Structural stability studies of graphene in sintered ceramic nanocomposites Fawad Inam a,n , Thuc Vo a , Badekai R. Bhat b a Northumbria University, Faculty of Engineering and Environment, Department of Mechanical and Construction Engineering, Newcastle upon Tyne NE1 8ST, United Kingdom b National Institute of Technology Karnataka, Department of Chemistry, Catalysis and Materials Laboratory, Surathkal, Srinivasanagar 575025, India Abstract The post-sintering structural stability of graphene in alumina nanocomposites synthesised by Spark Plasma Sintering (SPS) and Hot Pressing (HP) was compared. Raman spectroscopy, thermogravimetric analyses and electrical conductivity analyses were conducted to characterise degradation of graphene due to the utilisation of different sintering techniques and conditions. Scanning Electron Microscopy confirmed good dispersion of graphene in SPSed and HPed sample. Graphene in SPSed and HPed nanocomposite samples sintered using longer durations (60 min) were found to possess higher crystallinity, thermal stability and electrical conductivity as compared to SPSed samples sintered using shorter sintering durations (10–20 min). This was attributed to the thermally induced graphitisation caused by longer sintering durations, which was lacking in SPSed samples processed using shorter sintering durations and lower temperature. No additional effect of DC pulsed current on the structural stability of graphene for nanocomposites were observed for samples prepared by SPS. Keywords: A. Hot pressing; Graphene; Alumina; Structural stability; SPS 1. Introduction The addition of graphene in ceramics is becoming a widely researched area because of its superlative nature and significant positive contribution to ceramics' performance. Over the past few years, graphene has attracted enormous research attention for its very mechanical and thermal properties and exceptionally high electron mobility [1]. Significant improvements in the mechanical, thermal and electrical properties of ceramics nano- composites filled with graphene. Graphene has been added in ceramics likes cordierite [2], silicon carbide [3], silicon nitride [4], tantalum carbide [5], alumina [6], zirconium diboride [7], zirconia [8], boron nitride [9] to enhance not only electrical properties but also thermal conductivity, refractory, mechanical, antifriction, anticorrosive and biocompatibility properties for diverse applica- tions [10]. For example, a remarkable 235% improvement in fracture toughness was reported for by the addition of only 1.5 vol% of graphene in silicon nitride [4]. Zhou et al. [2] reported an increase of 8 orders of magnitude and 3.7 times in the electrical and thermal conductivity of cordierite ceramic respec- tively. Similarly, zirconium diboride filled with graphene, sintered at 1900 1C, was studied by Yadhukulakrishnan et al. [7] for possible high temperature barrier applications for space vehicles during the re-entry event. Whilst much of the emphasis is on the improvement of thermal, electrical and mechanical properties, there is no research about the graphene's structural health after sintering. For carbon nanotubes, structural and chemical degradation of carbon nanotubes during non-SPS techniques has been widely reported in the literature due to the relatively long sintering time (3–10 h) and high temperatures involved ( 4850 1C) in the sintering process [11–18]. Therefore, it is widely perceived in the ceramic community, that graphene would be degraded if non-SPS techniques are used for processing ceramic–graphene nanocomposites. There are very few research papers reporting the use of non-SPS techniques for processing ceramic–graphene or ceramic–graphene oxide nanocomposites with improved characteristics [19–23]. For example, Rutkowski et al. [19] used conventional hot-pressing for n Corresponding author. Tel.: þ44 191 227 3741. ACCEPTED MANUSCRIPT