Selective gas detection using uncoated SAW delay lines

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Selective gas detection using uncoated SAW delay
lines
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Introduction

• The attractive properties of acoustic wave
  sensing
• The principle of acoustic wave sensing
• The sensing properties are dependent on the test
  gas and material of the film
• The detection principle of the uncoated SAW gas
  sensor
• The limitation of the uncoated SAW gas sensor

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Experimental

• Substrates (001),lt110gtBi12SiO20 and AT, X-SiO2
• Untuned insertion loss 20 dB
• Operation frequency 20 MHz(Bi12SiO20) and 156
  MHz(SiO2)

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• Operating temperature 25-100oC
• Test cell 250 ml
• Flow rate 500 ml min-1
• The flow of the gases injected into the cell was
  perpendicular to the SAW substrate
• Reference gas Ar and N2
• Reference gas (at least two hours)?test gas (20
  min)?reference gas
• Noise level ???? 2 ppm

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Results and discussion

• Selective operation of the sensor is suggested by
  vanishing ?? or TCD for a given gas of two test
  gases
• When ?? or TCD are zero, the SAW response is zero

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The vanishing of ??(T)

• The large ?? , the higher is the sensor response
  and lower is the minimal detectable level
• ?? n?rg(T)-?tg(T)

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• The vanishing ?? for EtOH is achieved at a given
  temperature without the vanishing of ?? for CH4
  at the same temperature
• The SAW responses accord with behaviour of ??(T)

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• The SAW response for EtOH vanishes at 73oC, while
  the SAW response for CH4 reaches 1800ppm at the
  same temperature
• It confirms the selective operation towards EtOH

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Vanishing of TCD(T)

• The crossing zero is at 77oC
• The SAW response is increased or decreased by the
  proper selection of the operation temperature

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• Vanishing of the SAW Ar response achieved at 73oC
• The sign change of the response to Ar is related
  to the sign change of the TCD

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• For the optimum temperature, The SAW response
  towards He is higher than that for Ar

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• The injection of He(??lt0) decreases the
  temperature of substrate
• The injection of Ar(??gt0) increases the
  temperature of substrate
• The sensor working around the turnover
  temperature is able to distinguish between Ar and
  He
• The uncoated SAW sensors have not the problems of
  film stability and aging