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MWCNT/alumina composites were prepared by a flocculation method combined with acid treatment of the MWCNTs and spark plasma sintering. The effects of varying the processing conditions, namely the acid treatment temperature (50–80 °C), MWCNT content (0–10 vol.%) and sintering temperature (1200–1600 °C) on the composites’ microstructure, electrical conductivity and mechanical properties were investigated. This found that flocculation processing brings about a good dispersibility of the MWCNTs, especially with higher acid treatment temperature, which enhances their reinforcing and grain refining effect. When combined with a low sintering temperature, this significantly improves the mechanical properties of the composites.As a result, a composite prepared using a sintering temperature of 1200 °C and an acid treatment temperature of 80 °C exhibited a fracture toughness greater than that of a monolithic alumina, even with a MWCNT content as high as 10 vol.%. [ABSTRACT FROM AUTHOR]

Alumina composites reinforced with multiwalled carbon nanotubes (MWCNTs) at up to 3.7 vol% are prepared by a precursor method followed by a spark plasma sintering. We systematically and quantitatively investigate the effects of acid-treatment time of the MWCNTs on not only bending strength and fracture toughness of the composites but also on the mechanical strength and dispersibility of the MWCNTs, the grain size of the alumina matrix, and the interfacial strength between MWCNT and alumina. The main objective of this study is to evaluate how these parameters influence the mechanical properties with the aid of multiple regression analysis. We demonstrate that the matrix grain size, the mechanical strength of the MWCNTs, and the interfacial strength have little impact on the mechanical properties for the composites prepared in this study. On the other hand, the dispersibility of MWCNTs has the significant influence on the mechanical properties. Both the dispersibility of the MWCNTs and the mechanical properties of the composites increase as the acid-treatment time increases up to 2 h at low MWCNT content (0.9 vol%). Conversely, at a higher amount of MWCNTs, the degradation in the mechanical properties is shown to be associated with the deterioration of MWCNTs' dispersibility. As MWCNT agglomerates are anticipated to act as imperfections, they may override the effects of the strength of MWCNTs, matrix grain size, and interfacial strength. By means of the multiple regression analysis, we quantitatively show that improving MWCNTs' dispersibility is one of the most important factors in enhancing the mechanical properties of MWCNT/alumina composites. [ABSTRACT FROM AUTHOR]

Microstructure–property relationships of pressureless-sintered multi-walled carbon nanotube (MWCNT)/alumina composites have been investigated using four types of MWCNTs having different mechanical characteristics as well as almost the same diameter and length; the influences of grain size, location of MWCNTs, fracture patterns, dispersion states of MWCNTs, and crack bridging characteristics of MWCNTs on the composites׳ mechanical properties have been explored. It has been found that the improvement in bending strength and fracture toughness observed for all types of the composites was primarily attributable to the grain refining effect by adding the MWCNTs and to the energy-dissipation by a MWCNT debonding and pullout, respectively. The number and size of clustered MWCNTs were increased in response to the increase in the MWCNT content, which resulted in the decrease in mechanical properties. The pressureless-sintered composites made with MWCNTs having larger load-bearing ability led to ~25% and ~45% improvement in bending strength (742.6±13.1 MPa) and fracture toughness (5.83±0.19 MPa m 1/2 ), respectively, than a MWCNT-free alumina. [ABSTRACT FROM AUTHOR]