Membrane coating, also referred to as the active layer or the separation layer, is crucial as it delivers and controls all of the desired membrane properties, such as high porosity, high flux, high selectivity with the correct pore size, and a high level of mechanical strength. The coating is usually made of an inorganic material such as alumina, silicon carbide, zirconia, or titania. Membrane coating is a layer of a colloidal suspension, which is placed evenly on the porous support structure.
The layer must be placed evenly within the membrane support. If it is not managed to put an even membrane coating layer, it will lead to inhomogeneity, which results in an uneven porosity and roughness on the membrane surface. This will make different parts of one membrane layer perform differently. Therefore, it is critical to obtain high-quality and high-performing membrane layers, which can meet the demands from various harsh and arduous industries.
At LiqTech, we have more than 20 years of experience in producing ceramic membranes, which enables us to always obtain outstanding consistent coating results.
The Coating Materials
Based on more than 20 years of experience, our unique membrane coating material contains the ultra-hard material silicon carbide powder. Learn more about silicon carbide and its advantages here. In addition to the silicon carbide powder, dispersing agent, binder, defoaming agent, and plasticizer can also be used in the coating suspension as raw materials.
Since these materials change the coating suspension's rheological properties, it is essential to add the right amount. For example, if the coating material's viscosity is changed, it will lead to changes in the thickness and uniformity of the membrane layers. A high level of viscosity delivers thicker membrane layers. This will heighten the likelihood of defect formation on the surface of the membrane. On the contrary, a low level of viscosity delivers silicon carbide particles' infiltration within the membrane macropores of membrane support. Therefore, it is of utmost importance to obtain the right formulation of silicon carbide coating suspension to reach the perfect morphology of the membrane layers.
The silicon carbide support and the membrane layer can be seen in a scanning electron microscope (SEM). When the ceramic membrane consists of multiple layers with a gradual decrease in pore size distribution from the support to the membrane layer, it is called asymmetric.
Silicon Carbide Membrane Fabrication Process
The silicon carbide membrane coating fabrication consists of multiple steps. But basically, these can be summed up in four overall steps, being 1) Preparation of colloidal suspension, 2) Extrusion, 3) Coating, and 4) Sintering the membrane layer.
In the first step, it is crucial to prepare the homogeneous suspension of silicon carbide powders. This is because the fabrication of the desired membrane relies on the complete dispersion of SiC particles in the solvent.
In the second step, the SiC mix is extruded to the correct shape and cut at the right length. This should be done when the suspension is still wet to obtain even the most complex membrane support structures.
In the third step, the prepared suspension is used to coat the membrane support using coating techniques such as dip-coating or spray-coating.
In the fourth and last step of the silicon carbide membrane fabrication process, the membrane layer is sintered at extremely high temperatures. The membrane is sintered at a high temperature up to 2100 ˚C under an inert atmosphere. The high sintering temperature adds to the high manufacturing costs of ceramic silicon carbide membranes and makes the industrial-scale production of silicon carbide membranes costly. Still, the high sintering temperature is necessary in order to achieve the desired physical and chemical properties of a silicon carbide ceramic membrane. Click here to learn more about how a ceramic membrane is made.
Overall, the silicon carbide membrane fabrication process provides a strong adherence to the membrane support and adds robustness, and durability to the membrane layers. This ensures silicon carbide membranes of high quality with an extremely long lifetime.
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