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when heated resistively to a very high temperature. The accelerated electrons
(that are emitted) are bombarded on a metal target (anode) to generate X-rays.
The advantage associated with this method is that it works even in non-ultrahigh
vacuum ambiences, which contain various gaseous molecules. But this method
has several limitations: 1) a slow response time; 2) consumption of high energy;
and 3) a limited lifetime. Recent research has reported that field emission,
compared to thermoionic emission, is a better mechanism of extracting electrons.
This is because electrons are emitted at room temperature and the output
current is voltage controllable. In addition, giving the cathode the form of tips
increases the local field at the tips and, as a result, the voltage necessary
for electron emission is lowered. An optimal cathode material should have a
high melting point, low work function, and high thermal conductivity. CNTs
can be used as a cathode material for generating free flowing electrons. Electrons
are readily emitted from their tips either due to oxidized tips or the curvature when
a potential is applied between a CNT surface and an anode. Yue et al. [8]
generated continuous and pulsed X-rays using a CNT-based field emission
cathode. The field emission currents were found to follow the Fowler-Nordheim
regime, where the applied voltage and constants are dependent on the cathode
geometry and work function. Plotting current density against 1/E (cm/mV)
yields a straight line for a current of field emission origin. Yu et al. obtained a
total emission current of 28mA from a 0.2-cm-area CNT cathode. By program-
ming the gate voltage, pulsed X-rays with a repetition rate greater than 100 kHz
was readily achieved. The X-ray intensity was sufficient to image human organs
at 14 kVp and 180mAs. Recently, a dynamic radiography system using CNT-
based field emission has been proposed by Cheng et al. [9]. X-ray radiation
with continuous variation of temporal resolution as short as nanoseconds can
be readily generated by their system. The advantages of CNT-based X-ray
devices are fast response time, fine focal spot, low power consumption, possible
miniaturization, longer life, and low cost. Besides, it minimizes the need of
cooling required by the conventional method. Miniaturized X-ray devices
can be inserted into the body by endoscopy to deliver precise X-ray doses
directly at a target area without damaging the surrounding healthy tissues,
as malignant tumors are highly localized during the early stage of their develop-
ment. With time, the cancer spreads to neighboring anatomic structures.
Other processes such as chemotherapy and conventional radiation doses kill
the cancer but may also kill healthy tissues. This is not desired from a health
point of view.
18.3.1.2. Sensors.
Sensors are devices that detect a change in physical
quantity or event. There are many studies that have reported use of CNTs
as pressure, flow, thermal, gas, and chemical and biological sensors, as mentioned
at the beginning of this section. Liu and Dai [10] demonstrated that piezoresistive
pressure sensors can be made with the help of CNTs. They grew SWNTs
on suspended square polysilicon membranes. When uniform pressure was
applied on the membranes, a change in resistance in the SWNTs was observed.
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