Effects of La_2O_3 additions on properties of Ba_(0.6)Sr_(0.4)TiO_3-MgO ceramics for phase shifter applications

摘要

Influence of La_2O_3 additions on the microwave dielectric properties of Ba_(0.6)Sr_(0.4)TiO_3 (BST) mixed with magnesium oxide (omega = 60 percent) composites was investigated. With increasing quantities of lanthanum oxide x (wt percent), 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 La_2O_3 doping. SEM observations show that the BST grain sizes decrease with increasing amounts of La_2O_3. The dielectric properties of BST-MgO-La_2O_3 indicate that La_2O_3 additives shift the Curie point towards lower temperatures. Optimum doping amount of La_2O_3 can reduce the high frequency loss tangents of BST-MgO compound which also can ensure the moderate dielectric constant and tunability. Excessive doping of La_2O_3 tends to significantly lower dielectric constant and deteriorate tunability. When the doping amount of La_2O_3 is 0.2 wt percent, 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 percent (under electric field 3.57 kV/mm), which is suitable for ferroelectric phase shifter.