Preparation Of Barium Titanate Nanopowders And Dielectric Ceramics Using Organic Acid Assistant

BaTiO₃ with a perovskite structure is noteworthy for their exceptional dielectric, piezoelectric, electrostrictive, and electrooptic properties with corresponding electronics applications, and is mostly used to make multilayer ceramic capacitor (MLCC) materials. Sol-gel process has been intensively studied, because it is so effective to give ceramic powders of high purity, small size and good homogeneity. But the BaTiO₃ nanopowders synthesized by sol-gel process however are easy to form agglomeration. Therefore, in this paper, better dispersed BaTiO₃ nanopowders and ceramics with well dielectric properties were prepared by sol-gel process using undersaturated organic acid, organic monoacid and diacid assistant. Moreover, Ni, Ag, and Fe₃O₄ nanoparticles were synthesized by hydrothermal or solvothermal method using oleic acid (OA) as surfactant. And nanosized Ag doped BaTiO₃-based ceramic with high dielectric constant were prepared. Furthermore, on the base of our original X7R materials, Ag-doped X7R BaTiO₃-based ceramic with high dielectric constant were prepared.1. BaTiO₃ powders were prepared by sol-gel process using OA as surfactant, and the powders were all nanometer scale with a particle size ranging from 20 to 50nm. With the increase of the concentration of OA, the dispersibility of the powders was improved. After sintering, the ceramics were completely consisted of the tetrahedron BaTiO₃ phase. The relative density of the ceramic ([Ti(OBu)₄]/[OA] ratio of 1:1) calcined at 900℃for 2h and sintered at 1250℃for 2h was 98.5%, room temperature permittivity reached 4569, and the dielectric loss was 0.027.2. BaTiO₃ powders were prepared by sol-gel process using five kinds of organic acid as surfactant, and the powders were all nanometer scale with a particle size ranging from 20 to 55nm. As the length of carbon chain of the organic acid increases, the size of the powders first increases and then decreases, and the grain size and the relative density of the ceramics increased except the ceramic prepared using stearic acid as the surfactant. Specially, the powders prepared using dodecanoic acid as surfactant had the minimum size, the best dispersibility, and the shape tended to be spherical, and the as-prepared ceramic was fairly uniform and of better grain size (1.05μm). The permittivity of the ceramic reached 4319 and dielectric loss was 0.022.3. BaTiO₃ powders were prepared by sol-gel process using two kinds of organic diacid (hexanedioic acid and decanedioic acid) as surfactant, and the powders were all nanometer scale with a particle size ranging from 50 to 65nm. The relative density of the ceramic prepared using decanedioic acid as the surfactant was 97.1% and the room temperature permittivity reached 3270. Compared with monoacid, as the carboxyl number increases, the size of the powders obviously increased and the grain size, the relative density and the whole permittivity of the as-prepared ceramics also increased. As the length of carbon chain of the diacid increased, the grain size decreases slightly and the relative density of the ceramics increased. Moreover, compared with OA, the size of the powders prepared using diacid as surfactnat was bigger, and the relative density w as lower. The bigger original particle size was resulted from the bigger "organic acid micro-capsules" which was more stable and could contain more BaTiO₃ precursor. After calcining, the structure is preserved to produce bigger original particle. The bigger the original particle is, the bigger the agglomeration4. Uniform spherical monodispersed Ni, Ag and Fe₃O₄ nanoparticles were synthesized by hydrothermal or solvothermal method. No phases other than BaTiO₃ and Ag were present in nanosized Ag-doped ceramics, which suggersted that no reaction took place between Ag and BaTiO₃. The phase of silver metal can be observed when the content of Ag was higher than 5mol%. The relative density of the ceramics increased first and then decreased, finally increased again. The dielectric constant increased all the time and the dielectric loss first decreased and then increased. When the content of silver is 1mol%, the ceramic had the better dielectric properties. 5. X7R BaTiO₃ ceramics with different content of Ag has been prepared by the traditional solid-state method. No reaction took place between Ag and BaTiO₃ during sintering process, and silver metal dispersed uniformly in the ceramics. A small amount (<0.5mol%) of silver can improve the relative density and dielectric constant of X7R BaTiO₃ ceramics; larger addition (0.5～2mol%) of silver may decrease it; then addition of more silver (>2mol%) can increase theε_r(room temperature dielectric constant) greatly. The silver doping can't directly affect the TCC (Temperature Coefficient of Capacitance) characteristics, and can't obviously affect the dielectric loss. When the content of silver is 0.5mol%, the ceramic had the better dielectric properties.