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The importance of using the most pertinent mathematical description of the mass load versus frequency relation for quartz crystal thin-film thickness monitors is reviewed. The different usable crystal load ranges of the so-called frequency- and period-measurement techniques in comparison with the Z-Match technique are calculated for most of the commonly used deposition materials. A new thin-film thickness monitoring procedure is described, which takes the influence of the acoustic film properties on the mass load versus frequency slope into consideration without need for the explicit knowledge of the acoustic impedance ratio z of the deposited film and the quartz crystal. It is shown how the effective z value in the composite resonator built by the quartz crystal and the deposited foreign layer can be derived from a measurement of a quasiharmonic overtone resonance frequency in addition to the commonly practiced exclusive measurement of the fundamental resonance frequency. The presently established relation between the mass load or the deposited film thickness and the corresponding resonance frequency of the loaded crystal is enhanced by a more rigorous one-dimensional composite resonator description. The introduced so-called series resonance relation is of significance for the accurate determination of the specific acoustic impedance ratio z of the deposited film and the crystal. Furthermore, the deviations of the frequency spectra of plano-convex shaped crystals from that of the one-dimensional (infinite plate) geometry are investigated and taken into account. The achieved accuracy in the z determination from the first and third quasiharmonic frequencies justifies the applicability of the two-frequency or auto slope accommodation technique introduced in this paper.

This article describes two types of flow-through cell retention devices based on the concept of layered piezoelectric resonators. A single-chamber device is compared to a novel optimized steam-sterilizable prototype ultrasonic cell separator with improved acoustic design and an integrated cooling circuit, eliminating the problem of local temperature increase caused by the high amplitudes necessary to achieve the separation of animal cells with low acoustic contrast. This setup yields highly reproducible results and is ideal for studying the long-term effects of ultrasonic sound fields and separation efficiency. The novel two-chamber system has the potential for scaleability due to the reduction in thermal and acoustic flow, increased field stability, and separation efficiency. Finally, the effect of power input on separation and cell viability is reported. Such flow-through cell retention systems could be used as systems to retain biomass within the fermentor or as a substitute for centrifugation, with the major advantage of eliminating high-speed rotational motion.

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A general transfer matrix description of arbitrarily oriented layered piezoelectric structures is presented. Except for the restriction to two electrodes, it is the most general one-dimensional treatment possible. The description is derived as an exact solution of the fundamental differential equations and the boundary conditions at the terminating surfaces and at the electrical port. It allows the calculation of the electrical admittance appearing between the electrodes for any frequency, as well as the determination of the entire resonance frequency spectrum using only simple matrix multiplications. It covers the general case of multimode excitations and its coupling results. Thus it is not restricted to a single displacement direction and can be used for the rigorous analysis of layered structures containing, e.g., doubly rotated Y-cut quartz crystals.