Effects Of Sintering Additives On The Structure And Microwave Dielectric Properties Of BaAl₂Si₂O₈ Ceramics

BaAl₂Si₂O₈ ceramics are microwave dielectric ceramic materials with low dielectric constant,high quality factor and near-zero temperature coefficient of resonant frequency,which can be applied in the field of microwave communication.However,the BaAl₂Si₂O₈ceramics have a higher sintering temperature?>1400°C?,and the crystal structure is hexagonal phase.In this paper,In order to lower the sintering temperature and prepare monoclinic phase BaAl₂Si₂O₈ ceramics,solid phase reaction method was used to prepare BaAl₂Si₂O₈ ceramics with binary composite sintering additives?ZnO-B₂O₃,CuO-B₂O₃ and Bi₂O₃-B₂O₃,?and glass additives?ZnO-B₂O₃-SiO₂,MgO-B₂O₃-SiO₂,Li₂O-B₂O₃-SiO₂ and Li₂O-B₂O₃?,respectively.The sintering temperature,structure and microwave dielectric properties of BaAl₂Si₂O₈ ceramics with different sintering additives were investigated.And then,a sintering additive was discovered which can significantly reduce the sintering temperature of BaAl₂Si₂O₈ ceramics and improve the microwave dielectric properties.The research content of this paper mainly includes the following aspects:?1?ZnO-B₂O₃ as a composite sintering additive can slightly lower the sintering temperature of BaAl₂Si₂O₈ ceramics from 1400°C to 1350°C,increase the bulk density and effectively promote the transformation of hexacelsian-to-celsian.Moreover,there is only one crystal structure of celsian in the sample when 1?x?4,where x is mass fraction.The BaAl₂Si₂O₈ ceramics with 1wt%ZnO-B₂O₃ sintering additive sintered at 1350°C for 3h shows the high Q×f of 40608 GHz,?_r of 6.45 and?_f of-22.46×10^-6/°C.?2?The sintering temperature of BaAl₂Si₂O₈ can be significantly reduced to 1250°C by adding the CuO-B₂O₃ binary composite sintering additive.In addition,this additive increased the bulk density of BaAl₂Si₂O₈ ceramics and promoted all of the hexacelsian transform into the celsian when 1?x?2.5,where x is mass fraction.The BaAl₂Si₂O₈ ceramics obtained the high Q×f of 30198 GHz and good?_f of-14.78×10^-6/°C sintered 1250°C for 3h as x=1.And the?_r is 6.47.?3?After adding Bi₂O₃-B₂O₃ binary composite sintering additive,The sintering temperature of BaAl₂Si₂O₈ can also be significantly decreased to 1250°C.When 1?x?4,where x is mass fraction,the crystal structures of all BaAl₂Si₂O₈ ceramic samples are a sort of single phase of celsian.Moreover,the BaAl₂Si₂O₈ ceramics with 3wt%Bi₂O₃-B₂O₃ sintered at 1250°C for 3h shows the good microwave dielectric properties:?_r=6.2,Q×f=21972GHz,?_f=-17.06×10^-6/°C.?4?The sintering temperature of BaAl₂Si₂O₈ ceramics with ZnO-B₂O₃-SiO₂ glass addition can be remarkably reduced from 1400°C to 1250°C.The ZnO-B₂O₃-SiO₂ glass can increase the bulk density and promote the transformation of hexacelsian-to-celsian when x=2and 4,where x is mass fraction.However,the second phase of Zn2SiO4 was observed in the XRD patterns when x?6.The good microwave dielectric properties of?_r=6.41,Q×f=34237GHz and?_f=-25.34×10^-6/°C of BaAl₂Si₂O₈ ceramic with 2wt%ZnO-B₂O₃-SiO₂glass addition were obtained sintered at 1250°C for 3h.?5?The addition of MgO-B₂O₃-SiO₂ glass can slightly reduce the sintering temperature of BaAl₂Si₂O₈ ceramics to 1375°C,increase the bulk density and effectively promote the transformation of hexacelsian-to-celsian when x=2,where x is mass fraction.And as 2?x?8,all of the hexacelsian transformed into celsian.The BaAl₂Si₂O₈ ceramics with 2wt%MgO-B₂O₃-SiO₂ glass addition sintered at 1350?for 3h shows the high Q×f of 36100GHz,and the?_r and?_f are 6.59 and-23.99×10^-6/°C,respectively.?6?A small amount of Li₂O-B₂O₃-SiO₂ glass addition can increase the bulk density of BaAl₂Si₂O₈ ceramics and promote the transformation of hexacelsian-to-celsian.When0.1?x?2,where x is mass fraction,Only the celsian was observed in the XRD pattern.The Li₂O-B₂O₃-SiO₂ glass can significantly lower the sintering temperature of BaAl₂Si₂O₈ceramics.As x=0.3,the BaAl₂Si₂O₈ ceramics sintered at 1275°C for 3h shows the good microwave dielectric properties of?_r=6.74,Q×f=34570GHz and?_f=-15.97×10^-6/°C.As x=2,the sintering temperature can be further reduced to 1025°C,but the microwave dielectric properties of the sample are not good.?7?The Li₂O-B₂O₃ glass exhibits the better effects that lower the sintering temperature of BaAl₂Si₂O₈ ceramics.In addition,The Li₂O-B₂O₃ glass can effectively promote the transformation of hexacelsian into celsian when 1?x?7,where x is mass fraction.The BaAl₂Si₂O₈ ceramics with 5wt%Li₂O-B₂O₃ glass addition shows the good microwave dielectric properties of?_r=6.34,Q×f=24366GHz and?_f=-27.42×10^-6/°C sintered at 950°C for3h.When x=7,the sintering temperature can be further reduced to 875°C,but the microwave dielectric properties of the sample are deteriorated.?8?The way of Li₂O-B₂O₃ glass addition was changed.It has the better effects that lower the sintering temperature of BaAl2Si2O8 ceramics by adding Li2O-B2O3 glass before the first ball-milling.And the bulk density was significantly increased,too.The BaAl2Si2O8 ceramics with 4wt%Li2O-B2O3 glass addition sintered at 875°C for 3h shows the good microwave dielectric properties of?_r=6.62,Q×f=19056GHz,?_f=-24.78×10^-6/°C.According to the experimental results,it can be found that these kinds of sintering additives can promote the transformation of hexacelsian-to-celsian.The ZnO-B2O3 sintering additive can increase the Q×f value to maximum of 40608 GHz.The CuO-B2O3 sintering additive can decrease the absolute value of?_f to minimum of-14.78×10^-6/°C.In addition,the low-temperature sintering of BaAl2Si2O8 ceramics was realized after adding Li2O-B2O3 glass.The sintering temperature was reduced to below 950°C and 875°C by adding Li₂O-B₂O₃ glass before the second ball-milling and the first ball-milling,respectively.