The purpose of this research is to develop and improve the process of massive growth of carbon nanotubes (CNTs) via chemical vapor deposition (CVD). Beside the growth of CNTs and their potential applications, CNTs based polymer composites properties were also explored. The thesis may be divided into two major sections. In the first section a comprehensive introduction to carbon nanomaterials specifically CNTs (which includes the structure, types, growth mechanism and techniques, characterization techniques and properties) is described. Then the CVD growth procedure adopted in our lab to grow different carbon nanomaterials in particular Multiwall Carbon nanotubes (MWCNTs) under differential experimental conditions is discussed. We have grown upto 3mm thick MWCNTs carpet on Si substrate with MWCNTs diameter in the range 20nm-80nm. The individual length of MWCNTs is as long as few hundreds of micrometer. MWCNTs based structures were also grown on patterned surfaces. The patterning of the surfaces is performed by soft photolithography. These MWCNT structures have very interesting applications e.g. a). The vertical cylinders were use to produce SiC hollow cylinders, and b). CNT based fins grown on Si substrate were used to enhance the convective heat transfer properties. Several treatments (thermal annealing, acid treatment and plasma treatment) were also performed on MWCNTs in order to modify their characteristics. These procedures are useful for purification, functionalization and graphitization of MWCNTs. The second section about CNT based polymer composites starts with the brief introduction to polymer composites, processing techniques, major issues in mixing the CNTs in different polymers and finally the mixing tools used for better dispersion. The optical characterization of PDMS based MWCNTs composites films are studied. These films can have application in optical limiting devices. Furthermore, the transparency of these films is also used to calculate a unique parameter absorption cross section of a single MWCNT. The absorption cross section of individual MWCNTs having widely different aspect ratios scales with their volume. The approximation of absorption cross section per carbon atom is also in close agreement with that of graphite. The electrical conductivity phenomena in epoxy based carbon nanomaterials (CNMs) composites are also discussed. A total number of 16 types of different CNMs were used. Several conduction behaviors have been found e.g. from highly conductive CNTs which showed linear Ohmic curve, to non-linear diode-like trend to completely insulating one. The best performances have been reached by the shortest and thinner MWCNTs (both as grown and slightly functionalized with COOH groups), which can underline that small fillers can be better dispersed inside the composite and create a better conductive net within the matrix. We have also applied physical models such as the percolation theory and the fluctuation mediated tunnelling theory to the most conductive nanocomposites, with poor agreement between experimental data and theoretical prediction. Finally, we applied a recently revised model based on tunnelling-percolation theory and obtained a good fit between experimental and theoretical results.

Growth, Characterization & Applications of Carbon Nanomaterials / Shahzad, MUHAMMAD IMRAN. - (2014).

Growth, Characterization & Applications of Carbon Nanomaterials

SHAHZAD, MUHAMMAD IMRAN
2014

Abstract

The purpose of this research is to develop and improve the process of massive growth of carbon nanotubes (CNTs) via chemical vapor deposition (CVD). Beside the growth of CNTs and their potential applications, CNTs based polymer composites properties were also explored. The thesis may be divided into two major sections. In the first section a comprehensive introduction to carbon nanomaterials specifically CNTs (which includes the structure, types, growth mechanism and techniques, characterization techniques and properties) is described. Then the CVD growth procedure adopted in our lab to grow different carbon nanomaterials in particular Multiwall Carbon nanotubes (MWCNTs) under differential experimental conditions is discussed. We have grown upto 3mm thick MWCNTs carpet on Si substrate with MWCNTs diameter in the range 20nm-80nm. The individual length of MWCNTs is as long as few hundreds of micrometer. MWCNTs based structures were also grown on patterned surfaces. The patterning of the surfaces is performed by soft photolithography. These MWCNT structures have very interesting applications e.g. a). The vertical cylinders were use to produce SiC hollow cylinders, and b). CNT based fins grown on Si substrate were used to enhance the convective heat transfer properties. Several treatments (thermal annealing, acid treatment and plasma treatment) were also performed on MWCNTs in order to modify their characteristics. These procedures are useful for purification, functionalization and graphitization of MWCNTs. The second section about CNT based polymer composites starts with the brief introduction to polymer composites, processing techniques, major issues in mixing the CNTs in different polymers and finally the mixing tools used for better dispersion. The optical characterization of PDMS based MWCNTs composites films are studied. These films can have application in optical limiting devices. Furthermore, the transparency of these films is also used to calculate a unique parameter absorption cross section of a single MWCNT. The absorption cross section of individual MWCNTs having widely different aspect ratios scales with their volume. The approximation of absorption cross section per carbon atom is also in close agreement with that of graphite. The electrical conductivity phenomena in epoxy based carbon nanomaterials (CNMs) composites are also discussed. A total number of 16 types of different CNMs were used. Several conduction behaviors have been found e.g. from highly conductive CNTs which showed linear Ohmic curve, to non-linear diode-like trend to completely insulating one. The best performances have been reached by the shortest and thinner MWCNTs (both as grown and slightly functionalized with COOH groups), which can underline that small fillers can be better dispersed inside the composite and create a better conductive net within the matrix. We have also applied physical models such as the percolation theory and the fluctuation mediated tunnelling theory to the most conductive nanocomposites, with poor agreement between experimental data and theoretical prediction. Finally, we applied a recently revised model based on tunnelling-percolation theory and obtained a good fit between experimental and theoretical results.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11583/2551359
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