Browsing by Author "Koskinen, Jari, Prof., Aalto University, Department of Chemistry and Materials Science, Finland"

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Ceramic Composites with Solid Lubricants Processed by Pulsed Electric Current Sintering
(2021) Cura, M. Erkin
School of Chemical Engineering | Doctoral dissertation (article-based)
Friction is a system response to the interaction between moving surfaces and wear is the costly outcome due to material degradation. Their elimination constitutes a technological challenge, for which ceramic matrix composites with solid lubricants present a potential solution. This thesis focuses firstly on studying the tribological and mechanical properties of ceramic matrix composites modified with solid lubricants and synthesised by pulsed electric current sintering (PECS), and then on developing a new self-lubricating ceramic composite with W20O58, while exploiting unique advantages of PECS.Nanostructured composites of alumina hardened zirconia with various high temperature solid lubricants were synthesised by PECS, and their tribological performance was studied. Formation of oxygen deficient lubricious metal oxides were first observed during wear of Al2O3 + Mo composites at 400 °C. Tungsten oxide of the same type was then implemented as a solid lubricant additive in ZrO2 + W20O58 composites. Oxygen deficient metal oxides, i.e., Magnéli oxides of molybdenum and tungsten with easy shear planes were synthesised by vacuum annealing. Alumina hardened zirconia composites with solid lubricants had similar overall hardness as pure alumina, owing to preserved nanosized grain structure of the matrix. The CoF of the hard matrix was reduced by 20 – 24 % at 25 °C and by 60 – 65 % at 400 °C. Improvement in friction properties of Al2O3 matrix was limited at 25 °C when modified by Mo. At 400 °C, the CoF of the composite, when compared to alumina, was dropped by 60 – 65 %, and wear rate by two orders of magnitude with the addition of Mo. At high temperature tests, formation of Mo4O11 was observed, when Mo content was below 10 wt%. The lowest CoF was measured from Al2O3 + 5 vol% Mo against alumina at 400 °C. Low friction was explained by in-situ formation of Magnéli oxide phases as a result of surface temperature and applied normal load. Hard ZrO2 matrix was modified by W20O58 and the composites were tested against alumina at 25 °C. Relatively high pressure was applied during consolidation of the composites for preventing undesired phase transformations. The lowest CoF and wear rate was obtained for n-ZrO2 + 10 vol% WO2.9 under 10 N normal load. Lastly, Al2O3 + cBN composites were studied with both PECS and an ultrahigh pressure method. Despite the lower applied pressure, PECS produced composites with better fracture toughness and significantly higher wear resistance. Overall, PECS proved to be a versatile tool for synthesis of self-lubricating and wear resistant ceramic matrix composites. Formation of Magnéli oxide phase, as well as using one from the start as a solid lubricant, improved the friction properties of ceramic matrix significantly. Implementation of PECS resulted in achieving better mechanical properties.
Multilayer carbon hybrid based electrodes for direct electrochemical detection of analgesics and biomolecules - Development of an electrochemical sensor for determination of analgesics in blood samples
(2021) Wester, Niklas
School of Chemical Engineering | Doctoral dissertation (article-based)
There is increasing interest in point-of-care (POC) determination of drugs and biomolecules, as well as real-time monitoring of these analytes, with wearable and in vivo sensors. Rapid response POC tests can improve patient safety in situations like poisoning and enable personalized treatments whereas real time detection of physiological processes such as neurotransmission events can improve our understanding of neurophysiology and enable novel treatments for neurological disorders. Electrochemical detection is an attractive technology for both POC in vitro diagnostics and wearable sensors alike, due to its low price, simple instrumentation and small required sample volume. Many analgesic drugs show highly individual dose response and thus a dose that causes dangerous side effects in one patient may lack analgesic effect in another. Analgesics are also some of the most abused prescription drugs. Opioid overdoses kill tens of thousands of people annually in the United states alone. Both intentional and unintentional poisoning of over-the-counter analgesics, such as paracetamol, also regularly occur. Therefore, a quantitative POC test for determination of analgesics in capillary blood samples, could be a powerful diagnostic tool for poisoning. However, the POC determination of small drug and biomolecules in unprocessed blood samples has proven to be difficult. In this thesis, we have investigated the use of tetrahedral amorphous carbon (ta-C) and single-walled carbon nanotube (SWCNT) thin films in electrochemical sensing. We have carried out extensive physicochemical characterization of these electrode materials. We combine synchrotron based x-ray spectroscopy with electron microscopy, diffraction and conventional spectroscopic techniques. Both the studied materials were found to support facile electron transfer and show low background currents enabling highly sensitive detection of dopamine and morphine. They, however, lack the fouling resistance and selectivity required for detection of dopamine and analgesics in complex biological samples. Modification with carbon nanomaterials or polymer membranes significantly improved the selectivity as well as reduced the electrode fouling, allowing for detection of dopamine and analgesics in blood samples without precipitation of proteins. We show that such multilayer electrodes can be tailored for the requirements of detection of neurotransmitters and analgesic drugs. Finally, we demonstrate a process for mass production of electrochemical test strips. These sensor strips were shown to be capable of detecting clinically meaningful concentrations of paracetamol from a 20 µL blood sample in less than 5 min. Based on these results we propose an electrochemical assay for quantitative detection of paracetamol.