Surface Acoustic Wave Vapor Sensors Based on Resonator

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Surface Acoustic Wave Vapor Sensors Based on
Resonator
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Introduction

• Wohltjen, 1979
• Mass loading
• S (change in signal)/(change in analyte
  concentration)
• SAW resoator sensors have lower noise levels and
  hence yield lower detection limits than delay
  line sensors

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• ZnO/Si resonators for organic vapor detection
  (Martin)
• 200 MHz quartz SAW resonators (Bowers)

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• Sensors 200, 300, and 400 MHz quartz SAW
  resonators
• Coating methods LB method and spray-coating

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Experimental

• Coating material poly(vinyle tetradecanal)
• SAW resonators 200, 300, and 400 MHz two-port
  SAW resonator(500 refectors) on ST-cut quartz
  Al IDTs (split, 50?) SiO2 overcoat
• All devices have the same path length (10 ?) and
  aperture (65 ?)
• 4 pin round TO-6 headers, Au wire bonding
• Temperature 25oC

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• SAW delay lines 158 MHz SAW dual delay lines on
  ST-cut quartz Al IDTs SiO2 overcoat
• TO-8 header

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• Frequency data collection
• resolution of 0.02 Hz for resonators
• resolution of 2 Hz for dual delay lines
• noise measurement 4 min at 10 points/min, stand
  deviation of the residuals of the linear
  least-squares line
• baseline noise10 min at 5 points/min before
  vapor exposure

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• Mass sensitivity determination
• flow rate 120 mL/min
• the device was returned to the board and allowed
  to stabilize for at least an hour
• sensor was firstly exposed to clean carrier gas
  for 45 min
• clean carrier gas (45 min)? vapor (5 min)? clean
  carrier gas (10 min)? vapor (5 min) ? clean
  carrier gas (10 min) ? control experiment(clean
  carrier gas, 75 min)

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Results and discussionMass sensitivities

• For soft materials(rubbery amorphous polymers,
  ??107 dyn/cm2, ??1 g/cm3), the second term is
  negligible
• For stiffer materials(glassy amorphous polymers,
  ?1010 dyn/cm2, ??1 g/cm3), the second term is
  10-15 of the absolute value of the first term

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• Choice of a polymer coating
• the material is suitable for deposition by the LB
  method
• the material is above its static glass transition
  temperature at RT
• the material is a sorbent for organic vapors

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Vapor sensitivities

• The strength of sorption is dependent on the
  choice of the sorbent material and the sensors
  operation temperature
• The vapor sensitivity is determined by the
  coating thickness, the partition coefficient, and
  physical properties of the film during sorption

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• The coating thickness that can be applied without
  quenching oscillation will decrease as the device
  frequency is increased
• The coating thickness producing more than 500-600
  kHz of frequency shift damp out the oscillator

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• The higher frequency devices tolerate less
  coating than lower frequency devices
• If SAW sensors of various frequencies are all
  coated with the same material to the same degree
  of frequency shift, then their vapor
  sensitivities should be identical

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• Interfacial adsorption bulk adsorption
• Interfacial adsorption vapor is absorbed at
  polymer/solid, gas/ polymer, and gas/solid
• The interfacial adsorption increases with
  frequency increasing
• The principle mechanism for sensor response is
  bulk absorption of vapor into the film

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Noise and detection limits

• The limiting factor in the noise is the quartz
  chip rather than the oscillator electronics
• Thermal conditions play a key role in determining
  noise levels
• The 300 and 400 MHz resonators are noiser than
  200 MHz resonator
• The worst noise levels on 200 MHz resonator are
  well below the worst levels on delay lines
• The lowest deiection limits is achieved with the
  lowest noise device