## 1 Introduction
However, a cycle heating experiment showed that the continuous graphene framework is destroyed by the grain growth which takes place because of an incomplete crystal structure formation during the sintering process. The SPS process cannot be used for a batch production of zirconia/graphene composites because of the equipment complexity and limited geometry of specimens. Thus, there is a need for new simple and cost-effective fabrication routes for ceramic/graphene composites, and especially zirconia/graphene composites, where a deep vacuum and/or special equipment are not necessary. For instance, a fast room temperature and air fabrication of 3 YSZ/rGO composites by flash sintering (FS) has been recently reported in [14]. Dense composites with a homogeneous distribution of rGO in the zirconia matrix, enhanced electrical conductivity and mechanical properties compared to pure 3 YSZ were obtained. The powder bed sintering approach is a promising alternative to overcome the existing disadvantages of listed equilibrium and non-equilibrium sintering approaches. The essence of this approach is heating the sample fully covered with the mother powder of the same composition or some inert powder such as SiC. In this case, the direct contact with the furnace chamber atmosphere is eliminated. A powder bed can perform various functions: limiting diffusion of the material's components from the sample bulk, protecting sensitive components from oxidation, etc. The described approach prevents evaporation and oxidation of the ceramics and composite components and allows respective reactions in the sample bulk to occur. In works [19; 20], Si\({}_{3}\)N\({}_{4}\)-based ceramics were successfully sintered at 1650-1800 \({}^{\circ}\)C under a nitrogen atmosphere. In [19], samples were placed into a graphite container with a Si\({}_{3}\)N\({}_{4}\)-BN powder bed of 1:1 ratio. The authors reported that the use of a nitride powder bed allows successful sample sintering at a nitrogen pressure of 1 atm with minimal weight loss, limited thermal decomposition of Si\({}_{3}\)N\({}_{4}\) during heating and high density >95% compared to for the sample sintered under the same inert atmosphere but without a powder bed. The positive effect of the application of the powder bed technique was also proven for SiC ceramics [21]. It was shown that the mother powder bed serves as an additional source of the species. Several attempts were also made to apply powder bed sintering to solid electrolyte production. For instance, perovskite-type Li\({}_{3/8}\)Sr\({}_{7/16}\)Ta\({}_{3/4}\)Zr\({}_{1/4}\)O\({}_{3}\) was successfully sintered in a mother powder bed at 1300 \({}^{\circ}\)C for 15 h in an alumina crucible [22]. The technique was shown to be effective to densify the pellet and reduce impurity phases of SrTa\({}_{2}\)O\({}_{6}\) and Sr\({}_{2}\)Zr\({}_{2}\)O\({}_{7}\) and lithium losses. As a result, an about three-time increase in the ionic conductivity was observed compared to conventionally sintered ceramics. The use of the powder bed for zirconia/graphene composites may protect graphene additive from oxidation during sintering and hence could allow one to combine the advantages of equilibrium and non-equilibrium sintering approaches: high crystallinity, fine microstructure, simple technical realization and elimination of such limitations as sample geometry or the necessary minimum level of the material conductivity. Thus, the goal of the present work was to develop a suitable powder bed sintering method for the ceramics/graphene composite production, determine the relationship between the amount of introduced graphene and phase formation, microstructure, conductivity and mechanical characteristics and investigate the powder bed effect on the obtained properties.