Transcript The Quartz Crystal Microbalance and its Applications

The Quartz Crystal Microbalance
and its Applications
By: Monica Melo
March 25, 2004
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 870°C
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
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and for suppression of all unwanted modes
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:
1. They oscillate in the thickness shear mode –
most sensitive to the addition or removal of
mass
(perfect for microweighing!)
2. They are insensitive to temperature change near
room temperature
(at the conditions of an analytical laboratory!) 8
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
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deposited material
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 instrument’s 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.
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