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Abstract: A novel forming method-aqueous gelcasting was used to prepare 90wt.% (0.75ZnAl2O4–0.25TiO2)–10wt.% MgTiO3 (ZTM) microwave dielectric ceramics and GPS antenna. The effects of aqueous gelcasting and dry pressing on the phase compositions, microstructures and microwave dielectric properties of ZTM ceramics were investigated. The samples’ cracking problem happening in the process of drying and binder removal was successfully overcome. The phase compositions are completely the same no matter what forming method is adopted, but the ZTM ceramics prepared by aqueous gelcasting are denser than that prepared by dry pressing. Fewer pores and more uniform microstructures are observed in the ZTM ceramics prepared by aqueous gelcasting. Therefore, much better microwave dielectric properties are obtained in the ZTM ceramics prepared by aqueous gelcasting. Finally, a GPS antenna was successfully fabricated by aqueous gelcasting using the ZTM material, which meets the requirements of GPS application. [Copyright &y& Elsevier]

Abstract: The effects of aqueous gelcasting and dry pressing on the sinterability and microwave dielectric properties of 90wt.% (0.75ZnAl2O4–0.25TiO2)–10wt.% MgTiO3(ZTM) ceramics have been investigated. It is found that aqueous gelcasting could effectively decrease the sintering temperature of ZTM ceramics by 100°C and acquire more excellent microwave dielectric properties of ZTM ceramics compared with conventional dry pressing. X-ray diffraction (XRD), environment scanning electron microscope (ESEM) and energy-dispersive X-ray spectroscopy (EDX) were used to analyze the phase compositions and microstructures of ZTM ceramics. The results illustrate that the phase compositions are completely uniform no matter what sintering temperature and forming method are adopted. However, the densities, ɛ r and Q × f values are greatly affected by different forming methods, whereas there are few effects on the τ f values. It is observed that ZTM ceramics prepared by aqueous gelcasting exhibit greater densities, more excellent and stable microwave dielectric properties compared with that prepared by dry pressing at the relative low sintering temperatures. However, when the sintering temperature becomes higher, the opposite phenomenon would gradually appear. [ABSTRACT FROM AUTHOR]

Perovskite Ba 0.67 Sr 0.33 TiO 3 (BST) ceramics with improved dielectric performance were fabricated by a novel direct coagulation casting via high valence counter ions (DCC-HVCI) method using 0.5 mol% MgO as the coagulation agent. The influence of sintering temperature on micromorphology, crystal structure, and dielectric performance of BST ceramic components was systematically evaluated. The densification rate of BST samples could reach the maximum at a heating temperature around 1305 °C and end up approximately at 1380 °C. As increasing the sintering temperature from 1400 to 1550 °C, the relative density of BST ceramics decreases from 98.82 ± 0.16% to 96.72 ± 0.06%, and the mean grain size drops gradually from 9.96 to 4.68 μm. The increased sintering temperature promotes the lattice expansion due to Ti4+ replacement at B site by Mg2+, as well as the generation of oxygen vacancies. In addition, as increasing sintering temperature, the dielectric permittivity (ε r) of BST ceramic components decreases first and then slightly increases, while the dielectric loss (tanδ) of ceramics shows the opposite trend. The results indicate that the initial tanδ increase is mainly attributed to the lattice expansion of BST ceramics, and the subsequent loss reduction results from the easily localized conduction electrons on Mg2+ sites reducing the carrier concentration. The samples sintered at 1550 °C show the lowest dielectric loss (tanδ≈0.013), which is significantly reduced by an order of magnitude compared with that of dry-pressed samples (tanδ≈0.262). [ABSTRACT FROM AUTHOR]