Effect of acid-treatment and colloidal-processing conditions on the room temperature mechanical and electrical properties of 3YTZP/MWNT ceramic nanocomposites

1 Effect of acid-treatment and colloidal-processing conditions on the room temperature mechanical and electrical properties of 3YTZP/MWT ceramic nanocomposites R. Poyato 1,2 , A. Morales-Rodríguez 1,2 , F. Gutiérrez-Mora 1,2 , A. Muñoz 2 , Á. Gallardo- López 1,2 . 1. Inst. Ciencia de Materiales de Sevilla (CSIC-US). Américo Vespucio 49. 41092 Sevilla. Spain. 2. Dpto. de Física de la Materia Condensada. Univ. de Sevilla. Apdo. 1065. 41080 Sevilla. Spain. Abstract Different colloidal powder processing routines have been used to prepare composites of 3 mol% Y 2 O 3 -ZrO 2 (tetragonal zirconia polycrystals, 3YTZP) with 2.5 vol.% multiwall carbon nanotubes (MWNT) with the aim of achieving a homogeneous distribution of the MWNTs in the ceramic, eliminating agglomerates but also minimizing carbon nanotubes (CNTs) damage during processing. Modifications of the acid treatment applied to the nanotubes, including subjecting them to stirring or ultrasonic agitation, and use of acid or basic pH during composite powder mixing have been approached. No MWNT damage during processing was detected by Raman spectroscopy. CNTs bundles were found in all the composites forming different patterns depending on the processing route. Similar values of hardness were obtained for all the composites, while different anisotropy in fracture propagation was found when studying parallel and perpendicular directions to the sintering pressing axis on the cross sections of the composites due to the MWNT preferential alignment. The CNT bundles were found to act as fracture short paths. A similar anisotropic behavior was observed for the electrical conductivity. These results have been correlated to the different microstructures obtained in the composites prepared with different processing routines. 2 1. Introduction For more than two decades, the scientific community has paid great attention to carbon nanotubes (CNTs) which have remarkable Young´s modulus (up to 1.2 TPa), strength, chemical and thermal stability and outstanding resilience. These attractive properties have motivated many researchers to focus on ceramic composites containing CNTs with the aim of improving the properties of the matrix. Nowadays, the development of industry and society requires composite materials possessing not only excellent mechanical properties but also outstanding functional properties 1 . However, despite the outstanding properties and functionalities of the CNTs, their chemical inertness and their entangling due to van der Waals forces hinder the fulfilment of their potential benefits on these composites. Non-uniform dispersion of CNTs can result in the presence of large agglomerates and poor densification 2-4 . Thus, composite powder processing and consolidation are key points in order to achieve fully dense composites with homogeneous CNT distribution through the ceramic matrix. Several processing approaches have been proposed in literature. Physical mixing under wet conditions by ultrasonication or ball milling 5-7 , and in-situ growth of CNTs on the surface of ceramic powders 8,9 were proposed by different authors. More recently, other researchers focused on the incorporation of chemically modified CNT into the ceramic matrix with the aim of improving the surface compatibility of the CNTs with the host matrix. Both covalent and non-covalent functionalization of CNTs have been investigated in order to modify the inert hydrophobic CNT surface 2,10,11 . Although other attempts such as oxidation in air or basic etching have been approached 10 , acid treatment of CNTs is one of the more widely used methods for covalent functionalization 2,12-17 . The rationale behind this technique is the shortening of the CNTs and the formation of carboxyl groups on their surfaces 15 when dispersed in aqueous media, resulting in a highly stabilized suspension due to an electrostatic effect 2 , which would facilitate the homogeneous dispersion of the CNTs in the matrix. Also, an enhancement of mechanical interlocking with the ceramic matrix is expected as a consequence of the chemical surface roughness created by the attaching of functional groups to the CNTs surface. Acid treatments by reflux of the CNT-acid mixture 11-13 , by 3 mechanical stirring 10,16 or ultrasonication 12,15 of this mixture, with different temperatures 16 or times 10 , have been studied searching for an enhancement of the benefits while avoiding damage to the CNTs. The colloidal method is considered one of the most effective means of homogeneously incorporating chemically modified CNT suspensions into a ceramic matrix. Although some authors prepared the acid-treated CNTs/ceramic particle slurry just by magnetic stirring in ethanol 14,15 , other groups proposed advanced colloidal techniques including pH control. Estili et al 12,13 and more recently Wang et al 16 proposed colloidal processing at acidic pH to prepare Al 2 O 3 /MWNT composites. In these conditions, the CNTs and the ceramic particle surfaces are negatively and positively charged, respectively, and a flocculation process (also called heterocoagulation process) between them takes place. Other authors proposed processing at basic pH 2,11,17 , where the surface charges are both negative and charge stabilization is promoted in the aqueous solution. All these studies attempted surfactantfree processing, as opposed to previous works 18,19 incorporating surfactants to modify the surface charge of the particles and /or CNTs. However, the possibility of incomplete surfactant removal or their effect on the composite properties motivated the avoidance of surfactants. Even though the advanced colloidal techniques including CNT acid treatment and aqueous solution pH control have been used for several years to prepare Al 2 O 3 /CNT composites 2-4,11-13 , it has not been until recently that these techniques have been applied to the processing of zirconia/CNT composites 14,15,17 . Yi et al 14,15 sonicated multiwalled carbon nanotubes (MWNTs) at 40ºC in sulphuric acid and dispersed the MWNTs and zirconia particles in ethanol under magnetic stirring, followed by ultrasonic agitation. Kasperski et al 17 submitted double walled carbon nanotubes (DWNTs) to a soft functionalization at room temperature using a mixture of nitric, sulphuric and chloridric acids and used basic pH to disperse the zirconia and DWNT mixture. Both studies described a decrease in hardness and an increase in electrical conductivity for CNT contents up to 4.5 wt%. In this study, we have prepared 3YTZP composites with 2.5 vol.% MWNT by spark plasma sintering of powders prepared using different colloidal processing routines, with the main objective of differentiating the benefits or disadvantages of each 4 of them in terms of optimizing MWNT distribution through the ceramic matrix. These processing routines include subjecting the MWNTs to two different acid treatments and ulterior composite powder mixing in aqueous solution with either basic or acid pH. The effect of each routine on the composite microstructure, hardness, fracture behavior and electrical conductivity has been analyzed . 2. Experimental procedure 2.1. Materials processing 2.1.1. MWNT acid treatment Commercially available purified MWNTs (Nanolab Inc., Waltham, MA, USA) with 1-5 µm length and average external diameter 15 ± 5 nm (supplier data) were selected as starting material. The acid treatment of the MWNTs was carried out using a mixture of concentrated sulphuric acid (98%) and nitric acid (70%) in the ratio 3:1, with the aim of disentangling and cutting the raw MWNT ropes. Two different set of conditions were used. In the first one, the MWNTs were suspended in the acid mixture for 2 h under magnetic stirring (referred to as MWNT-ST). In the second one, the MWNTs were suspended in the acid mixture and subjected to ultrasonic agitation by means of an ultrasonic bath (240 W) for 2 h (referred to as MWNT-US). Both processes were carried out at room temperature. The acid-treated MWNTs were collected on ∼20 nm pore filter membranes, washed in high purity ethanol for several times and freezedried in order to avoid possible re-agglomeration. 2.1.2. 3YTZP/MWNT powder colloidal processing The 3 mol% yttria stabilized tetragonal zirconia powder (3YTZP), with 40 nm particle size and 99% purity, was obtained from a commercial source (Nanostructured and Amorphous Materials Inc., Houston, TX, USA). The colloidal processing of the 2.5 vol.% 3YTZP/MWNT composite powders was carried out in aqueous solution following two different routines. In both cases, the use of surfactants that could introduce undesirable impurities which could affect the sintering process and/or the composites’ properties was avoided. In the first routine, a basic charge stabilization 5 technique described elsewhere2 was followed. In a first step, the ceramic powder and the MWNTs were independently suspended in a highly basic aqueous solution (pH= 12), where both their surfaces are negatively charged, and subjected to ultrasonic agitation for two different times (15 or 45 min) by means of a high power ultrasonic probe (Model KT-600, Kontes Inc., Vineland, NJ) at 20 kHz and 95% amplitude. In a second step, the composite powder blends were mixed in aqueous solution and subjected to ultrasonic agitation for 5 min. The composite slurry drying was carried out by using a hot plate at 80°C with continuous stirring. In the second routine, a heterocoagulation technique 16 was followed. In this case, the ceramic powder and the MWNTs were independently suspended in acid aqueous solution (pH = 5.2), where their surfaces have opposite charges, and subjected to ultrasonic agitation for 15 min by means of an ultrasonic probe. In a second step, the suspensions were gently mixed by introducing the MWNT suspension into the ceramic powder suspension, and then vigorously stirred. When magnetic stirring of the suspension was finished, the flocculation process between the MWNTs and the 3YTZP particles started immediately and it was considered completed once the supernatant became transparent. The slurry was immediately frozen by immersion in liquid nitrogen and subsequently dried by lyophilization at -80 °C using a Cryodos 80 lyophilizer. Nylon filters were used in order to avoid composite powder loss during the process. The different processing routines used to obtain the composite powders are summarized in Table 1. Notation of the composites includes information about the corresponding routine: stirring/ultrasonic agitation during acid treatment (ST/US) and charge stabilization/heterocoagulation technique (stab/het). Information about tip sonication time is included only in the notation of the composite prepared using ultrasonic agitation for 45 minutes. 2.1.3. Spark Plasma Sintering (SPS) 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. All the composites were sintered at 1250 °C for 5 min under 75 MPa with 200 °C/min heating and 50 °C/min cooling ramps. A sheet of graphite paper was placed between the powders and the die/punches to ensure their electrical, mechanical and thermal contact. 6 A graphitic paper was also placed along the inner wall of the die for easy removal of the sample. The temperature was measured by means of an optical pyrometer focused on a bore hole in the center of the graphite die. The sintered composites of ~15 mm in diameter and ~2 mm in thickness were ground to eliminate the surface graphite. The density of the sintered composites was measured by the Archimedes method using distilled water as immersion medium. The theoretical density of the composites, ρ=6.0 g·cm -3 , was calculated using the rule of the mixtures, considering ρ=6.1 g·cm -3 for 3YTZP and ρ=2.1 g·cm -3 for the MWNTs. 2.2. Microstructural characterization. Phase identification was carried out using X-ray diffraction (XRD, model D8 Advance A25, Bruker Co., MA, USA). The Raman spectra of the as-received MWNTs, the acid-treated MWNTs (MWNT-ST and MWNT-US) and the fracture surfaces of the sintered composites were obtained 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. Six to eight spectra were taken for each sample. Microstructural observations of sintered composites were carried out on fracture and polished surfaces by high-resolution scanning electron microscopy (HRSEM), using a Hitachi S5200 microscope (Hitachi High Technologies America Inc., USA) and a FEI-Teneo microscope (FEI, USA) with the aim to analyze the distribution of the MWNTs in the matrix, and to characterize the ceramic grains morphology. The possible presence of MWNT agglomerates in the composites was analyzed by low magnification conventional SEM using a JEOL 6460LV microscope ( JEOL USA Inc., MA, USA). Cross-section (c.s.) and in-plane (i.p.) slices, i.e. surfaces parallel and perpendicular to the SPS pressing direction, were polished with diamond paste up to 1 µm for morphological studies. Additionally, the polished c.s. surfaces devoted to characterize the 3YTZP grains were thermally etched at 1150 ºC for 20 min in air to reveal the grain boundaries. The average grain size of the 3YTZP tetragonal phase was estimated by the 7 linear intercept method from SEM micrographs, measuring about 300 grains from each composite. 2.3. Mechanical and electrical characterization. Vickers indentation tests were carried out with a diamond Vickers microindenter (Wilson VH1150, USA) under 10 kgf, to estimate hardness at room temperature. At least 15 valid measurements were carried out on mirror-like finished surfaces for each sample, in well-separated and randomly selected regions. The tests were performed on both c.s. and i.p. surfaces. The hardness value, H V , was calculated from the indentation load, P, and the diagonal of the Vickers imprint, a.       = 2 854.1 a P H V (1) The crack fracture lengths starting from the corners of the Vickers imprints were measured in order to analyze the crack propagation. At least 15 imprints for each composite were evaluated for statistical purposes. The electrical conductivity was estimated using the capacitive method (metallization of two faces of the sample and measurement between the two electrodes). The measurements were performed on parallelepiped specimens at room temperature with direct currents using a model 2000 Keithley multimeter. A colloidal silver paste was applied on both sides of the samples and the electrodes were fired at 600 ºC for 30 min under argon flow to avoid degradation of the MWNTs during the process. Two different electrode configurations were used to measure the conductivity in the directions parallel (σ || ) and perpendicular (σ ⊥ ) to the SPS pressing axis. 3. Results and discussion. 3.1. Microstructure. The XRD patterns of the sintered composites are shown in Fig. 1. In all the samples it is clear the presence of the zirconia tetragonal phase (JCPDS 01-078-1808) as the main one, with a contribution of the zirconia monoclinic phase (JCPDS 01-081- 8 1314, main peaks at 2θ = 28.2 and 31.4º). No remarkable differences were observed on the amount of the secondary monoclinic phase in the composites prepared using the different processing routes. The existence of this secondary phase in the 3YTZP matrix has been related to the presence of the CNTs surrounding the 3YTZP ceramic grains. It has been published that the transformation from the tetragonal to the monoclinic phase during cooling after SPS can be promoted by an increase in the CNT content 20 or by improving their dispersion in the case of composites with the same CNT nominal content 21,22 . In the XRD pattern corresponding to the composite prepared with MWNTs subjected to tip-sonication for 45 minutes a peak at 2θ = 26.6º is observed, which corresponds to the main peak of graphite (JCPDS 00-026-1076). Thus, it can be concluded that sonicating the CNTs with the high energy probe during such a long time has damaged them. It has been previously reported that long time sonication can not only enhance dispersion of CNTs but it can also induce defects and even scission of the tubes 23 . This composite was not further analyzed. Possible structural defects generated on the MWNTs during the acid treatment or spark plasma sintering were evaluated by Raman spectroscopy. The spectra corresponding to the as-received and acid-treated MWNTs, and to the sintered composites are shown in figures 2 and 3, respectively. The intensity ratio of the MWNTs characteristic peaks D and G (situated in the 1200-1700 cm -1 range), I D /I G , was calculated from the spectra. A high ratio is usually regarded as evidence of structural defects in the CNTs, as D-Band is usually associated to disordered graphite and crystalline defects 20 . The high ratio observed for the as-received MWNTs (I D /I G =0.95 ± 0.06) points to the existence of a non-negligible amount of defective CNTs, although it is not as high as previously published values 11,14 . No significant change in the I D /I G ratio is observed for the acid-treated MWNTs (1.03 ± 0.01 for ST and 0.86 ± 0.17 for US), revealing no further deterioration of the CNT structure during the acid treatment. Regarding the sintered composites, remarkable changes of I D /I G ratio are not observed either (Table 2), so it can be concluded that neither the acid treatment nor the SPS process induce significant damage to the MWNTs. Some variability in the I D /I G values has been observed (minimum and maximum values shown in Table 2). However, it is not as significant as the wide variability reported by previous works, which related this effect to an uneven damage of the CNTs or to the presence of CNT agglomerates, which 9 would alter the homogeneity of the composites 17-22 . A ~10-15 cm -1 shift towards higher frequencies is observed in the G-Band position for the sintered composites when comparing with its position for the as-received and acid-treated MWNTs. This fact has been attributed to compressive strains imposed to the MWNTs by the ceramic matrix 24 . The ceramic grain size in the different composites, together with the density values, are shown in Table 2. Relative densities higher than 97% and similar mean grain sizes (~210-230 nm) were observed in all the composites regardless the different processing routines used. HRSEM images of fracture surfaces of the studied composites are shown in Fig. 4. It can be observed that all the composites present highly densified microstructures with very low porosity. Using this high magnification it is possible to observe MWNT bundles or small MWNT accumulations located at the ceramic grain boundaries. It can also be observed the intergranular character of the fracture mode and a similar grain size in all the composites. Low-magnification SEM micrographs (Figs. 5a-d) illustrate the MWNT distribution into the ceramic matrix in the polished c.s. surfaces of the composites. The microstructural characterization of ceramic/CNT composites by BSE (backscattered electrons) has been shown to be the most direct method to evaluate the CNT dispersion 2,22 . The presence and distribution of CNT bundles and agglomerates can be clearly visualized because of the high contrast due to the average atomic number difference between the ceramic 3YTZP matrix and C on CNTs. Figures 5a-d show a general view of the composites polished surfaces, whereas Figures 5e-h show details at higher magnifications on these surfaces. It is highly remarkable that some large MWNT agglomerates (arrowed in Fig. 5d) can only be found in the composite prepared with MWNTs subjected to an ultrasonic bath during the acid treatment and processed by heterocoagulation. The microstructure of this composite is thus composed of areas with dark MWNT bundles distributed throughout the matrix together with some large agglomerates (up to ~40 µm long). Most of these agglomerates appear elongated and preferentially oriented in the direction perpendicular to the SPS compression axis, as it has been previously reported 2,22,25 . These results suggest that the processing routine used to prepare this composite is not adequate to achieve the goal of obtaining an optimized and agglomerate-free microstructure. Although some processing steps that have been shown 16 deviation following a more tortuous path of the unaligned CNT bundles. The absence of this anisotropy in the composite prepared with the MWNTs subjected to ultrasonication during the acid treatment and further heterocoagulation is a consequence of the shorter CNT bundles without significant preferential orientation. The remarkable electrical anisotropy observed in all the composites points to a preferential orientation of the CNT bundles in the direction perpendicular to the SPS loading axis. The lower conductivity values in the composites prepared with the MWNTs subjected to acid treatment including ultrasonic agitation is a consequence of the shortening of the CNTs during this treatment. Our results indicate that the best processing routine for obtaining highly conductive 3YTZP/MWNT composites with homogeneous microstructure is the one which combines MWNT stirring during the acid treatment and further powder processing by charge stabilization. Acknowledgments The authors acknowledge the financial support provided by the Spanish Ministerio de Economía y Competitividad, under project MAT2015-67889-P, cofunded by European FEDER fundings. XRD and microscopy studied were performed at CITIUS facilities (Universidad de Sevilla). 17 Figure captions Figure 1. X- ray diffraction patterns of the composites. Figure 2. Raman spectra of the as-received MWNTs and the MWNTs subjected to the two different acid treatment routines. The spectra were normalized to the G band for easier comparison. Figure 3. Raman spectra of the sintered composites prepared with (a) MWNT-ST and (b) MWNT-US. Raman spectra of the two different acid-treated MWNTs were included for comparison. Figure 4. HRSEM micrographs of fracture surfaces of composites denoted as (a) ST- Stab, (b) ST-Het, (c) US-Stab and (d) US-Het. Figure 5. Backscattered electron scanning (BSE) microscopy images from the composites cross sections, showing dark MWNT distributed throughout the matrix and some agglomerates (arrowed). (a), (b), (c) and (d) correspond to low magnification images; (e), (f), (g) and (h) correspond to higher magnification images. (a) and (e) ST- Stab, (b) and (f) ST-Het, (c) and (g) US-Stab and (d) and (h) US-Het. SPS pressing axis is indicated in (a) by arrows. Figure 6. SEM micrographs illustrating typical Vickers indentations and crack paths on cross section surfaces of the composites denoted as (a) ST-Stab, (b) ST-Het, (c) US- Stab and (d) US-Het. SPS pressing axis is indicated in (a) by arrows. Figure 7. Fracture crack originated in ST-Stab composite by Vickers indentation in the directions perpendicular (a) and parallel (b) to the SPS pressing axis. (c) High resolution SEM image showing a single CNT bundle bridging a crack. 18 References 1 Q. Zhang, J.Q. Huang, W.Z. Qian, Y.Y. Zhang, F. 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Processing routines followed in this study Compote MWT acid treatment agitation procedure MWT tip sonication time pH conditions during composite powder mixing Slurry drying ultrasonic agitation stirring 15 min 45 min Basic Acid Hot plate Lyophilization ST-stab x x x x ST-het x x x x US-stab x x x x US-45-stab x x x x US-het x x x x Table 2. Experimental and relative densities, grain size and Raman I D /I G ratio of the composites. Composite ρ ρρ ρ (g·cm -3 ) ± 0.1 ρ ρρ ρ rel (%) (%) (%) (%) ± 2 d (nm) I D /I G I D /I G range (min-max) ST-stab 5.8 96.7 227 ± 16 0.83 ± 0.09 (0.74-0.93) ST-het 5.9 98.3 207 ± 15 1.03 ± 0.26 (0.73-1.26) US-stab 6.0 100 228 ± 12 0.94 ± 0.16 (0.83-1.16) US-het 5.8 97.4 226 ± 36 0.89 ± 0.09 (0.77-0.95) Table 3. Vickers hardness (H v ), crack length and electrical conductivity (σ σσ σ) of the composites. Composite H v (GPa) Crack length ( µ µµ µ m) σ σσ σ (S·m - 1 ) i.p. surface c.s. surface Parallel direction Per pendicular direction σ || σ ⊥ ST - stab 10.9 ± 0.4 10.8 6 ± 0.20 42 ± 5 80 ± 9 16.4 28.8 ST - het 12.2 ± 0.4 12.25 ± 0.14 46 ± 5 60 ± 6 7.1 20.6 US - stab 12.6 0 ± 0.14 12. 56 ± 0.24 41 ± 5 65 ± 9 0.3 2.2 US - het 12.5 ± 0.3 12.9 5 ± 0.19 52 ± 6 56 ± 5 3.5 8.6 Figure 1 30 40 50 60 70 80 90 Monoclinic ZrO 2 phase Tetragonal ZrO 2 phase Graphite US-Het US-45-Stab US-Stab ST-Het Intensity (a.u.) 2 θ ST-Stab