MgWO4, ZnWO4, NiWO4 and CoWO4 microwave dielectric ceramics

AWO4 (where A = Zn2+, Mg2+, Ni2+ and Co2+) tungstate ceramics were synthesised from a standard ceramic route, and their sintering behaviour and microwave dielectric properties were investigated. All AWO4 powders formed single-phase materials, except MgWO4, and they all had a wolframite-like monoclinic P2/c (13) structure, not the tetragonal scheelite structure found in CaWO4. MgWO4 had a poor Qf which deteriorated with further sintering, due to discontinuous grain growth at relatively low temperatures of 1000 °C and over. At higher temperatures, ZnWO4, NiWO4 and CoWO4 have high Qf values at 1200 °C of 62,800, 24,900 and 38,600 GHz, respectively, which will all almost certainly be improved with firing at higher temperatures, particularly in the case of CoWO4. Although not at all well sintered, ZnWO4 and CoWO4 have high Qf values of 34,000 and 28,900 GHz, respectively, at temperatures below the LTCC limit.