دانلود رایگان مقاله لاتین پراکندگی نانولوله کربنی بر خواص کامپوزیت سیمان از سایت الزویر
عنوان فارسی مقاله:
پراکندگی نانولوله های کربنی چند جداره و اثرات آن بر خواص کامپوزیت های سیمان
عنوان انگلیسی مقاله:
Dispersion of multi-walled carbon nanotubes and its effects on the properties of cement composites
سال انتشار : 2016
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مقدمه انگلیسی مقاله:
1. Introduction
Multi-walled carbon nanotubes (CNT) are carbon ordered structure and have unique mechanical properties. Young's modulus is estimated to be as high as 0.45 TPa and their tensile strength may reach 3.6 GPa [1]. Because of these outstanding mechanical properties, addition of CNTs as reinforcement to cementitious materials can potentially improve their properties and has received much interest among researchers. Pristine CNTs are insoluble in water and are hydrophobic which causes difficulties for dispersion in water. Agglomeration of CNTs originates from several causes. Due to their nano-scale diameter, the surface area of CNTs as well as the surface attraction between CNTs is high. Dispersion difficulties also stem from the CNT propensity to form bundles that are tightly bound by high van der Waals forces. In addition, the high CNT aspect ratio (length/diameter) combined with their flexibility causes highly entangled agglomerates in liquid phases. Incorporation of CNTs into cement also confronts two main challenges: bonding and dispersion. Strong bonding between CNTs and the cement matrix as well as effective dispersion of CNTs is essential for an effective load-transfer to the CNTs which may act as reinforcement to improve mechanical properties of cement composites. It is also believed that if bonding between CNTs and the cement matrix can be controlled, the CNTs can provide substantial mechanical reinforcement [2] and can thus bridge microcracks and improve the ability of the cement composite to withstand fracture. Conversely, agglomeration of CNT inside cementitious materials could function as a local defect and might be detrimental to the reinforcing role of CNTs. A combination of physical (sonication) and chemical (surfactant usage) dispersion methods is the most recommended way to exfoliate CNT agglomerates in mixing water of cement composites [3]. Surfactants not only aid with exfoliation of CNT bundles, but may also play a role inbonding with the cement matrix. Li et al. [4] claim that interfacial interactions between surface-modified CNTs and hydration products will produce high bonding strength, and increase the loadtransfer efficiency from the cement matrix to the reinforcement. Although sonication provides an efficient way to suspend CNTs in aqueous media, it is always related to a certain degree of damage on CNT surfaces [5] and may also cause rupture to shorten CNTs. An optimum sonication duration and, correspondingly, delivered ultrasound power to a unit volume of suspension is introduced as a compromise between the dispersion degree and the level of damage to CNT surfaces. There is still an open question concerning the optimum sonication duration, that leads to the best performance of CNT incorporation into cement composites from the aspect of mechanical properties. CNTs have been dispersed in mixing water by sonication for widely varying durations, from a few minutes [6e9] to a few hours [4,10e12]; investigation of its optimum, however, has received less attention. To date, many researchers have applied combinations of dispersion with different surfactants followed by sonication to fabricate cement composites containing CNTs, but have obtained conflicting results. Many works, for instance [4,7,10,13e17], reported improvement, while other studies [8,12,18e21] reported a lack of improvement in the mechanical properties of CNTcontaining cement composites. The scatter of the results implies that dispersion of CNTs in aqueous suspensions could be reached by usage of surfactant and sonication, but effective dispersion of CNTs in the cement matrix could not necessarily be guaranteed. It is likely that CNTs randomly re-agglomerated after mixing with solid phase (cement and aggregates). A systematic investigation of this CNT re-agglomeration after mixing with cement is presently lacking in the literature. A study is needed to shed light on the reagglomeration concept by characterizing the degree of dispersion of CNTs in the mixing water and in the cement matrix of cement composites.
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