SURFACE DECONTAMINATION TREATMENTS OF UNDOPED BATIO3 .1. POWDER AND GREEN BODY PROPERTIES

Five commercial barium titanate powders were subjected to various surf ace decontamination treatments. calcination at 500 degrees C, followed by leaching either in pure water or acetate buffer solution. They wer e then de-agglomerated by ultrasonic treatment and finally characteris ed by XRD, TGA, XPS, EGA, pyrolysis/FT-IR, BET specific surface area a nd particle size analysis. The main contaminating elements in raw powd ers were carbon and sulphur, present as barium carbonate (between 0.7 and 1.2wt%) and barium sulphate (about 0.5 wt%). Barium sulphate is th ermally stable up to 1400 degrees C and is not affected by the treatme nts. Barium carbonate decomposes in two steps, between 500 and 900 deg rees C and 1200 and 1400 degrees C by reaction with TiO2 and BaTiO3 to release CO2. Carbonation is slightly reduced by calcination, but subs equent water cleaning promotes re-carbonation. Acid cleaning substanti ally decreases the BaCO3 content, down to 0.07-0.7 wt%. The remaining quantity corresponds to the most stable form which decomposes near 120 0-1400 degrees C and is still partially present at the surface of the particles. Forming of ceramics green bodies by slip casting of aqueous suspensions stabilised by ammonium poly(acrylate) was then investigat ed. The dispersing solution compositions have been optimised for each powder and treatment, in order to minimise the liquid/solid ratio of t he slip. In all cases, the properties of the slips and of the cast spe cimens have been improved by the cleaning treatments. higher solid vol ume fractions in the slip up to 60% for the best case and higher green densities. rn particular green densities as high as 70% have been obt ained which is very close to the theoretical maximum packing density o f log-normally polydispersed spheres. This enhancement can be ascribed to the beneficial effect of decarbonation on the deagglomeration of t he powders.