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Sensing Properties of a Novel Temperature Sensor
Based on Field Assisted Thermal Emission
Zhigang Pan 1 , Yong Zhang 1, *, Zhenzhen Cheng 1 , Jiaming Tong 1 , Qiyu Chen 1 ,
Jianpeng Zhang 1 , Jiaxiang Zhang 1 , Xin Li 2 and Yunjia Li 1
1
2
*
State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710-049,
China; [email protected] (Z.P.); [email protected] (Z.C.); [email protected] (J.T.);
[email protected] (Q.C.); [email protected] (J.P.Z.); [email protected] (J.X.Z.);
[email protected] (Y.L.)
Vacuum Micro-Electronic & Micro-Electronic Mechanical Institute, School of Electronics and Information
Engineering, Xi’an Jiaotong University, Xi’an 710049, China; [email protected]
Correspondence: [email protected]; Tel.: +86-29-8266-8793
Academic Editors: Christos Riziotis, Evangelos Hristoforou and Dimitrios Vlachos
Received: 20 January 2017; Accepted: 22 February 2017; Published: 27 February 2017
Abstract: The existing temperature sensors using carbon nanotubes (CNTs) are limited by low
sensitivity, complicated processes, or dependence on microscopy to observe the experimental results.
Here we report the fabrication and successful testing of an ionization temperature sensor featuring
non-self-sustaining discharge. The sharp tips of nanotubes generate high electric fields at relatively
low voltages, lowering the work function of electrons emitted by CNTs, and thereby enabling the
safe operation of such sensors. Due to the temperature effect on the electron emission of CNTs,
the collecting current exhibited an exponential increase with temperature rising from 20 ◦ C to
100 ◦ C. Additionally, a higher temperature coefficient of 0.04 K−1 was obtained at 24 V voltage
applied on the extracting electrode, higher than the values of other reported CNT-based temperature
sensors. The triple-electrode ionization temperature sensor is easy to fabricate and converts the
temperature change directly into an electrical signal. It shows a high temperature coefficient and
good application potential.
Keywords: temperature sensor; carbon nanotubes; ionization; emission
1. Introduction
Temperature sensors are among the most-widely used sensors in consumer and industrial
temperature measurement. The development goal of a temperature sensor usually includes high
sensitivity, small footprint, and low power consumption. The potential of a carbon nanotubes (CNTs)
based temperature sensor offers great opportunities towards extreme miniaturization and low power
consumption [1–5], thanks to the unique nanoscale structure and electrical property [6–11]. Recent
studies showed that CNTs could be used to construct temperature sensors. For example, a thermometer
can be realized by measuring the thermal expansion of gallium inside a CNT [1], since the height
of a continuous unidimensional column of liquid gallium inside a carbon nanotube varies linearly
and reproducibly in the temperature range 50–500 ◦ C. Nevertheless, this methodology requires
microscopic measurement of the height of the gallium. Alternatively, the CNT-based temperature
sensors [2–5] can also be implemented by measuring the conductivity variation of CNT induced by
the thermal interaction. However, the application of this type of sensor is potentially limited by its
complex fabrication process and low sensitivity. A novel ionization temperature sensor based on
CNTs electrodes [12] was capable of overcoming the limitations of the above two types of temperature
sensors, but it was only used to detect temperature in N2 at 100 V extracting voltage; meanwhile,
Sensors 2017, 17, 473; doi:10.3390/s17030473
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its carbon nanotubes have large diameter and small spaces between nanotubes (see supplementary
material
Figure
which resultsmaterial
in smallFigure
field enhancement
factor,
current, andfactor,
low
nanotubes
(seeS1),
supplementary
S1), which results
insmall
smalloutput
field enhancement
◦
sensitivity—especially
below
(see supplementary material
andsupplementary
S2, Figure S2). material
In this
small output current,
and 70
lowCsensitivity—especially
below Tables
70 °C S1
(see
paper,
a
triple-electrode
CNT-based
ionization
temperature
sensor
is
fabricated.
Differently,
the
CNTs
Tables S1 and S2, Figure S2). In this paper, a triple-electrode CNT-based ionization temperature
array
is
grown
by
a
low-temperature
thermal
chemical
vapor
deposition
(TCVD)
process,
enabling
sensor is fabricated. Differently, the CNTs array is grown by a low-temperature thermal chemical
small
diameters
and(TCVD)
larger spaces
As a result,
the electric
around
CNT
vapor
deposition
process,between
enablingnanotubes.
small diameters
and larger
spaces field
between
nanotubes.
tips
and
the
emission
current
density
of
the
tips
can
be
enhanced
[13].
In
addition,
the
decrease
As a result, the electric field around CNT tips and the emission current density of the tips canofbe
the
electrode[13].
separation
can increase
the electric
strength
of the sensor.
The fabricated
CNT
enhanced
In addition,
the decrease
of the field
electrode
separation
can increase
the electric
field
temperature
is capable
detectingCNT
temperature
in ambient
at low
working
voltage.
strength ofsensor
the sensor.
The of
fabricated
temperature
sensoratmosphere
is capable of
detecting
temperature
The
sensing mechanism
of the voltage.
CNT sensor
explained insensing
terms ofmechanism
the emission
in temperature
ambient atmosphere
at low working
The istemperature
of of
theCNT
CNT
and
the discharge
properties
sensor
is explained
in termsofofair.
the emission of CNT and the discharge properties of air.
2. 2.
Materials
and
Methods
Materials
and
Methods
Figure
1 shows
the
schematic
illustration
ofof
the
presented
CNT-based
temperature
sensor.
The
Figure
1 shows
the
schematic
illustration
the
presented
CNT-based
temperature
sensor.
The
sensor
is
comprised
of
three
electrode
plates;
i.e.,
a
CNT-based
cathode,
an
extracting
electrode,
and
a
sensor is comprised of three electrode plates; i.e., a CNT-based cathode, an extracting electrode, and
collecting
electrode.
The
two
ventilating
holes
in
the
cathode
make
the
gas
diffuse
more
easily
[11].
a collecting electrode. The two ventilating holes in the cathode make the gas diffuse more easily [11].
AA
hole
in in
thethe
extracting
electrode
is is
used
to to
extract
discharge
particles.
rectangular
ditch
inin
the
hole
extracting
electrode
used
extract
discharge
particles.A A
rectangular
ditch
the
collecting
electrode
could
reduce
reflection-induced
loss
of
positive
ions
and
collect
more.
In
operation,
collecting electrode could reduce reflection-induced loss of positive ions and collect more. In
theoperation,
extractingthe
voltage
Ue is set
higherUthan
the collecting voltage Uc , where
the potential
of the
extracting
voltage
e is set higher than the collecting
voltage
Uc, where
thecathode
potential
is of
0 V.
In
this
configuration,
two
electric
fields
E
and
E
are
generated
with
reversed
field
direction.
1
2
the cathode is 0 V. In this configuration, two electric fields E1 and E2 are generated with reversed
The
currents
Ic and
Ie were
recorded
high-precision
multimeters (NI
PXI-4071,
National
field
direction.
The
currents
Ic andbyIe two
were
recorded by digital
two high-precision
digital
multimeters
(NI
Instruments
Corporation,
Austin,
TX,
USA).
One
minute
after
the
voltages
are
applied
to
the
electrodes
PXI-4071, National Instruments Corporation, Texas, United States). One minute after the voltages
and
discharge
ofelectrodes
air is stable,
tenthe
current
values
ten ten
current
values
of Ieofare
recorded
and
arethe
applied
to the
and
discharge
of of
airIcisand
stable,
current
values
Ic and
ten current
averaged
as
currents
I
and
I
,
respectively.
c
e averaged as currents Ic and Ie, respectively.
values of Ie are recorded
and
The
presented
CNT
temperature
sensor
waswas
fabricated
according
to the
steps.
Three
Si
The presented CNT temperature
sensor
fabricated
according
to following
the following
steps.
Three
substrates
were
firstly
patterned
by
photolithograph
to
form
the
pattern
of
the
cavity
with
different
Si substrates were firstly patterned by photolithograph to form the pattern of the cavity with
sizes.
The patterned
then etched
anetched
inductively
plasma
(ICP) plasma
etcher. The
different
sizes. Thesubstrates
patternedwere
substrates
were by
then
by ancoupled
inductively
coupled
(ICP)
etched
structure
included
the
cathode
with
two
circular
holes
of
4
mm
in
diameter,
the
extracting
etcher. The etched structure included the cathode with two circular holes of 4 mm in diameter, the
electrode
with
a circular
hole
of 6 mmhole
in diameter,
the collecting
electrode
withelectrode
a rectangular
extracting
electrode
with
a circular
of 6 mm and
in diameter,
and the
collecting
with a
ditch
structure
with
the
size
of
8
mm
×
6
mm
×
200
µm
(length
×
width
×
depth)
(Figure
1a–c).
rectangular ditch structure with the size of 8 mm  6 mm  200 µm (length  width  depth)(Figure
A 1a–c).
metallization
layer of Ti/Ni/Au
was thenwas
sputtered
on both sides
of the
extracting
electrode and
the
A metallization
layer of Ti/Ni/Au
then sputtered
on both
sides
of the extracting
electrode
inner
walls
of
the
cathode
and
collecting
electrode.
Vertically
aligned
multi-walled
carbon
nanotubes
and the inner walls of the cathode and collecting electrode. Vertically aligned multi-walled carbon
(MWCNTs)
subsequently
grown by thermal
vapor
deposition
on (TCVD)
one sideon
nanotubeswere
(MWCNTs)
were subsequently
grownchemical
by thermal
chemical
vapor (TCVD)
deposition
◦ C [14], with around 20 nm in diameter, 5 µm in length, and separated with a
ofone
the side
cathode
at
700
of the cathode at 700 °C [14], with around 20 nm in diameter, 5 µm in length, and separated
distance
200 nmofin200
between
1d).
The three
assembled
50 µm-thick
with a of
distance
nm in(Figure
between
(Figure
1d). electrodes
The three were
electrodes
were with
assembled
with 50
polyester
films.
µm-thick polyester films.
Figure 1. Cont.
Sensors
Sensors2017,
2017,17,
17,473
473
33 of
of 77
Figure
Schemeofofthe
thetriple-electrode
triple-electrode
carbon
nanotube
(CNT)-based
sensor.
(a) cathode
the cathode
Figure 1.
1. Scheme
carbon
nanotube
(CNT)-based
sensor.
(a) the
after
after
substrate
patterning,
Ti/Ni/Au
sputtering,
and
CNT
growing;
(b)
the
extracting
electrode
substrate patterning, Ti/Ni/Au sputtering, and CNT growing; (b) the extracting electrode and (c)and
the
(c)
the collecting
electrode
substrate
patterning
andTi/Ni/Au
Ti/Ni/Au
sputtering;(d)
(d) Scanning
Scanning electron
collecting
electrode
after after
substrate
patterning
and
sputtering;
electron
microscope
microscopeimage
imageof
ofthe
theCNT
CNTfilm;
film;(e)
(e)The
Thetest
testset-up.
set-up.
3.
3. Results
Results
The
studied as depicted in Figure 1e.
The effect
effect of
of temperature
temperature on the discharge current of air was studied
The
fabricated
sensor
was
placed
inside
a
test
chamber
filled
with
ambient
The fabricated sensor was placed inside a test chamber filled with ambient air
air heated
heated by
by aa resistive
resistive
wire.
wire. The temperature inside the chamber
chamber was closed-loop
closed-loop controlled
controlled by tuning
tuning the
the voltage
voltage of
of the
the
resistive
wirewire
and measuring
the temperature
using a k-typeusing
thermocouple
(chromel–nisiloy).
resistiveheating
heating
and measuring
the temperature
a k-type
thermocouple
◦ C.
The
steady state of the
within thewas
time
of 1 min,
in a range
of 20of◦ C1 to
100in
(chromel–nisiloy).
Thetemperature
steady statewas
of realized
the temperature
realized
within
the time
min,
a
After
temperature
reached
state, Uc of reached
1 V and U
values
24 and
100 Vvalues
were
e withstate,
rangethe
of 20
°C to 100 °C.
Aftersteady
the temperature
steady
Ucbetween
of 1 V and
Ue with
applied,
the discharge
current
Ic and
were
measured
respectively
the measured
detection temperature
betweenand
24 and
100 V were
applied,
andIethe
discharge
current
Ic and Iefor
were
respectively
◦ C to
and
the
study
of
emission
properties
of
the
CNTs
cathode.
When
temperature
T
rose
from
20 When
for the detection temperature and the study of emission properties of the CNTs cathode.
◦
100
C, currents
Ic and
Ie 20
exhibited
an°C,
exponential
with T (Figure
2a,b). When
extracting
temperature
T rose
from
°C to 100
currents Icincrease
and Ie exhibited
an exponential
increase
with T
voltage
U
increased
from
24
V
to
100
V,
currents
I
and
I
exhibited
an
increase
with
U
.
Sensitivity
(Figure 2a,b).
When extracting voltage Ue increased
e
c from
e 24 V to 100 V, currents Ic and
e Ie exhibited
curves
to temperature
show verycurves
similartoshape,
as shown
in Figure
2. This indicates
the in
sensor
is
an increase
with Ue. Sensitivity
temperature
show
very similar
shape, as that
shown
Figure
sensitive
to temperature
and sensor
is thus capable
of detecting
temperature
change.
Values
of Ie of
were
almost
2. This indicates
that the
is sensitive
to temperature
and
is thus
capable
detecting
twice
of thosechange.
of Ic . The
temperature
coefficients
of Figure
2a were
according
to equation
temperature
Values
of Ie were
almost twice
of those
of Ic.calculated
The temperature
coefficients
of
SFigure
= ∆I/(∆T
IFS ), where
∆T is according
the variation
temperature,
∆I is the
variation
of Icis
, and
is the full
2a ·were
calculated
to of
equation
S = ΔI/(ΔT·I
FS),
where ΔT
the IFS
variation
of
1 at 100 ◦ C and 24 V U (Table 1),
scale
range of IΔI
highest
coefficient
Smax
obtained
as 0.04
K−of
temperature,
is the
variation
of Ic, and
IFSwas
is the
full scale
range
Ic. The highest coefficient
Smax
c . The
e
higher
than theasvalues
of−1other
temperature
sensors
[1–5].
was obtained
0.04 K
at 100reported
°C and CNT-based
24 V Ue (Table
1), higher
than the
values of other reported
CNT-based temperature sensors [1–5].
Table 1. Performance comparison of the temperature sensors.
No.
No.
This
paper
Article
1
This
paper
Article 2
Article 1
Article 3
Article
Article24
Article35
Article
Table 1. Performance comparison of the temperature sensors.
Test Range (◦ C)
Highest Temperature Coefficient (K−1 )
Test 20–100
Range (°C)
50–500
20–100
−269–147
50–500
20–70
−269–147
20–75
20–60
20–70
−1
Highest Temperature
4.0 × 10–2 Coefficient (K )
–3
2.24.0
× 10
× 10–2
−7.0 × 10–4
2.2 × 10
–3
−1.3 × 10–2
–2 –4
−7.0
× 10
−1.7
× 10
–3 –2
−5.0
× 10
−1.3
× 10
Reference
Reference
[1] [2]
[1]
[3]
[4] [2]
[5] [3]
[4]
Article 4
20–75
−1.7 × 10–2
–3
[5]
Article
5
20–60
−5.0 × 10current density and is obtained
The
relationship
between
the logarithm lnje (je is the cathode
by
je = Ie /Sarea ) and the reciprocal –1/T was studied (Figure 2c). Sarea is the total cross-sectional area
relationship
thewas
logarithm
lnje according
(je is the cathode
current
density
is obtained
by
of all The
nanotubes
on thebetween
film, and
calculated
to Figure
1d (Figure
S3and
in reference
[15]);
−6 m
2 . Eight –1/T
= Ie=/S3.04
area) and
reciprocal
studied
(Figure
2c).were
Sarea is
the total
cross-sectional
of all
Sjearea
× 10the
leastwas
squares
straight
lines
fitted
to the
curves of lnjearea
−1/T
at
nanotubes
on
the
film,
and
was
calculated
according
to
Figure
1d
(Figure
S3
in
reference
[15]);
S
area
=
different Ue , and the coefficients R-Sq were calculated (Figure 2c)
−6
2
3.04  10 m . Eight least squares straight lines were fitted to the curves of lnje-1/T at different Ue,
R-Sq = 1 −(Figure
(Σ(yi −2c)
f i )2 )/(Σ(yi − yav )2 )
(1)
and the coefficients R-Sq were calculated
R-Sq = 1  ((yifi)2)/((yiyav)2)
(1)
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Sensors 2017, 17, 473
4 of 7
where yi denotes lnje , f i denotes the fitting value to lnje , yav denotes the mean value of all lnje , and
subscript
coefficients
R-Sq show
that the
straight
lines
are the
wherei ydenotes
i denotesthe
lnjesequence
, fi denotesnumber.
the fittingThe
value
to lnje, yav denotes
the mean
value
of all lnj
e, and
subscript
denotes
the at
sequence
number.
The coefficients
R-Sqemission
show thatbehavior
the straight
are the The
best fit
line toi the
curves
various
Ue , indicating
a thermal
oflines
the sensor.
best fit line
to the current
curves at
variousje U
e, indicating
a thermal
emission
the sensor.
The 2d),
relationship
between
density
and
the average
electric
field E0behavior
was alsoofobtained
(Figure
relationship
between
current
density
j
e and the average electric field E0 was also obtained (Figure
E0 = U
/d,
where
d
denotes
electrode
separation
between
cathode
and
extracting
electrode.
j
increased
e
e
E0 = Ue/d,awhere
d denotesthermal
electrode
separation
between
cathode and extracting electrode. je
with 2d),
E0 , showing
field-assisted
emission
behavior
[16].
increased with E0, showing a field-assisted thermal emission behavior [16].
(a)
(b)
(c)
(d)
Figure 2. Effect of temperature on current at different Ue. (a) Collecting current–temperature
Figure 2. Effect of temperature on current at different Ue . (a) Collecting current–temperature
characteristic and (b) Cathode current–temperature characteristic at 24–100 V Ue; (c) lnje increased
characteristic
and (b) Cathode current–temperature characteristic at 24–100 V Ue ; (c) lnje effect
increased
linearly with −1/T at 24–100 V Ue, suggesting the thermal emission behavior and a considerable
linearly
with
−
1/T
at
24–100
V
U
,
suggesting
the
thermal
emission
behavior
and
a
considerable
e
of temperature on current density; (d) je increased with E0, suggesting the field emission behavior. effect
of temperature on current density; (d) je increased with E0 , suggesting the field emission behavior.
4. Discussion
4. Discussion
This section analyzes the experimental results to understand the temperature sensing
mechanism
the present
temperature
sensor.
Ue is applied,
electrons are
emitted
from
This sectionofanalyzes
theCNT
experimental
results
to When
understand
the temperature
sensing
mechanism
the nanotube tips and collide with the gas molecules, generating positive ions [17]. A fraction of the
of the present CNT temperature sensor. When Ue is applied, electrons are emitted from the nanotube
generated positive ions are extracted from the cathode region through the extracting hole towards
tips and collide with the gas molecules, generating positive ions [17]. A fraction of the generated
the collecting electrode and form the collecting current Ic. Consequently, Ic is a part of the discharge
positive
ionsI [18],
are extracted
current
given by: from the cathode region through the extracting hole towards the collecting
electrode and form the collecting current Ic . Consequently,
Ic is a part of the discharge current I [18],
I = eN0ed
(2)
given by:
I = eN0 eαd
(2)
Sensors 2017, 17, 473
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where
is the
charge, N0 is the number of electrons leaving the cathode in one second,
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473
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7
α is the first ionization coefficient. Emission electrons and secondary emission electrons of the CNT
where econtribute
is the electron
N0non-self-discharge
is the number of electrons
leaving
the cathode
in one second,
and α
cathode
to Ncharge,
state, N
determined
by the emission
0 . In the
0 is mainly
is the first
coefficient.
electrons
and
secondary
emission
electrons
the CNT
electrons
[18].ionization
It is known
that onlyEmission
those electrons
with
energies
greater
than the
sum ofofFermi
energy
contribute
to Φ
N0.can
In the
non-self-discharge
state,nanotubes.
N0 is mainlyWhen
determined
by the emission
EFcathode
and work
function
be emitted
by the carbon
Ue is applied,
the work
electrons
[18]. It is emitted
known that
only
those
electrons
withΦ energies
than the sum of Fermi
function
of electrons
by the
CNT
is reduced
from
to Φeff . Φgreater
eff is the effective work function,
energy
E
F and work function Φ can be emitted by the carbon nanotubes. When Ue is applied, the
and it decreased with increasing E1 at a given temperature (Figure 3a). More electrons with energies
work function
ofΦ
electrons
emitted by the CNT is reduced from Φ to Φeff. Φeff is the effective work
greater
than EF +
eff could be emitted by CNTs. It is well known that when temperature rises to
function,
and
it
decreased
with
1 at a given temperature (Figure 3a). More electrons with
◦
T2 and T3 from T1 (100 C ≥ T3 increasing
> T2 > T1 ),Erespectively,
the probability of an electron gaining more
energies greater than EF + Φeff could be emitted by CNTs. It is well known that when temperature
energy increases (Figure 3b) [16]. As a result, more electrons leave the cathode at higher temperature
rises to T2 and T3 from T1 (100 °C ≥ T3 > T2 > T1), respectively, the probability of an electron gaining
and higher Ue , and form larger emission currentI0 (I0 = eN0 ) [19].The emission current density JSchottky
more energy increases (Figure 3b) [16]. As a result, more electrons leave the cathode at higher
could be expressed as follows [16]:
temperature and higher Ue, and form larger emission currentI0 (I0= eN0) [19].The emission current
density JSchottky could be expressed as follows [16]: 2
JSchottky = B0 T exp(−Φeff /kT)
(3)
JSchottky = B0T2exp(−Φeff/kT)
(3)
where B0 is the Richardson constant and k is the Boltzmann constant. Since JSchottky is the main source
where B0 is the Richardson constant and k is the Boltzmann constant. Since JSchottky is the main source
of je , it is approximated in this work that JSchottky ≈ je , and exponential increase Ie with temperature
of je, it is approximated in this work that JSchottky ≈ je, and exponential increase Ie with temperature
could be obtained. The higher the temperature and Ue are, the larger are the cathode currents. The
could be obtained. The higher the temperature and Ue are, the larger are the cathode currents. The
experimental
withthe
theabove
aboveanalysis.
analysis.
experimentalresults
resultsininFigure
Figure22are
arein
in good
good agreement
agreement with
(a)
(b)
Figure 3. (a) The effects of the E1 on Φeff; (b) The effect of temperature on the probability of an
Figure
3. (a) The effects of the E1 on Φeff ; (b) The effect of temperature on the probability of an electron
electron gaining energy.
gaining energy.
Additionally, the first ionization coefficient α was also affected by temperature [17],
Additionally, the first ionization coefficient α was also affected by temperature [17],
α = exp(–Ui/(Eλ))/λ
(4)
α = exp(
Ui/ (Eλ))/λ
where λ is the mean free path of an electron,
Ui is−the
first ionization potential of a gas molecule, and (4)
E is electric field.
where λ is the mean free path of an electron, Ui is the first ionization potential of a gas molecule, and E
λ = kT/(πr2P)
(5)
is electric field.
2
λ = kT/(πr
P)molecules, P is gas pressure. α denotes the (5)
where k is the Boltzmann constant, r is the radius
of gas
impact ionization ability of an electron with a gas molecule, which depends on E, λ, and Ui. Here, E
where k is the Boltzmann constant, r is the radius of gas molecules, P is gas pressure. α denotes the
does not change when electrode separation and applied voltage are given, and the first ionization
impact
ionization
ability of
with
molecule,
which
on E, λ, and
Ui . Here,
potential
Ui is a constant
forana electron
certain gas.
Asaagas
result,
α changes
withdepends
λ. Since pressure
P increases
E does
change when
separation
voltage
given,
andwith
the first
ionization
with not
temperature
T at aelectrode
fixed volume
of theand
testapplied
chamber,
λ doesare
not
change
temperature
potential
U
is
a
constant
for
a
certain
gas.
As
a
result,
α
changes
with
λ.
Since
pressure
P
increases
with
i
according to Equation (5). Therefore, rising temperature could not affect α in the experiment of this
temperature
T
at
a
fixed
volume
of
the
test
chamber,
λ
does
not
change
with
temperature
according
work. If the temperature sensor operates in an open environment, λ increases with temperature T at
to constant
Equationpressure
(5). Therefore,
risingthe
temperature
could not affect
in the
experiment
of this
work.
P, and then
effect of temperature
on α αand
discharge
current
should
be If
theconsidered.
temperature sensor operates in an open environment, λ increases with temperature T at constant
pressure P, and then the effect of temperature on α and discharge current should be considered.
Sensors 2017, 17, 473
6 of 7
5. Conclusions
In summary, a triple-electrode CNT-based ionization sensor was fabricatedby using micro
fabrication technology. A vertically aligned multi-walled nanotube array was grown by thermal
chemical vapor deposition on one side of the cathode. The relationship of currents Ic and Ie versus
temperature was investigated in a wider test range of 20–100 ◦ C and at 24–100 V Ue . The sensor in this
work had a sensitivity of 0.04 K−1 at 24 V Ue , which is the highest sensitivity compared to the existing
CNT-based resistive temperature sensors. The discharge current Ie and Ic increased with temperature,
due to the increased number of electron emission at a given volume of gas mixture.
Supplementary Materials: The Supplementary Materials are available online at http://www.mdpi.com/14248220/17/3/473/s1.
Acknowledgments: We thank Y.H. Wang for contributions to a part of the experiments. This work is partially
supported by the Specialized Research Fund for the Doctoral Program of Higher Education (20130201110007), the
Natural Science Basic Research Plan in Shaanxi Province of China (2014JZ017), the Director Fund (EIPE14125) from
the State Key Lab of Electrical Institution and Power Equipment of Xi’an Jiaotong University, the Fundamental
Research Funds (xkjc2015012) for the Central Universities and the National Natural Science Foundation of China
(51577142). We thank International Center for Dielectric Research of Xi’an Jiaotong University, Xi’an, China.
Author Contributions: Z.P. performed temperature effect experiment and wrote the paper. Y.Z. designed the
project, supervised the experiments and modified the manuscript. Z.C., J.T., Q.C., J.P.Z. and J.X.Z. analyzed the
data. Y.L. modified the manuscript and improved the English in the text. X.L. grew nanotube films.
Conflicts of Interest: The authors declare no conflict of interest.
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