KaohsiungTo investigate the effect of reoxidation on the grainboundary acceptor-state density of reduced barium titanate, n-doped BaTiOa ceramics are sintered in a reducing atmosphere (2% H2 + 98% N2) and then annealed in oxygen.After annealing at 1150°C for different times, the experimental results show a relationship between temperatureaveraged acceptor-state density and annealing time as N, = N,,Bt'/" with n between 2 and 3. An inherent acceptorstate density N,, = 4.25 X 10" cm-' is obtained with an increase rate B = 4.8 X 10" cm-2-min-1/3, when n reaches 3. The inherent grain-boundary acceptor states in the reduced n-doped BaTi03 ceramics are believed not due to adsorbed oxygen ions. [

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Barrier layer capacitors, by introducing thin CuO layer at the grain boundaries of semiwnducting (Ba Ti )O cermnics, are fabricated and stu%g@z?der&? dthcds are adopted in the fabrication of the barrier layer capacitors. The first is a "single-firing" method by segregating CuO into the grain boundaries. The second method is a "double-firing" method, in which the CuO is diffused fran the surfaces of the semiconducting cermics at tigh temperature. The third method is also a "double-firing" process. However, the CuO is "carried" fran the surface into the grain boundaries by a low-melting ccnposition at lower t a perature. The characteristics of the fabricated capacitors, by different methods, are canpared and studied. I m u c r I mBarrier layer capacitor ( B E ) is one of the barrier layer devices which are based on the uses of barrier layer exists between two adjacent grains.Il1The barrier layer is formed by alternating the stoichiometry at the grain boundary or segregating a different phase there.[2,3] Barrier layer capacitor keeps electrical energy in the dielectric layer, i.e., barrier layer. Since the thickness of barrier layer is very thin, high capacitance density can be achieved in a small volume.[4] The first cannercia1 barrier layer capacitor was described in a 1950 patent.[5] A detailed description of the production in the first ccmnercialization of grain boundary barrier layer capacitor was given by Waku. [6] In this Study, three different methods are adopted in the fabrication of barrier capacitors. The semiconducting ceranics with low grain resistivity are Nb 0 -doped (Ba Sr )(Zr, lTibnp2 (abbreviated as BS~T? hsulatllig p o be introduced into the grain boundaries is CuO. The first method is a "single-firing" process, i.e., segregating insulating phase in the grain boundaries. [7,8] The second method used in fabricating barrier layer capacitor is a "double-firing" process. Where CuO is diffused fran the surfaces of BSZT into its grain boundaries at temperature higher than the melting point of CuO. The third method is so-called carrier-assisted diffusion (C.A.D.) since CuO is carried into the grain boundaries of BSZT from its surfaces at tenperature lower than the melting point of CuO.[9] The electrical and dielectric properties of the fabricated barrier layer capacitors are studied and ccmpared to understand the advantages and disadvantages of each method. EXF'ERlMENTAL PROCEDURES Single-Firing MethodSelection of the c a p sition : Taking advantages of both bariun titanate and strontim titanate, (Ea Srl-x)TiO was selected as an initial cumsition forXbarrier dyer capacitor. To increase the stability of dielectric constant against biasing field and reduce the loss tangent at low frequency, Zr02 y s also added.[lo] The carposition (bo 8Sr0.2)(Zr finally reached to make it close to ?he morphortropic phase bamdary for optimal electrical properties.[lll To -rave the semiconducting properties of BSZT ceramics, 0.2 mol% Nb 0 was added, and 1 m1% Ti0 was also added as sin?e?ing promoter. The addition 8f Nb205...

Based on the oxygen-vacancy diffusion, a core-shell model of grain resistance is proposed, for the reduced La-doped (Ba, Sr)TiO3 ceramics, to describe the charge-carrier distribution. Complex impedance analysis is adopted to determine the grain resistance and grain boundary resistance. After sintering at 1400°C in reducing atmosphere, the low grain resistance is attributed to the effective electron compensation in the grain core, while the higher grain boundary resistance is thought to be due to the lower charge-carrier concentration in the grain shell. In addition, reoxidation leads to further decrease of charge-carrier concentration on the grain shell by diffusing inward to the grain core.