Towards highly electrically conductive and thermally insulating graphene nanocomposites: Al₂O₃–graphene

Abstract

Highly electrically conductive materials with low heat transfer rates are of very high importance for high temperature fuel cell technologies and the refractory material industry. We aim to develop such materials with high electrical conductivities/high thermal resistivities by creating composite materials of graphene and Al₂O₃. Here we describe a novel and facile method for the synthesis of Al₂O₃–graphene composites. Graphite oxide, which was prepared by the Hofmann method, was reduced by active hydrogen generated by the reaction of aluminum with a solution of sodium hydroxide. This reaction led to the formation of a nanocrystalline composite of graphene and aluminum hydroxide. The Al(OH)₃–graphene composite was then calcined and pressed into pellets. Sintering of the pellets yielded a nanostructured Al₂O₃–graphene composite. We characterized the properties of the Al(OH)₃–graphene and Al₂O₃–graphene composite materials in all steps to get an understanding of the process of the nanocomposite formation. The materials were analyzed by XRD, high resolution XPS, Raman spectroscopy, SEM, SEM-EDS, STEM, STA and AFM. The resistivity and thermal conductivity of the final Al₂O₃–graphene composite were measured. The Al₂O₃–graphene nanocomposite is a promising conductive material for high-temperature applications.

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Introduction

The discovery of graphene¹ in 2004 started a large boom in research on this modified 2D carbon. Since this discovery, graphene has been synthesized by various techniques:^2,3 by mechanical exfoliation, by CVD, exfoliation and chemical or thermal reduction of graphite oxide, by laser-based approaches,^4,5 etc. Bottom-up methods such as CVD led to the formation of graphene with a low defect concentration, whereas top-down methods led to much higher defect concentrations. The quality of graphene and graphene derivatives is the most important factor that can significantly change the properties of graphene. Thanks to its outstanding electrical, electrochemical, optical and mechanical properties graphene can be used for various applications.^6–8 Highly electrically conducting materials with high thermal resistivities are of high importance for industries concerned with heat management issues. Electronically conductive ceramics can be used for resistors, electrodes and heating elements. Highly conducting ceramics in bulk are suitable for electrode materials as electrocatalysts and inert substrates for electron transfer, therefore they can be used for high temperature fuel cells and molten salt processes.^9,10

Ceramic–graphene composites are used as highly efficient chemical sensors.¹¹ Carbon based nano-composites were studied due to their outstanding electrical properties. A single-wall carbon nanotube–MgAl₂O₄ composite was synthesized by Rul et al.¹² Multi-wall carbon nanotube based composites have been reported with SiC,¹³ TiN,¹⁴ borosilicate glass,¹⁵ SiO₂,¹⁶ ZrO₂,¹⁷ Si₃N₄,¹⁸ etc. In the last few years, many papers on the topic of graphene–ceramic or graphene–polymer composites have been reported as well. Graphene usually increases the electrical and thermal conductivities of graphene based composites; moreover, graphene can also improve the resulting mechanical properties. A silicon–graphene composite was prepared for reversible lithium storage.¹⁹ Graphene in Si₃N₄ is able to enhance the toughness of bulk ceramics.²⁰ TiO₂–graphene composites have been successfully synthesized.²¹ Alumina based graphene composites were reported by He et al.²² Fan et al. prepared Al₂O₃–graphene nanosheet composites by spark plasma sintering.²³ Al₂O₃–graphene nanocomposites can be used as conductive materials for high-temperature applications.²⁰ It can be assumed that grain boundaries are the principle influence on the physical properties, where graphene plays a key role thanks to its thermal and electrical conductivity. Moreover, aluminum carbide can be formed in traces at the grain boundaries, which can act as a barrier for phonons and thus the thermal conductivity can remain without significant changes. The mechanical properties of the alumina–graphene composite can be improved thanks to the outstanding mechanical properties of graphene. The graphene in the aluminum oxide has a significant effect on its electrical properties due to the high electrical conductivity of graphene and the resistivity of the formed composite can be tuned by varying the concentration of graphene within the alumina–graphene composite materials. The electrical transport and resistivity of the Al₂O₃–graphene composite is mainly determined by the graphene sheet boundaries, and the resistivity on its interface dominates the overall resistivity of the formed composite material. From this point of view the number of layers in graphene, defect density and concentration of remaining oxygen functional groups have only a minor influence on the resistivity of the composite material. The fabrication of graphene composite materials is also accompanied by the introduction of defects within the graphene structure due to material compaction by pressing and also the different lattice expansion coefficients of graphene and aluminum oxide during high temperature sintering.

Most graphene based composites are simply prepared by mixing and homogenizing the two different precursors. In contrast to the techniques and materials mentioned above we prepared an Al(OH)₃–graphene composite in one step from graphite oxide resulting in the formation of a nano-structured and completely homogenous material. Graphene was synthesized by the reduction of graphite oxide, which was prepared according to the Hofmann method.²⁴ This method is based on the oxidation of graphite using a strong oxidant (KClO₃) in a strong acid (HNO₃). The final graphite oxide contains high amounts of oxygen and hydrogen. Graphite oxide was reduced by active hydrogen, which was generated by reacting fine aluminum powder with sodium hydroxide solution. In addition to active hydrogen, the second product of the reaction was nanocrystalline Al(OH)₃, which is uniformly distributed in the sample.

Experimental section

The graphite oxide (HO–GO) was prepared according to a method described by Hofmann.^24,25 Graphite (5 g) was added to a mixture of sulfuric acid (98%, 87.5 ml) and nitric acid (68%, 27 ml). The resulting mixture was then cooled in an ice bath while potassium chlorate (55 g) was gradually added to it. Subsequently, the reaction mixture was stirred for 96 h at room temperature. Upon completion, the mixture was poured into deionized water (3 l) and decanted. The graphite oxide thus produced was re-dispersed in hydrochloric acid solution (5%) to remove sulfate ions and repeatedly centrifuged and re-dispersed in deionized water until a negative reaction for the presence of chloride and sulfate ions (with AgNO₃ and Ba(NO₃)₂ respectively) was achieved. The graphite oxide slurry was then dried in a vacuum dryer at 60 °C for 48 h before use.

For the synthesis, N,N-dimethylformamide (DMF), sulfuric acid (98%), nitric acid (68%), potassium chlorate (98%), barium nitrate (99%) and hydrochloric acid (37%) were purchased from Penta, Czech Republic. Natural graphite (<50 μm) was obtained from KOOH–I–NOOR Grafit, Czech Republic. Aluminum powder (>99.5%, 325 mesh) was delivered by Alfa Aesar and carbon dioxide (99.99%) by SIAD, Czech Republic.

The graphite oxide was dispersed in 100 ml of 1 M NaOH solution by 30 minute ultrasonication. 50 mmol of Al was placed in the graphite oxide dispersion. Hydrogen evolution started immediately after the addition of the metal (see eqn (1)).

Al + NaOH + 3H₂O → Na[Al(OH)₄] + H₂ (1)

The hydrogen formed reduced graphite oxide to graphene. After completion of the reaction the mixture was neutralized by bubbling CO₂ through it. In this reaction solid Al(OH)₃ is formed and Na⁺ remains dissolved (see eqn (2)).

2Na[Al(OH)₄]+ CO₂ → 2Al(OH)₃ + Na₂CO₃ + H₂O (2)

The formed composite, which was composed of graphene and Al(OH)₃, was filtered out and repeatedly washed with deionized water until a negative reaction for the presence of carbonate ions using Ba(NO₃)₂ was observed. The next step was drying the material under vacuum at 50 °C for 48 hours.

After characterization of Al(OH)₃–graphene the powder was calcined for 2 hours at 700 °C in a dynamic hydrogen atmosphere (100 cm³ min⁻¹). The heating and cooling rate of the furnace was set to 7 K min⁻¹. The obtained powder was uniaxially pressed at 0.5 GPa and sintered in a dynamic hydrogen atmosphere (100 cm³ min⁻¹) for 2 hours at 1000 °C in a molybdenum boat with the same heating and cooling rates as during the calcination. In another set of experiments the GO (0.05 g, 0.1 g, 0.25 g and 0.5 g) was dispersed in 400 ml of 1 M NaOH and 5.3 g of Al was added. After 24 hours the aluminum hydroxide was precipitated using CO₂. Subsequently the Al(OH)₃–graphene mixture was separated by decantation and repeated centrifugation. The composite was dried in a vacuum oven (48 hour/50 °C) and annealed at 700 °C for 2 hours in a hydrogen atmosphere. The obtained powder was uniaxially pressed at 0.5 GPa and sintered in a dynamic hydrogen atmosphere for 2 hours at 1000 °C.

All samples were analyzed by X-ray powder diffraction (XRD). Data were collected at room temperature with a PANalytical X'Pert PRO diffractometer in Bragg–Brentano parafocusing geometry using CuK_α radiation from 5° to 80° and compared with the graphite precursor. The particle size was calculated from the Scherrer equation.

Differential thermal analysis (DTA) and thermogravimetric (TG) curves of the Al(OH)₃–graphene composite were recorded simultaneously on Setaram STA apparatus (Setsys Evolution model) with a heating rate of 7 K min⁻¹ from ambient temperature to 700 °C to reproduce the calcination process. Measurements were performed in a dynamic argon atmosphere (50 cm³ min⁻¹). To determine the content of graphene in the final composite, STA measurements were performed in a dynamic oxygen atmosphere to 700 °C with a heating rate of 10 K min⁻¹.

High-resolution XPS spectra were measured with an ESCA ProbeP from Omicron Nanotechnology. Carbon conductive tape homogenously covered with graphene was used for the measurements. An Al X-ray source with a monochromator was used for the excitation.

An inVia Raman microscope (Renishaw, England) in backscattering geometry with a CCD detector was used to perform Raman spectroscopy. A DPSS laser (532 nm, 50 mW) with a 50× magnification objective was used for the measurement. The instrument was calibrated with a silicon reference which gives a peak position at 520 cm⁻¹ and a resolution of less than 1 cm⁻¹. The Al₂O₃–graphene composite was suspended in isopropanol (1 mg ml⁻¹) and ultrasonicated for 15 minutes (75 W). After the sedimentation of Al₂O₃, the suspension with graphene was dropped onto a silicon wafer and dried.

The morphologies of graphite oxide, the Al(OH)₃–graphene composite and the Al₂O₃–graphene composite were investigated by scanning electron microscopy using a Tescan Lyra dual beam microscope with an FEG electron source. Elemental composition and mapping measurements were performed with an energy dispersive analyzer (EDS) X-MaxN equipped with a 20 mm² SSD detector from Oxford instruments, using the Aztec software package. For these measurements powder samples were placed on carbon conductive tape.

Transmission electron microscopy was performed using a scanning transmission electron (STEM) detector on a Tescan Lyra dual beam microscope operated under an accelerating voltage of 30 kV. For the investigation of the graphene in the composite, the suspension with isopropanol (1 mg ml⁻¹) was ultrasonicated for 15 minutes (75 W) before use. After the sedimentation of Al₂O₃, 1 μL of graphene suspension was dropped on a copper carbon mesh (200 mesh grid). Bright field and dark field modes were used for observation. For atomic force microscopy (AFM) measurements the suspension with graphene was dropped on a freshly cleaved mica substrate. AFM measurements were carried out on an NT-MTD Ntegra Spectra from NT-MDT in tapping mode.

For the resistivity measurements of Al₂O₃–graphene 40 mg of the material was first compressed into a capsule (1/2′′ diameter) under a pressure of 400 MPa for 30 s. The resistivity of the resulting capsule was measured by the four-probe technique using the Van der Pauw method.²⁶

The high temperature thermal conductivity was calculated from the diffusivity, which was measured by laser flash analysis (LFA) on Linseis LFA 1000 apparatus with an Nd-YAG Laser from ambient temperature to 200 °C.

Results and discussion

The graphite precursor, graphite oxide (HO–GO), Al(OH)₃–graphene composite and the Al₂O₃–graphene composite (after the first calcination and after the final stage of sintering) were analyzed by XRD to investigate their phase compositions. X-ray patterns of graphite and graphite oxide (HO–GO) showed a difference between the (002) diffraction line, which is connected to the oxidation process (Fig. 1a). This diffraction line shifted from 2θ = 26.4° to 12.3°. Using the Bragg equation we calculated the distance between the graphene layers in HO–GO to be 7.20 Å, while in graphite the interlayer distance is 3.40 Å. After the reduction this peak almost disappeared, which confirmed successful exfoliation and reduction. From Fig. 1b it is obvious that Al(OH)₃ is present in addition to graphene. A detailed analysis showed two different polymorphs of Al(OH)₃: gibbsite and bayerite. Using the Scherrer equation the average particle size was calculated to be ∼24 Å for gibbsite and ∼60 Å for bayerite. After the calcination all of the Al(OH)₃ was transformed to nanocrystalline Al₂O₃ (see Fig. 1c) with a resulting particle size of ∼34 Å. After the final stage of sintering, the average crystalline size was calculated to be ∼80 Å with a pellet density of ∼1.9 g cm⁻³. The relatively low density is caused by the small particle size of Al₂O₃ and by the variable shape of graphene which hinders more efficient compacting of pellets.

Fig. 1 XRD patterns of (A) graphite and graphite oxide, (B) the Al(OH)₃–graphene composite and (C) the Al₂O₃–graphene composite after calcination (CALC) and after sintering (SINT).

The calcination process was studied by simultaneous thermal analysis (STA). DTA and TG curves are shown in Fig. 2a. Two endothermic peaks were observed on the DTA curve: the first at ∼224 °C is small and can be associated with the partial dehydration of Al(OH)₃ to AlO(OH). The second larger effect with a maximum at ∼294 °C is caused by complete dehydration and leads to the formation of pure Al₂O₃. The TG curve shows the weight loss during heating. Assuming that the weight loss is entirely due to the release of water we can calculate the ratio of Al₂O₃ to graphene in the sample. 30.8 wt% of water corresponds to a composition of 89.1 wt% Al₂O₃ and 10.9 wt% graphene.

Fig. 2 STA analysis of the calcination process in an inert atmosphere (A) and the thermal stability of the Al₂O₃–graphene nanocomposite in an oxygen atmosphere (B).

Moreover, DTA and TG analyses of the final composite material in pure oxygen were performed and the thermal stability of Al₂O₃–graphene was determined (see Fig. 2b). During heating, all graphene was burnt and white Al₂O₃ powder was left. The weight loss of Al₂O₃–graphene was ∼11.1 wt%, which corresponds to the same amount of graphene in the composite. This composite is stable up to ∼400 °C in an oxygen atmosphere.

Consequently, the Al₂O₃–graphene composite was characterized by X-ray photoelectron spectroscopy (XPS) to determine the chemical composition and to get a better insight into the bonding characteristics of carbon in graphene. For this purpose high resolution XPS of the C 1s peak at an energy of ∼285 eV was performed.

The XPS survey spectra (Fig. 3a) were measured to confirm all of the elements present in the composite. Several peaks can be observed on the survey spectra: Al 2p at 74.2 eV, Al 2s at 119.0 eV, C 1s at 285.1 eV and O 1s at 533.5 eV. More detailed information on the degree of reduction was obtained from high resolution XPS performed for the C 1s peak in Fig. 3b. The deconvolution of the C 1s peak was carried out for the C=C bond at 284.5 eV, the C–C/C–H bonds at 286 eV, the C–O bond at 287.2 eV, the C=O bond at 288.6 eV, the O–C=O bond at 289.8 and the π–π* interactions at 290.8 eV. The results are summarized in Table 1.

Fig. 3 Wide-range XPS spectra of the Al₂O₃–graphene nanocomposite (A), detail of high resolution XPS spectra of the C 1s peak (B), where fittings of the individual C1s spectra show the possible carbon bonding present in graphene oxide, (C) the Al 2p and Al 2s peak and (D) a detailed view of the O 1s peak.

Table 1 Quantitative comparison of high-resolution X-ray photoelectron spectra of the C 1s core level in the Al₂O₃–graphene composite

Groups	Concentration (%)
–C=C	61.8
C–C/C–H	20.5
C–O	11.7
C=O	3.4
O–C=O	1.7
π–π*	0.9

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In addition to C 1s, high resolution spectra of the Al 2p, Al 2s and O 1s peaks were also measured (see Fig. 3c and d). Al 2p was observed at 74.2 eV while the Al 2s peak has a maximum at 118.8 eV. The O 1s peak at 532.5 eV is relatively broad as it is associated with different types of bonds. The presence of a carbonyl group with a carbon–oxygen double bond (C=O) and a C–O single bond, like in a hydroxyl group, are expected in graphene. Moreover, an Al–O bond is also present. All of these bonds have their maxima between 530.5 eV and 533.3 eV.

A careful analysis of the O 1s peak was performed to calculate the band gap (see Fig. 4).²⁷ Our obtained value of ∼6 eV is low compared to the band gap of 8.8 eV reported for pure Al₂O₃.²⁸ This can be explained in terms of the highly defected structure of Al₂O₃ that originated in the strong reducing atmosphere during sintering.

Fig. 4 The band gap of the Al₂O₃–graphene composite from the O 1s peak obtained by high resolution XPS.

The Raman spectra of HO–GO and graphene in the Al₂O₃–graphene composite (Fig. 5) show two major bands: the G-band at ∼1580 cm⁻¹ corresponds to vibrational modes in the planar hexagonal lattice of the graphitic materials and the D-band at ∼1340 cm⁻¹ is attributed to the defects in this lattice. There are two minor bands at higher wavenumbers. The 2D band appears at ∼2690 cm⁻¹ and the D′′ band appears at ∼2930 cm⁻¹.²⁹ The ratio between the D and G band intensities (D/G ratio) can be used as an indication of the degree of disorder in a carbon structure. The D/G ratio was calculated to be 0.97 for HO–GO and 0.99 for graphene, which means that the D/G ratio remained almost unchanged. A significant change can be observed on comparison of the backgrounds of both spectra. The much higher background in HO–GO is connected to the luminescence, which is significantly reduced after the successful reduction.

Fig. 5 Raman spectra of graphite oxide and graphene.

The morphological and topographical characterization of the three stages of material fabrication was carried out using SEM. The corresponding images are shown in Fig. 6. Fig. 6a shows graphite oxide prior to exfoliation, Fig. 6b and c show the composites Al(OH)₃–graphene and Al₂O₃–graphene, respectively. It was observed from these images that graphite oxide had been successfully exfoliated because the graphene structure was similar to that of the exfoliated graphene oxide, where separate sheets could be clearly observed (see the wrinkled morphology of the sheets).

Fig. 6 Scanning electron microscopy of (A) graphite oxide, (B) the Al(OH)₃–graphene composite and (C) the Al₂O₃–graphene composite.

The surface of the Al₂O₃–graphene pellet was further characterized by SEM-EDS performed on areas of 5 × 5 μm to determine the amount of graphene in the structure. An average of 11.8 wt% of carbon was calculated which is in very good agreement with the results obtained by STA. The distribution of individual elements in the composite can be seen in Fig. 7.

Fig. 7 SEM-EDS of the surface of the Al₂O₃–graphene pellet. The elemental composition maps of carbon, aluminum and oxygen show the distribution of the individual elements in the sample.

A typical AFM image of Hofmann graphite oxide is in Fig. 8a. For more details of graphene separation from the composite see the Experimental section. The image shows a few layers of graphene. The cross section reveals approximately 5-layers with twisted edges. The AFM measurements generally show the presence of multi-layered graphene. The STEM images (see Fig. 8b) also show the presence of few-layered graphene where the number of layers differs from the edges to the middle of the flake. The approximate size of the flakes is around 3–5 μm. The wrinkled structure of graphene is related to the defects induced by the oxidation of the starting graphite material and the subsequent reduction. The defects within the graphene structure are formed during the reduction of the oxygen containing functional group on the graphene oxide surface.

Fig. 8 Typical AFM image of a graphene flake (A) and the respective cross-section height profile (right) and STEM images of a graphene flake (B) in dark field (left) and bright field (right).

Furthermore, the transport properties were measured. The average electrical resistivity of the Al₂O₃–graphene composite was measured by the four-point probe method to be 2.9 ± 0.2 kΩ cm. The obtained values show that 11 wt% of graphene in the aluminum oxide composite is sufficient to obtain the percolation limit. The electrical conductivity of the obtained composite also indicates a homogenous distribution of graphene within the aluminum oxide which was also corroborated by EDS and electron microscopy.

In addition to these results a series of samples with various graphene concentrations, 0.6 wt%, 0.8 wt%, 1.5 wt% and 2.1 wt%, were investigated. The concentration of carbon was measured by elemental combustion analysis. These samples were obtained by the reduction of 0.05 g, 0.1 g, 0.25 g and 0.5 g GO with 5.3 g of Al and the subsequent precipitation of aluminum hydroxide and further thermal treatment according to the procedures presented in the Experimental section. Only the concentration of 2.1 wt% was sufficient to obtain a reasonable value of electrical resistivity (765 ± 1.3 kΩ cm). The resistivity of the sample with 1.5 wt% graphene was 35.2 ± 0.4 MΩ cm and the resistivities of the other samples with lower concentrations were over 100 MΩ cm. Hence, the presence of graphene in the alumina–graphene composite has a significant effect on the electrical conductivity, with the percolation limiting concentration being about 2 wt% of graphene within the alumina. The graphene itself exhibits an extremely high conductivity which contradicts the results observed for the composite material. From this observation we can conclude that the resistivity of the composite depends on the homogenous distribution of graphene and the resistivity of contacts between the separated highly conductive graphene sheets. The possible formation of aluminum carbide on the alumina–graphene interface and defects present in the graphene have only minor effects on the conductivity of the composite material. The dominant factor affecting the overall resistivity of the composite is the resistivity on the interfaces of the graphene sheets. This effect is schematically shown in Fig. 9.

Fig. 9 A schematic drawing of the propagation of electric current and heat represented by phonons through the alumina–graphene composite material.

The thermal conductivity was measured by the laser flash method for the Al₂O₃–graphene composite and also for pure Al₂O₃ without graphene for comparison (see Fig. 10). The pure aluminum oxide used for comparison was obtained through the heat treatment of the graphene composite in an oxygen atmosphere. The calcination and sintering steps were performed using a similar temperature program except that a pure oxygen atmosphere was used instead of hydrogen. This synthesis led to alumina oxide with a density of 2.0 g cm⁻³. The low values of thermal conductivity obtained for both investigated samples are related to the low density and very high porosity of the samples prepared by uniaxial pressing, and the microstructure of alumina with nanosized grains. The extremely small size of the grains led to a strong scattering of phonons and significant suppression of the thermal conductivity. The Al₂O₃–graphene composite material exhibits much lower values of thermal conductivity compared to the pure oxide. The homogenous distribution of graphene sheets within the material is accompanied by an increase in the phonon scattering on grain boundaries and aluminum oxide–graphene interfaces. In addition to these experiments the thermal conductivity of the alumina–graphene composite with various graphene concentrations was investigated. The results are shown in the ESI (Fig. S1†). The dependence of the thermal conductivity and electrical resistivity on the carbon percentage loading is shown in Fig. S2.† From these additional experiments we can conclude that there is no observable evidence that the heat conductivity depends on the percentage loading of graphene in the composite material. The mechanism of thermal conductivity is schematically shown in Fig. 9. The heat transfer by phonons is strongly influenced by many other factors such as the material porosity, grain boundaries and their size. The higher thermal conductivities obtained for the new set of samples are mostly associated with the lower degrees of porosity (sample density 2.2–2.5 g cm⁻³). Actually, the presence of graphene reduces grain growth, sintering and the suppression of porosity. This is demonstrated by the lower density and heat conductivity of the sample with a high percentage loading of graphene (11 wt%). Graphene particles also act as phonon scattering centres on the alumina–graphene and also graphene–graphene interfaces. This allowed the synthesis of a material with a high electrical conductivity whilst simultaneously preserving good thermal insulating properties.

Fig. 10 Thermal conductivity of Al₂O₃–graphene and pure Al₂O₃ measured by LFA.

Conclusions

In this study we demonstrated a simple procedure for the synthesis of an aluminum oxide–graphene nanocomposite material with excellent conductivity and temperature insulating properties. The composition of the material was characterized by XPS, EDS and STA. The data obtained from various methods are in good agreement. The obtained concentration of graphene in the composite was up to 11 wt% and was sufficient to reach the percolation limit. The limiting concentration for electrical current percolation is about 2 wt% of graphene within the composite material. The structural properties of the prepared material were characterized using SEM, Raman spectroscopy and X-ray diffraction. The obtained data indicate uniformly distributed graphene within the nanostructured alumina. Surprisingly this material exhibits a significantly lower thermal conductivity compared to pure alumina prepared in a similar way. These findings can be used for the development of high temperature thermally insulating and electrically conducting materials suitable for various high temperature applications.

Acknowledgements

This research was supported by Specific University Research (MSMT no. 20/2013). M. P. thanks NAP (NTU) fund.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45069d

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