Nanotube Flow Sensors
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.
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.
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
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
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).
Flow Through Carbon
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.
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