Factors affecting the fluidity of oxide powders during heating were studied using an apparatus built after the model of a rotating viscometer.
A propeller made of clay refractory was suspended by a torsion spring to put into a revolving cup containing loose powders which were heated up to about 1100° at a rate of 10-12°C/min. under constant motion. The torque exerted upon the propeller, which is in inverse proportion to the fluidity of powders, was measured in frequent intervals by the degree of twist of the spring. The speed of rotation of the cup was 30rpm. The samples were silica, alumina, the mixture of alumina and chromium oxide, aluminium hydroxide for chromatography, clay, calcium carbonate, and calcium sulphate.
The torque seemed to be affected by the following factors arranged in decreasing order of influence:
(1) The evolution of gases such as water vapour, carbon dioxide, and sulphur dioxide generated by the vaporization of adsorbed water or by the thermal decom-position of particles. In this case the decrease of torque will take place due to the loosening of packing caused by the efflux of gases.
(2) The removal of water film from the surface of particles, which gives rite to the formation of electrical double layers leading to the increase of the repulsion force between the particles (W. A. Weyl, Ceramic Age, 60, Nov., 23, 1952).
(3) The thermal motion of the ions at the particle surface with increasing temperature, which brings the powders to sintering. Changes of the torque caused by this effect, however, is complex because of the peculiar agglomeration of the particles in the container.
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