Thermopile Sensors and
Applications to the
Detection of Chemical and
Biological Reactions and
Airborne Pollutants
David J. Lawrence
Dept. of Integrated Science and Technology
James Madison University
Harrisonburg, VA
Team Members
„
Faculty Members
George L. Coffman*, W. Gene Tucker* and
Thomas C. DeVore+
„
ISAT MS Students
Noble Egekwu*, Greg Paulsen*
„
BS Graduates
Jason Bliss*, Austin Bennett*, Megan Riley*, Nick
O’Grady*, Dan Aleman*, Patrick Olin*, Tiffany Jenkins*,
John Gotwald*, David Berry+, and Maura Goodrich+
*Department of Integrated Science and Technology
+Department of Chemistry
Objective
„
„
„
„
To design and microfabricate thermopile heat
sensors that can be used in conjunction with
chemical or biological coatings to detect the
presence of chemical or biological agents in the
air.
The coating is applied over the heat sensing
area of the thermopile.
Heat is released when the chemical or biological
agent to be detected binds to the coating.
This heat is detected as an increase in the
output voltage of the thermopile.
Outline
„ Thermocouples
and thermopiles
„ 36-junction thermopile design and
microfabrication
„ Thermopile characterization
„ Sensing of ammonia and acid vapors
„ Detection of biological reactions
„ Cantilevered silicon thermocouples and
thermopiles
Thermocouples and Thermopiles
The Seebeck Effect
„
„
A temperature gradient along a conductor
produces a potential difference.
E.g., for a metal or n-type semiconductor:
Voltage ΔV
+
−
Temperature ΔT
−
The Seebeck Effect
Voltage ΔV
+
−
−
Temperature ΔT
„
The Seebeck coefficient is defined as the
potential difference developed per unit
temperature difference, i.e.
ΔV
S=
ΔT
⎛V ⎞
⎜ ⎟
⎝K⎠
Seebeck Coefficient = S
„
By convention, the sign of S is the potential of
the cold side with respect to the hot side.
Metal
Bismuth (Bi)
Nickel (Ni)
Aluminum (Al)
Copper (Cu)
Gold (Au)
Chromium (Cr)
Antimony (Sb)
S (μV/K)
−79
−18.0
−1.7
+1.7
+1.8
+18.8
+43
Thermocouples
„
„
A thermocouple consists of two junctions between two
dissimilar conductors A and B.
The Seebeck coefficient for the junction is equal to the
difference in the coefficients of the two conductors, i.e.
SAB = SA − SB
„
For antimony (Sb) and (Bi):
SSbBi = SSb − SBi
= 43 +79 = 122 μV/K
Sensing
Junction
Reference
Junction
Thermocouple
Thermocouples & Thermopiles
„
„
„
A thermocouple consists of two junctions between two
dissimilar conductors.
A thermoelectric voltage is generated whenever there is a
temperature difference between the sensing junction and
the reference junction.
Thermopiles consist of multiple sensing/reference
junctions in series, producing a greater output voltage than
a single thermocouple.
Sensing
Junctions
Reference
Junctions
Thermocouple
Thermopile
36-Junction Thermopile Design
and Microfabrication
36 - Junction Thermopile
„
„
„
„
„
Bismuth - Antimony
junctions
9 mm X 12 mm
60 µm line width
Sensing junctions on
polyimide or PET
drumhead, reference
junctions positioned over
aluminum substrate.
Chemical coating applied
over sensing junctions.
36 - Junction Thermopiles
„
„
„
These devices are microfabricated in arrays of three.
They are attached to circuit boards using conductive
epoxy.
Wires are attached to the contact pads.
Polyimide Film
PET Film
Thermopile Cross-Section
Bismuth
Aluminum
Substrate
„
Antimony
Polyimide
or PET
Membrane
“Drumhead”
Membrane materials investigated:
Kapton®
polyimide
Mylar® polyethylene terephthalate
Melinex® polyethylene terephthalate
„
Protective polystyrene layers are applied to some
devices.
Sensor Fabrication
„
Aluminum substrates -- 1” x 1.5”
1.5”
1”
Sensor Fabrication
„
„
Polyimide or PET films are attached to the
substrates.
Film creates a “drumhead” over the holes to
support the sensing junctions.
Polyimide or PET
Aluminum Substrate
“Drumhead”
Sensor Fabrication
„
„
Substrates were treated with oxygen plasma to
clean and roughen the surface.
After cleaning, photoresist was applied to the
substrates in a spin coater.
Plasma Etcher
Spin Coater
Plasma Etcher
Sensor Fabrication
„
Photoresist layer
was used to
pattern subsequent
metal coating.
Photoresist
Polyimide or PET
Mask Aligner
Mask Aligner
Sensor Fabrication
„
„
Samples were exposed
to UV light in mask
aligner through Mask #1.
Developed and then
rinsed with water.
Developed Photoresist
Mask #1 ( Bi )
9 mm
9 mm
Sensor Fabrication
„
Bismuth was
deposited on
samples by thermal
evaporation.
Deposition Chamber
Bismuth
Metal Deposition
„
Two Techniques:
Evaporation
Magnetron Sputtering
Sensor Fabrication
„
Lift-off was performed by soaking
and sonication in acetone, leaving
desired bismuth stripes on the film.
Bismuth
Sensor Fabrication
„
Photolithography, deposition of antimony, and
lift-off were performed with careful alignment of
Mask #2 to the previous bismuth pattern to
complete the thermopiles.
Mask #1 ( Bi )
Mask #2 ( Sb )
36 - Junction Thermopile
„
„
„
„
Bismuth - Antimony
junctions
9 mm X 12 mm
60 µm line width
Sensing junctions on
polyimide or PET
drumhead, reference
junctions positioned over
aluminum substrate.
36 - Junction Thermopiles
„
These devices are microfabricated in arrays of three.
They are attached to circuit boards using conductive
epoxy.
„
Wires are attached to the contact pads.
„
Polyimide Film
PET Film
Thermopile Characterization
Sensor Uniformity Test
1.0E-03
9.0E-04
(V)
„
A warm (37°C) aluminum plate is placed 7.2 cm above a
three-sensor array.
The responses of the three sensors to this radiant heat
source are monitored.
8.0E-04
Output Voltage
„
6.0E-04
7.0E-04
5.0E-04
4.0E-04
3.0E-04
2.0E-04
1.0E-04
0.0E+00
0
5
10
Time
15
(s)
20
Thermopile Sensitivity
Two techniques have been used to
determine the thermopile sensitivity:
Solvent (hexane) evaporation
∆Hvap = 31.56 kJ/mole
Acid – base reaction
Mg(OH)2 + HCl
Hexane Evaporation
Measured volumes of hexane are applied to the sensing junctions
and the device output voltage is monitored as the solvent
evaporates.
Time
0
10
20
(s)
30
Sensor Output
(V)
0.00
-0.01
-0.02
-0.03
-0.04
-0.05
0.5 μL
1.0 μL
40
Hexane Evaporation
Integrated Sensor Output
(V-s)
2.5
2.0
1.5
Polyimide
1.0
PET
0.5
Slope = 5.7 V-s/J
0.0
0.0
0.2
0.4
0.6
0.8
Energy to Vaporize Hexane (J)
1.0
Acid – Base Reaction
„
„
„
„
„
„
An aqueous suspension of magnesium hydroxide
nanoparticles was allowed to dry over the sensing
junctions.
⇒ magnesium hydroxide coating
0.5 μL droplet of 0.0010 M HCl was applied to this
coating.
Mg(OH)2 + 2HCl ⇔ MgCl2 + 2H2O
Calculated heat release = 2.9 x 10−5 J.
Integrated thermopile output signal = 4.4 x 10−4 V-s.
⇒ sensitivity = 15 V-s/J
Sensing of Ammonia
and Acid Vapors
Chemical Detection
The ideal reactive coating for a thermal
chemical sensor would:
generate a large heat change for each
pollutant molecule reacting with the coating,
providing high sensitivity,
react quickly, giving a rapid response time,
and
react only with the pollutant of interest,
providing selectivity.
Ammonia Detection
„
„
„
„
Thermopiles coated with copper oxalate were used
for the ammonia sensing experiments.
CuC2O4 + 2NH3 ⇔ Cu(NH3)2C2O4
ΔHr = − 58 kJ/mole
The copper oxalate was ground and the powder
suspended in isopropyl alcohol. Drops of the
suspension were placed over the sensing junctions
and allowed to dry.
⇒ 0.1 to 0.2 mg copper oxalate coating
A flow tube test apparatus was used to expose the
sensor to air streams containing various
concentrations of ammonia.
Copper Oxalate Coated
Thermopile
The sensing
junctions of this
thermopile are
coated with copper
oxalate to enable
ammonia detection.
Copper Oxalate + NH3(g)
N-H stretching
oxalate bands
Changes in the IR spectrum observed from adding NH3 to CuOX.
Flow Tube Apparatus
„
„
„
„
„
Air flow is perpendicular to the
sensor array (200 mL/min).
Ammonia vapor is drawn into
a syringe from the headspace
above aqueous ammonia
solution (0.15 M).
Ammonia is diluted in syringe
with room air as desired.
This vapor injected through a
septum into the air stream at a
location 28 cm upstream from
the sensor (~ 1s injection).
After injection the ammonia is
further diluted by the air flow.
Ammonia Sensing
Typical responses to short term exposures to 0.060 and 1.4
ppm of ammonia from headspace above ammonia solution.
50
Sensor Output (μ V)
„
40
1.4 ppm
30
0.060 ppm
20
10
0
-10
-20
0
10
20
Time
30
(s)
40
Ammonia Sensing
„
The integrated response of a copper oxalate coated
sensor to a range of ammonia concentrations.
Integrated Sensor Output
(μV-s)
500
400
300
200
100
0
-2.0
15ppb – 180ppm
¾ Reaction Kinetics
-1.0
0.0
1.0
Log [NH3 (ppm)]
2.0
3.0
Ammonia Sensing
Challenges
„
„
„
„
Chemical coatings can be sensitive but not
selective.
E.g., any amine will react with copper oxalate.
We have found there to be significant heat
exchanges associated with the adsorption of
water vapor on copper oxalate coatings.
We need to conduct further tests in a higher
humidity environment.
Acid Vapor Detection
Thermopiles coated with magnesium
hydroxide were used for acid vapor sensing
experiments.
„ When placed in the flow tube test apparatus,
the devices responded to pulses of acid
vapor from the headspace over aqueous
nitric, sulfuric, and hydrochloric acids.
„ For example, for HCl,
Mg(OH)2 + 2HCl ⇔ MgCl2 + 2H2O
„
HCl Sensing
Typical responses to short term exposures to room air,
humidified air, and 2000 ppm of HCl vapor from headspace
above hydrochloric acid.
(μ V)
250
Sensor Output
„
200
2000 ppm HCl
150
room air
100%RH air
100
50
0
-50
0
20
40
60
Time
80
(s)
100
120
Detection of Biological Reactions
Detection of Biological Reactions
Motivation: Is the detection of airborne
anthrax spores possible?
„ Began exploring this experimentally by
working with a “model system” in aqueous
solution.
„ Biotin − Avidin
„
Biotin − Avidin Interaction
Biotin is a vitamin responsible for cell
growth and the metabolism of fats and
amino acids.
„ Avidin is a protein made up of four
identical subunits that have a high binding
affinity for the vitamin Biotin.
„ When Biotin binds to the Avidin protein,
heat is given off:
ΔHr = − 5.26 kJ/mole biotin
„
Biotin − Avidin Testing
„
Biotin and Avidin were each dissolved in
separate Tris buffer solutions
Biotin - 1.44 μg/μl
Avidin - 100 μg/μl
Sensing junctions were covered with a 1 μl
droplet of the Avidin solution (1.5 x 10−9
mole).
„ 1 μl droplet of Biotin solution was placed
on the Avidin solution with a syringe
(6 x 10−9 mole).
„
Test Chamber for Experiments
with Liquid Droplets
„
Can be humidified to minimize evaporation.
Biotin − Avidin Testing
B: Biotin into Avidin
A: Tris into Avidin
B-A
Detection of Biological Reactions:
Next Steps
Can we detect an antibody-antigen reaction?
„ Bacillus collagen-like protein of anthracis
(BclA) is a surface antigen on anthrax spores.
„ We will place a droplet containing the antiBclA antibody on the sensing junctions of one
of our devices.
„ Can we detect heat released when BclA is
added?
„
Biological Sensing Challenges
„ Biological
(protein) sensor coatings can
be more selective than chemical
coatings, but they generate less heat.
„ There can be difficulties binding the
desired sensing protein to the sensor
surface.
„ Once bound, the sensing protein may
become denatured. This may affect its
recognition by an antibody.
Cantilevered Silicon
Thermocouples and Thermopiles
Silicon as a Material for
Thermocouples and Thermopiles
„
Silicon has a large Seebeck coefficient.
Sp ≥ 300 μV/K
Sn ≤ −200 μV/K
„
„
„
Silicon is quite strong, but brittle.
Silicon can withstand high temperatures and
many chemicals.
Silicon has a high thermal conductivity.
~ 1000X that of polyimide or PET
10 − Junction Cantilevered Thermopile
on Silicon Substrate
„
„
„
N – type silicon substrate „ Nickel – p type silicon junctions
Four complete devices are shown. Each is 5 mm X 10 mm
Cantilevered sensing junctions (3 mm x 0.3mm x 25 μm thick)
Sensor Fabrication
Boron Diffusion
Windows
Contact Windows &
Si Etch Windows
Nickel Pattern
Etching
Through-Wafer
Etching
Completed devices:
Cross-section:
SiO2/BSG (insulator)
N-type Si
Nickel
P-type Si
Characterization: Hexane Evaporation
A 0.5 μl droplet of hexane was applied to a single sensing
junction and the device output voltage was monitored as the
solvent evaporated. The droplet spread all along the cantilever,
reaching the reference junction and resulting in substantial error.
Amplified Output (V)
Time (s)
10 12
0.00
-0.05
-0.10
-0.15
-0.20
-0.25
14 16
18 20 22
24
Characterization: Laser Heating
He-Ne Laser (632.8 nm)
Lens
Sample Under Test (The thermocouple junctions
are painted flat black.)
TC Output
(V)
Characterization: Laser Heating
0.0030
0.0025
0.0020
τ ~ 76 ms
0.0015
0.0010
0.0005
0.0000
0
1
2
Time
3
4
5
(s)
Laser power = 6.22 mW
„ Sensitivity = 0.43 V/W (for a single junction)
„
Concluding Remarks
„
„
„
„
„
„
„
„
Bi-Sb thermopile sensors are easily fabricated and could
be inexpensively mass produced.
Copper oxalate coated devices can detect ammonia
headspace vapor over an aqueous solution in the low or
sub ppm range.
Sensitivity to water vapor must be further investigated.
New chemical coating materials must be developed.
Aging of coatings must be investigated.
Experiments on the detection of biological reactions are
ongoing.
Silicon-based thermopiles may offer advantages in some
applications.
This thermal sensing platform may complement
detectors based on other sensing mechanisms.
Acknowledgments
Thanks to:
„
„
„
„
Mark Starnes, JMU machinist.
Joseph D. Rudmin for assistance with electronics.
Funding by NIST under grant number
60NANB2D0108 (Critical Infrastructure Protection
Program) through the Institute for Infrastructure
and Information Assurance (IIIA) at JMU.
The Materials Science REU Program funded by
the US Department of Defense ASSURE
Program, grant #DMR-0353773.
Thank you!
Hexane Evaporation (small droplets)
Integrated Sensor Output
(V-s)
2.5
2.0
1.5
1.0
Polyimide
PET
Polyimide
PET
0.5
0.0
0.0
0.2
0.4
0.6
0.8
Energy to Vaporize Hexane (J)
1.0
Hexane Evaporation (effect of
polystyrene coating)
Integrated Sensor Output
(V-s)
2.5
2.0
1.5
Polyimide
1.0
PET
PI with PS coating
0.5
PET with PS coating
0.0
0.0
0.2
0.4
0.6
0.8
Energy to Vaporize Hexane (J)
1.0
Hexane Evaporation (comparison of
large and small windows)
Integrated Sensor Output
(V-s)
2.5
2.0
1.5
Polyimide
1.0
PET
0.5
PI, Large Windows
0.0
0.0
0.2
0.4
0.6
0.8
Energy to Vaporize Hexane (J)
1.0
Ammonia Sensing
For all but the two highest ammonia
concentrations, the water vapor concentration
delivered to the sensor was nearly constant.
„
Water Vapor Concentration
(ppm)
Integrated Sensor Output
(μV-s)
500
400
300
200
100
0
-2.0
-1.0
0.0
1.0
Log [NH3 (ppm)]
2.0
3.0
3000
2500
2000
1500
-2.0
-1.0
0.0
1.0
Log [NH3 (ppm)]
2.0
3.0