Preparation and dielectric properties of Mn-doped Ba0.6Sr0.4TiO3-MgTiO3 composite ceramics

Barium strontium titanate (Ba0.6Sr0.4TiO3, BST) nano-powders were prepared using Ba(NO3)(2), Sr(NO3)(2), oxalic acid dehydrate, and tetrabutyl titanate (Ti(OC4H9)(4)) as precursors by the chemical co-precipitation method. The product was characterized by thermogravimetry-differential scanning calorimetry (TG-DSC) thermal analyses, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The experimental results indicated that the resulting Ba0.6Sr0.4TiO3 nano-powders were homogeneous with agglomerated nature. The Ba0.6Sr0.4TiO3-MgTiO3 (BST-MT) bulk composite ceramics doped by Mn were obtained by the traditional solid phase method. The XRD patterns demonstrated that Mn-doped BST was unable to change the perovskite crystalline structure of BST materials. SEM photographs revealed that the crystalline grains became larger with increasing the content of doping Mn (<1.5% (x, molar fraction)) and then the size of grains decreased after the Mn content exceeded 1.5% in the BST ceramics, suggesting the effect of Mn doping on the morphologies of BST-MT composites. The dielectric properties of BST-NIT composite ceramics doped with 0.1%-2.0% (x) Mn were investigated systematically. Two effects of Mn doping on the dielectric properties of the BST-MT composite ceramics were observed. At low Mn doping concentrations (<1.5%), Mn mainly acted as an acceptor dopant to replace Ti at the B site of ABO(3) perovskite structure, leading to a diffused phase transition. It was also observed that the grain size increased drastically as the Mn content increased and thus caused the decrease of dielectric loss. At higher Mn doping concentrations (>1.5%), the grain size decreased and the suppression of permittivity and the drastic increase of the dielectric losses were observed, which indicated a "composite" mixing effect.