Preparation and characterization of barium titanate electrolytic capacitors from porous titanium anodes

BaTiO₃ is widely used as the dielectric in ceramic chip capacitors and multilayer capacitors, because of its high dielectric constant and ferroelectric properties. Multilayer capacitors provide fairly high capacitance per unit volume (volumetric efficiency); however, processing difficulties in the preparation of ultrathin layers limit further enhancement. Tantalum solid electrolytic capacitors, on the other hand, provide very high volumetric efficiencies, because of the large surface area of the sintered, porous tantalum anode on which the dielectric Ta₂O₅ is electrochemically deposited. Recent developments in electrochemical methods to deposit BaTiO₃ on titanium substrates provide an opportunity to fabricate barium titanate electrolytic capacitors using sintered, porous titanium anodes. The high dielectric constant of BaTiO₃ and the high surface area of the sintered, porous anode provide a good combination to achieve larger volumetric efficiencies. Current work involves the fabrication and characterization of barium titanate electrolytic capacitors. Effects of electrochemical processing parameters on the formation of BaTiO₃ on the surface of sintered titanium anodes are described. Influence of the purity of titanium powder, the porosity of the sintered anode, and the post-deposition heat treatment on the dielectric properties of the fabricated capacitors is discussed. Complete penetration of the electrolyte solution and a thin uniform coating of TaTiO₃ over the entire titanium surface was achieved using high-porosity (35%-40% of theoretical density) sintered titanium anodes. Samples treated for 8 h in 0.5M Ba(OH)₂·8H₂O electrolyte solutions at 100°C with an applied cell voltage of 12 V show the formation of a dense, uniform BaTiO₃ coating on the surface of the titanium anode. High-purity, chloride-free titanium powder provides smaller dissipation factors at low frequencies. Heat treatment at 400°C significantly increases the capacitance at all frequencies, whereas the heat treatment lowers the dissipation factors at low frequencies. Calculated volumetric efficiencies are comparable to those typically obtained for tantalum solid electrolytic capacitors but are not as high as expected for barium titanate electrolytic capacitors. Penetration of the colloidal-carbon (external) electrode was limited to a depth of ∼300 μm, which might have caused the lower volumetric efficiencies.