Mechanical and thermal properties of reduced graphene oxide reinforced aluminum nitride ceramic composites

Abstract

High density reduced graphene oxide (rGO) reinforced aluminum nitride (AlN) composites were successfully fabricated by the one-step spark plasma sintering (SPS) method. Raman spectra showed that the raw material of GO was thermal reduced to rGO during the SPS process, and the reduction of GO can be strongly affected by the carbonaceous atmosphere. With the rGO content increasing (0–2 wt %), the dispersion of rGO and relative density of the composites decreased, decreasing the elastic modulus and hardness accordingly. However the flexural strength had a slight increase when rGO content was ≤1 wt%, and the fracture toughness increased from 3.5 to 5.2 MPa m^1/2 with the rGO due to the crack bridging and pulling out of rGO. The thermal conductivity of the composites was low and sharply decreased from 92.5 to 37.4 W m⁻¹ K⁻¹ with the addition of rGO, which are attributed to the low crystalline quality and high vacancy defects in rGO and the increase of interfacial thermal resistance.

Introduction

Graphene, as an allotrope of carbon consisting of a single layer of sp² bonded carbon atoms, has recently attracted tremendous attention due to its excellent mechanical, thermal, and electrical properties, such as high Young's modulus (1 TPa), high fracture strength (125 GPa), extreme thermal conductivity (5000 W m⁻¹ K⁻¹) and super charge-carrier mobility (exceeding 200,000 cm² V⁻¹ s⁻¹) [1], [2]. However, it is factually difficult to obtain a high yield of single-layered graphene without subsequent layer agglomeration. Graphene nanoplates (GNPs), composed of a few graphene layers, have thinness in the range of 2.5–100 nm and possess similar properties to that of single-layer graphene but are much easier to produce and less prone to agglomeration and entanglement as compared to single-layer graphene. Thus, researchers paid wide attentions to the mechanical, thermal and electrical properties of GNPs reinforced composites with polymer, metal or ceramic as matrix [3], [4], [5], [6], [7], [8], [9], [10].

In the past, numerous works have been focused on GNPs reinforced polymer matrix composites. The results indicated that GNPs can significantly improve the mechanical and thermal properties of GNPs/polymer composites. In recent years some researchers have gradually started focusing on the GNPs/ceramic composites, such as GNPs/Si₃N₄ [4], [11], [12], GNPs/Al₂O₃ [13], [14], GNPs/ZrO₂ [15], [16], and so on. Overlapped GNPs were located at the boundaries of ceramic grains, and hindered the grain growth and changed the shape of the grains. When doping a certain amount of GNPs, crack deflection, branching and bridging resulted by GNPs made the fracture toughness higher for the composites in comparison to the monolithic ceramic. However, the relative density, elastic modulus, and hardness usually decreased with the GNPs, due to the agglomeration and the retardation of sintering.

As for thermal and electric properties, GNPs lead to an anisotropic response of composites, which have the low through-plane thermal and electrical conductivities and the high in-plane thermal and electrical conductivities. This effect is related to the nanostructure of the preferred orientation which produces a less resistive network in the in-plane direction and the anisotropic conductivities [17], [18]. In the case of relatively low thermal conductivity matrices (polymer, Al₂O₃, ZrO_2, etc.), GNPs remarkably improved the thermal conductivity. But for the high thermal conductor matrices (SiC, AlN, etc.), the effect of GNPs on the thermal properties of composites was little studied and not clear by now. As for the carbon nanotubes/fiber reinforcement, Yu et al. [19] obtained thermal conductivity with 30% increase at room temperature for C_f/SiC composite. However, Wang et al. [20] reported the density, mechanical properties and thermal conductivity significantly decreased when doping CNTs into the high thermal conductive AlN ceramic. Shi et al. [21] also found that the interface in carbon nanofibers/AlN composites significantly affected the interfacial thermal transfer characteristics.

Aluminum nitride (AlN) ceramic, as an important III–V group wide-band-gap semiconductor, has been extensively investigated and applied to electronic devices as substrate and package materials due to its excellent properties of high thermal conductivity, high electrical resistivity, low dielectric constant and low thermal expansion coefficient [22]. However, its inherent brittleness and relative low strength compared with other structural ceramics such as Si₃N₄, ZrO₂, etc., restrict the application of the AlN ceramics [23], [24], [25]. For this reason, we expected that in combination with GNPs, the AlN ceramics might have a desirable development in the mechanical properties and even in the thermal properties. To date, as far as we know, there have not yet been reports on the fabrication and properties of GNPs reinforced AlN composites. So it is necessary to investigate the effect of the incorporation of GNPs on the mechanical and also the thermal properties of GNPs/AlN composites.

It is known that GNPs can be obtained by the reduction of graphene oxide (GO), which is a very attractive precursor for the large-scale production, due to the easy synthesis and processable solution. However, GO has heavily disordered and oxidized domains and vacancy defects, needing a reduction step (forming reduced graphene oxide, rGO) to partially restore the conductivity [26]. Spark plasma sintering (SPS) has been reported as a relatively new and efficient method to obtain high density ceramics and composites with shorter sintering time and lower sintering temperature. Some studies have reported that GO can be thermally reduced by one step SPS process, avoiding the degradation during conventional sintering with long time. Ramirez et al. [18] reported that GO reduction could be successfully achieved in a single step during the densification of GO/Si₃N₄ composites by SPS, and the rGO sheets were homogeneously embedded in the ceramic matrix. Rincón et al. [14] recently also considered that the material sintering and the GO thermal reduction occurred simultaneously during the fabrication process of Al₂O₃–3YTZP–graphene composite with SPS. Therefore, in our work, GO, which was latter thermally reduced during SPS processing, was used as raw material to fabricate the rGO/AlN composites.