Effect of NdAlO3 on microstructure, dielectric properties and temperature-stable mechanism of (Sr, Ca, Nd)TiO3 ceramics at microwave frequency

Abstract

Microwave dielectric ceramics (1 − x)(Sr0.3Ca0.427Nd0.182)TiO3–xNdAlO3 (abbreviated as SCNTAx hereafter, 0.1 ≤ x ≤ 0.4) were prepared by conventional mixed oxide route, and their phase composition, microstructure and microwave dielectric properties were investigated as a function of the x value and sintering temperatures. A single tilted orthorhombic perovskite structure in space group Pnma was refined in the studied composition range. For microware dielectric properties, the decreasing relative permittivity was strongly affected by the ionic polarizability of Nd3+ and Al3+ in SCNTAx ceramic systems. Also, the quality factor of SCNTAx solid solution had strongly depended on apparent densities and average grain sizes. As expected, the promising ceramic of SCNTAx (x = 0.25) sintered at 1520 °C for 4 h was found to possess good microwave dielectric properties: a relative permittivity (er) of 55.6, a quality factor (Q × f) of 25,600 GHz (at 4.249 GHz) and a temperature coefficient of resonant frequency (τf) of 6.7 ppm/°C. Especially, the τf values of SCNTAx ceramics were not strongly depended on tolerance factor (t) with increasing of the NdAlO3 content, while these τf values were essentially correlated with the B-site bond valence and octahedral tilting. Wherein, either decreasing the B-site bond valence or increasing the octahedral tiltings (θ and φ) led to a decrease in τf value for the present ceramics.