Dispersion And Alignment Of Carbon Nanotubes And Digital Image Processing Quantitative Analysis

Multi-walled carbon nanotubes(MWNTs) are widely used in modern composite materials because of their excellent structure, mechanical performance, and electric/heat conductivity. To further improve their electric/heat conductivity the MWNTs are modified and composited with other materials having high electric/heat conductivity(such as Ni and Fe). In order to further improve the performance and extend the scope of the application of such composites, various functionalised CNT materials have been extensively studied, and aligned CNTs and nanotube bundles are a type of these materials. Aligned CNTs can make the composite material anisotropic and further improve the thermal and electric conductivity in the alignment direction.As the MWNTs have extremely complex morphology, their dispersity in water or organic solvents is extremely poor because of Van der Waals interaction. Further, it is difficult to prepare a stable dispersion for the MWNTs and therefore this insolubility limits their practical applications.Current methods for MWNT dispersion include: mechanical dispersion, ultrasonic dispersion, chemical modification, dispersion by dispersants, polymer coating, and metal coating. Chemical and ultrasonic treatments damage the structure of the MWNTs and with prolonged ultrasonic treatment, their outer graphite layer is gradually stripped off, making them shorter and thinner and finally transforming them into amorphous carbon. However, the destruction caused by the chemical methods is greater than that caused by ultrasonic treatment, and the ultrasonic dispersion is suitable for low-viscosity mixed systems, while the mechanical dispersion is suitable for solid particles or high-viscosity mixed systems. The heat generated in the entire ultrasonic process aggravates the thermal motion of molecules, causing secondary aggregation of the MWNTs. Moreover, as the morphology of the MWNTs is varied by introduction of different metallic nanoparticles into the MWNTs, the entire dispersion process is rendered even more complex by variation in the level of temperature sensitivity among these metal materials. However, perfect dispersion effects are not realized by these common dispersion methods. To get better dispersion effects, multiple dispersion methods can be made feasible and therefore the MWNTs were first dispersed in solution using the ultrasonic dispersion in the compound method, to form stable MWNT dispersion. The MWNT dispersion was then added into resin which was dispersed using the ultrasonic and mechanical methods. We used dispersants and polymers to further improve the dispersion effects during this process as the MWNTs dispersion effects in solution are fundamental for improved dispersion(i.e., dispersion in matrix) and performance of MWNTs composites.We studied the dispersity of multi-walled carbon nanotubes(MWNTs) combined with different metallic particles(Ni and Fe). An ultrasonic-assisted water-bath dispersion process was used to disperse the metal-coated MWNTs in different solutions and the dispersity was measured using an ultraviolet-visible spectrophotometer. The dispersity and morphology of the MWNTs were characterized using field-emission scanning electron microscopy(FE-SEM) together with digital image processing technology. Effects of dispersant type(sodium dodecyl benzene sulfonate(SDBS), oleic acid, and polymer(TNEDIS)) and surfactant dosage on the dispersity of the metal-coated MWNTs were investigated under controlled and uncontrolled temperatures and results were compared with those from the untreated MWNTs. The results showed that the negative effects of temperature on the ultrasonic dispersion process could be eliminated through a temperature-controlled system. Moreover, the TNEDIS, SDBS, and oleic acid were arranged in the descending order of the dispersion effect degree. The untreated MWNTs, Ni-coated MWNTs, and Fe-coated MWNTs were arranged in the descending degree of dispersity order. Since the metal coating makes the MWNTs harder and more fragile, the metal-coated MWNTs are more likely to fracture during the ultrasonic dispersion process.Aligned carbon nanotubes(CNTs) composites can be prepared by post-growth alignment or through the use of CNT bundles. Post-growth alignment requires stringent processing conditions(high-voltage electric field / high-strength magnetic field).We studied the morphological changes in the aligned CNT bundles during the ultrasonic dispersion process. Mechanical dispersion and ultrasonic dispersion greatly affect the morphology of highly aligned CNT bundles. While dispersion, the particle size of the CNT bundles reduces with time, and the integrated degree of orientation is reduced.In this study, we use field-emission scanning electron microscopy to study the dispersion and morphology of aligned CNTs during the ultrasound dispersion of aligned multi-walled CNT bundles(AMWNTs) in a water/acetone solution. We also use a UV-visible spectrophotometer to study the degree of dispersion during the whole process. Further, the dispersion properties of the AMWNTs were compared to those of ordinary multi-walled CNTs(MWNTs). Achieving a balance between dispersion and required orientation was the main aim of this study. Further, we also studied effects of additional factors such as the type and amount of surfactant on dispersion.The degree of dispersion is an important parameter to quantitatively study properties of carbon nanotube composites. Dispersion is related to the degree of agglomeration of the objects. The higher degree of agglomeration of the objects is, the lower/worse dispersion it is. Dispersion is also often related to the presence of a distribution of dimensions/shapes in the system(poly-vs mono-dispersed systems). At present, there are many technologies for the dispersion of carbon nanotubes, but the dispersion results, and thereby the performance of a carbon nanotube composite, depend on the dispersion conditions. This complexity makes it difficult to quantitatively analyze the relationship between the morphology and properties of carbon nanotube composites. A quantitative estimate of the degree of dispersion of carbon nanotubes is key to solving this problem. With continuous innovation of methods for dispersing carbon nanotubes, it is necessary to constantly update the methods for characterization of the dispersion effect. Among the many methods for studying dispersion, scanning electron microscopy(SEM), transmission electron microscopy(TEM), and atomic force microscopy(AFM) are the most commonly used, intuitive, and convincing methods. However, they have the disadvantage of not being quantitative.To overcome this disadvantage, the fractal theory and digital image processing method can be used to provide a quantitative analysis of the morphology and properties of carbon nanotube composites. The fractal theory and digital image processing method is a quick and simple way to obtain quantitative parameters from irregular complex images, such as SEM, TEM, and AFM photos. Fractal theory was put forth by Mandelbrot in the mid-1970 s and has since become an emerging discipline that reveals the regularity of many irregular objects in nature(e.g., map of the earth, and shape of hills and rivers). It is widely applied to almost all fields of the natural and social sciences, and has become a topic of wide research interest. It has been shown that the dispersions of carbon nanotubes in solution / matrix / membrane materials exhibit fractal property. There are a variety of methods for computing the fractal dimension. The commonly used methods include the box dimension algorithm, the differential box counting algorithm, and the fractal Brown motion algorithm. However, there are differences in the physical meanings of these algorithms, their applicability to various research objects, and the corresponding relationships between the obtained fractal dimension and the dispersion properties. Therefore, it is necessary to establish a unified evaluation system.In this study, the box-counting method and the differential box-counting method were used to quantitatively characterize the dispersion of carbon nanotubes. On the basis of the results, we propose a new method for the quantitative characterization of the degree of dispersion of carbon nanotubes. This hierarchical grid method can be used as a supplementary method, and can be combined with the fractal calculation method. Thus, the accuracy and effectiveness of the quantitative characterization of the dispersion degree of carbon nanotubes can be improved.The degree of alignment is another important quantitative parameter of carbon nanotube composites. The degree of alignment is related to the degree of arrangement of carbon nanotubes in one direction. It is also related to the degree of anisotropy of the materials. In this paper, by using the thought of calculus, we simplified the parameters of carbon nanotube alignment model. We design a semi-automatic, easy-operated program to quantitative measure the degree of alignment of the carbon nanotubes. The direction and the degree of alignment of carbon nanotubes can be quantified characterization. We can get the orientation angle and length of the carbon nanotubes by the step of loading image, scale conversion, and measurement of carbon nanotubes. The result can be show real-time in this picture; it is also can be automatically saved as EXCEL files for further calculations. We studied schematic diagrams and SEM images of carbon nanotubes with different alignment degree through this program.We also presents an automatic quantitative measure method to get the alignment degree of carbon nanotubes. Two-dimensional Fourier transform are used to the image of carbon nanotubes, spectrum was further calculated to get the quantitative information of alignment degree of carbon nanotubes, including orientation angle and the degree of alignment of the carbon nanotubes.In this paper, we mixed different metal-coated carbon nanotubes with epoxy resin to prepare carbon nanotube-epoxy resin composites. And we studied the dispersion properties and electrical conductivity of these composites. We have added electric field and magnetic fields in the composite manufacturing process to align carbon nanotubes in the composites. Then we study the effects of electric field and magnetic field on conductive properties of carbon nanotube composites.