Novel high dielectric constant and low-loss tetragonal tungsten bronze dielectrics Ba_5LnMgNb_9O_(30) (Ln=La, Nd, Sm, Gd and Yb)

摘要

Five novel dielectric ceramics Ba_5LnMgNb_9O_(30) (Ln=La, Nd, Sm, Gd and Yb) were synthesized using a conventional solid-state reaction method. The phase structure, microstructure and dielectric properties of the prepared ceramics were investigated. Results show that the ceramics exhibit tetragonal tungsten bronze structure with fully occupied A-sitcs. Only tungsten bronze structure can be observed in the ceramics of which In refers to La, Nd, Sm and Gd, while tungsten bronze structure and YbNbO_4 were found in the ceramic of which Ln refers to Yb. All the ceramics exhibit high dielectric constant (ε_r=272~429), with the ceramics of which Ln refers to La, Nd, Sm and Yb possessing low dielectric loss with the order of 10~(-3) at 1 MHz. By decreasing the rare-earth ionic radius, the transition temperature increases from -114 °C to 45 °C and then decreases to -58 °C, while the maximum dielectric constant increases to 460 at Ln=Sm and subsequently decreases. The phase transition is a diffuse phase transition (DPT) in the ceramics, while the diffuse exponent y increases gradually as the ionic radius of rare earth element decreases from that of La~(3+) to that of Gd~(3+), and decreases thereafter. The temperature coefficients of dielectric constant of the ceramics with Ln=La, Nd, Sm and Yb are -1229 ppm/°C, -1542 ppm/°C, -1171 ppm/°C and -1067 ppm/°C respectively in the range of 20-80 °C at 1 MHz.