Synergistic Regulation And Application Of Thermal Expansion And Temperature Frequency Coefficient Of Ba BaZn₂Si₂O₇-based Ceramics

In aerospace and other extreme environment applications,dielectric resonator antenna(DRA)should not only meet the requirements of high gain,lightweight,wide bandwidth and miniaturization,but also put forward higher requirements for thermal stability indexes such as thermal expansion coefficient of materials and temperature coefficient of resonant frequency(?_f)due to the existence of temperature changing scenarios.However,microwave dielectric ceramics with near zero thermal expansion coefficient and resonant frequency temperature coefficient are very rare in nature.Therefore,it is necessary to study the internal correlation between thermal expansion coefficient of ceramics and microwave dielectric property to broaden the application field of microwave dielectric ceramics.Luneburg lens is a spherical lens composed of inhomogeneous dielectric materials.Its applications in antennas are mainly focused on gain enhancement and beam scanning.However,there are few studies on the application of metamaterial structures in dielectric resonator antenna.In this paper,we systematically studied the rare properties of Ba O-Zn O-Si O₂ microwave dielectric ceramics,such as anti-reductive characteristics,weak ferroelectricity and negative coefficient of thermal expansion(NCTE),and further discussed the potential applications of the above properties in DRA.Considering the technical difficulties in the design of DRA with high thermal stability,high gain and wide band index,an integrated DRA with Luneberg lens was fabricated by using SLA ceramics 3D printing technology.Based on the technical problems in the materials and devices mentioned above,the rare properties of Ba₂Zn Si₂O₇,Ba Zn Si₃O₈ and Ba Zn₂Si₂O₇ microwave dielectric ceramics,such as anti-reductive characteristics,weak ferroelectricity and NCTE,were deeply explored.the Ba2Zn Si2O7 ceramic presented good microwave dielectric properties(?_r=7.32,Q×f=27200 GHz,?_f=-41.5 ppm/°C)under the N₂–1 vol%H₂ atmosphere.The crystal structure of Ba Zn Si₃O₈was fully investigated by synchrotron radiation and XRD Rietveld refinment(monoclinic,space group:P2₁/a).The as-prepared ceramics show coexistence of ferroelectricity and good microwave dielectric properties(?_r=6.55?Q×f=52400 GHz??_f=-24.5 ppm/°C),which had great potential applications in the field of ferroelectric tuning.The phase transition from monoclinic phase to orthorhombic occurred in Ba Zn₂Si₂O₇ceramic at 280°C.Based on the characteristics of the phase transition of the Ba Zn₂Si₂O₇ceramic,it is found that the high temperature phase Ba Zn₂Si₂O₇ has a rare negative thermal expansion,and the permmitivity in the?_r-T spectrum shows a‘step'change at the phase transition temperature point.It provides a new idea for the regulation of CTE and temperature coefficient of resonant frequency(?_f).The mechanism of the transition temperature point of Ba Zn₂Si₂O₇ ceramics was further explored by the?_r-T spectrum.In the case of Ba_1-xSr_xZn₂Si₂O₇ ceramics,the introduction of Sr²⁺will move the transition temperature of Ba Zn₂Si₂O₇ ceramics towards low temperature,and the phase transition‘step'in?_r-T spectrum will become flat.Meanwhile,the introduction of Mg²⁺will move the transition temperature of Ba Zn₂Si₂O₇ ceramics towards low temperature in Ba Zn_2-yMg_ySi₂O₇ceramics.Ba_1-xSr_xZn₂Si₂O₇ ceramics are rare low dielectric microwave dielectric ceramics with dual regulation of thermal expansion coefficient and?_f value.With the gradual substitution of Sr²⁺for Ba²⁺,the CTE of B Ba_1-xSr_xZn₂Si₂O₇ ceramics at room temperature?200°C was adjusted from+6 ppm/°C to-0.9ppm/°C,while the?_f value was regulated from-73 ppm/°C to-289 ppm/°C.Ba_0.4Sr_0.6Zn₂Si₂O₇ ceramic is a microwave dielectric ceramic with a rare NCTE(?_r=8.6,Q×f=8122 GHz,?_f=-321.3 ppm/°C,?_L=-10.0 ppm/°C),which originates from the stretched[Zn O₄]tetrahedral chain to the twisted one along b axis with temperature.Because of its low permittivity and good Q×f value,Ba_0.4Sr_0.6Zn₂Si₂O₇ ceramic can be used as a new type of CTE regulator in low dielectric microwave dielectric ceramics.By studying the relationship between the microwave dielectric properties and the CTE of Zn_1.8Si O_3.8–Ba_0.4Sr_0.6Zn₂Si₂O₇–Ca Ti O₃ system,a new dual regulation method for the thermal stability of microwave dielectric ceramics was proposed.The CTE and?_f value of the main phase ceramics are controlled by introducing the NCTE Ba_0.4Sr_0.6Zn₂Si₂O₇ ceramic and the positive?_f Ca Ti O₃ ceramic.0.95(0.9Zn_1.8Si O_3.8–0.1Ba_0.4Sr_0.6Zn₂Si₂O₇)–0.05Ca Ti O₃has the best comprehensive thermal stability and microwave dielectric properties(?_r=7.32,Q×f=31400 GHz,?_f=-7.4 ppm/°C,?_L=+2.1 ppm/°C).On this basis,a DRA with high thermal stability,wide band width and high gain was fabricated by using SLA ceramic 3D printing technology.The integrated loading of the Luneberg lens on the DRA makes the antenna work in a higher resonant mode and obtain a wider bandwidth.Meanwhile,the aggregation effect of the Luneberg lens on the beam significantly improves the gain of the DRA.The average gain of DRA is 8.06 d Bi at 10.45–11.39 GHz and 10.3 d Bi at 12.27–13.45 GHz.