The Quartz Crystal Microbalance and its Applications

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The Quartz Crystal Microbalance and its
Applications

• By Monica Melo
• March 25, 2004

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What is a Quartz Crystal Microbalance?

• A quartz crystal microbalance is a sensor i.e.. a
  class of analytical devices that are capable of
  monitoring specific chemical species continuously
  and reversibly
• A device that is based on the piezoelectric
  characteristics of the quartz crystal
• The piezoelectric effect forms the basis for the
  quartz crystal microbalance

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The Piezoelectric Effect

• The appearance of an electric potential across
  certain faces of a crystal when it is subjected
  to mechanical pressure
• The effect has a converse i.e. when an electric
  field is applied on certain faces of the crystal,
  the crystal undergoes mechanical distortion
• The effect is explained by the displacement of
  ions in crystals that have a nonsymmetrical unit
  cell

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The Piezoelectric Effect

• The compression causes a displacement of the ions
  of the unit cell, causing an electric
  polarization of the unit cell
• These effects are accumulative and an electric
  potential difference appears across certain faces
  of the crystal
• When an external electric field is applied to the
  crystal, the ions of each unit cell is displaced
  by electrostatic forces
• The result is the mechanical deformation of the
  whole crystal

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The Crystal Structure of Quartz

• Quartz is crystalline silica (SiO2) at
  temperatures below 870C

Figure 2 The ?-quartz crystal lattice structure
Figure 1 The Natural Form of Quartz
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The Quartz Crystal Resonator

• A quartz crystal resonator is a precisely cut
  slab from a natural or synthetic single crystal
  of quartz
• A resonator can have many modes of resonance, or
  standing wave patterns at the resonant
  frequencies
• The quartz crystal resonator must be cut at a
  specific crystallographic orientation and have
  the proper shape
• This allows for selection of a specific mode of
  resonance and for suppression of all unwanted
  modes

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The Quartz Crystal Resonator

• Commonly, quartz crystal resonators are cut in
  one of two types AT-cut or BT-cut
• The angle is measured relative to the z-axis of
  rotation and the thickness is in the y-direction
  in a rectangular, square or disc shape

Figure 3 The Ideal Cuts of the Quartz Crystal
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The Quartz Crystal Resonator

• These cuts are ideal because
• They oscillate in the thickness shear mode most
  sensitive to the addition or removal of mass
• (perfect for microweighing!)
• They are insensitive to temperature change near
  room temperature
• (at the conditions of an analytical laboratory!)

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The Operation of a Quartz Crystal Microbalance

• Electrodes are affixed to either side of the
  quartz resonator and connected to a voltage
  source
• The quartz crystal is made to vibrate at the
  frequency of the exciting voltage

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Mass Determination

• The crystal in most quartz crystal microbalances
  in an essential part of an oscillator circuit
• The material to be weighed is deposited onto the
  quartz crystal plate (resonator) as a thin film
• A quartz crystal microbalance does not actually
  measure the mass
• It measures the areal density or mass thickness
  of the deposited material

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Mass Determination Sources of Error

• The mass is calculated from a frequency change on
  the quartz due to the deposited material
• A complicated formula is used and the display
  shows only the mass of the deposited material
• Errors occur because of the instruments
  inability to distinguish between a frequency
  change due to the deposited mass or other
  disturbances such as stress changes or temperature

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Applications

• Microweighing
• Detection of toxic gases such as sulfur dioxide,
  ammonia, hydrogen sulfide, carbon monoxide, and
  aromatic hydrocarbons
• Detection of biomolecules by antigen/antibody
  attachment to quartz resonators

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References

1. Bottom, Virgil E. Introduction to Quartz Crystal
   Unit Design. Van Nostrand Reinhold Company. New
   York. 1982.
2. Harris, Daniel C. Quantitative Chemical
   Analysis. 5th Edition. W.H.Freeman and Company.
   New York. 1999.
3. Heising, Raymond A. Quartz Crystals for
   Electrical Circuits Their Design and
   Manufacture. D.Van Nostrand Company, Inc. New
   York. 1946.
4. Ikeda, Takuro. Fundamentals of Piezoelectricity.
   Oxford University Press. Oxford. 1990.
5. Miessler, Gary L. D.A. Tarr. Inorganic
   Chemistry. 2nd Edition. Prentice-Hall, Inc. New
   Jersey. 1999.
6. Skoog, Douglas A., F.J. Holler T.A. Nieman.
   Principles of Instrumental Analysis. 5th Edition.
   Saunders College Publishing. Philadelphia,
   1998.