Surface acoustic wave humidity sensor based on a thin PolyXIO film

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Surface acoustic wave humidity sensor based on a
thin PolyXIO film
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Introduction

• Most surface wave (SAW) chemical gas sensors rely
  on variation of the SAW phase velocity with the
  adsorption of gas at chemical interfaces.
• The changes in the physical properties of the
  chemical interface which affect the acoustic wave
  velocity and/or attenuation can be exploited to
  conduct a sensor.

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• A good humidity sensor should have a good
  sensitivity, with wide humidity range, quick
  response, good reproducibility, easily
  interfaceability, tough durability with ling
  life, resistance to contaminants, insignificant
  dependence on temperature and simple structure
  and low cost.
• Most of the SAW humidity sensors have been based
  on the changes in the conductivity of a thin
  polymer film.

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Theoretical background

• SAW dual delay line (DL) oscillators have the
  advantage of overcoming sensitivity to
  undesirable effects such as temperature and
  pressure.
• The SAW velocity may be perturbed by a number of
  factors, the mass loading (m), conductivity (?)
  and elasticity (c) effects.
• The mass loading leads to a decrease in phase
  velocity proportional to the mass of deposited
  species and no change in attenuation.

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• The acoustoelectric effect is based upon the fact
  that the electric field associated with SAW
  induces currents in adjacent conducting media.
  This leads to a step change in the velocity and
  an attenuation peak analogous to those observed
  for bulk semiconducting media.
• The viscoelastic effects is important for polymer
  films and will require more attention.

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• A bulk modulus Kb ( Kb? j Kb?) and a shear
  modulus G (G? j G?)can be used to specify the
  mechanical properties of a linear, isotropic
  polymer.
• The real parts represent the component of stress
  in phase with strain, giving rise to energy
  stored in the film.
• The imaginary parts represent the component of
  stress 90o out of phase with strain, giving rise
  to power dissipation in the film.

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• The elastic ( or glassy) polymer is no energy
  loss due to deformation or traslation processes,
  i.e. Kb? G? 0.
• Varying any physical quantity that can affect the
  time relaxation of the polymer, a transition from
  an elastic (glassy) to a viscoelastic (rubbery)
  regime will take place.

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• Adsorption of vapors is seen to lead to polymer
  plasticization or vapor induced softening, giving
  rise to a change in regime from acoustically thin
  to thick.
• The SAW response of acoustically thick layers is
  governed by cross film displacement gradients and
  subsequent shear wave resonances, leading to an
  increase in phase velocity and a peak in
  attenuation with increasing vapor absorption.

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Experimental

• Substrate YZ-LiNbO3
• Device dual DL
• IDT N 20, fo 50 MHz
• Humid air generator and controller nebulizer
• Coating polyXIO
• Method casting
• Thickness 0.15?m

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Results and discussion

• This is considerably faster than the response of
  the commercial hygrometer.

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• The behavior is linear in the range 0-10 RH,
  becomes increasingly nonlinear for higher values
  and finally reaches a minimum and turn around to
  positive frequency shifts for 80 RH.

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• This behavior is very reproducible.

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• The behavior is a gradual increase, with a jump
  at the same value of RH as the turnaround in
  velocity.

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• The conductivity is flat up to 60 RH and
  increases rapidly. It is the possibility of an
  acoustoelectric interaction in the high RH range.

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• Expect for the very highest accessible values of
  RH, the difference is flat and equal to zero,
  demonstrating unequivocallythat the ae
  interaction is zero over most of the range.

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• The acoustoelectric interaction can only give a
  monotonic variation, so that it cannot explain
  the turnaround effect in the frequency shift.
• The acoustoelectric interaction plays a
  negligible role.

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• 0 - 10 RH mass loading for an acoustically thin
  film
• 10 - 100 RH viscoelastic effects predominate
• A turnaround characteristic in velocity film
  goes over to the acoustically thick (rubbery) at
  high RH
• A sharp increases in attenuation vapor absorbed
  swelling of the film

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• The reproducibility is good and there is a
  moderate hysteresis effect of the order of 5.

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• The sensitivity of the sensor to small
  concentration (? 600 ppm) for light molecular
  weight gases, i.e. CO, CO2, O2, CH4, is few Hz,
  which is about 0.05 of the RH sensitivity.