Malvern Dynamic Mechanical Properties of Ceramics
The performance of ceramic materials i.e. the strength of the final product is dependent upon the dynamic mechanical properties of the powdered material and the dispersed slurry (slip).
Of particular importance is the particle size and size distribution of the ceramic powder and rheological properties of the resulting suspension. The particle size defines the time and temperature required to attain full density during sintering (finer particles require shorter sintering times).
A direct relationship also exists between particle size and the pore size observed in the green body. Large particles tend to pack inefficiently, leading to large pore sizes. These pores are found to remain during sintering, thus reducing the strength of the final product. Packing can be improved by reducing the particle size. The use of polydisperse ceramic powders can also be advantageous, as the small particles present in these powders will fill the voids between the larger particles, thus reducing the overall pore size. Finally, the presence of large agglomerates must be avoided as these can lead to defect formation during sintering, as agglomerated grains tend to grow more quickly than well-dispersed particles - again reducing the strength of the fired product. Measuring rheological properties of the bulk material in suspension is often used as an indicator of quality of dispersion and existence of agglomerates.
Malvern Instrument provides solutions for particle sizing and rheology in ceramics, allowing the particle size, size distribution and rheology to be measured over a wide dynamic range. The techniques are sensitive to the presence of large particles and agglomerates. Small changes in the size distribution width can also be recognized. This allows for more rigorous product control, ensuring the quality of the final ceramic product.
Ensuring that the dispersion stability is controlled can prevent agglomeration leading to voids within the green body. Dispersion stability can be achieved either via charge or steric stabilization. Steric stabilization requires the addition of polymers to the green body, which can cause shrinkage during the firing of the product. Charge stabilization requires careful control of the pH and conductivity of the suspension during dispersion and final processing. By measuring zeta potential and rheological properties, the optimum dispersion stability conditions can be determined for a given ceramic suspension.