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

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

36-Junction Thermopile Design and Microfabrication

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 Fabrication

Aluminum substrates -- 1” x 1.5”

1.5”

1”

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

Plasma Etcher

Sensor FabricationPhotoresist layer was used to pattern subsequent metal coating.

Photoresist Mask Aligner

Polyimide or PET

Mask Aligner

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

Metal DepositionTwo Techniques:

EvaporationMagnetron Sputtering

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

Thermopile Characterization

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

-0.05

-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.

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.2 0.4 0.6 0.8 1.0

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

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, 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.

-20

-10

0

10

20

30

40

50

0 10 20 30 40

Time (s)

Sens

or O

utpu

t ( μ

V)

0.060 ppm

1.4 ppm

0

100

200

300

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.

-50

0

50

100

150

200

250

0 20 40 60 80 100 120

Time (s)

Sens

or O

utpu

t (

μV)

room air100%RH air

2000 ppm HCl

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/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).

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 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.

Cantilevered Silicon Thermocouples and Thermopiles

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

Completed devices:

Nickel

N-type Si

P-type SiSiO2

/BSG (insulator)

Cross-section:

Characterization: Hexane Evaporation

-0.25-0.20-0.15-0.10-0.050.00

10 12 14 16 18 20 22 24

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

0.00000.00050.00100.00150.00200.00250.0030

0 1 2 3 4 5

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.

Thank you!

Hexane Evaporation (small droplets)

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.2 0.4 0.6 0.8 1.0

Energy to Vaporize Hexane (J)

Inte

grat

ed S

enso

r Out

put

(V-s

)

PolyimidePETPolyimidePET

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.2 0.4 0.6 0.8 1.0

Energy to Vaporize Hexane (J)

Inte

grat

ed S

enso

r Out

put

(V-s

)

Polyimide

PET

PI with PS coating

PET with PS coating

Hexane Evaporation (effect of polystyrene coating)

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.2 0.4 0.6 0.8 1.0

Energy to Vaporize Hexane (J)

Inte

grat

ed S

enso

r Out

put

(V-s

)

Polyimide

PET

PI, Large Windows

Hexane Evaporation (comparison of large and small windows)

0

100

200

300

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 SensingFor all but the two highest ammonia concentrations, the water vapor concentration delivered to the sensor was nearly constant.

1500

2000

2500

3000

-2.0 -1.0 0.0 1.0 2.0 3.0

Log [NH3 (ppm)]

Wat

er V

apor

Con

cent

ratio

n (p

pm)