Temperature Coefficient Of Resonant Frequency And Anti-reductive Properties For Ba₂Zn_?1+x?Si_?2+y?O_?7+x+2y?Microwave Dielectric Ceramics

High-frequency,high-precision multi-layer ceramic capacitors(MLCC)are the key electronic components of wireless communication,so the demands are particularly large.In order to improve its cost-effective,many manufacturers advocate the utilization of Ni and other base metals to replace Ag and other precious metals as the MLCC internal electrodes.However,the reduction atmosphere must be applied to avoid the oxidation of Ni electrode in the co-firing process.Therefore,traditional oxide microwave dielectric ceramics,especially the low-permittivity microwave dielectric ceramics,are facing new challenges.In order to enhance the operating frequency and precision of MLCC,the microwave dielectric ceramics must possess low permittivity(?r<15),stable resonant frequency against temperature and excellent anti-reductive properties.In this paper,Ba2Zn(i+x)Si(2+y)O(7+x+2y)were investigated.Ceramic specimens were prepared by conventional solid-state method.Based on the rare coexistence of weak ferroelectricity and low ?r,microwave dielectric properties,the evidence of weak ferroelectricity was verified.The influence of Zn2+ content and Ba/Si ratio on the temperature coefficient of resonant frequency,the stability of phase structure and anti-reductive properties were studied.Finally,the ceramics with good anti-reductive properties were prepared productively.A new mechanism for controlling temperature coefficient of resonant frequency(?f)of Ba2Zn(i+x)Si(2+y)O(7+x+2y)low ?r microwave dielectric ceramics was established,which broke the traditional methods to modify the ?fvalue relied mainly on Ti-based dopants.Ba2ZnSi2O7 ceramics with a monoclinic structure(C 2/c)were prepared using the conventional solid-state method at 1200 ? for 3 h in air.The as-prepared ceramics show coexistence of high-Tc ferroelectricity and good microwave dielectric properties(?r = 8.09,Q×f = 26634 GHz,and ?f =-51.4 ppm/?).An abnormal ?r peak is observed at approximately 510 ? and above 750 ?.The weak ferroelectricity is verified from intrinsic remanent polarization with 0.005 ?C/cm2 and ferroelectric domains with 90° and 103° directions.In-situ observation of ferroelectric domains proves that the Curie temperature(Tc)of the Ba2ZnSi2O7 ceramics is 750 0C or higher.Apart from the previously reported Ba2Zn(i+x)Si2O(7+x)(x = 0)with a monoclinic structure(C 2/c),the end-member compositions at x =-1 and 1 exhibit single-phase?-BaSiO3 with an orthorhombic structure(P212i2i)and BaZnSiO4 with a hexagonal structure(P63),respectively.And they both possess also a coexistence of weak ferroelectricity and low-permittivity microwave dielectric properties.The strip-type domain structure and intrinsic saturated remanent polarization(?0.006 ?C/cm2 for x =-1 and?0.009 ?C/cm2 for x = 1)above the electric field of 50 kV/cm are observed.Two ?r anomaly peaks occur at lower temperature(145 ? for x =-1 and 124 ? for x = 1)and higher temperature(500 ? for x =-1 and 479 ? for x = 1).A reduction in Zn2+ content mainly decreases the intensity of the ?r anomaly peak at lower temperature and increases the ?r(or frequency)stability against temperature.The Zn2+-rich BaZnSiO4 phase has a ?f value of-181 ppm/?,whereas the ?f value of Zn2+-free BaSiO3 phase decreases to-35.4 ppm/?.The Zn2+ deficiency in Ba2Zn2O7 composition could inhibit the presence of BaZnSiO4 phase and improve the ?f value,whereas excessive Zn2+ cations prompt the formation of the BaZnSiO4 phase to deteriorate significantly the ?f value.In the study of the anti-reductive properties of Ba2Zn(i+x)Si(2+y)0(7+x+2y)ceramics,the phase structure stability in different atmosphere and the influence of Ba/Si ratio on the ?r-T and anti-reductive properties were investigated focally.The Ba2Zn(i+x)Si2O(7+x)ceramics(x=+1,0)maintain stable phase structure in Air,O2 and N2 atmosphere.However,the BaZnSiO4 phase gradually changes to Ba2ZnSi2O7 phase,and Ba2ZnSi2O7 phase gradually is converted into BaSiO3 due to the volatilization of Zn2+ in the N2-lvol%H2 reductive atmosphere.The phase structure of Ba2Si(2+y)0(6+2y)(y=0,1.0,1.2,2.0)are stable in four atmosphere which corresponding to the BaSiO3,Ba2Si308,Ba5Si8O2i and BaSi2O5 single phase.The Ba5SisO21 phase is a low ?r,microwave dielectric ceramic with abnormal positive ?f value,which is expected to become a novel ?f regulating dopants with excellent anti-reductive properties.