New System Of Low Loss Microwave Dielectric Ceramics With ABCO₄ Structure

The rapid developments of microwave communication systems require dielectric materials with high performances. The purpose of this work was to search low loss microwave dielectric ceramic new system. The excellent microwave dielectric ceramics were developed in ABCO₄ oxides with K₂NiF₄ structure, and the relation between microwave dielectric characteristics and structure were investigated.Single-phase SrRAlO₄(R=Nd and Sm) ceramics were prepared by a solid state reaction method, and they could be densified at 1450¹475℃ in air for 3 hours. Obvious grain growth with increasing sintering temperature was observed, and it decreased the amount of grain boundary and contributed to the significant increase of Qf value. Good microwave dielectric properties were achieved: 1) s=17.8, Qf=25,700GHz, τ_f=4ppm/℃ for SrNdAlO₄ ceramics; 2) s=18.8, Qf=54,880GHz, τ_f= 4ppm/℃ for SrSmA104 ceramics.Good sintering character was also observed in CaRAlO₄(R=Y, Nd and Sm) ceramics, however, some inhomogeneous areas and small amount of second phase were observed in CaYAlO₄ and CaSmA104 ceramics. The dielectric constant of CaRA1O₄(R=Y, Nd and Sm) ceramics could be well interpreted by Clausius-Mosotti (C-M) equation, while the changes of Qf values and τ_f values were related to the stretching and compressing of octahedral in K₂MF₄ structure: Rietveld results showed that the stress in octahedral of CaSmAlO₄ ceramics was the smallest, which ensured its largest Qf value comparing to CaYAlO₄ and CaNdAlO₄ ceramics. Meanwhile, the space in octahedral decreased from CaNdAlO₄ to CaYAlO₄, which reduced the vibration space for Al³⁺ . As a result, CaNdAlO₄ had the largest τ_f value, followed by CaSmAlO₄, and CaYAlO₄ had the smallest τ_f value. Good microwave dielectric properties were obtained in CaRAlO₄ system: 1) ε= 18.2, Qf = 17,980GHz, τ_f =-52ppm/℃ for CaNdAlO₄ ceramics; 2) ε = 18.2, Qf= 51,060GHz, τ_f =-3 ppm/℃ for CaSmAlO₄ ceramics; 3) ε = 18.9, Qf = 39,960GHz, τ_f = 6ppm/℃ for CaYAlO₄ ceramics.The relative tolerance factor difference between ABCO₄ and ABO₃ structure,△t% could be a criteria to judge the stability of K₂NiF₄ structure. If △ t% > 4.3%, the K₂NiF₄ structure was unstable, such as CaLaAlO₄ and SrYAlO₄ ceramics. To further investigate and improve the stability of CaLaAlO₄, the (Ca_1-xSr_x)LaAlO₄ ceramics were prepared and characterized. With increasing x values, the ceramics became more stable. When x=0.95, only small amount of Ca₃Al₂O₆ second phase was observed, and good microwave dielectric properties was achieved: ε=16.7, Qf=28,171GHz, τ_f=-52.4ppm/℃.