Anisotropic thermal conductivity of silicon nitride ceramics containing carbon nanostructures
Introduction
The thermal conductivity (κ) of the diverse carbon allotropes, including the latest discovered carbon nanostructures (C-n), extents over an extraordinary wide range of values, from the very low figures of amorphous carbon to the peak value reported for graphene and carbon nanotubes (CNTs). Unusual large κ values have been theoretically predicted for two dimensional and one dimensional carbon crystals, but experimental results are sometimes contradictory as discussed in a recent review.1
Early studies on millimeter sized CNTs mats2, 3, 4, 5, 6 gave relatively low κ values, 5–30 W m−1 K−1, but they lacked to provide absolute values for single CNTs as the numerous tube–tube junctions and intrinsic defects within nanotubes became the dominant barriers to thermal transport in the mat. Consequently, to probe thermal transport free from interactions, microfabricated suspended devices with both multi-walled (MWCNTs) or single-walled (SWCNTs) nanotubes were developed.7, 8, 9, 10 These measurements reported a wide range of room temperature κ values for individual CNTs, although commonly quoted values were ∼3000 and ∼3500 W m−1 K−1 for MWCNTs and SWCNTs, respectively. Both values were well above the bulk-graphite limit but in the order of the theoretical predictions.1 From these studies, a phonon mean free path at room temperature (Λ) of ∼700–750 nm was calculated, which is shorter than the typical length for CNTs (>2 μm), meaning that the phonon transport is still diffusive but close to the ballistic transition. Furthermore, κ for MWCNT decreased with the size of MWCNT bundles, becoming, for 200 nm, similar to the bulk measurement on a mat sample, which infers that the interactions of phonons between multi-walled layers also affected thermal conduction.10
Recently, experimental studies done on large-area suspended graphene layers attained κ values exceeding 3000 W m−1 K−1 near room temperature.1 The evolution of the thermal properties of few layer graphene has been examined as a function of the number of atomic planes (n), showing that κ decreases with n, even dropping below the bulk graphite limit for n > 4 due to the onset of the phonon-boundary scattering from the top and bottom interfaces; but the graphite value was recovered for thicker films.11 This κ(n) dependence agrees with current non-equilibrium molecular dynamics calculations for graphene nanoribbons.12
In spite of the numerous experimental and theoretical studies on thermal conductivity of C-n, relatively few works can be found on the thermal transport in composites containing these nanostructures, and ever fewer devoted to ceramic matrices. Particularly, no papers could be found on ceramics containing either graphene nanoplatelets (GNPs) or nanosheets. For CNTs, κ has been measured for various ceramic matrix composites without any conclusive result regarding the effect of the C-n.13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 Of all these works, only one based on SWCNTs/Al2O3 nanocomposites25 investigated both the through-thickness and in-plane thermal conductivity, showing a decrease in the transverse and no changes in the in-plane thermal diffusivity with SWCNTs content.
Improved thermal conductivity has been reported in the case of low thermal conductivity matrices containing MWCNTs, like in aluminoborosilicate glass13, 14 (16 W m−1 K−1 for aligned MWCNTs versus the 1.2 W m−1 K−1 of the matrix) or MWCNTs/SiO2 composites15, 16 that showed κ values of 4.1 W m−1 K−1 for 10 vol.% MWCNTs compared to 2.4 W m−1 K−1 of the silica matrix. For the high thermal conductor AlN, the two found sources17, 18 gave opposite results, apparently depending on the processing method. Wang et al.17 reported reduced thermal-conductivity for MWCNTs/AlN composites fabricated by hot pressing, which they attributed to the CNTs poor dispersion and degradation, whereas Datye et al.18 obtained enhanced thermal conductivity for spark plasma sintered (SPS) composites prepared by direct in situ growth of MWCNTs on AlN powders. Contradictory results were also found for MWCNTs/Al2O3 composites as both thermal conductivity decrease19 and notably enhanced thermal properties20 were reported, although in the later case, the CNTs were directly grown on Al2O3 powders by chemical vapor deposition and, therefore, the possible effect of the residual metallic catalysts cannot be disregarded.
As the properties of Si3N4 ceramics are strongly affected by the development of large elongated β grains, special attention should be paid to the α/β phase ratio when comparing the thermal properties of different Si3N4 materials.26 Present authors22 already reported a reduced thermal conductivity for Si3N4 composites with 5.3 vol.% of MWCNTs densified using SPS and, in a similar way, Corral et al.23 observed a 62% decrease in the room temperature thermal conductivity over the monolithic material for a composite with just 2 vol.% of SWCNTs. One work claimed improved thermal conductivity for Si3N4 composites containing nanotubes but without giving a precise account of the β-phase content.24
Under this perspective, the aim of the paper is to deeply analyze thermal conduction in Si3N4 with different types of C-n. The effect of both CNTs and GNPs is presented and evaluated as a function of the C-n content, their possible orientation respect the heat flux, testing temperature and α/β Si3N4 phase ratio. The final objective is to extract the real effect of the C-n over the composites and to compare the effectiveness of the CNTs and GNPs in raising thermal conduction. Additionally, these kind of composites have shown extraordinary electrical27, 28 and tribological29, 30 responses, which make them interesting for engine components and MEMs, being suitable for electro-discharging machining, as well.31
Section snippets
Materials and methods
The preparation of the Si3N4 monolithic materials and composites containing different C-n is described elsewhere.22, 28 In short, ceramic matrix powders of composition 93 wt.% of α-Si3N4 (SN-E10, UBE Industries, Japan) plus 5 wt.% of Y2O3 (HC-Starck) and 2 wt.% of Al2O3 (SM8, Baikowski Chimie, France), as sintering aids, were attrition milled in isopropanol media. CVD synthesized MWCNTs of 30 nm diameter and 1–5 μm length (Nanolab Inc., USA) in concentrations ranging from 0.9 to 8.6 vol.%, and GNPs
Result and discussion
FESEM observations of the composites containing MWCNTs and GNPs give interesting information on their microstructure (see Fig. 2 as an example). Both C-n show a homogenous dispersion in the composites as observed on the images of the fracture surfaces, where nanotubes and nanoplatelets are seen protruding from the matrix. One particularity of the later composite that can be clearly perceived in Fig. 2b is the preferential in-plane orientation of the GNPs owing to the fabrication process, as has
Conclusions
The addition of carbon nanostructures to Si3N4 composites fabricated by spark plasma sintering strongly affects their anisotropic thermal response, which depends on the type of nanostructures. The addition of both carbon nanotubes and graphene nanoplatelets reduces the thermal conductivity in the through-thickness direction leading to an increased anisotropy in ceramic composites. This increase is significantly larger for nanoplatelets additions as it enhances thermal conduction for the
Acknowledgements
This work was supported by the Spanish Ministry of Science and Innovation (MICINN) and the CSIC under projects number MAT2009-09600 and i-Link 0119, respectively. J. Gonzalez-Julian and C. Ramirez acknowledge the financial support of the JAE (CSIC) fellowship Program. Dr. E. Garcia also acknowledges the financial support of the Ramon y Cajal Program of the MICINN. Experimental supports of S. Molina (ICMM, CSIC) in preparing and viewing GNP/Si3N4 transmission specimens and Dr. L.A. Perez Maqueda
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