Thermophysical Properties of High-Frequency Induction Heat Sintered Graphene Nanoplatelets/Alumina Ceramic Functional Nanocomposites Iftikhar Ahmad, Tayyab Subhani, Nannan Wang, and Yanqiu Zhu (Submitted December 6, 2017; in revised form March 27, 2018) This paper concerns the thermophysical properties of high-frequency induction heat (HFIH) sintered alumina ceramic nanocomposites containing various graphene nanoplatelets (GNP) concentrations. The GNP/alumina nanocomposites demonstrated high densities, fine-grained microstructures, highest fracture toughness and hardness values of 5.7 MPa m 1/2 and 18.4 GPa, which found 72 and 8%, superior to the benchmarked monolithic alumina, respectively. We determine the role of GNP in tuning the microstructure and inducing toughening mechanisms in the nanocomposites. The sintered monolithic alumina exhibited thermal conductivity value of 24.8 W/mK; however, steady drops of 2, 15 and 19% were recorded after adding respective GNP contents of 0.25, 0.5 and 1.0 wt.% in the nanocomposites. In addition, a dwindling trend in thermal conductions with increasing temperatures was recorded for all sintered samples. Simu- lation of experimental results with proven theoretical thermal models showed the dominant role of GNP dispersions, microstructural porosity, elastic modulus and grain size in controlling the thermal transport properties of the GNP/alumina nanocomposites. Thermogravimetric analysis showed that the nanocom- posite with up to 0.5 mass% of GNP is thermally stable at the temperatures greater than 875 °C. The GNP/ alumina nanocomposites owning a distinctive combination of mechanical and thermal properties are promising contenders for the specific components of the aerospace engine and electronic devices having contact with elevated temperatures. Keywords ceramics, interfaces, microstructure, nanocomposites, sintering, thermal analysis 1. Introduction Development of thermally stable functional ceramics with excellent thermal management is imperative for the specific components of numerous advanced devices, working at ele- vated temperatures, such as aeroengine components, insulation for rocket electronics assemblies and missile nozzle parts (Ref 1). It is unfortunate that the monolithic ceramics like alumina, silicon nitride and several others carry insufficient thermophys- ical properties, therefore unsuitable for aforesaid components (Ref 2). In recent years, carbon-based nanostructures in diverse morphologies are considered for tuning the thermal properties of monolithic alumina (Ref 3). Most of the CNT/alumina nanocomposite investigated for thermal conductivity recorded a liner drop as a function of CNT increments with the exception of Kumari et al. report, who claimed better thermal properties despite the issues of poor sinterability and high porosity in the CNT/alumina nanocomposites (Ref 4). 2D graphene nanoma- terial has drawn significant attention, as promising reinforcing filler, due to outstanding mechanical, electrical and thermal properties (Ref 5). In contemporary composite technology, the graphene has particularly been focused, as a toughening agent, to overcome the intrinsic brittleness of many ceramics materials (Ref 6-8). So far, alumina- and silicon nitride-based nanocom- posites reinforced with small graphene additions (1.0 and 1.5 vol.%) have demonstrated noteworthy fracture toughness of 72 and 235%, respectively, following pull-out, crack-bridging, grain-anchoring toughening mechanisms (Ref 9-11). Besides excellent mechanical properties, graphene offers exceptionally higher thermal conductivity, ranging from 2000 to 4000 W/mK at ambient temperature, which shows its ability for enhancing the inherently inferior alumina thermophysical properties (Ref 12-15). Furthermore, theoretical models have predicted that the thermal properties of graphene-based alumina nanocomposites can outperform those composite materials reinforced with carbon nanotubes, metals nanoparticles and other fillers (Ref 16). The leading advantage of graphene is the convenient dispersion into ceramic matrices without much effort than nanomaterials owning fibrous and particle mor- phologies (Ref 17). In alumina nanocomposites microstruc- tures, the graphene generally occupies the position at inter- granular spaces; thus, the role of grain boundary structure and the interface is important for tweaking the thermal properties of the ceramic nanocomposites; therefore, diverse mixing pro- cesses and different sintering routes were practiced aiming firm graphene/alumina interfacial contacts and defect-free microstructures (Ref 18-24). Graphene/ceramics nanocompos- ites have been investigated for improving the electrical and mechanical properties; however, fewer addressed thermophys- ical properties (Ref 25-27). In available reports, decrease in the Iftikhar Ahmad, Center of Excellence for Research in Engineering Materials, King Saud University, P.O. Box 800, Riyadh 11421, Kingdom of Saudi Arabia; Tayyab Subhani, Composite Research Center, Department of Materials Science and Engineering, Institute of Space Technology, Islamabad, Pakistan; and Nannan Wang and Yanqiu Zhu, College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, UK. Contact e-mails: ifahmad@ksu.edu.sa and iffi1070@yahoo.com. JMEPEG ÓASM International https://doi.org/10.1007/s11665-018-3395-6 1059-9495/$19.00 Journal of Materials Engineering and Performance