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Microstructural Development and Properties of (1-x) wt% Ba0.55Sr0.45TiO3-x wt% MgO Bulk Ferroelectrics Costas G. Fountzoulas Weapons and Materials Research Directorate, Army Research Laboratory, APG, MD, 210055069. ABSTRACT A study was made of the relationship between the microstructure and electrical properties of the bulk ferroelectric composite materials: (100-x) wt% Ba0.55Sr0.45TiO3-x wt% MgO. The ferroelectric composites (x= 40, 60) were fabricated by dry pressing from two powder precursors: 1) Ba0.55Sr0.45TiO3 (BST) calcined in the solid state with sol-gel MgO (SgMgO), and 2) hydrothermal BST and MgO calcined in the solid state. The pellets were sintered at 1250, 1350 and 1450oC. The sintered microstructure, chemistry, and phase with crystallinity was characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray diffraction (XRD), respectively. The density and porosity were determined by the Archimedes technique. Dielectric properties of permittivity and loss tangent were characterized by the parallel plate capacitor method. INTRODUCTION Ferroelectrics are multicomponent materials with a wealth of interesting and useful properties, such as being piezoelectric. These properties derive from their noncentrosymmetric crystal-lattice structure in which spontaneous polarization is observed. The most widespread and potentially important use for ferroelectrics exists in the area of nonvolatile random access memories (NVRAM). As the ferroelectric device elements become smaller and smaller with the dimension of a ferroelectric in the submicrometer range and even lower, the properties become size dependent and the grain size effect should be taken into consideration in order to optimize the properties [1]. The grain size dependence of the dielectric permittivity in barium titanate ceramics [2] and thin films [3] is known for many years. The critical materials parameters for many microwave device designs are: low dielectric constant, high tunability, low dielectric loss tangent and small temperature coefficient of permittivity. However, ferroelectric materials are inherently lossy. To improve the loss tangent of ferroelectrics, investigators at the Army Research Laboratory (ARL) have developed an innovative approach, where the ferroelectric material (BaxSr1-xTiO3) is mixed with a low loss temperature stable dielectric such as MgO [4, 5, 6]. In addition, the MgO acts as a grain growth inhibitor, thereby eliminating exaggerated grain growth and narrowing the grain size distribution. This results in more uniform microstructure, which improves the temperature stability and dielectric loss characteristics [7]. Another important parameter for composite systems is the connectivity between the two phases. In particular, (Ba, Sr)TiO3/MgO systems containing more than 40% MgO begin to suffer from electrical isolation, thus limiting and ultimately eliminating the tunability of the composite [5]. The connectivity between the ferroelectric grains can be significantly influenced by the processing pr