Index IntroductionSynthesis TechniquesCarbon Arc DischargeLaser AblationChemical Vapor Deposition (CVD)Characterization of Carbon Nanotube-Based Pressure SensorsScanning Electron MicroscopyResistance-Pressure RelationshipsPressure Sensing MechanismApplicationConclusionI Pressure sensors are very essential and are used in various fields like aerospace, barometry, industries, automobiles, medical devices, etc. Carbon nanotube is one of the materials used to design the pressure sensor with the suitable substrate material such as silicon used for a certain application. In this theoretical study, the synthesis, characterization and analysis of CNTs for pressure sensing was carried out. Due to its high gauge factor (200 to 1000), high sensitivity, temperature independence, and many other advantages, CNT serves as a highly effective material for use as a piezoresistive element in piezoresistive pressure sensors. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay Introduction The discovery and development of the nanoworld has played a huge role in scientific and technological progress in the universe. Nanomaterials have received the most attention since their discovery and can be distinguished and classified based on their dimensionality (0D, 1D, 2D and 3D). They are also the most studied for their unique electrical, mechanical, optical and magnetic properties and a wide range of applications they provide. Lately there has been interest in various nanomaterials including nanowires, carbon nanotubes, polymer nanofibers, metal nanoparticles, and graphene has been used to fabricate new flexible pressure and strain sensors. These nanomaterials have potential applications such as flexible touch-on displays, soft robotics, electronic skin, and energy harvesting. Until now, pressure sensors work on force-induced changes in capacitance, piezoelectricity, triboelectricity and resistivity. Pressure sensors are one of the promising sensing devices in sensing technology and are part of a large field of mechanical sensors. Pressure sensors convert the force exerted on the object of interest into a measurable electrical signal that can be interpreted. Most of them are produced based on inductive, capacitive and piezoresistive phenomena which can be used to control and monitor pressure changes in many applications. Inductive pressure sensors require complex manufacturing techniques because it is difficult to bring the materials into coil shape. A capacitive pressure sensor uses a thin diaphragm as a condenser plate, and changes in the diaphragm due to pressure are converted into a signal by the transducer. Potential applications of capacitive sensors are in touch screen panels. Another type of pressure sensor is piezoelectric where the pressure applied to the piezoelectric element is seen as a bidirectional transducer that can convert stress to voltage and vice versa. Piezoresistive pressure sensor, on the other hand, pressure causes changes in resistance across the piezoresistive element. The operation of these nanomaterial-based pressure sensors has the advantage of ease of fabrication of the devices with relatively low power consumption during operation. Among other nanomaterials, carbon nanotubes (CNTs), graphene and its composites with piezoresistive elements have been studied by numerous researchers with differentsynthesis approaches. They are distinguished by their attractive thermal, electrical and mechanical properties, as well as their low density and high specific surface area at the nanoscale. Furthermore, the advantage of using CNT-based piezoresistive pressure sensors instead of polysilicon-based pressure sensors is that the response of the CNT sensor is independent of temperature and does not require it to be manufactured at high temperature. The performance of these sensitive materials depends on critically by their microstructures which in turn are influenced by the processing techniques that prepare them for the desired products. Regardless of whether chemical or physical training has been followed, it is of great importance to understand how materials work at the nanoscale for future technological applications. Since many nanomaterial-based pressure sensors are available in this study, a main focus will be given to the synthesis, characterization and potential application of CNT-graphene-based pressure sensors. Synthesis techniques Nanomaterials can be synthesized in two approaches, the bottom up method (CVD, electrochemical, sol-gel, solvothermal, etc.) where the material is synthesized atom by atom from below as opposed to the top down method (milling, ablation laser, lithography, etc.) in which the material is synthesized from the mass. Generally, the most commonly used techniques for producing carbon nanotubes are (i) carbon arc discharge technique, (ii) laser ablation technique, and (iii) chemical vapor deposition (CVD) technique . These techniques have been successful in fabricating large quantities of CNTs. Carbon Arc Discharge In the carbon arc discharge technique, two carbon electrodes are used to generate an arc via direct current. The electrodes are placed in a vacuum chamber to which an inert gas is supplied and the purpose of the gas is to increase the rate of carbon deposition. Initially the electrodes are separated until the pressure in the chamber stabilizes. Once the pressure has stabilized and the power (approximately 20 V) has been applied, the positive electrode is brought closer to the negative electrode to strike an arc. During the arc a high temperature plasma is formed and once the arc stabilizes with the electrodes held about a millimeter apart while the CNTs are deposited on the negative electrode. The power is then turned off and the tool is allowed to cool once it reaches a specific length. Important parameters to note in this technique are (i) the arc current and (ii) the optimum pressure of the inert gas in the chamber. This technique can produce high-quality CNTs, and some studies by Ebbesen and Ajayan have shown high-quality MWNTs with diameters ranging from 2 to 20 nm and lengths of several microns. They reported helium pressures of 500 Torr with current set to 18 V, and TEM analysis revealed that the MWNTs produced by the arc discharge technique were bound together by strong van der Waals forces and that the nanotubes consisted of two or more carbon shells. Laser AblationThe technique uses the same principle as arc discharge, however intense laser pulses are used to ablate a carbon target. CNTs are formed in the presence of an inert gas and a catalyst with the ablation of a carbon source. This technique fabricates high-quality CNTs with pronounced chirality [15] but it and arc discharge have drawbacks compared to CVD such as uncontrollable process parameters, high by-product yields along with the desired CNTs becoming difficult to separate. Another drawback is that both techniques use harsh conditions to produce these CNTs and scale-up is also an issuefactor, so they in turn become expensive as more energy is needed to synthesize them in large quantities. Important parameters determining the amount of CNTs produced that need to be taken into consideration are the amount and type of catalyst, temperature, pressure, type of inert gas and cooling systems near the carbon target. Chemical Vapor Deposition (CVD) This is the preferred technique due to its simplicity in producing CNTs, it is economically feasible as growth occurs at low temperatures and room temperature to allow for scale-up of CNTs. The tubes are synthesized by imparting energy to hydrocarbons. The energy breaks down the molecule into reactive radical species with a temperature range of 550 to 750℃, and the reactive radicals diffuse into the substrate and bind to it. This causes the formation of CNTs. The substrate is usually coated with transition metals such as Ni or Fe which acts as a catalyst and ethylene, acetylene, methane are usually used as hydrocarbon sources. The metal catalyst reduces the hydrocarbons into simple compounds and then the metal nanoparticles dissolve the carbon until the solubility limit is reached. The dissolved carbon material precipitates and progresses outward to produce a network of crystallized cylindrical structures. Characterization of Carbon Nanotube-Based Pressure Sensors Scanning Electron Microscopy The surface morphology of pure CNT can be analyzed using a scanning electron microscope. The SEM image of the pure CNT is shown in Figure 1 below. The scale bar is 5 μm, as seen in Figure 1 the surface morphology of the pure CNT-based sample is not uniform. The CNTs are randomly aligned on the sample surface. Some of the CNTs appear to be straight (red arrow) and curved (white arrow) in shape, but most of them even have a circular shape (blue arrow), which shows that carbon nanotubes are flexible in nature. The flexibility of CNTs makes them suitable materials for sensing technology, particularly in pressure sensors. Resistance-Pressure Relationships The resistance-pressure relationships for pure CNTs are shown in Figure 3. It can be seen from Figure 3 that as the external uniaxial pressure increases from 0 kNm−2 to 0.183 kNm−2, the DC resistances of the pressure sensor Pure CNT decrease from 1.5 kΩ to 0.3 kΩ, respectively. This shows an 80% decrease in direct current resistance for CNTs. The thickness of the fabricated sample is a significant factor that affects the overall performance of the sample and has an impact on the resistivity and conductivity of composite materials. Therefore, it is important to highlight the dependence of the sample thickness on the applied external pressure. Less pressure is needed to compress and deform the thinner sample and vice versa. Even under lower applied external pressure, a large increase in charge carrier concentration can completely fill the localized energy states present between the HOMO-LUMO levels, which can lead to higher electrical conductivity and thus lower resistance of the samples. Furthermore, the external uniaxial applied pressure can be transferred equally to every point of the thinnest CNT samples. Therefore, under the same applied external pressure, this effect increases the average coordination number, which leads to a greater decrease in the resistance of the thinner sample compared to the thicker one. Pressure Sensing Mechanism The basic principle of the CNT-based piezoresistive pressure sensor is the measurement of change in resistance across the CNTs due to applied pressure. But the difference..