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Overview of Chemical Gas Sensors. Ashok K. Batra + Department of Physics, PO Box 1268, Alabama A&M University, Normal, AL 35762. NSF/RISE Workshop/Short Course on Development and Study of Advanced Sensors and Sensor Materials July 9 - July 13 2007. + [email protected]

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slide1

Overview of Chemical Gas Sensors

  • Ashok K. Batra+
  • Department of Physics, PO Box 1268,
  • Alabama A&M University, Normal, AL 35762

NSF/RISE Workshop/Short Course

on

Development and Study of Advanced Sensors and Sensor Materials

July 9 - July 13 2007

+ [email protected]

slide2

Overview of Chemical Gas Sensors

Outline

  • Chemical Sensor
  • Categories of Sensors
  • How does it work?
  • What we are doing…
slide3

Nature’s Creation

Five Senses: Eyes, Ears, Skin, Nose and Tongue

Camera……………………………………mimics………Eyes

Microphone and Tape Recorder………mimics…………..Ears

Tactile Sensors……………………………………mimic………Skin

Chemical Sensors………………….mimic……….. Nose & Tongue

slide4

A Typical Sensor…

  • An interactive material which interacts with environment and generates a response.
          • +
  • Device which reads the response and converts it into an interpretable and quantifiable term.
slide5

Current

Voltage

Light Intensity

Mass

Refractive Index

Capacitance

Resistance

slide7

Some Definitions and Terms

Adsorption: is a process that occurs when a gas or liquid or solute (called adsorbate) accumulates on the surface of a solid or more rarely a liquid (adsorbent), forming a molecular or atomic film (adsorbate).

Chemisorption: a type of adsorption whereby a molecule adheres to a surface through the formation of a chemical bond.

slide8

Expected Qualities of an Excellent Chemical Sensor

3Ss

  • Sensitivity
  • Stability
  • Selectivity
  • Minimum Hardware Requirements
  • Good Reversibility
  • Identification and Quantification of Multiple Species
  • Quick Response
slide9

Application of Gas Sensors

  • Safety
  • Indoor Air Quality
  • Environmental Control
  • Food
  • Industrial Production
  • Medicine
  • Automobiles

Chemical sensors play an increasingly important role in our everyday life: environmental monitoring, industrial process control, quality control of food and beverages, hazardous chemicals, explosives detection and workplace monitoring are just a few examples of their widespread use. In all cases the driving force behind the development of sensor technology is the need for immediate and accurate analyses.

slide10

Four General Groups

Chemical Sensors

  • Electrochemical Sensors
  • Mass Sensors
  • Optical Sensors
  • Chromatography and Spectrometry
slide11

Substrate

Sensing Material

Electrode

Electrochemical Sensor

Based on Metal Oxides

- Stoichiometry

- Microstructure

- Thickness

- Phase

- Temperature

Sensitivity = Rgas / Rair

slide12

Design of a Typical Thick Film Semiconductor Sensor

“Heater-substrate-film-combination electrode” structure.

- Tin Oxide

- Tungsten oxide

- Zinc Oxide

- Indium oxide

-------------

The combination of the sensor operating temperature and composition of the metal oxide yields different responses to various gases

Materials Science & Engineering B 139 (2007) 1-23

slide14

Chemical Sensors

New Silicon-Based Metal-Oxide Chemical Sensor

Microfabricated Metal-Oxide Chemical Sensor

slide15

Chemical Sensors

A Scheme of SnO2 acting as Semiconductor Sensing Material

2e- + O2 2 O-

O- + CO CO2

slide16

Chemical Sensors

Role of Additives (Dopants)

Additives are used for sensitizing and increase the response to particular gases

i.e. enhance the sensitivity, selectivity, decrease the response time and operating temperature of sensitive layer.

Sensitizing SnO2 with Cu; under oxidizing condition Cu is present as CuO which is p-type; p-n junction is formed which results in electron depletion at interface.

Exposure to H2S converts CuO to Cu2O that exhibits metallic character and thus increases the conductivity of the system

slide17

Electrochemical Sensors

Additives Distribution Ways on Semiconductor Gas Sensors

slide18

Electrochemical Sensors

Chemical sensitive absorbent is deposited on a solid phase that acts as an electrode

When chemical vapors come in contact with the absorbent. The chemical absorbs into the polymer, causing it to swell.

The swelling changes the resistance of electrode, which can be measured

The amount of swelling corresponds to the concentration of the chemical vapors

slide19

Catalytic Bead Sensor

It is comprised of a passive and active element.

The active element is coated with a catalyst platinum and passive is coated with an inert glass to act as a reference element

Both the elements are heated to a prescribed temperature.

When a combustible gas contacts the elements, the vapor combusts on the active element, and the active element increases in temperature. As a result, the resistance of the platinum coil changes. Two elements are connected to a Wheatstone bridge circuit, so changes are measured in voltage

ActivePassive

slide20

Mass Sensors

Surface Acoustic Wave Sensor

The velocity and attenuation of the signal are sensitive to the viscoelasticity and mass of the thin film which can allow for the identification of the contaminant.

Heating element under the chemical film can also be used to desorb chemicals from the device.

A Signal pattern recognition system is needed.

slide22

Mass Sensors

Based on Cantilever: Nanotechnology

slide23

Optical Sensors

Infrared Sensors

Infrared sensors can be used to detect gases, which have unique infrared absorption signatures in the 2-14 μm range.

The uniqueness of the gas absorption spectra enables identification and quantification

slide24

Optical Sensors

Colorimetry

Work by analyzing the color of the contaminated water that has been mixed with a particular reagent

slide25

Surface Plasmon Resonance (SPR) Sensors

SPR- A Charge Density Oscillation that may exist at the interface of two media.

The SPR technique is an optical method for measuring the refractive index of very thin layer of material adsorbed on a metal

Optical Setup for SPR

P-reflectivity

Photons at certain angle are able to excite SP on the adsorbate side of the metallic slab; whenever plasmon is excited, one photon disappears, producing a dip in the reflected light; angle is dependent on refractive index of the adsorbate.

Angle of Incident

SPR Curves for Different Molecules

slide26

Surface Enhances Raman Spectroscopy (SERS)

  • Optical Sensors

SERS is based on finding the chemical composition of a sample by irradiating it with laser and measuring the light that scatters from it.

Surface Enhanced ( ~1014) Raman Scattering is observed for molecules found close to silver or gold nanoparticles because of surface plasmon resonance. Thus sensitivity increases many folds.

Plasmons are collective oscillations of the free electron density, often at optical frequencies

slide27

Chromatography

Chromatography: Separation of Molecules

Liquid Chromatography

Sorbent

Gas Chromatography

slide28

Spectrometry

Ion Mobility Spectrometry

  • Time-of-Flight Measurement

When the gas has entered the spectrometer, it will be ionized

by a radioactive source

The resulting positive and negative charged species are accelerated over a short distance

Time-of-Flight is determined

slide29

Spectrometry

Mass Spectrometry

The principle is similar to the ion mobility spectrometer, except vacuum is required

Gas mixture is ionized, and charged fragments are produced

These fragments are sorted in a mass filter according to their mass to charge ratio.

The ions are detected as electrical signal with an electron multiplier

slide31

Nano Structured Materials

Next Generation of Sensors ?

Because of the small size of nanotubes, nanowires, or nanoparticles, a few gas molecules are sufficient to change the electrical properties of the sensing elements. This allows the detection of a very low concentration of chemical vapors.

Nanotechnology based chemical sensors provide high sensitivity ( 3-4 orders), low power and low cost portable tools for in-situ chemical analysis. Operate at room temperature.

SnO2; ZnO; In2O3; WO3; SnO2:Pd; TiO2

Sensors & Actuators B 122 (2007) 659-671

slide32

Optochemical Sensors

For H2, O2, O3, CO, CO2 and H2O detection in Air

  • Absorbance and Reflectance
  • Refractive Index
  • Photoluminescence
  • Photothermal
  • Photoacoustic and related
  • Surface Plasmon Resonance (SPR)
  • Chemiluminescence

Trends in Analytical Chemistry 25(2006) 937-948

slide34

Development of Nanoparticles-based Chemical Gas Sensor

  • A. K. Batra+, J. R. Currie*, Anup D. Sharma and R. B. Lal
  • Department of Physics, PO Box 1268,
  • Alabama A&M University, Normal, AL 35762
  • *Instrumentation & Advanced Sensor Group,
  • NASA/Marshall Space Flight Center, AL 35812

NSF/RISE Workshop/Short Course

on

Development and Study of Advanced Sensors and Sensor Materials July 9-July 13, 2007

+ [email protected]

slide35

A Typical Semiconductor Sensor…

An interactive material which interacts with environment and generates a response.

+

Device which reads the response and converts it into an interpretable and quantifiable term.

Sensing material captures a molecule of vapor with a certain selectivity that induces physical change in the material because of captured molecule\'s chemical interaction with the material.

slide36

Semiconductor Gas Sensors: Mechanisms

Detect Gases Due to Change in Their Resistance or Conductance

Changes in Conductance can result from combination of several physical properties of film:

  • Bulk defects ( interstitials and oxygen vacancies)
  • Surface defects ( donor type oxygen vacancies)
  • Catalytic elements ( breakdown of an incoming gases by catalyst on the surface of the sensing film)
  • Microstructure and grain boundaries ( smaller grains, large number of grain boundaries, high surface to volume ratio)
  • Interface and three phase boundaries (changes in interface conductance due to incoming gases at triple point)
slide37

Our Approach

  • Microstructure and grain boundaries ( smaller grains, large

number of grain boundaries, high surface to volume ratio).

By use of Nanoparticles in fabrication of the thick-films.

…high surface reactivity + larger density of molecules which can adsorb on the surface … contribute to larger effect on electrical conductivity… enhances sensor sensitivity.

●Binary composites: SnO2:WO3; SnO2:In2O3; SnO2:ZnO

slide38

Weighing Raw

Materials

Mixing

And

Milling

Powder

Pressed to

Discs

Sintering

Lapping

And

Grinding

Electrical

Characterization

Ultrasonic

Cleaning

Electroding

SnO2 Sample Processing Steps

Pellet / Sample Preparation

slide39

Tablet Pressing Fixture

2.5 cm

2.5 cm

Pressing Sleeve

Pressing Sleeve

1.3 cm

1.3 cm

Anvil

Anvil

5 mm

5 mm

5 mm

5 mm

Die

Die

4 cm

Piston

Piston

2.9 cm

2.9 cm

Pressing Ram

1.4 cm

1.4 cm

1.2 cm

1.2 cm

1.2 cm

1.2 cm

slide40

electrodes

Sensor Configuration

The SnO2 Sensor (pellet) shows shaded electroded regions on the top surface having a finite gap between these two physical regions.

slide41

Test Sample

GPIB-USB2 Interface

Keithley 617

Electrometer

Agilent 34401A

6 ½ Digit Multimeter

Gateway E-3301

CPU running

National Instruments

LabVIEW Ver. 6.0

Hot Plate w/

Thermal Chamber

QuadTech 7600

LCR Meter

Test Facility for VOC Chemical Sensors

slide42

Our Study

  • Sensor elements were bombarded with helium particles to change the surface characteristics. In our study, a VOC sensor pellets of Tin Dioxide (SnO2) are fabricated then bombarded at various (2 MeV ) helium doses.
  • A noteworthy result has been observed that the device has a decrease in response time when bombarded at about 1016 ion/cm2.
  • The response time decreases with increase of fluence.
  • A trend is seen whereby capacitance tends to

decrease as fluence is increased.

slide43

Decrease in Response Time

Response Time is based on 1 (62.3% of saturation).

slide44

IPA Response Time

Decrease in Response Time Observed

IPA Response Time*

*Response Time is based on 1 or (62.3% of saturation).

slide45

Why look at Thick Films?

  • Bulk
  • Not stable; consume power; not compatible with silicon technology; and high operating cost.
  • Thin-Film
  • Compatible with micro-sensors; rapid response; compact; low operating cost; mechanically weak; Dopants homogeneity???.
  • Thick-Film
  • More Robust than Thin Films.
  • Lighter than Bulk Materials.
  • Dopants Homogeneity.
  • Consume less power.
  • Compatible with silicon technology.
slide46

Thick Film

Deposition

Techniques

Screen

Printing

Liquid

Phase

Epitaxy

Melt

Spinning

Dip

Coating

Solution

Casting

slide47

Why Screen Printing?

  • Low Cost of Production.
  • Ease of Fabrication.
  • Compatible with semiconductor technology.
  • Films with large surface to volume ratio.
  • Porous films.
slide48

Nanopowder

Solvent

Mixing (Paste)

Screen-printing

Drying

Annealing

Thick-Film

Functional & Porous Film

Process of Thick-film Preparation by Screen-printing

Organic Binder

slide49

Squeegee

Ink/Paste

Nylon Screen

Frame

Substrate

Vacuum Chuck*

Hinge

Dowel Pins

Base Plate

45°

Vacuum

Pump

*Fabricatedby Mr. Garland Sharp

Fabrication of Thick-Film using Screen Printing Technology

Screen Printing Set-up

slide50

Design of a Typical Semiconductor Sensor

“Heater-substrate-film-combination electrode” structure.

S. Film

Electrode

slide51

Acknowledgements

The authors gratefully acknowledge the support of the present work through SMDC grants # W9113M-05-1-0011, and NSF RISE grant # HRD-0531183.

One of the authors (RBL) would like to acknowledge NASA Administrator’s Fellowship program.

Further, technical assistance of Mr. Garland Sharp and members of the Center for Irradiation of Materials (CIM) at Alabama A & M University is greatly appreciated.

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