Quartz crystal microbalance

Quartz crystal microbalance (English Quartz Crystal Microbalance, QCM) are micro scales, whose sensor based on a quartz resonator. The piezoelectric properties of quartz is used. The resonance frequency of the quartz resonator is dependent on the mass of the adsorbed on the surface of the material. Unlike the name suggests, QCM - based systems are not only used as the scale or micro- scales, but also serve as sensors for other measurands.

General

Crystals belong to the family of crystals with piezoelectric effect. Piezoelectric effects are, inter alia, in applications in the field of sensors, actuators, frequency generation, motors, etc. are used. In this case, a relationship between electrical voltage and mechanical deformation of the quartz is used, whereby the suitability derived as a crystal oscillator. In a feedback circuit, it is the very exact frequency determining element. At resonance frequencies in the 4- to -6- MHz range resolutions are possible up to 1 Hz.

The frequency of oscillation of the crystal is dependent inter alia on the thickness of the crystal. In the undisturbed state results in a frequency which is inversely proportional to the thickness of the quartz. You can now all the other external influences constant and changes the thickness of the crystal ( by applying a film ), this change is directly correlated to a change in frequency. In simple words, the change in frequency precisely quantified and so used for exact mass determination are ( Sauerbrey equation).

By determining the mass and vibration damping with QCM sensors, other quantities of interest can be calculated, for example, for applications in biophysics (adsorption of vesicles ) for humidity measurements, the film thickness determination, to analyze the phase boundary region solid / liquid and the viscosity determination.

Materials

The by far most important material for QCMs is crystalline α - quartz, usually in the AT or BT cut. Alternative materials for specific applications, for example, for higher temperatures, langasite ( La3Ga5SiO14, LGS) and gallium orthophosphate are ( GaPO4 ). General and measuring systems with alternative materials are referred to as QCM.

Humidity measurement with a quartz crystal microbalance

Moisture measurement gas is passed through a hygroscopic coated quartz oscillator, thereby changing the frequency. As follows: first, the measurement gas is passed over the sensor, whereby the hygroscopic active layer a balance between adsorption and desorption of the humidity setting, the amount of moisture adsorbed in Equilibrum is proportional to the moisture content for some time. Also proportional to the adsorbed amount of moisture is, the frequency change of the quartz crystal. This frequency change is determined and converted by a measuring electronics / microcontrollers in humidity values ​​. Speed, a special sensitivity to water vapor and high resolution up to the trace range are typical of QCM humidity sensors.

Swell

• Measurement
• Laboratory balance