to meet the matching degree of co-firing with Ag electrode,and the low temperature sintering kinetics of CaAl₂B₂O₇-based ceramic materials was systematically explored.Firstly,the effects of calcined temperature and sintering temperature on microwave characteristics were studied by non-stoichiometric design of Al element:CaAl_2xB₂O₇（x=1,0.75,0.5,0.25）.It was found that the appropriate calcined temperature increased（x=0.25:750^oC;x=1:800^oC）,and the optimum sintering temperature also increased from 930^oC to 1020^oC with the increase of Al₂O₃ content.Excellent microwave dielectric properties:ε_r=5.80,Q×f=63338 GHz（@13.57 GHz）,ρ=2.42 g/cm³,τ_f=-29±2 ppm/^oC,were synthesized at x=0.5 when calcined at 800^oC and sintered at 940^oC,however,this ceramic cannot be co fired with Ag electrode because the sintering temperature is close to the melting point of Ag.To make CaAl₂B₂O₇ based microwave dielectric ceramics be useful in LTCC field and,Widen the sintering temperature window（widen from 10^oC to 25^oC）.the sintering additives Li₂O-B₂O₃-Si O₂（LBS）and LiF were added to CaAl₂B₂O₇（x=0.5）.The XRD results showed that the doped LBS glass and LiF did not react with the ceramic matrix,and the crystal phase composition was still CaAl₂B₂O₇ and Ca B₂O₄.Moreover,this low-firing ceramic now can be co-fired with Ag,where composite ceramic can match with silver paste.Combination great microwave dielectric properties were obtained at875^oC:ε_r=6.08,Q×f=37295 GHz（@15.78 GHz）,ρ=2.43 g/cm³,τ_f=-22.36 ppm/^oC with 0.5wt.%LiF addition;And with 1wt.%LBS glass addition,ceramic withε_r～5.91,Q×f～39443 GHz（@16.53 GHz）,τ_f～23.5 ppm/^oC,ρ～41g/cm³ were synthesized when sintered at 850^oC.Finally,in order to analyze the low-temperature sintering kinetics of CaAl_2xB₂O₇（x=1,0.75,0.5,0.25）ceramics and without additives,ceramics（x=0.5）with LiF and LBS glass dopant,meanwhile,to quantitatively analyze the influence of Al content and sintering aid on sintering temperature,.Specifically,the sintering behavior of the samples during heating process was analyzed,and the thermal shrinkage curve was also studied.The E_a value of the samples was calculated based on the constant heating rate model of Arrhenius equation.According to the thermal shrinkage curve,E_a value and variations of sintering temperature,it is found that the reduction of Al content reduces the E_a value,and the system can be sintered at a lower sintering temperature.According to the calculation results and shrinkage curves,it can be seen that the doped LiF（E_a=672 k J/mol）and LBS（E_a=530 k J/mol）glass are much lower when compared with CaAl_2xB₂O₇（x=0.5）（E_a=1342 k J/mol）ceramics,therefore,LiF and LBS glass can reduce the sintering temperature.Finally,the"step phenomenon"in the thermal shrinkage curve is attributed to the formation of CaAl₂B₂O₇ phase by the chemical reaction between the unreacted Ca B₂O₄and Al₂O₃.