What can you say about the shapes and distribution of particle sizes generated by the two methods? Can you explain any of your observations?
The first thing to appreciate is that the two images were not on the same scale. The ‘cast and crush’ was at shown at approximately twice the magnification of the ‘gas atomisation’ sample. An assessment can be made by eye but for convenience two sections of the images are shown here at the same scale.
Clearly the two samples are very different. The cast and crush is very angular whilst the gas atomised is composed almost entirely of spheres. The reason for this is in the production process. In cast and crush the solid catalyst is broken down by crushing and grinding. As this happens the solid fractures along lines of weakness, frequently crystalline grain boundaries. This naturally produces angular fragments. In the gas atomisation process tiny quantities of material are split off from the stream of molten catalyst by jets of high pressure gas. As they are still liquid they form into approximately spherical droplets which then solidify.
The cast and crush process seems to generate a greater variation in the particle sizes than does gas atomisation. It also seems to generate both the largest and the smallest particles.
Could the method of production have any affect on the effectiveness of the catalyst?
It was stated earlier that the cooling rate affects the phases of nickel aluminides present in the catalyst. The cooling rate of the gas atomisation process will be far higher than the cast and crush method as heat can only be lost through the surface and small particles have a far higher surface area to volume ratio. The original picture shows that the cast and crush sample was cast as a cylinder several centimetres thick whilst the gas atomised material forms droplets less than 50 μm (1/20th mm) in diameter. You do not have enough information to state whether this is a good or a bad thing. However the gas atomisation process has the greatest scope for altering cooling rates and particle size distribution, through changing the shape, size and speed of the gas jets.
