Peter Hauptmann

Role of mass accumulation and viscoelastic film properties for the response of acoustic-wave-based chemical sensors.

The sensitivity of acoustic-wave microsensors coated with a viscoelastic film to mass changes and film modulus (changes) is examined. The study analyzes the acoustic load at the interface between the acoustic device and the coating. The acoustic load carries information about surface mass and film modulus; its determination has no restrictions in film thickness. Two regimes of film behavior can be distinguished:  the gravimetric regime, where the sensor response is mainly mass sensitive, and the nongravimetric regime, where viscoelasticity gains influence on the sensor response. We develop a method, which allows the assignment of the sensor signal to a gravimetric or a nongravimetric response. The critical value can be determined from oscillator measurements. The related limits for the coating thickness are not the same for the coating procedure and mass accumulation during chemical sensing. As an example, we present results from a 10 MHz quartz crystal resonator.

Application of ultrasonic sensors in the process industry

Continuous process monitoring in gaseous, liquid or molten media is a fundamental requirement for process control. Besides temperature and pressure other process parameters such as level, flow, concentration and conversion are of special interest. More qualified information obtained from new or better sensors can significantly enhance the process quality and thereby product properties. Ultrasonic sensors or sensor systems can contribute to this development. The state of the art of ultrasonic sensors and their advantages and disadvantages will be discussed. Commercial examples will be presented. Among others, applications in the food, chemical and pharmaceutical industries are described. Possibilities and limitations of ultrasonic process sensors are discussed.

The quartz crystal microbalance: mass sensitivity, viscoelasticity and acoustic amplification

Abstract Quartz crystal resonators (QCR) respond to surface mass and material properties of a film coated on their surface. The acoustic load acting at the surface of the resonator is a more general parameter to describe this dependence. It can be represented by a mass factor and an acoustic factor. The quotient of resistance increase and frequency shift can be used for the determination of the acoustic factor, if the loss tangent of the coating is known. Viscoelastic properties of sensitive coatings can enhance the mass sensitivity of quartz crystal microbalance (QCM) sensors. Acoustic factor and acoustic amplification effective during chemical sensing are not the same. We further suggest a sensor concept, which is based on a bilayer arrangement. Acoustic amplification with a viscoelastic film and chemical sensitivity is separated. With a proper selection of materials, the first layer realizes acoustic amplification while the (chemical) sensitive layer acts as a pure mass detector. Major sensor design parameters are the shear modulus and the thickness of the first layer; major challenge is the preparation of a homogeneous and uniform first film.

SPICE model for lossy piezoceramic transducers

A transmission line equivalent circuit for piezoelectric transducers has been modified to provide modeling of lossy piezoceramic transducers. A lossy transmission line is used to model the mechanical losses. The equivalent circuit parameters are derived from analogies between electrical transmission lines and acoustic wave propagation. Implementation of the equivalent circuit model in SPICE is shown. Simulations and measurements in the time and frequency domain of a low-Q material and a multilayered ultrasonic sensor using a low-Q piezoceramic transducer are presented.

INTERFACE CIRCUITS FOR QUARTZ-CRYSTAL-MICROBALANCE SENSORS

The utilization of quartz-crystal-microbalance sensors in liquids yields new requirements to the applied interface circuits. In the present article, the fundamentals of the measuring principle and advantages and drawbacks of suitable interface circuits are discussed. Special requirements of oscillators as interface circuits are outlined. Possible solutions to those requirements are investigated and two recently developed oscillator circuits are presented.

New design for QCM sensors in liquids

The use of quartz-crystal microbalance (QCM) sensors in liquids for analytical purposes requires a well-designed transducer. Therefore a theoretical model is necessary to characterize the influence of liquid properties like density, viscosity, conductivity and dielectric behaviour. The high damping of the liquid loading results in a large loss of the quartz-crystal quality factor. Special electronics and a suitable sensor cell are necessary to obtain good frequency stability. The paper describes a new device based on an operational transconductance amplifier (OTA) with a very high bandwidth. Experimental results illustrate the ability of the sensor system to detect small amounts of special analytes in liquids.

Transduction mechanism of acoustic-wave based chemical and biochemical sensors

Acoustic-wave-based sensors are commonly known as mass-sensitive devices. However, acoustic chemical and biochemical sensors also face so-called non-gravimetric effects, especially if they work in a liquid environment. The one-dimensional transmission-line model (TLM) is a powerful tool, which considers the influence of geometric and material properties on the sensor transduction mechanism, most importantly the influence of viscoelastic phenomena. This paper demonstrates the concept of modelling acoustic microsensors on quartz crystal resonators. Particular attention is paid to special cases which allow for simplifications or specific solutions of the TLM, like the acoustic load concept (ALC), the BVD model or the Sauerbrey equation. Deviations from the one-dimensional assumption of the TLM are suspected to significantly contribute to the acoustic sensor response in biosystems. We therefore introduce a generalization of the ALC to get access to two- or three-dimensional effects, which are up to now not considered in the TLM. As examples, signatures of interfacial phenomena or non-uniform films are discussed.

Sensor system for the detection of organic pollutants in water by thickness shear mode resonators

Abstract In this study, a sensor system is presented for the detection of organic pollutants in drinking water based on the quartz-crystal-microbalance (QCM). Hydrophobic polymers and macrocyclic calixarenes were tested as solid sensitive layers on QCM. Polymer materials with glass transition temperatures below the operating temperature supply reversible and fast sensor responses, and the microporous calixarene layers are very sensitive and selective towards analytes with high dispersion parameters.

Ultrasonic sensors for process monitoring and chemical analysis: state-of-the-art and trends

Abstract Ultrasonic sensors are used in a large variety of ways. New fields of ultrasonic: sensor and ultrasonic sensor system applications are process monitoring and control, automotive techniques and chemical analysis. These applications have enjoyed a rapid increase of interest in recent years. The development of new ultrasonic sensors or systems was and is essentially accelerated by the progress in electronics, by new piezoelectric materials, by exploitation of new technologies and by the need for new or more accurate analysis methods in many industrial branches. A review of ultrasonic sensors based on piezoelectric materials and resonators is presented. First, the physical background for ultrasonic wave propagation and corresponding technical applications is given. A definition of the ultrasonic sensor system is introduced later because an ultrasonic sensor alone makes no sense. For an efficient use of this sensor principle, a well-developed transmitter and receiver electronics and intelligent data-acquisition electronics are necessary. Secondly, it is shown that ultrasonic sensors can be divided into four groups depending on how the ultrasonic signal has been changed on its path during propagation or the transducer properties are changed by interaction with the surroundings. The present state of established sensors for flow, distance and level is discussed. Ultrasonic sensors for process monitoring are described. New application fields for these sensors can be predicted. Finally, ultrasonic microsensors are introduced. A description of their state-of-the-art and application examples are given. To conclude, the use of new technologies for the manufacture of miniaturized ultrasonic sensors and future developments are discussed.

Determination of polymer shear modulus with quartz crystal resonators

The dependence of the electrical response of polymer-coated acoustic wave sensors on the viscoelastic properties of the coating material is used for the determination of the complex shear parameter of thin polymer films. We discuss the acoustic behaviour and the electrical response of a coated quartz crystal vibrating in the thickness shear mode at its fundamental frequency as well as its third and fifth harmonic. The changes in material properties were induced by temperature changes. Both a glassy and a rubbery polymer consistency were investigated. We also discuss the concept of impedance analysis and parameter fitting to extract the coating shear parameters and we introduce a double layer concept for the direct calculation of the polymer shear parameters.