Jarmo Laakso

Cavitation erosion in hydraulic machinery, such as in turbines and pumps, often leads to significant reduction of the service life of the affected components, with serious consequences for their maintenance costs and operation efficiency. In this study, the potential contribution of acoustic emission (AE) measurements to the assessment of cavitation damage is evaluated from experiments in a cavitation tunnel. Stainless steel samples were exposed to cavitation and damage was characterized from pitting tests carried out on mirror-polished samples. The pits were measured using an optical profilometer and cavitation damage was characterized by pit diameter distribution. In parallel, AE time signal was measured directly from behind the samples. A dedicated signal-processing technique was developed in order to identify each burst in the AE signal and determine its amplitude. The AE amplitude distribution compares well with PVDF and pressure sensor measurements from literature. It is concluded that AE signal analysis can be used to monitor the formation of pits without visual examination of the damaged surface. This provides a basis for possible future applications of nonintrusive cavitation erosion monitoring in hydraulic machines, provided the findings remain true in a more complex environment.

The antibacterial effect of silver can be exploited in the food and beverage industry and medicinal applications to reduce biofouling of surfaces. Very small amount of silver ions are enough to destructively affect the metabolism of bacteria. Moreover, superhydrophobic properties could reduce bacterial adhesion to the surface. In this study we fabricated superhydrophobic surfaces that contained nanosized silver particles. The superhydrophobic surfaces were manufactured onto stainless steel as combination of ceramic nanotopography and hydrophobication by fluorosilane. Silver nanoparticles were precipitated onto the surface by a chemical method. The dissolution of silver from the surface was tested in an aqueous environment under pH values of 1, 3, 5, 7, 9, 11 and 13. The pH value was adjusted with nitric acid and ammonia. It was found that dissolution rate of silver increased as the pH of the solution altered from the pH of de-ionized water to lower and higher pH values but dissolution occurred also in de-ionized water. The antimicrobial potential of this coating was investigated using bacterial strains isolated from the brewery equipment surfaces. The results showed that the number of bacteria adhering onto steel surface was significantly reduced (88%) on the superhydrophobic silver containing coating.

As cathode materials, LiFePO₄ particles have been investigated after a supercritical carbon dioxide (scCO₂) treatment. We first produced LiFePO₄ through a hydrothermal method. Then, scCO₂ was introduced to change the morphology of the particles. Three effects have been found after the scCO₂ treatment; the breakup of the aggregated LiFePO₄ platelets into separate plates, the purification of LiFePO₄, and the formation of porous LiFePO₄. Electrochemical measurements showed that the performance of LiFePO₄ greatly improved after the scCO₂ treatment. Finally, a theoretical model was used to explain the mechanism of the morphology change.