INFICON RQCM - Quartz Crystal Microbalance Research System - Manual (Page 39)

RQCM – RESEARCH QUARTZ CRYSTAL MICROBALANCE
CRYSTALS, HOLDERS AND FLOW CELL 4-1
4 CRYSTALS, HOLDERS AND FLOW CELL
An essential part of the RQCM system is the sensing crystal. Careful handling of both the crystal
and the crystal holder must be observed to ensure proper and reproducible measurements.
Furthermore, the sensing crystal, the crystal holder and the connecting cable must be orientated
and connected correctly in order for the RQCM to work properly. This is especially true if you
design your own crystal or holder.
If you have purchased a INFICON crystal, holder and cable, the installation is simple. Follow the
instructions below.
If you plan to build your own crystal or holder or cable, see Section 4.3.2.
4.1 1 INCH DIAMETER CRYSTALS
IINFICON
pioneered the standard AT-cut, 5 MHz, 1-inch diameter crystals for use in liquid
applications. The AT-cut quartz is chosen for its superior mechanical and piezoelectric properties,
and the angle of cut can be adjusted to obtain a zero temperature coefficient at a desired operating
temperature. The 1 inch diameter was chosen to allow enough distance between the active area
of the crystal and the mounting o-ring. This improves the overall stability of the crystal by
reducing the frequency changes due to mounting stress.
4.1.1 ELECTRODE CONFIGURATION
Figure 8 below shows INFICON’s 1” crystal electrode patterns. The left figure shows the ½ inch
diameter front electrode (also called sensing electrode) with an extended electrode that wraps
around the edge of the crystal and extends into a semicircle shown in the top half of the right
figure. The lower half of the right figure shows the ¼ inch diameter rear electrode (also called
contact electrode).
This configuration enables both electrical contacts to be made on the backside of the crystal
allowing measurement in conductive liquids.
The oversized front electrode (½ inch in diameter as oppose to the ¼ inch diameter rear electrode)
was chosen to ensure a more consistence deposition across the active area of the crystal. The
exposed area of the front electrode is 0.212 in 2 (137 mm 2 ), but the active oscillation region
(displacement area) is limited to the overlapping area of the front and rear electrodes (0.053 in 2 or
34.19 mm 2 ).