Composition Design,Preparation And Bending Strength Optimization Of Aluminate Microwave Dielectric Ceramics

Microwave dielectric ceramics are the key basic materials for modern communication technology.The rapid development puts forward higher requirements for their performances,such as higher Q×f value.In addition,the production process of cutting process and fracture failure due to the miniaturisation of increasingly thin ceramic pieces will produce loss of ceramic samples,therefore,the strength of microwave dielectric ceramics also have more urgent requirements.In this paper,aluminate microwave dielectric ceramics were chose as the objects.Firstly,Ca Ti O₃-La Al O₃ and Mg₂Ge O₄-Mg Al₂O₄ were designed and synthesized.Furthermore,the phase structure,microscopic morphology,microwave dielectric properties and flexural strength of ceramics were systematically and thoroughly analyzed.（1）0.65Ca Ti O₃-0.35La Al O₃（CTLA and CTLAs）microwave dielectric ceramics were prepared by a solid state reaction method using different Al₂O₃（normal Al₂O₃ powder:α-Al₂O₃ crystal type;acidic Al₂O₃ powder:γ-Al₂O₃ crystal type）as the raw materials.XRD shows that the ceramics are both cubic chalcocite structures,and the CTLA ceramics sintered at 1475°C are closer to zero withτ_f=-0.61 ppm/°C than that of the CTLAs ceramics withτ_f=18.19 ppm/°C,with flexural strengths of214 MPa and 277 MPa respectively.The Si₃N₄ enhancement increases the flexural strength of the ceramic by up to 389 MPa,but the microwave dielectric properties of the ceramic were rapidly deteriorated,as x=1wt%Si₃N₄,the ceramics had the microwave dielectric performances ofε_r=38.62,Q×f=12928 GHz,τ_f=-4.21ppm/°C.（2）The effects different shape Al₂O₃（normal granular and whisker-like）on the flexural strength and microwave dielectric properties were investigated and compared.The results shown that the Al₂O₃ whisker enhances the flexural strength of the ceramics more effectively than normal granular Al₂O₃.The maximum flexural strength of ceramics doped with Al₂O₃ whiskers was 414 MPa,increasing 93.48%that of pure CTLA ceramics,while the maximum bending strength of Al₂O₃ particles was 347MPa.However,both doped Al₂O₃ particles and Al₂O₃ whiskers would deteriorate the microwave dielectric properties of ceramics,while the degree of deterioration of the Al₂O₃ whiskers was more than Al₂O₃ particles.CTLA+3wt%Al₂O₃ had a Q×f value of up to 35764 GHz and CTLA+3wt%Al₂O₃whisker had a Q×f value of up to 32986GHz.（3）Although the addition of Al₂O₃ was effective in improving the flexural strength of the ceramics,it also deteriorated the microwave properties of the ceramics.Therefore,the microwave dielectric properties of CTLA ceramics were improved by doping with the rare earth oxide Ce O₂.The CTLA+0.5 wt%Ce O₂ ceramic sintered at1525°C was found to have improved microwave performance ofε_r=40.87,Q×f=49207.02 GHz,τ_f=-1.74 ppm/°C,along with an high flexural strength of 474 MPa.（4）In order to solve the problem of high sintering temperature of CTLA ceramics,the Ba Cu（B₂O₅）（BCB）was added to reduce the sintering temperature of the ceramics.Although BCB could reduce the sintering temperature of the samples,it also severely deteriorates their microwave properties.CTLA+2wt%BCB ceramics sintered at 1250°C had the microwave dielectric properties ofε_r=35.17,Q×f=6470.74 GHz,τ_f=1.17 ppm/°C and flexural strength of up to 259 MPa.（5）Mg₂Ge O₄-Mg Al₂O₄ ceramics were designed and synthesised to obtain aluminate microwave dielectric ceramics with excellent microwave dielectric performances.The samples were experimentally confirmed to be composite structures,where Mg₂Ge O₄ is olivine structure and Mg Al₂O₄ is spinel structure.The composite ceramic has excellent microwave properties ofε_r=8.0,Q×f=150,000 GHz,τ_f=-34ppm/°C and a flexural strength of 275 MPa.