Quartz crystal microbalance

Quartz crystal microbalances ( English Quartz Crystal Microbalance , QCM) are microbalances with a sensor based on an oscillating quartz . The piezoelectric property of quartz is used here. The resonance frequency of the quartz oscillator depends on the mass of the material adsorbed on the surface. Contrary to what the name suggests, QCM-based systems are not only used as scales or microbalances, but also serve as sensors for other measured variables.

Quartz oscillator of a QCM humidity sensor with gold electrodes (left: front view, right: rear view)

General

Crystals belong to the family of crystals with a piezoelectric effect. Piezoelectric effects are used in applications in the areas of sensors , actuators , frequency generation , motors , etc. A relationship between electrical voltage and mechanical deformation of the quartz is used, which also derives its suitability as an oscillating quartz. In a feedback circuit, it is the very precise frequency-determining element. The circuit in which the sensor is installed (interface) can be implemented by an oscillator circuit, network analysis or shock excitation. Resolutions of up to 1 Hz are possible for resonance frequencies in the 4 to 6 MHz range.

The frequency of the oscillation of the quartz depends, among other things, on the thickness of the crystal. In the undisturbed state there is a frequency that is reciprocal to the thickness of the quartz. If one now leaves all other external influences constant and changes the thickness of the quartz (by applying a film), this change correlates directly to a change in the frequency. In simple terms, the change in frequency can be precisely quantified and used to determine the exact mass (Sauerbrey equation). The Sauerbrey equation only provides good results for the special case of a thin and rigid layer. A more detailed theory is the so-called "transmission line model".

If the thickness of the quartz is kept constant, the mechanical natural frequency of the quartz crystal can also be influenced by temperature fluctuations, air humidity fluctuations and deformation (external force / residual stresses).

By determining the mass and vibration damping with QCM sensors can be calculated other quantities of interest, for example, for applications in biophysics (adsorption of vesicles ) for measuring humidity, the film thickness determination, to study the phase boundary at the Solid / liquid, for sublimation and Viscosity determination .

materials

By far the most important material for QCMs is crystalline α-quartz, mostly in the AT or BT cut . Alternative materials for special applications, for example for higher temperatures, are langasite (La ₃ Ga ₅ SiO ₁₄ , LGS) and gallium orthophosphate (GaPO ₄ ). In general, measurement systems with alternative materials are also referred to as QCM.

Moisture measurement with a quartz crystal microbalance

To measure the humidity, gas is passed over a hygroscopic coated quartz oscillator , which changes its frequency . In detail: first of all, the measuring gas is passed over the sensor for some time , whereby an equilibrium between adsorption and desorption of the moisture is established in the hygroscopically active layer, the amount of moisture adsorbed in the equilibrium is proportional to the moisture content. The frequency change of the quartz oscillator is also proportional to the amount of adsorbed moisture. This change in frequency is determined and converted into humidity values by measuring electronics / microcontroller . Speed, a special sensitivity to water vapor and a high resolution down to the trace range are typical for QCM humidity sensors.

swell

↑ ^a ^b Piezoelectric Sensors (= Springer Series on Chemical Sensors and Biosensors . Volume 5 ). Springer Berlin Heidelberg, Berlin, Heidelberg 2007, ISBN 978-3-540-36567-9 , doi : 10.1007 / 978-3-540-36568-6 ( springer.com [accessed September 18, 2019]).
↑ Günter Sauerbrey: Use of quartz crystals for weighing thin layers and for microweighing . In: Z. Phys. . 155, No. 2, 1959, p. 206. bibcode : 1959ZPhy..155..206S . doi : 10.1007 / BF01337937 .
↑ Kay Keiji Kanazawa: Some basics for operating and analyzing data using the thickness shear mode resonator . In: The Analyst . tape 130 , no. 11 , 2005, ISSN 0003-2654 , p. 1459 , doi : 10.1039 / b506691n ( rsc.org [accessed September 18, 2019]).

[:0-1] Piezoelectric Sensors (= Springer Series on Chemical Sensors and Biosensors . Volume 5 ). Springer Berlin Heidelberg, Berlin, Heidelberg 2007, ISBN 978-3-540-36567-9 , doi : 10.1007 / 978-3-540-36568-6 ( springer.com [accessed September 18, 2019]).

[sauerbrey-2] Günter Sauerbrey: Use of quartz crystals for weighing thin layers and for microweighing . In: Z. Phys. . 155, No. 2, 1959, p. 206. bibcode : 1959ZPhy..155..206S . doi : 10.1007 / BF01337937 .

[3] Kay Keiji Kanazawa: Some basics for operating and analyzing data using the thickness shear mode resonator . In: The Analyst . tape 130 , no. 11 , 2005, ISSN 0003-2654 , p. 1459 , doi : 10.1039 / b506691n ( rsc.org [accessed September 18, 2019]).