Cavitation Driven Exfoliation of Graphene and Two Dimensional Materials

10 Feed Preparation, Process Chemistry, and Contamination Control

10.1 Graphite Feedstock Qualification

Natural graphite grade, crystallinity, particle size, morphology, ash, moisture, metal content, and prior processing can substantially alter exfoliation behavior. Large crystalline flakes can provide large nanosheets but may clog small channels. Fine graphite disperses readily but may already contain defects and can produce smaller platelets. Expanded graphite lowers interlayer resistance but adds pretreatment and may introduce residual intercalants. Synthetic graphite offers greater consistency but may differ in crystallite size and cost. A feed specification should include, at minimum, particle size distribution, ash, moisture, elemental impurities, XRD crystallinity, and a reference exfoliation test.

d95 is the 95th percentile feed particle size, Dh is the minimum hydraulic diameter, and κ is a conservative blockage ratio selected from geometry and particle shape. For rigid irregular flakes, κ may need to be well below 0.1.

10.2 Premixing and Deagglomeration

Dry powder should not be introduced directly into a high pressure microchannel unless the feeding and wetting system is designed for that duty. Premixing should eliminate floating agglomerates, wet particle surfaces, dissolve the stabilizer, and produce reproducible viscosity. Low speed agitation, rotor stator mixing, recirculation through a coarse bypass, or vacuum wetting can be used. The premix should be sampled before exfoliation so that subsequent changes are not confused with variation in feed dispersion.

10.3 Contamination Sources

Metal contamination can originate from sonotrodes, valves, nozzles, pumps, bearings, rotor stator surfaces, and collection vessels. Polymer fragments can originate from seals, hoses, and filters, while silica or ceramic particles can originate from interaction chambers. Contamination should be monitored by ICP MS, ash analysis, SEM with elemental analysis, and blank runs. Equipment material selection must account for abrasion as well as corrosion. A process that produces excellent Raman spectra but introduces unacceptable nickel, iron, titanium, or ceramic contamination is unsuitable for electronic, biomedical, or catalytic products.