The effect of Ag electrode processing on (Nb, Ba) doped TiO 2 ceramics
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Since the characteristics of the electrode made from Ag paste greatly affect the dielectric properties of (Nb, Ba) doped TiO2 ceramics, the processing condition, i.e., baking temperature, was investigated. Low melting glass binder contained in Ag paste reacted with TiO2 ceramics to form an interface layer between Ag electrode and TiO2 ceramics during baking. The interface layer was identified as Bi2Ti2O7 by x-ray diffraction (XRD), and the thickness of the Bi2Ti2O7 layer was estimated from line profiles of EPMA and dielectric properties. The interface layers were found to increase with baking temperature. Increased baking temperature lowered the relative dielectric constant and dielectric dissipation factor of TiO2 ceramics, while it raised the resistivity. Controlling the baking condition of the Ag paste electrode on TiO2 ceramics resulted in reasonably good dielectric properties and excellent temperature stability.
I. INTRODUCTION 1
Yan and Rhodes reported ultrahigh capacitance by doping Ta (or Nb) and Ba into TiO2 ceramics. Space charge polarization occurring in regions of boundary segregation layers was thought to be the reason for the ultrahigh capacitance. Wu and Chen2"4 studied the effects of Ba and Nb on the dielectric properties of TiO2 ceramics and concluded that commercial Ag paste electrodes formed nonohmic contacts with «-type TiO2 ceramics because the low melting glass binder contained in Ag paste might act as a barrier to conduction electrons. How the electrode-ceramics interface acted as such a barrier was not yet clear. The present paper examines the electrode-ceramics interface in detail with electron probe microanalysis (EPMA), x-ray diffraction (XRD), and scanning electron microscopy (SEM). The baking temperature of the Ag paste electrode is found to be a crucial parameter. II. EXPERIMENTAL PROCEDURES
The preparation of TiO2 powder doping with Ba and Nb started with reagent grade chemicals like TiCl4 (Merck), NbCl5 (Merck), and BaCl2 2H2O (Pipette). The chemicals were dissolved into clear solution, respectively, and then mixed together with proper ratio. Powders were coprecipitated by dropping the mixed solution into a coprecipitation bath containing (NH4)2CO3 and NH4OH.4 Molar ratios of dopants to TiO2 were Ti :Ba:Nb = 100:1:0.25. After the coprecipitated powders were furnished, they were calcined either at 950 °C or 600 °C for 1.5 h. The phase of powder calcined at 950 °C was rutile, and that calcined at 600 °C was anatase. The powders were ball-milled with ZrO2 1530
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 7, Jul 1990
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balls in de-ionized water in a plastic jar for 3 h with addition of 1.0 wt.% polyvinyl alcohol. After ballmilling, the powders were dried, pulverized, and forced to pass through a #120 sieve for granulation. The granulated powder used was 0.3 g per sample and was then dry pressed at about 160 MPa. The diameter of all green compacts was 10 mm, and the thickness was 1.48 mm and 1.80 mm for the rutile compact and anatase compact, resp
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