Model for interpreting surface crystallization using quartz crystal microbalance: theory and experiments

© 2016 American Chemical Society.Surface crystallization of calcium sulfate was investigated using a dissipation crystal quartz microbalance (QCM-D) together with microscopy to understand the mechanical property changes occurring during the growth process. The use of optical microscopy and SEM revealed that needle-shaped crystals grow as clusters on the QCM sensors surface, not in uniform layers. As crystallization growth progressed, QCM-D revealed inversions between negative and positive frequency shifts. This behavior, a function of the growth of crystals in clusters, is not adequately predicted by existing models. As such, a new mass-to-frequency conversion model is proposed herein to explain the observed frequency inversions. This model is derived from a lumped element approach with point-contact loading and Mason equivalent circuit theory. Critically, the physical phenomena occurring form the basis of the model, particularly addressing the three sources of impedance. When a crystal nucleates and grows, its inertial impedance is considered along with a Kelvin-Voigt link with a hydration layer. A comparison between the proposed model and experimental data, of both frequency and dissipation data for the first four harmonics, shows good agreement for the supersaturations (S = C/C∗) of S = 3.75, S = 3.48, and S = 3.22. Additionally, significant improvements over existing models for the case of surface crystallization are observed. The proposed model was therefore able to explain that frequency inversions are caused by a shift from inertia-dominated to elastic-dominated impedance, occurring as a result of crystal growth. Using the nucleation induction time and nucleation rates, determined with imaging, an additional understanding of the crystals mechanical properties (stiffness and dampening) was obtained

A 13.56 MHz inductive power transfer system operating with corroded coils

This paper describes experiments which investigate the effects resulting from corrosion of air-core coils for high frequency inductive power transfer (HF-IPT). A group of coils were treated by exposing them to corrosive conditions for thirty days. Afterwards, the coils were measured with an impedance analyser and the coil with the lowest Q-factor was selected for further experiments. The treated coil was tested at the transmit side of a HF-IPT system, where the system DC-to-DC efficiency was measured and compared against an equivalent system using an untreated transmit coil. The total losses measured increased when the system was operating with the treated coil across a broad loading range, and thermal images were used to establish the additional losses on the treated coil. Analysis of the treated coil identified widespread damage to the surface of the coil. However, it was specific aggressive corrosion only found locally which was able to significantly reduce the Q-factor of the treated coil by 20%