Catálogo de publicaciones - libros

Oscillators are the standard interface circuits for quartz crystal resonator sensors. When applying these sensors in gases a large set of circuits is available, which can be adapted to particular applications. In liquid applications viscous damping accompanied by a significant loss in the factor of the resonator requires specific solutions. We summarize major design rules and discuss approved solutions. We especially address the series resonance frequency and motional resistance determination and parallel capacitance compensation. We furthermore introduce recent developments in network analysis and impulse excitation technique for more sophisticated applications. Impedance analysis especially allows a more complete characterization of the sensor and can nowadays be realized with sensor interface circuitry. The performance of electrical circuitry depends essentially on the stability of the acoustic device. We therefore begin with a discussion of selected quartz crystal properties, disturbances from temperature and mechanical stress, and analyze AT and BT cut from the sensor point of view.

Oscillators are the standard interface circuits for quartz crystal resonator sensors. When applying these sensors in gases a large set of circuits is available, which can be adapted to particular applications. In liquid applications viscous damping accompanied by a significant loss in the factor of the resonator requires specific solutions. We summarize major design rules and discuss approved solutions. We especially address the series resonance frequency and motional resistance determination and parallel capacitance compensation. We furthermore introduce recent developments in network analysis and impulse excitation technique for more sophisticated applications. Impedance analysis especially allows a more complete characterization of the sensor and can nowadays be realized with sensor interface circuitry. The performance of electrical circuitry depends essentially on the stability of the acoustic device. We therefore begin with a discussion of selected quartz crystal properties, disturbances from temperature and mechanical stress, and analyze AT and BT cut from the sensor point of view.

The chapter summarizes the standard model of how acoustic multilayers interact with a quartz crystal microbalance (QCM). In a first step, it is shown how the three formulations around (the mathematical description, the description in terms of acoustic reflectivities, and the equivalent circuit) model correspond to each other. Special emphasis is given to the small-load approximation, which states that the shifts of frequency and bandwidth are about equal to the real and the imaginary parts of the stress-speed ratio (the load) at the crystal surface. The (laterally averaged) stress-speed ratio can be computed for many types of samples (including anisotropic and heterogeneous materials). The small-load approximation is therefore of outstanding importance when employing the QCM in complex environments. The second part of the chapter provides the predictions of the standard model for various geometries. This includes the discussion of slip, of the comparison of optical and acoustic thickness, of electrode effects, of the frequency dependence of the viscoelastic parameters, and of the consequences of a finite contact area. Viscoelastic modeling of QCM data has some pitfalls, which are pointed out. A separate section is devoted to the shortcomings of the small-load approximation (which can be very noticeable) and the amendments to the model accounting for these.

The chapter summarizes the standard model of how acoustic multilayers interact with a quartz crystal microbalance (QCM). In a first step, it is shown how the three formulations around (the mathematical description, the description in terms of acoustic reflectivities, and the equivalent circuit) model correspond to each other. Special emphasis is given to the small-load approximation, which states that the shifts of frequency and bandwidth are about equal to the real and the imaginary parts of the stress–speed ratio (the load) at the crystal surface. The (laterally averaged) stress–speed ratio can be computed for many types of samples (including anisotropic and heterogeneous materials). The small-load approximation is therefore of outstanding importance when employing the QCM in complex environments. The second part of the chapter provides the predictions of the standard model for various geometries. This includes the discussion of slip, of the comparison of optical and acoustic thickness, of electrode effects, of the frequency dependence of the viscoelastic parameters, and of the consequences of a finite contact area. Viscoelastic modeling of QCM data has some pitfalls, which are pointed out. A separate section is devoted to the shortcomings of the small-load approximation (which can be very noticeable) and the amendments to the model accounting for these.

In this chapter we discuss the results of theoretical and experimental studies of the structure and dynamics at solid–liquid interfaces employing the quartz crystal microbalance (QCM). Various models for the mechanical contact between the oscillating quartz crystal and the liquid are described, and theoretical predictions are compared with the experimental results. Special attention is paid to consideration of the influence of slippage and surface roughness on the QCM response at the solid–liquid interface. The main question, which we would like to answer in this chapter, is what information on the structure and dynamics at the solid–liquid interface can be extracted from the QCM measurements. In particular, we demonstrate that the quartz crystal resonator acts as a true microbalance only if, in the course of the process being studied, the nature of the interface (its roughness, slippage, the density and viscosity of the solution adjacent to it, and the structure of the solvent in contact with it) is maintained constant.

So far most of the QCM data were analyzed on a qualitative level only. The next step in QCM studies requires a quantitative treatment of the experimental results. Theoretical basis for the solution of this problem already exists, and has been discussed in this review. Joint experimental and theoretical efforts to elevate the QCM technique to a new level present a challenge for future investigators.

In this chapter we discuss the results of theoretical and experimental studies of the structure and dynamics at solid-liquid interfaces employing the quartz crystal microbalance (QCM). Various models for the mechanical contact between the oscillating quartz crystal and the liquid are described, and theoretical predictions are compared with the experimental results. Special attention is paid to consideration of the influence of slippage and surface roughness on the QCM response at the solid-liquid interface. The main question, which we would like to answer in this chapter, is what information on the structure and dynamics at the solid-liquid interface can be extracted from the QCM measurements. In particular, we demonstrate that the quartz crystal resonator acts as a true microbalance only if, in the course of the process being studied, the nature of the interface (its roughness, slippage, the density and viscosity of the solution adjacent to it, and the structure of the solvent in contact with it) is maintained constant.

So far most of the QCM data were analyzed on a qualitative level only. The next step in QCM studies requires a quantitative treatment of the experimental results. Theoretical basis for the solution of this problem already exists, and has been discussed in this review. Joint experimental and theoretical efforts to elevate the QCM technique to a new level present a challenge for future investigators.

The quartz crystal microbalance can serve as high-frequency probe of the microcontacts formed between the crystal surface and a solid object touching it. On a simplistic level, the load can be approximated by an assembly of point masses, springs, and dashpots. The Sauerbrey model, leading to a decrease in frequency, is recovered if small particles are rigidly attached to the crystal. In another limiting case, the particles are so heavy that inertia holds them in place in the laboratory frame. The spheres exert a restoring force onto the crystal, thereby increasing the stiffness of the composite resonator. The resonance frequency increases in proportion to the lateral spring constant of the sphere–plate contacts. A third limiting case is represented by particles attached to the crystal via a dashpot. Within this model (extensively used in nanotribology) the dashpot increases the bandwidth. The momentum relaxation time τ (“slip time”) is calculated from the ratio of the increase in bandwidth and the decrease in frequency, Δ.

The force–displacement relations in contact mechanics are often nonlinear. A prominent example is the transition from stick to slip. Even for nonlinear interactions, there is a strictly quantitative relationship between the shifts of frequency and bandwidth, Δ and ΔΓ, on the one hand, and the force acting on the crystal, (), on the other. Δ and ΔΓ are proportional to the in-phase and the out-of-phase component of (), respectively. Evidently, () cannot be explicitly derived from Δ and ΔΓ. Still, any contact-mechanical model (like the Mindlin model of partial slip) can be tested by comparing the predicted and the measured values of Δ and ΔΓ. Further experimental constraints stem from the measurement of the amplitude dependence of the resonance parameters.

Contacts mechanics in the MHz range is much different from its low-frequency counterpart. For instance, static friction coefficients probed with MHz excitation are often much above 1. Contact mechanics at short time scales should be of substantial practical relevance.

The quartz crystal microbalance can serve as high-frequency probe of the microcontacts formed between the crystal surface and a solid object touching it. On a simplistic level, the load can be approximated by an assembly of point masses, springs, and dashpots. The Sauerbrey model, leading to a decrease in frequency, is recovered if small particles are rigidly attached to the crystal. In another limiting case, the particles are so heavy that inertia holds them in place in the laboratory frame. The spheres exert a restoring force onto the crystal, thereby increasing the stiffness of the composite resonator. The resonance frequency increases in proportion to the lateral spring constant of the sphere-plate contacts. A third limiting case is represented by particles attached to the crystal via a dashpot. Within this model (extensively used in nanotribology) the dashpot increases the bandwidth. The momentum relaxation time (“slip time”) is calculated from the ratio of the increase in bandwidth and the decrease in frequency, Δ/(− Δ).

The force-displacement relations in contact mechanics are often nonlinear. A prominent example is the transition from stick to slip. Even for nonlinear interactions, there is a strictly quantitative relationship between the shifts of frequency and bandwidth, Δ and Δ, on the one hand, and the force acting on the crystal, (), on the other. Δ and Δ are proportional to the in-phase and the out-of-phase component of (), respectively. Evidently, () cannot be explicitly derived from Δ and Δ. Still, any contact-mechanical model (like the Mindlin model of partial slip) can be tested by comparing the predicted and the measured values of Δ and Δ . Further experimental constraints stem from the measurement of the amplitude dependence of the resonance parameters.

Contacts mechanics in the MHz range is much different from its low-frequency counterpart. For instance, static friction coefficients probed with MHz excitation are often much above 1. Contact mechanics at short time scales should be of substantial practical relevance.

Mass-sensitive devices such as the quartz crystal microbalance (QCM) or the surface acoustic wave device (SAW) are very advantageous for chemical sensing. As their name implies, they react towards mass changes on their respective sensitive areas, which makes them almost universally applicable since every analyte has a mass. Detection limits can be as low as 1 ng for QCM and in the picogram range for SAW. Of course, selectivity also has to be introduced into the sensor system. For this purpose molecular imprinting, where the sensitive layer is generated by polymerising it directly on the respective device surface, is gaining increasing attention. A reason for this is the very straightforward synthetic approach, where the analyte-to-be is used as a template that determines the structure of the interaction sites within the polymer by self-organisation processes. In this chapter, we give an introduction into the electronic background of mass-sensitive devices as well as into molecular imprinting. In the second half, we will introduce selected strategies for actual chemical sensing of analytes covering a size range from molecular to micrometre as well as both pure compounds and mixtures.

Mass-sensitive devices such as the quartz crystal microbalance (QCM) or the surface acoustic wave device (SAW) are very advantageous for chemical sensing. As their name implies, they react towards mass changes on their respective sensitive areas, which makes them almost universally applicable since every analyte has a mass. Detection limits can be as low as 1 ng for QCM and in the picogram range for SAW. Of course, selectivity also has to be introduced into the sensor system. For this purpose molecular imprinting, where the sensitive layer is generated by polymerising it directly on the respective device surface, is gaining increasing attention. A reason for this is the very straightforward synthetic approach, where the analyte-to-be is used as a template that determines the structure of the interaction sites within the polymer by self-organisation processes. In this chapter, we give an introduction into the electronic background of mass-sensitive devices as well as into molecular imprinting. In the second half, we will introduce selected strategies for actual chemical sensing of analytes covering a size range from molecular to micrometre as well as both pure compounds and mixtures.