Overview of  Chemical Gas Sensors
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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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Ashok k batra department of physics po box 1268 alabama am university normal al 35762

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]


Ashok k batra department of physics po box 1268 alabama am university normal al 35762

Overview of Chemical Gas Sensors

Outline

  • Chemical Sensor

  • Categories of Sensors

  • How does it work?

  • What we are doing…


Ashok k batra department of physics po box 1268 alabama am university normal al 35762

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


Ashok k batra department of physics po box 1268 alabama am university normal al 35762

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.


  • Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Current

    Voltage

    Light Intensity

    Mass

    Refractive Index

    Capacitance

    Resistance


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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.


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Expected Qualities of an Excellent Chemical Sensor

    3Ss

    • Sensitivity

    • Stability

    • Selectivity

    • Minimum Hardware Requirements

    • Good Reversibility

    • Identification and Quantification of Multiple Species

    • Quick Response


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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.


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Four General Groups

    Chemical Sensors

    • Electrochemical Sensors

    • Mass Sensors

    • Optical Sensors

    • Chromatography and Spectrometry


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Substrate

    Sensing Material

    Electrode

    Electrochemical Sensor

    Based on Metal Oxides

    - Stoichiometry

    - Microstructure

    - Thickness

    - Phase

    - Temperature

    Sensitivity = Rgas / Rair


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    chemical Sensors


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Chemical Sensors

    New Silicon-Based Metal-Oxide Chemical Sensor

    Microfabricated Metal-Oxide Chemical Sensor


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Chemical Sensors

    A Scheme of SnO2 acting as Semiconductor Sensing Material

    2e- + O2 2 O-

    O- + CO CO2


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    • Electrochemical Sensors

    Additives Distribution Ways on Semiconductor Gas Sensors


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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.


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Mass Sensors


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Mass Sensors

    Based on Cantilever: Nanotechnology


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Optical Sensors

    Colorimetry

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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Chromatography

    Chromatography: Separation of Molecules

    Liquid Chromatography

    Sorbent

    Gas Chromatography


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Recent Advances


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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]


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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.


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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)


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    electrodes

    Sensor Configuration

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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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.


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Decrease in Response Time

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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    IPA Response Time

    Decrease in Response Time Observed

    IPA Response Time*

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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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.


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Thick Film

    Deposition

    Techniques

    Screen

    Printing

    Liquid

    Phase

    Epitaxy

    Melt

    Spinning

    Dip

    Coating

    Solution

    Casting


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Why Screen Printing?

    • Low Cost of Production.

    • Ease of Fabrication.

    • Compatible with semiconductor technology.

    • Films with large surface to volume ratio.

    • Porous films.


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Nanopowder

    Solvent

    Mixing (Paste)

    Screen-printing

    Drying

    Annealing

    Thick-Film

    Functional & Porous Film

    Process of Thick-film Preparation by Screen-printing

    Organic Binder


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Design of a Typical Semiconductor Sensor

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

    S. Film

    Electrode


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    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.


    Ashok k batra department of physics po box 1268 alabama am university normal al 35762

    Thank you .


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