Photoluminescent nanoparticles for temperature and pressure sensing
Résumé
Pressure and temperature are two relevant parameters in the elastohydrodynamic lubrication (EHL) regime. This regime is encountered in many machine component and is characterized by the occurrence of extreme conditions within a very small lubricant volume. For instance, the high pressures (up to 3 GPa) result in important elastic deformation of the rubbing bodies and induce important elevation of the lubricant viscosity. In addition, temperature rise occurs due to sliding conditions. To date, various experimental methods have been developed to measure pressure and temperature within the contact, including electrical resistance, infrared emissivity, and Raman spectroscopy. The aim of the current work is to develop a new in situ technique for simultaneous measurement of these two parameters. This technique exploits the photoluminescence (PL) sensitivity of some semiconductor nanoparticles (NPs) to changes in pressure and temperature. By monitoring photoluminescence wavelength variation of NPs dispersed in the lubricant and excited by an appropriate radiation, local measurements could be carried out, allowing the two parameters to be mapped throughout the contact.So far, the work has been mainly focused on pressure and temperature calibration of the selected NPs, namely the II-VI semiconductor CdSe/CdS/ZnS, and on the identification of the additional factors which could influence their response. In the literature, the sensitivities of CdSe-based NPs to pressure and temperature have been reported separately1,2. The original feature of the present work is that coupled pressure & temperature calibrations were conducted using a high pressure diamond anvil cell, in which the temperature can be controlled precisely. Measurements were conducted over temperature and pressure ranges found in actual EHL contacts, i.e., 0 – 1.3 GPa and 20 – 100 °C, respectively. While the sensitivity to pressure and temperature is essentially due to the so-called quantum confinement property, that is, the variation of the PL properties with the size of NPs, additional factors can affect the response of the NPs via some competing processes to the PL. An important one is the energy transfer between differently-sized NPs. Energy transfer efficiency is affected by the distribution of distance between NPs in space and time, and therefore, parameters such as NPs concentration and lubricant viscosity are likely to have some influence on the response of NPs. In order to minimize the influence of these factors, interaction effects were minimized by reducing NPs concentration. In the future, pressure and temperature calibration curves obtained with the selected concentration will be used to carry out measurements in an EHL tribometer, in which pressure, temperature and shear stress can vary simultaneously.