Mechanical and electrical properties of low SWNT content 3YTZP composites

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Mechanical and electrical properties of low SWNT content 3YTZP composites R. Poyato1,*, J. Macías-Delgado1,2, A. García-Valenzuela1,2, Á. Gallardo-López1,2, A. Morales-Rodríguez1,2, A. Muñoz2, A. Domínguez-Rodríguez2. 1 Inst. Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla), Avda. Américo Vespucio 49, 41092 Sevilla (Spain). 2 Dep. Física de la Materia Condensada, Univ. de Sevilla, apdo. 1065, 41080 Sevilla (Spain). Abstract Fully dense 3 mol% Y2O3-ZrO2 (3YTZP) composites with low single wall carbon nanotube content (0.5, 1 and 1.5 vol% SWNT) were prepared by colloidal processing and Spark Plasma Sintering. SWNT were distributed at ceramic grain boundaries and also into agglomerates. Characterization of SWNT agglomerates indicated that increase in SWNT vol% does not imply an increase in agglomeration. SWNT agglomerate density was related to the evolution of hardness and fracture toughness with SWNT vol%. Electrical properties of the composites were characterized in a wide temperature range, and percolation threshold was estimated. A model allowing separation of the individual SWNT bundles contribution to resistance from the resistance due to junctions between bundles was proposed for composites with a percolating SWNT network. Keywords: composites; carbon nanotubes; processing; mechanical properties; electrical properties * Corresponding author. E-mail address: [email protected] Phone number: +34 954 48 95 34 FAX number: +34 954 46 06 65 *Manuscript Click here to view linked References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 2 Introduction Within the past 20 years, the scientific and engineering communities have paid great attention to carbon nanotubes (CNTs) due to their attractive properties. CNTs can show a Young’s modulus up to 1.2 TPa, a tensile strength around a hundred times higher than steel and an elevated resilience. They also possess tunable surface properties, well-defined hollow interiors, and can be either metallic or semiconducting depending on diameter and chirality.1,2 Based on these fascinating properties, many potential applications such as scanning probe tips, drug delivery systems, electronic devices, sensors and actuators, high-strength composites, catalyst support, field emission displays, transparent conducting films, and so on were proposed.3,4 In these years, both the research articles and patents on this area increased rapidly. Recently, Zhang et al.5 analyzed the publications topic tendency from 2001 to 2011, showing the study of composites with CNTs to be one of the emerging areas, with an increase from 13 to 27% of the total publications. Interest in these composites is based on the possibility of transferring some of the attractive properties of CNTs to the resulting composites6. Among advanced ceramics, yttria-doped zirconia is considered a technologically interesting material due to its superior mechanical properties and good ionic conductivity.7,8 In recent years, several works were devoted to the study of zirconia/CNTs composites, pursuing an enhancement of the mechanical properties. However, up to date, this enhancement has not been clearly demonstrated. A decrease of Vickers hardness when increasing CNT content was reported by different authors,9-13 even for composites with low CNT vol%.14-16 Although some authors related this effect to the observed decrease in composite density when increasing CNT content,9,11 it has been also reported for fully densified composites with SWNT contents up to 10 vol%.13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 3 The decreasing trend was also linked to higher presence of agglomerated CNTs17 in composites with high amount of nanotubes, since it is assumed that agglomeration becomes more relevant due to dispersion difficulties during processing.18 However, recent investigations on Al2O3/SWNT composites have pointed out that the presence of agglomerates does not play a fundamental role on the decrease in hardness.19 On the contrary, it was rather explained by the presence of higher SWNT quantities at the grain boundaries. The detachment between SWNTs within thick bundles has been pointed out as the origin of the lower fracture toughness obtained for high SWNT content 3YTZP/SWNT composites when compared to monolithic ceramic.13 In this context, the study of low SWNT vol% composites, in order to asses whether a lower SWNT content at the grain boundaries results in an enhancement of the mechanical properties, appears as a challenge. The study of the electrical properties of ceramic/CNT composites has recently also come into focus, as adding CNTs to a ceramic matrix also modify the electrical conductivity of the resulting composite. Most of the studies have focused on the analysis of the electrical percolative behaviour,11,16,20-26 since a step raise in electrical conductivity is the common trend observed in composites once percolation of the CNT network is achieved. Percolation thresholds from 0.64 to 5.5 vol% have been published for composites with alumina or zirconia matrix.11,16,21-25 This wide range of reported values can be related to the different processing techniques used to prepare the materials. Shin and Liang22,23 and Fonseca et al.24 reported percolation thresholds of 5.5 and 3 vol% CNTs for 3 mol% Y2O3-ZrO2/MWNT and 8 mol% Y2O3-ZrO2/SWNT composites, respectively. In these studies, composite powders were processed by ball milling the mixture of CNTs and ceramic powder. Recently, composites obtained from powders prepared in a similar way have been shown to present a high amount of CNT 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 4 agglomerates,27 which would result in a lower CNT content at the ceramic grain boundaries than the nominal one. Thus, a higher amount of CNT would be required in order to achieve a percolating network, resulting in an overestimated percolation threshold. These results highlight the need to correlate the CNT agglomerate density and the percolation threshold in ceramic/CNT composites. Regarding the ac conductivity on ceramic/CNT composites, the published studies are scarce.24-26 Only Fonseca et al.24 and González-Julián et al.26 reported electrical properties in ac conditions in a wide temperature range. A mixed ionicelectronic conductivity was described for 8 mol% Y2O3-ZrO2/SWNT composites.24 Charge transport along the nanotube shells and hopping conduction across nanotubenanotube junctions were suggested as the two contributions to electrical conductivity in Si3N4/MWNT composites.26 To the best of our knowledge, studies on ac conductivity and charge transport contributions in 3YTZP/SWNT composites have not been published up to date. In this paper, 3 mol% yttria doped zirconia composites containing low SWNT content (0.5, 1 and 1.5 vol%) were prepared by a combination of aqueous colloidal processing and Spark Plasma Sintering, with the aim of obtaining a homogeneous SWNT distribution throughout the ceramic matrix and minimizing the presence of agglomerates. The SWNT agglomerate density was characterized and related to the evolution of hardness and fracture toughness with SWNT vol%. Electrical properties of the composites were characterized in a wide temperature range. Conductivity measurements at room temperature allowed determination of the percolation threshold. Modelling of the impedance properties of the composite with a percolating SWNT network was carried out, and an equivalent circuit which separates the individual 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 5 SWNT bundles resistance contribution from the resistance due to junctions between bundles was proposed. 2. Experimental procedure Raw materials and processing Monolithic polycrystalline 3YTZP and SWNT/3YTZP composites with different carbon nanotube content (0.5, 1 and 1.5 vol%) were prepared from 3 mol% yttria stabilized tetragonal zirconia powder (3YTZP, 40 nm particle size and 99% purity) supplied by Nanostructured and Amorphous Materials Inc. (Houston, TX) and HIP-co purified SWNTs provided by Carbon Solutions Inc. (Riverside, CA). Acid treatment of the SWNTs was carried out using a mixture of concentrated sulfuric acid (98%) and nitric acid (70%) in the ratio 3:1, with the aim of disentangle and cut the raw SWNTs ropes.28 This treatment also introduces carboxyl groups on the walls and ends of SWNTs enabling their dispersion in basic medium. SWNTs were suspended in the acid mixture for 24 h at room temperature and the suspension was sonicated for 8 h. SWNTs were collected on ∼20 nm pore alumina filter membranes, washed in high purity ethanol for several times and freeze-dried in order to avoid possible re-agglomeration. Colloidal processing of composite powders with the different SWNT contents was carried out using ammonia solution as a basic medium.28 Ceramic powder and acidtreated SWNT suspensions were subjected to ultrasonic agitation using a sonication bath before and after mixing. Composite powder blends were dried on a hot plate assisted by stirring, and pH and homogeneity were controlled during the process. Finally, composite powders were homogenized in an agate mortar. Ceramic sintering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 6 SPS (Model 515S, SPS Dr Sinter Inc., Kanagawa, Japan) was used to sinter the materials in a 15-mm diameter cylindrical graphite die/punch setup in vacuum atmosphere. The sintering processes were carried out at 1250 ºC with a hold time at peak temperature of 5 min, under uniaxial pressure of 75 MPa. Heating and cooling rates were 300 and 50 °C/min, respectively. Bulk densities were measured using Archimedes’ method, with distilled water as immersion medium. Theoretical density values for composites were calculated by the rule of mixtures assuming density values of 6.10 g·cm-3 for 3YTZP and 1.80 g·cm-3 for SWNTs. Microstructural, mechanical and electrical characterization Structural integrity of SWNTs in the composites after SPS sintering was assessed by Raman spectroscopy on fracture surfaces using a dispersive microscope (Horiba Jobin Yvon LabRam HR800, Kyoto, Japan) equipped with a 20 mW He-Ne green laser (532.14 nm). The microscope used a 100x objective and a confocal pinhole of 100 µm. The Raman spectrometer was calibrated using a silicon wafer. Microstructural studies of composites fracture and polished surfaces were performed by high-resolution scanning electron microscopy (HR-SEM), using a Hitachi S5200 microscope (Hitachi High-Technologies America Inc., USA), to analyze the distribution of SWNTs in the 3YTZP matrix, and to characterize the ceramic grains morphology. Distribution and morphology of SWNT agglomerates were characterized by low magnification conventional SEM (Model JEOL 6460LV, JEOL USA Inc., MA, USA). Cross section slices, i.e. surfaces parallel to the SPS pressing direction were polished with diamond paste up to 1 µm for morphological studies. Additionally, polished surfaces devoted to characterize the 3YTZP grains were thermally etched at 1200 °C for 20 min in air to reveal grain boundaries. The morphology characterization 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 7 was made measuring 200 grains or agglomerates, respectively, to obtain the equivalent planar diameter as size parameter, d (or D)=2(area/π)1/2, and the shape factor, f (or F)=(4π·area)/(perimeter)2. Standard deviation of distributions was also evaluated. Hereafter, lowercase letters will refer to 3YTZP grains parameters and uppercase letters to agglomerates ones. Agglomerate surface density was evaluated from the area fraction covered by them in low magnification SEM micrographs. ImageJ software was used for morphological analysis. Vickers indentation tests were carried out to evaluate the hardness and fracture toughness of sintered 3YTZP and composites at room temperature. Tests were performed on sample surfaces polished to 1 µm diamond paste using a Vickers indenter with a range of loads up to 2 kgf (Duramin Struers, Germany). Twelve indents were made on each sample avoiding boundary effects (i.e. keeping the appropriate distance from sample edges and between indentations marks) and were analyzed using a confocal microscope LEICA DCM 3D. Vickers hardness, Hv, was calculated from the indentation load, P, and the diagonal of Vickers imprints, a: Hv=1.854(P/a2). Fracture toughness, KIC, was calculated by using the equation given by Anstis et al.29 where c is the crack length measured from the centre of the imprint and E the elastic modulus. The crack length was measured 24 h after the indentation, once the cracks were fully developed.               =2/3 2/1 016.0 c P H E K v IC (1) Electrical characterization was carried out by Impedance Spectroscopy using an Agilent 4294A analyzer in the frequency range from 100 to 2x106 Hz, at temperatures from 25 to 450 °C. Measurements were carried out in argon atmosphere to avoid oxidation of the samples and subsequent degradation of the SWNTs during the process. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 8 Colloidal silver paste was applied on both sides of the samples and electrodes were fired at 600 °C for 30 min under argon flow. Equivalent circuit approach was adopted for the data analysis with fitted curve using Z-view software and equivalent circuit model. 3. Results and discussion 3.1 Microstructural characterization Table 1 displays the density values together with the global results of morphological parameters of as-sintered samples. A full densification was obtained in all the sintered materials. Similar mean grain size (~230 nm) and shape factor (~0.75) were measured in the different composites and no significant differences were observed compared to monolithic zirconia grains, except for slightly narrower size distributions (smaller σ<d>). Fig. 1 shows the Raman spectra measured in the composites, monolithic 3YTZP ceramic and as-received SWNTs. The composites spectra show SWNT characteristic radial breathing mode (RBM) band near 150–200 cm−1 and G-band near 1500–1600 cm−1. In the G-band it can be observed the lower-frequency broad shoulder centred around 1570 cm−1 (Breit-Wigner-Fano or BWF lineshape), which is characteristic of metallic SWNT28. The composites spectra are very similar to the SWNT spectrum before processing of the composites, clearly confirming the absence of significant damage to SWNTs during powder processing and sintering. A G-band shift towards higher frequencies, by ∼ ∼∼ ∼20 cm−1, is observed in the three composites, which can be attributed to residual stresses in the SWNTs imposed by the constraining ceramic matrix28. D-band (centred on 1350 cm−1), associated to disordered graphite and crystalline defects, is also observed. ID/IG ratio calculations give similar values for the three composites (6.6, 6.5 and 7.2% for 0.5, 1 and 1.5 vol% 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 9 SWNT, respectively) pointing to a similar amount of crystalline defects in the nanotubes. On the other hand, peaks at 165, 260, 320, 465, 610, and 643 cm−1 are observed in the spectra acquired in the composites, corresponding to the six Raman bands theoretically predicted for tetragonal zirconia.30 HR-SEM micrographs of characteristic fracture surfaces of the composites are presented in Fig. 2. SWNT bundles are located at the ceramic grain boundaries and debonded CNTs from the matrix can also be observed. Whereas the 3YTZP grains surrounded by SWNT bundles are scarce in the composite with 0.5 vol% SWNT (fig 2(a)), the amount of SWNT bundles at the grain boundaries increases in the composites with higher SWNT vol% (figures 2(b) and 2(c)). CNTs are mainly well distributed on the 3YTZP matrix; however, some agglomerates are also found in these high-resolution observations. The presence of these agglomerates or clusters has been previously reported in CNT/ceramic matrix composites.13,19,27,31,32 Low-magnification SEM micrographs (Fig. 3) illustrate the arrangement and morphology of SWNT agglomerates in the studied composites. The maximum agglomerate size ranges from 30 to 60 µm (Table 2), which is similar to previously published values for Al2O3/MWNT composites also prepared by colloidal processing32. Although it is assumed that the tendency of forming agglomerates is due to Van der Waals interactions between nanotubes, they are not expected to lead to such important agglomerate sizes in composites with a rather low SWNT content32 as in this study. Other forces that might play a key role in the behaviour of the 3YTZP/SWNT powder mixtures have been suggested, and it was shown from thermodynamic considerations that long, thin rods mixed with spheres can induce phase separation and demixion32. Similar mean agglomerate size, about 7–9 µm, and a marked elongation (F ~ 0.45) are found for the three composites. These morphological characteristics are similar 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 16 fraction of the grain boundaries is covered by SWNTs reduces the ionic conductivity resulting in an increase of the grain boundary resistivity for temperatures up to 350 ºC, and thus, an increase of the arc. For higher temperatures this effect is overcome and the grain boundary conductivity increases with temperature. Equivalent circuit approach of these data in order to obtain conductivity values for the different contributions presents a high complexity and will be addressed in future studies. Figure 7 shows the Arrhenius plots for total conductivity in the studied composites, together with monolithic zirconia. Total resistivity was calculated from resistance values obtained from intercepts on the real, Z’, axis. The slope of these diagrams was used to calculate the activation energy of the conducting species (Table 3). For composites with 0.5 and 1 vol% SWNT, as well as monolithic zirconia, a value close to 0.8 eV was obtained. This result is consistent with the activation energy for oxygen vacancy conduction in zirconia-based ceramic oxides.44,45,46 Thus, these two composites behave as ionic conductors without SWNT percolation. For the composite with 1.5 vol% SWNT, a completely different behaviour was observed, with activation energy of 65 meV, revealing that SWNT contribution to charge transport dominates for temperatures up to 350 ºC. In this case, an ionic-electronic mixed conductor is obtained due to the percolated SWNT network. A similar result was reported for composites of 8 mol% Y2O3/ZrO2 with SWNTs24, with activation energy of 30 meV. 4. Conclusions Fully dense 3 mol% yttria doped zirconia matrix composites containing 0.5, 1 and 1.5 vol% SWNT, with equiaxed grain microstructure and almost analogous ceramic grain size (250 nm), were prepared by colloidal processing and SPS. SWNTs were homogeneously distributed at grain boundaries and also into large SWNT agglomerates. Similar agglomerate mean size and shape were found for the three composites. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 17 Increasing SWNT vol% leads to similar percentage of the total SWNT content contained in agglomerates (~30%), and an increasing SWNT content at the grain boundaries. Similar Vickers hardness and slightly enhanced fracture toughness were obtained for the composites in comparison with the monolithic 3YTZP ceramic. The increase in the surface density of agglomerates does not play a fundamental role in their evolution when increasing SWNT content. Percolation threshold of the carbon nanotubes in the 3YTZP matrix was estimated to be between 0.74 and 1.1 vol% SWNT corresponding to nominal contents of 1 and 1.5 vol%, respectively. Composites with 0.5 and 1 vol% SWNT behaved as ionic conductors without SWNT percolation, with ~0.8 eV activation energy. Room temperature conductivity in the composite with 1.5 vol% SWNT revealed the existence of a percolated SWNT network in this material, which presented a mixed ionic-electronic conduction with 65 meV activation energy. Modelling of impedance properties in this composite using an equivalent circuit allowed separation of the individual SWNT bundles contribution to resistance from the resistance due to junctions between bundles. The existence of junctions involving large diameter bundles or SWNT agglomerates resulted in higher junction resistivity in comparison with the SWNT bundles resistivity. The remarkable decrease of junctions’ resistivity with increasing temperature up to 160 ºC was successfully described in terms of the fluctuation-induced electron tunnelling across nanotube-nanotube junctions. Acknowledgements The authors acknowledge the financial support provided by the Spanish Ministry of Science and Innovation (MAT2012-34217) and Junta de Andalucía (P12-FQM- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 18 1079). Microscopy studies have been performed in facilities belonging to the CITIUS (Universidad de Sevilla). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 19 Figure captions Figure 1: Raman spectra measured in the composites including the RBM frequency range and the D and G-Bands frequency range. Raman spectra measured in the monolithic 3YTZP ceramic and in the SWNTs have been included for comparison. Figure 2: HRSEM micrographs of fracture surface of the composites with different SWNT contents (a) 0.5 vol%, (b) 1 vol%, and (c) 1.5 vol%. Figure 3: SEM micrographs showing the SWNT agglomerate distribution on the crosssections of composites with different SWNT contents (a) 0.5 vol%, (b) 1 vol%, and (c) 1.5 vol%. Figure 4: (a) Vickers Hardness and (b) fracture toughness for monolithic 3YTZP and SWNT/3YTZP composites and comparisons with the bibliography values. Figure 5: (a) Impedance plots acquired in the composite with 1.5 vol% SWNT from room temperature to 180 ºC and (b) SWNT bundles and junctions resistivity in this temperature range. The inset in (a) shows the equivalent circuit used to fit the impedance data. Figure 6: Impedance plots acquired in the composite with 1.5 vol% SWNT from 200 to 400 ºC. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 20 Figure 7: Arrhenius plots of the electrical conductivity for monolithic 3YTZP and 3YTZP/SWNT composites. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 21 Table 1: Theoretical and relative density and morphological parameters for ceramic grains in monolithic 3YTZP and SWNT/3YTZP composites. SWNT vol% ρ ρρ ρ th (g·cm-3) ρ ρρ ρ r (%) <d> (nm) σ σσ σ <d> (nm) f 0 6.10 100 2 15 85 0.7 5 ±0.08 0.5 6.08 100 230 80 0.74±0.08 1 6.06 100 215 75 0.73±0.06 1.5 6.04 99.4 240 80 0.76±0.05 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 22 Table 2: Surface density of SWNT agglomerates, percentage of carbon nanotube content in agglomerates and at grain boundaries calculated for each composite and morphological parameters of agglomerates. SWNT vol% ρ ρρ ρ s (%) A (%) A-SWNT vol% GB-SWNT vol% <D> (µ µµ µm) σ σσ σ <D> (µ µµ µm) Dmáx (µ µµ µm) F 0.5 0.18 ± 0.07 36 0.18 0.32 9 7 60 0.4 ± 0.3 1 0.26 ± 0.03 26 0.26 0.74 7 4 30 0.5 ± 0.2 1.5 0.40 ± 0.05 27 0.40 1.10 8 5 40 0.5 ± 0.2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 23 Table 3: Mechanical and electrical properties measured for the studied materials. SWNT vol% Hv (GPa) KIC (MPa·m 1/2 ) σ σσ σ RT (S·cm - 1 ) Eact (eV) 0 12.7±0.4 4.3±0.3 ---- 0.86±0.02 0.5 13.4±0.3 4.2±0.3 ---- 0.84±0.08 1 12.6 ± 0.2 4.4 ± 0.2 ---- 0.76 ± 0.07 1.5 12.9 ± 0.3 4.6 ± 0.3 6x10 - 6 0.065 ±0.0 02 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 24 Table 4: Equivalent circuit values for the fitting of the impedance properties of the composite with 1.5 vol% SWNT. T (ºC) ρ ρρ ρ J (k Ω ΩΩ Ω ·cm) CJ (pF) ρ ρρ ρ B (k Ω ΩΩ Ω ·cm) CB (pF) 25 158±13 272±3 10.4±0.9 137±3 40 135±11 279±3 9.1±0.8 139±4 60 113±10 293±4 7.8±0.7 139±4 80 93±7 302±4 6.9±0.5 137±4 100 83±6 318±4 6.4±0.5 136±4 120 76±6 340±6 6.5±0.6 132±4 140 70±5 351±5 6.1±0.4 132±4 160 66±6 400±8 6.4±0.6 135±4 180 56±5 475±1 6.6±0.6 128±5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 25 References 1 Baughman RH, Zakhidov AA, de Heer WA. Carbon nanotubesthe route toward applications. Science 2002; 297: 787-92. 2 Zhang Q, Huang JQ, Zhao MQ, Qian WZ, Wei F. Carbon nanotube mass production: principles and processes. Chem Sus Chem 2011; 4: 864-89. 3 Endo M, Strano MS, Ajayan PM. Potential applications of carbon nanotubes. Top Appl Phys 2008; 111: 13-61. 4 Schnorr JM, Swager TM. Emerging applications of carbon nanotubes. Chem Mater 2011; 23: 646-57. 5 Zhang Q, Huang JQ, Qian WZ, Zhang YY, Wei F. Small 2013; 9: 1237–65. 6 Padture NP. Multifunctional composites of ceramics and single-walled carbon nanotubes. Adv Mater 2009; 21: 1767–70. 7 Deville S, Gremillard L, Chevalier J, Fantozzi G. A critical comparison of methods for the determination of the aging sensitivity in biomedical grade yttria-stabilized zirconia. J Biomed Mater Res B Appl Biomater 2005; 72: 239–45. 8 Minh NQ. Ceramic fuel cells. J Am Ceram Soc 1993; 76: 563–88. 9 Duszová A, Dusza J, Tomasek K, Blugan G, Kuebler J. Microstructure and properties of carbon nanotube/zirconia composite. J Eur Ceram Soc 2008; 28: 1023–27. 10 Garmendia N, Grandjean S, Chevalier J, Diaz LA, Torrecillas R, Obieta I. Zirconiamultiwall carbon nanotubes dense nano-composites with an unusual balance between crack and ageing resistance. J Eur Ceram Soc 2011; 31: 1009–14. 11 Shin JH, Hong SH. Microstructure and mechanical properties of single wall carbon nanotube reinforced yttria stabilized zirconia ceramics. Mater Sci Eng A 2012; 556: 382–7. Figure 2 Click here to download high resolution image Figure 3 Click here to download high resolution image Figure 4 Click here to download high resolution image Figure 5 Click here to download high resolution image Figure 6 Click here to download high resolution image Figure 7 Click here to download high resolution image 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 1: Raman spectra measured in the composites including the RBM frequency range and the D and G-Bands frequency range. Raman spectra measured in the monolithic 3YTZP ceramic and in the SWNTs have been included for comparison. Figure 2: HRSEM micrographs of fracture surface of the composites with different SWNT contents (a) 0.5 vol%, (b) 1 vol%, and (c) 1.5 vol%. Figure 3: SEM micrographs showing the SWNT agglomerate distribution on the crosssections of composites with different SWNT contents (a) 0.5 vol%, (b) 1 vol%, and (c) 1.5 vol%. Figure 4: (a) Vickers Hardness and (b) fracture toughness for monolithic 3YTZP and SWNT/3YTZP composites and comparisons with the bibliography values. Figure 5: (a) Impedance plots acquired in the composite with 1.5 vol% SWNT from room temperature to 180 ºC and (b) SWNT bundles and junctions resistivity in this temperature range. The inset in (a) shows the equivalent circuit used to fit the impedance data. Figure 6: Impedance plots acquired in the composite with 1.5 vol% SWNT from 200 to 400 ºC. Figure 7: Arrhenius plots of the electrical conductivity for monolithic 3YTZP and 3YTZP/SWNT composites. Figure Captions