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Pressure Sensors are very essential, and are used in various fields such as aerospace, barometry, industries, automobiles, medical, 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 CNT for pressure sensing has been carried out. Due to its high gauge factor (200 to 1000), high sensitivity, temperature independency and many other advantages, CNT serves as a highly effective material to be used as a piezoresistive element in piezoresistive pressure sensors.
The discovery and development of the nanoworld has played a huge role in scientific and technological advancement in the universe. Nanomaterials have received most attention since their discovery and they can be distinguished and classified based on their dimensionality (0D, 1D, 2D, and 3D). They are also the most studied due to their unique electrical, mechanical, optical, and magnetic properties and a wide range of application they provide. Lately there have been interest in various nanomaterials including nanowires, carbon nanotubes, polymer nanofibers, metal nanoparticles and graphene have been used for the fabrication of novel flexible pressure and strain sensors. These nanomaterials have potential applications such as touch-on flexible displays, soft robotics, electronic skin, and energy harvesting. Till now, pressure sensors operate on force induced changes in capacitance, piezoelectricity, triboelectricity and resistivity. Pressure sensors are one of the promising sensing devices in the sensing technology and is part of a vast field of mechanical sensors. Pressure sensors convert the force exerted on the object of interest into a measurable electric signal which can be interpreted. Most of them are produced on the base of inductive, capacitive, and piezoresistive phenomena that can be used to control and monitor pressure changes in many applications. Inductive pressure sensors require complex fabrication techniques as it is challenging to bring materials into coil shape. A capacitive pressure sensor employs a thin diaphragm as one plate of the capacitor and changes experienced by the diaphragm due to pressure is converted to a signal by the transducer. Potential applications of capacitive sensors are in the touch screen panels.
Another type of pressure sensor is the piezoelectric in which pressure applied to the piezoelectric element is seen as bi-directional transducer which can convert stress into voltage and vice versa. Piezoresistive pressure sensor on the other hand the pressure causes changes in resistance across piezoresistive element. The operation of these nanomaterial-based pressure sensors has an advantage of the ease of fabrication of the devices with relatively low energy consumption in operation. Among other nanomaterials carbon nanotubes (CNTs), graphene, and its composites with piezoresistive elements have been investigated by numerous researchers with different synthesis approaches. They stand-out due their interesting thermal, electrical and mechanical properties, also their low density and high specific surface area at a nanoscale. In addition, the advantage of using CNTs based piezoresistive pressure sensors instead of polysilicon-based pressure sensors is that the response of CNT sensor is independent of temperature and they do not require to be produced at high temperature. The performance of these sensing materials depends critically on their microstructures which in turn is affected by the processing techniques which prepares them for desired products. Whether a chemical or physical preparation was followed, it is of great importance to understand how materials works at a nanoscale for future technological applications. Since there are many nanomaterial-based pressure sensors available in this study a prime focus will be given to synthesis, characterization, and potential application of CNT-graphene based pressure sensors.
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 the bottom opposed to top down method (milling, laser ablation, lithography, etc. ) in which the material is synthesized from the bulk. Generally, the most common 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 fabrication of large quantities of CNTs.
In carbon arc-discharge technique two carbon electrodes are used to generate an arc by DC current. The electrodes are placed in a vacuum chamber which is supplied an inert gas and the purpose of the gas is to increase deposition speed of carbon. Initially the electrodes are separated until the pressure is stabilized in the chamber. Once pressure stabilized the power supply is turned on (about 20 V) the positive electrode is brought closer to the negative to strike an arc. Upon arcing, a high temperature plasma formed and once the arc stabilizes with the electrodes kept about a millimeter apart while the CNTs deposit on the negative electrode. The power supply is then cut off and the instrument is allowed to cool once a specific length is reached. Important parameters to be noted in this technique are (i) the arcing current and (ii) the optimum inert gas pressure in the chamber. This technique can produce high quality CNTs and some studies by Ebbesen and Ajayan showed high quality MWNTs with diameters in the range 2 to 20 nm and lengths of several microns. They reported helium pressures of 500 Torr with current set to 18 V and analysis by TEM revealed that the MWNTs produced by arc-discharge technique were bound together by strong van der Waals forces and the nanotubes consisted of two or more carbon shells.
The technique uses the same principle as the 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  but it and arc-discharge present drawbacks compared to CVD such as uncontrollable process parameters, high yields of by-products along with desired CNTs which becomes challenging to separate. Another set back is that both techniques utilizes harsh conditions to produces these CNTs and scaling-up is also a factor thus in turn they become expensive as more energy is required to synthesize in large quantities. Important parameters that determine the amount of CNTs produced that must be noted are the amount and type of catalyst, temperature, pressure, type of inert gas, and cooling systems near the carbon target.
This is the most preferred technique due to its simplicity to produce the CNTs, economical viable as growth is at low temperatures, and ambient temperature to allow scaling-up of the CNTs. The tubes are synthesized by imparting energy to hydrocarbons. The energy breaks the molecule into reactive radical species with temperature range of 550- 750℃ and the reactive radicals diffuse to the substrate and bonds there. This results in the formation of CNTs. The substrate is usually coated with transition metals such as Ni or Fe which acts as catalyst and ethylene, acetylene, methane is usually used as hydrocarbon sources. The metal catalyst reduces the hydrocarbons into simple compounds and thereafter the metal nanoparticles dissolves the carbon unlit its solubility limit is reached. The dissolved carbon material precipitates and progress outside to produce a network of crystallized cylindrical structures.
The surface morphology of pure CNT can be analysed using a scanning electron microscope. The SEM image of pure CNT is shown in the Figure 1 below. The scale bar is 5 μm, as seen in the Figure 1 the surface morphology of pure CNT-based sample is not uniform. The CNTs are randomly aligned on the surface of the sample. Some of the CNTs seem to be straight (red arrow) and curved in shape (white arrow), but most of them are even circular in shape (blue arrow), which shows that the carbon nanotubes are flexible in nature. The flexibility of CNTs makes them suitable materials for sensing technology especially in pressure sensors.
The resistance-pressure relationships for pure CNT is 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 pure CNT pressure sensor decrease from 1. 5 kΩ to 0. 3 kΩ respectively. This shows 80% decrease in direct current resistance for CNT. Thickness of the fabricated sample is a significant factor which affects the overall performance of the sample and has an impact on the resistivity and conductivity of the composite materials. Therefore, it is important to point out the sample thickness dependence on the external applied pressure. Smaller pressure required to compress and deform the thinner sample and vice versa. Even under a smaller external applied pressure, large increase in charge carrier’s concentration may completely fill the localized energy states present between the HOMO-LUMO levels which may lead to larger electrical conductivity and hence smaller resistance of the samples. Furthermore, the external uniaxial applied pressure can be equally transferred to every place throughout the thinner samples of CNT. Therefore, under the same external applied pressure, this effect increases the mean coordination number, which leads to a more decrease in the resistance of the thinner sample than the thicker one.
The basic principle of CNT based piezoresistive pressure sensor is the measurement of change in the resistance across the CNTs due to the pressure applied. But the difference is more importantly in the number of CNTs used as piezoresistors, their orientation and their location on the diaphragm. In this study, we focus on the sensor design that consists of piezoresistive CNT element resting on top of a diaphragm.
A contact is established with the SWNT utilizing Platinum electrodes, thus measuring the resistance of the nanostructure. The application of pressure underneath the sensor causes a deflection of the silicon membrane and this causes a change in resistance of the Carbon nanotube. The optimal location to place the CNT would be the region of maximum strain on the diaphragm. As a result, the calculation of strain distribution and deflection in accordance with the applied pressure becomes pivotal.
The material used to design the diaphragm on which the CNTs are placed, plays a very important role in deciding the application of a pressure sensor and the range of pressure the sensor can sense. However, the list is not exhaustive with respect to the material used to design the diaphragm and it depends on the application area and the range of pressure to be sensed. Apart from the piezoresistive CNTs and diaphragm materials, other materials such as Perylene C, SiO2, TiO2, etc. , are used as insulating layer to protect the diaphragm and improve the adhesion of electrodes to the substrate.
Pressure sensors can vary drastically in technology, design, performance, application suitability and cost. They are used to control and monitoring in thousands of everyday applications. The CNT is one of the most materials used to develop pressure sensors. The resistance to pressure analysis of the pure CNT has been carried out. It has been observed that as the pressure increases; the resistance decreases. The reason for using CNT for sensing application is that CNT are flexible in nature and that has been analyzed using the SEM technique. Carbon nanotubes can be synthezed using different technique such as carbon-arch discharge, laser vaporization and chemical vapor deposition.
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