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 fe rroelectric properties. Multilayer capacitors provide fairly high capa citance per unit volume (volumetric efficiency); however, processing d ifficulties in the preparation of ultrathin layers limit further enhan cement. Tantalum solid electrolytic capacitors, on the other hand, pro vide very high volumetric efficiencies, because of the large surface a rea of the sintered, porous tantalum anode on which the dielectric Ta2 O5 is electrochemically deposited. Recent developments in electrochemi cal methods to deposit BaTiO3 on titanium substrates provide an opport unity to fabricate barium titanate electrolytic capacitors using sinte red, porous titanium anodes. The high dielectric constant of BaTiO3 an d the high surface area of the sintered, porous anode provide a good c ombination to achieve larger volumetric efficiencies, Current work inv olves the fabrication and characterization of barium titanate electrol ytic capacitors. Effects of electrochemical processing parameters on t he 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 die lectric 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-poros ity (35%-40% of theoretical density) sintered titanium anodes. Samples treated for 8 h in 0.5M Ba(OH)(2). 8H(2)O electrolyte solutions at 10 0 degrees 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 dissipati on factors at low frequencies. Heat treatment at 400 degrees C signifi cantly increases the capacitance at all frequencies, whereas the heat treatment lowers the dissipation factors at low frequencies. Calculate d volumetric efficiencies are comparable to those typically obtained f or tantalum solid electrolytic capacitors but are not as high as expec ted for barium titanate electrolytic capacitors. Penetration of the co lloidal-carbon (external) electrode was limited to a depth of similar to 300 mu m, which might have caused the lower volumetric efficiencies .