In situ reduction of graphene oxide nanoplatelet during spark plasma sintering of a silica matrix composite

https://doi.org/10.1016/j.jeurceramsoc.2014.04.031Get rights and content

Abstract

Well dispersed silica-graphene nanoplatelet (GNP) and silica-graphene oxide nanoplatelet (GONP) composites were fabricated by optimising their processing conditions. Different processing methods, including colloidal and powder processing, were investigated using different solvents. High temperature and inert environment during SPS resulted in the reduction of the GONP during sintering. The in situ reduction of GONP in the SPS was investigated for various sintering times and temperatures, and characterised using Raman spectroscopy and XRD analysis. The composite sintered at 1200 °C, 50 MPa pressure and 15 min dwell time confirmed the recovery of the crystalline graphitic phase of GNP after reduction without crystallising the silica matrix. GONP was found to inhibit the crystallisation of the silica matrix at higher sintering times and temperatures possibly due to increased viscosity and reduced mobility of the silica particles bound to GNP.

Introduction

Graphene has remarkable thermal,1 electrical2 and mechanical3 properties. It is being considered as a multifunctional nano-filler in composites for a wide range of potential applications. The vast majority of work in the last decade has focused on polymer–graphene composites, e.g., Refs. 4, 5, 6, 7, 8, 9, 10, 11 and much fewer investigations have considered inorganic matrices, e.g., Refs. 12, 13.

A considerable amount of work has been published on CNT reinforced inorganic matrix composites,14, 15, 16, 17, 18, 19 but their applications are still very limited. The major difficulties associated with producing CNT based composite systems are: (1) CNTs tend to agglomerate unless they are surface functionalised20; (2) CNTs are potentially toxic21 because of the presence of undesired metal catalyst as well as their nano-size. Graphene on the other hand is easier to produce and disperse even without surface functionalisation.22, 23 It is less toxic than CNTs because of its two-dimensional geometry (in the range of a few microns) and the absence of nano-sized catalyst.

Recently authors have reported improvements in the structural and functional properties of inorganic matrix composites with the addition of GNP/GONP. For example, Walker et al.24 reported an increase of 235% in the fracture toughness of silicon nitride–GNP composites. Recently Porwal et al.12 reported an improvement of 40% in the fracture toughness of alumina–GNP composites and a change in mechanism of crack propagation from inter-granular to trans-granular with the addition of GNP. Miranzo et al.25 reported an increase of 100% in the thermal conductivity of silicon nitride–GNP composites. Fan et al.26 reported a percolation threshold of 0.38 vol% and electrical conductivity of 1000 S/m for alumina–GNP (2.35 vol%) composites. In a review on graphene reinforced ceramic matrix composites,13 various synthesis routes for producing good quality GNP/GONP and ceramic nano-composites were discussed and the resulting properties of the composite materials were analysed. It should be noted that the reinforcement of inorganic matrix composites with the addition of GNP/GONP mainly depends on three factors: (1) amount of GNP/GONP loading; (2) interface between GNP/GONP and inorganic matrix; and (3) quality of GNP/GONP and its dispersion in inorganic matrix. The presence of GNP/GONP in inorganic matrix composites might dramatically affect many properties such as coefficient of friction, wear resistance,27 coefficient of thermal expansion and thermal shock resistance. In view of these potential advantages along with the intrinsic properties of the inorganic materials such as high temperature stability, light weight and high corrosion resistance, inorganic–GNP/GONP composites are potentially attractive for a variety of applications.

So far the improvement in the properties of graphene nano-composites has been limited because of the difficulty in producing single layer graphene flakes. The strong van der Waals forces between graphene layers leads to GNP formation when using liquid phase exfoliation or milling methods. On the other hand these forces can be overcome by oxidising the graphene which avoids re-agglomeration of graphene flakes but in turn compromises its excellent electrical properties. Oxidised graphene is easy to disperse in aqueous solvents and can be used to produce GONP reinforced glass/ceramic composites. Authors have reported reduction of GONP either by using chemical or thermal methods.28, 29 Spark plasma sintering (SPS) uses pulsed direct current in an inert/vacuum environment to achieve very high heating rates, thus reaching the sintering temperatures in just a few minutes. The inert and high temperature conditions in SPS are sufficient for reducing GONP to GNP during sintering.

In the present work, the processing conditions for preparing well dispersed silica-GNP/GONP composites are discussed. Colloidal and powder processing routes were investigated with different solvents including ethanol, deionised (DI) water, n-methyl pyrrolidone (NMP) and di-methyl formamide (DMF). SPS was used to consolidate the powders. SPS avoided any damage/decomposition of GNP/GONP by minimising the sintering times and reduced GONP to GNP during sintering. In situ reduction of GONP in SPS was investigated for various sintering times and temperatures using Raman spectroscopy and XRD analysis. Finally, the conditions for preparing well dispersed and reduced silica-GONP composites were optimised avoiding crystallisation of the nano-composites.

Section snippets

GNP/GONP

GNP was synthesised using the liquid phase exfoliation method described elsewhere.12, 30 Briefly, graphite flakes (Sigma Aldrich) were dispersed in NMP and sonicated (CV33 flat probe sonic tip, 50 W, 25 kHz) for 20 h in an ice cooled bath. After sonication, the suspension was centrifuged (Centurion Scientific) at 500 rpm for 45 min to separate the un-exfoliated graphite. The prepared GNP suspension was very stable with a yield of 4–8 wt%. The GNP was characterised using TEM (JEOL JSM-2010, 200 kV),

Results and discussion

Fig. 1 shows the shrinkage rate, temperature and pressure profile of the silica-GONP (2.5 vol%) composite. To optimise the processing conditions, the nano-composites were sintered at 1200 °C18 with a 6 min dwell time. The short dwell time minimised any structural damage of the GNP/GONP during high temperature processing.

Table 1 lists the processing methods, solvents, sintering conditions, and bulk and relative densities of the pure silica, silica-GONP (2.5 vol%) and silica-GNP (5 vol%) composites

Conclusion

The conditions for preparing well dispersed silica-GNP and silica-GONP nano-composites were optimised using powder and colloidal processing routes in different solvents including ethanol, DI water, NMP and DMF. The best results were obtained for the silica-GONP (2.5 vol%) composite produced using a colloidal processing route and ethanol as the solvent. Raman spectroscopy and XRD results confirmed that the GONP was reduced to GNP during high temperature processing in SPS. Composites sintered at

Acknowledgement

The authors would like to thanks European Union's Seventh Framework Programme managed by REA – Research Executive Agency http://ec.europa.eu/research/rea (Marie Curie Action, GlaCERCo GA 264526) for their support and funding for this research.

References (40)

  • C. Ramirez et al.

    Synthesis of conducting graphene/Si3N4 composites by spark plasma sintering

    Carbon

    (2013)
  • S. Stankovich et al.

    Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide

    Carbon

    (2007)
  • U. Khan et al.

    Size selection of dispersed, exfoliated graphene flakes by controlled centrifugation

    Carbon

    (2012)
  • D. Zhan et al.

    Electronic structure of graphite oxide and thermally reduced graphite oxide

    Carbon

    (2011)
  • A.A. Balandin et al.

    Superior thermal conductivity of single-layer graphene

    Nano Lett

    (2008)
  • A.K. Geim et al.

    The rise of graphene

    Nat Mater

    (2007)
  • C. Lee et al.

    Measurement of the elastic properties and intrinsic strength of monolayer graphene

    Science

    (2008)
  • S. Stankovich et al.

    Graphene-based composite materials

    Nature

    (2006)
  • U. Khan et al.

    Improved adhesive strength and toughness of polyvinyl acetate glue on addition of small quantities of graphene

    ACS Appl Mater Int

    (2013)
  • J.J. Vilatela et al.

    Nanocarbon composites and hybrids in sustainability: a review

    ChemSusChem

    (2012)
  • Cited by (32)

    • Additive manufacturing high performance graphene-based composites: A review

      2019, Composites Part A: Applied Science and Manufacturing
    • Ceramic matrix nanocomposites

      2018, Comprehensive Composite Materials II
    • Understanding and quantification of grain growth mechanism in ZrO<inf>2</inf>‑carbon nanotube composites

      2017, Materials and Design
      Citation Excerpt :

      Although, authors have reported improvements in the mechanical, electrical and thermal properties of ceramic-CNTs composites, degradation of CNTs at high temperatures has been an issue for the development of these multifunctional composites [14,15]. The development of rapid sintering techniques such as spark plasma sintering (SPS) has made it possible to incorporate them in different ceramic matrices with minimal damage to their graphitic structure [16–26]. Other than improvement in mechanical and functional properties, CNTs have also been reported to reduce the grain size of Al2O3 ceramics whilst increasing their density [16,27,28].

    • Crystallization kinetics and microstructure evolution of reduced graphene oxide/geopolymer composites

      2016, Journal of the European Ceramic Society
      Citation Excerpt :

      The degree of thermal reduction of graphene oxide in the rGO/KGP composites during isothermal soaking can be evaluated by Raman. The D band intensity is characteristically disordered and the G band is typical of a graphitic structure [30–32]. As seen in Fig. 5, the composites show characteristic Raman peaks at ∼1348 cm−1 (D band), ∼1600 cm−1 (G band) and 2D peak at ∼2703 cm−1 [30–32].

    View all citing articles on Scopus
    View full text