Effects of La2O3 additions on properties of Ba0.6Sr0.4TiO3-MgO ceramics for phase shifter applications

Influence of La2O3 additions on the microwave dielectric properties of Ba0.6Sr0.4TiO3 (BST) mixed with magnesium oxide (w = 60%) composites was investigated. With increasing quantities of lanthanum oxide x (wt%), the lattice constant of BST-MgO material decreases to a minimum at x = 0.4 and then increases up to a maximum at x = 1.0. The XRD patterns analysis reveal that the solubility of MgO in BST is influenced by the amount of La2O3 doping. SEM observations show that the BST grain sizes decrease with increasing amounts of La2O3. The dielectric properties of BST-MgO-La2O3 indicate that La2O3 additives shift the Curie point towards lower temperatures. Optimum doping amount of La2O3 can reduce the high frequency loss tangents of BST-MgO compound which also can ensure the moderate dielectric constant and tunability. Excessive doping of La2O3 tends to significantly lower dielectric constant and deteriorate tunability. When the doping amount of La2O3 is 0.2 wt%, BST-MgO composite has the following properties: dielectric constant = 94.05, tan delta = 0.012 (at 2.853 GHz) and the dielectric constant tunability = 16.26% (under electric field 3.57 kV/mm), which is suitable for ferroelectric phase shifter. (c) 2005 Elsevier Ltd. All rights reserved.