Pressureless sintering of carbon nanotube–Al₂O₃ composites

Abstract

Alumina ceramics reinforced with 1, 3, or 5 vol.% multi-walled carbon nanotubes (CNTs) were densified by pressureless sintering. Commercial CNTs were purified by acid treatment and then dispersed in water at pH 12. The dispersed CNTs were mixed with Al₂O₃ powder, which was also dispersed in water at pH 12. The mixture was freeze dried to prevent segregation by differential sedimentation during solvent evaporation. Cylindrical pellets were formed by uniaxial pressing and then densified by heating in flowing argon. The resulting pellets had relative densities as high as ∼99% after sintering at 1500 °C for 2 h. Higher temperatures or longer times resulted in lower densities and weight loss due to degradation of the CNTs by reaction with the Al₂O₃. A CNT/Al₂O₃ composite containing 1 vol.% CNT had a higher flexure strength (∼540 MPa) than pure Al₂O₃ densified under similar conditions (∼400 MPa). Improved fracture toughness of CNT–Al₂O₃ composites was attributed to CNT pullout. This study has shown, for the first time, that CNT/Al₂O₃ composites can be densified by pressureless sintering without damage to the CNTs.

Introduction

Carbon nanotubes (CNTs), first reported by Iijima,¹ have excellent mechanical properties, including an ultra-high elastic modulus of ∼1 TPa and a high tensile strength of >10 GPa.2, 3 In addition, CNTs have good chemical stability. Therefore, CNTs are attractive candidates for reinforcement of various materials. Many studies have focused on using CNTs to enhance the mechanical properties of metal, ceramic, or polymer based composites.⁴ In some cases, CNTs can even replace carbon fibers.5, 6 In particular, attention has been paid to enhancing mechanical properties of polymers2, 7, 8, 9, 11, 12, 13, 14, 15 and metals.2, 6, 10, 16 Relatively few reports discuss CNT-reinforced ceramics, with the majority of those reports discussing alumina (Al₂O₃).2, 10, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28

Several key problems exist in the fabrication of CNT–Al₂O₃ composites.20, 23 The first problem is dispersion of CNTs in the Al₂O₃ matrix. Usually, CNTs are synthesized by catalytic chemical vapor deposition (CCVD),4, 25, 29 which results in a very high aspect ratio of 30–10,0004, 25 and strong Van der Waals forces between the tubes. As a result, both single- and multi-walled CNTs form twisted aggregate structures³⁰ that are difficult to disperse. To obtain a CNT–Al₂O₃ composite with a homogeneous CNT distribution, several approaches have been investigated. One is the in situ synthesis of CNTs on ceramic powders, which has been reported by several research groups.4, 10, 19, 28 In this process, ceramic powders are mixed with fine transition metal particles, such as Fe, Co or Ni, which serve as the catalyst for CNT growth. Gases that serve as carbon sources, such as CH₄–H₂4, 10 or C₂H₂–Ar–H₂,18, 28 are then flowed through the powders. The gases decompose and CNTs grow from the transition metal particles, which results in the formation of CNT–ceramic composite powders. However, the CNTs are not homogeneously distributed in the as-synthesized state. Consequently, the synthesized composite powders require further mixing by ball or attrition milling.¹⁰ In situ synthesis also leads to a second problem, which is deposition of residual amorphous carbon during growth.10, 31 The presence of residual carbon adversely affects the strength of the CNT–Al₂O₃ composites.

Another approach to disperse CNTs in ceramic powders is to prepare slurries of Al₂O₃ and CNTs using techniques such as ultrasonic agitation or ball milling with surfactant additions.8, 21, 30 However, the density of CNTs ranges²³ from 1 to 2 g/cm³ compared to ∼4 g/cm³ for Al₂O₃, which can lead to segregation during drying due to the different sedimentation rates of the two materials. In addition, CNTs have sp² hybrid covalent bonding⁶ whereas Al₂O₃ has a significant ionic bonding character. The difference in bond character affects the surface charges that develop when the two materials are dispersed in polar solvents as well as their response to surfactant additions. The differences in density and bonding tend to promote segregation of CNTs from Al₂O₃ particles in suspensions and during drying. Consequently, CNT aggregates have been observed in CNT–Al₂O₃ composites produced by hot-pressing powders in which CNTs were dispersed by ultrasonic agitation and ball milling.²¹

Previous studies32, 33 have shown that densification of ceramics can be inhibited by the presence of inclusions, such as CNTs, in the matrix. This is why all CNT–Al₂O₃ composites reported to date have been fabricated by hot-pressing4, 19, 20, 21, 22, 23, 24, 25, 26, 27 or spark plasma sintering (SPS).8, 18, 30 However, hot-pressing and SPS are limited to the formation of simple geometries and moderate sizes. Development of pressureless sintering processes is needed to enable the commercially viable fabrication of complex geometries to near-net shape using standard ceramic powder processing methods. As with other composite systems, it should be possible to develop a pressureless sintering process for CNT–Al₂O₃ composites. The CNTs may have a beneficial effect on the resulting composite by reducing the grain size of the matrix through physical pinning of grain boundaries. Generally, reducing the grain size increases the strength of the resulting composite compared to the nominally pure matrix material.

The objectives of the present study are to minimize the segregation of CNTs from Al₂O₃ during powder processing and to densify CNT–Al₂O₃ compacts by pressureless sintering.