0 Introduction

0.1 General

Graphene, the two-dimensional (2D) form of carbon, was first isolated at the University of Manchester and reported in 2004 by Professor Andre Geim and Professor Kostantin Novoselov [1]. The importance of this discovery was acknowledged by the award of the 2010 Nobel Prize for Physics, and initiated intense global activity in research and commercialization of variants of 2D carbon materials [2], [3].

Carbon is an extremely versatile element, being found as different forms, or allotropes, in synthetic or naturally-occurring minerals such as graphite and diamond, which exhibit markedly different properties. Graphite is a layered material, and comprises a stack of single layers of carbon atoms which are weakly bonded between the layers.

Graphene was originally isolated by Geim and Novoselov from mined graphite, the material used in pencil lead, and was extracted by repeatedly peeling off layers using adhesive tape - a process known as mechanical exfoliation. This exfoliation process successively reduced the carbon layers down to a single layer, denoted as graphene. The term graphene strictly refers to a single layer of carbon atoms arranged in a crystalline hexagonal arrangement, as illustrated in Figure 1.

Subsequently, a diversity of new manufacturing methods have been developed for producing 2D planar carbon materials, each process producing different variants of particles, liquid dispersions or sheets.

The term “graphene” has entered the vocabulary as a label to refer to batches of 2D carbon particles which can contain flakes of differing layer thicknesses, not just a single layer.

This PAS uses the term “graphene flakes” to refer to powders or liquids containing graphene.

NOTE Graphene is defined in PD ISO/TS 80004‑13.

Figure 1 — Illustration of the hexagonal arrangement of carbon atoms in a single layer of graphene

0.2 Manufacturing routes

A wide diversity of manufacturing processes have been, and are being, developed to produce powders containing graphene flakes and 2D carbon particles. In very general terms, they can be divided into two approaches: top-down (subtractive) and bottom-up (additive).

Top-down processes include exfoliation of a solid precursor such as highly ordered natural (mined), or synthetic, graphite. Bottom-up processes include thermal or chemical conversion of gases or liquids.

A schematic of the two generic manufacturing approaches is shown in Figure 2.

Figure 2 — Illustration of the stages in typical top-down and bottom-up approaches for manufacturing graphene flakes

Manufacture by the exfoliation route is typically in a liquid (liquid phase exfoliation) and can be enhanced by mechanical means, or through intercalation of a chemical element or species between the carbon layers to separate them. Some variants utilize both methods, including sonication, as a means of inducing mechanical stress.

Solid precursor materials can also be converted into graphene flakes by thermal or chemical processing techniques.

Bottom-up manufacturing processes include:

• thermal cracking of hydrocarbon gas;
• chemical vapour deposition (CVD); and
• production from a hydrocarbon liquid phase.

The primary manufacturing stage might be followed by secondary processing, such as a compatibilization/surface functionalization treatment.

The manufacturing route has a significant effect on the graphene flakes produced (see Characterisation of the Structure of Graphene[4]). Factors such as the type and purity of the precursor, the manufacturing route, and any secondary processing, could influence the characteristics of the graphene flakes produced and introduce additional chemical species and defects. Graphene flakes are generally supplied as dried powder, or dispersed as a liquid-based suspension.

0.3 Graphene in which form?

Graphene flakes are a candidate additive or modifier for various intermediates, including material systems such as inks, polymers, composites and paints. In addition, graphene flakes can be used as a functional additive, special layer or coating in a range of engineering applications, including energy storage and sensor devices [5].

Potential users of graphene flakes are faced with a wide variety of commercially available products, which differ in terms of the graphene particle type and composition. To match the application or need with the optimal material, and to compare different suppliers and batches, a potential user requires sufficient, and comparative, information on the basic material characteristics.

Some key parameters that a potential end-user might wish to exploit in an intermediate material include:

a) mechanical properties (e.g tensile strength, stiffness);
b) electrical conductivity;
c) barrier properties;
d) thermal conductivity;
e) bio-compatibility; and
f) morphology (e.g. surface area, porosity).

For any one application, it is possible that only one or two of these properties are relevant to a user. This poses a challenge to intermediate materials producers – which properties are required to sufficiently characterize the graphene flakes for the materials selection process?

The materials selection process is illustrated for a range of possible applications and intermediates in Figure 3. This PAS relates only to the characterization of graphene flakes as powders or dispersions (stage A).

Figure 3 — Stages impacting graphene material selection – Stage A production; Stage B application

NOTE This PAS refers only to stage A.

PAS 1201:2018

Properties of graphene flakes. Guide

Published:

31 Aug 2018

Withdrawn:

3 Feb 2023