Graphene, the two-dimensional form of carbon, has received a great deal of attention across academia and industry due to its extraordinary electrical, mechanical, thermal, chemical, and optical properties. In view of the potential impact of graphene on numerous and diverse applications in electronics, novel materials, energy, transport, and healthcare, large-scale graphene production is a challenge that must be addressed. In the past decade, top-down production has demonstrated high potential for scale-up. This review features the recent progress made in top-down production methods that have been proposed for the manufacturing of graphene-based products. Fabrication methods such as liquidphase mechanical, chemical and electrochemical exfoliation of graphite are outlined, with a particular focus on nonoxidizing routes for graphene production. Analysis of exfoliation mechanisms, solvent considerations, key advantages and issues, and important production characteristics including production rate and yield, where applicable, are outlined. Future challenges and opportunities in graphene production are also highlighted. V C 2018 American Institute of Chemical Engineers AIChE J, 00: 000-000, 2018 a) Scanning Electron Microscopy (SEM) image of expanded highly orientated pyrolytic graphite from Cooper et al. (b-c) TEM images of mono-and multilayer graphene from Qian et al. d) SEM image of GO from Voiry et al. e) High resolution TEM (HRTEM) image of single layer reduced graphene oxide with indications of holes (red arrow) and oxygen functional groups (blue arrow) from Voiry et al. Reproduced with permission from Ref. 15-17.

Shear-assisted liquid exfoliation is a primary candidate for producing defect-free two-dimensional (2D) materials. A range of approaches that delaminate nanosheets from layered precursors in solution have emerged in recent years. Diverse hydrodynamic conditions exist across these methods, and combined with low-throughput, high-cost characterization techniques, strongly contribute to the wide variability in performance and material quality. Nanosheet concentration and production rate are usually correlated against operating parameters unique to each production method, making it difficult to compare, optimize and predict scale-up performance. Here, we reveal the shear exfoliation mechanism from precursor to 2D material and extract the derived hydrodynamic parameters and scaling relationship that are key to nanomaterial output and common to all shear exfoliation processes. Our investigations use conditions created from two different hydrodynamic instabilities-Taylor vortices and interfacial waves-and combine materials characterization, fluid dynamics experiments and numerical simulations. Using graphene as the prototypical 2D material, we find that scaling of concentration of few-layer nanosheets depends on local strain rate distribution, relationship to the critical exfoliation criterion, and precursor residence time. We report a transmission-reflectance method to measure concentration profiles in real-time, using low-cost optoelectronics and without the need to remove the layered precursor material from the dispersion. We show that our high-throughput, in situ approach has broad uses by controlling the number of atomic layers on-the-fly, rapidly optimizing green solvent design to maximize yield, and viewing live production rates. Combining the findings on the hydrodynamics of exfoliation with this monitoring technique, we unlock targeted process intensification, quality control, batch traceability and individually customizable 2D materials on-demand.

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Convective heat transfer, due to axial flow fans impinging air onto a heated flat plate, is investigated with infrared thermography to assess the heated-thin-foil technique commonly used to quantify two-dimensional heat transfer performance. Flow conditions generating complex thermal profiles have been considered in the analysis to account for dominant sources of error in the technique. Uncertainties were obtained in the measured variables and the influences on the resultant heat transfer data are outlined. Correction methods to accurately account for secondary heat transfer mechanisms were developed and results show that as convective heat transfer coefficients and length scales decrease, the importance of accounting for errors increases. Combined with flow patterns that produce large temperature gradients, the influence of heat flow within the foil on the resultant heat transfer becomes significant. Substantial errors in the heat transfer coefficient are apparent by neglecting corrections to the measured data for the cases examined. Methods to account for these errors are presented here, and demonstrated to result in an accurate measurement of the local heat transfer map on the surface.