Reducing the γ'-Particle Size in CMSX-4 for Membrane Development
Colloidal emulsions for lipophilic drugs can be fabricated using premix membrane emulsification. The state of the art is the application of membranes made from, for example, polycarbonate or polyester, which, however, are prone to fouling and cause waste, due to the low number of cycles. With the use of metallic membranes made from the nickel based single crystalline superalloy CMSX-4, these key disadvantages are eliminated. However, instead, the pore size and the resulting droplet size distribution need to be adjusted and improved. This can be realized by tailoring the size of the γ'-particles, which is controllable by the time and temperature used during precipitation heat treatment and the quenching method after homogenization heat treatment. Therefore, we utilized different heat treatment protocols, varying the cooling rate (water quenching and air cooling) after homogenization heat treatment and the holding time and temperature during precipitation heat treatment. Then, we investigated the γ/γ'-microstructure, including the γ'-morphology and γ'-particle size. We show that water quenching has a significant impact on the γ/γ'-microstructure and often leads to irregular-shaped and poorly aligned γ'-particles after precipitation heat treatment. In comparison, air cooling, followed by a subsequent precipitation heat treatment, results in well-aligned and cubic shaped γ'-particles and is, therefore, favorable for membrane fabrication. A reduction in precipitation temperature leads to morphology changes to the γ'-particles. A reduction of the holding time during precipitation heat treatment diminishes the γ'-particle growth, resulting in smaller γ'-particles. Additionally, a suitable heat treatment protocol for membrane fabrication was identified with a γ'-edge length of 224 ± 52 nm and well-aligned, cubic shaped γ'-particles.