Structural evolution,sintering behavior and microwave dielectric properties of Al(1-x)(Si0.5Zn0.5)xPO4 ceramics

Al_(1-x)(Si_0.5Zn_0.5)_xPO₄ (0 ≤x≤0.6) ceramics were prepared via solid state reaction process. Their structural evolution, sintering behavior and microwave dielectric properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and with a network analyzer. Phase pure AlPO₄ could not be obtained for the undoped composition. However, microscopically homogeneous single phase solid solution formed within the compositional range of 0 < x ≤ 0.05. Immiscibility with two-phase structure within apparently single phase solid solution could be observed for other doped compositions (0.1 ≤ x ≤ 0.6). Small level of doping with (Si_0.5Zn_0.5)³⁺ (x≤0.4) stabilized the orthorhombic cristobalite-like AlPO₄ (C-AlPO₄) phase; while further doping led to the transformation from orthorhombic α-C-AlPO₄ into cubic β-C-AlPO₄ phase. The doping with (Si_0.5Zn_0.5)³⁺ considerably improved the sinterability and reduced the sintering temperature to ?900 °C when x = 0.6. The dielectric permittivity slightly increases and the Q × f value decreases with the increasing doping concentration. All composition demonstrate low permittivity (?_r ?4) and negative value of τ_f. Good combined microwave dielectric properties with ?_r ?3.9, Q × f?25,000 GHz and τ_f ? -25 ppm/^oC could be obtained for the x = 0.2 composition after sintering at 1200 °C/2h.