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Carbon Nanotube Flow Sensors

Published on Mar 12, 2016


Direct generation of measurable voltages and currents is possible when a fluids flows over a variety of solids even at the modest speed of a few meters per second. In case of gases underlying mechanism is an interesting interplay of Bernoulli's principle and the See beck effect: Pressure differences along streamlines give rise to temperature differences across the sample; these in turn produce the measured voltage.

The electrical signal is quadratically dependent on the Mach number M and proportional to the Seebeck coefficient of the solids.

This discovery was made by professor Ajay sood and his student Shankar Gosh of IISC Bangalore, they had previously discovered that the flow of liquids, even at low speeds ranging from 10 -1 meter/second to 10 -7 m/s (that is, over six orders of magnitude), through bundles of atomic-scale straw-like tubes of carbon known as nanotubes, generated tens of micro volts across the tubes in the direction of the flow of the liquid. Results of experiment done by Professor Sood and Ghosh show that gas flaw sensors and energy conversion devices can be constructed based on direct generation of electrical signals. The experiment was done on single wall carbon nanotubes (SWNT).These effect is not confined to nanotubes alone these are also observed in doped semiconductors and metals.

The observed effect immediately suggests the following technology application, namely gas flow sensors to measure gas velocities from the electrical signal generated. Unlike the existing gas flow sensors, which are based on heat transfer mechanisms from an electrically heated sensor to the fluid, a device based on this newly discovered effect would be an active gas flow sensor that gives a direct electrical response to the gas flow. One of the possible applications can be in the field of aerodynamics; several local sensors could be mounted on the aircraft body or aerofoil to measure streamline velocities and the effect of drag forces. Energy conversion devices can be constructed based on direct generation of electrical signals i.e. if one is able to cascade millions these tubes electric energy can be produced.

As the state of art moves towards the atomic scales, sensing presents a major hurdle. The discovery of carbon nanotubes by Sujio Iijima at NEC, Japan in 1991 has provided new channels towards this end. A carbon nanotube (CNT) is a sheet of graphene which has been rolled up and capped with fullerenes at the end. The nanotubes are exceptionally strong, have excellent thermal conductivity, are chemically inert and have interesting electronic properties which depend on its chirality. The main reason for the popularity of the CNTs is their unique properties. Nanotubes are very strong, mechanically robust, and have a high Young's modulus and aspect ratio. These properties have been studied experimentally as well as using numerical tools. Bandgap of CNTs is in the range of 0~100 meV, and hence they can behave as both metals and semiconductors.

A lot of factors like the presence of a chemical species, mechanical deformation and magnetic field can cause significant changes in the band gap, which consequently affect the conductance of the CNTs. Its unique electronic properties coupled with its strong mechanical strength are exploited as various sensors. And now with the discovery of a new property of flow induced voltage exhibited by nanotubes discovered by two Indian scientists recently, has added another dimension to micro sensing devices.

CNT Electronic Properties

Electrically CNTs are both semiconductor and metallic in nature which is determined by the type of nanotube, its chiral angle, diameter, relation between the tube indices etc. The electronic properties structure and properties is based on the two dimensional structure of Graphene. For instance if the tube indices, n and m, satisfies the condition n-m=3q where q is and integer it behaves as a metal. Metal, in the sense that it has zero band gap energy. But in case of armchair (where n=m) the Fermi level crosses i.e. the band gap energy merges. Otherwise it is expected the properties of tube will be that of semiconductor. The table below (Table 1) is the observations of experiments done on nanotubes by Scanning tunneling microscope (STM) and Scanning tunneling spectroscopes (STS).

Fluid Flow Through Carbon Nanotube

Recently there has been extensive study on the effect of fluid flow through nanotubes, which is a part of an ongoing effort worldwide to have a representative in the microscopic nano-world of all the sensing elements in our present macroscopic world. Indian Institute of Science has a major contribution in this regard. It was theoretically predicted that flow of liquid medium would lead to generation of flow-induced voltage. This was experimentally established by two Indian scientist at IISc. Only effect of liquid was theoretically investigated and established experimentally, but effect of gas flow over nanotubes were not investigated, until A.K Sood and Shankar Ghosh of IISc investigated it experimentally and provided theoretical explanation for it. The same effect as in case of liquid was observed, but for entirely different reason. These results have interesting application in biotechnology and can be used in sensing application. Micro devices can be powered by exploiting these properties.

Effect Of Liquid Flow Through Nanotube

P Kral and M Shapiro published a paper in Physical review letters, that dealt with development of voltage / current when liquid flows through CNTs. Generally an electric current in a material is produced when flow of free charge carriers is induced in the material. According to Kral and Shapiro, the generation of an electric current in a nanotube is essentially due to transfer of momentum from the flowing liquid molecules to acoustic phonons in nanotube so as to have a dragging effect on the free charge carriers in the nanotube. The outcome, according to these workers, is a linear dependence of the induced electric current on the flow velocity.

Another mechanism involved, as per these authors involves a direct scattering of the free carriers from the fluctuating Coulombic fields of the ions or polar molecules in the flowing liquid. They argued, however, that the latter mechanism creates a current that is five orders of magnitude smaller than the current that results from the phonon-induced electron drag.

These predictions were experimentally verified by A K Sood and Shankar Ghosh of IISc. In sharp contrast, Sood and coworkers have found that the behaviour is highly sub linear where the induced voltage fits a logarithmic velocity dependence over nearly six decades of velocity. Strong dependence of induced voltage on polarity and ionic nature of the liquid and relatively weak dependence on its viscosity was revealed from this experiment.

Carbon Nanotube Flow Sensors

Verification on Theorotical Prediction:

Experimental setup, observed results and hence the inference gathered from the observations, dependencies of voltage induced on various factors that has been observed are explained in following sections+

Specification of materials and instuments used:

The SWNT bundles used for the experiment were prepared by electric arc method, followed by purification process. The dimensions and physical properties of samples are listed below:

Diameter of nanotube 1. 5nm

Resistivity 0.02Q-m

Sensor using this tube was prepared by densly packing nanotubes between two metal electrodes. The dimensions of Sensor:

Along the flow 1mm

Thickness 0.2mm

Width 0.2mm

The specimen was placed inside glass tube of inner dia 0.03 and length 0.9m, kept vertically. Using KEITHLEY 2000 multimeter used voltage mesurment. Hydrochloric and Glycerin to increase polarity and viscosity of fluid media respectively.

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