A test method for evaluating the quality of graphene flakes, such as reduced graphene oxide (rGO) and graphene nanopowder (GNP), was developed in this study. The pelletizer was selected for a sampling tool, which enables us to formulate the flake sample as a measurable sample. Various parameters were measured from the pelletized sample in order to elucidate the best parameter for representing the quality of the graphene flakes in terms of their electrical properties. Based on the analysis of

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... robe measurement data on the pelletized sample, the best intrinsic parameter is volume resistivity (or volume conductivity) rather than resistivity (or conductivity). Additionally, the possible modification of a sample before and after the pressurization was investigated by electron microscopy and Raman spectroscopy. No significant modification was observed. The volume conductivity in the two types of the graphene was different from their individual conductivities by one order of magnitude. Based on the results of X-ray photoelectron spectroscopy and Raman spectroscopy measurements, the volume conductivity of the graphene flake samples was governed by the oxygen content in the sample. Our achievements will promote the effective use of powder-type graphene products for further applications. Graphene has attracted much attention due to its extremely high mobility and ballistic transport of electrons, which enable its utilization as a next-generation electronic material 1,2 . To date, two types of graphene, namely, films and powders have been developed for specific applications. The film-type graphene fabricated by the chemical vapour deposition (CVD) or epitaxy methods has been studied for replacing the indium-tin oxide (ITO) glass that is widely used in the current display industry 3-5 . Furthermore, graphene films can be used to fabricate flexible transparent conducting films that can be used as flexible substrates in printed electronics or wearable electronics 5-7 . The powder-type graphene that is mainly produced by reduction of graphite oxide monolayer is the other type of graphene that may enable more practical applications, for example serving as an energy material with a high density as well as an outstanding electrical conductivity 8-10 . The use of powder-type graphene can be extended to the formation of functional composites due to its lower density and high mechanical properties 11,12 . During the first decade after the discovery of graphene, most work in academic and industrial research has been done on the film-type graphene. However, technical obstacles must still be overcome to enable the economically viable use of film-type graphene as a transparent conducting electrode in display industry because currently, homogeneous electrical conductivity cannot be obtained over the entire surface 13,14 . Recently, more effort has been made in utilizing the powder-type graphene in industrial applications due to another advantage of this novel material. Since powder-type graphene exhibits greater versatility for use with other matrices, it can be a good candidate for use as an anodic material for secondary batteries, as an additive to activated carbon for supercapacitors, and in other applications 15, 16 . In particular, the introduction of chemical exfoliation for the fabrication of powder-type graphene can reduce its production cost and enhance large scale productivity, possibly accelerating the use of powder-type graphene product in industrial applications in the near future 8,17-19 .