Thermopile Sensors and Applications to the Detection of Chemical
Transcript of Thermopile Sensors and Applications to the Detection of Chemical
Thermopile Sensors and Applications to the Detection of Chemical and Biological Reactions and Airborne Pollutants
David J. LawrenceDept. of Integrated Science and TechnologyJames Madison UniversityHarrisonburg, VA
Team MembersFaculty MembersGeorge L. Coffman*, W. Gene Tucker* andThomas C. DeVore+
ISAT MS StudentsNoble Egekwu*, Greg Paulsen*
BS GraduatesJason 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
ObjectiveTo 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 thermopiles36-junction thermopile design and microfabricationThermopile characterizationSensing of ammonia and acid vaporsDetection of biological reactionsCantilevered silicon thermocouples and thermopiles
The Seebeck
EffectA 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
The Seebeck coefficient is defined as the potential difference developed per unit temperature difference, i.e.
⎟⎠⎞
⎜⎝⎛
ΔΔ
=KV
TVS
−+ −Voltage ΔV
Temperature ΔT
Seebeck
Coefficient = SBy convention, the sign of S is the potential of the cold side with respect to the hot side.
Metal
S (μV/K)
Bismuth (Bi)
−79Nickel (Ni)
−18.0
Aluminum (Al)
−1.7Copper (Cu)
+1.7
Gold (Au)
+1.8Chromium (Cr)
+18.8
Antimony (Sb)
+43
ThermocouplesA 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
Thermocouple
Reference Junction
Sensing Junction
Thermocouples & ThermopilesA 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.
Thermocouple Thermopile
Reference Junctions
Sensing Junctions
Bismuth - Antimony junctions9 mm X 12 mm60 µm line widthSensing junctions on polyimide or PET drumhead, reference junctions positioned over aluminum substrate.Chemical coating applied over sensing junctions.
36 -
Junction Thermopile
36 -
Junction ThermopilesThese 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
Membrane materials investigated:Kapton® polyimide Mylar® polyethylene terephthalateMelinex® polyethylene terephthalate
Protective polystyrene layers are applied to some devices.
AluminumSubstrate
Bismuth AntimonyPolyimide or PETMembrane
“Drumhead”
Sensor FabricationPolyimide 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 FabricationSubstrates 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
Sensor FabricationPhotoresist layer was used to pattern subsequent metal coating.
Photoresist Mask Aligner
Polyimide or PET
Sensor FabricationSamples were exposed to UV light in mask aligner through Mask #1.Developed and then rinsed with water.
9 mm
9 mm
Mask #1 ( Bi )
Developed Photoresist
Sensor FabricationBismuth was deposited on samples by thermal evaporation.
Bismuth
Deposition Chamber
Sensor FabricationLift-off was performed by soaking and sonication in acetone, leaving desired bismuth stripes on the film.
Bismuth
Sensor FabricationPhotolithography, deposition of antimony, and lift-off were performed with careful alignment of Mask #2 to the previous bismuth pattern to complete the thermopiles.
Mask #2 ( Sb
)Mask #1 ( Bi )
Bismuth - Antimony junctions9 mm X 12 mm60 µm line widthSensing junctions on polyimide or PET drumhead, reference junctions positioned over aluminum substrate.
36 -
Junction Thermopile
36 -
Junction ThermopilesThese 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
Sensor Uniformity TestA 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.
0.0E+001.0E-042.0E-043.0E-044.0E-045.0E-046.0E-047.0E-048.0E-049.0E-041.0E-03
0 5 10 15 20
Time (s)
Out
put V
olta
ge
(V)
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
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-0.04
-0.03
-0.02
-0.01
0.000 10 20 30 40
Time (s)
Sens
or O
utpu
t (
V)
0.5 μL
1.0 μL
Measured volumes of hexane are applied to the sensing junctions and the device output voltage is monitored as the solvent evaporates.
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Energy to Vaporize Hexane (J)
Inte
grat
ed S
enso
r Out
put
(V-s
)
Polyimide
PET
Hexane Evaporation
Slope = 5.7 V-s/J
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 + 2H2OCalculated heat release = 2.9 x 10−5 J.Integrated thermopile output signal = 4.4 x 10−4 V-s.⇒ sensitivity = 15 V-s/J
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, andreact only with the pollutant of interest, providing selectivity.
Ammonia DetectionThermopiles coated with copper oxalate were used for the ammonia sensing experiments.CuC2O4 + 2NH3 ⇔ Cu(NH3)2C2O4
ΔHr
= −
58 kJ/moleThe 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 coatingA 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)
Changes in the IR spectrum observed from adding NH3
to CuOX.
N-H stretching oxalate bands
Flow Tube ApparatusAir 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 SensingTypical responses to short term exposures to 0.060 and 1.4 ppm of ammonia from headspace above ammonia solution.
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Time (s)
Sens
or O
utpu
t ( μ
V)
0.060 ppm
1.4 ppm
0
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200
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400
500
-2.0 -1.0 0.0 1.0 2.0 3.0
Log [NH3 (ppm)]
Inte
grat
ed S
enso
r Out
put
(μV
-s)
Ammonia SensingThe integrated response of a copper oxalate coated sensor to a range of ammonia concentrations.
15ppb –
180ppm
Reaction Kinetics
Ammonia Sensing
ChallengesChemical 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
+ 2H2
O
HCl
SensingTypical responses to short term exposures to room air, humidified air, and 2000 ppm of HCl vapor from headspace above hydrochloric acid.
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Time (s)
Sens
or O
utpu
t (
μV)
room air100%RH air
2000 ppm HCl
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/μlAvidin - 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).
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 anti-BclA 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.
Silicon as a Material for Thermocouples and Thermopiles
Silicon has a large Seebeck coefficient.Sp ≥ 300 μV/KSn ≤ −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 junctionsFour complete devices are shown. Each is 5 mm X 10 mmCantilevered sensing junctions (3 mm x 0.3mm x 25 μm thick)
Sensor Fabrication
Boron DiffusionWindows
Contact Windows &Si Etch Windows
Nickel PatternEtching
Through-WaferEtching
Characterization: Hexane Evaporation
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Time (s)
Am
plifi
ed O
utpu
t (V)
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.
Characterization: Laser Heating
He-Ne Laser (632.8 nm)
Lens
Sample Under Test (The thermocouple junctionsare painted flat black.)
Characterization: Laser Heating
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Time (s)
TC O
utpu
t (
V)
τ
~ 76 ms
Laser power = 6.22 mWSensitivity = 0.43 V/W (for a single junction)
Concluding RemarksBi-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.
AcknowledgmentsThanks 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.
Hexane Evaporation (small droplets)
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Energy to Vaporize Hexane (J)
Inte
grat
ed S
enso
r Out
put
(V-s
)
PolyimidePETPolyimidePET
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Energy to Vaporize Hexane (J)
Inte
grat
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enso
r Out
put
(V-s
)
Polyimide
PET
PI with PS coating
PET with PS coating
Hexane Evaporation (effect of polystyrene coating)
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Energy to Vaporize Hexane (J)
Inte
grat
ed S
enso
r Out
put
(V-s
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Polyimide
PET
PI, Large Windows
Hexane Evaporation (comparison of large and small windows)
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Log [NH3 (ppm)]
Inte
grat
ed S
enso
r Out
put
(μV
-s)
Ammonia SensingFor all but the two highest ammonia concentrations, the water vapor concentration delivered to the sensor was nearly constant.
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Log [NH3 (ppm)]
Wat
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apor
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cent
ratio
n (p
pm)