# Thermal Investigations on Carbon Nanotubes by Spectroscopic Techniques
## 1 Introduction  
The growing request for renewable energy sources has accelerated the development of novel techniques used to produce new devices for energy thermal storage. Nanotechnology can represent an important solution to these problems, as it can instigate the synthesis of advanced materials with improved performances, to be used in various applications, such as energy storage, molecular biology, and water treatment. Carbon nanotubes (CNTs) are considered to be among the most important nanostructured materials due to their particular chemical composition: indeed, they show a high correlation between structure and properties, that holds for different structures [1; 2]. Since their discovery in the early 1990s, the electro-mechanical properties of carbon nanotubes have been extensively studied. Thanks to their important properties (such as good elasticity and mechanical strength, good electronic and thermal properties, and high specific surface), CNTs have been used effectively for numerous applications such as energy storage devices, photovoltaic devices, chemical sensors, and actuators [3; 4; 5; 6]. CNTs represent monodimensional carbon allotropic forms, in which the carbon atoms present are arranged in such a way as to create a cylindrical shape. Carbon materials can be divided based on their atomic bond (sp, sp\({}^{2}\), and sp\({}^{3}\) hybridizations) [7; 8; 9]. Furthermore, as regards the preparation of CNTs, among the numerous synthesis methods developed [10; 11; 12; 13; 14; 15; 16], laser ablation and graphite arc discharge are those most used to produce CNTs from the vapor phase. The carbon nanotubes obtained from these two synthesis methods have good characteristics but are characterized by the presence of defects linked to impurities. The problem of the synthesis methods is related to the high temperatures used [17; 18], which result in considerableproduction costs. In order to solve this problem, chemical vapor deposition (CVD) techniques have been used to obtain the growth of CNTs on various substrates [19; 20; 21; 22; 23; 24]. The different properties of the carbon nanotubes CNTs, determined both experimentally and with computational methods, are shown in Table 1; carbon nanotubes and graphene are among the most mechanically resistant materials because of the presence of strong covalent links (C=C) between carbon atoms with sp\({}^{2}\) hybridization. In fact, the carbon nanotubes CNTs have a high mechanical tensile strength (150 GPa): this value is about 20 times higher than that of steel although CNTs are five times lighter [25; 26]. On the other hand, as regards the thermal properties of CNTs, they show high values of thermal conductivity, whose value at room temperature is equal to 3500 W/(mK). This value is highly dependent on the quality and alignment of the carbon nanotubes. In addition to high thermal conductivity, CNTs show high thermal stability in air. Therefore, thanks to these very important thermal properties such as high thermal stability and excellent thermal conductivity, carbon nanotubes-based materials (CNTs) can be effectively used for energy thermal storage applications. Furthermore, the electromechanical properties of the CNTs have been assessed using both theoretical and experimental methods: in this sense, computational method has proved fundamental for the characterization and design of the CNTs [27; 28; 29; 30]. Moreover, the different forms of CNTs can be grouped both as metallic and semiconductor materials and in relation to the arrangement of the hexagonal rings in an armchair, zigzag or chiral position. As regards the number of walls, they can be divided into single-walled (SWCNTs), double-walled (DWCNTs), and multi-walled (MWCNTs). The disposition of carbon atoms in the structure of CNTs and the presence of defects strongly influence the mechanical strength of the carbon nanotubes [31; 32]. In addition to the property, the range of experimental measurements and the computational calculations, the table reports the unit of measurement of each property.