Double-walled carbon nanotube/zirconia composites: Preparation by spark plasma sintering, electrical conductivity and mechanical properties

Abstract

Double-walled carbon nanotube/yttria-stabilized zirconia composite powders with carbon contents up to 6.3 wt.% are prepared by soft covalent functionalization of carbon nanotubes (CNTs) followed by mixing with a nanometric yttria-stabilized zirconia (YSZ) powder. Composites are densified by spark plasma sintering (SPS). The composites present an electrical conductivity (0.09-0.88 S.cm -1 ) which is among the highest values reported in the literature. Both the fracture strength (V f ) and the single-edged notched beam fracture toughness (K Ic ) are measured for the first time on CNT/YSZ composites. The best mechanicalproperties (V f = 694 MPa; K Ic = 7 MPa.m 1/2 ), obtained for low carbon contents (up to 1.2 wt.%), are among the highest reported up to now. However, the fracture toughness is lower than that of the YSZ ceramic (K Ic = 10.3 MPa.m 1/2 ) whose mechanical properties are outstanding, taking into account its very fine microstructure.

Enhanced carbon nanotube dispersion in 3YTZP/SWNTs composites and its effect on room temperature mechanical and electrical properties

“…The results of flexural strength of composite C4 of this study, s F ¼ 600 ± 50 MPa, are analogous to the flexural strength for 1 vol% SWNT/Al 2 O 3 (in the range 520e610 MPa) [33] (composite tested with the same experimental set up and same specimen dimensions). They are also comparable to the value s F ¼ 526 ± 84 obtained for 1.2 vol% DWNT/ 3YTZP [42].…”

“…The differences observed in the Raman spectra for the composites in this work could be explained as damage related to the presence of CNT agglomerates which would induce local heating during SPS or assuming that agglomerates are considered as defective SWNTs by Raman spectroscopy, increasing D band in composites with more agglomerates. A wide variability of I D /I G values has been observed for some composites (C1, C3 and C5), also reported by other authors in DWNT/3YTZP composites and attributed to an uneven damage of the SWNTs [42]. This variability could also be related to the presence of agglomerates, which alter homogeneity of the composites, since composites C2 and C4 which do not show SWNT agglomerates exhibit very similar Raman spectra with a narrow I D /I G range ( Table 3).…”

“…Therefore, the effect of the use of an ultrasonic probe for dispersion is noticeable, since the composites turn electrically conductive, with conductivity values 0.4, 2.5, and 1.1 Sm À1 for C2, C4 and C5 respectively. These values have the same order of magnitude than doped semiconductors or sea water, are higher than reported by some authors with the same carbon nanotube content and comparable to higher (1.7 vol%) CNT content DWNT/ 3YTZP composites [42]. If we compare C1 and C3 (effect of lyophilization of the composite powder), we cannot see any difference between both composites, since both are isolating.…”

“…It can reach even values >1, as in C3. The occurrence of a D band higher than the G band has already been reported in the literature, for instance in MWNT [25] and very recently in DWNT/3YTZP composites when DWNT content is very high (18 vol%) [42]. The increase of the D band has been explained in the latter case by damage to the DWNTs during SPS due to the mixed ionic-electronic conductivity of the YSZ composite at high temperatures which can induce local heating of the CNTs.…”

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

“…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][13][14][15][16][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.…”