Intragranular carbon nanotubes in alumina-based composites for reinforced ceramics

Abstract

The traditional methods for the synthesis of reinforced alumina-based matrix composites with carbon nanotubes (CNTs) have presented serious difficulties for obtaining well dispersed and homogeneously distributed CNTs within the matrix. Besides this, the CNTs are typically found in the grain boundaries of the matrix. These features involve a non-optimal reinforcement role of the CNTs. With the aim of maximizing the efficiency of the reinforcement of the CNT, this work reconsiders a sol-gel based procedure for ceramic composites fabrication with a twofold objective: to achieve a good dispersion of the CNTs and to promote the intragranular location of the CNTs. The mixing of precursors and CNTs has been developed under the presence of high power ultrasounds, followed by a rapid in-situ gelation that "freezed" the nanotubes inside the gel. The chemical and physical relationships between the ceramic matrix and the embedded reinforcing phase has been researched. First results confirm the success of the synthesis procedure for the preparation of alumina-based composite powders starting from a commercial boehmite sol and multiwalled carbon nanotubes. X-ray diffraction and Raman analyses confirmed the formation of the α-Al 2 O 3 and the persistence of the non-damaged nanotube structure. N 2 physisorption and electron microscopy were used to check the evolution of the nanostructure and to confirm the presence of intragranular carbon nanotube within the 2 polycrystalline powder. Therefore, the alumina-based composite powder prepared by this new procedure is a good candidate for the preparation of reinforced ceramic matrix composites.

Mechanical characterization of sol-gel alumina-based ceramics with intragranular reinforcement of multiwalled carbon nanotubes

“…Samples were prepared following a procedure based on a report by Barrera-Solano et al [15,21].First, a commercial boehmite (AlOOH) sol (Nyacol Nano Technologies, Inc., density = 1.14 g/cm 3 ) and the required amount of OH-functionalized MWCNTs (Nanostructured & Amorphous Materials, Inc., purity > 95%, inner diameter: 3-5 nm, outer diameter: 8-15 nm, length: 0.5-2 μm, regarding the supplier, confirmed by TEM) were directly mixed. Then, the MWCNTs were dispersed into the sol with the application of high power ultrasounds (~15 min, 50 W) to achieve maximum dispersion.…”

“…This feature was monitored by XRD and published in a specific research work that can be accessed in ref. [15], together with more details about the synthesis procedure and additional structural characterization of the composite powders. Carbon contents of the samples were 0 (pure alumina sample, reference), 1.0, 2.0, and 5.0 wt.%.…”

“…Secondly, the reinforcing phase is inefficiently located within the matrix. Typically, the reinforcing phases are located around the grain boundaries [2,13], but this placement severely limits possible improvements achieved in the fracture and wear properties given that the toughening mechanisms of the composite, such as crack-bridging and the CNTs pulling out, may be limited or may not directly appear [9,14,15]. Lastly, weak bonds between the carbon allotropes and the ceramic matrix have been stated as a reason for unsuccessful reinforcements [16].…”

“…In this regard, large efforts are being invested into achieving homogeneous and stable liquid suspensions of low-dimensional reinforcing phases to ensure homogeneous dispersion of the precursors [2,19]. Among all the synthesis strategies, the sol-gel method has been considered for ceramic fabrication [20,21], and more recently, for the fabrication of alumina-based CMCs with CNTs [9,14,15]. The aim is to achieve a good dispersion of the CNTs by a rapid, controlled gelation though the use of liquid precursors, control of the pH, and the assistance of high-power ultrasound [22].…”

“…The aim is to achieve a good dispersion of the CNTs by a rapid, controlled gelation though the use of liquid precursors, control of the pH, and the assistance of high-power ultrasound [22]. In addition, this method may promote intragranular placement of the CNTs, as they are present during the very first steps of the formation of the ceramic grains [9,14,15]. Finally, the sol-gel method may also enhance the formation of interfacial bonds between the CNTs and the ceramic matrix through the use of functionalized CNTs and metallic hydroxides or alkoxides as precursors [23].…”