Investigation On The Structure And Dielectric Properties Of Barium Strontium Titanate Microwave Ferroelectric Materials Doped With Magnesia

As a microwave ferroelectric material, Barium strontium titanate (BST) has characteristics such as proper dielectric constant, small dielectric loss and good tunability. BST has the potential applications in the fields of photoelectricity information and microwave communication. In order to meet the requirements of different applications, the microwave dielectric properties of BST should be improved. Recently, investigation of BST doped with oxides has been developed fast.Microwave ferroelectric ceramic Ba_0.6Sr_0.4TiO₃ doped with different amount of magnesia (MgO) were prepared by two different doping methods. The microscopic structure characteristic, phase distribution, and micro-area ingredient of the composites were investigated by means of SEM, EPMA and XRD, and the dielectric properties of the samples were tested. The mechanism of the improvement of MgO-doped BST was also discussed.The two doping methods were direct mixture and position substitution. The measurement results showed that, when we used the doping method of mixing directly, Mg²⁺ did not appear in the BST crystal domain by substituting. Meanwhile, the dielectric constant and dielectric loss of material were both reduced. The main reason is that MgO is not a ferroelectric material and has low dielectric properties; When we used the doping method of position substitution, it could not find the structure of ion substitution as assumed, and MgO still presented in the composites as a separate phase. But we found a new compound appearing in the material and it approached the (Ba,Sr)TiO₄ of rhombic on the structure. With the amounts of MgO increasing, the dielectric constant of samples that we fabricated by the mean of position substitution was reduced, but the dielectric loss increased. The enhancement of loss could be explained as that the new compound caused the structure of ceramics to get bad such as the density reduced, the hole and the lacuna increased.From the experimented results of the two methods, it was found that Mg²⁺ did not appear in the crystal domain of BST of cubic structure. This was different from the popular view in literatures that Mg²⁺ could substitute Ba²⁺, Sr²⁺ or Ti⁴⁺.